Practical Power Plant Engineering. Zark Bedalov
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, minimum/maximum temperatures, humidity, rainfall, number of lightning days, are normally included in the tender document for the vendors to design the equipment accordingly.
2.3.3 Reliability Criteria
Before we start putting things together, let us clarify the design reliability criteria, discussed in Chapter 21.
In order to ensure adequate availability and continuous plant operation, the power distribution system must be designed to tolerate and override certain failures of the equipment. Generally, the system will be designed for “a single contingency failure” of the principal distribution equipment. This is sometimes called “a single outage contingency.” In other words, the design will fully cover for a single failure of one major piece of equipment, such as transformer, pump, motor, but not for a simultaneous failure of two pieces of similar major equipment.
Therefore, each pump system, which is considered a critical primary part of the operating plant shall include 2 × 100% units. In some cases, for larger pumps, 3 × 50% pumps could be used. For sump pumps, roof fans, heating, ventilating air conditioning (HVAC), etc., which are considered the plant auxiliary services, there shall be no immediate substitutes. The switchgear busbars from the plant distribution transformers will be bussed together through bus tie breakers to allow for feeding the plant loads from a single transformer, in case of an outage of one transformer. Failure of some smaller distribution transformer may be tolerated by reconnecting the load to alternate sources of power supply.
Recovery from power outages will be either by having spares cable connected or piped, or by switching capability to feed power from alternate sources such as closing the bus tie circuit breaker.
No contingency consideration will be given to the failures of power cables, lines, or pipes, which can be replaced or fixed relatively quickly, except for the high voltage (HV) single conductor cables used in power plants for 138 kV and higher, where one additional spare phase is added and laid out next to the operating cables. Therefore, during the plant design, one may ask: “What if…?” But not: “What if…, and if…?”
2.4 Connection to Power Utility
The investors will likely not be interested in building this industrial facility unless they have spoken to the utility and were assured that there is available generating capacity to power the project.
Let us talk to the Utility to acquire the information we need to build our plant power distribution system. Here are some of the issues to be clarified by the Utility engineers:
Power agreement: firm or interruptible
Tariffs, for power demand and time of use
Line voltage and its daily and weekly profile
Frequency off‐limits
Power factor tariffs and penalties
Source impedance, inclusive of the transmission line impedance (conductors)
Double‐ or single‐circuit incoming transmission line
Generating capacity, firm power, how many units are available, and their ratings
Method of line protection
Lightning level (number of lightning days/yr) in the area
Let us review each of the aforementioned issues:
Power agreement: The plant owner will sign a power agreement with the Utility. If the plant needs power 24 hours a day, every day, the owner will look at signing a power agreement for uninterruptible firm power supply, if available. An interruptible power supply allows the utility to occasionally cut the power supply in a specific amount or in total. Naturally, this contract comes with a lower tariff. The plant owner will likely insist on an uninterruptible power supply (UPS).
Tariffs: