Power System Frequency Control: Modeling and Advances

By Li Li

Power System Frequency Control: Modeling and Advances evaluates the control schemata, secondary controllers, stability improvement methods, optimization considerations, microgrids, multi-microgrids, and real-time validation required to model and analyze the dynamic behavior of frequency in power systems. Chapters review a range of advanced modeling and analytical considerations for single to multi-area networks using traditional and hybrid sources, including renewable sources, FACT devices and storage. The work also considers broad aspects of upstream and downstream control mechanisms which enable novel solutions in the area of automatic generation control in power system networks.

Highly recommended for power system engineers, researchers and practitioners with interests in load frequency control, automatic generation control, linearized models of isolated microgrid, and multi-microgrid, and hybrid LFC scheme, this book is an ideal resource on the topics discussed.

• Explains the function and purpose of power network frequency loops, including primary control, secondary control and emergency control loops
• Models LFC schemes from single-area to multi-area interconnected power systems
• Demonstrates five fundamental controller designs alongside their main error principles
• Evaluates dynamic response analysis with transient performance stated for modern indices
• Discusses performance studies such as state-space modeling, random loading, sensitivity and stability analyses

• Language: English
• Publisher: Academic Press
• Release date: Feb 23, 2023
• ISBN: 9780443184277

Book preview

1: Fundamentals of load frequency control in power system

Dillip Kumar Mishraa; Li Lia; Jiangfeng Zhangb; Md. Jahangir Hossaina    a School of Electrical & Data Engineering, University of Technology Sydney, Ultimo, NSW, Australia

b Department of Automotive Engineering, CLEMSON University, Clemson, SC, United States

Abstract

Automatic generation control (AGC) plays a significant role in the control process in a power network to achieve the equilibrium between generation and load in the most cost-effective manner. Moreover, the AGC approach deals with frequency regulation and control of the power exchange and economic dispatch. This chapter discusses the fundamentals of load frequency control with mathematical modeling and error functions. In addition, the transfer function of the single and multiarea AGC models is presented.

Keywords

AGC; LFC; Primary loop; Secondary loop; Emergency loop; Turbine

1.1: Basic concepts

In power systems, automatic generation control (AGC) plays a significant role in the control process in a power network to achieve the equilibrium between generation and load in the most cost-effective manner. AGC approach deals with frequency regulation and control of the power exchange and economic dispatch. It also adjusts the MW power of multiple generators at different power plants corresponding to the load change [1].

Principally, the AGC is a closed-loop-based scheme used to control both the frequency and exchange power. More importantly, two parameters, such as frequency and voltage, are crucial to the power network to balance the generation and load. Therefore, to control the frequency deviation, one of the essential control loops, i.e., load frequency control (LFC), has been used [2]. However, another loop, i.e., automatic voltage regulator (AVR), has been employed to manage the voltage level. The abovementioned control loop, namely LFC and AVR, is the extension of AGC, where both the frequency and voltage have been considered the control parameter. In most cases, researchers have assumed that the LFC is termed AGC [3]. Because frequency control is more important than the voltage, as far as frequency deviation is concerned, it can only cope with ± 0.5 hertz with the power frequency. And less than or more than that, the power network may be collapsed, or a blackout may happen. Hence, over the past few decades, a large body of research has been carried out to focus on load frequency control. And several advancement methods have been established to improve the dynamic response of the AGC during disruptive events [4].

An interconnected network means two or more areas are connected in a single platform. Similarly, in the power system context, the interconnection means the two or more control areas are incorporated through the tie-lines. The control areas are regulated by their control actions in the form of coherence principles, which must be assumed while doing the simulation. On top of that, a control signal can be derived from frequency change and tie-line power deviation, and this is termed an area control error (ACE). It indicates the need for generation either lowered or raised [5].

This chapter introduces the basics of AGC in modern area power networks, various control loops, individual models, and different structures. Besides, the facts about the AGC and its role, objectives, primary functions, and specific features are outlined. The operation of the power network and its control range of frequency change with their droop characteristics are described.

1.2: AGC in a modern area power network

AGC offers an efficient scheme for monitoring the change in frequency and tie-line power to balance the generation. It can recognize generation levels by transmitting commands to the generating units which are under control mechanisms. The AGC response is completely trusting in commands and how they respond [6]. Eventually, the command responses rely on according to the unit types, fuels, control methods, and equilibrium points.

The changes experienced by AGC over the past decade remain unprecedented on account of security measures, supervisory control, and data acquisition (SCADA) and coordinated control of load switching are the key factors of the energy management system (EMS), as shown in Fig. 1. The entire process of AGC is achieved remotely through the control center from each generating station, whereas the power generation is regulated by the governing system (turbine-governor) in the generation location. The detailed description of each unit is presented in Ref. [7].

Fig. 1

Fig. 1 Modern EMS.

