Current Transformers Flashcards
Explain the basic operation of a current transformer (CT).
The primary current induces a magnetic flux into some ferrite-ish core…
…this flux is then reflected on to the secondary windings
What is the ‘Dielectric Loss Angle’ (DLA)?
Measurement of the difference in angle between VP and the resultant current (ideally 90o).
This measures the power loss via the insulation, and can be an early indicator of insulation
breakdown.
A CT has a (high/low) impedance in the network
low
What is the ‘knee point’?
Minimum Vs causing saturation.
Do we want CTs to operate in their saturation region?
Why/why not?
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.
A CT in saturation will have ______ visible on its secondary output
distortion
A non-symmetrical sine wave (e.g. from a fault) and a DC offset (increases/decreases) the chance of a CT entering saturation.
Increases!
What is the X/R ratio?
The ratio between reactance and resistance in the HV primary of a CT.
The higher the X/R ratio, the (more/less) distorted the secondary output.
more
Explain ‘remanence’ in terms of a CT.
The flux remianing in a core when the MMF is removed
Why is it dangerous to open circuit a CT? How might this be done?
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.
What is the golden rule of working on CTs?
Always short circuit the secondary!!
Protection CTs in Powerlink are rated at __x their full load current.
HV transmission secondary CT outputs are __A.
20x
1A
What does ‘2.5P 400 F20’ mean in the context of a protection CT?
2.5 = max. % error in Is
P = protection class ‘P’
400 = minimum VS output
F20 = maximum 20x Ifault whilst remaining within accuracy range
What might be a reason to remove the DLA link from a CT?
When would you never do this?
One may remove the DLA link for winding insulation testing on a de-energised CT.
NEVER remove on an energised CT.
What is a ‘DLA’ test? Define and explain.
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).
What are some things to check on a CT during maintenance?
- oil (quantity + quality)
- winding insulation
- general visual (bots, nuts, oil leaks, etc…)
This device incorporates CTs, VTs, disconnectors, earth switches, and circuit breakers.
What is this device?
Hybrid switchgear
What is the first step in isolating a CT, and why?
Isolate the protection trip output first. This ensures protection doesn’t operate when CTs are shorted.
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?
The secondary voltage Vs increases, causing the magnetic field to increase and thus saturating the core. Generall 2ohms.
Draw the equivalent model for a CT.
How does CT saturation occur?
- 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.
Why should the secondary of a CT always be earthed?
- there’s a big capacitance between the primary/secondary conductors and the insulation
- earthing prevents a big floating voltage because of this capacitance.
What are the steps in isolating a CT?
- isolate protection trip outputs (i.e. CB)
- identify CT links on drawings
- check surrounding terminals/screws for tightness
- confirm CT arrangement (e.g. high impedance scheme has different procedure)
- short CT via shorting leads.
