Control Strategy Of Converter Research Articles

Control strategy of DC/DC converter for photovoltaic-storage-DC microgrid based on improved LADRC

Aiming at the problem that the DC bus voltage is easily affected by nonlinear noise of the converter, photovoltaic output power and load disturbance, a bidirectional DC/DC converter control strategy based on the improved Linear Active Disturbance Rejection Control is designed. Firstly, establish the mathematical model of the bidirectional DC/DC converter, and analysis of existing problems in traditional LADRC. On this basis, a new state variable is introduced to observe the differential of the total disturbance to improve its rapidity; Then a lead-lag correction link is connected on the total disturbance observation channel to suppress the problem of high-frequency noise amplification. Finally, the Bode diagram is used to analyze the performance indicators of the improved LADRC. The simulation results show that, compared with the traditional LADRC, the control strategy in this paper reduces the bus voltage amplitude fluctuation by 63%, the adjustment time by 59%, and the inductor current ripple by 66%, which is more superior to suppressing the DC bus voltage fluctuation.

Nonlinear current control strategy for grid-connected voltage source converters

This work presents an enhanced proportional-integral- (PI) based current control strategy for voltage source converters (VSCs). While conventional solutions often employ the error between the desired and actual values of the current, the proposed method involves the nonlinear processing of the error signal. In particular, the square root value of the error is used to improve the steady-state performance. On the other hand, the quadratic value of the error signal is required to increase the dynamic response of the control system. This combined approach results in faster response, higher noise rejection, and achievement of proper stability margins. Unlike conventional nonlinear techniques, the introduced solution requires a simple tuning procedure and one can perform the stability analysis based on conventional figures of merit such as the gain margin (GM) and phase margin (PM) of the open-loop transfer function. The strategy is assessed by means of a grid-connected inverter used as an interface between a dc power source and the ac grid. It can be promptly extended to a wide range of practical applications involving current control loops as well. Experimental results are presented and thoroughly discussed to demonstrate the effectiveness of the proposed control technique.

Impedance Characteristics and Harmonic Analysis of LCL-Type Grid-Connected Converter Cluster

This study addresses the output impedance model of the LCL-type grid-connected converter considering the dead-time effects and the digital control delay. The model shows that the digital control delay will affect the accuracy of the output impedance of the grid-connected converter, and the dead-time effects are only equivalent to superimposing a disturbance voltage on the original output impedance model. The derived output impedance model is verified by comparing it with the switching model in the PSIM simulation environment. The harmonic interaction between converter cluster and utility grid is modeled and analyzed based on the output impedance model. A combined harmonic suppression strategy is incorporated into every converter control scheme to suppress the harmonic interaction. Simulation results are presented to demonstrate the correctness of harmonic interaction analysis and the effectiveness of the proposed suppression strategy.

Dynamic response and low voltage ride-through enhancement of brushless double-fed induction generator using Salp swarm optimization algorithm.

A brushless double-fed induction generator (BDFIG) has shown tremendous success in wind turbines due to its robust brushless design, smooth operation, and variable speed characteristics. However, the research regarding controlling of machine during low voltage ride through (LVRT) need greater attention as it may cause total disconnection of machine. In addition, the BDFIG based wind turbines must be capable of providing controlled amount of reactive power to the grid as per modern grid code requirements. Also, a suitable dynamic response of machine during both normal and fault conditions needs to be ensured. This paper, as such, attempts to provide reactive power to the grid by analytically calculating the decaying flux and developing a rotor side converter control scheme accordingly. Furthermore, the dynamic response and LVRT capability of the BDFIG is enhanced by using one of the very intelligent optimization algorithms called the Salp Swarm Algorithm (SSA). To prove the efficacy of the proposed control scheme, its performance is compared with that of the particle swan optimization (PSO) based controller in terms of limiting the fault current, regulating active and reactive power, and maintaining the stable operation of the power system under identical operating conditions. The simulation results show that the proposed control scheme significantly improves the dynamic response and LVRT capability of the developed BDFIG based wind energy conversion system; thus proves its essence and efficacy.

