GE G30 Instruction Manual page 362

SYSTEM SETUP
5.5.4.2 General transformer settings
SETTINGS  SYSTEM SETUP  TRANSFORMER  GENERAL
 GENERAL

The general transformer settings apply to all windings. Settings specific to each winding are shown in the following
section.
REFERENCE WINDING
other windings are normalized (magnitude compensation) to this winding, and the per unit value of the differential
currents, the restraint currents, and the corresponding differential characteristics (pickup, break points, and so on) are all
based on the CT Nominal of this winding. The reference winding can be selected either automatically or manually. When
automatic reference winding selection is chosen, the winding with the smallest margin of CT primary current with respect
to the winding rated current is selected as the reference winding (see details in Magnitude Compensation section later in
this chapter).
In some applications where CT primary of the winding is much higher than the winding rated current, automatic
selection of reference winding can result in excessive magnitude compensation factor, and as a result can cause
the winding current clipping. Therefore, with the automatic selection of the reference winding, further checking is
required to ensure no winding currents after magnitude compensation can exceed 46 pu. If this happens, use
manual selection of reference winding to avoid this. See the T35/T60 Reference Winding Selection and CT Ratio
Mismatch Application Note (GET-8548) for information.
PHASE COMPENSATION
5
If set to "Internal (software)", the transformer phase shift is compensated internally by the relay algorithm.
If set to "External (with CTs)", the transformer phase shift is externally compensated by the CT connections. The Delta
connected CTs on the Wye winding compensate the phase shift of the currents between the transformer primary and
secondary windings (see the following figure). This is the old way of phase compensation.
And also to measure the currents on both sides of the transformer that correspond to the transformation ratio, divide the
CT primary by SQRT 3. This compensates for the increase of SQRT 3 caused by the Delta CT connection.
Therefore, the Actual Value of current and power cannot be used for measurements in this case.
5-148
REFERENCE WINDING:

Automatic Selection
PHASE COMPENSATION:

Internal (software)
— Reference winding is the winding to which all currents are referred. That is, the currents from all
— Selects the type of phase compensation to be performed by the relay.
Figure 5-68: Wye winding and Delta CT connection
Range: Automatic Selection, Winding 1, Winding 2
Range: Internal (software), External (with CTs)
G30 GENERATOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS