0 Introduction Power system faults are random and diverse. Traditional protection systems generally respond correctly to simple faults; however, this is not always the case for complex or special faults, depending on factors such as the rigor of the design and the performance of the protection devices. Although complex and special faults are rare, even a single occurrence, if the protection fails to operate, can escalate the accident and potentially cause system collapse, leading to widespread power outages. The LFP-900 series protection system, due to the advanced principles [1-3] and comprehensive performance of its basic relays (power frequency change series relays, oscillation blocking and V1, I0 polarized distance relays, etc.), coupled with advanced hardware and programming techniques, takes into account complex and special faults as much as possible. Currently, several thousand LFP-900 series protection systems are operating in power systems and have undergone various fault tests, including some complex and special faults, and have responded correctly and quickly. This article only introduces some complex and special faults encountered in operation. Actual fault reports include not only fault waveforms but also textual descriptions of the operating elements and their timing; this article only provides fault waveform diagrams. In the diagram, Fx, Sx, Ta, Tb, Tc, and ch represent sending, receiving, tripping A, tripping B, tripping C, and reclosing, respectively. T = -60 ms indicates 60 ms before the starting element operates. [b]1 Performance during oscillation blocking[/b] Traditional oscillation blocking results in protection failure or delayed operation if a fault occurs during the blocking period. Five such faults have been recorded in Chinese history, with consequences ranging from the collapse of a provincial system to widespread power outages in a region, causing severe losses. The LFP-900 series protection system, in principle, does not require oscillation blocking for the main protection. The oscillation blocking for distance protection consists of four parts: ① traditional oscillation blocking; ② asymmetrical fault opening element; ③ symmetrical fault opening element; ④ non-full-phase opening element. This ensures that the protection will not fail to operate during the blocking period, and will not malfunction even with oscillation and faults outside the protection zone. Example 1: The Guangxi Tianshengqiao-Pingguo II 500 kV line was equipped with LFP-901A protection (permissive type). On March 15, 1997, at 23:24:51, an intermittent A-phase ground fault (A0) occurred (see Figure 1). Initially, there was a disturbance, lasting approximately one cycle, which triggered the activation element. The oscillation blocking was released for 160 ms. Approximately 180 ms later, the actual A0 fault occurred, and the main protection immediately activated. The traditional oscillation blocking of the distance protection was in the blocking period, but there was an asymmetrical opening element. The original record shows that the distance protection tripped at 201 ms (the distance protection's own operating time is 21 ms). This proves that even if a fault occurs during the blocking period, the protection can operate normally and will not escalate the accident. In addition, the original record shows that the power frequency change impedance protection, power frequency change directional high-frequency protection, and zero-sequence directional high-frequency protection operated quickly (operating time less than 10 ms). [img=280,371]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9914/image14/0701.gif[/img] Fig.1 Fault record of Tianshengqiao—Pingguo line when ground fault occurs on phase A Example 2 The Hunan Fengtan—Maojiatang 220 kV line is equipped with LFP-901A protection. At 17:42:37 on June 22, 1995, an A0 fault occurred (as shown in Fig. 2). The protection tripped phase A, reclosed after 795 ms, and the fault disappeared. An A0 fault occurred again after 1210 ms (subsequent line inspection revealed that the insulator string had fallen to the ground). At this time, the reclosing was accelerated by 200 The milliseconds (ms) have disappeared. The distance protection's traditional oscillation blocking is in the blocking period. Due to the operation of an asymmetrical open element, the original record shows that the distance protection tripped three phases at 1238 ms (the distance protection's own operating time is 28 ms). This further proves that even if a fault occurs during the blocking period, it can still operate normally without delay. In addition, the original record also shows that the main protection operates quickly (operating time less than 5 ms). [img=290,320]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9914/image14/0702.gif[/img] Fig.2 Fault record of Fengtan—Maojiatang line when ground fault occurs on phase A [b]2 Protection performance of high resistance ground faults[/b] High resistance ground faults are generally caused by discharge to trees or bamboo. The initial fault current is very small and does not cause much harm to the system. However, the fault develops gradually, and the current increases accordingly, which will