ABB Relion 670 Series Applications Manual page 272

Section 10
Unbalance protection
removed from the service. Otherwise, it can lead to cascading failure of the capacitor units and
severe damage to the capacitor bank in addition to the hazard of explosion or fire.
CurveType : Select the type of time delay to be used for trip signal. It can be set either Definite
time or Programmable .
• If this setting is set as
• If this setting is set as
programmable curve.
tDefTrip : Set the definite delay for trip signal in seconds. The typical value is 5 seconds.
k : Select the time multiplier for programmable curve parameter k.
tACrv : Select the programmable curve parameter A.
tBCrv : Select the programmable curve parameter B.
tCCrv : Select the programmable curve parameter C.
tDCrv : Select the programmable curve parameter D.
tPCrv : Select the programmable curve parameter P.
tMin : Select the minimum time delay to trip output for programmable curve in seconds.
CrvSat : Set the tuning parameter that is used to compensate for the undesired discontinuity
created when the denominator in the equation for the customer programmable curve is equal
to zero. For more information, see
tReset : Select the reset time delay to warning, alarm and trip outputs in seconds.
10.4.4 Setting examples
It is important to know the effect of the failure to set up the neutral voltage unbalance
protection function. In the externally fused, fuseless, or unfused capacitor banks, few elements
are in parallel and many elements are in series. The failed element within the capacitor bank is
short-circuited by the weld that naturally occurs at the point of failure (the element failure due
to short-circuit). This short-circuit results in out of service of the group of elements and
increases the voltage on the remaining groups. Several capacitor elements may breakdown
before the external fuse (if it exists) removes the entire unit.
The following effects can be seen under this condition.
• All the elements or units in parallel with the faulty element or unit are shorted out.
• The total capacitor bank reactance decreases.
• The total capacitor bank current increases.
• The voltage across the healthy series elements or units increases.
• The increased voltage is evenly distributed among the healthy series elements or units.
Internally fused capacitors comprise units with many elements in parallel and individual fused
capacitor elements. The fuse of the faulty elements blows, disconnecting the faulty element
from the other parallel elements (the failed element opens). The risk of successive faults is
minimized because the fuse isolates the faulty element within a few cycles. The degree of
unbalance introduced by an element failure is less than that which occurs with the externally
fused units (since the amount of capacitance removed by blown fuse is less). Hence, a more
sensitive unbalance protection scheme is required when internally fused units are used.
266
Definite time , the definite delay is considered for the trip operation.
Programmable , the delay for the trip operation is based on the
Technical manual .
The timers related to TRIP output start only if the setting
Trip enabled .
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1MRK 511 401-UEN Rev. K
BlockTrip is set to
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Bay control REC670
Application manual