Designing a protection coordination loss assessment technique for DG integrated MV networks

Abstract

Conventional distribution system is generally passive, which means power flow is unidirectional, from its grid substation to the loads. With addition of Distributed Generation (DG) in to the system, several issues, such as power flow bi-directionality and increase of fault current in the system influence the existing protection scheme, which is originally designed for a passive network, which has limitations. Protection issue may become a crucial factor and more attention is required in designing the protection system at increased DG penetration levels. If it is possible to calculate loss of protection coordination quantitatively, utility can clearly identify the measures to be taken on their protection system in advance. The objective of this research study is to introduce a technique, which can be used to assess the protection coordination loss quantitatively, for a DG integrated network. Study is focused on the typical Medium Voltage (MV) network, mainly 33 kV and 11 kV in Sri Lanka. MV distribution system of Mawanella area of Ceylon Electricity Board (CEB), Sri Lanka is selected for the case study where, a total capacity of 6.4 MW of mini-hydro plants is integrated to the system. Fault current contribution to the system heavily depends on DG type, capacity and location. The effect of these decisive factors on the protection system is analyzed using the Power World Simulator. A tabulation method is used to record the data. Based on the table, decision on protection coordination loss in the system is segregated, protection coordination is maintained or not. Using the segregated data, protection coordination loss percentage is presented to get an overall idea for the system. The proposed technique can be used to analyze the loss of protection coordination quantitatively for existing networks where DGs are already in operation. The same can also be used as a powerful tool at planning stage at the time of studying new DG proposals. The proposed technique can be integrated as a new dimension to the currently practicing tools of over voltage check and short circuit capacity check.