Abstract: The three-phase dual-active bridge (DAB) is a dc–dc converter, which provides galvanic isolation, inherent soft-switching capability, and small filter size. In this study, the dynamic behavior of three-phase DAB is analyzed and a dynamic control strategy is developed. Furthermore, a compensation technique is implemented to compensate unbalanced transformer phase currents. The latter is often caused by asymmetric leakage inductances. State space averaging and first harmonic approximation models, both for the steady state and transient analysis, are developed to describe the dynamic behavior of the three-phase DAB. The accuracy of the models is compared with a detailed circuit simulation and the benefits of each model are identified. When the transferred power of the DAB changes fast, the transformer currents can become unbalanced, leading to oscillations in the output current. A unique control method is presented, which allows settling of the transformer currents within one-third of the switching period. Additionally, the transformer currents stay symmetrical and oscillations are avoided. Based on this fast current control, an outer voltage controller is designed. The comparison of the control system using the fast current control and the conventional quasi-steady-state control demonstrates the potential advantages of the new approach under dynamic conditions. In practice, it is difficult to achieve completely symmetrical short-circuit impedances in a high-power medium-voltage transformer. Asymmetric leakage inductances, however, result in unbalanced phase currents and higher dc current ripple in a three-phase DAB. The new control scheme that is developed here can be extended to compensate any unbalances in the transformer. This approach enables effectively the balancing of the three-phase currents. The new control schemes are experimentally verified.

Abstract: This paper presents a detailed operation analysis, controller design, and realization of a high-power, bidirectional quasi-Z-source inverter (BQ-ZSI) for electric vehicle applications. The circuit analysis shows that with a bidirectional switch in the quasi-Z-source network, the performance of the inverter under small inductance and low power factor can be improved. Based on the circuit analysis, a small signal model of the BQ-ZSI is derived, which indicates that the circuit is prone to oscillate when there is disturbance on the dc input voltage. Therefore, a dedicated voltage controller with feed-forward compensation is designed to reject the disturbance and stabilize the dc-link voltage during a non-shoot-through state. An 85-kW prototype has been built. Both simulation and experimental results are presented to prove the functionality of the circuit and the effectiveness of the proposed control strategy.

Abstract: The protection of the sensitive unbalanced nonlinear loads from sag/swell, distortion, and unbalance in supply voltage is achieved economically using the dynamic voltage restorer (DVR). A simple generalized algorithm based on basic synchronous-reference-frame theory has been developed for the generation of instantaneous reference compensating voltages for controlling a DVR. This novel algorithm makes use of the fundamental positive-sequence phase voltages extracted by sensing only two unbalanced and/or distorted line voltages. The algorithm is general enough to handle linear as well as nonlinear loads. The compensating voltages when injected in series with a distribution feeder by three single-phase H-bridge voltage-source converters with a constant switching frequency hysteresis band voltage controller tightly regulate the voltage at the load terminals against any power quality problems on the source side. A capacitor-supported DVR does not need any active power during steady-state operation because the injected voltage is in quadrature with the feeder current. The proposed control strategy is validated through extensive simulation and real-time experimental studies.

Abstract: Voltage sags are one of the main problems in transmission and distribution grids with high penetration of distributed generation. This paper proposes a voltage support control scheme for grid-connected power sources under voltage sags. The control is based on the injection of reactive current with a variable ratio between positive and negative sequences. The controller determines, also, the amount of reactive power needed to restore the dropped voltage magnitudes to new reference values confined within the continuous operation limits required in grid codes. These reference values are chosen in order to guarantee low current injection when fulfilling the voltage support objective. Selected experimental results are reported in order to validate the effectiveness of the proposed control.

