Appropriate generation rate constraint (GRC) modeling method for reheat thermal units to obtain optimal load frequency controller (LFC)

This paper focuses on choosing a suitable dynamic model for simulation of generation rate constraint (GRC) for load frequency control (LFC) of an interconnected realistic reheat thermal-thermal power system. Two different dynamic models for simulation of the GRC are investigated in details which are named as open loop and closed loop GRC models. These models have been used widely in literature interchangeably without any further description to the reason of selected modeling method and its suitability. This paper makes an attempt to help in choosing more effective and appropriate GRC structure pertained to reheat thermal units based on dynamic simulations to obtain optimal LFC. Reheat thermal units are examined with different GRC models. Then, integral of time multiplied squared error (ITSE) performance index is minimized by an improved particle swarm optimization (IPSO) algorithm to optimize proportional-integral-derivative (PID) controller parameters. All simulations are performed in MATLAB/SIMULINK environment. The results of eigenvalue analysis, dynamic simulations, and robustness analysis reveal the appropriate GRC modeling method for thermal units to attain high-performance and optimal LFC design.

I. Introduction

Load frequency control (LFC) is an important function in frequency regulation of modern power Systems. In order to get a precise realization and accurate insight of the LFC issue, it is essential to take into account the important constraints and main inherent requirements such as physical constraints which affect the power system dynamics. Generation rate constraint (GRC) is a physical constraint that means practical limit on the rate of the change in the generating power due to physical limitations of turbine. GRC has major influence on realistic power system performance due to its non-linearity characteristic. In practice, the rate of active power change that can be attainable by thermal units has a maximum limit [1]. So, the designed LFC for the unconstrained generation rate situation may not be suitable and realistic. The main reason to consider GRC is that the rapid power increase would draw out excessive steam from the boiler system to cause steam condensation due to adiabatic expansion. Since the temperature and pressure in the high pressure (HP) turbine are normally very high with some margin, it is expected that the steam condensation would not occur with about 20% steam flow change unless the boiler steam pressure itself does not drop below a certain level [2]. Thus, it is possible to increase generation power up to about 1.2 pu of normal power during the first tens of seconds. After the generation power has reached this marginal upper bound, the power increase of the turbine should be restricted by the GRC. Since the GRC has significant impact on the dynamic response of the power system LFC, effective inclusion of this constraint in a real frequency control scheme will greatly improve control performance [3]. The power system may face large momentary disturbance if GRC is not considered in the controller design [4].