Field Demonstration and Evaluation of Centralized Voltage Control System for Distribution Network

With the expansion of introduction of renewable energy sources (RES) such as photovoltaics (PV), voltage fluctuations (fluctuated voltage by PV output ups and downs depending on sunshine changing) in distribution network can be increasing. As a method of suppressing voltage fluctuations, intensive control of voltage control equipment by the introduction of a centralized voltage control system (CVCS) should be mentioned. In this paper, through a field demonstration, the effect of suppressing voltage fluctuations by introducing the CVCS was evaluated and confirmed. INTRODUCTION In Japan, the introduction of renewable energy has been promoted using a feed-in tariff (FIT) system. With this system, the amount of power from RES to which FIT is applied accounted for 5.4 % of the total demand in 2015 [1] (Fig. 1), and the 71.9 % of it was supplied from PV sources [2], which were installed mostly in the distribution network. With this situation, voltage fluctuations in the distribution network are increasing, and it is getting difficult to maintain the voltage within the target range simply by conventional autonomous decentralized voltage regulators. Fig. 1 Amounts of RES introduced. Traditionally, the autonomous control method has been adopted for voltage regulators installed on a few points in a distribution network, and their controlled variables are determined independently. This makes the cooperation control of some regulators difficult, and leads to voltage violation enlarged by PV output fluctuations. On the other hand, sensor-built-in switches and voltage regulators, which can measure voltage and current values in the network and communicate with a distribution automation system (DAS), are getting popular. This measurement data is expected to improve the network monitoring and operation to ensure power quality. This study focuses on the centralized control method [3] based on information and communications technology to maintain the network voltage within the target range using the measurement data. Specifically, we report on a field demonstration and evaluation results of the CVCS. NETWORK CONFIGURATION The standard configuration of distribution network in Japan is shown in Fig. 2. It consists of a 6.6 kV middlevoltage distribution lines and 100/200V low-voltage distribution lines. The average length of a distribution line is different for each electric power company. In Tokyo Electric Power Company (TEPCO), the average length of the 6.6 kV distribution line is about 3 km, and the average length of a 100/200V distribution line is about 50 to 100 m. The 6.6 kV distribution line is divided into 3 to 6 sections taking account of load distribution with automated switches. For voltage control of a distribution network, there is a load ratio control transformers (LRT) in the distribution substation and some step voltage regulators (SVR) in the distribution lines. Each regulator performs autonomous control according to only its secondary voltage and its passing current, without other point voltages and other regulators movement. For this reason, it is difficult to control the voltage considering the entire distribution network, and it is getting difficult to cope with voltage fluctuations caused by the reverse power flow from introduction of RES. Fig. 2 Standard configuration of distribution network in Japan. 5,590 GWh 18,119 GWh 28,602 GWh 43,235 GWh 0.66 % 2.14 % 3.48 % 5.42 %