Grid-connected Voltage Source Converters Research Articles

Real-Time Coordinated Control of a Grid-VSC and ESSs in a DC Distribution System for Total Power Loss Reduction Considering Variable Droop Using Voltage Sensitivities

Recent advances in direct current (DC) distribution systems that involve renewable energy sources (RESs) and energy storage systems (ESSs) have shown that DC voltage control improves operational stability. This paper proposes an efficient real-time DC voltage control strategy that reduces total power losses while maintaining the DC voltages within acceptable ranges via coordination of the grid-connected voltage source converter (GVSC) and the ESSs. A new strategy determines whether to invoke cooperative GVSC/ESS control or ESS voltage control alone. The GVSC and ESSs exhibit proportional-integral (PI) and variable droop controllers, respectively. In particular, the variable droop controllers are developed with consideration of the voltage sensitivities. The ESS outputs are then compensated by reference to the droop coefficients and the available ESS output powers. Case studies confirm that our voltage control strategy permits higher minimum DC voltages (and thus smaller total power losses) when consecutive events occur, compared with conventional strategies using PSCAD/EMTDC.

SWRDFT Controller for Single Phase Grid Tied PV system with Low Voltage Ride through Capability

Voltage control and power quality (PQ) management are crucial aspects for the control of grid connected voltage source converters (VSC). In this paper, a novel control technique for solar photovoltaic (PV) tied single-phase grid is described based on a Sliding Window Recursive Discrete Fourier Transform (SWRDFT). This article considers a single-phase, double-staged PV interconnection. The major goal of control is to operate the VSC as per the standard grid codes and ensure quality power is delivered to the grid even under abnormal grid conditions. The system is designed to work at unity power factor during typical operation. Reactive power injection is mandated as per a country's grid code and helps to regulate the voltage profile and low voltage ride through (LVRT) during grid voltage fluctuations. Furthermore, the paper focuses on two important aspects viz. load compensation and LVRT operation carried out in the event of grid voltage sags. During night time operation, in the absence of PV, the VSC and dc link capacitor operate as a shunt compensator. The proposed controller completes both the tasks as demonstrated by simulation and hardware results. Additionally, a comparison study using state of the art fundamental extraction techniques based on Second / Third order generalized Integrator (SOGI/TOGI) is presented.

Performance-Based Tuning for a Model Predictive Direct Power Control in a Grid-Tied Converter With L-Filter

Model predictive control (MPC) is a powerful and widely used technique to address the control challenges in power converters as the grid interface for renewable energy systems. This technique combines closed-loop control with error and control effort minimization; however, its design is challenging, and we know little about how the controller parameters affect the closed-loop performance of grid-connected voltage source converters. In this study, we applied an MPC direct power control with modulation for a grid-connected power converter with an inductive filter. For the controller design, we proposed an initial set based on the power converter’s nominal setup. Then, we define the range of settings to guarantee stability by analyzing the closed-loop poles of the system. The fine-tuning to improve the performance can be identified visually using the performance maps built from simulations of the control system, simultaneously sweeping the time horizons of the predictive model and the weight factors of the cost function. Experimental results on a low-power bench demonstrate the excellent performance of the designed controller, following and even outperforming the classical proportional-integral (PI) controller and other advanced control techniques.

Robust Stability of Grid-Connected Voltage Source Converter with Phase-Locked Loop

The robust stability of one single voltage source converter (VSC) connected to a bus via the line is studied in this letter. Considering arbitrary bounded perturbations from uncertainties, the perturbed system model is firstly proposed. Then, one switched system is constructed to investigate the stability of the perturbed system, and it is proved that the stability region of the switched system is the subset of that of the perturbed system. Finally, the stability region of the switched system is obtained by the inverse-time (backwards) integration starting from the unstable equilibrium point, which is also the conservative stability region of the perturbed system.

