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Continuous, Remote On-line Partial Discharge (OLPD) Monitoring of Complete Medium Voltage

(MV) Network

Lee Renforth, Riccardo Giussani,

Marc Foxall and Chia Heng Tan

HVPD Ltd, Manchester, UK

Laurent Aouad

Enertopia, France

Corresponding author riccardo.giussani@hvpd.co.uk

PAPER OVERVIEW

This paper presents results from a new condition monitoring (CM) technique for the on-line partial discharge (OLPD) condition monitoring of complete medium voltage (MV: 3.3–36 kV) networks. The new OLPD solution presented provides for reliable and cost-effective insulation condition monitoring across complete power generation, industrial and petrochemical industry MV networks. This new technique provides for cost-effective OLPD monitoring coverage, with the sensor points of attachment at the central MV switchboards and by employing multiplexer technology to monitor multiple MV circuits from central OLPD monitoring ‘hubs’ located at the main MV switchboards at a facility.

The remote OLPD monitoring technique was first published by HVPD and Chevron at the IEEE Petroleum and Chemical Industry Technical Conference in September 2012 (IEEE-PCIC 2012) [1]. This was followed up with a second paper by the two companies on the application at the IEEE-PCIC 2013 conference in September 2013 [2]. These two papers described the application of a new, remote, on-line partial discharge (OLPD) monitoring technique for in-service ‘Ex/ATEX’ medium voltage (MV) motors located in hazardous gas zones in the oil, gas and petrochemicals industry.

The technique employs the use of high current, wideband HFCT sensors and monitoring units located remotely from the Ex/ATEX HV motor to be monitored, at the switchboard end of the HV motor feeder cables i.e. in a ‘non-Ex/ATEX’ area. This enabled the OLPD monitoring of the in-service, ‘Ex/ATEX’ 10 kV motors due to the HFCT sensors having a good, low frequency response (down to 100 kHz), thus making them capable of detecting PD activity in the entire 10 kV motor feeder circuit, including the switchgear panel, the feeder cable and also the stator winding of the motor at the remote end of the cable.

The paper goes on to describes how the same OLPD monitoring technique can be applied in all the circumstances where the remote MV/HV assets need to be monitored for partial discharge activity. This applied to remotely connected MV/HV plant including motors, generators, transformers and secondary switchgear panels where such plant is located in an area that is

not easily accessible or is subject to restrictions such as certain areas of nuclear power plants.

Two case studies are presented wherein the HV stator insulation condition and reliability of in-service 10 kV Ex/ATEX motors and 11 kV Gas Turbine Generators, have been tested and monitored for partial discharge (OLPD) both on-line (in-service) and off-line (out-of-service). The case studies emphasize the importance of diagnostic baseline testing and calibrations followed carrying out extended, continuous on-line partial discharge monitoring of the in-service rotating machines where it through the detection of underlying trends in partial discharge activity over time (measured over months and years).

The authors also show how the data from a number of OLPD monitoring systems can be merged into a suitable format via a user interface containing colour-coded, plant condition data superimposed onto a ‘mimic’ of the MV network’s single-line diagram (SLD). This complete network OLPD monitoring solution provides real-time, insulation condition status of the HV cables, switchgear, motors, generators and transformers across the network for easy review by the plant manager in the control room.

The paper concludes with the proposal that significant cost and operational benefits can be gained from this new approach to the on-line partial discharge (OLPD) condition monitoring of complete power generation, industrial and petrochemical industry MV networks. The data from this continuous condition monitoring technology can be used to support condition based management (CBM) schemes and to direct preventative maintenance interventions to repair plant/cables ahead of insulation failure from PD activity. In this way the plant operator can have an ‘early warning’ against ‘incipient’ (latent) insulation faults and through carrying out a repair can thus avoid an unplanned outage of the MV network.

Index Terms ‒ partial discharge, PD, on-line partial discharge, OLPD, remote PD monitoring, condition monitoring of MV networks, PD monitoring of MV networks in nuclear plant, Ex/ATEX HV motors, hazardous area condition monitoring, restricted area condition monitoring

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Fig. 1: The Ex/ATEX restricted areas of the Petrochemical Industry have a lot in common with the restricted areas in the Nuclear Industry

I. LIST OF ABBREVIATIONS AND ACRONYMS

CBM Condition Based Maintenance

CM Condition Monitoring

HFCT High Frequency Current Transformer

HV High Voltage

MV Medium Voltage

OLPD On-line Partial Discharge

O&M Operation and Maintenance

PD Partial Discharge

PDEV Partial Discharge Extinction Voltage

PDIV Partial Discharge Inception Voltage

TEV Transient Earth Voltage

RC Rogowski Coil

XLPE Cross-linked polyethylene

PILC Paper-insulated lead covered

EPR Ethylene propylene rubber

PVC Polyvinyl-chloride

VSD Variable Speed Drive

ATEX Atmosphere Explosive

SNR Signal-to-noise

POA Point of attachment

II. PARTIAL DISCHARGE (PD)

“Partial discharge (PD) localized electrical discharge that only partially bridges the insulation between conductors and which can or can not occur adjacent to a conductor” [3]

