Abstract: A cooperative distributed secondary/primary control paradigm for AC microgrids is proposed. This solution replaces the centralized secondary control and the primary-level droop mechanism of each inverter with three separate regulators: voltage, reactive power, and active power regulators. A sparse communication network is spanned across the microgrid to facilitate limited data exchange among inverter controllers. Each controller processes its local and neighbors' information to update its voltage magnitude and frequency (or, equivalently, phase angle) set points. A voltage estimator finds the average voltage across the microgrid, which is then compared to the rated voltage to produce the first-voltage correction term. The reactive power regulator at each inverter compares its normalized reactive power with those of its neighbors, and the difference is fed to a subsequent PI controller that generates the second-voltage correction term. The controller adds the voltage correction terms to the microgrid rated voltage (provided by the tertiary control) to generate the local voltage magnitude set point. The voltage regulators collectively adjust the average voltage of the microgrid at the rated voltage. The voltage regulators allow different set points for different bus voltages and, thus, account for the line impedance effects. Moreover, the reactive power regulators adjust the voltage to achieve proportional reactive load sharing. The third module, the active power regulator, compares the local normalized active power of each inverter with its neighbors' and uses the difference to update the frequency and, accordingly, the phase angle of that inverter. The global dynamic model of the microgrid, including distribution grid, regulator modules, and the communication network, is derived, and controller design guidelines are provided. Steady-state performance analysis shows that the proposed controller can accurately handle the global voltage regulation and proportional load sharing. An AC microgrid prototype is set up, where the controller performance, plug-and-play capability, and resiliency to the failure in the communication links are successfully verified.

Abstract: Voltage problems are challenging in modern power systems with a high penetration of renewables integrated via power electronics. This paper extends the transient time-scale classification to identify voltage dynamics in modern power systems. Voltage dynamics in the current control time-scale is firstly proposed and then the mechanism of terminal voltage change is elaborated. After that the significant influences of the voltage source converter (VSC), current control (CC) loop and voltage feed forward (VFF) on the voltage dynamics in the current control time-scale are discussed. The VSC current control loop provides positive damping on the terminal voltage, while the VFF scheme results in an additional loop that deteriorates terminal voltage dynamic performance and stability. In addition, a sensitivity analysis was carried out to investigate the influence of critical parameters on stability. Finally, simulation results of a current-controlled VSC attached to different strength of AC grids (including a weak grid) are presented to validate the phenomenon and the influencing factors of voltage dynamics in the current control time-scale.

Abstract: With the wide application of voltage source converters (VSCs) in power system, dc-bus voltage control instabilities increasingly occurred in practical conditions, especially in weak ac grid, which poses challenges on stability and security of power converters applications. This paper aims to give physical insights into the stability of dc-bus voltage control affected by ac-bus voltage control in VSC connected to weak grid. The concepts of damping and restoring components are developed for dc-bus voltage to describe the stability of dc-bus voltage control. The impact of ac-bus voltage control on dc-bus voltage control stability can be revealed by investigating the impact of ac-bus voltage control on damping and restoring components essentially. Furthermore, the detailed analysis for the impact of ac-bus voltage control on damping and restoring components is presented considering varied ac system strengths, operating points, and ac-bus voltage control parameters. The simulation results from 1.5-MW full-capacity wind power generation system are demonstrated which conform well to the analysis. Finally, the experimental results validate the analysis.

Abstract: This paper presents a closed-loop space vector modulation (SVM)-based sliding mode controller (SMC) for a three-level grid-connected neutral point clamped (3L-NPC) inverter. The nonlinear SMC based on Gao's reaching law has been designed to control the grid current and inject desired amount of active and reactive power into the network. Due to using single dc source at the NPC inverter dc bus, neutral point voltage is controlled through redundant switching states and instantaneous dc voltage feedback integrated into SVM technique. Meanwhile, there is no external voltage controller involved, thus no associated fine tuning issues are existed. The performance of the proposed hybrid controller to inject a desired active/reactive power to the grid is investigated through external perturbations such as change in the line current amplitude/phase shift, ac voltage fluctuation, as well as dc voltage variation. Full converter state-space model was developed and simulated. Experimental results are provided to verify the fast dynamic performance, low content of line current THD%, and good voltage balancing of dc bus capacitors of the NPC inverter.

