Voltage Source Converter Based HVDC System Research Articles

Enhancement of fault ride-through capability of three-level NPC converter-based HVDC system through robust nonlinear control strategy

Voltage source converter-based high-voltage direct current (VSC-HVDC) transmission systems are becoming increasingly popular for long-distance power transmission due to their numerous advantages. However, the fault ride-through (FRT) capabil-ity of VSC-HVDC systems during AC faults remains a key challenge that needs to be addressed to ensure their uninterrupted operation under challenging grid conditions. This paper presents and analyzes a novel nonlinear control scheme based on a robust sliding mode control approach to improve the FRT capability of VSC-HVDC systems. The proposed control scheme based on the modification of the converter’s active power to make it more sensitive to any changes in voltage magnitude at the points of common coupling (PCCs) on the AC side. This helps to maintain the DC voltage sta-bility and prevent the converter from tripping during AC faults. The performance of the proposed control scheme is evaluated using a series of ex-perimental investigations using a digital signal processor (DSP) within Processor-in-the-Loop (PIL) quasi-real-time simulation. The PIL simulations closely examine the system’s response to AC faults with varying remaining voltages and durations. The results demonstrate that the proposed control scheme can effectively improve the FRT capability of VSC-HVDC systems and ensure their stable operation during AC faults.

The VSC-HVDC Transmission System Performance Assessment

As the use of renewable resources increases the transfer of power from remote area to consumer is a big challenge. Therefore, Voltage Source Converter High Voltage Direct Current (VSC-HVDC) transmission is gaining more popularity for integration of renewable energy generators to the consumers using smart grid. Even after many advantages of HVDC transmission system it is not used popularly due to unexpected instability in the system. However recent developments in semiconductor devices and control techniques the HVDC link operation in transmission system becomes easier. In this paper VSC based HVDC system is designed with AC-DC converter model for power flow assessment of both sides of HVDC link. To analyze the proposed model a simulation model of 75km DC line with voltage source converter (VSC) based AC-DC converter model on both side of DC line is designed in a MATLAB environment. HVDC link efficiency is calculated by analyzing power flow through AC lines at both ends of HVDC line.

Robust control strategy of VSC-HVDC systems based on feedback linearization and disturbance compensation method

The Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) technology has become the development direction of new energy grid-connected, DC power transmission and island power supply technology in the future due to its superior characteristics. The effective and reliable control method is important to realize the stable operation of VSC-HVDC system during disturbances. However, the common adopted control scheme for VSC-HVDC system lacks of superior dynamic response when system suffers external disturbances. Although some relatively new control methods have been continuously proposed and applied to the VSC-HVDC system, such as no-beat control, fuzzy control, sliding mode control and repetitive control, etc., they usually depend on detailed and exact system model. This paper focuses on the feedback linearization and disturbance compensation methods in VSC-HVDC system. Firstly, the mathematical model of VSC-HVDC system in dq coordinate system with modeling errors and external disturbances is derived. Secondly, in order to improve the dynamic response and robustness of the control, the feedback linearization and disturbance compensation method is used and designed to realize the decoupling control of the VSC-HVDC system. Finally, the VSC-HVDC system with conventional control and the proposed control is compared and analyzed on the PSCAD/EMTDC simulation platform. The results show that the proposed controller based on feedback linearization and disturbance compensation principle has better dynamic performance and higher robustness of regulation performance.

Data-Mining Techniques Based Relaying Support for Symmetric-Monopolar-Multi-Terminal VSC-HVDC System

Considering the advantage of the ability of data-mining techniques (DMTs) to detect and classify patterns, this paper explores their applicability for the protection of voltage source converter-based high voltage direct current (VSC-HVDC) transmission systems. In spite of the location of fault occurring points such as external/internal, rectifier-substation/inverter-substation, and positive/negative pole of the DC line, the stated approach is capable of accurate fault detection, classification, and location. Initially, the local voltage and current measurements at one end of the HVDC system are used in this work to extract the feature vector. Once the feature vector is retrieved, the DMTs are trained and tested to identify the fault types (internal DC faults, external AC faults, and external DC faults) and fault location in the particular feeder. In the data-mining framework, several state-of-the-art machine learning (ML) models along with one advanced deep learning (DL) model are used for training and testing. The proposed VSC-HVDC relaying system is comprehensively tested on a symmetric-monopolar-multi-terminal VSC-HVDC system and presents heartening results in diverse operating conditions. The results show that the studied deep belief network (DBN) based DL model performs better compared with other ML models in both fault classification and location. The accuracy of fault classification of the DBN is found to be 98.9% in the noiseless condition and 91.8% in the 20 dB noisy condition. Similarly, the DBN-based DMT is found to be effective in fault locations in the HVDC system with a smaller percentage of errors as MSE: 2.116, RMSE: 1.4531, and MAPE: 2.7047. This approach can be used as an effective low-cost relaying support tool for the VSC-HVDC system, as it does not necessitate a communication channel.

