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This paper comprehensively analyzes the stability of a grid-interfacing inverter with the LCL -filter in the discrete domain, where the LCL -filter, along with the controller, is modeled in a polar coordinate. System open-loop and closed-loop poles are analytically studied and expressed in the z -domain. Through the poles movement and distribution analysis, the relationship between system stability and the ratio of resonance frequency over sampling frequency is mathematically revealed and calculated as well as the system control gain limit. Moreover, this paper demonstrates that a grid-voltage feedforward regulator would significantly alter the inverter stability in a weak power system. By means of Jury stability criterion, the stability status under different filter resonance frequency is given. The selection of resonance frequency and filter parameters makes a considerable difference on system behavior. Finally, to improve the robustness against grid inductance variation, a conservative design recommendation of filter parameters and control gain is given. Through the tests on a laboratory-scale prototype, the theoretical analysis is validated by experimental results.

Three-phase floating H-bridges can be inserted between the utility grid and an induction motor to inject a voltage in series with the grid voltage. The magnitude and phase of this voltage is used to control the motor voltage under steady-state operation so as to lower the motor power losses over its entire load range. This is made possible because the three-phase H-bridge can be used to both increase and decrease the motor voltage relative to the grid voltage as required. This feature affects the performance of the overall system, more specifically, the motor power conversion efficiency is improved, lowering its operating temperature and, hence, improving reliability and lifetime expectancy. A variety of motor voltage control options exist, the controller presented is suitable for applications using loads such as fans and pumps, where variable frequency is not required. Readily available machine nameplate data are used to identify the motor output power associated with its maximum efficiency operating point when operated under rated voltage. This data are then used as the basis to control the motor voltage according to the square root of the measured motor input power. The controller and the performance of the three-phase H-bridge are described. The benefits of the variable voltage control are assessed by comparing the motor performance with the machine operated at its rated voltage. The chosen controller also results in the H-bridge dc voltages being relatively low and constant over a wide load range, hence lowering the power losses and electrical stress in the power electronics. Both experimental results and theoretical predictions are used to illustrate the performance of both the three-phase H-bridge and the motor.

The design and the experimental validation of a continuous-time model predictive control (CTMPC) for a permanent magnet synchronous motor (PMSM) drive with disturbance decoupling is discussed. The CTMPC approach uses Taylor series expansion to derive a closed-form solution to the problem of model predictive control even though the system behaviour is described by a non-linear model. This type of controller requires an exact knowledge of the system model to guarantee an accurate prediction of the system behaviour, while the PMSM is usually subjected to model uncertainties and external disturbances such as the load torque. Moreover, in the proposed approach, the predicted speed tracking error is directly used to determine the required voltage command without the need for a cascaded control scheme. As a result, the load torque is seen as unmatched disturbance which makes exact disturbance decoupling more challenging. To overcome such a problem, a non-linear disturbance observer is designed and combined with the CTMPC method to enhance the prediction accuracy under parameter variation and unknown load torque. The feasibility of the proposed approach is experimentally investigated, and good transient and steady-state performances are obtained.

Electrical grid voltage sags are a significant industrial power quality concern. According to a survey result across the US, voltage sags and short-duration power outages are responsible for 92% of power quality problems faced by industrial customers. These power interruptions often impose severe cost penalties in plant shutdowns for many industries. A series compensation scheme for an induction motor is presented with inherent voltage sag ride through capability. The system utilizes a system of three-phase floating capacitor H-bridge converters located in each phase between the utility grid and a squirrel-cage induction motor. By injecting a series voltage in each phase, the proposed system can manipulate the voltage supplied to a motor, increasing its tolerance of grid voltage sags. The voltage injection scheme has an inherently leading grid power factor under steady state and, hence, generates VARs into the grid, over a wide range of load conditions. The paper develops mathematical analysis of the proposed system to quantify both the voltage sag ride-through and the reactive power generation that results. The analysis shows that voltage sag tolerance of the proposed system is closely related to the motor fundamental power factor. Moreover, it is found that unity power factor operation of the system, as seen from the grid, is possible and that the reactive power generation capability can also be accurately quantified. A 5-hp experimental testbed is used to validate both the grid voltage ride-through capability feature and the reactive power generation characteristics.

