GE 745 Instruction Manual page 249

CHAPTER 7: COMMISSIONING
7.4.4 Transformer Type Selection
7.4.4.1 Description
7.4.4.2 Automatic Transformation
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
 With the voltage signal still ON, read the displayed system frequency
under
 Lower the voltage amplitude while displaying the system frequency.
 Verify that the frequency is displayed correctly with voltage levels down
to less than 3 V RMS input (when the lower limit is reached, the system
frequency will be displayed as 0.00 Hz).
 Verify that at less than 1.0 V, frequency is displayed as 0.00 Hz.
The 745 automatically configures itself to correct for CT ratio mismatch, phase shift, etc.,
provided that the input CTs are all connected in wye. The following example illustrates the
automatic setting feature of the 745.
The automatic configuration routines examine the CT ratios, the transformer voltage
ratios, the transformer phase shift, etc., and apply correction factors to match the current
signals under steady state conditions.
Consider the case of a Y:D30° power transformer with the following data (using a 1 A CT
secondary rating for the relay):
• Winding 1: 100 MVA, 220 kV, 250/1 CT ratio (rated current is 262.4 A, hence CT ratio
of 250/1)
• Winding 2: 100 MVA, 69 kV, 1000/1 CT ratio (rated current is 836.8 A, hence CT ratio
of 1000/1)
The 1000/1 CT ratio is not a perfect match for the 250/1 ratio. The high-side CT produces a
secondary current of 262.5/250 = 1.05 A whereas the low-side CT produces a current of
0.837 A. The 745 automatically applies an amplitude correction factor to the Winding 2
currents to match them to the Winding 1 currents. The following illustrates how the
correction factor is computed:
CT ( ideal
2
The mismatch factor is therefore:
Ideal CT Ratio
------------------------------------- -
Actual CT Ratio
Winding 2 currents are divided by this factor to obtain balanced conditions for the
differential elements.
If this transformer were on line, fully loaded, and protected by a properly set 745 relay, the
actual current values read by the relay would be:
• Winding 1: 262.5 A ∠0° (this is the reference winding)
• Winding 2: 836.8 A ∠210° (30° lag due to transformer and 180° lag due to CT
connections)
A2 METERING FREQUENCY

V
250 220 V
1
----- -------- - ------------- -
) CT × ×
= =
1
V 1 69 V
2
797.1
-------------
0.7971
= =
1000
METERING
SYSTEM FREQUENCY.

797.1
=
(EQ 7.5)
(EQ 7.6)
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