GE 745 - 5.2 Auto-configuration; 5.2.1 Introduction; 5.2.2 Dynamic CT Ratio Mismatch Correction

5–6 745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
AUTO-CONFIGURATION CHAPTER 5: SETPOINTS
5.2 Auto-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, 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)
• Auxiliary 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
5.2.2.1 Use of Standard CT Ratios
• Standard CT ratios: CT
2
/ CT
1
=V
1
/ V
2
• Tapped relay windings / interposing CTs (inaccurate/expensive)
Solution:
•Wx Nom 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)
CT
2
(ideal) CT
1
V
1
V
2
-----
×
500
1
---------
220 kV
69 kV
-----------------
×
1594.2
1
-----------------
== =