# Load Frequency Optimal Active Disturbance Rejection Control of Hybrid Power System
## 1 Introduction  
With the advancement of the power industry, the power system has transformed into an intricate and extensive network consisting of numerous regions. Usually, when the electricity production meets consumption, the frequency will keep at a stable value. Since the loads of the electricity power system have varying values, the outputs of the power station should be adjusted to bring the frequency back to its set value as quickly as possible. An automatic control strategy should be constructed, which is called load frequency control (LFC). The LFC is an efficient and accurate control frame. When the power system is affected by an external disturbance, the frequency deviation is quickly reduced, and both the frequency and tie line exchange power are kept back to the set value by using this control strategy. Finally, the power system obtains a stable working state quickly [1]. This control strategy guarantees the maintenance of the rated values for frequency and tie-line power exchange, and a harmonious equilibrium between load demand and power generation supply can be achieved [2]. The primary concerns of the LFC mechanism can be categorized into two aspects: the structure design of the controller itself and the parameter tuning. The objective is to enhance the dynamic analysis performance of the power system, facilitating a quicker recovery of system frequency and stability in the presence of disturbances. Various LFC methods have been proposed for traditional thermal power systems [3; 4; 5; 6; 7]. The PID and its variations have been extensively implemented across diverse domains [8]. A significant amount of time is dedicated to the adjustment of controller parameters. Intelligent optimization algorithms have been employed to optimize the parameters of the PIDand its variations to enhance the performance [9; 10]. Nevertheless, the inherent uncertainties present in power systems impede the achievement of desirable control outcomes. Thus, these controllers usually possess high orders, which present challenges in terms of practical implementation. To tackle these challenges, the utilization of sliding mode control has been implemented in order to mitigate system frequency deviations. This is primarily due to its prompt response and minimal overshoot. An enhanced sliding mode controller based on adaptive dynamic programming was proposed in [11]. This controller enables the online updating of weights to adaptively supplement control signals while also considering load variations and uncertainties in system parameters. Model predictive control is also widely utilized in LFC systems due to its adaptable modeling, rapid response characteristics, enhanced disturbance rejection capability, and superior stability performance [12; 13]. For example, a distributed model predictive control approach is presented as the LFC control mechanism in an interconnected power system [12]. This strategy aims to achieve efficient coordination among various regions. Active Disturbance Rejection Control (ADRC), a model-free control strategy, has garnered significant attention [14]. The ADRC controller, when compared to traditional PIDs and their variations, has several advantages. Firstly, it is independent of precise system mathematical models, thus requiring only minimal system information. Secondly, it demonstrates attributes such as rapid response, minimal overshoot, and strong robustness. The fundamental concept of ADRC involves reducing the reliance on model information and incorporating an estimation of the disturbance into the control strategy. The disturbances estimation takes into consideration the actual perturbations in the plant, uncertainties in the parameters of the model, and dynamics that are not accounted for in the model. A reduced-order linear ADRC controller is proposed for a three-control area system [15]. It demonstrates superior performance in terms of disturbance rejection when compared with PID-based controllers.