Transformer Selection and Calculation

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Transformer Selection and Calculation

2011-06-13 by seoer1

An existing project, you need 90 sets of 35 air-conditioning, power consumption is 1.4KW / Taiwan

71, need 70 sets of air-conditioning, power consumption 2.84KW / Taiwan;

15 120 need air conditioning, power consumption is 4.8KW, Taiwan;

I can only calculate the total power consumption (not considering the case of using both coefficient) is 396.8KW, the problem is to

choose how much capacity of theElectrical Transformer? ? Customers also need to set aside part of the lighting power consumption?

?

Customers have a 250KW (or 250KVA) and a 80KW (or 80KVA) of the transformer (about this, I do not understand the electrical side,

users do not know), do not know can not be used to meet the above requirements.

90 * 1.4 = 126kW

70 * 2.84 = 198kW

15 * 4.8 = 72kW

126 +198 +72 = 396kW

0.8 power factor calculated in accordance with air conditioning, 396/0.8 = 495kVA

You need at least 495kVA transformer. Also, consider the reactive power output of the transformer itself and the loss of capacity should

be increased by 20%. (Note: do not consider the transformer but also with other than air conditioning load conditions such as lighting,

etc.) then the transformer needs at least 495 * (1 +20%) = 594kVA

Then the model is closest to your requirements 630kVA transformer.

Even if you tie two transformers is not enough, and it looks like you twoDistribution Transformers that much difference, does not have

parallel conditions. Also with so many air-conditioning, need to increase the reactive power compensation devices to provide a large

number of air conditioning required reactive power, power factor or low Power Administration to be fine.

keywords:distribution transformer

Application of Standard Dry Type

Transformers in an Electrical System

Most utilities will only provide a customer with one service orelectrical system. This system may be either single-phase or three-phase. Single-phase installations will normally be 120/240V AC, 3wire systems. Three-phase installations could be 240 volt, 3 wire ,480 volt 3 wire, 600 volt, 3 wire, 208Y/120 volt, 4 wire , or280Y/277 volt,4 wire. These are the most popular installations andtheir selection can be either on customer preference or availabilityof the system from the serving utility. With this many choicesavailable , you may wonder why anyone would need a transformer,so let us offer an example. A new industrial plant moves into townand requires an electrical service in their new building. They have

a great many motors in use at their company, so they decide itwould be more economical to use 480 volt three-phase motors. Forthis reason they request a 480Y/277 volts three-phase system.This takes care of their motor loads at 480 volts and their officeand plant lighting loads at 277 volts. However, to operate theiroffice machinery and incandescent lighting they require 120 volts.They also have some small horsepower motors they want tooperate at 208 volts. Since the utility will only provide them with480Y/277 volt three-phase system , they require a dry-typedistribution transformer to provide the 208 and 120 volt loads.


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This is the most typical of applications for dry-type distributiontransformers. Other applications could be matching the voltage of amotor which does not match your system, isolating a computer or

solid state device from system voltage due to voltage drop in anextremely long run of wire. The more important thing is torecognize what transformers can and cannot do . Below is a tableof some of those things.

Operation Yes No

Change Voltage *

Change Frequency *Convert single -phase current to three-phase current *

Protect equipment (isolate line voltage from loadvoltage)

*

Stabilize fluctuating line voltage (constant voltage)*

Note

Compensate for voltage drop *

Improve power factor *

*Note: There are special purpose constantvoltage transformers that can do this.

Selection of a Transformer


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When a customer calls you for help in the selection of atransformer these are things you need to know:

1 What is the voltage of this load? The transformer you select musthave an output voltage which matches his load voltage. (dont getconfused between system and utilization voltage - See Section V,Paragraph D.)

2 Is the load single-phase or three-phase? Remember thetransformer cannot change phases. Three-phase loads must befed from the three-phase transformer/banks.3 What is the power requirement for this load? Weultimately needto arrive at a KVA value. If only amps are known, use the full loadchart or the following formulas.

