Copyright © IFAC Large Scale Systems: Theory and Applications. Os aka, Japan. 2004 ELSEVIER IFAC PUBLICATIONS www.elsevier.comllocatelifac LOW-ORDER ROBUST DECENTRALIZED CONTROLLER DESIGN FOR A HYDC POWER LINK: A COMBINED RICCATI - GENETIC ALGORITHMS APPROACH A. Farag·· H. Werner ••• Tedmical University Hamburg-Harburg Institute of Contml Engineering Eissendorfer Str 40, 21077 Hamburg, Germany Phone +4940428783015, Fax +4940428782112 •• a.farag@tu-harburg.de ••• h. werner@tu-harburg. de Abstract : In this paper an iterative technique based on a combination of Riccati solver and Genetic Algorithm (G A) is applied to the problem of designing a decentralized robust low-order controller for a High Voltage DC power link. The design problem is split into a convex subproblem which can be solved effi ciently by a Ricatti solver, and a non-convex subproblem which is solved using GA. For a 35th order model of the HVDC link considered here a first order controller that achieves robustness over a wide range of operating conditions is designed by imposing a Hoc norm constraint. An additional pole region constraint is used to trade nominal performance against robustness. Copyright @2004 IFAC Keywords: Fixed structure controller, robust control, decentralized control, algebraic Ricatti equation, genetic algorithms. 1. INTRODUCTION . High Voltage Direct Current (HYDC) power links are used in electrical power grids as a sup- plement to AC transmission. Power transfer by means of HYDC is used in case of (i) inter- connecting asynchronous AC systems with dif- ferent power frequencies, (ii) high voltage ca- bles longer than about 30-80km and (iii) long overhead lines with lengths in excess of about 600km (Kundur, 1994). The system comprises two AC/DC power electronic cOllverters separated by an equivalent impedance (ZDC). On the AC side there are AC filters, while the AC grids can be represented by an equivalent impedance (ZAC), as shown in Figure 1. Fig. 1. High Voltage DC scheme A state space model for this plant is proposed in (Aten et al ., 2001); its high dynamic order (35 state variables) reflects the large number of passive elements. This model has been linearized for nominal AC voltages (lpu) and nominal firing angles (rectifier firing angle a = 21° , inverter extinction angle I = 25.6°) and extensively val· idated against nonlinear EMTDC simulation for small changes. The uncertain time delays due to