The abnormal operating conditions of transformers mainly include overload, overcurrent caused by external short circuits, neutral point overvoltage caused by external ground faults, oil level reduction due to oil tank leakage, or temperature rise due to cooling system failures. Additionally, large-capacity transformers, due to their high rated working magnetic flux density (which is proportional to the voltage-to-frequency ratio), may experience over-excitation faults when operating under overvoltage or low-frequency conditions. To address these issues, large transformers generally adopt the following protection methods:
Gas protection responds to internal faults and oil level reduction within the transformer. Oil-immersed transformers with a capacity of 0.8 MVA or above, and workshop oil-immersed transformers with a capacity of 0.4 MVA or above, should be equipped with gas protection. When minor gas is generated or the oil level drops, the protection should instantly trigger an alarm signal. When a large amount of gas is produced, it should trip the circuit breakers on all sides of the transformer. Load-tap-changing oil-immersed transformers should also have gas protection installed on their tap-changing devices.
Differential protection or instantaneous current protection responds to phase-to-phase short circuits in the windings or outgoing lines, grounding faults in high-grounding current systems, and inter-turn short circuits in the windings. The protection trips the circuit breakers on all sides of the transformer instantaneously.
Overcurrent protection responds to external phase-to-phase short circuits and serves as backup for gas protection and longitudinal differential protection (or instantaneous current protection). After protection action, it operates with a time delay to trip the circuit breaker.
Zero-sequence current protection responds to external ground faults in transformers within high-grounding current systems. In high-grounding current systems of 110 kV or above, if the transformer neutral point may be grounded during operation, zero-sequence current protection should be installed for step-up or step-down transformers with two-sided or three-sided power sources. It serves as backup protection for the main transformer protection and adjacent components.
A device that uses the zero-sequence current generated during grounding to activate protection is called zero-sequence current protection. Dedicated zero-sequence current transformers are used on cable lines to achieve grounding protection. The zero-sequence current transformer is installed around the three-core cable, and the current relay is connected to the secondary winding of the transformer. During normal operation or in the absence of a ground fault, the vector sum of the three-phase currents equals zero, so the secondary winding current of the zero-sequence transformer is also zero (only a small unbalanced current exists), and the current relay does not operate. During a ground fault, the secondary winding of the zero-sequence transformer generates a large current, triggering the current relay to issue a signal or trip the circuit.
Overload protection responds to symmetrical overloads of the transformer. For transformers of 400 kVA or above, when operating in parallel or as a backup power source for other loads, overload protection should be installed based on potential overload conditions. For autotransformers and multi-winding transformers, the protection device should respond to overloads in the common winding and on all sides. In most cases, the overload current is three-phase symmetrical, so overload protection only needs to monitor one-phase current using a current relay, which triggers an alarm after a certain delay. When selecting the installation side for protection, it must be able to reflect overload conditions for all sides of the transformer. In substations without regular staff, overload protection can trip the circuit or disconnect part of the load if necessary.
Over-excitation protection responds to over-excitation of the transformer. In modern large transformer designs, to save materials, reduce costs, and lower transportation weight, the core’s rated working magnetic flux density is designed to be relatively high, typically around 1.7–1.8 T, close to the saturation magnetic flux density (1.9–2 T). Therefore, over-excitation can easily occur under overvoltage conditions. Additionally, because the magnetization curve is relatively "hard," during over-excitation, core saturation reduces excitation impedance, causing the excitation current to increase rapidly. When the working magnetic flux density reaches 1.3–1.4 times the normal value, the excitation current can reach the rated current level. Furthermore, since the excitation current is non-sinusoidal and contains many high-order harmonics, eddy current losses in the core and other metal components (proportional to the square of the frequency) can cause severe overheating of the core, metal components, and insulation materials. If the over-excitation multiple is high and lasts too long, the transformer may be damaged. Therefore, transformers with a high-voltage side of 500 kV should ideally be equipped with over-excitation protection. The purpose of installing over-excitation protection is to detect over-excitation conditions, issue timely signals, or trip the circuit to ensure that the transformer's over-excitation does not exceed allowable limits, preventing damage due to over-excitation.
