Basler Electric BE1-59N - Manual (Page 9)

SECTION 1 • GENERAL INFORMATION
PURPOSE
The available fault current for single-phase-to-ground faults is very limited for ungrounded systems and
systems, which 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. However, it remains important to detect and isolate single-
phase-to-ground faults in order to prevent their evolution 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 Ground Fault Overvoltage Relay is especially
suited to this task.
APPLICATION
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 distribution transformer's primary voltage rating is
equal to, or greater than, the generators rated line-to-neutral voltage. The distribution transformer
secondary is rated at 200/240 Vac or 100/120 Vac, and a resistor is connected across the secondary
winding. When reflected through the transformer, the resistor is effectively a high resistance.
R
p
= R
s
x N 2
Where: R
p
is the effective primary resistance
R
s
is the actual value of the secondary resistor
N is the turns ratio of the distribution transformer
To determine resistor sizing:
• Determine the thermal limit P (in watts) of the voltage transformers.
• Use R
Vpri
V
=
2
sec) (
to determine resistance in ohms, where Vsec = 120 x 3 = 360V. This assumes a
120V VT secondary in a worst-case condition.
• Size the watt rating of the resistor for 1/3 of the VT thermal limit to keep the resistor from excessive
heating. A smaller watt rating can be used if tripping for a ground condition.
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
provides isolation for the protection scheme and reduces the voltage to a convenient level.
The BE1-59N Ground Fault Overvoltage Relay is connected 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
transformer primary, while ground faults near the neutral will result in lower voltages. The overvoltage
relay setpoint 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 BE1-59N Ground Fault Overvoltage Relay monitors the fundamental frequency (50 or 60 Hz) voltage,
which accompanies a ground fault, but is insensitive to the third harmonic voltage present during normal
operation. One hundred percent protection of the generator stator windings is obtainable with the optional
overlapping undervoltage element. The under voltage element is tuned to the third harmonic voltage
which is present in the generator neutral under normal conditions. The undervoltage element detects the
reduction of the normal third harmonic voltage, which 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.
9171400990 Rev K BE1-59N General Information 1-1