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CHAPTER 6: SETPOINTS
345 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
Courtesy of NationalSwitchgear.com
Consider a typical Wye/Delta power transformer with the following data:
•
Connection: Y/d30° (i.e. delta winding phase currents lag the corresponding wye
winding phase currents by 30°)
•
Transformer rated load: 5/7/9 MVA
•
Winding 1: 13.8 kV nominal, 500/5 CT ratio
•
Winding 2: 4.16 kV nominal, 1500/5 CT ratio.
The following sections will illustrate auto-configuration principles using in the example.
The current transformer ratings per each winding are entered as:
S2 SYSTEM SETUP > CURRENT SENSING > W1 PHASE CT PRIMARY
S2 SYSTEM SETUP > CURRENT SENSING > W2 PHASE CT PRIMARY
The transformer type is entered as:
S2 SYSTEM SETUP > TRANSFORMER > TRANSFORMER TYPE
S2 SYSTEM SETUP > TRANSFORMER > XFMR RATED LOAD
S2 SYSTEM SETU > TRANSFORMER > W1 NOM F-F VOLTAGE
S2 SYSTEM SETUP > TRANSFORMER > W1 GROUNDING
S2 SYSTEM SETUP > TRANSFORMER > W2 NOM F-F VOLTAGE
S2 SYSTEM SETUP > TRANSFORMER > W2 GROUNDING
AUTOMATIC CT MISMATCH COMPENSATION
The 1500/5 CT on winding 2 does not perfectly match the 500/5 CT on Winding 1. The
perfectly matched winding 2 CT primary at nominal winding 2 voltage is calculated as
follows:
where:
CT
= Winding 1 CT primary rating
1
V
= Winding 1 nominal voltage
1
CT
= Winding 2 CT primary rating
2
V
= Winding 2 nominal voltage
2
Thus, for any load, the Winding 2 CT secondary current is higher (per unit) than the Winding
1 CT secondary current. The mismatch factor is 1658.6 / 1500 = 1.1057.
The 345 relay calculates the magnitude correction factor for Winding 2 as follows:
The 345 calculates and automatically corrects the CT mismatch to a maximum mismatch
factor of 16.
Hence, the measured currents from winding 2 will be automatically compensated by the
relay applying the correction factor of 0.904. The currents from winding 1 are not
multiplied by any correction factor, as they are always used by the relay as a magnitude
reference.
Based on 100% transformer loading (5MVA), the measured winding currents are:
Winding 1 CT: measured
- winding 1 nominal current
S2 SYSTEM SETUP
: "500 A"
: "1500 A"
: "Y/d30"
: "5 MVA"
: "13.8 kV"
: "Within Zone"
: "4.16 kV"
: "Not Within Zone"
Eq. 1
Eq. 2
Eq. 3
6–51
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