For a more sustainable supply of electricity, consumers are required to be increasingly rely on combinations of various types of power generators. Among them, distributed generators which utilize renewable energy sources, such as solar and wind powers, are highly expected. However, introducing these distributed power supplies into microgrids can cause fluctuation in frequency and voltage of the overall system. To overcome these fluctuations, it is desired to develop a control method and to understand the dynamic characteristics of microgrids for more stable power supply.
Yuko Hirase and colleagues have proposed a control system by mathematically investigating the impact of frequency and voltage deviations. The proposed approach, called the virtual synchronous generator (VSG), let static inverters mimic the inertial forces, and enhances the penetration of more and more renewable energy into the existing power grids. Their research work is now published in journal, Applied Energy.
In their studies, the VSG is mounted on a common type of inverter and operates like a synchronous generator. The measured voltages are system voltages and currents. The pulse width modulation signals are calculated in the VSG control and then sent to the bridge inverter. The mathematical analysis results were obtained using Fourier transform method, and the results were verified via simulations and experimental tests.
The authors focused that the high degree of the swing equation, which can be a destabilizing factor in the system, is closely related to the delay elements that are derived from an actually existing mechanical device such as a governor. In the VSG, it is possible to eliminate the delay elements virtually. They also found that frequency and voltage fluctuations are caused by the power fluctuations of the loads and distributed generators connected to the system, which can lead to the damage to other equipment connected to the same microgrids.
The study by Yuko Hirase and colleagues shows that grid instability is more pronounced by introducing renewable energy into remote areas where fossil fuel transport costs are expensive. Therefore, in such situations, using the VSGs, which positively affects the frequency/voltage stability and performance, will promote the introduction of renewable energy sources in power systems and microgrids.
The synchronous generator (SG) in the blue frame and the VSG inverter in the green frame operate in parallel to supply the load (LD) in the pink frame.
It is programmed so that the power consumption of the LD deviates (±20kW, 50kW of the base load is supplied from the SG).
The variation speed of the LD power (0.8Hz) was chosen so that the system instability would occur greatly.
The orange frame shows the monitor, which does not have any control function.
The scale of the frequency meter is from 55 Hz to 65 Hz and the median is 60 Hz. During the VSG operation, the button in the upper middle of the indicator lights up in green, and goes off while stopped.
In the monitor, the yellow and blue arrows are displayed according to discharge/charge of the storage battery.
In first 10s, only SG supplies the LD. During this, SG characteristics and LD fluctuation interfere, so the system frequency fluctuates greatly. After 10 s from start, the VSG start its operation and is connected to the grid. In the middle 10 s, VSG stabilization effect works, and the frequency fluctuations is greatly suppressed. After 10 s from start, the VSG is stopped and the system frequency starts fluctuation again.
Yuko Hirase, Kensho Abe, Kazushige Sugimoto, Kenichi Sakimoto, Hassan Bevrani, and Toshifumi Ise. A novel control approach for virtual synchronous generators to suppress frequency and voltage fluctuations in microgrids. Applied Energy, Volume 210 (2018), Pages 699-710.
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