1. Application
Equipment operating at lower than nominal voltages could overheat due to the increased amperage necessary to produce the same amount of power at the lower voltage. Undervoltage protection (27-element) is used to protect equipment from thermal stress created from lower than rated voltages. 27-elements almost always incorporate time delays to prevent nuisance tripping caused by transients or sags in the system voltage.
Undervoltage protection (27) is a little more complicated than overvoltage (59) protection described in the previous chapter because dynamic testing is necessary for accurate test results. It can also be difficult to determine the correct application. The undervoltage settings are often related to the nominal voltage setting of the relay, which could be line or line-ground voltages depending on the system, number of potential transformers (PTs), and/or the PT connection as discussed in the Instrument Transformers section starting on page 34.
After you have determined whether the relay measures phase-to-phase or phase-to-neutral voltages, you should review the relay’s 27-element and nominal voltage settings to make sure that they are correct. For example; if a relay is connected to a system with two PTs, the nominal voltage is likely to be between 115-120V and would be phase-to-phase voltages. A 27-element setting above 110V will likely cause nuisance trips.
Sometimes a 27-element will be applied to monitor breaker status or to determine whether a bus or line is de-energized. These applications will have lower voltage settings (approximately 30V) and are used for control applications.
Undervoltage (27) protection is the opposite of overvoltage protection. When the protected equipment is de-energized, the input voltage should fall below the undervoltage setting and cause an undervoltage trip. The trip indication from the 27-element can be a nuisance for operators; and even prevent the breaker from closing in some control schemes.
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Relay voltage inputs are supplied by PTs and an open circuit PT fuse could cause a 27-element to trip. Would you want your entire plant to shut down because someone accidentally touched a PT circuit? There are many methods for dealing with these problems and some are automatically applied by more sophisticated relays. You may:
• Interlock 27-element operation with a breaker signal to block tripping unless the breaker is closed.
• Interlock 27-element operation with a small 50-element setting to block tripping unless a preset amount of current flows.
• Only allow 27-element operation within a voltage window between pickup and minimum voltage. For example; if the voltage window is between 30 and 90V. The 27-element will not operate if the voltage is above 90V or below 30V.
• Interlock 27-element operation with a loss-of-fuse or PT Fuse Failure protection to block tripping if a PT fuse opens.
• Block 27-element operation if the positive sequence voltage is less than a predefined value to prevent nuisance trips if a PT fuse opens.
The test voltage must be initially higher than the 27-element pickup setpoint or the 27-element will always be on. Therefore, some kind of prefault voltage must be applied for timing tests.
The input voltage is continuously turned off and on during relay testing and 27-elements often interfere with tests or are just a plain nuisance. I often disable 27-element while testing generator protection and save it until the very last test after it is re-enabled. Remember, disabled elements should be tested AFTER the settings have been re-enabled.
2. Settings
A) Enable Setting
Many relays allow the user to enable or disable settings. Make sure that the element is ON or the relay may prevent you from entering settings. If the element is not used, the setting should be disabled or OFF to prevent confusion.
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4. Timing Tests
If the time delay uses a definite time curve, apply 90% of the pickup voltage and measure the time between test start and output contact operation. The time delay is compared to the setting and manufacturer's tolerances to make sure they match.
If the time delay is an inverse curve, perform the timing test by applying a multiple of pickup voltage and measure the time between the test start and output contact operation. Repeat the test for at least one other point to verify the correct curve has been applied.
Remember that prefault voltage higher than the pickup setting is required for successful 27-element testing!
A) Timing Test Procedure with Definite Time Delay and All Three Phases Required
• Determine which output the 27-element operates and connect timing input to the output.
• Determine if a breaker status contact is used to disable 27-element protection and ensure it is in the correct state.
• Determine if input current is required for 27-element operation and apply nominal 3Ø current as per the wiring diagrams in Figures 2-3, 2-4, and 2-5.
• Set the prefault voltage to nominal 3Ø voltage.
