[align=left][b]1 Introduction[/b] With social progress and the increasing number of users in local power grids, the power supply of local power grids can no longer meet the needs of users, making interconnection with the large power grid an inevitable trend in development. Interconnection can optimize energy allocation, improve energy utilization efficiency, and enhance the overall system safety level. However, some new problems have also emerged in this process, such as: whether the grid structure after interconnection is reasonable; whether the power supply capacity meets the needs of the load, and whether the power supply layout is reasonable; whether there are reliable protection and automatic safety devices; whether there are reliable data acquisition and communication systems, and whether there is a dispatcher training simulation DTS, etc. This paper analyzes the cause of protection maloperation caused by electromechanical transients after a fault occurs in the interconnected power grid by using the maloperation accident of the 35kV side bus tie protection of the 110kV Jishiwan substation, which is interconnected with the Neijiang power grid and the southern Sichuan system, and proposes measures to improve its behavior. [b]2 Introduction to Protection Malfunctions[/b] 2.1 Configuration of Protection for 110 kV Jishiwan Substation and its Outgoing Lines The configuration of the 35kV bus tie 300 switch protection and the corresponding Jidan line 311 switch protection of the 110kV Jishiwan Substation is as follows (actual measured values ​​on site): (1) Composite voltage blocking directional overcurrent protection of the 35kV side of the No. 2 main transformer of the 110kV Jishiwan Substation: CT ratio is 700 A / 3.5 A; V1=70 V, V2=6 V; 35kV side 302 switch trips in 2s; 35kV bus tie 300 section switch trips in 1.5s, direction pointing towards the bus. (2) Jidan line 311 switch protection: Section I: 3120 A, 1740 V; Section III: 200 A, 1s; reclosing time is 0.8 s. 2.2 Malfunction of the Bus Tie 300 Switch On December 9, 1998, a three-phase short circuit occurred in the porcelain insulator of the 35kV surge arrester at Ziyang Nitrogen Fertilizer Plant. This caused the overcurrent protection of the 311 switch on the 35kV side of the Jishiwan substation (Jishan line) to trip at t=1.0s, and reclosing failed. Meanwhile, the composite voltage-blocked overcurrent protection of the 300 switch on the 35kV side of the No. 2 main transformer tripped the 35kV bus tie 300 switch at 1.5s. The primary wiring of the Jishiwan substation is shown in Figure 1. [/align][img=359,224]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2002-8/79-1.jpg[/img][b]3 Accident Analysis[/b] 3.1 Verification of Protection Setting Values ​​The setting values ​​of the current protection of the 311 switch on the faulty line, Ji Niang Line, were verified: For instantaneous overcurrent protection, the maximum short-circuit current at the end of the Ji Niang Line under the maximum system operating mode is 1.9442kA, which meets the selectivity requirements. For definite-time overcurrent protection, the short-circuit current at the end of the Ji Niang Line under the minimum system operating mode is 1.2204kA. Stage III can completely operate to clear the fault, and its sensitivity is 6.102, which meets the requirements. Since the 311 switch protection of the Ji Niang Line is located at the end of the power grid, the operating time does not need to be coordinated with the next line. Here, the operating time is selected as 1s. The following verification was performed on the 35kV side composite voltage blocking directional overcurrent protection of the No. 2 main transformer: The 35kV side composite voltage blocking directional overcurrent protection of the No. 2 main transformer at the Jishiwan Substation responds to external faults of the No. 2 main transformer and serves as backup protection for differential protection and gas protection during internal faults. Its setting value is 700 A. When a three-phase short-circuit fault occurs at the end of the Jizhan line and the current protection of the Jizhan line 311 switch fails to operate, it should be able to trip the 35kV bus tie 300 switch, thereby isolating the fault. Under maximum operating conditions, when a three-phase short circuit occurs at the end of the Jizhan line, the current flowing through the bus tie 300 switch is 1.0597 kA, exceeding the setting value (700 A) of the 35kV side composite voltage blocking directional overcurrent protection of the No. 2 main transformer, thus tripping. The operating time limit is set to 1.5s to ensure selectivity. Therefore, the setting value and operating time limit of the 35kV side composite voltage blocking directional overcurrent protection of the No. 2 main transformer meet the requirements. 3.2 Impact of the electromagnetic transient process of the system after the fault on the protection: The electromagnetic transient process of the system after the fault was simulated using EMTP. The fault current flowing through the 311 switch and the 300 switch of the 35kV side outgoing line of Jishiwan substation is shown in Figures 2 and 3. [img=334,270]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2002-8/79-2.jpg[/img][img=359,239]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2002-8/79-3.jpg[/img] As can be seen from the two images above, the short-circuit current flowing through the 311 switch on the Ji-Nitrogen line is less than the setting current Izd=3120A of the instantaneous overcurrent protection, so the instantaneous overcurrent protection does not operate. However, this current value is much greater than the setting current Izd=200A of the time-limited overcurrent protection of the 311 switch on the Ji-Nitrogen line. This protection should operate at its setting time t=1.00s, and the short-circuit fault in the system is cleared. At t=1.5s, the current flowing through the 302 switch of the Jishiwan substation is zero (ignoring the influence of the load), and will not cause the 300 switch to operate. It can be seen that simply analyzing the electromagnetic transient process after the system fault is not enough to find the cause of the 300 switch tripping. 