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# Electrical Machines PowerPoint PPT Presentation

Electrical Machines. LSEGG216A 9080V. Parallel Operation & Auxiliary equipment. Introduction. Identify. 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

Electrical Machines

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LSEGG216A

9080V

## Parallel Operation&Auxiliary equipment

• Identify

### 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

### Parallel Operation

Both transformers must have:

• Same voltage

• Frequency

• Roughly the same VA rating

• Same polarity

### Parallel Operation

Single Phase

High circulation currents will flow

Incorrect Polarity

Single Phase

Correct Polarity

### 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

Types

PRIMARY

SECONDARY

Star

Star

No Phase Shift

Reference

Types

PRIMARY

SECONDARY

Delta

Delta

No Phase Shift

Reference

Types

PRIMARY

SECONDARY

Star

Delta

Reference

30

### 3 Phase Transformers

Types

PRIMARY

SECONDARY

Delta

Star

V2 Lags V1 by 30

30

Reference

### Paralleling 3 Phase Transformers

Must be from the same vector group

Must have the same effect on voltage:

-Δ

paralleled with

-Δ

Δ-

paralleled with

Δ-

Provided that the line voltages are equal

Δ-

paralleled with

-Δ

### Paralleling 3 Phase Transformers

Must be from the same vector group

Must 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 Equipment

• Protection

• Over temperature

• Over pressure

• Low oil level

• Oil quality

• Reduction of water

• Tap changers

### 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

### Transformer Construction

Conserver Tank

Sealed Tank

Tank always stays full

### 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

### 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

### Buchholz relay

On a slow accumulation of gas, due to slight overloadaccumulates 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 relay

rapidgas accumulation, flow of oil operates a switch attached to a vane

Test port to allow the accumulated gas to be withdrawn for testing

### Buchholz relay

Dry air test

Gas testing

Oil to conserver

Oil & Gass from Tx

Oil for testing

### Tap Changers

• Two types:

Can be located on the primary or secondary of the transformer

• Basically a multi-position switch

• Open circuits output between positions

• Contacts not rated to open circuit under load

Transformer Open circuited

• Manual

• Automatic

Sliding switch

Transformer windings

Common terminal