Voltage Transformers

Question 1

What is the purpose of a voltage transformer on a HV transmission network?

Answer 1

a) provide a suitable (lower) voltage to metering + protection + control devices
b) isolate these devices from the main system

Question 2

The three (3) types of VTs are:

1) electromagnetic
2) ____________
3) non-conventional

Answer 2

CVD - Capacitor Voltage Divider

Question 3

Electromagnetic VTs have a _____ excitation current, and an exciting impedance that remains _____ for varying voltages. Its flux density is (below/at/above) saturation.

Answer 3

low; constant; below

Question 4

What causes voltage and phase errors in a VT?

Answer 4

Induction + interaction between the core and windings; magnetising current; impedance of the core and windings

Question 5

What is a non-conventional VT, and why are they used?

Answer 5

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).

Question 6

VTs have a much _____ current draw that CTs because they have a ______ impedance.

Answer 6

Lower; higher

Question 7

HV VTs are connected between phase and _____.

Answer 7

ground

Question 8

What is the secondary winding voltage out of a VT, always?

Answer 8

63.5V

Question 9

Define the term ‘burden’ in the context of HV voltage transformers.

Answer 9

The secondary load of a transformer.

Question 10

Metering transformers must remain accurate within ____ - 120% of the rated system voltage.

Answer 10

80%

Question 11

Why is it important for a VT to accurately be able to accurately replicate a transient voltage to the secondary?

Answer 11

To operate protection devices

Question 12

What are some causes of transient voltages?

Answer 12

• energising transformers
• faults
• clearing faults
• switching inductive loads

Question 13

Electromagnetic VTs have (good / bad) reproduction of transients to the secondary side.

Answer 13

Good!

Question 14

What might a transient look like on the secondary of a CVT?

Answer 14

Complicated due to capacitance + inductive inside the CVT… may cause a harmonic of the main system voltage (this can inadvertently trip protection)

Question 15

What is ferroresonance?

Answer 15

Resonance caused from inductive elements (i.e. transformers) getting saturated.

Question 16

What might cause ferroresonance on a CVT?

Answer 16

a) a lightly loaded CVT
b) a secondary burden that’s easily saturated
c) charge trapped in capacitors after fault

Question 17

What is the maximum voltage drop in a secondary metering circuit?

Answer 17

0.05% - 0.1%

Question 18

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?

Answer 18

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).

Question 19

Why should you NEVER short-circuit a VT?

Answer 19

Causes a huge current that will blow the VT

Question 20

Why should you NEVER work on a VT where the primary is isolated but the secondary is live?

Answer 20

The secondary may back-energise the primary and electrocute you