Control Of Voltage Source Converter Research Articles

Large-signal Stability Analysis of Grid-connected DC Microgrid with VSC

The grid-connected DC microgrid under the background of high penetrations of renewable energy sources and high penetrations of inverter-interfaced devices has the characteristics of strong nonlinearity and poor immunity, which leads to the problems of large-scale voltage fluctuation and instability when the grid-connected DC microgrid suffers large disturbances, and it seriously influences the security and stable operation of the system. Therefore, a large disturbance stability criterion which is based on the mixed potential theory (MPT) for the DC microgrid model with a grid-connected voltage-source converter (VSC) is proposed. This paper also analyzes the mechanism of the conservatism of the stability criterion, verifies the accuracy of the large disturbance stability standard, and analyses the influencing elements and basic mechanism of the large-signal stability (LSS) of the nonlinear systems, which provides a benchmark to design the parameters of the control of VSC and main circuit. Finally, the effectiveness and accuracy of the analysis are verified by electromagnetic transient simulation.

VSC control strategy for HVDC compensating harmonic components

This paper proposes a voltage source converter (VSC) control strategy for the high voltage direct current (HVDC) transmission systems compensating the harmonic components. For VSC-HVDC AC system accounted for a relatively large share of the 5th, 7th, 11th, 13th and other low harmonics. A selective harmonic current control strategy based on vector proportional integral (VPI) regulator in dq coordinate system is proposed on the basis of PI control. In which, PI is used to control the DC component of the current error, while VPI is used to suppress the multiplicative fluctuations of the current error. Unlike the proportional-integral resonant (PIR) regulator, the VPI contains a second-order numerator that achieves the ideal 0° phase delay of the closed-loop transfer function of the control system at the set resonant frequency point, so its control accuracy for harmonic currents is better than that of the PIR. A two-terminal VSC-HVDC system is built using Simulink software, and current compensation simulations are performed for three control methods: conventional PI, PIR and PI parallel VPI schemes. The superior harmonic suppression performance of VPI is verified by comparing the harmonic content.

Adaptive fault ride through control of VSM Grid-forming converters

Recently, many high voltage direct current (HVdc) transmission lines are being constructed with the intention of becoming “backbones” of future power grids. Grid-forming (GFM) controlled Voltage Source Converters (VSCs) are anticipated to provide significant stability advantages compared with Grid-following (GFL) VSCs in HVdc converters connected to weak ac grids or to grids with high penetration of renewable inverter-based resources (IBRs). This paper proposes an adaptive fault ride through controller for Virtual Synchronous Machine (VSM) GFM control, which has significantly improved fault ride through capability over the more conventional GFM control with cascaded or switchable current limiting. The performance of the proposed adaptive control is investigated using Electromagnetic Transient (EMT) simulation. The results show that the proposed adaptive control has superior post disturbance recovery from ac faults as well as phase angle shifts, and overcomes the instability reported in GFM converters connected to strong ac networks.

Control for Grid-Connected VSC With Improved Damping Based on Physics-Informed Neural Network

The rapid penetration of renewable energy sources into the power system makes the grid-connected voltage source converter (VSC) highly dynamic and uncertain. This necessitates designing new adaptive control for VSCs to ensure satisfactory system performance, reliability, and stability. This paper introduces a physics-informed artificial neural network (ANN) controller for the grid-connected VSC to improve the system performance and dampen the voltage oscillation due to the sudden change in power demand. The employed ANN structure is a feed-forward multilayer Neural Network trained offline by the Levenberg-Marquardt-based backpropagation algorithm. Results are presented for different dynamic scenarios to show the satisfactory operation of the proposed controller. The small-signal stability analysis is presented to validate the system's stability. Further, the performance of the proposed ANN controller is compared with the widely-used PI-controller and model predictive controller. The results prove that the proposed controller has a better dynamic performance in damping the voltage oscillation.

