ASHRAE HVAC APPLICATIONS IP CH 56-2015 ELECTRICAL CONSIDERATIONS.pdf

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1、56.1CHAPTER 56ELECTRICAL CONSIDERATIONSTerminology 56.1Safety . 56.1Performance 56.2Electrical System Components and Concepts . 56.2Power Quality Variations . 56.7Billing Rates 56.13Codes and Standards. 56.15RODUCTION, delivery, and use of electricity involve count-Pless decisions made along the way

2、, by hundreds of people andcompanies. This chapter focuses on the decisions to be made aboutthe building and equipment. Creating a building that works meansincluding the best designs available, communicating needs andcapabilities, and planning ahead.For an owner-occupied building, the benefits of a

3、properly de-signed building return to the owner throughout the buildings life.For tenant-occupied spaces, good design means fewer problemswith tenant and building system interference (e.g., lighting or appli-ances in one suite disrupting computers in a neighboring suite).Because HVAC the pressure th

4、at willproduce a current of 1 A against a resistance of 1 ; equal to 1 J/s.Also called the electromotive force (emf).Current (I): movement of electrons through a conductor; mea-sured in amperes.Ampere (A): practical unit of electric current flow. If a 1 resis-tance is connected to a 1 V source, 1 A

5、will flow.Alternating current (ac): a current that reverses at regular,recurring intervals of time and that has alternately positive and neg-ative values. The values vary over time in a sinusoidal manner.Direct current (dc): a current where electrons move steadily inone direction.Watt (W): unit of r

6、eal electrical power, equal to the power devel-oped in a circuit by a current of 1 A flowing through a potential dif-ference of 1 V.Volt-ampere (VA): amount of apparent power in an alternatingcurrent circuit equal to a current of 1 A at an emf of 1 V. It is dimen-sionally equivalent to watts. Volt-a

7、mpere is equal to watts when volt-age and current are in phase.Volt-ampere-reactive (VAR): unit for reactive power. The sym-bols Q and sometimes N are used for the quantity measured in VARs.VARs represent the power consumed by a reactive load (i.e., whenthere is a phase difference between applied vo

8、ltage and current).Power factor: for an ac electric power system, the ratio of thereal power to the apparent power, or W/VA.Three-phase power: supplied by three conductors, with the cur-rents (or voltages) of any two 120 out of phase with each other.Y (or “wye”) connection: a configuration of wiring

9、 so that eachwinding of a polyphase transformer (or three single-phase trans-formers) is connected to a common point, the “neutral.”Delta-connected circuit: a three-phase circuit that is mesh con-nected, so the windings of each phase of a three-phase transformerare connected in a series for a closed

10、 circuit (i.e., in a triangle or“delta” configuration).Fundamental voltage: produced by an electric ac generator andhas a sinusoidal waveform with a frequency of 60 cycles per second,or 60 Hz (in the United States). Other countries may have a similarwaveform but at 50 cycles per second of 50 Hz.Cycl

11、e: the part of the fundamental waveform where the electricalpotential goes from zero to a maximum to zero to a minimum, andback to zero again (i.e., one complete wave; see Figure 1). At 60 Hz,there are 60 cycles in 1 second.RMS (root-mean-squared) voltage: an effective way to com-pare ac to dc value

12、. For a pure sinusoidal waveform, RMS value isequal to 0.707 times the peak magnitude.System voltage: the RMS phase-to-phase voltage of a portion ofan ac electric utility system. Each system voltage pertains to a partof the system bounded by transformers or end-use equipment.Service voltage: the vol

13、tage at the point where the electric sys-tems of the supplier and the user are connected.Utilization voltage: the voltage at the terminals of the utilizationequipment.Nominal system voltage: the rated system voltage level (i.e.,480 volts) at which the electrical system normally operates. Toallow for

14、 operating contingencies, utility systems generally operateat voltage levels within 5% to +5% of nominal system voltage.2. SAFETYThe greatest danger from electricity is that it is taken for grantedand not taken seriously as a hazardous energy source. Electricity canproduce bodily harm and property d

15、amage, and shut down entireoperations. The type of damage from electricity ranges from a mildshock to the body to a major electrical fire. Electrical safety is im-portant in all occupational settings. See information on safety codesin the Electrical Codes section.The preparation of this chapter is a

16、ssigned to TC 1.9, Electrical Systems.Fig. 1 Fundamental Voltage Wave56.2 2015 ASHRAE HandbookHVAC Applications3. PERFORMANCEIn the United States, the National Electrical Code(NEC; NFPAStandard 70) is generally accepted as the minimum safety require-ments for wiring and grounding in a structure. Oth

17、er countries havesimilar requirements. The NEC ensures building design is safe, butmay not provide the performance that a modern building requires.Rapid changes in electronic technologies have rendered many tra-ditional electrical distribution practices obsolete and must bereplaced with new designs.

