DC Link Voltage Regulation Research Articles

An Improved FCS-MPC Strategy for Low-Frequency Oscillation Stabilization of PV-Based Microgrids

With the rapid promotion of photovoltaic (PV) power generation, the local consumption of distributed PV power generation at medium and low voltage levels widely forms PV-based microgrids. The continuous increase in the penetration rate of uncertain PV power generation requires it to actively participate in the frequency and voltage regulation of weak microgrid systems. Finite control set-model predictive control (FCS-MPC) for voltage source converter (VSC) shows remarkable advantages in fast dynamic response and robustness compared with traditional cascaded linear control methods. However, the traditional FCS-MPC method for VSC without considering the DC-link voltage dynamic of the PV generator will face oscillations of power and frequency in PV-based microgrids. In this paper, to suppress the low-frequency oscillation issue caused by PV generations, an improved FCS-MPC (I-FCS-MPC) strategy considering the DC-link dynamics of PV generation is proposed, in which the AC side and DC side of the VSC are comprehensively considered to enhance the dynamic performance of the PV-based microgrids. Moreover, a DC-link voltage regulation mechanism is introduced, enabling the PV generators to operate in the maximum power point tracking (MPPT) mode. Finally, experiments are conducted to verify the effectiveness of the proposed control strategy comprehensively.

Efficiency Optimization and Resilience Improvement in Wireless Motor System With Flexible DC-Link Voltage Regulation

This paper proposes a novel dc-link voltage regulation (DCVR) strategy for the series-series compensated wireless motor (SSWM) system. The steady-state voltage control is presented to optimize the system efficiency and the feedforward voltage boost control is proposed to improve system resilience. The rated capacity identification (RCI) scheme is proposed to design proper parameters for the SSWM system. To achieve steady-state voltage control, the system efficiency characteristic is analyzed in detail and the efficiency optimization can be achieved without an auxiliary dc/dc converter. The influence of impedance matching, the power capacity of the system, and overmodulation of the motor drive are all investigated. Then, the dynamic model of the system is established to analyze the stability of dc-link voltage. In the proposed feedforward voltage boost control, the dc-link voltage is boosted according to the motor operation state to increase the instantaneous power capacity. The dc-link voltage stability is improved and the system resilience is enhanced against power disturbance. Besides, at the end of the acceleration process, the dc-link voltage is smoothly adjusted to the optimal dc-link voltage. The effectiveness of the proposed RCI scheme and DCVR strategy is verified by experiments on a three-phase wireless permanent magnet synchronous motor platform.

Nonlinear control design and stability analysis of hybrid grid-connected photovoltaic-Battery energy storage system with ANN-MPPT method

The problem of controlling a grid-connected solar energy conversion system with battery energy storage is addressed in this work. The study’s target consists of a series and parallel combination of solar panel, DC/DC converter boost, DC/AC inverter, DC/DC converter buck-boost, Li-ion battery, and DC load. The main objectives of this work are: (i) PV voltage regulation: the PV panel voltage must track a given reference voltage corresponding to the maximum power point. The optimal reference voltage is generated by an optimizer based on the artificial neural network (ANN); (ii) DC link voltage regulation: The DC link voltage must track a reference voltage as closely as possible. (iii) PFC requirement: The grid currents must be sinusoidal with the same frequency and in phase with the voltage grid. The periodic nature of solar energy and the frequent fluctuations in load demand reduce battery life and charging performance. To deal with these limitations and ensure the battery’s safety, a battery energy management algorithm is developed with the following objectives: (iv) CC mode: When the battery’s state of charge is less than 100% (SOC<100%), it indicates that the battery is not fully charged, this aim has been adopted. The battery current should follow a constant reference as closely as possible to the permissible limits battery current; (v) CV mode: When the battery is completely charged, this mode is switched on. The voltage at the battery terminals then achieves its reference signal, which corresponds to the charge state’s of the battery SOC=100%. In addition, the energy management system which generates several energy flow scenarios, in this work, the focus is to balance the energy flows between the load and the different energy sources to minimize the system costs, to ensure the stability of the grid and to improve the power quality. To do this, a mathematical modeling of the overall system was performed. Subsequently, backstepping controllers are then synthesized in order to ensure the control objectives. The closed loop control convergence is formally analyzed using Lyapunov’s stability theory and its performances are illustrated by simulation. As a result, the simulation results indicate that the proposed controllers perform admirably in achieving their objectives.

