Current Transformer Technical Specifications

Current Transformers (CTs): Types, Specifications, and Applications

🔹 Introduction

Current Transformers (CTs) are essential components of electrical power systems. Their primary purpose is to step down high current values into standardized, lower values (usually 5A or 1A), making them suitable for measurement, protection, and monitoring equipment. Without CTs, it would be impossible to directly connect sensitive instruments like meters and relays to high-current circuits safely.

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🔹 Types of Current Transformer Designs

CTs are manufactured in different designs based on construction and installation:

1. Window Type CT

   • The primary conductor (busbar or cable) passes through a “window” in the CT core.

   • Compact and widely used in switchgear and panel boards.

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2. Ring Type CT

   • Circular design, often used around conductors.

   • Easy to clamp around existing cables.

   • Typically used in retrofitting and metering applications.

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🔹 Applications of Current Transformers

CTs are broadly classified into two categories based on their function:

1. Measurement CTs

   • Provide accurate current values for instruments such as ammeters, energy meters, and power analyzers.

   • Designed with high accuracy and intended to saturate under fault conditions, protecting instruments from damage.

   • Common accuracy classes: 0.1, 0.2, 0.5, 1.

2. Protection CTs

   • Supply current to protective relays and circuit breakers.

   • Must remain accurate even during fault currents (very high currents).

   • Accuracy classes: 5P, 10P (P = Protection, number indicates % allowable error).

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🔹 Key Specifications of Current Transformers

1. Rated Transformation Ratio

$$\text{Ratio} = \frac{\text{Rated Primary Current}}{\text{Rated Secondary Current}}$$

• Primary current: Actual current in the system (1A to 7500A).

• Secondary current: Standardized at 5A (or 1A on request).

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2. Rated Burden

• Expressed in Volt-Amperes (VA).

• Represents the maximum load CT can drive at rated accuracy (e.g., 1–100 VA at 0.8 PF).

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3. Accuracy Class

• Defines permissible error in measurement.

• Measuring CTs: 0.1, 0.2, 0.5, 1, 3, 5.

• Protection CTs: 5P, 10P.

• Example: A 0.2 class CT measuring 100 A may show 99.8A to 100.2A.

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4. Current Error (Ratio Error)

Error due to transformation inaccuracy:

$$\% \, \text{Error} = \frac{(K \times I_s) - I_p}{I_p} \times 100$$

Where:

• $K$ = Rated transformation ratio

• $I_p$ = Actual primary current

• $I_s$ = Actual secondary current

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5. Accuracy Limit Factor (ALF)

• Ratio of the maximum primary current up to which CT complies with composite error limits.

• Important for protection CTs to ensure proper relay operation.

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6. Phase Displacement

• Angular difference between primary current vector and secondary current vector.

• Positive displacement: Secondary leads primary.

• Expressed in minutes (′).

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7. Composite Error

• Error caused by harmonic content in secondary current.

• Critical for relays that depend on waveform accuracy.

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8. Bore Diameter

• Minimum diameter: 40 mm.

• Larger bore in ring-type CTs allows flexible installation.

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9. Highest Withstand Voltage

• Maximum voltage CT can handle without breakdown.

• Ring type: 4kV at 50 Hz (1 min).

• Wound type: 3kV at 50 Hz (1 min).

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🔹 General Technical Specifications of CTs

Parameter	Typical Value / Range
Standards	IS/IEC 60044-1 (2003), BS 3938:1973
Insulation Class	E (120°C max)
Operating Frequency	50 Hz
Primary Rating	1A – 7500A
Secondary Rating	5A (1A optional)
Burden	1 – 100 VA
Accuracy Classes	0.1, 0.2, 0.5, 1, 3, 5, 5P, 10P
Ambient Temp.	-20°C to +45°C
Storage Temp.	-50°C to +80°C
Thermal Short-Circuit (Ith)	40×In (wound type), 60×In (busbar type)
Dynamic Short-Circuit (Idyn)	2.5×Ith
Instrument Security Factor (FS)	2.5, 5, 10

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🔹 Accuracy Limits of Measuring CTs

Classes 0.1 to 1 (High Accuracy – Metering)

• Very low ratio and phase errors.

• Suitable for laboratory and billing applications.

Classes 3 and 5 (General Indicating Instruments)

• Less accuracy but cost-effective.

• Used in ammeters and general monitoring.

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🔹 Applications of CTs in Power Systems

1. Metering & Monitoring

   • Energy meters, kWh billing meters, and load analyzers.

2. Protection

   • Overcurrent, differential, and distance protection relays.

3. Instrumentation

   • Control panels, switchboards, and SCADA systems.

4. Safety

   • Isolates high-voltage circuits from measuring instruments.

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🔹 Conclusion

Current Transformers are indispensable in modern electrical systems. By converting large primary currents into standardized, safe secondary values, they enable accurate measurement, billing, and reliable protection. The selection of CTs should always consider primary current rating, burden, accuracy class, and application type (measurement or protection).

With proper CT selection and specification compliance, power systems achieve efficiency, safety, and long-term reliability.

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⚡ Pro Tip for Engineers:
Always choose a measurement CT with low accuracy class (0.2, 0.5) for metering and a protection CT (5P, 10P) for relay circuits.

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