PD is caused by discontinuities in the insulation system (due to poor manufacturing or bad design, void, crack, etc.), and as a general rule-of-thumb, occurs in systems operating at voltages above 3 kV. Failure of high voltage (HV) insulation is the main cause of MV and HV system failures, with IEEE statistics

indicating that electrical insulation deterioration causes up to 90% of electrical failures of certain HV equipment.

PD can occur in voids in solid insulation (paper, polymer, etc.), along the interfaces of multi-layer solid insulation systems, in gas bubbles in liquid insulation, or around an electrode in a gas (corona discharge). PD can often be observed with the commissioning of new equipment, particularly in cable joints and terminations that are made up on-site, being caused by improper installation or poor design and/or workmanship. Poor workmanship can lead to infant mortality of MV/HV networks, with a disproportionate percentage of insulation failures being observed within the first 1–3 years of service, compared to the rest of the service life of the cables/plant.

PD activity can initiate under normal working conditions in HV equipment where the insulation condition has deteriorated with age, has been aged prematurely by thermal or electrical over-stressing or due to improper installation (leading to infant mortality). Premature ageing of the insulation system can be enhanced by several factors/stresses, including: thermal, electrical, ambient and mechanical. Fig. 2 provides an overview of these factors.

Fig. 2: TEAM Stresses Affecting Rotating Machines

After initiation, the PD can propagate and develop into electrical trees and interfacial tracking until the insulation is so weakened that it fails completely with breakdown to earth or between the phases of a 3-phase system. Depending on the discontinuity type and location in the insulation system, a failure can take anything from a few hours up to several years to track through, to produce a complete earth or phase-to-phase fault.

While some discharges can be extremely dangerous to the health of the insulation system (e.g. discharges within polymeric cables and cable accessories), other can be relatively benign (e.g. such as corona into air from sharp, exposed points on HV overhead networks or on the outside surfaces of outdoor cable sealing ends). The key to diagnostic OLPD testing is to be able to differentiate between the dangerous and the benign; this differentiation becomes more difficult as the voltage of the system increases.

To take into account these variable factors, continuous electrical and mechanical condition monitoring (CM) of rotating HV machines is now becoming more widely applied in the oil and gas industry.

III. CONDITION MONITORING (CM)

The term ‘on-line’ partial discharge (OLPD) monitoring refers to the diagnostic testing of the medium voltage (MV) and high voltage (HV) insulation of in-service networks (cable, switchgear, etc.) and plant (rotating machines, transformers, etc.), in their

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normal operational mode. The technique can be applied to all types of MV and HV assets including motors, generators, cables, switchgear and transformers. The technique is complementary to the use of out-of-service partial discharge testing that is often referred to as ‘off-line’ PD testing. The advantage of the OLPD testing of in-service assets is that the MV and/or HV component s are tested under both normal working conditions and abnormal variations related to changes in the associated electrical, thermal and mechanical operational stresses.

Out-of-service, ‘off-line’ PD testing requires the assets to be de-energised and a portable power source used to energise each component of the asset. This technique is useful as the test engineer has control over the voltage and can therefore measure the partial discharge inception voltage (PDIV) during the test voltage ramp-up (typically to 2U0) and the partial discharge extinction voltage (PDEV) on voltage ramp-down. The techniques of on-line and off-line PD testing are considered by many people in the field of MV and HV insulation diagnostics as complementary as the former can be used to pre-sort any machines that require an outage for off-line diagnostic testing, PD site(s) location and repair.

The OLPD insulation condition assessment of operational, in-service HV motors, generators, cables and switchgear is now becoming more widely accepted as a useful diagnostic aid in the worldwide industry. The application and uptake of the technology in the industry has increased significantly over the past decades as operators see the benefits that the technology brings, being capable of providing an ‘early warning’ against incipient (MV and HV insulation faults i.e. those faults yet to occur but which manifest themselves through high PD activity.

In the petrochemical industry, it is estimated that OLPD electrical condition monitoring (CM) technology is presently applied in only around 10% of the HV motors and generators. It is interesting to compare this ten-percent-estimate to the percentage of rotating HV machines in the industry that have mechanical (vibration and bearing wear) CM technology fitted (this is estimated at over 75%). This disproportion could be due to the fact that historically it was mechanical engineers who had been responsible for the design, build (and condition monitoring) of the motors and generators.