Black Start and Voltage Establishment Strategy for PMSG-Based Wind Turbine

During the black start, backup ac power sources such as diesel generators can offer line-side voltage reference for wind turbine and keep line-side converter of wind turbine work properly so that the dc-link capacitor voltage within converter can be established without overcharge. This study proposes a black start control strategy and line-side voltage establishment method for PMSG-based wind turbines with no ac power source. Unlike the traditional control strategy of full power grid-connected converters, the dc-link voltage within back-to-back full power converters of wind turbines can be controlled by generator-side converters, and the line-side voltage can be established by line-side converters with the help of fixed loads. The mechanical power can be balanced by pitch angle control, and the power unbalance between mechanical power and electrical power will be reflected in the rotor speed of PMSG. By this method, a single wind turbine can establish the line-side voltage with no extra backup ac power source, offering voltage reference for the other wind turbines during the black start of a wind power plant.

Distributionally Robust Optimization for Generation Expansion Planning Considering Virtual Inertia from Wind Farms

• The stability and reliability requirements in generation expansion planning are jointly considered. • A distributionally-robust-optimization-based generation expansion planning model considering the virtual inertia support from wind farms is proposed. • Frequency stability is guaranteed by inertia-based frequency requirements. • Balance between reliability and investment cost is achieved. High penetration of renewable energy generation imposes two significant challenges to power systems: the stability problem caused by low inertia and the reliability problem caused by generation uncertainties. Although these challenges have been widely recognized, their impacts on the optimal generation capacity mix have not been explicitly revealed and quantified. Luckily, with advanced converter control strategies, renewable generators may also provide the so-called virtual inertia similar to conventional inertia provided by thermal generators. This paper proposes a novel distributionally-robust-optimization-based generation expansion planning model considering the virtual inertia support from wind farms. The proposed model minimizes the generation expansion cost under uncertainty while maximizing the probability with which the system can fully absorb renewable energy generation. The constraints include expansion limits, unit commitment constraints, frequency requirements, virtual inertia provision, and distributionally robust joint chance constraints. The proposed formulation is recast into a mixed-integer second-order conic model and solved efficiently via commercial solvers. Case studies are carried on a 9-bus system and the IEEE 118-bus system to demonstrate the validity and scalability of the proposed method and highlight the importance of incorporating inertia response services

Control Optimization of Modular Multilevel Resonant DC Converters for Wide-Input-Range MVdc to LVdc Applications

In this article, the output voltage control strategy of modular multilevel resonant dc converters (MMRDC) for medium-voltage wide-input-range applications is studied. First, the feasibility and influence of the switching frequency regulation and modulation index regulation are investigated in detail. It is found that the switching frequency choice has a significant effect on the voltage stress of arm inductors and the transformer. Then, based on these considerations, the modulation index regulation plus switching frequency regulation based design and control optimization is proposed. With the proposed design and control method, the switching frequency range for adapting to a wide input range is further reduced, especially under light load operating conditions. Besides, the high voltage stress on arm inductors is avoided and the high voltage <i>dv</i>&#x002F;<i>dt</i> on the transformer is also eliminated. Finally, a laboratory prototype with 8&#x2013;16 kV input and 375 V&#x002F;60 kW output has been built and the experimental validation under a wide input range has been done. The efficiency over 96.4&#x0025; under the wide input range proved the effectiveness of the optimized control methods.

Employing Machine Learning for Enhancing Transient Stability of Power Synchronization Control During Fault Conditions in Weak Grids

Grid-connected converters are exposed to the loss of synchronisation with the grid during severe voltage sags particularly when operating under weak grid condition which introduces voltage and frequency volatility. This paper presents employing machine learning methods besides modifying the converter control scheme to enhance the transient stability of power synchronization control (PSC). For early detection of synchronization instability of PSC to provide adequate time for taking correcting control actions, an encoder stacked classifier is proposed which is trained to be robust against data corruption and added noise. Then, by integrating the proposed instability detection scheme to the synchronization loop of PSC, a phase freezing mode is introduced to avoid losing synchronism during grid faults. It is disclosed that the frozen synchronization loop, which is activated by the proposed instability detection scheme, can ensure synchronization stability of PSC. Time-domain simulations are conducted to confirm the presented findings.

