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Doctoral school of energy- and geo-technology January 15–20, 2007. Kuressaare, Estonia

Modern Arrangement to Reduction of Voltage Perturbations

R. Strzelecki Gdynia Maritime University, Department of Ships Automation

rstrzele@am.gdynia.pl

Abstract The contents of the paper encompasses general problems and the most important issues of power supply quality improvement in AC systems. In the context of above, consideration is given to evaluation of bilateral interactions of receivers with electrical power distribution system and methods of their reduction. Discussed are also the basis of operation of the most important compensation-filtration devices and their applications, that are joined to the system in parallel or in series. The main emphasis is placed on application of modern power electronic devices.

Keywords

Improvement of power quality, Custom power Mitigation methods, Power electronics,

Introduction

Rational decision relating to the power supply quality improvement (feeder voltage) can be made only as result of evaluation of different techno-economic aspects of possible actions and solutions [1]. This decision process, in simplified form is presented in the Figure 1. [2]. In addition it requires to answer for the following questions:

• To what extend voltage disturbance in feeder influence work of power supply devices (in it different loads) of the feeder?

• Which devices must be protected from power supply disturbances?

• What is the reason of supply disturbances and how do these disturbances propagate?

• What methods and technical solutions that mitigate voltage disturbances are available to apply?

Fig. 1. The major aspects to find the best techno-economic solution

The first and the second question result from different sensitivity of devices to supply disturbances [3]. Many contemporary devices, in

respect to applied technical solution, are resistant to some of disturbances. For these devices, lower voltage quality in feeder is less important. In the electrical power system, first of all the „sensitive points” must be protected, where supply disturbances cause large economic loses or even may lead to a failure.

The answers for the third and the fourth question refer to the choice of the points where the devices mitigating supply disturbances are joined and what type of the devices were used [4,5]. The disturbances may be mitigated by both reduction of the cause (source of the disturbances) and reduction of the effects in selected nodes (bus bars) of the electrical power system. In the first case, the quality of voltage in the whole system is improved. However, it usually requires the application of compensation-filtration device of relatively high rating power. In the second case improved is only the voltage quality for power receivers that are joined to the selected node. In this case adequate devices that mitigate voltage disturbances have lower rating power. It involves usually favorable relation cause-result and disturbances mitigation as their distance to the source increases.

1 Influence of Load on the Power Quality

1.1 Investigation Results

Fig. 2. Single line diagram of a simple power

distribution system (ZS1=ZS2=0.1+jω⋅3⋅10-4Ω)

Fig. 3. Voltage sags caused by a starting an squirrel-

cage induction motor (Load-1. Symmetrical R-L impedance: S=43 kVA, cosϕ=0.54; Load-2. Symmetrical ohmic resistance: P=15 kW; Load-3. Induction motor IM: S=16 kVA, cosϕ=0.8)

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Fig. 4. Unbalance in the bus voltages caused by unbalanced load (Load-1. Motor IM1: S=16

kVA, cosϕ=0.8; Load-2. Motor IM2: S=16 kVA, cosϕ=0.8; Load-3. 1-phase impedance: S=25 kVA, cosϕ=0.54).

Fig. 5. Effect of the DC offset in loads (Load-1. Symmetrical R-L impedance: S=43 kVA, cosϕ=0.54; Load-2. Symmetrical ohmic resistance: P=15 kW; Load-3. Single phase, half-period diode rectifier with R loads: P=14 kW)

Fig. 6. Harmonic distortion caused by a 3-phase full-bridge rectifier load (Load-1. Symmetrical R-L impedance: S=43 kVA, cosϕ=0.54; Load-2. Symmetrical resistance: P=15 kW; Load-3. Three-phase, full-bridge diode rectifier with R loads: P=23 kW).

Fig. 7. Notch in bus voltages (Load-1. Symmetrical resistance: P=20 kW; Load-2. Absence; Load-3. Three-phase, full-bridge diode rectifier with R-L loads: P=29 kW) .

1.2 Other Important Influences of Loads

Light and Heavy Load Influence. Conducted experiments indicate that the main worry of the customers is too low voltage in feeder. In accordance with IEC 60038 standards, the rated voltage in the feeder LV should amount to 230 V ±10%. The requirement of allowable voltage deviation ±10% applies also to the feeders MV (up to 110 kV). The most common reason for voltage drop is too heave load on the feeder.

