Voltage Source Converter HVDC System Research Articles

Large-Signal Stability Analysis of DC Side of VSC-HVDC System Based on Estimation of Domain of Attraction

The existing research on voltage-source converter HVDC (VSC-HVDC) system stability mostly focuses on small-signal stability analysis methods. However, when large-signal disturbances occur, the intrinsic nonlinearity of the VSC-HVDC system becomes inevitable so that the small-signal studies are no longer valid. In this paper, a large-signal stability analysis method of the VSC-HVDC system is proposed based on the estimation of domain of attraction (DOA). This method can not only analyze the asymptotic stability of the operating point, but also answer the question of how large deviations from the operating point can be tolerated by the system. By using Takagi-Sugeno (T-S) fuzzy model and Lyapunov stability theorem, the DOA of the VSC-HVDC System at the operating point is estimated, and the large-signal stability of VSC-HVDC System is analyzed. On this foundation, the influence of system parameters on the large-signal stability is studied. The results can be used to guide the adjustment of system parameters to solve the problem of system instability. The time-domain simulations conducted in Matlab/Simulink are used to test the stability of the system under large-signal disturbance in the range of DOA. The simulation results verify the feasibility and effectiveness of the proposed method.

Modeling and design of an adaptive control for VSC-HVDC system under parameters uncertainties

The growing demand for electricity and the increasing integration of clean energies into the electrical grids requires the multiplication and reinforcement of high-voltage direct current (HVDC) projects throughout the world and demonstrates the interest in this electricity transmission technology. The transmitting system of the voltage source converter-high-voltage direct current (VSC-HVDC) consists primarily of two converter stations that are connected by a dc cable. In this paper, a nonlinear control based on the backstepping approach is proposed to improve the dynamic performance of a VSC-HVDC transmission system, these transport systems are characterized by different complexities such as parametric uncertainties, coupled state variables, neglected dynamics, presents a very interesting research topic. Our contribution through adaptive control based on the backstepping approach allows regulating the direct current (DC) bus voltage and the active and reactive powers of the converter stations. Finally, the validity of the proposed control has been verified under various operating conditions by simulation in the MATLAB/Simulink environment.

Real-Time Implementation of Robust Loop-Shaping Controller for a VSC HVDC System

Voltage source converter (VSC) based HVDC systems are one of the most promising technologies for high voltage bulk power transmission. The reliability and stability of a VSC-based HVDC system greatly depends on the design of a proper controller for the inner decoupled d-q current loop. One of the major causes of instability in a properly tuned controller is due to system parameter variation. This paper presents the design of a fixed parameter robust controller for the inner decoupled d-q current loop for a VSC-based HVDC system to deal with the uncertainties due to system parameter variations. The method of multiplicative uncertainty is employed in the robust design to model the variations in the system parameters. The robust control design was realized through a graphical procedure known as the loop-shaping technique. The graphical loop shaping technique is a much simpler and more straightforward method compared to the traditional H∞-based algorithms for robust controller design. The designed robust controller was experimentally verified using a real-time hardware in loop (HIL) system and was tested on a VSC HVDC system. The performance of the designed robust controller is compared to that of a traditional PI controller. It has been observed that a classical PI controller is effective for a given operating point, and its performance deteriorates when the operating point changes or when the system parameters change. The studies conducted using real-time hardware in the loop (HIL) system prove that the designed loop-shaping-based robust controller provides very good performance and stability for a wide range of system parameter variations, such as changes in resistance and the inductance of the VSC HVDC system compared to the PI controller tuned using conventional methods.

A Phase Change Material Integrated Press Pack Power Module With Enhanced Overcurrent Capability for Grid Support—A Study on FRD

The capability of dynamic grid support of a voltage source converter-high voltage direct current (VSC-HVdc) system depends on the maximum allowable current of the converter. This study attempts to increase the capability by overloading the power semiconductor device for a short period of time. Keeping the device temperature low is a major challenge due to the extra loss and small local thermal capacity. This study investigates a solution to increase the transient thermal capacity by making use of the latent heat of a phase change material (PCM). This article presents an integration scheme for a press pack power module, focusing on the fast recovery diode in an IGBT-diode package. Experiment and finite element analysis are used to evaluate the concept and analyze the factors affecting the design. It is shown that as heat is absorbed by the integrated PCM, the device temperature rise during a power surge is effectively tempered, providing a high short-term overcurrent rating of the device and an increased dynamic grid support capability of the VSC-HVdc system.

