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BE1-CDS240 Input and Output Functions
3-1
SECTION 3 • INPUT AND OUTPUT FUNCTIONS
GENERAL
BE1-CDS240 relays have four sets of three-phase current measurement inputs, an ground current
measurement input, a three phase voltage measurement input, up to twelve contact sensing
(programmable) inputs, four trip coil monitor inputs, up to fourteen general purpose output contacts, and
one dedicated, fail-safe alarm output contact. Each input and output is isolated and brought out to
separate terminal blocks. This section describes the functioning and setup of these inputs and outputs.
POWER SYSTEM INPUTS
Power system inputs as described in the introduction, are sampled 144 times per cycle by the BE1-
CDS240. The BE1-CDS240 measures the voltage and current from these samples and uses those
measurements to calculate other quantities. Of the 144 samples, 24 samples per cycle are used to find
fundamental, 2
nd
, and 5
th
harmonic components. Frequency is measured from zero crossing detectors on
both the voltage and current inputs (details below). Measured inputs are then recorded every quarter
cycle. The BE1-CDS240 measures the frequency and varies the sampling rate to maintain 144 samples
per cycle. Frequency compensation applies to all power system measurements. Power system inputs are
broken down in the following paragraphs into Current Measurement, Voltage Measurement, Frequency
Measurement, and Measurement Functions Setup. After the power system inputs are discussed, the
paragraphs on Power Calculations discuss calculating power quantities.
Current Measurements
Currents from the secondary of CTs in power system equipment are applied to internal current
transformer primaries. The internal current transformers provide isolation from the monitored line currents
and step the currents down to internal circuit levels. Secondary current from each internal CT is converted
to a voltage signal, and filtered by an analog, low-pass, anti-aliasing filter.
Current Measurement Functions
The input waveforms are sampled by an analog-to-digital converter (ADC) at 144 samples per cycle and
digitally filtered in a DSP (digital signal processor) using an FIR (finite impulse response) anti-aliasing
filter algorithm. This high sampling rate allows the analog filter to have a high cutoff point, which virtually
eliminates errors from the analog filter elements. The output from the digital filter is sample data reduced
to 24 samples per cycle. From the digitally filtered samples, the relay extracts the magnitude and angle of
the fundamental, second harmonic, and fifth harmonic components of each three-phase input and the
fundamental of the optional independent ground current input.
Compensated Restraint and Operate Currents for Differential Protection
As described in Section 1,
General Information
, it is necessary to provide phase angle, zero-sequence,
and tap compensation to the measured currents prior to their use by the differential protection. The
current measurement function calculates compensated currents for restraint and operate currents and for
second and fifth harmonic operate currents. Tap compensation setup is covered in Section 4,
Protection
and Control, Differential Protection, 87 Differential Function
.
Neutral and Negative-Sequence Current Measurement
The BE1-CDS240 relay also calculates the neutral and negative-sequence components from the
fundamental component of each of the three-phase current inputs. The relay can be set to accommodate
either ABC or ACB phase-sequence in calculating the negative-sequence component.
Fast-Dropout Current Detector
The relay includes a separate, fast-dropout current measurement algorithm for use by the breaker failure
function and the breaker trip speed monitoring function. This measurement algorithm has a sensitivity of
10% nominal rating and detects the interruption of current in the circuit breaker in less than a ½ cycle.
Virtual Circuit Measurement Functions
The BE1-CDS240 provides two sets of “virtual circuits.” These circuits are formed by computing the
phasor sum of the actual analog current inputs. (
Note:
The current circuits being summed
must
have the
same CT ratio.) The virtual circuits are user configurable to sum either two or three of the analog inputs.
For each virtual circuit, the BE1-CDS240 calculates the summation and provides a sum for A, B, C phase
currents, negative-sequence current, and neutral current. The virtual currents can be used throughout the
Summary of Contents for BE1-CDS240
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