Single-phase dual-input split-source inverter for photovoltaic systems

This paper proposes dual-input configuration of split-source inverter (abbreviated as DSSI) to transfer the power of two photovoltaic (PV) modules simultaneously or individually. The proposed DSSI keeps the continuity of the input current and the voltage boosting capability offered by the conventional split-source inverter (SSI). Thus, independent as well as efficient tracking of the maximum power points (MPPs) is realized. In addition, low capacitance is utilized for power decoupling in single-phase circuits. Compared to using one SSI per PV source to feed a common load, the proposed DSSI uses lower number of switches at reduced total volt-ampere stresses. Therefore, decreased cost, size, and control circuitry are realized. Simple unipolar modulation technique is presented. Accordingly, the modes of operation and the Volt-ampere stresses of the switches are explained. A closed loop control system is explained to fulfill operation at MPP along with regulation of DC-link voltage and load current. Comparative study with other dual-input topologies is presented. Experimental results as well as real-time simulation results for dual- and single-input scenarios are presented. Also, the dynamic performance under step change in the radiation is investigated. Finally, the capability of operation under different power factors is verified.

An Investigation on the Integration of a Hybrid Offshore Wind-Wave Energy Conversion System With the Distribution Network

This paper introduces the hybridization of multiple linear permanent magnet generator (LPMG)-based wave energy conversion systems with a doubly-fed induction generator (DFIG)-based wind energy conversion system. A detailed investigation of the impact of the integration of the wind-wave hybrid system with the distribution network is presented in this study. The control scheme for the multiple LPMGs ensures the optimum extraction of the wave power and the ability to maintain voltage balance at the outputs of the LPMGs. Similarly, the back-to-back converters of the DFIG are controlled for the maximum power point tracking of the wind turbine and the dc-link voltage regulation. The turbine rotor blade pitch angle is controlled to minimize the power fluctuations caused by the integration of LPMGs at the common DC bus. The time-domain simulation results confirm the effectiveness of the proposed system by showing a stable operation when integrated with the distribution test feeder. Finally, the steady-state analysis shows that the voltage profile of the distribution network is also improved, and the step voltage regulator of the distribution feeder can be removed in the presence of the proposed system without violating the voltage limits at any node of the selected distribution network. Further, the response of the distribution network is also evaluated under an electrical fault condition in the presence of the wind-wave hybrid system.

Enhanced Grid Frequency and DC-Link Voltage Regulation in Hybrid AC/DC Microgrids Through Bidirectional Virtual Inertia Support

AC mains frequency and dc-link voltage are two of the most important parameters that determine the stability of hybrid ac/dc microgrids. Frequency and voltage deviations beyond the acceptable threshold can lead to unscheduled load curtailments, cascading failures, or even power system blackouts. To enhance the stability of hybrid ac/dc microgrids, this paper proposes a hybrid microgrid with bidirectional virtual inertia support, where virtual inertia is introduced to slow down dc voltage and ac frequency changes, thus enhancing system stability. The proposed approach allows significant reductions in voltage and frequency fluctuations. With a standard hybrid microgrid configuration, inertia is delivered to both ac and dc subgrids via the change of bi-directional interlinking converter control. Specifically, the bidirectional interlinking converter uses the dc voltage of the dc grid and ac frequency of the ac grid as inputs through a proportional integral (PI) controller and regulates the active power between two sub grids so that inertia and other supportive functions are implemented. To validate the merits of the proposed approach, experimental results are provided. According to experimental results, the frequency nadir, rate-of-change-of-frequency (RoCoF), and dc bus voltage deviation can be improved by the proposed strategy of bidirectional virtual inertia support in the hybrid ac/dc microgrid.

