Operation Of Voltage Source Converters Research Articles

An emergency control considering current limitation and transient stability of virtual synchronous generator

AbstractWith the increasing application of renewable energy sources (RESs), more and more grid‐connected voltage‐source converters (VSCs) in the grid need to operate in a “grid‐forming” manner. However, when large disturbances occur in the grid, grid‐connected converters may lose synchronization and cause the current to exceed set limits. To ensure the stable and safe operation of VSC during large disturbances and to improve the power supply capacity during voltage dips of RES, this paper proposes an emergency control method that takes into account the virtual synchronous generator (VSG) current limit and transient stability based on a large‐signal mathematical model of the VSG during fault and normal states. The method divides the operation area of the VSG into non‐emergency area and emergency area. If the operating point is in the non‐emergency region after a fault, only the active power reference value is changed to avoid current exceeding the limit and transient instability; if the operating point is in the emergency region, the current exceeding the limit and transient instability are avoided by changing the active power reference value and adding in the virtual impedance. Finally, the effectiveness of the proposed control strategy is verified by simulation.

Evaluation of linear stability operating boundary for voltage source converter under grid faults

Voltage source converters have been used in high voltage direction current systems, which provides various advantages such as reduction in installation cost and space, easy connectivity with renewable energy sources and enhanced power quality. One of the most important issues is how to ride through the grid faults to ensure the stability operation of voltage source converters. In practice, however, there are practical considerations of physical restrictions such as the linear stability operating boundary, beyond which the current would be distorted, even the system would be instable. In this paper, a quantitative analysis and evaluation of the linear stability operating boundary is presented for operating the voltage source converter under grid faults. The proposed method indicates that the operating boundary mainly depends on the circuit parameters and control strategies. Simulation results have provided to verify the proposed quantitative analysis and control strategy. It is expected to provide a useful guidance for designing and controlling the voltage source converter within the linear stability operating boundary, which enhances the system and increased the reliability of the system.

Bi-level robust scheduling for multi-terminal DC distribution networks considering uncertainties of loads and renewable energies

The randomness of loads and renewable energies affects the security of DC distribution networks. In this paper, a bi-level robust scheduling method is proposed to improve the security and economy of droop-controlled DC distribution networks. The upper-level scheduling aims to minimize the variance of node voltage deviation from rated value with the optimization of voltage source converter (VSC) operation reference power. The lower-level scheduling aims to minimize the power purchase cost with the optimization of energy storage systems (ESSs). Based on the linear power flow model of droop-controlled DC distribution networks, the upper-level scheduling model is converted to a least square programming model and the lower-level scheduling model is converted to a linear programming model, which can be solved with high computation efficiency. In the scheduling model of VSC operation reference power, the uncertain scenarios are directly established based on the linear power flow model of droop-controlled DC distribution networks to improve the calculation speed of robust algorithm. The adjustable confidence intervals of load variations are used to coordinate the robustness and conservatism. The VSC operation reference power and ESS charging/discharging power are coordinated through iterative optimization. The simulation results of two DC distribution networks show that the proposed scheduling method can effectively improve the security and economy of DC distribution networks.

DC grid controller for optimized operation of voltage source converter based multi-terminal HVDC networks

Generally, multi-terminal dc grids provide full control over the dc powers that converter terminals exchange with their respective host ac grids. Nevertheless, the same level of controllability does not exist over the power flow in the individual dc lines of a highly meshed HVDC network, and this increases the risk of overloading the dc cables that present lower resistances. To address the highlighted shortcoming, this paper presents a generic dc grid controller that uses nonlinear constrained optimization to optimize the performance of multi-terminal HVDC networks for any desirable operational objective, within system physical constraints such as dc cable and converter thermal limits, and minimum and maximum dc voltage limits. The presented dc grid controller performs online optimization at regular intervals to dynamically estimate and update the set-points of the converter terminals as system operating conditions vary. The technical viability of the proposed controller is assessed using a generic seven-terminal HVDC network that uses voltage sourced modular multilevel converters. Simulations from the scenarios that prioritize dc grid power loss or operational cost minimization show that the presented dc grid controller exhibits good performance during normal operation as converter terminals set-points vary, and during dc grid reconfiguration following simulated successive outages of the dc cables. It has been shown that the performance of the proposed DC grid control is not affected by the parameter variation such as changes of resistance with temperature.

