Modelling and Recoupling the Control Loops in a Heavy-Duty Gas Turbine Plant

Abstract

This paper presents a dynamic simulation of a single shaft heavy-duty gas turbine plant, suitable for gas-steam combined cycles. The plant is operated at maximum gas turbine exhaust temperature, using variable inlet guide vanes (VIGV) as control. In the first section, a non-linear lumped parameter mathematical model is described: it includes a control system representative of those controls normally utilised by industry today. Some dynamic responses of a controlled plant taken as an example are presented. In the second section, a different control system is proposed, operating with no interaction between the speed and exhaust temperature loops. To this aim, a linear model in the frequency domain of the uncontrolled plant is obtained, starting from the non-linear model in the time domain. Assuming that each one of manipulated variables influences only one of the controlled variables (VIGV only the exhaust gas temperature and the fuel mass rate only the load), the transfer functions of two new blocks have been obtained. To compensate for the system non linearity, the calculations are repeated for different load levels. The new control feature can offer advantages in the time response of the regulated plant, especially in the operating range where the airflow can be modulated by the VIGV at constant fuel firing temperature.