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Examoogle • User • info@examoogle.com • EE-2024-8821
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Ratio error in CT is given by
(KnIs−Ip)/Ip
(KnIp−Ip)/Ip
(KnIs−Ip)/Is
(KnIs−Is)/Ip
(KnIs−Ip)/Ip
Quick Summary: Ratio error in a Current Transformer (CT) is the difference between the actual transformation ratio and the rated (nominal) transformation ratio, expressed as a fraction of the actual primary current. It represents the deviation of the actual ratio from the ideal turns ratio due to the excitation current required to magnetize the core.
Ratio error in a Current Transformer (CT) is the difference between the actual transformation ratio and the rated (nominal) transformation ratio, expressed as a fraction of the actual primary current. It represents the deviation of the actual ratio from the ideal turns ratio due to the excitation current required to magnetize the core.
Ratio Error=IpKnIs−Ip
Ip=KnIs−Iecosδ
In an ideal CT, the primary current Ip is exactly Kn×Is, where Kn is the nominal ratio. However, in practice, a portion of the primary current (the exciting current) is used to overcome core losses and magnetize the core. This leads to a vector difference between KnIs and Ip. The error is defined as Ratio Error=IpKnIs−Ip.
Ratio error is primarily caused by the magnetizing and loss components of the exciting current.
The error is typically expressed as a percentage by multiplying the result by 100.
It depends on the secondary burden, power factor of the secondary circuit, and the core material properties.
For protection class CTs, ratio error is defined at higher currents (e.g., 5 to 20 times rated current).
Allows for standardized measurement of CT performance.
Helps in selecting appropriate CT classes for metering vs protection applications.
High ratio errors lead to inaccurate energy metering.
Can cause mal-operation of sensitive relay settings during fault conditions.
Energy billing and revenue metering.
Differential protection schemes in transformers and generators.
The rated transformation ratio Kn is defined as the ratio of rated primary current to rated secondary current.
Options B, C, and D are dimensionally incorrect or represent variations that do not define standard ratio error.
A is correct — The ratio error in a CT is defined by the formula IpKnIs−Ip.
Always remember that in CTs, 'Ratio Error' relates to the magnitude of current, while 'Phase Angle Error' relates to the phase displacement between primary and secondary currents.