Voltage Source Converter HVDC Research Articles

Power Compensation Control for Interconnection of Weak Power Systems by VSC-HVDC

Using voltage-source converter high-voltage direct current (VSC-HVDC) to interconnect weak ac power systems has drawn much attention. Under the traditional vector control for the VSC that the response of AC voltage loop cannot match that of the active power loop may limit power transmission capability of the VSCs that interconnect with weak power systems. To enhance the power transmission capability and stability, this paper proposes a power compensation control that is based on the analysis of characteristics of power–voltage (PV) curves. This method can improve the damping control performance of the active power and the ac voltage by adjusting the compensated values adaptively. A small-signal model of the integrated system is established to study the performance of the proposed control method. The impact of the compensation coefficients and steady-state operating points on the system stability is studied. Motivated by an on-going back-to-back project in China, corresponding electromagnetic transient simulation models are constructed in MATLAB/Simulink environment to verify the effectiveness of the proposed method on the performance of operating limits, damping performance, disturbance withstanding capability, and fault ride-through capability.

Protection scheme for VSC‐HVDC transmission lines based on transverse differential current

This study proposes a novel transverse differential protection for voltage sourced converter-high-voltage, direct current (VSC-HVDC) to clear faults on DC-line fast. The proposed scheme depends on the transient current of positive pole and negative pole at the same side. Transverse differential current can be used to distinguish DC and AC faults, novel criteria are built using the ratio of difference and sum current of bipolar. According to the variation characteristics of transient fault current, internal faults and external faults can be detected. The scheme can protect the whole transmission line without dead zone, simplify the algorithm and improve the speed of operation. A typical VSC-HVDC model in PSCAD/EMTDC is built. The tolerance to grounding resistance of the scheme is researched in simulation. The results demonstrate the feasibility and reliability of the proposed scheme. The scheme can be used as supplementary criteria for primary protection or backup protection for VSC-HVDC transmission lines.

An intelligent algorithm for fault location on VSC-HVDC system

This paper proposes a novel intelligent algorithm for fault location on the voltage sourced converter-high voltage direct current (VSC-HVDC) system. This method with single-ended measurement makes full use of frequency, time and energy to capture the fault features via Hilbert-Huang Transform (HHT). The time delay, characteristic frequency, energy attenuation and high-frequency energy are used as the input of ε-support vector regression (SVR) to get fault distance. Then bat algorithm (BA) optimizes the parameters of the model with cross validation comparing with other optimization algorithm. Furthermore, high-frequency variance contribution rate (HVCR) is adopted to identify the fault area. The VSC-HVDC simulation system is constructed to verify the reliability and accuracy of the method and the expected accuracy of the proposed method is ±500m. The results demonstrated that the proposed method still has reliability and accuracy for hybrid transmission line with a small amount of data.

DC-Current-Limiting Characteristics of YBCO Tapes for DC Currents of 50 A to 10 kA

The amplitude and rise time of fault currents can be reduced by installing a resistive-type superconducting fault current limiter (SFCL) in a voltage-source converter high-voltage DC (VSC-HVDC) system. Such an SFCL can provide sufficient time for a mechanical-type dc circuit breaker to interrupt a fault current. The objective of this paper is to elucidate the dc-current-limiting characteristics of Yttrium Barium Copper Oxygen (YBCO)-coated conductor tapes for a wide range of prospective currents (ranging from 50 to 10 kA). The factors investigated include critical currents (I c ) of the tape of 102 and 225 A, circuit time constants of 1 and 3 ms, and rated dc voltages of 250 and 500 V. The experimental results show that the limited current remains nearly constant with prospective current when the prospective current is higher than ten times I c . The circuit time constant affects only the peak value and rise rate of the limited current. A single tape (I c = 102 A) can limit a 10-kA prospective current to a peak value of 555 A. The limited current for a rated voltage of 500 V is always higher than that for a rated voltage of 250 V.

An Enhanced DC Voltage Droop-Control for the VSC--HVDC Grid

This paper introduces an enhanced droop-based dc-voltage control method, including dead-band, for applications to the high-voltage direct-current (HVDC) grid that utilizes the voltage-sourced converter (VSC) technology. The proposed droop-control structure also autonomously imposes energy balance between the HVDC grid and its host ac system. The droop-control method (1) divides the VSC stations into four groups, (2) activates the droop-control of each group based on a prespecified voltage margin, and (3) introduces an improved power-voltage characteristic for desirable VSC station dynamic performance. Feasibility and performance of the proposed control method are evaluated based on time-domain simulation studies in the PSCAD platform, using the IEEE-39-Bus system that imbeds a five-terminal VSC–HVDC grid. Each VSC station is a monopolar modular multilevel converter (MMC). The study results show that the proposed droop-control method enables the HVDC-AC system to reach a new steady state after transient events.

