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PROTECTION STRATEGIES FOR IMPROVING QUQLITY OF SUPPLY

G H Topham

Eskom, South Africa

ABSTRACT

Electricity supply quality has becorne an important and integral part of the utility business. Current topics receiving attention include, inter alia, the incorporation of quality of supply parameters into electricity supply agreements, utility and user mitigation technologies, the development of standards and the quantification of quality of supply parameters thrlough measurement. Voltage dips are one of the most common events on the utility power system which can adversely affect the quality of supply. The occurrence of voltage dips IS costly, both to the customer and the national economy. In South Africa, the loss suffered by major industrial customers due to dips is more than R 1,2 billion per annum (Coney (1)).

This paper explores the influences of protection operation on quality of supply, focusing on voltage dips, and proposes some possible strategies to improve quality of supply through the adoption of different protection technologies, philosophies and application and setting practices.

The strategies focus, inter alia, on hul t clearance times, auto-reclose policies, setting philosophies and adaptive protection techniques. The paper also presents the results of a study in which voltage dip measuremenls over a 2,5 year period are correlated with 2624 fault events on the Eskom transmission system over the same period, in terms of the type (and hence speed) of protection employed.

INTRODUCTION

Power system stability, safety and minimizing equipment damage have traditionally been the focal issues of power system protection. The impact of power system faults and associated protection remedial action on customers operations has not, until recently, received much attention. One of the major consequences of the operation of protection equipment following a power system fault is short duration dips experienced by customers. Reclosure, following a protection operation, onto a sustained fault, causes yet another dip further exacerbating the customers supply quality. A strategy to improve the quality of supply in terms of dips, therefore, needs to focus on fault clearance times and reclosing philosophy.

QUALITY OF SUPPLY PARAMETERS

The proliferation of power electronic equipment and the increased sensitivity of industrial equipment to electricity supply disturbances has brought pressure to bear on utilities to pay more attention to quality of supply issues. Quality of supply is measured in terms of a number of parameters including voltage fluctuations (notches, dips, undervoltages, spikes, surges and overvoltages), frequency variations, voltage unbalance, harmonic distortion and voltage flicker (Eskom Quality of Supply Group (2)). By far the most prevalent disturbance type contributing to a degradation of quality of supply is the short time voltage depressi-q, commonly referred to as a dip or sag.

VOLTAGE DIPS

Voltage dips are largely caused by power system faults such as insulation flashovers (utility initiated dips) and the connection and operation of large loads such as motors (customer initiated dips). Power system causes of dips cannot be completely eliminated, but can to a certain degree be controlled by mechanical intervention and indeed by adopting sensible protection philosophies and practices. Utility interventions to decrease the depth of voltage dips as well as other quality of supply phenomena could require the building of additional transmission lines. This action in itself increases the exposure of the utility network to power system faults and hence dips. On the other hand, intervention behind the meter at customers premises to increase the immunity of customers plant to voltage dips, only aids a particular customer (i.e. is very localized).

Voltage Dip Parameters

Voltage dips are characterized by a number of parameters. These are:

the frequency of occurrence; the duration (the time when the measured r.m.s. voltage is less than 0,9 per unit); the magnitude of the depression (the extent below nominal to which the measured r.m.s. voltage drops); phase shift; and, propagation.

Protection equipment and philosophies can have a direct influence on the first two of these parameters.

Developments in Power System Protection, 25-27th March 1997, Conference Publication No. 434, 0 IEE, 1997

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Correlation of protection employed and the latter three parameters has not, as yet, been fully investigated.

fault events on the transmission system. A subset of this data was used together with the quality of supply database information in a correlation exercise to

dips with combinations Of these determine the linkages between measured voltage dip parameters affect different c w m m s differently, duration and the type of protection involved in clearing

the correlated fault. The subset of the transmission data depending on the sensitivity of the customers equipment, but generally, the duration and depth Of the depression, coup1ed with the frequency Of Occurrence

included the fault date and time, the transmission line, and the types of protection at each line end, The results of the correlation exercise are presented later in this paper.

CLASSIFICATION OF PROTECTION USED ON THE ESKOM TRANSMISSION NETWORK

are the main concerns.

