Multi-terminal VSC-HVDC for smart DC network: Control and simulation

Different control schemes adopted in a multi-terminal VSC HVDC (MTDC) system will change the power injections into the connected AC grid and thus affect its dynamic performance. This paper investigates a multi-machine AC system model integrating a 4 terminal VSC HVDC system for wind farm connection and onshore power sharing. A series of control strategies are created for the MTDC grid including voltage margin control and different droop control settings. The DC system power sharing and the corresponding effects on the AC system are mainly considered. This work is carried out in DIgSILENT PowerFactory using case study simulations of the integrated system model.

In this work, a comparative analysis of direct current control (DCC) and indirect current control (ICC) schemes has been discussed for a single-stage grid-connected PV system. The use of a VSI in a single-stage grid-connected PV system gives rise to the issue of switching ripples in the grid current which yields increased THDs in grid current. Therefore, it is imperative to develop a suitable current control scheme for appropriate switching of the VSI. Both DCC and ICC schemes are simulated in MATLAB/Simulink environment. The results obtained indicate that the ICC scheme yields lesser THDs in grid current than the DCC scheme. The THD yielded by indirect current control scheme remains within the acceptable limits given by IEEE-519 standards.

Although the control of modular multi-level converters (MMCs) in high-voltage direct-current (HVDC) networks has become a mature subject these days, the potential for adverse interactions between different converter controls remains an under-researched challenge attracting the attention from both academia and industry. Even for point-to-point HVDC links (i.e., simple HVDC systems), converter control interactions may result in the shifting of system operating voltages, increased power losses, and unintended power imbalances at converter stations. To bridge this research gap, the risk of multiple cross-over of control characteristics of MMCs is assessed in this paper through mathematical analysis, computational simulation, and experimental validation. Specifically, the following point-to-point HVDC link configurations are examined: (1) one MMC station equipped with a current versus voltage droop control and the other station equipped with a constant power control; and (2) one MMC station equipped with a power versus voltage droop control and the other station equipped with a constant current control. Design guidelines for droop coefficients are provided to prevent adverse control interactions. A 60-kW MMC test-rig is used to experimentally verify the impact of multiple crossing of control characteristics of the DC system configurations, with results verified through software simulation in MATLAB/Simulink using an open access toolbox. Results show that in operating conditions of 650 V and 50 A (DC voltage and DC current), drifts of 7.7% in the DC voltage and of 10% in the DC current occur due to adverse control interactions under the current versus voltage droop and power control scheme. Similarly, drifts of 7.7% both in the DC voltage and power occur under the power versus voltage droop and current control scheme.

For better performance, different current control schemes are used in LCL-type grid-connected converters. However, these schemes are mostly used in three-phase grid-connected converters and multiple current sensors are needed. To address these issues, a current decoupling control scheme for single-phase grid-connected converters in rotating reference frame is proposed in the paper. Because an extended state observer is adopted in the control scheme, only one current sensor is needed and the complicated coupling relationships between the $d$ axis components and $q$ axis components can be eliminated completely. Moreover, to improve the dynamic and steady-state performance of converters, a state feedback closed-loop system structure and nonlinear controllers are adopted. The current decoupling control scheme is validated in an LCL-type single-phase grid-connected converter. The experimental results show that a fast-dynamic response under load change and voltage sag can be achieved besides preferable static performances with the proposed control scheme.

A new bidirectional dc-ac power electronic interface (PEI) consisting of a dc link, an HVdc terminal based on modular multilevel converters (MMCs), medium-frequency transformers (MF-XFMRs) - higher frequencies are also possible - and ac-ac converters is proposed for interconnecting two ac systems, or dc and ac systems. The advantages of each component make the proposed topology convenient for applications that require compactness, flexibility, and reliability. For example, integration of offshore and onshore wind farms with the power grid, ac and/or dc grids interconnections, and supplying power to conventional and future deep-sea oil and gas facilities. The PEI main overall topology, operating principles and design equations are described. Size and efficiency are evaluated for the proposed topology for different operating frequencies and compared with the conventional approach. A case study of a deep-sea electric power system (DEPS) is illustrated through timedomain simulations for different operating conditions to demonstrate the feasibility of the proposed ideas.