Small-signal Stability Analysis and Improvement with Phase-shift Phase-locked Loop Based on Back Electromotive Force Observer for VSC-HVDC in Weak Grids

Voltage source converter based high-voltage direct current (VSC-HVDC) transmission technology has been extensively employed in power systems with a high penetration of renewable energy resources. However, connecting a voltage source converter (VSC) to an AC weak grid may cause the converter system to become unstable. In this paper, a phase-shift phase-locked loop (PS-PLL) is proposed wherein a back electromotive force (BEMF) observer is added to the conventional phase-locked loop (PLL). The BEMF observer is used to observe the voltage of the infinite grid in the stationary <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\alpha\beta$</tex> frame, which avoids the problem of inaccurate observations of the grid voltage in the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$dq$</tex> frame that are caused by the output phase angle errors of the PLL. The VSC using the PS-PLL can operate as if it is facing a strong grid, thus enhancing the stability of the VSC-HVDC system. The proposed PS-PLL only needs to be properly modified on the basis of a traditional PLL, which makes it easy to implement. In addition, because it is difficult to obtain the exact impedance of the grid, the influence of short-circuit ratio (SCR) estimation errors on the performance of the PS-PLL is also studied. The effectiveness of the proposed PS-PLL is verified by the small-signal stability analysis and time-domain simulation.

Analysis of Additional Damping Control Strategy and Parameter Optimization for Improving Small Signal Stability of VSC-HVDC System

The voltage source converter based high voltage direct current (VSC-HVDC) system is based on voltage source converter, and its control system is more complex. Also affected by the fast control of power electronics, oscillation phenomenon in wide frequency domain may occur. To address the problem of small signal stability of the VSC-HVDC system, a converter control strategy is designed to improve its small signal stability, and the risk of system oscillation is reduced by attaching a damping controller and optimizing the control parameters. Based on the modeling of the VSC-HVDC system, the general architecture of the inner and outer loop control of the VSC-HVDC converter is established; and the damping controllers for DC control and AC control are designed in the phase-locked loop and the inner and outer loop control parts respectively; the state-space state model of the control system is established to analyze its performance. And the electromagnetic transient simulation model is built on the PSCAD/EMTDC simulation platform to verify the accuracy of the small signal model. The influence of the parameters of each control part on the stability of the system is summarized. The main control parts affecting stability are optimized for the phenomenon of oscillation due to changes in operation mode occurring on the AC side due to faults and other reasons, which effectively eliminates system oscillation and improves system small signal stability, providing a certain reference for engineering design.

Time Delay of Wide Area Damping Control in Urban Power Grid: Model-Based Analysis and Data-Driven Compensation

Due to the rapid development of economies, large urban cities consume an increasing amount of energy and have a higher requirement for power quality. Voltage source converter based high voltage direct current (VSC-HVDC) is a promising device to transmit clean power from remote regions to urban power systems, while also providing wide area damping control (WADC) for frequency stabilization. However, the time-delay naturally existing in the VSC-HVDC system may degrade the performance of WADC and even result in instability. To address this issue, this paper develops a time-delay correction control strategy for VSC-HVDC damping control in urban power grids. First, a small signal model of WADC is built to analyze the negative impacts of time delay. Then, a data-driven approach is proposed to compensate for the inherent time delay in VSC-HVDC damping control. The extensive training data will be generated under various disturbances. After offline training, the long short-term memory network (LSTM) can be implemented online to predict the actual frequency deviation based on real-time measurements. Finally, the proposed method is validated through MATLAB-Simulink in a two-area four-machine system. The results indicate that the data-driven compensation has a strong generalization ability for random delay time constants and can improve the performance of WADC significantly.

Strategy for grid low-frequency oscillation suppression via VSC-HVDC linked wind farms

The effective suppression of low-frequency grid oscillations by the offshore wind power in a voltage source converter-based HVDC (VSC-HVDC) output system can enhance the operational stability of the overall power system. In most existing studies, low-frequency oscillations are suppressed by controlling active power. However, active regulation can reduce the utilization of new energy sources. Therefore, this report proposes a reactive power suppression strategy based on frequency feedback. First, a mathematical model of the VSC-HVDC system is established to derive the mechanism of reactive power suppression of low-frequency oscillations in the receiving-end converter. Second, DC voltage control and AC-side reactive power control are adopted in the sending-end converter of the VSC-HVDC system, and DC control and AC voltage control are adopted in the receiver station. Finally, the single infinity system is subjected to a simulation analysis using MATLAB/Simulink. The simulation results demonstrate that this strategy can effectively suppress low-frequency oscillations of the grid, while simultaneously reducing the investment cost of reactive power equipment and improving the wind turbine utilization efficiency.