A robust control scheme for the dual inverter drive using a floating capacitor bridge is proposed which allows decoupled control of the motor and the capacitor voltage. Indirect vector control is implemented in the rotor flux reference frame and the voltage references generated are then converted into the stator current reference frame. This allows for simpler assignment of inverter voltage references and eliminates the need for a dedicated PI controller to operate the main bridge at unity power factor. Feed forwarding of the stator current is also introduced which makes the capacitor voltage immune to any changes in the stator current. This allows the capacitor to be sized taking into consideration only the voltage ripple due to switching effects. These claims have been demonstrated both through simulation and experimentation. The effect of switching frequency and capacitance on the voltage ripple has also been studied.

Switched reluctance motor (SRM) has various advantages which makes it an excellent candidate for many applications; nevertheless, its main drawback is torque ripples. This paper aims to enhance the SRM operation by reducing its torque ripples without the need for expensive and sophisticated physical changes in the motor materials or design. The SRM converter firing angles are optimized to produce the lowest possible torque ripples. The response surface method (RSM) is used to obtain the SRM optimization function that relates torque ripples with firing angles, and grey wolf optimizer (GWO) is used to minimize this function. The objective function convergence speed using the proposed GWO is compared with genetic algorithm (GA), and it is found to be faster than GA. Simulation and experimental results show the effectiveness of the proposed approach in enhancing SRM operation by providing the SRM converter firing angles that result the minimum feasible torque ripples.

An application of H-bridges that enable soft start and power factor correction of cage induction machines is presented. The proposed power electronics employs floating capacitor H-bridges connected in series with utility grid and a cage induction motor to provide series voltage compensation. Injecting a series voltage in each phase, the proposed system can be used to control the motor voltage during starting and hence limit the motor starting current. In addition, an algorithm is proposed to accurately predict the worst case scenario for bridge dc capacitor voltage before motor starting. Thus, a dc capacitor overvoltage problem can be avoided. Experimental results show that the proposed system can successfully soft start a standard squirrel cage induction machine under different modes and load conditions. A simple representation of the worst case scenario for dc capacitor voltage before motor starting is established.

This paper presents a robust continuous nonlinear model predictive control (CNMPC) for a grid-connected photovoltaic (PV) inverter system. The objective of the proposed approach is to control the power exchange between the grid and a PV system, while achieving unity power factor operation. As the continuous nonlinear MPC cannot completely remove the steady-state error in the presence of disturbances, the nonlinear disturbance observer-based control is adopted to estimate the offset caused by parametric uncertainties and external perturbation. The stability of the closed-loop system under both nonlinear predictive control and disturbance observer is ensured by convergence of the output-tracking error to the origin. The proposed control strategy is verified using a complete laboratory-scale PV test-bed system consisting of a PV emulator, a boost converter, and a grid-tied inverter. High performance with respect to dc-link voltage tracking, grid current control, disturbance rejection, and unity power factor operation has been demonstrated.

This paper discusses the impact of small dc capacitance in a motor drive using a floating capacitor H-bridge topology. The proposed topology is intended for applications where variable frequency control is not required. Special attention is paid on investigating the second-order dc capacitor voltage ripples, whose influence on the induction motor as well as on the motor drive itself is of importance. This issue is addressed in this paper through rigorous mathematical formulations. It is found that by inverting the second-order dc ripple voltage using the conventional SVPWM, no harmful harmonics will be generated in the motor line voltage. This demonstrates the possibility of drastically reducing the dc capacitance of the proposed system. Induction motor as well as H-bridge performance with respect to different dc capacitance values is demonstrated by experiments, which lays the foundation for cost reduction and reliability enhancement of the proposed system.

This paper presents a robust continuous-time model predictive direct power control for doubly fed induction generator (DFIG). The proposed approach uses Taylor series expansion to predict the stator current in the synchronous reference frame over a finite time horizon. The predicted stator current is directly used to compute the required rotor voltage in order to minimize the difference between the actual stator currents and their references over the predictive time. However, as the proposed strategy is sensitive to parameter variations and external disturbances, a disturbance observer is embedded into the control loop to remove the steady-state error of the stator current. It turns out that the steady-state and the transient performances can be identified by simple design parameters. In this paper, the reference of the stator current is directly calculated from the desired stator active and reactive powers without encompassing the parameters of the machine itself. Hence, no extra power control loop is required in the control structure to ensure smooth operation of the DFIG. The feasibility of the proposed strategy is verified by the experimental results of the grid-connected DFIG and satisfactory performances are obtained.

