2 Functions
136
7UM61 Manual
C53000-G1176-C127-3
2.22 Sensitive Earth Fault Protection (ANSI 51GN, 64R)
The highly sensitive earth fault protection detects earth faults in systems with isolated
or high-impedance earthed starpoint. This stage operates with the magnitude of the
earth current. It is intended for use where the earth current amplitude gives an indica-
tion of the earth fault. As an example of this is with electrical machines in busbar con-
nection 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 with a network
earth fault, the earth fault current is negligible due to the low machine capacitance.
The current may be measured using toroidal CTs or CTs in Holmgreen connection.
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 connec-
tion). If this protection feature nevertheless is to be used for earth fault protection, an
additional, external current transformer is required as intermediate transformer.
Note: The same measured current input is used for sensitive earth current protection
as well as for the directional or non–directional stator earth fault protection with
busbar-connection. The sensitive earth fault protection thereby uses the same mea-
sured values if address 150 S/E/F PROT. is set to directional or non-dir.
U0&I0.
2.22.1 Functional Description
Application as
Rotor Earth Fault
Protection
Alternatively, sensitive earth fault protection can be used as rotor earth fault protection
when a system frequency bias voltage is applied to the rotor circuit (see Figure 2-48).
In this case, the maximum earth current is determined by the magnitude of the bias
voltage U
V
and the capacitive coupling of the rotor circuit.
A measured value supervision is provided for this application as rotor earth fault pro-
tection. The measurement circuit is assumed closed as long as the earth current, even
with intact insulation, exceeds a parametrizable minimal 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 earth current current is numerically filtered so that only the fundamental
component of the current is used for the measurement. This makes the measurement
insensitive to earth fault transient phenomena and harmonics.
The protection consists of two stages. A pickup is detected as soon as the first param-
etrized threshold value IEE> is exceeded. The trip command is transmitted subse-
quent to the T IEE> delay time. A pickup is detected as soon as the second param-
etrized threshold value IEE>> is exceeded. The trip command is transmitted
subsequent to the T IEE>> delay time.
Both stages can be blocked via a binary input.
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