optical ct

Optical Measurement of Currents in Power Converters

SASCHA LIEHR

Masters Degree Project in Electrical Measurement Technology report no. XR-EE-MST 2006:001 Stockholm, Sweden 2006

Optical Measurement of Currents in Power Converters

Masters thesis projectSascha Liehr Supervisor: Hans Sohlstrm

Microsystem Technology Group School of Electrical Engineering Royal Institute of Technology Stockholm, March 2006

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AbstractConventional current measurement in high-voltage and high-EMI (Electromagnetic Interference) environments generally require complex devices due to the necessary insulation and shielding of the device and the signal line. This paper investigates the possibility of instead using a Faraday effect-based opto-magnetic field sensor technique for fault detection in IGBT-switched current lines. Firstly, possible techniques, optical and non-optical, are reviewed with a special focus on optical sensing techniques. An optical sensing technique using a highrotation Faraday film sensor is chosen. Then a FEM simulation of the magnetic field pattern encompassing a parallel conductor geometry is conducted and its favourable results on the magnetic field pattern are presented. Characterization and sensitivity determination of a present Faraday YIG sensor are conducted. The favourable magnetic field behaviour predicted by the simulation is then confirmed in experiments. The sensor electronics have been redesigned and electronic signal processing circuitry for failure handling has been added. Finally, first tests in an application-similar set-up with switched currents were conducted. The proposed sensing technique gave promising first results for reliable and instant current fault detection in high-EMI environments.

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Contents1. Introduction ................................................................................................................. 1 1.1. Motivation and Specification of the Problem.................................................... 2 2. Investigation of applicable Techniques ...................................................................... 4 2.1. Non-Optical Current Transformers .................................................................. 4

2.1.1. Current Transformer .............................................................................4 2.1.2. Rogowski Coil.......................................................................................5 2.1.3. Search-Coil Magnetometer ...................................................................6 2.1.4. Flux-Gate Magnetometer ......................................................................7 2.1.5. Shunt .....................................................................................................7 2.1.6. Hall Sensor ............................................................................................8 2.1.7. Magnetodiode........................................................................................8 2.1.8. Magnetotransistor..................................................................................9 2.1.9. Magnetoresistor...................................................................................10 2.1.10. SQUID Magnetometer ......................................................................11 2.1.11. Optically Pumped Magnetometer .....................................................11 2.1.12. Nuclear-Precession Magnetometer ...................................................11 2.1.13. Conclusion of the Applicability of Non-Optic Magnetic Field Sensors ..........................................................................................................112.2. Optical Current Transformers (OCTs)........................................................... 12

2.2.1. Introduction .........................................................................................12 2.2.2. OCTs based on the Faraday Effect .....................................................15 2.2.2.1. Explanation of the Faraday Effect................................................15 2.2.2.2. Magnetic Field Sensing ................................................................23 2.2.2.3. Magnetic Concentrator with Optical Measurement .....................27 2.2.2.4. Bulk Optics...................................................................................29 2.2.2.5. Optical Fibre Sensing Elements ...................................................31 2.2.2.6. Unlinked Type..............................................................................34 2.2.3. Interferometric Principles ...................................................................37 2.2.4. OCTs based on Bragg Gratings ..........................................................39 2.2.5. Micromechanical Sensors with Optical Readout ................................42 2.2.6. Other Optical Current Sensing Principles...........................................44 2.2.7. Conclusion Optical Current Transformers..........................................442.3. Conclusion of the Technology Investigation ................................................... 45

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3. FEM Simulation of the Magnetic Field Pattern ...................................................... 47 3.1. Introduction ....................................................................................................... 47 3.2. Normal Working Condition.............................................................................. 49 3.3. Case of Failure ................................................................................................... 53 3.4. Conclusion .......................................................................................................... 57 4. Experiments ............................................................................................................... 59 4.1. The Sensors ........................................................................................................ 59 4.2. Characterization of the Sensors ....................................................................... 60

4.2.1. Sensitivity............................................................................................60 4.2.2. Temperature Behaviour.......................................................................65 4.2.3. Modification of the Electronics...........................................................67 4.2.4. Noise ...................................................................................................70 4.2.5. Characterization of the Sensor Sensitivity ..........................................704.3. Validation of the Simulation Results ............................................................... 77

4.3.1. Measurement Set-up ...........................................................................77 4.3.2. Conclusion of the Simulation and Measured Results .........................814.4. Phase Shift Measurements................................................................................ 81 4.5. Current Fault Measurements........................................................................... 83

4.5.1. Test Conductor Set-up ........................................................................83 4.5.2. Magnetic Field Measurement Results.................................................83 4.5.3. Design of the Detection Electronics....................................................85 4.5.3.1. Delay measurement of the detection electronics..........................88 4.5.4. Current fault detection ........................................................................895. Conclusion ................................................................................................................. 91 6. Outlook....................................................................................................................... 92 Acknowledgements ........................................................................................................ 93 References...................................................................................................................... 94 Symbols and Abbreviations ......................................................................................... 101 List of Figures.............................................................................................................. 104 List of Tables................................................................................................................ 108

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Chapter 1

Introduction

1. IntroductionThe electrical power industry is an important and growing branch of industry. Not only in electrical power plants, power converters and transformer stations, but also in industrial high-power applications metering, monitoring and control of high currents is essential. The main challenge with conventional current transformers in high-voltage environments is the need for safe separation between the main circuit carrying the current to be measured and the control circuit in which the measured signal will be utilized. Therefore, the sensor has to be galvanically insulated. Another challenge for measurements in high-current and high-field environments is the influence of EMI (Electromagnetic Interference) on the sensor and its signal. Consequentially, there is a need to shield the sensor and the signal line from EMI in order to get a reliable signal. Shielding and insulation however, results in complex and massive structures that make conventional current transformers in high-current and high-voltage applications very costly. There is a need and trend to develop alternative sensing technologies to overcome these obstacles. Different approaches, predominantly using optical sensing and/or signal transmission techniques, have been developed during the last 20 years. There is potentially a large market for this technology but commercial success is still being waited for. The thesis work presented here arose in this context. A fast and reliable current sensor technique to immediately detect a current fault in a conductor, rather than a precise current metering sensor is investigated. The project is launched by the company ABB and is accomplished at the Royal Institute of Technology (KTH) in Stockholm. The sensor technology part of the project is the subject of my thesis and is carried out at the Microsystem Technology research group at KTH. The purpose of the work is to find a suitable technique in order to reliably and instantly (within 3s) detect a current fault in a high-voltage environment with a high level of Electromagnetic Interference. In chapter 2, possible techniques with the focus on optical techniques are described, and one is chosen. A finite element method simulation of the magnetic field encompassing the conductor and its results are presented in chapter 3. Some first experiments with the chosen sensing technique and tests in an application-similar set-up and environment are presented in chapter 4.

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Chapter 1

Introduction

1.1. Motivation and Specification of the Problem In electrical high power converters and other high-power applications, IGBTs (Insulated Gate Bipolar Transistors) are widely used to rapidly switch large currents at high voltage potentials. IGBTs are capable of switching high currents (>3000A) at voltages of more than 2500V. These devices must be connected in series in order to provide sufficient voltage handling capability. In such designs, the failure mode of the series connected devices becomes a crucial consideration. One solution is to utilize IGBT components with encapsulation, e.g. ABB StackPack, that exhibit short-circuit failure mode. Standard industrial IGBT modules on the other hand may be problematic because the bond wires may burn causing internal arcing in the device. However, it is advantageous to use standard industrial type IGBT devices as they represent the main-stream component type, which are economical and available from several manufacturers. It has been proposed by ABB to implement standard IGBT devices in series connection (>10) of parallel pairs of devices in a stack as shown in Fig. 1.

Fig. 1: Proposed geometry with parallel IGBTs

In normal operation, the devices share the total current and both devices perform switching according to the desired PWM (Pulse Width Modulation) pattern. If one device fails and stops conducting, a sensor should immediately detect that fault and command the remaining parallel device to conduct continuously. As switching losses are then eliminated in the device, it will be capable of conducting the total current in one device. For that case, additional IGBT stages are installed in the stack to prevent excess voltage. The stack can remain in duty depending on how many additional stages are installed in the stack. The defect IGBTs only have to be exchanged at the next check routine. Using this technique is expected to be considerably cheaper than using the expensive StackPacks. The company ABB has launched a project at KTH to test the proposed technique. A small-scale set-up with an IGBT firing control is being built at the department of 2

Chapter 1

Introduction

electrical engineering within the thesis work of Martin Skoglund [Skog06]. The setup will be able to switch currents up to 180A at voltages of 500V. An essential part of the technique is to reliably and rapidly detect a fault of one of the IGBTs to command the remaining device to conduct continuously. This current fault detection is the objective of this work. A desired maximum time delay for the detection of a fault is 3s. A suitable sensor technology has to be found to detect a fault in the objective of the actual application. The expected currents in the full-scale application are in the range of several kA and will be switched at potentials of 10-50kV. Hence, a considerable level of EMI is expected. The sensor has to be immune to EMI and also be insulated from the high voltage potentials. An appropriate sensing technique will have to be chosen, tested and integrated in the mentioned set-up. An optical sensing technique using the Faraday effect seems a promising option and will be focused on. The measurement part of the project is therefore conducted at the Microsystem Technology research group of KTH due to competences in magneto-optic field sensing.

