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Dynamic Performance of Distance Relayson Series Compensated Transmission Lines using RTDS 1Anil Kumar and 2Meera KS* 1,2CPRI, Bangalore, India

Series compensation is installed in power system networks to increase power transfer capacity, improve the system stability, reduce system losses, improve voltage regulation and for achieving flexible power flow control. Distance relays are widely used as main or backup protection of transmission lines including series-compensated transmission lines. The performance of conventional distance relays is affected by series capacitors and cause certain protection issues. This paper briefly discusses the problems like voltage inversion, current inversion, overreach and under reach during the fault conditions specific to series compensated lines. The behavior of capacitor protection techniques is discussed with simulations performed using Real Time Digital Simulator (RTDS) simulator for a typical 400 kV system having series compensation. The analysis is based on Transmission Line fault simulations, internal and external to the 400-transmission line where the Fixed Series Compensation (FSC)is installed.

Keywords: Distance relay, series capacitor, MOV (Metal oxide varistor), voltage inversion, current inversion, overreach, Real Time Digital Simulator (RTDSTM). INTRODUCTION To meet the increased demand on the consumption of electric power, it is required to construct new transmission lines. But construction of new transmission line is not much appreciable due to cost and environmental constraints. It is possible to meet the excess power demand with the existing lines by installing the series compensation. The Fixed Series Compensation (FSC) uses series capacitors to cancel a portion of the inductive reactance of the transmission line, and thereby reduce the overall impedance of the line, resulting in increased power flow. The power flow in the system can be regulated by switching these series capacitors in and out. Series capacitors are installed at one or both line ends. Line ends are typical capacitor locations, as it is possible to use space available in the substations. Another possibility is to install the series capacitors at some midpoint location on the transmission line. Series capacitors located at the line ends cause more protection problems than those installed at the midpoint of the line. The compensation levels adopted are generally in the range of 30 to 70 percent. As the transient response of the series capacitors are not

predictable, they present unique challenges for directional, distance, and differential elements. Capacitors will normally be associated with the metal oxide varistors (MOVs)to reduce the overvoltage and spark gaps and bypass switch across the MOV to bypass the capacitor when the MOV energy level exceeds a threshold. Distance protection relays have been widely used for protecting transmission lines due to their simple operating principle and operate independently under most circumstances (Network Protection & Automation Guide third ed. ALSTOM, 2002). The line protection scheme must perform correctly with the series capacitor still in operation. The impedance as seen by the relay will depend on the series capacitor and its associated protection depending on factors like conduction or non-conduction of MOV, operation or non-operation of bypass switch.

*Corresponding author: T Meera KS, CPRI, Bangalore, India. Email: meera@cpri.in

International Research Journal of Power and Energy Engineering

Vol. 3(2), pp. 092-098, November, 2017. © www.premierpublishers.org, ISSN: 3254-1213x

Conference Paper

Kumar and Meera 093 The capacitor, MOV or bypass switch coming in the fault loop depends on the ratings of capacitor and energy limits of MOV. Some of the problems that distance relay encounters in the presence of series capacitors are current inversion, voltage inversion, overreach and under reach. Thus, it is required to evaluate the performance of the relay for these conditions. The power system is modeled with the series capacitor at one end of the transmission line on RTDSTM and by simulating various fault conditions, tests are carried out. The faulted voltage and current signals at the relay location is fed to the relay (also modeled on RTDS) and the performance analyzed. Voltage and current Inversion The first challenge that a distance relay located on a series compensated line faces is the dynamic changes that occur in the total impedance presented by the series capacitor and its protection devices. The state of the series capacitor, whether it is in service/by-passed, or partly in service and partly by-passed, complicates the reach settings of zone 1 elements. Series capacitors located at line ends are more likely to create voltage and current inversions because of the absence of line impedance between the relay location and series capacitor (R.J. Marttila, 1992). Voltage inversion: This phenomenon is experienced on series compensated lines, if the impedance between the fault point and the relaying point is capacitive but the overall impedance between the power system source and fault point is still inductive i.e. the reactance of the capacitor is greater than the reactance of the line section up to fault point. During voltage inversion the voltage at the relaying point will be of the opposite sign with the source voltage, causing the voltage inversion. A voltage inversion on the power system results in a distance element (polarized by voltage) to incorrectly identify the fault direction. Current inversion: For a fault on series compensated line the impedance between the source and fault point can be capacitive. Under this circumstance, the fault current will be capacitive instead of inductive. This phenomenon is known as a current inversion and leads to false directional decision in distance relays. The presence of series compensation also generates sub harmonics that can cause distance elements to overreach. System Model

