A Word of Caution - For using High Set Element

While using the definite time delay protection in case of overcurrent and earthfault relay you should always be cautious. Following things should be avoided.

1)    Setting High Set element current of HV side relay corresponding to LV bus fault reflected on HV with instantaneous operation (Time Delay = 0 ms)

2)    Setting LV side forward direction High Set instantaneous (Time Delay = 0 ms). This setting may cause the relay to mal-operate in case of very near LV bus fault due to transients. It is better practice to set time delay for this element as 50 ms.

3)    Setting LV side reverse direction High Set current setting very less along with instantaneous operation. This will definitely cause operation of transformer LV breaker along with feeder breaker leading to unselective tripping. 

4)    As mentioned in section “Settings …. When T/F operating separately” that as LV Bus fault current decreases time discrimination between HV and LV relay increases. Converse of this is also true. This happens more predominantly in case of use of HS for LV relay. Hence if fault level increase it is necessary to review TMS settings.

Thus it is general tendency to keep High Set current value to some unrealistic high value and thereby keeping this feature in bypass state. In spite of all such odds it is recommended to use High Set element cleverly as it may provide valuable information during transformer tripping on fault and may assists in analysing the fault more correctly and decision making accordingly.

Here we have shown 2 methods of using High Set Element. Method 1 will work correctly. However method 2 will not work. Because in 2^nd method relay R3 to which IDMT characteristics assigned will operate before to HighSet element and we will not get expected operating characteristics. The figures shown are self-explanatory.

Fig-1	Fig-2
Correct Use of Highset	Incorrect Use of Highset