Abstract: A neutral-point-clamped three-level inverter with small dc-link capacitors is presented in this paper. The inverter requires zero average neutral-point current for stable neutral-point voltage. The small dc-link capacitors may not maintain capacitor voltage balance, even with zero neutral-point current. This may happen due to nonlinearities present in the circuit. This requires a fast control of the neutral-point voltage. A simple carrier-based modulation strategy which allows modeling of the neutral-point voltage dynamics as a continuous function of power drawn from the inverter is proposed. This continuous model shows that the neutral-point current is proportional to the power drawn from the inverter, and it enables the use of a well-established classical control theory for the neutral-point voltage controller design. A simple proportional integral controller is designed for the neutral-point voltage control on the basis of the continuous model. The design method for optimum performance is discussed. The implementation of the proposed modulation strategy and the controller is very simple. The controller is implemented in a 7.5-kW induction machine-based drive with only 14 μF dc-link capacitors. Also, the experimental results show that fast and stable performance of the neutral-point voltage controller are achieved and thus verify the validity of the proposed control approach.

Abstract: This paper presents a control strategy for an islanded medium voltage microgrid to coordinate hybrid power source (HPS) units and to control interfaced multilevel inverters under unbalanced and nonlinear load conditions. The proposed HPS systems are connected to the loads through a cascaded H-bridge (CHB) multilevel inverter. The CHB multilevel inverters increase the output voltage level and enhance power quality. The HPS employs fuel cell (FC) and photovoltaic sources as the main and supercapacitors as the complementary power sources. Fast transient response, high performance, high power density, and low FC fuel consumption are the main advantages of the proposed HPS system. The proposed control strategy consists of a power management unit for the HPS system and a voltage controller for the CHB multilevel inverter. Each distributed generation unit employs a multiproportional resonant controller to regulate the buses voltages even when the loads are unbalanced and/or nonlinear. Digital time-domain simulation studies are carried out in the PSCAD/EMTDC environment to verify the performance of the overall proposed control system.

Abstract: This paper proposes an adaptive control method of three-phase inverters for stand-alone distributed generation systems (DGSs). The proposed voltage controller includes two control terms: an adaptive compensating term and a stabilizing term. The adaptive compensating control term is constructed to avoid directly calculating the time derivatives of state variables. Meanwhile, the stabilizing control term is designed to asymptotically stabilize the error dynamics of the system. Also, a fourth-order optimal load current observer is proposed to reduce the number of current sensors and enhance the system reliability and cost effectiveness. The stability of the proposed voltage controller and the proposed load current observer is fully proven by using Lyapunov theory. The proposed control system can establish good voltage regulation such as fast dynamic response, small steady-state error, and low total harmonic distortion under sudden load change, unbalanced load, and nonlinear load. Finally, the validity of the proposed control strategy is verified through simulations and experiments on a prototype DGS test bed with a TMS320F28335 DSP. For a comparative study, the control scheme of feedback linearization for multi-input and multioutput is implemented, and its results are presented in this paper.

Abstract: This paper introduces a seamless grid interconnection control strategy for distributed generations (DG) with renewable energy. Based on the performance analysis of conventional voltage and current control loops, sophisticated control strategies, which can overcome the limited capability of the DG inverter control under mode transition conditions for unknown power plants, are needed in order to protect critical loads and operate the DG inverter without grid fault trip. The proposed control strategy consists of a current controller and a feedforward voltage controller to minimize the grid overvoltage. The feedforward voltage control loop is added to the d-q axis current control loop. The proposed control strategy reduces the overvoltage stress of the renewable energy and the critical load under the grid fault. In addition, the seamless operation of the DG inverter will also enhance the stability and reliability of the grid. Real-time digital simulator-based hardware-in-the-loop experimental results and simulation results show that the DG inverter achieves the seamless mode transition under grid overvoltage conditions.