Fast Bus Voltage Control of Single-Phase Grid-Connected Converter With Unified Harmonic Mitigation

This paper presents a comprehensive analysis of the harmonic sources of the single-phase grid-connected voltage source converter (VSC), which leads to an alternative approach for DC bus voltage control of the VSC under grid voltage distortion with a significant switching dead time. A selective current harmonic controller with the zero-reference structure plays a vital role in rejecting the harmonic components in the grid reference current created by the bus voltage control loop and the harmonic components in the grid voltage and in the VSC caused by the switching dead time. Therefore, the bus voltage control can adopt a conventional proportional-integral regulator tuned at a fast bandwidth. The proposed control scheme implemented in the multiple unbalanced synchronous reference frames was validated with a bidirectional 2-kVA VSC under grid voltage distortion and a significant dead time. Furthermore, the proposed control scheme exhibited the transient response and grid current quality superior to the conventional bus voltage control methods with a notch filter and a low-pass filter. The proposed control scheme has inherent frequency adaption.

Control of a Three-Phase Grid-Connected Voltage-Sourced Converter Using Long Short-Term Memory Networks

With the rise of inverter-based resources (IBRs) within the power system, the control of grid-connected converters (GCCs) has become pertinent due to the fact they interface IBRs to the grid. The conventional method of control for a GCC such as the voltage-sourced converter (VSC) is through a decoupled control loop in the synchronous reference frame. However, this model-based control method is sensitive to parameter changes causing deterioration in controller performance. Data-driven approaches such as machine learning can be utilized to design controllers that are capable of operating GCCs in various system conditions. This work explores a deep learning-based control method for a three-phase grid-connected VSC, specifically utilizing a long short-term memory (LSTM) network for robust control. Simulations of a conventional controlled VSC are conducted using Simulink to collect data for training the LSTM-based controller. The LSTM model is built and trained using the Keras and TensorFlow libraries in Python and tested in Simulink. The performance of the LSTM-based controller is evaluated under different case studies and compared to the conventional method of control. Simulation results demonstrate the effectiveness of this approach by outperforming the conventional controller and maintaining stability under different system parameter changes.

A PLL‐equivalent model for low‐frequency stability analysis of voltage source converter connected to weak grid

AbstractThis study focuses on stability of weak grid connected voltage source converter (WG‐VSC) in Low‐frequency mode (LFM) (around 1–10 Hz), which is dominated by interactions among phase‐locked loop (PLL), outer loop control and weak grid condition. In order to clearly reveal LFM mechanism of WG‐VSC, a simple but effective PLL‐equivalent model has been proposed. First, a generic small signal model consisted of PLL and ‘outer loop & WG’ two parts is derived. Then, the transfer function of complicated ‘outer loop & WG’ part is divided into two parallel channels, representing the impact of active side control and reactive side control on LFM of WG‐VSC respectively. Finally, based on the multi‐mode decomposition theory, a simple but effective second order PLL‐equivalent (E‐PLL) model is obtained for LFM stability analysis of WG‐VSC. The effectiveness of E‐PLL model has been verified by simulation results in PSCAD/EMTDC and experimental results based on a hardware platform.

PLL Synchronization Stability of Grid-Connected VSCs Under Asymmetric AC Faults

This paper investigates the phase-locked loop (PLL) synchronization stability of grid-connected voltage source converters (VSCs) under asymmetric ac faults. Negative sequence current control has been widely adopted to enhance the stability under asymmetric ac faults and the PLL plays a vital role in the maintenance of synchronization in VSCs. However, the positive and negative sequence current both have an important effect on the synchronization stability of PLL under asymmetric ac faults. Furthermore, the influence of negative sequence current and circuit parameters on PLL synchronization stability is different under different asymmetric ac fault types, which further increases the complexity of analysis. Hence, the unified synchronization stability coefficient (SSC) is proposed to evaluate synchronization stability of the VSC in this work. The parametric impacts of the PLL, the influence of positive/negative sequence current, and the circuit parameters on PLL synchronization stability are revealed analytically during asymmetric ac faults. Time-domain simulations are performed in Matlab/Simulink, and the obtained results validate the theoretical findings.

Frequency-Dependent Network Analysis and Stability Enhanced Design for Voltage-Source Converters Under Weak Grid Conditions

Phase–locked loop (PLL) is commonly used for the grid-connected voltage-source converter (VSC) to synchronize with the grid. Under weak grid conditions, it has been found that the frequency-dependent characteristic of the network impedance inherently introduces a positive feedback loop into PLL. Previous studies focused on the PLL analysis intuitively believe that the positive feedback loop will deteriorate the dynamic performance of the PLL. However, this conclusion is not rigorous since only the dynamic of the PLL is considered and the theoretical analysis is insufficiency to reveal the influence of the positive feedback loop on system stability. In this paper, the impact of the frequency-dependent network on VSC stability is quantified in terms of the overall system based on the damping torque analysis. It is found that the influence of the positive feedback loop on stability depends on the parameters of the PLL and the damping of the dominant eigenvalues. Besides, an angular frequency feedback scheme is proposed to eliminate the adverse effect of the frequency-dependent network and enhance the stability of the VSC under weak grid conditions. Time-domain numerical simulations and reduced-scale experiments are performed to validate the effectiveness of the proposed scheme.