Voltage Dip Classification

South Africa historically uses a two dimensional scatter plot of the magnitude of voltage depression versus dip duration to -present dip data ~ (see figure 1). Superimposed on the plot are five key areas or windows. The first is the area in which a voltage dip would have little or no effect on customers plant. This area is where the depth of the dip is not more than 10% and the duration of the dip is less than 20 ms. The other four areas on the plot which are labelled A, B, C and D, give a simplistic classification of the severity of the dip.

Magniludcof Dip ( X below nominal)

Figure 1: Dip Window Plot

ESKOM QUALITY OF SUPPLY MEASUREMENTS

In 1994 Eskom embarked on a drive to measure the depth, duration and frequency of occurrence of dips at selected measurement points throughout the network. To date, quality of supply measuring equipment has been installed at 176 Iocations on the Eskom transmission network and at selected customer locations in the distribution network. Data captured is loaded into a central database, providing a platform on which to base studies and investigations into quality of supply issues. The data is also used as a basis for making dip mitigation and capital investment decisions, and as an input into the setting up of supply contracts with customers.

TRANSMISSION LINE FAULTS

Transmission line faults are the predominant disturbances in the Eskom transmission system, with single-phase-to-ground faults representing some 87 % of all fault types. The System Operations Department in Eskoms Transmission Group maintains a database of

Prior to the mid 1980s, the transmission line protection utilized by Eskom was largely of the electromechanical and early analogue electronic types. For the purposes of the correlation exercise, the protection schemes using these relay types have been termed Phase 1 . From the mid-1980s to the mid-1990s , Phase 2 schemes were installed and the relays used were largely second generation analogue electronic devices. These relays are further classified according to whether or not they employ switched measurement techniques (i.e. only one measuring element). Also included in this category are the early analogue electronic relays used by Eskom for the protection of series compensated lines. Phase 3 schemes, installed from the mid- 1990s, comprise numerical-based relays.

The majority (more than 88 YO) of Eskoms transmission line protection schemes use dual main protection relays. Twelve distinct relay types, applied in various combinations in single and dual main arrangements, were used in Phase 1 type schemes. Four different relay types were used in Phase 2 type schemes, predominantly in an identical dual main protection arrangement. Two different relay types are used in Phase 3 type schemes with the two main protections being either identical or comprising one of each relay type.

PROTECTION SPEED OF OPERATION

The speed of operation of protection equipment together with the speed of operation of the associated circuit- breaker determines the fault clearance time. Before the correlation exercise mentioned above was undertaken, a number of expected results were postulated. These were:

clearance at both line ends of a meshed transmission line is required to effect full restoration of the voltage as measured at a location within the zone of sensitivity of the particular fault. Therefore, the dip duration is largely dependent on the line end with the slowest protection time.

it was anticipated, based on manufacturers specifications of relay operating times, that the best performance was to be expected from Phase 2 non- switched protection relay based schemes, followed by Phase 3 numerical relay based schemes, followed by Phase 1 (electromechanical and analogue

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electronic relay) based schemes and then by Phase 2 switched relay based schemes.

CORRELATION OF QUALITY OF SUPPLY MEASUREMENT AND FAULT DATA

As the raw data required to perform the correlation between the measured quality of supply dip data and the recorded transmission system fault data was located in two different databases, some dala manipulation was required. The transmission line fault data was obtained in Microsoft Excel@ form and comprised 2624 fault events. The quality of supply data was accessed from the separate Prealism database and contained 28676 measurement records. The data of interest for the correlation exercise was imported from the two separate databases into Microsoft Access@. A query w , ~ generated to perform the correlation. As the time references of the individual recording devices in the transmission network are not synchronized, a correlation algorithm based on an exact match of the date and hour stamps and a time stamp difference of less than or equal to 1 minute was used to generate a set of correlated results. As the main focus of the exercise was primary protection operations, correlated events where the measured dip duration was greater than 1510 ms were discarded. Figure 2 show!$ the data processing methodology used.

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Figure 2: Data correlation metholdology

CORRELATION RESULTS

The results were grouped according to the types of protection employed at each line end. The main groupings were as follows:

Phase 1 at each line end; Phase 2 at each line end;

0 Phase 3 at each line end; 0 Phase I at one line end and 2 at the either line end;

and, a combination of Phase 2 and 3 at each line end.

As the whole quality of supply initiative focuses on the customer, the worst dip duration for the three measured phases (in the case of multi-phase dips) was used in further processing of the results.

