Electrical Machines

LSEGG216A9080V

Parallel OperationParallel Operation&&

Auxiliary equipmentAuxiliary equipment

IntroductionIntroduction• Identify

Parallel Parallel OperationOperationWhy 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

Parallel Parallel OperationOperationBoth transformers must have:

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

Parallel Parallel OperationOperationSingle Phase

Incorrect Polarity

High circulation currents will flow

Parallel Parallel OperationOperationSingle Phase Correct Polarity

Parallel Parallel OperationOperationTesting for Polarity

Place a voltmeter across the transformer terminal

and the supply line

Correct polarity =Incorrect polarity =

0 VoltsTransformer voltage or higher

3 Phase 3 Phase TransformersTransformers

TypesStar

PRIMARY SECONDARY

No Phase Shift

Star

Reference

3 Phase 3 Phase TransformersTransformers

TypesDelta

PRIMARY SECONDARY

No Phase Shift

Delta

Reference

3 Phase 3 Phase TransformersTransformers

TypesDelta

PRIMARY SECONDARY

V2 Leads V1 by 30

Star

30Reference

3 Phase 3 Phase TransformersTransformers

TypesDelta

PRIMARY SECONDARY

V2 Lags V1 by 30

Star

30

Reference

Paralleling 3 Phase Paralleling 3 Phase TransformersTransformers

Must be from the same vector groupMust have the same effect on voltage:

-Δ paralleled with -Δ

Δ- paralleled with Δ-

Δ- paralleled with -Δ Provided that the line voltages are equal

Paralleling 3 Phase Paralleling 3 Phase TransformersTransformers

Must be from the same vector groupMust have the same effect on voltage:

-Δ paralleled with -Δ

Δ- paralleled with Δ-

Δ- paralleled with -Δ

Δ- -30 & -Δ +30

The voltage phase difference would be 60

Auxiliary Auxiliary EquipmentEquipment• Protection

• Voltage adjustment

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

• Tap changers

Transformer Transformer ConstructionConstruction

• Most large transformers are oil filled

• The oil level must be high enough to cover the transformer

• Oil must be kept dry and clean

Transformer Transformer ConstructionConstruction

Sealed Tank

Conserver Tank

Tank always stays full

Transformer Transformer ConstructionConstruction

Pressure relief valve allows over pressure oil to escape

Over temperature thermostat trips circuit breaker if

transformer gets too hot

Buchholz relay monitors oil between transformer and

conserver tank

Buchholz Buchholz relayrelay

• 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

Monitors

Buchholz Buchholz relayrelay

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

Buchholz Buchholz relayrelay 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

Buchholz Buchholz relayrelay

Oil & Gass from Tx Oil to conserver

Oil for testing

Gas testingDry air test

Tap ChangersTap ChangersTwo types:• Off Load• On Load

Can be located on the primary or secondary of the transformer

Off LoadOff Load• Basically a multi-position switch

• Open circuits output between positions

• Contacts not rated to open circuit under load

Transformer Open circuited

On LoadOn Load• Manual• Automatic

Transformer windings

Common terminal

Sliding switch

On LoadOn LoadInductors carry

current when contact is between taps

On LoadOn LoadContact moves to new tap