Abstract This paper analyzes the impact of series capacitor compensation on line relay protection, including its influence on the measured phasors and the operating characteristics of the protection, based on the Shanxi Yangcheng to Jiangsu power transmission project. It discusses the fault response of several typical protection principles in series capacitor compensated lines, addresses the design considerations for relay protection in such lines, explores the design principles of the protection, and finally proposes several suggestions for the design of line relay protection based on the Yangcheng to Jiangsu power transmission project. Keywords Series capacitor compensation protection, beyond-Mohm distance protection, reactance-type distance protection 0 Introduction Series capacitor compensation devices (referred to as series compensation, and lines containing series compensation are referred to as series-compensated lines) can increase stability margin, improve grid load distribution, and enhance the power flow transmission capacity of long-distance transmission lines. In the initial construction phase of the Shanxi Yangcheng to Jiangsu power transmission project, this type of compensation device will be used to save investment. However, the introduction or removal of series compensation will change the line impedance, affecting the accurate measurement of protection based on impedance characteristics. This paper analyzes the impact of series capacitor compensation on relay protection based on this project and proposes a design scheme for the line protection. 1. Impact of Series Compensation Capacitors on Line Protection 1.1 Impact of Series Compensation Capacitors on Phasor Measurements of Relay Protection Based on the equivalent calculation of the system network of the Yangcheng-Jiangsu power transmission project, both voltage reversal and current reversal are possible. 1.1.1 Voltage Reversal In non-series-compensated lines, the short-circuit current flowing from the power source lags behind the power source potential, and the bus voltage and power source potential are basically in phase. However, in a series-compensated system, if the inductive reactance from the power source to the protection installation point is greater than the capacitive reactance, when a fault occurs near the series compensation point (as shown in Figure 1, point F1 fault), the voltage phase applied to the relay will be 180° out of phase with the power source potential, meaning the voltage measured by the protection will be reversed. Voltage reversal will also occur in the fault sequence network diagram. [IMG=Simple Series Compensation System]/uploadpic/THESIS/2008/1/2008010711070730075T.jpg[/IMG] Figure 1 Simplified Series Compensation System The current direction of distance protection or directional protection will not change due to series compensation. This change in voltage direction will affect the correctness of protection operation, but it will not affect protections that do not use the measured fault voltage as a reference (such as current differential protection). 1.1.2 Current Reversal On a series-compensated line, with the bus voltage at the beginning of the line as the reference, the short-circuit current may lead the potential, with a phase change of about 180°, i.e., current reversal occurs. When the negative sequence impedance of the power supply is less than the capacitive reactance, the negative sequence current measured by the protection will also be in the opposite direction. For protections that use current as a reference, such as distance protection, directional protection, and current differential protection, the normal selectivity will be affected when the current reverses. 1.2 The Influence of Series Compensating Capacitors on Typical Protections 1.2.1 The Influence of Series Compensating Capacitors on Distance Protection When the MOV of the series compensating capacitor protection bypasses the series compensating capacitor, the distance protection naturally adapts. Therefore, the following mainly discusses the case where the series compensating capacitor is not bypassed. For the fault at point F1 in Figure 1, the measured voltage of the line protection relay is taken from the bus-side voltage transformer (TV). When |XC| < |ZS| (where XC is the capacitive reactance of the capacitor and ZS is the impedance from the protection installation point to the S-terminal power supply), the voltage reverses. The operating characteristics of the memoryless ohmic relay and the variable ohmic relay with finite memory polarization are shown in Figure 2(a). During a fault within the zone, the variable ohmic relay can operate during the dynamic period but cannot operate during the steady state period. When |XC| > |ZS|, the current reverses. The operating characteristics of the ohmic relay and the variable ohmic relay are shown in Figure 2(b). During a fault within the zone, the variable ohmic relay cannot operate during either the dynamic or steady state period. [IMG=MN line M-side protection operation