0 Introduction Computer technology provides automated and intelligent means for the control and dispatch of power systems. However, with the development of electronic devices towards integration and high speed, the operating voltage of electronic equipment has decreased, and the signal voltage has also become very low, leading to increasingly lower amplitude and energy of interference signals that can cause damage or malfunction. Switching operations, circuit breakers, lightning, and system short circuits in substations are common sources of interference. Among these sources, the transient electromagnetic fields generated by switching operations pose the greatest threat to electronic equipment. During the opening and closing of the moving contacts of a switch, a large number of electric arcs (including pre-ignition and reignition) are generated. The duration of the arc ranges from a few μs to a few ms, with a rise time on the order of nanoseconds. The typical overvoltage value generated by switching operations is approximately twice the phase voltage amplitude. In special cases containing resonant circuits, the overvoltage generated by transient electromagnetic fields can reach 6.5 times the phase voltage amplitude. There have been reports of control system damage or secondary equipment malfunction caused by the operation of high-voltage disconnecting switches [1-4]. Therefore, it is essential to understand the characteristics of the transient electromagnetic field generated by this operation and its protective measures. [b]1 Generation of Transient Electromagnetic Fields during Switch Operation[/b] When a high-voltage disconnecting switch is closed, the gap gradually narrows until the air insulation between them breaks down, generating the first arc. The potential on the unloaded busbar changes from its initial zero value to the instantaneous value of the voltage after a brief oscillation process. As the high-frequency oscillating current decays, the oscillation ends, the arc extinguishes, and the busbar is disconnected from the power supply again. The potential on the busbar remains at the value at which the arc extinguished. When the potential difference between the contacts increases to the breakdown voltage due to changes in the power supply voltage, an arc is generated again and extinguished again until the moving contact and the stationary contact make contact. The process of opening a high-voltage disconnecting switch is the reverse of the closing process. The extinction and reignition of the electric arc generate a series of high-frequency current and voltage waves on the busbar. At this time, the busbar acts like a high-frequency antenna, radiating energy into the surrounding space in the form of transient electromagnetic fields. Simultaneously, the transient processes on the busbar can be directly coupled to the low-voltage circuit through equipment connected to the busbar (such as CTs and PTs). In substations, the transient electromagnetic fields generated by different disconnecting switches and different operating methods are different. The EPRI group in the United States conducted field measurements of transient electromagnetic fields in 115, 230, and 500 kV air-insulated substations (AIS) and 345 and 500 kV gas-insulated substations (GIS). The waveforms of the measured transient electromagnetic fields of AIS at different voltage levels are shown in Figure 1. [img=227,158]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gdyjs/2001-4/35-1.gif[/img] [img=222,157]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gdyjs/2001-4/35-2.gif[/img] Figure 1 Transient electromagnetic field waveforms of AIS at different voltage levels. The measurement point is on the ground below the middle position of the busbar. The 115 kV busbar is 48 m long, and the 230 kV and 500 kV busbars are 6 to 9 m long[3]. As can be seen from Figure 1, the higher the voltage level of the substation, the larger the maximum peak value of the transient field it generates, but the waveform change is not significant with the change of system voltage. Table 1 shows the measured values ​​of the electromagnetic field in AIS under manual (115, 230 kV) and motorized (500 kV) conditions by the EPRI group. [img=314,177]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gdyjs/2001-4/36-1.gif[/img] As can be seen from Figure 1 and Table 1, the transient electric field is unipolar, rising to its maximum value within several hundred ns and lasting for 0.02–10 ms. Unlike the transient electric field, the magnetic field is bipolar, also rising to its peak value within several hundred ns, but its amplitude decays to 0 within 10–15 μs. The transient electric field waveform reflects the effect of the bus voltage, while the magnetic field waveform reflects the effect of the bus current. The transient magnetic field reaches a positive peak value in half a cycle and a negative peak value in the opposite direction in the other half of the cycle, with a large difference in the magnitude of the positive and negative peak values. It can also be seen that the main frequency variation range of the transient field in substations of different voltage levels is 0.5–3 MHz. The waveforms of the transient electric and magnetic fields generated by the operation of the 500 kV GIS switch recorded by the EPRI group are shown in Figure 2. [img=275,374]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gdyjs/2001-4/36-2.gif[/img] Typical data of transient electromagnetic fields measured by Russell et al. when disconnecting switches are closed in 345 and 500 kV GIS are shown in Table 2. [img=285,232]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/gdyjs/2001-4/36-4.gif[/img] The test data vary depending on the internal structure of the GIS and the testing equipment, but extensive field measurements of 345 kV and 500 kV GIS show that the frequency of radiated interference is approximately 0.5–100 MHz, with the strongest radiated electromagnetic field intensity around 20 MHz; the amplitude of the radiated electric field intensity is 1–50 kV/m; the amplitude of the magnetic field intensity is 1–5 A/m; the pulse duration is several ns–1 ms; and the rising edge is several ns–1 μs. Comparison of the measured data shows that the main frequency of transient phenomena in GIS is many times higher than that in AIS, and the frequency components of its electric and magnetic fields may be >20 MHz; the amplitude is smaller than that in AIS; and the total duration is also shorter, with some very high frequencies having a duration >0.4 ms [1–4]. This is because SF6 gas has extremely strong deionization properties, and its gas breakdown process and arc disappearance process