Power Sys Protect Lec9b - Voltage Transforner

7/29/2019 Power Sys Protect Lec9b - Voltage Transforner

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POWER SYSTEM PROTECTION

Lecture 9ba

Voltage (Potential) Transformer

Compiled by:Engr. Gener G. RestubogREE, MSEE(Units), MEEE.


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A VT is basically a conventional transformer with primary and

secondary windings on a common core. Standard VTs are single-phase units designed and constructed

so that the secondary voltage maintains a fixed relationship

with primary voltage.

The required rated primary voltage of a VT is determined by

the voltage of the system to which it is to be connected and

by the way in which it is to be connected (e.g., line to line,

line to neutral).

Most VTs are designed to provide 120 V at the secondary

terminals when nameplate-rated voltage is applied to theprimary.

Standard ratings are shown in Table 3-5 and Table 3-6.

Special ratings are available for applications involving unusual

connections.


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Table 3-5Ratings and characteristics of VTs with 100% of rated primary voltage across

the primary winding when connected line to line or line to ground


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Table 3-6Ratings and characteristics of VTs primarily for line-to-line servicea


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Accuracy

VTs are capable of continuous and accurate operation when

the voltage applied across the primary is within 10% of the

rated primary voltage.

Standard accuracy classifications of VTs range from 0.3 to 1.2,

representing percent ratio corrections to obtain a true ratio.

These accuracies are high enough so that any standard

transformer is adequate for most industrial protective

relaying purposes as long as it is applied within its open-air

thermal and voltage limits. Standard burdens for VTs with a secondary voltage of 120 V

are shown in Table 3-7.


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Thermal Burden Limits

Thermal burden limits, as given by transformer

manufacturers, should not be exceeded in normal practice

because transformer accuracy and life will be adversely

affected.

Thermal burdens are given in voltamperes and may be

calculated by simple arithmetic addition of the voltampere

burdens of the devices connected to the transformer

secondary.

If the sum is within the rated thermal burden, thetransformer should perform satisfactorily over the range of

voltages from 0% to 110% of the nameplate voltage.


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Polarity Markings

Polarity on VTs is normally identified by marking a primary

terminal H1 and a secondary terminal X1.

Alternatively, these points may be identified by distinctive

color markings.

The standard voltage relationship provides that the

instantaneous polarities of H1 and X1 are the same..


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Connection

Where balanced system load and, therefore, balanced

voltages are anticipated, VTs are usually connected in open

delta.

Where line-to-neutral loading is expected, VTs are more oftenconnected wye-wye, particularly where metering is required.

Many protective devices require specific delta or wye

voltages; therefore, specific requirements should be studied

before choosing the connection scheme. Wye-delta or delta-wye connections are occasionally used

with certain special relays, but these connections are

infrequent in industrial use.


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Where ungrounded power systems are used, VTs connected

wye-broken-delta are sometimes used for ground detection.

When so connected, the transformers can seldom be used

for any other purpose.

Broken-delta connections used on ungrounded systems

should normally include a loading resistor in the secondary tomitigate possible ferroresonance between the system

capacitance and the VT.


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Fuse Application

The application of fuses to VT circuits has been a subject ofdiscussion for many years.

The main purpose of a VT primary fuse is to protect the

power system by de-energizing failed VTs.

General practice now calls for a current-limiting fuse orequivalent in the system. Figure 3-11 shows a typical VT with

fuses.


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VT secondary fusing practices cannot be so clearly defined.

It is usually impossible to select primary fuses that protect

the transformer from most overloads or faults in the external

secondary circuit.

Secondary fuses selected to interrupt at loadings below the

thermal burden rating can provide such protection. Where branch circuits are tapped from VT secondaries to

supply devices located at a distance from the VT, it may be

desirable to fuse the branch at a reduced rating.


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VT secondary fusing practices cannot be so clearly defined.

It is usually impossible to select primary fuses that protect

the transformer from most overloads or faults in the external

secondary circuit.

Secondary fuses selected to interrupt at loadings below the

thermal burden rating can provide such protection. Where branch circuits are tapped from VT secondaries to

supply devices located at a distance from the VT, it may be

desirable to fuse the branch at a reduced rating.


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References

1. IEEE Std 242-2001 - IEEE Recommended Practice for

Protection and Coordination of Industrial and

Commercial Power Systems -

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Power Sys Protect Lec9b - Voltage Transforner

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