Stability Of Voltage Source Converters Research Articles

Stability and control of VSC-based HVDC systems: A systematic review

The technological development in the area of power electronics has paved the way for the construction of High Voltage Direct Current (HVDC) systems. The utilization of HVDC grids alongside conventional High Voltage Alternating Current (HVAC) grids poses several challenges, especially, from stability and control points of view. Indeed, moving towards such systems in the context of conventional Alternating Current (AC) power systems cannot be possible without ensuring the overall stability of hybrid HVAC/HVDC grids. This paper analyzes different aspects of the stability of Voltage Source Converter (VSC)-based HVDC grids and presents various methods of improving stability based on a systematic and comprehensive review. In addition, this paper provides a concise classification of various control methods to improve the operation of such grids and the advantages of each method.

Effects of PLL Frequency Limiters on Synchronization Stability of Grid Connected VSC and Strategy to Realize Global Stability

This paper investigates the synchronization stability of voltage source converter (VSC) connected to infinite bus system with frequency limiter of phase-locked loop (PLL). First, three different types of PLL frequency limiters are modelled based on piecewise-smooth dynamical system (PSDS). Then the effects of different frequency limiters and frequency limit values on the region of attraction (ROA) of the system are studied and the properties of the PSDSs are analyzed. It is found that PLL with anti-windup limiter with back calculation can achieve global stability with appropriate limit value, and the principle of setting limit value based on a specific ROA estimation is proposed. Based on ROA estimation with Lyapunov function, a method of setting frequency limit value which guarantees global stability is proposed. The validity of the proposed strategy is verified through MATLAB/Simulink 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.

Assessment and enhancement control for small‐signal voltage stability of VSC‐HVDC systems supplying passive industrial loads

AbstractThis paper presents a comprehensive assessment for the small‐signal voltage stability of voltage source converter (VSC)‐HVDC systems supplying industrial loads represented by the induction motors (IMs) and provides a stability enhancement control for the VSCs. First, the steady‐state voltage stability is analysed to seek the factors that affect the existence of the stable equilibrium point. Then, the small‐signal voltage stability analysis is performed based on the developed linear model. Eigenvalue analysis is carried out to capture the critical factors that induce instability phenomena correlative to the system voltage. It demonstrates that decreasing either the proportional gain K of the VSC's PI controller or the electrical distance between the VSC and IM is detrimental to voltage stability. Next, the enhancement control strategy is elaborated. It adopts virtual reactance and virtual admittance control for the VSC to improve the voltage control capability. Finally, the effectiveness and robustness of the proposed enhancement control are validated via time‐domain simulations in a multi‐IM system supplied by VSC‐HVDC.

A novel method for synchronization stability enhancement of grid connected converters based on equal area criterion

Penetration of inverter-based distributed energy (DG) resources into power grid has notably risen in recent years. Hence, new grid code standards require DGs to remain connected to the main grid even during faults and inject a certain amount of reactive power. However, in weak grids and during severe voltage faults, converters may fail to maintain their synchronization with the main grid. A main cause for this synchronization instability is the improper operation of synchronization unit which is phase locked loop (PLL). In literature, imitating the transient stability analysis of synchronous generators, swing equation and Equal Area Criterion (EAC) are applied to study the synchronization of grid-connected converters. In this paper, a new method based on EAC and swing equation is proposed to keep the damping coefficient in the swing equation positive, and consequently increase the maximum buffer area in EAC. Hence, the synchronization stability of voltage source converters (VSCs) connected to weak grids is enhanced. When the fault is detected, the proposed technique simply adds a certain gain of frequency deviation to the PLL and does not need any additional information of grid changes. The simulation results in MATLAB verify the viability of the proposed controller.

Impedance Analysis of Voltage Source Converter Using Direct Power Control

The impedance analysis has been proposed and proved to be an effective way to analyze the grid-connected stability of Voltage Source Converter (VSC) based energy conversion systems. Most of the existing impedance analyzes are discussed based on vector oriented control, while there are few discussions about the impedance analysis of Direct Power Control (DPC). In this paper, an impedance modeling approach of VSC using the DPC is firstly proposed and analyzed, which fills the gap of impedance analysis of the DPC based converters and provides a basis for studying the corresponding grid-integration stability problems. Since there is no need for PLL and Park transformation, the impedance matrix is built directly using the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha \beta$</tex-math></inline-formula> reference frame and features symmetrical characteristics. The proposed impedance matrix can be transformed into a positive-sequence impedance and analyzed by using SISO Nyquist criteria. The stability of a weak-grid connected VSC using DPC is analyzed in this article. The main factors affecting system stability, including the grid and control parameters, are considered in the analysis. The effectiveness of the proposed method is demonstrated by the simulations carried out in the Matlab/Simulink Simscape Power Systems as well as a hardware-in-loop system.

