Dc-link Voltage Control Research Articles

Energy management with improved frequency sharing based control for battery/ultracapacitor hybrid energy system in the presence of delay

Hybrid energy storage system (HESS) consists of different electrical storage devices with complementary characteristics to satisfy both the energy and power requirements of the load. This study presents an improved frequency sharing algorithm for battery/ultracapacitor (UC) HESS in the presence of delay. Time delay is inevitable with the digital implementation of control algorithms. Presence of delay deteriorates the transient response of the system by limiting the bandwidth of inner current loops. The DC-link voltage will settle only after settling of the slower loop, i.e. battery loop, hence the presence of delay will make the system response slower in conventional methods. However, the improved algorithm shares battery current loop error during transients with faster UC current loop and hence DC-link voltage settling becomes faster. Furthermore, for safe and efficient operation, an energy management algorithm (EMA) has been designed that ensures nominal UC voltage and other operational constraints as described in the study. To analyse the effect of UC voltage control loop on the operation of the DC-link voltage control loop during transients, UC voltage control loop with different bandwidths have been designed and tested. Microcontroller based implementation was done for experimental verification of the improved control algorithm and EMA.

Control System Design of a Three-Phase Active Front End Using a Sliding-Mode Observer

This article proposes a sliding-mode-observer (SMO)-based control strategy to regulate the dc-link voltage for a three-phase two-level active front end (AFE). The SMO is designed for the voltage control loop to estimate the external load which is abruptly connected to the AFE dc-link and consequently causes the dc-link voltage fluctuation. The estimated load value is used to compensate the voltage loop controller, therefore, the voltage loop gains more robustness against the load perturbation and its disturbing effect is greatly reduced. The effectiveness and advantage of the proposed control strategy has been verified through theoretical analysis, simulations, and the real-application experiments conducted on a 5 KVA laboratory AFE. The results show that the proposed control strategy provides obvious improvement of the dc-link voltage control performance comparing with the conventional PI controller and demonstrates stronger robustness against the operating point variations caused by the changes in external load and dc-link capacitance.

Modeling and Stability Analysis of Weak-Grid Tied Multi-DFIGs in DC-Link Voltage Control Timescale

The DC-link voltage control (DVC) timescale (i.e., the frequency dynamics covering converter outer controls) instabilities in wind generation have gained increased attention recently. This paper presents DVC timescale modeling and stability analysis for multi doubly-fed induction generators (DFIGs) connected to weak AC grids. A reduced-order, small-signal model of a grid-tied multi-DFIG system, designed for DVC dynamics analysis, is firstly proposed. The model allows for the dynamic interactions among the DC-link voltage control, active power control (APC), terminal voltage control (TVC) and phase-locked loop (PLL). Eigenvalue and participation factor analyses are conducted to explore the potential instabilities and correlated critical factors for such a multi-machine system. The sensitivity studies find that instability can occur at high levels of power generations or low short-circuit ratio (SCR) conditions. In addition, the dominant mode is identified to be highly related to the PLL, and its modal damping is decreased when the bandwidths of PLLs in different generators are close. Detailed model-based time domain simulations verified the analysis above.

Meta-heuristic optimization algorithms based direct current and DC link voltage controllers for three-phase grid connected photovoltaic inverter

This paper deals with improving the effectiveness of the control system for the DC/AC grid connected photovoltaic (PV) inverter. The three-phase DC/AC grid connected PV inverter control system consists of two main control loops: (i) external loop to control the DC link voltage. (ii) An internal control loop for regulating the inverter current by controlling both direct and quadrature currents (Id, Iq) that are provided by the inverter phase-locked loop (PLL). The main component of each control loop is the proportional-integral (PI) controller, whose optimal gains are challenging tasks. The optimal PI controller gains should be selected to achieve multi-objectives such as increasing inverter efficiency, stability and power factor while decreasing the total harmonic distortion (THD) in the inverter output current as well as voltage. For this purpose, various meta-heuristic techniques (MHTs) are suggested with a comprehensive comparative study such as: (i) Whales Optimization Algorithm (WOA), (ii) Grey Wolf Optimization (GWO) algorithm, (iii) Antlions Optimization algorithm (ALO) and (iv) Moth Flame Optimization (MFO) algorithm. The numerical results attained through MatlabTM-Simulink reveal that the WOA technique is more superior than the three others studied MHTs in enhancing the inverter efficiency, stability and power factor with minimum THD.

