Protection Testing for Limited Budgets – CMC 310

As the technology behind protection equipment has progressed, simple single-phase and transformer-based test devices are no longer up to the job. The transmission, transformer, motor or generator protection standards to be met in these systems tend to be numerical and three- or multi-phase in nature…

The maintenance and routine testing of the medium-voltage (MV) distribution systems of electrical utility companies, or industrial electric power plants, demands test equipment that is not only easy to use, but – most importantly – is economical too. As the technology behind protection equipment has progressed, simple single-phase and transformer-based test devices are no longer up to the job. The transmission, transformer, motor or generator protection standards to be met in these systems tend to be numerical and three- or multi-phase in nature. Yet, in contrast to high-voltage and extra-high voltage environs, these standards are less complex.

Why Use a Sledgehammer to Crack a Nut?

  A common feature of MV or industrial systems is the number of different tasks that have to be completed with a relatively low repetition rate. It is, therefore, no surprise that simple test equipment, without fully automated processes and at a price that even small budgets can afford, is very much in demand. Fortunately, OMICRON has the answer with the CMC 310: a test solution perfectly honed to meet these needs. While the standard models boast hardware that leaves nothing to be desired in terms of current and voltage amplitude and power output and precision, the control software is extremely user-friendly and has been optimized for the testing of response times, tripping limits and tripping characteristics. The protection-specific parts of the user-guided CMControl testing software have been enhanced with tools for wiring and polarity testing, energy meters and measuring transducers.

Switch On and Go

  Having the time to create comprehensive and automated test plans for the standard tasks in this field is often nothing more than a rare luxury. What counts is obtaining the necessary test signals quickly and with total flexibility in the application. Three-phase, electronically controlled and burden-independent signal sources allow the test variables to be precisely adjusted to suit the desired value – a significant advantage. As a result, there is a clear trend away from simple control transformers in secondary testing, including those for industrial systems.

Control Software Dedicated to Simplicity

  The test equipment used to date has either been expensive, high-end solutions packed with testing functions, or very basic devices based on control transformers. The latter in particular would not always be capable of meeting the required levels of performance in terms of precision, number of phases, measuring inputs or user-friendliness.

  Testing engineers would often speak of their desire for “something in between”. While the control software needs to cover the entire application spectrum, it also needs to be intuitive such that it can be mastered in a short period of time without any external training. Pen and paper to record the results are strictly a no-go, making the software-supported and automatic generation of test logs an indispensable function of any solution. It must also be possible for these log files to be exported for further processing, either by printing, filing in a database or file system, or saving onto a USB stick.

  The CMC 310 has been specially designed to satisfy all these needs. It can be operated via the dockable CMControl touchscreen control panel or alternatively via a laptop computer or Android tablet (Figure 1).

Figure 1: CMControl control software with various test tools

Example of Use: Testing Q-V Protection

  The rules governing the connection of power generation plant to the medium-voltage grid stipulate that undervoltage-controlled reactive power protection must be installed at the grid connection point. In the event of a short-circuit, this protection system disconnects the power generation plant (such as a wind generator) from the grid as soon as a predefined voltage value (reactive power) is undershot at the feed-in point. To prevent the Q-V protection from erroneously tripping, a minimum current threshold – for example, 10% Irated of the co-current system – is applied as a criterion (release current), depending on the characteristic type.

  A three-phase test set is needed to test this protection, which involves a relatively demanding test divided into several stages. In the following guide, you will see how this can be performed manually or semi-automatically, utilizing the user-guided CMControl software. The testing of other protection functions, such as frequency, voltage or overcurrent protection, is easy by comparison.

  As a general rule the following test steps have to be carried out:

1. Inspection to ensure proper connection (power direction)

  As experience has shown, polarity errors when connecting the current or voltage transformer are not an uncommon occurrence. A quick check to ensure that the connection wiring is in order is, therefore, highly advisable. Normally the test is performed by simulating a negative active power and reactive power on the generator, with feed-in occurring at the connection terminals of the transducer (secondary side). It must be ensured that the test variables listed below correspond to the load reference arrow system. The test variables (operating point in the third quadrant) have been selected so that activation does not occur if the wiring is correct (Figure 2).

Figure 2: Test statistics in the third quadrant

2. Testing of the release current (in the event of a tripping characteristic, as per Figure 3)

  Using the “Response Performance” test tool, a rising current ramp is initiated that starts with an output value below the set point tolerance. The step time here is t > trip time, the increment is approximately ¼ of the tolerance of the response value (Figure 4).

Figure 3: Constant reactive power monitoring (Source: FNN Lastenheft Q-U-Schutz [FNN Forum Q-V Protection], 2010)

Figure 4: Testing the response limit of the release current

3. Undervoltage test

  Again using the “Response Performance” test tool, this time a descending voltage ramp is generated. The starting value must be above the setpoint plus tolerance. Normally the voltage criterion tolerance for Q-V protection is 1 V (L-L) (Figure 5).

Figure 5: Testing the undervoltage trip value

4. Testing of the reactive power characteristic (as per Figure 6)

  During this test the angle between current and voltage in the first quadrant is gradually increased to the tripping point. In the second quadrant, the angle is reduced accordingly. The test is carried out with apparent power values for which the current is above the minimum current. The automatic ramp changes the angle in stages of ¼ of the angle tolerance (normally 2°) (Figure 7).

Figure 6: Reactive power characteristic with minimum current threshold (Source: FNN Lastenheft Q-U-Schutz [FNN Forum Q-V Protection], 2010)

Figure 7: Testing the reactive power characteristic at a secondary apparent power of 40 VA

5. Testing of the reactive power threshold (in the event of a tripping characteristic as per Figure 3)

  For relays with a tripping characteristic as per Figure 3, the reactive power is gradually increased at several points along the x axis (for instance, active power at -100 W, -40 W, 40 W and 100 W) until the tripping threshold is reached, starting with, for example, 6 var (Figure 8).

Figure 8: Testing the reactive power threshold at a secondary active power of -100 W

6. Testing the voltage logic

  The voltage logic is tested by simulating a two-pin error (L1-L2, L2-L3, L3-L1). The Q-V protection must not trip during this test. Only three-pin errors may cause the Q-V protection to trip. Stationary variables are output for this and the following tests performed, for which the “Time” test tool is suited (Figure 9).

Figure 9: Testing the voltage logic with a two-pole error (L1 and L2 < 85 V)

7. Testing the trip times (NAP off, single off)

  The Q-V protection can have two time-delay elements of different amounts (for instance, t1 = 0.5 s and t2 = 1.5 s). In this example, the break time t1 can be used to switch off the circuit-breakers of the individual machines in a wind generator while t2 switches off the circuit-breakers of the entire plant at the grid connection point. Both delay times are checked in isolation one after the other. During this test the off contact for t1 is connected to binary input 1 (trip) of the test device, then the off contact for t2 is connected (Figure 10).
A log in either html or xml format is generated once the last test step has been completed.

Figure 10: Testing the trip times

Summary

  A three-phase, software-controlled test device is essential for the efficient and reliable testing of a Q-V protection system. Yet complex test programs that enable fully automatic test procedures do not need to be part of the package. It is entirely possible to perform the tests manually or semi-automatically with simple tools.

  This offers a way out from what appears to be an impossible dilemma: how to reconcile shrinking budgets with higher secondary testing requirements. Solving this conundrum is the concept on which the CMC 310 has been based. The price is significantly lower than high-end models, yet the simple manual control covers every requirement in large swathes of medium-voltage and industrial grids.


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