Basler BE1-CDS220, Instruction Manual

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1-6 General Information BE1-CDS220
PRIMARY APPLICATIONS
The BE1-CDS Current Differential System provides percentage restrained differential protection along
with multiple overcurrent elements and is intended for use in any low impedance current differential
protection application including transformer, generator, motor, reactor, and bus protection. Its unique
capabilities make it ideally suited for applications with the following requirements.
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Applications that require low burden to extend the linear range of CTs.
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Applications where dedicated CTs for the differential are not available. Unlike traditional
differential relays, dedicated CT circuits are not required because each CT input is isolated from
the others and phase shift compensation can be accomplished internally.
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Applications that require high accuracy across a wide frequency range such as motor, generator,
and generator step-up transformer protection or in cogeneration facilities.
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Applications that require the flexibility provided by wide settings ranges, multiple setting groups,
and multiple coordination curves in one unit.
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Applications that require the economy and space savings provided by a multifunction, multiphase
unit. This one unit can provide all of the protection, as well as, the local and remote indication,
metering, and control required on a typical circuit.
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Applications that require harmonic restraint to aid security for the differential.
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Applications that require communication capability and protocol support.
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Applications where the optional case configurations facilitate modernizing protection and control
systems in existing substations.
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Applications where the capabilities of a digital multifunction relay are required, yet drawout
construction is also desirable.
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Applications where bus protection is provided by a high speed overcurrent blocking scheme on
the transformer bus mains instead of dedicated bus differential circuit.
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Applications where the capabilities of intelligent electronic devices (IEDs) are used to decrease
relay, and equipment maintenance costs.
DIFFERENTIAL PROTECTION APPLICATION CONSIDERATIONS
The principle of current differential relaying is simple in concept. Measure the current flowing into the
protected zone and the current flowing out of the protected zone. These should match exactly (sum to
zero). If they do not, there is a fault within the protected zone. The mismatch in current that results from
the instantaneous summation of the currents into and out of the zone of protection is called the differential
current or the operate current. While the concept is simple, differential protection has several difficulties
which present challenges to the application of this type of protection. The BE1-CDS Current Differential
System provides several features that allow it to easily address these complications and enable it to be
used in all differential applications including transformer protection.
Problem 1: False Differential Current Due to Poor CT Performance
General
The principle of current differential relaying requires accurate measurement of the currents entering and
exiting the zone of protection. During fault conditions where high current is flowing through the zone of
protection, a CT may saturate and not faithfully reproduce the current flowing in the primary system. This
will cause a false differential current to be seen by the differential relay.
BE1-CDS Solution:
The percentage restrained differential characteristic, shown in Figure 1-1, is applied in this application.
The differential current required to cause a trip is a percentage of the restraint current. The restraint
current is a measure of the current flowing into or through the zone of protection. Thus for higher levels of
restraint current, where the CTs may be subject to saturation, higher levels of differential current must be
seen to cause a trip. The percentage restraint is often called the slope characteristic.