Multivariable Control for Regulating High Pressure Centrifugal Compressor with Variable Speed and IGV Toufik Bentaleb a , Alessio Cacitti b , Sergio De Franciscis b , Andrea Garulli a Abstract— The objective of this paper is to develop a multi- variable control system for a class of centrifugal compressors, which exploit as control signals both the rotational speed and the Inlet Guide Vane (IGV). Linear Quadratic Gaussian control with Integral action (LQGI) and Model Predictive Control (MPC) are investigated. The LQGI and MPC controllers are compared to a standard proportional integral (PI) controller, to regulate the discharge pressure of the compressor. The control algorithms are simulated and compared in different operating scenarios. Results demonstrate that the proposed multivariabe control schemes provide better performance than the single- loop PI controller, thus motivating the use of IGV for control purposes. Index Terms— Multivariable control, Optimal control, Model predictive control, Linear quadratic regulator, Power plant. I. I NTRODUCTION High-pressure multistage centrifugal compressors are an essential part of the process machinery in the oil and gas industry across a wide variety of applications. Centrifugal compressors in a connected process system are very sensitive to changes in the inlet conditions [1], such as the suction pressure, the temperature, as well as the inlet gas density. This type of gas compressors requires quick response and reliable control systems to increase their capacity. Process regulation (for example discharge pressure control) is usually performed by acting on the shaft speed. More recently, a variable Inlet Guide Vane (IGV) has been used for control purposes. The IGV system allows wide capacity control of the centrifugal compressor with reduced energy losses. In centrifugal compressors, IGV is used to control the mass flow rate with negligible change in compressor ratio and shaft speed [2]. When both the turbine speed and the IGV are used to regulate the process, a multivariable control system is necessary. In this paper two types of multivariable control, namely a linear quadratic regulator (LQR) and a model predictive controller (MPC), are presented, and compared to a standard PI controller based on speed regulation. The LQR is widely and effectively used in many industrial applications [3], [4], including control of the compressor stations for natural gas pipelines [5], [6]. Since complete state information is not available, it is necessary to use a LQG control scheme, which includes Kalman filter estimation of the state vector. The LQG scheme includes an integral action to compensate the reference tracking error. In recent years, MPC has confirmed a Dipartimento di Ingegneria dell’Informazione, Universit` a di Siena, Italy. E-mail: bentaleb@dii.unisi.it; garulli@ing.unisi.it. b Nuovo Pignone Ge Oil & Gas, Florence, Italy. E-mail: sergio.defranciscis@ge.com; alessio.cacitti@ge.com. UV T1 P1 Gas Inlet V1 Ps P d V2 DV T2 P2 Gas Outlet Inlet guide vane Shaft Speed N Centrifugal Compressor Gas Turbine Fig. 1. Compression system: gas turbine-driven centrifugal compressor, equipped with IGV, and two volumes at suction and discharge. itself as a successful approach to multivariable control due to its advantages over traditional controllers [7]. In particular, it is widely employed in the oil and gas industry to deal with power plant control [8], [9], [10]. Although there is an increasing research interest in nonlinear MPC [11], [12], [13] most of this literature is dedicated to systems described by analytical models, while the model of the reference application considered in this paper contains non analytical parts (e.g. look-up tables). Hence, in this study linear MPC is employed, based on a linearized model of the plant. The paper is structured as follows: in Section II, the power plant model is presented. Section III deals with the PI, LQGI, and MPC control schemes and the details about the implementation of each controller. Simulation results are presented in Section IV to compare the performances of the PI, LQGI and MPC controllers, while some concluding remarks are given in Section V. II. MODELING This section describes the dynamic model of a com- pression system. The plant model, depicted in Figure 1, includes a gas turbine, a centrifugal compressor, driven by the turbine, equipped with an adjustable inlet guide vane (IGV), two volumes, and two actuator valves. The two volumes are placed one in suction and the other in discharge of the compressor. The valves are placed upstream the first volume, in compressor suction, and downstream the second volume. Model parameters are shown in Table I for a specific operating point considered in the simulations. 2014 IEEE International Conference on Control Applications (CCA) Part of 2014 IEEE Multi-conference on Systems and Control October 8-10, 2014. Antibes, France 978-1-4799-7408-5/14/$31.00 ©2014 IEEE 486