Difference Between High and Low Impedence Differential Protection

The high impedance differential protection relay, in general, is an instantaneous overcurrent relay. It works on the principle that whenever the differential current thro the relay is more than the set current, the relay would trip instantaneously. But, there could be differential currents, even without a genuine internal fault within the protected zone, due to:

i) CT ratio errors

ii) Differential CT Saturation

iii) Differential CT Saturation due to Aymmetrical Fault Current

iv) Different lead lengths, etc.

As differential protection is employed only for critical equipment in the plant, its tripping causes panic amongst the operating personnel that something is wrong within the critical equipment and one cannot put back the equipment into service again, without conducting an array of tests and ensuring that the equipment is fit to be put in service. But, this would take time & effort and might cause plnat downtimes. Hence, it is all the more imperative that the differential protection operates only for genuine internal faults and does not operate for spurious differential currents (due to any of the above reasons) or for external faults. Hence, to de-sensitise the differential protective relay, it is common practice to introduce a stabilising resistor in the relay circuit such that the relay does not operate for spurious differential currents. As the introduction of the resistance increases the effective impedance of the relay circuit, this scheme is called the "High Impedance Differential Protection Scheme".

Whereas, in modern day numerical differential relays, it is possible to introduce some slope in the relay charectersitics, which takes care of such spurious differntial currents, without the need for any external resistance. Such a scheme is called "Low Impedance Differential Protection Scheme".

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Quote EE65

"The high impedance differential protection relay, in general, is an instantaneous overcurrent relay. It works on the principle that whenever the differential current thro the relay is more than the set current, the relay would trip instantaneously. But, there could be differential currents, even without a genuine internal fault within the protected zone, due to:

i) CT ratio errors

ii) Differential CT Saturation

iii) Differential CT Saturation due to Aymmetrical Fault Current

iv) Different lead lengths, etc.

As differential protection is employed only for critical equipment in the plant, its tripping causes panic amongst the operating personnel that something is wrong within the critical equipment and one cannot put back the equipment into service again, without conducting an array of tests and ensuring that the equipment is fit to be put in service. But, this would take time & effort and might cause plnat downtimes. Hence, it is all the more imperative that the differential protection operates only for genuine internal faults and does not operate for spurious differential currents (due to any of the above reasons) or for external faults. Hence, to de-sensitise the differential protective relay, it is common practice to introduce a stabilising resistor in the relay circuit such that the relay does not operate for spurious differential currents. As the introduction of the resistance increases the effective impedance of the relay circuit, this scheme is called the "High Impedance Differential Protection Scheme".

Whereas, in modern day numerical differential relays, it is possible to introduce some slope in the relay charectersitics, which takes care of such spurious differntial currents, without the need for any external resistance. Such a scheme is called "Low Impedance Differential Protection Scheme"."

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