8-8
745 Transformer Management Relay
GE Multilin
8.4 DISPLAY, METERING, COMMUNICATIONS, AND ANALOG OUTPUTS
8 COMMISSIONING
8
8.4.3 VOLTAGE INPUT
1.
Connect an AC voltage to the voltage input (if the input voltage feature is enabled) to terminals C11 and C12. Set the
level at the expected VT secondary voltage on the VT for your installation.
2.
Remove all current signals from the relay.
3.
Verify the voltage reading in
A2 METERING
ÖØ
VOLTAGE
ÖØ
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. Ear-
lier 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
ÖØ
FREQUENCY
Ö
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 dis-
played as 0.00 Hz). Verify that at less than 1.0 V, frequency is displayed as 0.00 Hz.
8.4.4 TRANSFORMER TYPE SELECTION
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.
a) 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 correc-
tion factor to the Winding 2 currents to match them to the Winding 1 currents. The following illustrates how the correction
factor is computed:
(EQ 8.5)
The mismatch factor is therefore:
(EQ 8.6)
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.
The above results can be verified with two adjustable sources of three-phase current. With a single current source, how the
relay performs the necessary phase angle corrections must be taken into account. Table 5–1: Transformer Types on page
5–10 shows that the Y-side currents are shifted by 30° to match the Delta secondary side. The 30° phase shift is obtained
from the equations below:
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
=
=
Содержание TRANSFORMER MANAGEMENT RELAY 745
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