Re-examination of small-signal instability in weak grid-connected voltage source converters

Weak grid sub-synchronous oscillation (SSO) is an important research topic. Although some analysis and mitigation schemes have been conducted, misperceptions still exist. This paper first aims to update the understanding of weak grid instability of voltage source converters (VSCs) and then points out that instability risks originate in the super-synchronous frequency range rather than in the sub-synchronous frequency range. Moreover, the resonance frequency can be even larger than the double fundamental frequency under certain parameter conditions. This paper also refines the impacts of VSC control parameters on weak grid instability mechanism and resonance frequency. Based on the results, the classification of this phenomenon should be revised.

Modeling of Single-Phase Photovoltaic Generators for System-Wide Harmonic Power Flow Studies

An important power quality concern of integrating single-phase photovoltaic (PV) generators to low-voltage (LV) residential systems, is their impact on harmonic distortions due to their power electronic-based interface through a voltage source converter (VSC). Unfortunately, most existing single-phase PV models are focused on the analysis of an individual device, and do not provide a modular circuit model that can be directly integrated into a commercial harmonic power flow simulation tool for large-scale power system studies. In response, this paper derives a single-input single-output harmonic model for single-phase PVs that is independent of grid parameters, thus enabling multiple of these devices to be considered simultaneously in system-wide harmonic power flow analyses. The model is validated with electromagnetic transient simulations and then, characterization studies are conducted to determine the expected harmonic impact of these PV generators on a real LV system. Results demonstrate these devices can increase system damping at typical low order harmonics and are unlikely to introduce new resonances at this frequency range. In addition, it is essential to model the VSC controller effect for a more accurate assessment of its harmonic impact. Sensitivity studies reveal how different VSC parameters affect this behavior.

VSC control strategy for HVDC compensating negative sequence and decaying DC components

This paper proposes a voltage source converter (VSC) control strategy for the high voltage direct current (HVDC) transmission systems compensating the negative sequence and decaying DC (DDC) components. The mathematical model of the grid fault current is established firstly, and the DDC component is extracted using the property that the integral of the periodic signal is 0. Secondly, the fundamental wave component of the grid fault current signal is separated in the rotating frame system to obtain the compensation current command of VSC-HVDC. Finally, the performance of the proposed control strategy is compared with the conventional inverter control strategy when compensation the negative sequence and decaying DC (DDC) components by the experimental platform, and the experimental results show that the flexible HVDC transmission system with the proposed control strategy has the desired ability of compensating the negative sequence and DDC components simultaneously.

The transient instability mechanism and stability‐enhanced LVRT control for VSC riding‐through severe grid voltage sag

AbstractAs the grid‐connected interface device of wind and photovoltaic power generation, the voltage source converter (VSC) must pass the low voltage ride‐through (LVRT) test. However, existing literature demonstrates that under weak grid conditions, there is a possibility of severe fluctuations in the terminal voltage of VSC during transiting from the pre‐sag normal operation mode to the LVRT mode. Unfortunately, it will make VSC frequently enter and exit LVRT mode. In order to address the tricky problem of transient instability of VSC riding‐through severe grid voltage sag under the weak grid, in this paper, the full‐order large‐signal model of VSC is established first. Then, based on the established model, the analysis indicates VSC will face the risk of losing stability under the weak grid during LVRT due to current transients. By studying the impacts of low short circuit ratio (SCR) on the basin of attraction of the post‐sag equilibrium point, the intrinsic mechanism of VSC losing stability under weak grid conditions is revealed. Moreover, considering that the transient stability is related to the current control, the stability‐enhanced LVRT control ensuring both small‐signal stability and transient stability of VSC during LVRT is proposed. Finally, the correctness of the above theoretical analysis is verified by real‐time simulation

A Quantitative Stability Evaluation Method of VSC for Optimizing Control Parameters and Extending Stability Boundary to Avoid Oscillations

Oscillations caused by the interaction between voltage source converters (VSCs) and weak grids are vital threats to the stability of power systems. Determining the appropriate parameters for the control of VSCs is essential to prevent the occurrence of oscillations in advance. To achieve this goal, a quantitative evaluation method of system stability for VSCs is proposed in this article to specify the stability boundary of control parameters. Then, an active damping controller for current control and a parameter optimization method for the phase-locked loop (PLL) is proposed, and the related parameters are designed based on the guidance of the proposed evaluation method. With planting the parameters optimization in the control of VSC, the stability boundary of control parameters of VSC is extended, so that the stability of VSC can be significantly improved. Finally, simulations are presented to verify the effectiveness of the theoretical analysis.