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.

A Unified Control Structure for Grid Connected and Islanded Mode of Operation of Voltage Source Converter Based Distributed Generation Units Under Unbalanced and Non-Linear Conditions

This manuscript develops a unified control structure for Distributed Generation (DG) units based on Voltage Source Converters considering unbalanced and non-linear operating conditions. This control structure works for both the Islanded and the Grid-connected modes of operation of the Micro-Grid (MG). The objective of this control scheme is to regulate the line currents of the DG unit in such a manner that the voltage at the Point of Common Coupling (PCC) remains balanced despite the line currents of the DG unit being unbalanced and distorted. Multiple adaptive P-R controllers have been proposed for the current control loop of the Voltage Source Converter (VSC). These controllers have been implemented with resettable integrators so as to limit the DC components in the post fault current of the VSC. The Battery Energy Storage System (BESS) is interfaced to the DC link of the VSC through bi-directional dc-dc converters. An improved control structure for the bi-directional dc-dc converter has been developed. The effectiveness of these control structures have been presented and tested in PSCAD/EMTDC in an IEEE 34 node distribution system model being fed by two identical DG units.

Control of VSC for enhancement of power quality in off‐grid distributed power generation

In this study, a control algorithm based on non-linear adaptive second-order Volterra filter (NLVF) is utilised to operate the voltage-source converter (VSC). The main role of VSC is to supply compensating current to improve the power quality in wind-based off-grid distributed power generation system. This off-grid system consists of three-phase induction generator, VSC and non-linear loads. The proposed control algorithm is adaptive as well as non-linear in nature. The characteristic of Volterra series is exploited to generate the reference source current for VSC operation. The NLVF estimates the active and reactive weights of fundamental load current for reference current generation. It mitigates the power quality problems created by disturbances in non-linear load or wind oscillations on off-grid system. The solution of power quality problems such as the presence of harmonics in supply current, load unbalance and reactive power compensation is obtained. Nevertheless, voltage and frequency control are also achieved. The VSC is used as a shunt compensator in the system. The entire off-grid system is designed and validated through the laboratory prototype development. The objective of the system is achieved and the steady state and dynamic performance is found satisfactory.

Comprehensive Analysis and Design of a Quasi Two-Level Converter Leg

Quasi two-level operation of voltage source converters, intended to operate either within high/medium voltage DC networks or low output frequency applications, has caught the attention of both academia and industry. Sequential insertion of the so-called submodules (cells) into the circuit alleviates potential problems related to high voltage slopes (dv/dt), whereas the ease of stacking them together provides the means for satisfactory performance in terms of voltage scalability. Although resembling the modular multilevel converter, quasi two-level converter operates in a different manner, leading to the completely different sizing, operating and balancing principles. So far, no publication has analyzed the transition process occurring within the quasi two-level converter leg, which can be considered crucial from the cell capacitance sizing perspective. Therefore, this paper provides thorough and generalized mathematical description of converter leg dynamics during the voltage transition process. Comprehensive method giving the possibility for cell capacitance determination is presented. With the aim of validating the conducted analysis, simulation results covering the operation of a few different topologies, with varying number of cells, are presented.

