Interview Questions on Transformer Protection

• Q. What parameters or technical particulars are important to be considered while designing protection scheme of the transformer? Answer:

1. Network Diagram showing the position of the transformer in the system
   
2. kVA or MVA rating of the transformer
3. Fault Level at the transformer
4. Voltage Ratio
5. Winding Connections
6. Per Unit Impedance
7. Neutral Point Earthing Resistance
8. Value of the System Earthing Resistance
9. Whether Indoor or Outdoor
10. Dry or Oil Filled
11. Length and area of cross section of the connecting leads between CTs and Relay Panel

• Q. What can be the reasons for the failure of the transformer? Answer: Transformer is one of the most expensive equipments in the Power System. Failure of the transformer can cause outage of power supply for even days. However, multiple things can go wrong with the transformer: There can be two types of failure - Internal Faults and External Faults.
• Internal Faults (the faults within the transformer):
  

1. Core Failure
2. Winding Failure
3. Lamination Failure
4. Bushing Failure
   
5. Terminal Board Failure
6. Tap Changer Failure (OLTC failures)
   
7. Transformer Oil
8. Oil level low
9. Moisture absorption
10. Transformer cooling system
11. Failure of insulation between Lamination and Core Bolt
12. Badly made joints and connections.
13. Frequent exposure to Lightning strokes

• External Faults (the faults outside the transformer):

1. OverLoad ... increases copper losses and consequence temperature rise.
   
2. Through Faults or System Faults ... not detected by differential protection of transformer, however, if through faults persist for longer duration, transformer gets damaged by thermal and electromagnetic mechanical stress ... detected by overcurrent relays.
   
3. Short Circuits
   
4. Open Circuits
5. Earth Faults
6. Over Voltage
7. Reduced system frequency
   

• Q. How Transformer Protection provided for internal faults? Answer:

1. Bucholz Relay (1st Stage Alarm and 2nd Stage Trip)
2. Sudden Pressure Relay (1st Stage Alarm and 2nd Stage Trip)
3. Pressure Relief Valve (1st Stage Alarm and 2nd Stage Trip)
4. Pressure Switch (1st Stage Alarm and 2nd Stage Trip)
5. Oil level Gauge Indicator (Alarm)
   
6. Vacuum Level Indicator (Alarm)
7. Oil Temperature Indicator (Alarm)
   
8. Gas Temperature Indicator (Alarm)
   
9. Silicon Breather (Passive - no alarm)
   
10. Smoke Detector (Alarm)
    

• Q. How Transformer Protection provided for external faults? Answer:

1. Differential Protection
2. Restriced EarthFault Protection
3. Over Current Protection
4. Over Fluxing Protection
5. Over current relays with under voltage blocking
6. Zero sequence protection.
7. Negative phase sequence protection
   

• Q. What are the tests done to minimize internal faults? Answer:

1. Dissolved Gas Analysis (DGA): Faults due to arcing, high current breakdown of oil, localized sparking, partial discharge of oil, localized overheating or hot spots, etc. causes thermal degradation of oil and used for analysis.
2. Thermography of Transformer: Hot Spots on the transformer bushing, circulation of oil in radiator tubes, distribution of heat on transformer body, etc.
3. Capacitance Tan Delta: to detect oil degradation.
4. Oil Sample Test: to check the oil quality.

• Q. What are the reasons for Oil degradation? Answer:
  

1. Degradation
2. Oxidation
3. Contamination

4. a) Dielectric breakdown voltage indicates the presence of electrically conductive contaminants in oil.

   b) Interfacial tension and acid number (sometimes called neutralization number or acidity) are affected by oxidation and contamination.

   c) Water content is temperature dependent.

   d) Power factor is also temperature dependent.

• Q. What protection are provided to transformer depending on their kVA or MVA ratings?

• Protection	Transformer Rated below 500 kVA	Transformer Rated between 500 kVA - 5MVA	Transformer Rated above 500 kVA
  Differential	No	No	Yes
  Restricted Earth Fault	No	No	Yes
  Back Up Over Current	No	Yes	Yes
  Back Up Earth Fault	No	Yes	Yes
  Over Fluxing	No	No	Yes
  Fuses	Yes	No	No
  Bucholz, PRV, OSR	No	Yes	Yes
  Oil and Winding Temperature Indicator	No	Yes	Yes
  Oil level monitoring equipment	No	Yes	Yes

• References:

1. Basler: Transformer Protection Application Guide http://www.basler.com/html/html/transformer.htm
2. IEEE Guide for Protective Relay Applications to Power Transformers (IEEE Std C37.91-2000)
3. Dissolved Gas Analysis Wiki http://en.wikipedia.org/wiki/Dissolved_gas_analysis
4. Thermography of transformer. http://us.fluke.com/usen/Solutions/TI/Thermal-Imaging-Electrical-Systems.htm
5. Implementation of Thermography in Power utilities www.ndt.net/article/afndt2008/papers/tahir.pdf (good material on what is thermography and its basic concept).
6. Capacitance Tan Delta test http://cr4.globalspec.com/thread/32684#comment341158