Testing of Induction Type and Microprocessor-Based Overcurrent Relay Using Relay Testing Kit to Plot Inverse Characteristics

Overcurrent relays are protection devices used to disconnect electrical circuits when the current exceeds a predetermined value, thus protecting equipment from potential damage caused by overcurrent. These relays can either be of induction type or microprocessor-based type, with different characteristics and principles of operation. The purpose of this lab test is to use a Relay Testing Kit to assess these relays and plot their inverse time-current characteristics.

Objective:

To test the Induction Type Overcurrent Relay and Microprocessor-Based Overcurrent Relay using a Relay Testing Kit and to plot their inverse characteristics (the relation between the operating time of the relay and the magnitude of the overcurrent).

Equipment/Materials Required:

  1. Induction Type Overcurrent Relay or Microprocessor-Based Overcurrent Relay (to be tested)
  2. Relay Testing Kit (usually consists of a power supply, ammeter, voltmeter, and time measurement tools)
  3. Test Panel for relay testing
  4. Power Supply (AC supply, usually 415V or as required)
  5. Ammeter (for measuring the test current)
  6. Time Counter (to measure the operating time of the relay)
  7. Voltage Source (if required for voltage inputs)
  8. Test Leads and Connectors
  9. Data Logger (if available, to record test data)

Types of Overcurrent Relays:

  1. Induction Type Overcurrent Relay: These relays use an electromagnetic induction principle to trip when the current exceeds the set threshold. The operating time is inversely proportional to the current level (inverse time-current characteristic).

  2. Microprocessor-Based Overcurrent Relay: These relays are controlled by a microprocessor and typically offer more flexibility in setting time-current characteristics. The microprocessor allows for programmable trip times based on the current.

Theory of Operation:

Induction Type Overcurrent Relay:

  • The operating mechanism of an induction type overcurrent relay is based on an electromagnetic induction. When the current flowing through the relay exceeds the preset value, the induction disc (or moving element) begins to rotate.
  • The time taken for the relay to trip is inversely proportional to the magnitude of the overcurrent. That is, the higher the overcurrent, the faster the relay trips (inverse time characteristic).

Microprocessor-Based Overcurrent Relay:

  • The microprocessor-based relay uses a microprocessor or digital signal processor (DSP) to process the input signals and determine the fault condition. The relay can be configured to operate based on a variety of predefined characteristics, including IEC or ANSI standards for time-current curves.
  • These relays are often more versatile and can provide characteristics like definite time, inverse time, and very inverse or extremely inverse time-current curves.

Procedure for Testing the Induction Type/ Microprocessor-Based Overcurrent Relay:

1. Setup the Relay Testing Kit:

  1. Power Supply Configuration:

    • Connect the relay testing kit to a suitable AC power supply (typically 415V).
    • Ensure that the relay is correctly installed in the test panel and that all connections are secure.

  2. Ammeter Connection:

    • Connect an ammeter in series with the relay to measure the current that will be fed into the relay during testing.

  3. Time Counter Setup:

    • Connect a time counter to monitor and record the time taken by the relay to trip during the test.
    • Some relay testing kits have an in-built time counter for this purpose.

2. Perform the Test for Different Current Levels:

  • Set the Overcurrent Protection Setting:
    • For Induction Type Relay, adjust the current setting (pickup setting) to a known value based on the relay's rated current.
    • For Microprocessor-Based Relay, set the relay’s current threshold and time characteristic (e.g., inverse time, very inverse, etc.).
  • Apply Incremental Test Currents:
    • Begin by applying a current lower than the relay’s set current (pickup current) and gradually increase the current in steps.
    • For each current value: Record the current applied and the corresponding time taken for the relay to trip.

3. Testing the Induction Type Overcurrent Relay:

  • Start the test by applying the first test current just above the pickup value (starting current).
  • Increase the current in increments and measure the time taken by the relay to operate.
  • Record the current level and time taken to trip for each step.
  • The operating time of the relay should decrease as the current increases, following an inverse time-current characteristic curve.
  • Repeat this process for several test currents across the range of interest, and note the trip times at each step.

4. Testing the Microprocessor-Based Overcurrent Relay:

  • Set the relay’s time-current characteristic curve (Inverse, Very Inverse, Extremely Inverse, Definite Time, etc.) using the relay tester or the relay’s settings menu.
  • Apply the same testing procedure: Start with a current just above the pickup current, then increase in steps.
  • Measure the time to trip at each current step. The microprocessor-based relay should follow the set characteristic curve.

5. Plotting the Inverse Characteristics:

  • For both the Induction Type Relay and the Microprocessor-Based Relay, plot the time (seconds) vs. current (in multiples of the pickup current) on a graph.

    • The x-axis represents the current multiple (e.g., 1.0, 1.5, 2.0, etc., times the pickup current).
    • The y-axis represents the time to trip (in seconds).

  • The graph should show an inverse characteristic: As the current increases, the time to trip decreases.

    • For an Induction Type Relay, this curve will typically follow a standard inverse, very inverse, or extremely inverse characteristic, depending on the model.
    • For a Microprocessor-Based Relay, the curve will depend on the characteristic selected (such as IEC Inverse, IEC Very Inverse, or Definite Time).

6. Documentation:

  • Record all the data in a table with the following columns:
    • Test Current (in Amps)
    • Current Multiples (as a factor of the pickup current)
    • Time to Trip (in seconds)
  • Include the plotted graph for each relay type.

Precautions:

  1. Safety First: Ensure all equipment is properly grounded to avoid electrical shock hazards.
  2. Ensure Correct Connections: Always verify that the connections are correct before applying current.
  3. Set Proper Relay Settings: Ensure that the correct settings are applied to the relay for testing (current pickup and time characteristics).
  4. Monitor for Heat: If the relay operates at high currents for extended periods, monitor it for overheating.

Conclusion:

Testing the Induction Type and Microprocessor-Based Overcurrent Relays using a Relay Testing Kit allows you to assess their response under different overcurrent conditions. By applying various test currents, you can measure the time taken for the relay to trip and plot the inverse time-current characteristics. This test helps to verify that the relay functions according to its design specifications, providing essential protection for electrical systems.

  • Induction Type Relay: Shows inverse characteristics, with a faster trip time as current increases.
  • Microprocessor-Based Relay: Offers flexibility in choosing time-current curves and can simulate more complex protection schemes.