Improving Stability of Low-Inertia Systems Using Virtual Induction Machine Synchronization for Grid-Following Converters

This paper presents a novel strategy for the synchronization of grid-following Voltage Source Converters (VSCs) in power systems with low rotational inertia. The proposed synchronization unit is based on emulating the physical properties of an induction machine and capitalizes on its inherent grid-friendly properties such as self-synchronization, oscillation damping, and standalone capabilities. To this end, the mathematical model of an induction machine is analyzed and reformulated to obtain the unknown grid frequency by processing the voltage and current measurements at the converter output. This eliminates the need for a Phase-Locked Loop (PLL) unit, traditionally employed in grid-following VSC control schemes, while simultaneously preserving the system- and device-level control. Furthermore, we provide the appropriate steps for obtaining an index-1 DAE representation of the induction-machine-based synchronization unit, suitable for stability analysis. Our analysis shows that replacing the PLL unit with the virtual induction machine-based synchronization may considerably improve the stability of systems with grid-following converters and facilitate the frequency containment. Furthermore, we validate the performance of the proposed synchronization unit through simulations and provide recommendations for its tuning.

Impact of the Differences in VSC Average Model Parameters on the DC Voltage Critical Stability of an MTDC Power System

Voltage source converters (VSCs) are generally assumed to be identical when studying the DC voltage critical stability of a multi-terminal DC (MTDC) system, which in reality is not the case and can lead to erroneous results. In this study, the differences in VSC average model parameters, such as VSC control loops and operating states, were considered, and the mechanism of how these differences affect the DC voltage critical stability of an MTDC power system was clarified. In an MTDC power system with DC voltage droop control, it was found that these differences can be considered as two types of added dynamic impedances, which affect DC voltage critical stability by changing the construction of the DC network. Moreover, a method was developed to numerically determine the effects of these differences on DC voltage critical stability. It was verified that these effects are related to not only the dynamic characteristics of the added dynamic impedances but also the parameters and constructions of the DC network. By considering these differences, accuracy of DC voltage critical stability can be improved. Finally, an experimental platform of a four-terminal DC power system was established to validate the findings.

The parameters optimization and stable boundary expansion method of current controller for VSC to avoid low frequency oscillations

The low frequency oscillations that are raised by the interactions between the voltage source converter (VSC) and the weak grid, are becoming a critical threat to power system stability. Identifying the proper parameters for VSC is essential for preventing the appearance of oscillation in advance. To achieve this target, this paper proposes a quantitative evaluation method for the system’s stability to clarify the stable boundary of current control parameters. Then, an active damping controller is proposed and supplemented in the control of VSC, to expand the stable boundary of current control parameters. Thus, the VSC can stably operate within a wider range of parameters selection, the oscillation can be damped without weakening the dynamic control capability of the VSC. The simulation tests are performed to verify the correctness of the quantitative evaluation method and the effectiveness of the proposed stability boundary expansion control.

Comparative assessment of single-loop droop controlled grid-forming converter and its damping enhancement

As the penetration of power electronic interfaced power sources increases rapidly, grid-forming controlled voltage source converters (VSCs) are deemed as an effective alternative to synchronous generators. As a typical grid-forming control scheme, droop control is commonly utilized due to its simplicity of construction and stability of operation. VSCs driven by droop controls, such as multi-loop droop control (MLDC) and single-loop droop control (SLDC) are capable of actively establishing voltage and frequency of grid-forming converters. However, the transient performance of SLDC at different faults is less explored to date. In this work, an in-depth comparative assessment of large-signal stability against the established MLDC is first presented by means of phase portrait. It is revealed that, both control strategies have similar transient behaviors with their basic scheme. However, with the introducing of the low-pass filters which can emulate the inertia, SLDC generally exhibits a larger power angle overshoot during the fault and therefore has poorer transient stability as compared that of MLDC. To address this issue, this work improves the damping of SLDC by incorporating differential compensation link into it. Furthermore, a parameter design guideline that balances damping and inertia, hence the transient stability and frequency stability, is proposed. The analysis is supported by real-time hardware-in-the-loop experimental results.

