1. Introduction
A significant change in the dynamics of electrical power networks has occurred, mostly as a result of changes in energy sources, grid structure, and power consumption patterns. The rising prevalence of renewable energy sources in newly installed power systems has become a particularly noticeable indicator of this transition, resulting in modifications to the grid’s attributes. These advancements have made maintaining steady voltage levels and frequencies a crucial objective in control system design. Frequency and voltage variations can result in a negative impact on integrated loads, reducing their dependability and durability [
1]. These variations in voltage and frequency have a direct impact on a system’s power losses, as well as its active and reactive power. Even small voltage variations can have a significant effect on reactive power. When the voltage deviates more than the typical ±5% from its rated value, it seriously affects the efficiency and operational life of appliances or other connected power system components [
2,
3].
In particular, it is possible to effectively reduce the impact by managing voltage changes in a proper way. Voltage control can be used in the generation, transmission, and distribution phases of power networks. Several kinds of reactive power compensation devices can be used in the transmission and distribution phases. Several examples exist in the literature, including tap changers in transformers, different filtering devices, flexible AC transmission systems (FACTSs), etc. Automatic voltage regulators (AVRs) are often used to manage voltage variations at the generating stage as they show high efficiency in controlling voltage on the synchronous generator’s generation side. However, adjusting the AVR settings to account for many uncertainties and differences in AVRs, sensors, gains, and other elements is an essential and crucial issue for the proper operation of these devices [
4].
Actually, numerous AVR controllers based on the Proportional–Integral–Derivative (PID) have been introduced in [
5]. The PID controller, as a preferred solution in industrial applications due to its straightforward structure and the well-understood impact of its parameters on the output of controlled systems, has a prevalent advantage despite some limitations existing [
3,
6]. Various PID controller designs have been addressed for AVR systems, but traditional PID tuning methods like trial-and-error, Ziegler–Nichols, and root locus often fall short in handling uncertain system parameters and load disturbances [
7]. To address these shortcomings, a conventional PID controller was designed for AVR systems, employing several metaheuristic algorithms to determine the best controller parameters [
7,
8,
9,
10]. These controllers’ performance has been assessed and compared with other studies.
Several algorithms in the literature for designing PID controllers include the pattern-searching algorithm (PSA) [
11], particle swarm optimization (PSO) [
12], artificial bee colony (ABC) [
13], tree-seed algorithm (TSA) [
14], grasshopper optimization algorithm (GOA) [
15], whale optimization algorithm (WOA) [
16], improved WOA (IWOA) [
17], genetic algorithm (GA) [
18], sine–cosine-based algorithm (SCA) [
19], symbiotic organism searching (SOS) algorithm [
20], salp-swarm-based algorithm (SSA) [
21], bacteria-foraging-based optimization algorithm (BFOA) [
22], ant–lion optimization (ALO) algorithm [
23], differential evolution algorithm (DE) [
13], etc. The PID possesses only three tunable parameters represented by the gains of its terms (
in the P-term,
in the I-term, and
in the D-term). The literature shows a weak disturbance rejection capability with high sensitivity to the process uncertainties of conventional PID control methods.
The conventional PID controller has an integer-order manner, but there has been a shift towards Fractional-Order PID (FOPID) controllers. The FOPID-based controllers are more flexible due to the additional included FO operator parameters. Optimization-algorithm-based FOPID AVR controllers have been proposed in the literature, such as the marine predator algorithm (MPA) [
4], PSO [
24,
25], GA [
25], ABC [
26], Chaotic Ant Swarms (CASs) [
27], Multi-objective Extremal Optimizer (MOEO) [
28], Cuckoo Searching (CS) [
29], SCA [
30], Improved NSGA-II [
31], salp swarm optimizer (SSO) [
32], etc. Similar algorithms have been presented using [
33,
34] PIDD2 based on PSO [
35]. The FOPID has five tunable parameters represented by the gains (
,
, and
) and FO operators (
in the FO-I term and
in the FO-D term). The slime mold optimization algorithm has been employed in [
36] for optimizing FOPID parameters.
Recent advancements in the FOPID include variable-order modifications [
37]. Additionally, state-feedback PID controllers have been developed [
5,
38], with a robust two-degree-of-freedom (2DOF) state-feedback PI controller [
5] that eliminates steady-state errors using integral control, in addition to outperforming the one-degree-of-freedom (1DOF) PI controller. Further enhancements to the 2DOF state-feedback PI controller employed a dynamic weighted state-feedback method [
20], offering flexibility under varying system conditions. For the AVR, a distinct robust state-feedback controller was proposed, considering bounded system uncertainties and external disturbances in its design [
39]. To tackle AVR system uncertainties, a non-fragile PID controller, optimized with a genetic algorithm, and a model predictive controller using Angle of Arrival (AOA) optimization were also introduced in [
40,
41], respectively.
