Transmission grid strength analysis and operation of inverter-based microgrids

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Renewable energy resources interfaced using inverters contribute insignificant inertia and short-circuit capacity to the power system. As a result, frequency response and voltage regulation may be adversely affected. For this reason, this work focuses on evaluating transmission system strength and operation of microgrids with inverter-interfaced resources. The strength of the transmission grid with large wind farms is investigated based on the voltage phase angle stability. A novel approach for evaluating the strength of low-inertia systems that only relies on phasor measurement unit (PMU) data is proposed and implemented to the Panhandle region of the ERCOT system. The proposed metric evaluates the system strength by calculating the sensitivity of the voltage phase angle to the change of active power flow without using network parameters. The advantages of the proposed method are demonstrated using PMU field data collected from the ERCOT system. This work also focuses on microgrids with inverters. Detailed electromagnetic models of grid-forming and grid-following inverters are discussed, and their equivalent models are developed to improve computation efficiency for steady-state analysis. The equivalent circuit model is developed based on the voltage and current control loops with a current limiter. The current limiting algorithm for the steady-state solver is developed and incorporated into the short-circuit study process. Moreover, voltage imbalance mitigation control of grid-forming inverters supplying unbalanced loads is analyzed. The performance of a grid-forming inverter with voltage balancing capability is demonstrated by blackstarting an islanded microgrid with an unbalanced load and an induction motor. Simulation results showed that the inverter can maintain balanced voltage at nominal value and limit the large unbalance current required to start the motor load. However, the magnitude of the negative-sequence current an inverter can supply is limited by its relatively low rated current. Therefore, we investigate the range of negative-sequence current the inverter can supply and derive formulas to determine the minimum capacity required to supply unbalanced loads interfaced with a delta-wye grounded transformer. Finally, we analyze the power-sharing control of grid-forming inverters in unbalanced circuits. Even though conventional droop controls, designed to share active and reactive powers, are well-developed, they cannot contribute to assigning negative-sequence currents to each inverter. This work presents an adaptive negative-sequence virtual impedance control which allows inverters to inject negative-sequence current proportionally to their capacities. Simulation results show that inverters with the proposed method regulate both positive- and negative-sequence currents as desired.


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