Grid-side Voltage Source Converter Research Articles

A Modified RBF Neuro-Sliding Mode Control Technique for a Grid Connected PMSG Based Variable Speed Wind Energy Conversion System

A modified control scheme based on the combination of online trained neural network and sliding mode techniques is proposed to enhance maximum power extraction for a grid connected permanent magnet synchronous generator (PMSG) wind turbine system. The proposed control method does not need the knowledge of the uncertainty bounds nor the exact model of the nonlinear system. Since the neural network is trained online, the time to estimate good weights can affect the dynamic performance of the process during the startup phase. Therefore an appropriate way to smoothly and explicitly accelerate the neural network rate of convergence during the startup phase is proposed. Furthermore, a flexible grid side voltage source converter control structure which can handle both grid connected and standalone modes based on conventional proportional integral (PI) control method is presented. Simulations are done in Matlab/Simulink environment to verify the effectiveness and assess the performance of the proposed controller. The results analysis shows the superiority of the proposed RBF neuro-sliding mode controller compared to a nonlinear controller based on sliding mode control method when the system undergoes parameter uncertainties.

Frequency Stability Enhancement of Integrated AC/VSC-MTDC Systems With Massive Infeed of Offshore Wind Generation

This paper investigates how the operation of the onshore AC grid and the offshore voltage source converter-based multiterminal dc (VSC-MTDC) grid can be cooptimized within the unit commitment (UC) framework to enhance system frequency stability following various contingencies. Frequency dynamic constraints corresponding to three typical types of outages (i.e., loss of a synchronous unit, loss of a grid-side VSC, and loss of a wind farm-side VSC) are established. A novel frequency dynamics constrained UC for integrated AC/VSC-MTDC systems is then proposed. Case studies on three integrated AC/VSC-MTDC systems demonstrate that major frequency disturbances caused by credible contingencies occurring in either the AC grid or VSC-MTDC grid can be effectively stabilized by the proposed approach.

An improved nonlinear robust control design for grid-side converter of VSC-HVDC connected to wind power generation system

This article proposes an improved nonlinear (IN) robust control strategy for the grid-side voltage-source converter (GSVSC) of a VSC-based high-voltage direct current (VSC-HVDC) transmission system connected to a large wind farm by the using Hamiltonian function method. With the help of variable transformation, the nonlinear model with parameter uncertainties and external disturbances of the GSVSC is changed into the port-controlled dissipative Hamiltonian (PCDH) system. Based on the PCDH system, an IN robust control law is established. In order to demonstrate the effectiveness and robustness of the IN robust control, the backstepping power control (BPC), which is developed by using the backstepping design procedure in the sense of Lyapunov stability theorem for the GSVSC to satisfy the control objectives of a stable HVDC bus and the grid connection with a unity power factor, is selected as a comparison object. Generally speaking, the system used by the backstepping method must have special strict feedback of the lower triangular structure, but the wind farm connected to the power grid with the VSC-HVDC is a multivariable structure and highly coupled nonlinear system. So, the BPC cannot effectively maintain and utilize the nonlinear characteristics of the VSC-HVDC system, while this nonlinear physical structure characteristics is very useful for the design of a nonlinear controller. The greatest advantage of the Hamilton function method is that it can effectively keep and utilize the nonlinear characteristics of the system. The simulation results indicate that the proposed IN control designed by using the Hamilton function method gains the advantage over the BPC under a variety of operating conditions.

Passivity‐based linear feedback control of permanent magnetic synchronous generator‐based wind energy conversion system: design and analysis

This study designs a passivity-based linear feedback control scheme of a permanent magnetic synchronous generator-based wind energy conversion system, which attempts to achieve a maximum power point tracking (MPPT) at generator-side voltage source converter (VSC) and enhance fault ride-through (FRT) capability at grid-side VSC simultaneously. A storage function is constructed based on the passivity theory, in which the actual role of each term is meticulously investigated while the beneficial ones are remained so as to significantly improve the transient responses. Then, an auxiliary input is employed in the form of linear feedback control to ensure a desired tracking error convergence. Moreover, the closed-loop system stability is thoroughly analysed, together with a detailed physical interpretation of the storage function. Three case studies are undertaken including step change of wind speed, stochastic wind speed variation, and FRT. Simulation results verify that the proposed approach can effectively achieve MPPT and dramatically improve the FRT capability under various operation conditions against that of vector control and feedback linearisation control.