Positive-sequence virtual-flux control of grid-connected converter during unsymmetrical voltage dips

One of the major problems in direct power-controlled grid-connected voltage source converters is that during voltage dips, the converter current increases to compensate for the reduced grid voltage. The most common voltage dips are unsymmetrical, and they cause unbalance and distortion in the converter current. This paper introduces a new, simple but effective algorithm which limits the current in a direct power-controlled grid-connected voltage source converter during voltage dips. A positive-sequence virtual-flux based control scheme is employed and this makes the current balanced and sinusoidal during unsymmetrical voltage dips. The proposed control clearly demonstrates the performance which is illustrated through various simulations and experimental work. The current during unsymmetrical voltage dips is limited in magnitude and is balanced, with low distortion. The Implementation of this control scheme will enable voltage source converters to stay connected to the grid during voltage dips as required by most grid codes.

A Multivariable Phase-Locked Loop-Integrated Controller for Enhanced Performance of Voltage Source Converters Under Weak Grid Conditions

This article proposes a new control strategy in stationary frame to improve the operational robustness of voltage source converters (VSCs) under weak grid conditions. First, dynamics of the phase-locked loop (PLL) are incorporated into the model of a grid-connected VSC in stationary reference frame. It is rigorously shown that the three-phase PLL introduces cross-coupling terms between the two control channels. Second, based on the augmented model, a multivariable controller is proposed which combines the current control feedback gains with PLL feedback gains. Third, the gains of the controller are designed using an optimal control theory approach. It is shown through theoretical analysis, computer simulations, and a laboratory experimental setup that the proposed controller improves the stability margins of the VSC during weak grid conditions. More specifically, under a given identical operating conditions, while the conventional controller’s responses become increasingly oscillatory for short-circuit ratio (SCR) below 1.95 and eventually becomes unstable at SCR=1.26, the proposed controller exhibits smooth and stable responses for SCR as low as 1. The proposed controller also produces cleaner sinusoidal currents during unbalanced grid voltages.

A Systematic Control Design Method with Active Damping Control in Voltage Source Converters

This paper proposes a systematic control design method for active damping control of grid-connected voltage source converters (VSCs). The proposed control method considers the conventional cascaded control loops and improves them by including additional states feedback-based active damping. In such a way, all control gains are lumped into one control gain matrix based on the proposed formulation. The lumping of all control gains into one matrix leads to a linear optimization problem, so different techniques can be used to calculate control gains. This work calculates them by using a simple but effective optimal control theorem as a noteworthy feature. The proposed control method can overcome the challenges of designing multiple control loops, evaluating wide time scale dynamics, and tuning required control parameters. Moreover, direct relationships between the proposed tuning parameters and system well-known stability and performance indicators such as maximum damping factor, minimum damping ratio, and the control efforts are identified, providing good physical insight. Finally, the proposed control structure and optimal gain calculations ensure power converter robustness against uncertainties in the grid’s short-circuit ratio (SCR) and different operating-point conditions. When the grid’s SCR changes from 10 (strong grid condition) to 1 (ultra-weak grid condition), the system under the proposed control method maintains good stability margins and simultaneously provides a fast dynamic response by facilitating the implementation of a high-bandwidth phase-locked loop (PLL). The performance of the proposed control strategy was investigated analytically and practically by conducting eigenvalue analysis, simulations, and experiments.