Limitations of voltage source converter in weak ac networks from voltage stability point of view

A weak ac grid could be envisioned as a voltage source behind a large synchronous impedance; thus the phase and magnitude of its terminal voltage vary significantly with the magnitudes of active and reactive powers it sinks or sources. Such characteristics present formidable challenges to control and stability of voltage source converters in weak ac grids. Therefore, this paper presents a theoretical analysis which establishes the maximum active power transfer that the voltage source converter (VSC) can exchange with a weak ac grid. The presented analysis reveals that the maximum active power that the VSC can exchange with a weak ac grid is determined by voltage stability limit. The validity of the presented theoretical analysis is confirmed by simplified simulation, and further corroborated by detailed simulations from a VSC model, developed in MATLAB-SIMULINK that incorporates all necessary control systems.

Improved Operation Strategy with Alternative Control Targets for Voltage Source Converter under Harmonically Distorted Grid Considering Inter-Harmonics

This paper proposed an improved control method for grid-connected voltage source converter (VSC), when the grid voltage consisted of the integer harmonics and inter-harmonics. Control object of the proposed control can be alternated to achieve the sinusoidal current or smooth output power, which enhances the operation adaption of VSC under the harmonically distorted grid. On the basis of a PI regulator in the fundamental current control loop, the novel control strategy was proposed with a supplementary controller which consisted of a prepositive high-pass filter and a modified proportional-derivative controller. In the proposed control, the inter-harmonics could be suppressed without detecting frequency, while the traditional resonator was effective in the premise of knowing the harmonics frequency. Also, the influence of control gain on the steady performance and the stability of VSC was analyzed, and the influences on the fundamental control caused by the proposed controller were also analyzed to verify the practicability of the proposed control strategy. Finally, the effectiveness of the proposed strategy was verified by the experiments.

Analysis of Modal Resonance Between PLL and DC-Link Voltage Control in Weak-Grid Tied VSCs

Weak ac grid operation is known to challenge the stability of voltage source converters (VSCs). This paper provides an interpretation for the cause of such instability in view of a particular modal resonance within VSC's control, i.e., the resonance between dc-link voltage control (DVC) and synchronizing control-phase-locked loop (PLL). To characterize this modal interaction, a two degrees-of-freedom mass–spring–damper model is first proposed. Subsequently, based on a multi-modal decomposition approach, the amount of interaction is quantified by the incremental damping and frequency drift superimposed on each individual mode. Analytical results indicate that when the natural frequencies of PLL and DVC mode are close, strong interaction will push the lower-frequency mode to move toward the decreased frequency and damping direction, while causing the higher-frequency mode to go the opposite. Moreover, weaker ac grid operation will amplify such frequency/damping excursion and, thus, will render the lower-frequency mode unstable. Further, when the ac voltage control (AVC) is disregarded, a dynamic stability margin is analytically derived, and operation beyond the margin will result in monotonic drift. With the inclusion of AVC, owing to its introduced additional negative damping, by contrast, instability will occur in the presence of oscillations. Both eigenvalue analysis and simulations are conducted to verify the results.

Steady-state Analysis of Voltage Source Converter (VSC) Connected to Weak AC System and Optimal Control Strategies

The power transfer limitation of voltage source converter (VSC) connected to weak AC system is restricted by three main elements: steady-state characteristic, small signal stability, and transient stability. In this paper, according to steady-state analysis, constraint condition and mathematical model of VSC system are established to figure out the appropriate operating point. A novel outer-loop control diagram is proposed. Meanwhile, a small signal model of VSC system is established. Based on participation factor analysis, the dynamic performance of phase-locked loop (PLL) have significant impacts on the system stability assessment. Therefore, an advanced PLL control appending damping factor is proposed. Simulations validated the following conclusions: 1) The optimal AC voltage of point of common coupling is associated with the equivalent AC system voltage, impedance, and the current limitation. The novel outer-loop controller can enhance the power transfer limitations. 2) The small signal stability of VSC can be enhanced by appending an alternative damping factor control on PLL. The advanced PLL has robustness which can endure a great active power disturbance in AC system. Simulations and calculations have been performed to validate the findings.

Multivariable Droop Control of Synchronous Current Converters in Weak Grids/Microgrids With Decoupled &lt;italic&gt;dq&lt;/italic&gt;-Axes Currents

Weak grids/microgrids are gaining considerable attention due to the installation of renewable power generation resources in areas that are remote from load centers. This paper presents a multivariable-droop synchronous current converter control strategy for current management of weak grid/microgrid applications, and offers a systematic and straight-forward design approach using the loop shaping technique. The controller integrates current regulation, management, and limitation that are applicable to weak grids and microgrids (MGs). The multivariable synchronous current converter enables superior decoupling of ${d}$ - and ${q}$ -axis currents in weak grids and/or resistive grids where these control variables are highly coupled due to converter operation at large load-angles. Therefore, stable connection of weak (high impedance) resistive MGs to utility grids is feasible. The multivariable droop concept also realizes real power sharing by simultaneous drooping of both frequency and voltage amplitude, which enhances load sharing accuracy in resistive and/or weak grids. Similar to its predecessor, synchronous converters, the controller can eliminate phase-locked loop (PLL) and provides a unified controller, PLL, and load manager in one simple control methodology. This can facilitate grid auto-synchronization, which can improve the stability of voltage source converters in weak grid applications. Also, the controller can participate in the power system frequency, and voltage regulation and control. To evaluate the performance of the controller, several simulations are conducted under various typical scenarios and conditions.