Voltage Source Converter Stations Research Articles

An implicit Z-bus based sequential power flow algorithm for VSC AC/DC systems

AC/DC power flow algorithms have been proposed and developed as essential analysis tools for the Voltage Source Converter (VSC) based AC/DC hybrid grid. However, prevailing AC/DC power flow algorithms, e.g., the Newton-Raphson based algorithm, suffer from limitations in terms of convergence, multiple solutions, and high computational complexity. To address these issues, this paper presents a novel sequential AC/DC power flow algorithm based on the implicit Z-bus (IZB) method. We firstly provide a general AC/DC power flow model in real vector space, which takes into account the interface data, internal connection and various control modes of VSC stations. Then, simple and compact IZB iterative mappings in real vector space are derived for the AC solver and DC solver. A sequential algorithmic architecture and data exchange mechanisms are designed considering losses calculation, capacity limits and control strategy switching. The impacts of tolerance settings, initial value selections and load levels on the convergence and accuracy of the algorithm are theoretically analysed. Notably, rigorous convergence analyses substantiate the superior attributes of the proposed algorithm over existing models, guaranteeing convergence to high-voltage solutions (feasible and stable solutions). Numerical tests are performed to compare the proposed algorithm with existing works in terms of accuracy, efficiency, scalability and reliability.

Control strategy used for AC fault ride-through in VSC–HVDC transmission systems

Fault ride-through (FRT) is a very important requirement for power grids based on voltage source converters (VSCs) to improve its operational availability of the AC grid. Grid codes require VSC stations to incorporate fault handling capabilities to prevent disconnection of converter stations from the alternating current (AC) grid for certain fault characteristics. In this paper, an FRT strategy is used in the two converter stations to handle the faults that may occur in the AC part of the grids and ensure their stability. The method improves the performance of the FRT compliance strategy during a voltage drop in the AC side. In addition, a current control that operates simultaneously on positive and negative sequence current is adopted. It also allows the injection of a reactive current to maintain the availability of the high-voltage direct current (HVDC) converter station during a fault. The performance and stability of the suggested FRT control is tested considering the direction of power flow with possible faults in balanced and unbalanced regime. For the validation of the efficiency of the FRT capacity, simulation tests are carried out with MATLAB-Simulink software, and experimental tests with a Smart-grid test bench.

Hierarchical Flexible Operation Approach on a VSC-MTDC Interconnected Hybrid Grid With a High Share of Renewable Power

Voltage source converter (VSC) based multi -terminal HVDC (VSC-MTDC) systems have emerged as flexible collector systems for multi-area interconnected AC/DC hybrid grids with high share of volatile renewables energy. However, it remains an open question as how to exploit the flexible and cooperative regulation of multiple VSC stations and enhance secure operation on the premise of information privacy protection of multiple independent dispatch areas. This paper proposed an Analytical Target Cascading (ATC)-based three-layer decentralized optimization approach on flexible operation of VSC-MTDC hybrid systems. The low layer is a chance-constrained stochastic optimal power flow (SOPF) problem for AC grids. To guard against the secure operation risk by a VSC's random unipolar atresia fault, a modified loss of load probability index (MLOLP) is proposed to improve the day-ahead generation-reserve scheme. The middle layer and the high layer are a multiperiod OPF problem for VSC stations and the MTDC grid, respectively, where a detailed power regulation model is built to fully exert the flexible cooperation of multiple VSC stations. Two case systems are tested to verify the effectiveness and superiority of the proposed flexible operation hierarchy.

MTDC Grids: A Metaheuristic Solution for Nonlinear Control

This scientific paper aims to increase the voltage source converter (VSC) control efficiency in a multi-terminal high voltage direct current (MTDC) network during dynamic operations. In the proposed study, the Mayfly algorithm (MA) is used to modify the control parameters of VSC stations. Traditional strategies that modify VSC control settings using approximate linear models fail to produce optimal results because VSCs are nonlinear characteristics of the MTDC system. Particle swarm optimization (PSO) may produce optimal outcomes, but it is prone to becoming stuck in a local optimum. To modify the proportional-integral (P.I.) control parameters of the VSC station, the Mayfly algorithm, a modified form of PSO, is used. The suggested algorithm’s objective function simultaneously optimizes both the outer and inner control layers. A four-terminal MTDC test system is developed in PSCAD/EMTDC to assess the benefits of the proposed algorithm. For VSCs, a comparison of classical, PSO, and proposed MA-based tuned parameters is carried out. The integral of time multiplied by absolute error (ITAE) criterion is used to compare the performance of classical, PSO, and a proposed algorithm for VSC controller parameters/gains. With an ITAE value of 6.8521 × 10−6, the MA-based proposed algorithm computes the optimal values and outperforms its predecessor to adjust the VSCs controller gains. For (i) wind farm power variation, (ii) AC grid load demand variation, and (iii) ultimate permanent VSC disconnection, steady-state and dynamic performances are evaluated. According to the results, the proposed algorithm based MTDC system performs well during transients.