Single -phase KVA= Volts (loads) x Amps (load)1000

Three=phase KVA = Volts (load) x Amps (loads) x 31000

Where 3 = 1.732

4 What is the frequency(hertz or Hz) of the load and line (source)?

Remember, transformers cannot change frequency. Generally, allU.S. power companies generate power at 60 Hz. Therefore, theload must also be rated 60 Hz.5 What is the supply or source voltage? Are primary taps required?6 Is there a special temperature rise or insulation system

requirement ? If not, quote our standard general purposetransformers.7 Is the transformer to be installed indoors or outdoors? Sometransformers, particularly small encapsulated units are rated forindoor or outdoor applications. Others sizes will require the

addition of a weather shield for outdoor use.

With the above information you should be able to quickly select atransformer from the catalog.

Note: Other considerations which may require special units mayinclude, but are not limited to: copper windings; low temperaturerise units; units for applications in ambient temperatures higherthan 40(C;units to be used at a high altitude above 3300 feet;special impedances; and many others. If requirements arise thatdo not fit the description of our standard units, be sure to contactyour Federal Pacific representatives for assistance.

Problem: What is the proper transformer for a customer to supplyan electric heater rated 100 amps, @240 volts, three-phase, 60Hz? His available supply voltage is 480 volts, three-phase, 60 Hz.

The transformer is installed indoors. 150 C with standard taps isrequired.

Solution: We have all of the information required with theexception of the load KVA. We know that:

Three-phase KVA = Volts (load) x Amps (load) x 3 = 240 x 100 x1.732 = 41.61000 1000

A 480 volt to 240 volt (Delta-Delta), three-phase, 45 KVA (which isthe next standard KVA rating) general purpose transformer isrequired


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Problem: A single line shows a 25 KVA, single-phase, 60 Hz,150(C rise, 240X 480V to 120/240 volt transformer fed from athree-phase volt system. Is it correct?

Solution: Yes, the transformer has series-multiple primarywindings so connection to 480 volt is acceptable. Remember, thatif the transformer is single-phase, the source can be single-phaseor three-phase. When the supplies three-phase, any two (2) lines

or one (1) line and neutral will be used as shown below . Figure 1applies for the 480 volt primary voltage in the above problem.

Problem: An industrial plant requests a transformer that ca step down

480 volt three-phase to 240 volt three-phase and supply 200 KVA of load

at 240 volt three-phase and 5 KVA of load at 120 volt single-phase. What

would you quote?

Solution: A general purpose 300KVA, 480volt to 240 volt three-phase

transformer with 120 volt lighting tap will work. Remember that the useof the 120 volt lighting tap requires a 30 % derating of the nameplate

KVA and that the 120 volt loads cannot exceed 5% of the nameplate

KVA. (See below)

Derating Calculations:

300 KVA x .30 KVA

300 KVA

-90KVA210KVA (Available capacity for 30 (loads)

120 Volt Lighting Tap Calculations:

300 KVA x .05 = 15 KVA (Available capacity for 120 volt loads)

The following must be known before a transformer can beselected:


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KVA the rating or capacity of the transformerPhaseLoad requirements (single-phase or three-phase)If the load is three-phase, both the supply and the transformermust be three-phase.If the load is single-phase, the supply can be either single or three-phase, but the transformer must be single-phase.Frequencyusually 60 Hz (Hertz).

Primary Voltage Designates the load voltage for wich the primarywinding is designated.Secondary Voltage Designates the load voltage for which thesecondary winding is designed.TapsAdjustment capability for voltage variations.Locationof Installation Indoor or OutdoorOtherConsiderationsMounting Requirements, Sound Levels,Impedance, Special Applications, K-Rating, Copper Windings,Electrostatic Shields, Temperature Rise, Insulation Class

Cost ComparisonSecondary Unit SubstationTransformers

How to choose transformer rating?Posted Mar 12 2011 by Edvard in Transformers with 1 Comment

