Constant DC-link Voltage Research Articles

Differential flatness-based strategy of Photovoltaic/Battery/supercapacitor hybrid source for stand-alone system

This work presents the differential flatness-based energy management strategy for stand-alone Photovoltaic (PV) system with high energy density - battery and high-power density- supercapacitor with DC load. The fluctuation in DC bus voltage is due to the variation in generation of PV power and load Power requirement. This causes the frequent charging /discharging profile of the battery which ultimately reduces the life cycle and increase the internal temperature of the battery. This drawback can be overcome by adopting the flatness-based control technique. In this technique, the supercapacitor converter maintains the DC link voltage constant and the battery converter provides the constant current with the distribution of solar power. Due to constant charging/discharging of battery the thermal run-away problem like leak, smoke, gas venting, rapid disassembly, flames etc., can be eliminated. Decoupling of load power and battery power significantly increase the battery life cycle and maintain the internal temperature of battery within the optimum of 20 °C to 40 °C. In order to achieve efficient charging/discharging of Supercapacitor (SC), Dynamic Temperature Dependent SC Model (DTDSCM) is considered. To verify the performance of the control technique, simulation is carried out in the Mat lab Simulink and it is tested for daily load profile.

Performance of Type-4 Direct Driven PMSG Based Wind Energy Conversion System under Nonlinear Load Demand

The wind power installed capacity in wind power generation increasing day by day and having a substantial amount of share in total electrical power generation. As a result, there is lot of research going on in wind power generation system for the improvement in its steady-state and transient operation by choosing better controllers. In this paper, the control schemes have been developed for the switching of power converters which interfaced with common back-to-back dc-link in Type-4 PMSG based WECS configuration. The proposed system has been tested under non-linear reactive load demand, the controllers of generator-side and grid-side converters operates independently to fulfill the active and reactive power demand, by maintaining the DC link voltage constant. The performance of proposed Type-4 PMSG based WECS system has been evaluated in MATLAB/SIMULINK software under dynamic operation and simulation results verify the proposed control scheme.

Advanced Decoupling Techniques for Grid-Connected Inverters With Multiple Inputs

The parallel connection of multiple distributed energy resources with a common DC-link structure is typically used in grid-connected applications which enables flexible operation maximizing power production of the inverter system under various operation conditions. However, it has brought drawbacks for DC-link power decoupling with the requirement of a larger capacitor bank, faster voltage regulation, etc., to maintain a constant DC-link voltage which increases the overall size and cost. In this paper, a DC-link decoupling technique using a nonlinear control algorithm is proposed to perform rapid DC-link voltage regulation for multi-input grid-connected inverters. With the implementation of a nonlinear observer, the power fed into the DC-link from multiple inputs is estimated by the proposed control algorithm and can be rapidly compensated by the inverter minimizing the DC-link voltage fluctuation. The effectiveness of the proposed nonlinear power decoupling control algorithm is verified by comparing the DC-link performance with a conventional control algorithm through both simulation results on a MATLAB platform and experimental verification on a grid-connected inverter prototype.

Modelling and Performance Analysis of Grid Connected PMSG Based Tidal Generation System

Globally, most national grid relies on fossil fuel with consequent economic burdens, environmental pollution and global warming. Hence, Tidal energy possesses the capacity to mitigate the emission of carbon dioxide because it is clean, abundant, less expensive and predictable. However, the power output from Tidal energy generation system (TEGS) fluctuates due to changes in tidal flow whereby causing power instability. This work is geared at fundamentally model, simulate and control TEGS since it encounters problems such as harmonics, frequency fluctuations and voltage sags. These problems are resolved using a nonlinear controller coupled with a high-speed simulation of circuit-oriented electrical and digital electronic systems. The generator power output is optimised by utilising various control methods as proposed to control the voltage and frequency which is a Voltage Oriented Control technique that relies on high level performance 𝑑𝑞‐coordinate controllers. The system simulation is implemented in the MATLAB/Simulink environment. The current controller and DC‐link voltage controller are designed using Internal Model Control method. The simulation results show the system performance in response to the load and dc‐bus voltage step changes while directly running a Simulink model as a controller. The performance and validity of the preferable control method is verified by the simulation results and corresponds to available literature. The simulated results show that the controller can stabilise the constant dc-link voltage under dynamic and steady-state performance of the system.

