Transformers are critical components in electrical power systems that play a crucial role in transmitting and distributing electricity. They are used to step-up or step-down voltage levels, allowing efficient transmission and distribution of electricity across different voltage levels. However, like any other equipment, transformers are susceptible to faults that can disrupt the normal operation of the power system. Hence, it is essential to have effective protection schemes to detect and mitigate transformer faults promptly.
Transformer faults can be classified into internal and external faults. Internal faults occur within the transformer windings, while external faults involve the transformer in a fault occurring elsewhere in the power system, such as on the transmission lines or distribution feeders. Some of the common transformer faults include:
Winding Faults: These faults can occur due to insulation breakdown in the winding, resulting in short circuits between the turns. Winding faults can be caused by thermal aging, overheating, mechanical stress, or insulation degradation.
Core Faults: Core faults can occur due to mechanical damage, such as core insulation failure or core clamping failure. These faults can lead to increased core losses and inefficient operation of the transformer.
Tap Changer Faults: Tap changers are used to vary the transformer turns ratio and adjust the output voltage. Faults in tap changers can result in voltage fluctuations or even complete failure of voltage regulation.
Overload Faults: Overloading a transformer beyond its rated capacity can lead to thermal stress, insulation degradation, and ultimately, transformer failure.
To protect transformers from these various faults, a combination of protective devices and schemes is employed. The primary objective of transformer protection is to detect and isolate faults quickly, ensuring the safety of personnel and minimizing damage to the transformer and the power system. The protection schemes typically include:
Buchholz Relay: The Buchholz relay is a gas and oil-operated relay that detects internal faults, such as those occurring within the transformer windings. It senses the presence of gas or oil due to fault-related arcing, overheating, or insulation breakdown.
Differential Protection: Differential protection is one of the most common and effective protection schemes for transformers. It compares the currents entering and leaving the transformer and operates if a fault causes a significant imbalance. The primary current inputs are typically connected to the transformer primary winding.
Overcurrent Protection: Overcurrent relays are used to detect and isolate faults caused by overcurrents in the transformer windings. These relays monitor the current flowing through the transformer and operate when the current exceeds a pre-set threshold.
Temperature Monitoring: Temperature sensors are used to monitor the temperature of various critical points in the transformer, such as the windings and oil. If the temperature exceeds a pre-defined limit, it indicates a fault or abnormal operating condition.
Numerical Example:
Let’s consider a practical scenario involving a 138 kV/11 kV, 50 MVA power transformer used in a distribution substation. The transformer has a star-connected primary winding and a delta-connected secondary winding. The primary side is protected using differential protection, and the secondary side is protected using overcurrent protection.
For the primary side differential protection, the operating current for the relay is set at 10% of the transformer rated current (50 MVA / 138 kV). Therefore, the operating current is 3,623 A (50e6 / (sqrt(3) * 138e3)). If there is a fault on the primary side, such as a short circuit, the differential relay will sense the imbalance and trip the circuit breaker to isolate the fault.
For the secondary side overcurrent protection, the transformer rated current is 4,558 A (50 MVA / (sqrt(3) * 11 kV)). Let’s assume an overload condition occurs, causing the current to exceed 120% of the rated current. This corresponds to 5,469 A (1.2 * 4,558 A). The overcurrent relay is set to operate at this current level and will trip the circuit breaker to protect the transformer.
It is important to note that these relay settings are subject to coordination with upstream devices to ensure proper selectivity and coordination within the protection system.
In summary, transformer faults can lead to significant damage and disruption in power systems. Implementing effective protection schemes, such as Buchholz relay, differential protection, and overcurrent protection, can detect faults promptly and ensure the safe operation of transformers. Careful coordination and selection of relay settings are crucial in developing a reliable and robust transformer protection system.