About the CT Sizing & Class Selection
Current transformer (CT) sizing determines the primary current ratio, accuracy class, burden, and knee-point voltage of CTs used for protection and metering in a solar plant's HT switchgear — ensuring accurate measurement and reliable relay operation.
How to calculate it
- 1Primary current: select the closest standard primary rating above the expected maximum load current. Standard ratios: 50/5, 100/5, 200/5, 400/5, 600/5, 800/5, 1000/5, 1200/5, 2000/5 A.
- 2Secondary current: 5 A (standard) or 1 A (for long wiring runs to reduce burden).
- 3Accuracy class for metering CT: 0.2 or 0.5 (per IS 2705 Part 3). Class 0.2 for revenue metering (DISCOM billing).
- 4Accuracy class for protection CT: Class 5P or 10P (error limit at 5× or 10× rated current), with accuracy limit factor (ALF) of 10 or 20.
- 5Burden: total connected burden of cables + relay inputs + meter inputs. CT must be rated for this burden (in VA).
- 6Knee-point voltage for differential protection CTs: Vk ≥ 2 × (Rct + Rl) × If_max ÷ n (Vk must be high enough to drive protection relay under fault conditions).
Worked example
An 11 kV feeder for a 1 MWp solar plant. Rated current at 11 kV = 1,000 ÷ (1.732 × 11) = 52.5 A. Select CT ratio 75/5 A (next standard above 52.5 A). For revenue metering: class 0.2, 15 VA burden. For overcurrent protection relay: class 5P20, 15 VA. Two separate CT cores in the same housing (one for metering, one for protection) — standard practice.
What you can calculate
- CT ratio
- 5P/10P class
- Knee-point
- IEC 61869
Standards & references
Frequently asked questions
Why are separate CTs used for metering and protection?
Metering CTs (class 0.2 or 0.5) are accurate at normal load currents (1–1.2× rated) but saturate under fault conditions — this is by design, to protect connected meters from overcurrent. Protection CTs (class 5P, 10P) must remain linear at high multiples of rated current (up to 20× rated) to ensure the relay sees the true fault current. Using a single CT for both would compromise either metering accuracy or protection reliability.
What happens if a CT secondary is left open-circuited?
A CT secondary must never be left open-circuited while primary current is flowing. With the secondary open, all the primary magnetising force drives the core into deep saturation, generating a very high secondary voltage (hundreds to thousands of volts) that can destroy the CT insulation, damage connected equipment, and electrocute personnel. Before disconnecting any instrument from a CT secondary, always short-circuit the CT secondary terminals first.
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