Characterisation of interaction in an offshore AC grid with large WPPs and VSC‐HVDCs

To exploit the full and flexible capability of offshore voltage-sourced converter high-voltage DC (VSC-HVDC) lines, interaction studies in the offshore AC grid are required but are not well understood or reported. This study examines the interaction dynamics of an offshore AC grid for interconnecting large wind power plants (WPPs). The conventional eigenvalue analysis method has limitations which make the interaction analysis of such systems difficult. Hence, in this study, an impedance-based analytical approach is employed to investigate the interaction phenomena. The impedance model of a VSC-HVDC converter for both direct and vector control with outer and droop controls are derived along with the impedance model of the full-converter wind generator. The interaction dynamics of the offshore grid is predicted through the well-established Nyquist criteria and is validated using time-domain simulations. The analysis shows that the system stability is decidedly influenced by the control configurations and tuning of the VSC-HVDC lines.

Wind power plant integration in voltage source converter HVdc grids with voltage droop control

This paper introduces the use of a diode-based HVdc rectifier for the integration of a large off-shore wind power plant into an existing HVdc voltage source converter-based grid. The proposed control system allows for the wind power plant to energize the complete HVdc grid, in order to contribute to service restoration in the case of a black-out. The system exhibits good response to wind power plant black-start operation, rectifier breaker trip and reclosure, and short-circuit ride through at the on-shore ac-grid. The technical feasibility of the proposed topology has been validated by means of detailed PSCAD simulations using frequency dependent parameters for cable modeling.

Novel Modeling of a Fast DC Breaker for a VSC-HVDC Transmission System Protection

The inability to quickly isolate the faulty sections of the direct current (DC) network damages the converters and the transmission system. Further delay in fault interruption can be escalated to the generation system thus shutting down the whole power system. Currently, Voltage Source Converter-High Voltage Direct Current (VSC-HVDC) systems provide the best mode of bulky power transmission over long distance. Though VSC-HVDC have their own internal protection, they still suffer from short-circuit faults especially on the DC side hence requiring a novel fast DC breaker. The novel design of the DC breaker in this paper utilizes a systematically calculated mutual inductance to divert the energy generated during fault to resistive elementson both sides of the DC breaker thus, preventing the transient voltage and backward current generated at the time of clearing the fault. The major challenge for DC breaker is that current is not alternating inferring that it has no natural zero current occurring point. Thus, the current has to be artificially forced to cross a current zero point so that the fault can be easily interrupted.This study aims at reviewing the currently available DC breakers with a fault clearing time of 5msby modelinga novel fast DC breaker therefore, providing the best possible protection against short-circuit faults on VSC systems. The results indicate that the proposed DC breaker model has a fault clearing time of 1ms. This is a major contribution towards the development a fast DC breaker.

风光火混合多馈入直流并网与传统交流并网对比分析

大规模新能源的开发与利用形成了许多区域性的能源基地，能源基地之间可由多种方式汇集到汇流站进行并网，然后通过电流源型直流输电(LCC)外送通道送电到受端电网。本文论述了由电压源换流器高压直流(Voltage Source Converter HVDC, VSC-HVDC)系统与LCC-HVDC系统组成的用于提高新能源消纳的混合多馈入直流(hybridmulti-infeed HVDC，HMIDC)并网的可行性，构建了该并网方式和传统交流并网方式的拓扑结构，通过DigSILENT软件，搭建了仿真模型，基于风电场故障扰动法对混合多馈入和传统多馈入暂态特性进行了分析。由仿真结果得到以下结论：风电场发生三相短路故障时，混合并网较传统交流并网，电压电流受到较小影响，恢复速率快，功率波动小，提高了暂态电能质量。 Large-scale development and utilization of new energy form a number of regional energy base; the energy base can be collected by a variety of ways to the convergence station and network, and then through LCC transmission channel to the Power grid. This paper discusses the feasibility to improve the consumption of new energy by the hybrid multi-infeed HVDC (HMIDC), which is composed of Voltage Source Converter HVDC (VSC-HVDC) system and LCC-HVDC system. The topology of the H-MIDC and the traditional AC grid-connection is constructed. Then, based on the wind farm fault disturbance method, the special analysis simulation model of the Hybrid MIDC and traditional AC grid-connection is built by DigSILENT software. The simulation results show that when the three-phase short-circuit fault of the wind farm occur, the H-MIDC is better than Ac Grid-connec- tion, the voltage and current are affected less, the recovery rate is fast, the power fluctuation is small, and the transient power quality is improved.

