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Auto-Configuration
Introduction
Dynamic CT Ratio
Mismatch Correction
GE Multilin
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 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, 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 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.
a) Use of Standard CT Ratios
•
Standard CT ratios: CT
•
Tapped relay windings / interposing CTs (inaccurate/expensive)
Solution:
•
Wx Nom Voltage, Wx rated Load, Wx CT primary setpoints
•
Automatic correction for mismatch: (CT
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:
CT
(ideal)
=
2
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.
The transformer type is entered as
. The 745 calculates and automatically corrects for CT mismatch
TRANSFORMER TYPE
to a maximum mismatch factor of 16. The following information is entered as
setpoints:
S2 SYSTEM SETUP
!"
WINDING 1
S2 SYSTEM SETUP
WINDING 1
!"
S2 SYSTEM SETUP
!"
WINDING 1
http://www.GEindustrial.com/multilin
Transformer Management Relay
/ CT
=V
/ V
2
1
1
2
V
) / (CT
×
2
2
V
500
220 kV
1
CT
×
×
------ -
---------- -
-------------------- -
=
=
1
V
1
69 kV
2
S2 SYSTEM SETUP
!
WINDING 1 NOM Φ-Φ VOLTAGE
WINDING 1 RATED LOAD
!"
!"
WINDING 1 PHASE CT PRIMARY
745
V
)
16
×
<
1
1
1594.2
-------------------- -
(EQ 5.1)
1
!
TRANSFORMER
!"
: "220 kV"
: "100 MVA"
: "500:1 A"
5–5
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