AC-DC Hybrid Power System Research Articles

Test Models for Stability/Security Studies of AC-DC Hybrid Power Systems with High Penetration of Renewables

Test Models for Stability/Security Studies of AC-DC Hybrid Power Systems with High Penetration of Renewables

Modified average‐value model for high‐performance fault transient simulation of AC‐DC hybrid power networks

Due to the complicated transients and the possibility of transition from a minor fault in AC subsystems to a severe blocking event of a high-voltage direct–current (HVDC) transmission line, the study of AC-DC hybrid power systems requires both efficient and accurate electromagnetic transient (EMT) simulation. However, the computational efficiency of such simulations is restrained by the switching events when detailed models of line-commutated converters (LCCs) are used. Recently the average-value modelling technique has been developed for accelerating the simulation while still accurately capturing the transient characteristics of line-commutated AC–DC converters. This study presents the application of the average-value model in the modelling of the LCC-HVDC system and gives a further discussion about the deficiency of the model. A modified interface and the corresponding soft-switching technique are proposed for enhancing the simulation performance of the LCC-HVDC model in the transition between normal and faulty operation. The model is tested on a series of LCC-based systems including a hybrid power system with four LCC-HVDC lines to verify its efficiency and accuracy. The results indicate that the developed model can accurately predict the transients of the system both in symmetrical and asymmetrical operation while being computationally efficient.

Parallel Restoration Method for AC-DC Hybrid Power Systems Based on Graph Theory

High voltage direct current (HVDC) transmission systems have been used for interconnection among multiple power grids. As a result, the conventional AC power systems are transforming into AC-DC hybrid power systems. In this paper, a parallel restoration method for AC-DC hybrid power systems is proposed. First, the effects of line-commutated-converter-based (LCC) and voltage-source-converter-based (VSC) HVDC systems on service restoration are explored. The startup characteristics of HVDC systems are analyzed. Second, an optimization model for service restoration of AC-DC hybrid power systems is developed, aiming at maximizing the generating power and minimizing the restoration time. The startup constraints of HVDC systems are incorporated into the proposed model. Third, a graph-theoretic algorithm is proposed to obtain a near-optimal parallel restoration scheme, verified by power flow calculations. An 11-bus test system is used to illustrate the effects of using HVDC systems for system restoration and a modified IEEE 39-bus test system is used to verify the effectiveness of the proposed parallel restoration method. The simulation results indicate that the proposed method can make full use of the benefits of HVDC systems, allocate cranking power flexibly during the parallel restoration, and reduce the restoration time.

Piecewise Normalized Normal Constraint Method Applied to Minimization of Voltage Deviation and Active Power Loss in an AC–DC Hybrid Power System

In this paper, we establish a mathematical model of the multi-objective reactive power optimization (MORPO) problem in an ac–dc hybrid power system. To get an evenly distributed set of Pareto solutions of this MORPO problem throughout a whole day, we propose a piecewise normalized normal constraint (PNNC) method. Based on this method, the Pareto frontier is divided into four arcs so that the fluctuation of angle between the tangent line at each Pareto point and its sub-Utopia line is reduced. By doing so, the original MORPO problem is transformed into four sets of single objective optimization problems, which can be solved efficiently by a nonlinear primal-dual interior-point method. Performance of the PNNC method is tested on a real ac–dc interconnection system. The results demonstrate that it has the capability to generate a more evenly distributed set of Pareto solutions when compared with the normalized normal constraint (NNC) and weighted sum (WS) methods. In addition, a fuzzy-based membership value assignment method is employed to derive 96 optimal compromise solutions corresponding to 96 time periods in a whole day; the numerical outcomes show that a saving in active power loss and an improvement in the voltage profile can be gained simultaneously.

Monte Carlo simulation and bootstrap method based assessment of available transfer capability in AC–DC hybrid systems

In practical power markets, available transfer capability (ATC) can provide important information for all parts of power market participants and the assessment of ATC should be carried out instantly. As high voltage direct current (HVDC) systems have been extensively used in modern power systems, less work has been done on evaluation of ATC in AC–DC hybrid power systems. This paper is dealing with the evaluation of ATC for the integration of HVDC link with an AC power system. The mathematical model of ATC for AC–DC hybrid power system is formulated. Considering the time-varying and uncertainties of the power system, several statistical indices are presented to evaluate ATC. A novel approach combined Monte Carlo simulation with bootstrap method is proposed to calculate these statistics. Sequential solution method is employed to deal with the AC–DC power flow and golden section search method is used to accelerate convergence of load flow. The algorithm is developed in the environment of MATLAB R2008a with MATPOWER. Case studies with a modified IEEE RTS-79 24-bus AC–DC hybrid power system and IEEE 3-area RTS-96 system are used to demonstrate the presented approach. The results show that the proposed method is effective and practical. Some new problems are suggested at the end of paper.

AC–DC optimal reactive power flow model via predictor–corrector primal‐dual interior‐point method

This study presents a quadratic optimal reactive power flow (ORPF) models for ac-dc hybrid power system. In the ac system, the load tap changing transformer is represented by an ideal transformer and its series impedance with an artificial node located between them. The ideal transformer is further replaced by the current injections at the two terminal nodes. The artificial node voltage and the current injections are used to express the relationship between the quantities at both sides. For the dc system, some variables are introduced to formulate the dc equations. Finally, utilising their couplings, the ac and dc equations expressed in different coordinates can be integrated into a unified quadratic ac-dc ORPF model, which is simply expressed in the block form. The predictor-corrector primal-dual interior-point method is used to solve the optimisation problem. The quadratic model results in the constant Hessians that all have elements of 1 or −1, and mostly being zero and multiplication is not required to form the full Hessian, thereby accelerating the entire optimal process significantly. Extensive numerical simulations have shown the validity and superior performance of the proposed model than the conventional one.