Practical Power Plant Engineering. Zark Bedalov
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Figure 2.1 Part of a plant one line diagram.
Decisions must be made on the main switchyard, the number of incoming transformers, and the selection of the plant busbar voltages for distribution of power to the major load centers for large and small loads and primary and secondary power lines to remote plant facilities.
Note: The voltages and frequency applied in this book will be those of the North American standards. The principles of calculations and application used here are equally applicable to the IEC system voltages used in the other parts of the world.
2.2 The Electrical Project
The activities presented in this book, some of it in this chapter as part of the electrical design, include the following:
Determine the site conditions and discuss the interconnection with local Utility.
Review of mechanical load flow diagrams, P&D diagrams and establish load (kW) estimates and voltage levels.
Prepare one‐line diagrams and plant design criteria.
Conduct system studies and determine electrical equipment ratings.
As the starting point, the mechanical engineers will develop 10–20 flow diagrams of the plant process. A small part of the ore handling flow diagram is shown in Figure 2.2. The process (instrumentation) engineers will develop 30–40 process piping and instrumentation diagrams (P&ID) to instrument and automate the plant, as shown on a small P&ID segment in Figure 2.3 for a feed pump. The P&ID gives us the indications on how the plant will be controlled, monitored, and operated.
Figure 2.2 Part of plant flow diagram.
Figure 2.3 A part of a P&ID diagram.
As a young design electrical engineer you have been assigned to be a part of a multidiscipline engineering team responsible to develop a project estimated to consume about 30 MW of power, or 37.5 MVA at 0.8 power factor (pf). The design team of electrical engineers and draftsmen led by an experienced senior lead electrical engineer is responsible to design the electrical power distribution system and procure the electrical equipment to power up and control the plant equipment. The plant distribution system will follow the national standards and voltages for 60 Hz (50 Hz) frequency as applicable to the location of the project.
The principal operating item in this facility and the biggest electrical load is a large 10 MVA semi autogenous grinding (SAG) mill operated as a variable speed drive cyclo‐converter. It receives the ore from the crushers, as 10–20 cm chunks of raw material and reduces it to 1–2 cm large gravel. The mill speed is regulated in accordance with the hardness of the ore, which may vary on a daily basis. This material is then conveyed to ball mills, which pulverize the product to allow it to be mixed with water and pumped around as slurry through the rest of the plant. The slurry will be then subjected to some chemical treatment processes to separate the metal from the ore.
What is the operating basis for this project? The owner will look at operating the plant 24 hours a day to maximize his early output in order to quickly pay off the loan to the financial institution. The only way to do that is to run the facility around the clock in two 12‐hour shifts. This operating regime is also favored by the local utility, as it allows them to run more generation around the clock as less costly base load and flatten the load cycle.
A housing complex for 100 people will be built at a site about 3 km away from the main process plant for the workers to build and operate the plant. This is an area away from the plant crushers. Operators are happy to work long shifts seven days a week on a basis of three weeks in and one week out. The housing complex (camp) will be a continuation of a construction camp for 300 people, which will be used for a full year prior to the start of production.
2.3 Site Conditions
2.3.1 Source of Power
Let us assume, the local utility has just built a new power plant on the coast, about 120 km away from the ore deposit and have extended a 230 kV transmission line to a city 50 km away from the deposit. This is the line to which the plant will be connected to. The line passes 20 km by the proposed mining site. To further simplify the matters, we will assume that the utility has sufficient spare capacity and is happy to furnish power to the new facility. This is a fortunate situation as it makes it feasible to import the power instead of generating it on its own.
The plant load will be relatively constant with ±10% variability. Utilities love constant load, which they can supply as a base load. The base load energy is less costly to produce in $/MWh.
The plant's electrical distribution system must operate in a stable manner within the prescribed tolerances of voltage and frequency as stipulated by the standards, in spite of the load variations. The load may be subject to changes, both MW and MVAR, caused by the operating cycle and duty of the plant large motors.
The plant owner must determine, based on the history of operation of the generating plant, if the source of power is reliable enough to meet the plant requirements. The plant process can tolerate short power outages without detrimental effects, but longer outages would be a concern with respect to the economies of the plant production.
Studies will have to be made to find out if a wind farm or a solar plant could be economical and possibly developed in the vicinity to supplement the imported power.
2.3.2 Ambient Derating Factors
The electrical equipment will be operating at an altitude of 1700 m. The equipment shall be derated for the altitude in accordance with the applicable ANSI C57.40 and IEC 282‐1.2 standards. The following derating factors are applicable for the 1700 m site altitude:
Voltage: 0.93
Current: 0.99
The voltage derating will be factored by the suppliers of the equipment, such as switchgear, transformer bushings, to ensure the equipment insulation is designed for lower air density at the specified altitude. The current (ampacity factor) derating for the plant cables is not significant, well within our conservative plant selection estimates.
Applicable ANSI or IEC standards for ampacity derating factors will be used for the power cables buried or installed in multiple duct banks, also discussed in Chapter 12.
The other site ambient conditions, such as road conditions,