Protection Relay Setting Calculation for MV Motor Feeder Relays

Introduction

This tutorial provides a comprehensive guide on calculating protection relay settings for medium voltage motor feeder relays. Understanding these calculations is essential for ensuring motor protection against various electrical faults. The tutorial covers several types of protection relays, including overload, under-voltage, and phase sequence protection.

Step 1: Gather Required Information

Before starting the calculations, collect all necessary data regarding the motor and system, including:

• Motor specifications (rated voltage, current, power factor)
• System configuration
• Protection relay characteristics and settings
• Manufacturer's data sheets for relays

Step 2: Calculate Excessive Start Time Protection Relay Settings

The Excessive Start Time Protection Relay (Relay 48) is crucial for preventing damage during motor start-up. Follow these steps:

1. Determine the motor's starting characteristics.
2. Calculate the maximum allowable start time based on motor specifications.
3. Set the relay to trip after this duration if the motor does not start.

Step 3: Set Thermal Overload Protection Relay

For the Thermal Overload Protection Relay (Relay 49):

1. Calculate the full load current (FLC) using the formula: [ FLC = \frac{Power (kW)}{Voltage (V) \times \sqrt{3} \times Power Factor} ]
2. Set the relay to operate at a percentage of the FLC, typically between 100% and 125%.

Step 4: Configure Negative Phase Sequence Protection Relay

The Negative Phase Sequence Protection Relay (Relay 46) helps protect against phase imbalance:

1. Establish the threshold for imbalance based on motor specifications.
2. Set the relay to trip when negative sequence current exceeds this threshold.

Step 5: Set Stall Detection Protection Relay

For Stall Detection Protection Relay (Relay 51LR):

1. Measure the stall current during motor operation.
2. Configure the relay to trip if the current exceeds the stall current for a predefined duration.

Step 6: Configure Phase Under Voltage Protection Relay

For the Phase Under Voltage Protection Relay (Relay 27):

1. Identify the minimum acceptable voltage for the motor operation.
2. Set the relay to trip if the voltage drops below this level.

Step 7: Configure Phase Over Voltage Protection Relay

For the Phase Over Voltage Protection Relay (Relay 59):

1. Determine the maximum voltage limit for the motor.
2. Set the relay to trip if the voltage exceeds this limit.

Step 8: Set Phase Time Delay Overcurrent Protection Relay

For the Phase Time Delay Overcurrent Protection Relay (Relay 51):

1. Calculate the current threshold based on the motor's FLC.
2. Determine the time delay setting according to system requirements.

Step 9: Configure Ground Time Delay Overcurrent Protection Relay

For the Ground Time Delay Overcurrent Protection Relay (Relay 51G):

1. Calculate the ground fault current threshold.
2. Set the relay to trip after a specified time delay if ground fault conditions are detected.

Step 10: Set Start Number Protection Relay

For the Start Number Protection Relay (Relay 66):

1. Define the maximum number of starts permissible within a certain time frame.
2. Set the relay to trip if this number is exceeded.

Step 11: Configure Loss of Load Protection Relay

For the Loss of Load Protection Relay (Relay 37):

1. Identify conditions that indicate a loss of load.
2. Set the relay to trip under these conditions.

Step 12: Monitor Sonde Temperature in Windings

Sonde temperature monitoring is essential for preventing overheating:

1. Install temperature sensors in motor windings.
2. Set alarm and trip parameters based on the temperature readings.

Conclusion

In this tutorial, we covered the essential steps for calculating protection relay settings for medium voltage motor feeders. Each relay serves a specific protection function, and proper configuration is vital for the motor's safety and performance. Review the settings periodically and adjust based on system changes or performance feedback to ensure ongoing protection.