1.3: Power network frequency loop

In an electric power network, frequency is one of the major factors for maintaining the load demand-generation balance. As discussed earlier, the permission frequency change in the system is ± 0.5 hertz, and more devotion prompts to blackout or power outage in the entire interconnected network. Hence, it is essential to keep within rigid limits. More importantly, it has been taken as a favorable performance index to show generation-load balance characteristics. On the other hand, it affects the reliable, secure, and efficient operation of an electric power network.

From the fundamentals, the rotation speed of the turbine is proportional to the frequency of the system irrespective of the constant pole. Hence, the change in the value of rotation speed can change the range of frequency level with the help of a speed governing the system, where the turbines (low pressure and/or high pressure) are coupled with each other. The speed changer plays a vital role in meeting the requirement of input steam to the turbine to balance the frequency and generation as well. The schematic diagram of the speed governing mechanism is shown in Fig. 2[3].

Fig. 2

Fig. 2 Speed governing system.

In response to the frequency change, the control action can be made, and it has been categorized into three operations: primary, secondary, and emergency. In the primary operation phase, the deviation is assumed as very small, which can be alleviated through the primary loop. In the response of substantial-frequency change, the secondary control action comes into play, which has the capability to change the frequency level in a broader range, which is called secondary operation. However, with the high-disruptive events, or extreme events, both the control as mentioned above, action cannot restore the system frequency. In that scenario, emergency control action should be applied to get back to its system frequency, which is called emergency operation. The three control actions with the frequency range change are portrayed in Fig. 3.

Fig. 3

Fig. 3 Three control operation of the power system.

1.3.1: Primary loop

The active power generated by the generator depends on the mechanical energy produced by the turbine, which is different from different input sources. In thermal and hydro plants, the turbine is called a steam turbine and a hydroturbine. Besides, the input source is steam for thermal and water for hydro, which are the responsible input to the turbine, and it produces mechanical power. In spite of that, the input (steam or water) can be controlled through the regulator by opening and closing the valve to meet the active power demand [8].

The simple block diagram of the primary loop is illustrated in Fig. 4. In this loop, speed governor acts as a speed sensor, which senses the speed or frequency from the feedback through steam/hydro turbine and subsequently gives a command to the hydraulic amplifier to change the valve level either raise or low according to the desired frequency. In practice, the primary loop achieves a local programmed control that offers backup power in the response of frequency change. Though, it has been verified that this loop cannot restore the frequency at a rigid level when the power system has more than one area. Thus, a secondary loop must be employed to regulate the set point via the speed changer and controller.

Fig. 4

Fig. 4 Primary loop.

1.3.2: Secondary loop

The secondary loop consists of the primary loop with a speed regulator and controller, which is shown in Fig. 5. The importance of this loop is feedback control, which is fed from the frequency deviation output, and it adds to the primary loop to precisely control the frequency change. Meanwhile, the speed changer motor acts as a regulator after providing the set point command from the controller. In an actual power system scenario, the integral or proportional plus integral controller has been used as a dynamic controller. With this mechanism, the AGC restores the frequency level in response to the disruptive events within a broad range. However, in case of extreme events, even the secondary controller cannot restore the frequency, which may lead to a power outage; hence, the emergency loop must be considered [9].

Fig. 5

Fig. 5 Secondary loop.

1.3.3: Emergency loop

The need for an emergency loop is in the extreme event cases such as major 3-phase faults, disasters, inevitable accidents, or other major catastrophes. In these cases, the frequency deviation is very large, which is very difficult to restore. Thus, load curtailment techniques shall be adapted to quickly manage the frequency deviation and evade the blackouts. Moreover, the emergency loop establishes better dynamic characteristics during the events with the load curtailment procedure. The main aims of the load curtailment are to cut down the least coupled load and make the reliable operation of the system from a contingency scenario to a normal operating state [9].

1.4: Individual model of the AGC system

The AGC system comprises mainly controller, governor, turbine, generator, and load units. Each unit has been modeled is as follows [1].

1.4.1: Generator model

The swing equation of synchronous generator with minute load change can be written as

si1_e    (1)

With a small speed deviation, we have

si2_e    (2)

Applying Laplace transformation in Eq. (2), we get

si3_e

   (3)

The parameters of the Eq. (3), H, Δ Pm , and Δ Pe are the inertia constant, mechanical power, and electrical power, respectively (Fig. 6).

Fig. 6

Fig. 6 Generator model.

1.4.2: Load model

The load on the power system is given by

si4_e    (4)

In this case Δ PL, D, and Δ ω are the non-frequency sensitive load change, damping co-efficient with frequency sensitive load change, respectively (Fig. 7).

Fig. 7

Fig. 7 Load model.

1.4.3: Turbine model

The turbine model can be modeled from the input source to the output of mechanical energy. The input source is different from a different plant such as thermal, hydro, and gas is the steam, hydro, and gas turbine correspondingly. Further, the input can be presented as the change in valve power, i.e., Δ Pv.