18、 Electrical power distribution decisionsmade during design affect occupants productivity for the life of thebuilding. Many improvements over the minimum requirements arerelatively inexpensive to implement during building construction.Power quality, like quality in other goods and services, is diffi-

19、cult to define. There are standards for voltage and waveshape, butthe final measure of power quality is determined by the perfor-mance and productivity of the building occupants equipment. Ifthe electric power is inadequate for those needs, then the quality islacking.Specifications for electric powe

20、r are set down in recognizednational standards. These are voltage levels and tolerances thatshould be met, on the average, over a long period of time. Electricutilities and building distribution systems generally meet such spec-ifications. Voltage drop in a building is a fundamental reason for cal-c

21、ulating the size of electrical conductors. Brief disturbances on thepower line are not addressed in these time-averaged specifications;new standards are being developed to address these concerns.Interaction between tenants electrical equipment is an ongoingproblem. Often, a large load in one tenants

22、 space can disrupt a smallappliance or computer in another part of the building. Voltage dropalong building wiring and harmonic distortion are often the causesof the problem. Dedicated circuits usually solve the voltage dropproblem, but harmonic distortion must be solved at the contributingloads. By

23、 eliminating much of the wiring common to both pieces ofequipment, the original performance of each is restored. With mod-ern electronic loads, the interaction might easily involve a large loadthat interferes with smaller, more sensitive equipment. Disturbancesmight travel greater distances or throu

24、gh nondirect paths, so diag-nostics are more difficult.For tenants of a building with ordinary power distribution, lostproductivity associated with power quality problems is an addi-tional operating expense. The disturbance may last only millisec-onds, but the disruption to business may require hour

25、s of recovery.This multiplication of lost time makes power quality a significantbusiness problem.Lost productivity may be the time it takes to restart a chiller, torepair a critical piece of equipment, or to retype a document.Another aspect of lost productivity is the stress on employees whosework i

26、s lost. The building owner may suffer loss, as well. Certainlythe building equipment itself may suffer from the same damage orlosses as tenant equipment. Sophisticated energy management sys-tems, security systems, elevator controls, HVAC A, B, and Cidentify phases on the high-voltage side of the tra

27、nsformer, and a, b,and c identify phases on the low-voltage side. Typically, three-phasevoltages present the higher voltage (phase-to-phase) first, followedby the lower voltage (phase-to-neutral). Single-phase voltagestypically present the lower voltage (phase-to-neutral) first, followedby the highe

28、r voltage (phase-to-phase). For example, 208/120 V isthree-phase and 120/240 V is single-phase.Y-Y connections (Figure 3) are rarely used because of balancingand harmonics problems.Y- connections (Figure 4) are typically used for stepping downfrom high to medium voltage.The -Y transformer (Figure 5)

29、 is commonly used as a generatorstep-up transformer, where the winding is connected to the gen-erator terminals and the Y winding is connected to the transmissionline. One advantage of the high-voltage Y winding is that a neutralpoint N is provided for grounding on the high-voltage side.The - transf

30、ormer (Figure 6) has the advantage that one phasecan be removed for repair or maintenance while the remainingphases continue to operate as a three-phase bank. The open con-nection allows balanced three-phase operation with the kVA ratingreduced to 58% of the original bank. These - connections are ty

31、p-ically used in distribution networks.An autotransformer has two windings connected in series (Fig-ure 7). Whereas a typical transformers windings are only coupledmagnetically via the mutual core flux, an autotransformers wind-ings are both electrically and magnetically coupled.An autotransformer h

32、as smaller per-unit leakage impedancesthan a two-winding transformer; this results in both smaller seriesFig. 3 Three-Phase Y-Y TransformerFig. 4 Three-Phase Y-TransformerFig. 5 Three-Phase -Y TransformerFig. 6 Three-Phase -TransformerFig. 7 Typical Autotransformer56.4 2015 ASHRAE HandbookHVAC Appli

33、cationsvoltage drops and higher short-circuit currents. It also has lower per-unit losses, lower excitation current, and lower cost, if the turns ratiois not large. An autotransformer is not isolated as well as a typicaltwo-winding transformer; transient overvoltages pass through theautotransformer

