Modeling and simulation of control strategy for voltage source converter multi-terminal DC system

A three-terminal DC transmission system based on modular multilevel converter (MMC) is developed to solve the problem of power supply for distributed islands or cities. The system, with flexible features, may generate high quality output voltage and reduce construction costs and power losses in the converter stations. However, the three-terminal DC system based on MMC faces more complex situations in operation and its control strategies. In this paper, the mathematic model of three-terminal DC system based on MMC is deduced. Then according to the structure of MMC, a method of carrier phase-shift SPWM is introduced. As the system-level control, the decoupling controller for three-terminal DC system based on MMC is designed. A multi-point DC voltage control strategy based on DC voltage error method, which doesn't need communication between converter stations, is proposed to improve the system reliability. By simulation with PSCAD/EMTDC, the feasibility and effectiveness of the proposed model and control strategies are verified.

The modular multilevel converter (MMC) containing full-bridge submodules (FBSMs) can increase the modulation index to reduce the energy storage requirement. The MMCs containing FBSMs can also adjust the dc voltage to fit special applications. However, limited research explores the energy storage characteristics of MMCs with variable dc voltages, and few effective approaches can ensure low-energy storage requirements. This study proposes an approximate analysis method for the energy storage requirement in MMC with variable dc voltages. Results reveal that the energy variation under low dc voltages restricts the reduction effects of the increased modulation index on the energy storage requirement. Then, on the basis of the fact that the low ac component of the arm current under low dc voltages provides a large current margin, an optimal circulating current (OCC) is proposed to optimize the energy variation under low dc voltages. In addition, a practical curve-fitting approach is proposed to facilitate the calculation of OCC in real-time controllers. With these approaches, the energy storage requirement with variable dc voltage becomes similar to that with constant rated dc voltage. Simulation and experimental results verify the analysis.

Optimal load flow normally considers active and reactive power loads as independent of the voltage level. In the paper the exponential form of a static load model relating the active and reactive power components to the busbar voltage, is considered and the effects of incorporating this load model in the optimal load flow solution of several test systems are studied. The effects of modelling active and reactive powers are considered separately as well as when taken together. The results are compared with the respective standard optimal load flow solutions for a number of standard test systems.The differences in fuel cost, total power loss and voltage values when compared with the standard optimal load flow solution are significant for some systems. The fuel cost and power generation differences are the highest when only the active power variation with the voltage is modelled. The heavier the system loading, the lower the fuel cost difference between conventional OPF, and the OPF incorporating power variations with voltages. The differences in power loss are more pronounced when both active and reactive powers are modelled. The higher the loading, the less pronounced the difference in power loss. More significant deviations in voltage take place when both active and reactive powers are modelled. The modelling of reactive power is more influential on the voltages than is the modelling of active power on the voltage deviations. The contrary is true for the differences in phase angle and incremental cost of power delivered, wherein the main factor is active power modelling. There are no differences in iteration numbers for small-size systems. The larger and more heavily loaded systems exhibit a difference in the required number iterations.

For stable operation of a power system reliable control of active & reactive power is necessary. Voltage source converter based HVDC system bears the advantages of being able to change the control strategy immediately with respect to the active and reactive power changes. In this paper a function based fuzzy control system is proposed for the control of active power, DC voltage at converter station and voltage at inverter station. Basically two input one output methodology based fuzzy logic controller has been proposed. The basic features of proposed system used in VSC based HVDC systems are: 1) only two rule base system is used.2) all controllers are not self correcting. At both stations error and change in error have been taken as input to the controller. Design of high pass filter with FFT analysis has also been proposed for a better dynamic performance of the function based fuzzy controller. Computer simulation using Simulink gives clear result of the performance of the proposed controller.

