
BE1-59N
APPLICATION
2
PURPOSE
The available fault current for single-phase-to-ground
Faults is very limited for ungrounded systems and
systems that are grounded through a high resistance.
This current limiting reduces the possibility of extensive
equipment damage, and eliminates the need for a
neutral breaker by reducing the fault current below the
level required to sustain an arc.
But it remains important to detect and isolate single-
phase- to-ground faults in order to prevent their evolu-
tion into more dangerous faults such as phase-to-
phase-to-ground and three-phase-to-ground faults.
Sensitive voltage relays can be used to detect ground
faults where the fault current is very small. The
BE1-59N is especially suited to the task.
HIGH RESISTANCE GROUNDING
A common method of grounding an ac generator is to
connect a distribution transformer between the neutral
of the generator and the station ground. The distribu-
tion transformer’s primary voltage rating is equal to, or
greater than, the generator’s rated line-to-neutral
voltage. The distribution transformer secondary is rated
at 200/240 Vac or 100/120 Vac, and a resistor is con-
nected across the secondary winding. When reflected
through the transformer, the resistor is effectively a
high resistance.
RP= RS X N2
where RP is the effective primary resistance
RS is the actual value of the secondary resistor
N is the turns ratio of the distribution transformer
Available single-phase-to-ground fault current at the
generator terminals is greatly reduced by the high
effective resistance of the distribution transformer and
secondary resistor. The distribution transformer pro-
vides isolation for the protection scheme and reduces
the voltage to a convenient level.
The BEl-59N ground fault overvoltage relay is con-
nected across the secondary resistor to detect the
increase in voltage across the distribution transformer
caused by a ground fault in the generator stator
windings. A ground fault at the generator terminals will
result in rated line-to-neutral voltage across the trans-
former primary, while ground faults near the neutral will
result in lower voltages. The overvoltage relay set point
must be higher than any neutral voltage caused by
normal unbalances in order to avoid nuisance trips.
This will allow a certain percentage of the stator
windings to go unprotected by the overvoltage relay.
The overvoltage relay function typically protects 90 to
95% of the generator stator windings.
The BEl-59N ground fault overvoltage relay monitors
the fundamental frequency (50 or 60 Hz) voltage that
accompanies a ground fault, and is insensitive to the
third harmonic voltage present during normal opera-
tion.
One hundred percent protection of the generator stator
windings is obtainable with the optional overlapping
undervoltage element. The undervoltage element is
tuned to measure the third harmonic voltage, which is
present in the generator neutral under normal operat-
ing conditions. The undervoltage element detects the
reduction of the normal third harmonic voltage that
accompanies a ground fault near the neutral point of
the generator.
An undervoltage inhibit feature is included with the
third harmonic undervoltage element. This feature
supervises the operation of the ground fault relay to
prevent operation during startup and shutdown by
monitoring the generator terminal voltage.
UNGROUNDED SYSTEMS
The BEl-59N ground fault overvoltage relay is used to
detect ground faults on ungrounded three-phase-three-
wire systems. The relay is connected as shown in
Figure 1. A set of voltage transformers is wired with a
grounded wye primary and a broken delta secondary.
The BEl-59N is connected across the broken delta. It is
often necessary to connect a resistor across the
broken delta to avoid ferroresonance.
Figure 1 - Ungrounded 3-Phase, 3-Wire System