Lecture-8
Voltage Transformers
Voltage Transformer Main Tasks
 To transform voltages from usually a high
value to a value easy to handle for relays
and instruments.
 To insulate the relays, metering and
instruments from the primary high-voltage
system.
 To provide possibilities of standardizing
the relays and instruments, etc. to a few
rated voltages.
Voltage Transformers Error
The error of transformation of such a
transformer is negligible for all practical
purposes in its entire operating range- from
zero to about 110 percent of its normal
setting. We may consider such transformers
to be error free from the point of view of
relaying. Voltage transformers are rather
expensive , especially at extra high voltages,
345 kv or above. Consequently they are
usually found on low, medium and high
voltage systems.
A possible problem with VT
As shown in figure 3.13, when a ground fault occurs on such a
system, the voltage transformers connected to the un-faulted
phases are subjected to a voltage equal to the phase-to-phase
voltage of the power system. This usually drives one of the
transformers well into saturation, and may below the protective
fuse due to the excessive magnetizing current drawn by the
transformer.
F
Vector diagram
Notes:
Output burdens of 500 VA per phase are
common.
Accuracy of voltage transformers
Protection of voltage transformers
Protection of voltage transformers
• The voltage drop in the secondary circuit is of importance. The
voltage drop in the secondary fuses and long connection wires
can change the accuracy of the measurement.
• It is especially important for revenue metering windings of
high accuracy (class 0.2, 0.3). The total voltage drops in this
circuit must not be more than 0.1%.
Typical values of resistance in fuses:
6 A ------ 0.048 ohm
10 A ------ 0.024 ohm
16 A ------ 0.0076ohm
25 A ------ 0.0042ohm
A 6–10 A is a typical value for safe rupture of the fuses.
Protection of voltage transformers
• The voltage drop in the leads from the VT to the associated
equipment must be considered as this, in practice, can be
alarming mainly in case of measuring circuits.
• This is the one that separates the metering circuits (with low
burden) from protective circuits (with higher burdens). Refer
Figure.
Secondary earthing of voltage transformers
• To prevent secondary circuits from reaching dangerous
potential, the circuits should be earthed.
• A VT with the primary connected phase-to-earth shall have the
secondary earthed at terminal n.
A set of VTs with one Y-connected and one broken delta secondary circuit
VT Earthing
Voltage Transformer Questions???
1) 1- What is Voltage transformer main tasks?
2) 2- Explain with aid diagram the voltage transformer types?
3) 3- Draw the V.T. equivalent circuit and the phasor diagram?
4) 4- Draw the different connections of voltage transformers?
5) 5-Discuss the Ferro-resonance in magnetic voltage transformer
and how we can damp the ferro-resonance?
6) 6- Explain with aid diagram the voltage transformer protection?
7) 7- Discuss the voltage transformer earthing?
Optical Instrument Transducer
• Main difference is the electronic interface
• This interface is required for sensing function and
for adapting new sensor technology to that of
secondary outputs currents and voltages
• Optical Transducer can be separated in two families:
Hybrid Transducer: use conventional electric circuit
technology coupled with converter systems
All optical transducers that based on fundamental,
optical sensing principles