Basler BE1-CDS220, Instruction Manual

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1-14 General Information BE1-CDS220
high, fifth harmonics. The BE1-CDS220 calculates the magnitude of the second and fifth harmonic
components of the differential current. If the differential current is made up of a significant proportion of
either of these two components, the user can select to inhibit the sensitive percentage restrained
protection from operating.
Since CT saturation can also cause the currents seen by the relay to be rich in harmonics, an
unrestrained instantaneous differential element is included to provide dependability for an internal fault
with CT saturation.
During inrush, the second harmonic component may not be equally shared on all three phases. Because
of this, misoperation can occur on a phase with low second harmonic content. Our unique method of
second harmonic sharing improves security by allowing the harmonic restraint elements to respond to the
ratio of operate current to the sum of harmonic current measured on all three phases. This is superior to
other methods of cross blocking since each phase element operates independently in its comparison of
operating current to harmonic current. Thus, security is enhanced without sacrificing dependability
because a faulted phase will not be restrained by inrush on unfaulted phases.
To further enhance security from false tripping on inrush, the operating characteristic responds only to the
fundamental component of this highly distorted current, reducing sensitivity to inrush current yet allowing
improved sensitivity to power system faults.
Problem 7: Digital Measurement Errors
General
Numerical relays sample the current and use digital signal processing algorithms to extract magnitude
and angle information to develop their operating quantities. Most methods used are accurate only when
the sampled power system quantity is operating at nominal frequency. Unless steps are taken to
compensate, accuracy falls off very quickly with deviations in the power system frequency.
Another problem that must be overcome with digital technology is the need to use anti-aliasing filters prior
to the sampling process to prevent harmonic components from affecting the measurement. Analog filters
introduce phase shift errors and are subject to attenuation drift which can introduce magnitude and angle
errors to the measurement.
These problems are more acute for differential protection since the operating quantity (differential current)
is derived from the difference between the measured quantities. Any error in magnitude or angle
measurement can result in large inaccuracies in the differential current measurement.
BE1-CDS220 Solution
Digital relay designers must decide on which of three solutions to use for this problem. They can allow
their relays to misoperate at off-nominal frequency operation. Or, disable the protection at off-nominal
frequency. Or, compensate to maintain accuracy at off-nominal frequency. The BE1-CDS220 uses
frequency tracking to adjust the sampling interval to maintain full accuracy across a wide frequency range
so that it is both secure and dependable in all applications. For example, tripping of important
transformers during a disturbance that causes the system to go unstable can have a catastrophic affect
on an already over-stressed power system. Generator and motor differential protection applications are
another situation where accuracy across a wide frequency range is important.
To eliminate the errors introduced by analog low pass filters, the BE1-CDS220 uses digital signal
processing technology and 144 samples per cycle over-sampling to provide digital low-pass filtering.
MODEL AND STYLE NUMBER DESCRIPTION
General
The BE1-CDS220 Relay electrical characteristics and operational features are defined by a combination
of letters and numbers that make up the style number. The model number, together with the style
number, describe the options included in a specific device, and appear in the clear window on the front
panel and on a sticker located inside the case. Upon receipt of a relay, be sure to check the style number
against the requisition and the packing list to ensure that they agree.