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 used to ensure the safe operation of the transformer and maintain stable power supply. This type of protection works by comparing the magnitude and phase of currents at both ends of the protected equipment. The core idea is that under normal conditions or during external faults, the currents entering the relay from both sides should cancel each other out, resulting in a near-zero value, which prevents unnecessary tripping. However, during an internal fault, the sum of the currents from both sides becomes significant, triggering the relay to act.
The basic principle of transformer differential protection is similar to other types of differential protection, such as those used in generators or transmission lines. It relies on the comparison of current values and their phase relationships. While the term "differential pressure" might be used in some contexts, it's not commonly applied in transformer protection. Instead, the system focuses on detecting differences in current flow between the primary and secondary sides of the transformer.
To improve reliability and reduce false trips, modern differential protection systems have evolved with advanced techniques like proportional restraint differential protection, double-slope differential protection, and even wavelet transform-based algorithms. These enhancements help the system better distinguish between real faults and transient imbalances caused by factors like magnetizing inrush currents or CT saturation.
In addition, there are specialized methods for identifying abnormal states such as TA (current transformer) disconnection or short circuits. One common approach involves using the ratio braking principle for differential current alarms, where the threshold adjusts based on the transformer’s load current. If the differential current exceeds the set threshold for more than 10 seconds, an alarm is triggered, but the protection itself remains active. The initial setting for this alarm is typically based on the transformer’s excitation current, often around 0.03 times the rated current (Ie).
This protection method also combines voltage changes with current data to detect TA issues. After the differential protection is activated, if any of the following conditions occur, it is considered a true fault: (1) A change in the voltage component of any phase is detected. (2) The negative sequence voltage on either side exceeds 6V. Otherwise, the system assumes the trip was due to a TA problem rather than an actual fault.
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