[b]0 Introduction[/b] Controllable series capacitor compensator (TCSC) is an important FACTS component. Due to its potential performance benefits, it is the preferred practical device in FACTS practices in various countries. Therefore, it is necessary not only to study in depth the impact of TCSC on existing relay protection and examine the adaptability of existing relay protection systems on TCSC lines, but also to explore new line protection principles and combine modern computer technology, control technology, and communication technology to construct high-performance relay protection devices suitable for the TCSC operating environment [1]. The operation status and simulation studies of TCSC projects in recent years have, to some extent, demonstrated the adaptability of existing relay protection on TCSC lines. However, there are still few examples of using fault component protection in existing TCSC projects. Reference [2] believes that existing relay protection that can be used for conventional series compensation lines can also be used for TCSC lines; researchers from well-known companies such as GE have also concluded from simulation studies of existing relay protection devices that existing single-phase and multi-phase line relays operate correctly with little or no performance degradation [3]. However, due to the complexity of relay protection, further in-depth research is still needed in this area. Based on the study of dynamic fundamental frequency impedance in reference [4], this paper conducts a relatively systematic analysis and study on the adaptability of fault component protection on TCSC lines. The fault component protection involved in this paper includes negative (zero) sequence power directional protection, power frequency fault component distance protection, and power frequency fault component directional protection. [b]1 The Influence of TCSC on Fault Component Protection[/b] 1.1 The Influence of TCSC on Negative (Zero) Sequence Power Directional Protection The negative sequence and zero sequence directional relays compare the phase of each component voltage and current [5]. Relative to the negative sequence component calculation network corresponding to Figure 1 (the zero sequence component calculation network is similar to Figure 1), the operating conditions of the negative sequence and zero sequence power directional relays are Equations (1) and (2). [img=192,45]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9917/image17/04g01.gif[/img] （1） [img=189,45]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9917/image17/04g02.gif[/img] （2） Where Zr2 and Zr0 are the analog impedances of the relay, and the impedance angles of Zr2 and Zr0 are equal to the negative sequence and zero sequence impedance angles of the power supply, respectively. [img=257,95]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9917/image17/0401.gif[/img] Fig.1 Circuit for negative sequence component calculation Reference [6] discusses in detail the influence of conventional series compensation on negative sequence and zero sequence power directional relays. In summary, for the negative sequence network shown in Fig.1, whether protecting against asymmetrical short circuits in the positive or negative direction, and regardless of whether the series compensation capacitor is between the relay and the short circuit point or in the opposite direction of the short circuit point, as long as the series compensation capacitor does not experience asymmetrical breakdown and its capacitive reactance Xc is less than the system inductive reactance Zs, the negative sequence power directional relay can respond correctly. In actual systems, the above constraints are usually met, so the negative sequence power directional relay will not malfunction. When using TCSC compensation with the same compensation degree, this condition is easier to meet based on the dynamic fundamental frequency impedance characteristics of TCSC, thus reducing the likelihood of relay malfunction. For zero-sequence networks, when the bus is connected to a large-capacity transformer and the neutral point is grounded, on conventional series compensation (FSC) lines, the capacitive reactance Xc may be greater than the system inductive reactance Zs, potentially causing the zero-sequence power directional element to fail to operate. However, on TCSC lines, if the TCSC capacitor is bypassed after a fault, the TCSC will shift towards inductive operation. The short-term capacitive impedance in the initial stage of the fault will at most cause a delayed operation, without causing a failure to operate. If the TCSC capacitor is not bypassed, failure to operate on the zero-sequence network may still occur, but the probability is lower than on FSC lines. On the other hand, when using conventional series compensation, if the series compensation capacitor is asymmetrically short-circuited, both negative-sequence and zero-sequence networks may experience failure to operate or maloperation under short-circuit conditions. On TCSC lines, the three-phase TCSC control may be inconsistent, also causing three-phase asymmetry and leading to maloperation of negative-sequence or zero-sequence power directional protection under short-circuit conditions. 