TRENDS & DEVELOPMENTS IN POWER TRANSFORMER PROTECTION

1. TRENDS & DEVELOPMENTS IN POWER TRANSFORMER PROTECTION A TALK DELIVERED BY- CH. P.K.PATRO, DGM(E), NALCO.

2. Plan Of Presentation • Introduction • Failure Modes Of Transformers • Protection Of Transformer • Numerical Relay Technology • Case Studies

3. Introduction • Power Transformer is a vital link in a power transmission system and impact of a transformer fault is more serious than a transmission line outage. Important Aspects- • High quality and reliable equipment/transformer. • Operating the transformer within specified limits of temperature and voltage. • Proper checking and maintainance of OLTC. • Providing suitable protective relays and monitoring devices.

4. Types of Transformer Generator transformer Sub station transformer Furnace transformer Rectifier transformer

5. Types of Transformer Failures Winding failures due to short circuits (turn-turn faults, phase-phase faults, phase-ground, open winding) Core faults (core insulation failure, shorted laminations) Terminal failures (open leads, loose connections, short circuits) On-load tap changer failures (mechanical, electrical, short circuit, overheating). Abnormal operating conditions (overfluxing, overloading, overvoltage) External faults

6. CAUSES OF FAILURES • Lightning Surges • Line Surges/ External Short Circuits • Over-loading • Deterioration Of insulation • Moisture Ingress • Poor maintenance • Poor Workmanship/Manufacturing

7. Failure Statistics • Lightning Surges- 13% • Line Surges/ Short Circuits- 20% • Over Loading- 4% • Moisture Ingress- 7% • Deterioration Of Insulation- 15% • Poor Workmanship/ Manufacturing- 03% • Inadequate Maintenance- 12% • Loose Connection- 6% • Others- 20%

8. Line Surges • Line Surge (or Line Disturbance) is the number one cause for all types of transformers failures. • This category includes switching surges, voltage spikes, line faults/flashovers, and other transmission and distribution (T&D) abnormalities. • This significant portion of transformer losses indicates that more attention should be given to providing surge protection, or testing the adequacy of existing surge protection.

9. Deterioration of Insulation • Insulation Deterioration is found to be the second leading cause of failure. The average age of the transformers that fails due to insulation deterioration is around 20 years against a general life expectancy of 30 to 35 years.

10. Factors Affecting Life Expectancy Of Insulation • Misapplication   • Vibration   • High Operating Temperature     • Overloading   • Care of Control Equipment   • Lack of Cleanliness   • Care of Idle or Spare Equipment   • Careless or Negligent Operation  

11. Inadequate Maintenance • Inadequate Maintenance was the third leading cause of transformer failures. • This category includes disconnected or improperly set controls, loss of coolant, accumulation of dirt and oil, and corrosion. • Inadequate maintenance has to bear the blame for not discovering incipient troubles when there was ample time to correct it

12. Poor-Workmanship/Manufacture • A low percent of the total failures are attributed to Poor Workmanship or Manufacturer’s Defects these days. • Loose or unsupported leads, loose blocking, poor brazing, inadequate core insulation, inferior short circuit strength, and foreign objects left in the tank are the reasons falling under the above category.

13. Overloading • It includes only those transformers that experienced a sustained load that exceeded the nameplate capacity. • Often, the overloading occurs when the plant or the utility slowly increases the load in small increments over time. The capacity of the transformer is eventually exceeded, resulting in excessive temperatures that prematurely ages the insulation. As the transformer’s paper insulation ages, the strength of the paper is reduced. • Then, forces from an outside fault may cause a deterioration of the insulation, leading to failure.

14. Moisture • The Moisture category includes failures caused by floods, leaky pipes, leaking roofs, water entering the tanks through leaking bushings or fittings, and confirmed presence of moisture in the insulating oil. 

15. Loose Connections • Loose Connections could be included in the Maintenance category. • This category includes workmanship and maintenance in making electrical connections. • One problem is the improper mating of dissimilar metals, although this has decreased somewhat in recent years. • Another problem is improper torquing of bolted connections.

16. Protection Of Transformer

17. External Faults Internal Faults Transformer External Faults

18. Auxiliary Faults • Mainly related to the Tank, Oil, etc • Detection by Pressure Switches / Temp. Switches • Linked with the trip circuit logic

19. Proection Philosophy • Main Protection (Internal Faults) • Back up Protection ( External faults) • Location • Size Transformer

20. LIST OF PROTECTIONS Back up Protections O/C + E/F ( Primary Side) O/C + E/F ( Secondary Side) Current Unbalance Thermal Overload Over Fluxing Over Voltage/Under Voltage Over Frequency / Under Frequency Synchro Check Directional O/C + Directional E/F Power (Forward / Reverse) Zero Sequence Voltage Voltage Unbalance Breaker Failure Main Protections Differential Restricted E/F Auxiliary Protections Oil Temperature Gas pressure Oil Level Vibration Cooling Fans trip

