What is the Transformer Protection Relay and How does It Work?

A transformer protection relay is a device used to monitor and protect power transformers from various faults and abnormal conditions. Its primary function is to detect faults promptly and isolate the transformer from the power system to prevent damage and ensure the safety of personnel and equipment.

Fig- Basic Idea about Differential Relay 

 

Transformer Protection Relay and Its Working Principle

The protection relay works by monitoring various electrical parameters of the transformer and initiating appropriate actions when predefined thresholds or abnormal conditions are detected.

Here is an overview of how a transformer protection relay works:

Current Monitoring: The relay continuously monitors the current flowing through the transformer windings. It compares the actual current with predefined settings to detect overcurrent or undercurrent conditions, which could indicate a fault or abnormal operation.

Voltage Monitoring: The relay also monitors the voltage across the transformer windings. It checks for overvoltage or under-voltage conditions, which may indicate issues such as insulation failure or improper voltage regulation.

Differential Protection: Differential protection is a crucial feature of transformer protection relays. It compares the currents entering and leaving the transformer to detect internal faults within the transformer. If there is a significant imbalance between the currents, indicating a fault, the relay operates to isolate the transformer from the system.

Buchholz Relay: Many transformer protection relays are equipped with a Buchholz relay. This relay is installed in the oil-filled conservator tank of the transformer and detects faults such as internal short circuits or insulation failure. It operates based on the gas and oil flow generated by the fault and triggers an alarm or initiates the tripping of the transformer.

Temperature Monitoring: The protection relay also monitors the temperature of the transformer windings and oil. It uses embedded temperature sensors or additional external sensors to measure the temperature. If the temperature exceeds predefined limits, the relay initiates appropriate actions like issuing an alarm or tripping the transformer to prevent overheating and damage.

Communication and Control: Many modern transformer protection relays are equipped with communication capabilities. They can communicate with a supervisory control and data acquisition (SCADA) system or a local control panel, providing real-time data and receiving commands for remote monitoring and control. They can also send alarms or trip signals to other protective devices or breakers.

Overall, the transformer protection relay continuously monitors key electrical parameters of the transformer and responds to abnormal conditions or faults by initiating appropriate actions, such as alarms, tripping the transformer, or communicating with other devices. This helps to prevent damage to the transformer and ensures the reliability and safety of the power system.

Types of Transformer Protection Relays Used in Electrical Substations

In electrical substations, various types of transformer protection schemes are employed to ensure the safe and reliable operation of transformers. Here are some common types of transformer protection used:

Differential Protection: This is the primary protection scheme used for transformers. Differential protection compares the currents entering and leaving the transformer windings. If an internal fault occurs, such as a short circuit, an imbalance in the currents is detected, and the protection system trips the transformer circuit breaker to isolate it from the fault.

Buchholz Relay Protection: Buchholz relay is a gas and oil-operated relay used for the protection of transformers with oil-filled tanks. It is installed in the connecting pipe between the transformer main tank and the conservator. The Buchholz relay detects internal faults, such as incipient winding faults and core faults, by monitoring the gas and oil flow within the transformer. It provides an alarm or trips the transformer depending on the severity of the faults.

Overcurrent Protection: Overcurrent protection schemes, such as overcurrent relays and fuses, are used to protect transformers against external faults, such as short circuits in the distribution network. These protection devices sense the magnitude of current flowing through the transformer and operate when the current exceeds a predetermined threshold.

Overload Protection: Overload protection prevents excessive heating of the transformer due to prolonged overloading. Thermal relays or temperature sensors are used to monitor the transformer's temperature and trip the circuit breaker if it exceeds the permissible limit.

Overvoltage Protection: Overvoltage protection devices, like surge arresters, are installed to safeguard transformers against transient voltage surges caused by lightning strikes or switching operations. Surge arresters provide a low-impedance path to divert excessive voltages to the ground, protecting the transformer insulation.

Restricted Earth Fault Protection: This protection scheme detects faults between the transformer winding and the earth. It utilizes current transformers (CTs) to measure the residual current in the earthed winding. If a fault occurs, indicating an insulation breakdown, the protection system operates to isolate the transformer.

Transformer Oil Temperature Protection: Temperature sensors are used to monitor the temperature of the transformer oil. If the temperature rises beyond a specified threshold, the protection system initiates an alarm or trips the transformer to prevent any damage.

Pressure Relief Device: In transformers with oil-filled tanks, a pressure relief device is used to protect against excessive internal pressure caused by faults or abnormal conditions. The device operates to release excessive pressure and prevent the transformer tank from rupturing.

These are some of the commonly used transformer protection schemes in electrical substations. The selection and combination of protection schemes depend on the specific requirements of the transformer and the substation. Different Types of Relays and Their working Procedure are discussed in detail in another episode.