IEEE 推荐规程中商业建筑 22 电力系统。负载特性(外文文献翻译)

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IEEE Recommended Practice for electric Power Systems in

Commercial Buildings-2

2. Load Characteristics

2.1 General Discussion

The electric power distribution system in a building exists solely to serve the loads Ñ the electrical utilization devices.

The power distribution system should accomplish that assignment safely and economically, provide sufficient reliability to adequately satisfy the requirements of the building (and its users), and incorporate sufficient flexibility to accommodate changing loads during the life of the building.

This chapter is intended to provide typical load data and a suggested method for determining individual and total connected and total demand load characteristics of a commercial building. The engineer should make provisions for load growth as well as building expansion in order to provide adequate electrical capacity or provision for electrical equipment expansion during the expected life of the building.

The steadily increasing sophistication of some of the load devices (complex communication systems; electronic data processing equipment; fire protection equipment; closed-circuit television security systems; heating, ventilation, and air-conditioning systems; centralized automated building control systems; etc.) increases the difficulty of determining initial load, forecasting future loads, and establishing realistic demand factors.

The electrical engineer should determine a building’s electrical load characteristics early in the preliminary design stage of the building in order to select the proper power distribution system and equipment having adequate power capacity with proper voltage levels, and sufficient space and ventilation to maintain proper ambients. Once the power system is determined, it is often difficult to make major changes because of the coordination required with other disciplines. Architects and mechanical and structural engineers will be developing their designs simultaneously and making space and ventilation allocations. It is imperative, therefore, from the start that the electric

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systems be correctly selected based on realistic load data or best possible typical load estimates, or both because all final, finite load data are not available during the preliminary design stage of the project. When using estimated data, it should be remembered that the typical data applies only to the condition from which the data was taken and most likely an adjustment to fit the particular application will be required.

While much of the electrical requirements of building equipment, such as ventilating, heating/cooling, lighting, etc., are furnished by other disciplines, the electrical engineer should also furnish to the other disciplines such data as space, accessibility, weight, and heat dissipation requirements for the electric power distribution apparatus. This involves a continuing exchange of information that starts as preliminary data and is upgraded to be increasingly accurate as the design progresses. Documentation and coordination throughout the design process is imperative.

At the beginning of the project, the electrical engineer should review the utility’s rate structure and the classes of service available. Information pertaining to demand, energy, and power factor should be developed to aid in evaluating, selecting, and specifying the most advantageous utility connection. As energy resources become more costly and scarce, items such as energy efficiency, power demand minimization, and energy conservation should be closely considered to reduce both energy consumption and utility cost.

1) Lighting- Interior (general, task, exits, and stairwells), exterior (decorative, parking lot, security), normal, and emergency

2) Appliances- Business and copying machines, receptacles for vending machines, and general use

3) Space Conditioning-Heating, cooling, cleaning, pumping, and air-handling units

4) Plumbing and Sanitation-Water pumps, hot water heaters, sump and sewage pumps, incinerators, and waste handling

5) Fire Protection-Fire detection, alarms, and pumps

6) Transportation-Elevators, dumbwaiters, conveyors, escalators, and moving walkways

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7) Data Processing-Desktop computers, central processing and peripheral equipment, and uninterruptible power supply (UPS) systems, including related cooling

8) Food Preparation-Cooling, cooking, special exhausts, dishwashing, disposing, etc.

9) Special Loads- For equipment and facilities in mercantile buildings, restaurants, theaters, recreation and sports complexes, religious buildings, terminals and airports, health care facilities, laboratories, broadcasting stations, etc.

10) Miscellaneous Loads-Security, central control systems, communications, audio-visual, snow melting, recreational or fitness equipment, incinerators, shredding devices, waste compactors, shop or maintenance equipment, etc.

2.1.1 Load Estimates

There are several load estimates that should be made during the course of a project including

1) A preliminary load estimate, generally based on a projection of available data on existing buildings of the same usage and the square footage or volume. This information is used in preliminary engineering studies for determining feasibility and cost and for very preliminary discussions with the utility.

2) An early design load estimate, of higher accuracy than (1) above, to determine the types of service required, to present more realistic information to the utility, to begin formal utility negotiations, and to determine the type of distribution system and voltages to be selected. At this point, the areas required for electrical rooms and substations will be determined. Once preliminary architectural decisions have been made, it may be difficult to obtain additional space, access, and floor loading requirements for the electric system. Typical figures that could be used for this type of estimate are included in this chapter.

3) The NEC specifies minimum service and feeder sizes based on the areas involved and the types of loads.

The intent is to prevent the design of an unsafe electric system, which could result from the undersizing of feeders, panelboards, and services (either erroneously or for cost saving purposes). In many modern buildings, the actual maximum demand load will be substantially less than that calculated under NEC methodology; but, where the

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NEC or equivalent code is in effect, the code calculations must be used in sizing service, feeders, switchboards, and panelboards.