Increase Stability and Efficiency in PV-Battery-Grid Systems Using PSO Algorithm

In this article, the meta-heuristic algorithm PSO with PI fuzzy logic controller is proposed to develop a new control strategy of bidirectional converter (CSBC), in order to improve the stability and increase the efficiency of energy flow exchange in grid-connected photovoltaic generator (PVG) Battery hybrid system. The proposed command aims to satisfy the DC Motor load demand, manages the power flows from different parts, injects surplus energy into the grid as and when need, ensure charge-discharge battery operation even under the fluctuating condition of power generation, and stabilize the DC bus voltage. This new control has been placed in the network topology, which consists of a PVG Photovoltaic Generator, grid-connected DC/AC converter, storage battery and DC motor load. A bidirectional buck-boost converter is used for optimum exploit of power from PVG along with battery charging/discharging control, and for feeding DC motor load. The effectiveness of the proposed control scheme is demonstrated by using MATLAB / Simulink program. The results obtained, show that the proposed control provided the system with the stability of Vdc voltage, and contributed to improving the speed produced by the DC motor load. The results have been compared with the conventional control.

A novel control strategy of resonant converter with modular rectifier based on SiC switching device

The resonant converter with modular rectifier (RMR) is a promising topology for DC grid applications. This paper studies the control strategy of RMR based SiC switching device including the output voltage and submodule (SM) capacitor voltage balancing control strategy. Firstly, different working modes of the RMR converter are analyzed, and the Critical Mode with the highest efficiency is introduced. A control method is proposed to guarantee the RMR operating the Critical Mode. The control parameters design is detailed. Secondly, the charging and discharging principles of SM capacitor voltages are introduced, then a balancing method is proposed: the SM capacitor voltages are sorted, then assign gate signals with smaller pulse width to SMs with lower capacitor voltages, and vice versa. The proposed control methods are verified by experimental results on a 300 V/1200 V 8 kW RMR prototype based SiC switching device. According to the experimental results, the RMR operates at the Critical Mode under different load and gain conditions. The SM capacitor voltages are well balanced. The peak efficiency is 98.2%.

A novel scheme for control by active and reactive power utilized in gearless variable speed wind turbine system with PMSG connected to the grid

Introduction. As a result of increasing fossil fuel price and state-of-the-art technology, more and more residential and commercial consumers of electricity have been installing wind turbines. The motivation being to cut energy bills and carbon dioxide emissions. Purpose. The main goal of this work is developing a control scheme for a variable speed wind turbine generator in order to produce utmost power from varying wind types, and variable wind speed. Novelty. This research paper presents an IGBT power converter control scheme for active power in relation to wind speed and reactive power by adjusting Q-reference (Qref) value in a gearless variable speed wind turbine with permanent magnet synchronous generator. Methods. An effective modelling and control of the wind turbine with the suggested power converter is executed by utilizing MATLAB/Simulink software. The control scheme consists of both the wind turbine control and the power converter control. Simulation results are utilized in the analysis and deliberation of the ability of the control scheme, which reveals that the wind turbine generator has the capability to actively sustain an electric power grid network, owing to its ability to independently control active and reactive power according to applied reference values at variable wind speed. Practical value. This research can be utilized for assessing the control methodology, the dynamic capabilities and influence of a gearless variable-speed wind energy conversion system on electric power grids. A case study has been presented with a (3×10 MW = 30 MW) wind farm scheme.

