Coordinated Power Control Strategy for the Multi-Terminal VSC-HVDC System

DC voltage control and power distribution are significant requirements for multi-terminal HVDC control system. The common voltage droop control has the drawback of failing to keep the dc voltage at a fixed value if active power is fluctuating, and voltage margin method actually controls dc voltage only by one converter at a time. A novel control strategy-Coordinated DC Voltage Control-is thus proposed. This strategy utilizes telecommunication technology to coordinate all the grid side converters to control dc voltage. A stable dc voltage as well as accurate power distribution according to the minimization of power loss or other optimal power flow algorithm can be achieved. The control effects are validated based on a five-terminal HVDC system modeling in PSCAD/EMTDC with three different scenarios, including steady state and dynamic state. The satisfactory simulation results prove the feasibility and robustness of the proposed control strategy.

A multi-terminal HVDC system includes the connection of different HVDC terminals to a common grid. Most of the MTDC networks are realized in voltage source converter high voltage direct current. The network is designed to either control DC voltage and the magnitude of AC voltage or active power and the magnitude of AC voltage. To demonstrate the basic behavior and functionalities of the MTDC VSC-HVDC network, a 400 kV DC-link test model is simulated on the PSCAD software tool. The results show that multi-terminal VSC-HVDC system has many advantages, such as better controllability, relatively strong independence between different terminals. Results show the multi-terminal VSC-HVDC system has good prospects for development.

The voltage-source-converter- (VSC-) based multiterminal VSC-HVDC power transmission system (VSC-MTDC) is an ideal approach to connect wind farm with power grid. Analyzing the characteristics of doubly fed induction generators as well as the basic principle and the control strategy of VSC-MTDC, a multiterminal DC voltage control strategy suitable for wind farm connected with VSC-MTDC is proposed. By use of PSCAD/EMTDC, the proposed control strategy is simulated, and simulation results show that using the proposed control strategy the conversion between constant power control mode and constant DC voltage control mode can be automatically implemented; thus the DC voltage stability control and reliable power output of wind farm can be ensured after the fault-caused outage of converter station controlled by constant DC voltage and under other faults. The simulation result shows that the model can fulfill multiterminal power transmission and fast response control.

In recent years, with the rapid development of the national economy, the national electricity consumption has increased significantly. Under the scenario of island power supply, the contradiction of insufficient transmission capacity is particularly prominent. Therefore, it is necessary to increase the transmission capacity of the existing lines or to build new transmission lines. As a new type of transmission technology, low-frequency transmission technology has the advantages of relatively increased transmission capacity, no circuit breaker technology bottleneck, and the energy release rate of low-frequency AC (LFAC) system is significantly lower than that of DC system. Therefore, LFAC transmission technology is a very competitive transmission technology in island power supply scenario. This paper analyzes the advantages and disadvantages of various transmission technologies, studies the control strategy of AC-AC converter and the coordinated control strategy of multi-terminal LFAC power grid. The master-slave control strategy of multi-terminal LFAC transmission system is proposed. The simulation model of four -terminal LFAC transmission system is established in PSCAD / EMTDC. Through the simulation analysis of the proposed control strategy, the feasibility and effectiveness of the control strategy are verified.

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.

An improved strategy for controlling DC voltage droop is proposed to solve the problems of conventional DC voltage droop control strategy, such as the received power of converter stations is distributed according to fixed proportion, the power variation amplitude is large, the power regulation ability is easy to lose when reaching the upper limit, and there is obvious voltage deviation. A three-terminal VSC-MTDC system model including wind farm was constructed by Matlab/Simulink. According to different conditions of power fluctuation caused by wind power output variation, simulation and analysis of VSC active power and DC voltage variation were carried out. Simulation results showed that the improved strategy for controlling DC voltage droop proposed in this paper made the converter station retain more power regulation margin when the converter station absorbed more active power, and could prevent the converter station from losing its power regulation ability when the power reached the upper limit, and realize reasonable power distribution among converter stations. Meanwhile, the proposed control strategy could maintain DC voltage constant, which verified the feasibility of its application in wind power grid-connected flexible DC transmission system.

