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.
