A.H.M.A. Rahim

Robust STATCOM voltage controller design using loop-shaping technique

Abstract Static synchronous compensator (STATCOM) is a shunt-connected converter, which can affect rapid control of reactive flow in the transmission line by controlling the generated a.c. voltage. This article presents a robust STATCOM voltage controller design for power system damping. The method of multiplicative uncertainty has been employed to model the variations of the operating points. The design is carried out applying robustness criteria for stability and performance. A loop-shaping technique has been employed to select a suitable open-loop transfer function, from which the robust controller function is constructed. The controller was tested for a number of disturbances including three-phase fault. The damping provided by the robust controller was compared with a fixed-parameter PID controller, whose gains were calculated through a pole-placement technique. It has been observed that the PID control is ineffective at operating points other than nominal. The robust design has been demonstrated to provide extremely good damping characteristics over a range of operating conditions.

Improvement of synchronous generator damping through superconducting magnetic energy storage systems

Stabilization of a synchronous generator through control of firing angle of the power converters in superconducting magnetic energy storage (SMES) systems is considered. An optimum strategy of the firing angle control is designed so as to eliminate the transients in minimum time. A nonlinear model of a synchronous generator, its governor and exciter systems, and an SMES system connected to the generator terminal is considered. The optimum firing angle control is derived retaining the nonlinearities of the system dynamics. Digital simulation results indicate that the proposed strategy controls the slowly growing as well as first swing instabilities very effectively. >

Control of subsynchronous resonant modes in a series compensated system through superconducting magnetic energy storage units

A simple and novel strategy for damping subsynchronous resonant oscillations through control of converter firing angles of a superconducting magnetic energy storage system (SMES) is proposed. The strategy is derived such that the current injected or drawn by the SMES compensates for any deviation in real and reactive power in the system. The proposed control has been tested on the IEEE second benchmark model for subsynchronous resonance studies. It has been found to eliminate the slowly growing transients resulting from the unstable modes very effectively. The control algorithm is simple and its realization will require very little hardware.

Dynamic performance improvement of an isolated wind turbine induction generator

A dynamic model of a variable speed stand alone wind generation system is developed. The transient performance of the system under variable wind gust and other disturbance conditions has been studied. It has been observed that a stand alone generator is very vulnerable to transient disturbances in the system. A variable susceptance excitation control system, installed at the generator terminal, has been shown to improve the dynamic performance of the system following transient disturbances. Additional pole-placement based PI and PID controllers in the excitation control circuit are shown to control the violently oscillating and even growing transients very effectively. The robustness of the proposed controller has been tested for various disturbances and also for a range of operation.

Stabilization of a High Voltage AC-DC Power System I. Evaluation of Control Strategies

Various optimal and suboptimal stabilizing signals for a single machine infinite bus system connected through an ACDC link have been investigated. The controls in order of effectiveness of transient control are the linear state regulator control, a quasioptimal proportional control, output regulator controls from terminal voltage and frequency, and frequency and acceleration. From implementation viewpoint the proportional control is preferred. This scheme can also be easily extended to Include the nonlinearities in the system dynamics.

An efficient method of identifying coherent generators using Taylor series expansion

A novel method of identifying the coherency of a group of generators is presented. The method requires values of generator rotor angles at three distinct instants of time in the early part of the transients. Also, a measure of the degree of electrical coupling between the generators is required. The angles are calculated during the faulted and part of the postfault period through Taylor-series expansions. The degree of coupling is calculated from an admittance distance measure. The proposed method has been tested on five different power systems of varying sizes. It is concluded that the results obtained by the proposed method match perfectly those obtained through simulation of the total system dynamics. >

Supercapacitor based energy storage system for effective fault ride through of wind generation system

This paper investigates the potential of supercapacitor based energy storage system for enhancing fault ride through capability of wind driven induction generator. A dynamic model of wind generation system including Static Compensator (STATCOM) is developed. The power control capability of the STATCOM is extended by incorporating supercapacitor based energy storage system. A vector control technique based on the decoupling of real and reactive power is implemented. Simulation results show that fixed speed induction generator achieves significant grid fault withstand capability when aided by supercapacitor based energy storage system. Induction generator regains prefault status immediately after the fault is cleared.

Aggregation of Induction Motor Loads for Transient Stability Studies

This study investigates the dynamic representation of a group of induction motors by one or more equivalent motors. The unknown parameters of the equivalent are estimated through a weighted-least-squares procedure. Verification of simulated results with experimental ones suggests that a fifth order dynamic model of the aggregate with a ninth order parameter model is appropriate. The least squares technique was found to provide good convergence characteristics subject to satisfaction of the following conditions good initial estimate of the parameters, proper selection of acceleration factors and homogenity of the motor group.

A robust STATCOM controller for power system dynamic performance enhancement

A robust controller for providing damping to power system transients through STATCOM devices is presented. Method of multiplicative uncertainty has been employed to model the variations of the operating points in the system. The design is carried out applying robustness criteria for stability and performance. A loop-shaping method has been employed to select a suitable open-loop transfer function, from which the robust controller is constructed. The proposed controller has been tested through a number of disturbances including three-phase faults. The robust controller designed has been demonstrated to provide extremely good damping characteristics over a good range of operating conditions.

Fast low voltage ride-through of wind generation systems using supercapacitor based energy storage systems

Dynamic model of a wind generation system with a supercapacitor based energy storage system is developed. The supercapacitor system has been interfaced with the wind generation system through a STATCOM. Decoupled P-Q control of STATCOM is implemented by introducing suitable transformation on its injected current. The controller parameters are obtained through a search based optimization technique. The effectiveness of the STATCOM supercapacitor energy storage was examined for very low voltage situations. Simulation studies demonstrated that the STATCOM supercapacitor control can achieve significant withstand capability for reasonable low voltage duration. System transients are adequately suppressed and normal conditions are quickly restored.