Electrical MachinesLSEGG216A9080V

Parallel Operation&Auxiliary equipment

IntroductionIdentify

Parallel OperationWhy not get a bigger one?Can carry many of the same size so maintenance is easier & cheaperIf one fails can still supply some loadCan turn one off if load reduces

Parallel OperationBoth transformers must have:Same voltageFrequencyRoughly the same VA ratingSame polarity

Parallel OperationSingle PhaseIncorrect PolarityHigh circulation currents will flow

Parallel OperationSingle PhaseCorrect Polarity

Parallel OperationTesting for PolarityPlace a voltmeter across the transformer terminal and the supply lineCorrect polarity =Incorrect polarity =0 VoltsTransformer voltage or higher

3 Phase TransformersTypesStarPRIMARYSECONDARYNo Phase ShiftStarReference

3 Phase TransformersTypesDeltaPRIMARYSECONDARYNo Phase ShiftDeltaReference

3 Phase TransformersTypesDeltaPRIMARYSECONDARYV2 Leads V1 by 30Star30Reference

3 Phase TransformersTypesDeltaPRIMARYSECONDARYV2 Lags V1 by 30Star30Reference

Paralleling 3 Phase TransformersMust be from the same vector groupMust have the same effect on voltage:-paralleled with--paralleled with--paralleled with-Provided that the line voltages are equal

Paralleling 3 Phase TransformersMust be from the same vector groupMust have the same effect on voltage:-paralleled with--paralleled with--paralleled with-- -30 & - +30The voltage phase difference would be 60

Auxiliary EquipmentProtection

Voltage adjustmentOver temperatureOver pressureLow oil levelOil qualityReduction of waterTap changers

Transformer ConstructionMost large transformers are oil filled

The oil level must be high enough to cover the transformer

Oil must be kept dry and clean

Transformer ConstructionSealed TankConserver TankTank always stays full

Transformer ConstructionPressure relief valve allows over pressure oil to escapeOver temperature thermostat trips circuit breaker if transformer gets too hotBuchholz relay monitors oil between transformer and conserver tank

Buchholz relay 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 circuitMonitors

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

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

Buchholz relay Oil & Gass from TxOil to conserverOil for testingGas testingDry air test

Tap ChangersTwo types:Off LoadOn LoadCan be located on the primary or secondary of the transformer

Off LoadBasically a multi-position switchOpen circuits output between positionsContacts not rated to open circuit under load

Transformer Open circuited

On LoadManualAutomaticTransformer windingsCommon terminalSliding switch

On LoadInductors carry current when contact is between taps

On LoadContact moves to new tap