Mitigation of Third Harmonic Current at the Generator Neutral using Selective Hybrid Grounding

This paper proposed a selective hybrid grounding to mitigate the high third harmonic (TH) current in generator neutral mainly due to triplen harmonic. In a medium voltage network, neutral earth resistance (NER) grounding method is commonly used to limit the fault current rating in the generator neutral. Due to triplen harmonic at generator neutral, NER has experienced overheating over prolonged periods during normal operating conditions. This paper explores the hybrid grounding solution utilizing the existing NER combined with grounding transformer to provide TH current mitigation and limiting fault current. It is a hybrid method that can selectively combine the low resistance and high resistance grounding. In this paper, investigation on the proposed selective hybrid method is presented. In this method, a high resistance, R<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">harmonic</inf> is imposed on the generator neutral during the normal condition and a low resistance, R<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sec</inf>, is imposed during the phase-to-ground fault. The two secondary resistors are connected to the generator neutral via grounding transformer and equipped with switching capability. The simulation was carried out on the simplified system network of Universiti Teknologi Petronas (UTP) using real parameters. Characteristics of voltage and current for the switching condition was analyzed and the optimum value of secondary resistance is also explored. It is found that the hybrid method able to mitigate third harmonic current in neutral during normal condition and maintain a safe fault current level.