Generator response following as a primary frequency response control strategy for VSC-HVDC connected offshore wind farms

As penetration of renewable energy sources increases, the proportion of synchronous generators with rotating masses directly coupled to the grid decreases. In the case of wind generation, the kinetic energy stored in the machine rotor is not available to perform the natural task of frequency support for the AC power system. New active control methods must therefore be developed to synthetically recouple this inertia characteristic to the grid. European offshore windpower is growing and the average distance to shore is increasing. Voltage Source Converter based HVDC (VSC-HVDC) is a promising method for transporting this power to the onshore AC grid over long distances. The fast power transfer capability of VSC-HVDC offers the opportunity for new control strategies to provide frequency support to the AC grid. The authors propose a method for using a VSC-HVDC transmission link to follow and amplify the traditional primary frequency response behavior of a small synchronous generator connected at the point of common coupling (PCC). In terms of frequency support, the onshore grid will then see the HVDC connected offshore windfarm (OWF) and small synchronous generator together as one large synchronous generator with rotor inertia directly coupled to the grid (i.e. synthetic inertia). The method will therefore enable the OWF to provide frequency support services equivalent to that of a traditional power plant at the same rating. Electromagnetic transient simulation is used to investigate the proposed active control strategy. Fiber-optic technology is considered as a method of remote communication between the OWF and the small synchronous generator (SSG). A second communication-less method is also considered which utilizes a cascaded droop frequency to DC voltage control at the Grid Side VSC (GS-VSC) and then DC voltage to frequency at the Windfarm Side VSC (WF-VSC). Simulation results are used to justify the proposed method.

Active ROCOF Relay for Islanding Detection

Intentional frequency perturbation by recently researched active islanding detection techniques for inverter based distributed generation (DG) define new threshold settings for the frequency relays. This innovation has enabled the modern frequency relays to operate inside the non-detection zone (NDZ) of the conventional frequency relays. However, the effect of such perturbation on the performance of the rate of change of frequency (ROCOF) relays has not been researched so far. This paper evaluates the performance of ROCOF relays under such perturbations for an inverter interfaced DG and proposes an algorithm along with the new threshold settings to enable it work under the NDZ. The proposed algorithm is able to differentiate between an islanding and a non-islanding event. The operating principle of relay is based on low frequency current injection through grid side voltage source converter (VSC) control of doubly fed induction generator (DFIG) and therefore, the relay is defined as “active ROCOF relay”. Simulations are done in MATLAB.

An Optimal Integrated Control Scheme for Permanent Magnet Synchronous Generator-Based Wind Turbines under Asymmetrical Grid Fault Conditions

In recent years, the increasing penetration level of wind energy into power systems has brought new issues and challenges. One of the main concerns is the issue of dynamic response capability during outer disturbance conditions, especially the fault-tolerance capability during asymmetrical faults. In order to improve the fault-tolerance and dynamic response capability under asymmetrical grid fault conditions, an optimal integrated control scheme for the grid-side voltage-source converter (VSC) of direct-driven permanent magnet synchronous generator (PMSG)-based wind turbine systems is proposed in this paper. The optimal control strategy includes a main controller and an additional controller. In the main controller, a double-loop controller based on differential flatness-based theory is designed for grid-side VSC. Two parts are involved in the design process of the flatness-based controller: the reference trajectories generation of flatness output and the implementation of the controller. In the additional control aspect, an auxiliary second harmonic compensation control loop based on an improved calculation method for grid-side instantaneous transmission power is designed by the quasi proportional resonant (Quasi-PR) control principle, which is able to simultaneously restrain the second harmonic components in active power and reactive power injected into the grid without the respective calculation for current control references. Moreover, to reduce the DC-link overvoltage during grid faults, the mathematical model of DC-link voltage is analyzed and a feedforward modified control factor is added to the traditional DC voltage control loop in grid-side VSC. The effectiveness of the optimal control scheme is verified in PSCAD/EMTDC simulation software.

Dual-Mode Control of AC/VSC-HVDC Hybrid Transmission Systems With Wind Power Integrated

This paper proposes a novel dual-mode control scheme for the control of an ac/voltage-source converter (VSC)-HVDC hybrid transmission system with the wind farm connected. First, a novel ac voltage and phase-angle control scheme is designed for the active power control of the wind farm side voltage-source converter (WFVSC) such that the switching of control schemes of WFVSC is no longer required when the ac lines are switched on or off the transmission system. This results in the proposed dual-mode control scheme being able to accommodate different operation modes of the ac/dc hybrid transmission systems. Second, an improved direct-current vector control scheme has been designed for the control of grid-side VSC (GVSC) to provide better control of the dc voltage of the GVSC. Moreover, simulation studies have been undertaken on a two-terminal VSC-HVDC and an ac hybrid transmission system under a variety of operating conditions, which have validated the feasibility performance of the proposed control scheme.

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.

