Unbalanced Current in the Differential Protection Circuit of UHV Transformers
Causes of Transformer Unbalanced Current
Unbalanced Current Due to Inconsistency Between CT Calculated Ratio and Actual Ratio
The ratios of transformers and current transformers (CTs) are produced according to relevant standards, and once they leave the factory, these ratios are fixed. In actual field operation, it is challenging to match the actual ratio of transformers and CTs to the calculated ratio, potentially causing unbalanced current and affecting the reliability of differential protection.
Unbalanced Current Due to CT Transmission Error
The unbalanced current flowing into the differential relay is related to the excitation current of the CTs on the high and low voltage sides of the transformer. The excitation surge current of the CT can directly flow into the differential protection device, causing it to malfunction.
Unbalanced Current Due to Transformer Excitation Surge Current
Transformers can be modeled as equivalent circuits with multiple electrical circuits plus one magnetic circuit. The excitation circuit can be equivalent to a fault branch in the internal fault of the transformer. When the transformer generates an excitation surge current, this current flows into the differential relay, creating unbalanced current. Given the large magnitude of excitation inrush currents, adjusting the differential protection settings alone may reduce the sensitivity of the differential protection during actual transformer faults, potentially causing malfunctions.
Unbalanced Current Due to Transformer Voltage Regulation
Voltage regulation by changing the transformer tap settings alters the transformer ratio. While this helps in the rational distribution of reactive power in the grid, it can disrupt the balance between the CT ratio and the transformer ratio since the CT ratio remains unchanged. Differential protection settings, typically based on the rated ratio, can be affected, generating unbalanced current.
Measures to Reduce the Impact of Unbalanced Current
Balancing Coil and Software Adjustments
To address unbalanced currents caused by the inconsistency between the calculated and actual CT transformation ratios, traditional methods involve the use of balancing coils. Modern microcomputer protection systems, however, can achieve precise balance adjustments of current amplitude through software.
Electronic and Photoelectric Transformers
In the context of the rapid development of smart grids, using electronic photoelectric transformers can help eliminate unbalanced currents caused by CT transmission errors.
Suppression and Identification of Excitation Surge Currents
To mitigate unbalanced currents caused by transformer excitation surge currents, various methods can be employed to suppress or identify these currents, ensuring the differential protection does not malfunction due to surge currents.
Compensation for Unbalanced Current Due to Voltage Regulation
In current engineering practice, unbalanced currents due to voltage regulation are often managed by adjusting differential protection settings when the transformer tap changes. However, this continuous adjustment can be inconvenient in practical fieldwork. New methods are needed to scientifically and reasonably compensate for unbalanced currents.
By implementing these measures, the impact of unbalanced currents on the differential protection circuit of ultra-high voltage transformers can be significantly reduced, enhancing the reliability and stability of the protection system.
More resources
Differential Protection of UHV Transformer
