What Are The Problems Arising In Differential Protection In Power Transformer And How Are They Overc

Differential protection is a vital component of power transformer protection schemes, but it can face several challenges and problems. Some common issues with differential protection in power transformers include:

1. Inrush Currents: During transformer energization, inrush currents can occur due to the magnetization of the transformer core. These currents can mimic fault currents and cause differential relays to operate incorrectly, resulting in nuisance tripping. Inrush current detection algorithms or harmonic blocking filters are commonly used to mitigate this problem.

2. Saturation: Saturation of current transformers (CTs) during high fault currents or through-flux conditions can lead to distorted differential currents and incorrect operation of differential relays. Proper CT saturation detection schemes, including harmonic restraint or percentage differential schemes, help prevent false tripping caused by CT saturation.

3. Mismatched CTs: Differential protection relies on CTs to accurately measure currents entering and leaving the transformer windings. Any mismatch in the performance or characteristics of CTs, such as different turns ratios or phase shifts, can lead to inaccuracies in differential current measurements. Regular calibration, testing, and ensuring CT consistency help mitigate this problem.

4. Transformer Inhomogeneity: In large power transformers, internal winding connections, tap changers, or phase-to-phase voltage differences can introduce asymmetry and affect the balance of differential currents. Compensating methods, such as neutral current injection or vector group correction, are employed to maintain balance and improve the performance of differential protection.

5. Ferromagnetic Inrush: Certain fault conditions or load fluctuations can induce ferromagnetic inrush currents in power transformers, resembling internal faults. Advanced algorithms, such as second-harmonic blocking or dynamic restraint, are used to distinguish between inrush and fault currents and prevent false tripping of differential relays.

6. Transient Voltage Events: High-frequency transient voltage events, such as lightning strikes or switching surges, can introduce temporary imbalances in transformer differential currents. Differential protection schemes may incorporate transient blocking or filter techniques to suppress the effects of transient events and avoid spurious operation.

7. Communication Delays: In differential protection schemes employing communication-assisted relays, delays in communication between relay units can affect the synchronization and coordination of differential protection elements. Redundant communication paths, faster communication protocols, and adaptive coordination algorithms help address communication delays and ensure reliable operation.

To overcome these challenges, modern differential protection schemes employ advanced algorithms, sophisticated relay technologies, and integrated protection systems. Additionally, regular maintenance, testing, and coordination with other protective devices enhance the reliability and performance of differential protection in power transformers.