Selection of Protection Relays in Medium Voltage Networks

Selection of Protection Relays in Medium Voltage Networks

Electrical energy is generated at low or medium voltage in power plants, then the voltage is increased using a power transformer and transmitted at high voltage. At step-down substations, the energy is reduced to medium voltage. Afterwards, it is further reduced to 400V low voltage via a distribution transformer and delivered to consumers.

If such a system is not properly electrically protected, any fault may cause damage not only at the source power plant but also all the way to the end user.

In electrical systems, protection is implemented to ensure uninterrupted electrical energy from generation to consumption. The main function of the protection system is to detect faults in the electrical installation quickly and reliably, and when necessary, to disconnect the faulty equipment (line, transformer, or generator) from the system.

Devices that perform this function are called protection relays.

What Does a Protection Relay Do?

When a fault occurs in the network, current and voltage transformers detect fault current or voltage and transmit this information to microprocessor-based protection relays.

The relay reads real-time system data and decides whether to trip the circuit breaker based on predefined settings. Protection relays detect the fault and send the necessary open/close signal to the circuit breaker.

They calculate the actual current and voltage values by scaling the measurements from instrument transformers according to their ratios.

When a fault current or abnormal voltage is detected, the relay sends a trip signal to protect the system. The relay settings are configured according to the required current and voltage thresholds.

During protection, relays are expected to operate selectively (selectivity), quickly, and reliably.

Selective Protection Principle

In this way, the continuity of electrical energy is ensured as much as possible.

In selective protection, the goal is not only to isolate the faulty section but also to maintain energy supply in the rest of the system.

For this purpose, protection relay settings must be correctly determined by expert technical teams.

ANSI standards define protection functions and their codes. Relays that perform the required ANSI functions are integrated into the system.

In medium voltage distribution networks, overcurrent and earth fault protection relays are commonly used in transformer protection panels, while more advanced digital relays are used in feeder and generation cells, offering functions such as current, voltage, earth fault, frequency, and directional protection.

Protection relays are classified according to their function, such as:

• Transformer protection
• Line protection
• Motor protection
• Capacitor bank protection
• Busbar differential protection
• Line differential protection
• Generator protection
• Distance protection

Relay Selection in Medium Voltage Systems

In medium voltage distribution systems, the type of protection required in the network is selected according to ANSI codes.

Technical datasheets of protection relays specify which ANSI protection functions they support. After selecting the appropriate relay, correct setting values are determined to ensure proper selective protection, and the relay is configured accordingly.

Common ANSI protection codes used in medium voltage systems are defined according to application needs and system design requirements.