Transformers are commonly used in power supply systems to increase or decrease working voltage. They are a crucial piece of equipment that cannot be lacking in the power supply system. Their failure can cause significant harm to the reliability of the power supply system and its optimized operation. Moreover, large-scale power engineering transformers are also precious machines. Therefore, it is necessary to install high-quality, reliable relay protection devices according to the transformer's volume level and criticality.
Transformer faults can be categorized into two types: those outside the car's fuel tank and those inside the car's fuel tank.
External faults of the car fuel tank are mainly caused by two-color short circuits and grounding faults on the waterproof sleeve and transformer grounding wire. Internal faults in the car fuel tank include two-color short circuits in the winding, grounding faults, inter-turn short circuits, and burnishing of the transformer core. When internal faults in the car fuel tank occur, the resulting electrical arcs not only damage the insulation layer of the winding and the transformer core but also, due to decomposition reactions of the insulation material and transformer oil, produce a large amount of gas, which could potentially lead to an explosion of the transformer fuel tank. For various faults generated by transformers, the protector should remove the transformer as soon as possible.
Field practice indicates that the two-color short circuits on transformer waterproof bushings and transformer grounding wires, as well as short-circuiting devices and winding inter-turn short circuits, are relatively common fault modes; however, the occurrence of two-color short circuits within the transformer's automotive oil tank is relatively rare.
The critical abnormal operating conditions of transformers include: overcurrent caused by external short circuits in transformers, overloading due to long-term load exceeding the short-circuit capacity, and reduced cooling capacity due to issues such as cooling fan failure or oil leakage. These abnormal conditions can lead to overheating of the winding and transformer core.
In addition, for star-wired transformers with neutral line grounding devices, a short circuit in external grounding devices may cause overvoltage of the transformer's neutral line, threatening the transformer's insulation. Under abnormal operating conditions such as overvoltage or low frequency, large capacity transformers can cause excessive excitation regulators, leading to overheating of transformer cores and other metal structures. When transformers are in abnormal operating conditions, relay protection devices should issue alarm signals based on the severity, allowing operation staff to timely address the situation and take appropriate measures to ensure transformer stability.
Transformer Installation Maintenance
(1) Coalbed Methane Maintenance: Defense against fuel tank failures and declining fuel levels in vehicles.
(2) Differential Protection or Current Differential Maintenance: Defense against transformer winding sheath leaks and transformer grounding line faults (Current Differential Maintenance is typically used for transformers below 10005kVA).
(3) Overcurrent Maintenance: Defends against external two-color short circuits (serving as a reserve maintenance for coalbed methane maintenance and differential protection).
(4) Zero-sequence current maintenance defense against overcurrent caused by short-circuiting of external grounding devices (in power grids with neutral grounding immediately to the ground).
Basic Principle of Transformer Differential Protection
To better grasp the basic principles of transformer differential protection, as illustrated in the following diagram, when the sum of forces is not zero when F_left ≠ F_right, the object is unbalanced. When F_left = F_right, the resultant force on the object is zero, and the object remains in a state of equilibrium and stationary.
Differential protection operates based on the fundamental principle of Kirchhoff's Current Law, which states, "At any arbitrary moment, the algebraic sum of the currents passing through any connection point in a power circuit is always zero." In differential protection, the transformer under maintenance is treated as a contact point. Current and voltage transformers are installed on all sides of the transformer, and the secondary sides of these transformers are wired using the differential wiring method, meaning that the same旋光性（polarization）ends of the transformers on each side are oriented towards the busbar, with the same旋光性 terminal connections made and then connected in series to the differential protection relay.
The current passing through the electromagnetic coil in automotive relays is the difference in the secondary current of the current voltage transformers on each side. In other words, the differential protection for automotive relays is connected to the differential protection control circuit. Theoretically, under normal conditions or when a fault occurs outside the maintenance area, the current injected into the transformer is the same as the current discharged (after conversion), and the current in the differential protection control circuit is zero. (To ensure consistent secondary current values between the high and low voltage current voltage transformers in the main transformer, different current transformers with varying ratios must be selected.) However, due to various factors, an unbalanced current exists in the differential protection control circuit. Therefore, the starting current value of the differential protection needs to be calculated and set. When the differential current exceeds the integral time constant of the differential protection device, the maintenance position is activated, and the circuit breakers on all sides of the transformer under maintenance are tripped, cutting off the power supply to the faulty transformer.
Several key reasons contribute to the unbalanced current in transformers:
Unbalanced current caused by transformer excitation inrush (Ily).
2. Unbalanced current caused by the differing current positions on either side of the transformer.
3. Unbalanced current caused by discrepancies between calculated ratio and actual ratio.
4. Unbalanced current caused by different models of current transformers on both sides.
5. Unbalanced current caused by load regulation at the transformer's tap connection.
