Evaluating Lightning Performance of Overhead Lines & Methods for Reducing Trip Rate

Utility Practice & Experience

The Turkish Electricity Transmission Corporation owns and operates the 154 kV and 400 kV transmission systems that form an integrated network that interconnects all of Türkiye. TEİAŞ is responsible for the safe, adequate, reliable and economical operation of this grid.

The south-west of Türkiye experiences an extremely high number of lightning strikes every year. This high GFD value has affected performance of transmission lines and it is therefore important that lightning studies be carried out regularly to ensure that this system offers a continuous supply to users. In evaluating an overhead line lightning performance, several steps must be followed and certain key information needs to be taken into account:

Trip Rate

Before a lightning study is commenced, trip rate is calculated based on the IEEE formula:

Trip rate = No. of Trippings / [(Date Now – Commissioning Date) years x Length of Line x 100 km]

Normally, a lightning study is not deemed necessary for lines with less than 1 trip per 100 km per year but will still be conducted if there are special requests or unique cases, e.g. new projects or important lines such as 400 kV. Up to now, TEIAS constantly calculates trip rate for its 154 kV and 400 kV lines to ensure that the OHL trip rate always remain within an acceptable range, i.e. as stated in IEC EN 60071-2(2018) Clause 5.2. 

Historical Analysis

Analyzing lightning trip data for any specific line includes:

* Total number of trippings;
* Lines that trip (Circuit 1 or Circuit  2) if the study is conducted on a double circuit line;
* Location  of trip i.e. distance from substations or exact location of tower i.e. tower numbers;
* Phases which trip i.e. Red, Yellow or Blue;
* Type of tripping i.e. double circuit (DCCT) or single circuit (SCCT).

Ground Flash Density

A GFD map is generated from the GKC (Geographical Knowledge Centre)  for further examination. Exact tower location is plotted on Google Earth and compared with the GFD in that area.

Fig. 1: GFD map for 154 kV Mugla-Marmaris 2 Line for 2015 to 2019 (buffer is 5 km radius).

Tower Altitude

Tower altitude is important since this will affect probability of lightning strikes on any specific tower. However, GFD plays the more important role.

Tower Footing Resistance

Towers with higher TFR values will have a higher risk of experiencing a  backflashover compared to towers with lower TFR.

Lightning Current

The GKC Lightning Detection System is capable of generating the actual lightning current with the exact time based on the GPS clock.

Software Simulation

Simulations are important in aiding selection and application of transmission line arresters on the system for best performance. A single stroke study can also be conducted on any single tower to analyze the surge pattern on each of the phases during lightning events.

The simulation will guide the System Operator in proposing the best method for improving line lightning performance. Mitigations can include reducing the  TFR, installing Line Surge Arresters or even re-locating earth wires. In some cases, there were also proposals such as re-locating previously installed LSAs.

Attend the 2022 INMR WORLD CONGRESS in Berlin where expert Ertugrul Partal will give a lecture on the procedures used for evaluating lightning performance of 154 kV overhead lines in Türkiye. He will also discuss and explain the methods and actions used to reduce OHL tripping due to lightning, thus improving line performance.