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 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 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 three-phase to neutral
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.