In this paper, a novel mixed current and voltage control scheme for an inverter-controlled arc welding machine is proposed. The proposed control scheme uses both a closed-loop current controller and an open-loop voltage controller to optimize the output current and voltage waveform based on the metal transfer procedure. An experimental comparison is made between the proposed and the conventional control scheme for the full output-current range.

The voltage droop control concept is the most appropriate strategy to provide primary control in multiterminal VSC-HVDC (MTDC) transmission systems. However, due to voltage drops in the dc lines/cables, the primary power balancing is influenced by the dc grid topology and line resistances. This phenomenon is also evident when determining the secondary control strategy in MTDC systems. In this paper, secondary control in MTDC is derived by analogy to the traditional secondary control in an ac grid system. In doing so, the influences of dc line voltage drops are systematically included in the control strategy to restore power flow to pre-disturbance levels at selected dc terminals. The effectiveness of the strategy is demonstrated with the help of a five terminal VSC-HVDC model in the EMTDC/PSCAD simulation tool.

The lack of control in voltage overshoot, transient response, and steady state error are major issues that are frequently encountered in a grid-connected photovoltaic (PV) system, resulting in poor power quality performance and damages to the overall power system. This paper presents the performance of a control strategy for an inverter in a three-phase grid-connected PV system. The system consists of a PV panel, a boost converter, a DC link, an inverter, and a resistor-inductor (RL) filter and is connected to the utility grid through a voltage source inverter. The main objective of the proposed strategy is to improve the power quality performance of the three-phase grid-connected inverter system by optimising the proportional-integral (PI) controller. Such a strategy aims to reduce the DC link input voltage fluctuation, decrease the harmonics, and stabilise the output current, voltage, frequency, and power flow. The particle swarm optimisation (PSO) technique was implemented to tune the PI controller parameters by minimising the error of the voltage regulator and current controller schemes in the inverter system. The system model and control strategies were implemented using MATLAB/Simulink environment (Version 2020A) Simscape-Power system toolbox. Results show that the proposed strategy outperformed other reported research works with total harmonic distortion (THD) at a grid voltage and current of 0.29% and 2.72%, respectively, and a transient response time of 0.1853s. Compared to conventional systems, the PI controller with PSO-based optimization provides less voltage overshoot by 11.1% while reducing the time to reach equilibrium state by 32.6%. The consideration of additional input parameters and the optimization of input parameters were identified to be the two main factors that contribute to the significant improvements in power quality control. Therefore, the proposed strategy effectively enhances the power quality of the utility grid, and such an enhancement contributes to the efficient and smooth integration of the PV system.

A three-phase four-wire active filter with neutral-point-clamped topology is adopted to eliminate harmonics, to compensate reactive power and to balance the source current for the balance and unbalance loads. Using the equal current control scheme, the balanced line currents are drawn from the AC mains. The neutral current of the linear or nonlinear loads is compensated by the neutral current of the active filter such that the source neutral line current is close to zero. The compensated current commands of the active filter are tracking using the carrier-wave current control scheme. Three voltage levels are generated on the AC terminal of the inverter. Experimental results based on a laboratory prototype are provided to verify the effectiveness of the proposed control scheme.

In voltage‐controlled voltage source inverters (VSIs)‐based microgrids (MGs), the inner control is of prime interest task for guaranteeing safe and stable operation. In this paper, an in‐depth investigation of the modelling, control design, and analysis of the voltage and current inner control loops intended for single‐phase voltage‐controlled VSIs is established. The main objective of this work is to provide a comprehensive study of the mathematical modelling, control design, and performance evaluation of the inner control's loops considering different proportional‐integral (PI) controller types with and without compensation, and to determine the optimal scheme that can offer better performance in terms of implementation simplicity, robustness, and transient and steady‐state responses. Thus, the mathematical closed‐loop models of designed outer voltage and inner current control schemes based on PI, P, and feedforward controllers with and without compensation are, first, derived. Following this, a systematic and effective control design for tuning the different PI controllers’ parameters is proposed. Furthermore, an analysis revealing the performance of the designed voltage and current control schemes is provided. This analysis enables us to choose a P controller and PI feedforward controller for the current control loop and the voltage control loop, respectively. The chosen P and PI controllers should be simple; meanwhile, they should offer a wide bandwidth. A simulation study is carried out in MATLAB/Simulink software to assess the performance of the adopted inner control scheme for both linear and non‐linear loads. In addition, an experimental setup, based on a TMS320F2837xD μ C, of a single‐phase VSI supplying linear and non‐linear loads is built to verify the effectiveness and the robustness of the adopted inner controller. The results demonstrated: (1) the necessity of introducing the compensation term, which is responsible for offering control improvement against voltage perturbation, (2) the high tracking performance of the chosen controller in terms of dynamic and steady‐state responses as well as its simplicity of implementation.

