A serial AC-DC integrated load flow algorithm for variable speed offshore wind farms is proposed. It divides the electrical system of a wind farm into several local networks, and different load flow methods are used for these local networks sequentially. This method is fast, more accurate, and many factors such as the different wind farm configurations, the control of wind turbines and the power losses of pulse width modulation converters are considered. The DC/DC converter model is proposed and integrated into load flow algorithm by modifying the Jacobian matrix. Two iterative methods are proposed and integrated into the load flow algorithm: one takes into account the control strategy of converters and the other considers the power losses of converters. In addition, different types of variable speed wind turbine systems with different control methods are investigated. Finally, the method is demonstrated using an 80-MW offshore wind farm.

Load flow analysis for variable speed offshore wind farms

The significant increase in planned offshore wind parks and the tendency towards large parks in considerable distances offshore, make the well established HVDC technology a favorable solution for the connection of these large & distant offshore wind parks to the main power grid onshore. It is therefore necessary to adequately model the HVDC transmission links and integrate them in the load flow analysis of the complete AC-DC system. In this paper, the well known Newton-Raphson method for the load flow analysis is modified to achieve compatibility for AC-DC systems with integrated DC links in the AC network. The elements of the residual vector and the Jacobian matrix for the AC network are kept unchanged and are merely complemented by a new vector and a new matrix, which represent the modifications due to the DC link. The modified Jacobian equation includes the DC real and reactive power at the AC-DC buses, and their dependency on the AC system variables. The modified Newton-Raphson method is evaluated on an AC-DC test system with a load flow computation in Matlab and the results are presented.

Modified Newton-Raphson load flow analysis for integrated AC/DC power systems

SUMMARY

The paper describes a new approach for the load flow calculations of integrated AC–DC system. A simple and reliable method for sequential modified Gauss and Gauss–Seidel power flow for AC–DC system is developed. This approach is based on applying nodal injection theory at all buses. The DC system is treated by the current injected to the buses where it is connected and its effect is reflected at internal buses by additional power injection. Iterations between AC and DC power flow algorithms are made to match boundary conditions between the two systems. In this approach, the active and reactive power and the AC voltages at the converter buses are considered as the interface between the AC and DC equations in each iteration step. The combined AC–DC equations are solved separately using sequential modified Gauss and Gauss–Seidel methods. The developed algorithm to solve the AC–DC power flow has been tested on the IEEE 9-bus test system. Copyright © 2011 John Wiley & Sons, Ltd.

A new approach for load flow analysis of integrated AC–DC power systems using sequential modified Gauss–Seidel methods

High voltage direct current (HVDC) power distribution systems have been extensively used in modern transmission system. The major concern of the HVDC transmission is the loadflow in the system. This paper is dealing with the development of loadflow solution for a HVDC link in a power system. The loadflow solution is based on the fast decoupled method which needs some iteration to obtain the result. The paper exposed us to the integration of HVDC link with an AC power system. Variables of the direct current link which have been chosen for simulation are the converter terminal DC voltages, converter transformer tap ratios, firing angle of the rectifier and current in the HVDC link. An algorithm has been developed to solve the HVDC link power system loadflow and the algorithm has been tested on the IEEE test system.

A modified approach for load flow analysis of integrated AC-DC power systems

Two-terminal high voltage direct current HVDC transmission links are in operation throughout the world. They are key elements in electrical power networks, their representation is oversimplified or ignored in most power system studies. It is therefore necessary to adequately model the HVDC transmission links and integrate them in the load flow analysis of the complete AC/DC system. In this paper, the well known Newton-Raphson method for the load flow analysis is modified to achieve compatibility for AC/DC systems with integrated DC links in the AC network. The modified Jacobian equation includes the DC real and reactive power at the AC/DC buses, and their dependency on the AC system variables. The modified Newton-Raphson method is evaluated on an AC/DC test system with a load flow computation in Power World Simulator PWS and the results are presented.

Notice of Violation of IEEE Publication Principles Modified load flow analysis for integrated AC/DC power systems

The development of an AC-DC load flow program based on the fast decoupled algorithm is dealt with. Variables of the DC link which have been chosen for the problem formulation are the converter terminal DC voltages, converter transformer tap ratios, firing angle of the rectifier, extinction angle of the inverter, and current in the DC link. Equations relating these seven variables and their solution strategy are discussed. The model developed is independent of a particular control mode of the DC link. The flow chart of the algorithm and the results of a sample run obtained using a MicroVAX-II computer are included. Use of multiple DC links in a large AC system is also described. >

H.A. Sanghavi

Papers

Load flow analysis of integrated AC-DC power systems