The real time digital simulator used in the tests reported in this paper is supplied by RTDS Technologies, Inc. Canada. The simulator performs fully digital electromagnetic transient power system simulation in real time, utilizing the Dommel Algorithm (Hermann W. Dommel, 1968) similar to non-real time EMTP-type programs. The RTDS has an Electromagnetic Transient type representation and so can

provide a very detailed and realistic model of the system, including all nonlinearities. The simulator with parallel processing architecture is specifically designed for power system simulations and ensures continuous real-time operation. This type of simulator is an ideal tool for designing, studying, and testing protection schemes (Real Time Digital Simulator (RTDS TM) user’s Hardware and Software manual set, RTDS Technologies, Canada). A. Power System Network model

The power system network chosen for simulation is based on recommendations of CIGRE Working Group 04 of Study Committee 34 (Evaluation of characteristics and performance of power system protection relays and protective systems, CIGRE Working Group 04 of Study Committee 34 (Protection), 1986). Fig 1 shows the 400 kV power system network simulated on RTDS, with source at either ends of the line. The parallel line is kept open by the circuit breakers at either end of the line. The data used for the system simulation are given in Appendix I. The transmission line is modeled using the distributed parameter Bergeron line model. The network behind the buses at either end are represented by voltage sources behind the impedance calculated based on the short circuit contribution from the network. The series capacitor on RTDS is modeled as shown in Fig 2. A compensation level of 40 % has been chosen. The Lightning arrester across the series capacitor has been modeled with its nonlinear characteristics and providing of the discharge voltage at @10 kA and the associated decay factor. a. Relay model

Fig 3 (a) and 3(b) shows the sampling, extraction of fundamental component of voltage and computation of line-to-line voltages. The currents are computed similarly. The

Fig.1: System modeled on RTDS

Fig. 2 MOV protected series capacitor

Int. Res. J. Power Energy Engin. 094 Relay logic diagram implemented on RTDS for computing the line-to ground impedance and line-to-line impedance is shown in Fig 3(c) and Fig 3(d). The impedance computed by the relay model is plotted on the operating characteristic (mho and quadrilateral characteristic) of the relay model. The impedance locus in the complex plane directly indicates whether the fault is within or outside the protected zone. Under normal operating conditions, the impedance locus remains outside the operating characteristic. For internal faults, the measured impedance locus moves into the operating characteristic and there lay (model) is expected to issue a trip command to the circuit breaker modeled in RTDS.

Fig 3(a) Voltage sampling for Relay model

Fig 3(b): Line to line Voltage computationsfor Relay model

Fig. 3(c): Line-to-Ground positive sequence impedance computation

Fig 3(d): Line-to-Line positive sequence impedance computation Simulation Results and Analysis The dynamic performance of the distance relay model is studied for various fault locations. The relay model is first validated by comparing the measured reactance and resistance (without series compensation) with the transmission line reactance and resistance from the relay location to the fault position. It was observed that after decaying oscillations in the reactance and resistance the values settled to the desired values, thereby validating the relay model. A. Single Line to Ground fault (SLG) – 0 % line length An SLG fault (R-phase) was created at 0 % of the line length, with a fault resistance of 20 ohms. Figure 4 (a) shows pre-fault and post-fault voltage and current waveforms at the relay location, Arrester currents & energies and also the status of the bypass switch as obtained from the real-time simulation tests. Fig 4(b) shows the Impedance seen by the relay with series capacitor in the fault loop. The results show that the fault current is 16.41 kA and the energy dissipation across MOV is 38.8 MJ which is less than the threshold energy limit of the MOV. So, the bypass switch will not operate resulting in the capacitor as well as MOV to remain in the fault loop. Due to the combination of MOV and the series capacitor branch in the fault loop, the impedance seen by relay is slightly capacitive as seen in figure 4(b). It can also be observed from fig 4(a) that voltage inversion is occurring in this case due to which, the locus of the impedance seen by the relay lies outside the zone (mho characteristic) leading to mal-operation of the relay.

Kumar and Meera 095

Fig 4 (a): Voltages and currents at relay location for a SLG fault at 0% of the transmission line

Fig 4(b): Impedance seen by the relay with series capacitor in the fault loop

.

Fig 4(c): Locus of Impedance seen at the relaylocation forfault at 0 % along the line. However, a relay with Quadrilateral characteristic having memory polarization and also higher resistive reach setting (Appendix II gives the settings calculations (Sub-committee on relay/protection under task force for power system analysis under contingencies, 2014), the locus of the impedance lies in zone 1 of the relay as shown in figure 4(d). Therefore, the relay picks up and trips in zone 1.

Fig 4(d): Locus of Impedance seen by the relay for a fault at 0% 0 % along the line

Int. Res. J. Power Energy Engin. 096 a. Three-phase bus fault (3 Phase) Figure 5 (a) shows the CVT voltages, CT currents, MOV currents & voltages and its energy dissipation, for a 3 phase fault at sending end bus terminals. This is seen as a forward fault by the relay leading to its mal-operation. Like a normal fault in forward direction a phase shift in CT currents which lags the CVT voltages is observed in figure 5(a). Locus of impedance as seen by i.e. shown in figures 5(b) and 5(c) respectively.

