ABSTRACT

Accepted voltage quality standards focus on two areas: • voltage regulation – the magnitude of the voltage • waveform quality – the shape of the voltage

waveform

The growing application of diverse embedded generation worldwide is creating new problems with voltage regulation. Increasing amounts of non-linear loads, in such form as variable-speed drives, are causing waveform quality to deteriorate.

Traditionally automatic voltage control (AVC) through on-load tap-changers (OLTCs) has been used to regulate busbar voltage, although its success has been limited. This paper highlights the deficiencies in these complex and restrictive schemes and outline the Transformer Automatic Paralleling Package (TAPP) scheme, which has gained acceptance worldwide.

The TAPP scheme has been available for some years in an analogue form, and is well liked for the simplicity and effectiveness of its operation. It has now been developed as a numeric AVC relay. This paper will show how further development of the control algorithm allows improved control of customers’ voltage, even in areas with low, or varying, system power factors, which in the past have been difficult to manage.

The numeric relay is now able to tackle the other area of concern with voltage quality – waveform quality. Although unable to greatly influence the waveform quality, it can provide information allowing the power utility to pinpoint areas of the network for further investigation.

The paper ends by proposing how voltage control should develop in the future to cater for increasing diversity of users and generators of electricity, making use of new technologies.

INTRODUCTION

It is incumbent upon power utilities to maintain customers’ supplies within specified parameters under varying conditions of load. In the vast majority of cases, the last point at which the power utility can influence the voltage is the on-load tap-changer (OLTC). A

number of factors influence the optimum tap change operating point:

• the power system voltage level, • loading, • power factor, and, • if transformers are operating in parallel, the

magnitude of any inter-transformer circulating current.

As power systems have developed and become more complex, the potential efficiency savings of a comprehensive voltage control system cannot be realised by the majority of existing AVC relays. Indeed their complexity has often resulted in voltage control not being implemented.

After examining how voltage control has developed to meet this need, the paper will describe a simple solution to remove the unneeded complexity, based upon the fundamental requirements of the power system.

THE BASICS OF VOLTAGE CONTROL

Simple AVC scheme

AVCRelay

VVT

ICT

Rline + jXline

Veff

FIGURE 1: Transformer connected to single load

The simplest form of AVC can be used where a single transformer supplies a single load (FIGURE 1). If the load is some distance from the transformer, there will be a voltage drop in the line. The AVC relay makes an estimate of the voltage at the load using a model of the

IMPROVED VOLTAGE QUALITY THROUGH ADVANCES IN VOLTAGE CONTROL TECHNIQUES

VP Thornley and NJ Hiscock

VA TECH Reyrolle A,C&P Fundamentals Limited

line and applies Line Drop Compensation (LDC) based on the conditions seen at the transformer:

( )linelineCTVTeff XRIVV .j. +−= (1)

targeteffdev VVV −= (2)

where Veff is the effective voltage at the single load, VVT and ICT are the measured voltage and current respectively, Rline + j.Xline is the model for the impedance of the line, Vdev is the voltage deviation from target, and, Vtarget is the target voltage.

The above represents the ideal situation: in reality there are usually a number of loads on a transformer distributed at different distances (electrically) from the transformer, so the model of the line will always be a compromise. It can be shown an optimum voltage control will establish a constant voltage point at the electrical mid-point of the network, thus achieving a minimum overall variation between the no-load and full-load conditions.

Parallel Transformers

It is normal practice for power utilities to parallel transformers to obtain a higher security of supply. In FIGURE 2, which shows an example with two transformers, the load on each transformer (discounting any circulating current) is half of the total load, so the model will produce half the required voltage boost.

AVCRelay

VVT

ICT,1

Rline + jXline

AVCRelay

VVT

ICT,2

Iload

Transformer 1 Transformer 2

Veff

FIGURE 2: Parallel transformers connected to

single load

If the effective terminal voltages of the paralleled transformers are not identical, a circulating current will flow around them. This will be highly reactive since the transformers are highly inductive. If two paralleled transformers operate the simple AVC scheme described above, eventually one transformer will be on the highest tap and the other on the lowest tap. The busbar voltage will be an average of their terminal voltages and a high amount of circulating current will flow between them.

