Secondary control in multi-terminal VSC-HVDC transmission system

Multi-terminal VSC-HVDC transmission is a promising solution to transport energy from the generating plants to the distribution stations, and the number and size of the corresponding power grids is expected to increase in the next years. Such a system presents challenging control problems that, so far, have been approached with classical control techniques.
\nHowever, large scale systems of interconnected nodes
\nfall within the framework of the emerging field of complex
\nnetworks. This paper analyzes the stability of multi-terminal
\nVSC-HVDC systems from a complex dynamical network perspective, and provides sufficient conditions to ensure bounded
\nsynchronization of its trajectories. The obtained results are
\nvalidated via numerical simulations.

In multi-terminal VSC-HVDC system, The most important prerequisite to ensure stable operation of the system is to maintain stability of the DC voltage, a coordinated power control strategy applies to multi-terminal VSC-HVDC system is proposed in this paper. The proposed control strategy can make the converter stations controlled by multiple fixed-point DC voltage distribute the power reasonably according to the capacity of station converters and the settings of operators, eliminating the DC voltage offset caused by change of load power, and also eliminating the adverse impact of DC line resistance on the accuracy of converters power distribution. Finally, the simulation is carried out on a three terminal physical prototype and also testified on a three terminal HVDC system in PSCAD/EMTDC, the effectiveness and validity of the proposed control strategy are verified.

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.

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HVDC (High Voltage DC) transmission systems are believed to be a key to integrate more renewable energy sources, especially from remote area and offshore environment, into power systems. In this paper, a steady state control strategy that can be used in a meshed HVDC grids is presented. The control strategy combines local primary controllers and a central secondary or supervisory controller. The objective of the primary controllers is to enable shared management of required changes in bus powers as a result of a disturbance in the system. The objective of the secondary controller is to change voltage and power settings of the primary controllers so that power flows in the transmission cables of the system are kept within the thermal limits after the disturbance. The primary controllers react first when there are disturbances in the system followed by the secondary controller which acts only when necessary.

Multi-Terminal VSC based HVDC (MTDC) is promising anticipation to transmit and integrate large offshore wind energy to on shore grids. This paper interprets the use of MTDC technology, seven terminal systems consisting of unanimously different local control for each VSC for integrating large offshore wind farms located hundreds of kilometer far from shore. Operating principal of proposed system has been described along with control of every VSC to accredit the solemn working and power sharing between various AC connected networks. EMTDC/PSCAD is used to simulate the seven terminal MTDC system, it consists of two different offshore wind farms connected to five different AC onshore grids through VSC's to exhibit the robust performance against power variation under load and faulty conditions. Local control schemes to the hilt have been implemented to study the behavior of VSC for aggregating offshore wind farms to onshore network.

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.

Lyapunov-based distributed secondary frequency and voltage control for distributed energy resources in islanded microgrids with expected dynamic performance improvement

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.

This paper explains a hybrid control approach to model a controller for voltage source converters (VSCs) in offshore wind farm (OSWF) applications, where the OSWFs are integrated to AC grid through the multi-terminal high voltage direct current (MT-HVDC) transmission system. Proper DC link voltage provides the effective power dispatch. So, a constant DC link voltage should be maintained to achieve the effective power dispatch between OSWFs and AC grid. A new control approach is a compound of proportional-integral and sliding mode control to regulate the DC link voltage on the grid side VSC. Also, the proposed control approach can control AC voltage and reactive power on wind farm and grid side VSCs respectively. Time- domain simulations executed in MATLAB/Simulink software are used to verify the proposed and conventional control approaches. Transient stability analysis is carried out for a case study. The three-terminal VSC-HVDC system has taken for study, where two OSWFs are fed to AC grid.

The extension and size of the power grid is expected to increase in the near future. Managing such a system presents challenging control problems that, so far, have been approached with classical control techniques. However, large scale systems of interconnected nodes fall within the framework of the emerging field of complex networks. This paper models multi-terminal VSC-HVDC systems as a complex dynamical network, and derives conditions ensuring bounded synchronization of its trajectories for a family of controllers. The obtained results are validated via numerical simulations.

In order to achieve the flexible and efficient utilization of distributed energy resources, microgrids (MGs) can enhance the self-healing capability of distribution systems. Conventional primary droop control in microgrids exhibits deviations in voltage and frequency and lacks research on voltage–frequency control during network reconfiguration. Therefore, this paper investigates the control strategy of secondary control for voltage and frequency during the process of reconstructing distribution networks to operate in the form of microgrids after faults. Firstly, the mathematical model of three-phase voltage-source inverters with droop control is analyzed. Then, inspired by the generator’s secondary frequency control strategy, an improved secondary control method based on droop control is proposed, and the effectiveness of the improved droop control strategy is verified. Finally, simulation examples of distribution networks with various dynamic boundary topology changes are designed to simulate a relatively stable control result, validating the proposed strategy.

This paper presents a smart DC network formed by a multi-terminal VSC-HVDC connecting various micro-grids, independent power generation stations, and passive loads. The corresponding deadbeat current control scheme based on coordinated strategy for multi-terminal VSC-HVDC systems is proposed. The current control scheme is easy to design, implement and has good dynamic response. the case of four-terminal VSC-HVDC interconnected with AC grids, wind farms, and passive loads is developed in MATLAB/Simulink to demonstrate the validity of the proposed control scheme and the united system. Simulation results have demonstrated that the proposed scheme enables optimal operation under conditions of voltage dip, power flow and load changes when used for a large DC smart network.

Topology of DC energy dissipation device based on dual capacitor five-level sub-module and energy-power dual closed-loop control strategy

The hierarchical control strategy enables seamless integration of power electronics-based distributed generators (DGs) into microgrids (MGs). This work presents a distributed coordinated secondary voltage and frequency control strategy of islanded MGs, utilizing distributed model predictive control (DMPC) approach. The Proposed approach is entirely distributed in nature, with plug and play features and requires minimal communication network with information exchange between immediate neighbors. This technique facilitates efficient information exchange and obviates the need for centralized control. Droop control is the primary controller for all the DGs, which leads to the deviations in DG voltage and operating frequency from standard nominal values. Secondary control enables the restoration of DG voltage and frequency to their nominal values. Input–output feedback linearization (IOFL) is used to transform nonlinear dynamics of inverter-interfaced DGs into first-order linear dynamics between input and output. Compared to some published works, the single-integrator dynamics eliminates the need to compute the derivative of the output voltage. Furthermore, DMPC has been used for all DG units as secondary controller, to achieve voltage and frequency regulation. This approach takes advantage of distributed architecture with optimal control actions and enhances reliability and flexibility of MG operations. Moreover, the proposed approach ensures that voltage and frequency of overall system are restored to its reference value while maintaining accuracy in real power sharing. Effectiveness of proposed control scheme is validated with simulations conducted on islanded MG test system using MATLAB/Simulink toolbox.