Abstract

In this paper,the subsynchronous resonance (SSR)for IEEE second benchmark system is investigated using Matlab based on the Power System Block-set (PSB)in conjunction with Simulink.The effects of both series compensation level and fault clearing time on SSR are investigated. The results obtained verify the effect of series compensation level on the subsynchronous frequencies. These studies are confirmed using both frequency analysis and eigenvalue techniques,which are carried out based on Matlab control system toolbox.In addition,the influence of both fault clearing time and series compensation level on maximum magnitudes of the turbine-generator shaft torques have been analysed using time domain simulation technique.

Introduction:
Series capacitors have been extensively used as a very effective means of increasing power transfer capability of transmission system,and improving transient and steady state stability limits of a power system.This is due to partially compensating the reactance of the transmission lines. However, the application of series capacitors may lead to the phenomenon of subsynchronous resonance. Under a disturbance,series capacitors may excite subsynchronous oscillations,when electrical resonant frequency (fer)of the network is close to natural torsional mode frequency of turbine-generator shaft.

Under such circumstance the shaft will oscillate at this natural frequency. This oscillation might grow to endurance limit in seconds resulting in shaft fatigue and possibly damage and failure.Therefore,there is a need to investigate and analyse subsynchronous resonance when planning inclusion of series capacitors for new or existing power system. Subsynchronous resonance is addressed in three categories,i.e.,induction generator effect,torsional
interaction and torque amplification.In all cases, subsynchronous resonance is due to the interaction of
a series capacitor with turbine-generator [1-3]. 

The first two types are caused by a steady state disturbance,while the third is excited by transient disturbances.Different approaches in subsynchronous resonance analysis are presented in the literature. References 1-3 refer to three analytical methods to analyse SSR:frequency scanning,eigenvalue analysis and the time domain simulation. Frequency scanning has been used to study induction generator effects [4].The eigenvalue analysis is used to identify frequencies of subsynchronous oscillations as well as the damping of each frequency from state space model of the entire system [1].This analysis has been regarded as a good measurement of the proper
operation of SSR countermeasures suggested in [5]. Time domain simulation has been performed in different types of programs such as EMTP (Electromagnetic Transient Program).These studies provide important information regarding peak shaft that is to be expected when a certain level of series compensation is applied [6].

In this paper,an important application of the second benchmark system [4]and Matlab [7]is presented to study subsynchronous resonance that may occur in series compensated transmission systems –in particular,the simulation of the torque amplification and eigenvalue analysis of the electrical network.
The investigation of the above phenomenon is carried out using Matlab simulation and analysis tools. The subsynchronous resonance damping under different levels of series compensation in the network,and the effect of fault clearing time on the torque amplification are investigated.

System configuration:
The system considered in this paper is the IEEE second benchmark model of which the single line diagram and data used in this study are shown in figure 1 [1]. A single generator of 600 MVA,22kV is connected to infinite busbar through transformer and two transmission lines.One line is series compensated

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