This will cause an unnecessary power loss within the transformers and the network, reducing their useful capacity and efficiency, and can result in the loss of one transformer due to overload, and a consequent severe overvoltage.

EXISTING VOLTAGE CONTROL SCHEMES

One of the main aims of all the schemes discussed below is to keep the transformers’ taps together, to remove circulating current. To a greater or lesser extent they all achieve this, however as will be seen some schemes create other problems in doing so.

Master-Follower

In this scheme, one AVC relay in each paralleled group of transformers is nominated as the master; all other AVC relays in the group are set as followers, or slaves. When the master issues a tap instruction to its tap-changer, the followers all issue identical instructions to their tap-changers. All the transformers keep in step. However, if the incoming voltages are different, a permanent circulating current will flow.

Negative-Reactance Scheme

This scheme makes use of the fact that circulating current is reactive. The reactive element of the measured current is used to bias the measured voltage in order cause the transformers to tend to a tap position where any circulating current is minimal.

Transformer Automatic Paralleling Package (TAPP)

TAPP is a modified negative-reactance AVC scheme that eliminates the main disadvantages of the negative-reactance scheme. The circuit (FIGURE 3) includes a negative-reactance control (or “coupling” element). The modification is in the three buswires, which enable the currents of all paralleled transformers in the substation to be summed.

FIGURE 3 (a) shows circulating currents only, that flow in opposite directions in the two transformers. As the circulating current does not flow through the LDC element, it will not affect the LDC control. This cures the first disadvantage of the negative-reactance scheme.

FIGURE 3 (b) shows load currents only, that flow in the same direction in both transformers. It can be seen that the load current presented to each of the LDC elements is the total load of the substation. If one transformer is switched out, the load seen by the LDC element remains unchanged, so no change to LDC settings is required.

As a further advantage, the switched out transformer continues to respond to changes in load so when it is switched back into service it is at the correct tap to ensure that the busbar voltage remains constant.

A crucial point to note is that the buswires are used only for the purposes of LDC. The coupling control will still be effective when the buswires are not used. A corollary of this is that the scheme will eliminate circulating currents not only on a common busbar, but also across a network.

The control algorithm for this scheme is: θje.. lineloadVTeff ZIVV −= (3)

trCTccv XIV j.= (4)

( ) ccvtargeteffdev VVVV −−= (5)

where the model for the line Rline + j.Xline is now represented in mag-arg form by Zline.ejθ, Iload is the summated load, Xtr is the reactance of the transformer and Vccv is the corrective coupling voltage.

Scheme Comparisons

The three schemes outlined above are compared in TABLE 1.

The ideal of voltage control is to achieve the correct system voltage with minimal circulating current under all system conditions. Master-Follower is cumbersome and complex, difficult to operate and rarely produces acceptable results. The negative-reactance scheme eliminates most of the failings of master-follower but still fails to provide an effective LDC.

While TAPP can achieve superior performance to both master-follower and standard negative-reactance schemes in nearly all circumstances, it suffers from some minor disadvantages itself, which in an ideal world would be eliminated.

The area that needs most attention is the LDC performance under varying power factors. All the existing AVC schemes do suffer from “droop” when the power factor is less than unity.

Couplingelement

LDCelement

Couplingelement

LDCelement

Couplingelement

LDCelement

Couplingelement

LDCelement

(a) circulating current

(b) load current

Transformer 1 Transformer 2

Transformer 1 Transformer 2

Iload

circulatingcurrent

FIGURE 3: TAPP scheme showing circulating and load currents

TABLE 1: Comparison of AVC Schemes

Mas

ter-

Follo

wer

Neg

ativ

e R

eact

ance

TAPP

Differing incoming voltages ! " " Differing transformers ! " " Differing tap spacings ! " " Paralleling across the network ! " " No re-configuration required for transformer switch-out ! ! "

Simple scheme to implement ! " " Linear LDC at “normal” system power factors " ! "

Linear LDC at all system power factors ! ! !

Differing CTs and CT phases " " !

