9328900990 Rev L BE1-951 Input and Output Functions 3-1
SECTION 3 • INPUT AND OUTPUT FUNCTIONS
INTRODUCTION
BE1-951 inputs consist of three-phase current inputs, an optional ground current input, three-phase
voltage inputs, optional auxiliary voltage input, and four contact sensing inputs. Five general-purpose
output contacts and one dedicated, fail-safe alarm output make up the BE1-951 outputs. Each input and
output is isolated and terminated at separate terminal blocks. This section describes the function and
setup of each input and output and provides the equations that the BE1-951 uses for calculating the
power quantities.
POWER SYSTEM INPUTS
Power system inputs as described in the introduction, are sampled 12 times per cycle by the BE1-951.
The BE1-951 measures the voltage and current from these samples and uses those measurements to
calculate other quantities. Frequency is measured from a zero crossing detector. Measured inputs are
then recorded every quarter cycle. If the applied voltage is greater than ten volts, the BE1-951 measures
the frequency and varies the sampling rate to maintain 12 samples per cycle. Frequency compensation
applies to all power system measurements. Power system inputs are broken down in the following
paragraphs: Current Measurement, Voltage Measurement, Frequency Measurement, Power
Measurement, and Measurement Functions Setup.
Current Measurement
Secondary current from power system equipment CTs is applied to current transformers inside the relay.
These internal transformers provide isolation and step down the monitored current to levels compatible
with relay circuitry. Secondary current from each internal CT is converted to a voltage signal and then
filtered by an analog, low-pass, anti-aliasing filter.
Current Measurement Functions
Input waveforms are sampled by an analog-to-digital converter (ADC) at 12 samples per cycle. The relay
extracts the magnitude and angle of the fundamental components of each three-phase current input and
the magnitude and angle of the optional independent ground current input.
Neutral and Negative-Sequence Current
Neutral and negative-sequence components are measured from the fundamental component of the three-
phase current inputs. The relay can be set to accommodate ABC or ACB phase-sequence when
calculating the negative-sequence component.
Fast-Dropout Current Detector
A separate, fast-dropout current measurement algorithm is used by the breaker failure function and the
breaker trip-speed monitoring function. This measurement algorithm has a sensitivity of 10 percent of
nominal rating and detects current interruption in the circuit breaker much more quickly than the regular
current measurement functions. This measurement algorithm only monitors phase current.
Voltage Measurement
Three-Phase voltage inputs are reduced to internal signal levels by a precision resistor divider network. If
the relay is set for single-phase or four-wire VT operation, the measuring elements are configured in wye.
If the relay is set for three-wire VT operation, the measuring elements are configured in delta.
Voltage Measurement Functions
Input waveforms are sampled by an analog-to-digital converter (ADC) at 12 samples per cycle. The relay
extracts the magnitude and angle of the fundamental components of each three-phase voltage input and
the magnitude of the optional auxiliary voltage input.
VT Connections
When four-wire VT connections are used, the relay measures the AN, BN, and CN voltages and
calculates the phase voltage quantities. Overvoltage and undervoltage functions (27/59) can be set to
operate on either the phase-to-neutral (PN) or phase-to-phase (PP) quantities. Three-wire VT
connections limit 27/59 operation to PP quantities. When single-phase VT connections are used, the
27/59 elements operate as appropriate for the single-phase voltage applied.
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