As a supplier of Three Phase Oil Immersed Power Transformers, I am often asked about how the protection devices of these transformers work. In this blog post, I will delve into the various protection devices used in three - phase oil - immersed power transformers and explain their working principles.
Over - current Protection
Over - current protection is one of the most fundamental protection mechanisms in a three - phase oil - immersed power transformer. When the current flowing through the transformer exceeds its rated value, it can cause overheating and damage to the windings.
There are two main types of over - current protection: instantaneous over - current protection and time - delayed over - current protection.
Instantaneous Over - Current Protection
This protection device operates immediately when the current exceeds a pre - set threshold. It is designed to quickly isolate the transformer from the power system in case of a severe short - circuit fault. The instantaneous over - current relay senses the current through current transformers (CTs) installed in the transformer's primary and secondary circuits. When the current reaches a value high enough to trigger the relay, it sends a signal to the circuit breaker, which then trips and disconnects the transformer from the power supply.
Time - Delayed Over - Current Protection
Time - delayed over - current protection is used to handle less severe over - current situations. Instead of tripping immediately, it allows the transformer to carry the over - current for a certain period of time. This is because some short - term over - currents may be normal, such as during motor starting. The time delay is set based on the transformer's thermal capacity and the expected duration of the over - current. Similar to instantaneous over - current protection, it also uses CTs to sense the current and a relay to send a trip signal to the circuit breaker when the over - current persists beyond the set time limit.
Over - voltage and Under - voltage Protection
Over - voltage Protection
Over - voltage can cause insulation breakdown in the transformer windings, leading to a short - circuit. Over - voltage protection devices monitor the voltage across the transformer terminals. Potential transformers (PTs) are used to step down the high - voltage to a level that can be measured by the protection relay. When the voltage exceeds a pre - determined safe limit, the over - voltage relay is activated. It then sends a signal to the circuit breaker to disconnect the transformer from the power system.
Under - voltage Protection
Under - voltage can also have a negative impact on the transformer and the connected loads. It may cause motors to stall and reduce the efficiency of electrical equipment. Under - voltage protection works in a similar way to over - voltage protection. The PTs measure the voltage, and when the voltage drops below a set value, the under - voltage relay sends a signal to the circuit breaker, which may trip the transformer or take other appropriate actions to protect the system.
Differential Protection
Differential protection is a highly sensitive protection method for three - phase oil - immersed power transformers. It compares the current entering and leaving the transformer. Current transformers are installed on both the primary and secondary sides of the transformer.
The operating principle is based on Kirchhoff's current law, which states that the sum of currents entering a node is equal to the sum of currents leaving the node. In a healthy transformer, the current entering and leaving should be in balance. However, in case of an internal fault, such as a short - circuit in the windings, the balance is disrupted.
The differential relay continuously calculates the difference between the primary and secondary currents. If this difference exceeds a pre - set value, it indicates an internal fault, and the relay sends a signal to the circuit breaker to trip the transformer. This protection method can quickly detect and isolate internal faults, minimizing damage to the transformer.
Temperature Protection
Temperature is a critical parameter for the safe operation of a three - phase oil - immersed power transformer. High temperatures can accelerate the aging of the insulation material and reduce the transformer's lifespan.
Oil Temperature Protection
The oil in the transformer serves as both a coolant and an insulating medium. Temperature sensors are installed in the oil to monitor its temperature. When the oil temperature rises above a certain limit, it may indicate a problem, such as an over - load or an internal fault. The temperature relay sends a signal to either alarm the operator or trip the transformer, depending on the severity of the temperature rise.
Winding Temperature Protection
In addition to oil temperature, the temperature of the transformer windings is also monitored. Winding temperature is estimated by measuring the oil temperature and taking into account the load current. Since the heat generated in the windings is proportional to the square of the current, the winding temperature can be calculated more accurately. Similar to oil temperature protection, when the winding temperature exceeds a safe limit, appropriate actions are taken to protect the transformer.
Gas Protection (Buchholz Relay)
The Buchholz relay is a very important protection device for oil - immersed transformers. It is installed in the pipeline between the transformer tank and the conservator.
When an internal fault occurs in the transformer, such as a short - circuit in the windings, it generates heat, which causes the oil to decompose and produce gas. The Buchholz relay has two chambers: a lower chamber and an upper chamber.
In the case of a minor fault, such as a slow - developing insulation breakdown, the gas generated accumulates in the lower chamber. As the gas accumulates, it displaces the oil, causing a float in the lower chamber to rise. When the float reaches a certain position, it activates a contact, which can be used to send an alarm signal to the control room.
In the case of a major fault, such as a severe short - circuit, a large amount of gas and oil flow rapidly towards the conservator. This causes a baffle in the upper chamber to move, which activates another contact. This contact is used to trip the transformer, disconnecting it from the power system to prevent further damage.
Conclusion
The protection devices of three - phase oil - immersed power transformers play a crucial role in ensuring the safe and reliable operation of the power system. Each protection device has its own unique function and working principle, and they work together to detect and respond to various faults and abnormal conditions.
As a supplier of Three Phase Oil Immersed Power Transformer, we are committed to providing high - quality transformers with advanced protection systems. Our transformers, such as the 11kv Distribution Transformer and 20kv Oil Immersed Distribution Transformer, are designed to meet the highest industry standards and offer reliable performance.
If you are interested in our products or have any questions about the protection devices of three - phase oil - immersed power transformers, please feel free to contact us for further discussion and procurement negotiation.
References
- Blackburn, J. L. (1998). Protective Relaying: Principles and Applications. Marcel Dekker.
- Gross, C. A. (1986). Electric Power Generation, Operation, and Control. John Wiley & Sons.
- Stevenson, W. D. (1982). Elements of Power System Analysis. McGraw - Hill.