Design and Evaluation of a Photovoltaic Inverter with Grid-Tracking and Grid-Forming Controls
Rye, Rebecca Pilar
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This thesis applies the concept of a virtual-synchronous-machine- (VSM-) based control to a conventional 250-kW utility-scale photovoltaic (PV) inverter. VSM is a recently-developed control scheme which offers an alternative grid-synchronization method to the conventional grid-tracking control scheme, which is based on the dq phase-locked-loop- (PLL-) oriented vector control. Synchronous machines inherently synchronize to the grid and largely partake in the stabilization of the grid frequency during power system dynamics. The purpose of this thesis is primarily to present the design of a grid-forming control scheme based on the VSM and the derivation of the terminal dq-frame ac impedance of the small-signal model of the inverter and control scheme. This design is also compared to the design of the conventional grid-tracking control structure, both from a loop design and terminal dq-frame ac impedance standpoint. Due to the inherent lax power-balance synchronization, the grid-forming control scheme results in 1 to 2 decades' lower frequency range of negative incremental input impedance in the diagonal elements, which is a favorable condition for stability. Additionally, the stability of the grid-forming control scheme is compared to the conventional grid-tracking control using the generalized Nyquist criterion (GNC) for stability under three modes of operation of active and reactive power injection. It is found that the connection is stable for both control schemes under unity power factor and fixed reactive power modes; however, the grid-forming control is able to inject twice the amount of active power under the voltage regulation mode when compared to the grid-tracking control.
General Audience Abstract
Concerns about the current and future state of the environment has prompted government and non-profit agencies to enact regulatory legislation on fossil fuel emissions. In 2017, electricity generation comprised 28% of total U.S. greenhouse gas emissions with 68% of this generation being due to coal combustion sources. As a result, utilities have retired a number of coal power plants and have employed alternative means of power generation, specifically renewable energy sources (RES). Most RES operate as variable-frequency ac sources (wind) or dc sources (solar) and are interfaced with the power grid through ac-dc-ac or dc-ac converters, respectively, which are power-electronic devices used to control the injection of power to the grid. Conventional converters synchronize with the grid by tracking the phase of the voltage at the point of common coupling (PCC) through a phase-locked loop (PLL). While power system dynamics significantly affect the performance of a PLL, and, subsequently, inverters' operation, the initial frequency regulation during grid events is attributed to the system's inherent inertia due to the multitude of synchronous machines (SM). However, with the steady increase of RES penetration, even while retaining the number of SM units, the net inertia in the system will decrease, thus resulting in prolonged responses in frequency regulation to the aforementioned dynamics. This thesis investigates the control of variable-frequency sources as conventional synchronous machines and provides a detailed design procedure of this control structure for photovoltaic (PV) inverter applications. Additionally, the stability of the connection of the inverter to the grid is analyzed using innovative stability analysis techniques which treat the inverter and control as a black box. In this manner, the inner-workings of the inverter need not be known, especially since it is proprietary information of the manufacturer, and the operator can measure the output response of the device to some input signal. In this work, it is found that the connection between the inverter and grid is stable with this new control scheme and comparable to conventional control structures. Additionally, the control based on synchronous machine characteristics shows improved stability for voltage and frequency regulation, which is key to maintaining a stable grid.
- Masters Theses