Multi-terminal Voltage Source Converter Research Articles

Adaptive Hierarchical Fractional-Order Sliding Mode Control Strategy for Multi-terminal VSC-HVDC System with Wind Farm

In this paper, an adaptive hierarchical fractional-order sliding mode controller (AHFSMC) for a multi-terminal Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) Integrated with a wind farm (WF) system is designed. Multi-terminal VSC-HVDC (MtVDC) connected to wind farms has received several attentions in the power sector because of its numerous benefits. The effectiveness of MtVDC, on the other hand, is dependent on the control scheme used. To achieve this goal, a hierarchical sliding mode control method by fractional-order calculus is used. The controller parameters are adjusted according to a suitable adaptation method to enhance the proposed controller's robustness compared to the system uncertainties coefficient. An appropriate Lyapunov-based approach is used to achieve the adaptation rule. This paper discusses a scheme to design additional controllers in MtVDC systems to damp electromechanical oscillations, one of several features of HVDC presently under active study. Numerical simulations validate the proposed control strategy's feasibility and efficiency. This novel approach is employed for the upgrading of system stability with the dynamic properties of the MtVDC in a variety of operative conditions.

Power Flow Solution on Multi-Terminal HVDC Systems: Supergrid Case

High Voltage Direct Current (HVDC) systems offer distinct advantages for the integration of offshore wind farms to inland grid system. HVDC transmission system based on Voltage Source Converter (VSC) enables multi-terminal use HVDC for the integration of large-scale wind power in the North Sea. That network requires a special formulation for power flow analysis as opposed to the conventional method employed on AC networks. This paper presents a sequential AC/DC power flow algorithm, which is proposed for the analysis of multi-terminal VSC HVDC (VSC-MTDC) systems. This sequential power flow method can be implemented easily in an existing AC power flow package and is very flexible when compared with unified methods. Gauss-Siedel is used to solve DC power balance equations, as it offers two keys advantages: very fast and simple computational implementation, and errors do not accumulate during the calculation. The algorithm is tested using the WSCC 3-machine, 9-bus system with a 3-terminal MTDC network and the results are compared with those obtained from DIgSILENT PowerFactoryTM demonstrating the validity of the proposed algorithm. As an aggregate value, a representative test case of the projected scheme for the phase I of the Supergrid project on the North Sea is presented. The proposed approach presented in this paper is used to calculate DC power flows for some scenarios.

Evaluation of a Passive-Based Controller for Power Monitoring in Networks of High Voltage Transmission Lines in Direct Current with Voltage Source Converters (VSC–HVDC)

In this paper a tracking Passivity--based Control (PBC) for a multiterminal Voltage Source Converter based High Voltage Direct Current Network (VSC--HVDC) is evaluated. First, the system (an actual multiterminal HVDC) described as a Port Hamiltonian (PH) System and the controller are presented and subsequently three scenarios are proposed to evaluate the closed--loop behavior, namely: (i) load changes; (ii) generation changes and, (iii) DC--bus changes. Numerical simulations are carried out and results shown the robustness of the closed--loop to cope with these common disturbances.

Robust Traveling Wave-Based Protection Scheme for Multiterminal DC Grids

The DC transmission line protection technology is crucial for the development of multi-terminal Voltage Source Converter (VSC)-based HVDC systems. This article proposes a robust non-unit traveling wave protection (TWP), which deals with the DC fault area identification and fault type discrimination for high impedance fault conditions. The authors applied the traveling wave (TW) reflection and refraction method for the line-mode network. The distinctive features of high-frequency components contained in the line-mode and pole-mode voltage TWs at different relay units are used for the algorithm modeling. Discrete Wavelet Transform (DWT) is selected as the time-frequency analysis tool. The performed simulations are conducted for a four-terminal VSC-HVDC system, and validate the protection feasibility and robustness. More precisely, the proposed protection scheme identifies the internal and external DC faults within 2 ms, and provides correct operation during high-impedance faults (HIF) with a 25 dB level noise interference. This protection scheme makes use of a VSC-assisted resonant current (VARC) direct current circuit breaker (DCCB), that successfully interrupts the fault currents in less than 10 ms after the fault inception. The authors also comprehensively compared the proposed scheme with the existing methods. The obtained results show that the proposed protection scheme is superior in terms of sensitivity and selectivity performance.

