Keeping the grid stable is often synonymous with keeping frequency within a narrow band. Deviations manifest as changes in the voltage and microgrids entirely powered by distributed solar generators are more sensitive to the issue than utility grids fed by a multitude of power sources.
Keeping the grid stable is often synonymous with keeping frequency within a narrow band. Deviations manifest as changes in the voltage and microgrids entirely powered by distributed solar generators are more sensitive to the issue than utility grids fed by a multitude of power sources.
Researchers Han Min Htut and Wijarn Wangdee, of King Mongkut’s University of Technology North Bangkok, in Thailand, have tackled the issue and proposed a new inverter control strategy. Their findings were published in Engineering Journal as the article Virtual Oscillator Control of Multiple Solar PV Inverters for Microgrid Applications.
A gradual increase in power electronics-interfaced generation methods on the grid has led to a paradigm shift in how grids respond to disturbances. A microgrid powered by very high penetration of small scale solar will have to face the challenge of stable operation, as all those maximum power point trackers (MPPTs) ensure attaining such an outcome is complicated. In the set-up the Thai-based scholars envisioned for their tests, the PV generation sources were connected to the grid with DC-DC boosters. Shading or other changes in irradiation can alter the input voltage for the DC-DC booster, subsequently changing output voltage.
Virtual oscillator control
The academics suggested use of a modified virtual oscillator control (VOC) and a cascaded sliding mode control (SMC) would help optimize microgrid management strategies. When PV output power is higher than the combined loads in the grid, inverters will not use their maximum power point trackers. However, they will switch back to using the algorithm when the power supply dips below demand. The control strategy enables stable operation of 100% solar microgrids even in islanding mode, without requiring energy storage to stabilize voltage frequency.
To achieve that, the Thai group proposed a hybrid controller with a switch between a ‘fast’ MPPT and a slower one for microgrid-integrated solar. In their setup, the power electronics feature a single controller regulating DC-link voltage and MPPT autonomously, without any need for system reconfiguration. Effectively, the two-stage converters can decrease DC-link voltage if the PV capacity cannot meet its droop control command – in which algorithms consider active power frequency and manage the active power output as a function of frequency deviation. Droop control lifts the nadir, the point of widest frequency deviation, and improves the recovery process for grid frequency when large loads are connected.
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