This article presents the concept of self-protective inverters using steady-state and dynamic reference models. The self-protection strategy inspects incoming setpoints from the utility operator or third-party aggregators using analytical reference models before engaging the setpoints to the inverter’s local controller. When a malicious setpoint passes the existing security layers, a smart inverter can examine the integrity of an incoming setpoint in real-time. The efficacy of the self-protection strategy has been tested using a laboratory setup, including a three-phase 3 kVA inverter and a 12 kW regenerative grid emulator. The results verify that the analytical steady-state and dynamic models can provide device-level protection for grid-interactive inverters by preventing harmful setpoints from getting engaged to the local controller.

This article compares two strategies for seamless (re)connection of grid-forming inverters to a microgrid powered by droop-controlled inverters. While an incoming inverter must be synced to the microgrid, seamless syncing and power-sharing are technical challenges for grid-forming inverters. In the first strategy, called the output-sync method, an incoming inverter is synced to the microgrid, and then the circuit breaker is closed for power-sharing. In the second strategy, called the controller-sync method, the inverter initially contributes at zero power following the microgrid frequency, and then the controller is transferred to the mode of power-sharing. Remarkably, the circuit breaker can be kept closed during the syncing process using the controller-sync method as the inverter starts from zero power contribution. In this novel strategy, two controller sets run in parallel, i.e., two parallel control paths are in place for obtaining the magnitude and the phase angle of the PWM reference signal. These parallel paths are synced for seamless transitions when only one of them is engaged to generate the PWM reference signal. The efficacy of these control strategies has been tested in a hardware setup of a microgrid fed by two 5kVA 208V droopcontrolled inverters, and the results are presented in this article.

The full-order state-space model is directly extracted from the pulse width modulation switching pattern and enables the stability analysis of concurrent variations in the three-phase circuit and control parameters. The system stability is a function of both the grid R/X ratio and grid inductance. Despite the grid-side inductor of the LCL filter is in series with the grid impedance, they have different impacts on the stability of a grid-tied PQ-controlled VSI, i.e., an increase in the filter inductance may improve the system stability in a weak grid. These findings are verified through simulated and experimentally obtained data.

The dynamic performance under parameter mismatch of a variable-frequency ac-dc converter is analyzed. A Lyapunov-based adaptive parameter estimation algorithm is developed, and included in the control scheme to address the performance degradation that occurs from the parameter mismatch between the hardware and the controller due to the variable-frequency and ambient operating conditions. Because of this adaptive parameter estimation technique, the proposed controller can effectively regulate the output dc-bus voltage and maintain a unity power factor. The validity and performance effectiveness of the proposed controller are verified experimentally through a laboratory-scaled three-phase 1.5 kW 270 V SiC-based two-level voltage-source-converter using a variable-frequency programmable grid emulator. The results demonstrate that by including the developed adaptive parameter estimation algorithm in the control scheme, the performance of the ac-dc converter can be improved under rapid input frequency changes and filter parameter variations. 

The dynamic performance under parameter mismatch of a variable-frequency ac-dc converter is analyzed. A Lyapunov-based adaptive parameter estimation algorithm is developed, and included in the control scheme to address the performance degradation that occurs from the parameter mismatch between the hardware and the controller due to the variable-frequency and ambient operating conditions. Because of this adaptive parameter estimation technique, the proposed controller can effectively regulate the output dc-bus voltage and maintain a unity power factor. The validity and performance effectiveness of the proposed controller are verified experimentally through a laboratory-scaled three-phase 1.5 kW 270 V SiC-based two-level voltage-source-converter using a variable-frequency programmable grid emulator. The results demonstrate that by including the developed adaptive parameter estimation algorithm in the control scheme, the performance of the ac-dc converter can be improved under rapid input frequency changes and filter parameter variations. 

In motor-drive systems, which generally consist of an ac–ac converter, a cable, and an ac electric motor, fast semiconductor devices (e.g., IGBTs, SiC MOSFETs, and recently GaN Transistors) are desirable in order to reduce switching losses and consequently obtain higher efficiency in a compact package. Fast switching means shorter voltage rise time, which can lead to the reflected wave phenomenon as well as high-frequency leakage currents through the system’s stray capacitors. Reflected waves cause voltage spikes at the motor terminals, and the high-frequency leakage currents cause electromagnetic interference (EMI). These two phenomena are commonly modeled as two decoupled single-line circuits, namely, differential mode (DM) and common-mode (CM). Herein, low-to-high frequency model is presented, where both differential and common mode phenomena can be studied. For more information, one may check:

• B. Mirafzal, G. Skibinski, and R. Tallam, “Determination of parameters in the universal induction motor model,” IEEE Transactions on Industry Applications, vol.45, no. 1, pp. 142 - 151, January/February 2009.

• B. Mirafzal, G. Skibinski, R. Tallam, D. Schlegel, and R. Lukaszewski, “Universal induction motor model with low-to-high frequency response characteristics,” IEEE Transactions on Industry Applications, vol.43, no. 5, pp. 1233 - 1246, September/October 2007. (IEEE Transactions IEEE IAS Society Paper Award)

• B. Mirafzal, G. Skibinski, and R. Tallam, “A failure mode for PWM inverter-fed AC motors due to the antiresonance phenomenon,” IEEE Transactions on Industry Applications, vol. 45, no. 5, pp. 1697-1705, September/October 2009.