4) Energy codes, primarily those enacted into law by political subdivisions, may provide budgets for the allocation of electric power. These are part of legislated energy conservation programs and are usually based on ASHRAE/IES Standards in the ASHRAE 90 Series. These codes develop overall energy conservation standards for the building including mechanical and electric systems and building insulation. While specifying the maximum energy usage for different areas of occupancy, they permit the increase of the allowable consumption in certain areas if other areas use less than the allowable limit. Most codes use power allocations as a base (e.g., unit power densities in W/ft2); others may include energy budgets in which time of usage is one of the variables.

5) The final load estimates are based on actual take-offs from the final electrical and mechanical drawings.

These include motor sizes, sizes of permanently connected appliances, lighting loads, estimated loads for receptacles, and heating equipment loads. Even these figures should be reviewed when the requirements of the actual equipment are furnished by contractors and manufacturers.

It is important to distinguish between loads expressed in voltamperes and watts (VA, W, kVA, kW, MVA, or MW). The energy codes are primarily concerned with real energy or watts, while the NEC often requires the use of apparent power in voltamperes.

2.1.3 Relation to Power Company

Just as the individual and collective load requirements of one building differ from all other buildings, each electric utility differs to some degree from every other utility in its rate structure, service policies, and requirements, which makes it important for the electrical engineer to contact the utility company early in the design phase. But, before beginning to discuss rate structure and availability of service, the engineer should develop a load survey to estimate initial and future loads and their electrical characteristics, in order to convey to the electric utility the following data:

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1) Initial demand and connected load, and possible expansion

2) Average usage or load factor

3) Seasonal and time-of-day variations

4) Power factor of total load

5) Ratings of largest loads and associated switching (that is, starting) requirements

6) Required reliability and expected continuity of service

7) Identification of interruptible loads, to permit consideration of demand limiting

8) Identification of loads sensitive to voltage and frequency transients

A detailed discussion on the various aspects of planning for utility service and the many factors affecting electric utility rates is presented in Chapter 4.

The electrical engineer should establish, in consultation with the electric utility, the special service classifications and incentive tariffs that are available to customers employing heat recovery, space-conditioning systems; thermal storage designs; solar energy; off-peak space-conditioning systems; or similar special systems to minimize electric power consumption.

The electrical engineer should analyze the features of rate structures that serve to penalize poor loads. Ratchet clauses cause utility customers to pay a demand charge on the highest demand established during a number of preceding months and is an incentive to control demand. Increased seasonal and time-of-day rates may result in higher electric rates during the high rate periods.

Several techniques are available to the electrical engineer to reduce the cost of electric power. These techniques include the following:

1) Load Limiters-Load limiters, demand limiters, programmable energy controllers, or load shedding controllers are devices programmed to control building loads in such a sequence or manner that the billing demand remains at an optimized value.

2) Power Factor Correcting Equipment-Many utilities have the authority to levy power factor penalties or surcharges on those users whose power factor is below some specified level, often 85% (but sometimes as high as 95%). Whenever economically feasible, synchronous motors should be selected or capacitors used to compensate for the lagging power factor, particularly caused by induction motors and

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certain lamp ballasts, e.g., “normal power factor” ballasts, to improve the overall power factor of the system.

3) Power Factor Improvement Techniques- Because the power factor of an induction motor is lowered considerably when the motor is loaded to less than 75%-80% of rated load (even though motor efficiency remains relatively high and constant down to about 25% load), proper sizing of induction motors for the respective application serves to improve the load power factor and minimize the investment in power factor correcting equipment. Power factor correcting capacitors are commonly installed to be switched with the respective motor starter, that is, connected at the motor terminals or at the motor control center. Capacitor correction may not be acceptable for all motor applications. (e.g., motors with electronic speed control or overhauling loads). The use of high power factor ballasts in lighting equipment can improve power factor significantly in buildings where lighting is an appreciable part of the total load.

4) High-Efficiency Motors-Use of high-efficiency motors, which utilize improved materials and modified (from standard motor) construction, may result in a considerable reduction in energy consumption. Due to the lack of uniform testing procedures among the various suppliers, the electrical engineer should exercise caution when evaluating various motor sources solely on the basis of published values of efficiency.

5) Motor-Speed Control-For certain motor applications, such as pumps and blowers, where energy can be saved by reduced speed operation when rated output is not needed, ac induction motors with solid-state, adjustable frequency controllers or multiple-speed motors may be economically justified. Use of adjustable frequency controllers may induce harmonics into the system. Necessary evaluation of the impact and possible solutions should be considered.

6) Regenerative Systems-Energy can be saved in some motor-driven applications where, under certain operating conditions, the load is capable of driving the motor by utilizing regenerative systems. A loaded descending elevator or an empty ascending elevator, for example, can return energy to the building power system. The designer should analyze system performance during abnormal conditions to prevent equipment

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malfunction and damage.

7) Programmed Loads-Certain loads can be programmed to save energy by being switched off during the hours when the space is unoccupied, or the systems are not required.

8) Switched Loads-The need to provide flexible lighting systems should be satisfied by the choice of luminaire systems and lighting circuitry design.

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