Design and control of shunt active power filter for power quality improvement of utility powered brushless DC motor drives

ABSTRACT The presence of nonlinear loads like brushless DC (BLDC) motor drives in the distribution system leads to poor power quality (PQ) indices like low power factor, high total harmonic distortion and creates electromagnetic interference. Passive filters and Power factor correction converters are often inserted between the utility and BLDC motor drives to improve the PQ parameters. This paper proposes the shunt active power filter (SAF) for the PQ improvement of medium and high power BLDC motor drives. In this paper, the design steps and control of an SAF for a 20 kW BLDC motor drive are presented. The control of SAF is realized by applying the instantaneous “p–q” theory with the sinusoidal current control strategy. The current control is realized by a hysteresis controller. In contrast to normal electrical loads, the motor drive loads are subjected to continuous speed and load variations. This will result in fluctuations in the DC-link voltage of the SAF capacitor. Hence, the sliding mode controller is used for DC-link voltage control. The proposed methodology is validated in simulations and the experimental prototype. The enhancement of various PQ parameters is analysed for different loading conditions of the motor in the distorted and non-distorted distribution system voltage conditions.

Robust Current Sensorless Control of VSC-Based MTDC Transmissions for Integrating Wind Farms

This article presents an observer-based robust current sensorless control (RCSC) scheme for voltage-source-converter (VSC)-based multiterminal direct-current (MTDC) transmission system integrated with wind farms. Perturbation observer-based point of common coupling (PCC) voltage controller and dc-link voltage controller are designed for wind farm station (WFS) and grid-side station (GSS) of MTDC, respectively, for grid integration of multiple wind farms. Perturbation states are defined to describe the interactions of voltage control loops of nonlinear VSC model, and the feedback linearization control is realized based on the state estimations provided by perturbation observers to achieve the decoupled voltage control of MTDC. Neither current sensors nor parameters of MTDC are required in the RCSC scheme, thus achieving the robust sensor fault-tolerant operation of the MTDC system. Simulation and experimental studies are undertaken on a three-terminal MTDC transmission system integrated with two wind farms to verify the control performance of the RCSC scheme under the conditions of wind speed variations, sudden load changes, parameter variations, and current sensor faults.

Experimental investigations on voltage sourced inverter interfaced photovoltaic based distributed generation system

ABSTRACT The voltage sourced-inverter (VSI) interfaced photovoltaic (PV) system with classic controller supplies the true power demand of the loads coupled at the Point of Interconnection (PoI). Thus, PV systems assets practically remain dormant at nighttime which reduces the device utilization factor of the VSI. Therefore, to subjugate the underutilization of VSI, this paper presents a synchronously rotating dq-frame and adaptive phase-locked loop (PLL) structure. The proposed system serves the following distinctive functions: (i) True power management function which is fundamentally responsible for sharing power requirement of the nonlinear load based on the renewable power accessibility at DC-link; and (ii) Active harmonic filtering (AHF) which is responsible for deliberately producing controlled harmonics in the output current and, in turn confronting the power quality (PQ) challenges. The envisaged control structure can be purposefully employed to transfer the power into the electric grid while providing grid current harmonic mitigation and balancing, reactive power elimination, and unity power factor (UPF) operation. In this respect, it is an inexpensive substitute to existing commercial shunt AHF meant for this job. The most distinctive feature of the presented structure is its computational effectiveness and easy implementation. Furthermore, insights are provided on the operating principle of a DC-link voltage control loop and power balance operation. The proposed PV system employs an incremental conductance (InC)-based technique to ensure optimal power evacuation from the PV arrays. Finally, the MATLAB/Simulink simulation and dSPACE-1104 based experimental results confirm the potential merits of the proposed structure to restrict the grid current distortions within the 5% and thereby complying with THD limit specified in the IEEE-519.

Pole Reduction Concept for Control of Synchronous Reluctance Motor-Based Solar Photovoltaic Water Pumping System for Improved Performance

This article deals with an improved control for synchronous reluctance motor (SyRM)-based photovoltaic (PV) water pumping system, eliminating the speed controller of position sensorless field-oriented control (FOC) of SyRM. The solar water pump drive involves two stages of power conversion. The reference quadrature axis current of FOC is estimated from available PV array power that is being continuously tracked for maximum power point. This eliminates the requirement of an additional speed controller. This improves the transient behavior of overall control under dynamic conditions such as varying solar radiation. A pole is eliminated from overall characteristic equation of open-loop dc link voltage control transfer function. As a result, a single proportional integrator controller satisfactorily controls the whole pumping system. The system is much easier to tune, and by default, immune to aging and variation in pump's constant. This improves fidelity and reduces the overall cost of the system. Performance of this system is validated through a developed laboratory prototype of solar PV array-fed SyRM drive for water pumping. The overall system performance is examined under different environmental conditions.