3.3 Impact of electromechanical transient process after fault on protection The initial power angle d(0) of the normally operating power system is about 30°. Figure 4 shows the system power angle change curves corresponding to various initial power angles when the overcurrent protection of the 311 switch of the 35kV Jidan line of the 110kV Jishiwan substation trips at t=1.0s. [img=350,207]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2002-8/80-1.jpg[/img] As can be seen from the changes in the power angle curve in Figure 4, for an accident, if the initial power angle d(0) of the system is ≥55°, the relative value of the power angle between the generator and the system will be greater than 180°. The generator enters an asynchronous operation state, the power angle d will continue to increase, and the generator and the system will eventually lose synchronization. From the d(t) curve in the above figure, its corresponding I(t) curve is plotted, as shown in Figure 5. [img=366,195]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2002-8/80-2.jpg[/img] As shown in Figure 5, after a three-phase short circuit occurs at the end of the Jizhan line, the overcurrent protection of the 311 switch on the 35kV Jizhan line of the 110kV Jishiwan substation trips at t=1.0s, and the oscillating current in the system changes. Because the relative power angle d between the equivalent generator and the system changes, an oscillating current I is generated in the system. Although Figure 4 shows that the system will be pulled back to synchronization after several cycles after the oscillation, it may still cause the current protection to malfunction. When a three-phase short circuit occurs at the end of the Ji-Nitrogen line, the current I=1958A flowing through the 300 switch on the 35kV side of the No. 2 main transformer is much greater than the setting current Izd＝700A of the overcurrent protection of the 300 switch. The overcurrent protection of the 300 switch will start. After the overcurrent protection of the 311 switch of the 35kV Ji-Nitrogen line of the 110 kV Ji-Shi-Wan substation trips at t=1.0s, due to the presence of oscillating current in the system[3], assuming that the initial power angle d（0）≈30° when the system is working normally, its oscillating current is shown in curve 1 of Figure 5. When t=1.0s, the oscillating current I=1089A is greater than the setting current of the overcurrent protection of the 300 switch. The overcurrent protection of the 300 switch will not reset, and the oscillating current I is always greater than 700A during t=1.0-1.5s. That is, the overcurrent protection setting current of the 300 switch is greater than that of the 300 switch. When t=1.5s, the overcurrent protection of the 300 switch will operate because the setting time tzd=1.5s. From the above analysis, it can be seen that the reason for the protection maloperation is that the relative value of the power angle between the equivalent generator and the system changes, resulting in an oscillating current in the system, which causes the 300 switch to maloperate. [b]4 Measures to prevent protection maloperation[/b] 4.1 Improve the original traditional protection As can be seen from the above analysis, after the interconnection system fails, the relative value of the power angle between the equivalent generator and the system changes, resulting in an oscillating current in the system, which causes the switch to maloperate. Most of the equipment in the local power grid is a traditional electromagnetic relay, so it can be improved from its setting value and setting time. (1) Shorten the operating time of the 311 switch protection; (2) Increase the setting current of the 300 switch overcurrent protection; (3) Extend the operating time of the 300 switch overcurrent protection. The specific coordination relationship is shown in Tables 1 to 4 below. Tables 1 and 2 give the setting current range of the 300 switch protection under different moments of inertia of the equivalent generator, while Tables 3 and 4 give the corresponding operating times of the 300 switch protection under the same conditions. [img=355,250]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2002-8/80-3.jpg[/img][img=354,255]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2002-8/80-4.jpg[/img] [img=350,252]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2002-8/81-1.jpg[/img][img=400,288]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dwjs/2002-8/81-2.jpg[/img] Based on different initial states of the system, the setting current value of the 300 switch overcurrent protection on the 35kV side of the Jishiwan substation can be determined by looking up in Tables 1 and 2, thereby avoiding the influence of oscillating current on the protection after a system fault and ensuring its correct operation. From Tables 3 and 4, the operating time of the 300 switch overcurrent protection on the 35 kV side of the Jishiwan substation and the setting time of the 35 kV outgoing line overcurrent protection of the 110 kV Jishiwan substation can be determined according to different initial states of the system. This can avoid the protection being affected by the oscillation current after the system fault and ensure the correct operation of the protection. 4.2 Using microcomputer protection that starts by detecting sudden changes The protection that starts by detecting sudden changes can correctly distinguish between system faults and system oscillations, and avoid the protection from maloperating due to system oscillations. 5 Conclusions This paper focuses on the actual situation of the maloperation of the 300 bus tie switch protection of the Jishiwan substation in the Neijiang power grid without knowing the cause. It analyzes the cause of the maloperation of the relay protection device of the interconnected power grid from the two aspects of electromagnetic transients and electromechanical transients after the fault, and draws the following conclusions. (1) The electromechanical transient digital simulation method using EMTP electromagnetic transient simulation calculation and compiled in this paper is suitable for the protection behavior analysis of the interconnected power grid and can effectively perform more accurate quantitative calculations; the proposed method and the compiled software have certain promotion and application value. (2) Through detailed calculations, a quantitative relationship was obtained between the operating time of the outgoing line protection of Jishiwan Substation and the setting time and setting value of the tie switch protection, providing a quantitative basis for solving the problem of frequent maloperation of this protection. The accident analysis method and calculation conclusions proposed in this paper have certain reference value for solving similar problems in interconnected power grids.