Research On Electromagnetic Compatibility For The Microprocessor Based Protective Relay

The microprocessor based prot8ctive reIay works in serious eIectromagneticenvironment of the substations. Its immunity IeveI and reliabiIity is moreimportant to the safe and reliable operation of the power systems. Themicroprocessor based prot8Ctive relay has exceeded the concept of the traditionaIproteCtive reIay. The trends for the microprocessor based protective relay are thecomputerization, artilicial intelligence, netWorking, the integration of protective relay,control & measuring and data communication technologies. The computer hardwareand software design has been more compIicated. On the Dther hand, theBay-Oriented Design techniques have wideIy been adopted in power systems. TheconduCted and radiat6d distuthance to the proteCtive reIay decentraIized instaltationin switChyard are much more int6nse. Above aII, the microprocessor based proteCtiverelay is more susceptibIe and frangibIe to eIeCtromagnetic compatibiIity.The eleCtrical fast transient l burst (EFT) is a f8st, repetitive waveforrn withenergy content only on the order of tens to hundreds of milIijoules. The EFTs mayhave infIuence on the dependabiIity and security of the numericaI prot6ctions, andcause the apparatus under functioning or over funCtioning. ln this paper, theinterf6rence coupling mechanisms and the propagation path and the methods forensuring EMC have been anaIyzed.Studying the frequency spectra of the EFT wavefOrm, we can conclude that thetransient below 100MHz is the main parts. When the EFTS enter the microprocessorbased pfoteCtive reIay, the conduCted coupling diSturbance is main, and theconduCted circuit modeIs are established. The digitaI oscilloscope is empIoyed toanaIyze the immunity of the microprocessor based proteCtive reIay quantitativeIy asan auxiIiary tooI. This method makes the EMC research visibIe.The circuit modeIs that the EFTs propagation path through power supply port,input port, encIosure port, output port, functional earth port and communication portof the microprocessor based protective reIay are discribed. A great deaI of tests showthat the power suppIy circuit, the AC current and voItage circuit, communicationand the binary input circuit are the main routes for transient.The filter, three-terminal chip capacitor and the capacitor can be used in the lowvoItage power suppIy circuit(5V. +l-15V and 24V) to restrain the EFTs affecting thedigital system. The ferrites can be used in AC current and voltage circuit andcommunication circuit, because they absorb the transient energy effectively.Furthermore, changing the arrangement of the primary winding coil and the secondarywinding coil and using the double screen fOr auxiliary current and voItage transfOrmercan be adopted to 'induce the common mode transient. These methods abovementioned have been successfully applied in the newly microprocessor basedprotective relay of Sifang Coporation, and have been proved better.A particuIar probiem with opto-coupIers is found that high leveI, but short transientcan coupie into the base of the receiving phototransistor and cause its saturation. lt isca1Ied "Transient Saturation". This saturation wiII effect on opto-couplers' isolation anddigitaI inputs' vaIidity The solution for opto-couplers' saturation is put fotward in thispaper.lt is pointed out that layout the transient suppressors for surges in power suppIypQrt and communication signaI line of protective reIay, such as varistors(voItagedependant resistors),zeners and spark gaps(gas discharge tubes) is necessary.The asynchronous seriaI I/O (UART) mode in the embedded microprocessor isdiscussed. lt is proved that the messages through the UART interface are not vaIidityduring EFT test. So the asynchronous seriaI l/O (UART) mechanism is not satisfied tothe demand for high speed, accurate and reaI-time controI applications, especia[ly thenumerical busbar protection.Many EMC problems in the numericaI protection apparatus of Si fang Corporationhave been soIved in this paper, and many heIpful concIusions for irnproving the EMCleveI of the numericaI protections have been dra...