characteristics XC|<|ZS| time]/uploadpic/THESIS/2008/1/2008010711071265671O.jpg[/IMG] (a) XC|<|ZS| time [IMG=MN line M-side protection operation XC|>|ZS|]/uploadpic/THESIS/2008/1/20080107110721874243.jpg[/IMG] (b) |XC|>|ZS| time The shaded area in the figure is the reactor operation boundary, and the shaded area is the non-operation area. Figure 2 When there is a short circuit fault at point F1, the protection operation characteristics of the MN line M-side For the series-compensated adjacent line, as shown in Figure 1, the voltage measured by the protection on the M-side of the MP line is reversed. Let the setting impedance of the protection relay on the M side of the MP line be ZY′. When |XC|<|ZY′|, the operating characteristics of the ohm relay and the variable ohm relay are shown in Figure 3(a). It can be seen that for a fault in the opposite direction, the variable ohm relay will malfunction in steady state and will not operate in dynamic state. When |XC|>|ZY′|, the operating characteristics of the ohm relay and the variable ohm relay are shown in Figure 3(b). It can be seen that for a fault outside the zone, the variable ohm relay will malfunction in dynamic state and will not operate in steady state. [IMG=When there is a short circuit fault at point F1, the protection operation characteristics of the M side of the MP line (a) when |XC|<|ZY′|]/uploadpic/THESIS/2008/1/2008010711072734343Q.jpg[/IMG] (a) when |XC|<|ZY′| [IMG=When there is a short circuit fault at point F1, the protection operation characteristics of the M side of the MP line |XC|>|ZY′]/uploadpic/THESIS/2008/1/2008010711073260024A.jpg[/IMG] (b) when |XC|>|ZY′| The shaded area in the figure is the reactor operation boundary, and the shaded area is the non-operation area. Figure 3 Protection operation characteristics of the M side of the MP line when there is a short circuit fault at point F1 1.2.2 Impact on negative sequence relays and directional protection Assume that an asymmetrical short circuit occurs at point F1 in Figure 1. If the voltage measured by the protection relay on the M side of the MN line is taken from the line TV, in the negative sequence network, when |Z2S|>|X2C|, I2J leads U2J, and the negative sequence directional relay operates. However, when |Z2S|<|X2C|, current reversal occurs, I2J lags behind U2J, and the negative sequence relay does not operate. When the voltage measured by the protection relay is taken from the bus TV, the negative sequence directional relay operates regardless of whether current reversal occurs, and it is unrelated to the series compensation capacitor. For the protection on the M side of the MP line in Figure 1, when a short circuit occurs at point F2, the operation of the protection depends on the relative magnitude of the total impedance of the series compensation line and the power supply at the R end and the series compensation capacitive reactance. 2. Series Compensation Line Protection Operation Principle 2.1 Distance Protection Setting Level Detector For the protection on the N side of the MN line in Figure 1, if no other measures are taken, the protection setting range is usually (80%～90%) (XNM-XC). GE proposed setting a level threshold detector, with the distance protection setting range set to 90%XNM. The protection action output is generated by an AND gate between the distance protection and the level detector, forming an overcurrent/distance protection combination. In principle, the level detector detects the value IZ-V, where Z is the level detector setting range and V is the relay voltage. Assuming Z is set to XNM, considering a fault at the M bus outlet where the series compensation capacitor is not bypassed by the MOV, the setting threshold value PL = IXNM - I(XNM - Xt) = IXt, where Xt is the parallel value of the series compensation capacitor and the MOV. When the series compensation capacitor is bypassed, the detection voltage of the level detector is IXt′, where Xt′ is the parallel value after the series compensation capacitor and the MOV are turned on. Clearly, Xt′... For a ground fault, Uop = U - (I + 3KI0)Zzd; for a phase-to-phase fault, Uop = U - IZzd. Where U is the phase-to-phase voltage; I is the phase-to-phase current; Zzd is the setting impedance, which is (80%~90%) (ZL-XC); I0 is the zero-sequence current; K is the zero-sequence current compensation coefficient; and Uz is the setting threshold, taken as the memory value of the operating voltage before the fault. Variable ohm relay: [IMG=Variable ohm relay]/uploadpic/THESIS/2008/1/2008010711073715535H.jpg[/IMG] Reactor-type relay: [IMG=Reactor-type relay formula]/uploadpic/THESIS/2008/1/2008010711074267906E.jpg[/IMG] Based on Figures 2 and 3, the following conclusions are shown in Table 1. Table 1. Characteristics of Various Protection Actions [IMG=Characteristics of Various Protection Actions]/uploadpic/THESIS/2008/1/2008010711074898558J.jpg[/IMG] Therefore, the distance protection for power frequency variation can be constructed using reactors based on the above conclusions. 