Computationally Effective Frequency Transient-Based Transmission Line Protection for Multiterminal VSC-HVdc

For the transmission line fault in multiterminal voltage source converter (VSC) based high-voltage direct current (HVdc), the frequency spectrum of internal fault is obviously higher compared with the external fault. In frequency transient-based protection, the fault is generally identified by the high-frequency component via the professional mathematical algorithm, such as Fourier and wavelet transforms. However, the burdensome computation would slow down the protection operation speed, and the details in the specific frequencies are far beyond the need for fault identification. In this article, a computationally effective protection is proposed without utilizing the complicated mathematical algorithm. Comprehensive index given from the supplemental inductor is developed, which shows the greater value for higher frequency spectrum, and requires fewer algebraic operations. The proposed protection based on the comprehensive index is with superior computational efficiency and maintains the advantage of frequency transient-based protection in fault identification accuracy. The protection performance is validated in four/three-terminal VSC-HVdc.

Potential Contribution of the Grey Wolf Optimization Algorithm in Reducing Active Power Losses in Electrical Power Systems

Active power losses have the potential to affect the distribution of power flows along transmission lines as well as the mix of energy used throughout power networks. Grey wolf optimization algorithms (GWOs) are used in electrical power systems to reduce active power losses. GWOs are straightforward algorithms to implement because of their simple structure, low storage and computing needs, and quicker convergence from the constant decrease in search space. The electrical power system may be separated into three primary components: generation, transmission, and distribution. Each component of the power system is critical in the process of distributing electricity from where it is produced to where it is used by customers. By using the GWO, it is possible to regulate the active power delivered by a high-voltage direct current network based on a multi-terminal voltage-source converter. This review focuses on the role of GWO in reducing the amount of active power lost in power systems by considering the three major components of electrical power systems. Additionally, this work discusses the significance of GWO in minimizing active power losses in all components of the electrical power system. Results show that GWO plays a key role in reducing active power losses and consequently reducing the impact of power losses on the performance of electrical components by different percentages. Depending on how the power system is set up, the amount of reduction can be anywhere from 12% to 65.5%.

Transient analysis in multi-terminal voltage source converter based direct current system: Median mode decomposition

Due to the capacitive discharging effect after the fault inception, the dc line current increases dramatically in the dc power system which is interfaced with the ac system through the voltage source converters (VSCs). Therefore, the fast response of the fault detection methods is important to interrupt the fault line with short delay. In recent years, dc fault detection methods based on the frequency-domain analysis have become popular, while their calculation burden is relatively high. In this study, the median mode decomposition (MMD) is introduced, which improves the traditional empirical mode decomposition (EMD). Accordingly, a new transient analysis method is developed with only computation in the time-domain. In contrast to the conventional EMD, the new MMD can peel off the intrinsic median mode function (IMMF) with less quantities of iterations. These characteristics are of significance in reducing the computation burden and accelerating the response speed of the detection algorithm in the actual use. Two multi-terminal dc (MTDC) systems are developed in PSCAD for the simulation verification, including a ring bus system and a radial bus system. In addition, a scale-down MTDC system is established for the experimental verification, which reveals sensitive response, high robustness against noise and 100 % reliability. From the comparison with four popular methods, the comprehensive performance of the proposed method has been underlined.

