Transmission & Distribution line equipment are stressed with a variety of transients, fluctuations and abnormalities. These abnormalities affect the dielectric strength of equipment. Dielectric tests are performed on these equipment to confirm that they work perfectly even after facing the abnormalities of the transmission line or grid. Among dielectric tests, the Lightning Impulse is the most important test that confirms the dielectric strength or withstanding capabilities of line equipment to the natural lightning. The Lightning Impulse has a wave characteristic of 1.2/50 µsec with 1.2 being front time (T1) with tolerance of 30% and 50 being tail time (T2), as shown in Fig 1, with tolerance of 20% as per IEC 60060-1 : 2010.
This article describes various cases of dielectric failure and probable causes of transmission & distribution line equipment during lightning impulse test.
Failure Analysis
Sample 1: 11 kV Gun type resin cast Potential Transformer (PT)
- Description: Potential Transformers (PTs) are a special type of transformers used in distribution system for metering purposes. These PTs come in variety of shapes and sizes with insulation material being either resin or porcelain. Here, we have taken 11 kV gun type resin cast transformer as shown in Fig 2.
- Test Set-Up: Primary terminal of 11 kV Potential Transformer connected to 1000 kVp, 100 kJ 10 stage Marx Impulse generator. Secondary terminals (a and n) shorted together and connected to earth along with primary neutral terminal N.
- Test Source and Test Procedure: 3 stages of 10 stage, 1000 kVp 100 kJ Marx Impulse generator, as shown in fig 3, was used to conduct the dielectric test. Three Impulse voltage of 75 kVp was applied to primary terminal of PT with primary and secondary neutral connected to earth.
- Result and Discussion: The sample withstood the first two impulse shots at 75 kVp with standard wave shape characteristic and no abnormalities as shown in Fig 4 but in third full wave aplication, one small deflection observed in tail of impulse voltage waveform as shown in Fig 5.
- Analysis: After the test, the sample and connection was inspected and it was found that the primary neutral got disconnected from earth after 2 shots due impulse voltage vibration.
Sample 2: 5 MVA, 33/11 kV 3 – phase Power Transformer
- Description: A power transformer is a device that is used to convert high voltage and low current to low voltage and high current between distribution and transmission lines.
- Power transformers have two or more windings, which are magnetically coupled through a Cold Rolled Grain Oriented (CRGO) or amorphous core. Here we have taken one 5 MVA power transformer as shown in Fig 6.
- Test Set-Up: 1U-Phase of High voltage terminal connected to impulse generator, 1V and 1W shorted together and connected to earth through a 0.5 ohm shunt. All LV terminals shorted together and connected to earth along with body and LV neutral. Voltage and current transients were recorded using LI recorder and Apitz software.
- Test Source and Test Procedure: 4 stages of 10 stage, 1000 kVp 100 kJ Marx Impulse generator was used to conduct the chopped impulse test. Test was conducted as per clause no. 13.3 of IS 2026 (Part-3) : 2018/ IEC 60076-3 : 2013.
- Result and Discussion: The voltage and current transients were normal at reduced voltage level as shown in Fig. 7, but when full voltage impulse of 170 kVp was applied, the voltage got chopped before peak and current shoot up suddenly as shown in Fig. 8.
- Analysis: After the test, the transformer was inspected and it was found that a plywood was used between adjacent phases, which punctured during test. This plywood was replaced by press board and the test was repeated. The transformer passed the test successfully after replacing the plywood with press board.
Sample 3: 630kVA, 33/.415kV 3- phase Distribution Transformer
- Description: 3- phase distribution transformers are the most important and critical parts of electrical distribution system. These are basically two winding star-delta (Dyn-11) connected transformers, which are used to convert 33 or 11 kV to 415 or 433 Volts. The distribution transformers generally range form 10 kVA to 2500 kVA. One such transformer is used here for analysis purpose as shown in Fig 9.
