Abstract:
Emergency load shedding for preventing frequency degradation is an established practice all
over the world. The objective of load shedding is to balance load and generation of a particular
Power System. In addition to the hydro and thermal generators each with less than 100 MW,
today, the Power System of Sri Lanka is comprised of three coal Power Plants: each has a
generation capacity of 300 MW, Yugadanavi combined cycle Power Plant (300 MW generation
capacity) and a considerably extended transmission network. To cater consumers with high
quality electricity, a reliable Power System is a must. Therefore, it has become timely necessity
to review the performance of the present CEB Load Shedding Scheme and suggest amendments
where necessary.
The objective of this research is to explore a better Under Frequency Load Shedding Scheme
(UFLSS) which can face probable contingencies and maintain stability of the system while
catering more consumers. The suggested UFLSSs can address the recent changes taken place in
the Sri Lanka Power System too.
A simulation of the Power system of Sri Lanka was designed with software PSCAD. Its validity
was checked through implementing actual scenarios which took place in the power system under
approximately-equal loaded condition and by comparing the simulated results and actual results.
Then a performance analysis was done for the Ceylon Electricity Board (CEB) Under Frequency
Load Shedding Scheme which is being implemented in Sri Lanka. Having identified the
drawbacks of the CEB Under Frequency Load Shedding Scheme, the new UFLSSs (LSS-I and
LSS-II) were suggested.
The Load Shedding Scheme – I (LSS-I) is designed based on power system frequency and its
derivative under abnormal conditions. Without doing much modification to the prevailing
UFLSS, and utilizing the available resources, the suggested LSS-I can be implemented.
The LSS-II gives priority for 40% of the system load for continuous power supply, and it is
comprised of two stages. During the stage-I, approximately 30% of the load is involved with the
Load Shedding action. During the stage-II, the disintegration of the power system is done. This
involves the balance 30% of the load. At 48.6 Hz the disintegration of the power system takes
place. By disintegrating the power system at the above mentioned frequency, all islands as well
as the national grid can be brought to steady state condition without violating the stability
constraints of the Sri Lanka power system. During disintegration of the Power System, special
attention must be paid for:
• Generation & load balance in each island and in the national grid.
• Reactive power compensation in islands and in the national grid.
• Tripping off of all isolated transmission lines (which are not connected to effective
loads).
Through simulations the effectiveness of the UFLSSs were evaluated. They demonstrate better
performance compared to that of the currently implementing CEB scheme. Results highlight that
the UFLSS should exclusively be specific for a particular Power System. It depends on factorssuch as power system practice, power system regulations, largest generator capacity, electricity
consumption pattern etc.