A power distribution system is generally constructed to include a high-voltage system (generally, 6600 volts) and a low-voltage system (for example, 100 volts to 200 volts), and a receiving end of a general consumer is connected to the low-voltage system. An electric power provider is required to maintain the voltage at the receiving end of a general consumer within an appropriate range (for example, in the case of receiving 100 volts, the voltage is maintained at 95 volts to 107 volts). Therefore, the electric power provider adjusts, in order to maintain the appropriate voltage at the receiving end of a general consumer, a control amount of a voltage control device (for example, an LRT (Load Ratio Control Transformer: transformer with on-load tap-changing device) or an SVR (Step Voltage Regulator: step voltage regulator)) connected to the high-voltage system. The voltage of the voltage control device is controlled by a voltage controller integrated therein or attached thereto.
A transformer-type voltage control device such as the LRT or SVR is installed for the purpose of changing the load-side voltage by a tapping operation based on LDC (Line Drop Compensator) control to adjust the voltage at all points on the load side within an appropriate range. The LDC control here is for calculating an adequate load-side voltage for adjusting the voltage at all points on the load side within an appropriate range, based on an assumption that as an electric current increases, the voltage at the end of a distribution line decreases, by using the voltage measured by the voltage control device and electric current information. The transformer-type voltage control device generally needs to suppress the change of a tap position to 30 taps or less per day on average, in order to prevent wear of the device.
The LDC control is based on the assumption that load distribution of the power distribution system is uniform, that is, the voltage at each point of the power distribution system changes in the same direction with passage of time. However, in recent years, there is a tendency that the load distribution of the power distribution system largely varies non-uniformly with passage of time, due to diversification of how electricity is used and popularization of a dispersed power system by means of photovoltaic power generation and the like. Therefore, it is difficult to estimate voltage conditions of the entire power distribution system only based on the voltage measured by the voltage control device and the current information, and maintenance at an appropriate voltage has become an issue.
Therefore, such a mechanism has been proposed that measurement information of the voltage and current at various points of the power distribution system is centralized into a so-called “central apparatus (centralized voltage controller)” via a communication network and integrated, and a target voltage is instructed from the central apparatus (the centralized voltage controller) to each of the voltage controllers.
Furthermore, in order to cope with a rapid fluctuation of the voltage associated with changes of a photovoltaic power generation amount due to the movement of clouds, application of a reactive-power-adjusting-type voltage control device such as an SVC (Static Var Compensator) or a power conditioner for photovoltaic power generation (hereinafter, “PCS (Power Conditioning System)”) or the like to the power distribution system has been studied. Regarding the reactive-power-adjusting-type voltage control device, when a capacity (VA) is increased, the cost and the installation space also increase. Therefore, in the power distribution system, a single device is not suitable to cope with large voltage fluctuations, and thus its basic use is to absorb voltage fluctuations in a matter of seconds.
However, even with a small capacity, dealing with large voltage fluctuations in an order of time, for example in a minute or more, has been expected, by cooperatively operating a plurality of reactive-power-adjusting-type voltage control devices by the central apparatus (the centralized voltage controller). For example, if a PCS is essential to photovoltaic power generation, it can be expected that an additional measure against the voltage problem, such as installation of a separate SVC, is made unnecessary by utilizing such cooperative control with respect to the plural power conditioning systems.
In this manner, in a state where a plurality of voltage control devices are installed in one power distribution line, it has been expected to apply a mechanism, in which the central apparatus (the centralized voltage controller) ascertains the voltage conditions of the entire power distribution system and issues an appropriate command to each of the voltage controllers, to the power distribution system in order to realize the cooperative operation between the voltage control devices.
However, the central apparatus (the centralized voltage controller) needs to regularly collect the voltage and current information at each point of the power distribution system, and the amount of information is very large. Therefore, in order to cope with a case in which the voltage fluctuates largely in several tens of seconds to several minutes, a high-speed communication network such as an optical network is required. A high-speed server and the like are also required for the central apparatus (the centralized voltage controller). Further, it is required to ensure, operate, and maintain an installation space of the central apparatus (the centralized voltage controller) and change facility data in accordance with the change of devices such as the voltage control device. At the time of introduction thereof, a scale merit is required, and for example, the system needs to be installed for each prefecture.
On the other hand, there are not many power distribution systems requiring centralized voltage control actually using the central apparatus (the centralized voltage controller) at present. It is anticipated that the number of power distribution systems requiring the centralized voltage control will increase considerably within the next 20 years. However, the rate thereof in the entire power distribution system is expected to remain a small part thereof.
Therefore, a voltage control method that can start from a small scale and can be used even in a large scale, without using a central apparatus (the centralized voltage controller) and a high-speed communication network, and has also small operation maintenance cost has been desired. As a method thereof, an autonomous and cooperative power-distribution-system voltage controller that realizes a cooperative operation among the voltage controllers by performing communication among a plurality of the voltage controllers with a small amount of information can be considered.