Tech View

Easy protection testing in MV distribution systems

  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 optimised 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 goHaving 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 todate 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.
Example of use: Testing Q-V protectionThe 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 per cent 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 utilising 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).
2. Testing of the release current (in the event of a tripping characteristicUsing the “Response Performance” test tool, a rising current ramp is initiated that starts with an output value below the setpoint tolerance. The step time here is t > trip time, the increment is approximately ¼ of the tolerance of the response value (Figure 4).
3. Undervoltage testAgain 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)
4. Testing of the reactive power characteristic 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-degree)
5. Testing of the reactive power threshold (in the event of a tripping characteristic
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).
6. Testing the voltage logicThe 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
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
A log in either html or xml format is generated once the last test step has been completed.
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.

Click to comment

You must be logged in to post a comment Login

Leave a Reply

Most Popular

To Top