Functional Description
3-2
Voltage Sensing And Conditioning
Monitored generator and bus voltages are sensed and scaled down to suitable internal circuit levels.
Internal solid state analog switches select whether the generator voltage differential levels represent line-
to-line or line-to-neutral values. The user determines these selections through menu options. The BE3-
GPR requires a minimum sensing voltage of 10 volts to be applied for normal operation.
Frequency Sensing
Filtered zero crossing detectors connected to outputs from the voltage conditioning circuits generate
timing waveforms. The microprocessor uses these timing waveforms to calculate generator line
frequency, and if the sync-check function is present, bus frequency and generator-bus phase difference.
Current Sensing And Conditioning
Monitored generator currents are sensed and scaled down to suitable internal circuit levels. These
internal current transformers provide isolation from the monitored currents. Two taps located on the
primary side accommodate connections to either one ampere or five ampere circuits.
Control (Pushbutton) Inputs
A three-pushbutton keypad provides the user with the capability to program the relay when used in
conjunction with the LCD based setup menu.
Hardware (Jumper) Inputs
When connected, the eight terminal block pins provide the following relay functions:
1. Program Enable. Allows the user to make changes to the relay settings using the front panel
keypad and the LCD menu. When the program enable is not installed
(jumpered), the keypad is locked out and changes can only be made using the
communications link.
2. Output Disable. Disables relay trip activity and prevents nuisance tripping on power system
buildup.
3. Fail-safe Enable. Changes the output relays coils so that they are energized in the normal state
(untripped ) and deenergized when operated (tripped). The power system then
detects loss of power as simultaneous trips on all of the outputs.
4. Trip Reset. When auto reset is disabled, momentary connection to a common pin causes
the clearing of tripped functions.
Analog To Digital Converter
Scaled down outputs from the voltage and current sensing circuits are sampled and digitized by the twelve
bit analog to digital converter under the microprocessor control. Sampling occurs 26 times per cycle with
60 hertz nominal operating power. Each sample is stored in microprocessor RAM and processed on a
cycle-by-cycle basis.
Microprocessor
Embedded software in the microprocessor controls the overall functionality of the relay and makes all
decisions based on programming and system inputs. The RMS values for current, voltage, and power are
calculated by the microprocessor from the analog to digital converter samples. The formulas used to
calculate these values are provided in the following paragraphs.
NOTE
Any loss of voltage sensing (
≤
10 volts) causes the:
Undervoltage output to trip if enabled.
Phase balance to trip if enabled and the undervoltage is disabled, and if configured for
three-phase, four-wire operation.
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