In case of large currents, unequal transient saturation of CT cores with different
remanence or different saturation factor may result in differences in the secondary
currents from the two CT sets. Unwanted blocking of protection functions during the
transient stage must then be avoided.
Current circuit supervision CCSRDIF (87) must be sensitive and have short operate
time in order to prevent unwanted tripping from fast-acting, sensitive numerical
protections in case of faulty CT secondary circuits.
Open CT circuits creates extremely high voltages in the circuits which
is extremely dangerous for the personell. It can also damage the
insulation and cause new problems.
The application shall, thus, be done with this in consideration,
especially if the protection functions are blocked.
3.11.1.2 Setting guidelines
Current circuit supervision CCSRDIF (87) compares the residual current from a three-
phase set of current transformer cores with the neutral point current on a separate input
taken from another set of cores on the same current transformer.
The minimum operate current, IMinOp, must be set as a minimum to twice the residual
current in the supervised CT circuits under normal service conditions and rated primary
current.
The parameter Pickup_Block is normally set at 150% to block the function during
transient conditions.
The FAIL output is connected to the blocking input of the protection function to be
blocked at faulty CT secondary circuits.
3.11.1.3 Setting parameters
Table 146: CCSRDIF (87) Group settings (basic)
Name
Values (Range) Unit Step Default Description
Operation Disabled
Enabled
- - Disabled Disable/Enable Operation
IBase 1 - 99999 A 1 3000 IBase value for current pickup detectors
IMinOp 5 - 200 %IB 1 20 Minimum operate current differential pickup in
% of IBase
1MRK504116-UUS C Section 3
IED application
573
Application manual
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