Mode selective damping of power system electromechanical oscillations for large power systems using supplementary remote signals

Abstract

This paper presents the design of local decentralized power system stabilizer (PSS) controllers, using selected suitable remote signals as supplementary inputs, for a separate better damping of specific inter-area modes, for large-scale power systems. System identification technique is used for deriving lower order state-space models suitable for control design. The lower-order model is identified by probing the network in open loop with low-energy pulses or random signals. The identification technique is then applied to signal responses, generated by time-domain simulations of the large-scale model, to obtain reduced-order model. Lower-order equivalent models, thus obtained, are used to design each local PSS controller separately for each of the inter-area modes of interest. The PSS controller uses only those local and remote input signals in which the assigned single inter-area mode is most observable and is located at a generator which is most effective in controlling that mode. The PSS controller, designed for a particular single inter-area mode, also works mainly in a frequency band given by the natural frequency of the assigned mode. The locations of the local PSS controllers are obtained based on the amplitude gains of the frequency responses of the best-suited measurement to the inputs of all generators in the interconnected system. For the selection of suitable local and supplementary remote input signals, the features or measurements from the whole system are pre-selected first by engineering judgment and then using a clustering feature selection technique. Final selection of local and remote input signals is based on the degree of observability of the considered single mode in them. To provide robust behavior, H_∞ control theory together with an algebraic Riccati equation approach has been applied to design the controllers. The effectiveness of the resulting PSS controllers is demonstrated through digital simulation studies conducted on a sixteen-machine, three-area test power system.

Introduction

As a consequence of deregulation of electrical energy markets worldwide and the extensions of large interconnected power systems, many tie lines operate near their maximum capacity, especially those connected to the heavy load areas. Stressed operating conditions can increase the possibility of inter-area oscillations between different control areas and can even lead to the breakup of the whole system [1]. Weakly damped low frequency inter-area oscillations, inherent to large interconnected power systems during transient conditions, are not only dangerous for the reliability and performance of such systems but also for the quality of the supplied energy. With the heavier power transfers ahead, the damping of these oscillations will decrease unless new lines are built (construction of new lines is restricted by environmental and cost factors) or other heavy and expensive high-voltage equipment such as series-compensation is deployed. Therefore, the achievement of maximum available transfer capability as well as a high level of power quality and security requires better coordinated protection and system stability control which leads to damping improvement.

Damping of low-frequency oscillations and improving power system stability via PI stabilizer is given in [2]. Ref. [3] has presented coordinated design of PSSs and TCSC via bacterial swarm optimization algorithm in a multimachine power system. Design of power system stabilizers using two level fuzzy and adaptive neuro-fuzzy inference systems is discussed in [4]. Comparative seeker and bio-inspired fuzzy logic controllers for power system stabilizers have been developed by [5]. It is found that if remote signals from one or more distant locations of the power system are applied to the controller design, the system dynamic performance can be enhanced in terms of better damping of inter-area oscillations [6]. The remote signals are often referred to as global signals to illustrate the fact that they contain information about overall network dynamics as opposed to local control signals which lack adequate observability of some of the significant inter-area modes [7]. The recent advances in wide area measurement (WAM) technologies using phasor measurement units (PMUs) can deliver synchronous control signals at high speed [8]. PMUs are deployed at strategic locations on the grid to obtain a coherent picture of the entire network in real time [8]. PMUs measure voltages and currents at different locations of the grid. Global positioning system (GPS) technology ensures proper time synchronization among several global signals [8]. The measured global signals are then transmitted via modern telecommunication equipment to the controllers.

According to [9], for a local mode, participation factor method gives correct results for predicting best PSS location. But for an inter-area mode this method may not give accurate prediction [9]. Participation factor method gives inaccurate or wrong predictions because this method does not take into consideration PSS control effect information. Participation index include part information of modal controllability and observability [10]. The residue method is derived from the modal control theory of linear time-invariant systems. The concept presented in [11] is to find the locations of local H-infinity based PSS controllers based on the amplitude gains of frequency responses of the best-suited measurement to the inputs of all generators in the interconnected system. The mode selective damping of power system electromechanical oscillations using supplementary remote signals is presented in [11]. Ref. [11] also proposes a technique for the selection of suitable local and remote feedback input signals to the controllers. For the selection of suitable local and supplementary remote input signals, the features or measurements from the whole system are pre-selected first by engineering judgment and then using a clustering feature selection technique. Final selection of local and remote input signals is based on the degree of observability of the considered single mode in them and that is determined from the amplitude gains of frequency responses of the features or measurements obtained using clustering technique. Ref. [11] has applied the proposed technique on a three machine system which is too small to provide any convincing justification for the method. The power system, in practical, is large in size. Therefore, the applicability of the technique to larger-sized systems becomes questionable. The much larger system with many choices of signal would provide a much more thorough case. In this study, the authors have applied the technique proposed in [11] to realistic large-scale multi-owner power systems such as the European interconnected power system.

This paper deals with the design of local decentralized PSS controllers using remote signals as supplementary inputs, for a better damping of inter-area oscillations in a large-scale power system, in a manner that each decentralized PSS controller is designed separately for each of the inter-area modes of interest. The basic architecture [11], [12] used in this work is shown in Fig. 1. It consists a set of n PSSs located at specially selected generators G₁, … , G_n together with m PMUs which are remote to the PSSs. The concept presented in this paper is based on the assumption that the PSS inputs are formed by measured variables coming from the whole system, i.e., also from remote generators [13]. In this way, each PSS receives more complete measurement information about the inter-area oscillations to be damped. It is possible to use any of the variables from the generators, e.g., generator rotor speeds or angles or variables not assigned to generators, e.g. selected tie-line power flows as remote input signals to the controller. As the global information is required only for some oscillatory modes, only a few PSS sites with the highest controllability of these modes need be involved in the supplementary global-signal-based actions. H_∞-based robust control technique is used to design the proposed PSS controllers. Digital simulation studies on a 16-machine, three-area test power system are conducted to investigate the effectiveness of the proposed controllers during system disturbances.

The rest of the paper is organized as follows: In Section 2, lower-order state-space model identification is presented. In Section 3, concept of mode selective damping is presented. Section 4 presents the selection of suitable local and remote input signals and locations for the PSS controllers. In Section 5, the design of robust H_∞-based dynamic output feedback PSS controller is presented. The application results for a dynamic model of sixteen-machine, three-area test power system are presented in Section 6. The conclusions are discussed in Section 7. References are given in Section 8.