Why is a biased differential relay preferred over a simple differential relay?

Biased differential relay is preferred because its operation is not affected by the trouble arising out of the difference in the CTs ratios for high values of external short circuit currents.

What is biased differential protection of transformers?

The purpose of biased differential protection is to detect faults with high selectivity and sensitivity. This means that it can trip with no intentional time delay for faults within its zone of protection, the reason why differential protection is often applied on high value equipment such as transformers.

What is percentage differential protection why it is superior to simple differential protection?

So generally the percentage differential relay scheme is more stable than a simple differential relay scheme. And stable means that the relay will operate only for real internal fault in the protected zone, and it will never operate for through faults or due to any other mismatch.

What are the advantages of using a percentage differential relay in differential protection of current transformer?

What is advantage of using percentage differential relay protection?

• It does not require CTs with air gaps or special balancing features.
• It permits a low fault setting to be used and this ensures maximum protection of the windings.
• It ensures complete stability under the most severe through fault conditions.

What is biased relay?

Definition: A relay in which the characteristics are modified by the introduction of some quantity, and which is usually in opposition to the actuating quantity.

What is percentage differential protection of transformer?

Definition: The percentage differential relay is defined as the relay that operates on the phase difference of two or more similar electrical quantities. The restraining coil controls the sensitive characteristic of the relay. It restricts the unwanted tripping of the transformer due to the imbalance current.

What is mean by percentage differential protection of transformer?

The percentage differential relay is designed to operate the differential current in terms of its fractional relation with actual current flowing through the circuit. It is used to protect the system under Current transformer saturation, unequal CT ratios, nuisance trip etc.

Why do we use percent differential relay?

What is meant by biased-differential protection?

The term biased-differential or percentage differential is based on the fact that the operating threshold increases with the through current. Because of that, comparison of upstream and downstream currents in the same phases is not convenient for transformer differential protection.

What is differential protection in power transformers?

Differential protection schemes are mainly used for protection against phase-to-phase fault and phase to earth faults.The differential protection used for power transformers is based on Merz-Prize circulating current principle.

What is the difference between bus and differential protection?

Remember that a differential relay is basically an instantaneous overcurrent relay that operates on the difference of current flowing into and out of the protected zone. For transformers the differential protection ( Figure 1) is basically the same as that for a bus but there are certain differences that we will look more closely at.

Do you prefer voltage based or current based differential protection?

But mostly voltage based relays are not preferred. We use to prefer current based differential protection, but it has some limitations such as Both CT Should be identical CT ratio, Identical burden, extention cable resistance nuisance trip etc.