Reclosing Current Limiting for DC Line Faults in VSC-HVDC Systems

The problem of reclosing current limiting in voltage source converter based high-voltage direct current (VSC-HVDC) systems is becoming more and more serious. A soft reclosing scheme for DC permanent faults is presented in this paper. Because the converter voltages of stations at both terminals of the disconnected faulty line may be different, the choice of which terminal to reclose first will affect the reclosing overcurrent. A method for selecting the terminal to reclose first is inves-tigated to achieve a minimum peak overcurrent during the reclosing process. In order to ensure that the hybrid DC circuit breaker (HDCCB) adapts to the needs of the reclosing process better, the traditional HDCCB is improved by adding a soft reclosing module (SRM). The energy dissipated in the arresters is significantly reduced when using the improved HDCCB. The improved HDCCB will be able to reclose multiple times safely and thus increase the possibility of successful reclosing. Moreover, the recovery time after the HDCCB is successfully reclosed is very short with the improved HDCCB and its control principles. Simulation results show that this proposed scheme is capable of limiting the reclosing overcurrent when the fault still exists.

Proposed a New Voltage Rebalancing Method for Pole-to-Ground Fault in Bipolar Two-Level VSC-HVDC

Two-level converter and modular multilevel converter (MMC) are the most commonly used converters in voltage-source converter-based high-voltage direct current (VSC-HVdc) system. Pole-to-ground fault is the most probable temporary dc fault in a bipolar VSC-HVdc system, which causes a voltage drop across the faulty pole and an overvoltage on the healthy pole. In order to restart the system after fault clearance, it must first eliminate the unbalanced voltage on the poles. The speed of voltage rebalancing is so important issue for fast restarting. In this article, a new voltage rebalancing method comprised of a simple auxiliary circuit is proposed in the bipolar two-level VSC-HVdc. The proposed scheme not only has fast restarting speed but also minimize power losses during restoration. Moreover, its control system is simple. The principles of the proposed scheme, design considerations, and control method of that are investigated. Finally, the proposed scheme is verified by the simulation and experimental results in a bipolar two-level VSC-HVdc.

Time-Varying Resistance Optimization for the Resistive Type Superconducting Fault Current Limiter Applied in VSC-HVDC System

When short-circuit fault occurs in a voltage source converter–based high-voltage direct current (VSC-HVDC) transmission system, the fault current curve is much more complex than conventional AC power grid. The high-voltage DC circuit breaker with large cut-off capacity is rather expensive; thus, some current limiting methods are proposed to limit the fault current. The superconducting fault current limiter (SFCL) is one of the promising devices for both AC and DC short-circuit current limiting application. The optimization criterion for SFCL applied in VSC-HVDC system is presented, and a four terminal VSC-HVDC system is adopted as an example. The short-circuit fault could be divided into condenser discharge and AC system feeding process, and the fault current curve is related to the time-varying RLC parameters of the grid. The thermal and resistance characteristics of the SFCL are analyzed, and the optimization method of the current limiting impedance and thermal characteristics of the SFCL is developed based on the time-varying fault current. Finally, the design of a 160-kV SFCL was completed based on the proposed method.

Nonlinear Subsynchronous Oscillation Damping Controller for Direct-Drive Wind Farms With VSC-HVDC Systems

Due to the subsynchronous interaction between the grid-side converter (GSC) of wind farms and the rectifier (REC) of voltage source converter-based high-voltage direct current (VSC-HVDC) transmission system, subsynchronous oscillations (SSOs) may occur in direct-drive wind farms with VSC-HVDC systems. Considering the nonlinearities and uncertainties of the system, a nonlinear SSO mitigation strategy is proposed in this article based on the feedback linearization theory and sliding mode control (SMC). The feedback linearization theory is used to eliminate the nonlinearities, and the SMC is adopted to improve the robustness against uncertainties and disturbances. The proposed feedback linearization SMC (FLSMC) takes the advantages of feedback linearization control (FLC) and SMC. The FLC transforms the nonlinear forms of the GSC and REC into the linear forms through the coordinate transformation and feedback. Considering that the FLC is sensitive to parameter uncertainties and external disturbances, the SMC is combined with the FLC to improve the system robustness. An eigenvalue analysis and time-domain simulations are carried out, which demonstrates that the FLC outperforms over the traditional proportional–integral control for the SSO mitigation and decoupling. Meanwhile, the FLSMC shows better robustness against parameter uncertainties and external disturbances over the FLC and traditional damping control.

