Voltage Source Converter Stations Research Articles

Decentralized and autonomous voltage balancing control approach for hybrid multi-terminal Ultra-HVDC system

The hybrid multi-terminal Ultra-HVDC (UHVDC) transmission system combines the advantages of line commutated converter (LCC) and voltage source converter (VSC). It can meet long-distance and large-capacity power transfer requirements from multiple power supplies to multiple power receiving system. To balance the voltages between series-connected high-end converter (HEC) and low-end converter (LEC) of the power-controlled VSC station in hybrid multi-terminal UHVDC system, this paper investigates the inherent mechanism of voltage imbalance issue in the power-controlled VSC station, and then proposes a decentralized and autonomous voltage balancing control approach, which can be easily implemented in the practical engineering. Then, a hybrid three-terminal UHVDC system is developed in PSCAD/EMTDC, based on the scheme of the under-constructed Wudongde hybrid three-terminal UHVDC project (±800 kV, 8000 MW), and the effectiveness of the proposed voltage balancing control approach is investigated under steady-state, power step-change and AC fault conditions. The results show that, with the proposed control approach, the voltage-balance between series-connected converters in the power-controlled VSC station can be realized under different operating conditions.

A communication-free reactive-power control strategy in VSC-HVDC multi-terminal systems to improve transient stability

This paper proposes a new reactive-power control strategy for High Voltage Direct Current multi-terminal systems with Voltage Source Converter stations (VSC-HVDC) to improve transient stability in electric power transmission systems. The proposed algorithm uses local measurements to estimate, in each converter station, a weighted average of the frequencies seen by the VSC stations of the HVDC multi-terminal system. This estimation is carried out making use of a (small) auxiliary local active-power modulation along with the DC-side voltage droop control in the VSC stations, where the latter also uses local measurements only. The estimated frequency is used as the set point for supplementary reactive-power control at each converter station. The proposed control law has been simulated in a test system using PSS/E and the results show that it improves transient stability, significantly, producing similar results to those obtained controlling reactive power using global measurements.

A Continuation Power Flow Model of Multi-Area AC/DC Interconnected Bulk Systems Incorporating Voltage Source Converter-Based Multi-Terminal DC Networks and Its Decoupling Algorithm

Existing continuation power flow (CPF) models mainly focus on the regional independent systems, which are not suitable for multi-area AC/DC interconnected systems because the market trading behaviors and security control for power allocation of tie-lines are ignored. This study presents a novel CPF model and its decoupling algorithm for multi-area AC/DC interconnected systems incorporating a voltage source converter (VSC)-based multi-terminal direct current (MTDC) network. This CPF model includes the following unique features: (1) In view of the bilateral power trading contracts among regional subsystems, the nonlinear constraint equations of directional trading active power via interface are derived, and the multi-balancing machine strategy is introduced to realize the active power balance of each subsystem. (2) An accurate simulation method for the security control behaviors of the power allocation in tie-lines is proposed, which includes a specific selection strategy for automatic generation control units and a generation re-dispatch strategy. These two strategies work together to prevent the serious overload in tie-lines during load growth and improve the voltage stability margin of the interconnected bulk systems. (3) The switching characteristic of reactive power control behaviors of VSC stations is simulated in the CPF calculation. In the end, a novel decoupling CPF algorithm based on bi-directional iteration is presented to realize the decomposition and coordination calculation. This decoupling algorithm preserves the precision and convergence of integrated CPF algorithms, and it has an apparent advantage on the calculation speed. Furthermore, this decoupling algorithm also can easily reflects the effects of the control mode changes of VSC stations to the voltage stability margin of AC system. Case studies and comparative analysis on the IEEE two-area RTS-96 system indicate the effectiveness and validity of the proposed CPF model and corresponding decoupling algorithm.