To illustrate the benefits of employing the OLPD electrical CM technology, it must be noted that HV motor cause-of-failure analysis studies presented in [4] state that stator winding failure contributes to around 37% of the total HV motor failures reported. The data from these published studies is illustrated in Fig. 3 below. With 37% of all failures due to HV stator winding failure, it would suggest that monitoring of the HV stator windings should be considered (in conjunction with the mechanical/vibration monitoring already mostly in place) to provide an ‘early warning’ against these stator winding faults.

Fig. 3: Data from High Voltage (HV) Motor Cause of Failure Analysis

Study [4]

IV. THE DRIVERS BEHIND THE CM AND THE CONDITION

BASED MAINTENANCE (CBM)

The main drivers of applying the OLPD test and monitoring technology are:

To improve the reliability of the networks by identifying the worst performing assets before they fail.

To collect and analyse insulation CM data to provide an early warning against ‘incipient’ insulation faults.

To provide plant condition data to support reliable life-extension programs of ageing networks.

To avoid unplanned outages and minimise downtime.

To provide qualitative information on the assets that facilitate Condition Based Maintenance (CBM) and a programmed ‘replacement and repair’ strategy.

MV and HV operators worldwide are increasingly deploying

OLPD technology to test, monitor and manage a wide range of assets including power cables, switchgear, transformers, and rotating HV machines [5]. The business drivers for carrying out OLPD testing and monitoring of the HV networks include:

Health & safety – most diagnostic OLPD test projects are carried out further to insulation faults and/or equipment component failures occurring within an HV network. In this case there is an immediate requirement to carry out condition ‘spot-tests’ on ‘sister’ plant to ensure there is no immediate risk of failure and thus no danger to staff or the public. This is particular relevant in the petrochemical and nuclear industries.

Supporting reliable life-extension projects ‒ As many in-service HV machines are reaching the end of their ascribed ‘design-life’ (typically of 25‒30+ years), operators have important asset management decisions to make as to whether to extend the life of these existing assets (using CBM techniques) or to replace the machines with new units (time-based replacement). This situation is particularly common in the petrochemical industry.

Avoiding unplanned outages and downtime – generation, petrochemical, nuclear and industrial process operators are focussed on the consequences of any plant failure-induced downtime on their generation capacity, process or service. This is the strongest financial driver for these production and process industries.

Studies on power generation, petrochemical and process

customer networks show that the main cost driver is to avoid any unplanned outages. This is simply due the cost of an interruption to the process being normally much higher than the capital replacement/repair cost of any cable/plant/machine in the network. That notion is also true in the case of the nuclear sector.

Oil, gas, and chemical processing facilities have HV networks which support ‘mission-critical’ processes with potentially very large ‘loss of business costs’ due to unplanned outages. Such MV and HV plant owners have little difficulty in justifying the cost of regular OLPD diagnostic measurements or complete permanent network OLPD monitoring installations.

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V. MODERN OLPD TESTING AND MONITORING

TECHNOLOGY

There is a wide range of technologies available in the market for OLPD testing and monitoring of in-service MV and HV cables, plant and rotating machines. These range from simple-to-use, handheld PD screening test units (for screening large numbers of HV assets quickly and easily) to continuous PD monitoring technology. It is now becoming increasingly popular for rotating HV machine owners to install permanent PD sensors and monitoring systems at suitable points in the HV networks that drive their industrial processes, chemical processing and oil and gas refining. PD detection is now commonly regarded as the best indicator of insulation degradation of in-service MV and HV cables and plant. By combining OLPD screening tests (Phase 1), OLPD diagnostic spot-tests (Phase 2) and extended, continuous OLPD monitoring (Phase 3) technologies, it is possible to carry out:

Routine, regular walk-by assessments of HV cables and rotating plant using simple, handheld PD test units

Diagnostic OLPD testing and location of the PD sites using portable diagnostic spot-test technology

Permanent monitoring of key substations, circuits and rotating machines on the network to trend PD activities with time/load cycle changes

This comprehensive approach to PD testing throughout the service life of an asset is shown in Fig. 4

Fig. 4: PD Testing Approach

VI. PARTIAL DISCHARGE SENSORS OPTIONS

There are many sensor options available for the on-line detection of PD activity in cables, switchgear, transformers plant and rotating machines. These sensors include SSC, RTD, High Voltage Coupling Capacitor (HVCC), High Frequency Current Transformer (HFCT), Rogowski Coil (RC), and Transient Earth Voltage (TEV) sensors. The four main types of on-line PD sensors used are shown in Table 1. The wideband frequency response of the three sensor types used to test rotating HV machines is shown in Fig. 5.

It can be noted from Table 1 that at 10 MHz the HVCC sensor is the most sensitive, followed by the HFCT sensor, the TEV sensor and the RC sensor. Fig. 5 shows the frequency response along with the approximate spectra of PD activity occurrence from 10 kHz to 100 MHz. The most suitable sensor solution for any application will depend on the cable/plant/machine to be

tested and the most suitable point of attachment (POA) for the sensor on the network.

