Review of Load Frequency Control Methods Part-I: Introduction and Pre-deregulation Scenario Sachin K. Jain S. Chakrabarti and S. N. Singh Department of Electronics & Communication Engineering PDPM Indian Institute of Information Technology, Design & Manufacturing, Jabalpur, India skjain@ieee.org Department of Electrical Engineering Indian Institute of Technology Kanpur Kanpur, India saikatc@iitk.ac.in, snsingh@iitk.ac.in Abstract— In an electric power system, which is one of the most complicated systems among any non-natural systems, Load Frequency Control (LFC) has always been an important and critical issue. Therefore, many configurations of power system model, variety of control strategies and a number of controllers have been proposed in the literature for making LFC more effective, robust, adaptive and efficient. In this work, which is divided into two parts, namely, ‘Part-I: Introduction and Prederegulation Scenario’ and ‘Part-II: Post-deregulation Scenario and Case Studies’ attempt has been made to provide literature review on LFC. This paper discusses fundamental aspects of LFC followed by some LFC methods that were proposed for prederegulation scenario. Significance of these methods in the present scenario has been highlighted. Keywords—Automatic Generation Control (AGC); Deregulated Power Market; Load Frequency Control (LFC); Power System; Real Power Control. I. INTRODUCTION A power system is a highly non-linear and large-scale multi-input multi-output (MIMO) dynamical system with numerous variables, protection devices and control loops, with different dynamic responses and characteristics. Hundreds of small and large interconnected generating units supply a variety of loads across the huge geographical area through highly meshed transmission lines, and distribution network. It is highly desirable to improve the performance and functions of power systems during normal and abnormal operations, but it is not an easy task, due to large interconnected network with constantly changing load, operational limits, rotor angle instability, voltage instability, frequency instability, economy in operation and physical disturbances. In real power systems, there is some overlap between the different forms of instability; one phenomenon may lead to others and as systems fail, more than one form of instability may ultimately emerge [1]. For example, during frequency excursions following a major disturbance, voltage magnitudes and power flow may change significantly. Various power system controls have evolved over the past few decades to keep the power system in a secure state and protect it from dangerous phenomena [2]-[5]. Some of these are generation excitation controls, prime mover controls, generator/load tripping, fast fault clearing, high speed re- closing, voltage control/reactive power compensation, load– frequency control and current injection etc. In the recent times, static VAR compensator (SVC), high voltage direct current (HVDC) converters, FACTS controllers and wide-area power system stabilization and control are increasingly being used to solve power system oscillation problems [6] but loadfrequency control still retains its importance because it forms an important, integral part of automatic generation control. Automatic generation control, a centralized integrated control, balances the total generation and load (plus losses) to reach the nominal system frequency (usually 50 or 60 Hz) and scheduled power interchange with neighboring systems. This work aims at emphasizing the challenges presented by deregulation (restructuring) of the power system to the LoadFrequency Control and the state of the art in exertion of these challenges. The entire work has been divided into two parts, namely, ‘Part-I: Introduction and Pre-deregulation Scenario’ and ‘Part-II: Post-deregulation Scenario and Case Studies’. Part-I discuss about the motivation and fundamentals of LoadFrequency Control (LFC), and comparison of LFC techniques proposed for pre-deregulation scenario. Part-II presents, impact of deregulation on power system, and review of LFC techniques for post-deregulation scenario. Finally, case studies of small power system are presented to explain the LFC approaches in deregulated environment. II. OVERVIEW OF LFC It’s amazing but true that all the generators (can be hundreds, thousands or even more) interconnected to a network run at the same speed. To understand how this speed can be controlled, let us consider an isolated generator, supplying a local load, as shown in Fig. 1. The generator experience two opposing torque; the mechanical torque (input), Tmech, in the direction of rotation that acts to increase rotational speed, and the electrical torque (load/output), Telect, in the opposite direction of rotation that acts to slow down the speed. In the steady state condition, both torques will be equal in magnitude and speed will remain constant. When there is a change in electric load on the generator (reflected instantaneously as a change in the electrical output torque (Telect)) and/or mechanical input to the turbine, equilibrium between the mechanical torque (Tmech) and the electrical torque (Telect) is disturbed, which in turn results in speed variation. This Telect Mechanical