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Chapter 2

Investigation of applicable Techniques

2. Investigation of applicable TechniquesIn this chapter, principles for measuring an electric current are presented and evaluated in terms of applicability to the purpose of sensing a current fault in the proposed geometry under the given demands. Most current sensor principles are based on some kind of magnetic field measurement technique. Therefore magnetic field sensing techniques are also investigated for current metering purposes. Thus the term current sensor is somewhat extended in this work also covering magnetic field sensors as potential current sensors. The following investigation is divided into two groups: conventional, or non-optical current transformers and optical current transformers. Considering that the capabilities of optical principles are interesting and promising, the following review has a stronger focus on optical technologies. 2.1. Non-Optical Current Transformers 2.1.1. Current Transformer The most-used device for measurements of alternating currents in electrical highand medium-voltage networks is the CT (Current Transformer). A current transformer transforms the current down to a reasonable level and provides an isolation barrier between the primary winding and the secondary winding at ground potential. The primary current of the transformer is translated to the secondary current (I1=nI2) by the turns ratio n and I2 is measured by an ampremeter or other conventional methods. The secondary winding, or measurement winding, have to be isolated from high voltages to prevent short circuits and the resulting heat has to be dissipated. For that reason the transformers are often filled with oil which causes a risk of explosion. These devices are reliable and have long life cycles but become very costly and massive with increasing voltages. An example of a current transformer with indication of its size is shown in Fig. 2.

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Chapter 2


Fig. 2: Conventional ABB transformer for high voltages [ABB06]

The current transformer is based on Ampres law, whereby the line integral of the r magnetic field H along any closed path equals the enclosed current I. r r H dl = I Equation (1) Most current transformers consist of a ferromagnetic core entirely enclosing the conductor. Hence, measurements are independent of the position of the current carrying conductor in the core. Measurement windings are wound around the core and a voltage is induced according to Faradays induction law. With low resistance in the measurement winding, the resulting current is proportional to the primary current and cancels most of the field in the core. At low frequency however, the driving voltage can no longer create a current that is proportional to the primary current. Also the core will be saturated. That implies that the current transformer can only be used for AC measurement. For DC measurements, more complex devices with Hall elements are often used. 2.1.2. Rogowski Coil A Rogowski coil is a core-less coil toroidally placed around a conductor forming a closed loop, Fig. 3.

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Chapter 2


Fig. 3: Scheme of a Rogowski coil [Xiao03]

It is also based on the induction law. A voltage proportional to the rate of change of the current is induced into the uniformly wound coil with constant cross-sectional area. Due to the absence of a magnetic core, the sensor shows good linearity, no saturation, as well as high current capability (up to 100kA) and bandwidth (0.1Hz to 100MHz) [Xiao03]. Similarly as for other induction-based sensors, the Rogowski coil can not detect currents of low frequency. In contrast to the conventional transformer with a closed iron core, the output is not completely independent of the primary conductor position. There are also problems with reproducibility and accuracy. 2.1.3. Search-Coil Magnetometer The search-coil magnetometer is also based on Faradays induction law. It typically consists of an iron magnetic core and a coil wound around it. When the magnetic flux through the coiled conductor changes, a voltage proportional to the rate of change is induced in the coil and can be measured between its leads. This sensor can measure fields over a very wide range from 1pT to 1kT depending on the design of the magnetic core. According to the induction law, only the change of the magnetic field can be detected. The useful frequency range of this sensor is typically between 1Hz and 1MHz [Lenz90], but also static fields can be detected with a search-coil magnetometer when the coil is rotated in the field. A principle of the search coil magnetometer is shown in the figure below.

Fig. 4: Scheme of a search-coil magnetometer [Lenz90]

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Chapter 2


2.1.4. Flux-Gate Magnetometer The most common type of a Flux-Gate magnetometer consists of two coils, a primary and a secondary, wrapped around a ferromagnetic core. The magnetic induction in the core changes with the external magnetic field. A signal, for example 10kHz, applied to the primary coil causes the core to oscillate between saturation points. The secondary winding outputs a signal coupled to the primary signal by the iron core. This signal is influenced by any change of core permeability (slope of the B-H curve) and appears as variation of the amplitude at the secondary output. The value for the magnetic field strength can be obtained by using a phase-sensitive detector and following signal processing. Phase-sensitive detection is a useful technique to recover small signals that are obscured by larger and/or background signals with the help of a reference or modulation signal.

Fig. 5: Fluxgate magnetometer operation [Caru98]

A fluxgate magnetometer can precisely measure direction and magnitude of constant or changing magnetic fields at sensitivities down to 1nT. The bandwidth however, is limited to some kHz and the dynamic range covers fields from 1nT to 1mT [Xiao03]. The Fluxgate magnetometer is therefore not suitable for fault detection. 2.1.5. Shunt Shunts are low resistance sensing elements that are directly inserted in the main current path. They operate on the principle of the Ohmic voltage drop and are suitable to measure currents. Shunts are relatively cheap and can be used to measure direct currents and alternating currents up to tens of MHz [Xiao03]. But since they have to be integrated directly into the circuit, efficiency decreases, especially at high currents and low voltages. Moreover, the output voltage is directly connected to the current to be measured. Shunts can therefore normally not be used with high voltage.

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Chapter 2


2.1.6. Hall Sensor This sensor is based on the Hall effect, discovered by Edwin H. Hall in 1879. He found a potential difference (Hall voltage) on the sides of a thin sheet of conducting material in a magnetic field perpendicular to the surface when a current flows along the sheet, Fig. 6.

Fig. 6: Scheme of a Hall Effect Sensor [Lenz90]

This voltage is the result of the Lorentz force that every electron that moves through a magnetic field experiences. This force is perpendicular to both the magnetic field and the direction of motion of the electron. Electrons moving in the sheet perpendicular to the magnetic field will therefore be deflected to one side of the sheet resulting in the Hall voltage. Hall elements made of semiconductors have a much larger effect than those made of metallic conductors. Nowadays, Hall sensors are produced at low costs due to standard CMOS technologies and are mostly made of silicon or III-V semiconductors. They have good temperature characteristics, bandwidths from static fields up to 100MHz, resolutions of 100nT and a dynamic range from 50T to 30T [Maci00]. 2.1.7. Magnetodiode A magnetodiode is basically a semiconductor diode where the p-region is separated from the n-region by an area of undoped silicon. The silicon layer lies between, for example, a Sapphire substrate and a SiO2-layer, Fig. 7.

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Chapter 2


Fig. 7: Structure of a Magnetodiode [Lenz90]

When a potential is applied between the p- and n-region, holes and electrons are injected into the silicon and move in opposite directions resulting in a current flow. In absence of a magnetic field, mainly recombination contributes to the resistance, especially at the surface Si-SiO2 and Si-Sapphire. When a magnetic field is applied, the electrons and holes are diverted in the same direction to one of the surfaces. Since the possibility to recombine at the Si-Sapphire surface is much greater than at the Si-SiO2 surface, the resistance is higher when the charge carriers are diverted towards the Si-Sapphire surface. Magnetodiodes have higher responses than Hall elements [Lenz90] at similar bandwidths and resolutions up to 0.5T, but they suffer from a high temperature dependency [Here93]. 2.1.8. Magnetotransistor The magnetotransistor is a version of an npn-transistor. Like a transistor, it consists of an n-doted emitter separated by a p-doped base from the n-doped collector. The difference is that there are two collectors instead of one, Fig. 8.