Abstract: In future low voltage grids, with multiple inverter interfaced sources connected, voltage regulation may become a necessary task. The potential exists for inverter interfaced sources to be deployed to regulate the voltage at the point of common coupling (PCC) of each inverter interfaced sources. The PCC voltage regulation is attainable with inverter interfaced sources by dynamically controlling the amount of reactive power injected to the power distribution grid by individual systems. In the current research, a closed-loop controller is proposed to regulate the PCC voltage of a solar photovoltaic (PV) system that is connected to a single-phase power distribution feeder (with R to X ratio greater than 1). The plant model of the PCC voltage controller of the PV system is derived considering both reactance and resistance of the network to which the PV system is connected. Three different compensators are evaluated to identify a suitable compensator for the closed-loop PCC voltage controller to regulate the PCC voltage at a given reference voltage. Simulation studies and experimental verification confirm that the theoretical approach taken to derive the control plant model of the PCC voltage controller is accurate and the procedure that is followed to design the controller is robust. The control design procedures illustrated in the current research leads to a PCC voltage control system with acceptable dynamic and steady state performance.

Abstract: This paper presents a decentralized self-adjusting reactive power controller for the autonomous operation of a multi-bus medium voltage (MV) microgrid The main objective of the proposed control strategy of each distributed generation (DG) unit is to compensate the reactive power of its local loads and to share the reactive power of the nonlocal loads among itself and other DG units The proposed control strategy includes an improved droop controller whose parameters are adjusted according to the reactive power of the local loads A virtual inductive impedance loop is augmented to the voltage controller to enhance the steady state and transient responses of the proposed reactive power management scheme The small signal analysis of the proposed method is presented to ensure stability of the system for different reactive power values The presented strategy considerably enhances the voltage profiles of the microgrid buses as compared with the conventional droop methods The proposed method does not require any communication link and minimizes the reactive power flow in the MV lines, thus reducing the losses of the overall microgrid The performance of the proposed control scheme is verified by using digital time-domain simulation studies in the PSCAD/EMTDC software environment

Abstract: This paper presents a robust single-loop direct voltage control strategy featuring effective suppression of uncertain resonant modes generated due power-factor correction (PFC) capacitors and residential capacitive loads in distributed generation (DG) microgrids. The proposed controller adopts an improved uncertainty modeling approach, which facilities the realization of a robust controller based on structured singular values analysis. The resultant controller is used as a direct voltage controller where no additional damping technique, either passive or active, is required. This feature reduces the sensor requirements in the DG interface controller and enhances the bandwidth characteristics of the closed-loop voltage-controlled converter. Mathematical and comparative analyses are provided to show the advantages of proposed -synthesis controller over the conventional controller in maintaining robust stability as well as robust performance of the microgrid in presence of parameter uncertainties and uncertain resonant peaks caused by connection of PFC capacitors. Systematic design approach for the proposed controller is presented. Time-domain simulation studies and comparative experimental results are presented to show the effectiveness and robustness of the proposed controller in microgrid applications.

Abstract: A Dynamic Voltage Restorer (DVR) is one of the most common custom power devices to compensate for the voltage sag and swell. The main functions of the DVR are the injection of voltage to the power line and maintain the pre-sag voltage condition in the load side. Different control strategies are available depending upon the compensation technique. In this paper, a simple method for the generation of reference voltage for a DVR is presented. This control scheme provides superior performance compared to conventional control methods because it directly measurers the rms voltage at the load point without involving any transformation process. Even under system disturbances, this control scheme maintains a constant voltage at the load point. The control scheme is rather simple, flexible in cost and has an excellent voltage regulation capability. The simulation was carried using MATLAB/ SIMULINK and the results of the simulation show that this proposed method is able to provide the desirable power quality in the presence of a wide range of disturbances.