Efficient Power Flow Management in Hybrid Renewable Energy Systems

ABSTRACT In a DC microgrid, the main aim of power management is to maintain the balance of active power between renewable sources, storage batteries, loads, and distribution grid which can be represented by stable DC-link voltage. This paper proposes an adaptive strategy for power flow management of a stand-alone hybrid renewable energy system (HRES) consisting of a wind turbine generator, photovoltaic cell, battery energy storage (BES), and load by optimizing the gain of the PI controller in the battery storage system using the particle swarm optimization-based firefly algorithm (PSO-FA). A part of the PSO algorithm is utilized for improving velocity and attractiveness of the firefly algorithm thereby increasing the rate of convergence in the PI controller. This control technique improves gain constant by generating control pulses for charging and discharging of the BES system in order to make optimum use of renewable energy sources and to stabilize the voltage of the DC-link. From the simulations done in MATLAB/SIMULINK, it is observed that conventional trial- and error-based PI tuning produces high overshoot, whereas the proposed method produces better performance with less overshoot. Maintaining constant DC-link voltage, the instantaneous power generation under various conditions of generation and load consumption is managed by monitoring the state of charge of battery. This approach can be used as a complement for improving the performance of PI controllers designed by conventional methods.

Integral sliding mode control for back‐to‐back converter of DFIG wind turbine system

In this study, an integral sliding mode control approach for controlling the power electronics converters of a doubly-fed induction generator (DFIG) wind turbine system is presented. The power electronics interface consists of back-to-back converters. The rotor side converter regulates the active and reactive powers at the DFIG stator through controlling the stator currents. The stator current dynamics, with respect to the rotor voltages, is developed from the conventional equations of the DFIG model. In this control configuration, the knowledge about the rotor currents is not required, which reduces the use of the current measurement sensors. The grid side converter ensures constant dc-link voltage while transferring the power from the DFIG rotor to the grid. The proposed control approach uses a composition of sliding mode and integral parts to improve the overall performance and robustness against parametric variations and uncertainties. A lab-scale DFIG wind turbine system is used to investigate the proposed control approach efficiency under various operating conditions. The experimental results show the effectiveness of the proposed control approach in achieving control objectives to operate the DFIG wind turbine system.

Active DC‐link balancing and voltage regulation using a three‐level converter for split‐link four‐wire system

The integration of the distributed generation to the unbalanced loads or the grid requires a three-phase four-wire inverter. The three-phase four-wire inverter could be of three-leg or four-leg topology. However, both the topologies have their drawbacks. The three-leg inverter topology with a split capacitor suffers from poor DC link voltage regulation and poor DC link voltage utilisation. The four-leg inverter topology suffers from poor electromagnetic compatibility. To solve these problems, the split link inverters with active balancing capabilities are studied by many researchers. The split link inverter with an active balancing inverter requires a constant DC link voltage and has less flexibility regarding the variation of the DC link voltage. In this study, a new converter topology is proposed, which provides the controllable DC link voltage to split link inverter with active balancing capability and also reduces the voltage stress to semiconductor switches to half DC link voltage. The detailed description of the proposed converter topology, design of the controller, and control hardware-in-the-loop verification are presented in this study.

Coupled Modeling and Advanced Control for Smooth Operation of a Grid-Connected Linear Electric Generator Based Wave-to-Wire System

The perpetual oscillations of ocean waves produce potential energy, which can be converted to electrical energy with the help of direct-drive linear generators. The fluctuating generated power poses a major challenge when it is supplied to the power grid. In this article, a supercapacitor provides the short-term energy storage to buffer and smooth out the power fluctuations. A new coupled model of a wave energy converter and a linear generator is proposed for its response characterization under varying system conditions. The developed model and an advanced control strategy are used to exhibit a smooth and stable operation of the wave-to-wire system. The generator-side converter is controlled to extract the maximum power from the waves and to minimize the generator losses by controlling its d -axis and q -axis currents. The grid-side converter is controlled to keep the dc-link voltage constant and to generate the required voltage waveforms at the point of common coupling. The performance of the proposed control strategy for the wave-to-wire system is investigated under different applied diffraction forces. The simulation results show that with the use of the proposed control scheme and the supercapacitor, the wave-to-wire system can operate in a smooth and stable operation under normal and fault conditions.

