An auxiliary voltage controller is proposed for the fault ride-through (FRT) control of voltage-controlled inverters used in distributed energy resources. The proposed auxiliary controller can be added to different existing voltage control strategies with minimum or no need to alter the original structure of the voltage controller. The proposed controller prevents the transient overcurrent during grid faults and controls the exchange of real and reactive powers according to the governing grid standard. A fault detection algorithm is also implemented to detect the fault instant and determine the interval of fault during which the real and reactive power references are adjusted accordingly. Complete stability analysis of the proposed controller is performed through a new model. Successful performance of the proposed controller in achieving FRT requirements of German and Danish codes are illustrated by simulations. Extensive real-time and experimental results are also presented and discussed.

Virtual Synchronous Machine (VSM) approach is proposed to integrate inverter-based distributed energy resource (DER) units into the power and energy system. VSMs emulate the model of a synchronous machine (SM) in the inverter's controller. Among different aspects, emulating the damper windings or devising an alternative damping strategy is critical. The damper has a major stabilizing effect against system disturbances and change. An effective damping method to improve the VSM responses is proposed in this paper. It is shown that the proposed method offers two degrees of freedom which allows flexible adjustment of response damping without compromising the inertial response. Mathematical analysis, comparative simulations and experimental results are presented to prove and validate this claim.

A new inverter control approach, called enhanced virtual synchronous machine (eVSM), is proposed based on the VSM concept. Unlike existing VSM approaches, however, the eVSM does not emulate the rotating inertia based on an assumption of unlimited energy storage but it deploys the physically existing inertia of the dc-link element. The eVSM adopts an innovative way of enlarging the inertia utilization range which obviates the need for having a large dc-link element or a dedicated battery storage system, while still providing the same inertia response of an equivalent synchronous machine. Theoretical developments and numerical results presented in this paper confirm that the proposed eVSM can present a stabilizing support to the grid similar to a synchronous machine despite the small size of its dc-link element. Moreover, its transient responses can be improved beyond those of conventional synchronous machines thanks to the possibility of more flexibly adjusting damping and governor functions.

This paper presents an enhanced current controller for improving the performance of a class of single-phase grid-connected inverters operating in weak and distorted grid conditions. An inverter designed to operate at normal (strong or stiff and clean) grid conditions may not perform satisfactorily during weak and distorted grid conditions. One major reason is the interfering dynamics of the synchronization or phase-locked loop (PLL). This paper proposes an enhanced control structure for a popular class of single-phase inverters to address this problem. The proposed idea is to include the PLL state variables into the main inverter controller thereby minimizing the undesirable interactions of the PLL with the other components. A method for optimally designing the controller gains is also proposed. Compared to the conventional one, the proposed controller is shown to have a more robust performance over a substantially wider range of weak and distorted grid conditions. Extensive simulation and experimental results are presented to validate the proposed controls.

A design approach for grid connected inverter controllers for distributed and renewable energy system applications is proposed where high number of controller gains can be designed optimally in a fairly systematic way. The linear quadratic regulator (LQR) problem is first modified to accommodate sinusoidal signal tracking through a meaningful state space transformation. The cost function is modified to explicitly include the tracking error, so that its weights are designed in a transparent and systematic way. The proposed technique is applied to the well-known control structure comprising the fundamental and harmonic resonant controllers. Secondly, the control structure is rearranged to reject the distortions from both the grid voltage and the reference signal, and the proposed design technique is applied to this new structure. It is shown that the desired features of active damping for LCL filters and robust performance against system uncertainties, harmonics, and disturbances are achieved. Thirdly, the controller and the systematic design procedure are extended for inverters with LLCL filters. Proposed control structures are designed and simulated and then implemented on a digital signal processor. Results confirm features of the method and its ability to address control system challenges in high power density and efficient inverter applications.

This article proposes an improved sensorless control strategy for interior permanent magnet synchronous motor (IPMSM) using high-frequency (HF) rotating signal injection method in the stationary reference frame. Conventional HF rotating signal injection methods face the problems of long convergence time and poor stability, due to the filters used in signal demodulation process. Therefore, an improved signal demodulation algorithm based on an enhanced phase-locked loop (ePLL) is proposed in this article. This method improves the stability and dynamic response of the system by eliminating additional filters and reducing the resulting delays. This ePLL, in addition to estimating the phase angle of sinusoidal signal, which contains the rotor position inside it, estimates its amplitude. It is shown that the proposed method has a high robustness against the uncertainties of the motor inductances. The proposed sensorless control method is verified by experiments on a 1.8-kW IPMSM drive platform.