Although there has been a significant amount of study conducted on PID controllers, the literature also showcases a wide range of control strategies. The use of a strong controller that combines
and
-synthesis techniques is suggested to enhance resilience against uncertainties and disturbances that are parametric and structured in nature [
42]. The research work [
43] introduced a model reference adaptive control method with a fractional order, which was optimized using a genetic algorithm. In addition, a neural network predictive controller for the AVR was optimized using an imperialist competitive algorithm [
44]. In [
45], the researchers created an Emotional Deep Learning Programming Controller (EDLPC) for AVR systems. The EDLPC incorporates an Emotional Deep Neural Network (EDNN) structure and an artificial emotional Q-learning algorithm. Furthermore, a recent investigation conducted in [
46] focuses on improving AVR systems by employing a deep deterministic policy gradient (DDPG) agent. This strategy prioritizes enhancing the AVR’s ability to quickly and effectively adapt to changes in its environment, such as variations in the load and alterations in the parameters, while also ensuring its resilience and stability.
An extensive overview of the several optimization strategies for the AVR control system tuning is shown in
Table 1. It draws attention to the wide variety of methods used in the literature to modify AVR controller structures. Each of these algorithms has a different working principle, which determines its effectiveness. The various employed objective functions are summarized in
Table 2.
In this study, a new modified hybrid FO (MHFO) AVR based on the FO tilt integral (FOTI) proportional derivative with a filter double derivative with a filter (PDND2N2) controller, namely FOTI-PDND2N2, is proposed. The proposed FOTI-PDND2N2 controller combines the benefits of using the FOTI controller with the PDND2N2 for ensuring better dynamic performance and steady-state response and for enhancing controller robustness and flexibility. Moreover, thanks to the authors’ knowledge, a new application of the growth optimizer (GO) is proposed in the paper for optimally tuning the controller parameters to obtain better system performance compared to the other controller or metaheuristic methods in the literature. The major contributions of this paper can be summarized as follows:
A new modified hybrid FO (MHFO) controller is proposed for AVR applications in this paper. The new proposed MHFO AVR method is developed based on the FO tilt integral (FOTI) proportional derivative with a filter double derivative with a filter (PDND2N2) controller, namely FOTI-PDND2N2. The newly proposed controller merges the benefits of the FOPID, PIDF, and TID controllers, leading to better performance and enhanced characteristics. The tuning process of the control parameters is made offline, which benefits the power and speed of recent microprocessor technologies.
The proposed FOTI-PDND2N2 controller combines the benefits of the FOTI controller with PDND2N2 for ensuring better dynamic performance and steady-state response, and for enhancing controller robustness and flexibility. Also, the inclusion of filters with derivative terms improves their responses, reduces noise, smooths the control action, and has better stability.
New practical applications of the recently developed growth optimizer (GO) method is introduced in this paper for optimally optimizing the proposed FOTI-PDND2N2 controller’s parameters in a simultaneous manner. Both the recent GO algorithm’s benefits and the associated benefits of the proposed FOTI-PDND2N2 controller are combined to provide a more robust and wide-ranging, stable AVR control method. Moreover, the GO algorithm guarantees the optimum parameter set together for achieving minimization of the defined objective function.
The remainder of the paper is organized as follows:
Section 2 provides the mathematical and structure representation of the AVR system. The proposed MHFO AVR controller is presented in
Section 3. The proposed design and optimization algorithm are detailed in
Section 4.
Section 5 presents the obtained performance evaluation and comparison results. Finally, the paper’s conclusions are provided in
Section 6.
2. Mathematical Representations of AVR Systems
The main elements of the AVR system include the generator, sensing, the AVR controller, the amplifier, and the excitation system, as shown in
Figure 1. The main objective of the AVR controller is the regulation of the generator’s output voltage with various load variations and disturbances. The control is achieved through controlling the generator’s excitation system based on the error signal fed into the AVR controller.
The AVR system is affected by the connected electrical loads at its terminals. When there is an increase in the connected loads, the AVR terminal voltage drops. In accordance, error voltage signal (between measured signal and reference setting ) increases in the positive value direction. This, in turn, increases generator excitation, reducing steady-state error voltage till reaching its minimum. In the steady state, generator excitation is preserved constant to maintain stable voltage supply for all of the connected loads, whereas in the load-reduction condition, terminal voltage increases, leading to a decrease in the error signal in the negative value direction. In the same way, excitation is decreased till having the minimized steady-state error.
The AVR system elements’ transfer functions (TFs) are normally represented by the Laplace transform of each block. Different elements of the AVR (including the generator, amplifier, sensing system, and exciter) are modeled based on linearized first-order TFs to facilitate the representation process. Their TFs (amplifier
, generator
, exciter
, and sensing system
) and their associated parameters’ range from the literature are as follows [
30,
50]:
where
,
,
, and
are gains and
,
,
, and
are time constants for the amplifier, voltage sensing, generator, and exciter, respectively. The AVR’s complete modeling using first-order TFs for its elements is represented in
Figure 2. The voltage error between desired reference
(1 p.u., normally) and sensed voltage
represents the controller input signal. The AVR controller functionality is to continuously minimize this error, leading to a zero steady-state value in efficient control design.
Based on
Figure 2, the complete AVR TF with the controller TF of
is represented by
. The TF input/output is expressed as:
More details about the characteristics of the AVR system response without the controller can be found in [
4,
48,
51]. The dynamics of the AVR system without the controller (with
= 1 in (
5)) exhibits very low values of the damping ratio for the existing complex poles, which indicates the need for enhancing the uncontrolled AVR system’s performance.