A New Press Pack IGBT for High Reliable Applications With Short Circuit Failure Mode

The modular multilevel converter (MMC or M2C) technology becomes the mainstream solution in today's voltage source converter-high voltage direct current (VSC-HVDC) transmission lines because of its unique performance. Recently press pack IGBT (PPI) has come into focus as a preferred realization for high power rating VSC-HVDC stations, it could offer the highest current rating IGBTs and advanced features in reliability. A new PPI is introduced in this paper, which is designed for several innovative improvements based on the application demands from VSC-HVDC systems. The chip is optimized for low on-state voltage with advanced trench technology, and well protected in a chip-stack by double side sinter process. The disc package is designed with hermetic sealed ceramic housing to avoid environmental erosion and could provide a robust feature of short circuit failure mode (SCFM), which is fully qualified under the worst-case scenario. In addition to the PPI device, system solutions had also been developed and investigated to support field developments, including electrical and mechanical design. An example of a power stack is analyzed in commutation loop, pressure distribution, and power losses.

A Novel Adaptive Control Approach Based on Available Headroom of the VSC-HVDC for Enhancement of the AC Voltage Stability

Increasing offshore wind farms are rapidly installed and planned. However, this will pose a bottle neck challenge for long-distance transmission as well as inherent variation of their generating power outputs to the existing AC grid. VSC-HVDC links could be an effective and flexible method for this issue. With the growing use of voltage source converter high-voltage direct current (VSC-HVDC) technology, the hybrid VSC-HVDC and AC system will be a next-generation transmission network. This paper analyzes the contribution of the multi VSC-HVDC system on the AC voltage stability of the hybrid system. A key contribution of this research is proposing a novel adaptive control approach of the VSC-HVDC as a so-called dynamic reactive power booster to enhance the voltage stability of the AC system. The core idea is that the novel control system is automatically providing a reactive current based on dynamic frequency of the AC system to maximal AC voltage support. Based on the analysis, an adaptive control method applied to the multi VSC-HVDC system is proposed to realize maximum capacity of VSC for reactive power according to the change of the system frequency during severe faults of the AC grid. A representative hybrid AC-DC network based on Germany is developed. Detailed modeling of the hybrid AC-DC network and its proposed control is derived in PSCAD software. PSCAD simulation results and analysis verify the effective performance of this novel adaptive control of VSC-HVDC for voltage support. Thanks to this control scheme, the hybrid AC-DC network can avoid circumstances that lead to voltage instability.

An Improved AC System Strength Measure for Evaluation of Power Stability and Temporary Overvoltage in Hybrid Multi-Infeed HVDC Systems

We developed a new strength measure, the hybrid multi-infeed effective short-circuit ratio (HMESCR), for evaluating hybrid multi-infeed high-voltage direct current (MIDC) systems. The HMESCR defines the strength of hybrid MIDC systems with line-commutated converter (LCC) high-voltage direct current (HVDC) and voltage-source converter (VSC) HVDC systems. Unlike previous measures, the HMESCR considers the capacity of VSC HVDC systems to provide reactive power and thus, enhance the strength of AC systems connected at different buses with LCC HVDC systems. To demonstrate how the HMESCR reflects power stability (PS) and temporary overvoltage (TOV), we used MATLAB/Simulink software to perform quasi-steady-state analyses and dynamic simulations. The HMESCR can be used to determine the strength of hybrid MIDC systems.

Impedance-based method for DC stability of VSC-HVDC system with VSG control

DC-side oscillation problems trouble the stable and secure operation of voltage source converter high voltage direct current (VSC-HVDC) systems. Therefore, it is of considerable significance to investigate the physical origins of these oscillations. Thus, this paper proposes an impedance-based method for VSC-HVDC system DC stability analysis. Differently, a new perspective for DC resonance interaction is introduced, which is judged by the interactive behavior of two subsystems impedances, namely converter impedance and DC network impedance. Also, the proposed method quantitatively estimates the damping of DC schemes of VSC-HVDC system. Based on the proposed method, it is found that the global stability of VSC-HVDC system can be guaranteed if and only if: 1) subsystems are passive and locally stable, 2) there is no resonance interaction between inductive impedance and capacitive impedance, and 3) damping of DC system is large enough. Moreover, another contribution is to apply the virtual synchronous generator (VSG) control technique in VSC-HVDC system to dampen DC-side oscillation caused by impedance interaction. Specifically, this VSG control enhances inertia and damping of VSC-HVDC system on DC-side. Finally, the feasibility of the proposed technique and the accuracy of the mathematical model are demonstrated through simulation and experimental results.

