Electrical Machines

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Electrical Machines

• LSEGG216A
• 9080V

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Parallel OperationAuxiliary equipment
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Introduction

• Identify

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Parallel Operation

• Why not get a bigger one?

• Can carry many of the same size so maintenance is
  easier cheaper
• If one fails can still supply some load
• Can turn one off if load reduces

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Parallel Operation

• Both transformers must have

• Same voltage
• Frequency
• Roughly the same VA rating
• Same polarity

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Parallel Operation

• Single Phase

High circulation currents will flow
Incorrect Polarity
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Parallel Operation

• Single Phase

Correct Polarity
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Parallel Operation

• Testing for Polarity

Place a voltmeter across the transformer terminal
and the supply line
Correct polarity Incorrect polarity
0 Volts Transformer voltage or higher
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3 Phase Transformers

• Types

PRIMARY
SECONDARY
Star
Star
No Phase Shift
Reference
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3 Phase Transformers

• Types

PRIMARY
SECONDARY
Delta
Delta
No Phase Shift
Reference
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3 Phase Transformers

• Types

PRIMARY
SECONDARY
Delta
Star
V2 Leads V1 by 30?
Reference
30?
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3 Phase Transformers

• Types

PRIMARY
SECONDARY
Delta
Star
V2 Lags V1 by 30?
30?
Reference
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Paralleling 3 Phase Transformers

• Must be from the same vector group

Must have the same effect on voltage
?-?
paralleled with
?-?
?
?-?
paralleled with
?-?
?
Provided that the line voltages are equal
?-?
paralleled with
?
?-?
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Paralleling 3 Phase Transformers

• Must be from the same vector group

Must have the same effect on voltage
?-?
paralleled with
?-?
?
?-?
paralleled with
?-?
?
?-?
paralleled with
?
?-?
?-? -30? ?-? 30? The voltage phase difference
would be 60?
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Auxiliary Equipment

• Protection
• Voltage adjustment

• Over temperature
• Over pressure
• Low oil level
• Oil quality
• Reduction of water

• Tap changers

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Transformer Construction

• Most large transformers are oil filled
• The oil level must be high enough to cover the
  transformer
• Oil must be kept dry and clean

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Transformer Construction
Conserver Tank

• Sealed Tank

Tank always stays full
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Transformer Construction
Buchholz relay monitors oil between transformer
and conserver tank
Pressure relief valve allows over pressure oil to
escape
Over temperature thermostat trips circuit breaker
if transformer gets too hot
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Buchholz relay
Monitors

• Presence of air bubbles due to faulty initial
  filling or faulty seal of gaskets
• Formation of gas due to burning of oil or solid
  insulation
• Lowering of oil level due to leak of the tank
• Oil flow from tank to conservator due to
  explosion or short circuit

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Buchholz relay
On a slow accumulation of gas, due to slight
overload accumulates in the top of the relay and
forces the oil level down. A float operated
switch in the relay is used to initiate an alarm
signal
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Buchholz relay
rapid gas accumulation, flow of oil operates a
switch attached to a vane
Test port to allow the accumulated gas to be
withdrawn for testing
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Buchholz relay
Dry air test
Gas testing
Oil to conserver
Oil Gass from Tx
Oil for testing
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Tap Changers

• Two types
• Off Load
• On Load

Can be located on the primary or secondary of the
transformer
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Off Load

• Basically a multi-position switch
• Open circuits output between positions
• Contacts not rated to open circuit under load

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Transformer Open circuited
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On Load

• Manual
• Automatic

Sliding switch
Transformer windings
Common terminal
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On Load
Inductors carry current when contact is between
taps
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On Load
Contact moves to new tap