rated (see Chapter 4 for the NFPA guidelines). The main transformers will be provided with the Deluge water mist protection system (see Chapter 22).
Furthermore, we decided that the transformers would have OnLTC in the range of ±10%. As we are 120 km away from the power plant, the voltage may not be stable. Daily 24‐hour load cycle will vary from hour to hour. The automatic tap changers will give us some form of voltage stability for the plant operation. We may also need some additional means of voltage regulation within the plant to enable us to further improve the voltage profile through power factor correction. To meet this requirement, additional capacitors and reactors may be considered (see Chapter 13).
2.6 Load Site Placement
At this conceptual phase of the engineering design, we have to determine the locations of loads within the plant as well as their kW ratings to determine the major power routes of the distribution system. Depending on the kW load magnitudes, we will determine the corresponding voltages for the distribution equipment.
| Loads (motor and feeders) up to 200 kW | 480 V, Motor voltage: 460 V |
| Loads (motors with VFDs) up to 500 kW | 480 V, Motor voltage: 460 V |
| Loads above 200 kW | 4.16 kV, Motor voltage: 4000 V |
We will obtain the load data from our mechanical engineers. Roughly, we expect that the plant‐connected load will be about 50 MW. The mining load is typically a motor load with 0.8 pf, but with power factor correction, we will get it over 0.9. This MW figure is assessed generally from the plant flow diagrams, based on the hardness of the ore, raw material processed, and product produced. In their flow diagrams, mechanical engineers may suggest a kW (HP) rating figure for each motor. However, the final kW (HP) ratings will be taken from the actual bids received from the suppliers. The tendency seems to be that suppliers more often overestimate rather than underestimate the load.
From the plant layout drawings, we determine the locations of the groups of loads: mining, crushing, conveying, grinding, process plant, tailings, and camp. The load centers will generally follow the flow of the ore. These locations may be kilometers apart from each other. The biggest groups of loads will be in the grinding and the process plant for conveying, pumping, agitation, and floatation. The camp, crushing, and tailings will be away from the process plant.
The power load centers may include either 480 V loads only, or both: 4.160 and 480 V.
Typical 4.16 kV controllers for large motors are rated at 400 A. Controllers rated 800 A are also available, but rarely used. Let us calculate the current of a 2000 kW, pf = 0.8 motor and verify if a 4.16 kV, 400 A controller can operate it safely:
The power will be fed from the main substation to the plant load centers by 15 kV overhead lines or medium voltage (MV) cables, depending on the relative location of the main substation. In North America, the 13.8 and 4.16 kV voltages are often called 15 and 5 kV, respectively.
2.6.1 Crushing
This facility may be close to the grinding plant, but at a certain distance, to limit the dust spreading to the rest of the plant. Also a large (seven‐day) ore stockpile will be placed between the crushing station and the grinding plant. Crushing usually operates a single 12‐hour shift, while the rest of the plant marches on two shifts. A load center will be required to feed the 4 kV and 460 V motors. We will use 1.5 MVA transformers (13.2 kV/480 V or 4.16 kV/480 V) with MV controllers for motors >200 kW and 480 V MCC starters for motors rated ≤200 kW.
2.6.2 Grinding and Conveying
This is the location of the largest load, the 8 MW, (10 MVA) cyclo‐converter motor. This load is too big for 4.16 kV and must be powered at 13.8 kV. It will have its own power feeder fed directly from the main 13.8 kV switchgear located at the main substation. For this variable frequency drive (VFD), the main power supply is converted to DC voltage and then back to AC voltage of varied frequency to operate at variable speed to suit the ore quality and hardness. This large piece of equipment is a complete package. It includes a number of smaller auxiliary loads, such as MCC, fans, lube pumps, lighting, and heating, all of them fed from the same power source feeder.
The grinding facility will also need a 4.16 kV load center for MV motors (Ball Mills) and a number of load centers for a large number of low voltage (LV) motors and drives. Drives mentioned here are typically referred to as the VFD operated motors. Two grinding ball mills may even use synchronous MV motors, which by controlling their excitations may help us improve the plant power factor (see Chapter 13).
The MV load center with 5 kV switchgear and MV motor controllers will be fed from two 13.2 to 4.16 kV, 10/15 MVA, Dy11, ONAN/ONAF, oil type transformers, which will be placed outdoors adjacent to the plant and have a joint provision for oil containment.
Oil containment is a concern in the plant due to a fire hazard. Dry transformers for feeding smaller loads are preferred due to their flexibility to be placed closer to the loads and for being less of a fire hazard. However, since dry transformers are commercially built only up to 3 MVA and at voltages up to 13.8 kV, oil immersed transformers will be ordered for this application. To conform to the NFPA guidelines, a fire‐rated blast wall will be provided between the two transformers (see Chapter 4 for details).
In addition, this plant will need several load centers for the LV motors and drives. This can be covered by a number of unit substations with 2/3 MVA, 13.2 kV to 480 V transformers, 480 V switchgear and 480 V MCCs. For explanation, why 13.2 kV is used and not 13.8 kV (see Chapter 11).
Process (pumping, flotation): This facility will need several load centers for LV motors and drives. These loads will be fed from a couple of unit substations with 2/3 MVA, 13.2 kV to 480 V transformers. Each unit substation will include a 480 V switchgear and 480 V MCCs for feeding the LV loads.
Camp: The camp is typically located 3–5 km away from the main substation. An overhead distribution line will be erected to the Camp directly from the main substation. This line may include several other 15 kV circuits that may be going in the same direction. The camp load center will include a 500 kVA, 13.2 kV/ 480 V load center with LV distribution boards. Additionally, the load center will be furnished with one or two small emergency standby diesel generating units connected to the main load center via an automatic transfer switch (ATS) to feed the selected camp essential loads. Two small generators are recommended specifically for the camp due to the remote and challenging living environment.
Tailings: Tailing pumping station is 6 km away. The facility will require a 15 kV overhead line to feed a small 13.8 kV/480 V load center.
Water supply pumping: The manufacturing and process equipment will need fresh water supply either from the natural sources or by trucking and storing in the tanks near the process plant.
Mining: This load may be supplied by a portable DG at an open‐pit mine location.
Auxiliary system: Add HVAC, lighting, cranes,