A systematic design methodology for DC-link voltage control of single phase grid-tied PV systems

PI controllers are commonly used for the DC-link voltage control of single phase grid-tied inverters. This DC-link voltage is characterized by double-line frequency ripples, which are natural by-product of single phase AC systems. These ripples, if not controlled properly, can adverse the performances of the grid-tied PV system at the AC side, particularly the grid current THD. On the other hand, random and sudden changes of the active power produced by PV panels, during sudden shadow or lighting of PV panels, can lead to high DC-link voltage fluctuations. This paper presents a systematic design methodology to tune the gains of the PI-based DC-link voltage controller so that the DC-link voltage fluctuations as well as the grid current THD are reduced to tolerable limits. Several simulation results are presented and discussed to show the effectiveness of the proposed design methodology.

Sliding Mode Controller with Integral Action for DC-Link Voltage Control of Grid-Integrated Domestic Photovoltaic Systems

The grid integration of photovoltaic systems is preferred over the islanded mode of operation as the former does not require additional storage element, hence less maintenance and no chemical pollution. To regulate power exchange between the grid and the photovoltaic system, a robust DC-link voltage controller capable of withstanding the intermittent nature of solar energy and sudden variations in load is inevitable. A proportional–integral controller is used for DC-link voltage control, exhibiting oscillations during steady state and overshoot during transients. However, the conventional sliding mode controller reduces the overshoot at the expense of increased steady-state error. This paper proposes a robust sliding mode controller for DC-link voltage control to reduce steady-state error by incorporating integral action to the conventional sliding mode controller. The harmful effect of chattering phenomenon is minimised by limiting the error in the control variable using a signum function. The added features of the work include an incremental conductance method for obtaining maximum power from the photovoltaic system, instantaneous pq theory-based self-tuning filter for the extraction of fundamental component and inverter switching pulse generation using hysteresis current control technique. The grid-integrated photovoltaic system along with all features is modelled and simulated in MATLAB/Simulink platform. The results of numerical simulations carried out for various system conditions illustrate that the proposed controller provides superior performance when compared to proportional integral controller and conventional sliding mode controller in terms of harmonic compensation, power flow balance and speed of response at all system conditions.

A Wireless Vehicle-to-Grid-to-Home Power Interface With an Adaptive DC Link

Electric vehicles (EVs) that offer grid services using the vehicle-to-grid (V2G) concept essentially require an interface that allows for directional power flow. This article presents a wireless bidirectional grid power interface for EVs that facilitate power flow between the grid, EV, and homes with nonlinear loads. The proposed wireless vehicle-to-grid-to-home power interface (WVGH-PI) uses the grid-side low frequency to dc converter, used in typical wireless bidirectional chargers only to facilitate the two-way energy flow, to improve the power quality by compensating for the harmonic power and reactive power of nonlinear household loads. To improve the overall efficiency of the interface, an adaptive dc-link voltage controller is also proposed. Operation of the proposed wireless interface is investigated under dynamic conditions and in different modes. The simulation and experimental results obtained from a 1-kW prototype are presented, benchmarking against conventional control, to demonstrate the validity of the proposed concept.

A Symmetrical Control Method for Grid-Connected Converters to Suppress the Frequency Coupling Under Weak Grid Conditions

The asymmetrical rotating (dq-) frame control leads to frequency coupling dynamics, which tends to bring in harmonic instability. In this article, based on the established complex signal impedance model, the influence of the grid impedance and the frequency coupling on system stability is analyzed. For the phase-locked loop (PLL) mainly affects the q-axis dynamics, it is proposed that the q-axis feedforward and the d-axis compensation control methods to decrease the asymmetric influence caused by the PLL. Similarly, for the dc-link voltage controller mainly affects the d-axis current reference, this article proposes a d-axis feedforward and q-axis compensation method to improve the asymmetry dynamics introduced by the dc-link voltage controller. When the proposed methods are adopted, the components introduced by the PLL and the dc-link voltage controller in the coupling terms are eliminated, thereby achieving the purpose of suppressing the frequency coupling phenomenon and improving the system stability. The theoretical analysis and the experimental results show that the proposed methods are effective.

