Technical Audit of Sahowala (Sialkot) 220kV Grid Station

Commissioned on August 11, 1999, the station has a total transformation capacity of 480MVA, connected to the grid through three 160MVA-220/132kV autotransformers, one 20/26MVA-132/11kV, and one 10/13MVA-132/11kV power transformers. The main feeding sources for this station are the Kala Shah Kaku 220kV and Gakkhar 220kV Grid Stations. The technical audit team, comprising transmission and protection experts, visited the station from February 25, 2014, to March 6, 2014. Their findings revealed several critical issues that require immediate attention. The loading condition of the transformers is a major concern, with the 220/132kV transformers operating slightly above the prescribed loading per NEPRA grid code. This excessive loading can reduce the expected useful life of the transformers. The audit team also identified several other issues, including the operation of tap changers of 132/11.5 kV transformers locally in the switchyard, insufficient existing sources in case of an outage, and pending major maintenance of circuit breakers. Additionally, the team found that five 220kV and forty-four 132kV isolators are operated manually in the switchyard due to weak mechanical strength of porcelains and improper alignment of few. The team also noted that the 11kV capacitor banks are not installed on 132/11kV transformers, and tie line/stub protection is not installed on both 220kV feeding circuits. The recommendations made by the audit team include the installation of an additional feeding source to address the insufficient existing sources, the procurement of spare parts for major maintenance of circuit breakers, and the replacement of manually operated isolators with appropriate ones. The team also recommended the installation of thermal overload protection relays on all 220/132kV and 132/11kV transformers, as well as the installation of HV winding temperature protection relays on 132/11kV transformers. The audit team also highlighted the importance of conducting regular tests, including leakage current measurement (LCM) test of lightning arresters, C&DF test for all CTs and CVTs, and thermal overload protection relays on all transformers. The team emphasized the need for proper load flow studies to analyze the system, considering the construction of new circuits and the upcoming commissioning of a new Gujrat 220kV Grid Station. The installation of a sequential event recorder (SER) and fault/disturbance recorder with CPU and printer, voltage, and power recorders is also recommended to ensure proper working of the system protection and to identify components that fail to operate. The team also suggested that the "System Protection and Telecommunication departments of NTDC" should look into the inoperative direct transfer trip (DTT) system and enable the "Carrier aided facility" at either ends of the transmission lines to clear faults rapidly.