THE POWER PLANT OF AN INDUSTRIAL PLANT
An industrial plant like a brewery, cement plant, sugar milling plant or a wood-based industrial complex may in some instances elect to put up a self-generating plant to support its power requirements. In the country, these power plants are either diesel or steam power plants. The steam power plants are commonly fuelled by coal, or a combination of bunker oil & wood wastes or a combination of oil & sugar cane refuse called bagasse.
There could be four reasons for the concept, i.e.:
1) there might be no existing power supplier in the vicinity where the industrial plant is to be erected,
2) the local electric cooperative in the area may not be capable to supply the needed power,
3) process steam may be intensive where co-generation with power could be most economically attractive,
4) the cost of self-generation may be very much cheaper than utility’s PPA-laden power rates.
While it is true that power lines of National Transmission Corporation (TransCO) are now criss-crossing the countryside, tapping to a distant 230 KV line may not be cost-effective for a medium size plant of say 10 or 20 MVA. A 69 KV TransCO sub-transmission line may be deemed best but may not be available in the area when the plant was conceptualized. On top of this, a number of Independent Power Producers (IPP’s) are recently courting many industrial plants with Build Own Operate (BOO) or Build Operate Transfer (BOT) schemes - somehow offering rates which are lower than that of the utilities. The idea is to install medium size generating plant (distributed generation) close to the industrial plant to supply the power requirements of the manufacturing plant. If the IPP’s can offer lower power rates but still can produce profits, then self-generation may be attractive.
DESIGN REQUIREMENTS
The appropriate size of the power plant can only be answered as a result of a thorough study of the circumstances surrounding the conditions of the system. For instance, if the power plant in concept is only for emergency supply in the events of power outages by the utility, then, 80% of maximum demand capacity in two (2) or three (3) generating units may be the right size for the system condition in an ATS configuration. In small systems where power interruptions can be tolerated, one or two generating units serving the loads may be sufficient, again with an Automatic Transfer System or ATS.
For medium size systems to act as “Island Generation”, stringent requirements have to be met by the power plant in concept. The determination of the size of the power plant and the number of units may pass through a careful study of the behavior of the plant served as reflected by its load profile and load duration curve. For instance, an electrical engineer who fully understands the behavior & peculiarities of a brewery or cement plant load should be there during the conceptualization of such a project.
For large island generation systems, the more tough requirements are needed in the process of selecting of the sizes and number of generating units. Theory of Probability on forced outages of generating sets is to be employed and inputted into the computations. It’s only then that the number & sizes of generating units can be established.
Power Plant Engineering Design is an art that includes selecting and placing the necessary generating equipment so that a maximum of return will result from optimum capital investments & maintenance costs over the working life of the plant (Life Cycle Cost). Most industries that self-generate usually make use of bunker oil-fed diesel engines or steam generators. Power may then be co-generated with steam where bulk of the power produced may be configured to become “free” or “by-product” by using a combination of straight condensing with topping or back-pressure turbines.
Normally in Steam Power Plants, a few diesel-driven generators are also used in combination with steam turbine-driven mainstays for purposes of starts-up or alternate power supply in times where the boilers are in maintenance during weekends.
In any case, the selection of sizes and number of generating units in a well-designed “island power generating plant” for a medium sized industry should consider the following factors:
Safety - this means safety in design and good workmanship in the execution or installation of the system components. A good design and workmanship will result to safety of personnel and property.
Economics - this means resulting to the best choice of arrangement that would deliver the most cost effective first cost (P/kw), and operational costs (P/kw-hr) as well. Economics however does not mean not to include the necessary spinning and ready reserve units required in an island generation power plant.
Maintainability - this means that the power plant can still meet load requirements even if part of the plant is under maintenance. In real life scenario, it always a fact for diesel power plants that one of the generating units is under maintenance at any given time. If ever one unit is under maintenance and the power plant can not serve plant requirements, then that plant is ‘not maintainable’. This is one area that the engineering manager and the finance manager are always in quarrel i.e., in interpreting costs.
Reliability of Service - The reliability of the power system is measured by its ability to serve all power demands made by its customers without failure over long periods of time. For instance, a common criterion is to install sufficient capacity to prevent interrupting power to customers oftener than one day in a 5-year period on a long time average basis. The second criterion is the appropriate size of the generating units that must be placed in the power plant to satisfy the base units and reserve requirements. This is so because the loss of a large unit will render the power plant incapacitated of this large capacity.
Reliability means capital investments. New Reliability requirements are now expressed in terms of “Reliability Nines” (four to six nines) and they don’t come cheap. The demand for more reserves would increase the cost of power. Inadequate reserve causes great inconvenience and economic loss to the business, both to the power plant and the customer plant. It is thus for the engineer to assess the balance of these two extremes.
Availability - “Operating Availability” is the statistical measure showing how physically ready the generating unit or plant is, in producing power whether needed by the system or not. It is a measure of overall unit or plant dependability. Availability Performance further shows the effects of forced outages and the unexpected glitches in operation & maintenance. If the power plant in concept has to operate like a utility company or an “island generation” where all the power requirements have to be supplied by the power plant, then the stringent requirements described above have to be met. This means that loss of power is not tolerated, i.e., to protect spoilage of the product that may prove to be too expensive.
PRINCIPLES IN POWER PLANT DIMENSIONING
The design engineer must consider the loads as seen by the power plant in terms of Load Profiles & Load Duration Curves. Having established them, the engineer is now in the position to design the shape of the power plant in concept.
But let us be reminded however, that the power plant has its own complexities where such complexities must be satisfied, to wit;
What made power plant operation complex is the inherent variability of the loads and the power plant must be ready and serve the load in any eventuality.
The production, delivery & sale of electricity all happen at the same time. The product of the power plant, which is “power”, is not completed until the instant it is needed and used, at the quantity exactly equal to the demand at that instant.
There is no simple way to “warehouse” an extra supply of “Kilowatts” or Kilowatt-Hours” against some future period of extraordinary demand, yet it is certain that such a period will occur.
The above simply means that there must be power at any time the customer needs, and this power must be sufficient with spinning reserves. It means further that the power plant supply must always be significantly greater than the demand at the instant and at any instance, for that matter.
There are no hard and fast rules governing the sizing of the power plant. According to the Standard Handbook for Electrical Engineers, 13th Edition, “a design of a generating plant derived from probability calculations is still subjective. It is still the “han-ay sa pinsada” of the engineer that counts.
The electrical engineer of the industrial plant therefore has to consider all the important factors of a reliable power plant that would be best suited to his requirements”. However, experiences and practices of North American and Japanese power engineers can help shed light to this thought process. CESEEPS Power Book 2 (Medium Voltage Systems & Applications in Industries) could be a great help.
Doods A. Amora (July 2007)