Optimized Active Power Management in Solar PV-Fed Transformerless Grid-Connected System for Rural Electrified Microgrid

Dynamic voltage instability is one of the major issues faced by grid-connected renewable energy systems due to fluctuations in the generation and sudden variation in the loads. The primary objective of this paper is to propose a method for constant power consumption from the grid to maintain a stable DC-link voltage during peak and nonpeak hours. It can be achieved by implementing an optimized active power management (OAPM) scheme between the photovoltaic (PV) and the grid by enabling a battery energy storage system (BESS). The intelligent constant power balance (ICPB) algorithm and detailed control strategies for dual active bridge (DAB) isolated DC–DC converter and grid-connected voltage source inverter (VSI) are discussed in this paper. Moreover, the high-gain step-up DC–DC converter (HSDC) is utilized to perform maximum power point tracking (MPPT) operation and to meet the required DC-link voltage. The accuracy of power transmission of DAB gets improved by imposing a curve-fitting interpolation (CFI) approach, thereby maintaining a constant DC-link voltage. Furthermore, an instantaneous sinusoidal current control (ISCC) scheme assures the feeding of active power with better power quality. The measured results are obtained and verified under different dynamic conditions of load and generation. Based on the validation, we conclude that the proposed OAPM scheme is most suitable for grid-connected renewable energy systems in the rural electrified microgrid.

DC Capacitor-Less Solid-State Variable Capacitor

H -bridge-based single-phase applications have second-order harmonics on the dc-link voltage. Conventionally, the second-order harmonics are filtered by bulky dc capacitors to maintain the constant dc-link voltage, which results in poor power density and low reliability. However, in applications, such as the solid-state variable capacitor (SSVC), the constant dc-link voltage is unnecessary since the dc link is disconnected from the load or source. This article proposes a dc capacitor-less SSVC, which removes the dc capacitors from the topology. The dc voltage is supported by the ac-side voltage directly. Only a 0.03 per unit (p.u.) capacitor is needed to filter the switching-frequency harmonics. This enables the proposed SSVC to use the snubber capacitors to achieve the function of the 0.03-p.u. filtering capacitor. Other capacitors can be removed from the dc-link busbar. The modulation, modeling, and control strategy of the proposed dc capacitor-less SSVC are provided in this article. The proposed dc capacitor-less SSVC is validated by the experiment.

Harmonics Reduction of Adjustable Speed Drive Using Transistor Clamped H-Bridge Inverter Based DVR With Enhanced Capacitor Voltage Balancing

Harmonics generated by nonlinear critical loads are major anxiety for the industries. A control technique for dynamic voltage restorer (DVR) is described to limit voltage harmonics introduced by adjustable speed drive (ASD) in the utility. Moreover, this control technique maintains constant dc-link voltage of ASD during any disturbances occurring at point of common coupling. Mathematical description to limit ASD voltage harmonics and to generate reference dc-link voltage of ASD is illustrated in the dq0 frame. Transistor clamped H-bridge (TCHB) multilevel inverter (MLI) is used as a voltage source inverter of DVR. An enhanced capacitor voltage balancing (ECVB) method for TCHB MLI for the fast dynamic performance of DVR is also discussed. The proposed control technique of DVR to limit ASD voltage harmonics is simulated using MATLAB software. Furthermore, a simulation for DVR protecting ASD during source harmonics, unbalance, sag, and swell is carried out and results of ASD performances are demonstrated. The working of the proposed controller of DVR with ECVB strategy based TCHB is reverified using experimental results. ASD voltage harmonics mitigation and fast dynamic response of ECVB based TCHB are the highlights of this article. The performance of the proposed strategy shows satisfactory results with a less complex control system.

Improved DC-Link Voltage Regulation Strategy for Grid-Connected Converters

In this article, an improved dc-link voltage regulation strategy is proposed for grid-connected converters applied in dc microgrids. For the inner loop of the grid-connected converter, a voltage modulated direct power control is employed to obtain two second-order linear time-invariant systems, which guarantees that the closed-loop system is globally exponentially stable. For the outer loop, a sliding mode control strategy with a load current sensor is employed to maintain a constant dc-link voltage even in the presence of constant power loads at the dc-side, which adversely affect the system stability. Furthermore, an observer for the dc-link current is designed to remove the dc current sensor at the same time improving the reliability and decreasing the cost. From both simulation and experimental results obtained from a 15-kVA prototype setup, the proposed method is demonstrated to improve the transient performance of the system and has robustness properties to handle parameter mismatches compared with the input-output linearization method.