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When an installation is to be supplied directly from a MV/LV transformer and the maximum

apparent-power loading of the installation has been determined, a suitable rating for the

transformer can be decided, taking into account the following considerations:

y The possibility of improving the power factor of the installation (see chapter L)

y Anticipated extensions to the installation

y Installation constraints (e.g. temperature)

y Standard transformer ratings

Apparent power [kVA] In (A)

- 237V -- 410V -

100 244 141


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160 390 225

250 609 352

315 767 444

400 974 563

500 1218 704

630 1535 887

800 1949 1127

1000 2436 1408

1250 3045 1760

1600 3898 2253

2000 4872 2816

2500 6090 3520

3150 7673 4436

Figure. 1 : Standard apparent powers for MV/LV transformers and related nominal output

currents

3-phase transformer

The nominal full-load current In on the LV side of a 3-phase transformer is given by:

where:

y Pa = kVA rating of the transformer

y U = phase-to-phase voltage at no-load in volts (237 V or 410 V)y In is in amperes

Single-phase transformer

For a single-phase transformer:


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where

y V = voltage between LV terminals at no-load (in volts)

Simplified equation for 400 V (3-phase load)

y In = kVA x 1.4

The IEC standard for power transformers is IEC 60076.

SOURCE:Schneider Electric

Transformer Ratings

5,900 views

Transformer Ratings

Transformer size or capacity is most often expressed in kVA. We require 30 kVA of power

for this system is one example, or The facility has a 480 VAC feed rated for 112.5 kVA.

However, reliance upon only kVA rating can result insafety and performance problems whensizing transformers to feed modern electronic equipment.

Use of off-the-shelf, general purpose transformers for electronics loads can lead to power

quality and siting problems:

y Single phase electronic loads can cause excessive transformer heating.

y Electronic loads draw non-linear currents, resulting in low voltage and output voltagedistortion.

y Oversizing for impedance and thermal performance can result in a transformer with asignificantly larger footprint.


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It is vital for the systems designer to understand all of the factors that affect transformereffectiveness and performance.

.

Thermal Performance

Historically, transformers have been developed to supply 60 Hz, linear loads such as lights,motors, and heaters. Electronic loads were a small part of the total connected load. A system

designer could be assured that if transformer voltage and current ratings were not exceeded,the transformer would not overheat, and would perform as expected. A standard transformer

is designed and specified with three main parameters: kVA Rating, Impedance, andTemperature Rise.

.

KVA Rating

The transformer voltage and current specification. KVA is simply the load voltage times the

load current. A single phase transformer rated for 120 VAC and 20 Amperes would be ratedfor 120 x 20 = 2400 VA, or 2.4 KVA (thousand VA)..

Impedance

Transformer Impedance and Voltage Regulation are closely related: a measure of thetransformer voltage drop when supplying full load current. A transformer with a nominal

output voltage of 120 VAC and a Voltage Regulation of 5% has an output voltage of 120VAC at no-load and (120 VAC 5%) at full load the transformer output voltage will be

114 VAC at full load. Impedance is related to the transformer thermal performance because

any voltage drop in the transformer is converted to heat in the windings.

.

Temperature Rise

Steel selection, winding capacity, impedance, leakage current, overall steel and windingdesign contribute to total transformer heat loss. The transformer heat loss causes the

transformer temperature to rise. Manufacturers design the transformer cooling, and selectmaterials, to accommodate this temperature rise.


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Transformer Heat Loss

Use of less expensive material with a lower temperature rating will require the manufacturerto design the transformer for higher airflow and cooling, often resulting in a larger

transformer. Use of higher quality materials with a higher temperature rating permits a more

compact transformer design.

Transformer Insulation Systems

.

K Factor Transformer Rating


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In the 1980s, power quality engineers began encountering a new phenomenon: non-linearloads, such as computers and peripherals, began to exceed linear loads on some distribution

panels. This resulted in large harmonic currents being drawn, causing excessive transformerheating due to eddy-current losses, skin effect, and core flux density increases.

Standard transformers, not designed for nonlinear harmonic currents were overheating and

failing even though RMS currents were well within transformer ratings.

