GE T35, Instruction Manual, Page: 350 / 610

5-148 T35 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
SYSTEM SETUP CHAPTER 5: SETTINGS
5
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).
PHASE COMPENSATION
— Selects the type of phase compensation to be performed by the relay.
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.
Figure 5-68: Wye winding and Delta CT connection
LOAD LOSS AT RATED LOAD
— Take this setting from the transformer nameplate. If not available from the nameplate, the
setting value can be computed as , where I
n(w)
is the winding rated current and R is the three-phase series
resistance. The setting is used as an input for the calculation of the hottest-spot winding temperature.
RATED WINDING TEMP RISE
— This setting defines the winding temperature rise over 30 °C ambient temperature. The setting
is automatically selected for the transformer type as shown in the following table.
The loss of life function calculates the insulation aging acceleration factor using the settings entered in this section, by
following equation:
Eq. 5-7
where Θ
H_R
is the rated hottest-spot temperature as per the table,
and Θ
H(t)
is the actual computed winding hottest-spot temperature
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.