With regard to the TAPP scheme the reason for the droop is seen by reference to FIGURE 3(b). Although circulating currents do not flow in the LDC element, transformer load currents do flow in the coupling circuit. Resistive currents have little effect on the coupling control. However, as the power factor of a load decreases, the increasing reactive load causes the coupling control to respond. It is normal with the TAPP method to take account of some reactive component in a load current and to shift the angle by 15° before applying it to the coupling circuit. FIGURE 4 shows a typical AVC response of the TAPP scheme at different power factors for an application with transformer impedance of 10% and required LDC boost at full load of 2.5%.

100

101

102

103

0 50 100

Load Magnitude/%

pf = 1.00pf = 0.97idealpf = 0.95pf = 0.90pf = 0.80

FIGURE 4: Busbar voltage for theTAPP scheme

DEVELOPMENT OF NUMERIC VOLTAGE CONTROL

There are many opportunities afforded by numeric AVC relays, however, the potential has not been exploited. Numeric AVC relays to date have implemented traditional analogue techniques together with a sophisticated user interface and the ability for remote communications. This section looks at ways in which numerical methods are now employed to bring further improvements to voltage control than those already provided by TAPP.

Voltage Regulation

Numerical techniques provide the method of removing poor performance for low power factors. Because it is possible to manipulate magnitudes and angles as the designer chooses, the angle shift required in the coupling circuit can be tailored to the system power factor. On most circuits, the system power factor will not vary widely. The user applies the normal system power factor to the AVC relay as a set point.

Embedded generation is becoming increasingly common, both synchronous and asynchronous. One feature of asynchronous machines is their ability to export real power while absorbing reactive power. Their characteristics can cause the power factor to swing substantially and a fixed power factor set point is not a viable option. An alternative scheme, known as the true circulating current scheme, is described below and can be used in these circumstances.

VVT

ICT,1

VVT

Transformer 1

Transformer 2

ICT,2

Icirc,1

Icirc,2

Iload,1

Iload,2

FIGURE 5: Multiple loads applied to numeric TAPP

With numerical techniques, the various components that make up the measured current ICT can be completely isolated, giving improved performance. FIGURE 5 shows the current seen by two AVC relays ICT,1 and ICT,2, with respect to their phase voltages VVT (when the transformer LV circuit breakers are closed the measured voltages will be identical). The load currents, Iload,1 and Iload,2, have the same power factor. Transformer 1 is on a

higher tap position than transformer 2, hence a circulating current will flow represented by Icirc,1 and Icirc,2 in the diagram. If the measured currents, ICT,1 and ICT,2, are summated, the network power factor can be found. The true load on each transformer and its contribution to circulating current can be established; therefore, compensation is always correct resulting in the complete elimination of droop in the AVC response.

However, as the currents must be summed for this technique to work, the ability to parallel across networks is lost. A technique that will eliminate droop and allow paralleling across networks is to use a combination of the fixed power factor set point and the true circulating current method.

Consider the network of FIGURE 6. Transformers 1 and 2 are at the same site and so have strong coupling; transformer 3 is some distance away and so the coupling will be weaker. Any mismatch in transformer open-circuit terminal voltages between transformers 1 and 2 will result in a high circulating current, while a difference between transformers 1 and 3 will result in a much lower circulating current because of the network

impedance.

The circulating current method fully eliminates circulating currents between transformers 1 and 2 and allows them to provide correct LDC under any power factor. A weaker compensation is also applied by measuring the difference between the transformer current and the power factor set point, Ires in FIGURE 7. Although the system power factor might fluctuate, the set point applied should be the average power factor. This prevents transformers 1 and 2 drifting from transformer 3 – any tapping action will naturally keep them together. If there is circulating current flowing between the two sites it will be small so the LDC should not exhibit droop. The control algorithm for this scheme is:

( ) trrestrcirctargeteffdev ZkIZIVVV ... −−−= (6)

where Ztr is the reactance of the transformer. Note that all quantities in this equation are scalar.