Accurate High-Frequency Modeling of the Input Admittance of PWM Grid-Connected VSCs

This article addresses high-frequency admittance modeling of current-controlled voltage source converters (VSCs). Recent studies have shown that harmonic instability may also occur at frequencies above the Nyquist frequency. To form an accurate multiple-frequency model in this frequency range, sidebands that originate from modulation and sampling must be examined. In this article, an accurate small-signal model is developed taking into account an adequate digital pulsewidth modulator (DPWM) representation, which allows to predict dependence of the frequency response on the steady-state dc (SS-dc) operating point. It is shown that when center-pulse sampling is implemented, pulsewidth modulation sidebands do not create additional loops leaving only sampling sidebands to be considered. Using the same approach as for SS-dc operation, a model that accurately represents admittance measurements during sinusoidal ac (S-ac) operation is developed. Its basis is a novel DPWM model suitable for S-ac regime, which allows to predict dependence of the VSC’s input admittance on the grid voltage magnitude. Experimental and simulated admittance measurements, performed on a single-phase two-level VSC during various SS-dc and S-ac regimes, match with the proposed models up to twice the sampling frequency.

Hybrid Si/SiC Switch Modulation With Minimum SiC MOSFET Conduction in Grid Connected Voltage Source Converters

In this article, a hybrid Si/Si carbide (SiC) switch (HyS) modulation with minimum SiC MOSFET conduction (mcHyS) is experimentally characterized, so as to derive its conduction and switching performance. These are later used to derive a silicon (Si) area analytical model for the HyS configuration. The chip area model is used to benchmark the mcHyS modulation concepts against single-technology switches and typical HyS modulation when considering the implementation of a 100-kW two-level voltage-source converter (VSC) deployed for three industrial applications: photovoltaic inverter, electric vehicle fast-charging station, and battery storage systems for grid ancillary service. The two additional switching events of the SiC MOSFET, which differentiate the mcHyS modulation from the typical HyS one, are proven to happen in soft switching; therefore, the mcHyS switching performances are not penalized. Furthermore, the analysis presented shows how the studied mcHyS modulation performs against the single semiconductor technology and the typical HyS solution in terms of cost and power conversion efficiency. More specifically, it is shown that the HyS solutions are particularly competitive versus the full Si-based VSCs when the application at hand often operates at low partial loads. Finally, a 10-kW two-level VSC assembled with mcHyS is tested, so as to compare its efficiency versus single-technology switches.

A Real-Time Enhanced Thevenin Equivalent Parameter Estimation Method for PLL Synchronization Stability Control in VSC

The real-time estimation of the AC power system equivalent parameters enables the grid-connected Voltage Source Converter (VSC) to achieve more effective stability control during a short-circuit fault. In this paper, a novel Thevenin Equivalent (TE) parameter estimation method based on local synchronous Phasor Measurement Units (PMUs) is proposed and applied to the Phase-Locked Loop (PLL) synchronization stability control of VSC. First, the relationship between the variation of the measured voltage and the error of the estimated impedance is strictly derived based on the rules of norm inequality. Then, according to the analysis of the upper bound of the system voltage change, a voltage threshold is proposed to select the PMU measurements to ensure the accuracy of the estimation during the large disturbance. Finally, using the estimated TE parameters, the current control instruction of VSC during the fault is calculated in real-time to ensure a synchronous and stable operation. The simulations reveal that the proposed estimation method can accurately track the TE parameters after a fault. Furthermore, the associated current control actions during a fault can ensure the PLL synchronization stability of VSC.

A Probabilistic Framework for the Robust Stability and Performance Analysis of Grid-Tied Voltage Source Converters

This paper proposed a probabilistic framework that could be used for the sensitivity assessment of grid-connected voltage source converters (VSCs), where uncertainties in the grid short circuit ratio (SCR) and operating point conditions, as well as control-loop interactions, were considered. The proposed method tried to broaden the available knowledge on the small-signal stability analysis of VSCs and provide a probabilistic point of view of this subject. It considered the probability of different operational conditions in order to obtain less conservatism and more accurate results. Based on uncertain inputs and the employed stability model, the proposed model produced statistical distributions of the critical mode and its damping factor and ratio, which were not accessible by existing deterministic methods. Crucial statistical information measures how much system stability and performance are maintained or changed over the system uncertainties and disturbances, as well as provides a clear insight into the system stability problem. For instance, as concluded in this paper, for the conventional control system design, fast dynamic parts of a VSC, such as the current controller and control delay, significantly impact the minimum damping ratio. Furthermore, slow dynamic parts, such as outer voltage control loops and the synchronization block, influence the maximum damping factor. For strong grids, the AC voltage magnitude controller (AVC) significantly impacts the maximum damping factor due to its lower bandwidth among all control loops. For weak grids, the damping factor of the critical mode is highly affected by interactions between the VSC, the power grid, and different control loops due to the synchronization mechanism. The other contributions of this paper were the introduction of robust stability and performance definitions and indices; explanations of the pros and cons of probabilistic assessment methods and their applicability; interpretation of the obtained results; and, finally, a link was provided between system stability and reliability, which will be crucial for future power system design.

