Dynamic load-shedding analysis for enhancement of power system stability for the Lesotho 132 kV transmission network

Abstract

In the past two decades, the Lesotho power system load demand increased from below 72 MW to 150 MW. This steep load increase means that the country's power generation of 72MW from hydro-power generators installed at 'Muela is highly lacking and that the country cannot operate without surplus power from the neighbouring states. The South African Eskom tie-line that feeds into Lesotho has thus become crucial for continuous supply of the balance of power and the stable operation of the Lesotho power system, particularly during loading conditions above the installed capacity. Perturbations that affect this tie-line impose stress on the power system and often lead to system collapse. The transient stability of the generating units at 'Muela Hydro-power during and after such network perturbations is key to sustaining continuity of supply to the highest volume of power consumers during the tie-line contingency. It is therefore paramount that inadvertent system collapses be minimised and for the techno-economic status quo to be improved. This study assessed the system loading, using the system data from the Lesotho Electricity Company for the years 2013 and 2017. Using DIgSILENT for simulations of the performed calculations, the result scans showed the system behaviour during the system disturbances and load changes for the different study cases to support the study outcome. DIgSILENT is a widely used software package in advanced power system calculations and analysis. The core of the proposed solution through the different studies was a successful implementation of dynamic load-shedding for the events that affected the power transfer to the Lesotho power system. Using the Lesotho and the South African grid code as the basis for the assessment of the system operation limits during emergency operation, this study examined and presented the results showing the impact of tripping different combinations of loads to achieve the supply/demand balance during the faulted system condition. On application of each dynamic load-shedding, the study drew attention to the impact that the dropping of each combination of loads had on the synchronous generators' operational limits, e.g. the active power, reactive power, excitation voltage and the frequency characteristics, which determined whether each case supported safe system operation for adoption by the Lesotho Electricity Company (LEC) and the Lesotho Highlands Development Authority (LHDA). The results showed that through the application of dynamic load-shedding, system collapse caused by disturbances on the tie-line is avoidable. The study's results are valid as post the load-shedding operation, the generating system met the voltage and frequency variations limits in IEEE Std C50.13 — 2014. The study further tested the system stability during faulted conditions through varying fault clearing times, to determine the critical fault clearing time (CFCT) based on the fault type and location. The faults on the critical Tweespruit-Mabote transmission lines and on Tweespruit and EMB busbars caused rotor angle swing. Failure to clear these faults within the critical fault clearing times caused the synchronous generators’ rotors to swing beyond the stable operating limits (pole-slip/ out-of-step). The results support the suggested time limits for fault clearance to permit continuity of service to more customers. Based on the results of the successful dynamic load-shedding scheme, this study continued to examine the pre-synchronism loading conditions which permitted successful closing of the tie-line breaker to re-join the two systems by considering the system load prior to closing the tie-line breaker. The pre-synchronising investigation extended to the peak active power oscillations limit to support the limits required to successfully resynchronize the two systems. Finally, the study examined the impact that different load restoration amounts had on the synchronous generators' operation limits, e.g. the active power and frequency oscillations. The conclusion reached, supported by the results, is that the application of dynamic loadshedding can improve the Lesotho power system stability. The resynchronising of the two systems can succeed if the system satisfied the following requirements prior to reconnection: system loading is less than 112.3% of installed generators' capacity, breaker load angle difference is less than 10.76°, voltage difference is less than 5% and frequency variation is less than 0.6 Hz.