Coordinated control of wind farm and VSC–HVDC system using capacitor energy and kinetic energy to improve inertia level of power systems

For large-scale offshore wind power integration to main grids over a long distance, the VSC–HVDC transmission is a typical way. However, the asynchronous characteristic of HVDC link leads to the frequency decouple of the offshore grid and the main grid, i.e., the offshore grid has little or no inertia support for the main grid. The high level penetration of wind energy makes the main grid an “inertia-less” system and impairs the overall stability of the system. This paper proposes a new coordinated control strategy which uses the electrical energy stored in the DC capacitors and the kinetic energy stored in wind turbine rotors to emulate the inertia of synchronous generators. By this control strategy, the DC link capacitors release or absorb energy following the droop DC voltage control of the grid side VSC (GSVSC), and the wind farm VSC (WFVSC) changes its output frequency according to the DC voltage. Thus, an artificial coupling of the frequencies of the two-side AC systems is obtained without remote communication. According to the WFVSC’s output frequency, the wind turbine power controller alters its power reference, and the wind turbine speed changes. Thus, the kinetic energy stored in wind turbine rotors is absorbed or released. As a result, the wind turbine is utilized to keep the main grid frequency stable. Based on the doubly fed induction generator (DFIG) wind turbine, this paper analyzes the influence of different additional power controllers and different control parameters of the proposed control strategy on the inertia time constant. Within the permissible range of the DC voltage variation, the proposed control strategy can provide a wide range of inertia time constant, which improves the overall stability of the main grid system. Simulation results of three operation conditions, i.e., sudden load changes, variation of the wind speed, and AC system faults, validated the effectiveness of the proposed coordinated control strategy.

Study on Stability Improvement of Wind Farm Integrated to Weak Grid by VSC-HVDC

Reactive and active power variation of large scale wind farms always lead to the instability of voltage and frequency. To solve this problem, this paper proposed a method of joining with frequency and voltage control loop of Voltage Source Converter HVDC (VSC-HVDC). Control loop of VSC-HVDC converter WFVSC (Wind Farm Side VSC) and GSVSC (Grid Side VSC) were detailed designed. By the frequency loop of WFSVSC, transient stability is greatly improved. Finally, based on the DIgSILENT/Power Factory simulation, the results show its validity and effectiveness.

Application of SMES Unit to Improve DFIG Power Dispatch and Dynamic Performance During Intermittent Misfire and Fire-Through Faults

The number of wind turbines connected to power grids has significantly increased during the last decade. This is mainly due to the convincing revolution in power electronic technology and the growing concern about greenhouse effect that is intensified due to the burning of fossil fuels. Variable-speed wind energy conversion systems (WECSs) such as doubly fed induction generators (DFIGs) are dominating the wind energy market due to their superior advantages over fixed-speed-based WECS which include more captured energy, less mechanical stress, and acoustical noise. DFIG is interfaced to the ac network through the grid-side voltage source converter (VSC) and rotor-side VSC to enable the variable-speed operation of the wind turbine and to provide reactive power support to the ac grid during disturbance events. Converter switching malfunction such as misfire and fire-through may influence the power dispatch capability of the DFIG. In this paper, a superconducting magnetic energy storage (SMES) unit is utilized to improve the power dispatch and dynamic performance of DFIG-based WECS during internal converter switching malfunctions such as misfire and fire-through faults. Simulation results without and with SMES connected to the system are presented, compared, and analyzed.

Design of backstepping power control for grid‐side converter of voltage source converter‐based high‐voltage dc wind power generation system

This study presents a backstepping power control (BPC) design for the grid-side voltage source converter (GSVSC) in a voltage source converter-based high-voltage dc (VSC-HVDC) wind power generation system. First, a dynamic model by taking parameter variations and external disturbances into account is derived on the basis of the space-vector theory to achieve the decoupling control characteristic of the GSVSC in the VSC-HVDC. Moreover, based on the backstepping design procedure, a BPC scheme is developed in the sense of Lyapunov stability theorem for the GSVSC to satisfy multiple objectives of a stable HVDC bus and the grid connection with a unity power factor. The salient feature of the proposed BPC is the introduction of additional error terms into the control laws to reduce the chattering phenomena in traditional backstepping control. In addition, the effectiveness of the proposed BPC scheme is demonstrated by numerical simulations on a doubly-fed induction generator wind farm with VSC-HVDC grid connection, and its advantage is indicated in comparison with a traditional proportional-integral control strategy under a wide range of operating conditions and the possible occurrence of uncertainties.

An Improved Control Strategy for Hybrid Wind Farms

This paper addresses the control requirements of hybrid wind farms, comprising a relatively large number of conventional induction machines (IMs) along with one or very few permanent magnet synchronous machines (PMSMs), capable of compensating the reactive power demanded by the IMs during faulty conditions as well as attenuating the active power variations due to wind gusts. Based on the superposition theorem and the feedback linearization technique, a controller is designed to independently regulate the positive and negative sequence currents of the PMSM voltage source converters (VSCs), overcoming several drawbacks of existing approaches in the presence of unbalanced voltages. In the proposed scheme, the grid-side VSC currents are controlled in order to improve the ride-through capability of IMs, so that the whole wind farm can fulfill demanding grid codes in the absence of extra equipment, such as static compensators. As shown by the test results, combining IM-based wind farms with PMSMs accomplishes several relevant goals: delivering the reactive power consumption of the IMs, increasing the rated active power of the installation, and smoothing mechanical power oscillations.