Extended control strategies of voltage source converter stations linked to converter dominated systems

The state of the art of control strategies of modular multi-level converter based high-voltage DC systems can be categorised into two classes according to the connecting system, i.e. the active control strategies such as the vector control and the passive control ones such as the voltage and frequency control. These two control modes need to be transformed to each other under some circumstances. The traditional method to deal with this problem is to switchover the control modes. However, this process takes time and there is risk to lose stability during this period. Thus, an extended control strategy to control the converter in both modes is wanted. An extended control strategy is designed by introducing supplementary feedback property into the traditional vector control strategy. The extended and traditional vector control strategies are compared under various cases by simulation. Simulations show that the system controlled by the traditional vector control strategy loses stability when synchronous machines drop out. However, the extended vector control strategy is superior under weak or even passive grid conditions.

Study of Resistive-Type Superconducting Fault Current Limiters for a Hybrid High Voltage Direct Current System.

In this paper, a hybrid high voltage direct current transmission system containing a line commutated converter and a voltage source converter is developed. To enhance the robustness of the hybrid transmission system against direct current short-circuit faults, resistive-type superconducting fault current limiters are applied, and the effectiveness of this approach is assessed. Related mathematical models are built, and the theoretical functions of the proposed approach are expounded. According to the transient simulations in MATLAB software, the results demonstrate that: (i) The superconducting fault current limiter at the voltage source converter station enables to very efficiently mitigate the fault transients, and owns an enhanced current-limiting ability for handling the short-line faults. (ii) The superconducting fault current limiter at the line commutated converter station is able to mildly limit the fault current and alleviate the voltage drop, and its working performance has a low sensitivity to the fault location. At the end of the study, a brief scheme design of the resistive-type superconducting fault current limiters is achieved. In conclusion, the application feasibility of the proposed approach is well confirmed.

A Decentralized Optimal Operation of AC/DC Hybrid Distribution Grids

In an ac/dc hybrid distribution grid (ac/dc HDG), different ac distribution grids are connected through a voltage source converter (VSC)-based dc grid to operate the entire system in a secure and economic manner. This paper presents a decentralized optimal operation approach for running ac/dc HDG which can either work individually or collaborate with each other. Using the analytical target cascading approach, a three-level hierarchy consisting of dc grids, VSC stations and ac grids, is formulated to decompose the operation problem of ac/dc HDG. The obtained subproblems for these grid levels, as well as the interactions between them, are modeled and solved with a nested solution process. The simulation results show the effectiveness of the proposed decentralized approach.

Interconnecting Very Weak AC Systems by Multiterminal VSC-HVDC Links With a Unified Virtual Synchronous Control

In this paper, a unified virtual synchronous control (UViSynC) is proposed for multiterminal voltage-source converter (VSC)-high-voltage dc (HVDC) (MTDC) systems, which can: 1) realize grid synchronization of the VSC stations by mimicking the rotor behavior of synchronous generators with the dynamics of dc grid and dc capacitor considered and 2) ensure the stable operation of the MTDC system that interconnects very weak ac grids. Unlike conventional master–slave control strategies that assign one master VSC to control the dc voltage, the UViSynC is capable of involving all VSC stations in dc voltage regulation. In this manner, each VSC station plays an equal role to control the dc voltage, which is a better way for dc voltage regulation compared with conventional master–slave control strategies. Grid synchronization mechanism for VSC station with UViSynC is particularly analyzed. Small-signal model of the MTDC system is developed to perform stability analysis and investigate the dynamic performance when interconnecting very weak ac systems. Simulation analyses based on MATLAB/Simulink and hardware-in-the-loop platform are conducted to verify the effectiveness of the UViSynC using a four-terminal VSC-HVDC test system.

Coordinated Control of DC Grid and Offshore Wind Farms to Improve Rotor-Angle Stability

This paper proposes a coordinated control strategy for voltage source converter (VSC) based dc grids and offshore wind farms connected by the dc grids. The strategy could enhance the stability of the onshore synchronous power system subjected to disturbances. Based on the mathematical model of dc grid connected power system and control Lyapunov function, the active and reactive control law for onshore VSC stations is presented. And to maintain the power balance of dc grid and reduce the rotor swing of wind turbines, the windfarms’ power modulation and distribution method is given. A layered control structure to implement the proposed strategy which combines the dc grids and windfarms control is also put forward. Simulations are run in a modified New England 39-bus system by DIgSILENT/Power Factory to validate the effectiveness of the proposed scheme.

