Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 59

The overall estimate for the connected load (Table 2.2) as part of the conceptual design broken down at voltage levels is as follows:

Of course, the figure given earlier is not the actual operating load. This is yet to be determined. We will arrive at that by using diversity and other load factors.

Having participated in the design of many projects, we have yet to know the actual operating load in relation to the connected load. It surely depends on the type of plant. We would expect that a batch plant would have a higher percentage compared to a mining plant. During the design, it is evident that the connected load far exceeds the expected operating load and far exceeds the power distribution transformer capability. The connected load must be “treated” with several diversity and load factors.

During the commissioning, we do observe the operating currents of the individual drives and note that the actual load factors are considerably lower than projected. This is partly due to the fact that mechanical engineers and suppliers continually oversize the plant motors due to their fears and rounding up to the higher frame sizes to insure flows are adequate and in line with expectations. We have yet to hear an owner complaining that we have overloaded the system, or that we have oversized the system. There always seems to be “a lot of fat” in the system.

Table 2.2 Connected load.

Voltage	MW
13.8 kV	9.5
4.16 kV	15
480 V	20
Cable and equipment losses	0.5
Growth (10%)	5
Connected load	50

Table 2.3 Load list.

ID	Name	kW	Volt	Serv.	LF	UF	(DF)	Load
20AG01	Tank#1 Agitator	11	480	1	0.7	1	0.7	7.7
15LT03	Lighting transformer	15	480	1	0.5	1	0.5	7.5
20PU21	Tank #1 Supply pump	30	480	1	0.8	1	0.8	24
20PU22	Tank #1 Supply pump	30	480	0	0.8	1	0.5	0
15FA11	Crusher building fan	30	480	1	0.6	1	0.6	18
15CR01	Crusher building crane	50	480	1	0.4	0.1	0.05	2.5
21BM01	Ball mill	2000	4160	1	0.7	0.6	0.7	840
21BM02	Ball mill	2000	4160	1	0.7	0.6	0.7	840
15WR01	Welding receptacle	60	480	1	0.5	0.1	0.05	3.0
	Connected	4226				Operating		1742.7

Abbreviations: ID, identification number; kW, connected load; Serv., unit in service (1) or on standby (0); LF, load factor (<1); UF, utilization factor (<1); DF, calculated diversity factor (Serv *LF*UF).

We were all told to size the plant motors to make them operate at not less than 80% of their nominal ratings. This is to save on the cost of motors as well as to operate them at their highest efficiency and thus reducing the cost of energy. My personal feeling is that when all is well and done, the operating load will be around 50–60% of the connected load.

Occasionally, it does happen that a motor is undersized and must be replaced. But, more often, the motors are oversized and operate with a low load factor. That scenario does not seem to concern the owners. Falling short certainly is a bad news.

Here, we go. Let us determine the projected operating load. First, we prepare the complete “Load List.” A partial example of the list is given in Table 2.3 for several plant loads.

Based on the small sample, the operating load is 41.2% of the connected total load. This is just an engineering guess on the paper, for now.

One can create his own tabulation to suit the particular plant and define the load factors for its own reasonable comfort suited to a type of the plant. In this case, we used the factors for the peaking duty: peaking operation for one hour duration. The load in the table is calculated by multiplying the connected load (kW) with the diversity factor (DF).

It was mentioned earlier that the process plant will be operating 24 hours, while the crushers will be working one 12‐hour shift only. Most of the maintenance and admin will be shut down too. Since we are concerned with the maximum plant load over one hour, the crushing plant is included. A welding receptacle or a crane or similar loads, for instance, have low utilization factors as maintenance work is not a continuous work.

Based on the load estimate for the 4.16 kV load, the plant transformers will be 13.2 to 4.16 kV, 12/15 MVA, Dyn1, ONAN/ONAF, BIL 110 kV, 55 °C rise at 30 or 40 °C ambient, depending on the environment. If the plant is in the Northern region, the ambient temperature can be <40 °C.