RETRACTED ARTICLE: Power flow control and power quality analysis in power distribution system using UPQC based cascaded multi-level inverter with predictive phase dispersion modulation method

Power quality is associated with the ability of utilities to provide electrical power without interruption. One of the major concerns in the electrical industry today is the issue of power quality over major loads. This work presents a Cascaded H-Bridge Multi-Level Inverter (CHBMLI) based unified power quality conditioner (UPQC) for the compensation of voltage sag/swell in the source side and current harmonics in the load side due to the non-linear loads. A predictive control based algorithm called as predictive phase dispersion modulation (PPDM) is used in the proposed system for the control of series and shunt active power filter of the UPQC. The shunt and series active power filter is connected back to back with a common DC link capacitor. The objectives of the proposed algorithm is to maintain a constant DC link voltage and to control the switching pulses of the CHBMLI according to the variations of the current in the load side and also the voltage unbalances in the source side. The performance of the UPQC is significantly improved with the proposed CHBMLI based UPQC than the conventional Voltage Source Inverters (VSI) based UPQC. The algorithm used in this work reduces the computational burden than the conventional model predictive controllers. The proposed CHBMLI based UPQC with PPDM technique is validated through simulation studies using MATLAB and a hardware prototype also realized and the results are investigated.

An Improved Adaptive Control Strategy in Grid-Tied PV System With Active Power Filter for Power Quality Enhancement

This article investigates the power quality enhancement of a grid-tied photovoltaic (PV) distribution system by employing a fuzzy logic proportional-integrator-derivative multiple complex coefficient filter multiple second-order generalized integrator frequency-locked loop (FLPID-MCCF-MSOGI-FLL) hybrid control scheme based shunt active power filter. The MSOGI-FLL reference current generation strategy is implemented to mitigate the current harmonics by extracting the fundamental constituents (FCs) from the nonlinear load currents, whereas an MCCF is employed to separate the FC from the distorted grid voltages and eliminates the voltage harmonics during extremely polluted grid voltage condition. The main objective of using FLPID is to maintain the stable power between dc and ac sides by regulating the dc-link voltage constant under transient conditions. To track the maximum power from the PV panel under varying environmental condition, the particle swarm optimization based perturb and observe technique is used in this article. The comparative analysis is analyzed to check the effectiveness of the proposed hybrid control scheme with existing and adaptive control techniques in respect of power quality, better dc offset rejection, better FC and frequency extraction, and grid synchronization. The system with the proposed control scheme is simulated on MATLAB/Simulink and validated in a real-time field-programmable gate array platform under different test scenarios. In conclusion, the harmonic content of grid currents and voltages is found well within the IEEE-519 standard limits.

Pulse Width Modulation of Three Phase Inverters Based on Linear Programming

Traditional three-phase voltage-source inverters supplied by constant dc-link voltage usually utilize the space vector PWM to achieve maximum output voltage. This paper deals with optimization of inverter leg voltages using linear programming method. System definition is based on the relationship between the known voltage vector, which is demanded by an upper control loop and the unknown leg voltages which enter the carrier-based PWM block as modulation signals. A slack variable is introduced to the system as minimization objective, defining border for all leg voltages. Optimization procedure minimizes leg voltage maxima, and thus, the maximum utilization of the dc-link voltage is ensured. The theoretical assumptions have been verified by simulations as well as by experiments on laboratory prototype.

MPPT Control for Grid Connected Wind Energy Conversion System Based Permanent Magnet Synchronous Generator (PMSG) and Five-Level Neutral Point Clamped Converter

This paper presents nonlinear control of a Wind Energy Conversion System (WECS) based Permanent Magnet Synchronous Generator (PMSG) and Five-Level Neutral Point Clamped Voltage Source Converter (5L-NPC-VSC). For WECS, the Maximum Power Point Tracking (MPPT) is a very important requirement in order to maximize the effectiveness. Therfore, MPPT approach and grid-connected control methods are discussed. On the other hand, the proposed control techniques are based on Sliding Mode Approach (SMA) and Vector Control (VC) to achieve the MPPT, to keep the dc link voltage constant and to the control active and reactive powers. The stability of the regulators is obtained using Lyapunov analysis. An extensive simulation study, using MATLAB/Simulink, is conducted on a 4-MW wind farm composed of two 2 MW PMSG based wind turbines.