The transfer function model of the turbine is presented as

si5_e    (5)

where Tt is represented as turbine time constant and the range between 0.2 and 2 s (Fig. 8).

Fig. 8

Fig. 8 Turbine model.

1.4.4: Governor model

The governor model can be defined as the feedback power Δ Pg with regulating parameter (R) to the valve power (Δ Pv). The transfer function model of the governor is presented as (Fig. 9)

si6_e    (6)

si7_e    (7)

Fig. 9

Fig. 9 Governor model.

1.4.5: Tie-line model

In an interconnected power network, the tie-line plays a vital role in increasing the number of areas. The frequency and tie-line both can present the mismatch characteristics of the connected areas. Thus, a proper power exchange can be done through AGC.

The change in tie-line power Δ Ptie,i, can be written as

si8_e

   (8)

where Tij represents the synchronizing coefficient. For instance, if there is a two-area power system, which is interconnected with each other through tie-line, i.e., Tij = T12. Similarly, for three areas, Tij = T12, T13, and T23.

For two area power system, Tij = T12, and the Δ Ptie12 is formulated as (Fig. 10)

si9_e

   (9)

Fig. 10

Fig. 10 Tie-line model.

1.5: Structure of the AGC system

The AGC can be formed in a single-area or multiarea power system. The formation depends on the number of control area which is connected to the single area through tie-line. Furthermore, a diverse source of power generation can also be added to a single area to construct a multiarea network. There are many advantages of a multiarea network and mainly focusing on the reliability and power interchange. The detail description of LFC and its interconnection is explained as follows.

LFC is termed as one of the breakthrough technology which has been employed over the past few decades to regulate the frequency and interchange power deviation in an interconnected power network by means of tie-line [1]. Here, the deviation in frequency is adjusted based on increasing or decreasing the individual generators in a group by designing suitable controllers. Many researchers have developed a variety of controllers for AGC and hence control the system frequency to maintain and to improve stability. The exchange power flow through the tie-line between the control areas can be achieved through frequency regulation during the normal and disruptive scenarios [2].

The main aim of the LFC is

•To observe and regulate the frequency and throughput power deviation in an interconnected power network.

•To monitor the tie-line exchange power between the control regions.

Even with a minute load change, the power system becomes an imbalance between generation and demand in their frequency range. This imbalance problem is due to kinetic energy extracted from the plant. Consequently, system frequency reduces. Seeing that power consumed by the load reduces slowly with the reduction of frequency. In bulk power networks, the balance can be achieved by all of them meeting at a point when the freshly added load is diverted by minimizing the power expended by the old load, which is directly related to the extraction of kinetic energy in the system. Undoubtedly, with the marginal frequency decrease we’re attaining this balance. In this way, some regulatory action takes place to meet the equilibrium point, and there is no requirement of governing operation. Notwithstanding, the level of the frequency deviation of this state is very high.

To manage the large deviation, the governor plays a significant role. When the governor is put into operation, the generator’s output becomes imbalanced. Therefore, the equilibrium point can be achieved by means of load demand and power generation balance technique through governing action. It controls both raising and lowering the generation, which results in meeting the equilibrium point to evade the large deviation. Moreover, kinetic energy plays a vital factor, as it reduced the considerable amount, but not ultimately. Consequently, frequency deviation reduces but still require to meet the equilibrium point. However, in this action, the deviation reduction is minimal compared to those mentioned above. There is an increasing concern that this case mostly acquired the equilibrium from 10 to 12 s after adding loads. This type of governing action is named as the primary control.

However, after the experience with governing operations, still, the system frequency does not match with the actual one. Thus, a new control technique must be needed to match with the actual frequency. In an initial attempt, engineers have applied an integral controller to further reduce frequency deviation, and this control action has been termed a secondary control loop. More importantly, after the operation of the governor, a secondary control loop plays into action to bring into the actual system frequency or closer to that.

1.5.1: Power system interconnection and its significance

The power system connects two or multiarea systems through tie lines; each area provides the control, and this tie-line between the areas permits the flow of power. While the change in load demand affects the output frequency in any interconnected regions of the system, tie-line power also gets disturbed. Hence, the disturbance affected by all neighbor areas is monitored through the control action of the respective region to establish the predefined value of system frequency and tie-line power [10]. Each system has its area frequency deviation, tie-line power change, and area control error (ACE). The ACE can be described with frequency deviation (△ Fi), tie-line power change (△ Ptie), and frequency bias parameter (βi), expressed in Eq. (10). It is noted that the ACE is used as the input of the controller.

si10_e    (10)

Given all that has been mentioned so far, one may suppose that an appropriate control system is needed in the LFC wherein the two main parameters, frequency and exchange power, restore the actual value during the disturbance. But in practice, the interconnected power system of load frequency control is more important than an isolated single area system. Although, it is equally important for both isolated and interconnected power systems as per the theoretical knowledge. Today’s power system has all interconnections with their contiguous regions, and LFC is developed by combined effort. Some fundamental working practices of an interconnected power network are