Efficient method for the real‐time contingency analysis of meshed HVDC power grids fed by VSC stations

An efficient method for the real-time contingency analysis of meshed high-voltage direct current (HVDC) power grids fed by voltage source converter (VSC) stations is introduced here. A linearised AC/DC grid model is initially determined considering the control strategies of the various VSC units. This lays the foundations for the determination of linear sensitivity factors with which the contingency analysis is carried out to evaluate the real-time N−1 criterion in AC/DC grids, as demanded by system control centres. Distribution and power-injection factors are subsequently derived for efficiently assessing the impact of AC/DC transmission line outages and load/generator disconnections on the HVDC grid. Conversion factors are also derived to estimate the impact of the loss of VSC stations on the AC/DC network, this being another inherent contribution of this work. The efficiency and validity of this timely approach, which finds practical applicability to the real-time operation of HVDC power grids, is confirmed using a meshed DC network fed by three VSC stations. The disconnection of AC and DC transmission lines, generators, and VSC stations are dynamically simulated using Simulink and their post-disturbance steady-state conditions are compared against those computed by the introduced method where it is confirmed that both solutions concur very well with each other.

A new unified approach for the state estimation and bad data analysis of electric power transmission systems with multi-terminal VSC-based HVDC networks

The aim of this paper is the proposal of a static state estimation approach suitable for electric power transmission systems containing fully controlled multi-terminal high voltage direct current networks. An integrated electric circuit model of the voltage source converter station, including its on-load tap-changer (OLTC) transformer, is used to determine the state variables associated with the AC–DC interface. These controllable variables are added to the state vector, together with the nodal voltages associated with the AC and DC networks, as well as the duty cycle of the DC–DC converters, for a unified SE solution in a single frame of reference. The SE problem is formulated based on Hachtel’s augmented matrix method in order to directly consider the operational constraints associated with the AC–DC network. In addition, a new technique for performing multiple bad data analysis is proposed in which the original level of measurement redundancy is maintained unaltered during the estimation process. The applicability of the proposed SE approach and bad data analysis is demonstrated by estimating the operating state of two integrated AC–DC multi-terminal transmission networks. The numerical results show that the proposal retains good convergence properties in terms of the number of iterations required to achieve a reliable state estimation as well as robustness to deal with bad data.

Evaluation and Enhancement of Control Strategies for VSC Stations Under Weak Grid Strengths

Faced with increasingly bulk power transfer by voltage source converter (VSC)-based HVDC systems, the stability of VSC stations is of interest. Two widely adopted control schemes, that is, to control the VSC as a current source (e.g., the current vector control) or to control the VSC as a voltage source (e.g., the virtual synchronous machine-based control), are analyzed near the power limit. Bottlenecks of the first control scheme are pointed out and a novel quasi-steady-state method to seek the corresponding critical short-circuit ratio is proposed. Distinctive impacts of the short-circuit ratio on the second control scheme are also analyzed. According to the analysis and relevant comparison, supplementary compensators are proposed to provide extra damping for the two control schemes so that they are able to cope with a wider range of grid strengths. The analytical results and enhancement strategies are validated by time-domain simulations. Suggestions on dividing strong and weak systems with respect to VSCs are finally put forward and which control scheme is preferred under different conditions is discussed.

Decoupled AC/DC Power Flow Strategy for Multiterminal HVDC Systems

AbstractThis paper proposes a decoupled AC/DC power flow approach for multi-terminal HVDC systems. The proposed method simplifies the power flow computation of multi-terminal HVDC systems while accurately reflecting the operation and control characteristics of VSC (voltage source converter) stations in a HVDC network. In the DC network, the power flow calculation is conducted based on a slack DC bus VSC station and power commends issued to other VSC stations from the power system control center. Then, in the AC power flow calculation, VSC stations are treated as special AC generators that can generate and absorb power from the AC grid in active and reactive power or active power and bus voltage control mode. For validation purpose, the conventional unified power flow method for multi-terminal HVDC systems is built. The paper compares the proposed method with the unified power flow method for an 8-bus multi-terminal HVDC system based on MATPOWER. Then, more case studies for different VSC control modes are conducted and evaluated for the 8-bus system. Afterwards, the proposed method is applied to the power flow study of a more practical and complicated multi-terminal HVDC system based on the IEEE 118-bus system.