A Modified PFC Rectifier Based EV Charger Employing CC/CV Mode of Charging

Automobile industry has displayed an inclination towards Electric Vehicles (EVs). However, EVs charging throws inevitable challenges due to inclusion of non-linear charger circuitry. The conventionally utilized AC-DC rectification in charger poses ruinous effects to Grid and EV structure in the form of harmonics interference and obnoxious spikes in current. Thus, repercussions of elevated THD can be witnessed in poor efficiency and deterioration of EV charger. Furthermore, harmonics in input inductor current produce harmonics in rectifier’s output voltage. This can lead to DC link voltage fluctuation and adversely affect DC/DC converter functioning. Henceforth, a Power Factor Correction (PFC) rectifier based charger has been proposed that eliminates unwanted harmonics from input current and reduces THD. Moreover, harmonics in rectifier’s output voltage are reduced and constant DC link voltage is obtained. Sinusoidal input current is maintained through Critical Conduction Mode (CrCM) and hysteresis current control application. These are achieved using inner current and outer voltage control loop method. The former produces sinusoidal current wave in phase with input voltage to improve power factor. Whereas, latter helps in achieving constant DC link voltage. Hence, THD factor of 1.30% and power factor of 0.9998 are recorded. In addition, model inculcates CC/CV charging algorithm to control overcharging of battery. Here, battery charges at Constant Current (CC) initially. Once, maximum voltage is reached, charging occurs at Constant Voltage (CV). It is governed by two isolated PI controllers. The collaborated work of PFC and CC/CV helps in recording model’s efficiency of 96.8%. Furthermore, a 2 kW charger prototype is analysed using real time simulation and validated through Hardware-in-loop (HIL) in OPAL-RT.

Immersion and Invariance Manifold Adaptive Control of the DC-Link Voltage in Flywheel Energy Storage System Discharge

In order to keep constant DC-link voltage of a flywheel energy storage system (FESS) discharge in a wide rotational speed range, the control structure of the FESS is comprised of an inner current loop and an outer DC-link voltage loop. Since the dynamic equation of the DC-link voltage in the FESS discharge is nonlinear, it is difficult for some controllers to make the DC-link voltage in discharge be constant as the rotational speed is varying in a large range. Considering the nonlinearity of the DC-link voltage in discharge and the fast discharge requirements of the FESS, an immersion and invariance manifold (I&IM) adaptive nonlinear controller for a constant DC-link voltage is proposed via methodology of immersed in the invariant manifold. The stability of the control algorithm and the influence of the parameter error on the stability are verified by the Lyapunov stability theory, and the influence of the parameters error on the steady state and transient characteristics of the closed-loop system is analyzed numerically. It is proved that the closed-loop system satisfies the global uniform asymptotic stability conditions at the equilibrium point, and the error of the model parameters does not affect the equilibrium point of the system. Finally, the effectiveness of the I&IM adaptive nonlinear controller were studied by simulation and experiment. The results show that the DC-link voltage in discharge remains stable when switching the system load in cases of different rotational speeds and loads.

Power Balance Modes and Dynamic Grid Power Flow in Solar PV and Battery Storage Experimental DC-Link Microgrid

In this paper, an energy management system, based on different power balance modes and dynamic grid power flow, is proposed to operate a DC-link microgrid based on a solar photovoltaic generator and battery storage, with the option to request variable power from the grid to meet the load demand. The energy management provides the required references, for each mode, based on the solar source availability, the battery status, the power losses, and the grid billing rate. A fuzzy logic system is developed to provide a dynamic grid power flow based on the grid price. Eight power balance modes are defined based on the power generation, storage, and grid affordability to meet the load demand. The objectives are to minimize the energy cost and increase the lifespan of the storage device. The microgrid is controlled to maintain a constant DC-link voltage and regulate the battery current depending on the mode of operation. The proposed energy management system, based on the power balance modes, is experimentally validated on a laboratory-scale DC-link microgrid for different conditions. The experimental results have shown the satisfactory performance of the microgrid and smooth transitions between the different power balance modes.

Nonlinear Control Based on Fuzzy Logic for a Wind Energy Conversion System Connected to the Grid

This paper presents modeling, analysis, and simulation of a variable-speed wind turbine control. Wind turbine emulator (WTE) based on induction motor (IM) is used to provide a controlled environment for wind power generation system control testing. The permanent magnet synchronous generator (PMSG) is tied to the grid via back-to-back converters. The aim of the study is threefold: the development of the WTE, extracting the maximum power, and feeding captured power to the grid. The methods of maximum power point tracking (MPPT) using Fuzzy logic is used to maximize the wind power capture at different wind speeds. The grid-tied inverter controlled by the fuzzy-PI controller is used for transferring power to the grid and maintaining DC-Link voltage constant. Simulation results performed on the Matlab/Simulink environment, verify the performance of the different control strategies and the usefulness of the approach .