Abstract: This study presents the analysis and design of a sliding-mode control of a buck converter operating in continuous conduction mode that minimises the energy during start-up and provides output voltage regulation in front of input voltage perturbations and load disturbances. A linear combination of inductor current and capacitor voltage errors with respect to their corresponding equilibrium values is analysed as a switching surface. A linear matrix inequalities (LMI)-based analysis to obtain optimum coefficients of the linear combination reveals that the best compromise between inrush current and output response rapidity is the current control given by the switching surface S(x) = iL − IE , where I E is the current coordinate of the equilibrium point. This surface is proposed for start-up and for rejecting input voltage perturbations, because it is demonstrated that the current control is inherently insensitive to input voltage variations. Output voltage regulation in front of load perturbations or input voltage variations is achieved once the converter is in a steady state by modifying S(x) with the insertion of a PI-correcting network. The resulting controller is implemented analogically and employs two switching surfaces, that is, one surface for start-up and another one for output voltage regulation. The theoretical predictions are verified by means of simulation and experimental results.

Abstract: In this paper a novel approach for controlling the output voltage of the isolated flyback converter is presented using sliding mode controller. Due to non-minimum-phase nature of the converter and presence of a right-half-plan-zero in voltage transfer function, indirect regulation of the output voltage is applied. In the designed controller, simultaneous use of output voltage and transformer current feedbacks improves dynamic response of the controller. The sliding mode controller is developed so that in the final control law, integral of the output voltage error is present and consequently the output voltage error will be zero in the steady-state. Also, in order to measure transformer magnetizing current, simple and efficient method is proposed. Finally, in order to evaluate response of the controller, flyback converter is a simulated using MATLAB / Simulink.

Abstract: Motor loads require high amount of reactive power for a short period during their startup. The high reactive power drawn from the system causes voltage dips at startup time and potentially overvoltage after motor startup is over. The voltage dip and overvoltage may cause the relays to trip and the system to go unstable. This phenomenon is more challenging in weak distribution systems and isolated systems such as microgrids due to the limited inertia of the master generator. This paper presents a dynamic voltage controller that coordinates all the reactive power sources in the system to provide the necessary reactive power during motor startup. The presented Model Predictive Control (MPC) based dynamic Volt/Var Control (VVC) scheme considers the dynamics of the microgrid in the VVC formulation to overcome the voltage dip caused by motor startup. This method uses predictions of voltage behavior of the system based on a simplified system model and tries to eliminate the effect of motor startup by coordinating the reactive power sources in the system.

Abstract: In this project to develop a reconfigurable electrical grid emulator, a Hardware Test-Bed (HTB) is being developed that emulates large scale power system generators and loads by using power electronic converters. Source converters in the HTB system are designed to emulate generators. A synchronous generator model is implemented in the converter to calculate the voltage references in the dq axis, and a voltage controller is added to achieve zero steady state error. A traditional cascade controller with inner current control and outer voltage control brings additional output impedance to the generator model, and causes voltage tracking error during transients. To minimize the controller output impedance and eliminate controller influence on the generator model, a single voltage loop with current differential feedback is proposed in this paper. Combined with rescaled generator parameters, circulating current elimination, and dead time compensation, simulation and experiments are performed in the HTB. The results verify the effectiveness of the controller and demonstrate the dynamic generator emulator behavior.

Abstract: The present invention relates to a constant voltage constant current (CVCC) controller, and associated control methods In one embodiment, a CVCC controller for a flyback converter can include: (i) a current controller configured to generate an error signal by comparing an output current feedback signal against a reference current; (ii) a voltage controller configured to receive an output voltage feedback signal and a reference voltage, and to generate a control signal; (iii) a selector configured to control the flyback converter to operate in a first or a second operation mode based on the control signal, and to further generate a constant voltage or a constant current control signal based on the error signal; and (iv) a pulse-width modulation (PWM) controller configured to generate a PWM control signal to control a main switch, and to maintain the output voltage and/or current of the flyback converter as substantially constant

