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Operation and characteristic
2.6.1
Each stage of the UC protection will operate when a current is under a threshold level, as
illustrated in Figure 2.6-1(a). The threshold levels of the UC protection are set by using
settings [UC1] or [UC2]. The operation of the UC protection, however, is blocked as the current
falls below 0.04A of the CT secondary, below which the loss of load cannot be distinguished
from that of a trip operation performed by other protective functions. This discrimination is
introduced by another under current element (UCDO) that is set at 0.04A. The UCDO
characteristic is drawn in Figure 2.6-1(b).
Operating zone
(a) UC1 and UC2
Figure 2.6-1 Element characteristics of UC and UCDO
A definite time delay can be programmed by applying a value of delay time using setting
[TUC1], the user can choose a value between 0 and 300s.
These features are described for the UC1 function, and the UC2 is the same as the UC1.
Miscellaneous functions
2.6.2
Signal of UC trip
(i)
The UC protection will issue a trip command for the trip function, but can be used to signal an
alarm, depending upon the application to which is has been applied, when Alarm is set for
scheme switches [UC1-UseFor] or [UC2-UseFor]. In practice, Trip is set as the default setting.
Blocking UC by CTF
(ii)
If the operation of the UC protection is not required when the failure of current transformer
(CTF) occurs, the user should set Block for scheme switch [UC1-CTFBlk] or [UC2-CTFBlk].
Scheme switch
2.6.3
Figure 2.6-2 shows the scheme logic for UC1. As noted earlier, the operation of UC1 is removed
when the input current falls below 0.04A. A flip-flop circuit and the UCDO element are used
to remove the operation of UC1 in each phase. Scheme switch [UC1-EN] Off is provided when
the operation of UC1 is not required.
Threshold value
0
I
- 109 -
6F2T0207 (0.01)
0.04A
0
I
(b) UCDO
GRE200 (5,6)
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