21. 27 59 50 51 50N 51N Less than 1 MVA External Faults 50/51 IDMT Over Current 50N/51N IDMT Earth Fault 27 Under Voltage 59 Over Voltage Transformer 50 51 50N 51N

22. More than 1 MVA • Internal Faults • Differential fault • 64 Restricted Earth Fault 87 64

23. More than 1 MVA • Internal Faults • Differential fault • 64 Restricted Earth Fault 87 64

24. TRANSFORMER PROTECTION 27 59 24 50 51 50N 51N 49 46 More than 10 MVA External Faults 50/51 IDMT Over Current 50N/51N IDMT Earth Fault 27 Under Voltage 59 Over Voltage 49 Thermal Over Load 46 Current Unbalance 24 Over Fluxing Transformer 50 51 50N 51N

25. Differential relay 87 Should trip for an internal fault. Trip time is always instantaneous. Should not trip for an external fault. Why normal O/C relay can not do this job ?

26. Fundamental of differential protection • Types of differential High impedance differential:: • Here a high impedance is added to relay circuit to prevent relay operation due to CT saturation under through fault conditions. • This is very sensitive and fast operating for internal faults. Biased differential : Here the operation depends upon differential current exceeding the bias current. The bias characteristics is variable so that it is applicable to a wide variation in transformer design and configuration. This bias slope is set to stabilize the protection for small differential currents, which flow due to tap changer variation and CT tolerance under through fault conditions.

27. Harmonic Restraint

28. RESTRAINTS IN DIFFERENTIAL RELAYS 2ND HARMONIC RESTRAINT WHEN A TRANSFORMER IS SWITCHED ON, THE DIFFERENTIAL RELAY MAY SENSE A DIFFERENTIAL CURRENT AND INITIATE TRIP. THIS IS DUE TO THE MAGNETISING CURRENT INRUSH ON ONE SIDE OF THE TRANSFORMER. THIS WILL RESULT IN A LARGE VALUE OF I1-I2 FOR ABOUT 4 OR 5 CYCLES. RELAY IS RESTRAINED FROM OPERATION AT THE TIME OF SWTICH ON BY THE 2ND HARMONIC RESTRAINT FEATURE.

29. RESTRAINTS IN DIFFERENTIAL RELAYS 5TH HARMONIC RESTRAINT WHEN OVER FLUXING OCCURS IN A TRANSFORMER THE DIFFERENTIAL RELAY MAY SENSE A DIFFERENTIAL CURRENT AND INITIATE TRIP. THIS IS DUE TO THE HIGH LEVEL OF FLUX PRESENT IN THE CORE CAUSING AN ARTIFICIAL I1-I2 FOR A SHORT TIME. RELAY IS RESTRAINED FROM OPERATION AT THE TIME OF OVER FLUXING BY THE 5TH HARMONIC RESTRAINT FEATURE.

30. Harmonics present in transformer charging in rush current

31. Evolution Of Protective Relaying. • Electromechanical single function • Static single function • Digital single function • Digital multifunction relays • Numerical multifunction relays • Numerical multifunction systems

32. Comparison of technologies

33. Comparison of technologies

34. Functions of Numerical Transformer Protection

44. CASE STUDIES

45. Case-1: Generator transformer • In one of our GTs, the Buchholtz alarm was appearing frequently.On inspection, it was found that, there was minor oil leakage from some of the bolts connecting the top cover with the tank just below the LV turret area. Oil fumes were also observed near the area. • Thermography was carried out for detecting localized heating. • Temperature of the above bolts was found to be abnormally high ie more than 100 deg cent. • On analysis it was found that the bolts were just below the LV turrets area. Eddy current due to localised induction was concluded. Additional earth paths were provided near the area after taking a S/D of the transformer and the problem was rectified.

46. Case-2 • On one occasion, in one GT when the oil and the winding temperatures were normal, the GT triped on Buchholtz protection with alarm & trip coming to-gether. • The buchholtz was getting reset automatically after the unit trip-out. • No gas was found in the buchholtz chamber. • Required tests and inspections were carried out and the transformer loaded again. • The above problem got repeated 02 times within a very short span of time afterwards. The situation was taking place during the afternoon hours of the day. • It was subsequently detected that, the above phenomena was happening when all the GT cooling oil pumps were getting started at a time on auto-mode due to rise in ambient temperature. • Oil pump starting was staggered.

47. Case-3 OSR operation • In one of our 200MVA Transformer installed in the 220KV Transmission line, repeated tripping was observed due to OLTC oil surge operation. • Transformer healthiness was found to be OK. • On thorough inspection it was found that, the tripping was spurious and was due to earth fault in the cable at the gland.

48. Case-4: Mal-trip due to moisture ingress. • Due to ingress of moisture in the field terminal boxes of buchholtz relay and PRD there has been some spurious trippings of transformers. • Canopy provided. Proper sealing made. • Additionally, during rainy season poly coverings are provided.

49. THANK YOU