Characterization of Microgrid Domain of Stability with Various Approaches of Power Sharing

M.Sc. Student: A. Aderibole | Advisors: Jimmy C. -H. Peng, and Hatem Zeineldin, Khalifa University, UAE | Project Duration: 2016 -2017

Microgrids are local networks consisting of generators and loads that are capable of operating in-parallel with, or independently from, the main electric grid. A microgrid mainly consists of low-inertia power electronics (i.e. renewable resoruces and energy storage) that have faster time responses than conventional synchronous generators. This makes it more susceptible to disturbances and variations in system parameters. As a result, control and stability are important considerations in the reliable operation of a microgrid. To this end, we analyzed the impact of equal and unequal active power sharing conditions on the dynamic stability of a hybrid microgrid (Figure 1) consisting of a diesel generator and renewable resources. Such network exhibits similar technical characteristics to the Ross Island microgrid project in Antarctica. The renewable resources are interfaced to the microgrid by a power electronics inverter. Note the isolated switch enables the microgrid to operate in grid-connected and islanded modes. 

 

 

Figure 1. A hybrid microgrid consisting of a diesel generator and dispatchable DC source (i.e. renewable resources like energy storage and wind farms). 

Power oscillations are inherent power sharing dynamics between interconnected synchronous machines and power electronics. When operating in isolation from the utility grid, they can destablise the microgrid if not properly damped. In this context, the low-frequency oscillations are noticeable in a hybrid microgrid due to interactions between the slower dynamics of the diesel generator and the faster response of the inverters. Here, the sensitivity of low-frequency eigenvalues to the changes in active power droop gains is studied. Subsequently, a “domain of stability” region is proposed to assess the stability of a microgrid under different power sharing conditions. Simulated results demonstrated that careful tuning of the droop controller in the inverter ensures the stability of the microgrid (Figure 2). 

 

This research concludes that equal active power sharing in a microgrid might not offer the best performance from a small-signal stability perspective. The next step is to provide theoretical derivations to describe the stability of multi-microgrids in an islanded distribution system. In particular, identifying solutions to compensate the poorly damped oscillations within the network.

Figure 2. Domain of stability of a hybrid microgrid under different droop controller settings. The horizontal axis refers to the droop gain of the diesel generator, while the vertical axis represents the droop gain of the inverter.

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