Basler BE1-24, Instruction Manual

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3-2 BE1-24 Functional Description
Third Harmonic Rejection Filter
The volts per hertz signal from the secondary of the sensing input transformer is passed through the low
pass, third harmonic (180 hertz) filter. This filter attenuates by 21 db, the peak distorting effect that the third
harmonic places on the 60 hertz fundamental. The microprocessor uses peak detection to calculate
magnitude (relative to pickup setting) for trip. In this way, greater sensitivity and more precise application
can be realized.
Microprocessor
The microprocessor determines if the actual volts per hertz condition exceeds the established limits of the
protected system by first measuring the actual frequency, and then determining the maximum allowable
voltage for each of the three functional settings which may be included in the relay. The microprocessor
operation is synchronized with the zero crossings of the input waveform. All timing functions within the relay
are incorporated within the microprocessor and the microprocessor program.
The microprocessor starts the sequence by measuring system frequency during the first negative half cycle.
During the second positive half cycle, it calculates the maximum permissible voltage level for the time delayed
trip setting. It then subsequently calculates the corresponding values for the (optional) instantaneous trip
setting and for the alarm setting on the successive positive cycles. (See Figure 3-2.) At the proper time this
digital information is supplied to an external digital-to-analog converter in preparation for magnitude
comparison.
Pickup for all three functions is individually adjustable over the range of 1.00 to 3.99 in 0.01 volts per hertz
increments. Pickup accuracy is dependent upon frequency as indicated below.
Frequency Range Accuracy at 25
(
C
40 - 72 Hz ±1 Hz
20 - 40 Hz ±2 Hz
3 - 20 Hz ±8 Hz
Figure 3-2. Calculation Sequence
Trip Level Detector
An analog comparator sequentially monitors the sensed input waveform to determine if system conditions
are within allowable operating limits. The microprocessor sequentially outputs the allowable levels for each
function to the digital-to-analog converter. The analog output is then compared to the actual system voltage
level. If the monitored voltage exceeds the limit, the appropriate function within the microprocessor is
initiated.
Digital-To-Analog Converter
The representative binary-coded-decimal (BCD) number derived from the first cycle frequency calculation
is combined with the BCD number from the respective front panel thumbwheel. The microprocessor then
calculates the value for trip, based on these two inputs, and passed the data to the digital-to-analog converter
(DAC). The DAC converts the digital signal from the microprocessor to an analog value which is used as the
reference for trip by the magnitude comparator. When the monitored waveform is at a level that is above the
previously established reference value, the appropriate action is initiated (i.e., instantaneous output
energized, or TIME TRIP or ALARM LED lit and timing initiated).