Classification of Relay Testing:
Classification of Relay Testing are usually conducted to demonstrate that:
- the relay will operate correctly to clear a fault; and
- the relay will remain inoperative on faults outside its specified zone.
To ensure correct operation of relay, it is advisable to set up a relay testing programme to suit the system needs. The system conditions should be simulated before testing a particular relay.
Broadly speaking the Classification of Relay Testing to be performed fall under the following heads:
- factory tests,
- installation or commissioning tests, and
- periodic maintenance tests.
It is the responsibility of the manufacturer to ensure that the gear conforms to standard specifications and, therefore, before it is accepted and commissioned it is to be tested. For a protection scheme or relay to be tested, the operational conditions which arise in transmission and distribution systems during faults must be faithfully reproduced. For this purpose the plant for supplying the current for these tests particularly in the case of high speed types of protection must be of considerable capacity and special design. Separate light and heavy current tests are carried out and the results correlated. Relay test benches are used for light current tests, and the heavy current tests are carried out with the relays and current transformers connected operationally.
The development in the factory will also include many other tests and trials ranging from material investigations and control, to effects of impact and vibration, resistance to atmospheric corrosion, effects of temperature and dust tightness of relay cases. Sometimes the development of a new relay design is also proved in the testing laboratory of the factory.
Installation or Commissioning Tests:
As the behavior of the equipment under fault conditions has been proved at the factory tests, the main object of commissioning tests is to prove that protective gear has been properly installed and wired and that it functions correctly and is ready for service. Also the presence of many other elements associated with the protective relays, which may have been supplied by different manufacturers, the comprehensive tests at the site of installation are of great importance. Further these Classification of Relay Testing provide a check over the components which have passed factory tests and have been transported undamaged for erection at the site.
The commissioning tests carried out on an installation can be summarized as follows:
Check the wiring diagrams used by the erectors.
- Make a general inspection of the equipment, checking all the connections, wires on relays terminals, labels on terminal boards,
- Measure the insulation resistance of all circuits.
- Test main current transformers for ratio and polarity, and check points on the magnetization curves.
- Test main voltage transformers for ratio, polarity and phasing.
- Inspect and test the relays by secondary injection.
- Check the equipment by primary injection to prove stability for external faults and to determine the effective current setting for internal faults.
- Check the tripping and alarm circuits for the equipment.
Periodic Maintenance Tests:
Periodic maintenance after the installation and commissioning of protective gear is of utmost importance. Moisture and dust are generally present as potential sources of trouble, and tend to produce deterioration in insulation, corrosion of conductors, high resistance between relay contacts, and stickiness of relay bearings and pivots. Infrequent operation of the gear in service these troubles often develop unchecked. Further connections may get loose in time and this, coupled with high contact resistance due to corrosion may lead to open circuits.
It is, therefore, essential to inspect and test protective gear at suitably chosen intervals. The frequency of maintenance inspections and tests depends on the quality of the equipment, importance of the supply and upon the conditions at the site where the relays are installed.
The protective equipment includes in addition to relays many ancillary pieces of equipment which may affect the efficacy of the protection if not tested and inspected regularly. Typical of the items, failure of which may prevent or delay fault clearance, are the small wiring connections, batteries, fuses, links and the auxiliary switches on circuit breakers.
A general inspection of the physical condition of all equipment, a check of accessible connections including fuses and links, secondary injection tests of relays and functioning tests of logic and trip circuits are included in maintenance tests. The operation of indicating and alarm circuits should also be proved. Maintenance programmes also provide for the occasional tripping of circuit breakers which otherwise are very rarely operated under normal circumstances, in order to check the tripping mechanism of the circuit breakers.
The programming and recording of maintenance tests is of great importance, and the various tests should be so scheduled in order to fit the system needs. The test sheets employed should specify exactly every test and inspection required.
A typical maintenance schedule is given below:
(a) Continuous Observation: The items mentioned below need continuous supervision and a trained person is needed for this purpose:
- Pilot supervision.
- Trip circuit supervision.
- Relay voltage supervision.
- Battery earth-fault supervision.
- Bus-bar protection CT circuit supervision.
(b) Daily Inspection: Relay flags and indicators are to be inspected in every shift.
(c) Once a Week: Carrier current protection testing.
(d) Monthly Tests: Inter-tripping channel tests without tripping any switches.
(e) Six Monthly: Inspections, tripping tests, insulation resistance tests, and battery biasing equipment check.
(f) Yearly: Check tripping angle of phase comparison method, secondary injection tests, Buchholz relay tests, and tests on earthing resistors.
(g) Two Yearly: Secondary injection tests.