Current Transformers (CTs): Types, Classifications, and Performance

Current Transformers (CTs) and Potential/Voltage Transformers (PTs/VTs) are the backbone of electrical measurement and protection systems. They serve as the “ears and eyes” of the power system, feeding scaled-down signals of high currents and voltages to relays, meters, and control equipment. Without CTs and VTs, direct measurement of high values (like 100A current or 132 kV voltage) would be impractical and unsafe.

This article explains why CTs are essential, their classification, performance parameters, and differences between Class T and Class C CTs—with formulas, comparisons, and practical notes for engineers.

Why CTs and PTs are Needed

Measurement Limitation: Directly connecting a 100A current to an ammeter would require a very large meter coil, which is impractical to install. CTs solve this by stepping down current (e.g., 100A → 5A).
Relay Protection: Protection relays cannot handle high voltages/currents directly. CTs and PTs scale down signals within safe operating limits for relays to function correctly.
System Safety: Isolates measurement and protection devices from high-voltage circuits.

👉 Analogy: CTs and PTs act like “ears” and “eyes” of the protection system, while relays are the “brain” that decides, and circuit breakers are the “hands” that act.

Classification of Current Transformers (CTs)

CTs are broadly classified into two categories:

Measurement CTs
Protection CTs

1. Measurement CTs

Designed for accurate measurement across a wide range (5% to 125% of rated current).
Must have high magnetizing impedance at low currents for accurate metering.
Used in: Energy meters, ammeters, billing meters, load surveys.
Limitation: Not expected to maintain accuracy during large fault currents.

2. Protection CTs

Designed to remain linear up to 20 times the rated current.
Required for relay accuracy during fault conditions.
Maintain high magnetizing impedance even at large current levels.
Used in: Overcurrent, differential, distance, and protection relays.
Accuracy Class: Ratio error within ±10%, phase angle error less critical than in measurement CTs.

Dual-Purpose CTs

Some CTs are designed for both measurement and protection, with special accuracy classes (e.g., 5P10). They must be accurate for both small loads and large fault currents.

CT Performance: Burden, Saturation & Ratio Error

1. CT Burden

Definition: Net impedance connected to CT secondary (meters, relays, wires).
If burden ↑ → Secondary voltage ↑ → CT core saturates → Non-linear response.
Nameplate Example: CT rated 100 V on secondary. If burden causes secondary voltage >100 V → CT saturates.

2. Saturation

Occurs when CT’s magnetic core cannot handle flux increase.
Results in large ratio errors and phase angle shift.
Protection impact: Relay may fail to trip during fault.

3. Ratio Error

\% \, \text{Ratio Error} = \frac{\frac{I_p}{N} - I_s}{I_s} \times 100

Where:

$I_p$ = Primary Current
$I_s$ = Secondary Current
$N$ = CT Ratio
Cause: Magnetizing current (Ie) → Difference between actual secondary current and expected ratio.
Measurement CTs: Strict limits on ratio + phase error.
Protection CTs: Ratio error tolerated up to ±10%.

Classification by Construction: Class T and Class C CTs

CTs are also classified by construction and leakage flux behavior:

🔹 Class T CTs (Tested CTs)

T = Tested
Construction: Wound type (one or more primary turns on core).
High leakage flux → Performance depends on actual tests.
Must be tested for linearity with different burdens.
Performance curve shows:
- At low burden (<0.1Ω) → Linear response.
- At higher burden (e.g., 4Ω) → Non-linear, high errors.
Application: Where CT performance must be confirmed by testing under actual burden.

🔹 Class C CTs (Calculated CTs)

C = Calculated
Construction: Bar type, very low leakage flux.
Performance predictable using standard excitation curves.
Example Spec: 500:5 C100
- 500:5 → CT Ratio
- C → Calculated
- 100 → CT maintains linearity up to 100V across secondary
- Secondary burden = $\frac{100}{5 \times 100} = 0.40 \, \Omega$
Used in protection schemes, where predictable accuracy is essential.

Comparison Charts

Measurement CT vs Protection CT

Feature	Measurement CTs	Protection CTs
Accuracy Range	5% – 125% rated current	Up to 20 × rated current
Magnetizing Impedance	High at low current	High at fault current range
Purpose	Metering, billing, monitoring	Relay operation, fault protection
Accuracy Class	Strict ratio + phase angle	Ratio error within ±10%
Saturation Behavior	Non-linear at fault currents	Linear up to fault current levels

Class T vs Class C CTs

Feature	Class T CT (Tested)	Class C CT (Calculated)
Construction	Wound type	Bar type
Leakage Flux	High	Negligible
Performance Curve	Must be tested	Predictable from excitation data
Accuracy	Dependent on test & burden	Guaranteed within standard limits

Practical Engineering Notes

Always keep CT burden as low as possible to avoid saturation.
Check CT’s accuracy class before using for protection or billing.
Use Class C CTs for predictable performance in protection systems.
Ensure CT secondary is never left open → can cause dangerous voltages.
For energy metering → use Class 0.2s or 0.5 accuracy CTs.
For protection → use 5P, 10P, or PS class CTs (depending on relay type).

Conclusion

Current Transformers are an indispensable part of electrical systems, enabling safe measurement and reliable protection.

Measurement CTs ensure billing and monitoring accuracy.
Protection CTs guarantee correct relay operation during faults.
Class T CTs require performance testing, while Class C CTs provide predictable behavior using excitation curves.

By understanding CT classification, burden impact, ratio error, and saturation, engineers can select the right CT for both metering and protection applications, ensuring accuracy and system safety.

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Current Transformer (CT) and its types; Why CT required; Classification of CT's