Autonomous Control of Soft Open Point for Distribution Network Reliability Enhancement

Power-electronics-interfaced soft open points (SOPs) emerge as a promising solution for the fault-tolerant operation of distribution networks. The conventional SOPs interface two AC terminals with a back-to-back circuit configuration. Nevertheless, the SOP will break down if a fault occurs and interferes with the converter responsible for DC-link voltage regulation. To address this problem, a smart SOP with enhanced reliability is developed in this paper. The proposed SOP can protect the system from the fault, which occurs at any of the two AC terminals. More importantly, the control of SOP is fully autonomous and does not need remote monitoring, manual activation, or operation mode transformation. This merit makes the proposed SOP more reliable and intelligent than the conventional SOP and normally-open point (NOP). Finally, the effectiveness of this proposal is validated by simulation and experimental results.

A smart control for self-reliant single-phase, grid-tied photovoltaic-battery storage system

This paper presents a grid-tied, solar energy conversion-battery energy storage (BES) system with an autonomous control method for critical load applications. In order to improve grid current dynamics and ensure that balance power is exchanged as an active power with utility, direct current management of the voltage source converter utilizes least mean fourth method. The intermittent nature of solar photovoltaics also affects dynamics and power quality of grid current. A potential solution to such a situation evolves by using a bi-directional, DC/DC converter which interfaces an optimized-sized BES system with a DC link. Dual-loop DC/DC converter control keeps BES system charging and discharging current within pre-set limits and also simultaneously regulates DC link voltage. State-of-charge control gets incorporated through a dual-loop control to prevent BES from overcharging/deep-discharging. Dual-loop voltage control with harmonic compensation is used to manage load voltage amplitude and frequency in islanded mode. Auto-grid synchronization is realized by using a first-order, frequency-locked loop (FLL) technique. Using FLL also reduces dc offset, harmonics, and inter-harmonics even in distorted grid circumstances. The presented investigation also proposes a step-by-step controller parameter design procedure based on the frequency domain stability restrictions. Experimental results using OPAL RT-5600 validate the expected system response.

Kalman Filter-Based Super-Twisting Sliding Mode Control of Shunt Active Power Filter for Electric Vehicle Charging Station Applications

Electric vehicle (EV) charging stations draw nonlinear currents, which makes the distribution network unbalanced, distorted, and results in power quality (PQ) issues. These PQ issues are mitigated in this work through high-performance control of the shunt active power filter (SAPF). In the proposed method, a linear Kalman filter (LKF) has been applied to grid voltage and load current signals for harmonic and disturbance robust estimation purpose. Unlike the conventionally used orthogonal vector model, in this work, phase angle vector model together with a simple to tune phase-locked loop (PLL) has been considered for the LKF implementation. DC-link voltage regulation and charging of the DC-link capacitor has been obtained by proposing a Luenberger observer-based super twisting sliding mode control (ST-SMC), which has fast dynamic response and lower voltage ripples compared to similar other existing control methods. This results in significant reduction in size, cost and loss together with lifetime enhancement of the DC-link capacitor. Rigorous sensitivity analysis is conducted to analyze the robustness of the developed method. The proposed control technique achieves fast response time and satisfy the harmonic requirements as specified in the IEEE Std. 519 under various grid and load disturbances. Comparative quasi-real time validation results are presented by using digital signal processor (DSP) based processor-in-the-loop (PIL) with another recently proposed control strategy to verify the performance enhancement by the developed method.

Fuzzy and PSO tuned PI controller based SAPF for Harmonic Mitigation

The development of a reliable power filter is essential for meeting the need for high-quality power. Current and voltage harmonics are a major contributor to poor power quality and must be eliminated. Shunt active power filters (SAPFs) can be installed to reduce the negative effects of harmonics. Fuzzy logic and proportional integral (PI) controllers excel at regulating DC link voltage in shunt active power filters (SAPFs). This research assessed how well Particle Swarm Optimization controls the DC link voltage in a Shunt Active Power Filter (SAPF), thereby mitigating harmonics. The appropriate PI control parameters are first determined using the Particle Swarm Optimization technique. The PSO-tuned PI parameters are combined with a fuzzy logic controller for the SAPF simulation. It has been demonstrated in simulations that a Shunt Active Power Filter (SAPF) with reference currents are generated in accordance with the instantaneous real and reactive power (p-q) theory, employing fuzzy and PSO -PI controller for diode rectifier type non-linear load. It is extremely effective for harmonic correction in distribution networks. The simulation work is carried out using MATLAB-SIMULINK.

