Impact of series compensation on the perfomance of distance protection on Eskom transmission grid.

The research will examine some of the challenges faced by remote protection relays when protecting a transmission line that includes series capacitors. Relay operation will include an analysis of the effect of series capacitors on relays for faults before and after the series capacitors.

LIST OF APPENDIXES

LIST OF TABLES

INTRODUCTION

Distance Protection

Distance Protection Philosophy
Distance Zones of Protection
Distance Relay Characteristics

Plain Characteristic

The balance point on the remote protection relay is defined by the zone range settings of the relay. Figures 1-4, 1-5 and 1-6 show an overview of the generations of the remote protection relay characteristics, with Fig.

Figure 1-2 Distance Zones of Protection [11]

Timer

The characteristic shape of the remote relay operating zones has been developed over the years. Figures 1-4 (a) represent the impedance characteristic of a directional control element, so the semicircle AQTS represents the combined characteristic of the directional and impedance relay.

Timer

Quadrilateral characteristic
Quadrilateral Distance Applications .1 Short Line Application

Load Encroachment Supervision Application
Power Swing Blocking Application
Single-Pole Trip Application

Permissive Distance protection Schemes

Permissive Under-Reaching Scheme
Permissive Over-Reaching Scheme
POR Scheme on Series Compensated Lines
Final Comparison Remarks on PUR and POR schemes

Distance Relay Settings

Background of the REL 531 relay
Zone 1 Settings
Zone 2 Settings

The monitoring operating point for the load impedance in the blocking region (see Fig. Load. We let hR be the range of the relay at A with the capacitor operating.

Figure 1-4 Plain Distance Relay Characteristics [18]

2. Series Compensation

Series Compensation of Transmission Lines

Improved Power Transfer Capability

The study included an analysis of how the transmitted power varies with the size of the series capacitor, assuming that the magnitude of the voltage on the transmitting bus would be V1 [kV] and that the magnitude of the voltage on the receiving bus would be V2 [kV]. It was further assumed that the series reactance of the high-voltage line is XL [Ω] and the series resistance of the line is zero.

Figure 2-1 Power Transmission Line with Series Capacitor

Series Capacitor Protection

Spark Gaps

Principle of Operation

Metal Oxide Varistors

Principle of Operation

Final Comparison Remarks on SG and MOV schemes

A complete study obtained from [13] of SC on the Eskom Hydra network is as shown in Appendix A. 2-4 show a typical series capacitor protected by the spark gap scheme consisting of the basic following elements: Spark Gap and by -pass switch. If the reintroduction attempt is made too soon, it is likely to cause re-ignition of the ionized SG, especially when the line current is high.

The gap scheme is sufficient for many applications, but when fast reinsertion after disconnection of external fault is required (i.e. less than 100ms after fault clearance), the relatively long deionization time of the gap is a disadvantage [4]. 2-5 shows a typical series capacitor protected by the MOV scheme consisting of the following basic elements: the MOV, the damping circuit and the bypass switch. According to Goldsworthy model [9], the apparent impedance of the SC and MOV combination, as a function of the current flowing in the line, can be represented in the equivalent circuit shown in Fig.

Therefore, for bank currents below the protection level SC ("The protection level is the fault current level at which the MOV starts to conduct" [14]), the series circuit presents a constant capacitive reactance equal to its full SC rating.

Figure 2-3 SC Protection Survey Statistics on the Eskom Hydra South Network [13]

Effects of Series Capacitors and its Protection

Behavior of Non Series Compensated line and its Protection
Behavior of Series Compensated line and its Protection
Voltage Inversion
Current Inversion

Mainly power transmission lines are inductive, as a result of which the internal fault currents in such a network will cause the phase currents flowing from the terminal to the protected line to lag behind the source voltage, assuming that the reference direction of the relay currents is from the busbar to the protected platoon Reversal of voltage and current are two problematic phenomena that challenge the relay logic in the positive identification of transmission line faults [4]. This is because the impedance seen by the relay is no longer a unique match of the physical distance from the relay location to the fault point.

The phenomenon occurs as a result of the relay at substation A looking forward in the line and seeing the impedance to the fault point as capacitive (XC > XLA) rather than inductive (XC < XLA), causing the voltage measured at the relay. point to be capacitive (ie the fault current leads the measured voltage at relay A by 90˚). 2-13, a three-phase fault just ahead of SC, if we assume the arrangement of (XC > XLA), VA and VA' voltages will be 180 degrees out of phase, where VA' is the normal forward fault voltage and VA voltage reversed in relation to VA' voltage [15]. 2-13, in order for the distance protection relays located at substation A to correctly identify the fault for what it is, a forward fault, the line side voltage data VA' should be utilized by the relay.

However, in cases of high resistance faults, the low fault currents will prevent the overvoltage series capacitor protection devices from operating, thus allowing the occurrence of the current inversion phenomenon.

Figure 2-10 Apparent Impedance for Non Series Compensated lines

System Under Study 1 System Layout

Studies Performed
Relay Setting Calculations
Response of Relay at Muldersvlei for a fault at ‘G’
Response of Relay at Droerivier for a fault at ‘G’
MOV Response for Faults In front and Behind SC
Response of Relays at Muldersvlei for a fault at ‘F’
Response of Relay at Droerivier for a fault at ‘F’

3-6 shows a dynamic impedance analysis of the relay response at Muldersvlei for a single phase to a ground fault at point G. 3-7 shows a dynamic impedance analysis of the relay response at Droerivier for a study where a three-phase fault was placed at point G, the point immediately before the Bacchus SC . 3-8 shows the dynamic impedance analysis of the relay response at Droerivier for a single-phase earth fault at point G.