Floating capacitor voltage source power electronic converters can be used as series voltage compensators for grid supplied induction motors to improve their steady-state operating performance, namely: actively reducing the motor power losses over its entire load range; lower the motor's operating temperature to improve lifetime expectancy; the voltage supplied to the motor can be used to avoid derating the motor power rating. These features are especially useful when the motor is connected to a grid whose nominal voltage differs from the machine's rated value or that may fluctuate over time (sag or swell). By injecting a voltage in series with the grid supply, floating capacitor converters can set the motor voltage at a fixed desired value, above or below the grid voltage, under transient or continuous steady-state conditions, and over the entire range of the motor load. These features allow the losses in the motor and power electronics to be controlled, e.g. minimized, hence improve the motor and system efficiencies. For applications where frequency control is not required, the proposed power electronics is more energy efficient; hence a viable cost effective solution. Experimental tests are used to compare the proposed system with several alternatives, using performances such as: power losses and efficiency of the motor, power electronics and the system as a whole.

For the grid-connected voltage source inverters, the feedforward scheme of grid voltage is commonly adopted to mitigate the current distortion caused by grid background voltages harmonics. This paper investigates the grid-voltage-feedforward active damping for grid connected inverter with LCL filter. It reveals that proportional feedforward control can not only fulfill the mitigation of grid disturbance, but also offer damping effects on the LCL filter resonance. Digital delays are intrinsic to digital controlled inverters; with these delays, the feedforward control can be equivalent to a damping resistor and a reactance paralleled with filter capacitor. The damping performance in different frequency regions are discussed through Bode diagrams. Compared to other widely used active damping strategies, no extra sensor is needed because the Point of Common Coupling (PCC) voltage is sampled for Phase Locked Loop (PLL). Simulation and experiment results are provided for verifying the theoretical analyses.

The proliferation of semiconductor-based non-linear devices in power systems has caused serious concern over power quality problems. This paper proposes a control strategy to coordinate multiple adjustable speed drives (ASDs) so that harmonic currents as well as reactive power generated by non-linear loads can be compensated. Under the proposed control strategy, the power quality improvement task is distributed to multiple adjustable speed drives and their contribution to the task is determined proportionally to their instantaneous power margins. The proposed control strategy can be a fairly flexible and economical solution for power quality improvement especially for the places where multiple machine drives are already-installed or required, for example, manufacturing factories or shipboard or aircraft power systems, because the adjustable speed drives can be fully utilized to improve power quality without additional compensators as well as conditioning end-use machines. Detailed simulation results using switching-level converter models verify the performance of the proposed coordination strategy.

As a powerful optimization algorithm, particle swarm optimization (PSO) has been widely applied to power system researches. However, most existing applications of PSO can only be implemented offline. The difficulties of online implementation mainly come from the unavoidable lengthy simulation time to evaluate a candidate solution. Recently, PSO was implemented online that can identify parameters in a motor control systems. In this paper, the real-time PSO (RT-PSO) based identification technique is applied to cancel current harmonics in power systems. By transforming the identification problem to optimization problem, RT-PSO can simultaneously identify four parameters associated with fundamental current from measurement. In this way, there is no need to identify the fundamental frequency separately or construct fundamental signal from identified harmonic information. The identification algorithm can be applied to three-phases independently, even for unbalanced system or single-phase system. The identified fundamental signal is then used as the reference for current harmonics cancellation. The RT-PSO based harmonic cancellation is realized with an active filter and used to compensate harmonic current created by a nonlinear load. Simulation results demonstrate that the RT-PSO algorithm can provide accurate identification of the fundamental current which in turn will result in good harmonic cancellation performance. As a capable online optimization technique, RT-PSO can be extensively applied to many optimization and control problems.

A new algorithm is proposed to measure the harmonic distortion without calculating individual harmonic components. This feature can greatly reduce the computational burden on many harmonic detection methods when it comes to power quality evaluation. In addition, the proposed algorithm can also facilitate power quality compensation such as shunt active filter control or active front-end coordination. Therefore, the proposed harmonic distortion measurement algorithm can provide a flexible and economical solution for many power quality problems. Simulation results verify the performance of the proposed algorithm.