An Overview of Soft Open Points in Electricity Distribution Networks

Soft open points (SOPs) are power electronic devices that are usually placed at normally open points of electricity distribution networks to provide flexible power control to the networks. This paper gives a comprehensive overview of both academic research and industrial practice on SOPs in electricity distribution networks. The topologies of SOPs as multi-functional power electronic devices are identified and compared, which include back-to-back voltage source converters, multi-terminal voltage source converters, unified power flow controllers, and direct AC-to-AC modular multilevel converters. The academic research is reviewed in three aspects, i.e., benefit quantification, control, and optimal siting and sizing of SOPs. The benefit quantification indices are categorized into feeder load balancing, voltage profile improvement, power losses reduction, three-phase balancing and DG hosting capacity enhancement. The control of SOPs is summarized as a three-level control structure, where the system-level and converter-level control are further discussed. For optimal siting and sizing of SOPs, problem formulation and solution methods are analyzed. Besides the academic research, practical industrial projects of SOPs worldwide are also summarized. Finally, opportunities of research and industrial application of SOPs are discussed.

Fault protection of multiterminal HVDC networks: Impact of inductance

Multiterminal Voltage Source Converter (VSC) High Voltage Direct Current (HVDC) networks are developing rapidly worldwide, especially in China with urgent needs of long-distance power transmission. Currently, the interrupting capacity of available DC circuit breakers (DCCBs) is a key obstacle to the development of multiterminal VSC HVDC networks. Superconducting fault current limiters (SFCLs) can compensate for the limitation of DCCBs. Conventionally, a resistive SFCL is regarded as essential and sufficient, while an inductive one is less related and optional. However, pure resistance cannot effectively suppress the rising rate of fault current at the very early stage of a fault, resulting in unnecessary blocking of the converter. Adequate inductance is required for such SFCLs to curb rapidly increasing fault currents within the first few milliseconds, which is critical to DCCBs to isolate fault without interrupting other DC transmission lines. In this paper, we compared and analyzed the performances of SFCLs with different resistances and inductances in a four-terminal VSC HVDC network using PSCAD/EMTDC software. The results indicate that even resistance and inductance show different current limiting effects, but they are both necessary to realize effective SFCL function for multiterminal VSC HVDC networks. The outcomes of this paper are of significance in FCL design for multiterminal VSC HVDC networks.

Deep Learning Based Hurricane Resilient Coplanning of Transmission Lines, Battery Energy Storages, and Wind Farms

In this article, a multistage model for expansion coplanning of transmission lines, battery energy storages, and wind farms (WFs) is presented considering resilience against extreme weather events. In addition to high-voltage alternating current lines, multiterminal voltage source converter based high-voltage direct current lines are planned to reduce the impact of high-risk events. To evaluate the system resilience against hurricanes, probable hurricane speed scenarios are generated using Monte Carlo simulation. The fragility curve concept is utilized for calculating the failure probability of lines due to extreme hurricanes. Based on each hurricane damage, the probable scenarios are incorporated in the proposed model. Renewable portfolio standard policy is modeled to integrate high penetration of WFs. To deal with the wind power and load demand uncertainties, a chronological time-period clustering algorithm is introduced for extracting representative hours in each planning stage. A deep learning approach based on bidirectional long short-term memory networks is presented to forecast the yearly peak loads. The mixed-integer linear programming formulation of the proposed model is solved using a Benders decomposition algorithm. A modified IEEE RTS test system is used to evaluate the proposed model effectiveness.

Optimal sparse control of interval networks: Voltage source converter based DC grid study

Decentralized optimal control of interval state space systems involves significant complications due to the required sparsity, robustness, and convergence rate. By adding an l1 term to the standard linear quadratic optimization, the obtained composite index, accommodating the sparsity, contains both differentiable and non-differentiable terms. Knowing that the choice of optimization techniques are problem-dependent, in this paper, several smart provisions such as “Active set”, “Coordinate descent”, and separately managing the differentiable part of the cost function are put together to form a relatively fast algorithm. In addition, a rule is introduced to adjust automatically the degree of sparsity and a constraint to confine the interval closed-loop system eigenvalues to the desired s plane region. As a result, a robust sparse decentralized control (RSDC) approach for interval networks is established. The RSDC method is utilized for the control of a multi-terminal VSC (Voltage Source Converter ) -based DC (Direct Current) grid with parametric uncertainty under various external random perturbations. The test results show that the proposed method with just 20 feedback links (out of available 100) performs superiorly to the alternative ones in terms of sparsity, convergence rate, and time-domain performances, verified through extensive simulations.