Fig. 5: Normalised Frequency Response of Four Types of PD Sensor

with PD Pulse Spectrum

Table 1: Partial Discharge Sensors Options

With reference to Fig. 5, the centre frequency and wide bandwidth of the new HFCT sensor ensures that lower frequency PD signals, which have been attenuated by dispersion within the power cable and propagation through the machine stator windings, can still be detected with a sensitivity sufficient to make reliable measurements. The HFCT sensor can also detect higher (>10 MHz) frequency components of the PD signal that appear closer to the point of origin of the PD activity, such as in the switchgear cable box or machine terminal box. This wideband frequency response of the HFCT means that the sensor can be located remotely from the rotating machine under test while still being able to detect sufficient PD pulse energy to ensure that the PD pulse is represented accurately in the time- and frequency-domain. This feature allows for the discrimination between PD types based on their location and pulse wave-shape.

It should be noted that, as discussed in the IEEE and IEC standards [7] and [8] (sections 11 and 5 respectively), the risk of misinterpreting PD signals always exists due to interference exhibiting characteristics similar to the PD signals. These standards discuss time- and frequency-domain methods of noise separation, types of interference that can be expected in OLPD testing, and the importance of distinguishing between the origins of the PD activity. The technique of remote OLPD monitoring developed by the authors has been built on PD classification

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knowledge rules described in [9] and [10] that use time- and frequency-domain parameters to distinguish between PD types and interference.

A brief description of the four main types of sensor used for the OLPD detection of in-service HV cables, switchgear, bus duct, transformers and rotating machines is given below.

A. High Voltage Coupling Capacitor (HVCC) sensors

A typical installation of three HVCC sensors, one per phase in a generator terminal box is shown in Fig. 6. These sensors require a galvanic connection to the HV terminals of the machine and provide the greatest measurement sensitivity of all the OLPD sensor options when installed there. HVCC sensors of different voltage and capacitances are available in the marketplace ranging from 6.6 kV to 36 kV in voltage, with typical available capacitance ratings of 80 pF, 500 pF, 1 nF and 2 nF. HVCC sensors are recommended for PD monitoring of larger rotating machines (e.g. >20 MW at 11 kV) and are now widely employed to monitor PD activity in HV generators and larger HV motors. A limitation in the use of these sensors is sometimes seen with smaller motors (<5 MW), supplied with concentric neutral, 3-core triplex cables, as it is sometimes difficult to fit the HVCC sensor in the smaller cable boxes on these machines. In such cases, HFCT sensors are preferred as they require less space to fit.

Fig. 6: Permanent On-line High Voltage Coupling Capacitor installation in an large 11 kV Generator Terninal Box

B. High Frequency Current Transformer (HFCT) sensors

Split-core, ferrite HFCT sensors have now become the de facto sensor of choice for the OLPD testing of in-service HV cables. The HFCT works inductively to detect PD currents in the either the cable earth/drain wire or in the cable’s conductor. Permanent HFCT sensor installations such as those shown in Fig. 7 are also becoming more popular with HV plant owners for the application of OLPD monitoring of smaller rotating machines (sub 10 MW) due to their ease of installation and lower cost compared to the more conventional HVCC sensors. The HFCT sensors are installed inside the terminal box and are installed to intercept the PD current on the conductor of each phase (i+) or the PD current on the earth drain/electrostatic shield (i-). Another advantage of the HFCT sensor, as discussed in the previous section of this paper, is that they have a suitable low frequency

response (down to approximately 100 kHz, as shown in Fig. 5) while also being capable of detecting high frequency PD signals (up to approximately 30 MHz).

Due to this wideband frequency response (from 100 kHz to 30 MHz), the HFCT sensors are suitable for permanent installation within either the machine terminal box or switchgear cable box. Installation of these sensors at the switchgear cable-end enables remote OLPD monitoring of rotating HV machines to be made as the HFCT sensors are able to detect the lower frequency PD pulses that have travelled down the HV cable from the machine.

Fig. 7: Permanent HFCT sensor inside cable boxes Left: HFCT sensor around Cable + Earth/Drain.

Right: HFCT sensor on Earth Drain Wire

It is known that attenuation is a function of frequency [10], which, for a PD pulse, is directly related to the distance propagated from the machine stator winding. With their low frequency response and wide measurement range, HFCT sensors can be used to monitor PD activity in the stator windings of rotating machines by connecting the sensor at the central switchboard at up to 2.5 km/1.5 miles (for XLPE cables) from the rotating machine under test, as illustrated by Fig. 8.