Abstract: A number of modulation strategies have been proposed in literature in the particular case of three-phase three-level NPC inverter, each one focusing on the optimization of specific aspects and performance of the converter. Nevertheless a comprehensive analysis of state-of-art techniques and their specific features suitable for a photovoltaic (PV) applications is still missing, such a study being highly desirable. Both carrier-based and space vector modulation techniques are analyzed in this paper, highlighting specific features and limitations, especially related to PV applications. Basic issues are considered and compared among modulation strategies, namely: switching losses, low-frequency oscillations of the neutral-point (NP) voltage, total harmonic distortion (THD) and weighted total harmonic distortion (WTHD) of the phase currents and line voltages, dynamic response of the neutral-point voltage control loop under imbalance conditions and modulation algorithm complexity. A hybrid modulation strategy assisted by an optimal neutral-point voltage controller is then proposed aiming at both the reduction of the switching losses with a limited deterioration of the output voltage/current quality and fast dynamics control of the neutral point voltage. Those features allow the development of reduced dc bus capacitance PV inverters with optimized efficiency and quality of the output waveforms. Simulation and experimental results based on a TMS320F28069 DSP controller and a PV inverter are presented to confirm the effectiveness of the proposal.

Abstract: Disclosed are exemplary embodiments of systems and methods for determining a power stealing capability of a climate control system controller. In an exemplary embodiment, a controller for use in a climate control system generally includes a capacitor chargeable by current flowing through an off-mode load of the climate control system. A voltage detect circuit detects a voltage across the capacitor. The controller includes a timer for determining a charge time of the capacitor from a first specific voltage to a second specific voltage based on input from the voltage detect circuit. The controller determines a resistance of the off-mode load based on the charge time and, based on the determined resistance, determines a level of current for power stealing through the off-mode load.

Abstract: This paper presents a control technique of cascaded H-bridge multilevel voltage source inverter (CHB-MLI) for a grid-connected photovoltaic system (GCPVS). The proposed control technique is the modified ripple-correlation control maximum power point tracking (MRCC-MPPT). This algorithm has been developed using the mean function concept to continuously correct the maximum power point (MPP) of power transferring from each PV string and to speedily reach the MPP in rapidly shading irradiance. Additionally, It can reduce a PV voltage harmonic filter in the dc-link voltage controller. In task of injecting the quality current to the utility grid, the current control technique based-on the principle of rotating reference frame is proposed. This method can generate the sinusoidal current and independently control the injection of active and reactive power to the utility grid. Simulation results for two H-bridge cells CHB-MLI 4000W/220V/50Hz GCPVS are presented to validate the proposed control scheme.

Abstract: A novel dual single-input single-output (DSISO) model for a three-phase two-level PWM rectifier is proposed, in order to simplify the complicated structure of multiple input variables and multiple control objectives. In the proposed model, the three-phase PWM rectifier is equivalent to two single-phase PWM rectifiers in the α-β stationary reference frame, which is easier to analyze than a three-phase PWM rectifier. Based on the DSISO model, the small-signal model is derived, the control-to-output transfer function is deduced, and the design principle of the voltage controller is given. The control strategy is analyzed to determine switching control signals in a switching period. Experimental results confirm the validity of the DSISO model and the feasibility of the control strategy based on DSISO model.

Abstract: In this paper, a primary-side control integrated circuits for single stage AC-DC flyback converter is realized. With the primary-side control, the system area and cost can be reduced. The proposed controller samples the feedback voltage on the auxiliary winding, which is proportional to the output voltage plus the turn-on voltage of the output diode. A simple knee point detector is adopted to ensure the sampling is near the instant when the inductor current is zero. Thus, the feedback voltage is more accurate under various load conditions because the diode voltage is not affected by the diode current. Since the auxiliary winding voltage can not accurately reflect the output voltage under low input voltage condition, the duty ratio is kept as constant under this condition. Thus, good load regulation can be achieved. Finally, the chip is integrated with TSMC 0.25 um CMOS high voltage mixed signal general purpose process.