For integrating large-scale offshore wind farm system more effectively, issues as insufficient inertia energy and dc voltage variation caused by traditional grid-forming (GFM) control in voltage source converter based HVDC (VSC-HVDC) have to be settled urgently. This article proposes an improved coordinated GFM control strategy based on hybrid energy to improve grid frequency and dc voltage stability. In terms of grid-side VSC control, an improved virtual inertia control method consisting of additional capacitor structure and matching GFM control is proposed, which can decouple dc voltage and energy of dc capacitor in VSC-HVDC. By establishing coupling relationship between additional capacitor energy and grid frequency, the matching GFM control can fully utilize its energy and provide better inertia support. In terms of wind farm control, a three-stage coordinated control based on hybrid energy is designed. During stage1 and stage2, fast inertial support and primary frequency regulation are achieved through rotor kinetic energy. The supercapacitor is controlled to quickly increase or decrease active power to suppress sudden output power change caused by exiting rotor kinetic energy control in stage3. Finally, a comparative simulation is performed using MATLAB/Simulink to verify the effectiveness and advantages of the proposed strategy.

In this paper, DC power and voltage characteristics in MMC-HVDC grid are analyzed. Coupling between DC voltage and power in MMC grid under power imbalance is proposed based on MMC converter response, and time-varying curve of DC voltage is analyzed. A simplified model of four-terminal MMC-HVDC grid is built, and simulations of DC voltage change under power imbalance is done. Analyses and simulation results show that, under fixed control response; a positive proportional relationship exists between power variation and DC voltage variation. Time-varying curve of DC voltage can be calculated by a first order differential equation.

We propose frequency and voltage control method for a hybrid high-voltage direct current (HVDC) system for integrating an offshore wind farm into transmission networks. Conventionally, DC voltage level is maintained as constant regardless of the wind velocity and the grid frequency, so AC voltage fluctuates with active power variation in a hybrid HVDC system due to the innate reactive power absorption of inverter side line-commutated converter (LCC). However, in the proposed method, the DC voltage changes according to the wind velocity. Furthermore, DC voltage is simultaneously regulated to participate in frequency control in a communication-free manner. These two characteristics can be achieved by using five droop control methods proposed here. Additionally, by determining droop constants using the proposed determination methods, AC voltage can be maintained as constant during active power fluctuations. Therefore, using the proposed method, an offshore wind farm can be stably connected to mainland transmission networks and successfully used as frequency supporting resources, because voltage characteristics of the AC network are not destabilized. A small-signal state-space (SS) model of the target system was also developed to investigate stability of the proposed control. The utility of the proposed method is demonstrated by case studies using PSCAD and SS models.

New Instrumentation for Electric Drives

This paper outlines the design of an offshore wind farm using a dc offshore grid based on resonant dc-dc converters. Multiphase resonant dc-dc converters are studied to step up the dc voltage from individual wind generators to a Medium Voltage (MV) dc bus, and from the MV bus to an HVDC line that will connect the wind farm to shore. Compared to an equivalent ac grid-based wind farm, a dc grid-based wind farm has slightly higher losses, but the weight of the magnetic components and cables is substantially lower. The analysis of operating permanent-magnet synchronous generators at variable and constant dc voltages shows that a fixed dc voltage has marginally higher efficiency than a variable dc voltage. However, using a variable dc voltage gives lower harmonics at the generator facing the voltage-source converter and the dc-dc step-up converter. An aggregated model of multiple parallel connected wind generators is developed and shown to accurately approximate a detailed PSCAD model during varying wind conditions and transients.

Microgrids consist of multiple parallel-connected distributed generation (DG) units with coordinated control strategies, which are able to operate in both grid-connected and islanded modes. Microgrids are attracting considerable attention since they can alleviate the stress of main transmission systems, reduce feeder losses, and improve system power quality. When the islanded microgrids are concerned, it is important to maintain system stability and achieve load power sharing among the multiple parallel-connected DG units. However, the poor active and reactive power sharing problems due to the influence of impedance mismatch of the DG feeders and the different ratings of the DG units are inevitable when the conventional droop control scheme is adopted. Therefore, the adaptive/improved droop control, network-based control methods, and cost-based droop schemes are compared and summarized in this paper for active power sharing. Moreover, nonlinear and unbalanced loads could further affect the reactive power sharing when regulating the active power, and it is difficult to share the reactive power accurately only by using the enhanced virtual impedance method. Therefore, the hierarchical control strategies are utilized as supplements of the conventional droop controls and virtual impedance methods. The improved hierarchical control approaches such as the algorithms based on graph theory, multi-agent system, the gain scheduling method, and predictive control have been proposed to achieve proper reactive power sharing for islanded microgrids and eliminate the effect of the communication delays on hierarchical control. Finally, the future research trends on islanded microgrids are also discussed in this paper.

