Difference between metering and protection current transformers (CTs):

Metering and Protection Current Transformers:

Metering and Protection Current transformers are different types of current transformers with varying properties.

• Current transformers are used for both metering and protection purposes.

• The combined metering and protection current transformer is shown in the below figure.

• By reducing the power system’s primary current to a lower value CT is used for measuring or metering purposes.

• By permit, the use of standard current rating for secondary equipment CT is used for Protection purposes.

Metering and Protection Current transformers, The CTs used for metering and protection purposes have different properties.

Following are the basic differences between the two.

Metering Current Transformers:

1. These are noticed by their yellow color in the circuit when used only for metering in LT sections.
2. The limits are well-defined.
3. These require good accuracy up to approximately 120% rated current.
4. Require low saturation level to protect instruments, thus use nickel-iron alloy core with low exciting current and knee point at low flux density.
5. Metering C.Ts are classified into various classes based on the highest permissible percentage ratio error at rated current.
6. The classes are defined as class 0.1, class 0.2, class 0.5, class 1, class 3, and class 5.

Protection Current Transformers:

There are two types of Protection current transformers available. One is the Normal protection CTs and the other is Special Purpose Protection CTs.

Normal Protection C.Ts: These are used for giving input to the relays like Overcurrent and Earth fault to provide protection for feeders and motors etc.

1. These are noticed by their brown color in the circuit.
2. Operation over a wide range of currents.
3. Accuracy is not as important as measuring CTs.
4. Require accuracy up to many times rated current, thus use grain-oriented silicon steel with high saturation flux density.
5. Protection C.Ts are classified into various classes
6. The classes are defined as 5P20, 10P20…

Here 5P20 means

5->%Error

P-> Protection Class

20-> 20 times of rated current

This means when 20 times of rated current is flowing through CT the error in secondary current may be ± 5%.

PS-Class [Special Purpose] Current Transformers:

Class PS transformers are special-purpose protective current transformers. These are used where the required characteristics of the current transformers cannot be conveniently expressed in terms of the ordinary protection current transformers like Class 5P, 10P, and 15P current transformers.

The typical applications involve differential and restricted earth fault protection schemes.

The knee point voltage is a very important factor in designing this type of transformer.

Let us see what the Knee Point Voltage is.

Knee Point Voltage is the sinusoidal voltage of rated frequency applied to the secondary terminals of the current transformer, all other windings being open-circuited, which when increased by 10 percent, causes the exciting current to increase by 50 percent.

The performance of the current transformer depends on the specified minimum knee point voltage.

Special Characteristics of PS-Class Current Transformers:

1. The turns ratio shall be numerically the same as the rated transformation ratio.
2. Error in turns ratio shall not exceed ±25 percent.
3. The knee-point voltage shall not be less than the specified rated knee-point voltage.
4. The exciting current shall not be greater than the maximum exciting current, at the rated knee point voltage, or at the specified fraction thereof.
5. When specified, the resistance of the secondary winding corrected to 75^oC, shall not be greater than the specified value.

For this type of CT knee point voltage test must be done in addition to the routine and type tests.