Energy Input Turbine Electrical Energy Output Generator Tmech Δf Speed Governor factual Controller Frequency Measurement Reference Frequency fref Fig. 1. Mechanical and Electrical Torque Balance in a Generating Unit. continues until system attains a new equilibrium point. Frequency, being a direct function of speed of the prime mover, also varies along with any imbalance between electrical and mechanical torque. For a lossless system, the relationship between imbalance and speed can be expressed as, Pmech Pelect M ds dt (1) where, Pmech is mechanical power input to the turbine, Pelect is electrical power output of the generator, s is rotational speed of the rotor and M is inertia coefficient. A. Need for LFC [1, 4, 7] For satisfactory operation of a power system, frequency should remain nearly constant. Frequency deviations can directly impact on a power system operation, system reliability and efficiency. Large frequency deviations can damage equipments, degrade load performance, overload transmission lines and interfere with system protection schemes. These large-frequency deviation events can ultimately lead to a system collapse [4]. Variation in frequency adversely affects the operation and speed control of induction and synchronous motors. The reduced speed of motor-driven generating station auxiliaries, associated with the fuel, the feed-water and the combustion air supply systems, such as fans, pumps, and mills, will bring down plant output. Considerable drop in frequency could result in high magnetizing currents in induction motors and transformers thereby increasing reactive power consumption. The extensive use of electric clocks and the use of frequency for other timing purposes require accurate maintenance of synchronous time which is proportional to integral of frequency. In domestic appliances, where refrigerators’ efficiency goes down, Television and Air Conditioners reactive power consumption increases considerably with reduction in power supply frequency. Therefore it is very important to maintain the frequency within allowable range. Due to the statistical nature of the load fluctuations, we cannot avoid continuous load changes but we can hope to keep the system frequency within sufficiently small tolerance levels by adjusting the generation continuously. This is achieved using automatic generation control, which requires LFC mechanism for its implementation. Shayeghi et al. [5] have mentioned main objectives of LFC as: 1. Ensuring zero steady-state error for frequency deviations. 2. Minimizing unscheduled tie line power flows between neighboring control areas. 3. Getting good tracking for load demands and disturbances. 4. Maintaining acceptable overshoot and settling time on the frequency and tie line power deviations. B. Defining LFC Load Frequency Control (LFC) is an ancillary service that is related to the short-term balance of energy and frequency of the power systems and acquires a principal role to enable power exchanges and to provide better conditions for electricity trading. The main goal of the LFC is to maintain zero steady state errors for frequency deviation and good tracking of load demands in a multi-area power system. At the generation unit level, speed governor controls the speed of the prime mover as the load on the generator change. This is referred as primary speed control function. The setting of the governor can be modified as per the load to keep prime mover speed and hence frequency constant. In a single machine system, it is not a big deal to control the frequency. The basic scheme of Load Frequency Control of single generating unit is shown in Fig. 1. In this scheme, the speed governor senses the change in speed (frequency) via the primary and supplementary control loops. A hydraulic amplifier provides the necessary mechanical forces to position the main valve against the highsteam (or hydro-) pressure, and the controller provides a steady-state power output setting for the turbine. For real interconnected system the control is not as simple as for single unit. III. LFC METHODS FOR PRE-DEREGULATION SCENARIO For almost a century, electricity business (Generation, Transmission and Distribution) was considered as a natural monopoly. The electricity business was operated by vertically integrated utilities, under regulatory control of government/ public bodies. It means generation, transmission and distribution of electricity were under control of a single entity. Hence, in an isolated power system, owned by single entity or utility, consisting of a group of generators and loads, where all the generators respond to changes in load or speed-changer settings, in unison, regulation of power imbalance in generation and supply is not a serious issue. The LFC task, in such systems, is limited to restore the system frequency to the specified nominal value. There are following possible ways to share the change in the load to maintain frequency [8]: 1. Either of generating units cater the change in load (Flat Frequency Regulation) known as control areas. A centralized system then uses automatic equipments to control the frequency of complete system. This concept is known as Multi Area Control. Fig. 2 shows the vertically integrated utility structure of power