However, as in the case of the relay response at Droerivier for a three-phase fault before SC, the same can be observed for a single-phase fault. 3-11 shows the dynamic impedance analysis of the relay response at Muldersvlei for a study where a three-phase fault was placed at point F, the point immediately after the Bacchus SC. 3-13 shows the dynamic impedance analysis of the relay response at Muldersvlei for a single-phase earth fault at point F.

3-17 describes the dynamic impedance analysis of the Droerivier relay response for a single phase to ground fault at point F.

Figure 3-2 Safety margin for zone 1 setting [20]

4. Current Supervised Zone 1

Background

Current Supervised Zone 1 Operating Philosophy

This determination of the current level according to the principle should not be less than 150% of the protection level of the electric SCs closest to the protected line. Now since the successful operation of the CSZ1 configuration is based on the MOV conducting enough current to ensure SC bypass, a current level setting of 150% of the selected SC protection level was shown to be sufficient in studies conducted by [14] to ensure that the MOVs are performing. In some cases, such MOV conduction has been shown to be sufficient to provide SC bypass, resulting in the location of the fault impedance being placed well away from the characteristic region of zone 1, as illustrated in Fig.

When investigating the likelihood of using the CSZ1 configuration for the relay at Muldersvlei to eliminate the impact of external series compensation on the distance protection performance, the following static short-circuit studies were carried out to achieve the RMS fault currents on the network depicted in Figure First, the current level setting was selected to 4.18 kA (i.e. 150% of the Komsberg SC MOV protection level). The three-phase fault studies carried out showed that the current sensed by the relay for a fault just after the current transformers (CTs) at Muldersvlei was 9.4 kA and at 80% of the Mul-Bac line it was 4.2 kA.

Consequently, applying the CSZ1 configuration in this study is shown to be sufficient to ensure that the remote protection's security is maintained.

Figure 4-1 CSZ1 Impedance Vector Diagram [14]

Impact of Bacchus SC on Current Supervised Zone 1

Moreover, even when the evaluation of currents using CSZ1 configuration on distance protection had to be disregarded as certain assumptions were made on fault current calculations performed in static short-circuit condition. 4-3 was performed in full EMT mode and the results showed that for a fault immediately behind the Komsberg 1 SC, when the MOV is in conduction, the reactance of the SC is reduced sufficiently so that the impedance loci are seen to settle without for the immediate range zone 1.

F ≥ Current Level Setting

CSZ1 Impedance Locus

Z1 Instantaneous Trip

Response of the Muldersvlei Relays with MOVs in and out of Service
Response of the Droerivier Relay with MOVs in and out of Service
Conclusion
Further Work Recommended
REFERENCES

4-5 illustrates the response of the relays at Muldersvlei when the MOVs on the series capacitors at Bacchus were taken out of service. The answer was that the impedance locus was seen passing through zone 1 and settling right into the characteristic range area of zone 2 of the relay at Muldersvlei. 4-8 illustrates the response of the relay at Droerivier when the MOVs were taken out of service.

4-9 illustrates the reaction of the relay at Droe River when the MOVs were brought back into service. 4-4, as in the case of the Muldersvlei relays, the Droerivier relay will also overreach for mistakes behind the Bacchus SC. Furthermore, if we compare the performance of the relay at Muldersvlei for a fault immediately behind the Komsberg 1 SC (a series capacitor located in the middle of the adjacent line of the one being protected), and that immediately behind the Bacchus SC (a series) capacitor located on the busbar of the adjacent line of the one being protected).

Therefore, the relays still fail to trip immediately for a fault on the adjacent line.

Figure 4-5 Response of relay at Muldersvlei with MOV out of service.

APPENDIXES

APPENDIX A

Current Rated Reactance Bank Rating Continuous 8 hours to 12 hours 30 minutes to 6 hours 10 minutes to 2 hours Transmission over Voltage.

Table A-1 Series Capacitor Data on the Eskom Hydra Network

APPENDIX B

APPENDIX C

APPENDIX D

APPENDIX E

SETTING OF THE

DISTANCE PROTECTION RELAY

REL 531

The distance protection function in REL 531 line protection consists of five independent zones each containing three measuring elements for phase-to-earth (Ph-E) faults and/or three measuring elements for phase-to-phase (PH-PH) faults. Operating mode for remote protection zone 1 for phase-to-phase faults. Possible setting = [ Off / On. Operating mode of time-delayed trip for the distance protection zone 1 for Ph-Ph faults. Possible setting = [ Off / On.

Operating mode of time-delayed shutdown for remote protection zone 1 for Ph-E faults Possible setting. This setting provides a phase-to-phase fault resistance coverage of: 50.00 ohm primary Time-delayed trip operating mode for the distance protection zone 2 for Ph-Ph faults. Operating mode of time-delayed trip for remote protection zone 2 for Ph-E faults Possible setting = [ Off / On.

It should be noted that the reverse direction will also be affected, as the opposite directions are a mirror image of the forward directions.

APPENDIX F

The interest of the studies being investigated is based on Zone 1 only, therefore Zone 1 and 2 settings will be carried out. Operation PP = On. a) The Zone 2 requirement for series comp. lines or protection on sections affected by it is greater than or equal to 150%. The Zone 2 requirement for series-compensated wiring or protection on affected sections is greater than or equal to 150%.

BIOGRAPHY