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.

A Novel Converter-Breaker Integrated Voltage Source Converter Based on High-Surge IGCT and Fault Self-Clearing Strategy for DC Grid

Existing multiterminal voltage source converter based high voltage dc projects are constructed by separate modular multilevel converter (MMC) and dc breaker (DCB) utilizing insulated gate bipolar transistors. However, the MMC requires high-surge thyristors and the DCB should be equipped with very large turn- off solid-state switch against fault. Their independent operations are complex and unreliable by two separate control systems. In this letter, a novel converter-breaker integrated voltage source converter (IVSC) based on high-surge integrated gate commutated thyristor is proposed, and to integrate the converter and breaker operation, the fault self-clearing strategy is developed. IVSC can get rid of thyristors and replace DCB with small turn- off capability load switch. This converter-breaker integration can result in advantages including lower manufacturing cost and volume.

Multiterminal Medium Voltage DC Distribution Network Hierarchical Control

In this research study, a multiterminal voltage source converter (VSC) medium voltage DC (MVDC) distribution network hierarchical control scheme is proposed for renewable energy (RE) integration in a co-simulation environment of MATLAB and PSCAD/EMTDC. A DC optimal power flow (DC OPF) secondary controller is created in MATLAB. In PSCAD/EMTDC, the main circuit containing the adaptive DC voltage droop with a dead band and virtual synchronous generator (VSG) based primary controller for the VSCs is implemented. The simulation of the MVDC network under the proposed hierarchical control scheme is investigated considering variations in wind and solar photovoltaic (PV) power. The network is also connected to the standard IEEE-39 bus system and the hierarchical scheme tested by assessing the effect of tripping as well as restoration of the REs. The results show that during random variations in active power such as increasing wind and PV power generation, a sudden reduction or tripping of wind and PV power, the primary controller ensures accurate active power sharing amongst the droop-based VSCs as well as regulates DC voltage deviations within the set range of 0.98–1.02 pu with an enhanced dynamic response. The DC OPF secondary control optimizes the system’s losses by 38% regularly giving optimal droop settings to the primary controllers to ensure proper active power balance and DC voltage stability. This study demonstrates that the hierarchical control strategy is effective for RE integration in the MVDC distribution network.

Virtual Flux Predictive Direct Power Control of Five-level T-type Multi-terminal VSC-HVDC System

This paper proposes a Virtual Flux Predictive Direct Power Control (PDPC) for a five-level T-type multi-terminal Voltage Source Converter High Voltage Direct Current (VSC-HVDC) transmission system. The proposed PDPC scheme is based on the computation of the average voltage vector using a virtual flux predictive control algorithm, which allows the cancellation of active and reactive power tracking errors at each sampling period. The active and reactive power can be estimated based on the virtual flux vector that makes AC line voltage sensors not necessary. A constant converter switching frequency is achieved by employing a multilevel space vector modulation, which ensures the balance of the DC capacitor voltages of the five-level t-type converters as well. Simulation results validate the efficiency of the proposed control law, and they are compared with those given by a traditional direct power control. These results exhibit excellent transient responses during range of operating conditions.

