How is knee point voltage defined?

50% rise in voltage causes 10% rise in current

10% rise in voltage causes 10% rise in current

What happens beyond the knee point?

Core saturates and the CT can no longer reproduce primary current faithfully

Secondary voltage drops to zero

Why does a measuring CT have a low knee point?

It saturates early at fault, protecting meters

To feed relays accurately at 30 times rated current

To reduce its accuracy at normal current

2070 Bhadra · 4 marks

Knee point voltage; measuring vs protective CT

Short answer

Knee point voltage is where a 10% rise in secondary emf causes 50% rise in exciting current, indicating saturation. Meeasuring CTs have low knee point, saturate early to protect meters; protective CTs have high knee point to avoid saturation during faults, ensuring accurate relay current.

What is 'knee point voltage'? Mention the difference between measuring and protective CT.

Knee point voltage

The knee point voltage of a CT is the point on its excitation (magnetisation) curve where a 10 % increase in secondary e.m.f. causes a 50 % increase in exciting current (IEC/IS definition). Beyond this point the core saturates, the exciting current rises rapidly and the CT can no longer reproduce the primary current faithfully.

 Vs (sec. emf)
  │            ___________ saturation
  │        _--
  │   Vk ●   knee point
  │     /
  │    /  linear region
  │   /
  └──┴────────────────── Ie (exciting current)

A protective CT needs a high knee point voltage so that it does not saturate during heavy fault current; it is chosen so that Vk ≥ K × If(sec) × (R_CT + R_lead + R_relay).

Measuring CT vs protective CT

Point	Measuring CT	Protective CT
Purpose	Feeds meters (ammeter, energy meter)	Feeds relays
Working range	Up to about 120 % of rated current	Up to 10–30 times rated current
Accuracy needed	High at normal current (class 0.1, 0.2, 0.5, 1)	Moderate, maintained up to fault current (5P, 10P, PS)
Knee point	Low – saturates early	High – no saturation during faults
Core	Small, high-permeability (nickel-iron)	Large cross-section, CRGO steel
Saturation	Desirable: protects meters during faults	Undesirable: relay must see the fault
Instrument security factor	Low (ISF ≤ 5)	Accuracy limit factor 10, 15, 20, 30

✓ Answer checked by Claude Opus 5.5 (high effort)

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