Multi-Sampled Grid-Connected VSCs: A Path Toward Inherent Admittance Passivity

This article investigates the capability of multi-sampled pulsewidth modulator (MS-PWM) control to improve the input admittance properties of voltage source converters (VSCs). Due to delays found in digital control systems, the VSC admittance features a negative real part above a certain frequency, which may result in lowly damped or even unstable grid dynamics. To prevent the occurrence of harmonic instability, recent standards require the admittance of every grid-connected VSC to behave as a passive network. In this article, it is shown that the MS-PWM control significantly improves passivity measures by reducing the overall system delay. The passive behavior is achieved effortlessly, using a single-stage control loop. This avoids the need for passive or active damping techniques, which are associated with increased losses and number of sensors, sensitivity to noisy measurements, ambiguity of damping filter design, and overall system complexity. The experimental verification, performed on a two-level single-phase VSC, shows a very good match between admittance measurements and analytic modeling even above the switching frequency. Grid-connected operation is tested to demonstrate the improvement of resonance damping obtained with MS-PWM control.

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.

Multivariable phase-locked loop free strategy for power control of grid-connected voltage source converters

A multivariable control strategy in a dq reference frame for grid-connected voltage source converters (VSCs) without using a phase-locked loop (PLL) is presented in this paper. First, common VSC controls such as vector current control (VCC) and grid voltage modulated direct power control (GVM-DPC) are analyzed and their main drawbacks are identified. Then, the multivariable control strategy is presented, including the implementation of saturation and anti-windup mechanisms, and limitation of overcurrents. Next, different uncertainty channels that may lead to instability in each approach are analyzed, showing the drawbacks related to the use of a PLL, which are avoided in our proposal. The effectiveness of the three strategies is compared by means of MATLAB/Simulink simulations, showing that the proposal presents a more robust behavior, specially in weak grids.

Steady-State and transient assessments of integral back-stepping current controllers in high-voltage direct current transmission systems

In the last five decades, proportional-integral (PI) controller remains dominant in control of high-power converters in industrial and utility power systems despite its bandwidth, hence, the performance is coupled to power circuit parameters and degrades in parts of operating range. Consequently, further mitigation measures such as fine-tuning or loop reshaping might be needed. In contrast, integral back-stepping (IBS) decouples controller bandwidth and performance from the system parameters; hence, leads to consistent performance throughout the operating range and simplifies tuning of controllers with complex structures with multiple loops. Therefore, this paper investigates the suitability of IBS for implementation of inner current controllers of the voltage source converter (VSC)-based high-voltage direct current (HVDC) transmission systems. The paper presents step-by-step synthesis of back-stepping current controller and supporting theoretical bases, and assesses its performance against the conventional PI current controller. Theoretical analysis and discussions and comparative simulations reveal that the characteristics of IBS technique enjoys unique characteristics: decouples the controller gains from the converter parameters; restricts closed loop poles to real axis; and exhibits larger closed loop bandwidth than the PI counterpart. It has been shown that the gains of IBS current controllers are explicitly specified in terms of the desired natural frequency and damping ratio, which are advantageous compared to that of the PI equivalent, in which the closed loop bandwidth are limited by converter parameters. For ease of illustration of similarities and differences, only inner current controllers of the converter stations were designed using integral back-stepping technique and compared against PI equivalents. The outer control loops such as dc link voltage, active and reactive power controllers are designed based on proportional-integral controllers. MATLAB-SIMULINK models of the point-to-point VSC-HVDC system, in which the current controllers implemented using integral back-stepping and proportional-integral controllers (active/reactive power and DC link voltage controllers) have been developed and subsequently used for performance comparison. Results of the one-to-one comparison show that the integral back-stepping current controller can match and even outperform its proportional-integral equivalent during normal and abnormal conditions, including current limitation during ac faults. Selected experiments conducted on grid connected voltage source converter that employs IBS current controller have confirm its viability.