1). What is Insulation Resistance (IR) Testing?
IR testing measures the resistance between electrical conductors & insulation using a megohmmeter. It is used to verify insulation health.
2). What is a good Insulation Resistance value?
- For LV systems >1 MΩ is acceptable;
- For HV systems, >100 MΩ is ideal.
Always refer to manufacturer standards.
3). What is the Polarization Index (PI)?
Polarization Index (PI) = IR at 10 min / IR (at 1 min)
A PI > 2 indicates good insulation.
It is used to assess insulation aging.
4). What does a low PI value indicate?
Moisture or contamination in the insulation system.
PI < 1.5 is considered poor.
5). What is the purpose of a BDV test for transformer oil?
To check the dielectric strength of the oil.
A low BDV indicates contamination (or) moisture.
6). What is the acceptable BDV value for transformer oil?
Minimum 30 kV for new oil.
>50 kV – Preferred in essential equipment.
7). What is DGA (Dissolved Gas Analysis)?
It analyzes gases dissolved in transformer oil to detect internal faults like
- Arcing,
- Overheating,
- Corona.
8). Which gases indicate arcing in DGA?
Acetylene (Câ‚‚Hâ‚‚) is the primary indicator of arcing.
9). What is Tan Delta Testing?
Tan Delta Testing measures dielectric losses in insulation.
A high tan delta indicates deteriorating insulation.
10). What is the recommended value for Tan Delta?
For transformers,
<0.005 is ideal.
Values >0.01
indicate insulation aging.
11). Why is Winding Resistance measured in Transformers?
Winding resistance measured in transformers to detect shorted turns (or) loose connections.
12). What is the significance of transformer turns ratio (TTR) testing?
The significance of transformer turns ratio (TTR) testing verifies the correct voltage ratio between windings and also ensures the winding structure is not damaged.
13). What is the purpose of Relay Testing?
The purpose of relay testing to verify that protection relays operate at the correct current/voltage/time settings.
14). What is Secondary Injection Testing?
Secondary Injection Testing is a method to test relay operation by injecting signals into its secondary side.
15). How do you perform Primary Injection Testing?
To perform Primary Injection Testing,
Inject a high current into the primary circuit to test
- CTs,
- Relays, and
- Protection Trip Circuits
together.
16). Why is Contact Resistance measured in Breakers?
Contact Resistance measured in breakers is to check for wear (or) oxidation in breaker contacts.
High resistance may lead to heating.
17). What is SFRA (Sweep Frequency Response Analysis)?
SFRA (Sweep Frequency Response Analysis) detects mechanical movement or deformation in transformer windings by analyzing frequency response.
18). What is the role of a Vacuum Interrupter Test?
The function of a Vacuum Interrupter Test is:
- To check the integrity of vacuum in VCBs.
- Done via contact timing (or) pressure measurement.
19). What is the importance of Earth Resistance Testing?
The Importance of Earth Resistance Testing:
- Ensures effective grounding.
- Should typically be <1 ohm in substations.
20). What is the procedure for Earth Resistance Testing?
Performed using a 3-point (or) 4-point method with an earth resistance tester.
21). What is a CT Ratio Test?
CT ratio test confirms the transformation ratio of the current transformers (CT) to ensure accurate protection & metering.
22). What is the purpose of a Breaker Timing Test?
- Verifies opening & closing time of circuit breakers.
- Ensures they trip within specified limits.
23). How often should Transformers be tested?
Major tests like
- Insulation Resistance (IR),
- BreakDown Voltage (BDV), and
- Dissolved Gas Analysis (DGA)
should be done annually.
- Visual inspection checks monthly.
24). What is the purpose of Thermography in Maintenance?
The purpose of thermography in maintenance utilizes infrared cameras to detect hot spots in the electrical panels (or) transformers without shutdown.
25). Why is Preventive Maintenance important?
Preventive maintenance is important as it reduces
- Unplanned outages,
- Extends equipment life, and
- Improves reliability.