C. Rogowski Coil sensors

Air-cored, inductive RC sensors have been used for many

years in the UK power generation, oil and gas and petrochemical industries to monitor PD in rotating machines. They were originally adapted in the 1960s by scientists at the UK’s Central Electricity Generating Board (CEGB) to monitor PD activity in large generators. These sensors have also been used in the UK North Sea offshore oil and gas industry; a number of designs have become Atmosphere Explosive (ATEX) approved for installation within hazardous gas zones. The RC sensors are normally permanently installed in the cable boxes of the motor or generator, and are attached at each phase, around the HV cable cores. While proven to have a very low spark-risk, the main drawback to the RC sensor is that it has a low sensitivity to PD signals ‒ typically around 30 to 100 times less than the HFCT and HVCC sensors respectively. This means that only very significant levels of PD activity are detectable by the RC sensor, with the early stage detection of PD activity being more difficult due to the poor signal-to-noise ratio of this sensor.

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Fig. 8: Measurement range for an HFCT sensor connected at the switchboard cable end for the three main cable insulation types (PVC, PILC, XLPE)

D. Transient Earth Voltage sensors

TEV sensors are used on the outside of the metal-clad plant

under test ‒ predominantly to test HV switchgear for internal PD activity. The occurrence of PD within the HV insulation system induces a high frequency voltage pulse (the ‘transient earth voltage’) on the inner surface of the earthed housing. These electromagnetic signals emerge onto the outer skin of the switchgear through breaks in housing such as vents, joints or seams. The TEV sensors are thus placed on the outside of the switchgear panel to capacitively couple to these induced PD signals originating from the inside.

There are many handheld PD screening devices in the market that incorporate TEV sensors; these devices have shown to be very useful in identifying and locating PD activity in HV switchgear panels and bus ducting (example shown in Fig. 9). Handheld PD test technologies for simple look-see OLPD screening are becoming more popular and can also be used for the testing of rotating HV machines equipped with built-in permanent PD sensors (HVCC, HFCT or RC).

VII. REMOTE ON-LINE PARTIAL DISCHARGE MONITORING OF

RESTRICTED AREAS

From the authors’ experience, of particular focus in the petrochemical industry is the reliability of the process-critical HV motors, which drive the industry’s core pumping and refining processes, located in hazardous gas zones. OLPD insulation CM technology provides a continuous assessment of the HV

insulation of these motors in-service and over time. Continuous OLPD monitoring of rotating HV machines has gained considerable acceptance and appreciation in the industry as an effective method to identify sites of localised stator insulation damage and degradation ahead of scheduled preventative maintenance outages [2]. The same situation can be seen in the nuclear industry where several areas have restricted access due to their hazardous nature.

Fig. 9: Handheld PD Test Unit with TEV Sensor

During Testing of Switchgear Panel

A strong argument for continuous OLPD monitoring is that it allows for the measurement of PD activity in an in-service motor to be conducted under normal and abnormal operating conditions, providing a much better diagnostic than the more

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traditional periodic, OLPD spot-tests. Continuous, diagnostic OLPD monitoring is necessary to ensure that any sporadic, or time-varying PD activity in the HV machine stator or in any component of the MV and HV system is detected. Such continuous CM data provides the necessary level of diagnostic information for planning and scheduling of CBM intervention strategies.

As these HV motors operate in explosive atmosphere (Ex) or atmosphere-explosive (ATEX) hazardous gas zones, any monitoring system situated in the gas zone requires special design measures to ensure that it meets the relevant standard for the gas group in question; similarly, the nuclear-certified equipment must undergo additional rigorous and costly customisation for service in the restricted-access areas.

While a number of vendors in the marketplace offer Ex/ATEX-rated PD sensor and monitor designs, another, more overriding, factor often is the logistical and operational limitations in installing the sensors in (and the cabling from) the Ex hazardous gas zone back to the control centre. A further limitation in some cases, due to the compact design of some Ex HV motor cable boxes, with limited space within the terminal box, is that it is not always possible to place conventional, permanent HVCC sensors inside the cable box. These technical and operational restrictions in deploying this sensor technology in the Ex/ATEX zones has led to the development of the new, remote OLPD testing and monitoring technique. This technique uses inductive, wideband, HFCT sensors located remotely from the motor under test i.e. at the switchgear line-up.

The basis for the remote OLPD monitoring technique is the development of innovative, high-current, ferrite-based HFCT sensors [11]. These sensors are used to test rotating HV machines on any voltage rating above 3.3 kV. The sensors provide wideband (100 kHz to 30 MHz) OLPD measurements, up to a maximum phase current of 1000 A. These split-core sensors are inductively coupled to each phase of the cable/motor feeder under test (they are attached around the HV cable and the earth/drain wire take-off point at the switchgear, as shown in Fig. 10). There is therefore no limit to the voltage of the rotating HV machine this inductive HFCT sensor can be used with.

