In this post we will discuss about the feature of Harmonic Restraining feature of Differential Protection. First we should know why Harmonic Restraining is needed?
Suppose we are going to energize the Transformer, obviously the Transformer will have an Inrush current which is around 6 times of full load current. Refer “Transformer Inrush Current” for detail on Transformer Inrush Current. Figure given below shows waveform of Inrush Current of Transformer.
Therefore, the Differential protection will operate. Thus we won’t ever be able to energize the Transformer or we need to bypass the Transformer Differential Protection when we are going to energize. Is it a good practice to Bypass Transformer Protection? You will say NO. So what we need to do for preventing the Differential Protection operation due to Inrush Current?
This requirement calls for Harmonic Restraining in Differential Protection of Transformer.
The inrush current of a Transformer, if analyzed, is rich in 2nd harmonic component. So we can use this fact to prevent the operation of Differential Protection. Therefore a 2nd harmonic Restraining is provided in Transformer Differential Protection. Normally the setting of 2nd Harmonic Restraining is kept at 20% which means that if the 2nd harmonic component in the differential current of Relay is more than 20% of differential current Idthen Differential Protection Relay will not operate as it will think that it is because of Transformer Inrush Current but if the 2nd harmonic component in the differential current of Relay is less than 20% of differential current Id then Differential Protection Relay will operate.
Similarly, during overfluxing of a Transformer the Transformer current is rich in 5th harmonic current. But it may happen so that Transformer is overfluxed for some short time period say around 2 sec. So for this short period of overfluxing should not cause the operation of Differential Protection. Note that during overluxing, the differential current Id will increase which will operate the Differential protection Relay.
Therefore to prevent the operation of Differential protection Relay due to overfluxing, 5thharmonic Restraining is provided in such a manner that if the 5th harmonic component in the differential current of Relay is more than 25% (say) of differential current Id then Differential Protection Relay will not operate but if 5th harmonic component in the differential current of Relay is less than 25% of differential current Id then Differential Protection Relay will operate.