Synchronous Generator Imitation Control and Dynamic Power Sharing for Distributed Power Generation Systems

In this paper, a control method called synchronous generator imitation control (SGIC) as well as a dynamic power sharing for distributed power generation systems (DPGSs) are presented. In order to imitate the behavior of a synchronous generator (SG), the SGIC method for the voltage source converter (VSC) station is proposed. The SGIC includes three loops, namely active power and frequency loop (Pf loop), reactive power and voltage loop (QU loop) as well as inner current loop. The Pf loop is used to emulate the swing equation of the SG. The exciter of the SG is mimicked by the QU loop. The inner current loop is developed for fast current and voltage regulations as well as current limiting. The system stability performances are analyzed through small-signal model, and the effectiveness of the proposed control is validated by PSCAD/EMTDC simulations. The results show that the Pf loop of the VSC emulates the motion equation of the SG rotor very well, which makes the VSC station to have inertia just as a SG; the VSC station can maintain a stable output voltage and regulate the reactive power at the same time; through the dynamic power sharing, precise power control, frequency offset elimination and system stability improvement are achieved; the system has the merits of fault ride-through capability as well as good dynamic performance.

Compensation of distortions in VSC-based DC–AC power systems using a modified vector control method

Offshore wind farms are always connected to onshore alternating current (AC) power systems using direct current (DC) submarine cables through a DC–AC voltage source converter station. In most cases, nonlinear loads are located on the AC side of the station. The distortion caused by such loads tends to flow into the AC system due to its low impedance, negatively impacting power quality and raising the power losses. This paper presents a novel vector control method (VCM) for the DC–AC station to simultaneously deliver the power demanded by the AC side and act as a shunt active power filter to suppress distortions. In this scope, the conventional VCM is further modified by developing several control loops with proper control and power parameters to obtain a sufficiently wide bandwidth. Additionally, the performance of three different types of low pass filters (LPF) is investigated due to the significant role of LPF in the success of the proposed method. Simulation and experimental outcomes are provided to validate the effectiveness of the proposed method

Research on the Coordinated control Strategy of fault crossing capability of offshore wind power multi-terminal flexible DC system

To improve the ride-through capability (RTC) of the multi-terminal offshore wind power flexible DC system in response to faults, this paper is based on the basic structure and control strategy of the voltage source converter (VSC) station of the multi-terminal offshore wind power flexible DC transmission system (MTDC), and aims at the AC grid connected to the system with three-phase short circuits and wind turbine output changes According to the situation, the mechanism of voltage drop caused by the fault is analyzed, and the DC chopper energy bleeder circuit is connected to the DC side to cooperate with the comprehensive control scheme for fault ride-through of multi-terminal offshore wind power flexible DC collection system. Through theoretical analysis and building a simulation model, the design of the control strategy of the flexible collection system and the simulation study of fault ride-through is carried out on the Shandong Peninsula Beihai Wind Farm. The results show that: the VSC station controller of the offshore wind power multi-terminal system can make the multi-terminal system wind power distribution and consumption reasonable, and realize the stable operation of the system.

Adequate Operation of Hybrid AC/MT-HVDC Power Systems Using an Improved Multi- Objective Marine Predators Optimizer