Adaptive Virtual Impedance Controller for Parallel and Radial Microgrids With Varying X/R Ratios

In this paper, an adaptive virtual impedance-based universal voltage source converter (VSC) control scheme is proposed to improve the stability, and power-sharing performance of VSC interfaced distributed energy resources in hybrid AC/DC microgrids with different feeder characteristics. The proposed control scheme modulates the VSC output impedance by tracking the active and reactive power transfer difference between the inverter terminal and the point of connection to the microgrid. Firstly, the stability limits of microgrid with resistive and inductive microgrid feeders are investigated by theoretical analysis. Subsequently, influence of the feeder parameters on the stability of microgrid is evaluated. From the theoretical analysis, the proposed control scheme shows resistive inverter output impedance at low frequency, while it shows more inductive behavior at system frequency for both the low and medium voltage microgrid feeders. The effectiveness of the proposed control strategy is demonstrated through a range of scenarios for two different microgrid types. The results show that the proposed controller can autonomously vary the output impedance of the VSC and provides significantly improved damping and power-sharing performance of the microgrid.

High-voltage ride-through strategy for wind turbine with fully-rated converter based on current operating range

During grid high-voltage ride-through (HVRT), a wind turbine (WT) with fully-rated converter (FRC) would experience an overvoltage at the DC link, which can threaten the safety of the DC link capacitors and the connected power modules. This paper investigates the process of the DC link voltage rise. Two concepts, current operating range (COR) and performance factor, are introduced to help propose a grid-side converter control strategy, in which the DC link overvoltage is greatly released or eliminated through injection of quantitative reactive current derived from the current margin of fully-rated converter and the grid voltage. The injected levels of reactive current into the grid are determined under different grid overvoltage conditions with reduced magnitude and occurrence of the DC link overvoltage. Thereby, the high-voltage ride-through capability of the fully-rated converter wind turbine is significantly improved. The detailed verification is obtained through simulations using DIgSILENT PowerFactory.

A quantitative harmonics analysis approach for sinusoidal pulse‐width‐modulation based Z‐source inverters

The shoot-through (ST) states are essential for the operation of sinusoidal pulse-width-modulation (SPWM) based Z-source inverters. However, the insertion of shoot-through state unavoidably leads to output harmonics which is greatly affected by its distribution, especially for bipolar modulation schemes. With quantitative analysis, this paper investigates the mathematical relationship between the output harmonics and shoot-through states for single-phase Z-source inverters. A modified double-Fourier-transformation-based calculation scheme is proposed to estimate the output harmonics with different shoot-through states insertion methods. Simulations and hardware experiments with a 200 W single-phase Z-source converter under two bipolar modulation control methods verified the accuracy of the proposed theory. The quantitative analysis would be helpful for the design of Z-source converter's control strategy and modulation scheme.

A New Method for Simplifying Complex DC Systems and Obtaining the Controller Droop Coefficients

DC grid has become an important application in power transmission. However, the exist works only have the method to obtain the droop coefficient in a radial topology. Also, this droop coefficient is for V-I droop control. There is no such like method to obtain the droop coefficient for the P-V droop control. This paper proposed a new method for a complex dc grid to obtain the droop coefficients. Firstly, based on the converter control strategies, the types of converter were classified. It will help to find out which kinds of converter can be participated into the dc-side voltage control. Secondly, a virtual node was defined, and the matrix of the dc grid was reshaped to fit the new added virtual node by using the proposed simplification method. An example was shown how this method worked. After that a new method of calculating the P-V droop coefficient was proposed. Finally, a simulation model was investigated to verify if the proposed method can operate in a real system. The results showed that the simplification method can ensure the mathematical relationships of the dc system. And simulations show good performances of the droop coefficient calculation method.

PLL-Free Voltage Oriented Control Strategy for Voltage Source Converters Tied to Unbalanced Utility Grids

Asymmetrical working conditions of the utility grid introduce the large-amplitude negative-sequence component to the output current of the voltage source converter (VSC), causing power semiconductor devices to suffer from thermal fatigue and thermal damage. Though the conventional phase-locked loop (PLL) based voltage oriented control (VOC) solution can suppress the steady-state negative-sequence current effectively, it has a weak suppression ability of transient overload current, and even aggravates the transient current asymmetry and causes more severe transient impact to the VSC. This paper first analyzes the transient performance of the conventional VOC strategy, especially its dynamic response time and the main factors for performance limitation. On this basis, the PLL-free VOC strategy for VSCs tied to unbalanced grids is proposed, and its critical parts, namely, the reference current calculation and the fast detection of the grid voltage sequence components, are implemented. Besides, to improve the frequency adaptability, a high-performance grid frequency detection strategy is developed based on the difference-frequency phase caused by the frequency variation. Finally, experiments are performed to verify the effectiveness and advancement of the proposed method. Specifically, the results proved the rapidity, accuracy, and frequency adaptability of the proposed method in suppressing the VSC negative-sequence current, both in transient and steady-state conditions.

