INVESTIGATION AND ANALYSIS OF LOW FREQUENCY OSCILLATIONS IN 500/220KV NETWORK OF PAKISTAN

Abstract

Generation expansion in the southern part of Pakistan by installing three coal-fired power plants of almost 3100MW, have not only enhanced the capability of the generation system but also have provided a cheap source of generating electricity. However, with the increase in power generation in the southern part of the country, 500/220kV network of Pakistan is facing the problem of low frequency oscillations while transporting energy from southern to northern part. This poses a great threat to system stability as system parameters like voltage, frequency, active and reactive power greatly fluctuate during the period of oscillations and if these oscillations are not controlled or properly damped, it can lead to partial or full blackout. Second major problem due to these oscillations is that it requires system operator to curtail cheap generation in the southern part of the country and to use expensive generation in the northern part, resulting huge financial loss by violating the economic merit order. The purpose of this research is to investigate the cause/s of low frequency oscillations by modeling the 500/220KV power system of Pakistan on DIgSILENT Power Factory and using a stability technique known as modal/eigenvalue analysis. In this technique, system eigenvalues are calculated at an operating point and then eigenvectors and participation factors are calculated to identify the contribution of generators that are participating in the critical oscillation modes. In this research modal/eigenvalue analysis is done under the condition of disturbance when 470MW & 110MVAR load is suddenly dropped in the K. Electric network resulting overloading of the three major 500kV circuits connecting southern to northern part of the country. Results of eigenvalue analysis reflect very low damping ratio of less than 1% of four critical oscillations modes including 0.49Hz, 0.50Hz, 0.51Hz and 0.69Hz. Also mode shape curvesviii clearly indicate the presence of inter-area oscillations. Participation factors in the critical oscillation modes reveals that the generator G10 has the highest contribution of 94.5%. Similarly G9, G13, G14, G5 and G6 have relatively high participation factors of 89%, 60.4%, 81.1%, 45.9% and 46.2% respectively. Further time domain investigation is done to reveal that generator G10 goes into deep leading phase by absorbing more MVars than the maximum allowed limit after the disturbance occured and hence is a major cause to initiate the oscillations of frequency 0.5Hz. Similar is the case with generators G5 and G6 that also go into the deep leading and unstable zone by absorbing more MVars than the maximum allowed limit. So they also contribute in the oscillations. Two improvements are proposed in this research in order to mitigate the causes of oscillations and to provide adequate damping. First is to bring down the voltages of the southern region to 510kV, so that the generators G5, G6 and G10 don‘t go in the deep leading and unstable phase. This is achieved by switching on the shunt reactors installed on the various 500kV southern AC transmission lines and by increasing the tap-changer position of unit transformer installed on various generators in the southern region. Also it is made sure that G1, G2 & G3 equally participate in absorbing reactive power (apart from G5, G6, G9, G10, G13 & G14) to keep the voltages of southern region under control. Second improvement is to activate the Power System Stabilizer, installed on the generators G5, G6, G9, G10, G13 and G14 in the southern region. After combining the two improvements, eigenvalue results reveal that % damping ratio of 0.49Hz mode, 0.50Hz mode and 0.51Hz mode have been significantly improved to 10.5891%, 9.5909% and 12.7663% respectively. Hence system response to damp low frequency oscillation has been significantly improved after combining the two proposed solutions.