A Novel Resilient Control of Grid-Integrated Solar PV-Hybrid Energy Storage Microgrid for Power Smoothing and Pulse Power Load Accommodation

Solar photovoltaics (PVs) are increasingly penetrating remote are a power systems. However, the adverse effect of pulse power loads and fluctuating PV power brings severe grid instability. Therefore, an effort is made to propose a hybrid energy storage system (HESS) that encompasses hydrogen/bromine redox flow battery (RFB) and supercapacitor (SC) for grid-integrated PV systems to facilitate high energy and power support. Furthermore, the system is equipped with an antiwindup mixed-order generalized integrator (AWMOGI), an improved sparrow search algorithm (ISSA) tuned tilt integral–derivative with filter (TIDF) controller, and a fast-acting adaptive dc-link power-based energy management control. AWMOGI extracts fundamental components of grid voltage and is immune to noise and grid disturbances. The ISSA-tuned TIDF controller has better error mitigation ability and quickly settles the actual signal to the reference for dc-link voltage control of the voltage-source inverter, PV maximum power point tracking, and HESS control that improves the overall system dynamics. The energy management system coordinates RFB and SC participation in handling sudden and average changes in power surges resulting in the first dc-link voltage regulation and less current stress on RFB. The proposed system and furnished control effectiveness are confirmed through MATLAB simulation and OPAL-RT real-time simulator test bench.

Harmonic Mitigation in a Hybrid Power System Integrated Shunt Active Power Filter Employing FLC and an Adaptive Current Control Technique

The main objective of this study is to mitigate current harmonic issues in hybrid renewable energy systems based on solar photovoltaic (SPV) and wind energy distribution. This is achieved with the help of a suitable controller-based shunt active power filter (SAPF). The SAPF is designed using a modified synchronous reference frame (MSRF) to reference current generation, an adaptive hysteresis current controller (AHCC) for switching pulse generation, and a fuzzy logic controller (FLC) for DC-link voltage regulation. The results of the proposed SAPF model developed in MATLAB/Simulink, show that the filter performs remarkably well in suppressing harmonics under different loading conditions. It is capable of fast corrective action under dynamic conditions and outperforms previous methods in terms of harmonic mitigation and dc-link voltage stabilization. The control techniques are compared on the basis of parameters such as harmonic compensation and dc-link voltage ripple reduction capability under changing load conditions. The results obtained through simulation represent the validity of the filter performance.

Simulation of Single-Phase on-Grid Photovoltaic Inverter for Power Injection and Active Power Filter

Currently, most photovoltaic (PV) sources are connected to the grid. This research discusses single-phase on-grid PV inverters. A two-stage inverter which consisted of a boost-type DC-DC converter and a single-phase inverter, was used. In addition, the inverter improved the power quality to deliver PV maximum power. The entire power generated by PV was to be delivered to PCC, and power quality in PCC was also improved. In this system, the grid only drew or supplied active power. The P&O algorithm, as a simple algorithm, was used to control the boost converter to obtain the maximum PV power. In a single-phase inverter, the DC link voltage regulation was carried out using the PI control (outer loop), while the hysteresis control was used to control the output current (inner loop). The voltage control regulated the power delivered from the PV to the PCC by maintaining a constant DC bus voltage at the specified value. With the current control, a single-phase inverter provided two compensations: reactive power and harmonics. In this research, a simulation to control a two-stage inverter was created by using PSIM. Irradiation for PV was varied between 0-1000 W/m2 for 5 seconds. The simulation results showed that the controls performed could work well, as shown by the maximum power injection from the PV to the PCC in which the grid current was sinusoidal (harmonic mitigation) and reactive power compensation was performed.