Abstract: This paper presents methods and circuits to achieve tight output voltage regulation and self-calibrated valley switching for primary-side-control quasi-resonant (QR) flyback converters. By tracking the slope of auxiliary winding voltage, the controller retrieves the output voltage at a fixed diode current with a simple analog method, and it thus suppresses the load and line effect on the output voltage. The proposed self-calibrated valley switching circuit is based on the perturb and observe (P&O) method to automatically detect the QR valley, and no extra off-chip component is required. A primary-side flyback controller that integrates the two proposed circuits has been designed and simulated using the SPICE model for VIS 0.5-μm 5-V/40-V high-voltage CMOS process. Simulation results of a 6.2-V/5-W QR flyback converter exhibit a 0.8% output voltage variation with the load varying from 10% to full and the input voltage from 127 V to 375 V, and the validity of the proposed valley switching technique is also verified.

Abstract: The increasing number of distributed generation units has led to the development of microgrids, to which the distributed generators are commonly interfaced by means of a voltage-source inverter (VSI). When the microgrid is operating independently of the power system, i.e., in islanded mode, two levels of control can be distinguished for these VSIs: power control and voltage control (frequency and amplitude). The set-point values for the voltage controller are obtained from the power controller. This paper investigates theoretically and experimentally the benefits of using several PID control structures for the voltage control. Theoretical insights into the dynamics of such a system emphasize the benefits of measuring current signals for control purposes and adding voltage measurements to the output of the controllers. Direct voltage control and cascade voltage control are compared both with and without forward compensation of the grid voltage. Simulation and experimental results are given showing that such PID-type controllers on a digital signal processor are simple yet effective strategies for an accurate voltage control in islanded microgrids.

Abstract: A controller is described for a 3-phase PMSM that uses an open winding and a dual inverter drive with a floating bridge. The drive has a voltage boosting function that: extends the high-speed range of the PMSM; operates the main inverter bridge at unity power factor above the base speed; compensates for battery voltage fluctuations. The main features of the controller presented are: (a) coordinated control of the two inverters to supply the motor with the demand voltage and frequency; (b) the floating bridge dc capacitor voltage can be regulated and its output voltage used to boost the machine terminal voltage when required; (c) the motor speed is controlled via torque control is independent of the floating bridge dc voltage control. Simulation results are used to clarify the motor-drive characteristics in the speed range extension region. Experimental results show that a significant speed range extension is achievable even when using a PMSM with a very low inductance.

Abstract: In this paper, a low cost microcontroller based control method utilizing the concept of capacitor charge balance is presented to achieve a near-optimal transient response for Buck converters. First, this paper presents a new derivation of practical charge balance equations based on simplified differential equations. The final implementation does not require complex calculations and accurate knowledge of the output filter LC parameter. The hardware implementation only requires the output voltage information so that no extra sensing circuitry is needed compared with voltage mode controller. Also, this algorithm can be simply extended to adaptive voltage positioning (AVP) application. Second, due to the simplicity of this algorithm, a low cost microcontroller unit (MCU) based controller can be implemented to shorten the developing period for users. Thirdly, unlike previous work, the proposed voltage based CBC (V-CBC) controller does not require accurate current sensor or fast analog-to-digital converter (ADC). Instead, to detect the critical time instant when the inductor current equals the new load current, a practical extreme voltage detector is introduced to capture the output voltage peak/valley information. Experimental prototype is built to verify the feasibility and advantage of the new method.

Abstract: This paper investigates the performance of Dynamic Voltage Restorer for compensating different voltage sag levels with various faults and to reduce the Total Harmonic Distortion during the mitigation process. The DVR is implemented with three phase voltage source inverter and is connected at the point of common coupling in order to regulate the load side voltage. The compensation is based on PI and Mamdani Fuzzy Controller. Extensive simulation studies under different magnitude of sag for faults on load side for balanced and unbalanced conditions are conducted using fault generator. Simulation result analysis reveals that DVR performs perfectly with PI and Fuzzy control approach. In addition, capability and performance of DVR for various energy storage capacities and injection transformer rating are also analyzed. The performance of these controllers are validated with simulation results using Matlab/Simulink.