Mitigation of impacts of lightning surges on low voltage side of power distribution substations

Abstract

Lightning surges are the major source that causes failure of power electronic equipment in low voltage (LV) power distribution systems, specially for the equipment with low immunity parameters and high sensitivity to surges. The 400/230V low voltage systems are usually affected by lightning mainly by the surges coming through the distribution transformers. In addition to that induced surges or direct strikes on load side could cause damage to distribution substations. Power utilities are concerned about the possibility of damage to distribution transformers caused by lightning strikes leading power supply failures and other losses including transformer damage, meter equipment damage and damage to the surroundings. Assessment of such occurrences is important in the design of suitable protection schemes and mitigation strategies. The number of incoming surges, their energy content as well as the number and the amplitude of power frequency follow currents determine the level of protection required for each substation. Hence, the assessment of impacts becomes more and more important for the design of lightning protection system. The methods used in Sri Lanka to assess the impacts of lightning are based on experience of the utility employees or the frequency of protection failures and power supply failures. Specially, when it comes to the low voltage (LV) side, the impacts are not assessed or examined, and in most of the cases, the low voltage side of a substation is not effectively protected from lightning. In this research the impact of lightning surges particularly on low voltage side of the power distribution substation was analyzed, and the possibilities of using protection mechanism in low voltage side of the power distribution substations were investigated in order to reduce or avoid human and installation damage caused by lightning. Study was limited to power distribution substations in the service area of Distribution Licensee No. 3 (Distribution Division 3 of Ceylon Electricity Board) where the total number of installed distribution substations were around 5000. iv Then by simulating the impact of lightning surges on a low voltage side of power distribution substation using PSCAD software, the behavior of the electrical parameters under occurrence of lightning was studied, and based on the observations and results, design parameters for the protection system were derived. Then the available protection methods for low voltage systems in the world were studied. In the next stage, to protect the low voltage side of the distribution substation economically and safely, application of several protection systems was investigated. Those protection systems included surge protective devices (SPD) and proper grounding mechanisms tried out on LV side of the selected distribution substations of DD3. Finally, based on the findings of investigation, simulation and pilot installation, a standard protection system was deployed, and results were analyzed to check whether the implemented protection system was effective in electrical terms and monetary terms. Continuous monitoring of protected distribution substations has been carried out and the results were produced in the report. By the results of the assessment of the impacts of lightning on LV side of power distribution substations, it was obvious that huge amount of visible and invisible losses is incurred to Ceylon Electricity Board. The results of the PSCAD simulation clearly shows that the LV side of the power distribution side is affected in the event of lightning strikes. Hence, it was concluded that the LV side of the power distribution side should be protected to eliminate the impacts of lightning strikes. Finally, this research proposes a standard protection mechanism to protect distribution substations from lightning and the effectiveness of the protection system was proved by the results obtained.