Fault current limiter of VSC-HVDC systems using variable resistive bridge controller

This paper proposes a fuzzy logic controller (FLC) based variable resistive bridge type fault current limiter (VR-BFCL) to minimize the impact of faults on voltage source converter high voltage direct current (VSC-HVDC) systems. FLC for VR-BFCL is developed to dynamically insert the appropriate current limiting resistance during contingencies to maintain the fault currents within the acceptable limit. The proposed controller effectively provides variable duties to generate and insert controllable resistance in the system based on fluctuation of direct current (DC) link voltage and point of common coupling (PCC) voltage. The efficacy of the proposed controller is justified through simulations work for balanced and unbalanced faults. The results demonstrate that oscillations in PCC voltage and DC link voltage can be maintained within the acceptable range by controlling fault current through the insertion of the appropriate resistance. It is also found that the proposed controller outperforms the existing method.

Analytic of VSC-HVDC Interconnected Power Systems from a Perspective of Frequency Security

In the interconnected power systems with VSC-HVDC (Voltage Source Converter-High Voltage Direct Current) transmission links, the failures of VSC-HVDC system can cause large power imbalance for both sending-end and receiving-end power systems. In this paper, the reliability model of the VSC-HVDC system is developed based on series-parallel structure recognition. The frequency dynamics of the power systems are characterized considering the frequency regulation of the power systems in response to the power imbalance arise from the component failures. Moreover, the Monte Carlo simulation method is applied to analyze the frequency dynamics in different contingencies. The indices measuring the frequency stability and the reliability of the interconnected power systems are introduced.

State Space Modeling and Stability Analysis of a VSC-HVDC System for Exchange of Energy

Nowadays, relevant actors are searching for solutions to produce energy with a low impact on the environment. Indeed, transmitting power via large distances with maintaining low losses is one of the main challenges. To improve electricity communication between countries and offshore wind, a new interconnections line must be built. Therefore, Voltage Source Converter High Voltage Direct Current (VSC-HVDC) transmission is incoming as the exceptive technology in order to address the challenges related to the integration of future offshore wind power plants. In spite of its many advantages, VSC-based HVDC transmission systems can experience unexpected instability and interaction phenomena: Small disturbances that occur continually in VSC-HVDC transmission systems due to the complex VSC based interconnections and an important number of components with non-linear nature that may cause failure. Thus, before installing the HVDC system, there is a significant need for studying a hybrid AC-DC system to guarantee the reliable and stable operation. This paper deals with the stability of a VSC-HVDC system by the use of a small signal stability method; such procedure enables to study the stability of a linearized VSC-HVDC system through state-space modeling and eigenvalue-based stability analysis.

Centralized Fuzzy Logic Based Optimization of PI Controllers for VSC Control in MTDC Network

Advancements in the field of power electronics led to global changes in the electrical energy generation, transmission, and distribution. The voltage source converter (VSC) based HVDC system is the future of the power system due to the advantages it offers in terms of renewable power generation, transmission, and integration. Currently, the two-terminal VSC–HVDC systems have been successfully commissioned. Multi-terminal VSC–HVDC system is the developing trend for higher power reliability, large scale integration, and smart operation. The performance of a VSC based multi-terminal direct current (MTDC) system greatly depends upon the tuning of the controller. Standard practice is to tune the PI controller using hit and trial method or based on the operator’s experience. However, the tuning process becomes complex when multiple grids are involved. Thus, for MTDC systems, the manual tuning of the controller does not yield the desired results. This paper presents a centralized fuzzy logic-based optimization technique for VSC control of the MTDC system to obtain the optimized parameters for the PI controllers. The optimized parameters ensure a better system performance in terms of fast settling time, better slew rate, minimum undershoot, and minimum overshoot response. The proposed technique is tested on a three-terminal MTDC network in SIMULINK / MATLAB.

