What is the principle of differential current protection for a main transformer, and how does it differ from general differential protection?
Differential protection for a main transformer is one of the most critical protective systems in power transformers. It plays a vital role in ensuring the safe operation of the transformer and maintaining the reliability of the entire power system. The basic principle of transformer differential protection is similar to other types of differential protection, which involves comparing the magnitude and phase of currents at both ends of the protected equipment.
In a typical setup, current transformers (CTs) on both sides of the transformer are connected in a way that allows the relay to compare the currents. Under normal operating conditions or during external faults, the difference between the two currents is minimal, resulting in almost zero current flowing into the relay—thus, no action is taken. However, when an internal fault occurs, the relay detects the sum of the currents from both sides, which represents the short-circuit current, triggering the protection mechanism.
While the term "differential current" and "differential pressure" may sometimes be used interchangeably, there is no actual difference between them in this context. The core concept remains the same: comparing the current values from both sides of the transformer.
To enhance the performance and accuracy of differential protection, advanced techniques such as proportional restraint differential protection, dual-slope differential protection, and wavelet transform-based methods have been developed. These innovations improve the system's ability to distinguish between real faults and false signals caused by factors like magnetizing inrush or CT saturation.
Additionally, the method of identifying abnormal states in the TA (current transformer) during differential current protection includes using a ratio braking principle. The alarm threshold for differential current changes based on the load current of the transformer. If any phase differential current meets certain criteria for more than 10 seconds, an alarm signal is triggered without blocking the differential protection.
The initial alarm setting should be adjusted according to the excitation current of the transformer. Typically, the alarm value is set to 0.03 times the rated current (Ie). The formula used is: Id = k * (I1 + I2 + ... + Im), where Id is the differential current, Ir is the braking current, and k is a proportionality constant. This approach helps in distinguishing between normal fluctuations and actual fault conditions.
Furthermore, the protection system combines the power frequency variation voltage component with current measurements to detect TA disconnection or short circuits. After the differential protection is activated, if any of the following conditions are met, it is considered a fault condition, and dynamic protection is initiated. Otherwise, the protection is assumed to have been triggered due to an issue in the differential circuit.
- A change in the voltage component of any phase is detected. - The negative sequence voltage on either side exceeds 6V.
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