Three-terminal Hybrid HVDC Transmissions Control Strategies for Bundled Wind-thermal Power Plants

This paper evaluates application feasibility of a Hybrid Multi-terminal HVDC system and wind-thermal-bundled plants simulated in DIgSLIENT PowerFactory environment. The proposed hybrid MTDC system consists of two line-communicated converters (LCC), which are connected to both wind farms and thermal power plants, and one voltage source converter (VSC) at the grid side. Control strategies for each converter are designed to handle this system under different disturbance conditions. Simulation results show that the wind power fluctuation can be compensated by the thermal-generated power. Results demonstrate the effectiveness of the proposed control strategies of the hybrid MTDC system compared to a conventional MTDC system. The proposed scheme combines advantages of both LCC and VSC HVDC systems and provides a new way to transmit wind power over long distances to the main grid.

Advanced Fault Ride-Through Management Scheme for VSC-HVDC Connecting Offshore Wind Farms

This paper proposes a novel configuration and transient management scheme for Voltage Source Converter-High Voltage Direct Current (VSC-HVDC) connecting Large Scale Wind Farm (LSWF). The proposed configuration aims to fully utilize the HVDC converters with shunt and series reconfiguration during steady state and fault conditions respectively. Thus, it targets the employment of the available converters controls capabilities to enhance the Fault Ride-Through (FRT) performance of the VSC-HVDC and achieve smooth power evacuation to the electric grid. The control strategy is built in such a way to guarantee three factors: 1) enhancing the voltage profile of the electrical grid during symmetrical and asymmetrical grid faults, 2) providing smooth power evacuation from the LSWF, and 3) mitigating the DC link oscillations caused by unbalanced faults. Hence, the proposed scheme demonstrates superior performance of the VSC-HVDC system during steady state and transient conditions. Comprehensive simulation studies are carried out to demonstrate the gorgeous performance of the proposed VSC-HVDC configuration using PSCAD/EMTDC. In addition to that, a lab-scaled experimental set up is constructed to validate the functioning of the proposed VSC-HVDC configuration.

Control Strategies of Offshore Wind Farms Connected Voltage Source Converter HVDC under Grid Fault

Control Strategies of Offshore Wind Farms Connected Voltage Source Converter HVDC under Grid Fault

Modeling and Analysis of VSC-Based HVDC Systems for DC Network Stability Studies

A modeling approach is presented in this paper in order to investigate instabilities originating from dc-network resonances in voltage-source converter HVDC (VSC-HVDC) systems. Through frequency-domain modeling, it is shown that the VSC-HVDC system can be represented as a feedback interconnection of two subsystems. These subsystems are defined in this paper as a dc network impedance connected to a VSC dc-side power-dependent admittance. The conditions in which this admittance has a negative conductance are investigated since they can amplify dc network resonances. It is found that when the VSC that controls the direct voltage injects power into the dc network, the mentioned conductance becomes negative above certain frequencies. In addition, the impact of weak ac grids, control system delays, and other direct voltage controllers on the studied conductance is investigated. The theoretical findings are verified through time-domain simulations.

The Effect of FRT Behavior of VSC-HVDC-Connected Offshore Wind Power Plants on AC/DC System Dynamics

Future power systems will contain more converter-based generation, among which are the voltage-source converter–high-voltage direct-current (VSC-HVDC)-connected offshore wind power plants (WPP). Their interaction with the onshore system influences power system dynamics in the transient stability timeframe. The respective protection and control methods which cause this interaction must be taken into account in grid-integration studies performed today. This paper gives insight into the effect of typically required fault ridethrough (FRT) and post-FRT measures of VSC-HVDC-connected offshore WPPs on the combined ac and HVDC system dynamics. Several important sensitivities are addressed, among which are: 1) FRT implementation, 2) the postfault active power-recovery rates, 3) the ac network dynamic characteristics, and 4) the HVDC topology. The analysis is first performed as a proof of concept on a small benchmark system, and subsequently generalized to a realistic dynamic model of the future Northwestern European power system. The results of this paper can be used as a reference for understanding the effects of large-scale VSC-HVDC-connected offshore WPPs on the stability of the onshore interconnected power systems.

Multi‐pole voltage source converter HVDC transmission systems

This study connects several modular multilevel converters to form multi-pole voltage source converter highvoltage dc (VSC-HVDC) links which are suited for bulk power evacuation, with increased resiliency to ac and dc network faults. The proposed arrangements resemble symmetrical and asymmetrical HVDC links that can be used for bulk power transfer over long distances with reduced transmission losses, and for the creation of multi-terminal supergrids currently being promoted for transitional dc grids in Europe. The technical feasibility of the proposed systems is assessed using simulations on symmetrical and asymmetrical tri-pole VSC-HVDC links, including the case of permanent pole-to-ground dc faults.