Fig. 8. Simple equivalent circuit diagram of a feeder

with load

Fig. 9. Typical voltage profile along a feeder

Fig. 10. Typical voltage profile along a feeder with

and without additional transformer

Fig. 11. Transformer capacity factor as function of

the feeder length and section

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Interharmonics and Subharmonics (Flicker). Interharmonics, always present in the power system, have recently become of more importance since the widespread use of power electronic systems results in an increase of their magnitude. In general, we can distinguish two basic mechanisms to generate current interharmonics. The first mechanism is generation of components, as results of current rapid changes in equipment and installations. Interharmonic disturbances are generated by loads operating in a transient state, either continuously or temporarily, or, in many more cases, when the amplitude modulation of currents occurs. The second mechanism is the asynchronous switching (i.e. not synchronized with the power system frequency) of semiconductor devices in static converters. In many kinds of equipment both mechanisms take place at the same time. The most effect of the presence of currents interharmonics is voltage fluctuations in rms voltage magnitude. These voltage fluctuations may result in light flicker (luminous flux fluctuation), possibly annoying for customers, even for the voltage variations of only a few tenths of a percent. Figure 12 illustrates the way in which a small voltage change produces a noticeable effect on the luminous flux of a bulb.

Fig. 12. Change in luminous flux resulting from a

voltage fluctuation

2 Reduction of Load Influence on the Voltage Profile

Fig. 13. Methods of the location of the compensators

2.1 Principle Compensation of the Load Influence

1SU∆

Lϕ

2SU∆

3SU∆

LϕLϕ

( ) min→− LS UU or ( ) min→=− SCL III

( ) 0== C*CL PIURe

Fig. 14. Illustration of the parallel compensation of load influence

1SU∆

Lϕ

( ) 0== C*SC PIURe

Fig. 15. Illustration of the series compensation of load influence

2.2 Review of Selection Problems

Load Unbalance Reduction. Reduction of supply voltages unbalance, produced by unbalanced load currents, can be obtained by reduction of supply system impedance. Yet it involves large investments, often economically unfounded. Another traditional way is suitable arrangement of unbalanced loads in the feeder. However is sometime is impossible and ineffective. To eliminate the influence of unbalanced loads, especially with high power, first of all balancing compensators are applied.

Fig. 16. The example of ineffective symmetrization

through load’s arrangement

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( )cbcc,b zzzU + Fig. 17. Scott transformer (a) and Steinmetz

compensator (b) series and parallel arrangements

Fig. 18. AC railway connections to a 3-phase grid

using a different arrangements

Load Reactive Power Reduction. Reactive power compensation is used to unload the feeder from the passages of reactive currents and the same reduction of voltage drops. The influence of compensation on the voltage profile is presented in the Figure 19.

( ) ( )

22ImLReLL III +=( ) ( )( )22

CImLReLF IIII −+=

CI

Fig. 19. Effect of the reactive power reduction on the

voltage profile

Fig. 20. The example of thyristor switch (a) and

controllable compensator (b)

Reduction of Load High-harmonics. There are many known methods, resulting in reduction of undesired harmonics, generated by loads.

Fig. 21. Example of the typical shunt LC filters

Fig. 22. Filtering performance of the shunt LC filters

Fig. 23. Typical application of the series LC filters

Fig. 24. Special filter “Lineator™” (Mirus Internat. Inc.) and test results

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Fig. 25. Series APF for reduction load current high-harmonic and test results

Fig. 26. Series active power filter for reduction

neutral current high-harmonic

D-STATCOM

Nonlinearload

PWMCurrent

controller

Referencecurrents

calc.

DCvoltage

controller

SE LL

FL

setDCV

NLi Tv Ci

-DCu

Fig. 27. Block diagram of a shunt active power filter

for load harmonics compensation

NLi

Tv Tv

Tv

Ci

Li

Fig. 28. Feeder current harmonics without and with

compensation

3 Mitigation of the Voltage Disturbance

3.1 Basic Concept and Methodological Questions

Fig. 29. Basic voltage quality concepts covering the

whole system

Fig. 30. Possible mitigation methods

3.2 Modern Protection and Reconfiguration Devices

( )FlSSNsag ZZZUU +⋅=

Fig. 31. Voltage sag due to a symmetrical 3-phase

short circuit in a MV radial grid

Fig. 32. Type of the semiconductor protection and reconfiguring devices

Fig. 33. Alternative SCB topology (a) and example of the SCB installation (b)

The best position to place a current limiter (SCB or SCL) is the output of the main incoming transformers, i.e. locations 1 and 2. Any fault in the

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ZL

PCCZF

IC

IL

network will be limited before it can cause any coordination problem. In this case the current tap setting of the downstream overcurrent relays can be set at lower values. A limiter at the bus-tie location 3 is the most effective as it has lower losses under normal operating conditions. Since the current flowing through this position for a fault at any part of this circuit is maximum, the rating device at this location must be very high.