At any time, if the system power factor is known it is possible to calculate the circulating current so the

AVCRelay

VVT,a

ICT,1

Rline,1 + jXline,1

AVCRelay

ICT,2

Iload,a

Transformer 1 Transformer 2

AVCRelay

ICT,3

Transformer 3

Rline,2 + jXline,2

VVT,a VVT,b

FIGURE 6: Coupling across a network

VVT

ICT,1

VVTTransformer 1 Transformer 2

ICT,2

Icirc,1

Icirc,2

Iload,a Iload,a

Ires,1

Ires,2

VVTTransformer 3

ICT,3 Ires,3sys pf sys pf

sys pf

FIGURE 7: Eliminating circulating currents across a network

numeric TAPP method will be very accurate. Therefore, an adaptive algorithm is needed that can:

(a) establish the optimum value k, and, (b) progressively estimate the true system power

factor over time.

Work is currently being undertaken to achieve this.

Transformer Switching

As numeric voltage control develops, functions will be added to further improve voltage regulation. The master-follower scheme suffers from inherent complexity in its operating method. In order not to fall into a similar trap with what, on the face of it, is a simple task, the underlying principle that is used when developing the scheme is to answer the question: “What would you do if you were there?” The development process is assisted by attempting to recreate what an engineer does if he operates the tap-changers manually. The result is a scheme that is based upon the fundamental measurands of the system, not on a complex arrangement of inter-transformer wiring and conditions.

This philosophy finds benefits in the area discussed here: switching transformers out of service. As was outlined previously, the TAPP method will ensure that the transformers are on the tap position to result in minimal voltage change when switching a transformer into service. Numeric communication between AVC relays allows the operator to prepare a transformer for

being switched out of service – again resulting in minimal voltage change.

If an operator had to switch out a transformer manually, with minimal voltage change, he would first put that transformer onto the tap position where it will cause minimal effect when switched out – transformer current at unity power factor. This way there is little voltage drop across the transformer, so switching it out will cause little change to busbar voltage. Of course, it is necessary to tap the other transformer to keep the voltage at the correct level. During this time, there will be circulating current flowing, but this is acceptable for a limited period until the transformer is switched out.

Implementation of this procedure automatically, consists of the following steps:

1. On the transformer to be switched out, set the target power factor to unity.

2. Instruct the other transformers in the busbar group to relax their coupling control.

3. When all voltages balance again, inform the operator that the transformer is ready to be switched out.

4. Switch out the transformer. 5. Return the other transformers to their normal

operating mode.

FIGURE 8 (the dotted line) shows the result of a simulation for a two-transformer substation (the worst case) with 30MVA 20% transformers when the total load on the substation is 30 MVA at power factor 0.97. This is compared to a “normal” switching operation, represented by the solid line.

10.8

10.9

11.0

11.1

11.2

FIGURE 8: Comparison of switching methods for transformers

Operator issues Prepare for Switch-out

Operator opens CB

±1.25% band

AVC responds after normal delay

Ready for Switch-out. Operator opens CB

AVC responds quickly to correct voltage

Trending and Prediction

Numeric relays are able to store and analyse vast amounts of data. Use of historical data enables the operators to make predictions of the likely system voltage and load at a given time on a given day. This can be used to provide a bias to the voltage control algorithm with the intention of reducing unnecessary tapping operations – if the relay predicts that the load is about to increase there is little point in tapping down.

The algorithm used for this prediction might range from a simple “What is the load likely to be in 10 minutes, based on yesterday’s levels?” to an adaptive technique that makes it decision based upon the predicted load and predicted incoming voltage for the time of day, day of week and season.

The advantages gained from applying these techniques are both to the customer – better regulation; and the network operator – less tap-changes mean less maintenance.

WAVEFORM QUALITY

There is now increasing interest in the quality of the waveform, not just in its RMS value.

A number of bodies have a stake in the power system, and they tend to have differing requirements:

1. users of power require the voltage waveform to be a sinewave,

2. suppliers of power require the current

waveform to be a sinewave, 3. suppliers of power and the network operator

prefer the power factor to be close to unity, and, 4. the network operator prefers a balanced three-

phase voltage and current.

The second requirement is a direct result of the first – a current waveform of “low” quality makes it difficult to produce a voltage waveform that is a sinewave. The third and fourth requirements are in the interests of system efficiency.

With numeric relays, it is possible to record various measurements of waveform quality. Simple measure-ments that can provide indication of how the network is performing include form factor, crest factor and the ratio of negative phase sequence to positive phase sequence. The AVC relays throughout the network provide ample opportunities to record this information.