This paper proposes a new direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind turbine system. The required rotor control voltage, which eliminates active and reactive power errors within each fixed time period, is directly calculated based on stator flux, rotor position, and active and reactive powers and their corresponding errors. No extra power or current control loops are required, simplifying the system design, and improving transient performance. Constant converter switching frequency is achieved that eases the design of the power converter and the ac harmonic filter. Rotor voltage limit during transients is investigated, and a scheme is proposed that prioritizes the active and reactive power control such that one remains fully controlled while the error of the other is reduced. The impact of machine parameter variations on system performance is investigated and found negligible. Simulation results for a 2 MW DFIG system demonstrate the effectiveness and robustness of the proposed control strategy during variations of active and reactive power, machine parameters, and wind speed

Direct Current (DC) transmission system is an important part of the interregional grid. However, the frequency impact of power grid caused by DC blocking is more serious with the continuous increase of scale of single DC transmission. The frequency coordination control system is applied to the receiving-end power grid in High Voltage Direct Current (HVDC) transmission system. In the cases of single-infeed HVDC blocking, multi-infeed HVDC blocking or fast power drop, the frequency coordination control system can reduce the frequency fluctuation of power grid by instantly promoting the transmission power of the rest normal DC systems and reducing power loss. The frequency coordinated control substation is an executive device of the system at the converter station. It keeps communicating with the coordinated control master station for real-time monitoring the operation of converter station, reports the fault and executes the remote power boost instructions. Taking the frequency coordinated control substation of 500 kV conventional DC systems as a research background, the load of converter station in a regional power grid is analyzed. The working mechanism of frequency coordination control system in the cases of current converter station fault or other station faults is analyzed. The actual effect of a frequency coordination control system on a converter station in East China power grid is investigated by testing the actual power, voltage and current variation of a conventional DC transmission system.

As the wind power generation technique is improved greatly, permanent magnet synchronous generator (PMSG) is paid more and more attention because of its multiple advantages. Nevertheless, conventional directly-drive PMSG wind generation system is connected to the grid by AC transmission. This paper introduces the DC connection structure of wind farm based on PMSG. According to VSC-HVDC (Voltage Source Converter based HVDC) transmission, the topology of series-parallel DC generation units is proposed. Regarding to the control strategy of the power generation and transmission system mentioned above, the DC voltage is controlled on generator-side for achieving the purpose of seriesparallel, and on grid-side active and reactive power decoupling method is applied to track maximum wind power by controlling the d-axis and q-axis current of the grid. The models of singlegenerator, series connection and parallel connection system are established on the PSCAD/EMTDC platform. Simulation results prove the validity of the topology scheme and the corresponding control strategy.

As the core control unit, the pole control unit of converter station determines the performance of the whole HVDC system. Aimed the back-to-back converter station, the control strategies are analyzed and discussed in this paper; characteristic curve for converter station control is patterned. On the basis of above work, the pole control unit is designed. Through the fast control for fire angle of converter valves and the slow control for taps of converter transformer, the projected control strategies are implemented. At last, the simulation results verified that this designed pole control unit can realize expected functions.

In this study, a novel droop control method for ac microgrids is proposed to enhance the performance of power regulation, which is composed of three parts. The angle droop and the frequency droop are adopted to control the active power in coordination, while the modified voltage droop is used to control the reactive power. There are several improvements in this proposed triple droop control method compared with the conventional droop control. First, the dynamics of active power regulation are enhanced because of the angle droop. Second, the stability of the system is upgraded and the droop coefficients can vary in greater range. Third, the accuracy of reactive power regulation is improved, especially in the grid‐connected mode, the DGs can output the specified reactive power with the modified voltage droop, which benefits the economic dispatch of the microgrid. Through these three combined droop control strategies, the performance of the whole system can be improved a lot whether in the grid‐connected mode or the islanded mode. Furthermore, the stability of the proposed triple droop control is analysed, which shows that the stability of the system is upgraded. The effectiveness of the proposed control method is verified by the hardware‐in‐loop tests.