Metering and Protection Current Transformers:
Current transformers are used for both measurement and protection purposes.
By reduce the power system primary current to lower value CT is used for measuring purpose.
By permit the use of standard current rating for secondary equipment CT is used for Protection purpose.
The CTs used for measurement and protection purposes have different properties.
Following are the basic differences between two.
- These are noticed by their yellow color in the circuit.
- The limits are well defined.
- These require good accuracy up to approximately 120% rated current.
- Require low saturation level to protect instruments, thus use nickel iron alloy core with low exciting current and knee point at low flux density.
- Metering C.Ts are classified into various classes based on the highest permissible percentage ratio error at rated current.
- The classes are defined as class 0.1, class 0.2,class 0.5,class 1, class 3, class 5.
- These are noticed by their brown color in the circuit.
- Operation over wide range of currents.
- Accuracy not as important as measuring CTs.
- Require accuracy up to many times rated current, thus use grain oriented silicon steel with high saturation flux density.
- Protection C.Ts are classified into various classes
- The classes are defined as 5P20, 10P20…
Here 5P20 means
P-> Protection Class
20-> 20 times of rated current
Means when 20 times of rated current is flowing through CT the error in secondary current may be ± 5%.