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This paper develops a new control strategy for voltage source converter (VSC) based multi-terminal high voltage dc (MT-HVDC) grids for enhancing the mutual frequency support between MT-HVDC interconnected asynchronous ac systems. The existing droop control structure of VSCs is augmented with an additional feedback voltage-based loop that generates a voltage shifting term to be added to the original dc voltage reference. The addition of this integrator-based controller nullifies the difference between voltage errors seen at adjacent converter stations. This action helps to overcome inaccuracies in the mutual frequency support caused by the uneven voltages across a dc system. The proposed strategy uses only local variables for the continuous-time control; thus, it does not rely on a high-bandwidth communication network, and its performance is not affected by the change of ac systems’ parameters. Modal analysis is carried out to investigate the stability of the proposed controller. Finally, comprehensive simulation studies are used to verify the proposed strategy’s performance and compare it with recently reported alternative controllers. [ABSTRACT FROM AUTHOR]

Line current differential relays (LCDRs) detect faults accurately and promptly, by comparing all currents flowing into the line. This type of relay has been identified in the literature as a reliable protection for lines in DC microgrids (MGs). LCDRs, however, lack the required resiliency against cyber intrusions, such as false data injection attacks (FDIAs) and time synchronization attacks (TSAs), due to their high dependence on communication infrastructure and/or the Global Positioning System (GPS). This paper first introduces coordinated attacks—i.e., several almost-simultaneous FDIAs or TSAs that are carried out independently to achieve a specific objective—as potential threats for MGs. Then, through a case study, it shows how coordinated FDIAs and TSAs can initiate a sequence of events that result in instability of an entire MG. Afterwards, an approach is presented to detect FDIAs and TSAs, and to distinguish them from real faults. The proposed method is comprised of passive oscillator circuits (POCs) installed in series with each converter. During faults, the resultant RLC circuit causes the POCs to resonate and generate a damped sinusoidal component with a specific frequency, i.e., $f_{d}$. However, $f_{d}$ is not generated during FDIAs and TSAs, since unlike faults, which are physical events that trigger the natural frequencies of a system, cyber-attacks happen in the cyber layer without provoking natural frequencies of the physical layer. Thus, an LCDR pickup without detecting $f_{d}$ denotes an FDIA or a TSA. Since $f_{d}$ is locally measured and analyzed by each LCDR, the proposed detection approach cannot be targeted by cyber-attacks. The proposed method is evaluated on a simulated ±2.5 kV DC MG. Numerical analysis confirms that the proposed method (i) is system-independent; (ii) detects FDIAs and TSAs in less than 1 ms; (iii) is sensitive to high-resistance faults; (iv) can determine fault types, and (v) reduces faults’ peak currents. [ABSTRACT FROM AUTHOR]

The load sharing in the DC microgrids is affected by the practical factors such as the cable resistances and changes in the system topology. This paper presents a new control algorithm for the application in the DC microgrids to achieve accurate load distribution between droop controlled converters. An identical steady state voltage feedback is used for all droop controlled converters to ensure accurate current sharing. The information about the common voltage is obtained by means of the proposed communication-less control strategy. The proposed strategy does not require prior information about the grid topology and parameters. Therefore, it is applicable to both single- and multi-bus microgrids. The conducted stability analysis demonstrates that the proposed approach and the conventional droop control deomstrate similiar dynamic performance. Furthermore, the proposed algorithm has a potential to enhance the operation of the Interlinking Converters in the hybrid AC/DC microgrids. A comprehensive nonlinear simulation study is conducted in the Matlab/Simulink environment. The simulation results verify the effectiveness of the proposed communication-less control scheme to enable proper load distribution between droop controlled converters as well as enhance the load sharing between AC and DC microgrids. Finally, the proposed control approach is verified using an OPAL-RT setup. [ABSTRACT FROM AUTHOR]

This paper presents a new control strategy for voltage source converter based Multi-Terminal High Voltage Direct Current (MTDC) systems. The proposed control approach ensures accurate power sharing between the droop-controlled converter stations. By communicating the power-sharing index between neighboring converters, the proposed approach achieves exact droop control operation independent of the DC system topology and line parameters. The pilot voltage droop based controller, which is an alternative communication-based approach for achieving precise power sharing, was used as a base case for comparison. Modal analysis is carried out to reveal the sensitivity of the system's eigenvalues to the changes in control parameters (e.g., power droop gain, proportional integral gains of the proposed controller) and the latencies in the communication. It is demonstrated that the proposed strategy remains in the stable operation even when excessive latencies are encountered in the communication. Nonlinear simulations are conducted in the MATLAB/Simulink environment in four- and five-terminal MTDC grids, validating the capability of the proposed controller to achieve the desirable performance under various operational conditions. [ABSTRACT FROM AUTHOR]

This paper presents an improved pilot voltage droop based control for the multiterminal high-voltage dc (MTDC) systems. The proposed control strategy maintains adequate power sharing and efficient voltage regulation among the converter stations. The incorporation of the average voltage sharing loop enables achieving superior dynamic performance while avoiding dc system average voltage deviation from nominal value. Furthermore, the frequency consensus algorithm is used for modifying the power reference of converters to achieve the desired frequency deviation sharing between the ac areas connected through the droop controlled voltage source converters. Finally, the incorporation of the frequency locked loop enables achieving fast grid synchronization and superior dynamic performance for the control of the converter stations. The simulations carried out in the MATLAB/Simulink environment revealed the effectiveness of the controller to provide enhanced dc voltage regulation and frequency deviation sharing during disturbances in the MTDC system. [ABSTRACT FROM AUTHOR]

Voltage source converter (VSC) based high voltage direct current (HVdc) transmission draws more attention in recent years for the grid connection of large-scale offshore wind farms (WF). This paper proposes nine switch converter (NSC) based configuration and control strategy for the VSC-HVdc connected offshore WFs to enhance the fault ride through operation of the system. The configuration allows having both shunt and series interfaces to the onshore grid, which allows the isolation of the faulty part of the network, continuous power delivery to the healthy portion, and significant reduction of the fault current. Reduced switch count NSC is utilized at the onshore station to provide simultaneous shunt and series compensation to the electric grid. The proposed control strategy ensures full power evacuation and the dc-link voltage regulation during network disturbances. The comparative analysis between the proposed and conventional VSC-HVdc systems carried out in MATLAB/ Simulink environment revealed the former's enhanced transient performance. [ABSTRACT FROM PUBLISHER]

This paper proposes a new fault ride-through (FRT) configuration for a fixed-speed induction generator-based–wind turbine (FSIG–WT). It utilizes a nine-switch converter (NSC) to enable shunt and series compensation in response to the system dynamics and grid faults. The steady-state control strategy and transient-management scheme are developed to ensure proper performance in steady-state, dynamic operation as well as enhanced FRT capability of the FSIG–WT. In addition to that, the developed control strategy tackles the balanced and unbalanced operation of the electric grid with an enhanced positive- and negative-sequence compensation control scheme. The recently reported grid synchronization scheme, namely, the frequency-locked loop (FLL) is implemented to achieve fast phase detection and transient response. Consequently, the performance of the FLL has been compared with the multiple reference–frame–phase-locked loop (MRF–PLL) to demonstrate its superior performance for enhancing transient response. The proposed topology, FLL, and control strategies are verified through a comprehensive simulation study to demonstrate the enhancement of the transient response and FRT capability of FSIG-WT in adherence to grid-code requirements. [ABSTRACT FROM PUBLISHER]