6–24 345 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
S2 SYSTEM SETUP CHAPTER 6: SETPOINTS
The 345 relay simplifies the process by performing the magnitude and phase shift
compensations automatically (internally). Upon receiving entered settings for the
protected transformer and winding CT ratings, the relay automatically calculates and
applies the correct magnitude of scaling to the winding currents, as well as the correct
phase shift, in order to prepare the currents for summation. In order to performing the
appropriate current compensation, all winding CTs must be connected in Wye (polarity
markings pointing away from the transformer).
Magnitude Compensation Reference: Percent differential protection by the 345 relay
uses the user-entered phase-to-phase voltage and the CT primary settings for Winding 1
as a reference for current magnitude compensation.
Phase Shift Compensation Reference: Percent differential protection by the 345 relay
uses either the Delta or the Zig-Zag currents (depending on the transformer setup) as a
phase shift reference to perform the phase shift compensation. When a Wye/Wye
transformer is selected in the setup, the currents from the first Wye winding are used as a
reference for the phase shift compensation.
EXAMPLE:
This section describes the process of automatic current magnitude and phase shift
compensation using a specific example that shows how CT ratios, transformer voltage
ratios, and transformer phase shifts are used to generate the correction factors. These
correction factors are applied to the current signals to obtain extremely accurate
differential currents. The menu paths in the example are shown graphically in the above
figure.
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: “500 A”
S2 SYSTEM SETUP > CURRENT SENSING > W2 PHASE CT PRIMARY: “1500 A”
The transformer type is entered as:
S2 SYSTEM SETUP > TRANSFORMER > TRANSFORMER TYPE: “Y/d30”
S2 SYSTEM SETUP > TRANSFORMER > XFMR RATED LOAD: “5 MVA”
S2 SYSTEM SETU > TRANSFORMER > W1 NOM F-F VOLTAGE: “13.8 kV”
S2 SYSTEM SETUP > TRANSFORMER > W1 GROUNDING: “Within Zone”
S2 SYSTEM SETUP > TRANSFORMER > W2 NOM F-F VOLTAGE: “4.16 kV”
S2 SYSTEM SETUP > TRANSFORMER > W2 GROUNDING: “Not Within Zone”
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:
Eq. 1
where:
CT
1
= Winding 1 CT primary rating
V
1
= Winding 1 nominal voltage
CT
2
= Winding 2 CT primary rating
V
2
= Winding 2 nominal voltage
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