• Set the 3Ø fault voltage 10% lower than the pickup setting. The test for example would be performed at 59.23V (VRated * Pickup * 0.90= 69.28V *0.95 * 0.90). Set your test-set to stop when the timing input operates and to record the time delay from test start to stop.
• Apply prefault test voltage. Apply fault voltage and ensure timing input operates.
Note the time on your test sheet. Compare the test time to the 27-element settings to ensure timing is correct.
• Lower the test voltage to any value below the first test and above the minimum voltage setting, if one exists. Apply prefault, and fault voltages. The time delay should be the same.
• Review relay targets to ensure the correct element operated.
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B) Timing Test Procedure with Definite Time Delay and Two Phases Required
• Determine which output the 27-element operates and connect timing input to the output.
• Determine if a breaker status contact is used to disable 27-element protection and ensure it is in the correct state.
• Determine if input current is required for 27-element operation and apply nominal 3Ø current as per the wiring diagrams in Figures 2-3, 2-4, and 2-5.
• Set the AØ and BØ fault voltage 10% lower than the pickup setting. The test for our example would be performed at 59.23V (VRated * Pickup * 0.90= 69.28V *0.95
* 0.90). Set your test-set to stop when the timing input operates and to record the time delay from test start to stop.
• Apply prefault test voltage. Apply fault voltage and ensure timing input operates.
Note the time on your test sheet. Compare the test time to the 27-element settings to ensure timing is correct.
• Review relay targets to ensure the correct element operated.
• Lower the test voltage to any value below the first test and above the minimum voltage setting, if one exists. Apply prefault, and fault voltages. The time delay should be the same.
• Repeat the steps above for BØ-CØ and CØ-AØ.
C) Timing Test Procedure with Definite Time Delay and Any Phase Required
• Determine which output the 27-element operates and connect timing input to the output.
• Determine if a breaker status contact is used to disable 27-element protection and ensure it is in the correct state.
• Determine if input current is required for 27-element operation and apply nominal 3Ø current as per the wiring diagrams in Figures 2-3, 2-4, and 2-5.
• Set the AØ fault voltage 10% lower than the pickup setting 59.23V (VRated * Pickup * 0.90= 69.28V *0.95 * 0.90). Set your test-set to stop when the timing input operates and to record the time delay from test start to stop.
• Apply prefault test voltage. Apply fault voltage and ensure timing input operates.
Note the time on your test sheet. Compare the test time to the 27-element settings to ensure timing is correct.
• Review relay targets to ensure the correct element operated.
• Lower the test voltage to any value below the first test and above the minimum voltage setting, if one exists. Apply prefault, and fault voltages. The time delay should be the same.
• Repeat the steps above for BØ and CØ.
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D) Timing Test Procedure with Inverse Time Delay and All Three Phases Required
• Determine which output the 27-element operates and connect timing input to the output.
• Determine if a breaker status contact is used to disable 27-element protection and ensure it is in the correct state.
• Determine if input current is required for 27-element operation and apply nominal 3Ø current as per the wiring diagrams in Figures 2-3, 2-4, and 2-5.
• Pick first test point from manufacturer’s curve. (Typically in percent of pickup) Set the 3Ø fault voltage at the test point. The first test for our example at 90% pickup would be performed at 59.23V (VRated * Pickup * 0.90= 69.28V *0.95 * 0.90). Set your test-set to stop when the timing input operates and to record the time delay from test start to stop.
• Apply prefault test voltage. Apply fault voltage and ensure timing input operates.
Note the time on your test sheet. Compare the test time to the 27-element settings to ensure timing is correct.
• Review relay targets to ensure the correct element operated.
• Perform second test at another point on the manufacturer’s timing curve. (E.g.
60%= 69.28V * 0.95 * 0.6 = 39.49V)
• Apply prefault test voltage. Apply test voltage, ensure timing input operates, and note the time on your test sheet. Compare the test time to the 27-element timing curve or formula to ensure timing is correct.
• Review relay targets to ensure the correct element operated.
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