Energy Power Consulting Engineers was awarded a project by Fiji Sugar Corporation to undertake power systems studies at all four of their sugar mills – Lautoka, Labasa, Rarawai and Penang. The scope of the studies included:-
Protection coordination study
Load Flow study
Load Flow fault level study
Transient stability study
The primary purpose of the studies was to verify the suitability of upgrading the existing power generation and distribution networks as part of the main equipment and technology upgrades being undertaken by Sugar Technology Mission and their Indian vendors at the Fijian Mills.
The studies were carried out using DigSilent Power Factory software applying the following methodologies:-
All the Mill generators were represented by the full Park's equations
The AVR's were represented by equivalent IEEE models
Governors were not represented as they were not expected to significantly affect the stability of the generators
The 6.6 and 11 kV systems from the connection point back to the nearest FEA transmission connection were fully represented.
The remainder of the networks were represented by an infinite bus behind source impedances representing prior and post fault conditions.
Mill loads were represented as a combination of constant impedance and equivalent induction motors.
The studies consisted of processing the data and setting up a system model using DigSilent Power Factory Software for the stability, Load Flow, Protection Coordination and Harmonic Studies. A stability study was run for a 120 ms 3 phase fault on the appropriate transmission bus for each mill.
The following outputs were recorded for the Stability studies:-
· Generator angles
· Generator Terminal voltages
· Generator Field voltages
· Generator Stator currents
· Generator Active and Reactive power
The Load Flow and Analysis studies were used to determine the following:-
· 6.6 kV Fault Levels
· 415 V Fault Levels
· Power Factor Correction Bank requirements
· Main Power Transformer OLTC settings (where fitted)
The following settings were provided for the protection relays
· FSC/FEA Interconnector relays
· Generator relays
· Feeder relays for all 11 and 6.6 kV mill equipment (complete switchboard)
· 415 Vac MCC Incomer circuit breaker settings
The studies highlighted a number of shortcomings especially with respect to switchboard fault levels being exceeded. A program has already been initiated to address these shortcomings and it is hoped that over the next few years all areas of concern will have been addressed. The studies have also highlighted the gains that can be made in overall Mill load flow by simply improving the real and reactive power control of the installed generation. The latter will take time to implement as old practices and control strategies are sometimes hard to change.