Transformer Differential Protection Relay

INTRODUCTION

1.1 Protection of transformers

The development of modern power systems has been reflected in the advances in

transformer design. This has resulted in a wide range of transformers with sizes from a

few kVA to several hundred MVA being available for use in a wide variety of

applications.

The considerations for a transformer protection package vary with the application and importance of the transformer. To reduce the effects of thermal stress and

electrodynamic forces it is advisable for the overall protection package to minimise the

time that a fault is present within a transformer.

On smaller distribution transformers effective and economically justifiable protection can

be achieved by using either fuse protection or IDMT/instantaneous overcurrent relays.

Due to the requirements of co-ordination with the down stream power system protection

this results in time delayed fault clearance for some low level faults. Time delayed

clearance of major faults is unacceptable on larger distribution, transmission and

generator transformers, where the effects on system operation and stability must be

considered. High speed protection is desirable for all faults.

Transformer faults are generally classified into four categories:

−

Winding and Terminal faults

−

Core faults

−

Abnormal operating conditions such as over voltage, overfluxing and overload

−

Sustained or uncleared external faults

All of the above conditions must be considered individually and the transformer

protection package designed accordingly.

To provide effective protection for faults within a transformer and security for normal

operation and external faults, the design and application of transformer protection must

consider factors such as:

−

Magnetising Inrush current

−

Winding arrangements

−

Winding connections

−

Connection of protection secondary circuits

The way that the protection of larger transformers is typically achieved is best illustrated

by examining the protective devices associated with common applications.

The Transformer Differential Protection Relay is an essential device used in power systems to safeguard transformers from internal faults. Transformers are critical assets in power distribution networks, and any fault within them, like short circuits or winding faults, can lead to severe damage and extensive outages. Differential protection relays protect against these types of faults by detecting imbalances in current flowing in and out of the transformer.

Principle of Operation

The transformer differential protection relay operates on the principle of comparing the currents at the transformer's primary and secondary sides. Under normal conditions, the current entering the transformer (primary side) is nearly equal to the current leaving (secondary side), when accounting for the transformer turns ratio and load. The relay calculates the difference (or differential) between these currents. If the difference exceeds a set threshold, it indicates an internal fault within the transformer.

The primary components of this protection scheme include:

Current Transformers (CTs): These are placed on both the primary and secondary sides of the transformer. They step down the high current flowing through the transformer windings to a lower, manageable level, which is then fed to the relay.
Differential Relay: The relay compares the currents from both sides. If the current difference is within permissible limits, the relay remains inactive. However, if a significant difference is detected, the relay quickly triggers a trip signal to isolate the transformer from the network.

Key Features and Functions

Sensitivity and Selectivity: Differential relays are highly sensitive to internal faults, responding only to imbalances caused by actual faults within the transformer. They remain unaffected by external faults or load changes, maintaining stability and selectivity.
Percentage Differential (Bias) Protection: To avoid unnecessary tripping from minor imbalances due to CT errors or slight differences in primary and secondary currents, percentage biasing is used. This means the relay operates only if the differential current exceeds a certain percentage of the load current, providing security during inrush currents or external faults.
Harmonic Restraint: Transformer energization can cause high inrush currents that mimic fault conditions. To prevent misoperation during inrush, harmonic restraint is applied, using harmonic components (like the second harmonic) to distinguish between inrush and actual fault conditions.
Time Delay: Some relays incorporate a brief time delay to avoid tripping on transient or temporary conditions. However, modern differential relays operate in milliseconds, ensuring the transformer is isolated quickly if a fault is detected.

Common Faults Detected by Differential Protection

Winding Faults: Short circuits within the windings are common faults detected by differential relays.
Interturn Faults: Short circuits between turns in the same winding result in high differential currents, activating the relay.
Phase-to-Ground and Phase-to-Phase Faults: Differential protection identifies faults occurring between phases or between phase and ground, which can lead to severe damage if not isolated immediately.

Advantages of Transformer Differential Protection Relays

Speed of Operation: These relays detect and isolate faults rapidly, minimizing the risk of damage.
Selective Protection: Only the faulty transformer is isolated, preventing unnecessary outages across the network.
Sensitivity to Internal Faults: Differential protection relays are designed to detect even low-magnitude internal faults that other types of protection may miss.

Limitations

Sensitivity to Inrush Currents: Inrush currents during transformer energization can potentially trigger false trips, though harmonic restraint addresses this.
Cost and Complexity: Differential protection systems are more expensive and complex to install due to CT placement on both primary and secondary sides.

Conclusion

Transformer Differential Protection Relays are crucial in modern power systems, providing reliable and sensitive protection against internal transformer faults. Their ability to quickly detect and isolate faults not only protects the transformer but also maintains the integrity and stability of the broader power system.

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