Control strategy for direct voltage and frequency stabilityenhancement in HVAC/HVDC grids

Direct voltage fluctuations due to the presence of relatively large DC reactors (as an essen-tial part of HVDC breakers), lack of inertia, and unwanted frequency fluctuations in theAC side of HVDC grids, have major consequences on the stability of HVAC/HVDC grids.The use of the DC Power System Stabilizer (DC-PSS) can damp and eliminate voltageoscillations caused by the presence of the DC reactors. However, DC-PSS cannot addressthe issues of inertia and unwanted frequency fluctuations. A method to improve inertiais proposed here that can operate well with the droop controller, and DC-PSS does notinterfere with power-sharing and does not interact with any of these elements. Since thepresence of a droop controller in HVAC/HVDC grids associates with power and directvoltage, the method proposed here can improve direct voltage fluctuations by eliminatingsevere power peaks. Moreover, this method does not change the voltage level of the entiresystem, so there is no need to change the set-points of controllers. In addition, all param-eters of the controllers are tuned by an intelligent algorithm, and the Participation factor(PF) scheme is used to find the proper placement of the proposed controller

An improved droop-based control strategy for MT-HVDC systems

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This paper presents an improved droop-based control strategy for the active and reactive power-sharing on the large-scale Multi-Terminal High Voltage Direct Current (MT-HVDC) systems. As droop parameters enforce the stability of the DC grid, and allow the MT-HVDC systems to participate in the AC voltage and frequency regulation of the different AC systems interconnected by the DC grids, a communication-free control method to optimally select the droop parameters, consisting of AC voltage-droop, DC voltage-droop, and frequency-droop parameters, is investigated to balance the power in MT-HVDC systems and minimize AC voltage, DC voltage, and frequency deviations. A five-terminal Voltage-Sourced Converter (VSC)-HVDC system is modeled and analyzed in EMTDC/PSCAD and MATLAB software. Different scenarios are investigated to check the performance of the proposed droop-based control strategy. The simulation results show that the proposed droop-based control strategy is capable of sharing the active and reactive power, as well as regulating the AC voltage, DC voltage, and frequency of AC/DC grids in case of sudden changes, without the need for communication infrastructure. The simulation results confirm the robustness and effectiveness of the proposed droop-based control strategy

Power Management and Protection in MT-HVDC Systems with the Integration of High-Voltage Charging Stations

Due to the significant increase of the long-distance electricity demand, effective use of Distributed Generations (DGs) in power system, and the challenges in the expansion of new transmission lines to improve the reliability of power system reliability, utilizing Multi-Terminal HVDC (MT-HVDC) technology is an applicable, reliable, and cost-effective solution in hybrid AC/DC grids. MT-HVDC systems have flexibility in terms of independent active and reactive power flow (reversible control) and voltage control. Interconnecting two AC grids with different frequencies and transmitting electricity for the long-distance with low power-losses, which leads to less operation and maintenance costs, can be done through the MT-HVDC systems. The integration of large-scale remote DGs, e.g., wind farms, solar power plants, etc., and high-voltage charging stations for Electric Vehicles (EVs) into the power grid have different issues, such as economic, technical, and environmental challenges of transmission and network expansion/operation of both AC and DC grids. In details, damping oscillation, voltage support at different buses, operation of grid-connected inverters to the off-shore and on-shore AC systems, integrating of existing converter stations in MT-HVDC systems without major changes in control system, evaluation of communication infrastructure and also reactive power and filtering units’ requirements in MT-HVDC systems are the technical challenges in this technology. Therefore, a reliable MT-HVDC system can be a possible mean of resolving all the above-mentioned issues. MT-HVDC systems need a control system that can bring stability to the power system during a certain period of the operation/planning time while providing effective and robust electricity. This thesis presents an improved droop-based control strategy for the active and reactive power-sharing on the large-scale MT-HVDC systems integrating different types of AC grids considering the operation of the hybrid AC/DC grids under normal/contingency conditions. The main objective of the proposed strategy is to select the best parameters of the local terminal controllers at the site of each converter station (as the primary controller) and a central master controller (supervisory controller) to control the Power Flow (PF) and balance the instantaneous power in MT-HVDC systems. In this work, (1) various control strategies of MT-HVDC systems are investigated to propose (2) an improved droop-based power-sharing strategy of MT-HVDC systems while the loads (e.g., high-voltage charging stations) in power systems have significant changes, to improve the frequency response and accuracy of the PF control, (3) a new topology of a fast proactive Hybrid DC Circuit Breaker (HDCCB) to isolate the DC faults in MT-HVDC grids in case of fault current interruption. The results from this research work would include supporting energy adequacy, increasing renewable energy penetration, and minimizing losses when maintaining system integrity and reliability. The proposed strategies are evaluated on different systems, and various case scenarios are applied to demonstrate their feasibility and robustness. The validation processes are performed using MATLAB software for programming, and PSCAD/EMTDC and MATLAB/Simulink for simulation

A Comprehensive VSG-Based Onshore FRT Control Strategy for OWFs with VSC-MT-HVDC Transmission