Fig. 8: Principle of a Magnetotransistor [Lenz90]

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Chapter 2


Without any external magnetic field, equal numbers of charge carriers reach the two collectors. When a magnetic field is applied perpendicular to the direction of motion of the charge carriers, they will be deflected toward one collector or the other. The two collector voltages are fed to a differential amplifier, whose output is proportional to the applied magnetic field. Magentodiodes are about 100 times more sensitive than Hall elements, have a bandwidth up to 1MHz [Lando] and are based on more standard fabrication technologies (CMOS) than the magnetodiode [Lenz90]. 2.1.9. Magnetoresistor The MR (MagnetoResistance) effect describes the relative change of resistance of a conductor at the presence of a magnetic field. According to the orientation of the magnetic field vector and the electric current vector, the effect is named either longitudinal MR effect (magnetic field and current parallel) or transversal MR effect (magnetic field and current perpendicular). Several effects are known today: The AMR (Anisotropic MagnetoResistance) effect occurs in magnetic materials such as Permalloy (Ni-Fe alloy). This material is given an easy direction in the direction of the current. When a magnetic field is applied, the resistance will change with the angle between the field and the direction of the current. When the magnetic field is applied perpendicularly to the current, the magnetic orientation will rotate in direction of the field. This rotation is dependent on the magnitude of the field. Rotation results in higher resistance since electrons that move in direction of the magnetization have a higher probability to be scattered. The GMR (Giant MagnetoResistance) effect occurs in stacks of very thin layers of Fe and Cr with antiparallel magnetization of neighbouring layers. When the magnetization of the single layers is rotated to parallel magnetization due to an external magnetic field, the resistance will significantly change. Resistance changes of up to 50% are possible compared to the AMR effect where the resistance changes 3% at maximum. Fields from static to 5MHz can be measured [Xiao03]. AMR and GMR are both used as read/write heads in hard disc drives. The CMR (Colossal MagnetoResistance) effect is the strongest magnetoresistive effect known. It occurs mostly in manganese-based perovskite oxides and changes the electrical resistance of the material in the order of magnitudes at the presence of a magnetic field. This relatively newly found effect is subject to current research. Typically, magnetoresistors for field sensing have a dynamic range of B=1T to B=1T, a resolution of 10nT and a bandwidth from dc to 10MHz [Maci00]. Magnetoresistors do however, not give information about the direction of the field.

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Chapter 2


2.1.10. SQUID Magnetometer The most sensitive magnetometer is the SQUID (Superconducting Quantum Interference Device) with a resolution of fT. The SQUID is based on the Josephson effect that arises in superconducting rings with a weak link (thin layer of insulator). Josephson discovered, that a current can flow through the weak link as an oscillating function of the magnetic field intensity. Typically, the ring is inductively coupled to a radio frequency circuit that both, supplies a known bias field and serves as detector output. Changes in the ring current alter the resonant frequency of the circuit. The change of the magnetic field can be measured by counting the maxima and minima. If there are two weak links in the circuit (DC SQUID), the voltage difference between them can be measured directly. This voltage also periodically changes with the change in magnetic field. [Lenz90] SQUIDs are very precise and expensive magnetic field sensors and need liquid Nitrogen cooling to allow superconductivity. They typically have a dynamic range from B=1pT to B=0.1mT, a resolution of 100fT and a low bandwidth from dc to 5Hz [Maci00] and are therefore not suitable for current fault detection. 2.1.11. Optically Pumped Magnetometer The optically pumped magnetometer is based on the Zeeman effect and precisely measures a scalar magnetic field. The measurand is the resonance frequency of a radio frequency source at which the electrons in a Caesium or Helium vapour change their spin angular momentum. The energy required to flip the electron spins, and thus the radio frequency, depends on the strength of the magnetic field. This sensor however, is relatively large, has a high power consumption, a sensitivity range from B=1nT to B=0.1mT and a low bandwidth from dc to 5Hz [Lenz90], [Maci00], which makes it unsuitable the purpose of this work. 2.1.12. Nuclear-Precession Magnetometer This magnetometer exploits the response of protons to a magnetic field in a hydrocarbon fluid, such as benzene. The precession frequency of the protons at a present magnetic field is proportional to the magnetic field strength and is picked up by a coil. Again, this magnetometer has a low sensitivity range (10pT to 100T) and is rather expensive. 2.1.13. Conclusion of the Applicability of Non-Optic Magnetic Field Sensors All presented sensing principles suffer from electromagnetic interference that is expected to be at a very high level in the application environment. Especially Lorentz force-based sensors, such as Hall element, magnetodiode and magnetotransistor are sensitive to EMI due to their typically low signal levels. The same holds for the search-coil magnetometer. The flux-gate magnetometer suffers 11

Chapter 2


from its low bandwidth. Shunts are not suitable owing their lack of insulation and the magnetoresitor does not detect the sign of the field. Costly high-precision magnetometers such as the SQUID, the optically pumped magnetometer and the nuclear precession magnetometer are not suitable due to their cost, dynamic range and low bandwidth limitations. Concluding this, there are no real alternatives to the conventional current sensors, such as current transformer and Rogowski coil among the presented non-optical principles. 2.2. Optical Current Transformers (OCTs) 2.2.1. Introduction A logical step to overcome the problems caused by electromagnetic interference on the sensor signal is to use a signal transmission that is immune to electromagnetic fields. Optical signal transmission using optical fibres are the best solution for that purpose. Normally, the optical signal is not influenced by electromagnetic fields. In a suitable designed sensor however, several properties of the light that is used as the signal carrier can be influenced, e.g. intensity, state of polarization, spectral properties and phase delay. Ideally, the sensor signal is directly generated by the interaction of the magnetic field with the sensor medium. Several such magnetooptical effects are known to occur in magneto-optic active materials. The most interesting magneto-optic effect in transmission for magnetic field sensing is the Faraday effect. It causes the polarization of linearly polarized light to rotate at the presence of a magnetic field when propagating in a material exhibiting the Faraday effect. It is widely used for magnetic field sensing and will be explained in detail in chapter 2.2.2.1. Another effect in transmission is the Zeeman effect, which causes the split of a spectral line into several components at the presence of a magnetic field. The most important effect in reflection is the MOKE (Magneto-Optic Kerr Effect). It occurs for example in thin magnetized metal films and exists in three different geometries: The PMOKE (Polar Magneto-Optic Kerr Effect) occurs with the magnetization direction perpendicular to the surface of the film, the LMOKE (Longitudinal Magneto-Optic Kerr Effect) with the magnetization in the film plane and also in the plane of incidence. In TMOKE (Transverse Magneto-Optic Kerr Effect) geometry, the magnetization direction is in the film plane but perpendicular to the plane of incidence. These effects are for example used in magneto-optic disks for reading the data with help of the Kerr effect but are not suitable for magnetic field measurements because of their non-linearity. Other magneto-optic effects are the Voigt effect, the Cotton-Mouton effect and the Majorana effect.

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Chapter 2


But not only direct magneto-optic conversion is used as a sensor principle, also different magneto-mechanic-optical or magneto-electric-mechanic-optical transducers have been presented for magnetic field sensing purposes. Therefore, OCTs (Optical Current Transducers) are here defined as sensors that directly or indirectly use optical sensing methods to measure electrical currents. The advantage of direct magneto-optic transducers using opto-magnetic active materials is the absence of additional disturbance variables caused by mechanical or electrical sensor parts such as hysteresis, saturation, induction, temperature influence and damping. Over the last 30 years, numerous current measurement systems based on optical devices have been developed. OCTs have numerous potential advantages over conventional current transformers (CTs), depending on their sensor principle. Potential advantages are: immunity to electromagnetic interference (EMI) high electrical insulation large bandwidth potentially high sensitivity ease in signal light transmission being compact and lightweight potentially low-cost no danger of explosion ease of integration into digital control systems no saturation hysteresis-free passive measurement (dependent on the principle)

However, in most fields of application, OCTs have to compete with mature technologies. Consequently, many customers simply desire sensor systems having good performance with reasonable price (except for special uses) and choose conventional technologies. Therefore, only few optical devices, mainly developed by the customer itself, i.e. electric power companies and electric power distributors, or major industrial companies, are field-tested and used. Optical fibre sensors have been studied extensively over the last years. Fig. 9 and Fig. 10 show the distribution of measurands and measurement technologies in optical fibre technology based on the 15th Optical Fiber Sensors Conference 2002 [Lee03].

