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Overcurrent protection principle

        Overcurrent protection is one of the most basic current protection ,used to respond to short circuit faults and severe overload faults.Protection of a wide range of objects:such as lines,generators,transformers and various loads.Overcurrent protection generally with action time,can be divided into fixed and inverse time two.

       The so-called fixed-time over-current protection is no matter how much fault current exceeds the set value,the action time is certain.If the action time is inversely proportional to the fault current,that is,the fault current exceeds the setting value,the faster the action time is called inverse time overcurrent protection.The following figure shows a block diagram of overcurrent protection.Figure start,time limit,signal,export all links are current relay,time relay,signal relay,intermediate relay.
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       When the line short circuit failure,through the current transformer TA,the current relay to detect the fault and start,after the time part of the delay,and then use the intermediate relay circuit breaker QF trip,while the signal relay through the light and sound signals.
 
       The main advantage of the overcurrent protection structure is simple,reliable,economical disadvantage is that with the increase in the number of grid extension,aside power time overcurrent protection device will be very long,and sometimes can not grow to the extent permitted.In addition,when the load current is large,the protection of the sensitivity is often unable to meet the requirements.
 

 

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