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CT failures as encountered in NDPL - A Dissertation

CT failures as encountered in NDPL - A Dissertation. Types of Failures. Three types of CT failures have been noticed in the recent days at NDPL: Thermal failure of CTs. Burden failure of CTs. 4. Case 1: Thermal Failure of CT.

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CT failures as encountered in NDPL - A Dissertation

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  1. CT failures as encountered in NDPL- A Dissertation

  2. Types of Failures • Three types of CT failures have been noticed in the recent days at NDPL: • Thermal failure of CTs. • Burden failure of CTs. 4

  3. Case 1: Thermal Failure of CT • The CTs were mounted on 11 kV side of 25 MVA 66/11 kV Transformer no.3 at DSIDC Narela • The load on the day of occurrence was 1000A • The transformer had tripped on Differential R phases • On opening the LV Breaker Cubicle, the R phase CT (Protection & Metering Core) was found cracked • The CT cracks were found in annular ring form, cracking the cast resin mould • Crack mark was also noticed on B phase CT • Localized hot spot was noticed on all three CTs • The problem was not in termination of the cable with CT • The fault was found within 2 years from installation • There is no history of overloading at this station 4

  4. Case 2: Thermal Failure of CT • The CTs were mounted on 11 kV side of 25 MVA 33/11 kV Transformer no.2 at Ashok Vihar • During maintenance of the transformer bay along with 11 kV incomer, crack marks were observed on all three CTs • Distinct hot spot along with cracked resin surface at the Y and B phase CTs were observed • On the R phase CT only Crack marks on the CT was observed. Localized hot spot was noticed on all three CTs • The problem was not in termination of the cable with CT • No such problem was noticed in half-yearly maintenance carried out six months earlier • There is no history of overloading at this station 4

  5. Burden Failure of CT • At 33 kV AIR Khampur, 16 MVA, 33/11 kV Tx.2 tripped on Y phase differential • On the basis of SCADA reports, it was found that there was a fault in Outgoing feeder in Y phase, where Hi Set start up was observed • Along with differential tripping, CT Saturation discrimination was triggered in the differential relay • On detailed investigation, it was revealed that the Differential Core of Y phase of the HV side CT was saturated • All the three HV side CTs were replaced for the maintaining identical knee point voltages, and the transformer was put on load • After 6 months, the same problem re-occurred at the same bay • This time the saturation was found in HV side B phase CT. • The CT being new, the reason for saturation of the CT was investigated • No anomaly was noted in CT circuitry. The wires were tested from yard to panel and the resistance was found to be around 4 ohms • The relay was suspected as having high burden, and was replaced, thus, affecting new CTs too • There has been no problem reported from the site after replacement of the relay 4

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