Transient Stability Constrained Optimal Power Flow in Mixed AC/Multi‐Terminal VSC HVDC System

This paper proposes a hierarchical transient stability constrained optimal power flow (TSCOPF) analysis in a power system with a multi‐terminal voltage‐source converter (VSC) high‐voltage direct current (HVDC) system and large‐scale wind farms. This aims at determining the reinforcement and operation of the power system using the multi‐terminal VSC HVDC (MTDC) system. By the method, the wind farms can be installed without any transient instability issues. On the AC grid side, transient rotor‐angle stability and transient AC voltage stability are included as additional constraints. On top of that, the constraints of DC bus voltage fluctuations and DC transmission line current deviations on the DC grid side are also taken into account. Time‐domain simulation approach is applied to investigate these transient stability constraints. The MTDC system reinforcement is designed considering wind power transmission and economic operation of the system model. The influences of the transient stability constraints on the system operation costs with high and low wind power output patterns are discussed. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

Optimal metaheuristic-based sliding mode control of VSC-HVDC transmission systems

The design of classical controllers for Voltage Source Converter High Voltage Direct Current (VSC-HVDC) transmission systems, is load-dependent and has to be adjusted for each operating condition. Thus, the robustness of such controllers becomes necessary to cope with operating condition continuous variations. Therefore, the design of hybrid optimal Artificial Intelligence Based-Sliding Mode Controllers (AI-SMCs) for VSCHVDC transmission systems is crucial research interest. These AI based controllers are proved to improve the system’s dynamic stability over a wide range of operating conditions considering different parameter variations and disturbances. For this purpose, a comprehensive state of the art of the VSC-HVDC stabilization dilemma is discussed. The nonlinear VSC-HVDC model is developed. The problem of designing a nonlinear feedback control scheme via two control strategies is addressed seeking a better performance. For ensuring robustness and chattering free behavior, the conventional SMC (C-SMC) scheme is realized using a boundary layer hyperbolic tangent function for the sliding surface. Then, the Modified Genetic Algorithm (MGA) and Particle Swarm Optimization technique (PSO) are employed for determining the optimal gains for such SMC methodology forming a modified nonlinear MGA-SMC and PSO-SMC control in order to conveniently stabilize the system and enhance its performance. The simulation results verify the enhanced performance of the VSC-HVDC transmission system controlled by both MGA-SMC and PSO-SMC compared to the C-SMC. The comparative dynamic behavior analysis for both the conventional SMC and the two meta-heuristic optimization based SMC control schemes are presented. Through simulation results, the effectiveness of the proposed metaheuristic optimization approaches and their applicability to VSC-HVDC system global stabilization and dynamic behavior enhancement are validated.

Control of Parallel AC Voltage Source Converter High-Voltage DC System Using an Adaptive PI Evolutionary-Based Controller

High-voltage direct current (HVDC) transmission system based on voltage source converter technology gains attraction in recent years for the grid integration due to many advantages like connecting wind farms to power grids, underground power links, connecting asynchronous grids, and providing efficient long-distance power transmission, etc. To acquire the best output from the above topologies, it is crucial to design a better controller strategy for the converter control. With this objective, this paper presents an adaptive PI controller based on evolutionary optimization for a parallel AC voltage source converter HVDC system. Looking at the extensive application of constant gain PI controller type, an adaptively changing gain parameter PI controller based on tan hyperbolic (tanh) function is proposed in this study. To enhance further its performance, a Modified Teaching Learning-Based Optimization (MTLBO) is used to find optimally the parameters of the proposed control strategy. To justify the effectiveness of the proposed controller, different test cases in terms of faults, parameter variation, and load & reference power variation are studied and analyzed. Comparative analysis with a conventional tuned PI controller, PI-MTLBO controller, and proposed API-MTLBO controller is demonstrated for indicating a substantial improvement in the damping system oscillations with better controllability and stability of the proposed approach.

Damping controller design based on FO-PID-EMA in VSC HVDC system to improve stability of hybrid power system

Wind energy sources have different structures and functions from conventional power plants in the power system. These resources can affect the exchange of active and reactive power of the network. Therefore, power system stability will be affected by the performance of wind power plants, especially in the event of a fault. In this paper, the improvement of the dynamic stability in power system equipped by wind farm is examined through the supplementary controller design in the high voltage direct current (HVDC) based on voltage source converter(VSC) transmission system. In this regard, impacts of the VSC HVDC system and wind farm on the improvement of system stability are considered. Also, an algorithm based on controllability (observability) concept is proposed to select most appropriate and effective coupling between inputs-outputs (IO) signals of system in different work conditions. The selected coupling is used to apply damping controller signal. Finally, a fractional order PID controller (FO-PID) based on exchange market algorithm (EMA) is designed as damping controller. The analysis of the results shows that the wind farm does not directly contribute to the improvement of the dynamic stability of power system. However, it can increase the controllability of the oscillatory mode and improve the performance of the supplementary controller.