34、more easily because the windings are connectedelectrically.Transformer Coolants and Insulators. Because heat is createdby the flow of electrical current through the windings, a liquid (e.g.,oil or silicone) is often used as a coolant inside the transformer.Such liquids are also good electrical insul

35、ators for the wire windingsand iron core. Dry transformers do not require a liquid for cooling,instead using ambient air for cooling as well as insulation. Dust,dirt, moisture, and other contaminants in the air can reduce its insu-lating capabilities and deteriorate exposed parts, and may cause pre-

36、mature failure of the transformer.Emergency and Standby Power SystemsEmergency Power Systems. IEEE Standard 446-1995 definesthese systems as independent reserves of electrical power that auto-matically take over if the usual supply experiences an outage or fail-ure, and sustain mission-critical syst

37、ems (i.e., those that, ifinoperable, could present a danger to health and safety, or to prop-erty). Local or national codes may also mandate specific systems asrequired emergency power systems.Standby Power Systems. Generally, these systems providepower back-up for loads that may be critical to prod

38、uction or prod-uct preservation, but do not present a danger to life or safety. Theyallow facilities to carry on with satisfactory operation during failureor outage of the usual supply source. NFPA Standard 70 distin-guishes between those that are legally required and those that areoptional.Emergenc

39、y and Standby Power Supplies. Diesel generatorsare still the dominant source of emergency power, particularly foremergency loads such as fire pumps or elevators that require largestarting currents and must meet the outages full capacity at start-up.Some natural gas engines meet the start-up requirem

40、ents for emer-gency systems, and are popular for smaller standby applicationssuch as homes, communication towers, and other operations wherediesel fuel is undesirable and/or impractical. Some turbines are alsoused for standby service, but most turbines are dedicated to com-bined heat and power (CHP)

41、 applications (see Chapter 7 of the 2012ASHRAE HandbookHVAC Systems and Equipment).The use and constraints of emergency and standby power sys-tems must be understood to ensure proper safety and operation ofthe electrical equipment they support. This is especially true withemergency systems and with

42、standby systems that are expanded tocarry the full facility load, including HVAC thesetend to be utility-specific and may exceed normal NEC or localcodes.Check the existing system for closed-transition or parallel-transfer switch issues and compatibility.Investigate emissions issues; standby emissio

43、ns permit require-ments are almost always less stringent than peaking requirements.If the emergency system includes a UPS or flywheel system, thenSize the UPS or flywheel for the maximum required load for aspecific time; remember that capacity of these systems is x kW fory min.Avoid motor loads as m

44、uch as possible.MotorsMotor Control and Protection. Chapter 45 of the 2012 ASHRAEHandbookHVAC Systems and Equipment addresses motor controland protection in detail, but is summarized and simplified here.Motor control must be effective without damaging the motor or itsassociated equipment. Control mu

45、st be designed to prevent inadver-tent motor starting caused by a fault in the control device. The con-trol should be able to sense motor conditions to keep the motorwindings from getting too warm.Motor protection involves sensing motor current and line voltage,and can include bearing vibration, win

46、ding temperature, bearingtemperature, etc. Motor temperature increase has two basic sources.Heating occurs when dirt or debris blocks airflow over or through themotor, or it comes from the motor current and is commonly referredto as I2R, where I is motor current and R is motor winding resistance.Bec

47、ause the current is squared, its contribution is exponential. R isquite small and contributes a linear function to heating (and thereforetemperature rise) in the motor. Motor windings can withstand tem-perature rise, depending on the motor winding temperature rating.The second source of motor temper

48、ature increase is lack of motorcooling. The primary source of cooling is moving air, usually from ashaft-driven fan. As a motor slows down, the fan runs more slowly;therefore, the less air movement, the less cooling. Because fan loadsare also exponential, a small decrease in motor speed greatly redu

49、cesairflow on the motor, reducing cooling. To compound the issue, theslower the motor runs, the greater the slip, and the greater the motorcurrent. This then becomes a vicious circle.Motor Starters and Thermal Overloads. Motor starters energize(start), deenergize (stop), protect, and control the motor. They senseFig. 9 Closed-Transition ATSFig. 10 Parallel-Transfer Switch56.6 2015 ASHRAE HandbookHVAC Applicationsmotor current based on a time curve: the shorter the time, the morecurrent they let through. They may also limit the number of motorstarts in a give