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Functions
2.15 Sensitive Earth Flt.(comp/ isol. starp.)
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
297
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 (forward direction), if Q
E
> 0 and I
Er
> setting value,
• Earth fault (reverse direction), if Q
E
< 0 and
I
Er
> setting value.
In
resonant-earthed
networks (with
arc suppression coil
) the following criteria apply:
• Earth fault (forward direction), if P
E
> 0 and I
Ea
> setting value,
• Earth fault (reverse direction), if P
E
< 0 and
I
Ea
> setting value,
In the latter case it must be noted that, depending on the installation site of the protection relay, a considerable
reactive component may be superimposed which, in the least favourable cases, can attain up to 50 times the
active component. The accuracy of the calculation algorithm, which is extremely high, is not sufficient if the
transformer is not able to transmit the primary values exactly.
The measurement input circuit of the relay version with earth fault detection is particularly designed for this
purpose and permits an extremely high sensitivity for the directional determination of the wattmetric residual
current. To be able to use this sensitivity, we recommend toroidal current transformers for earth fault detection
in resonant earthed systems. Furthermore, the angle error of the toroidal current transformer can be compen-
sated in the 7SD5. Since the angle error is non-linear, this is done by entering two operating points of the angle
error curve of the transformer. The device then calculates the error curve with the accuracy needed.
Earth Fault Location
In radial systems, locating earth faults is relatively simple. Since all feeders from a common bus (Figure 2-150)
deliver a capacitive charging current, nearly the total earth fault current of the system is available at the mea-
suring point of the faulty line in the earthed system. In the non-earthed system it is the residual wattmetric
current of the arc-suppression coil that flows via the measuring point. For the earth-faulted line or cable, a def-
inite „forward“ decision will result, while in the remaining circuits either „reverse“ is signalled or no measurement
is possible because the earth current is too small. In any case, the faulted line can be determined clearly.
Figure 2-150 Earth fault location in a radial network
In meshed or ring networks the measuring points at the ends of the faulted cable also receive a maximum of
earth fault (residual) current. Only in this cable will the direction „forward“ be indicated at both line ends (Figure
2-151). However, also the rest of the direction indications in the system may be useful for earth fault detection.
Some indications may not be output when the earth current is too low. Further information can be found in the
leaflet „Earth fault detection in isolated neutral or arc-suppression coil earthed high voltage systems“.