cause harm to the system. At this time, the tripping should be performed immediately. Traditional distance protection is powerless against high resistance ground faults and generally trips with a time delay of the zero-sequence current stage III. The LFP-901A protection system, with its power frequency change directional relay and zero-sequence directional current longitudinal main protection, exhibits high sensitivity to high-resistance grounding faults and can trip quickly. Its transient and steady-state protections complement each other, and it also has a zero-sequence current stage III as backup, enabling a relatively comprehensive and rapid response to high-resistance grounding faults. The LFP-901A has been used to handle dozens of high-resistance grounding faults; some typical faults are described below. Example 3: The Hunan Fengtan-Maojiatang 220 kV line, equipped with LFP-901A protection, has experienced multiple discharges to trees and bamboo. At 12:46:22 on May 25, 1995, phase C discharged from the tree (as shown in Figure 3). The initial current was very small. After the starting element (3I0) operated, it sent a signal. The sensitivity of the power frequency change direction element and the zero sequence direction element was insufficient, so they could not operate. The signal was not interrupted, but a signal was received. The protection was blocked. At 4755 ms, the current suddenly increased. The power frequency change direction element and the zero sequence direction element operated and stopped the signal. The phase selection element could also operate. The original record showed that the longitudinal protection (power frequency change direction high-frequency protection and zero sequence direction high-frequency protection) quickly disconnected the faulty phase (phase C) at 4796 ms. [img=270,355]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9914/image14/0703.gif[/img][align=left] Fig.3 Fault record of Fengtan-Maojiatang line when phase C discharges electricity to tree Example 4 At 12:54:28 on August 18, 1997, the B phase discharged electricity to the tree at Yueyou substation in Ningbo, Zhejiang Province, as shown in Fig.4. In the early stage of the fault, the short-circuit current was very small, just enough to make the starting element operate. Although the power frequency change directional element and the zero sequence directional element both operated and stopped the signal, the phase selection element, due to the maximum phase electric braking, had a slightly lower sensitivity than the starting element and did not operate, so it could not trip. After 5 cycles, the fault developed, the current suddenly increased, the phase selection element operated, and the original record longitudinal protection (high-frequency protection in the direction of power frequency change and high-frequency protection in the direction of zero sequence) disconnected the faulty phase (phase B) in 110 ms. [/align][img=273,302]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9914/image14/0704.gif[/img] Fig.4 Fault record of Mian-You line when phase B discharges electricity to tree Example 5 Jiangxi Zhelin Station, Zhe-Dou 220 kV line, discharged to the tree 3 times on May 6, 7 and 8, 1997. The protection operation was similar. The following is an explanation of one of the discharges on the 6th, see Fig. 5. The initial current is very small, insufficient to start any components. The fault gradually develops, causing the zero-sequence starting element (set value 0.75 A) to activate, triggering the transmitter. At this point, the power frequency change direction element and the zero-sequence direction element, due to insufficient sensitivity, fail to activate, preventing signal interruption. The receiver action blocks the protection. Approximately 160 ms after the starting element activates, the current increases sufficiently to activate the zero-sequence direction element, stopping the transmitter and resulting in no received signal. Because the current changes gradually, the phase selection element does not activate at this time, equivalent to phase selection failure. A three-phase trip should occur after 150 ms. The original record shows the longitudinal protection (zero-sequence directional high-frequency protection) activated; the backup protection tripped three phases at 317 ms. [img=263,359]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9914/image14/0705.gif[/img] Fig.5 Fault record of Zhe-Dou line when discharge to tree occurs Example 6 The Tianshengqiao-Pingguo II 500 kV line experienced two permanent B0 faults on January 22, 1997, at 21:53:39 and January 23, 1997, respectively, caused by farmers setting fire to the mountain. The fire and smoke caused permanent B0 faults. The Pingguo side on January 22 is selected as an example, as shown in Fig. 6. The current increases rapidly, therefore the starting element, the power frequency change direction element, the zero-sequence direction element, and the phase selection element all act quickly. The original record shows that the action time is 19 ms on the Tianqiao side and 24 ms on the Pingguo side, and the output is activated, quickly clearing the B-phase fault. When a fault occurs, the fault is cleared by the accelerated protection (the main protection mode is permissive). [img=262,320]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9914/image14/0706.gif[/img] Fig.6 Fault record of Tianshengqiao—Pingguo line when permanent ground fault occurs on phase B at Pingguo side [b]3 Performance of fault clearing when a single-sided power supply line experiences a fault.