Optimization of LCL Filter With Integrated Intercell Transformer for Two-Interleaved High-Power Grid-Tied Converters

Paralleling of voltage source converters (VSCs) with a limited maximum output power is essential when the power rating of a system must be increased. Interleaving (ILV) of the switching signals for parallel-connected VSCs provides a better harmonic spectrum than hard paralleling (HP), i.e., applying identical signals to all VSCs. This can be particularly beneficial for grid-tied converters, since a smaller LCL filter can be realized. To investigate the performance limitations and the impact of ILV operation on the LCL filter hardware, a comprehensive optimization procedure for LCL filters and intercell transformers (ICTs) for two paralleled VSCs is presented in this paper. In addition, a comparative evaluation of HP and ILV operation of VSCs is carried out. The developed optimization procedure and the hardware benefits of ILV operation were verified through testing on a 260-kW full-scale system. Compared with the commercial state-of-the-art weight-optimized LCL filter for HP, the developed LCL filter for ILV utilizing an integrated ICT reduces the total filter weight by 30% while guaranteeing harmonic standard compliance and meeting thermal requirements.

Reactive Power Synchronization Method for Voltage-Sourced Converters

There is a growing interest in the parallel operation of voltage source converters (VSCs) both in an isolated microgrid or connected to the utility grid. The most common solution in the literature for the paralellization of VSCs is the so-called droop control, which brings about a relationship between active power and frequency. In this paper, a different approach is proposed where reactive power is used instead of active power to ensure synchronous operation. Active and reactive power are independently controlled using a dq-frame representation based on the vector oriented control, which inherently provides current limitation capability. A detailed dynamic model of the system is used to demonstrate the relation between reactive power and frequency. Due to the intrinsic synchronizing mechanism, the proposed scheme can operate in both isolated and grid-connected modes. As opposed to droop control schemes, active power is not used for synchronization and thus synchronization is possible even if active power is not controllable. Simulation and experimental results, for a case study where a VSC is connected to a host grid, are presented to validate the proposal.

The Impact of Grid Impedance in Operation of Voltage Source Converter Integrated to Weak Grid

The Impact of Grid Impedance in Operation of Voltage Source Converter Integrated to Weak Grid

Power flow calculation method for DC distribution system with voltage source converter operation mode consideration

This paper focuses on the power flow calculation method for DC distribution system with voltage source converter operation mode consideration. To accurately calculating the dc power flow, the master-slave and droop control mode of VSC converter station is used. The structure and mathematical model of VSC converter are discussed. When performing DC power flow calculation, the converter station is equivalent to P node/Slack node/D node according to different control modes of the converter station. And the non-linear DC network equations is solved with the Newton-Raphson (NR) method. A study case which is based on 11 nodes DC distribution system is used to carry out the DC power flow method, and simulation results show that the proposed model and method are effective.

Direct Power Control of Three-phase Voltage Source Converters Using Feedback Linearization Technique

This paper describes the direct power control (DPC) of a three-phase voltage source converter (VSC) using feedback linearization technique. This technique transforms nonlinear model of the VSC into a linear one so that the controller design is easy and independent of the operating point. The problem is formulated in the DPC framework. The DPC with space-vector modulation (DPC-SVM) is adopted because of its constant switching frequency operation. The control scheme is implemented on a STM32F4DISCOVERY board. The validity of the scheme has been verified through experiment which shows that the proposed control strategy provides the VSC operation with wide operating range and fast transient responses.

Inducverters: PLL-Less Converters With Auto-Synchronization and Emulated Inertia Capability

Inspired by induction machines working principles, this paper presents the idea of phase-locked-loop-less (PLL-less) operation of grid-connected voltage source converters (VSCs) under new concept of inducverter. The proposed controller eliminates the need for a dedicated synchronization process and the related PLL, and therefore it offers a simpler and more reliable control strategy compared to the conventional methods. In addition, it represents an improved performance, as it provides real and true auto-start and auto-synchronization with a grid without the need for grid voltage information. A current damping/synchronization unit enables grid auto-synchronization by using local current information and can track grid voltage frequency, angle, and amplitude variations while feeding constant amount of power, which is of high-interest in frequency varying grids and also in the case of grid voltage angle jump. Another advantage of the inducverter is that it introduces virtual inertia to the grid to regulate frequency, which enhances frequency dynamics of smart grids. Beside the current synchronization unit, the proposed strategy has a single-loop controller core with control over both power and current, which is implemented in a hybrid dq and abc frame using a virtual adaptive lead or lag impedance. The controller also offers stable and high-performance synchronization and operation under unbalanced and/or distorted grid conditions. The work beside synchronous current converters gives a bird’s eye view to research in the new area of PLL-less and virtual inertia-based operation of VSCs and fulfill a unified set of controllers for the smart grid integration. Simulation, hardware-in-loop, and experimental results are presented to validate effectiveness of the controller.