2.3 Compensation in Directional Protection Taking the negative sequence direction as an example, the protection on the M side of the MN line is analyzed. In Figure 1, if there is a fault at point F1 in the positive direction, assuming the protection is connected to line TV, before compensation, V2=-I2ZJ, ZJ is the equivalent negative sequence impedance behind TV, ZJ=ZS-XC. When the capacitor is bypassed, ZJ=ZS, which is the inductive reactance characteristic; when the capacitor is not bypassed, such as ZS... |ZS-XC|, therefore in ZS |XC|, the protection measurement voltage is in the positive direction. If there is a fault at point F1 in the reverse direction, V2=ZSI2. When XC>ZS, the current reverses, and the voltage also reverses. The protection measurement voltage is still in the reverse direction, so the protection can operate correctly. 3 Series Compensation Line Relay Protection Design The system diagram of the Yangcheng to Jiangsu power transmission project is shown in Figure 4. [IMG=Yangcheng to Jiangsu power transmission system wiring diagram]/uploadpic/THESIS/2008/1/20080107110753544658.jpg[/IMG] Figure 4 Yangcheng to Jiangsu power transmission system wiring diagram The protection configuration on the Dongming side of the series compensation line is relatively simple and can be considered according to the protection design method of the usual line. A level detector can be set to improve the protection range. The protection configuration on the Sanbao side of the series-compensated line is more complex, as both voltage and current reversal can occur. When the protection measurement voltage is taken from the inner line TV of the capacitor, simple directional elements and sequence component directional elements are permissible. When the protection measurement voltage is taken from the outer line TV of the capacitor, the directional elements need to be compensated. Based on the previous analysis, when the series-compensated capacitor is not bypassed, distance protection with the protection measurement voltage taken from the inner line TV will experience current or voltage reversal when a fault occurs near the series-compensated capacitor. Distance protection with the protection measurement voltage taken from the outer line TV will also experience voltage reversal when a fault occurs in the opposite direction. 4. Relay Protection Design for Adjacent Series-Compensated Lines For the protection of the Yangcheng-Dongming three-circuit line, if the long-term series compensation configuration is not considered, a series compensation of 40% has little impact on the protection on both sides of the line. The Sanbao-Huaiyin line and the Sanbao-Renzhuang line are similarly affected by series compensation. The following explanation uses the protection of the Sanbao-Renzhuang line as an example. For the Renzhuang side protection, based on the previous analysis, its distance protection stage I is significantly affected by whether the series compensation capacitor is bypassed. To ensure its selectivity and coordination with downstream protection, a level detector can be used to improve the protection range. The protection most significantly affected by series compensation is likely the Sanbao side protection. Based on the previous analysis, this mainly occurs when there is a reverse fault and the series compensation capacitor is not bypassed, resulting in voltage or current reversal. When using the distance protection principle, the influence of the auxiliary current from adjacent lines must be considered. The relay's operating characteristics, after considering the equivalent capacitive reactance generated by the auxiliary effect, exhibit two situations as shown in Figure 3. Therefore, measures must be taken to ensure the correctness of dynamic and static protection operations. 5. Conclusion Based on the above discussion and analysis, the following suggested conclusions are made: a. From the perspective of protection principles, adding a level detector to the distance protection can prevent protection overrun and improve the protection operating range. b. Adding a reactive relay line to the power frequency variation distance protection is very effective in ensuring the directionality of the protection. c. The directionality of directional protection is related to the location of the line TV used for measuring the voltage. However, for directional protection where the measuring voltage is compensated by taking the TV outside the capacitor, the directionality of the protection can be correctly determined. d. For protection on the series compensation side of a series compensation line, if the measuring voltage is taken from the TV outside the series compensation line, the impact of a series compensation reverse direction fault on the protection is similar to that of a reverse direction fault on the Sanbao side of the Sanbao-Huaiyin and Sanbao-Renzhuang lines. Additionally, attention should be paid to the operating characteristics of the series compensation line protection when a fault occurs between the busbar and the line TV.