Once erection is completed, various pre-commissioning checks/ tests are performed to ensure the healthiness of the switchyard equipment prior to their energization. Various major electrical tests to be performed and their significance are given below:
|
Sr. No. |
Name of Test / Check point |
Purpose of test/ check |
|
1. |
Tan δ & Capacitance measurement of CT, each stack of CVT & total capacitance, CB voltage grading capacitor |
The purpose of the dissipation factor measurement of high voltage insulation is to detect incipient weaknesses in HV insulation. The most important benefit to be gained from this measurement is to obtain a “benchmark reference reading” on costly and high voltage equipment when the equipment is new and insulation is clean, dry and free from impurities. Tan delta & Capacitance values shall be comparable with factory test results and in no case shall be more than 0.5 %. |
|
2. |
Checks/ Tests applicable for CTs |
|
|
I. |
Polarity test for CT |
To ascertain whether the polarity markings are correct or not as per drawing. |
|
II. |
Magnetization characteristics of CT |
To prove that the turns of CTs secondary windings are not short circuited and to check healthiness of CT cores. The magnetizing currents at KPV (Knee point voltage) shall be less than the specified value. The ratio of secondary and primary voltage shall also be measured. |
|
III. |
Ratio test for CT |
The ratio errors of the primary to the secondary currents should within specified ratio errors. |
|
IV. |
IR measurement of CT (Primary & Secondary windings) |
Changes in the normal IR value of CT indicate abnormal conditions such as presence of moisture, dirt, dust, crack in insulator of CT and degradation of insulation. |
|
V. |
DGA test of CT oil |
This test shall be conducted after 30 days of commissioning. The purpose is to identify evolving faults in the CT and DGA values shall be comparable with factory values (if available) |
|
VI. |
Dew point & Purity measurement of SF6 gas (In case of SF6 filled CTs) |
Dew point & Purity of SF6 gas is to assess quality of SF6 gas filled in CTs |
|
3. |
Checks/ Tests applicable for Circuit Breakers |
|
|
i. |
Dew point measurement of SF6 gas |
Dew point of SF6 gas is to measure moisture content in SF6 gas which shall indicate whether CB evacuation is done properly or not. |
|
ii. |
Measurement of Circuit Breaker Operating Timings including PIR Timings |
To measure closing/ tripping/ CO timings. These timings should be within permissible limits and shall be comparable with factory values. Pole discrepancies and Break to Break discrepancies shall be less than specified values. |
|
iii. |
DCRM Contact Travel Measurement / DC injected currents and trip/ close coil currents. |
DCRM is the technique for measuring Contact Resistance during operation (Close/ Trip) of a circuit breaker with a delay Tco of 300ms. A DC current of at least 100 Amp is injected through the circuit breaker. The current and voltage drop are measured and resistance is calculated. The resistance and travel versus time data provides useful information on the condition of the circuit breaker contacts and is used as a diagnostic tool. DCRM test signatures shall be approved by Corporate AM. |
|
iv. |
Operational lockout checking for EHV Circuit Breakers |
To ensure various lockout operation of CB by simulating the actual conditions at the specified pressure of operating medium. |
|
v. |
Measurement of static contact resistance |
This test is conducted to evaluate healthiness of Main contacts. 100 Amp DC is injected and voltage drop is measured across each CB contact to compute contact resistance. |
|
vi. |
Checking the Anti- Pumping feature |
By giving simultaneous close/ trip commands, CB hunting shall not take place by operation of Mechanical/ Electrical anti pumping feature. |
|
vii. |
Checking the Anti- Condensation Heaters |
To check correct operation of Thermostat provided for anti condensation heaters. |
|
viii. |
Pole discrepancy relay testing |
To test tripping of CB in case of pole discrepancy more than 2.5 seconds or specified value. |
|
4. |
Checks/ Tests applicable for CVTs |
|
|
i. |
CVT polarity, Ratio test |
This test is conducted in the same manner as for CT to determine correct CVT polarity, ratio and phasor group. |
|
ii. |
Insulation resistance measurement of Primary & secondary winding |
Changes in the normal IR value of CVT indicate abnormal conditions such as presence of moisture, dirt, dust, crack in insulator of CVT and degradation of insulation. |
|
5. |
Checks/ Tests applicable for Isolators |
|
|
i. |
MILLIVOLT Drop test |
The voltage drop gives a measure of resistance of current carrying part and contacts by injecting minimum 100 A DC current. |
|
ii. |
50 operation tests |
To test operation of contacts etc with jumpers connected and contact resistance to be measured after 50 operations. There shall not be any change from the previous value. |
|
6. |
Checks/ Tests applicable for Surge Arrestors |
|
|
i. |
Third Harmonic Resistive Current (THRC) for surge arrestors |
To monitor healthiness of Surge arrestors by monitoring third harmonic resistive current from the leakage current. This test is to be conducted after charging of Las. The value of THRC shall be less than 30 µA. |
|
ii. |
IR measurement of each stack of LA |
Changes in the normal IR value of LA indicate abnormal conditions such as presence of moisture, dirt, dust, crack in insulator of LA and degradation of insulation. |
|
iii. |
Checking of operation of LA counter |
This test is done to check the healthiness of LA counter. |
|
7. |
Checks/ Tests for other areas/ equipments |
|
|
i. |
Earth resistance measurement |
To ensure value of earth resistance is below 1 ohm. |
|
ii. |
Secondary current injection test |
Conducted for testing of protecting devices, circuit breakers, trip coils, motor overloads etc. |
|
iii. |
Contact Tightness check of Bay contacts by Primary injection method |
Since complete bay contact resistance measurement is practically not possible because DC current may not be injected in CT primary, hence contact tightness check by primary injection method has been introduced to check overall contact tightness. |
|
iv. |
Stability check for Bus Bar |
This test is performed to check the proper operation of Bus Bar protection by simulating actual conditions. Any problem in CT connection, wrong cabling, relay setting can be detected by this test. |