This section of the paper describes the work currently being undertaken to turn these easily obtainable measurands into a tool to help to diagnose the source of waveform quality problems.

Form factor and crest factor are two measurements that vary with the form of a wave. Form factor is the RMS value of the waveform divided by the mean absolute deviation (MAD) – that is the mean of the rectified waveform. Crest factor is the peak value of the waveform divided by the RMS value.

For a pure sinewave the form factor is 1.111 and the crest factor is 1.414. As harmonics are superimposed, the values will vary. Although it is not possible to completely identify the nature of the harmonics from

1.25

1.30

1.35

1.40

1.45

1.50

1.55

1.05 1.10 1.15

Form factor

3rd @ 0 degs3rd @ 90 degs3rd @ 180 degs5th @ 0 degs5th @ 90 degs5th @ 180 degs7th @ 0 degs7th @180 degs7th @ 90 degs

FIGURE 9: Form factor vs. crest factor for different harmonic content

these two figures, they can give some indication of the type of harmonic.

FIGURE 9 compares the plots of form factor versus crest factor for 3rd 5th and 7th harmonic as their magnitude increases from zero to 10% of fundamental. Points to note from this plot are:

• All harmonics at 90° to fundamental will exhibit unchanged form factor. The crest factor will increase as the harmonic content increases.

• The plots of 5th at 180° and 7th at 0° show knee points. These coincide with the peaks of the waveforms being reversed, e.g. as shown in FIGURE 10.

• Where the peak of a harmonic coincides with the peak of the fundamental (e.g. 3rd at 180° or 5th at 0°) the crest factor will be higher than that for a sinewave; and vice-versa.

FIGURE 10: Composite waveform comprising fundamental plus 10% 5th harmonic at 180°

Given analysis of the information, steps can be taken to improve the quality – devices are now available that can actively improve waveform quality, e.g. dynamic voltage restorers (DVRs).

TRANSFORMER MANAGEMENT

It was once the case that power utilities were extremely reluctant to mix control and protection. That philosophy is now being reconsidered with the arrival of feeder managers – devices that handle the protection and control of feeders together with providing SCADA access. A similar trend can be expected with transformers as power utilities become more confident with the operation of numeric relays and wish to make greater use of the benefits this affords.

With transformer managers there are 3 elements which can be included in the device:

• transformer protection – differential, overcurrent, REF, Bucholz etc.

• automatic voltage control • transformer condition management

Knowledge of the tap-position by the protection enables tighter settings on the differential protection. However, the greatest benefits come in the integration of automatic voltage control and the control of transformers’ pumps and cooling devices. The load on the transformer can be used to predict temperature for the control of cooling devices. The load and temperature can be optimised for most efficient operation.

The control algorithm can be expanded to include this extra functionality, which is more easily represented in matrix form, figure (7). The values of the constants will be dependent on the transformer and, in fact, might not be constant, or indeed, linear.

( )

−−−=

100000.

,

,

freqheatcircheatloadheat

freqtrnettrLDCv

ndev

ndev

kkkkZkZkk

TV

( )

( )

( )

−

−

−

•

−

−

1,

1,

arg

ndev

ndev

sys

res

reqcirccirc

load

ettVT

TV

ffI

III

VV

(7)

FUTURE DEVELOPMENT OF VOLTAGE CONTROL

The means of implementing voltage control may well change in the future. Currently OLTCs are used extensively throughout the network and these are mechanical devices, the design of which has changed little in the past fifty years. Solid state devices will have a future in voltage control and it is possible that the DVR and OLTC functions will merge. From the point of view of the control system, this provides new opportunities, as the means of applying control will probably be more flexible, with a response time of less than one cycle (rather than 1 second as at the moment). It will be possible to actively improve waveform quality.

For these developments to succeed three conditions must be fulfilled:

1. the technology must be in place to enable power quality control,

2. there must be a willingness from power utilities to embrace these new technologies, and

3. the AVC system must have the ability to make full use of the control device.

We, as developers of AVC systems, are now preparing for this vision of the future, using our knowledge and experience of the fundamental requirements of power systems.