Fig. 10: High-Current HFCT Permanent Sensor Installation

Some typical voltage and MVA ratings of motors and generators that can be tested with this design of 1000 A, high-current HFCT sensor include: <12 MVA for 6.6 kV, <20 MVA for 11 kV, and <24 MVA for 13.8 kV. The high-current HFCT sensor uses a gapped ferrite core that has a high magnetic permeability,

which increases the 50/60 Hz saturation current rating of the sensor (the primary current that it can handle before measurements become unreliable due to saturation of the ferrite core).

The remote OLPD monitoring performed with HFCT sensors located at the central switchboard can also reduce the risk of Variable Speed Drive (VSD) and inverter drive pulses at the machine being classified as noise. This is often possible as the electromagnetic noise pulses from these electronic switching devices are often attenuated by the low-pass filtering effect of the power cable from the machines under test.

A significant amount of the high-frequency energy of the PD pulse is attenuated after travelling some distance along the power cable and the amount of attenuation (and pulse dispersion) is dependent upon the type of HV cable insulation and the distance to the measurement point (l) (the feeder cable length). The different dielectric properties of the main insulation types used in HV power cables (including cross-linked polyethylene (XLPE), paper-insulated lead covered (PILC), ethylene propylene rubber (EPR) and polyvinyl-chloride (PVC)) mean that transient PD pulses attenuate, and broaden at different rates. Equations (1), (2) and (3) below can be used to relate the parameters of the power cable to the pulse propagation constant and the cable length to provide the basis for the frequency-dependent PD pulse propagation model used by the authors.

e−γl (1)

γ = √(R + jωL)(G + jωC) (2)

= α(ω) + jβ(ω) (3)

where

𝛼(𝜔) pulse magnitude attenuation (dB/m)

β(ω) pulse dispersion (rad/m) R resistance (Ω/m) L inductance (H/m) G conductance (S/m) C capacitance (F/m)

j √−1

𝜔 frequency (Hz).

VIII. CASE STUDY 1 – OLPD TESTING & MONITORING OF

10 KV MOTORS

A crude oil-processing facility in Kazakhstan began operation in 1993, and has grown significantly since. There are approximately one hundred 10 kV motors at the facility, with about half of these now nearing an in-service age of 20 years. The site conditions are severe, with the ambient air temperature ranging from -30°C to +45°C. Additionally, almost all of the motors at the facility are Ex/ATEX-rated for continuous operation in hazardous gas zones. The motors are critical to production operations with limited spares maintained at the site. As the motors are reaching an in-service age of 20 years, a decision was made by the operator to employ OLPD diagnostic testing and analysis to measure the insulation condition of the 10 kV motor stator windings in order to develop a league table of condition data; this league table would then help to identify the worst performing machines will ensure that the motors continue to work reliably in-service until the next major maintenance overhaul.

The conventional HVCC sensor, mounted within the terminal box of the generator, had been used at the facility for many years to monitor the stator condition of the gas turbine HV generators

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on-site. This conventional OLPD monitoring approach has been applied because the gas turbines are located in a non-hazardous location. However, as most of the HV motors at the facility are located in hazardous gas zones, an alternative PD monitoring solution was sought for the OLPD monitoring of these motors. A solution using wideband, permanent HFCT sensors located at the switchgear-end of the HV cable feeder to monitor the PD activity in the Ex motor stator windings was successfully developed and reported in the IEEE-PCIC 2012 paper presented by the authors [1]. Further to the success of the remote OLPD monitoring trial, a new project was initiated in 2012, whereby permanent, remote OLPD monitoring systems (using HFCT sensors mounted at the switchgear-end of the 10 kV motor feeder circuits) were installed at the facility. Data from the continuous OLPD monitoring over the three-month period (from May to August 2012) from two selected 10 kV motors is presented here ‒ the OLPD trend plots for the 3 months of PD activity monitoring are shown in Fig. 11 and Fig. 12.

Fig. 11: Three-month OLPD trend monitoring data for Motor M-FF a 4 MVA, wet sour gas compressor motor, phase current = 267 A, 1487

rpm, located 521 m from the switchboard

The three month OLPD trend plots in Fig. 11 and Fig. 12 show the monitored PD activity (in nC/cycle) plotted against time (in days) for each phase of motors Ref M-FF and M-BB respectively. The trend plots show low and stable PD activity on motor Ref M-FF (Fig. 11) and a high and increasing trend in PD activity on motor Ref M-BB (Fig. 12). Table 2 and Table 3 provide details of the measured peak PD signals and cumulative PD activity upon each phase of motors Ref M-FF and M-BB respectively.

