Earth Fault Sensitivity; Solid Or Effective Neutral Earthing; High Impedance And Resistance Earthed Neutrals - Siemens 7SR18 Solkor Manual

7SR18 Applications Guide

7.5 Earth Fault Sensitivity

The method adopted for earthing the power system network will determine the amount of fault current available to
operate the differential elements. As mentioned above care must be taken when assessing the best combination
of settings to use.
Network feeder circuits of the type that this relay is likely to be applied on, e.g. 3 kV to 150 kV, may operate with
their neutral points either solidly earthed (typical for the 150 kV end of the range), unearthed (often employed in
the middle range of distribution voltage ratings) and impedance earthed at the lower end.
This must be considered when selecting and applying the relay, as outlined below: -
7.5.1

Solid or Effective Neutral Earthing

Solid earthing will normally result in earth fault levels of a similar magnitude or just above the three phase faults.
Low impedance earthed generators are normally designed to allow fault current of the order of magnitude of the
source incoming circuit rating, typical values being in the range of 100 A to 1600 A.
In either case, solid or low impedance, the standard basic relay, with fixed or variable settings should provide
adequate sensitivity for earth faults. Both the differential and the back-up non-unit protection will provide sensitive
protection.
For low impedance earthed networks it is only necessary to ensure that the current transformer primary rating and
ratio is compatible with the earth fault current, or is of a lower value.
Example: maximum fault current – 800 A
CT ratio £ 800/1 (or 5)
For low impedance earthed networks it is recommended that the differential sensitivity be no more than 80% of
the minimum earth fault current. The relay with variable differential settings should allow this to be met.
7.5.2

High Impedance and Resistance Earthed Neutrals

This method is often employed in medium voltage power system, where the fault current in each source neutral is
limited to a low value, for reasons of safety and to limit fault damage.
The fault current may be of the order of 100 to 1000 A.
In this type of network, with feeders typically rated 400 A to 800 A and CT ratios chosen appropriately, e.g.
800/400/1 or 5 A, the earth fault current may not be sufficient to operate relay models from the basic range. The
minimum relay setting is 10% of nominal current rating for both the differential (i.e. variable setting models) and
back-up protection.
For transformer feeders, an earth fault part way into the transformer winding would result in a much lower
proportion of maximum fault current. For Delta windings this is usually acceptable, whereas for star connected
windings a separate more sensitive restricted earth fault protection is normally provided.
For plain feeders, acceptable settings to apply to the relay are as follows: -
Example: Maximum earth fault from one source = 40 A
CT ratio £ 100/1 (or 5 A)
Relay settings P/F Differential (I
Bias slope 1 (S
Bias slope 2 (S
Bias Break Point (B2)
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) = 0.10 x I
S
) = 20%
1
) = 150%
2
= 2.0 x I
N
N
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