Power Transformer Protection

Rishi TandonAssistant Manager – WWO Wind World (India) Limited

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.

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….?

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 leakage4. Reduced Cooling

5. Phase to phase and Phase to Ground Fault 6. Turn to Turn Fault 7. Transformer Overload

General Scheme of Differential Protection

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)

Operating Characteristics

Calculation for Transformer Differential Protection

Given Details:

Power Transformer Details:Voltage = 132 / 33KVRating = 60MVATap Changer = +5% - 15%Vector Group = Yd1

Current Transformer Details:CT RatioFor HV Side = 300/1For LV Side = 1200/1

Calculation for Transformer Differential ProtectionHV 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) = 276.2AHV Multiplier = 300 / 276.2 = 1.086 = 1.09

LV CT secondary current = 1049.7 / 1200 = 0.87475ASo the LV multiplier = 1200 / 1049.7 = 1.143 = 1.14

Initial Setting = 200mA (20%)

Bias setting = Selectable SlopeBias Slope Limit = 4 times of full load current

HV ICT multiplier = 1.09LV ICT multiplier = 1.14

Differential Relay Operation

Differential Relay Operation

Differential Relay Operation

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)

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 GoodLV-E 1.18G Ohm GoodHV-LV 500M Ohm GoodTest results normal.

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 230VMeasured currentHV CT Differential core 12mAHV NCT current 12mACurrent measured at HV REF relay terminal 0.34mA Which is Nearly Zero

LV CT Differential core 12mALV NCT current 12mACurrent measured at LV REF relay terminal 12mASame 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

Case Study: Continued REF Stability Demo:

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.

Thank You