Surge Protection System for a 22 kV Pole-Mounted Transformer in a Region of High Lightning Density in South Africa

Abstract:
This study aims to create, simulate, and analyze a lightning surge protection system for a pole-mounted δ to wye 22/0.230 kV step-down transformer located in the Pongola city of KwaZulu Natal Province, South Africa. PSCAD software was used for simulation purposes. The resistivity and flash lightning density in the area are 10.2 Ω.m and 11-14 flashes km-2 yr-1, respectively. To represent the system, a model was designed with a transmission line impedance of 5.8 Ω, an RC pole model with a resistance of 10 MOmega and a capacitance of 21.7 pF. The pole footing resistance was calculated to be 0.9 Ω. Since the primary side is more prone to lightning strikes than the secondary side, different earthing designs were used for both sides. Ten five-meter-long electrodes were used on the primary side, placed 1m apart in a circular configuration. The primary and secondary side earth resistance were obtained to be 1.7 Ω and 4.4 Ω, respectively. To protect the system, a surge arrester clamp voltage of 95 kV was selected, just below the transformer power frequency voltage. In simulating the system, a double exponential current waveform (8/20 mus) of 30 kA amplitude was injected into it. Without the surge arrester, the system produced a voltage surge of 175 kV, which exceeds the impulse withstand voltage (100 kV) of the transformer. However, with the surge arrester, the system produced a voltage below 70 kV, indicating that the protection system is functional as per requirements. When the earth resistance of the primary side was increased randomly to a value of 10 Ω, the system ground voltage was disturbed, and as a result, the clamp voltage of the surge arrester shifted from 70 kV to 80 kV.