3.3 Compensation Devices

Compensating voltage disturbance devices can be generally divided similarly to the compensators of load influence, into shunt and series devices. However different is their control and therefore their characteristics. Shunt compensators effectively mitigate voltage disturbances in the PCC node, in the cases when their cause is load connected to the node. If the voltage disturbances ‘come from outside’ (from the supply source), then efficiency of series compensators is usually poor, and is the poorer, the feeder is ‘more rigid’ (i.e. the lower is impedance of the feeder). Let us consider, as an example, which is linear system presented in the Figure 34, where ∆US – disturbances of supply voltage US, IC – compensating current, ∆UL – voltage disturbances in load ZL, ZF – feeder impedance. For this system it is easy to notice that:

F

SCL Z

UIU

∆∆ =⇔= 0

Fig. 34. Illustration of the shunt compensation of the voltage source disturbance

Additionally we assume that short-circuit source current ISC =US/ZF is 10 times higher than the load current and that ∆US=0.1US. In this case full compensation of small voltage disturbances in the PCC node requires application of shunt compensator with rated power at least equal to load rated power. Compensation of voltage disturbances ‘injected’ on the supply source side is much more effective when applying series compensators.

Fig. 35. Illustration of the principle of series

compensation

Let us consider a simple example of application of series compensator without additional energy source, to stabilize voltage UL=UPCC in PCC node in symmetric linear circuit presented in the Figure 35a. In this case, because voltage injection UC is in

quadrature with line current, performance of the compensator is equivalent to connection to the feeder of series reactance with the value X=UC/IS. As it results from above efficiency of application of series compensator, without additional energy source, to regulate voltage UPCC=UL is strongly limited by the load phase angle φL. In the specific case of resistance load (cosφL=1) possible is regulation of the voltage UPCC only below source voltage US.

Fig. 36. Illustration of the series compensator limit

without additional energy sources

Fig. 37. Influence of the equivalent reactance X on

the current and voltages – examples

Because of presented above limitations associated with unfeasibility to generate or absorb real power, series compensators without additional energy source are the most effective when applied to compensate higher harmonics or imbalance voltage in the PCC node of the feeder. In these cases, compensator must be equipped only with small capacitor, of capacity depending on pulsation of instantaneous active power.

Fig. 38. Basic agreements of a series compensators

with additional energy source

Systems of series compensators with additional energy source, are predisposed especially for voltage sags compensation. Additional energy source frees the range of voltage UPCC in the node PCC from the load and allows to keep quality factors of this voltage at the demanded level, independently from

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the kind of US supply voltage disturbances. Allowable duration and depth of voltage disturbances US, which when compensated require active power P, obviously depends on energy capacity of additional energy source. With regards to their features of UPCC voltage parameters restoration back to demand parameters, such systems are called Dynamic Voltage Restorer (DVR)

Fig. 39. A typical location and operation principle of the DVR

QCU

PCU

ratedU

QCU

PCU

ratedU

QCU

PCU

ratedU

Fig. 40. Phasor diagrams illustration of the DVR

control strategies

Fig. 41. Comparison of the „in-phase” and „minimal

energy” control strategies

Fig. 42. Typical output voltages characteristic of a

commercial DVR

The DVR structure presented in the Figure 38, with shunt APF rectifier instead of AC/DC converter, can also serve a function of shunt-series or series-shunt compensators, popularly called Active Power Line Conditioner (APLC) or Unified Power Quality Conditioner (UPQC). Figure 43 presents three different topologies UPQC with load side-connected shunt compensator.