Power System Stability Analysis Using Feedback Control System Modeling Including HVDC Transmission Links

A general platform is introduced to study the dynamics of power systems with high voltage dc (HVDC) transmission links. Small-signal stability, voltage stability, and interaction phenomena of power systems with both line-commutated-converter HVDC (LCC-HVDC) and voltage-source-converter HVDC (VSC-HVDC) are addressed using the proposed platform. In this platform, the entire power system is modeled as a multivariable feedback control system (FCS) which consists of three interconnected blocks. The contents as well as the inputs and outputs of the blocks are selected such that the conventional analysis tools for power system stability are applicable, both in the time and frequency domains. In the FCS model, the relationships between different instabilities are clear, and participant agents of each instability can be determined. The model is developed in a modular and hybrid style, to make it feasible for a large power system. The proposed model is validated against an electromagnetic transient simulation program (PSCAD) using time responses.

Adaptive Sensorless Control of PMSGs-Based Offshore Wind Farm and VSC-HVdc Stations

This paper proposes sensorless control strategies for an offshore wind farm (OWF) and voltage source converter high voltage direct current (VSC-HVdc) stations. The extended Kalman filter provides an estimation of the speeds and rotor positions of permanent magnet synchronous generators (PMSGs) of OWF, the dc voltage at dc-bus and the dc voltages of the both VSC-HVdc stations. The OWF, composed of 150 variable speed wind turbines based on PMSGs, is considered in this paper, where each generator is connected in series by its own ac-dc converter to dc-bus. Only one dc-ac inverter converts the dc-bus voltage to a proper ac voltage of wind farm ac-bus. The HVdc transmission system based on a three-level neutral point clamped VSC is used for the interconnection between the OWF and the onshore grid via two dc cables. Detailed modeling and control strategies are developed for each individual component of the system. Simulation results show the effectiveness of the proposed method to manage the desired HVdc system operation.

Wind farms-fed HVDC system power profile enhancement using solid state transformer based flywheel energy storage system

As the power of wind farms (WFs) considerably proliferates in many areas worldwide, energy storage systems will be required to dynamically compensate the wind energy intermittency and increase power system stability. In this paper, a backup power conditioning strategy for wind energy-fed voltage source converter HVDC transmission systems is presented. An induction machine (IM) based flywheel energy storage systems (FESS) is integrated to the HVDC system via a solid state transformer (SST).The FESS is connected in parallel with the dc-link of the grid side converter; therefore during the healthy conditions, the excess wind energy can be stored in the flywheel and then restored during the energy-shortage periods. However, in faulty conditions, typically ac faults, the FESS will provide an alternating path for the wind energy instead of feeding the fault. Hence, excessive overvoltage across the DC-link capacitor due to trapped energy is avoided.. The proposed system including FESS with an interfacing SST is modeled, simulated, and analyzed in MATLAB/SIMULINK environment. The results verify the effectiveness of the proposed system.

Investigating the impact of unanticipated market and construction delays on the development of a meshed HVDC grid using dynamic transmission planning

This study presents a market-based dynamic transmission planning framework for the construction of a meshed offshore voltage source converter-high voltage direct current (VSC-HVDC) grid. Such a grid is foreseen for integrating offshore wind and electricity trade functions among the North Sea countries. The proposed model seeks to maximise the social welfare of all zones and to minimise the investment capital of transmission infrastructure subject to technical and economic constraints. It determines the optimal grid design, including grid topology and transmission capacities for each development stage. The transmission capacities are set in such a way that congestion revenues collected throughout the lifetime of the infrastructure project pay off the investment cost of building the grid. The model is used to investigate the impact of unanticipated delay constraints due to technical (e.g. unavailability of DC breakers), economic (e.g. supply chain shortages) and legal obstacles (e.g. heterogeneous permitting criteria). It is quantified how (i) longer delays result in larger social welfare losses; (ii) different countries will be affected differently by the delays and so have unequal incentives for solving them; (iii) the length of the delay affects the capacity of cross-border connections. Numerical results are interpreted in economic terms and allow appraisal of the effectiveness of the proposed approach.

DC Grid Control Through the Pilot Voltage Droop Concept—Methodology for Establishing Droop Constants

Converter control is an important aspect of voltage-source-converter high-voltage dc (VSC-HVDC) based multi-terminal dc grids. Variants of power drooped against dc voltage control have previously been proposed for this purpose. However, there is at this time no clear methodology available on how to select the size of the droop constants for distributing a power mismatch, following, e.g., a converter trip, on several converters. This paper proposes a methodology for establishing droop constants for a variant of power drooped against dc voltage called pilot voltage droop control. The methodology is derived from the basic control laws in a straight forward fashion and the accuracy is exemplified by time domain simulations in Digsilent's Powerfactory platform.