Power Flow Control in Multi-Terminal HVDC Grids Using a Serial-Parallel DC Power Flow Controller

© 2013 IEEE. Multi-terminal HVDC (MT-HVDC) grids have no capability of power flow control in a self-sufficient manner. To address this important issue, utilization of dc-dc high power and high-voltage converters is motivated. However, proposing suitable partial-rated dc-dc converters as well as their suitable modeling and control in both primary and secondary control layers as well as the stability analysis are the existing challenges that should be alleviated beforehand. This paper addresses the control of power flow problem through the application of a power converter with a different connection configuration, namely, serial parallel dc power flow controller (SPDC-PFC). The SPDC-PFC input is the transmission line voltage, and its output is transmission line current. Therefore, employing a full-power dc-dc converter is avoided as a merit. Additionally, in this paper, the common two-layer MT-HVDC grid control framework comprised of primary and secondary layers is efficiently modified in order to integrate the SPDC-PFC. A differential direct voltage versus active power droop control scheme is applied to the SPDC-PFC at the local control layer, guaranteeing dynamic stability, while an extended dc power-flow routine - integrating the SPDC-PFC - is developed at the secondary control layer to ensure the static stability of the entire MT-HVDC grid. The proposed control framework enables the SPDC-PFC to regulate the flow of current/power in the envisioned HVDC transmission line. From the static and dynamic simulation results conducted on the test CIGRE B4 MT-HVDC grid, successful operation of the proposed SPDC-PFC and control solutions are demonstrated by considering power flow control action. In more detail, the SPDC-PFC successfully regulates the compensated lines' power to the desired reference both in static and dynamic simulations by introducing suitable compensation voltages. In addition, good dynamic performance under both SPDC-PFC power reference and wind power-infeed change is observed

Static Modeling of the IDC-PFC to Solve DC Power Flow Equations of MT-HVDC Grids Employing the Newton-Raphson Method

© 2019 IEEE. Power transmission technology of the offshore wind farm (OWF)s is usually based on HVDC interconnection. Power flow controller (PFC)s are flexible power transmission devices which play important role in the DC power flow (PF) control especially in contingency conditions. So, these devices should be modeled to solve related MT-HVDC grid DC PF equations. In this context, an interline DC PFC (IDC-PFC) is considered as a sample PFC for modeling due to its advantages in comparison to other series and cascaded PFCs. The novelty of this work is solving the DC PF problem of the IDC-PFC compensated MT-HVDC grids by modeling of the IDC-PFC and employing the Newton-Raphson (N-R) method. An eight-bus MT-HVDC test grid is considered to authenticate the presented IDC-PFC modeling and verify the accuracy of the DC PF results obtained by employing the N-R method. The obtained static results verify the accuracy of the presented IDC-PFC modeling and the performance of the N-R method in solving flexible MT-HVDC grid PF equations. Hence, it is suitable to integrate them in the future power system analysis softwares

Power injection model of IDC-PFC for NR-based and technical constrained MT-HVDC grids power flow studies

© 2020 Elsevier B.V. Insufficient control flexibility in multi-terminal HVDC (MT-HVDC) grids is an important motivation to install suitable power electronic-based DC power flow controller (PFC)s to ensure grid controllability, security, and reliability. This article proposes a new static power injection model (PIM) for those (interline) DC-PFCs to enable DC power flow (PF) studies and ease integration of the PFCs in the power system analysis softwares within the well-accepted Newton–Raphson (NR) solver-based framework. HVDC lines shunt conductances are also taken in to account in this newly developed paradigm. For this purpose, the IDC-PFC, as well as other MT-HVDC grid physical/control state variables are modified in cooperation to attain predefined control objective(s). Furthermore, a novel general routine (solution procedure) is proposed to handle several system physical/control limitations during the NR-based DC PF problem solution. Static/dynamic simulations are executed on an eight-bus test MT-HVDC grid to show/confirm the accuracy, effective performance, and excellent convergence property of the proposed IDC-PFC model, NR-based DC PF solver, and technical constraints handling routine. In this situation, it is proved that the original structure and symmetry of the admittance matrix can still be kept, and a few modifications are needed to be done in the Jacobin matrix

Optimal placement and control variable setting of power flow controllers in multi-terminal HVDC grids for enhancing static security

© 2018 This research proposes an approach to select an optimal place and control variable setting for recently proposed Power Flow Controller (PFC)s including series, cascaded, and interline PFCs in Multi-Terminal High-Voltage DC (MT-HVDC) grids based on sensitivity analysis technique to enhance static security. To do so, appropriate static power injection models of the PFCs are obtained. Accordingly, sensitivity relationships between the defined power performance index and each PFCs control variable are obtained. It is for the optimal PFCs placement purpose in order to utilize maximum capacities of the transmission grid. Optimal settings for the PFC as well as the controllable voltage source converters are computed in turn by applying sequential quadratic programming solver to the developed security-based DC optimal power-flow problem which includes several corrective constraints. The study scope is single contingencies affecting HVDC lines with the objective of eliminating the consequent overloaded HVDC lines and thereby enhancing the MT-HVDC grid security by controlling the power flows within the MT-HVDC grid. Static simulations are performed considering two generic four-terminal and eighth-terminal MT-HVDC test grids in order to demonstrate the proposed methods effectiveness and authenticate its robustness. The obtained results indicate that the MT-HVDC grid can remain secure under single HVDC line contingencies in most cases by implementing the proposed method

Analytical modeling and inertia estimation of VSG-controlled Type 4 WTGs: Power system frequency response investigation

© 2018 This paper develops a model to study the inertial dynamics of single Type 4 Wind Turbine Generator (WTG)s that emulate the response of grid-friendly Virtual Synchronous Generator (VSG)s. In this vein, a simplified linearized model is developed for the WTG in the electromechanical time scale considering complete WTG dynamics to obtain/investigate the power System Frequency Response (SFR) characteristics. The objectives are to obtain virtual frequency motion equation, present the mechanism of inertia provision clearly, and feature physical interpretations of inertial frequency response. Accordingly, the available inertia constant and damping coefficient are estimated/expressed analytically in frequency-domain that synthetically embed the impacts of key control parameters as well as the initial operating points. It is realized that the available inertia constant and damping coefficient are adjustable and frequency dependent in contrast to that's manifested by a conventional synchronous generator. The developed virtual frequency motion equation is used in turn to establish/investigate the complete SFR model of a generic test power grid which features enormous wind power generation capacity factor. The theoretical analysis shows that the SFR attributes could get adjusted/enhanced by proper tuning of the WTG control parameters. Comprehensive and comparative time domain simulations are also performed in the MATLAB/Simulink platform. It is to verify the accuracy of the proposed models in the evaluation of SFR characteristics and authenticate the theoretical analysis