In response to this problem, IEEE C57.110-1986 developed a method of quantifying

harmonic currents. A k factor was the result, calculated from the individual harmonic

components and the effective heating such a harmonic would cause in a transformer.

Transformer manufacturers began designing transformers that could supply harmonic

currents, rated with a k factor. Typical K factor applications include:

y K-4: Electric discharge lighting, UPS with input filtering, Programmable logic

controllers and solid state controls

y K-13: Telecommunications equipment, UPS systems, multi-wire receptacle circuits in

schools, health-care, and production areas

y K-20: Main-frame computer loads, solid state motor drives, critical care areas ofhospitals

K factor is a good way to assure that transformers will not overheat and fail. However, K

factor is primarily concerned with thermal issues. Selection of a K factor transformer may

result in power quality improvement, but this depends upon manufacturer and design.

.

Transformer Impedance

Transformer impedance is the best measure of the transformers ability to supply an

electronic load with optimum power quality. Many power problems do not come from theutility but are internally generated from the current requirements of other loads.

While a K factor transformer can feed these loads and not overheat, a low impedancetransformer will provide the best quality power. As an example, consider a 5% impedance

transformer. When an electronic load with a 200% inrush current is turned on, a voltage sagof 10% will result. A low impedance transformer (1%) would provide only a 2% voltage sag

a substantial improvement. Transformer impedance may be specified as a percentage, oralternately, in Ohms () from Phase- Phase or Phase-Neutral.

.

High Frequency Transformer Impedance

Most transformer impedance discussions involve the 60 Hz transformer impedance. This isthe power frequency, and is the main concern for voltage drops, fault calculations, and power

delivery. However, nonlinear loads draw current at higher harmonics. Voltage drops occur at both 60 Hz and higher frequencies. It is common to model transformer impedance as a

resistor, often expressed in ohms. In fact, a transformer behaves more like a series resistorand inductor.


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The voltage drop of the resistive portion is independent of frequency, the voltage drop of theinductor is frequency dependent.

Standard Transformer impedances rise rapidly with frequency. However, devices designed

specifically for use with nonlinear loads use special winding and steel lamination designs tominimize impedance at both 60 Hz and higher frequencies. As a result, the output voltage of

such designs is far better quality than for standard transformers..

Recommendations for Transformer Sizing

System design engineers who must specify and apply transformers have several options when

selecting transformers..

Do It Yourself Approach

With this approach, a larger than required standard transformer is specified in order to supplyharmonic currents and minimize voltage drop. Transformer oversizing was considered

prudent design in the days before transformer manufacturers understood harmonic loads, and

remains an attractive option from a pure cost standpoint. However, such a practice today has

several problems:

y A larger footprint and volume than low impedance devices specifically designed for

non-linear loads

y Poor high frequency impedance

y Future loads may lead to thermal and power quality problems

Standard Isolation Transformer

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K-factor Rated Transformers

Selecting and using K-factor rated transformers is a prudent way to ensure that transformer

overheating will not occur. Unfortunately, lackof standardization makes the K factor ratinga measure only of thermal performance, not impedance or power quality.


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Percent Impedance

Some manufacturers achieve a good K factor using design techniques that lower impedance

and enhance power quality, others simply derate components and temperature ratings. Only

experience with a particular transformer manufacturer can determine if a K factor

transformer addresses both thermal and power quality concerns.

.

Transformers Designed for Non-Linear Loads

Transformers designed specifically for non-linear loads incorporate substantial design

improvements that address both thermal and power quality concerns. Such devices are low

impedance, compact, and have better high frequency performance than standard or K factordesigns. As a result, this type of transformer is the optimum design solution.

This type of transformer may be more expensive than standard transformers, due to higher

amounts of iron and copper, higher quality materials, and more expensive winding and

stacking techniques. However, the benefits of such a design in power quality and smaller size

justify the extra cost, and make the low impedance transformer the most cost effective design

overall.

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Transformer Selection and Calculation