A Comprehensive Power Flow Approach for Multi-terminal VSC-HVDC System Considering Cross-regional Primary Frequency Responses

For the planning, operation and control of multiterminal voltage source converter (VSC) based high-voltage direct current (HVDC) (VSC-MTDC) systems, an accurate power flow formulation is a key starting point. Conventional power flow formulations assume the constant frequencies for all asynchronous AC systems. Therefore, a new feature about the complex coupling relations between AC frequencies, DC voltages and the exchanged power via VSC stations cannot be characterized if VSC-MTDC systems are required to provide cross-regional frequency responses. To address this issue, this paper proposes a comprehensive frequency-dependent power flow formulation. The proposed approach takes the frequencies of asynchronous AC systems as explicit variables, and investigates the novel bus models of the interlinking buses of VSC stations. The proposed approach accommodates different operation modes and frequency droop strategies of VSC stations, and considers the power losses of VSC stations. The effectiveness and generality of the developed approach are validated by a 6-terminal VSC-HVDC test system. The test system presents the characteristics of the coexistence of numerous VSC operation modes, the absence of slack buses in both AC and DC subsystems, and diversified grid configurations such as point-to-point integration of renewable energy sources and one AC system integrated with multiple VSC stations.

Non‐pilot protection scheme for multi‐terminal VSC–HVDC transmission systems

High-voltage direct current (HVDC) transmission systems are nowadays ideal candidates for interconnecting renewable energy resources to the electrical grids. This study presents a communication-less protection scheme for HVDC transmission systems with a multi-terminal voltage source converter (VSC). The existence of a tie-busbar is also considered, which represents a challenge for protection selectivity functions without the aid of a communication channel. The proposed scheme identifies the faulted segment on either a DC busbar or a line segment based on monitoring the calculated rate of change of the current-to-voltage ratio. This criterion is sensitive to detect the faults as the current increases rapidly in a few milliseconds, while the voltage decreases in the same time frame remarkably. Once the fault detection criterion is satisfied, the faulted section can be identified by checking the summation and direction of all branch currents at each busbar individually without utilising communication links. For investigation purposes, the proposed scheme is tested via a four-terminal HVDC system with two tie-busbars. Different simulation cases are evaluated with a variety of fault conditions using MATLAB/SIMULINK. The results corroborate the reliability, security, and efficacy of the proposed protection scheme for multi-terminal HVDC transmission systems.

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.

Design and Transient Studies on Multi-Terminal VSC-HVDC Systems Interconnecting Offshore Wind Farms

In recent years, offshore wind energy has increased significantly. The continuous increase in the offshore wind power generation level brings the requirement of the offshore wind farms (OWFs) integration with an AC grid. The multi-terminal (MT) voltage source converters (VSC)-based high voltage direct current (HVDC) transmission system is an emerging technology and also the best option to interconnect the large-scale OWFs to the AC grid. This paper presents the design, modeling, and control of MT VSC-HVDC transmission system linked offshore wind farms. Different cases of MT VSC-HVDC transmission systems are developed, and its simulation studies are carried out using PSCAD/EMTDC. The test results show the transient performance of the MT VSC-HVDC transmission systems under various AC and DC fault conditions. The studies also include the influence of wind variabilities as in the form of gust and ramp pattern during steady state and fault conditions.

Research and application of a power-flow-calculation method in multiterminal VSC-HVDC power grid

For demonstrating a multiterminal voltage-source converter (VSC)-based high-voltage DC (HVDC) (VSC- HVDC) project, this study puts forward a technical route for calculating the power flow in a 500-kV VSC-HVDC power grid in comparison with that of an AC power grid. The Jacobian matrix used in the power-flow calculation was deduced through methods such as Newton-Laphson iteration and Taylor series expansion. Further, the operation effect of power- flow calculation on a true bipolar VSC-HVDC power grid was analyzed briefly. The elements of the Jacobian matrix corresponding to VSC were studied under the mode of droop control and the control strategy of VSC-HVDC power grid was analyzed in detail. The power-flow calculation model for VSC-HVDC power grid of the master-slave control mode was simplified using the PQ decomposition method of the power-flow calculation of an AC power grid. Moreover, a four-terminal model of the Zhangbei VSC-HVDC demonstration project was established and tested on MATLAB. The simulation results under two kinds of operating conditions were analyzed and compared to the results of BPA; the deviation between the power-flow results was studied. The results show that the proposed calculation method can provide a feasible support for calculating the power flow in VSC-HVDC grids.