What Are You Looking For?
Faults in Power Transformers
Internal Faults: These include phase-to-phase and inter-turn short circuits in the high and low voltage windings, and single-phase grounding.
External Faults: These include phase-to-phase faults and single-phase grounding on bushings and lead wires.
Abnormal Operating Conditions
Abnormal conditions can result from external short circuits or overloads causing overcurrent, and from oil leakage leading to a drop in oil level.
For transformers rated 400 kVA and above, overload protection should be installed based on potential overload conditions when operating in parallel or as a backup supply for other loads. For autotransformers and multi-winding transformers, the protection device should respond to overload conditions on common and individual windings.
The operating current for overload protection is set based on the transformer's rated current:
Idj=IeKKKj/KfnL
Where Ie is the rated current on the high-voltage side; KK is the reliability coefficient (1.2 for DL type, 1.3 for GL type); Kj is the connection coefficient (1.0 for incomplete star); Kf is the return coefficient (generally taken as 0.85); and nL is the transformation ratio.
The time delay for operation should be greater than the maximum time delay of the transformer’s backup protection by a margin Δt\Delta tΔt, typically taken as 5-10 seconds, to prevent misoperation from external short circuits or short-duration overloads.
Gas protection is designed to detect internal faults in the transformer’s oil tank and to monitor oil level drops. When a fault occurs within the transformer oil tank, gases produced from the decomposition of oil and insulation materials flow to the top of the oil conservator, triggering the gas protection.
Gas protection is quick-acting and highly sensitive, capable of responding to various short circuit faults inside the transformer oil tank, including minor faults such as winding inter-turn or phase faults, single-phase grounding in the high-voltage winding, localized heating in the core, poor contact at tap changers, and poor welding in conductors. However, it does not respond to external faults, so it cannot serve as the main protection for the transformer alone; it is typically used in conjunction with differential protection.
Gas protection can be classified into light gas protection, which generally signals an alarm, and heavy gas protection, which triggers a trip.
Differential protection, also known as longitudinal differential protection, is the main protection against various short circuit faults in the transformer windings, bushings, and leads. However, it is less sensitive to inter-turn short circuits within the transformer tank. Therefore, it is usually combined with gas protection to provide comprehensive protection.
The differential protection device consists of current transformers on both sides of the transformer and relays. Two current transformers are connected in series to form a loop, with the current relay connected in parallel. The current through the relay is the difference between the secondary currents of the two current transformers.
Under normal conditions or during faults outside the protection range, the secondary currents of the current transformers are equal in magnitude and phase, resulting in zero differential current through the relay. However, if a short circuit occurs within the protection range, the differential current is no longer zero, causing the relay to trip the circuit breaker for protection.
For smaller transformers, when the action time of overcurrent protection exceeds 0.5 seconds, instantaneous current protection can be installed on the supply side. This works in conjunction with gas protection to respond to various faults on the transformer windings and supply side bushings.
The operating current for protection can be chosen based on the following conditions, typically taking the larger value:
Idj=Id2.maxKkKj/nL
where Id2.max is the maximum current through the high voltage protection device during three-phase short circuits on the low voltage side; Kk is the reliability coefficient; Kj is the connection coefficient; and IN is the transformation ratio.
(2) It can also be set based on the excitation current when the transformer is energized under no-load conditions, generally taken as 3-5 times the rated current:
The sensitivity coefficient is usually verified under the minimum operating conditions, ensuring that the minimum short-circuit current flowing through the protection device during a two-phase short circuit on the high voltage side is not less than twice the operating current.
Backup protection responds to overcurrent caused by external short circuits and serves as the main protection for the transformer and as backup for adjacent busbars or lines. Transformers must be equipped with overcurrent protection. Depending on the transformer capacity and system short-circuit current levels, options include overcurrent protection, low voltage start overcurrent protection, combined voltage start overcurrent protection, and negative sequence current protection.
The operating current for overcurrent protection is set based on the maximum load current that the transformer may experience. The reliability coefficient is typically set between 1.2 and 1.3, with the return coefficient set between 0.85 and 0.95. For transformers operating in parallel, the load current after disconnecting the largest capacity transformer should be considered. For step-down transformers, the maximum load current during motor startup should be accounted for.
Low voltage start overcurrent protection ensures that the motor can start without triggering the protection under normal conditions, hence the operating current is set above the rated current of the transformer, with the reliability coefficient at 1.2 and the return coefficient between 0.85 and 0.9.
Combined voltage start overcurrent protection is set based on the rated current of the transformer.
For transformers in direct grounded neutral systems with a voltage level of 110 kV and above, ground backup protection should be installed as near backup for direct grounding faults and as remote backup for external adjacent components including busbars and line grounding faults.
Groun faults are a major form of fault in power systems. For transformers connected to direct grounded neutral networks at 110 kV and above, backup protection against ground short circuits caused by external single-phase faults should be installed.
The zero-sequence protection for transformer ground faults consists of zero-sequence overcurrent protection, zero-sequence overvoltage protection, and gap zero-sequence overcurrent protection.
