part 8 emachines EEP 3243

8/14/2019 part 8 emachines EEP 3243

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RECAP/ADDITION

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Autotransformer

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All the criteria are identical to standard transformer having a turns ratio, a.


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C onventional Transformer C onnected

as an Autotransformer The following rules apply :Current in any winding should not exceed its nominal currentrating.Winding voltage should not exceed its nominal voltage rating.Ampere-turns of the both windings are equal.


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C onts.S ame mutual flux links both windings.If current in winding flows from H1 to H2, the current in other winding must flow from X2 to X1 and vice versa.The voltages add when terminals of opposite polarity areconnected together. The voltages subtract when H 1 and X1 areconnected together.


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C ont.

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Voltage Transformer ( V T)Also called potential transformer which used for metering and protection in high-voltage circuits.

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C ont.They are designed to present negligibleburden to the supply being measuredand to have a precise voltage ratio toaccurately step down high voltages sothat metering and protective relayequipment can be operated at a lower potential.The nominal secondary voltage isusually rated for 69 V or 120 V at ratedprimary voltage.

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C ont.VT are used to measure or monitor the transmission linesand to isolate the meteringequipment from the main lines.1 terminal of the secondarywinding is always groundedand the distributedcapacitance between windingsmakes the winding to beisolated.

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C ont.S imilar construction withconventional transformer butgreat insulation between winding

to withstand HV.Basic Impulse Insulation (BIL)expresses the transformer'sability to withstand lightning and

switching surges.

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C urrent Transformer ( C T)CT is a measurement device designed to provide acurrent in its secondary coil proportional to thecurrent flowing in its primary.

CT are commonly used in metering and protectiverelaying in the electrical power industry where theyfacilitate the safe measurement of large currents,often in the presence of high voltages.

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C ont.The highly accurate current ratio and smallphase angle are achieved by keeping theexciting current small.The CT safely isolates measurement andcontrol circuitry from the high voltagestypically present on the circuit beingmeasured. The nominal secondary current isusually 5A, irrespective of primary current.

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C ont.The insulation betweenwinding must be great towithstand the full line-neutralvoltage, including line surgesand one of the secondaryterminal is grounded.

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C ont.Care must be taken that the secondarywinding is not disconnected from itsburden while current flows in theprimary, as this will produce a

dangerously high voltage across theopen secondary and may permanentlyaffect the accuracy of the transformer.For safety reasons, we must first short-circuit the secondary winding thanremove the component.

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Toroidal C T

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C ont.The primary is a singleconductor that simply passesthrough the center or less center of the ring.If the secondary possesses Nturns the transformer ratio is N.This transformer is simple and

inexpensive and widely used inLV and MV indoor installation.

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Problem

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END OF PART 8

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part 8 emachines EEP 3243