Current Transformers

Question 1

Explain the basic operation of a current transformer (CT).

Answer 1

The primary current induces a magnetic flux into some ferrite-ish core…

…this flux is then reflected on to the secondary windings

Question 2

What is the ‘Dielectric Loss Angle’ (DLA)?

Answer 2

Measurement of the difference in angle between V_P and the resultant current (ideally 90^o).

This measures the power loss via the insulation, and can be an early indicator of insulation

breakdown.

Question 3

A CT has a (high/low) impedance in the network

Answer 3

low

Question 4

What is the ‘knee point’?

Answer 4

Minimum V_s causing saturation.

Question 5

Do we want CTs to operate in their saturation region?

Why/why not?

Answer 5

No! In saturation, the primary current isn’t accuratetly reflected on to the secondary.

This is bad news for protection relays - they don’t operate appropriately.

Question 6

A CT in saturation will have ______ visible on its secondary output

Answer 6

distortion

Question 7

A non-symmetrical sine wave (e.g. from a fault) and a DC offset (increases/decreases) the chance of a CT entering saturation.

Answer 7

Increases!

Question 8

What is the X/R ratio?

Answer 8

The ratio between reactance and resistance in the HV primary of a CT.

Question 9

The higher the X/R ratio, the (more/less) distorted the secondary output.

Answer 9

more

Question 10

Explain ‘remanence’ in terms of a CT.

Answer 10

The flux remianing in a core when the MMF is removed

Question 11

Why is it dangerous to open circuit a CT? How might this be done?

Answer 11

If a burden (secondary load) is removed - e.g. a relay, some metering device, etc… - the CT will see an infinite impedence as its load. It will try to increases its voltage infinitely high (to produce enough current for an infinite impedence - impossible). This will cause overheading, insulation breakdown, shorts, and even explosions.

Question 12

What is the golden rule of working on CTs?

Answer 12

Always short circuit the secondary!!

Question 13

Protection CTs in Powerlink are rated at __x their full load current.

HV transmission secondary CT outputs are __A.

Answer 13

20x

1A

Question 14

What does ‘2.5P 400 F20’ mean in the context of a protection CT?

Answer 14

2.5 = max. % error in I_s

P = protection class ‘P’

400 = minimum V_S output

F20 = maximum 20x I_fault whilst remaining within accuracy range

Question 15

What might be a reason to remove the DLA link from a CT?

When would you never do this?

Answer 15

One may remove the DLA link for winding insulation testing on a de-energised CT.

NEVER remove on an energised CT.

Question 16

What is a ‘DLA’ test? Define and explain.

Answer 16

Diaelectric Loss Angle.

This test measures the difference in phase angle between an applied voltage and the resulting current through the insulation. Ideally, they should be 90 degrees apart (winding insulation acts as capacitor).

Question 17

What are some things to check on a CT during maintenance?

Answer 17

• oil (quantity + quality)
• winding insulation
• general visual (bots, nuts, oil leaks, etc…)

Question 18

This device incorporates CTs, VTs, disconnectors, earth switches, and circuit breakers.

What is this device?

Answer 18

Hybrid switchgear

Question 19

What is the first step in isolating a CT, and why?

Answer 19

Isolate the protection trip output first. This ensures protection doesn’t operate when CTs are shorted.

Question 20

What happens if the burden of a CT is greater than its rated burden? What is the general burden rating (in ohms) of a protection core?

Answer 20

The secondary voltage V_s increases, causing the magnetic field to increase and thus saturating the core. Generall 2ohms.

Question 21

Draw the equivalent model for a CT.

Answer 21

Question 22

How does CT saturation occur?

Answer 22

• load open-circuits
• secondary current must go somewhere; instead of going through load, goes through internal CT impedance
• all this current going through the CT’s internal magnetising impedance increases the magnetic flux density
• this saturates the core.

Question 23

Why should the secondary of a CT always be earthed?

Answer 23

• there’s a big capacitance between the primary/secondary conductors and the insulation
• earthing prevents a big floating voltage because of this capacitance.

Question 24

What are the steps in isolating a CT?

Answer 24

1. isolate protection trip outputs (i.e. CB)
2. identify CT links on drawings
3. check surrounding terminals/screws for tightness
4. confirm CT arrangement (e.g. high impedance scheme has different procedure)
5. short CT via shorting leads.