A decentralized control strategy is proposed to coordinate multiple shunt active power filters (APFs) to improve power quality at the point of common coupling (PCC) in terms of power factor correction and harmonic elimination. Under the proposed control strategy, each APF uses locally measured current signals to decide the amount of power quality improvement task, which is limited to its power rating. In this way, the overall power quality improvement task is automatically distributed to the APFs without control communications. Simulation results verify the performance of the proposed control strategy.

Microgrids consist of multiple distributed generators, which are normally integrated via power inverters and can employ various sustainable energy resources. Therefore, the microgrid is a new promising concept for future power systems with improved energy security, reliability, quality of power, and energy efficiency. To this end, microgrids should survive during severe power system abnormalities so that the sub-areas in the microgrids can be protected all the time. This paper focuses on a robust controller design scheme for inverter-interfaced distributed generators considering various operating modes and loading conditions of microgrids. Especially, during the island mode, the voltage and system frequency can easily change due to instant power mismatch between distributed generators and loads in the microgrid. This paper presents the design procedure of a robust optimal controller against exogenous disturbances such as voltage and frequency variation in microgrids. L 1 robust control theory is applied to optimize the robust controller and a Particle Swarm Optimization algorithm is used to solve the optimal problems.

As a powerful optimization algorithm, Particle Swarm Optimization (PSO) has been widely applied to power systems researches. However, because of the unavoidable evaluation time for tentative solutions, most of these applications have been implemented offline. Recently, PSO was realized in real time to identify parameters of a PMSM control systems. In this paper, the Real-Time PSO (RT-PSO) algorithm was applied to current harmonic cancellation problem. RT-PSO is used to identify the fundamental current from a measured current that contains harmonics. The identified fundamental current is then used as a reference current for current harmonics cancellation. Simulation studies show that the RT-PSO algorithm can provide accurate estimation of the reference current for harmonics cancellation control using active power filters. As a capable online optimization algorithm, RT-PSO can be extensively applied to many optimization and control problems.

A new control scheme is proposed to utilize the active front-end of a high performance adjustable speed drives (ASD) to solve power quality problems caused by conventional diode/thyristor drives. Under the proposed control scheme, a fully controlled active front-end ASD acts as a local power quality conditioner in order to compensate harmonic currents as well as reactive power generated by other neighboring drives. In this way, power quality problems caused by multiple motor drives can be solved locally. The proposed solution can fully utilize the capability of the active front-end of ASDs and does not require vast replacement of conventional diode/thyristor ASD sets. Therefore, it can provide flexible and economical solutions for power quality improvement compared to other custom power devices. Simulation results verify the performance of the proposed active front-end configuration and control schemes.

This paper discusses the whole design process of a position control system of a single-phase reluctance machine under mechanical and electrical constraints. A MIMO Neural network is used to model the nonlinear properties of the machine. Based on it, a neural network based control scheme is developed to precisely control the rotor position of the designed single-phase reluctance machine. Simulation results show that a MIMO neural network model can effectively capture the nonlinear characteristics of the designed machine and the proposed neural network control scheme can control the rotor position precisely.

To improve energy flexibility and deal with peak energy demand in shipboard power system, a bi-directional dc/dc converter is investigated for a notional U.S. Navy Medium Voltage DC (MVDC) shipboard power system. Surplus energy due to light electric load or ship-speed variation can be captured by energy storages distributed in 800V load zones and during heavy load or black starting condition, supplied to the rest of the 5kV MVDC system through the bi-directional dc/dc converters. This paper presents the controller optimization process using the Particle Swarm Optimization for an isolated-type bi-directional dc-dc converter. The control performance of the proposed controller is evaluated using small-signal average models and a large-scale simulation of the notional U.S. Navy MVDC system using the Real-Time Digital Simulator.

To address power quality problems in industrial power systems, a new control scheme for three-phase active power filter is proposed. The control scheme includes two functional modules such as a current harmonic cancellation module and a reactive power compensation module. The current harmonic cancellation module is based on a multiple adaptive feed-forward cancellation algorithm for selective current harmonic identification. The reactive power compensation module utilizes the measured voltage and the estimated fundamental current to calculate reactive power. After the above information has been calculated, the corrective current reference signals can be generated for the active filter. In this way, the two control functions are integrated together so that one active filter can accomplish two functions simultaneously. Simulation results under different operating conditions demonstrate that the proposed control scheme can successfully cancel harmonic current and compensate reactive power at the point of common coupling.