Investigation On Transient Electromagnetic Environment In Substation And EMC Of Microprocessor Protection Devices

EFT/B (Electrical Fast Transient Burst) always interferes with microprocessor protection devices in power system, because it has such features as short rise time, short duration, high amplitude and high repetition rate. Study of anti-jamming of EFT/B is at the beginning in China and concentrated mainly on the Countermeasures in practice. Investigations on the mechanisms of EFT/B forming, coupling and working are still in great demand. Based on the three factors of EMC (Electromagnetic Compatibility), EFT/B's forming and coupling mechanism are discussed in this paper. And Countermeasures on anti-jamming of EFT/B for microprocessor protection devices are studied through experiments.With regard to EFT/B's forming mechanism, the process of forming EFT/B caused by a switch operation is studied at first. Then, the restrike and extinction model of a disconnecting switch's arc is built with EMPT-ATP codes. EFT/B caused by a disconnecting switch's cutting off inductive or capacitive load is simulated in the primary circuit of power system. Countermeasures of MOV (Metal Oxide Varistor), RC and phase-control switch technology are used to inhibit the primary circuit EFT/B. EFT/B's forming mechanism in the secondary circuit is studied through experiments. It's discovered that EFT/B coupled into a relay's coil interferes with DC power supply of microprocessor protection devices badly. And a model of output relay is built. The method of electromagnetic shielding is simulated to cut off EFT/B's path from the relay's contact to its coil with ANSOFT codes.With regard to EFT/B's coupling mechanism, coupling path of EFT/B in the primary circuit is analyzed and the electromagnetic isolation measures are proposed as follows. Firstly, a model of the switch cabinet is built associated with ANSOFT. Countermeasures to attenuate voltage and current disturbance induced by EFT/B are simulated. Secondly, secondary cables with low transfer impedance are proposed to connect with microprocessor protection devices. Multi-layer electromagnetic shielding and high impedance of shielding layer are proposed to attenuate current induced in the shielding layer. Thirdly, an electromagnetic unit model of transformer is built associated with EMTP-ATP. Countermeasures are simulated to prohibit EFT/B from the primary circuit to the microprocessor protection device through electric voltage and current transformer.The quantitive study of EMI (Electromagnetic Interference) source and its coupling path is still in its primary state now. The algorithm based on data mining technology is first proposed to predict and analyze the electromagnetic environment of a substation. The strategy of anti-jamming measures can be established according to the appraisal model generated from data mining algorithm. Electromagnetic environment can be improved in substations. Besides, the microprocessor protection device can be fixed in the place where the disturbance is at minimum according to the prediction results.With regard to anti-jamming EFT/B of microprocessor protection devices, the reasons for logic confusion, program mistake, display error and malfunction are analyzed when EFT/B interferes with the microprocessor protection device at first. Then, the microprocessor protection device is under EFT/B immunity test. Improved distribution power supply, ferrite core, EMI filter and transient voltage suppressor can be used in the microprocessor protection device to control EFT/B through experiments. Lastly, EFT/B and surge immunity tests are performed on a vacuum on-line condition monitoring system made in laboratory with the EFT/B and surge generators developed in this paper. The system's immunity level is improved with the Countermeasures.At present, disturbance signal generators for EMC immunity test are mainly imported and are very expensive. An EFT/B generator is designed and made according to the standards of IEC61000-4-4 and GB/T17626.4 in this paper. A switching mode power supply is developed as the high-voltage DC power supply. A hydrogen thyratron is used as the main circuit switch. And the developed EFT/B generator is very cheap. A combination wave generator is also designed and made according to the standards of IEC61000-4-5 and GB/T17626.5. Spark global gap switch with triple electrodes is developed as the main circuit switch. Experimental results show that the output signals of the developed EFT/B generator and combination wave generator can be used in EFT/B and surge immunity test.