Calculated U0 is scaled to the same level as measured U0. The scaling is determined by the ratio of ULTVTR primary voltage and RESTVTR primary voltage within one UTVTR instance. The assumption is open-delta U0 measurement type.
Typical markings of an open-delta U0 transformer in a 20 kV network can be: 20kV/ sqrt(3) :100V/sqrt(3) :100V/3. In case of a solid earth fault, this transformer gives 100 V output which corresponds to 11.547 kV. If ULTVTR primary voltage is set to 20 kV and RESTVTR primary voltage is set to 11.547 kV when this transformer is used, URES_CLC and URES_MEAS output equal amplitude.
The VT connection mode must be "WYE".
In this case, ROVPTOV1 and ROVPTOV2 in Figure 2 can both use the same U0 reference level in their settings. URES_CLC and URES_MEAS in Figure 1 are scaled to the same level.
If voltage Analog 4 is connected to UTVTR for SECRSYN , the user has to use Calculated U0 , URES_CLC output from UTVTR for ROVPTOV protection.