Objectives
ƒ Examine CT performance
ƒ Calculate winding “tap” values
ƒ Determine 87T pickup points
- Determine variable percentage slope breakpoints
- Determine harmonic restraint values
ƒ Determine 87H pick up
ƒ Determine 87GD pick up
CT Performance
ƒ Class “C” CTs have established secondary
voltage/burden curves that are used to predict linear performance
ƒ Operating a CT at values above the “kneepoint” on the curve invite unfaithful replication of the primary waveform and saturation
ƒ Unfaithful replication (including saturation) between measurement CTs in a differential protection causes misoperation.
Class “C” CTs
ƒ Class “C” CTs are designed to “faithfully” replicate primary currents to within +/- 10%
ƒ Class designations designate maximum secondary CT circuit voltage at standard burden that can be applied to
maintain accuracy
- Zrated = Voltage Class / 20 * Rated Secondary Current
- Zrated = C800 / 20 * 5A = 8 Ω, ∴ 800V at 8Ω burden
- Zrated = C200 / 20 * 5A = 2 Ω, ∴ 200V at 2Ω burden
CT Performance Calculation
Use with Class “C” CTs
ƒ Determine maximum primary phase and ground fault current
ƒ Calculate secondary current based on CTR and CT connection type (wye or delta)
ƒ Determine secondary CT circuit burden
ƒ Determine maximum secondary voltage produced from burden with secondary fault currents
ƒ Check maximum secondary voltage against CT capability
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