[/b] When a single-phase fault occurs on a single-sided power supply line, if rapid phase selection and tripping are required, traditional protection systems are difficult to achieve, but LFP-902A can achieve this smoothly. Example 7: The Shapo substation in Guangxi was converted to a power-free substation. The Ma-Sha 220 kV line was equipped with LFP-901A and LFP-902A protection systems. At 6:45:32 AM on October 24, 1997, a phase C ground fault occurred. The operation of the protection systems on the Shapo side is shown in Figure 7. The grounding of the neutral point of the power-free transformer provided zero-sequence current (all three phases had equal magnitude and the same direction). At this time, the starting element (I0, ΔIφmax) operated. Due to its reasonable principle, the power frequency change element, the composite distance element, the zero-sequence direction element, and the phase selection element all operated. The original record showed that the longitudinal protection (power frequency change impedance protection and zero-sequence direction high-frequency protection) operated rapidly within 8 ms, disconnecting the faulty phase (phase C). Practice proved the good performance of the original design. [img=250,305]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9914/image14/0707.gif[/img] Fig.7 Fault record of Ma-Sha line when ground fault occurs on phase C Example 8 Fig.8 shows the operation of LFP-901A at Shapo substation. The starting element activates and sends a signal, the zero-sequence direction element activates and stops sending a signal, but the phase selection element cannot activate (ΔIφφ=0), phase selection fails, and three-phase tripping must be waited for 150 ms, but LFP-902A has already tripped in 8 ms. In later procedures, low-voltage phase selection was added to the no-power-supply side of LFP-901A, which can quickly trip by phase, but this set of devices is an early product. Figure 7 further illustrates another point: when both sets of protection systems are activated for reclosing simultaneously, the protection that did not trip will initiate reclosing due to a mismatch, without requiring other protection to activate it. The LFP-902A issues a closing command at 670 ms, while the LFP-901A issues it at 685 ms, a difference of 15 ms. The former is activated by its own protection mechanism, resulting in a faster response, while the latter is activated by a mismatch in the circuit breaker trip relay, resulting in a slightly slower response, but without causing secondary reclosing. [img=238,351]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9914/image14/0708.gif[/img] Fig.8 Fault record of Shapo substation [b]4 Performance of longitudinal protection when a fault occurs on a single-sided open-circuit charging line[/b] In traditional protection, there is accelerated protection when a fault occurs. However, when the open-circuit charging line is running, the circuit breaker on the opposite side has not yet closed, and the longitudinal protection has not yet been activated. If a fault occurs at the end of the line, the second stage of delayed tripping is the only option, thus failing to achieve rapid fault clearing. The longitudinal protection of LFP-900 has made careful arrangements for this situation in the design, and can also clear the fault at a relatively fast speed. Example 9: On March 16, 1997, at 00:03, a single-phase transient fault occurred 0.5 hours earlier. Due to a fault in the TLS-1B component on the Tianshengqiao side, the circuit breaker tripped three times and failed to close. On the Pingguo side, the circuit breaker was reclosed and operated as an open-circuit line. A permanent fault B0 occurred near the Tianshengqiao side (see Figure 9). The LFP-901A circuit scheme considered this type of fault. The original record permissive longitudinal protection (high-frequency protection in the direction of power frequency change) cleared the B-phase fault at a relatively fast speed (54 ms). The circuit breaker was reclosed to the fault, and the CF1 and CF2 accelerated the three-phase trip to clear the fault. [img=253,261]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dlxtzdh/dlxtzdh99/dlxt9914/image14/0709.gif[/img] Fig.9 Fault record of Tianshengqiao-Pingguo line when permanent ground fault occurs on phase B at Tianshengqiao side [b]References[/b] 1 Shen Guorong. Research on the principle of power frequency change direction relay. Automation of Electric Power Systems, 1983, 7(1) 2 Dai Xuean. A major breakthrough in relay protection principle - a comprehensive discussion on power frequency change relay. Automation of Electric Power Systems, 1995, 19(1) 3 Shen Guorong, Deng Shaolong, Zhu Shengshi. A new principle for distinguishing between oscillation and short circuit. Automation of Electric Power Systems, 1990, 14(1)