a) Typical UPQC topology

Superior controller

iS iL

u′LuT

uC1

iC2

UDC

Voltage

control

Current

control

iC2uC1

1 2

Model

b) UPQC topology with inversed inserted sources

Model

c) UPQC topology without dc-link

Superior controller

iS iL

u′LuT

UDC2

Current

control

Voltage

controlUDC1

iS+iC2

uC1

iS+iC2uC1

1 2

Model

iS

iS+iC2

u′LuT

iLuC1

Fig. 43. Basic UPQC arrangement with load side-connected shunt compensator

3.4 Immunization Devices

Many of sensitive loads, including IT equipments and connected with safety, defense, health and also devices connected with important technological processes must be protected against catastrophic failure in electrical grid, i.e. total lack of voltage. The most dangerous are primary failures, i.e. failures in parts of electrical grid occurring relatively case to load and those occurring directly in feeder supplying the load. In these cases compensation devices are not sufficient mitigation arrangements. And we must apply devices that immune sensitive loads to catastrophic failure – duplicated feeders from the grid or standby power supply devices. First solution where used are reconfiguration devices, i.e. STS, is only effective if the two connections are electrically independent, i.e. a predictable single failure will not cause both network connections to fail at the same time. It depends on the network structure, and, often, this requirement cannot be met without the use of very long (and expensive) lines. More often, especially for the loads of power up to several MW, the second solution is definitely more efficient –standby power supply devices. In this case use are various Engine Generating Sets (EGS) and, static Uninterruptible Power Supply (UPS) systems. Appropriately designed EGSs can meet most requirements for reserve power sources as well as

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continuous power supply. On the other hand, the main and crucially disadvantages of a ESGs , especially high power units, are noisy (the average noise level is from 70 – 95 dB), large and heavy, and they require large fuel storage, air intake and exhaust systems. Consequently, these generators are usually installed in separate buildings, relatively distant from occupied buildings. Not always is also acceptable delay in their starting time acceptable

Static UPS systems are now commonly used as standby power supplies for critical loads where the transfer time must be very short or zero. Modern 3-phase static UPS systems, with power electronic switches with turn-of capability, are easily available in ratings from 10 kVA up to 4000 kVA. As well as providing a standby supply in the event of an outage UPSs are also used to locally improve power quality. The efficiency of UPS devices is very high, with energy losses ranging from 3% to 10%. The basic classification of UPS systems, in accordance with norms IEC 62040-3 or EN 50091-3, specifies three main classes:

• UPS-VFD (output Voltage and Frequency Dependent from mains supply) also called standby UPS, line-preferred UPS or off-line UPS;

• UPS-VFI (output Voltage and Frequency Independent from mains supply) we also use names as: on-line UPS, double-conversion UPS and inverter-preferred UPS;

• UPS-VI (output Voltage Independent from mains supply) are commonly called also line-interactive UPS systems.

Basic UPS structures of the individual class are presented in the Figure 44. Note that similar typology to UPS-VI(b), also called delta-conversion UPS, have systems UPQC with inversed inserted sources (see Figure 43b).

Grid

ES

AC/DC DC/AC

Load

DC/AC

Grid

AC/DC

UPS-VDF or UPS VFI

LoadUPS-VI (b)

ES DC/AC

Load

Grid

DC/AC

Grid

AC/DC

Load

UPS-VI (a)

UPS-VI (c)

Fig. 44. Block diagrams of a typical structure of the

standard UPS

4 Summary

This paper discussed general issues concerning supply quality in AC systems, and especially methods and selected countermeasures for power quality improvement. Main attention is paid to cause-effect relation. Simple examples show characteristic influence of different typical loads on the supply disturbances in the load connection point and on their propagation distribution systems.

In the most broad part of the paper, the authors tried to present material in a way, which allows evaluate and compare potentials of various modern mitigation agreements, without getting into details of their structure and control. Information content in these sections can be considered as the answers for questions referring to the choice of connection point and general type of devices mitigating supply disturbances.

The authors hope that the whole paper was inspiring for the reader and will result in deeper study of dedicated literature.

References

1. Power Quality. Mitigation Technologies in a Distributed Environment, edited by Antonio Moreno-Munoz, Springer, July 2007 (in printed)

2. Didden M.: Techno-economic analysis of methods to reduce damage due to voltage dips. Ph.D. Thesis, Leuven/Belgium: Catholic University of Leuven, 2003.

3. Green T.C., Arámburo H.: Future technologies for a sustainable electricity system: The role of power electronics in future power systems. Cambridge/UK, Cambridge University Press, 2005.

4. Ghosh A., Ledwich G.: Power quality enhancement using custom power devices. Boston: Kluwer Academic Publishers, 2002.

5. Strzelecki R., Supronowicz H.: Power factor correction in AC supply systems and improving methods. Warsaw/Poland: Warsaw University of Technology Publishing House, 2000 (in Polish)

Science Work founded from the research resources in 2005-2008 of Polish Ministry of Science Higher

Education as a project Nr R01 002 01