Fig. 12: Three-month OLPD trend monitoring data for motor M-BB a

6.4 MVA, propane compressor motor, phase current = 447 A, 1500

rpm, located 460 m from the switchboard

Motor Ref Peak PD

(nC) PD Activity (nC/cycle)

M-FF

U Phase 6.9 9.5

V Phase 10.9 14.9

W Phase 9.9 10.5

Table 2: OLPD Measurements From Motor Ref M-FF

Motor Ref Peak PD

(nC) PD Activity (nC/cycle)

M-BB

U Phase 36.3 1153

V Phase 34.0 1135

W Phase 35.6 1155

Table 3: OLPD Measurements From Motor Ref M-BB

To further differentiate the condition criticality of the motors, analysis of the underlying OLPD activity trends was carried out to observe any increasing trends in the OLPD activity in the motor stator windings.

Table 5 gives an example of an OLPD condition criticality league table from a selection of six 10 kV Ex motors being monitored using permanent OLPD monitoring systems. Table 5 illustrates how a combination of the OLPD guideline levels (as shown in Table 4) and OLPD trending analysis (from the continuous OLPD monitor data shown in Table 2 and Table 3) can provide the necessary level of diagnostic data to rank the HV motor stator insulation from the highest risk of failure (motor Ref M-AA) to the lowest (motor Ref M-FF).

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Condition Assessment

Peak OLPD level (nC)

OLPD activity (nC/cycle)

Excellent < 2 < 50

Good 2–4 50–99

Average 4–10 100–249

Still acceptable 10–15 250–500

Inspection recommended

15–25 501–999

Unreliable > 25 > 1000

Table 4: On-line PD Guideline Levels for HV Rotating Machines

in 10‒12 kV Voltage Class

Ref Peak

PD (nC) PD activity (nC/Cycle)

PD monitor trend

Condition

M-AA 35.4 1596 Increasing Unreliable

M-BB 36.3 1153 Increasing Unreliable

M-CC 10.6 689 Stable PD Probable Inspection

M-DD 15.1 262 Stable PD Still

acceptable

M-EE 36.3 102 Stable PD Average

M-FF 10.9 15 Stable PD Excellent

Table 5: OLPD Condition Criticality League Table

It is proposed that this league table condition-based ranking approach can be used to support a CBM program where replacement candidates can be prioritised and spares or replacements procured. The continuous OLPD monitoring also identifies any variations related to operational, environmental and seasonal variations to provide trends in data to further support the CBM approach.

IX. CASE STUDY 2 – OFF-LINE & ON-LINE TESTING OF

11 KV GENERATORS

OLPD measurements were carried out on an 11 kV 20 MVA diesel generator on an offshore oil drilling facility in the Norwegian North Sea. The OLPD testing of the in-service generator showed very high levels of PD (of up to 200 nC).

Fig. 13 shows the PD waveforms as measured with temporarily installed HFCT sensors at the generator terminals. The OLPD diagnostic measurements carried out made showed the PD to be originating from the stator slot section as the PD pulses were phase-to-earth. Due to the high peak and activity levels of PD activity that were measured in these on-line tests, the generator was removed from service for further, off-line PD testing (using a HV 50/60 Hz power source) and a visual inspection/repair of the stator windings at the machine rewind factory. The results of this off-line testing corresponded with the on-line tests carried out with PD levels of 200 nC measured, as described below.

Fig. 13: Large partial discharge (PD) Waveforms from In-service 11 kV Diesel Generator

Upon removal of the generator’s rotor and out-of-service energisation, the PD waveforms measured from the on-line testing (as shown above in Fig. 13) and the off-line PD testing (in the machine rewind factory) were confirmed to be originating from the area within the stator winding where the slot section meets the end winding region. This is a location where significant electrical stresses can be present, particularly if there is insufficient clearances (in air) and/or HV insulation thicknesses.

Fig. 14 shows a photograph of the (very high) visible phase-to-earth PD activity (blue/purple sparks) occuring at the position where the windings exits the slot-section of the coils on this 11 kV, 20 MVA diesel generator (with its stator removed for off-line PD testing of the HV stator windings).

Fig. 14: Visible PD activity occurring near the end-winding region of the stator on a 11 kV, 20 MVA diesel generator

X. CONDITION MONITORING DATABASE

It is proposed that a comprehensive MV & HV network CM coverage can be achieved by pooling of the insulation CM data from multiple sensors and monitoring units, and networking it to a control centre. Such a complete network OLPD monitoring solution will allow the operator to identify any plant with a significant risk of insulation failure and schedule preventative maintenance interventions.

The processed data can be viewed via a user interface containing colour-coded plant condition data superimposed onto a ‘mimic’ of the MV network’s single-line diagram (SLD). Logging, comparison and trending of the CM data provides real-time insulation condition status of the HV cables, switchgear, motors, generators and transformers across the network. Additionally, it is recommended that to avoid false alarms caused by intermittent noise spikes and network switching, any increasing trends in PD activity should be investigated before preventative maintenance interventions are being carried out.

