Introduction to Current Transformers

In power systems, measuring high currents directly can be dangerous and impractical. That’s where Current Transformers (CTs) come in — they “step down” large currents to smaller, manageable values for meters and protection relays. Think of them like binoculars for current: they help devices “see” high currents safely and clearly.

What is a Current Transformer?

A Current Transformer is an instrument transformer that converts high current flowing in a power line to a much smaller current (usually 5A or 1A) proportional to the actual load current. These small currents are easier to measure and safely route to protective devices and meters.

CT Analogy: The Garden Hose and the Flow Meter

Imagine water gushing through a thick garden hose (representing a high-voltage line). You can’t directly measure such forceful flow with a tiny flow meter. But if you use a smaller, controlled bypass pipe (the CT), you can measure a small, proportional sample of the flow — and calculate the real thing. That’s what CTs do for electric current.

Construction and Working Principle of CT

A Current Transformer has:

Primary winding – connected in series with the main circuit.
Magnetic core – guides the magnetic flux.
Secondary winding – delivers a scaled-down current to instruments or relays.

How It Works

It operates on the principle of electromagnetic induction. The high current in the primary produces a magnetic flux in the core, which induces a proportional current in the secondary winding. Since the primary has just one or a few turns and the secondary has many, the current steps down accordingly.

Key Point: A CT always works with its secondary circuit closed (never open-circuit the secondary) to avoid dangerously high voltages.

CT Ratio (Current Ratio)

The CT Ratio defines the scaling factor between the primary and secondary currents.

$CT Ratio=IPrimaryISecondary\text{CT Ratio} = \frac{I_{Primary}}{I_{Secondary}}$

For example, a 1000/5 CT means when 1000 A flows through the primary, 5 A flows through the secondary. This ratio helps meters and relays interpret actual line current.

Example

Primary Current: 600 A
CT Ratio: 600/5
Secondary Current: $600120=5 A\frac{600}{120} = 5\,A$

Burden of a Current Transformer (CT)

The burden of a current transformer is defined as the product of the square of the secondary current and the total impedance connected across its secondary terminals.

$\times Z$

Where:

= Secondary current (typically 5 A or 1 A)
= Total impedance (in ohms, ) of the connected load including wires, meters, and protection devices

Why It Matters

If the burden is too high, the CT can’t produce enough voltage to push current through it, leading to inaccurate readings or saturation. Always stay within the rated burden (e.g., 15 VA, 30 VA).

CT Saturation

Saturation occurs when the CT’s magnetic core is pushed beyond its limit, distorting the output current.

Analogy: Sponge Saturation

A CT core is like a sponge absorbing magnetic flux. If you pour too much, the sponge (core) can’t absorb more and starts spilling — that’s saturation. In CTs, this “spill” results in inaccurate, distorted secondary current, which can cause maloperation of protective relays.

Causes:

Excessive burden
High fault current
Poor CT design

Accuracy Class of CTs

CTs are classified based on how accurately they reproduce the primary current.

For Measuring CTs:

Class 0.1, 0.2, 0.5, 1.0, 3.0 → percentage error under rated conditions.
- Class 0.5 = ±0.5% error at rated burden

For Protection CTs:

Class 5P, 10P → “P” for Protection, number for composite error (5%, 10%) at a specified multiple of rated current.
Example: 10P10 = 10% error at 10 times rated current

Types of CT Cores

Core Type	Description	Application
Bar Type	Straight conductor as primary, molded around	Substation & switchgear CTs
Wound Type	Separate windings for both primary and secondary	Low-voltage industrial systems
Toroidal (Window) Type	Primary conductor passes through center opening	Portable or clamp-on CTs

Metering vs. Protection CTs

Feature	Metering CT	Protection CT
Accuracy Range	Low error at normal currents	Accurate at high fault currents
Core Saturation	Lower saturation point	Higher saturation point
Purpose	Energy metering, instrumentation	Relay operation & fault detection
Typical Accuracy	Class 0.2, 0.5	Class 5P, 10P, PS

Safety Note: Never Open-Circuit a CT Secondary

When a CT secondary is open while the primary carries current, it produces high voltage across the open ends — dangerous to equipment and personnel. Always keep secondary terminals shorted or loaded properly.

