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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Ratio error in PT is give by
(KnVs−Vp)/Vp
(KnVp−Vp)/Vp
(KnVs−Vp)/Vs
(KnVs−Vs)/Vp
(KnVs−Vp)/Vp
Quick Summary: The ratio error of a Potential Transformer (PT) is the difference between the actual transformation ratio and the rated transformation ratio, expressed as a fraction of the actual secondary voltage. It indicates how much the actual voltage ratio deviates from the nominal value due to internal voltage drops across winding resistance and leakage reactance.
The ratio error of a Potential Transformer (PT) is the difference between the actual transformation ratio and the rated transformation ratio, expressed as a fraction of the actual secondary voltage. It indicates how much the actual voltage ratio deviates from the nominal value due to internal voltage drops across winding resistance and leakage reactance.
ϵ=VpKnVs−Vp — Ratio error of PT
Kn=VsVp — Nominal transformation ratio
In an ideal PT, the secondary voltage Vs should be exactly Vp/Kn, where Kn is the rated transformation ratio. In a practical PT, the magnetizing current and the voltage drops across the primary and secondary impedances cause the actual ratio to differ. The error is calculated as the relative difference between the transformed secondary voltage (KnVs) and the primary voltage Vp, normalized by Vp.
The ratio error occurs primarily due to the current flowing through internal winding resistance and leakage reactance.
For an ideal PT, the ratio error is zero.
PTs are designed to have very low ratio and phase angle errors for accurate metering.
Ratio error is usually expressed as a percentage by multiplying the result by 100.
High accuracy for voltage measurement
Provides electrical isolation between high voltage and metering circuits
Subject to saturation at high flux densities
Affected by burden connected to the secondary
Voltage measurement in high voltage power systems
Power monitoring and energy metering
Relay protection circuits
The rated transformation ratio Kn is defined as the ratio of rated primary voltage to rated secondary voltage.
Option B is incorrect as it uses an incorrect form of the voltage term in the numerator.
Option C is incorrect because the denominator must be Vp according to the standard definition of ratio error.
A is correct — The ratio error of a Potential Transformer is defined as the relative difference between the actual secondary voltage transformed by the nominal ratio (KnVs) and the primary voltage (Vp), divided by the primary voltage (Vp).
Always remember that 'Ratio Error' relates to the magnitude, while 'Phase Angle Error' relates to the displacement between primary and secondary voltage vectors in instrument transformers.