Large-Signal Stability Analysis for VSC-HVDC Systems Based on Mixed Potential Theory

Stability studies of voltage-source converter HVDC (VSC-HVDC) systems are mainly based on small-signal linearization techniques. However, when VSC-HVDC systems become unstable due to large-signal disturbances, the small-signal analysis method would be no longer available. In this paper, the large signal stability of VSC-HVDC system is studied based on mixed potential theory. A simplified state-space averaging model of VSC-HVDC system is established under the synchronous rotating reference frame. Based on this model, the stability prediction criteria for VSC-HVDC system under large-signal disturbances is obtained through Brayton-Moser's nonlinear potential method. On this foundation, the influence of system parameters on the large-signal stability is studied and the boundaries of stability are located as well. Time-domain simulations conducted in Matlab/Simulink are used to test the stability of the working points next to the stable boundary, which aim to prove the correctness of the obtained stability criterion. Aiming at the unstable situation, the time-domain simulation results prove the feasibility of the method of adjusting the system parameters according to the stability criterion to make the system stable, which further verifies the correctness of the stability criterion.

An improved control method for Voltage Source Converter-High Voltage Direct Current system

This paper aims to enhance the accuracy and response speed of the active-disturbance rejection control (ADRC) controller of the voltage source converter-high voltage direct current (VSC-HVDC) system. For this purpose, an improved control method was proposed for the VSC-HVDC system based on the least squares support vector machine (LSSVM) optimization of the ADRC controller. First, the ADRC regulators were designed for the active/reactive power outer loop in VSC-HVDC inverter control system. Then, the LSSVM optimized regression model was created for the ADRC regulators. Finally, the proposed method was successfully simulated on MATLAB. The simulation results show that the LSSVM-optimized ADRC method can effectively enhance the anti-interference ability of the VSC-HVDC. The research findings shed new light on the signal analysis of the VSC-HVDC system.

Coordinated control strategy of multi‐terminal VSC‐HVDC system considering frequency stability and power sharing

Aiming at the power sharing among converter stations and the frequency stability of weak AC system connected by converter stations in multi-terminal voltage source converter high-voltage direct current (VSC-HVDC) system, a coordinated control strategy without communication is proposed, which can realise the bidirectional active power support between DC system and weak AC system. The strategy divides the voltage of DC system and the frequency of weak AC system into open-time operation area and time-limited operation area, respectively. When DC voltage and AC frequency are in the open-time operation area, respectively, unbalanced power in the system can be flexibly allocated according to the power margins of converter stations, which reduces DC voltage fluctuation and the influence of DC voltage errors (line length and other factors) among the converter stations on power regulation. When the AC frequency is in the time-limited operation area, the DC system can provide power support for the weak AC system to maintain frequency stability. When the DC voltage is in the time-limited operation area, the weak AC system loses part of the frequency quality to provide short-term power support for the DC system to maintain the stability of VSC-HVDC system as far as possible. The simulation results have verified the feasibility and effectiveness of the proposed control strategy.

Active and Reactive Power Compensation Control Strategy for VSC-HVDC Systems under Unbalanced Grid Conditions

This paper presents a power compensation strategy to suppress the double frequency power ripples of Voltage source converter high-voltage direct current (VSC-HVDC) systems under unbalanced grid voltage conditions. The mathematical control equations of the double frequency ripple power of VSC under unbalanced operating conditions are firstly derived and established, where the dynamic behaviors of the double frequency ripples in active and reactive power are regarded as being driven by current-relevant components and voltage-relevant components, respectively. Based on the equations, a power compensation control strategy of VSC-HVDC is proposed via the passivity-based control with disturbance observer to suppress both the current-relevant and voltage-relevant components in the power ripples. With this control strategy, the double frequency ripples in active and reactive power are suppressed simultaneously and system performance is significantly enhanced with the implementation of the disturbance observer in the passivity-based control. Theoretical stability analysis and simulation cases show the effectiveness and superiority of the proposed strategy.