Wave excitation force forecasting using neural networks

Many wave energy conversion applications require future knowledge or forecasting of the wave excitation force values. Most wave energy converter (WEC) control strategies need to forecast the time-series excitation force for wave energy harvesting maximization. The main aim of this study is to forecast the wave excitation force experiences by a two-body heaving point absorber WEC (as a case study) using three forecasting neural network methods. The wave excitation force is calculated based on the hydrodynamic characteristics of the considered device in the frequency and time-domain simulations. The nonlinear autoregressive neural (NAR) network, group method of data handling (GMDH) network, and Long Short-Term Memory (LSTM) network are fitted to the wave elevation time-series data to forecast the future values of the excitation force. The performance of the examined methods is evaluated for various irregular incident waves that are created using different wave spectrums. Moreover, sensitivity analyses to sampling period and algorithms input parameters are performed to investigate the accuracy and generalizability of the discussed methods at different conditions. Each data set is divided into training and test sets. The results show that the performance of all discussed methods is satisfactory in training data sets and short-term ahead forecasting, but the NAR network method provides a relatively better agreement with test target data compared to other methods.

Model Predictive Control Scheme of a Four-Level Quasi-Nested Converter Fed AC-Drive, with dc-Link Voltage-Drift Compensation

In this paper, a Model Predictive Control (MPC) strategy is introduced for its application in a four-level quasi-nested topology, feeding an Interior Permanent Magnet Synchronous Machine (IPMSM) AC-drive. The proposed control strategy is capable to synthesize the required output space vectors to ensure perfect tracking of the AC-drive speed reference under different loading conditions, while also ensuring voltage balance between the dc-link capacitors. The proposed converter topology is based on a reduced number of components compared to other mature converter topologies, such as the neutral-point clamped converter (NPC) or the active neutral-point clamped converter (ANPC) topologies, when compared in terms of the number of output voltage levels, since this quasi-nested topology does not require passive clamping devices such as diodes or active switches. Moreover, no floating dc-link capacitors with asymmetrical voltage levels are employed, thus simplifying the dc-link capacitor voltage balance mechanism. This work presents the switching operation principles and MPC control law when supplying an IPMSM AC-drive load are addressed in detail. Simulation and validation results using a Hardware in the Loop (HIL) prototype under different operation conditions are presented in order to validate the proposed converter topology and control strategy.

A novel smooth switching control strategy for multiple photovoltaic converters in DC microgrids

A novel smooth switching control strategy for multiple photovoltaic converters in DC microgrids

Towards Optimal Coordination Between Regional Groups: HVDC Supplementary Power Control

With Europe dedicated to limiting climate change and greenhouse gas emissions, large shares of Renewable Energy Sources (RES) are being integrated in the national grids, phasing out conventional generation. The new challenges arising from the energy transition will require a better coordination between neighboring system operators to maintain system security. To this end, this paper studies the benefit of exchanging primary frequency reserves between asynchronous areas using the Supplementary Power Control (SPC) functionality of High-Voltage Direct-Current (HVDC) lines. First, we focus on the derivation of frequency metrics for asynchronous AC systems coupled by HVDC interconnectors. We compare two different control schemes for HVDC converters, which allow for unilateral or bilateral exchanges of reserves between neighboring systems. Second, we formulate frequency constraints and include them in a unit commitment problem to ensure the N-1 security criterion. A data-driven approach is proposed to better represent the frequency nadir constraint by means of cutting hyperplanes. Our results suggest that the exchange of primary reserves through HVDC can reduce up to 10% the cost of reserve procurement while maintaining the system N-1 secure.