Wideband dq-Frame Impedance Modeling of Load-Side Virtual Synchronous Machine and Its Stability Analysis in Comparison With Conventional PWM Rectifier in Weak Grid

The load-side virtual synchronous machine (LVSM) enables the load to actively participate in the grid regulation, but it might still induce oscillations in a weak grid. Considering the dc-link voltage controller, power loops, ac voltage and current loops, the control delay, and sampling filters, the wideband dq-frame impedance model of the LVSM is first established, and it is found that Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dd</sub> exhibits negative resistor impedance within 10 Hz. In addition, the dq-frame impedance of the LVSM is approximately inductive above 10 Hz. Then, using the dq-frame impedance-based approach, the comparative study shows that the smaller the proportional gain and integral gain of the dc-link voltage controller are, the more stable the conventional voltage source rectifier (VSR) is in a weak grid. However, the larger the proportional gain is, the smaller the integral gain is, the more stable the LVSM is in a weak grid. Furthermore, the voltage feedforward decreases the stability margin of the VSR in a weak grid, while the virtual moment of inertia J and the damping gain D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> affect the stability of the LVSM, and the smaller J and D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> are, the more stable the LVSM is in weak grid. Finally, simulations and experimental results verify the impedance model and the stability analysis.

Comparative Assessment of Control Strategies Used in a Hybrid AC-DC Micro-Grid Consisting of Composite Energy Sources

In the present paper, comparative assessment of two important available intelligent control strategies namely the Mamdani fuzzy logic controller and the T–S fuzzy controller used for DC-link voltage control in DC micro-grid is carried out, first. Then, a new inverter control technique is proposed to maintain the inverter voltage as well as the DC-link voltage in a hybrid AC-DC micro-grid. The DC micro-grid under study comprises of photovoltaic system, permanent magnet synchronous generator-based wind energy system, battery energy storage system, fuel cell, electrolyzer and DC loads. The complete DC Micro-grid along with its correlated control algorithms are examined and accessed through MATLAB/Simulink environment.

VSC Controllers for Multiterminal HVDC Transmission System: A Comparative Study

Voltage source converter (VSC)-based multiterminal high-voltage direct current (HVDC) systems received widespread attention throughout the world for grid integration of renewable energy resources in recent years. This paper presents a comparative performance analysis of different VSC-based outer control and inner current controllers for the multiterminal HVDC system. It employs either lead-lag (LL) or proportional–integral (PI) controllers for outer DC link voltage control, whereas it uses PI or model predictive controllers (MPCs) for inner current control. Hence, it designs four combinations of controllers (LL–MPC, LL–PI, PI–MPC, and PI–PI) to control the outer DC link voltage and inner current of the VSC-based multiterminal HVDC system. Besides, it proposes an integral time squared-error (ITSE)-based optimization technique to tune the parameters of the employed PI controllers that selects optimal parameters at minimum ITSE under extreme operating condition. The combination of the mentioned controllers forms the main control unit of the multiterminal HVDC transmission network for regulation of the DC link voltage and the power flow. Moreover, this article evaluates the controller performance in terms of maximum overshoot, steady-state error, settling time, rise time, and total harmonic distortion. It implements the proposed controllers in a typical VSC-HVDC system and multiterminal HVDC transmission system in MATLAB/SIMULINK platform. Presented results confirm the efficacy of the four-type controllers. The optimized PI–MPC controller provides overall better performance relative to other controllers.

DC‐link voltage control strategy of Z‐source inverter for high‐speed permanent magnet motor

In order to widen the flux-weakening range of motors, this study proposed a DC-link voltage control strategy for high-speed permanent magnet motor drive systems powered by Z-source inverter. In this strategy, the DC-link voltage, also known as the output voltage of the impedance network, varies with the inverter output voltage and remains optimal at all times, whether in steady state or transient state. As for the shoot-through duty ratio and the modulation index, the two main control variables in the Z-source inverter are under unified control to ensure the stability of the system, while achieving fast tracking and reducing the loss. In addition, a new sliding mode control system based on the indirect control of the capacitor voltage is used to control the DC-link voltage to suppress the system fluctuation caused by the change of the given value or the motor load. Besides, various conventional voltage Z-source inverter pulse width modulation strategies can be applied to this technology. Simulations and experiments verified the effectiveness of the proposed strategy.

