Visit to Plant

Coal Mining Area Shaped Into Solar Town (page 3)

Output lowered due to restriction by 'bank reverse power flow'

2013/11/25 13:47
Kenji Kaneko, Nikkei BP CleanTech Institute
Print Page

Output lowered due to unexpected 'bank reverse power flow issue'

Meanwhile, an unexpected issue came up in the discussions on grid interconnection with the electric power company. If connected with a 6.6kV grid, less than 2MW output can be connected and flown back to the grid under the rules imposed on grid interconnection. In light of the 6.6kV grid in the Takesaki District, SB Energy, at first, was planning a maximum reverse power flow of nearly 2MW. However, after discussions with the electric power company, it turned out that reverse power flow is only allowed up to 1.75MW. This is an issue called "bank reverse power flow."

"Bank" indicates the grid between the distribution transformer stations and the utility customers. Bank reverse power flow is a phenomenon where reverse power flow occurs in distribution transformers at distributing substations due to the overall grid voltage boosted by the integrated power flown back to the grid because of the increased deployment of solar power generation. And it could cause problems with electric power security.

An assessment by the electric power company revealed that the risk of bank reverse power flow would increase if more than 1.75MW flew back to the grid in Koyagimachi. As a result, the maximum output of the PV inverters had to be reduced to 1.75MW despite the solar panels’ maximum output of 2.6MW.

Installing solar panels with an output exceeding the PV inverters’ output capacity is called "overload," which in itself is not unusual. Should the solar panels generate power at their maximum capacity on a sunny day, the output that exceeds the PV inverters’ capacity would be wasted. However, when the panels’ output lowers in the morning and late afternoon or on a cloudy day, overload has the benefit that it could improve the utilization rate of PV inverters thanks to the panels’ output capacity, which outperforms that of the PV inverters.

Partly because a sufficient amount of land can be procured in Takesaki District, a certain degree of overload had been assumed from the beginning. Yet the gap between the outputs of panels and PV inverters grew wider than anticipated because of the output being restricted to 1.75MW due to the issue of bank reverse power flow.

The bank reverse power flow issue is moving toward resolution after the Ministry of Economy, Trade and Industry revised its order after the completion of "SoftBank Nagasaki Koyagi Solar Park." It has been confirmed that efforts such as adding a protection system to the distributing substations can make power flow back to distributing transformers. However, the additional cost to install such systems should be borne by the power producer that connects with the grid and makes generated power flow back to the grid.

About four months after completion, the plant is working smoothly with "the amount of power generation showing an upswing compared with the initial estimate," Yuki said. As the solar power plant can be easily observed from the road, not only the residents of Koyagimachi but also people from outside the region come to look at the plant.

"We are planning to plant different kinds of azaleas along the fence surrounding the solar power plant in early spring 2014," Ojima said.