Mine Power Factor Correction Study
An Australian metalliferous mine, expanding production and so adding load to an already extensive electrical infrastructure, required an additional 50MVA transformer, 11kV substation and additional power from the grid. The grid electricity supply agreement placed constraints on total power, power factor and harmonic content - requiring a power factor of 0.95 lagging or better and limits on Total Harmonic Distortion and individual harmonics. The site already had a Harmonic Filter system (pictured) whose additional benefit was to provide power factor correction, albeit fixed and always in service. The filtering was there for harmonics produced by several sources on site, including a variable speed mine winder. As part of the project, PCE carried out various power studies to verify proposed equipment ratings, including a detailed Power Factor Correction (PFC) study.
Understanding Power Factor
The reasons for improving power factor are well documented but a rough mechanical comparison is the angle being made to the riverbank of a barge tow rope as shown below. If the angle is too large, then the rope tension needed to move the barge may exceed the rope strength. If the angle is reduced by using a longer rope, the required tension is reduced low enough to save the rope. However, the rope must be made extremely long to reduce the tension to a minimum, so there is a compromise in how long is enough. A similar compromise is made in adding capacitors in the electrical world, to reduce the supply current to a minimum.
Power Factor Correction Study
The first aims of the PFC study were therefore to show -
- Whether additional PFC was required at all; and
- If so, how large a system was required.
Other questions followed, for example -
- Whether the existing filter system would provide adequate filtering - connected where it was in the expanded system;
- Whether the new 11kV switchboard was an acceptable place for connection of the new PFC;
- What step sizes would be suitable and
- Whether the new PFC would be adversely affected by harmonics.
Each question typically required a set of load flow calculations using a software model of the mine system. In this case “Power Tools for Windows” software was used.
To be realistic, the model required verification as far as possible. An important verification step was to model the pre-expansion system, adjusting load factors, diversity factors and pf of all the modelled loads, to obtain similar currents and pf as measured on site.
Then the proposed expansion loads were added to the model, with typical load factors and pf’s. Various candidate PFC sizes were tested in these load flow studies. Results of interest included site pf when at peak mine winder load and at low loads.
Constraints on the PFC configuration included -
- Rating of the supply circuit breaker,
- Cost of adding too many steps,
- 5-minute discharge time,
- Avoiding large step sizes,
- Limiting frequency of on/off cycles, and
- Having MVAr combinations, say 1, 2, 4, that allow all switching increments e.g. 1,2,3,4,5,6,7.
After an acceptable 13MVAr, 6 step system was established, a follow up Harmonics study was necessary to show that power factor capacitor ratings would not be exceeded because of predicted harmonics.
Values for series inductances to the capacitors could be proposed from this study. Series inductances are used to limit inrush currents and to determine resonant frequencies of the PFC banks. These are typically tuned away from the frequencies of known harmonic sources.