Power management system for hybrid AC/DC microgrid based on high frequency isolated interlink converter

The hybrid AC/DC microgrid systems have been popular and being developed as the next generation power systems because of the comprehensive combination of both AC and DC microgrid systems. Power management scheme is one of the most critical operation aspects for hybrid microgrids because the system is operating with various generation sources and loads such as renewable energy sources, energy storage systems and AC and DC loads. Therefore, in this research, control strategies and power management scheme is considered for all possible operation modes of standalone and grid-connected conditions. In the existing microgrid system in Electrical Power Engineering department in Yangon Technological University, rooftop PV plant and battery are cooperating to supply the electricity. The inverter in this system allows the unidirectional power flow from DC to AC and there is no specific power management system for grid-connected and standalone operation mode. To provide bidirectional power flow in the existing system, the configuration of converter and control strategies for power management system are developed in this research. To provide the bidirectional power flow between AC bus and DC bus, the bidirectional interlink power converter with high frequency isolation is applied. The control system including the power balancing between generation and demand, DC link voltage control and AC link voltage and frequency control is considered. By applying droop control method in the developed system, power flow balancing between AC bus and DC bus is maintained. And also, using high frequency isolation transformer in interlink converter provides fast response of the system performance and maintaining continuous power supply within each AC system and DC system during disturbance condition in one subsystem. The performance of the proposed power management system is demonstrated by using MATLAB/Simulink.

DC-link loop bandwidth selection strategy for grid-connected inverters considering power quality requirements

DC-link voltage and output current control loops are two cascaded loops in the control structure of grid-connected inverters. A high DC-link voltage loop bandwidth (DCL-BW) provides more disturbance rejection capability for the control loop and is preferred from control system perspective. However, for stability issues, this BW is limited and must be sufficiently less than that of the current control loop. Among the different control schemes, instantaneous active reactive control (IARC) method provides the highest possible DCL-BW (i.e., 0.02 × switching frequency). Having this degree of freedom in the controller design, a proper methodology should be defined for selection of DCL-BW. In this paper, different factors including the output current harmonic distortion, low-order harmonics on DC link voltage, and the inverter nominal rating are introduced as affecting parameters for DCL-BW selection. Accordingly, a DCL-BW selection strategy based on output current harmonic distortion is proposed. The proposed method enhances the power quality indices of the grid-connected inverters, which especially is useful for few kVA power inverters. Finally, to validate the conducted analysis, experimental and simulation results are provided.

Assessment of Grid-Connected Wind Turbines with an Inertia Response by Considering Internal Dynamics

This paper presents a small-signal analysis of different grid side controllers for full power converter wind turbines with inertia response capability. In real wind turbines, the DC link controller, the drivetrain damping controller and the inertial response might present contradictory control actions in a close bandwidth range. This situation might lead to reduced control performance, increased component stress and non-compliance of connection agreements. The paper presents an analysis of the internal wind turbine dynamics by considering different grid-side converter control topologies: standard current control used in the wind industry, standard current control with inertia emulation capabilities and virtual synchronous machines. Comments are made on the similarities between each topology and the negative effects and limits, and possible remedies are discussed. Finally, the conclusion poses that the inclusion of a DC link voltage controller reduces the ability of a converter to respond to external frequency events without energy storage. The degradation increases with the DC link voltage control speed.

Cascaded Control of Back-to-Back Converter DC Link Voltage Robust to Grid Parameters Variation

The article elaborates on the mathematical modeling and control structure design of a grid-connected back-to-back voltage source inverter with a complex dc link and an LC filter for the current harmonics reduction. A cascaded, three-loop control structure is designed for controlling the converter current, the grid current, and the dc link voltage. The derived control structure, utilizing two sets of current sensors and a dc link voltage measurement, is based on proportional-integral controllers and a Truxal–Guillemin model-based controller for the dc link voltage control. The chosen control structure provides consistent results for great variations of the grid impedance. Robustness is tested for various scenarios of variations in grid filter parameters, controller parameters, chosen operating point for linearization, and grid impedance. Results are validated by simulations and experiments on a 7.5 kW converter and show highly robust performance for $\pm$ 20% parameter change and a large span of grid impedance variation.