This paper presents an Improved Multi-Objective Marine Predators Optimizer (IMMPO) for optimal operation of hybrid AC and multi-terminal-high voltage direct current (AC/MT-HVDC) power systems. The proposed IMMPO incorporates an external repository to conserve the non-dominated preys. Furthermore, fuzzy decision making is employed to select the best compromise operating point for the hybrid AC/HVDC power systems. In these systems, the active and reactive power controllability of the voltage source converters (VSCs) are activated besides the full control in AC grids via the committed generators, transformer tap settings and VAR compensations. The modelling of the VSC losses is integrated in its quadratic function of the converter current. The optimal operation of AC/MT-HVDC power systems is handled as a multi-objective problem for minimizing the total fuel costs, the environmental emissions of the generation units and the total losses over the AC, HVDC transmission lines and VSCs stations. For solving this problem, several recent optimization algorithms are applied on a modified standard IEEE 30-bus. Also, a real part of the Egyptian West Delta Region Power Network emerged with VSC-HVDC grids is considered as a practical case study. The simulation results demonstrate the effectiveness and preponderance of the proposed algorithm with great stability indices over several competitive algorithms. Nevertheless, the proposed IMMPO is successfully extracting well-diversified Pareto solutions while a compromise operating point is effectively produced to satisfy the operator requirements.

Extended State Observer Based Nonsingular Terminal Sliding Mode Control for Voltage Source Converter Station With Uncertain Disturbances

This paper proposes a generalized design method of extended state observer (ESO) based nonsingular terminal sliding mode (NTSM) control for voltage source converter (VSC) station. The non-linear loads and external disturbances and parameter perturbations of the converter station are considered as external disturbances which are bounded. The ESO is designed to reject the external disturbances which include not only the unmeasured system states but also the modeling uncertainties. At the same time, the non-singular terminal sliding mode control method based on ESO is proposed to solve the problems of slow dynamic response of output voltage and high waveform distortion rate in converter station. The proposed control method is validated through numerical simulation shows a closed-loop dynamics and better transient performance, and the controller can effectively suppress the differential peak values of active power, reactive power and DC voltage in the initial stage of system start-up. Under conditions of non-singularity of the controller, the influence of system disturbances in the chattering of the controller is effectively eliminated and the robust performance of the system is enhanced.

Equal Loading Rate Based Master–Slave Voltage Control for VSC Based DC Distribution Systems

With the increasing penetration of renewable resources and the DC nature loads, voltage source converter (VSC) based DC distribution systems take an important position in the future smart grid architecture due to many distinguishing features. However, DC distribution systems face frequent power disturbances and multi-mode swithcing problem, how to achieve better DC voltage control and reasonable power sharing performance under various disturbances is still a challenge. After analyzing the control structure and master–slave (M–S) method, an equal loading rate based M–S method is proposed based on the power sharing capability of the VSC station. Moreover, error analysis of the proposed M–S method is conducted and a correction method for the proposed M–S method is developed. The proposed M–S method not only preserves the advantage of M–S method, but also changes the power instructions accordingly based on the operation state of the whole DC distribution system. Moreover, the proposed M–S method is simple and easy to implement. Lastly, simulations of a three-terminal VSC based DC distribution system are conducted in PSCAD/EMTDC to validate the effectiveness of the proposed M–S method.

A Selective Fault Clearing Scheme for a Hybrid VSC-LCC Multi-Terminal HVdc System

A selective fault clearing scheme is proposed for a hybrid voltage source converter (VSC)-line commutated converter (LCC) multi-terminal high voltage direct current (HVdc) transmission structure in which two small capacity VSC stations tap into the main transmission line of a high capacity LCC-HVdc link. The use of dc circuit breakers (dc CBs) on the branches connecting to VSCs at the tapping points is explored to minimize the impact of tapping on the reliability of the main LCC link. This arrangement allows clearing of temporary faults on the main LCC line as usual by force retardation of the LCC rectifier. The faults on the branches connecting to VSC stations can be cleared by blocking insulated gate bipolar transistors (IGBTs) and opening ac circuit breakers (ac CB), without affecting the main line’s performance. A local voltage and current measurement based fault discrimination scheme is developed to identify the faulted sections and pole(s), and trigger appropriate fault recovery functions. This fault discrimination scheme is capable of detecting and discriminating short circuits and high resistances faults in any branch well before 2 ms. For the test grid considered, 6 kA, 2 ms dc CBs can easily facilitate the intended fault clearing functions and maintain the power transfer through healthy pole during single-pole faults.

