rahul tanwar presentation 1 (p.t)
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SUBMITTED BY:- RAHUL TANWAR
ELECTRICAL DEPT.
08EBNEE043
B-SET ,JAIPUR
SUBMITTED TO:-
PUSHPENDRA SINGH
H.O.D. ELECTRICAL DEPT.,
B.S.E.T, JAIPUR
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CONTENTS Introduction 1
Construction of P.T 3
Circuit diagram 5
Phasor diagram 7
High voltage P.T 8
Error in P.T 9
Reduction of error in P.T. 10
Characteristics of P.T. 11
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POTINTIAL TRANSFORMER
P.T. are used to operate voltmeters. The normal secondary voltage rating is 110v.
The P.T. may be considered as parallel transformer with
its secondary winding operating nearly under open circuit
conditions. The primary winding current in a P.T. certainly depends
upon the secondary circuit burden.
In a P.T. full line voltage is impressed upon its terminal.
Under normal operation the line voltage is nearly
constant and, therefore, the flux density and hence the
exciting current of a P.T. varies only over a restricted range
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POTINTIAL TRANSFORMERThe power loading of a P.T. is a
very small and consequently the
exciting current is of the sameorder as the secondary winding
current while in a power
transformer the exciting current is
a very small fraction of secondary
winding load current.
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Construction of P.T P.T. has larger core and conductor sizes.
The output of a P.T is always small and the size is quit
larger.
The temperature rise is small and hence there are nothermal problems caused by overloads as in power
transformer.
The load range from 2 to 3 times for low voltage P.T
and up to 30 or more times for same high voltage
transformer.
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Construction of P.T. CORE: The core may be of small or core type of
construction.
WINDINGS: The primary and secondary winding are
coaxial to reduce the leakage reactance to minimum.
INSULATION: Cotton tape and varnished cambric are used
as insulation for coil construction. Hard fiber separators
are used between coil.
BUSHINGS: Oil filled bushings are usually used for oil filledP.T as this minimizes the overall size of the transformer.
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Equivalent circuit of P.T
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Single phase P.T.
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Phasor diagram of a P.T
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High voltage P.T. Conventional type P.T used for high voltages of 100kv and
above, are very large in size and costly to build because of
insulation requirements.
Exe. A 110kv P.T has an overall height of about 7.5 metre
and weight nearly 5 tones.
Insulating casing:
Moulded rubber potential transformer: Cascaded transformers:
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Errors in P.T. Ratio (Voltage) error: The actual ratio of transformation varies
with operations and the errorin secondary voltage may be
defined as:
% Ratio error = Kn-R*100/R. Phase angle error: In an ideal voltage transformer there should
not be any phase difference between winding voltage and the
secondary winding voltage reversed. However, in an actual
transformer there exists a phase difference between Vp and Vsreversed.
Phase angle =(Is/n) *(Xicos - Risin) + (*Ie Xp Im Rp)/nVs.
= (Is/Vs)* (Xs cos - Rs sin)+(Ie Xp Im Rp)/nVs
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Reduction of errors in P.T. Reduction of magnetising and loss components: A
considerable improvement in the performance can be
made by reducing the value of Im and Ie. Such a reduction
requires short magnetic path, good quality core material,
low flux density in core and suitable precautions in the
assembly and interleaving of core.
Reduction of resistance and leakage reactance: Winding
resistance can be minimized by using thick conductors andby adopting the smallest length of mean turn.
Turn compensation: At no load the actual ratio exceeds
the turn ratio by an amount (IeRp+ImXp)/Vs.
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Characteristics of P.T Effect of secondary current or VA: If we increase the
secondary current is increased, and therefore, the primarycurrent increase.
Effect of power factor of secondary burden: If the powerfactor of secondary circuit burden is reduced, angle isincreased. This makes current Ip to shift toward currentIo. The voltage Vp and Vs come more nearly in to phase
with Ep and Es respectively since the voltage drop arealmost constant. The result is an increase in Vp relative toEp. But Vp is constant and, therefore , Ep reduce relativeto Vp.
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Characteristics of P.T. Effect of frequency: For a constant voltage, the flux is
inversely proportional to frequency. Increase in frequency
reduces the flux and, therefore, Im and Ie are decreased
and, therefore, the voltage ratio decrease.
Effect of primary voltage: There is no wide variation of
supply voltage to which the primary winding of the P.T. is
connected.
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REFERENCES
A.K Sawhney (2009), ISBN o-81-770-016-0.
EXTERNAL LINK http://en.wikipedia.org/Potential Transformer
http://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Transformer
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