This paper describes measurements, causes, and countermeasures of voltage reduction, spread of voltage unbalance, and voltage flicker, which are technical issues of voltage regulation in distribution system in recent years. The voltage measured in the distribution feeder with large reverse power flow from photovoltaic systems is explained by relationship between voltage rise by active power and voltage reduction by reactive power. The spread of voltage unbalance, which is also related to the large reverse power flow, is caused by unbalanced mutual impedance of three-phase distribution wires arranged horizontally or vertically. Frequency feedback method with step injection of reactive power, which is one of islanding detections installed into PCS, is attributed to voltage flicker in the distribution system.

According to the increase of the amount of renewable energy, voltage control of the distribution system is becoming more important. Thus we propose a voltage control method of power conditioning systems (PCSs) in the customers serving as virtual power plant (VPP) with the step voltage regulator (SVR). The proposed method is verified by the simulation and experimental results. As a result, it is clarified that PCSs are able to control voltage moderately and cooperate with SVR under several voltage fluctuation conditions.

An optimization of distribution system with Distributed Generators (DGs) is a big challenge in recent power systems. In our previous study⁽²³⁾ a real-time control method has been proposed to track an optimal operating point that is the common optimum for the global problem to maximize the total generations of DGs and for the local individual problems to maximize the profits of local DG generations. By extending the previous study, this paper proposes an advanced method to improve the robustness of the control. Instead of solving the local optimizations in the individual DGs, the target nodal voltages and P&Q prices are provided as the individual control targets. Based on multi-agent architecture, an effective distributed control is performed effectively. A management agent performs an optimal power flow computation to determine the optimal voltage profiles and the shadow prices of real and reactive powers, which are transferred to the local individual agents through a common blackboard memory. The proposed method is effective to mitigate the voltage violation problem and maximize the total PV power generations. The method also resolves a present problem that some specific PVs tend to be deactivated due to voltage violations.

Recent years, many issues about voltage and power quality arise in distribution lines by the spread of distributed power supplies. To cope with this, many voltage control techniques of the voltage regulator for distribution line have been developed for each problem, but there are few examples which were compiled about their application point and future problems in the form that covered all these techniques, while comparing the characteristic and cost. In this report, for the purpose of contributing to the future technology development and the operation of the power distribution, we compile the characteristic, advantages and disadvantages of these main techniques of the voltage regulator which are really used, and propose some new techniques and products which we have developed in order to cope with the voltage control that will become more difficult in future.

Promotion of photovoltaic power generation (PV) is accelerated in the world, however, large voltage fluctuation by massive PVs in distribution system may cause frequently operation of voltage regulation device such as LRT and SVR. Since voltage regulation by LRT and SVR have mechanical tap-change operation, equipment life of LRT and SVR would be shortened by frequently operation. One of the measurements is to utilize batteries in the distribution system, which are owned by system operator or customer, however, since installation cost of batteries is expensive, development of the method to operation batteries effectively is important challenge. This paper proposes novel operation method of batteries for contributing to reduction tap-change operation of LRT and SVR considering distribution system loss. Moreover, this paper reveals the required capacity of batteries to avoid additional installation of SVR by numerical case studies with actual demand and PV profile.

Transmission system operators (TSO) are responsible to manage the power quality within a proper range and to reduce power loss. Voltage reactive power control (VQC) is mainly utilized to manage the power flow. In the conventional VQC scheme, the static condenser (SC) and shunt reactor (ShR) is generally adjusted based on the target value determined for the heavy load condition in each voltage class. On the other hand, the installation of the massive photovoltaic (PV) systems into transmission systems cause the complex power flow such as the short-term fluctuation of power flow and reverse power flow from the lower transmission system to the upper transmission system. Thus, in the future power system with PV systems, the target values of the VQC devices need to be updated in the short-time period considering the interaction between the upper and lower transmission system to manage the power quality and to the reduce power loss efficiently. This paper proposes the optimization methodology for VQC devices in two-voltage class of transmission system using tabu-search and optimal power flow based on the central VQC methodology. To verify the effectiveness of the proposed methodology, the numerical simulation was performed based on the revised IEEJ EAST 30-machine system model.

In a power system, when a large amount of renewable energy sources (RES) are introduced, there is a possibility that the voltage can't be maintained because of output fluctuation. In order to maintain voltage even for such cases, the voltage estimation without additional device is necessary. Hence, this paper presents a voltage estimation methodology applicable to High Voltage (HV) system using regression tree.

Voltage fluctuations caused by output fluctuations of renewable energies are becoming an issue where a large amount of renewable energies interconnect to a localized area. We have developed a reactive power control system which calculates reactive power command in order to suppress voltage fluctuations in a localized area utilizing a reactive power output function of a large-capacity battery system installed in Minami-Soma substation. The system has started operation in February 2016. Verification results of the system which have been reported in workshops are summarized.

Recently, distributed generation using renewable energy sources has increased in power systems to solve environmental problems and resource exhaustion problems. However, conventional reliability evaluation methods of power systems cannot easily evaluate power systems with massive penetration of distributed generations because of the loop switching operation. This paper proposes the reliability evaluation method with reference to the concept of reliability controls. This method controls power system states by the loop switching operation and evaluates power system reliability of controlled state. Therefore, proposed method can evaluate power system while considering the effect of loop switching operation. Moreover, this system helps the decision of power system operators because it quantitatively selects the best operation from the reliability evaluation result.

It is expected to promote the introduction of renewable energy (RE), such as photovoltaic (PV) and wind power (WP), from the view points of carbon dioxide reduction and diversification of energy sources. However, as the introduction of RE into the conventional power grid, output curtailment of RE will increase due to supply and demand balance constraints. In this study, we have developed a model and data which enable long-term power supply and demand analysis based on some scenarios. Using this model and data, we analyzed the transition of the amount of RE generation and the amount of RE curtailment per installed capacity. As a result, the change in the facility utilization rate of PV and WP is clarified quantitatively based on the influence of RE curtailment. The amount of PV curtailment was significantly increased in Kyushu and Tohoku area, where RE is particularly widespread. The results of this paper can be used to evaluate how and when it is necessary to take measures such as the introduction of storage batteries.

Penetration level of large-scale wind farm into the existing power system has been increasing significantly, which impacts on the transient stability during a network fault. This is because wind farm can be disconnected from the grid according to a grid code of the system when instantaneous voltage drop due to a network fault occurrs in the grid system. This paper proposes a new method to stabilize the system frequency drop and enhance the transient stability of the grid system by using a battery which is based on the new virtual synchronous generator control performed by the cooperative operation between the battery and LFC hydro power plant.

The grounding of a Back-to-Back system is necessary in order to determine the valve insulation level. The commonly used method is direct grounding of the negative side of the DC voltage but this method requires the full of the DC voltage. The authors proposed a neutral point grounding method with a reactor. The authors studied the required value of the reactor. Then the authors studied the miniaturization method of the reactor.

Surface charge accumulation phenomena on insulating epoxy spacer are measured under DC-GIS simulated environment. Homo-charges are accumulated near the electrodes, and charge injection from the electrodes is assumed to be dominant in surface charge accumulation. The surface charge is saturated at a time constant of 1000 hours below 20°C, and the saturated charge amount tends to increase with room temperature rising.