Instrument transformers working principle; Current Transformer ; Potential Transformers

Current Transformer (CT) – Definition, Theory, Errors & Accuracy Classes

🔹 What is an Instrument Transformer?

Instrument transformers are used to step down high voltages and currents of a power system to safe values for metering and protection.

• Voltage Transformer (VT/PT): Reduces system voltage to 110 V (standard).

• Current Transformer (CT): Reduces system current to 5 A or 1 A (standard).

👉 Why?
Meters and protective relays are not designed to handle system-level high voltages and currents directly.

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🔹 Definition of CT (Current Transformer)

A Current Transformer (CT) is an instrument transformer in which the secondary current is proportional to the primary current and ideally differs in phase by zero degrees.

CTs are used to safely measure high currents and to operate protective relays.

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🔹 CT Accuracy Classes (Categories)

• Metering CTs: Designed for high accuracy within normal load conditions (typically up to 125% of rated current).

• Protection CTs: Designed to withstand high fault currents without saturation, ensuring reliable relay operation.

⚡ Example: A CT with 400/1 A ratio

• If 400 A flows in the primary (1 turn), then secondary delivers 1 A (with 400 turns).

• Ratio = 400:1

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🔹 Theory of Current Transformer

Unlike a power transformer:

• In power transformers, primary current depends on secondary load.

• In CTs, the primary is directly connected in the line — so primary current is independent of secondary burden/load.

Mathematical Relation:

• Primary AT = Secondary AT (ideally)

• $N_p I_p = N_s I_s$

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🔹 Errors in CTs

CTs are not perfect. Errors occur due to core excitation and losses.

1. Ratio Error (Current Error):
   Difference between actual secondary current and the ideal transformed current.

   $$\text{Ratio Error} = \frac{K_t I_s - I_p}{I_p} \times 100\%$$

   Where $K_t = \frac{N_s}{N_p}$

2. Phase Angle Error:
   Deviation in angle between primary and secondary currents (ideally zero).

   Represented in phasor diagrams as angle β.

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🔹 Causes of Error

• Core excitation current (Io) is needed for magnetization.

• Io has two components:

  • Im (Magnetizing current)

  • Iw (Core loss current)

• Since some primary current is consumed in exciting the core, not all gets transformed → errors appear.

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🔹 How to Reduce CT Errors

• Use high permeability, low hysteresis magnetic materials.

• Increase cross-sectional area of the core.

• Minimize joints and flux leakage paths.

• Maintain appropriate burden (VA rating) on the CT

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Potential Transformer (PT) or Voltage Transformer (VT)

🔹 Definition of PT

A Potential Transformer (PT), also called a Voltage Transformer (VT), is an instrument transformer that steps down the system voltage to a lower, standardized level (usually 110 V) for safe measurement and protection.

Just like CTs handle current, PTs handle voltage.

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🔹 Why PT is Needed?

• Power system voltages range from 11 kV to 765 kV or higher.

• Meters and relays cannot work directly on such high voltages.

• PT reduces these voltages to a safe level (commonly 110 V) for metering and protection circuits.

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🔹 Construction & Working Principle

• A PT works like a step-down power transformer, but it is specially designed for high accuracy within a limited burden (VA rating).

• The primary winding is connected across the high-voltage line.

• The secondary winding provides a scaled-down voltage (e.g., 110 V) proportional to the primary voltage.

Mathematical Relation:

$$\frac{V_p}{V_s} = \frac{N_p}{N_s}$$

Where:

• $V_p$ = Primary (system) voltage

• $V_s$ = Secondary voltage (110 V)

• $N_p, N_s$ = Number of primary & secondary turns

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🔹 Types of PTs

1. Electromagnetic PT (for voltages up to ~132 kV)

2. Capacitive Voltage Transformer (CVT/CVTs) (for higher voltages above 132 kV)

   • Uses a capacitor divider + auxiliary transformer.

   • Preferred for EHV/UHV systems as they are cost-effective and lighter than large electromagnetic PTs.

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🔹 Accuracy Classes

Like CTs, PTs also have accuracy classes depending on whether they are used for:

• Metering → High accuracy at normal voltages.

• Protection → Must faithfully reproduce voltage during faults, even if accuracy is slightly lower.

Standard Accuracy Classes: 0.1, 0.2, 0.5, 1.0 (for metering) and 3P, 6P (for protection).

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🔹 Errors in PTs

Similar to CTs, PTs are not ideal and have:

1. Ratio Error:

   $$\text{Ratio Error} = \frac{K_v V_s - V_p}{V_p} \times 100\%$$

   Where $K_v = \frac{N_p}{N_s}$

2. Phase Angle Error:
   Small angular difference between actual system voltage and secondary voltage vector.

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🔹 How to Reduce PT Errors

• Use high-quality magnetic core materials.

• Maintain correct burden (VA rating).

• Use shielding & proper insulation for high-voltage systems.

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✅ Key Takeaways (PT vs CT)

• CT: Steps down current (5 A or 1 A standard).

• PT/VT: Steps down voltage (110 V standard).

• Both are essential for metering & protection.

• Metering types → prioritize accuracy.

• Protection types → prioritize performance under faults.

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