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Chapter 2


Fig. 9: Distribution of papers according to measurands [Lee03]

Fig. 10: Distribution of papers according to used technologies [Lee03]

The diagrams shown above illustrate that there is a big interest in optical current/voltage sensors. Fibre grating technologies have a great share of the publications (Fig. 10) partly because this technology was at an intense research phase at that time. However, OCTs not only utilize fibre sensing elements. Also other geometries and principles or hybrid devices have been proposed. Optical Current Transducers will in the following be divided into five main groups: OCTs based on the Faraday effect Interferometric principles OCTs based on Bragg gratings Micromechanical sensors with optical readout Other optical current sensing principles 14

Chapter 2


These main groups will be presented in the following chapters. This classification is however, not accurately defined but rather used to give an overview over the existing principles. 2.2.2. OCTs based on the Faraday Effect 2.2.2.1. Explanation of the Faraday Effect The Faraday effect is a magneto-optical effect that causes a change of the state of polarization of light. Thus, the concepts of polarization and birefringence are briefly explained to give a better understanding of the Faraday effect and the problems arising in sensor applications. Polarization Light can be regarded as a plane wave and, like all electromagnetic waves, has the electric and magnetic fields perpendicular to the direction of propagation. r Conventionally, only the electric field vector E is described when speaking about polarization, since the magnetic field vector is always perpendicular and proportional to it. The two components of the electric field vector are defined as x and y components. For a simple harmonic wave, these components vary sinusoidally with the same frequency. However, their amplitude and phase might differ, compare Fig. 11.

Fig. 11: Linear, circular and elliptic polarization [Wiki06]

Special cases of polarization are linear polarization, which only occurs when both components have the same phase (Fig. 11 a)) and circular polarization which supposes that the two components are exactly 90 out of phase and have exactly the same amplitude, Fig. 11 b). The direction of rotation of the vector depends on which of the two components is 90 ahead of the other one. These cases are called right-hand circular polarization and left-hand circular polarization. All the other 15

Chapter 2


cases, where the two components differ in amplitude or phase are called elliptical polarization, Fig. 11 c). Birefringence Birefringence, or double refraction, is the decomposition of a ray of light into an ordinary ray and an extraordinary ray when it passes through an optically anisotropic material, depending on the state of polarization of the light. One can distinguish between two different kinds of birefringence: linear birefringence and circular birefringence. Linear birefringence occurs in an optically anisotropic material with different speeds of light propagation for different geometrical axes due to material anisotropy or geometrical constraints in an optical waveguide. The difference of the corresponding indexes of refraction n = nslow nfast is the linear birefringence. Linear polarized light passing through a linear birefringence medium experiences a phase difference in of = 360 n l 0Equation (2)

where l is the length of the light path and 0 is the wavelength of the light. This phase difference causes a change of the polarization state and is for example used to change the state of polarization (/4-plate). This effect may also occur in optically isotropic materials due to mechanical stress and electric and magnetic fields. Circular birefringence occurs in a material where the speed of propagation of the light is different for left-hand polarized and right-hand polarized light. The material is then called an optically active material. The difference of the two different indices of refraction, nc, is the circular birefringence n c = n right n left . Circular birefringence rotates the polarization of linearly polarized light by the angle=360 n c l 2 0Equation (3).

In addition to the magnetic circular birefringence, a linear birefringence can be induced by a magnetization perpendicular to the light propagation direction. This effect is called Voigt or Cotton-Mouton effect, whereas the latter is often denoted to a molecule orientation effects to a magnetic field in fluids. There may also be a magnetic field dependent difference in optical absorption between the linear or the circular polarization states: MLD (Magnetic Linear Dichroism) and MCD (Magnetic Circular Dichroism).

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Chapter 2The Faraday effect


The Faraday effect is named after Michael Faraday who discovered this phenomenon in 1845. It describes the rotation of polarisation of light propagating in the direction of a magnetic field. When a beam of light is sent through a material exhibiting the Faraday effect, the polarisation of the light will be rotated by the angle in dependency of the magnetic field strength parallel to the light.

Fig. 12: The Faraday effect [Sohl93]

The Faraday effect is proportional to the magnetisation of the material, r = Lk M dl Equation (4) where is the polarisation rotation, M is the magnetisation, l is the length of the light path and k a constant dependent on the propagating material, the wavelength and the temperature. In paramagnetic and diamagnetic materials, the magnetisation and thus, also the polarisation rotation is practically proportional to the magnetic field strength [Sohl93]. The rotation can then be described in terms of the magnetic field strength H and the Verdet constant V, r = LV H dl Equation (5) The Verdet constant V is the specific rotation of a material and is defined as the angle over the magnetic Field times the length (/Tm).V= B lEquation (6)

V is determined by the magnetic properties of the material. B is the component of the magnetic flux density parallel to the light propagation direction. The Faraday effect arises from the interaction of the electron orbit and the electron spin with the magnetic field. The general principle can be understood as right17

Chapter 2


handed and left-handed circularly polarized light causing charges in a material to rotate in opposite senses. Each polarization therefore produces a contribution to the orbital angular momentum with opposite sign. A magnetic field gives rise to a spinpolarization along the magnetic field direction and the spin-orbit interaction then leads to an energy contribution for the two circular polarizations having the same magnitude but with opposite sign [Blun01]. This leads to right-handed and lefthanded polarizations having different refractive indices in the material. A linearly polarized wave can be seen as the sum of two circularly polarized waves with equal amplitude but opposite direction of rotation. As these two waves propagate with different speeds through the material, they will acquire a phase difference proportional to the travelled distance. In terms of their sum, these two beams, when they emerge, have a phase lag between them implying that the emerging beam has a rotated plane of polarization by an angle which is equal to half the phase change. The superposition of the left- and right-hand polarized components can be seen in Fig. 13.

Fig. 13: Polarization before and after polarization rotation

This effect is non-reciprocal, meaning a light beam passing a medium twice in opposite direction acquires a net rotation twice that of a single pass. It should be noticed that according to the material, the Verdet constant is temperature- and wavelength-dependent. Faraday Materials and Geometry A difference between on one hand diamagnetic and paramagnetic materials, and on the other hand ferrimagnetic and ferromagnetic Faraday materials has to be made. Diamagnetic (e.g. SF-57, SiO2, BK-7) and paramagnetic (e.g. TGG (Terbium Gallium Garnet), FR-5) Faraday materials have specific, but relatively low rotations. In contrast, ferrimagnetic and ferromagnetic materials have Faraday rotations orders of magnitude higher than diamagnetic and paramagnetic materials. The most

18

Chapter 2


prominent ferrimagnetic materials are RIGs (Rare Earth Iron Garnets). YIG (Yttrium Iron Garnet), Y3Fe5O12, and substituted YIGs are the most widely-used RIGs. Strictly speaking, in all materials, the Faraday rotation is proportional to the magnetization component along the direction of the optical propagation. For the diamagnetic and paramagnetic materials, the shape of the modulator element is not important in determining the magnetization. Also the magnetization is linearly dependent on the applied field, and equation (5) can be used. In ferri- and ferromagnetic materials the picture is more complex. In these materials, volumes of equal direction of magnetization, the so-called magnetic domains, will form. Thin transition regions, the Bloch walls, separate these domains of different magnetization directions. The domain size is determined by the magnetostatic energy balance that depends on the material properties and the sample geometry. In thin bulk samples and epitaxially grown films with uniaxial anisotropy perpendicular to the surface, the domains can form two-dimensional patterns extending through the entire thickness of the film, Fig. 14.

Fig. 14: Two-dimensional domain pattern: schematic and measured (YIG film, no field) [Sohl43]

These iron garnet films are sometimes also referred to as uniaxial garnet films. Larger bulk iron garnet crystals however, exhibit a more complex three-dimensional domain structure causing a higher non-linearity in the net Faraday effect and are less interesting for magnetic field sensing purposes. Hence, the definition of the Verdet constant V is not always suitable. Diamagnetic and paramagnetic materials have a specific rotation in every point of their volume and this rotation is independent of their geometry. The rotation for diamagnetic and paramagnetic materials can therefore be described with the Verdet constant V. Ferrimagnetic materials however, do not have a specific (constant) rotation, they exhibit a domain structure where all domains are fully saturated in different directions. Moreover, the rotation of those materials also depends on the geometry of the medium. It is therefore actually wrong to speak of a Verdet constant for those materials, many authors however do.

19

Chapter 2


The ferri- and ferromagnetic materials, such as RIGs, exhibit a very high Faraday rotation and are therefore interesting for sensor application. Iron garnets have widely been used for bubble memory and displays. At present, the main application is in optical isolators. These materials have therefore been studied extensively and are commercially fabricated. Bulk RIG crystals are grown similarly to silicon using the Czochralski technique. A YIG seed crystal, for example, is dipped into the melt and is slowly pulled upwards under constant rotation. Dopants can be added to the melt. These crystals normally exhibit a cubic magnetic anisotropy. Uniaxial anisotropy with stable direction of spontaneous magnetization, or easy axis, can occur in thin bulk samples and epitaxially grown garnet films, compare Fig. 14. Thin garnet films with a desired uniaxial anisotropy perpendicular to the film plane are commonly grown epitaxially on a GGG (Gadolinium Gallium Garnet) substrate. The common process is LPE (Liquid Phase Epitaxy), but also SPE (Solid Phase Epitaxy) [Jang04] has been proposed recently. An external magnetic field applied perpendicular to such a film causes the domains with magnetization in direction of the field to grow at the expense of the other domains, Fig. 15.

