Maintenance Testing - Relay Testing Fundamentals

Relay Testing Fundamentals

D) Maintenance Testing

Maintenance tests are performed at specified intervals to ensure that the relay continues to operate correctly after it has been placed into service. In the past, an electromechanical relay was removed from service, cleaned, and fully tested using as-found settings to ensure that its functions had not drifted or connections had not become contaminated. These tests were necessary due to the inherent nature of electromechanical relays.

Today, removing a relay from service effectively disables all equipment protection in most applications. In addition, digital relay characteristics do not drift and internal self-check functions test for many errors. There is a heated debate in the industry regarding maintenance intervals and testing due to the inherent differences between relay generations.

In my opinion, the following tests should be performed annually on all digital relays:

• Perform relay self-test command, if available.

• Check metering while online and verify with external metering device, or check metering via secondary injection. (This test proves the analog-to-digital converters.)

• Verify settings match design criteria.

• Review event record data for anomalies or patterns.

• Verify all inputs from end devices.

• Verify all connected outputs via pulse/close command or via secondary injection.

(Optionally, verify the complete logic output scheme via secondary injection.)

E) Troubleshooting

Troubleshooting is usually performed after a fault to determine why the relay operated or why it did not operate when it was supposed to. The first step in troubleshooting is to review the event recorder logs to find out what happened during the fault. Subsequent steps can include the following, depending on what you discover in the post-fault investigation.

• Change the relay settings accordingly.

• Change the event record or oscillography initiate commands.

• Re-test the relay.

• Test the relay’s associated control schemes.

• Replay the event record through the relay or similar relay to see if the event can be replicated.

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www.valenceonline.com C) Microprocessor Relay Testing Techniques

Simple microprocessor relays are almost identical in operation to the solid-state relays they replaced and the test techniques for these relays are identical to the techniques previously described.

Complex microprocessor relays included a large number of settings and interlinked elements which created confusion in the relay testing industry because a relay tester could spend an entire week testing one relay and barely scratch the surface of the relay’s potential. The confusion increased when relay manufacturers claimed that relay testing was not required because the relay performed self-check functions and the end user would be informed if a problem occurred. Some manufacturers even argued that the relay could test itself by using its own fault recording feature to perform all timing tests. Eventually a consensus was reached where the relay tester would test all of the enabled features in the relay. Relay testers began modifying and combining their electromechanical test sheets to account for all of the different elements installed in one relay but the basic fundamentals of relay testing didn’t change very much.

One of the first problems that a relay tester experiences when testing microprocessor relay elements is that different elements inside the relay often overlap. For example, an instantaneous (50) element set at 20A will operate first when trying to test a time-overcurrent (51) element at 6x (24A) its pickup setting (4A). The relay tester instinctively wants to isolate the element under test and usually changes the relay settings to set one output, preferably an unused one, to operate only if the element under test operates. Now they can perform that 6x test without interference from the 50-element. While these techniques will give the test technician a result for their test sheet, the very act of changing relay settings to get that result does not guarantee that the relay will operate correctly when required because the in-service relay settings and reactions have not been tested.

Relay testers often use the steady-state and simple-dynamic test procedures described previously to perform their element tests on microprocessor relays which create another problem. These complex relays are constantly monitoring their input signals to determine if those signals are valid. The steady-state and simple-dynamic test procedures are often considered invalid system conditions by the relay and the protection elements will not operate to prevent nuisance trips for a perceived malfunction. For example, if a relay tester tries to perform a standard electromechanical impedance test (21) on a digital relay, the relay will likely assume that there is a problem with a PT fuse and block the element; or the switch-on-to-fault (SOTF) setting could cause the relay to trip instantaneously. Relay testers who encountered this problem often disable those blocking signals to perform their tests and, hopefully, turned the blocking settings back on when they were complete.

Again, the act of changing settings is fine if you need a number for a test sheet but will not guarantee that the relay will operate correctly when it is required.

Chapter 4: Relay Testing Fundamentals

Modern test equipment allows the relay tester to apply several different techniques to overcome any of the situations described above. When an instantaneous element operates before a time element timing test can operate, that is usually a good thing if the relay is programmed correctly. Instead of modifying the settings to get a result, the technician can modify the test plan to ensure that all time-element tests fall below the instantaneous pickup. If a ground element operates before the phase element you are trying to test, apply a realistic phase-to-phase or 3-phase test instead and the ground element will not operate.

If the loss of protection element prevents a distance relay from operating, apply a balanced 3-phase voltage for a couple of seconds between each test to simulate real life conditions.

If switch-on-to-fault operates whenever you apply the fault condition; use an output to simulate the breaker status, apply prefault current, or lower the fault voltage so that you can lower the fault current when testing impedance relays. All of these possibilities are easily applied with modern test equipment to make our test procedures more intelligent, realistic, and effective.

Modern test equipment also allows the following additional test methods.

i) Computer-Assisted Testing

Because modern test equipment is controlled by electronics, computer-assisted testing became available. Standard test techniques could be repetitive on relays that were functioning correctly. Computer programs were created that would ramp currents and voltages at fixed rates in an effort to make relay testing faster with more repeatable results because every test would be performed identically. Computer-assisted testing has evolved to the point where the software will:

• connect to the relay

• read the relay settings

• create a test plan based on the enabled settings

• modify the settings needed to isolate an element and prevent interference

• test the enabled elements

• restore the relay settings to as found values

By following the steps above, computer-assisted relay testing can replace the relay tester on a perfectly functioning relay and can theoretically perform the tests faster than a human relay tester can. This type of testing works extremely well when performing pickup and timing tests of digital relays because these relays are computer programs themselves.

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However, it is very unlikely that the basic test procedures described by computer-assisted testing, whether initiated by computers or humans, will discover a problem with a digital relay. Most digital relay problems are caused by the settings engineer and not the relay. If a computer or human reads the settings from the relay and regurgitates them into the test plan, they will not realize that the engineer meant to enter a 0.50A pickup but actually applied 5.0A which will make the ground pickup larger than the phase pickup. Each element will operate as programmed and, when tested in isolation, will create excellent test results but may not be applied in the trip equation which was probably not tested by the automated program. An excellent relay technician could create additional tests to perform the extra steps necessary for a complete test;

but will that technician be more capable or less capable as they rely more heavily on automation to perform their testing?

ii) State Simulation

State simulations allow the user to create dynamic tests where the test values change between each state to test the relay’s reaction to changes in the power system. Multiple state simulations are typically required for more complex tests such as frequency load shedding, end-to-end tests, reclosing, breaker-fail, and the 5% under/over pickup technique described later in this chapter.

iii) Complex Dynamic State Testing

Complex dynamic state testing recognizes that all faults have a DC offset that is dependent on the fault incidence angle and the reactance/resistance ratio of the system.

Changing the fault incidence angle changes the DC offset and severity of the fault and can significantly distort the sine wave of a fault as shown in Figure 4-5. This kind of test requires high-end test equipment to simulate the DC offset and fault incidence angle and may be required for high speed and/or more complex state-of-the-art relays.

Figure 4-5: Complex Dynamic Waveform (Compliments of Manta Test Systems)

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Applying logic testing will not find every problem but it will allow the relay tester to feel reasonably confident that the relay has been set correctly, there are no obvious logic errors, and the relay will operate when required and is connected properly.

In document Testing Relay (Page 65-71)