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.

This paper presents a novel control strategy employing the artificial neural network-based direct torque control (DTC) strategy and direct power control (DPC) strategy for permanent magnet synchronous generator (PMSG)-based wind energy conversion system (WECS). The Aeolos 3 kW wind system ratings are considered in this paper for real time system validation. The proposed control strategy combines artificial neural network (ANN)-based DTC as machine side control (MSC) strategy and ANN-based DPC as grid side control (GSC) strategy in order to track the maximum power and to regulate the active power, respectively. To generate the switching pulse for the converter in generator side and grid side, a neural network-based technique is employed. Furthermore, the DC link voltage is optimised using the GSC topology. The proposed controller provides a stable voltage and power which is an effective solution for grid integration and constant power flow from the generator system to the grid system. The proposed coordinate control strategy is validated using MATLAB/Simulink software to analysis the working condition. The experimental test bench is implemented using dSPACE to study the real time analysis of the proposed control strategy.

It is highly critical that renewable energy-based power generation units provide continuous and high-quality electricity. This requirement is even more pronounced in standalone wind–diesel systems where the wind power is not always constant or available. Moreover, it is desired that the extracted power be maximized in such a way that less fuel is consumed from the diesel engine. This paper proposes a novel method to design decentralized model-predictive controllers to control the frequency and power of a single standalone generation system, which consists of a wind turbine subsystem mechanically coupled with a diesel engine generator subsystem. Two decentralized model-predictive controllers are designed to regulate the frequency and active power, while the mechanical coupling between the two subsystems is considered, and no communication links exist between the two controllers. Simulation results show that the proposed decentralized controllers outperform the benchmark decentralized linear-quadratic Gaussian (LQG) controllers in terms of eliminating the disturbances from the wind and load power changes. Furthermore, it is demonstrated that the proposed control strategy has an acceptable robust performance against the concurrent variations in all parameters of the system as compared to the LQG controllers.

This paper is focused on the application of a multi-terminal line-commutated converter-type low frequency AC transmission system (MTLF) using a cycloconverter by applying a new power control scheme for multi-terminal operation. With the virtual synchronous generator (VSG) control scheme, the transmitting power among the multi-terminal system can be accomplished without a communication link for frequency synchronization in each terminal. The details of the proposed control scheme are explained in order to understand the advantages of this method. The configuration of a two-phase low frequency AC transmission system (LFAC) is adopted to examine with the proposed control scheme. Simulation results are provided to illustrate the proposed control scheme with respect to the LFAC system’s performance.

This paper focuses on the control strategy of a VSC-HVDC system supplying power to offshore platforms consisted mainly of induction machines together with diesel engines. The offshore platform VSC emulates the dynamic behavior of a synchronous generator as well as the governor and field regulator to render similar voltage and frequency regulation. The utilization of droop control of dc voltage couples the frequency of onshore and offshore systems, so that the inertia of the offshore platform can be increased. Finally an equivalent of an offshore platform system is simulated in DIgSILENT/PowerFactory to validate the proposed control strategy. The simulation compared the dynamic response of the system with the proposed control strategy and with the conventional control strategy in some operation scenarios such as motor starting, load disturbance, onshore system faults and offshore system faults. The results show that the offshore platform system operates more stably and reliably with the proposed control strategy.

Trends in the use of renewable energy sources to power buildings do not bypass objects for which maintaining a power supply is critical. This also applies to railway signal boxes. The aim of the research work was to test the multisource power supply system for a railway signal box with power electronic converter systems and a DC bus, built as part of the research project. The assumption for powering the railway signal box building was to use renewable sources, energy storage devices, and a 3 kV DC traction network as the second required power supply grid. Both power grids were connected by power electronic converters, and the power values of the converters were set based on the calculated power balance values using the values measured at the system nodes and the set constraints. The tests primarily tested the response of the power supply system to changes in load power and power generated by the photovoltaic source, as well as the charge level of the energy storage devices. The correctness of the control algorithm’s operation was assessed based on the recorded power values in the power supply system nodes. The tests were carried out for 60 scenarios that covered all normal and emergency operating conditions. During the tests, delays in response to changes in the power supplied to the converters and the values of circular power flow between the power grid connections were recorded. The recorded delays ranged from 2 to about 50 s and the circular power flows did not exceed 1500 W. Based on the results of the tests, it was found necessary to improve the power measurement system in the power supply system nodes and to improve the quality of communication and the transmission time of measurement data transmission time.

