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.
Insulation resistance measurement
Secondary injection tests
Current transformer tests
Primary injection test
- Measurement of insulation resistance
This test should be performed using a 1000V Insulation Resistance Meter. It is difficult to be precise as to the resistance value that should be obtained. The climate can influence the results – a wet day tends to give lower values, while on a dry day much higher values can be obtained.
- Secondary injection tests
These tests are intended to reproduce the operating conditions of each relay and are limited to the protection as such. It is therefore important to read and understand the instruction manual of the relay (application, operation, technical characteristics, installation, and maintenance). ).
To perform these tests, it is necessary to electrically isolate the relay using test plugs or physically remove the relay from its housing.
Although the relays should have been carefully tested by the manufacturer, it is necessary to carry out on-site checks after they are mounted on the panels to ensure that they have not been damaged in transit to the installation. The tests performed on relays depend largely on the type of relay.
Secondary injection tests are mandatory to ensure that the protection relay equipment is operating within its predefined settings.
Relay inputs and outputs must be disconnected before performing these tests. The test equipment provides the relay with current and voltage inputs corresponding to different faults and different operating situations. The starting values are reached by gradually changing the amplitudes of these inputs while simultaneously measuring the operating time of the relay.
Contacts and trigger targets should be monitored during these tests to ensure that the relay is operating according to the manufacturer’s specifications and the settings made.
If the curves and characteristics of a relay are to be tested at many points or angles, it is convenient to use test equipment capable of performing a test automatically. Modern protective relay test equipment has the ability to perform automatic tests using software, for which the testing process is much faster and more accurate.
In addition, the time during which the relay is out of service is minimized.
This equipment is capable of providing current and voltage injection as well as phase shift when testing directional protection. It thus allows testing of a wide variety of relay types such as overcurrent, directional overcurrent, reverse power, remote, and under / overvoltage units.
It is very important to record all test results, preferably on special forms for each type of relay.
The parameters used before the start of the tests, which had been applied in accordance with the protection coordination study. This information should include current pickup, hourly dialing, and instant settings.
Operating times for different multipliers, as measured by calibration tests. These should be checked against the data provided by the manufacturer.
Test data for instantaneous units.
Finally, the equipment used in the test should be recorded with all relevant observations, as well as details of the personnel involved in the test.
It is important to note that the mentioned tests correspond to steady-state conditions beforehand and the equipment used to perform them is rather conventional. Due to technological advances, it is now possible to use more sophisticated equipment to perform tests using signals very similar to those existing in the event of a failure.
Since the relays are necessary to respond to the transient conditions of disturbed power systems, their actual response can be assessed by simulating the signals sent to the relays under such conditions. Several manufacturers offer equipment for performing dynamic and transient state simulation tests.
A dynamic state test is one in which the test amounts of phase butter exhibiting multiple power system states are synchronously switched between states. The characteristics of the power supply system, such as high frequency and decrease in direct current, however, are not represented in this test.
A transient simulation test signal can represent, in terms of frequency content, amplitude, and duration, the actual input signals received by a relay during power system disturbances.
- Current transformer tests
Before commissioning a protection scheme, it is recommended to test the following functionalities of current transformers:
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.
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.
- Primary injection test
This test checks the entire protection system, including the current transformers. The main objectives of this test are to check the CT transformation ratios and all the secondary circuit wiring of the protection and measurement CTs in order to confirm the operation of the trip, signaling, and alarm circuits.
The test current is generally between 100 and 400 A. The two high current terminals of the primary injection equipment marked must be temporarily connected to the terminals of the CT under test.