Basler Electric BE1-87B - Manual (Page 28)

Figure 2-8. Voltage Appearing Across Full Winding of CT
Characteristics
Operation of the BE1-87B relay is initiated as a function of the instantaneous voltage developed across
terminals 5 and 7 (single-phase model) of the relay. However, an output will not be produced unless the
total secondary fault current that flows through the relay after the SCRs fire is greater than the pickup
current setting. In this fashion both voltage and current are required to produce an output. The voltage
selector switch setting and current selector switch setting determine the voltage and current requirements
needed to produce an output.
Differential Voltage Pickup
The BE1-87B pickup voltage selector switch allows for a setting range of 50 to 400 volt rms symmetrical
in 50-volt increments. This sets the voltage level at which the back to back internal SCRs will trigger. The
SCRs will trigger whenever the instantaneous value of the applied voltage is equal to twice the peak value
of the rms symmetrical pickup voltage setting. Refer to Equation 3.
NOTE
The voltage V
R
is equal to the peak voltage of a fully offset voltage wave
having an rms symmetrical value equal to the pickup voltage selector switch
setting. Because of this the SCRs will fire whenever an rms symmetrical
voltage greater than twice the voltage selector switch setting is applied, or
whenever the corresponding peak voltage is exceeded on an instantaneous
basis as in the case of an internal fault. When the SCRs fire, provided the total
secondary fault current that flows in the relay is greater than the pickup current
setting, the relay will produce an output.
Differential Pickup Current
Pickup current is defined as the rms value of a symmetrical sinusoidal current that must flow into the 87B
in order to provide a contact output. The BE1-87B has a pickup current selector switch that allows for a
range of 0.25 to 2.5 amperes rms in 0.25 ampere increments. However, the relay internally operates off of
the instantaneous peak of this sinusoidal current. For example, if the relay is set at 1 amp, the current
sensing element monitors for instantaneous current riding above 1.414 amps.
Alarm Voltage Pickup
The BE1-87B is equipped with a differential voltage alarm function. The function is used to detect steady
state voltage imbalances across the input sensing circuit of the relay (Terminals 5 and 7, 3 and 7, and 1
BE1-87B N2
N1
V
N2
CT1
N2
CT2 CT3
VF=
N1
N2
VR
V = VOLTAGE DEVELOPED ACROSS FULL WINDING
V = MAXIMUM INSTANTANEOUS VOLTAGE
NI = TOTAL NUMBER OF CT TURNS
N2 = NUMBER OF CT TURNS USED (TAP SETTING)
D2853-10
5-25-99
F
R
R
VF
2-10 BE1-87B Application 9282300990 Rev P