This application is greatly simplified with the BE1-CDS240. The user can connect all CTs in wye and
specify that the delta transformer winding has a ground source. The BE1-CDS240 will apply delta
compensation to the wye winding to obtain phase shift and zero-sequence compensation for that current
input. For the delta winding with ground source, it will numerically remove the zero-sequence components
from that current input just as would traditionally be done with a zero-sequence trap. The setup
parameters for each of the current input circuits are described in Section 3,
Input and Output Functions,
Power System Inputs
. The parameters specifying CT connection, transformer connection and ground
source for each current circuit determines the angle and zero-sequence compensation used on each
current input.
Problem 6: Transformer Energization Inrush and Overexcitation
NOTE
The BE1-CDS240 relay does not automatically remove the zero-sequence
components from all currents presented to the differential protection functions as
that can unnecessarily reduce the relay's sensitivity to internal ground faults by
33%. Also, since the zero-sequence components are removed from all phases, it
can result in confusion during single-phase testing by introducing differential
current in phases not under test.
General
When the zone of protection includes a transformer, excitation and energization inrush currents appear as
differential current because they flow into the zone of protection and not back out again. Normally,
transformer excitation losses are small. However, when the transformer is overexcited or upon initial
energization or sympathetic inrush conditions, they can cause sensitive differential protection to operate.
BE1-CDS240 Solution
Overexcitation and inrush are nonlinear and the currents have high harmonic content. Inrush is
characterized by high, even harmonics such as the second harmonic. Overexcitation is characterized by
high, fifth harmonics. The BE1-CDS240 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 thus 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.
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BE1-CDS240 General Information
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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 that can introduce magnitude and angle
errors to the measurement.
Summary of Contents for BE1-CDS240
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Page 292: ...ii BE1 CDS240 Human Machine Interface 9365200990 Rev F This page intentionally left blank ...
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Page 349: ...Figure 12 5 Horizontal Rack Mount Front View 9365200990 Rev F BE1 CDS240 Installation 12 5 ...
Page 361: ...Figure 12 17 Typical DC Connection Diagrams 9365200990 Rev F BE1 CDS240 Installation 12 17 ...
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