Field Measurement Studies For Shunt Coefficient Of Lightning Shield Line

When grounding design and upgrade is done on power plants and substation, objective values, such as the earthing resistance, step voltage and apparatus contact voltage of earthing device are needed to be set. If the objective value of earthing resistance is too large, system won’t release the short-circuit current, which brings on the potential rise of the power plant and substation, producing great contact voltage and step voltage, thus damaging the safe and steady operation of the power system, moreover, it endangers the personal safety; otherwise if the object values of earthing resistance is set too small, in which case the investment cost will definitely increase greatly, thus leads to an unnecessary waste. The objective value of earthing resistance in power plant and substation is depended on short-circuit running current across power plant and substation. It is obvious that the value of short-circuit current has direct relationship with the safe and steady operation of the power system and the security of electrical instrument. However, it’s difficult to calculate the shunt coefficient in lightning shield line, for the determining the coefficient is the major problem that puzzles many engineering designers, mainly because some parameters in the formulas is hard to determine, so as to make it impossible to calculate or the inaccuracy of the result. Thus, it has great meaning researching on the shunt coefficient in lightning shield line.This paper discusses the three existing methods and the common problems on the three methods in calculating the shunt coefficient in lightning shield line. Sequence Component Method is complicated, while the parameters needs considered is too many, some of which is even impossible to value, and still some don’t have deserved consideration; Phase Component Method has a complex calculation model, some of the parameters in the process is hard to set; Simple Calculation Method supposes the ideal conditions in computing process, some parameters’value is not according to the practical situation, impractically.Take the presented problems of the existing calculation method into account, shunt coefficient in lightning shield line is hard to determine in practical earthing project, which makes it impossible to compute the objective values of earthing resistance, step voltage and apparatus contact voltage.etc. To solve this problem, this focus on the research on field measurement to determine the shunt coefficient, which means to test the distributary earthing resistance in the lightning shield line by current-voltage method in the field, to find the respective relationship between distributary earthing resistance and the average line spacing, the earthing resistance of design of grounding of offside substation and those of each transmission-line tower, and through analysis on plenty of related data, establish the practical math model of shunt coefficient in lightning shield line, analyze the relationship between shunt coefficient in lightning shield line , distributary earthing resistance , the average line spacing, earthing resistance of both short circuit substation and design of grounding of offside substation, the parameters of design of grounding of offside substation itself, and earthing resistance of each transmission-line tower, etc. An empirical equation,to calculate the shunt coefficient of lightning shield line, of practicality, precision and simplicity is deduced, which provides a reference for the designing and upgrading of the power plants and subatation.