Switching stabilising control of VSC‐HVDC transmission systems

Power systems are a class of typical cyber-physical systems with switching-hybrid dynamics. With the innovation of power electronics technology and the implementation of power grid development strategy, the application of voltage source converter-based high voltage direct current (VSC-HVDC) transmission has been paid more and more attention. Firstly, the general small-signal model of the VSC-HVDC system is established, and its stability analysis is carried out by integrated modal analysis, participation factor analysis, and sensitivity analysis methods. It is clarified that the system controller parameters will have a significant impact on the stability of the VSC-HVDC system. Then, based on the switching system theory, the stabilising switching law of the bidirectional converter is derived, which makes the system energy attenuate fastest, and the stabilising switching controller is designed according to the switching law. Last, the effectiveness of the controller is verified by simulation, and the superiority of the stabilising switching control strategy is proved from the theoretical and simulation aspects by comparison with the traditional current vector control strategy.

A strategy for access points selection and capacity configuration for the VSC-HVDC system considering overload scenarios alleviation

Appropriate access points of the voltage source converter-based high voltage direct current (VSC-HVDC) system are very important for better utilization of controllability provided by the integration of VSC-HVDC systems into AC grids. In this paper, the reasonable access points selection and capacity configuration of the VSC-HVDC system in hybrid AC/DC networks are investigated considering the efficient remission of AC line overloads. First of all, a comprehensive sensitivity factor (CSF)-based approach is designed with the overall consideration of multiple typical overload scenarios, for the evaluation of different access point pairs and selection of the candidate schemes. Then, a multi-objective optimization model, which takes into account line overload index and economic investment of the VSC-HVDC system, is established to obtain an economical and effective solution for the capacity configuration and final scheme determination of the VSC-HVDC system. Hence, an overall strategy for access points selection and capacity configuration of the VSC-HVDC in hybrid AC/DC networks is proposed from the perspectives of actual feasibility and performance on alleviating overloads as well as economy of constructing the VSC-HVDC system. Finally, a case study is performed for the proposed strategy to validate the reasonable access points as well as capacity configuration.

Quench and Recovery Characteristics of Solenoid and Pancake SFCLs Under DC Impact Current

With the rapid development of the voltage source converter-based high voltage direct current (VSC-HVdc) power systems, as one of the methods for the suppression and interruption of short circuit current, resistive type dc superconducting fault current limiter (SFCL) is getting more and more attention. As SFCL will quench during the current limiting process, it is necessary to complete the quench and recovery process before the automatic reclosing of VSC-HVdc power systems. In this article, simulation and experimental analysis on SFCL are carried out. The superconducting tape used in the SFCL is the AMSC 8602 YBCO superconducting tape. It is concluded that the simulation results obtained by thermal conduction model using lumped parameter method are basically consistent with experimental results. With the increase of dc impact current and dc impact duration, recovery time of SFCL increases. Recovery time of the solenoid SFCL and the pancake SFCL under different placement modes differs, which is related to the heat dissipation space and the accumulation of bubbles.

Fault Ride-Through Capability Enhancement of Type-4 WECS in Offshore Wind Farm via Nonlinear Adaptive Control of VSC-HVDC

This paper proposes a perturbation estimation-based nonlinear adaptive control (NAC) for a voltage-source converter-based high voltage direct current (VSC-HVDC) system which is applied to interconnect offshore large-scale wind farms to the onshore main grid in order to enhance the fault ride-through (FRT) capability of Type-4 wind energy conversion systems (WECS). The VSC-HVDC power transmission system is regraded as a favourable solution for interconnecting offshore wind farms. To improve the FRT capability of offshore power plants, a de-loading strategy is investigated with novel advanced control of the VSC-HVDC systems. The proposed NAC does not require an accurate and precise model and full state measurements since the combinatorial effects of nonlinearities, system parameter uncertainties, and external disturbances are aggregated into a perturbation term, which are estimated by a high-gain perturbation observer (HGPO) and fully compensated for. As the proposed NAC is adaptive to system model uncertainties (e.g., mismatched output impedance of the converters and the line impedance of transmission line), time-varying disturbance (e.g., AC grid voltage sags and line to ground faults), and unknown time-varying nonlinearities of the power-electronic system (e.g., unmodelled dynamics existed in valve and VSC phase-locked loop system), a significant robustness can be provided by the de-loading strategy to enhance the FRT capability. Simulation results illustrated that the proposed strategy can provide improved dynamic performance in the case of operation with a variety of reduced voltage levels and improved robustness against model uncertainties and mismatched system parameters comparing with conventional vector control.

