Pre-commissioning tests should be carefully scheduled so that they take place in a logical and efficient order so that no equipment is disrupted again in subsequent tests. Tests before commissioning and in-service checks of the protection system (photo credit: projectech.com.au)
Before starting the tests, it is essential to ensure that the assembly of the article to be tested has been completed and verified.
The most important tests before commissioning and in-service checks of the protection system can be summarized as follows:
Analysis of wiring diagrams to confirm the polarity of connections, positive and negative rotation, etc.
A general inspection of the equipment, physically checking all connections, at the relay and panel terminations
Measurement of the insulation resistance of protective equipment
Inspection and secondary injection test of relays
Testing current transformers
Checking the operation of the protection system trip and alarm circuits
In addition, the list of all tests to be performed should be arranged in chronological order with all the precautions to be taken into account. Some of the more common tests are briefly described below.
Pre-commissioning tests
Insulation resistance measurement
Secondary injection tests
Current transformer tests
Primary injection test
Overlapping TCs
When CTs are connected for a fault on a circuit breaker to be covered by both protection zones, the cover connections must be carefully checked. This should be done by visual inspection.
If this is not possible or difficult, a continuity test between the appropriate relay and the secondary terminals of the appropriate CT should be performed.
Correct connection of CTs
There are often several combinations of CTs in the same ring and it is important to ensure that the CTs are correctly connected to their respective protection. Sometimes all CT scanners have the same ratio but very different characteristics, or the ratios are different but the CT scanners are close to each other which can be confusing.
Polarity
Each CT scanner should be tested individually to verify that the polarity indicated on the primary and secondary windings is correct. The measuring instrument connected to the CT secondary should be a high impedance voltmeter or moving coil ammeter, with zero centers. A low voltage battery is used in series with a push button to energize the primary.
When the circuit breaker is closed, the measuring instrument should make a small positive deviation, and when opening the circuit breaker, there should be a negative deviation, if the polarity is correct.
