This paper proposes a multi-option power flow approach for hybrid AC/DC grids. A unified AC-DC unit is introduced which can be used in two different cases: case a) AC grids with embedded VSC-based MTDC grids, and case b) asynchronous AC grids connected via a common VSC-based MTDC grid. In the proposed method for each MTDC grid (regardless of the number of converters), a new state variable is introduced to handle any kind of converter loss models. For the case (b), the AC-DC unit can be used in two different analyses, namely, a1) the separated analysis and a2) the integrated analysis. Both a1) and a2) can be used in the practical analysis of the real-size power systems. However, it is shown that the separated analysis not only offers a shorter computational time but it is also very suitable for the future connection between large existing AC systems and other AC systems or remote renewable energy sources through the DC grids. The proposed methodology is implemented in MATLAB software and verified using commercial software SIMPOW.

A Multi-Option Unified Power Flow Approach for Hybrid AC/DC Grids Incorporating Multi-Terminal VSC-HVDC

Commutation failures, which may lead to outage of HVDC systems, usually occur in line-commutated HVDC systems, especially in the inverter stations. As a method to reduce the risk of commutation failures, additional angle deducted from the firing order at the inverter station has its inherent limitations. In this paper, by analyzing the inverter transient process of normal commutation and commutation failure especially, it is shown that direct-current control can be used to inhibit the commutation failures and present an improvement strategy of control system on the inverter side. Based on the analysis of commutation failure prevention (CFPREV), an approach of direct current value setting is proposed, the selections of parameters in calculating process are CFPREV-based in order to implement it conveniently, and the dc current predictive control strategy is applied by modifying the conventional control system at the inverter. The validity is proved by simulation of the CIGRE HVDC model and multi-infeed HVDC model based on CIGRE HVDC, where various ac system faults in PSCAD/EMTDC are set manually. The results show that the proposed algorithm is very effective in inhibiting commutation failures or repetitive ones, and even would enhance fault-ride-through capability of HVDC system with commutation failures.

Direct-Current Predictive Control Strategy for Inhibiting Commutation Failure in HVDC Converter

A promising configuration for future smart grids is an AC/DC hybrid topology that enables the integration of AC/DC energy resources and modern loads, thus permitting the consequent formation of AC/DC hybrid microgrids (HMGs). An understanding of AC/DC HMGs and their operational premise during islanding will certainly pave the way toward the realization of a future smart grid that includes a plug-and-play feature. However, the planning and operation of such isolated and hybrid systems are reliant on a powerful and efficient power flow tool. To this end, this paper proposes a novel unified, generic, and flexible power flow algorithm for isolated AC/DC HMGs. The power flow subproblems related to AC and DC subgrids are described mathematically by a set of linear and nonlinear equations and are solved simultaneously using a Newton trust-region method. The proposed algorithm is generic in the sense that it includes consideration of the unique characteristics of islanded AC/DC HMGs: a variety of possible topologies, droop controllability of the distributed resources (DRs), and bidirectionality of the power flow in the interlinking converters (ICs). The new power flow formulation is flexible and permits the easy incorporation of any changes in DR operating modes and IC control strategies. The developed algorithm was tested and applied for analyzing selected operational and control aspects of isolated AC/DC HMGs, including inaccurate power sharing and interlinking converters characterized by differing control strategies. The proposed load flow program can form the basis of and provide direction for further studies of isolated AC/DC HMGs.

A Unified Approach to the Power Flow Analysis of AC/DC Hybrid Microgrids

This paper proposes a unified load flow (LF) model for AC–DC hybrid distribution systems (DSs). The proposed model can be applied in hybrid DSs with mixed configurations for AC/DC buses and AC/DC lines. A new classification of DS buses is also introduced for LF analysis. A set of generic LF equations has been derived based on comprehensive analysis of the possible AC–DC hybrid system configurations. Three binary matrices, which are used as a means of describing the configuration of the AC and DC buses and lines, have been employed in the construction of the unified power equations. These matrices enable a single configuration at a time to be activated in the power equations. The proposed LF model is generic and can be used for both grid-connected and isolated hybrid DSs. The new model has been tested using several case studies of hybrid DSs that include different operational modes for the AC and DC distributed generators. As a means of evaluating the effectiveness and accuracy of the proposed model, the LF solution was compared to the solution produced by PSCAD/EMTDC. A comparison of the results reveals the efficacy of the proposed model.