are extremely rapid. Although there are other sources of interference in the substation, the most serious problem is the transient electromagnetic field generated by the operation of the high-voltage disconnector. The slow movement of the switch contacts not only causes the insulation medium between the components of the high-voltage system to break down multiple times, but also the residual charge causes the breakdown voltage to exceed the operating voltage of the system. Each breakdown releases a large amount of energy at a frequency of 100 kHz to several MHz [2]. Such a high frequency and amplitude transient voltage with a rise time of only ns causes strong interference to computers and other electronic equipment in the substation. With the implementation of integrated automation technology, some electronic equipment is installed in the substation, or even near the switch, making the interference problem more prominent. [b]2 Interference Pathways and Anti-interference Measures[/b] The transient electromagnetic field generated by high-voltage switch operation interferes with secondary equipment in different coupling or conduction forms: (1) Capacitive coupling: Interfering with secondary equipment through electrostatic induction; (2) Inductive coupling: The generated alternating magnetic flux links with the secondary circuit, generating an induced electromotive force in the secondary circuit; (3) Resistive coupling: When high-frequency current passes through the grounding leads of CT, CVT, etc. connected to the busbar, it causes high-frequency current to flow through the cable sheath, generating an interference voltage on its core wire; (4) High-frequency radiation: Using the busbar as an antenna, interfering with secondary equipment in the form of high-frequency radiation. To achieve good electromagnetic compatibility, the following measures must be taken: (1) Control the interference source so that it does not generate interference, or weaken the amplitude of the interference source to reduce the interference to a certain level; (2) Prevent interference from coupling to sensitive equipment or reduce the degree of interference; (3) Improve the anti-interference capability of vulnerable equipment. With the continuous development of high-performance power electronic devices with higher breaking power, some foreign companies have announced the successful development of power electronic devices based on silicon carbide (SiC) substrates. The withstand voltage and heat capacity of the substrate can be greatly improved, while the loss of the components is greatly reduced, thus greatly improving the breaking power of the components [5]. If the high-voltage mechanical switches (oil circuit breakers, SF6 circuit breakers, vacuum circuit breakers, etc.) of the power system are replaced by high-power electronic switches, the source of interference is eliminated. The traditional methods to cut off the interference path or reduce the degree of coupling are shielding, grounding, isolation and filtering. The secondary cables in the substation adopt metal-sheathed shielded cables with grounded ends, which have a significant suppressive effect on the interference generated by capacitive coupling or inductive coupling. However, the secondary cables still need to be far away from the source of interference; for different transmission signals, multi-layer shielded cables or different grounding methods must be used. With the mature application of optoelectronic technology in the power system, fiber optic high-voltage sensors and current sensors have been successfully applied at home and abroad. Since optical fibers do not introduce electromagnetic fields into the equipment, the propagation of interference is cut off from the path. The grounding of secondary equipment includes safety grounding and working grounding. The safety grounding of secondary equipment should share the same grounding network as the primary equipment, and the grounding wire should be as short as possible to reduce potential transient overvoltages. For functional grounding, low-frequency circuits (f < 10 MHz) typically use single-point grounding, while high-frequency circuits (f > 10 MHz) use multi-point grounding. Even with multi-point grounding, the leads should be as short as possible and grounded as close to the nearest point. Secondary equipment within the substation must undergo various immunity tests. Hardware measures such as isolation transformers and surge absorbers can be used to suppress interference. The reliability of the control system is directly related to both the hardware and software systems; therefore, corresponding measures should be taken in the software, such as using software filtering, setting up watchdog timers to prevent crashes, and implementing fault-tolerant design. Software design largely depends on the analysis and summarization of various on-site conditions. The software should be improved through research and analysis of the impact of interference on program operation results to obtain a highly reliable software system that meets the requirements. [b]3 Conclusions[/b] a. The transient electromagnetic field generated by the high-voltage disconnector is short in duration, large in amplitude, and contains a large number of high-frequency components. It causes strong interference to the control equipment in the substation through coupling or radiation, and the interference to secondary equipment is the most serious; b. Shielding, grounding, filtering, and isolation measures should be taken according to specific circumstances to eliminate or reduce interference to secondary equipment; for electronic equipment, both hardware and software measures should be taken to reduce interference to ensure the reliability of operation. [b]References[/b] 1 Boggs SA, Chu F Y. Disconnect switch induced transient and trapped charge in gas-insulated substation. IEEE Trans on PAS, 1982, 101(10): 3593 2 Russell B Don, Harvey SM, Nilsson S L. Substation electromagnetic interference, Part I Characterization and description of the transient EMI problem. IEEE Trans on PAS, 1984, 103(7): 1863 3 Wiggins CM, Wright S E. Switching transient fields in substations. IEEE Trans on PD, 1991, 6(2): 591 4 He Jingliang. Electromagnetic compatibility of power systems. Beijing: China Water Resources and Electric Power Press, 1993 5 Mansour H Abdel-Rahman. Calculation of very fast transient caused by disconnector switching in GIS, ISH 97 6 Zheng Jianchao. Current Status and Prospects of Cutting-Edge Power Technologies. China Electric Power, 1999, 32(10): 25-7. Yang Yinmei. Electromagnetic Compatibility Issues in Substations. Power System Technology, 1997, 21(5): 67.