1.2 Influence of TCSC on Distance Protection for Power Frequency Fault Components For distance relays for power frequency fault components, the relay operates when the voltage change ΔUop at the end of the setting value is greater than the setting threshold voltage Uz; otherwise, it does not operate. According to reference [7], for the system shown in Figure 2, when using a conventional series compensation capacitor, if the protection range is set according to K(Zzd-Xc), the relay can correctly respond to short circuits of F3 outside the forward zone, F2 in the reverse direction, and F4 near the zone. However, whether the relay can operate correctly for a short circuit of F1 within the zone depends on factors such as the short-circuit current, the compensation degree, and the distance between F1 and the protection device. If set according to Zzd, the relay can correctly respond to short circuits of F2 outside the reverse zone and F4 within the zone. However, a short circuit of F3 outside the forward zone may result in overrunning. [img=295,75]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9917/image17/0402.gif[/img] Fig.2 Diagram for analysis of fault component distance protection relay. When the transmission line uses TCSC compensation, if the TCSC is bypassed after a fault, for relays with protection range set according to K(Zzd-Xc), the protection range will be greatly reduced when the capacitor is short-circuited in the forward zone, that is, the range of non-operation during a short circuit at the far end of the zone will increase. When set according to KZzd, the relay operation characteristics are similar to those of a line without series compensation. If the TCSC is not bypassed after a fault, the relay operation characteristics are more complex. As can be seen from the study of the dynamic fundamental frequency impedance characteristics of TCSC in reference [4], when the capacitor of TCSC is not bypassed, the reactance of TCSC alternates between capacitive and inductive characteristics during the process from transient to steady state, which makes the setting of protection difficult. If the protection range is set according to K(Zzd-Xc), the advantage of the reactance part having small capacitive reactance or inductive characteristics during transient is not fully utilized, making the protection range too small. If it is set according to KZzd, it is very likely to malfunction after entering steady state during short circuit outside the zone. Considering both bypass and non-bypass cases, for the sake of reliability, it may be more appropriate to make a more conservative setting according to K(Zzd-Xc). 1.3 The effect of series compensation capacitor on fault component directional relay The fault component directional relay measures the phase of the voltage and current fault components, and operates when the phase is reversed [7]. For the system shown in Figure 3, the phase angles measured by the directional element ΔFφφ+ which reacts to the fault component in the three positive directions are: [img=176,45]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9917/image17/04g03.gif[/img] (3) [img=288,104]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9917/image17/0403.gif[/img] Fig.3 Diagram for analysis of directional power frequency fault component relay The phase angles measured by the directional element ΔFφφ- which reacts to the fault component in the three negative directions are: [img=140,44]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9917/image17/04g04.gif[/img] Where φφ=AB,BC,AC; CZd is the compensation impedance, which is 35% to 45% of the line impedance; Zd is the analog impedance. When conventional series compensation is used for transmission lines, reference [7] believes that only when |Xc| < min(|Zs1|, |Zs1′|) will the fault component directional element be unaffected by the series compensation capacitor. |Zs1| and |Zs1′| are the positive sequence impedances of the power supply during forward and reverse short circuits, respectively. When TCSC compensation is used for transmission lines, if the TCSC is bypassed after a fault, the TCSC will quickly become inductive, only exhibiting capacitive reactance for a very short time in the initial stage of the fault, and much smaller than the capacitive reactance during normal operation. Therefore, it is very beneficial to improve the performance of relays. If the TCSC is not bypassed after a fault, as shown in the study of reference [4], regardless of how the impedance of the TCSC changes, its capacitive reactance modulus is always less than that of the TCSC capacitive reactance before the fault. Therefore, under the same compensation degree, the TCSC line is more likely to satisfy the condition |Xc| < min(|Zs1|, |Zs1′|) than the FSC line, and the relay performance on the TCSC line is more reliable. Moreover, from the impedance change characteristics of the TCSC, it is known that the capacitive impedance of the TCSC in transient state is smaller than that in steady state. Therefore, utilizing the transient impedance characteristics of the TCSC will be more beneficial to improving the performance of the directional relay. 2 Conclusion The impact of TCSC on fault component protection will vary depending on the different construction principles. Compared with the protection on the FSC line, the performance of fault component protection on the TCSC line will not be reduced. References 1 Mankoff L L. Protective Relaying and Associated Control for FACTS Applications. In: FACTS Conference. EPRI (USA): 1992 2 Pereira M. Digital Protection of Advanced Series Compensators. In: Developments in Power System Protection Conference Publication. 1993 3 Adamiak M, Patterson R. Protection Requirements for Flexible AC Transmission System. In: CIGRE. 1992 4 Yu Jiang, Duan Xianzhong, Wang Weiguo, et al. Dynamic Fundamental Frequency Impedance Characteristics Analysis of TCSC. Automation of Electric Power Systems, 1999, 23(14) 5 Zhu Shengshi. Principles and Operation of High Voltage Power Grid Relay Protection. Beijing: China Electric Power Press, 1995 6 Huazhong Institute of Technology (ed.). Principles and Operation of Power System Relay Protection. Beijing: Electric Power Industry Press, 1980 7 Wang Weiguo, Yin Xianggen, Duan Xianzhong, et al. Influence of fixed series compensation capacitor on power frequency fault component relay protection. Automation of Electric Power Systems, 1998, 22(12).