GE G60 Instruction Manual page 606

PHASE DISTANCE THROUGH POWER TRANSFORMERS
• The current signals require compensation if the transformer is located between the CTs and the intended reach point.
If this is the case, set the CT connection setting to transformer connection and vector group as seen from the CTs
toward the reach point. Otherwise, set the CT connection setting to "None."
• The voltage signals require compensation if the transformer is located between the VTs and the intended reach point.
If this is the case, set the VT connection setting to transformer connection and vector group as seen from the VTs
toward the reach point. Otherwise, set the VT connection setting to "None."
• The reach setting is entered in secondary ohms and as such must take into account location and ratios of VTs and CTs,
as well as voltage ratio of the involved power transformer
The following equations explain the setting rules. Consider two applications as shown in the figure.
where
Z = intended reach impedance for Zone 1 (primary ohms)
X
Z = intended reach impedance for Zone 3 (primary ohms)
H
Z = positive-sequence impedance of the transformer
T
V , V = transformer rated voltages
X H
n = transformation ratio of the CTs
CT
n = transformation ratio of the VTs
VT
Z1: Z1 reach setting (secondary ohms)
Z3: Zone 3 reach setting (secondary ohms)
8.3.1.2 Zone 1 setting in application A
As the transformer is located between the potential source and the reach point for Zone 1, the reach impedance must
include the positive-sequence impedance of the transformer. In addition, the primary impedance must be re-calculated for
the voltage level of the VTs and CTs, and eventually, re-calculated to secondary quantities:
8
8.3.1.3 Zone 3 setting in application A
As the transformer is not located between the potential source and the reach point for Zone 3, the reach impedance must
not include the positive-sequence impedance of the transformer. Because both VTs and CTs are located on the same side
as the intended reach point, no correction for the transformer ratio is required. The primary impedance must be only re-
calculated to secondary quantities:
8-16
Figure 8-6: Phase distance protection looking through a power transformer
CHAPTER 8: APPLICATION OF SETTINGS
G60 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
Eq. 8-35
Eq. 8-36