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5 SETPOINTS
5.2 AUTO-CONFIGURATION
For transformer differential protection, it is necessary to correct the magnitude and phase relationships of the
CT secondary currents for each winding, in order to obtain near zero differential currents under normal operat-
ing conditions. Traditionally, this has been accomplished using interposing CTs or tapped relay windings and
compensating CT connections at the transformer.
The 745 simplifies CT configuration issues by having all CTs connected Wye (polarity markings pointing away
from the transformer). All phase angle and magnitude corrections, as well as zero-sequence current compen-
sation, are performed automatically based upon user entered setpoints.
This section describes the process of auto-configuration by means of a specific example, showing how CT
ratios, transformer voltage ratios, and the transformer phase shifts are used to generate correction factors.
These correction factors are applied to the current signals to obtain extremely accurate differential currents.
Consider a WYE-DELTA power transformer with the following data:
Connection:
Y/d30° (i.e. DELTA winding phases lag corresponding WYE winding phases by 30°)
Winding 1:
100/133/166 MVA, 220 kV nominal, 500/1 CT ratio
Winding 2:
100/133/166 MVA, 69 kV nominal, 1500/1 CT ratio
onload tap changer: 61 to 77 kV in 0.5 kV steps (33 tap positions)
Aux. Cooling:
two stages of forced air
a) PROBLEM 1:USE OF STANDARD CT RATIOS
•
Standard CT ratios: CT
•
Tapped relay windings / interposing CTs (inaccurate/expensive)
Solution:
•
WxNom Voltage, Wx rated Load, Wx CT primary setpoints
•
Automatic correction for mismatch: (CT
Example:
Even ignoring the onload tap changer, the 1500/1 CT on Winding 2 does not perfectly match the 500/1 CT on
Winding 1. A perfectly matched Winding 2 CT ratio (at nominal Winding 2 voltage) is calculated as follows:
V
×
CT
(ideal)
CT
=
----- -
2
1
V
where CT
= Winding 1 CT ratio
1
V
= Winding 1 nominal voltage
1
CT
= Winding 2 CT ratio
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 1594.2 / 1500 = 1.063.
GE Power Management
/ CT
= V
/ V
2
1
1
2
× V
) / (CT
2
2
500
220 kV
1594.2
1
×
=
--------- -
------------------ -
=
----------------- -
1
69 kV
1
2
745 Transformer Management Relay
5.2 AUTO-CONFIGURATION
5.2.2 A TYPICAL POWER TRANSFORMER
5.2.3 DYNAMIC CT RATIO MISMATCH CORRECTION
× V
) < 16
1
1
5.2.1 DESCRIPTION
5
5-3
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