Multi-Objective Optimization of VSC Stations in Multi-Terminal VSC-HVdc Grids, Based on PSO

The objective of this paper is to enhance the control performance of voltage source converter (VSC) stations within multi-terminal high voltage direct current (M-HVdc) grids under dynamic conditions. This paper presents the application of particle swarm optimization (PSO) to tune the control parameters of VSC stations. VSCs are non-linear components of the M-HVdc grids. Conventional techniques use approximated linear models to tune the control parameters which do not produce optimal results. Thus, PSO is employed to optimally tune the control parameters of VSC stations. The algorithm of the proposed objective function applies simultaneous optimization of inner and outer control layers. Dynamic simulations of a four-terminal HVdc system are developed in PSCAD/EMTDC to extol the merits of the presented optimization technique. Results show the steady state and dynamic control performances, both for classically and PSO-based tuned parameters, during load demand change from ac grids, wind power change, and eventual permanent VSC disconnection.

Reactive-Power Coordination in VSC-HVDC Multi-Terminal Systems for Transient Stability Improvement

This paper proposes a new control strategy for the reactive-power injections of voltage-source converters (VSCs) in high-voltage direct-current multi-terminal systems to improve power system transient stability. A reactive-power supplementary signal is provided for each converter. Its value is proportional to the frequency deviation of its corresponding AC bus with respect to the weighed-average frequency of the multi-terminal system stations. The purpose is to increase (decrease) the electromagnetic torque of generators close to those terminals, in which the frequency is above (below) the weighed-average frequency used. The AC frequency for all VSC stations is always available locally for synchronization purposes and could be used by a central controller. Simulations have been carried out using PSS/E, and the results have shown that transient stability can be improved using this strategy. Since this approach uses global measurements of all VSC stations, the impact of the communication delays has been analyzed, concluding that the negative effect is small, for realistic latency values.

Unified reference controller for flexible primary control and inertia sharing in multi‐terminal voltage source converter‐HVDC grids

Multi-terminal dc (MTDC) grids are expected to be built and experience rapid expansion in the near future as they have emerged as a competitive solution for transmitting offshore wind generation and overlaying their ac counterpart. The concept of inertia sharing for the control and operation of MTDC grids, which can be achieved by the proposed unified reference controller. The control objectives of the MTDC grids voltage source converter (VSC) stations are no longer limited to the stabilisation of MTDC grid, instead, the requirements of ac side are also met. The interaction dynamics between the ac and dc grid is analysed to illustrate the proposed concept. In addition, the voltage source converter stations can work in different operation modes based on the proposed unified control structure, and can switch among the operation modes smoothly following the secondary control commands. Simulation results exhibit the merits and satisfactory performance of the proposed control strategy for stable MTDC grid operation.

Multi‐terminal medium voltage DC grids fault location and isolation

Voltage source converters (VSCs) are highly vulnerable to DC fault current; thus, protection is one of the most important concerns associated with the implementation of multi-terminal VSC-based DC networks. This study proposes a protection strategy for medium voltage DC distribution systems. The strategy consists of a communication-assisted fault location method and a fault isolation scheme that provides an economic, fast and selective protection by means of using the minimum number of DC circuit breakers. This study also introduces a backup protection which is activated if communication network fails. The effectiveness of the proposed protection strategy is analysed through real-time simulation studies by use of the hardware-in-the-loop approach. Furthermore, the effects of fault isolation process on the connected loads are also investigated. The results show that the proposed strategy can effectively protect multi-terminal DC distribution networks and VSC stations against different types of faults.