Study on Coordination of Resistive SFCLs and Hybrid-Type Circuit Breakers to Protect a HVDC System With LCC and VSC Stations

Directing at the robustness increase of a high voltage direct current (HVDC) system with line commutated converter (LCC) and voltage source converter (VSC), this paper proposes to coordinate resistive superconducting fault current limiters (SFCLs) and hybrid-type DC circuit breakers (HDCBs) for dealing with the DC line fault. The theoretical modeling of the SFCLs is presented, and the DC fault current characteristics are analyzed. Considering that the fault interruption involves multi-stages, the coordination of the SFCL and the HDCB in the LCC/VSC station is elaborated. Using PSCAD/EMTDC, the simulation model of a 320 kV HVDC system is built. Changing the SFCL size and the HDCB operation delay is simulated, and the performance indexes such as the fault current level, breaking time and dissipated energy are compared. The proposed coordination is verified to be very effective in handling the fault. Especially for the VSC station, it enables to visibly limit the fault current rising, shorten the transient voltage duration, and facilitate a significant decrease in the dissipated energy of the HDCB. For the LCC station, the proposed coordination can serve as a competitive backup to ensure a timely fault breaking. Consequently, the DC line fault is removed more rapidly and reliably.

Hierarchical and Robust Scheduling Approach for VSC-MTDC Meshed AC/DC Grid With High Share of Wind Power

This paper proposes a novel hierarchical and robust scheduling paradigm for the emerging voltage source converter (VSC) based multiterminal high voltage direct current (VSC-MTDC) meshed AC/DC hybrid system with high share of wind power. Considering the multilevel structure of the VSC-MTDC meshed AC/DC system, the scheduling problem is decomposed into three interactive levels according to the hierarchical analytical target cascading (ATC) technique. The low level is a two-stage adaptive robust security-constrained unit commitment problem for the AC system, which is solved by the column-and-constraint generation (C&CG) algorithm; the middle level is a day-ahead power transmission optimization problem for the VSC stations, where a day-ahead operation model for the VSC station is presented to fully use its flexible and controllable power adjustment capability to promote wind power accommodation; and the high level is a multi-period optimal power flow problem for the DC grid. An integrated ATC and C&CG algorithm is proposed to enable the hierarchical and robust scheduling. The parallel solution is used to accelerate the hierarchical scheduling. Simulations show that this formulation is effective in dealing with wind power uncertainty, promoting wind power accommodation, and improving the calculation efficiency.

Optimal placement of direct current power system stabiliser (DC‐PSS) in multi‐terminal HVDC grids

This study focuses on the selection of the most effective location of direct current power system stabiliser (DC-PSS) in multi-terminal high-voltage direct current (MT-HVDC) grids that is applied to voltage source converter (VSC) to voltage stability enhancement and oscillation damping. Due to the adverse interaction between having multiple number of DC-PSS, it is necessary to select the best location for installing DC-PSS in MT-HVDC grid. In this study, the participation factor (PF) method is used. This method assists to find which power converters are more sensitive and have a major impact on the instability of the entire system. In this approach, the whole of the system is modelled as a multi-input multi-output (MIMO) dynamic system. PFs related to each power converter are intended by the oscillatory mode sensitivity analysis against the components of the MIMO matrix of the system. The PF analysis determines which VSC station is more participating in the small-signal instability of the system. In order to endorse the efficiency of the proposed method, time-domain MATLAB simulations are conducted in Cigre MT- HVDC grid benchmark.

A Multi-Terminal HVdc Grid Topology Proposal for Offshore Wind Farms

Although various topologies of multi-terminal high voltage direct current (MT-HVdc) transmission systems are available in the literature, most of them are prone to loss of flexibility, reliability, stability, and redundancy in the events of grid contingencies. In this research, two new wind farms and substation ring topology (2WF-SSRT) are designed and proposed to address the aforementioned shortcomings. The objective of this paper is to investigate MT-HVdc grid topologies for integrating large offshore wind farms with an emphasis on power loss in the event of a dc grid fault or mainland alternating current (ac)grid abnormality. Standards and control of voltage source converter (VSC) based MT-HVdc grids are defined and discussed. High voltage dc switch-gear and dc circuit topologies are appraised based on the necessity of dc cables, HVdc circuit breakers, and extra offshore platforms. In this paper, the proposed topology is analyzed and compared with the formers for number and ratings of offshore substations, dc breakers, ultra-fast mechanical actuators, dc circuits, cost, flexibility, utilization, and redundancy of HVdc links. Coordinated operation of various topologies is assessed and compared with respect to the designed control scheme via a developed EMTDC/PSCAD simulation platform considering three fault scenarios: dc fault on transmission link connecting the wind farm to mainland power converters, dc fault within substation ring of VSC-HVdc stations, and ultimate disconnection of grid side VSC station. Results show that 2WF-SSRT is a promising topology for future MT-HVdc grids.