Application of Superconducting Fault Current Limiter and Chopper Controlled Resistor for Protection of VSC-HVDC System

The Voltage Source Converter based HVDC (VSC-HVDC) transmission system involving overhead lines lack the inherent ability to overcome severe over currents and over voltages during dc line fault conditions. Presently, the fast acting commercial HVDC circuit breakers are uneconomical and are in prototype status. Due to this, the VSC-HVDC system finds applications in back to back and underground cable transmissions where the occurrence of dc faults is almost zero. In this context, the present paper analyses the combined application of Superconducting Fault Current Limiter (SCFCL) along with Chopper Controlled Resistors (CCR) to overcome and recover from the effects of dc line faults. The resistive SCFCL is modeled in MATLAB and the VSC-HVDC system is analyzed using PSCAD/EMTDC environment. The simulation results show that during dc line faults, the SCFCL is effective in limiting the over currents and the CCR overcomes the over voltages. The combined simulated protection strategy restores the complete VSC-HVDC system without restarting of the dc link.

Analysis of the Current Limiting Performance of a Novel DC-SFCL in the VSC-HVDC System

With increasing demand of the power load and the development of the new energy resources, voltage-sourced converter-based high-voltage direct current (VSC-HVDC) transmission system has been widely utilized. However, its development is seriously limited due to short circuit. To address this, high-temperature superconducting direct current fault current limiter (DC-SFCL) is designed. In this paper, a novel topology of DC-SFCL is firstly presented along with its principle and mathematical model. Secondly, the thermoelectric model of its superconducting current-limiting unit (SCLU) is established. In addition, simulation of VSC-HVDC system containing this novel DC-SFCL topology is analyzed using Simulink software. Finally, experimental measurements are designed and conducted to verify the correctness of the theoretical model.

Control of VSC-HVDC Connected Offshore Windfarms for Providing Synthetic Inertia

This paper presents a generator emulation control (GEC)-based method to provide synthetic inertia with a voltage source converter-based high-voltage direct current (VSC-HVdc) converter station, and furthermore, a coordinated control strategy for VSC-HVdc connected offshore windfarms (OWFs) to achieve enhanced ability of serving synthetic inertia. Given that the increasing penetration of renewable energy sources may lead to a significant reduction of system inertia and will thus result in a greater risk of instability, an extended control loop to emulate both the inertia and the droop characteristics of a synchronous generator is first added, with which the VSC-HVdc converter station can provide active power support by using the stored energy in the dc-link capacitors to increase the inertia of power system. To provide larger synthetic inertia with reasonable volume of dc-link capacitors, a coordinated control strategy for VSC-HVdc connected OWFs is further proposed to utilize the OWFs to supply additional energy needed by the GEC strategy. The validity of the proposed control methods is verified exemplarily by simulation results of a three-terminal VSC-HVdc system with two connected OWFs.

A novel supplementary frequency-based dual damping control for VSC-HVDC system under weak AC grid

The Voltage-Source-Converter based High Voltage Direct Current (VSC-HVDC) system which is connected to weak AC grid, under the widely used Vector Current Control (VCC), may experience oscillations and even dynamic instability due to the Phase-Locked-Loop (PLL) dynamics. This paper proposes a novel supplementary control, i.e., supplementary frequency-based dual damping control (SFDDC), to enhance the stability margin for VSC-HVDC system under weak AC grid conditions. Based on the tracked frequency from PLL, the SFDDC approach introduces the power-based and AC voltage-based damping components to mitigate the instability induced by the larger PLL gains. A small-signal model of VSC-HVDC system is developed, and validated by the detailed electromagnetic transient (EMT) simulation. Then, the SFDDC dynamics are evaluated and the feasible region of the introduced damping coefficients are calculated. Finally, this paper investigates the maximum available power (MAP) and evaluates the critical short-circuit ratio (CSCR) for VSC-HVDC station, under the proposed control approach. The results show that, by proper selection of the dual damping coefficients within recommended range, the proposed SFDDC approach can effectively mitigate the instability issue induced by PLL, improve the maximum power transfer capability and enable the VSC-HVDC station operate in extremely weak AC grid conditions, even at SCR = 1.0. In addition, without control strategy switching, the over-current can be limited to an acceptable value by the proposed controller during the solid three-phase to ground fault scenario.