Malaysia’s power system operates in a region of intense lightning activity that adversely impacts performance of certain lines operated by the country’s grid operator, Tenaga Nasional Berhad (TNB). Among the lines most seriously affected by lightning are the 132 kV Kuala Krai to Gua Musang line located in the northeast and the 500 kV Ayer Tawar to Bukit Tarek line which runs along the western coast of the Malay peninsula.
This article from 2014, contributed by Dr. Iryani Mohamed Rawi, discussed a program of selective installation of externally gapped line arresters to reduce the rate of lightning related trippings on these two lines.
Different approaches were applied in each case since the 132 kV line had already been equipped with EGLAs since 2007 yet continued to experience lightning flashovers. By contrast, the 500 kV line was part of an ongoing TNB project to reduce the rate of trippings due to lightning through cost-effective installation of line arresters.
Background on 132 kV Kuala Krai-Gua Musang (KKRI-GMSG) Line
This 113 km double circuit line was commissioned in 1995 and consists of 295 towers. Between 2001 and 2012, it recorded a total of 53 trippings, most of the double circuit type. This translates into a tripping rate of 4.26/100 km-year – considered unacceptably high when compared to TNB’s average for such lines of 1.8/100 km-yr. Acceptable tripping rates for 132 kV lines at most other utilities are reportedly between 1.5 and 2.8/100 km-yr.
A study was therefore undertaken with the goal of explaining the seeming ‘ineffectiveness’ of more than 120 line arresters (TLAs) installed on this line between 2007 and 2012. In theory, an increasing population of TLAs should have reduced line flashover rate. Yet, for this line, flashover rate increased.
Background on 500 kV Ayer Tawar-Bukit Tarek (ATWR-BTRK) Line
The 145 km 500 kV ATWR-BTRK line consists of 352 towers and in recent years experienced trippings at a rate of 1.532/100 km-year. This rate was also considered too high, as an acceptable rate in the case of such important lines should be less than 1/100 km-year.
In both cases, it was regarded as necessary to analyze historical line performance in order to find the best way to reduce nominal tripping rates to more acceptable levels
Geographic Profiles of Lines
The 132 kV KKRI-GMSG line is located in a ‘mixed area’ comprising the town of Kuala Krai, flat land and dense jungle around Gua Musang. A total of 154 towers (52%) are located in flat areas while 139 towers (47%) are in jungle. The remaining two towers are near substations. In the case of the 500 kV ATWR-BTRK line, 40% of towers are located in hilly areas (i.e. at higher altitudes).
To better understand soil conditions along the two lines, tower footing resistance (TFR) values were determined for both. Figs. 2 and 3 are charts showing altitude and TFRs for the different tower locations along each.
As evident from Fig. 2, TFR for the 132 kV line is typically higher at lower altitudes and vice versa.
Fig. 3 shows that TFR values for the 500 kV line increase in higher altitude areas. This usually means very poor lightning performance since the line becomes the main target for lightning.