Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 55

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.