Types of three-phase transformers – basic types of construction
2. Shell-type transformer
In shell-type transformers, the core surrounds a considerable portion of the windings. This means more of the core is visible compared to the other type. The whole winding consists of discs stacked with insulation spaces between the coils.
Such a transformer may have the shape of a simple rectangle.
A strong mechanical support must be given to the cores and coils of the shell-type transformer. A transformer with good support (bracing) will not produce any humming noise while working and will also reduce vibration of the laminated plates.
Usually, these transformers are placed in tightly-fitted sheet-metal tanks filled with special insulating oil.
The general arrangement of the core-type transformer with respect to the core is shown in figure 3.7 and 3.8.
Types of three-phase transformers:
Mostly quoted from: http://www.trafoworld.com/en/transformers/different_
types_transformer/#DTH
There are several different types of three-phase transformers available in the world market. They are mainly divided by power and voltage supplied, but also depend on their application:
• small distribution transformers • distribution transformers • cast resin transformers • dry-type transformers
• large distribution transformers • medium power transformers • large power transformers.
Small distribution transformers
Single-phase transformers, usually made with wound core system and rectangular windings.
Power range: 50 to 200 kVA
Main use: Distribution in rural areas and countryside
Main advantages: Small production costs with the possibility of good automation
Distribution transformers
Usually three-phase transformers, immersed in liquid oil as dielectric insulation and enclosed in a tank with cooling system. Recently made hermetically sealed for reduced maintenance and better quality.
Power range: 250 to 2 500 kVA
Main use: Distribution of energy in cities and centres with different houses Main advantages: Great extension of use in different outdoor applications
Cast Resin Transformers CRT
Usually three-phase transformers but instead of being immersed in oil, the high voltage side is cast into a resin which will be its dielectric insulation.
Power range: 250 to 4 000 kVA
Main use: Underground systems, mines and skyscrapers
Main advantages: Fireproof and explosion-proof, particularly adapted for indoor applications
Dry type transformers
Usually three-phase transformers but instead of being immersed in oil, the HV side is dipped into an insulating varnish which will be its dielectric insulation along with open air.
Power range: 250 to 4 000 kVA
Main use: Underground systems, mines and skyscrapers.
Main advantages: Fireproof and explosion-proof, particularly adapted for indoor applications
Large distribution transformers
Three-phase transformers, usually immersed in liquid oil as dielectric insulation and enclosed in a tank with cooling system.
Power range: 2 500 to 20 000 kVA
Main use: Grid interconnections, industrial applications, special applications such as furnace or railway.
Main advantages: Big power with the potential of 35 kV distribution Medium power transformers
Three-phase transformers, adapted for grid interconnections for short distance transmission lines up to 220 kV.
Power range: 250 to 4 000 kVA Main use: Interconnecting grids Main advantages: Big power and high voltage Large power transformers
Three-phase transformers, adapted for grid interconnections for long distance transmission lines above 220 kV.
Power range: 250 to 1 000 MVA
Main use: Interconnecting grids and main power station.
Main advantages: Big power and high voltage
Transformers (star-star, delta-delta, star-delta, delta-star) Star-star transformer
Figure 3.9: Diagrammatic star-star by means of three single-phase transformers
Figure 3.10: Schematic star-star by means of single three-phase transformer Characteristics of a star-star connection:
• Used for small high-voltage applications.
• It gives a VL of √3 times the VPH.
• Most economical due to minimum insulation required.
• Smallest number of turns (windings) required.
• Lowest insulation and smallest number of turns per phase.
• No phase shift between VPRIM and VSEC.
• Possibility of a star point forming a neutral on both the primary and the secondary sides.
L1 L2 L3
L1 L2 L3 a1 a2 b1 b2 c1 c2 a1 a2 b1 b2 c1 c2
L1
L2 L3
L1
L2 L3
Delta-delta transformer
Figure 3.11: Diagrammatic delta-delta by means of three single-phase transformers
Characteristics of a delta-delta connection:
• Relatively large, low line voltage applications.
• No possibility of a neutral on either side.
• System that carries large currents for low voltages.
• Phase current under balanced conditions only 57,7% of the line current.
• Continuous service even if one phase is removed (open VEE system).
• No phase displacement between primary and secondary.
Star-delta transformer
Figure 3.12: Diagrammatic star-delta by means of three single-phase transformers L1
L2 L3
L1
L2 L3 L1 L2 L3
L1 L2 L3 a1 a2 b1 b2 c1 c2 a1 a2 b1 b2 c1 c2
L1 L2 L3
L1 L2 L3 a1 a2 b1 b2 c1 c2 a1 a2 b1 b2 c1 c2
Characteristics of a star-delta or delta-star connection:
• Combines advantages of both star and delta in one transformer.
• Used in power distribution systems (where voltage must be stepped up or down).
• No neutral displacement under unbalanced loads.
• The star side can provide a neutral.
• Polarity conscious due to an inherent electrical 30° phase shift between primary and secondary.
Delta-star transformer
Figure 3.13: Diagrammatic delta-star by means of three single-phase transformers L1
L2 L3
L1
L2 L3 L1 L2 L3
L1 L2 L3 a1 a2 b1 b2 c1 c2 a1 a2 b1 b2 c1 c2 L1
L2 L3
L1
L2 L3
Calculations: (Efficiency at 100%)
It is extremely helpful always to make a sketch of the information given. Fill in your calculated values as they are calculated. It helps to keep track of where you are in the process.
It may be a three-phase transformer but remember that it is made up of three single-phase transformers placed opposite each other. Therefore ANY calculation that is done using the transformer formulae must be done using phase values only.
Example 1:
A three-phase delta-star transformer has a winding ratio of 1040:23.
a) Calculate the value of the line voltage on the secondary side if the primary side is connected to a line voltage of 11 kV.
b) Calculate the line current on the secondary side if the transformer had a rating of 15 kVA.
Answer 1:
a) On the supply side VL = VPH = 11 kV
The transformer formula may only be used with phase values
b)
Example 2:
A 20 kVA star-delta transformer has primary and secondary line voltages of 6 kV and 400 V respectively. Ignore all losses and calculate:
a) The secondary line current of the transformer.
b) The primary line current of the transformer.
c) The windings ratio rounded off to the nearest turn.
d) If the power factor is 0,97, calculate the effective (true) power of the transformer on the secondary side.
11kV 1040 : 23
VPH
Papparant
Answer 2:
a)
b)
c)
d)
Example 3:
A 10 kW delta-connected load with a lagging power factor of 0,8 is connected to a delta-star transformer with a windings ratio of 137:3. The transformer is connected to a 11 kV supply.
a) Draw a schematic representation of the system.
Ignore all losses and calculate:
b) The secondary line voltage of the transformer.
c) The secondary line current.
d) The current flowing through each phase of the load.
e) Primary line current.
f) The apparent power that the transformer must deliver.
Take note Remember that 0,97 is substituted, and not cos 0,97.
Papparant
(Prim) Papparant
Answer 3:
a)
b) On the primary side VL = VPH = 11 000 V (delta)
(Remember...all values must be phase)
c) The load is rated as 10 kW and cos θ = 0,8
d) The load is in delta
e) On the secondary star side of the transformer IL = IPH = 17,3 A (star)
Remember, calculation with transformer ratios must be done with PHASE values only.
f) Deliver means on the secondary side Take note
Whenever a question refers to supply voltage, it means a line value.