Different current mode controls are widely adopted in commercial PWM controllers for Point-of-Load converter application. To understand the unique properties of various current mode control schemes and study the subtle differences between them, this paper investigates and compares four current mode control schemes (peak-current control, valley-current control, constant on-time control and constant off-time control) for Point-of-Load power converter and Voltage Regulator applications. Performance attributes under investigation include high bandwidth voltage loop design, adaptability to wide input voltage range, adaptive voltage positioning design and audio susceptibility. The pros and cons of these schemes are identified and explained by the unified small signal equivalent circuit model. From dynamic performance point of view, this work provides a current mode control scheme selection criteria and feedback design guideline. The theoretical analyses are verified by both simulation and experimental results.

The paper discusses the problem of speed and rotor flux magnitude estimation for the induction motor (IM) drive and presents a sliding mode observer (SMO) that is constructed based on the IM model in the synchronous reference frame. To implement the control scheme of the IM drive, generally, the speed and the flux magnitude are needed. In a control scheme with speed and flux regulation, they are used for d and q axis feedback. If the current control scheme uses a decoupling compensator, the flux and speed are needed to compute the decoupling voltages. The paper develops a SM observer for the flux magnitude based on the synchronous reference frame model of the IM. The method assumes that the d-q voltages and currents are available. The observer requires knowledge of the motor speed - instead, the SMO is fed with a speed estimate (assumed inaccurate). Using a specific gain design, the SMO is made insensitive to the input speed inaccuracy. Using the equivalent controls of the observer, the initial speed estimate is corrected to obtain two secondary speed estimates. A weighted average speed estimate that combines the secondary estimates is also shown. The theoretical findings are supported with simulations.

ABSTRACTTo overcome the inaccurate power sharing and circulating current issues caused by the mismatched feeder impedance, this paper proposed an adaptive virtual complex impedance‐based power sharing control scheme, where both virtual inductance and virtual resistance are introduced in the control loop. Although communication is employed in this scheme, it is just used to facilitate tuning virtual impedance to compensate for the mismatched voltage drop across the feeders instead of tuning control’s closed loops; therefore, the power sharing performance is not affected by the communication time delay. Once the virtual impedances are tuned at a given load, the scheme ensures good power sharing performance even if the communication is interrupted. Moreover, a detailed state‐space model is developed, and the system stability analysis is conducted. Finally, the effectiveness of the proposed scheme is validated. The simulation results show that the reactive power error is reduced from 12.2% to 0.18% with the proposed control scheme, and the scheme exhibits good power‐sharing performance under conditions of load changes and communication interruptions. In addition, corresponding experimental results have also been obtained using RT‐LAB.

The Optimal Power Flow (OPF) problem has been widely studied by researchers and electric industry practitioners for many years. Most of these studies use DC power flow equations to represent the transmission system. However, future power systems will have to deal with new challenges, including integration of renewable energy resources, HVDC links, FACTS devices, etc. The DC formulation may not be appropriate to comprehensively model the behaviour of the new system, for which AC formulations might be required. In this paper, the integration of point-to-point and multi-terminal VSC-HVDC in three nonlinear and DC OPF formulations is analysed using three different test systems. The Knitro and CPLEX solvers are used as optimisation tools. The results show that the generation dispatch resulting from the DC formulation could overload the VSC-HVDC and HVAC transmission branches in the real operation. Moreover, it is demonstrated that the nonlinear formulations can produce good quality solutions that preserve the secure operation of the transmission system.

Accurate aggregated modelling of wind farm systems in modified sequence domain for stability analysis