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).

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