Relay Feedback Identification based Load Frequency Control and Controller Design

. The main objective of power system operation and control is to maintain the continuity of supply with required quality and reliability. In power system operation one of the issue is Load frequency control. Generally, the dynamics of the system ( i.e the over all dynamics of governor ,turbine, load and machine) for isolated or inter-connected power system is of higher order. The controller design for a higher order system is a tedious process. In order to over come this difficultly relay feedback identification approach has been adopted. In this approach the higher order system has been reduced to lower order system by the method of IMC-PID tuning control. Robust controller has been design for identified model for single area power system in order to achieve satisfactory closed loop performance. The same concepts have been readily extended to multi area power system.


Introduction
The problem of control the real power output of generating units in response to changes in system frequency and tie-line power inter change with in specified limits is known as load frequency control [1]. The deviation of the frequencies and tie-line power arise because of unpredictable load variations, which occur due to a mismatch between the generated and the demanded power [2]. To ensure the quality of the power supply, a load-frequency control is required to minimize the transient deviations and ensure zero steady-state errors of these two variables [3]. The objective of the control strategy in a power system is to generate and deliver power in an interconnected system as economically and reliably as possible while maintaining the frequency and voltage within permissible limits [4][5].To maintain stability of system were required best & simple control method like IMC-PID Tuning, it is most used method in industrial controller in the process of maintain stability of real power by controlling load frequency variation [6][7][8][9] .
In this paper relay feedback identification based load frequency control is proposed and design by using IMC-PID tuning method (Internal model control is one of the popular tuning method for PID controller). The proposed approach is flexible in the performance and robustness of the closed -loop systems. The proposed control structure has two tuning parameters. The method can be extended to multi -area power systems easily.

IMC-DESIGN
The concept adopt from IMC method for load frequency controller design [2]. It is common control structure used in process control. The IMC structure shown in fig-2, where P(s) is to be controlled, is the plant model,Q(s) is IMC-controller were Design .

IMC -Model design steps are followed:
Step-1: From concept of IMC parameterization the plant model is : is the system with all pole's and zero's of system present at left side of S-plane(minimum phase system), is the system with zero's present at right side of Splane(non-minimum phase system), .
Step-2: Design of set-point tuning of IMC -Controller.
where, f is the filter of IMC model Here, λ is tuning parameter of IMC-Controller, r is higher degree to get better perform of IMC -Controller, is parameter to adjust the unstable pole's & pole's near to the plant which to control.
Step-3: To cancel the poles the condition to follows: Step-4: Thus, Total close loop response can be determine.

IMC-PID design for stable third-order system:
To determine the transfer function of IMC-based PID Controllerfor Third order stable plant, where, Step-1: Solve the IMC controller transfer function, From equation (2) Step-2: Solve the typical feedback controller by using the modification, Thus transfer function is Step Step-4: To maintain robustness of the closed loop system , Adjust the value of "λ", up to the system performance is better & flexibility.

Numerical Analysis
We adopt, the numerical value from [2], Consider the power system with non-reheat turbine, the plant parameter are: The plant model without droop characteristics: The LFC IMC Design the λ=0.9 were adjusted, Then the PID controller.

The plant model with droop characteristics:
For λ=0.5, The parameter of PID Controller were change value ,time constant were remain same, Thus new PID controller were:

SIMULATION MODEL
Here, the simulation model consist Excitation system model:

Conclusion
A Relay feedback identification of load frequency control of the power system is addressed in this paper. The controller parameters were tuned with IMC approach setting to improve performance of system stability and robustness. The beauty of the tuning method is that the controller parameters are expressed in terms of plant parameters. Further, the proposed technique is flexible and can easily applied to a multi area power system.