system with three control areas. Area-2 G Area-1 G1 G2 G3 The real system is much more complex and has large number of generating units and diverse load centers interconnected in highly meshed way via transmission lines. There may also be some Independent Power Producers (IPPs). Therefore, in real power system, the controlling of generationsupply imbalance is not as simple as it appears above. To make the control task simpler, the most widely used technique is to divide whole system into many relatively smaller systems D Area-3 D1 D2 + D A. Tie-Line Bias Control The LFC uses an indicator called Area Control Error (ACE) to send control signals to appropriate units under its control. A Turbine ΔPmech1 1 M 1s D1 ΔPtie Δ1 + T/s - + + Governor + Turbine ΔPmech2 - 1/R2 B2 Fig. 3. Tie Line Bias Supplementary Control Loops for Two Area Control. D A multi-area power system comprises areas that are interconnected by high voltage transmission lines or tie-lines. The trend of frequency measured in each control area is an indicator of the trend of the mismatch power in the interconnection and not in the control area alone. Therefore, in addition to regulating area frequency, the supplementary control should maintain the net interchange power with neighboring areas at scheduled values. This is generally accomplished by adding a tie-line flow deviation to the frequency deviation in the supplementary feedback loop. Fig. 3 shows the block diagram of tie line bias supplementary control loops for two area control. - K/s G Fig. 2. Vertically Integrated Utility Multi Area Structure of Power System. - ACE2 G TRANSMISSION IPP - Governor G Tie Line 1/R1 K/s D TRANSMISSION B1 ACE1 G TRANSMISSION 2. All units share the change in load (Parallel Frequency Regulation) Control signals based on any of the above two methods can be issued to the speed governor of generating units to balance the load change. Primary control action is not usually sufficient to restore the system frequency, especially in an interconnected power system and the supplementary control loop is required to adjust the load reference set point through the speed-changer motor. The supplementary loop performs a feedback via the frequency deviation and adds it to the primary control loop through a dynamic controller. G Tie Line 1 M 2 s D2 Δ2 unit will react to a raise/lower control signal by raising/lowering its generation output. The ACE can be represented as, ACE Ta Ts B * f a f s (2) where, Ta and Ts are actual and scheduled interchange (in MW), respectively, fa and fs are actual and scheduled frequency, and B is ACE frequency bias (in MW/Hz). A positive value of ACE indicates an excess area generation and a negative value of ACE indicates generation deficiency in the area. The above equation (2) is referred to as tie-line bias control. The effects of local load changes and interface with other areas are properly considered as two input signals. Each control area monitors its own tie-line power flow and frequency at the area control centre. The ACE signal is computed and allocated to the controller. Finally, the resulting control action signal is applied to the turbine–governor unit. Power to compensate the tie-line power change initially comes from all areas to respond to the step load increase in any area, and results in a frequency drop sensed by the speed governors of all generators. However, after a few seconds (at steady state), the additional powers against the local load changes come only from respective area. Glover and Schweppe [9] has proposed an advanced LFC based on discrete time, linear-plusdeadband feedback control law. B. Optimal Control Elgerd and Fosha [10], [11] first time addressed optimal control concept for frequency control design of interconnected power systems using a state variable model to develop new feedback control laws. They developed an optimal feedback controller and suggested feasible ways of improving dynamic response and stability margins of the load-frequency control. However, it was found that above approach requires knowledge of the new steady-state operating point, that is not available for implementation, and hence, the control is not a feasible optimum control. A modified Kalman filter was introduced in [12] for the identification of the incremental power demand in order to optimally compensate for loadfrequency deviations. Miniesy and Bohn [13] presented optimum load-frequency continuous control using differential approximation and a Luenberger observer based methods. C. Multi-level and Decentralized Control The concept of multi-level and decentralized control was originated because of structure of the power system as a large interconnected system. In these controls, a power system is viewed as an interconnection of several lower-order subsystems. The local controller controls each subsystem according to the interaction variables provided by a second level controller (coordinator). The coordinator improves the interaction variables to satisfy interaction feasibility by means of an iteration technique. Many multi-level control schemes have been suggested [14]-[17] for such interconnected systems as a means of providing a more reliable control strategy. The controller proposed in [14] minimizes the deviations