ABB REG670 - Page 475

where i
L1
, i
L2
and i
L3
are instantaneous sampled values of the three-phase currents
from LV side of the excitation transformer. Note that this calculation is done twenty
times per power system cycle. After that the average DC current value I
DC
is
calculated over one power system cycle by using the following formula:
20
1
20
DC
DC
i
I =
å
GUID-5D825381-66DE-404D-B3E9-471B13A394A5 V1 EN (Equation 102)
The average DC current value I
DC
is used further within the rotor overload function
for the operating characteristic calculations and for the service value.
• When CT_Location = HV_winding is selected, it means that the used CT is
located on the primary, high-voltage side (HV) of the excitation transformer, see
Figure220. In that case the measured HV side three-phase currents (I
L1
, I
L2
, I
L3
)
are first transferred to the LV side currents (i
L1
, i
L2
, i
L3
) by using the rating data
of the excitation transformer. The excitation transformer ratings are specified by
the following three setting parameters:
UrLV : LV winding rated ph-ph voltage in Volts.
UrHV : HV winding rated ph-ph voltage in kV.
PhAngleShift : HV side to LV side phase angle shift in degrees (it has value of +30º for Dy1 excitation
transformer and -30º for Dy11 excitation transformer)
Once the measured currents are transferred from the HV side to the LV side of the
excitation transformer the same calculations are performed as in case when CT is
located on the LV side.
The measured current used by the function is available as a service value.
Overload characteristics
Rotor winding temperature increases with the current. Thus, it is logical to apply over
current elements with inverse time-current characteristics for overload protection.
The function operating characteristic is designed in accordance with the American
standard IEEE-C50.13. This standard specifies the following operating points:
Table 261: Operate time in accordance with IEEE-C50.13 standard
Current (% of IBase)
Trip time (s)
113 120
125 60
146 30
209 10
1MRK502052-UEN B Section 8
Current protection
469
Technical manual