Fig. 15: Domain wall motion in thin films, schematic and recorded [Sohl93]

The resulting net polarization rotation can be approximated using the area ratio between the two kinds of domains. The high frequency behaviour of these domains depends on domain wall damping or domain wall resonance in the same way as the magnetic susceptibility and is in the range of several hundred MHz for low damping film materials [Wolf92]. A magnetic field applied in the plane of such a film rotates the magnetization of both types of domains in direction of the applied field equally, Fig. 16. Thus, magnetization rotation, as opposed to domain wall motion is the dominant response. These films can be employed in an optical waveguide geometry 20

Chapter 2


[Deet93/1]. Fig. 16 shows a possible planar waveguide geometry and the magnetization rotation of the domains at an applied magnetic field.

Fig. 16: Wave guide structure

While the polar geometry with a wide area of light-material-interaction gives a good average of the Faraday effect on the domain pattern in the film, the waveguide geometry gives only a one-dimensional average of the few domains along the path of light in the film. This geometry therefore gives a less linear signal. Thus, different principles have been proposed to increase the interaction length or to linearize the effect by using a bias field [Sohl91]. Also other problems, such as precise mode coupling to the planar waveguide arise with this geometry. Only a few principles have been proposed for sensor application. The rotation of magnetisation of the domains however, is a faster process than domain wall movement and therefore enables higher bandwidths of the planar geometry up to at least 1GHz [Deet93/1] respectively to 600MHz for a lowdamping film exhibiting domain wall movement [Wolf92]. Domain wall rotation also shows less hysteresis than domain wall motion [Deet93/1] and is not effected by possible imperfections in the material such as lattice dislocations [Sohl93] since no domain wall motion occurs. The most commonly used Faraday materials are, on one hand diamagnetic and paramagnetic glasses (e.g. SF-57 and FR-5) or TGG with relatively low Verdet constants, and on the other hand rare earth iron garnets (RIGs) and ferromagnetic materials with considerably higher Faraday rotations. Diamagnetic and paramagnetic glasses are transparent over a wide range of wavelength and are used in bulk form owing to their low Verdet constants. RIGs such as YIG have a limited transmission spectrum in the near-infrared region, Fig. 17. They are transparent, i.e. exhibit no noticeable loss in application, for wavelength of >1.1m [Sohl93].

21

Chapter 2

Investigation of applicable TechniquesI = I 0 exp( t )

I intensity of light t penetration depth absorption coefficient

Fig. 17: Absorption of light in YIG depending on [Paro84]

However, due to their high Faraday rotation, they can be used in thin film geometry and have therefore also been used at shorter wavelengths. Table 1 gives an overview over the magnitude of Faraday rotation for the most common Faraday materials.Table 1: Faraday rotation for some common materials [Deet93/2]

Rare earth ions and other dopants can be added to improve the temperature behaviour of YIG crystals. Also the Faraday rotation can be altered by adding mainly rare earth ions. Numerous compositions with several substitutes have been studied. Bi0.98Gd0.92La0.03Y1.07Fe4.72Ga0.28O12 films for example achieve a good temperature linearity and high rotation in sensor application [Itoh99]. Bismuth is commonly added to achieve higher rotations. The Faraday rotation of RIGs is thus very much dependent on the substitutes and their concentration, but also the temperature and the wavelength of the light, Fig. 18.

22

Chapter 2


Fig. 18: Wavelength-dependency of the Faraday rotation for common materials [Dona88]

2.2.2.2. Magnetic Field Sensing The magnetic field in a Faraday medium can be measured by determining the rotation of polarization , that occurs after a linearly polarized light beam passed the Faraday medium. This can be done by measuring the intensity of the light beam after passing a second polarizer (or analyzer). The intensity of this light beam is a function of the angle of rotation and thus the magnetic field strength. (Fig. 19)

Fig. 19: General form of a Faraday current sensor [Sohl93]

23

Chapter 2


Utilizing a folded design with a reflective surface at the end of the Faraday material as shown in Fig. 20, leads to twice the polarization rotation for a given length of the Faraday material.

Fig. 20: Schematic of a folded design giving twice the Faraday rotation

The characteristic of the sensor is determined by the orientation of the two polarizers to each other. The angle between the transmission axes of the polarizers determines by which value the transmitted intensity varies with a varying magnetic field. The angles can be chosen anywhere between 0 and 90 giving the same results for all other quadrants. The resulting intensities can be calculated employing Malus law: considering a linearly polarized light beam incident on a polarizer, its perpendicular component of the beam is blocked. Therefore, the amplitude of the light transmitted by the polarizer isE( ) = E0 cos( )Equation (7)

Hence, the intensity of the transmitted light is given byI ( ) = I 0 cos 2 ( )Equation (8),

where E0 is the electric field vector, and I0 is the intensity of the incident beam [Siro99]. Three angles of orientation of the analyzer to the polarizer are especially interesting for the sensor application, namely 0, 45 and 90. These three cases are shown and discussed in Table 2 to Table 4.

24

Chapter 2Table 2: Characteristics of a 0-Sensor


I = I 0 cos 2 ( )This sensor gives the same signal for positive and negative fields and has a quadratic characteristic for small fields. + This sensor is easy to fabricate since only one polarizer is needed in a folded design - It gives no indication of the direction of the field - Only small signal change for small fields

Table 3: Characteristics of a 45-Sensor

I = I 0 cos 2 ( 45 )This sensor gives different signals for positive and negative fields. + This sensor is has a linear characteristic for a wide range of field + It detects the direction of the field

25

Chapter 2Table 4: Characteristics of a 90-Sensor


I = I 0 cos 2 ( 90 )This sensor gives the same signal for positive and negative fields. + This sensor is relatively immune to drift - It gives no indication of the direction of the field - It gives only a small and non-linear signal for low fields Another possibility to read out the state of polarization is to use a polarization separating prism (Wollaston prism) and two detectors, as shown in Fig. 21. The two orthogonal linearly polarized beams are detected separately. This technique has the advantage that optical losses in the fibres and sensor material can be compensated. The rotation of the polarization can directly be obtained by comparing the two sensor signals.

Fig. 21: Polarization state detection with polarization splitter [Sohl93]

A similar loss compensation has been demonstrated using conventional polarizers, but with light propagation in both directions through the sensor [Holm95]. All Faraday detection principles are fundamentally based on intensity detection. The structure, materials and the path of light of the different Faraday sensors however, differ greatly. The following overview is meant to give a brief and general idea of the major classes of optical Faraday transducers. Some examples for each group are presented.

26

Chapter 2


2.2.2.3. Magnetic Concentrator with Optical Measurement In this approach, a magnetic concentrator encloses the conductor, but instead of this magnetic loop forming the core of a transformer, the field inside the concentrator is measured optically in an air gap, Fig. 22. Due to the air gap, the field in the core is limited and core saturation is avoided. With a larger air gap however, the field in the gap becomes dependent on the position of the current conductor.

Fig. 22: Schematic of Faraday current sensor using a magnetic concentrator

Many sensors using this approach have been proposed. Using multiple-reflection in a glass bulk mounted in a ferromagnetic field concentrator increases the total effective optical path and therefore the sensitivity [Li97], [Yi00], [Yi02]. In order to prevent reflection-induced phase shifts that interfere with the polarization rotation, two methods have been proposed: critical angle reflection and dual-quadrature reflection [Ning95]. One example [Yi00] for multiple critical angle reflections is shown in Fig. 23.

Fig. 23: Faraday glass element showing 7 critical angle reflections [Yi00]

Matsushita Electric Industry (Panasonic) developed a small optical magnetic field sensor using a new garnet composition in 1999 [Itoh99], Fig. 24. This sensor is used in a magnetic core flux concentrator and shows high linearity (1% for alternating fields from 0.3 to 42mT) and temperature stability (2% from 20C to +80C). 27

Chapter 2


According to Forrest et al. [Forr96], this sensor and earlier versions with less linearity have been sold in thousands of units a year to a Japanese utility company for use as a current fault sensor.