The real-time evaluation of CM data can be done via the CM Database. The top-level of its interface, shown in Fig. 15, is

Segment Waveform

Time us

765

Volts (

mV

)

1,000

500

0

-500

-1,000

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based on the SLD and allows individual substations to be selected and viewed.

Fig. 15: Top level overview – substations can be selected and viewed from central layouts

Within the second-tier substation layer, the operator can access detailed asset condition data (based on PD activity) with alert levels and trends, as shown in Fig. 16. The condition of individual plant items is displayed in form of colour-coded plant condition indicators. Individual switchroom within the network can be accessed to view the CM data for the plant equipment located there. An overview of plant condition is given using colour-coded indicators. Each indicator represents a single OLPD sensor installed within the currently selected switchroom; the colour-codes indicate the insulation condition criticality.

Fig. 16: Substation view – detailed PD information is provided by substation, with alert levels and trends

Significant cost and operational benefits can be gained from complete power generation, industrial and petrochemical industry MV networks. The data from continuous CM technology can be used to support CBM schemes and to direct preventative maintenance interventions to repair plant/cables ahead of insulation failure from PD activity. In this way, the plant operator can receive an early warning against incipient insulation faults and carry out necessary repair so an unplanned outage of the MV network can be avoided.

XI. APPLICABILITY OF REMOTE OLPD MONITORING TO

NUCLEAR SECTOR

The authors suggest that there is an opportunity for the knowledge transfer between the petrochemical and nuclear industries due to inherent similarities between the hazardous (Ex/ATEX) zones of the former and the restricted-access areas of the latter. It is believed that the remote monitoring of PD activity can be applied with the same degree of success in nuclear plants to monitor the insulation condition of MV/HV plant located in hazardous/restricted areas; the motors, generators, transformers and secondary switchgear panels can be monitored from the central switchboard, without the need of accessing these areas.

This approach will help to improve the safety at the facility: firstly, by removing the necessity of entering the restricted zone, and, secondly, by allowing the nuclear plant operators to ensure that there is no immediate risk of MV/HV insulation failure that could pose a serious threat to the plant’s operation.

To conclude, ensuring a failure-free operation of a nuclear plant should be paramount to keep the working environment safe for the staff and to prevent any catastrophic events that could directly or indirectly harm the public and the environment.

REFERENCES

[1] L. Renforth, R. Armstrong, D. Clark, S. Goodfellow, and P. S. Hamer, “A New Technique for The Remote Partial Discharge Monitoring of The Stator Insulation of High-Voltage Motors Located in ‘Ex’ (Hazardous) Locations,” in IEEE Petroleum and Chemical Industry Technical Conference (PCIC), 2012.

[2] L. Renforth, P. S. Hamer, D. Clark, S. Goodfellow, and R. Tower, “Continuous, remote on-line partial discharge (OLPD) monitoring of HV EX/ATEX motors in the oil and gas industry,” Ind. Appl. Soc. 60th Annu. Pet. Chem. Ind. Conf., pp. 1–8, 2013.

[3] BSI, “IEC 60270 High voltage test techniques partial discharge measurements.” 2001.

[4] P.O'Donnell, "Report of Large Motor Reliability Survey of Industrial and Commercial Installations Parts I, II, and III - Annex H," IEEE Trans. Ind. Appl., 2007.

[5] S. Haq, B. Mistry and R. Omranipour, "How Safe is the Insulation of Rotating Machines Operating in Gas Groups B, C & D?", IEEE-PCIC, Toronto 18-20 September 2011.

[6] “IEEE Std. 1434: Trial Use Guide to the Measurement of Partial Discharges in Rotating Machinery", 2000.

[7] “British Standards Institution. BS EN: 60034-27, Off-Line Partial Discharge Measurements on the Stator Winding Insulation of Rotating Electrical Machines", London, 2011.

[8] R. Mackinlay and C. Walton, 2001, "Diagnostics for MV cables and switchgear as a tool for effective asset management", International Conference And Exhibition

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On Electricity Distribution (CIRED) Amsterdam, The Netherlands, 18-21 June 2001.

[9] M. Seltzer-Grant, D. Denissov, R. Mackinlay, F. Petzold, L. Renforth and H. Schlapp, "On-Line Continuous Monitoring for In Service Distribution Class Cables and Switchgears", Proceedings of the 8th International Conference on Insulated Power Cables, Versailles, France, 19-23 June 2011.

[10] M. J. Foxall, A. P. Duffy, J. Gow, M. Seltzer-Grant and L. Renforth, "Development of a new high current, Hybrid 'Ferrite-Rogowski', high frequency current transformer for partial discharge sensing in medium and high voltage cabling", in 59th International Wire & Cable Symposium, Rhode Island Convention Centre, Providence, RI, USA, 7-10 November 2010.