ESS를 이용한 저압 배전계통의 EV 수용성 향상에 관한 연구

Abstract

The Korean government actively takes actions of the energy policy, by which the capacity of RES(Renewable Energy Resource) is to increase up to 20% of all energy sources by the year 2030. Along with the Korean government policy, Jeju Special Self-Governing Province of Korea executes a policy(so called "Carbon Free Island Jeju by 2030") of distributing 2GW of RES and converting all vehicles into EV(Electric Vehicle) by the year 2030. In Korea, the RES capacity is not only increasing but also the fast/slow charger in buildings, homes, and public places are rapidly spreading. Especially as the installation of the slow charger for the EV users in the individual customer becomes dominant in the LVDS(Low Voltage Distribution System), it begins to act as a malicious load in LVDS designed according to transformer capacity. Therefore, a special caution is required to increase the inflow rate of the EV charger in the customer not to cause a low voltage in the secondary feeder as well as not to violate the capacity limit of the pole transformer. In addition, when it comes to the increase of RES capacity in the LVDS, it also needs a caution to prevent the overvoltage in the secondary feeder. Traditionally, in order to solve aforementioned problems, the first reinforcement action is to replace the pole transformer by a larger capacity, and to expand the secondary feeder. The second reinforcement action is to install pole transformers closer to EVs. However, the traditional approaches require time and high cost. Thus, it is understandable that the traditional approaches are not suitable for future electric networks. Therefore, this thesis proposes an innovative control algorithm of the ESS(Energy Storage System) that is to contribute to increase the capacity of EVC(Electric Vehicle Charger) in the LVDS without reinforcement of the existing facilities. An ESS is used in various forms from the transmission system to distribution system and microgrid such as frequency regulation in distribution substations, power stabilization of fluctuating RES, peak-cut for charging at light load and discharging at heavy load, and emergency power generation, etc. In particular, the introduction of the ESS in a LVDS without voltage regulation can have a number of advantages. However, as the capacity of EV and renewable energy is continuously increased in LVDS, research is needed to increase the capacity that can be connected using existing facilities as well as voltage issues. In the proposed control algorithm, in order to solve the problems of voltage drop/rising and feeder capacity by introducing a large number of EV chargers and RES in the LVDS, an advantage of ESS bidirectional power flow is considered at the end of the secondary feeder in order to maintain the voltage and the power within allowable limit(voltage: 220±6%, feeder capacity: pole transformer rated capacity). The proposed control algorithm shows its ability to expand the capacity of EV up to maximum twice the capacity of the pole transformer with the various conditions while the voltage and feeder capacity are maintained with allowable limit. Its ability is assessed and discussed by the simulation consisting of an ESS, PV, EVC, and load in the environment of PSCAD/EMTDC software and hardware system.