Abstract: In conventional control systems for a static synchronous compensator (STATCOM) based on a cascaded H-bridge multilevel converter, the action of individual voltage controllers that balance capacitor voltages can alter the output of a cluster voltage controller and a current controller due to a coupling effect. In this paper, first, the conditions that eliminate this coupling effect are derived, and then, a control system that enforces the derived decoupling conditions is proposed. It is also shown that an additional advantage of the proposed decoupled control system is that it enables the linearization of a cluster voltage control loop. Experimental studies on a single-phase seven-level STATCOM demonstrate the effectiveness of the proposed decoupled control system in improving the transient performance of the STATCOM.

Abstract: In this paper, a new cascaded nonlinear controller has been designed and implemented on the packed U-cell (PUC) seven-level inverter. The proposed controller has been designed based on a simplified model of PUC inverter and consists of a voltage controller as an outer loop and a current controller as an inner loop. The outer loop regulates the PUC inverter capacitor voltage as the second dc bus. The inner loop is in charge of controlling the flowing current, which is also used to charge and discharge that capacitor. The main goal of the whole system is to keep the dc capacitor voltage at a certain level results in generating a smooth and quasi-sine-wave seven-level voltage waveform at the output of the inverter with low switching frequency. The proposed controller performance is verified through experimental tests. Practical results prove the good dynamic performance of the controller in fixing the PUC capacitor voltage for various and variable load conditions and yet generating low-harmonic seven-level voltage waveform to deliver power to the loads. Operation as an uninterruptible power supply (UPS) or ac loads interface for photovoltaic energy conversion applications is targeted.

Abstract: Arm voltage and submodule (SM) capacitor voltage balancing is a key factor for the safe and reliable operation of modular multilevel converters (MMCs). The arm voltage balancing is achieved through a zero-sequence voltage controller in carrier pulse-width modulation (CPWM). In this study, a dual space-vector pulse-width modulation (SVPWM) technique is proposed for an MMC, which eliminates the external controller for arm voltage balancing. In this approach, the three-phase top and bottom arms are independently controlled using SVPWM. In addition, the capacitor voltage balancing can be achieved using redundant switching vectors. However, this will increase the computational load on the space-vector modulator. Therefore, an external capacitor voltage-balancing approach is proposed to minimize the computational complexity. The proposed approach uses the direction of load current instead of the arm current in SM selection process. As such, the required number of current sensors is reduced to 50% in a three-phase system. The proposed modulation and voltage-balancing approach are simulated and experimentally verified on the MMC system with three-level flying capacitor (3L-FC) SMs. Simulation and experimental results show the successful balancing of the arm voltage and SM capacitors voltage.

Abstract: In this study, a fast terminal sliding-mode control scheme is proposed as a new approach for the voltage tracking control of the DC-DC boost converter affected by disturbances, such as the variations in the input voltage and the load resistance. Some experiments are performed on a test bench to show the effectiveness of the proposed approach. The fast reference tracking capability with small overshoot and robustness to the disturbances of the designed controller is verified by the experimental results. Moreover, the results demonstrate that the proposed controller has a better performance related to conventional sliding mode and proportional-integral controllers in terms of the settling time and robustness to the disturbances.

Abstract: The influence of state feedback coupling in the dynamics performance of power converters for stand-alone microgrids is investigated. Computation and pulsewidth-modulated (PWM) delays are the main factors that limit the achievable bandwidth of current regulators in digital implementations. In particular, the performance of state feedback decoupling is degraded because of these delays. Two decoupling techniques to improve the transient response of the system are investigated, named nonideal and ideal capacitor voltage decoupling, respectively. In particular, the latter solution consists in leading the capacitor voltage on the state feedback decoupling path in order to compensate for system delays. Practical implementation issues are discussed with reference to both the decoupling techniques. A design methodology for the voltage loop that considers the closed-loop transfer functions developed for the inner loop is also provided. A proportional resonant voltage controller is designed according to Nyquist criterion taking into account application requirements. For this purpose, a mathematical expression based on root locus analysis is proposed to find the minimum value of the fundamental resonant gain. Experimental tests performed in accordance to UPS standards verify the theoretical analysis.