Fig. 24: Schematic of OCT probe [Itoh99]

A similar current sensor based on Bi-doped YIG is used for fault section detection in Japanese underground transmission lines [Toya93]. This sensor is mounted in a ring core of laminated silicon steel plates around the conductor in order to measure the electric current. Possible disadvantages of using magnetic core concentrators with an air gap are sensitivities to currents in nearby conductors and dependency on the position of the conductor owing to a not entirely closed concentrator as well as nonuniformity of the magnetic field in the air gap. To overcome these drawbacks, alternative configurations allowing the light to pass through the Faraday material in a direction perpendicular to the magnetic field have been proposed. Using such a structure enables a design with a smaller air gap. Toshihiko et al. proposed a sensor with a transverse configuration of the light beam and the magnetic field in a small YIG bulk in 2001 [Yosh01], Fig. 25. The magnetic field changes the 3D local magnetization according to the domain characteristics in the crystal. The Faraday rotation of the light beam passing in zdirection is direct proportional to the integration of the magnetisation in zdirection over the 3D zone within the YIG crystal.

28

Chapter 2


Fig. 25: Measuring set-up for a transverse configuration in YIG [Yosh01]

In 2003, the same group proposed a similar scheme where a uniaxial garnet film was obliquely inserted in a narrow core gap and the light beam passes through the film in the transverse direction to the core [Yosh03], Fig. 26. The authors claim that in film form, the incident light beam undergoes an equal amount of Faraday rotation independent of the incident angle to the film plane.

Fig. 26: Scheme of a current sensor in transverse configuration [Yosh03]

These transverse schemes [Yosh01], [Yosh03] allow a very high sensitivity and a better insulation from surrounding currents due to a considerably smaller air gap width. Moreover, this transverse scheme is easier to construct than a longitudinal one. 2.2.2.4. Bulk Optics This type of OCT is analogous to an optical implementation of a conventional CT. It consists of a Faraday material completely enclosing the conductor, Fig. 27. Numerous designs have been proposed with light beams encircling the currentcarrying conductor exactly once or several times. These sensors are fabricated from diamagnetic or paramagnetic single-glass blocks with relatively low Verdet constants and do not suffer from problems such as intrinsic birefringence and bendinginduced linear birefringence occurring in optical fibre sensing elements (compare chapter 2.2.2.5).

29

Chapter 2


Fig. 27: Schematic of a Faraday OCT using bulk optics

The first proposed geometry by Takahachi et al. [Taka83] shows a square-shaped sensing element closing the optical path via three reflection corners, Fig. 28. In this sensor, the reflection-induced phase-differences cancel each other through doublequadrature reflections.

Fig. 28: Square-shaped OCT [Taka83]

A triangular-shaped sensing element has been introduced by Chu et al. [Chu92] using critical angle reflections to preserve the state of polarisation, Fig. 29. This design is easier to manufacture but the demand on the angular tolerance is very high (angles for internal critical angle reflection C are within 0.01).

Fig. 29: Triangular sensing element [Chu92]

30

Chapter 2


Later, an openable form emulating a conventional current clamp has been developed [Ning93]. Further designs have been published, including a circular ring using multiple criticalangle reflections of a light beam encircling the conductor five times [Ning91/1] and therefore increasing the current sensitivity by the same factor, Fig. 30.

Fig. 30: Circular geometry with optical path [Ning91/1]

Several bulk OCTs have been developed for commercial application, especially in Japanese power systems. This design however, is sensitive to shock and vibration. 2.2.2.5. Optical Fibre Sensing Elements This kind of current sensor basically consists of an optic fibre wound a number of times around the current-carrying conductor forming a coil, Fig. 31. The fibre itself exhibits a Faraday effect and acts as the sensing material.

Fig. 31: Schematic of an optical fibre sensing element

Although the Verdet constant of a fibre is not very high, a measurable rotation can be achieved with a long fibre wound around the conductor many times. A good approximation of the closed line integral of the field is achieved. These fibres are typically single-mode silica fibres and do not require a precision matching or alignment. The sensitivity can be adjusted by adding dopants to the core or by varying the number of turns. In order to remain the state of polarization in the fibre between the sensing region and the light source/detectors, polarization-maintaining 31

Chapter 2


single-mode fibres are used. These fibres have a core of elliptical cross-section or index of refraction anisotropy introduced by dopants or uniaxial stress. However, the sensitivity of the sensing part of the fibre is influenced by intrinsic and stress-induced linear birefringence due to bending of the sensing fibre or vibrations. Furthermore, since linear birefringence is temperature-dependent, the sensor will be sensitive to external temperature perturbations. In order to overcome the induced linear birefringence, a number of solutions have been presented [Ning95]. For example can the linear birefringence be removed by annealing the fibre coil [Rose96]. Another possibility is to suppress the influence of the bending-induced birefringence by using a fibre with a large degree of circular birefringence. A twisted fibre or a SEB (Spun Elliptically Birefringence) fibre is used so that the Faraday rotation is superimposed onto this circular birefringence. Optical current sensors using optical fibres as sensing elements have been produced by ABB for AC currents [Bohn02] (Fig. 32) and DC currents [Bohn05].

Fig. 32: Principle and picture of a fibre-optic AC-current sensor [Bohn02]

Siemens developed a similar AC current transducer [Will02]. Toshiba has developed a silica fibre OCT [Taka97] with a reflecting arrangement. This type has the double sensing length and can be used to suppress the influence of vibration-induced birefringence since the influence on the clockwise-propagating beam will be in anti-phase to the effect on the counterclockwise-propagating beam, Fig. 33.

32

Chapter 2


Fig. 33: Reflective-type wound-fibre OCT [Yu02]

NxtPhase Corp. has developed a commercial current sensor [Blak03], [Sand02] using the Faraday effect in a different architecture than the polarimetric technique. This sensor uses the Sagnac interferometer design and is based on the changes of the velocities of right- and left-hand polarized light waves propagating in the magnetic field around the conductor, Fig. 34. These two light waves travel with different velocities through a coil of a polarization-maintaining sensing fibre and the time-difference is measured. It is easier to accurately measure changes in light velocity than changes in polarization state. Since this is a velocity measurement scheme, there is no need to anneal the fibre. First field tests of this sensor started in 2000.

Fig. 34: NxtPhase sensor NXTC principle [Blak03]

Also Takahashi et al. developed a Sagnac interferometer-type fibre-optic current sensor [Taka04]. Faraday sensor techniques using fibres as sensing elements made great progress during the last few years. Some of the proposed sensors exhibit sensitivities of the highest standards and are produced for precise current metering.

33

Chapter 2


2.2.2.6. Unlinked Type This type of sensor, also referred to as witness sensor, is based on magnetic field sensing techniques. Instead of forming a closed loop around the conductor, this sensor only measures the magnetic field at a point near the conductor making it rather a magnetic field sensor than a current sensor, Fig. 35. However, this sensor can be used as a current sensor if the system is calibrated. Other applications are condition monitoring and fault-detection in electric power systems.

Fig. 35: Schematic of a Faraday-effect sensor, unlinked type geometry

In order to achieve a sufficient sensitivity for measuring magnetic fields at a point, the Faraday material needs to have a very high Faraday rotation. Therefore predominantly materials with high rotations such as YIG and substituted YIG are used. One example for such kind of sensor is proposed by Sohlstrm [Sohl93] using a Bisubstituted YIG film as Faraday material in a sensor head to measure magnetic fields, Fig. 36. The polarized light beam passes twice through a YIG crystal of 130m thickness being reflected at a mirror. This sensor can, after calibration, also be applied to measure electric currents.

Fig. 36: Principle of a magnetic field sensor head [Sohl93]

Unlinked type OCTs have been commercially produced by NGK Insulators, Ltd. in Japan [Imae92]. This sensor is based on a YIG crystal and is used as a ground fault current detector. The principle is shown in the picture below.

34

Chapter 2


Fig. 37: Principle of an optical magnetic field sensor [Imae92]

A current sensor using a TGG crystal to measure an electric current has been presented by Cruden et al. [Crud95]. In 1998, the same group proposed an unlinked type OCT made of a FR-5 glass piece [Crud98] (dimensions 22525mm3), Fig. 38.

Fig. 38: Reflective type OCT [Crud98]

In order to enhance the sensitivity of a YIG crystal-based magnetic field sensor, ferrite flux concentrators are used in [Deet96], [Roch00] and [Robl02]. The bandwidth of the sensor by Deeter [Deet96] is in the order of 10MHz and a noise equivalent field of 6pT/Hz, Fig. 39.

Fig. 39: Sensor head with flux concentrators [Deet96]

In 2003, Bai et al. proposed an OCT for integrated power electronic modules [Bai03] using Bi-doped YIG. A scheme of this point-sensor is shown in Fig. 40.

35

Chapter 2


Fig. 40: Sensing tip [Bai03]

An enhanced version [Inou95] of the current sensor presented earlier in the magnetic concentrator section [Toya93] is used as a magnetic field sensor in the vicinity of the conductor to detect the section of a fault current. Also the sensor head of the current transducer previously presented in the magnetic concentrator section [Itoh99] (Fig. 24) is used as a witness sensor with inferior characteristics. This device, produced by Matsushita, uses a Bi-substituted RIG-film as Faraday material, Fig. 41.

