Different reliability aspects for planning of local 110 kV grid

摘要

The principal operation rule for Transmission System Operators (TSO) is to ensure the reliability of the power system. The Finnish laws oblige the TSO to develop the network according to the needs of the customers and to keep the reliability on a high level. The Finnish transmission network is operated according to the N-1 criteria.The current Finnish practice for evaluating the Customer Interruption Costs (CIC) of Energy Not Supplied in the transmission network is calculated with an equation that takes into account the starting moment (weekday, weekend, summer, winter...), the duration of the interruption and the power of the disconnected load. Different customer groups have different weighting coefficients for starting time and duration. Energy Not Supplied is calculated simply by multiplying the interrupted power and the duration time. In this study, three different approaches and case studies for the reliability of the local grid are presented. The evaluation is based on two main concepts: constant fault probability and Customer Interruption Costs.In the first studied case, a reliability analysis with event-trees was carried out to study three different possibilities to connect a third power transformer to a substation. Two of the studied substation schemes had two busbars and one scheme had three busbars. Sequences of different combinations of line faults and of protective equipment failures were analysed. The grid consequences of event tree failure branches were divided into prolonged voltage dips and load interruptions. The analysis showed that the differences in reliability were negligible for an industrial customer that is connected with two lines to the substation. The three-busbar substation was evaluated against the expected savings in the CIC. The CIC savings were negligible and did not justify adding a third busbar. In the second studied case, the evaluating method of probable CIC was applied to study the cost-benefit of installing a protective automation to a sectionalizing circuit breaker. A sectionalizing circuit breaker can halve the probabilistic CIC. The expected savings were significant but remained too low to justify the investment.In the third studied case, two local grid reinforcement alternatives of a 110 kV local network were compared with their probable future CIC-values. Grid reinforcement with a new substation was compared to grid reinforcement with only new transmission lines. The difference in probabilistic CIC between the alternatives was observed.The presented evaluating method of probabilistic CIC for estimating the reliability of the local grid can be used to compare the network investments. CIC coefficients take into account the different needs of quality of service for each customer. The evaluation method is useful when different network reinforcement alternatives are compared to each other.