Abstract: This paper presents an autonomous wind farm voltage controller based on model predictive control. The reactive power compensation and voltage regulation devices of the wind farm include static Var compensators, static Var generators, wind turbine generators and on-load tap changing transformer, and they are coordinated to keep the voltages of all the buses within the feasible range. Moreover, the reactive power distribution is optimized throughout the wind farm in order to maximize the dynamic reactive power reserve. The sensitivity coefficients are calculated based on an analytical method to improve the computation efficiency and overcome the convergence problem. Two control modes are designed for both voltage violated and normal operation conditions. A wind farm with 20 wind turbines was used to conduct case studies to verify the proposed coordinated voltage control scheme under both normal and disturbance conditions.

Abstract: In this paper, a simple and efficient rule based energy management system for battery and supercapacitor hybrid energy storage system HESS used in electric vehicles is presented. The objective of the proposed energy management system is to focus on exploiting the supercapacitor characteristics and on increasing the battery lifetime and system efficiency. The role of the energy management system is to yield battery reference current, which is subsequently used by the controller of the DC/DC converter. First, a current controller is designed to realize load current distribution between battery and supercapacitor. Then a voltage controller is designed to ensure the supercapacitor SOC to fluctuate within a preset reasonable variation range. Finally, a commercial experimental platform is developed to verify the proposed control strategy. In addition, the energy efficiency and the cost analysis of the hybrid system are carried out based on the experimental results to explore the most cost-effective tradeoff.

Abstract: This paper investigates the voltage distribution in form wound windings of large electrical machines due to fast transient voltage steps, usually produced by (multi-level) voltage source inverters. Measurements are conducted on a stator core of a large prototype machine with voltage step rise times between 0.1 and 10 The measurement results of the voltage distribution in one phase string of the stator winding are analyzed and compared to previous simulation results of an equivalent π-section lumped circuit model based on parameters determined by finite element method (FEM) calculations. Good agreement is found for the oscillating overvoltages between the coils of the winding due to the fast rising transients. Furthermore, it is shown that the effect of a non-equally distributed voltage in the winding is diminishing in the investigated machine winding for voltage step rise times longer than 10 μs.

Abstract: Linear small-signal models of a dc–dc converter often ignore switching dynamics; thus such models are insufficient to fully explore the performance objective under large signal transients. Linear/nonlinear hybrid controllers are promising alternatives; however, they require structurally different hardware resources along with extra antiwindup arrangements. This paper proposes a geometric tuning method in a digitally current-mode-controlled buck converter under continuous-conduction mode. This considers a proportional-integral voltage controller along with a load current feedforward in the digital domain, while the inductor current has a traditional analog implementation. This resembles a first-order switching surface with near load-invariant regulation; thus a (fixed) small integral gain is sufficient to minimize the steady-state error. Using phase-plane geometry, the objective is to tune the controller gain in a way to achieve proximate time optimal recovery using a fixed-frequency pulse-width modulator and also to retain the large-signal stability. The effects of parameter variation and finite sampling are analyzed. The proposed tuning is implemented using an FPGA device.

Abstract: Although many high-power voltage source converters are of three-phase three-wire type, there may also be other types of configuration with which tripplen harmonics in the ac voltage and/or current are of great concern. This paper analyzes the harmonic characteristic of the output voltage of modular multilevel converter (MMC) in cases where the third-order harmonic voltage and current are inevitable, such as in three-phase four-wire configurations. The interaction among the capacitor voltage, arm current, and the modulating signal will generate multiple harmonics in the output voltage, in which the third-order harmonic is the most dominant. The third-order harmonic in the MMC output voltage will cause excessive amount of third-order harmonic in the output current in grid-connected applications with three-phase four-wire configuration. Analytical expression for the output voltage is derived for open-loop case, considering both the fundamental-frequency reference signal and the circulating current-control signal in the modulating signals. A feedforward compensation (FFC) method is proposed for suppressing the influence of the third-order harmonic in the output voltage for grid-connected applications. Simulation and experimental results verified the correctness of the analysis and effectiveness of the proposed FFC method.

