Why Transformer Rating In kVA, Not in KW?

Why Transformer Rating In kVA, Not in KW?

In Simple words, 

There are two type of losses in a transformer:

1. Copper Losses

2. Iron Losses or Core Losses or  Insulation Losses

Copper losses ( I²R)depends on Current which passing through transformer winding while Iron Losses or Core Losses or  Insulation Losses depends on Voltage.

That’s why the Transformer Rating may be expressed in kVA,Not in kW.

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Different types of transformer

Different types of transformer

Types of Transformers:

There are two basic Types of Transformers

1. Single Phase Transformer
2. Three Phase Transformer

Below are the more types of transformer derived via different functions and operation etc.

Types of Transformers w.r.t Cores:

• Core Type Transformer
• Shell Type Transformer
• Berry Type Transformer

Types of Transformer w.r.t uses:

• Large Power Transformer
• Distribution Transformer
• Small Power Transformer
• Sign Lighting Transformer
• Control & Signalling Transformer
• Gaseous Discharge Lamp Transformer
• Bell Ringing Transformer
• Instrument Transformer
• Constant Current Transformer
• Series Transformer for Street Lighting

Types of Transformer w.r.t Cooling:

• Self Air Cooled or Dry Type Transformer
• Air Blast-Cooled Dry Type
• Oil Immersed, Self Cooled (OISC) or ONAN (Oil natural, Air natural)
• Oil Immersed, Combination of Self Cooled and Air blast (ONAN)
• Oil Immersed, Water Cooled (OW)
• Oil Immersed, Forced Oil Cooled
• Oil Immersed, Combination of Self Cooled and Water Cooled (ONAN+OW)
• Oil Forced, Air forced Cooled (OFAC)
• Forced Oil, Water Cooled (FOWC)
• Forced Oil, Self Cooled (OFAN)

Types of  Instrument Transformer:

• Current Transformer
• Potential Transformer
• Constant Current Transformer
• Rotating Core Transformer or Induction regulator
• Auto Transformer

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Different Parts of Transformer

Different Parts of Transformer

1.	Oil filter valve	17.	Oil drain valve
2.	Conservator	18.	Jacking boss
3.	Buchholz relay	19.	Stopper
4.	Oil filter valve	20.	Foundation bolt
5.	Pressure-relief vent	21.	Grounding terminal
6.	High-voltage bushing	22.	Skid base
7.	Low-voltage bushing	23.	Coil
8.	Suspension lug	24.	Coil pressure plate
9.	B C T Terminal	25.	Core
10.	Tank	26.	Terminal box for protective devices
11.	De-energized tap changer	27.	Rating plate
12.	Tap changer handle	28.	Dial thermometer
13.	Fastener for core and coil	29.	Radiator
14.	Lifting hook for core and coil	30.	Manhole
15.	End frame	31.	Lifting hook
16.	Coil pressure bolt	32.	Dial type oil level gauge.

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Introduction to Electrical Transformer

Introduction to Electrical Transformer

What is a Transformer?

In Very Simple words.

Transformer is a device which:

1. Transfer Electrical power from one electrical circuit to another Electrical circuit.
2. It’s working without changing the frequency.
3. Work through on electric induction.
4. When, both circuits take effect of mutual induction.
5. Can’t step up or step down the level of DC voltage or DC Current.
6. Can step up or step down the level of AC voltage or AC Current.

• Without transformers the electrical energy generated at generating stations won’t probably be sufficient enough to power up a city. Just imagine that there are no transformers.How many power plants do you think have to be set up in order to power up a city? It’s not easy to set up a power plant. It is expensive.
• Numerous power plant have to be set up in order to have sufficient power. Transformers help by amplifying the Transformer output (stepping up or down the level of voltage or current).
• When the number of turns of the secondary coil is greater than that of primary coil, such a transformer is known as step up transformer.
• Likewise when the number of turns of coil of primary coil is greater than that of secondary transformer, such a transformer is known as step down transformer.

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Why we are use 11KV / 22KV / 33KV / 66KV / 110KV / 230KV / 440KV this type of ratio. Why can’t we use other voltage ratio like 54KV / 99KV etc?

Why we are use 11KV / 22KV / 33KV / 66KV / 110KV / 230KV / 440KV this type of ratio. Why can’t we use other voltage ratio like 54KV / 99KV etc?

When an alternator generates voltage, we always use a multiple of 1.11 because for a pure sine wave the FORM FACTOR is the  ratio of rms value of voltage or current with the avg. value of voltage or current and for pure sine wave rms value of current is Imax/root '2' and avg. value is 2Imax/pie and which comes out to be 1.1;

We can't have a combination of other then a multiple of 1.11*.
So we can see all the voltages are made inevitably multiple of this value (1.1, which is the form factor of ac wave).

Also it provides us the best economic construction of step up and step down transformers.


* In the case of a Square Wave ie. a digital wave, the RMS and the average value are equal; therefore, the form factor is 1.

 

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How to choose transformer rating?

 How to choose transformer rating?

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:

• The possibility of improving the power factor of the installation
• Anticipated extensions to the installation
• Installation constraints (e.g. temperature)
• Standard transformer ratings.

3-phase transformer

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



where:

• P_a = kVA rating of the transformer
• U = phase-to-phase voltage at no-load in volts (237 V or 410 V)
• I_n is in amperes

Single-phase transformer

For a single-phase transformer:



where

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

Simplified equation for 400 V (3-phase load)

• I_n = kVA x 1.4

The IEC standard for power transformers is IEC 60076.

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Working principle of Transformer

Transformer

Working principle of Transformer

Transformer works on the principle of mutual induction of two coils or Faraday Law’s Of Electromagnetic induction. When current in the primary coil is changed the flux linked to the secondary coil also changes. Consequently an EMF is induced in the secondary coil due to Faraday law’s of electromagnetic induction.

Explanation

The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism), and, second that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.

 

 

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