Fig 5 (a): Voltages and Currents at Relay location for a 3 Phasebus fault at sending end

Fig 5(b): Locus of Impedance seen by the relay for a 3 Phase bus fault

Fig 5(c): Locus of Impedance seen by the relay for a 3 Phase bus fault b. Three-phase fault (3 Phase) – 100 % line length A 3-phase fault was created at 100 % of the line length with zero fault resistance. Fig 6 (a), 6(b) and 6(c) shows the plots of CT currents and CVT voltages and the impedances

Fig 6 (a): Voltages and Currents at Relay location for a 3 Phase fault at 100% line length As seen by relays located at the sending end with mho and quadrilateral characteristic. For this case, the fault was correctly identified by relays (mho as well as quadrilateral characteristic) as a fault in Zone 2.

Kumar and Meera 097

Fig 6(a): Locus of Impedance seen by the relay for a fault at 100 % line length

Fig 6(b) Locus of Impedance seen by the relay for a fault at 100 % line length CONCLUSIONS

The main objective of series compensation in transmission lines is to increase power transfer capacity. Protection relay application in series compensated networks is not straight forward and hence a careful evaluation needs to be performed. It is difficult to foresee the impact of dynamic behavior of the series capacitor compensated transmission network when setting distance protection schemes. The setting of distance protection schemes using standard practice is not adequate, without a clear understanding of the behavior of the protected network itself. The dynamic behavior of distance relays with a real-time digital simulator (RTDS) is done in real time so as to study their performance under actual power system conditions.

Few of the results of the performance of distance relays evaluated for a typical system with series compensation transmission network has been discussed. The impact of the MOV protected series capacitor response on the relay operation has also been highlighted. These studies clearly show that detailed dynamic studies are required if distance relays on series compensated lines are to be set with confidence. ACKNOWLEDGMENT The authors would like to thank the authorities of CPRI for having permitted to publish the paper. REFERENCES Evaluation of characteristics andperformance of power

systemprotectionrelays and protective systems, CIGRE Working Group 04 of Study Committee 34 (Protection), January 1986.

Hermann W. Dommel (1968). Digital Computer Simulation of ElectromagneticTransients in Single and Multi-Phase Networks,” Paper 68 TP 657-PWR presented at IEEE Summer Power Meeting, Chicago, IL.

Network Protection & Automation Guidethird ed. ALSTOM, 2002.

R.J. Marttila, Member (1992). Performance of distance relaymho elements on MOV-protected series-compensated transmission lines. IEEE Transactions on Power Delivery, Vol. 7. No. 3, pp 1168-1178.

Real Time Digital Simulator (RTDS TM) user’s Hardware and Software manual set, RTDS Technologies, Canada.

Sub-committee on relay/protection under task force for power system analysis under contingencies, “Model setting calculations for typical IEDs line protection setting guide lines protection system audit check list recommendations for protection management,” 2014.

Accepted 23 October, 2017 Citation: Kumar A and Meera KS (2017). Scada Based Remote Monitoring and Data Acquisition for Energy Management. International Research Journal of Power and Energy Engineering, 3(2): 092-098.

Copyright: © 2017. Kumar and Meera. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.

Int. Res. J. Power Energy Engin. 098 APPENDIX I A. Source VS = 400kV ∟0o, 50 Hz Source Impedance ZS = 145.45∟86.18oΩ B. Transmission Line Positive Sequence Resistance, (r1) = 0.02897 Ω/Km Positive Sequence Reactance, (x1) = 0.3072 Ω/Km Zero Sequence Resistance, (r0) = 0.2597 Ω/Km Zero Sequence Reactance, (x1) = 1.0223 Ω/Km Zero Sequence Susceptance (b0) = 2.347 µmho/Km Positive Sequence Susceptance (b1) = 3.630 µmho/Km Line length =117.85 Km C. Load Real Power, P = 1000 MW Reactive Power, Q = 30 MVAR D. Fixed Series Compensation: 40 %, Xc =14.48Ω APPENDIX II Total positive sequence impedance, Z1

1 = 36.364 ∟84.61º Total zero sequence impedance, Z0

1 = 124.30 ∟75.74º Total positive sequence impedance (secondary) Z1 = CT/PT ratio x Z1

1= 10∟84.61º Ω Distance relay characteristics are set based on the NRPC guidelines for transmission line protection. Zone1 protection = 0.8*Z1= 8∟84.61º Ω

Zero compensation factor, K0 = (𝑍0−𝑍1)

3𝑍1 =0.812

Time delay for zone1, td1 = 0 S Total zero sequence impedance (secondary) Z0 = 34.184 ∟75.74º Ω Zone2 protection = 1.2*Z1 = 12∟84.61º Ω Time delay for Zone2, td2 = 0.35 S Zone 3 protection = 2*Z1 = 20∟84.61º Ω Time delay for Zone3, td3 = 1 S Resistive reach, Rreach = 30 Ω Directional angle for Distance protection zones, rgDir = -30º Negative restraint angle for Distance protection zone, ArgNeg Res= 115º