This paper proposes a flexible enhanced power quality strategy for a multi-function distributed generation (DG) in an aggregated system. Being a multi-function DG system, the grid-interfaced inverter can provide very sinusoidal current and harmonic compensating current simultaneously. Different from the classic strategy where the compensating current is directly calculated by the measured harmonic content in the load current, the proposed power quality control strategy presents a more flexible way using a voltage-controlled approach based on the virtual impedance theory. As a result, each individual DG can provide harmonic compensation functionality independently and the power quality of grid current can be enhanced. This paper first develops an aggregated DGs model using continuous timedomain equivalent circuit. The classic and proposed power quality control strategies can then be derived from the developed models. The proposed control strategy is performed by the virtual harmonic impedance method, which can promote the output current of each individual DG itself and compensate harmonics caused by the other DGs. The proposed control strategy can be seamlessly incorporated into the conventional PR control scheme, and maintain both the stead-state and transient performance within the control bandwidth. Moreover, the proposed control strategy is less sensitive to the variable of the grid frequency. Experimental results are provided to validate the correctness of the proposed power quality strategy.

In this paper, a feedforward proportional carrier-based pulsewidth modulation (PWM) is proposed for the dc link control of the single-phase cascaded H-bridge multilevel photovoltaic inverter. The conventional carrier-based PWM techniques, such as the phase-shifted PWM and the level-shifted PWM, require multiple individual dc voltage controllers for the individual maximum power point tracking (MPPT). Thus, the control structure and the controllers' designs are quite complex. Several noncarrier-based PWM techniques, such as the feedforward space vector modulation, the sorting-algorithm-based modulation, and the lookup table-based modulation, have the capability of individual dc voltage regulation. Although these noncarrier-based PWM techniques require only one total dc voltage controller in the outer voltage loop, the implementations of them are much more complicated than those of the carrier-based PWM techniques. In this paper, with the proposed feedforward proportional carrier-based PWM, each individual modulation signal is directly generated by multiplying the current loop controller's output and the modulation signal's feedforward proportional factor. With the feedforward PWM and the total dc voltage control, the individual dc voltage regulation is achieved. This carrier-based PWM combines the strengths of both the traditional carrier-based PWM and the noncarrier-based PWM. It is not only easy for implementation, but also requires only one total dc voltage controller in the outer voltage loop. The detailed description, derivation, and analysis of this modulation are included in this paper. In addition, the simulations and experiments demonstrate the performance of the dc link control and the individual MPPT using the proposed PWM and the control strategy.

In the three-phase photovoltaic (PV) cascaded inverter, the output power of PV arrays is not equal due to the difference of solar radiation, temperature and other factors, which leads to the over modulation of PV cascaded inverter. In order to solve the above problems, the carrier phase-shifted PWM (CPS-PWM) control strategy based on third harmonic injection is proposed in this paper, taking a single-stage high-frequency-link isolated cascaded PV inverter topology with clamping circuits as an example. It can not only improve the utilization of DC voltage, but also enlarge the power balance constraint range between the units in the phase of cascaded PV inverter, so as to ensure that all units can avoid over modulation under some serious power imbalance cases, and will not increase the third harmonic component injection of grid current. Firstly, the topology and working principle of the proposed inverter based on CPS-PWM control strategy are introduced. Then, the improved CPS-PWM control strategy which can improve the DC voltage utilization of the PV cascaded inverter is analyzed, and the control strategy of intra-phase power balance is emphatically analyzed in the overall control strategy of PV cascaded inverter system, and it is discussed that the proposed control strategy can also enlarge the power balance constraint range between the units of the cascaded inverter. Finally, the effectiveness of the proposed PV cascaded inverter based on the improved CPS-PWM control strategy is verified by the simulation and experimental results.