Transformer Protection Schemes

Transformer Protection Schemes: An Overview

Transformer protection is a crucial aspect of power network transmission and distribution systems. Transformers play a vital role in electrical power systems, stepping up or stepping down voltage levels as required for transmission and distribution. However, they are susceptible to various faults and abnormalities that can cause significant damage to the equipment and disrupt the power supply. To mitigate these risks, engineers employ transformer protection schemes designed to monitor and detect faults, and quickly isolate the transformer from the network to prevent further damage.

There are several transformer protection schemes commonly used in high-voltage transmission and distribution systems. These schemes combine different protective devices and methods to ensure comprehensive protection for the transformer. The main objective of any transformer protection scheme is to provide fast and selective protection while avoiding unnecessary tripping during normal operations. Let’s explore some of the common protection schemes used for transformers:

1. Differential Protection:
   Differential protection is the primary and most commonly used scheme for transformer protection. It relies on the principle of comparing the currents entering and leaving the transformer windings. In a healthy condition, the magnitude of these currents should be the same, implying that no fault exists within the transformer. However, during internal faults, the current balance is disrupted, triggering the differential protection relay to operate and isolate the transformer.

The differential protection scheme provides excellent sensitivity and selectivity for detecting internal faults such as winding-to-winding faults, winding-to-earth faults, and winding-to-core faults. It can be applied to both power and distribution transformers, with various relay configuration options available based on the protected transformer’s characteristics.

2. Overcurrent Protection:
   Overcurrent protection serves as a backup to the differential protection scheme. It is responsible for detecting external faults that occur in the vicinity of the transformer, such as faults on the transmission or distribution lines connected to the transformer. Overcurrent relays measure the current flowing through the transformer’s windings and compare it with a preset threshold. If the current exceeds this threshold, indicating a fault, the relay operates and initiates the fault clearing process.

Overcurrent protection should be designed with proper coordination to avoid unnecessary tripping. The coordination process ensures that only the closest relay to the fault operates, minimizing the extent of the network outage and reducing the impact on system stability.

3. Buchholz Relay:
   The Buchholz relay is primarily used for detecting and protecting transformers against incipient faults that could lead to catastrophic failures. It is installed in the oil-filled conservator tank of the transformer and detects faults such as internal winding faults, low oil level, or excessive gas accumulation inside the transformer. When such abnormalities occur, the relay initiates an alarm and/or trip signal to isolate the transformer.

4. Pressure Relief Device:
   Pressure relief devices are employed to safeguard transformers against excessive internal pressure due to abnormal operating conditions or a fault within the transformer. These devices include pressure relief valves and rupture discs that vent the generated pressure, preventing catastrophic failures such as transformer explosions.

These are just a few examples of the common transformer protection schemes utilized in high-voltage transmission and distribution systems. It is essential to consider various factors such as transformer rating, system stability, fault analysis, and coordination requirements when designing and implementing transformer protection schemes.

Application Example:

Let’s consider a practical scenario where a 100 MVA, 115 kV/13.8 kV power transformer is subjected to a fault. The transformer is protected by a differential protection scheme and backup overcurrent protection.

Differential Protection Setting:

• Pick-up current: 20% of transformer rated current (TRC)
• Fault clearing time: 0.2 seconds
• Relay operating time: 0.1 seconds

Overcurrent Protection Setting:

• Pick-up current: 200% of transformer rated current (TRC)
• Fault clearing time: 0.5 seconds
• Relay operating time: 0.2 seconds

Assuming a fault occurs on the high-voltage side of the transformer, drawing a fault current of 30 kA, we can analyze the response of the protection scheme.

1. Differential Protection:
   The differential relay operates if the fault current exceeds the pick-up current, which is 20% of the transformer rated current (TRC):
   Pick-up current = 0.2 * TRC = 0.2 * 100,000 A = 20,000 A

Since the fault current (30 kA) exceeds the differential pick-up current (20 kA), the differential relay will operate within its operating time (0.1 seconds) and initiate the fault clearing process.

2. Overcurrent Protection:
   The overcurrent relay operates if the fault current exceeds the pick-up current, which is 200% of the transformer rated current (TRC):
   Pick-up current = 2 * TRC = 2 * 100,000 A = 200,000 A

In this scenario, the fault current (30 kA) is lower than the overcurrent pick-up current (200 kA). Therefore, the overcurrent relay will not operate, as the fault lies within the differential protection zone.

This application example illustrates how