Fig. 41: Schematic of the opto-magnetic field sensor probe [Itoh99]

A different principle related to the Faraday effect is presented in [Dido00]. This principle is the same as in other conventional Faraday effect sensors but instead of measuring the average area of the up and down magnetized domains, the position of the domain wall is measured. A yttrium iron garnet crystal is mounted between two small magnets with opposite magnetisation directions. This special material forms two domains only (characterized by mutually opposite magnetisation direction) with opposite Faraday rotations, Fig. 42.

Fig. 42: Principle of a two-domain sensor principle [Dido00]

36

Chapter 2


A polarized light beam coming from the source (1) illuminates the crystal plate (2). This plate, closely located to the current-carrying conductor (3), forms a twodomain structure by means of two permanent magnets (not shown). The light, propagating through the analyzer (4), is detected by a position-sensitive photo receiver (5). For calibration the analyzer has to be rotated to a position where the light passing through the lower domain is extinguished for zero magnetic field. Measurements of the magnetic field (electric current) can now be made by detection of the position of the domain wall, Fig. 43.

Fig. 43: Dark and bright zones indicating opposite Faraday rotation

The boundary between bright and dark domains changes in accordance to the value of the magnetic field. Also ac currents can be measured evaluating the zone of intermediary brightness as shown below, Fig. 44.

Fig. 44: Intermediate zone at 100kHz

This relatively new sensor principle is able to measure high currents from dc up to hundreds of kHz and is relatively temperature-independent. 2.2.3. Interferometric Principles In an interferometer, the difference in length between two optical paths is measured. In order to use this principle to measure a current or a magnetic field, it must be transferred into a path length variation. In most cases, this change is accomplished through magnetostriction. This principle was first proposed by Yariv et al. [Yari80] in 1980: a magnetostrictive material is mechanically coupled to a fibre. When this material is exposed to a magnetic field, a change in shape occurs, which induces strain in the fibre resulting in a change of length of the fibre. This change in 37

Chapter 2


optical path length can be measured by bringing the fibre into one arm of a MachZehnder-Interferometer. Numerous different designs have been realized. A sensor exploiting the magnetostriction of a ferromagnetic core with a single-mode fibre coiled onto it is presented in [Pere02], Fig. 45.

Fig. 45: Fibre-optic current sensor [Pere02]

The best principle has been proven to be an optical fibre coated with magnetostrictive materials such as nickel, metallic glasses or ceramic magnetostrictive materials[Jarz80], [Sedl96], Fig. 46.

Fig. 46: Fibre coated with magnetostrictive material in an interferometer [Sedl96]

However, this sensor also reacts to all other kinds of parameters that can cause a change in optical path length, e.g. temperature. Many of these problems have however been solved, but still, the remaining non-linearity, hysteresis and saturation of magnetostrictive materials limit the applicability of this technique. Other sensor principles using interferometric detection have also been studied. For example by Ning et al. [Ning91/2], the optical path length change is caused by the strain of a piezoelectric element driven by the output-voltage of an ordinary current transformer around the current-carrying conductor ,Fig. 47.

38

Chapter 2


Fig. 47: Piezoelectric-optic current sensor [Ning91/2]

Another approach to change the optical path length is by creating a strain in the fibre caused by the Lorentzian force. This sensor utilizes a conductor-coated sensing fibre in one arm of a Mach-Zehnder interferometer [Okam90]. The DC or AC current flowing through the coating causes the metal-coated fibre to extend or vibrate elastically, which can be sensed in terms of an optical-phase change (Fig. 48).

Fig. 48: Lorentzian force current sensor [Okam90]

2.2.4. OCTs based on Bragg Gratings This principle is actually an interferometric one, but is grouped separately due to differences in detection mechanisms and structure. In recent years, several sensors that measure the mechanical strain of a material in a magnetic field with a Bragg grating have been proposed. A Bragg grating is an optical grating that works as an optical filter. A FBG (Fibre Bragg Grating) is a periodic or aperiodic perturbation of the effective refractive index and/or the effective absorption coefficient in the core of an optical fibre. Light propagating in the core will be reflected by the interfaces between regions having different refractive indexes. But the reflected light is generally out of phase and is extinct. However, for a certain wavelength, the Bragg wavelength Bragg, the light reflected by the periodically varying index of refraction will be in equal phase and added constructively. This leads to the reflection of light in a very narrow range of wavelength. Other wavelengths are nearly not affected and pass the fibre. When such a fibre is strained, the grating constant changes and consequently the reflected wavelength changes. That can be detected as a function of strain and thus the magnetic field. 39

Chapter 2


Many different types of FBG current sensors have been developed. A simple example is proposed by Feng et al. [Feng00] using a magnetostrictive rod with a FBG fibre mounted on it to measure the change in Bragg-wavelength, caused by the lengthened rod, Fig. 49.

Fig. 49: FBG sensor scheme, redrawn after [Feng00]

A different approach to measure a current is chosen by Fisher et al. [Fish97] using the output signal of a conventional current transformer to actuate a piezoelectric cylinder on which a FBG fibre is bonded, Fig. 50.

Fig. 50: Scheme of a FBG sensor [Fish97]

The output voltage of the current transformer is proportionally translated into the variation of diameter of the cylinder causing a linear current-frequency shift dependence. Cavaleiro et al. [Cava98] use a different approach. Their sensor is based on the temperature sensitivity of a fibre Bragg grating. A current passing through a thin conductive coating on the surface of the FBG causes the temperature change. This current is the secondary current of a Rogowski coil measuring the electrical current of the current-carrying conductor, Fig. 51.

Fig. 51: Bragg sensor with temperature induced phase shift [Cava98]

40

Chapter 2


By monitoring the temperature-induced Bragg wavelength shift, the value of the electrical current can be recovered. All these principles require a rather expensive wavelength-detection technique and suffer from temperature interference. However, a sophisticated fibre Bragg sensor has been introduced by Chiang et al. [Chia03], overcoming the temperature sensitivity and using a simple and cheap detection technique. This sensor consists of a fibre Bragg grating bonded on two joined metal alloys (Terfenol-D and MONEL 400). At zero magnetic fields, the sensor shows a single reflection peak. But when a magnetic field is applied, the Terfenol-D is stretched due to huge magnetostriction, while the dimensions of the MONEL 400 remain unchanged resulting in two separated reflection peaks, Fig. 52.

Fig. 52: Reflection peak separation [Chia03]

The reflection peak of that part of the grating on Terfenol-D shifts to a longer wavelength, while that of the other half on MONEL 400 remains on its original position. The magnetic field thus causes a split in the reflection spectrum making it easy to measure, using a simple photo detector to detect its intensity. Moreover, the almost identical thermal expansion coefficients of the Terfenol-D and MONEL 400 make the peaks shift in the same direction by the same amount keeping the overlap of the two reflection spectra constant and therefore the sensor insensitive to temperature changes. However, Terfenol-D-based sensors are limited in their bandwidth to only a few kHz [Gree90]. A potential advantage of a Bragg grating based system is the relative ease of multiplexing, which can potentially reduce the costs of the source and electronics when multiple devices are to be used. [Culs04]

41

Chapter 2


2.2.5. Micromechanical Sensors with Optical Readout With the rise in MEMS industry in the 1990s many well-known and widely understood principles have been applied in order to measure magnetic fields and thus, electric currents. Most principles use mechanical deformation basing on standard silicon-technology and optical readout to ensure electromagnetic immunity. However, no commercialisation of this technology has been seen yet. Some of the principles are presented below. Heredero et al. [Hede99] proposed a micromachined optical fibre current sensor measuring the magnetic field around a conductor. The sensing element consists of a square silicon membrane that has a cylindrical permanent magnet fixed on its central region. The vibration of this structure at the presence of the magnetic field gradient generated by an AC current is measured with white-light interferometry. There is a great advantage in feasibility and price for LEDs and detectors since there is no limitation for a central wavelength and standard single-mode optical fibres can be used. The actuator design is a simple one-mask process. The principle is shown in Fig. 53.

Fig. 53: Micromachined optical fibre current sensor [Hede99]

This sensor is mechanically limited by dynamic range and bandwidth and is not able to detect homogenous magnetic fields. The same group proposed a DC and AC sensor with a similar structure using a dual-wavelength fibre Bragg grating technique working in quadrature in order to interrogate the microcavity [Hede03]. Another micromechanical magnetometer has been introduced by Yang et al. [Yang02]. This sensor optically detects the deflection of a ferromagnetic-coated beam in a magnetic field, Fig. 54.