New control method for VSC-MTDC stations in the abnormal conditions of power system

Low complexity, insensitivity on the system parameters, lesser number of electrical sensors, fast and accurate dynamic response, the ability of connection to weak AC system and high power quality are some of the major criterions for selection of control method of the voltage source converters (VSC). Considering these issues, in this paper, a new control method is presented for application in the VSC stations. This control method is based on recursive least squares with variable forgetting factor (VFF-RLS) which is a numerical estimator. Estimation of fundamental components of distorted AC grid voltage and their symmetric positive components are done by VFF-RLS estimators. Desired power injection by the proposed control method is done with corresponding to estimated signals. Simulation results in the MATLAB/SIMULINK platform and experimental results using TMS320F2812 digital signal processor validate the superiority of the proposed control method.

A Self-Regulation Strategy for the Power Fluctuation of the Islanded Voltage Source Converter (VSC) Station Delivering Large-Scale Wind Power

When the power source of a voltage source converter (VSC) station at the sending end solely depends on wind power generation, the station is operating in an islanding mode. In this case, the power fluctuation of the wind power will be entirely transmitted to the receiving-end grid. A self-regulation scheme of power fluctuation is proposed in this paper to solve this problem. Firstly, we investigated the short-time variability characteristic of the wind power in a multi-terminal direct-current (MTDC) project in China. Then we designed a virtual frequency (VF) control strategy at the VSC station based on the common constant voltage constant frequency (CVCF) control of VSC station. By cooperating with the primary frequency regulation (PFR) control at the wind farms, the self-regulation of active power pooling at the VSC station was realized. The control parameters of VF and PFR control were carefully settled through the steady-state analysis of the MTDC grid. The self-regulation effect had been demonstrated by a twenty-four-hour simulation. The results showed that the proposed scheme could effectively smoothen the power fluctuation.

Optimal Power Flow With Emerged Technologies of Voltage Source Converter Stations in Meshed Power Systems

It is no doubt that the optimal power flow (OPF) has great importance in electric power systems. It aims at assigning the adequate operating levels in order to meet the required demands with the objective of minimizing combined economic and environmental concerns. Integration of emerged technologies of voltage source converter (VSC) stations in AC meshed power systems changes foremost their corresponding operation and control features. The VSC stations are usually connected with each other through HVDC lines and consequently a multi-terminal direct current (MDC) system is established. This paper presents an improved manta ray foraging optimizer (IMRFO) for solving the OPF in electric power systems with and without emerged technologies of VSC stations. The proposed IMRFO aims at minimizing the total fuel costs, the total environmental emissions, and the total electrical losses. The MRFO simulates the foraging behaviors of the manta rays. MRFO is improved to handle multi-objectives by incorporating an outward store for the non-dominated Pareto individuals. The form of the fitness function is adaptively varied by iteratively changing their weights. Furthermore, a technique for order preference by similarity to ideal solution (TOPSIS) is applied to extract a suitable operating point among the resulted Pareto set. Several applications of the proposed IMRFO are presented for conventional IEEE 30-bus system, as an AC meshed power system, and modified IEEE 30-bus with emerged VSC stations, as a hybrid AC/MDC meshed power system. Simulation results declare that the proposed algorithm has great effectiveness and robustness features compared to the others. Also, various well-distributed Pareto solutions are obtained based on the proposed algorithm with adequate techno-economic-environmental characteristics.

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.

Power‐system level classification of voltage‐source HVDC converter stations based upon DC fault handling capabilities

To date, numerous concepts for converter station designs for use in voltage source converter (VSC)-based high-voltage direct current (HVDC) systems have been proposed. These differ not only in converter circuit topology, sub-module design, and control scheme but also in AC-or-DC switchgear and other auxiliary equipment. In the main, the existing literature categorises these converter stations according to just the converter circuit technologies and controls. However, for the development of network codes and to enable systematic network studies, a system-focused and technology-independent classification is needed. As such a classification does not yet exist, this study proposes a new framework, which categorises VSC station designs according to their capabilities during a DC-side fault and the method by which post-fault restoration may be achieved, given that these are the main differentiating factors from a system perspective. The classification comprises six converter station types and three time-intervals through which to fully characterise a design. Many well-known forms of converters are used as case studies, and simulation results are used to exemplify the classification framework. The outcome is a generic and technology-independent way of characterising converter station designs that is useful in wider power-system analysis but also for putting proposed converter stations into context.