Local control method and simulation analysis of hybrid AC/DC low-voltage distribution networks with high-proportion photovoltaics

Driven by the goal of energy transformation and carbon neutralization, more and more distributed photovoltaics (PVs) are integrated into low-voltage distribution networks (LVDNs). However, the integration of high-proportion PVs may lead to overload problem of distribution transformer (DT) and voltage violation problem, bringing many challenges to the safe and stable operation of LVDN. DC distribution network has the advantages of large transmission capacity and flexible power regulation, which can also realize the interconnection and power transfer between AC low-voltage lines. Therefore, structures and control methods suitable for hybrid AC/DC LVDN are proposed to alleviate the above problems caused by PVs. Firstly, the structure of hybrid AC/DC LVDN is analyzed, including the structure with parallel DC line within distribution area and the structure of DC-interconnected distribution networks between distribution areas. Secondly, a local control method of voltage source converter (VSC) is proposed for the comprehensive improvement of overload problem of DT and voltage violation problem. Finally, an actual LVDN is used to verify the effectiveness of the proposed structure and control method of hybrid AC/DC LVDN. According to the simulation results, the proposed structure of hybrid AC/DC LVDN and VSC control method can improve the voltage level and overload problems of LVDN, decrease the power loss and increase proportion of PV accommodation.

Artificial Neural Network-Based Constrained Predictive Real-Time Parameter Adaptation Controller for Grid-Tied VSCs

This article proposes a real-time algorithm for identifying the grid parameters, which is concurrently used for online tuning of the predictive controller in each iteration, in a grid-tied active front end (AFE) voltage source converter (VSC) applications. The algorithm is designed by inspiring from the concepts of the extended Kalman filter (EKF) and the model predictive control (MPC). The performance of the algorithm highly depends on the weighting factors of the algorithm. The artificial neural network (ANN)-based algorithm is used to find the optimal set of weighting factors among the ones in a parameter search block. An offline particle swarm optimization (PSO) is run to provide the data source for the parameter search block. The algorithm identifies not only the inductance but also the resistance of the grid. In addition, the hard constraints on the amplitude of the input and output variables are guaranteed. The validation of the proposed approach is performed experimentally and compared with the state-of-the-art conventional methods. The experimental results show that the proposed method could effectively stabilize the system in weak grid conditions and under wide impedance variations. In addition, the accuracy of the proposed impedance identification method is 96%.

Hosting capacity enhancement of hybrid AC/DC distribution network based on static and dynamic reconfiguration

AbstractThe hybrid AC/DC distribution networks will have a crucial role in the future of smart grids due to their adaptiveness to a variety of DC loads and energy resources. Thus, it is necessary to assess and enhance the Hosting Capacity (HC) of these hybrid networks for the integration of Renewable Energy Sources (RESs). One way to increase the HC without further expansion in the grid is using the potential of Network Reconfiguration (NR). This paper proposes an HC enhancement model that includes AC/DC distribution NR. Two types of NR are considered, namely static, that is, embedding the NR capabilities at the planning stage, and dynamic, that is, using remotely controlled switches as an Active Network Management (ANM) scheme. Besides, two unique potential sources of hybrid AC/DC distribution networks that could be used for the HC enhancement are included, that is, reactive power control of Voltage Source Converters (VSCs) and the mesh operation capability. The problem is formulated as a mixed‐integer linear optimal power flow that aims to maximize the RESs HC under thermal and voltage constraints. Also, stochastic programming is utilized for managing the RESs uncertainties. The simulation results show the accuracy and efficiency of the proposed formulation from various perspectives.

