Mitigation of undervoltage in distribution networks using an automatic load shedding scheme to curtail voltage collapse.

Abstract

Undervoltage is still a power quality problem globally with several interventions like reactive power compensation in generation stations, transmission, and distribution lines. This issue still prevails in Uganda's power system network that has experienced five blackouts within five months in 2020. Most utility companies have resorted to automatic undervoltage load shedding schemes as a safety net to mitigate undervoltage. This study purposely sought to develop an automatic undervoltage load shedding relay in Physical Security Information Management (PSIM) software for mitigation of undervoltage in the distribution network. The study specifically was to design part of Uganda power system distribution network in Digital Simulation of Electrical Network software (DIgSILENT), develop an automatic undervoltage load shedding relay, and lastly validate the undervoltage load shedding relay solution. The research employed quantitative design, case study method, and experimental design. DIgSILENT and PSIM software were used for data analysis, interpretation and validation. Quantitative data was majorly used in this research. The validation result revealed percentage voltage deviation (0- 3.45) % that was within the acceptable error margin. The study results showed total distribution and transmission power output loss after network simulation with DIgSILENT software was 7.2% as compared with 21% loss from Umeme Limited (approx.17%) and Uganda Electricity Transmission Company Limited (approx.4%). It showed a balanced power system network after executing simulation implying that the generated power meets the electrical load demand. There was 10 busbars (47.6%) out of 21 network busbars that qualified for disconnection after supply side fault (Bujagali Hydro power) moving normal operating point (0.95-1.05) p.u to (0.85-0.95)p.u. Demand side faults at Rwenzori, Century bottling, and Roofing mill moved normal operating point to (0.89- 1.06), (1.0-1.1), and (0.82-1.02) p.u respectively. The demand side then settled to (1.0-1.07), (1.02-1.07), and (1.0-1.07) p.u respectively after the different faults were isolated. Validated PSIM results revealed 0.094% of total harmonic distortion (THD) compared to 3% (THD) for the 22kV to 400kV range according to Western Power Distribution in United Kingdom. This is evident by 98% active power flow from the generation station to end-users. The study recommend the use of undervoltage load shedding relay in PSIM solution that disconnected weak busbars, reduced total harmonic distortion, improved voltage regulation, and increased power transfer efficiency.