Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 57

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, sumps, eat tracing, etc., to the load estimate. These items generally are not shown on the process flow diagrams.

 Plant standby generator: A standby DG 1 MW, will be connected to the 13.8 kV bus at the main substation to supply power to the plant essential load. It will automatically be energized and loaded in case of a total loss of the main supply. This may happen due to problems at the utility or a loss of the 230 kV line. The standby generator will supply power to 30% of lighting, heating, some pumping, and other essential and life critical services. It will not provide power to the process, but it will maintain the charge of the plant DC and control systems (see Section 2.10).