GE Multilin 745

Metering745
Transformer Management Relay
Commissioning
http://www.GEindustrial.com/multilin
7–8
GE Multilin
2. Remove all current signals from the relay.
3. Verify the voltage reading in
A2 METERING !" VOLTAGE !" SYSTEM LINE-TO-LINE
VOLTAGE. The reading should be equal to the Input voltage × VT Ratio.
The displayed system voltage is always the line-to-line voltage regardless
of the input VT signal. Earlier versions of the 745 may display the same
voltage as the selected input, i.e. phase-to-neutral if the input is a phase-
to-neutral signal and phase-to-phase if the input is phase-to-phase.
4. With the voltage signal still ON, read the displayed system frequency under
A2
METERING !" FREQUENCY ! SYSTEM FREQUENCY.
5. 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.
Transformer Type
Selection
a) Description
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.
b) Automatic Transformation
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:
(EQ 7.5)
The mismatch factor is therefore:
(EQ 7.6)
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)
• Differential current: < 0.03 × CT as the two winding currents are equal once
correctly transformed inside the relay.
• The loading of each winding would be 100% of rated.
NOTE
CT
2
ideal()CT
1
V
1
V
2
------ -
×
250
1
-----------
220 V
69 V
----------------- -
× 797.1== =
Ideal CT Ratio
Actual CT Ratio
----------------------------------------------
797.1
1000
----------------- 0.7971==