| As per “Design manual for electrical power project” and “Specification for designingplant service of fire power plant”(DL/T5153-2002), for high voltage motors, followingprotection is applied:(1)Overload protection;(2)Over current protection;(3)Instantaneousover current protection;(4) Single phase to ground protection;(5)Differential protection;(6)Under voltage protection;(7) Other protection.Thermal overload protection is one of the main protections for motor. So many reasonsmight cause a motor thermal overloaded. For now most relay producers are using thermalcapacity model for this protection. Curve of the model has3sections; we can make oursettings by adjust any of these curves. Differential protection is another main protection formotor greater than2000kW. It detects phase to phase and motor internal faults; it used2groups of CT and differential relay which form a circuit for protection. For those less than2000kW, current instantaneous trip protection is the main protection for phase to phase andmotor internal faults. Over current protection is similar to current instantaneous tripprotection but the difference is over current protection has time setting which currentinstantaneous trip protection don’t. Most power plants now using non-grounded system fortheir middle-voltage systems, when a phase to earth fault occur, it still has current gettingthrough the feeder, then we should set up single phase to earth protection. As perspecification DL/T5153-2002, phase to ground current great than5A should set up singlephase to earth protection. As per the same specification, low voltage protection is needed forsome of motors, when necessary these motors should be tripped to remaining the busbarvoltage.Sometimes, general configuration and setting is not applicable in practice. When suchcircumstances appear, how can we fix them? We should base on the basic theory ofprotection and do some special study on it, and give the right solution to make thoseproblems gone. |
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