Abstract: This paper presents a control scheme for single phase grid connected photovoltaic (PV) system operating under both grid connected and isolated grid mode. The control techniques include voltage and current control of grid-tie PV inverter. During grid connected mode, grid controls the amplitude and frequency of the PV inverter output voltage, and the inverter operates in a current controlled mode. The current controller for grid connected mode fulfills two requirements – namely, (i) during light load condition the excess energy generated from the PV inverter is fed to the grid and (ii) during an overload condition or in case of unfavorable atmospheric conditions the load demand is met by both PV inverter and the grid. In order to synchronize the PV inverter with the grid a dual transport delay based phase locked loop (PLL) is used. On the other hand, during isolated grid operation the PV inverter operates in voltage-controlled mode to maintain a constant amplitude and frequency of the voltage across the load. For the optimum use of the PV module, a modified P&O based maximum power point tracking (MPPT) controller is used which enables the maximum power extraction under varying irradiation and temperature conditions. The validity of the proposed system is verified through simulation as well as hardware implementation.

Abstract: Mixed-signal current-mode control (MCMC) implementation has been gaining popularity, because of the tuning flexibility using the digital voltage controller $G_{c}(z)$ along with the fast-changing analog current controller. Generally a continuous-time frequency-domain approach is adopted for the design of $G_{c}(z)$ ; however, this method often fails to capture sub-harmonic, more generally the fast-scale instability due to finite discretization effects. This paper derives approximate discrete-time models and proposes a unified framework to derive closed-form stability conditions and discrete-time small-signal models of a synchronous buck converter under MCMC. Considering the effects due to finite output-voltage sampling and the effective series resistance (ESR) of the output capacitor, the stability boundary in MCMC is found to be significantly restricted compared to its analog counterpart. Further, design methods are proposed to enhance stability boundary with improved transient performance by tuning the controller directly in the digital domain. A buck converter prototype is made, and the MCMC technique is realized using an FPGA device. Analytical predictions are verified experimentally.

Abstract: This study presents fuzzy-proportional–integral (PI)-based sensorless frequency and voltage control strategy for doubly fed induction generator connected to a dc microgrid system. A significant reduction of the costs can be achieved by this topology because only a single dc/ac converter and a diode rectifier are required instead of the traditional back-to-back converter topology. To improve the performance of the rotor current controller, a fuzzy PI-based control algorithm is used. Furthermore, a simple sensorless control technique based on the detection of the stator frequency is employed to detect the rotor position. The sensorless control technique can operate without the knowledge of the machine parameters. The main aim of this study is to keep the stator frequency as well as the dc bus voltage at the desired value under different load and wind speed conditions without using position sensor. Simulation and experimental studies were performed to verify the dynamic and steady-state performances of the proposed control strategy The results show that the proposed strategy not only has an excellent steady state and dynamic performance, but also it is robust against the variation of system parameters such as wind speed and load.

Abstract: This article presents the implementation of a new voltage controller for a single-phase two-winding self-excited induction generator that is suitable for renewable energy applications, such as bio energy and diesel engine. The proposed voltage controller regulates the terminal voltage within ±5% at varying resistive, inductive, and dynamic loads. The proposed voltage controller uses only one shunt capacitor (Csh) controlled using back-to-back connected thyristors to vary the reactive power for voltage control of the self-excited induction generator. The power loss in this shunt capacitor (Csh) is 2 W for a 5-kW self-excited induction generator, which is much less compared to its counterparts, such as the static compensator. The proposed controller is useful for remote applications for low power generation up to 5 kW. The proposed controller also regulates the system voltage for small variations in the prime mover speed due to its variable input mechanical power as well as during load perturbations.

Abstract: With increasing photovoltaic (PV) penetration in low voltage networks (LVNs), voltage regulation is a challenge. Active power curtailment (APC) is one possible solution for mitigating over voltages resulting from active power injection in LVNs. There is an inherent unfairness in the APC scheme. When generation is high and consumption is low, the voltages at the end of the feeder tend to be the highest. This results in high curtailment of active power output of the inverters located at the end of the feeder and low or even no curtailment for the inverts located closer to the transformer. A secondary voltage controller has been implemented to mitigate this unfairness in APC based voltage support schemes. The focus of this work is to quantify this unfairness and develop methods that enable residential PV owners serviced by the same feeder to participate equally in voltage regulation in the LVN.

