Voltage
Transformers
1. Provides
isolation from high voltages
2. Must
operate in the linear region to prevent accuracy problems - Do not over specify
VT
3. Must be
capable of driving the burden, specified by relay manufacturer
4. Protection
class VT will suffice
Typical
Working Points on a B-H Curve
Two main
basic types are available
Electromechanical VT`s
1. Similar to
a power transformer
2. May not be
economical above 132kV
Capacitor VT`s (CVT)
1. Used at
high voltages
2. Main
difference is that CVT has a capacitor divider on the front end.
Electromagnetic Voltage Transformer
Capacitive Volatage Transformer(CVT)
Ferro-resonance
1. The
exciting impedance of auxiliary transformer T and the capacitance of the
potential divider form a resonant circuit.
2. May
oscillate at a sub normal frequency
3. Resonant
frequency close to one-third value of system frequency
4. Manifests
itself as a rise in output voltage, r.m.s. value being 25 to 50 per cent above
normal value
5. Use
resistive burden to dampen the effect
Voltage Transformer Earthing
Primary Earthing
1. Earth at
neutral point
2. Required
for phase-ground measurement at relay
Secondary Earthing
1. Required
for safety
2. Earth at
neutral point
3. No
relevance for protection operation
VT Construction
5
Limb
Used
when zero sequence measurement is required (primary must also be earthed)
Three
Single Phase
Used
when zero sequence measurement is
required (primary must also be earthed)
3
Limb
Used
where no zero sequence measurement is required
V
Connected (Open Delta)
No
yellow phase
Cost effective
Two
phase-phase voltages
No
ground fault measurement
Protection
of VT’s
1. H.R.C.
Fuses on primary side
2. Fuses may
not have sufficient interrupting capability
3. Use MCB
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