1.0 Harmonics in a Transformer Feeder:
Harmonics will appear in a transformer at following events :
a) Whenever a transformer is energised, there would be a high level of 2nd harmonics in the current drawn by the transformer - due to very high inrush current for magnetisation. This will create a differential current of the order of 15%.
b) Whenever a transformer is over excited (very high V/F ratio), the proportion of 5th harmonics will reach highr levels than what it was at the time of energisation. This can be equivalent to a differential current of greater than 30% ..
These harmonic currents will be on the primary side only -creating a condition as though there is a differential fault.
A transformer differential relay should have a facility whereby it will ignore a fault condition created due to presence of harmonics under above conditions (ie) the relay should be restrained from tripping when the current wave form has harmonics due to either energisation or over excitation.
Following paragraphs describe the features provided in a transformer differential relay MD32-T to manage the effects of harminics in a transformer feeder.
1.1 2nd Harmonic restraint level 2H( 10% to 30%):
The relay will measure the 2nd harmonic content and compare with the set point 2H. As long as the 2nd harmonics level in the current is more than the set value 2H, it will be considered as an event related to trasformer switch on, and the operation of the relay is restrained.
Consequently the relay will not trip during transformer switch on, even if harmonics are present.
1.2 5th Harmonic restraint level 5H (20 % to 40%):
A transformer will reach an over excitation level under following conditions:
a) when the load is thrown off suddenly (the primary voltage will rise resulting in a temporary over voltage condition)
b) when the frequency goes down
Under either of the above conditions, the 5th harmonic level will increase beyond 30% of total differential current.
It should be noted that both above conditions are external to the transformer and the differential relay should not trip.
The relay will measure the 5th harmonic content and compare with the set point 5H. As long as the 5th harmonics level in the current is more than the set value 5H, it will be considered as an event related to over excitation , and the operation of the relay is restrained.
Consequently the relay will not trip during over excitation of transformer , even if harmonics are present.
2.0 For a differential relay protecting a normal transformer feeder, the above two features are adequate to prevent mal-operation of relay due to presence of harmonics during charging and over excitation.
It should be noted that , while harmonic restraint is very important to avoid spurious tripping of transformer during energisation, it can not be always set at a very low value - say 8% .
There can be cases where a high level of harmonics (20 % to 30%) can be present even during reagular operation of the transformer - like a thyrister load or a furnace transformer or when the transformer is exporting power etc.
In such cases, it may be required to increse the restraint levels 2H and 5H to say 30%. Only then the relay would be restrained from tripping during normal operation even though the harmonics are high . At the same time the relay will trip on actual fault - since the differential current at actual fault will be predominantly of fundamental frequency and as a percentage , the harmonics will be less than 10%.
On the other hand if 2H and 5H are set at 30%, there is a danger that the relay will trip whenever the transformer is energised - since the harmonic levels will be around 15%.
MD32-T has following additional settings tH, R2H and R5H to take care of above complications (ie) a feeder where harmonis are very high under normal operating conditions.
2.1 Harmonic restraint level reduction time tH:
We can set a time tH in MD32-T relay from 0.05 to 99 sec.
We can connect an auxiliary contact of the breaker to terminals 1 & 3 of the relay. Time delay tH will start from the instant the contact closes at 1&3.
During this time we can reduce the actual restarint levels 2H and 5H by a factor .
This would mean that, for the duration tH from the contact closure, the levels of 2H and 5H will be automatically reduced.
2.2 2nd harmonic restraint level reduction during tH (R2H):
This is a factor by which the restraint level 2H will be automatically reduced for a period tH. This factor can be set from (0.5 to 1)2H in MD32-T.
2.3 5th harmonic restraint level reduction during tH (R5H):
This is a factor by which the restraint level 5H will be automatically reduced for a period tH. This factor can be set from (0.5 to 1)5H in MD32-T.
By the above three additional settings following are achieved:
a) a higher level can be set for harmonic restraint . This will ensure that there is no restraint and the relay will trip on actual fault in a transformer feeder with high harmonic levels under normal operating conditions.
b) Once a high level of restraint is set for normal operation, the tripping of relay at the time of transformer energisation , due to 2nd harmonics and 5th harmonics can be avoided, by automatic reduction of restraint levels R2H and R5H for a period tH