in frequency and scheduled tie-line power resulting from sudden disturbances. The coordinator, a second level controller, improves the interaction variables to satisfy interaction feasibility by means of an iteration technique. The work reported by Premakumaran et al. [16] utilizes certain possible beneficial aspects of interconnection to permit more desirable system performances in the LFC of a two-area power system. This work also includes the effects of excitation system and governor controls of the power-system model studied in this work. Shirai [18] has proposed a decentralized LFC scheme, where, both governor and voltage controls are used in each area, the disturbances occurred in each area can be absorbed by only the controller installed in own area. There have been many works since than on decentralized LFC. Davison and Tripathi [19] has proposed two minimum order controllers for the tie-line/inter-area power flow and frequency control problems as an optimal decentralized control. Feliachi [20] has also proposed an optimal decentralized load frequency control scheme, however, this uses a fixed mode evaluation algorithm based on eigenvalue dynamics to determine its feasibility. Geromel et al. [21] proposed numerical procedure that solves a Riccati equation iteratively and the proposed method allowed to design the LFC with pre-specified structures. D. Adaptive Control Since, power system parameters, are a function of the operating point, many adaptive control schemes were reported to keep the system performance near its optimum, by tracking the operating conditions and using the updated parameters to compute the control [22]-[27]. Kanniah et al. [22] used microprocessor for application of self-tuning regulators in power system LFC. This adaptive control strategy combined a control algorithm with a parameter estimation algorithm for online update of the parameters of a model of the system. Pan and Liaw [23], [24] proposed an adaptive controller using a PI adaptation that only required parameters for the control are the available information of the states and output of the model as well as the plant output. Rubaai and Udo [25] proposed a multilevel adaptive technique based on self-tuning regulator. As claimed by the authors, the proposed technique permits optimality at all levels while providing a decomposed solution. Wang et al. [26] proposed a robust, adaptive control based on a combination of the robust control approach and an adaptive control technique that considered power system LFC with system parametric uncertainties. In [27], a neural network (NN) was used to act as the control intelligence in conjunction with a standard adaptive LFC scheme. In this approach a NN is operated in parallel with a full load frequency adaptive control scheme to monitor the system frequency, as the controller issues its control commands. If there appears a condition where the frequency values are corrupted or the system is not sufficiently excited, then the NNs will be able to provide the power set points that may be directly communicated for necessary system frequency control. IV. SIGNIFICANCE OF THESE METHODS IN PRESENT SCENARIO Introduction of deregulation of power industry worldwide has changed the way the power system is operated. Elements of power system are now owned by different utilities, which compete with each other in the market for selling their product (energy) and other services. LFC is treated like an ancillary service. Therefore, it appears that earlier methods of load frequency control are absolutely obsolete. However, this is not completely true. Although, power system has been deregulated, it has only changed the management of the system, basic control requirements remain same even in present scenario. As evident from companion paper [28] and a review article by Shayeghi et al. [5], these methods provide firm platform for development of advanced techniques that are suitable for deregulated scenario. Many recent works on LFC in deregulated power system are reported based on the concepts, such as, adaptive control, multi-level and decentralized Control, etc. Therefore, significance of these methods in present scenario cannot be overlooked. V. CONCLUSION Load frequency control has been an important issue since the establishment of first power system. Major objectives of LFC includes following the changes in load demands and regulating frequency. Specific LFC methods also help in maintaining tie line power inter-changes to specified values. This paper has reviewed key literature of pre-deregulation scenario in the area of LFC and has highlighted some of the significant developments that are worthy enough even in present deregulated scenario. Salient features of various LFC schemes reported in the literature have been discussed under four categorizing, namely, Tie-Line Bias Control, Optimal Control, Multi-level and Decentralized Control, and Adaptive Control. Significance of these methods has been established in post-deregulation using recent research outcomes. It is believed that this work, presented in two parts, will serve as a valuable resource to practicing engineers and researchers are working in this important area. 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