Differential protection

Also Includes REF Stability

1. Power Transformer Protection
Rishi Tandon
Assistant Manager – WWO
Wind World (India) Limited

2. Introduction
Transformer is a static device used to transform the electrical energy
from one alternating current circuit to another without any change in
frequency.

Power Transformer is a vital link in a power transmission system and
distribution
 A Power Transformer is an expensive part of a power network.
 The impact of a transformer outage due to fault is more serious than a
transmission line outage.

3.  To Protect the Transformer from severe damages.
 Time required to rectify the Transformer in case of damages is more and

it is very difficult.
 Operation of a power network when the power transformer is out of
service is always difficult.
 A Power Transformer fault therefore often is a more severe disturbance
for the network than an overhead line fault which usually can be
repaired rather quickly.
Why Protection Required….?

4. Types of Fault
1.Incipient faults
2. Internal faults
3. External fault
What are causes for the faults in a transformer ?

1.Insulation Breakdown
2.Over heating due to over excitation
3.Oil contamination and leakage
4.Reduced Cooling
5.Phase to phase and Phase to Ground Fault
6.Turn to Turn Fault
7. Transformer Overload

5. General Scheme of Differential Protection

6. Protection Functions
Bias Differential Protection (87)
Harmonic Restraint Protection
Over Fluxing i.e V/f Protection (24)
Over Voltage (59)
Under Voltage (27)
Neutral Unbalance (64R)
Restricted Earth Fault (64REF)
Back up O/C & E/F (50/51/67)

7. Operating Characteristics

8. Calculation for Transformer Differential Protection
Given Details:
Power Transformer Details:
Voltage = 132 / 33KV
Rating = 60MVA
Tap Changer = +5% - 15%
Vector Group = Yd1
Current Transformer Details:
CT Ratio
For HV Side = 300/1
For LV Side = 1200/1

9. Calculation for Transformer Differential Protection
HV rated current = 60MVA / (132 * 1.732) = 262.4A.
LV rated current = 60MVA / (33* 1.732) = 1049.76A
Mean Tap value = [(+5) + (-15)] / 2 = -5%
HV current at –5% tap = (60MVA) / (1.732 * 132KV *0.95) = 276.2A
HV Multiplier = 300 / 276.2 = 1.086 = 1.09
LV CT secondary current = 1049.7 / 1200 = 0.87475A
So the LV multiplier = 1200 / 1049.7 = 1.143 = 1.14
Initial Setting = 200mA (20%)
Bias setting = Selectable Slope
Bias Slope Limit = 4 times of full load current
HV ICT multiplier = 1.09
LV ICT multiplier = 1.14

10. Differential Relay Operation

11. Differential Relay Operation

12. Differential Relay Operation

13. Models of Transformer Differential Protection
• ABB (RET 650)
• GE (T60)
• Alstom MiCOM (P632 & P633)
• Siemens (7SA611)
• ERL (T-Pro)
• NR (PCS-9708)
• SIFANG (CSC 326)

14. Case Study:
100MVA Power Transformer #2 Trip on Restricted Earth
Fault-LV at 220/33kv GSS, Bhu (Jaisalmer)
Incident : Transformer tripped on REF LV. Trip followed by fault in 33kV
feeder.
Post tripping checks :
After tripping transformer got isolated from HV & LV and testing started by team to
investigate root cause.
Following test conducted :
1. Meggering done for HV-E, LV-E, HV-LV.
HV-E 16.7G Ohm Good
LV-E 1.18G Ohm Good
HV-LV500M Ohm Good
Test results normal.

15. Case Study: Continued
2. REF stability test conducted by 1ph method and found relay pickup for
external fault on LV side. Observation of REF stability are as under:
Voltage applied HV R-ph 230V
Measured current
HV CT Differential core 12mA
HV NCT current 12mA
Current measured at HV REF relay terminal 0.34mA Which is Nearly Zero
LV CT Differential core 12mA
LV NCT current 12mA
Current measured at LV REF relay terminal 12mA
Same observation for Y & B phases.
3. BDV of oil sample collected from top as well as bottom are found ok. Test
values are as below:
BDV Bottom Sample~ 75.7KV,
Top Sample~ 65.1KV

16. Case Study: Continued
REF Stability Demo:

17. Case Study: Continued
4. No other protection operated, Buchholz & PRV Healthy.
Conclusion :
From above few points and test results it is concluded that REF LV relay is
sensing current for fault external to transformer i.e. After LV side CT. Hence
REF relay tripped for external fault. After confirming from above tests
transformer considered healthy and there is no internal fault, Transformer
charged by 20:10 hrs.

18. Thank You