- Test Set-Up: 1U-Phase of High voltage terminal connected to impulse generator, 1V and 1W shorted together and connected to earth through a 0.5 ohm shunt. Tap position was set to 1. All LV terminals shorted together and connected to earth along with body and LV neutral. Voltage and current transients were recorded using LI recorder and Apitz software.
- Test Source and Test Procedure: 4 stages of 10 stage, 1000 kVp 100 kJ Marx Impulse generator was used to conduct the chopped impulse test. Test was conducted as per clause no. 21.3(a) of IS 1180 (Part-1): 2014.
- Result and Discussion: The voltage and current transients were normal at reduced voltage level i.e., 50% – 70% of 170 kVp as shown in Fig. 10 but when full voltage impulse of 170 kVp was applied, there were kinks in the voltage waveform and a number of oscillations were observed in the current waveform at different time intervals as shown in Fig. 11. One more full voltage was applied and the kinks in the voltage waveform and oscillations in the current waveform increased as shown in Fig. 12.
- Analysis: The transformer was inspected after the test and it was observed that there was insulation failure in the tap changer leading to leakage current, which was reflected in voltage and current waveform. Sample 4: 22 kV, 400 A Gang operated air break (GOAB) switch
- Description: A number of isolators, disconnectors circuit breakers and fuses are used in the transmission and distribution line to separate any part of the grid from the live part of the line. In case of any abnormality is detected or for maintenance of a specific section like switchyard or substation, fuses are used to protect a certain circuit from the abnormalities of the grid resulting in reduction in loss of life as well as property. One such 22 kV, 400 a GOAB switch has been taken for analysis purpose as shown in Fig. 13.
- Test Set-Up: The GOAB fuse was mounted on a metallic structure and kept on insulated platform to isolate it from the ground. The switch was in closed position and line terminal of one phase was connected to impulse generator, rest all the terminals along with frame was earthed.
- Test Source And Test Procedure: 3 stages of 10 stage, 1000 kVp 100 kJ Marx Impulse generator was used to conduct the Lightning Impulse test. The test was conducted as per clause no. 7.2 of IEC 62271-1: 2021 & IEC 62271-103 : 2021.
- Result and Discussion: 15 impulses of each polarity was to be applied of 125 kVp, correction factor was applied as per IEC 60060-1: 2010. The switch withstood the reduced impulse of positive polarity but failed to withstand the full voltage impulse, as shown in Fig. 14. Three consecutive flashovers were observed resulting in failure of the sample. The test was repeated on other two poles also but they also failed to withstand. The switch was dismantled and the test was conducted on individual insulator. The insulator withstood the 15 impulses of each polarity of 125 kVp as shown in Fig. 15.
- Analysis: The fuse was inspected after the test and it was found that the metal assemblies used on the insulator were reducing the arcing distance of the insulator due to which it was unable to withstand the impulse voltage of 125 kVp.
Conclusion
It is concluded that the transmission and distribution line equipment fail due to human error, manufacturing error, assembling error and design error as seen in sample 1, sample 2, sample 3 and sample 4 respectively.
All these errors can be detected and corrected by lightning impulse voltage test as if the equipment fail during operation, there will be huge loss of life and property.
Gangeshwar Singh completed his graduation in Electrical Engineering from JSS Academy of Technical Education, Noida, Uttar Pradesh. Currently he is working as an Engineering Officer in the High Voltage Testing Laboratory at CPRI, RTL, Noida and have four years of experience in the field of testing and certification in High voltage and Ultra high voltage equipment.
Satish Kumar completed his graduation in Engineering from Janardan Rai Nagar Rajsthan Vidypeeth University, Udaipur, Rajasthan. Currently he is working as an Engineering Officer in the High Voltage Testing Laboratory a CPRI, RTL, Noida and have Thirty years of experience in the field of testing and certification in high voltage.
Manoj Kumar Jaiswal completed his Post Graduation in Electrical Engineering from Indian Institute of Technology (IIT), Roorkee. Currently he is the Unit Head and Joint Director of CPRI, RTL, Noida and have Thirty years of experience in the field of testing, certification and consultancy in high voltage, Ultra high voltage, energy meters, LED and cables.
