Siemens 7SG164 Ohmega 400 Series Manual page 21

7SG164 Ohmega 400 Series Impedance Functions
This is a convenient factor to use in the phase comparison logic as the actual value of Z0 is not
required provided the ratio Z 0 / Z1 and the phase angles of Z0 and Z1 are available. The K0 factor
described above must of course take into account the phase angles of Z1 and Z0 in the calculation for
K0.
The advantage of using the method described above is that the ratio Z0/Z1 is a relatively simple
calculation and can be obtained using any convenient dimensions (e.g. Primary Ohms, Secondary
Ohms, Ohms/Km etc) provided the zero phase sequence value and the positive phase sequence
value are expressed in the same units.
When the reach setting Z1, the ratio Z0/Z1, the line angle, and the angle of Z0 are entered the relay
calculates the composite value.
K0 = 1/3 (Z0/Z1 -1)
and this value is taken into account for a polarised mho characteristic using the complex expression.
(IZ + K0 IN Z1 ) - V within 90° of V + Vp where I = Phase current
Z1 = Positive phase sequence impedance of zone setting
K0 = 1/3 (Z0/Z - 1)
IN = Earth fault current
V = Phase-earth fault voltage
Vp = Polarising voltage
©2010 Siemens Protection Devices Limited Chapter 2 Page 7 of 21