This paper proposes a communication-free control strategy at the offshore wind farm (OWF) level to enhance onshore fault ride-through (FRT) grid code compliance of the voltage source converter (VSC)-based multi-terminal high voltage direct current (MT-HVDC) grid. In this proposal, the emerging virtual synchronous generator (VSG) concept is employed to equip the Type 4 wind turbine generator (WTG)s with inherent grid forming ability. Accordingly, it is proposed to switch the offshore HVDC converters control mode from grid forming to grid feeding during onshore FRT period to realize direct wind power in-feed reduction as a function of the severity of MT-HVDC grid's overvoltage. The related dynamics are mainly characterized by the high-speed current control loop, so improved OWF response is achieved during onshore FRT period as conventional voltage/frequency modulation strategies are not employed. New analysis/amendments are also proposed to study and improve the transient active power reduction sharing between the WTGs in first few power cycles under wind wake effect. Finally, with the objective of a smooth transfer of HVDC converters and WTGs in several proposed operation states, a set of state machines are proposed considering whole WTG's dynamics. Comprehensive time-domain simulations are performed with averaged electromagnetic transient models to demonstrate the improved onshore FRT behavior in terms of minimizing the electrical stress at both MT-HVDC grid and OWF levels

HVDC grids stability improvement by direct current power system stabilizer

High-voltage direct current breaker is among the essential components of high-voltage direct current grids. Such a breaker generally needs a direct current reactor to reduce the fault currents rate. However, direct current reactors have destructive effects on the multi-terminal high-voltage direct current grid dynamic stability, and in such a system, despite the variety of controllers, the system dynamics are highly sensitive to the operating point. Therefore, additional damping control will be needed. This paper proposes a modification to be applied to the traditional droop controller of high-voltage direct current grids to cope with the influence of these large reactors, improving the direct voltage stability and decreasing power variations in the transient events by introducing a direct current power system stabilizer. The proposed method for direct voltage control has been investigated through the analytical model of the system. Stability improvement has been studied following the application of the proposed method by investigating zeros, poles, and frequency response analysis. Moreover, a method is proposed for optimal design and optimal placement of direct current power system stabilizer. The system analysis and time-domain simulations demonstrate a decent damping improvement attained by the proposed method. All simulations and analytical studies are conducted on Cigré DCS3 test high-voltage direct current grid in MATLAB/Simulink

Comparative stability analysis of droop control approaches in voltage-source-converter-based DC microgrids

Droop control has been widely applied in DC microgrids (MGs) due to its inherent modularity and ease of implementation. Among the different droop control methods that can be adopted in DC MGs, two options have been considered in this paper; I-V and V-I droop. I-V droop controls the DC current depending on the DC voltage whilst V-I droop regulates the DC voltage based on the output current. The paper proposes a comparative study of V-I/I-V droop control approaches in DC MGs focusing on steady-state power sharing performance and stability. The paper presents the control scheme for current-mode (I-V droop) and voltage-mode (V-I droop) systems, derives the corresponding output impedance of the source subsystem including converters dynamics and analyzes the stability of the power system when supplying constant power loads. The paper investigates first the impact on stability of the key parameters including droop gains, local control loop dynamics and number of sources and then performs a comparison between current-mode and voltage-mode systems in terms of stability. In addition, a generalized analytical impedance model of a multi-source, multi-load power system is presented to investigate stability in a more realistic scenario. For this purpose, the paper proposes the concept of “global droop gain” as an important factor to determine the stability behaviour of a parallel sources based DC system. The theoretical analysis has been validated with experimental results from a laboratory-scale DC MG

Design, analysis and control of DC/DC converter based DC wind farms

This thesis discusses the design, operation and control of DC wind farms that use high power DC/DC converters, DC cables and DC collection networks. DC wind farms are proposed as alternatives to traditional AC wind farms due to the potential to reduce the system size, improve the speed of dynamic response and improve the system efficiency. DC wind farms involve different types of high-power DC/DC converters in different stages of power conversion. Isolated DC/DC converters are chosen as the wind turbine converters in which the intermediate transformer design is of great importance. A general and comprehensive medium frequency transformer modelling and design methodology is presented in this thesis, which considers the efficiency, leakage inductance and thermal management. The proposed methodology is applied to transformers for single phase and three phase DC/DC converters. Isolated Single Active Bridge DC/DC converters are appealing topologies for medium voltage applications. The operation of these DC/DC converters is complex and important for the converter control design. The comprehensive operational principles of three-phase single active bridge converters under changing duty cycle are investigated. Eight operating modes are identified with detailed derivation of power flow and current dynamics. The converter performances are evaluated and compared theoretically and experimentally. Then, the control of wind turbine converters in DC wind farms is designed considering both DC-link and network dynamics. To deal with the oscillations caused by smoothing reactors, a power system stabilizer based control design is developed and implemented. Furthermore, a multi-variable feedback control design using pole-placement technique is proposed. This method is able to achieve the minimum oscillatory time without compromising the dynamic performance of the DC-link voltage. Finally, taking into account the low capacitance issue in wind farms, the voltage stability of DC wind farms is investigated and different stabilizing methods are designed and analyzed. The impedance models of aggregated wind turbine converters, DC cables and the station DC/DC converter with control action are derived, in order to study the interactions between the station converter and the DC wind farm. A new equivalent capacitor control strategy to enhance the system capacitance is proposed and analyzed through various case studies.Open Acces