GE Multilin
745 Transformer Management Relay
5-5
5 SETPOINTS
5.2 AUTO-CONFIGURATION
5
5.2AUTO-CONFIGURATION
5.2.1 INTRODUCTION
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 operating 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 compensation, 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, trans-
former 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 typical 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
The following sections will illustrate auto-configuration principles using this example.
5.2.2 DYNAMIC CT RATIO MISMATCH CORRECTION
a) USE OF STANDARD CT RATIOS
•
Standard CT ratios: CT
2
/ CT
1
=
V
1
/
V
2
•
Tapped relay windings / interposing CTs (inaccurate/expensive)
Solution:
•
WxNom Voltage, Wx rated Load, Wx CT primary setpoints
•
Automatic correction for mismatch: (CT
2
×
V
2
) / (CT
1
×
V
1
)
<
16
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:
(EQ 5.1)
where
CT
1
= Winding 1 CT ratio
V
1
= Winding 1 nominal voltage
CT
2
= Winding 2 CT ratio
V
2
= Winding 2 nominal voltage
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.
The transformer type is entered as
S2 SYSTEM SETUP
Ö
TRANSFORMER
ÖØ
TRANSFORMER TYPE
. The 745 calculates and
automatically corrects for CT mismatch to a maximum mismatch factor of 16. The following information is entered as set-
points:
S2 SYSTEM SETUP
ÖØ
WINDING 1
Ö
WINDING 1 NOM
Φ
-
Φ
VOLTAGE
: "220 kV"
S2 SYSTEM SETUP
ÖØ
WINDING 1
ÖØ
WINDING 1 RATED LOAD
: "100 MVA"
S2 SYSTEM SETUP
ÖØ
WINDING 1
ÖØ
WINDING 1 PHASE CT PRIMARY
: "500:1 A"
S2 SYSTEM SETUP
ÖØ
WINDING 2
Ö
WINDING 2 NOM
Φ
-
Φ
VOLTAGE
: "69.0 kV"
S2 SYSTEM SETUP
ÖØ
WINDING 2
ÖØ
WINDING 2 RATED LOAD
: "100 MVA"
S2 SYSTEM SETUP
ÖØ
WINDING 2
ÖØ
WINDING 2 PHASE CT PRIMARY
: "1500:1 A"
For a 3-winding transformer, the setpoints under
S2 SYSTEM SETUP/WINDING 3
must also be set.
CT
2
(ideal)
CT
1
V
1
V
2
------
×
500
1
----------
220 kV
69 kV
-------------------
×
1594.2
1
------------------
=
=
=
