ABB RELION REX640 Technical Manual page 522

Section 4
Protection functions
516
GUID-69094BC0-5DBF-4B2F-ACB2-B8681A1F8FEB V1 EN
Figure 282: Typical example of the third-harmonic voltage measured at the
generator neutral and terminals under different conditions
The operating equation of the protection is described in the following equation.
U U Beta U
H T + H N − ⋅
3 _ 3 _
GUID-5BDE8AC2-F69D-4617-8368-F7B85F38114F V1 EN
The third harmonic voltages Ū
imaginary parts. Ū is approximately in the opposite direction to that of the Ū
3H_T
however the actual no-fault angle between those two phasors depends on the type of
generator earthing. For example, the angle is about 145 degrees for a high-resistance-
earthed unit generator.
0
H N =
3 _
and Ū are the phasor with its real and
3H_T 3H_N
1MRS759142 C
(Equation 71)
,
3H_N
REX640
Technical Manual