Functions
2.29 Sensitive Earth Fault Protection (ANSI 51GN, 64R)
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
211
2.29 Sensitive Earth Fault Protection (ANSI 51GN, 64R)
The sensitive earth current protection detects earth faults in systems with isolated or high-impedance earthed
starpoint. This stage operates with the magnitudes of the earth current. It is therefore useful in applications
where the magnitude of the earth current is an indicator of the earth fault. This may be the case e.g. in electrical
machines in a busbar configuration in an isolated power system where during a machine earth fault of the stator
winding, the entire network capacity supplies the earth fault current, but during a network earth fault, the earth
fault current is negligible due to the low machine capacitance. The current can be measured using torroidal
current transformers or Holmgreen connection.
In the 7UM62, the sensitive earth fault detection feature can be allocated to either input I
ee1
or I
ee2
. This choice
is made during configuration (see Section 2.4).
Because of the high sensitivity this protection is not suited for detection of high earth fault currents (above
approx. 1 A at the terminals for sensitive earth current connection). If this protection feature nevertheless is to
be used for earth fault protection, an additional, external current transformer is required as intermediate trans-
former.
Note: The sensitive earth current protection may use the same current measuring input (I
ee2
) used for the di-
rectional or non–directional stator earth fault protection with busbar-connection. This means that the sensitive
earth fault protection thereby uses the same measured values if address 150 S/E/F PROT. is set to
directional or non-dir. U0&I0.
2.29.1 Functional Description
Application as Rotor Earth Fault Protection
Also, the highly sensitive earth fault protection can be used for rotor earth fault detection when a system fre-
quency bias voltage is applied to the rotor circuit (see Figure 2-90). In this case, the maximum earth current is
determined by the magnitude of the bias voltage U
V
and the capacitative coupling to the rotor circuit.
A measured value supervision is provided for this application as rotor earth fault protection. The measurement
circuit is assumed closed as long as the earth current, even with intact insulation, exceeds a parametrizable
minimum value IEE< due to the rotor-earth capacitance. If the value is undershot an alarm is issued after a
short delay time of 2 s.
Measurement Method
Initially, the residual current is numerically filtered so that only the fundamental wave of the current is used for
the measurement. This makes the measurement insensitive to short-circuit transients and harmonics.
The protection consists of two stages. A pickup is detected as soon as the first parametrized threshold value
IEE> is exceeded. The trip command is transmitted subsequent to the T IEE> delay time. A pickup is detected
as soon as the second parametrized threshold value IEE>> is exceeded. The trip command is transmitted sub-
sequent to the T IEE>> delay time.
Both stages can be blocked via a binary input.
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