Flexible Control of Power Flow in Multiterminal DC Grids Using DC–DC Converter

This paper proposes an efficient control framework that utilizes dc-dc converters to achieve flexible power flow control in multiterminal dc (MTDC) grids. The dc-dc converter employed in this paper is connected in cascade with the dc transmission line, and is therefore named cascaded power flow controller (CPFC). In this paper, a two-layer control strategy is developed for the operation and control of voltage source converter stations and CPFC station in MTDC grids. At the primary control layer, a novel differential voltage droop control is developed, while at the secondary control layer, a modified dc power flow algorithm—employing the new CPFC framework—is implemented. The overall control strategy enables the CPFC to regulate the power flow in the dc transmission line. The primary control guarantees the transient stability of the CPFC, and the secondary control system ensures the desired steady-state operation. The proposed voltage droop control framework helps the MTDC grid to remain stable in the event of a communication failure between the primary and secondary control layers. Static analysis and dynamic simulations are performed on the CIGRE B4 dc grid test system, in order to confirm the effectiveness of the proposed control framework for power flow regulation in MTDC grids.

Synchronous Generator Emulation Control Strategy for Voltage Source Converter (VSC) Stations

The voltage source converter (VSC) station is playing a more important role in modern power systems, but the dynamic behavior of the VSC station is quite different from that of the synchronous generator. This paper presents the synchronous generator emulation control (SGEC) strategy for the VSC-HVDC station. The SGEC strategy is divided into the inner control loop and the outer control loop. The inner controller is developed for fast current and voltage regulations. An inertia element is introduced into the frequency-power droop to determine the command reference of the frequency, and the inertia response and the primary frequency regulation are emulated. In addition, the secondary frequency regulation can be achieved by modulating the scheduled power in the SGEC strategy. The time-domain simulation results demonstrate the VSC station with the proposed control strategy can provide desired frequency support to a low-inertia grid. Therefore, the SGEC strategy provides a simple and practical solution for the VSC station to emulate the behavior of a synchronous generator.

A Generalized Voltage Droop Strategy for Control of Multiterminal DC Grids

This paper proposes a generalized voltage droop (GVD) control strategy for dc voltage control and power sharing in voltage source converter (VSC)-based multiterminal dc (MTDC) grids. The proposed GVD control is implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid, and constitutes an alternative to the conventional voltage droop characteristics of voltage-regulating VSC stations, providing higher flexibility and, thus, controllability to these networks. As a difference with other methods, the proposed GVD control strategy can be operated in three different control modes, including conventional voltage droop control, fixed active power control, and fixed dc voltage control, by adjusting the GVD characteristics of the voltage-regulating converters. Such adjustment is carried out in the secondary layer of the hierarchical control structure. The proposed strategy improves the control and power-sharing capabilities of the conventional voltage droop, and enhances its maneuverability. The simulation results, obtained by employing a CIGRE B4 dc grid test system, demonstrate the efficiency of the proposed approach and its flexibility in active power sharing and power control as well as voltage control. In these analysis, it will be also shown how the transitions between the operating modes of the GVD control does not give rise to active power oscillations in the MTDC grids.

Generic inertia emulation controller for multi‐terminal voltage‐source‐converter high voltage direct current systems

A generic inertia emulation controller (INEC) scheme for multi-terminal voltage-source-converter (VSC)-based high voltage direct current (HVDC) systems is proposed in this study. The INEC can be incorporated in any grid-side VSC station, allowing the multi-terminal HVDC (MTDC) terminal to contribute an inertial response to connected AC systems during system disturbances, in a fashion similar to synchronous generators. The DC-link capacitors within the MTDC are utilised by the INEC scheme to exchange stored energy with the AC system by varying the overall DC voltage level of the MTDC network within a safe and pre-defined range. A theoretical treatment of the INEC algorithm and its implementation and integration within a conventional VSC control system are presented, and the impact on the total DC capacitance required within the MTDC network to ensure that DC voltages vary within an acceptable range is discussed. The proposed INEC scheme is validated using a MATLAB/SIMULINK model under various changes in demand and AC network faults. The model incorporates a multi-machine AC power system connected to a MTDC transmission system with multiple converter-interfaced nodes. The effectiveness of the INEC in damping post-fault oscillations and in enhancing AC grid frequency stability is also investigated.