Power quality improvement of large power induction motor drive using three-level front-end converter

PurposeThe purpose of this study is to provide the enhancement of power quality of a high power-rated voltage source inverter driven induction motor with a three-phase, three-level neutral point clamped converter placed at the front end, while a passive power filter is connected in shunt with it. The improvement in power quality can be achieved by reducing the total harmonic distortion in source current. The controllers were designed for the linearization of the high-power induction motor drive. A control method is presented for the regulation of the common DC-link voltage.Design/methodology/approachThe induction motor is modeled using its dynamic equations, and a decoupling controller is designed to linearize the nonlinear dynamics of the drive through feedback. The common DC-link voltage of the proposed front-end connected converter is monitored and controlled through a control method which feeds the pulse width modulated inverter that drives the induction motor. A passive power filter is designed to meet the reactive power requirement of the system in addition to improve the power quality.FindingsSimulations were carried out for the proposed topology of the drive mechanism, and the outcomes were analyzed by a comparative analysis of the drive system both in the presence of the passive filter as well as in the absence of the filter. The total harmonic distortion is found to be reduced enough to meet the standards with the designed filter, and the reactive power is also compensated considerably. The input power factor at the supply side is maintained almost to unity, and the DC-link voltage of the proposed circuit topology is maintained at the desired level. The overall performance of the drive system was found to be useful and economical.Originality/valueA new topology of a front-end connected three-level neutral point clamped converter to a high power-rated induction motor drive is proposed. The drive is fed by a pulse width modulated inverter with a common DC-link with the front end connected converter. A passive filter is designed with respect to the reactive power requirement of the system and connected in shunt to the converter at the supply side. Control schemes are designed and used for the drive system and also for the regulation of the common DC-link voltage of the proposed front end connected converter.

Type-2 fuzzy logic PID control for efficient power balance in an AC microgrid

This study suggests the Type-2 fuzzy logic proportional-integral-derivative (T2-FLPID) control strategy, optimally tuned using particle swarm optimization (PSO), for exigence control and efficient power management in an AC microgrid by efficient power sharing among various components and regulating DC link voltage under operational uncertainties. Proposed control strategy is assessed relative to Type-1 FLPID (T1-FLPID), and conventional PI controller, both also tuned with PSO, for better appreciation of the proposition. The varying operating conditions investigated here include variations in solar irradiance, wind speed, load demand, and grid isolation scenario. Results, analyzed and showcased both qualitatively and quantitatively, indicate that the proposed T2-FLPID controller excels the other two controllers in terms effective maintenance of power balance via seamless coordination of power among various components and providing the least values of error indices and computational time.

Flywheel Energy Storage Supported Adaptive Energy Management Strategy for Solar-powered Electric Vehicle Charging Station

The large-scale integration of solar photovoltaic systems and electric vehicles into power systems result in technical challenges due to the volatile nature of the generation and electric vehicle load. The paper presents an energy-storage supported adaptive DC-link voltage regulation based energy management strategy for improving hybrid AC/DC microgrid stability. The proposed volatility based control approach improves hybrid microgrid stability under volatile electric vehicle loading and renewable energy fluctuations. The adaptive energy management strategy limits the overstress on flywheel energy storage depending on the flywheel SoC profile. Different load profiles and source intermittency are considered to analyze the effectiveness of the proposed strategy. The combined control strategy of the interlinking converter and flywheel energy storage in power exchange mode operation and independent mode operation achieve energy balance with the change in solar irradiation and the addition/disconnection of electric vehicles. In addition, real-time experiments are performed to validate the proposed energy management strategy under various volatile conditions. The hybrid AC/DC microgrid with proposed energy management strategy provided a frequency improvement of 0.44% and voltage improvement of 7.5%.

Power quality and stability improvement of microgrid through shunt active filter control application: An overview

Owing to avoid harmonic and power quality issues, the concept of eliminating their impacts on microgrid systems has gained a lot of interest. Incidentally, shunt active filters (SHAFs) is selected as the most reliable solutions against the concern problems and become the first choice of researchers. However, the performance of SHAF is strictly dependent upon the controller's action, design, and stability. Looking at the necessity, the detailed working of parallel/series filters for current and voltage source-based non-linear load application is discussed and compared. This paper reviews and collects information related to various SHAF control techniques for improving microgrid performances. The effectiveness of the controller is examined and justified by considering the non-linearity reduction, dc-link voltage balancing, current and voltage regulation, and improving the synchronization techniques. In this review, the most advanced control techniques are discussed and contrasted systematically to highlight their strengths and weaknesses. In addition to that, by considering different control architectures, the possible control outcomes and shortfalls are also summarized in different tabular forms. The survey can hypothetically serve as a standard and establishment of material for selecting the most significant methods for smoothening the SHAF operation for complex microgrid systems.