Enhancing Small Signal Stability and Reactive Power-sharing Accuracy in Autonomous Microgrids by a New Decentralized Reactive Power Controller

Load sharing without communication between voltage source converters is most important for reliable operation of microgrids. The common approach to achieve this is using the conventional droop technique. Although this method shares a common active load, reactive power sharing is strongly affected by system parameters. In addition, there is an inherent trade-off between the reactive power-sharing accuracy, voltage regulation, and small signal stability margin. This article presents a novel decentralized reactive power controller for parallel operation of voltage source converters in an autonomous microgrid, which leads to the proper reactive power sharing, voltage regulation, and small signal stability margin in microgrids. The proposed methodology and other reactive power-sharing controllers are simulated in IEEE 37-bus test system, and the multi-objective optimization method is used to optimum tuning of each controller and comparison against each other. Simulation results show better performance of the proposed technique in comparison to other reactive power controllers. Furthermore, since the frequency and magnitude of voltage source converters is changed when the demand is altered in the droop method, the conventional load flow cannot be employed to calculate the operation point; a novel method to calculate the operation point in a voltage source converter based microgrid is proposed in this article.

Adaptive voltage droop scheme for voltage source converters in an islanded multibus microgrid

In this study, a novel voltage droop scheme for the parallel operation of voltage source converters (VSCs) in an islanded multibus microgrid is proposed. In this scheme, the voltage droop coefficient is defined as a function of respective VSC active and reactive power outputs. Thus, each VSCs voltage reference is adaptively drooped as a non-linear function of its active and reactive power outputs. This approach leads to reduction in the reactive power sharing dependence on real power control and system parameters such as mismatched connecting and line impedances. A multiobjective index is introduced for evaluating the scheme performance. The index is used as an objective function in an optimisation problem that is employed to obtain optimal parameters of the scheme. The detailed analysis shows that this scheme has a superior behaviour compared to the conventional voltage droop method, in view of the reactive power sharing and loads voltage control under all loading conditions. Simulation and experimental results show the good performance of the method for three paralleled VSCs in a multibus microgrid system.

Exponentially convergent estimator to improve performance of voltage source converters

In this study, the authors introduce an exponentially convergent estimator of load power to improve the performance of voltage source converter (VSC) applications. This estimator, based on a non-linear reduced-order observer, is used to estimate the load power and its time derivative. The estimated signals are feedforwarded to build a non-linear controller in order to improve the whole system performance. The main advantage of the proposed estimator is that it presents linear error dynamics which allows to design the observer by using linear techniques. In this way, the observer transient performance (i.e. convergence rate) is clearly established in the whole VSC operation range. The feedback controller is based on input-output feedback linearisation considering the energy as output so that the DC-voltage internal dynamics is avoided and linear control strategies can be also used in the controller design. Several tests - including performance in presence of switching frequency, parameter uncertainties and disturbances in the AC input voltage - validate the proposed technique.

Control and modulation methods of voltage source converter

Control and modulation methods of voltage source converterControl and modulation methods of Voltage Source Converter (VSC) have been presented in the paper. Model of VSC with three value transistor branch state function is introduced to describe operation of VSC. Predictive-corrective control method of VSC system is presented. Two variants of Space Vector PWM methods for VSC system are developed. Algorithm of cancelation of negative influence of dead time on the AC voltages is implemented in the proposed modulation methods. Correctness of introduced method is validated by simulation and experiment investigations.