42

Chapter 2


Fig. 54: Principle and SEM image of the magnetometer [Yang02]

This principle is subjected to effects like squeeze-film damping, which considerably limits its performance. A similar sensor was proposed by Goedeke et al. [Goed04]. Here, the deflection of a cobalt-coated cantilever is measured optically. In [Kepl04], the sensing element is a U-shaped cantilever that bears a thin film lead. The magnetic field strength is converted into a movement of the cantilever when a current flows through the film, the resonant frequency of the cantilever being 5kHz, Fig. 55.

Fig. 55: Schematic of the sensor with optical readout of the cantilever position [Kepl04]

The front plane of the bended cantilever acts as a deflecting mirror enabling an optic readout system. The dynamic range of this sensor can be tuned by altering the current in the film. Another intensity type principle has been tested for fault detection on electric power transmission lines by Carome et al. [Caro91]. It consists of a pair of multimode optical fibres directly mounted end-to-end. One fibre is fixed while the other is deflected by a magnetic field. The miss-alignment results in change in intensity transferred from one core to the other and can be measured as function of the magnetic field.

43

Chapter 2


2.2.6. Other Optical Current Sensing Principles Following, some principles that allow to measure electric currents are briefly presented. These principles are rather unusual and might not be commercially used for current sensing. A new type of OCT based on a new physical effect, the thermal lens coupled magneto-optical effect in ferrofluid, is presented in [Chen98], [Ning91/1]. When a laser beam is focused on an absorptive ferrofluid thin film, an effective concave lens is resulted, which diverges rays in the beam and makes them interfere. A consequently appearing pattern of diffraction rings can be detected by two fibres, which detect the current-corresponding variation of light intensity of the diffraction rings. An OCT using liquid crystals and chromatic modulation has been presented by Pilling et al. [Pill93], Fig. 56. The CT attenuates a certain portion of the optical spectrum incident upon it by an amount dependent on the current flowing in the power line. The spectrum is analysed by a double-layer photodiode and the current in the power line can thus be indirectly measured by evaluating the ratio of the photodiode shortcut currents.

-

Fig. 56: LC modulator based on chromatic modulation [Pill93]

This principle however, suffers from temperature dependency and a very high response time. 2.2.7. Conclusion Optical Current Transformers Concluding the development in optical current sensing, one can say that OCTs are on the way to commercialisation and many have already been field-tested and commercially installed. There is an industrial need for two kinds of current sensors. One sensor with high accuracy for revenue metering of high currents and one fast, high dynamic sensor with a wide bandwidth for protecting the installation in case of overload currents [Mohr02]. For revenue metering, there are already numerous companies field-testing or commercially installing high-accuracy current metering systems. Most major electric 44

Chapter 2


companies like ABB, Siemens and GE show efforts in developing optical current metering systems. Great efforts are especially made in Japan but optical current sensors are still far from being widely used. It can be seen that during the last years there has been a shift from bulk-glass sensors to all-fibre sensors reflecting the progress in temperature- and birefringence-compensation techniques. For fault detection sensors, there has not been a steady commercial development. Most principles for fault detection fall into the group unlinked type (chapter 2.2.2.6.) since those sensors are simpler, potentially cheaper and sufficiently precise. These devices are commonly based on magnetic field sensing and primarily use iron garnets with high Faraday rotation as sensing medium. Many technologies have been proposed, predominantly in the first half of the 1990s, but only few are commercially installed. Again, Japanese companies seem to have made the greatest effort in developing these technologies. Several thousands of optical current fault sensors are reported to have been produced and sold within Japan since the late 1990s. Panasonic (Matsushita) alone reported the shipment of several thousand optical sensor heads for fault detection [Yu02]. However, there has not been an open market for optical current fault sensors. Noteworthy is the recent development of hybrid opto-mechanic systems, many of which are based on reflective intensity modulation. These devices have, however, not been industrialized yet, but are potentially cheap owing to standard optic components and batch processing production techniques. 2.3. Conclusion of the Technology Investigation Considering all presented techniques, it can be concluded that none of the nonoptical current or magnetic field sensors exhibits the demanded reliability in highEMI environments without additional shielding and isolation and/or fulfils the required bandwidth, dynamic range and size. Optical current measurement systems are favourable for that purpose because of their inherent insensitivity to EMI and galvanic isolation. Most of the indirect OCTs however, suffer from inherent problems such as bandwidth limitation, saturation and hysteresis due to their mechanical components or other interferences associated with energy conversions. Some examples for limitations are the low resonance frequencies of most MEMS sensors or saturation, hysteresis and actuation frequency limits of magnetostrictive components in Bragg sensors. The most suitable sensor principle for this application are OCTs with intrinsic optomagnetic effects, i.e. the Faraday effect. Considering the purpose of current fault detection, a simple and cheap unlinked type sensor in the vicinity of the conductors is sufficient since no precise current measurement using fibre-coils or magnetic concentrators is necessary. 45

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The best solution for that seems to be a Faraday sensor with point measurement of the magnetic field employing high-rotation materials such as RIGs. Two such sensors are available at the Microsystem technology department of KTH and will be used for some first measurements and fault detection tests in this work. The Faraday materials and properties of these sensors however, are unknown, but will be determined in chapter 4.2.

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Chapter 3

FEM Simulation of the Magnetic Field Pattern

3. FEM Simulation of the Magnetic Field Pattern3.1. Introduction The chosen technique using Faraday point-sensors to detect a fault in the described system with parallel IGBTs gives the possibility to use a single sensor to detect a fault in each of the current lines. This is possible when the sensor is brought into the exact middle between the two conductors (Fig. 57) and the sensor has a 45-characteristic (compare chapter 2.2.2.2.) with polarizer and analyzer having a 45 rotation to each other. The magnetic field around an infinitely long, thin and straight conductor can be calculated usingH= I 2 rEquation (9)

where I is the current in the conductor and r is the distance from the conductor. The case for the application with two conductors with rectangular cross-section is more complex.

Fig. 57: Magnetic field between parallel conductors

The magnetic fields caused by both conductors in the exact middle will cancel each other when the current in both conductors flows in the same direction due to the symmetry of the geometry, Fig. 57. When one of the IGBTs fails, only one of the lines will be conducting, resulting in a positive or negative field at the sensor depending on which of the IGBTs failed. This can be detected with a 45-sensor. In order to reliably detect the current or in this case a current fault in the conductors, the magnetic field and its pattern around the conductors has to be known. 47

Chapter 3


The conductors in the actual application will have a non-circular cross section and will be placed in proximity to each other. The current in the conductors might change with a frequency of tens of kHz. That makes it difficult to calculate the magnetic field and its distribution, since eddy currents and skin effect will occur and significantly influence the magnetic field. Therefore, a FEM (Finite Element Method) simulation of the two conductors and the surrounding magnetic field was made. From this we can get some essential information. Firstly, it is essential to know the expected magnitude of the field between the two conductors in order to conclude the necessary sensitivity of the sensor to detect a fault. Secondly, the distribution of the field between the conductors is important to know in order to determine the best location of the sensor. Lastly, the frequency behaviour of the magnetic field is very important. The frequency response has to be flat up to hundreds of kHz in order to detect faults with minimum delay. The simulation has to be done for two cases: The case when both conductors are carrying the same current in the same direction and the case of a fault where only of the conductors is conducting. Since at higher frequencies, skin effect and induction are expected to have a very large influence on the current density distribution in the conductor and thus the magnetic field, the field has to be investigated for a range of frequencies. The FEM software COMSOL Multiphysics (formerly Femlab) was used to simulate the two conductors and the magnetic field around them. The simulation can be done in 2D with a cross-section of the conductors perpendicular to the direction of the currents as the 2D plane. A 2D model implies that the length of the conductors is infinite which is not strictly true in application. In principle, for a correct simulation with finite conductor lengths a much more complicated 3D simulation would be necessary, but since the 2D simulation is expected to give a sufficiently good indication of the resulting magnetic fields, a 2D model has been used. In order to model the two copper conductors and the resulting field around them, the Femlab model Quasi-Statics, Magnetic, Perpendicular Induction Currents, Vector Potential from the Electromagnetics Module is used. To set and maintain the correct currents in the conductors, an additional Weak Form, Point model had to be added that integrates the actual current density over the conductor and recalculates the input potential for each conductor.

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Geometry and dimensions for the simulation are chosen in the order of the presumable actual application. The cross section of the conductors carrying a current perpendicular to the simulation plane (z-direction) is 10010mm2, Fig. 58.

Fig. 58: Sketch and dimensions of the two conductors

In normal operating condition, both conductors carry the same current I0. In order to simulate the case of fault in one of the conductors, the current in the right conductor was s