Voltage Transformers Flashcards
What is the purpose of a voltage transformer on a HV transmission network?
a) provide a suitable (lower) voltage to metering + protection + control devices
b) isolate these devices from the main system
The three (3) types of VTs are:
1) electromagnetic
2) ____________
3) non-conventional
CVD - Capacitor Voltage Divider
Electromagnetic VTs have a _____ excitation current, and an exciting impedance that remains _____ for varying voltages. Its flux density is (below/at/above) saturation.
low; constant; below
What causes voltage and phase errors in a VT?
Induction + interaction between the core and windings; magnetising current; impedance of the core and windings
What is a non-conventional VT, and why are they used?
Basically a CVT but instead of an intermediate transformer inside, the output of the capacitor array is fed straight in to electronics.
Small, and less error prone (no flux linkage issues as in regular electromagnetic VT).
VTs have a much _____ current draw that CTs because they have a ______ impedance.
Lower; higher
HV VTs are connected between phase and _____.
ground
What is the secondary winding voltage out of a VT, always?
63.5V
Define the term ‘burden’ in the context of HV voltage transformers.
The secondary load of a transformer.
Metering transformers must remain accurate within ____ - 120% of the rated system voltage.
80%
Why is it important for a VT to accurately be able to accurately replicate a transient voltage to the secondary?
To operate protection devices
What are some causes of transient voltages?
- energising transformers
- faults
- clearing faults
- switching inductive loads
Electromagnetic VTs have (good / bad) reproduction of transients to the secondary side.
Good!
What might a transient look like on the secondary of a CVT?
Complicated due to capacitance + inductive inside the CVT… may cause a harmonic of the main system voltage (this can inadvertently trip protection)
What is ferroresonance?
Resonance caused from inductive elements (i.e. transformers) getting saturated.
What might cause ferroresonance on a CVT?
a) a lightly loaded CVT
b) a secondary burden that’s easily saturated
c) charge trapped in capacitors after fault
What is the maximum voltage drop in a secondary metering circuit?
0.05% - 0.1%
Why can’t you protect the secondary side of a VT with fuses on the primary side? What does secondary protection look like for a VT?
A secondary short might only produce a few amps on the primary, but will burn out the VT + secondary circuits. Therefore, must have dedicated secondary side protection (generally a mcb inside the marshalling box).
Why should you NEVER short-circuit a VT?
Causes a huge current that will blow the VT
Why should you NEVER work on a VT where the primary is isolated but the secondary is live?
The secondary may back-energise the primary and electrocute you
