This paper proposes three-phase steady-state models for distributed generators (DGs) with different negative control strategies. An augmented rectangular load flow method is used for three-phase distribution systems. For two types of DGs with power electronic devices: photovoltaic power generators and doubly-fed induction generators, the nodal voltage equations and output power equations are derived. Moreover, under unbalanced conditions, the strategies for mitigating the effects caused by negative sequence components are formulated as functions of the nodal voltage and the injected current. Furthermore, the models are incorporated into an augmented rectangular load flow method with nodal voltage and injected current as state variables in three-phase systems. Case studies are conducted to verify the correctness and accuracy of the proposed models by comparing the results obtained from MATLAB and PSCAD.

This paper proposes a novel primary frequency regulation (PFR) scheme for Multi infeed HVDC (MIDC) system. First, a dynamic model for frequency response (FR) in MIDC system is carried out to involve LCC-HVDCs in frequency regulation. Then, a disturbance observer (DO) is proposed to estimate both the power imbalance and the full order states of the system. By combining a Kalman filter (KF), this DO gets an accurate estimation quickly with local frequency measurement. On the basis of the estimated disturbance and state variables, feedforward control schemes can be concluded to fully utilize the fast adjustability of LCC-HVDCs. In this paper, an model predictive control (MPC) based scheme is designed to coordinate LCC-HVDCs under constraints of HVDC capacity and frequency boundaries. As a result, the optimal power-sharing property of LCC-HVDCs is achieved to enhance the PFR dynamic performance of the AC main system. Further, the stability of the proposed method is analyzed using Lyapunov method. Finally, numerical simulations are concluded to verify the effectiveness and the merits of the proposed method, by comparing it to typical droop controls.