Outage management of hybrid AC/DC distribution systems: Co-optimize service restoration with repair crew and mobile energy storage system dispatch

To achieve the most efficient restoration of hybrid AC/DC distribution system, this paper proposes an outage management through co-optimizing service restoration with repair crew (RC) and mobile energy storage system (MESS) dispatch. Firstly, this paper proposes a hybrid AC/DC distribution system restoration (DSR) model considering network reconfiguration, distributed generation (DG) islanded operation and voltage source converter (VSC) based DSR, which means adjusting VSC control mode to realize uninterrupted power supply of loads. In the DSR model, a restoration coordination model that can coordinate network reconfiguration and the control modes of VSCs, DGs and MESS discharge stations is developed to ensure the radial configuration of the system. Moreover, to accurately calculate the power flow of the system during a failure of VSCs, a new power flow model considering VSC’s operation status is proposed in the DSR model. Then, a bi-level co-optimization method is developed based on the DSR model and improved dispatch models of RC and MESS. The upper level co-optimizes hybrid AC/DC distribution system restoration with RC and MESS dispatch to maximize the restored loads. The lower level further optimizes the power outputs of VSCs and DGs to minimize the voltage deviation of the restored loads. Finally, simulations studies are performed to verify the effectiveness and superiority of the proposed method.

Grid-Forming Control of Voltage Source Converters Based on the Virtual-Flux Orientation

The increasing penetration of renewable generation in power systems is causing growing concern about system stability. Grid-forming (GFM) control has been appointed as the technology required for achieving a high penetration of renewables in the grid, as it successfully contributes to the power system stability. This paper proposes a novel grid-forming control scheme for voltage source converters (VSC) achieving the operational characteristics of a virtual synchronous machine (VSM). The proposed scheme is based on the orientation of a defined virtual flux to a reference axis obtained from the emulation of the synchronous generator swing equation. The virtual flux is obtained by the integration of the VSC internal voltage and it is oriented to the reference axis by means of a flux controller that also controls the flux magnitude. In this way, the VSC synchronism is maintained, operating as a voltage source connected to the grid. Moreover, the flux orientation in turn allows to control the VSC active power, while the flux magnitude control allows to regulate the VSC reactive power or terminal voltage in isolated mode. The paper also demonstrates that the stability of GFM converters is negatively affected when emulating high inertia values. So, a stabilizer is also proposed for damping these modes. Finally, due to fact that GFM converters behave as voltage sources, their low voltage ride through capability becomes an issue, because they naturally respond to faults with high short-circuit currents, that cannot be withstand by power electronics converters. Therefore, a method for limiting fault currents is proposed as well in the paper. The proposed grid-forming control scheme has been implemented in a real-time control platform and validated using comprehensive simulation models in a real-time simulator for assessing its grid-forming capability.

Investigation on Multisampling Deadbeat Current Control With Time-Delay Compensation in Grid-Connected Inverter

The control of voltage source converters (VSCs) is now implemented on digital microprocessors. This digitalization has the drawback of time delay in the control loop. The goal of this research work was to investigate improvements that can be obtained from the combination of model-based and model-free time-delay compensation approaches. Deadbeat control (DBC) from model-based techniques and the method of moving the control variable’s sampling instants, or the pulse-width modulation (PWM) updating instants, from model-free time-delay compensation techniques, were put together as the proposed new method of time-delay compensation in this study. These controllers were thoroughly examined in terms of control algorithm design, system stability analysis, and sensitivity analysis of plant parameter perturbations. In addition, thorough Simulink-based computer simulations were conducted in this work to assess the performance of each controller. The proposed method compensated about <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$80~\mu \text{s}$ </tex-math></inline-formula> as compared with the time delay compensated by the conventional single-sampling method. This research work was limited to simulations only; hence, conducting experiments to further validate this research work could be a direction for further research.

An Overview on the Recent Advances of the Voltage Source Converter Control Modes in Terms of their Roles in Transmission Grid Ancillary Services

Voltage Source Converters (VSC) are expected to be one of the major actors in future AC transmission grids. Their role includes the interconnection of renewable energy, from distributed sources such as residential PV to large Offshore Wind Farms (OWF) using HVDC. With the ongoing decommissioning of traditional power plants with their synchronous generators, VSCs must now bring support functions to the grid. The aim of this paper is to review the different grid services that can be achieved through VSCs and discuss their compatibility with the state-of-the-art VSC controls especially the so-called grid-following and grid-forming controls. After presenting the upcoming issues of future grids and classifying the main VSC controls, this paper deals with the different additional control laws which make it possible to support the AC grid. Some recommendations regarding future grid services classifications and power and energy requirements are given. A comparative table is also proposed to assess the role of the different control modes.