Siemens siprotec 7SA6, Manual

Page: 372 / 876

Functions
6-182
7SA6 Manual
C53000-G1176-C156-2
power components. Thus for determination of the direction of the earth fault, active
and reactive components of the earth fault current as well as the direction of the active
and reactive power are evaluated.
In networks with isolated starpoint the following criteria apply:
•
earth fault forwards, when P
Er
> 0 and I
Er
> set value,
•
earth fault backwards, when P
Er
< 0 and I
Er
> set value.
In resonant-earthed networks (with arc suppression coil)
the following criteria apply:
•
earth fault forwards, when P
Ea
> 0 and I
Ea
> set value,
•
earth fault backwards, when P
Ea
< 0 and IEa > set value.
In the latter case it must be noted that, dependent upon the location of the protective
relay, a considerable reactive component may be superimposed which, in the most
unfavourable cases, can attain 50 times the active component. Even the extremely
high accuracy of the calculation algorithm is then inadequate if the current transform-
ers do not exactly convert the primary values.
The measurement input circuit of the relay version with earth fault detection is partic-
ularly designed for this purpose and permits an extremely high sensitivity for the direc-
tional determination of the wattmetric residual current. In order to utilize this sensitivity
it is recommended that core balance current transformers be used for earth fault de-
tection in compensated networks. As even the core balance transformers have an
angle error, the protection system allows the setting of correction parameters which,
dependent upon the current amplitude, will correct the error angle.
Earth Fault
Location In radial networks, location of the faulted line is relatively simple. Since all circuits on
a busbar (Figure 6-98) carry a capacitive partial current, the measuring point on the
faulted line in an isolated network sees almost the full prospective earth fault current
of the network; in compensated networks the wattmetric residual current from the Pe-
tersen coil flows through the measuring point. For the faulted line or cable, a definite
“forward” decision will result, whilst in the remaining circuits a “reverse” indication will
be given unless the earth current is so small that no measurement can be taken. In
any case the faulted cable can be clearly determined.
Figure 6-98 Faulted line location in radial network