Abstract: This paper proposes a two-level voltage controller for large-scale power systems. The aim of the controller is to maintain a near-optimal voltage profile in the transmission network by coordinating discrete reactive power (var) control devices in the system. The controller is targeted for implementation in electric power transmission utility companies where most of the voltage controls are coordinated by switching of discrete var devices such as shunt capacitor and reactor banks and transformer banks load tap changers (LTCs). By effectively dividing the controller responsibilities between local substation controls and a central coordinator at control center level, the design is aimed at voltage control of large-scale power systems. At local level, the substation controllers maintain their respective substation bus voltages by local power-flow-like computations using mostly local PMU measurements. The central coordinator computes and provides the voltage set-points to the substation controllers and also coordinates by enabling or disabling the local controllers as needed. The controller is being designed towards prototype implementation in Southern California Edison (SCE), starting with local controllers at specific substations in the first stage. It is tested on real-time dynamic models and large-scale power-flow simulations of SCE transmission network.

Abstract: Optimization techniques are increasingly used in research to improve the control of three-phase induction motor (TIM). Indirect field-oriented control (IFOC) scheme is employed to improve the efficiency and enhance the performance of variable speed control of TIM drives. The space vector pulse width modulation (SVPWM) technique is used for switching signals in a three-phase bridge inverter to minimize harmonics in the output signals of the inverter. In this paper, a novel scheme based on particle swarm optimization (PSO) algorithm is proposed to improve the variable speed control of IFOC in TIM. The PSO algorithm is used to search the best values of parameters of proportional-integral (PI) controller (proportional gain (kp) and integral gain (ki)) for each speed controller and voltage controller to improve the speed response for TIM. An optimal PI controller-based objective function is also used to tune and minimize the mean square error (MSE). Results of all tests verified the robustness of the PSO-PI controller for speed response in terms of damping capability, fast settling time, steady state error, and transient responses under different conditions of mechanical load and speed.

Abstract: This paper proposes an improved multiloop control strategy for a three-phase four-leg voltage source inverter (VSI) operating with highly unbalanced loads in an autonomous distribution network. The main objective is to balance the output voltages of the four-leg inverter under unbalanced load conditions. The proposed control strategy consists of a proportional-integral (PI) voltage controller and a proportional current loop in each phase. The voltage controller and the current control loop are, respectively, used to regulate the instantaneous output voltage and generate the pulse width modulation (PWM) voltage command with zero steady-state tracking error and fast transient response. A voltage decoupling feedforward path is also used to enhance the system robustness. Since the outer voltage loop operates in the synchronous reference frame, tuning and stability analysis of the PI controller is far from being straightforward. In order to cope with this challenge, the stationary reference frame equivalent of the voltage controller in the rotating frame is derived. Subsequently, a systematic design based on a frequency response approach is provided. Simulation results are also carried out using the DIgSILENT PowerFactory software to verify the effectiveness of the suggested control strategy.

Abstract: The three-phase modular cascaded solid stated transformer (SST) is an important element in the micro-grid systems as its outstanding adventures compared with conventional power transformer. It consists of three stages, the three-phase cascaded modular rectifier stage, the dual active bridge (DAB) converter stage and the three-phase inverter stage. Three output-paralleled DAB converters offer three dc-buses, which forming the interfaces of renewable energy. However, unbalanced modular voltage or transferred power causes overvoltage or overcurrent issues, which increase the stress of the semiconductor switches and even the instability of SST system. Focusing on the imbalance issues, this paper proposes a systematic control strategy for the three-phase modular cascaded SST. A d-q vector-based common-duty-ratio controller is applied to the rectifier stage aiming at balancing the modular current in each phase. Voltage controller for DAB stage achieves the balanced dc-link and dc-bus voltage, and proportional-resonant controller is applied to inverter stage. The effect of this proposed control method has been verified theoretically. In addition, a scaled-down experimental prototype is built to prove the performance.

Abstract: This paper presents a cascade output voltage control strategy for an uncertain DC/DC boost converter adopting an adaptive current controller in its inner loop. Considering the non-linearity, load uncertainties and parameter uncertainties of the converter, the proposed controller is designed following the conventional cascade voltage controller design method. The proposed method makes the following three contributions. First, a coordinate transformation is introduced for the inner loop, enabling avoidance of the singularity problem caused by the estimates of uncertain parameters. Second, a slight modification to the adaptation law is performed to guarantee closed-loop stability in the presence of the time-varying component of the load current. Third, the outer-loop controller is devised such that its performance can be adjusted without any parameter information. The closed-loop performance is demonstrated through simulations and experiments using the DSP28335 with a 3 kW DC/DC boost converter.