Applications of CTs

Energy Metering – in residential, commercial, and industrial panels
Protection Relays – for overcurrent, differential, and distance protection
Monitoring Systems – SCADA, power quality meters
Switchgear and Substations – 11kV, 33kV, and above

Common CT Markings

Marking	Meaning
P1, P2	Primary terminals
S1, S2	Secondary terminals
K, L	Older marking (K = S1, L = S2)
Arrow	Direction of current flow

Understanding Current Transformer (CT) Saturation, Accuracy Class, Knee Point Voltage & Testing Methods

Introduction

Current Transformers (CTs) are crucial components in power systems for protection, measurement, and metering. To ensure accurate and reliable performance, it is important to understand key characteristics such as CT saturation, accuracy class, knee point voltage, and testing methods.

CT Saturation: What It Is and Why It Matters

What is CT Saturation?

CT saturation occurs when the magnetic core of the CT becomes magnetically saturated and cannot produce a proportional secondary current in response to the primary current. This usually happens during short-circuit conditions or when the burden exceeds the CT’s capability.

Causes of Saturation:

High fault currents
Excessive burden (load impedance)
Incorrect CT sizing
Remanent flux in the core

Consequences of Saturation:

Underestimation of actual fault current
Malfunctioning of protective relays
Delay in relay operation
Loss of waveform fidelity for metering

CT Excitation Curve

The excitation or magnetization curve plots the secondary voltage (V) vs magnetizing current (I) of a CT:

    ^
V   |                       *
    |                      *
    |                    *
    |                 *
    |              *
    |          *
    |      *
    | *
    +-----------------------------> I
                 Knee Point

The knee point is where a small increase in voltage causes a large increase in magnetizing current.

CT Accuracy Class Explained

CTs are classified based on the accuracy with which they replicate primary currents under specified conditions.

1. Metering CTs:

Classes: 0.1, 0.2, 0.5, 1.0, 3.0
Application: Energy meters, ammeters, instrumentation
Example: Class 0.5 means ±0.5% error at rated current.

2. Protection CTs:

Classes: 5P, 10P (as per IS/IEC 60044-1 / IEC 61869-2)
“5P20” means:
- 5P = 5% maximum composite error
- 20 = Accuracy up to 20 times rated current

3. Class PX (IEC/IEEE Standard):

Special protection class for differential protection
Characterized by parameters: Knee Point Voltage (Vk), Excitation Current (Ie), Resistance (Rct), and Burden (Rb)

Knee Point Voltage (Vk)

Definition:

According to IEC 60044-6, the knee point is the voltage at which a 10% increase in voltage causes a 50% increase in magnetizing current.

Formula:

$Vk=If\times(Rb+Rct)$
Where:

If = Minimum fault current for protection relay
Rb = Burden resistance
Rct = CT secondary winding resistance

Importance:

Ensures CT does not saturate before relay operates
Critical in differential and distance protection schemes

CT Testing Methods

1. Polarity Test

Ensures correct polarity of primary and secondary windings
Use a battery and galvanometer (kick test)

2. Ratio Test

Measures the actual transformation ratio
Method: Primary injection or clamp-on meter comparison

3. Excitation (Magnetization) Test

Determines the knee point voltage
Plot V-I curve by applying increasing AC voltage

4. Insulation Resistance Test

Measures insulation between core and winding
Done using a megger (typically 1 kV or 5 kV)

5. Winding Resistance Test

Measures DC resistance of CT secondary
Important for computing accurate voltage drop and knee point

6. Burden Test

Verifies total connected burden (relay, wires, etc.) does not exceed CT rating

FAQs on Current Transformers

Q1. What is the function of a CT?
A: To step down high line current to a measurable low value for metering and protection.

Q2. What happens if CT secondary is left open?
A: It can generate dangerously high voltages — always avoid this condition.

Q3. What does 10P10 mean in a CT?
A: A protection CT with 10% composite error at 10 times rated current.

Q4. How to calculate CT burden?
A: Multiply the square of secondary current by the impedance (VA = I² × Z).

Q5. Can CTs measure voltage?
A: No. For voltage measurement, Voltage Transformers (VTs) or Potential Transformers (PTs) are used.