Stable Maximum Power Extraction and DC Link Voltage Regulation for PMVG-Based WECS

This study discusses the power maximization and dc-link voltage regulation problems of a permanent magnet vernier generator (PMVG)-based wind energy conversion system. To do this, first, the dynamical model of PMVG-based wind turbine is developed and presented. Then, the overall control structure is configured utilizing a back-to-back converter with a machine-side converter (MSC) and a grid-side converter (GSC). At this time, the sliding-mode control scheme with the modified enhanced exponential reaching law is proposed for both MSC and GSC control to achieve maximum power extraction and stable dc link voltage, respectively. Furthermore, the sliding manifold’s stability condition is derived using the Lyapunov function, which guarantees a better transient performance and tracking accuracy. Finally, the proposed control scheme’s superiority and efficiency are demonstrated using theoretical simulations on 5 kW PMVG-based and 1.5 MW permanent magnet synchronous generator-based wind turbine systems and empirical findings derived from a grid-connected 5 kW PMVG-based wind turbine in the experimental setup.

Transient Stabilization Control of Electric Synchronous Machine for Preventing the Collapse of DC-Link Voltage

Electric synchronous machine (ESM) control is a power synchronization strategy that utilizes the measurement of dc-link voltage to generate a frequency reference. By using ESM, the grid-connected inverters with indeterminant renewable energies can work in grid-forming mode, and realizes a closed-loop control of the dc-link voltage. However, under weak grid conditions, the ESM provides a sluggish performance for the regulation of dc-link voltage and is prone to the transient instability issues, including dc-voltage collapse and synchronization instability, which hinders the application of ESM. This paper reveals the instability mechanism of dc-link voltage under large disturbances, and proposes a passivity-based control for the transient stabilization of ESM. Firstly, the transient stability of ESM is analyzed from the view of port-Hamilton system. The corresponding energy function comprises the energy stored in dc-link capacitor and potential energy in ac side. Then, a 2-degree-of-fredom controller is proposed to stabilize the dc-link voltage by enhancing the energy dissipation. It combines a phase-shift controller and supplementary voltage controller, fully utilizing the control flexibility of the inverters. Dissipation analysis proves the large-signal stability of the proposed control. Simulations and hardware-in-the-loop (HIL) experiments validate its transient performance in a single-inverter-infinite-bus (SIIB) system and a multi-inverter test system.

Internal Energy Based Grid-Forming Control for MMC-HVDC Systems With Wind Farm Integration

The virtual synchronous generator (VSG) control is regarded as an effective solution for operating converters in weak grid conditions due to its excellent grid support functions. However, successful implementation of the VSG control of an AC/DC converter relies on the existence of a steady DC voltage at the DC side, while this is not easy to achieve for the cascaded converter system of this work, i.e., the Modular Multilevel Converter-based High-Voltage Direct-Current (MMC-HVDC) transmission systems with offshore wind farm integration. To address this issue, a novel grid-forming control strategy with real-time inertia support and direct DC-link voltage regulation is proposed for the Receiving End Converter (REC) of the MMC-HVDC. The intrinsic power balancing regime of the internal energy stored in MMC's submodule (SM) capacitors is utilized for grid synchronization rather than emulating the swing equation as requested by the VSG control. For being more robust to sudden power variations, proper insertion strategies of the REC are developed to decouple the DC-link voltage of MMC-HVDC from the voltage of SM capacitors. Furthermore, in terms of the grid fault-ride-through issue of the REC, an associated FRT control strategy is proposed aiming for power angle stability and the stability of SM capacitor voltage. Finally, simulation results in PSCAD/EMTDC show that the proposed control can provide fast inertia support with satisfactory control of DC-link voltage and FRT capability, etc.