Basler Electric BE1-851 - Manual (Page 43)

SECTION 3 • INPUT AND OUTPUT FUNCTIONS
General
The BE1-851 inputs consist of three-phase current inputs, neutral current inputs, and four contact sensing
inputs. Five general-purpose output contacts and one dedicated, fail-safe alarm output make up the BE1-
851 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.
Current Inputs
Secondary current from power system equipment CT is applied to current transformers inside the relay.
These internal transformers step down the monitored current to levels compatible with relay circuitry and
provide isolation. Secondary current from each internal CT is converted to a voltage signal and then
filtered by an analog, low-pass, anti-aliasing harmonic filter.
Current Measurement Functions
The power system analog quantities for phase, neutral, and negative-sequence currents are calculated
and used by all of the current dependent functions of the relay. There is no separate positive or zero-
sequence value calculated. The filter response for phase and neutral calculations can be independently
programmed. Operation of the current measurement function is governed by settings for nominal
frequency, digital signal processing algorithm, current transformer ratio, and normal phase rotation.
Nominal Frequency
Input waveforms are sampled by an analog-to-digital converter at 24 samples per cycle. A nominal
frequency of either 50 or 60 Hz must be selected in order for the analog-to-digital converter to sample
analog quantities at appropriate time intervals to achieve 24 samples per cycle.
Digital Signal Processing
The digital signal processing (DSP) setting governs how the phase and neutral operating quantities are
measured. The negative-sequence current is derived from the phase currents and is not independently
settable. The three choices are Fundamental, RMS, and Average. This is independently settable for the
phase and neutral quantities. Each setting causes the relay to respond differently in the presence of
significant harmonics and for operation at significantly off-nominal frequency. Accuracy characteristics for
each algorithm (Fundamental, RMS, and Average) are shown in Figure 3-1. This figure is for a 60 Hz
nominal system with frequencies between 55 and 65 Hz. A 50 Hz nominal system would have similar
characteristics.
The fundamental setting (F) uses a Fourier filter to extract the fundamental frequency component of the
measured current and reject the harmonic frequency components. This setting is best suited for most
protection purposes due to its superior transient overreach and fast dropout characteristics. It is also
recommended for applications where harmonic rejection is desired. For example, in a neutral circuit
where the third harmonic component is additive and can result in unwanted tripping.
The RMS setting uses a true RMS calculation to include harmonic components of the measured current.
The presence of significant levels of harmonics can cause heating in protected equipment and increased
sensitivity in electro-mechanical devices. This setting is recommended for equipment applications that
require thermal overload protection. It is also recommended for applications where the transient
overreach and sensitivity characteristics provide better coordination with induction disk type overcurrent
protection systems.
9289900990 Rev R BE1-851 Input and Output Functions 3-1