A Generalized Approach to the Load Flow Analysis of AC–DC Hybrid Distribution Systems

The proliferation of technologies operating on dc power has motivated the system planners toward integration of dc and ac grids. The optimal power flow (OPF) analysis is widely used to determine the economically efficient operating points of the power grids. The OPF problem in ac–dc grids is a non-convex optimization problem due to the nonlinear power flow equations and the operating constraints imposed by the ac–dc converters. In this paper, we study the OPF problem in ac–dc grids to address the non-convexity of the problem. The objective of the ac–dc OPF problem is to jointly minimize the generation cost and the losses on the lines and converters. The optimization problem is subject to the ac and dc power flow constraints, the limits of the voltages and line flows, and the operating limits of the converters. We use convex relaxation techniques and transform the problem to a semidefinite program. We derive a sufficient condition for zero relaxation gap to obtain the global optimal solution. Simulations are performed on an IEEE 118-bus test system connected to sample dc grids. We show that the zero relaxation gap condition holds for the case study and the global optimal solution can be obtained.

Semidefinite Relaxation of Optimal Power Flow for AC–DC Grids

An improved approach based on sequential method for the AC-DC power flow calculation is proposed in this paper. This approach solves the convergence problem caused by voltage violations at AC buses during the power flow calculation for the DC subsystems. The convergence property can be significantly improved by adjusting the converter transformer tap position flexibly. In order to adjust the tap position of the converter transformer flexibly, three mainly modifications are proposed. Firstly, the equations for whole DC systems are decoupled into individual DC systems so as to easily figure out which DC system's tap position needs adjustment. Secondly, the tap ratio of a converter transformer is selected as an alternative state variable to replace the cosine of the control angle when necessary. Thirdly, the Newton-Raphson method is utilized to solve DC subsystems instead of the method using the linear equations. Furthermore, a theoretical analysis of the advantages of the proposed approach is also presented. Numerical simulations and practical applications show that the proposed approach meet the requirement of different system operating conditions and has advantages in terms of convergence and speed. The proposed approach has been successfully integrated into the Energy Management System (EMS) for China Southern Power Grid.

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An Improved Approach for AC-DC Power Flow Calculation With Multi-Infeed DC Systems

A three-phase dynamic phasor (TPDP) model of high-voltage direct current (HVDC) systems, especially the converter, is proposed. The proposed model can rapidly solve the dynamic characteristics of the converter for symmetrical and asymmetrical operating conditions: 1) the symmetrical faults occur at the ac side or the dc side and 2) the asymmetrical faults occur at the ac side of the converter. Switch functions are used to indicate the commutation process of the converter. By using the sequence component method, it is observed that the vd should contain the 0th and 2nd components and ia,ib,ic should contain the 1st and 3rd components. The TPDP model of the HVDC converter is proposed based on these observations. The interface and the details to establish the proposed model are presented and a method to represent the commutation failure in the TPDP model is also proposed. Simulation results show that the proposed model is effective and acceptable.

Modeling and Analysis of HVDC Converter by Three-Phase Dynamic Phasor

The Multi-infeed Interaction Factor (MIIF) is an important index for quantifying the interaction between the converters of a Multi-infeed (MI) HVDC transmission system. Using an analytical formulation of power flow, the impact of ac and dc side system parameters on the MIIF is analyzed. The analysis is conducted using a two-infeed HVDC test system derived from the CIGRE HVDC Benchmark model. Results show that the ac grid effective short ratios at the MI converters, length of the tie line connecting the converter ac buses, and the power flow distribution in the ac network are the main factors affecting the MIIF value. The magnitude of the voltage drop at the converter bus does not influence the MIIF value.

Influence of the MIIF index on operation of multi-infeed HVDC systems

Discussion of the solvability of HVDC systems power flow with a sequential method

An improved continuation power flow method for the AC/DC system is proposed in this paper. The three level control strategies and the coordination of the control modes for DC systems are presented. Firstly, the load-type continuation power flow is introduced in brief. Secondly, the concept of the three-level control and their coordination are proposed. The reactive power control strategy is classified into two categories: 1) Reactive Power under Control and 2) Voltage Magnitude under Control. If the control angle is out of its range, the converter transformer tap should be changed to restore the angle. The voltage or reactive power demand is achieved by coordinating the three level control strategies. Finally, the details of the proposed method are proposed. Numerical simulations show that the approach presented in this paper has advantages in terms of convergence and speed.

Chongru Liu

Papers

An Improved Approach for AC-DC Power Flow Calculation With Multi-Infeed DC Systems

Modeling and Analysis of HVDC Converter by Three-Phase Dynamic Phasor

Influence of the MIIF index on operation of multi-infeed HVDC systems

Discussion of the solvability of HVDC systems power flow with a sequential method

Improved continuation power flow method for AC/DC power system