The average maximum dip duration for all correlated records was 79,70 ms. The average dip duration for faults cleared by Phase 1 protection was 85,79 ms. For faults cleared by Phase 2 protection the average dip duration was 69,2 1 ms. These results were as expected. An unexpected result was the average dip duration of 57,42 ms for faults cleared by Phase 3 protection which is markedly better than the average for all events. The reason for this is, at this stage, not clearly understood. Some possibilities include favourable conditions for fast breaker clearance (all correlated events relate to two particular lines in the Natal area), or more consistent protection operating times as compared to Phase 2 relays. Future investigations will aim at establishing the explanation for the results. A selection of the correlation results is shown graphically in Figure 3 .

100 . --

Category of protection types

Figure 3: Chart showing correlation results

The results indicate that the choice of relay technology employed has a definite influence on the voltage dip performance in the vicinity ofthe faulted transmission line. In contradiction to expectation, it

even at only one line end can reduce the overall voltage dip duration. From a financial point of view, it is

protection equipment purely for the sake of reducing

A total of 46 16 correlated records was obtained. An important point to note is that the query did not result in a one-to-one correlation as the effect of individual fault

location. However, averaging the results still provides a good indication of the correlation between measured dip

provides a summary of the correlation results.

events was, in Some cases, at more than one reasonable to conclude that upgrading the protection

duration and protection speed Of )peration. Table I obviously not feasible to upgrade all older types of

voltage dip durations.

TABLE 1: Numerical results of correlation study

I Phase 1 at both line ends 85,79 I 26,59, 3063 I Phase 2 at both line ends 69,2 1 22,14 1067

Phase 3 at both line ends 57,42 16,59 62

Phase 1 at end I and Phase 2 at end 2 65,98 21,93 404

Phase 2 & 3 at both line ends

Phase 2 Non-switched protection at both line ends

52,s

68,62

9,67

21,47

20 I 972 I 43 I 23,18 I Phase 2 Switched protection at both line ends 90,93 I

However, it would make sense to keep protection technology upgrading as a possibility when considering the options available to reduce voltage dip durations for specific customer quality of supply mitigation projects. When cost justifying refurbishment, quality of supply is an important aspect which should be brought into the equation.

The settings employed on transmission line protection relays will also have an influence on the overall fault clearance times and hence the voltage dip duration. Revising settings with the aim of improving relaying speed of operation could be an economical solution to improving quality of supply, although, the results would perhaps be somewhat marginal compared to other options. If this option was considered, it would be important to not place the security of the protection in jeopardy when revising the settings, as this could worsen the quality of supply rather than improving the situation.

RECLOSING PHILOSOPHIES

As was mentioned earlier in this paper, another voltage dip parameter of importance is the number of dips. Some customers equipment, which can tolerate single dips of a certain depth and duration, cannot withstand multiple dips within a short space of time. Eskoms reclosing philosophy to date has primarily been one of reclose at all costs (1). This clearly is not to the advantage of the customer. An improvement to this approach is therefore required.

Adaptive Reclosing

An improved approach to reclosing would be to only permit a reclose if the probability of success was determined to be of an acceptable level. An adaptive auto-reclose relay has been developed by Reyrolle in conjunction with the University of Bath in the UK, and with input from Eskom (Laycock (3)). The relay is based on neural network technology. A data capture unit was

installed at a particular location in the Eskom network to gather data regarding the currents and voltages during a single pole open condition following a single pole trip. This data was used to train the adaptive reclose relay. A field trial relay was installed during the first quarter of 1996 on the Eskom network and is currently under evaluation.

The auto-reclose relay monitors the line voltages during the open pole condition. Through a signature analysis, the recloser can determine when it is safe to reclose thereby minimizing the chance of reclosing back onto an uncleared fault and thereby minimizing the number of voltage dips which would be experienced by customers in the vicinity.

CONCLUSIONS

Voltage dips experienced by customers are costly and a review of protection philosophies and practices, and the installation of appropriate protection equipment, can have a positive impact on minimizing the effect of the dips.

REFERENCES

1. Coney RG, 1996, The impacts of protection philosophy and performance on quality of supply Southern African Conference on Power System Protection

2. Eskom Quality of Supply Group, 1994, Power Quality Reference Guide

3. Laycock, WJ, 1994, Adapting reclosure of HV circuits to system conditions, Southern African Conference on Power System Protection.