Determination of Control Input/Output Signals for Damping Low Frequency Oscillations

This paper presents the two approaches of control feedback signal selection for wide area controller to damp the inter-area oscillations in two-area system namely geometric approach and residue approach. With the help of simulation studies, the best approach is selected. The result obtained show that from both the measures we get the same control signal but the magnitude is relatively larger in case of residue approach. Keywords—Control loop selection, Geometric measures, interarea oscillations, power system stabilizer, Residue measures, wide-area control. I.INTRODUCTION With the increase in use of new technologies, complex type of instability is occurring in power system. In particular, oscillatory instability has drawn concern. With the bulk power transfer, such system exhibits low frequency interarea modes of oscillations. Their frequency range is 0.1-0.7 Hz. Today, the damping of inter-area oscillations can be achieved using Power System Stabilizer(PSS),the static VAR compensator (SVC) modulation and FACTS devices with local control modes[3],[5]. With the increase in interconnections among the Power system and coupling between the machines, with the occurrence of fault in one area will tend to affect the larger area. Therefore, a multi controller must be designed that can minimize the interactions between the controllers and at the same time damp the low frequency oscillations. We have considered a two area system with two generators in each area. Modal analysis of the system is done to obtain the modes responsible for oscillations [1]. Two control loop selection measure is applied to the system in order to select the most effective feedback signal for damping inter-area oscillations. As the conventional PSS is only limited to damp the local area modes effectively, therefore, we require a wide area controller which requires a global control signal that can damp out the oscillations increases the reliability of the system [5]. This paper concentrates on two approaches for control signal selection which can be used to select the control signal [5]. However, the objective for control signal selection is to select pair of signals that can provide maximum controllability/observability or to minimize the interaction between the controllers [4]. Based on the results, the better method will be selected that is robust and efficient and provide better results for damping the oscillations. The paper is organized as follows: Section II is demonstrates the system modelling. Then the signal selection methodologies are presented in Section III. Section IV depicts the studied case with the results given in Section V, the conclusion is provided in Section VI. II. SYSTEM MODELLING Power system is a non-linear system which is described by ordinary non-linear differential of the following from: ? ̇? = f(x, u) (1) y = g(x, u) (2) where x ɛ R , u ɛ R and y ɛ R are the state, input and output vectors, respectively. n is the dimension of the system, m is the number of inputs, and p is the number of outputs. For measurement and control signal selection, a linearized model is used which obtained by the modal analysis of the network. As the perturbations are small, a non-linear function f(x, u) can be expressed in terms of Taylor’s series. To obtain the small signal model around an operating point, several software such as MASS/PEALS, POSSIM/MANSTAB, and SDYN can be used. In this paper linearized is obtained using the MATLAB/simulink. The linearized form of the eq.(1) and (2) are given by: ∆? ̇? = A∆x + B∆u ∆y = C∆x + D∆u (3) Where ∆x is the state vector of dimension n ∆u is the input vector of dimension r ∆y s the output vector of dimension m A is the state matrix of dimension n × n B is the input matrix of dimension n × r C is the output matrix of dimension m × n D is the feed-forward matrix of dimension m × r The electromechanical modes are calculated with eigenvalues of A{ λi = σi ± jωi }. Frequency (Hz) and damping ratio of the i mode is calculated using fi = ωi/2π and ζi = −σi/√σi 2 + ωi 2. III. SIGNAL SELECTION METHODOLOGIES In this section, two unique techniques are utilized to select the feedback control signal for wide area PSS in order to damp inter-area oscillations. The same analysis is done on a different system in [5]. We’ll discuss them one by one as follows: International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 http://www.ijert. org IJERTV5IS020251 (This work is licensed under a Creative Commons Attribution 4.0 International License.) Published by : Vol. 5 Issue 02, February-2016