[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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IOP<br />
Operating Region<br />
Slope 2<br />
(SLP2)<br />
2 nd Harmonic<br />
Blocking<br />
4 th Harmonic<br />
Blocking<br />
5HB1<br />
S2<br />
S3<br />
S4<br />
2HB1<br />
S1<br />
87BL1<br />
S5<br />
Slope 1<br />
(SLP1)<br />
60%<br />
DCBL1<br />
FIGURE 3.8.16 Differential-element blocking logic.<br />
O87P=0.3<br />
25%<br />
Restraining Region<br />
(A) IHBL=Y<br />
(B) IHBL=N<br />
IRS1=3<br />
FIGURE 3.8.14 Percentage restraint differential characteristic.<br />
S +<br />
IRT<br />
87R1<br />
87BL1<br />
87R2<br />
87BL2<br />
87R3<br />
87BL3<br />
87R<br />
87R1<br />
87R2<br />
87R3<br />
87BL1<br />
87BL2<br />
87BL3<br />
87R<br />
Min<br />
Max<br />
DCR<br />
_<br />
+<br />
DCBL1<br />
FIGURE 3.8.17 Differential-relay blocking logic.<br />
3.8.7 Differential-Element Performance during Inrush Conditions<br />
Differential Current<br />
S -<br />
0.1<br />
Following is a study of the performance of the differential elements for three field cases of transformer<br />
energization. These cases are special because they cause some of the traditional differential elements to<br />
misoperate.<br />
FIGURE 3.8.15 DC blocking logic.<br />
the maximum of the absolute values of the two one-cycle sums and calculate the dc ratio, DCR, by<br />
dividing the minimum one-cycle sum value by the maximum one-cycle sum value. When DCR is less<br />
than a threshold value of 0.1, the relay issues a blocking signal, DCBL1. Then, the relay blocking condition<br />
is according to Equation 3.8.25.<br />
By defining DCR as the ratio of the minimum to the maximum values of the one-cycle sums, an<br />
accounting is made for differential currents having positive or negative dc-offset components. In addition,<br />
the resulting DCR value is normalized.<br />
Relay tripping requires the fulfillment of Equation 3.8.19 but not Equation 3.8.13 or Equation 3.8.25.<br />
3.8.6.4 Relay Blocking Logic<br />
Figure 3.8.16 depicts the blocking logic of the differential elements. If the even-harmonic restraint is not<br />
in use, switch S1 closes to add even-harmonic blocking to the fifth-harmonic and dc blocking functions.<br />
In this case, the differential elements operate in a blocking-only mode. Switches S2, S3, S4, and S5 permit<br />
enabling or disabling each of the blocking functions. The output (87BL1) of the differential-element<br />
blocking logic asserts when any one of the enabled logic inputs asserts.<br />
Figure 3.8.17 shows the blocking logic of the differential relay. The relay can be set to an independent<br />
blocking mode (IHBL=Y) or a common blocking mode (IHBL=N).<br />
3.8.7.1 Case 1<br />
Figure 3.8.18 shows a transformer energization case while A-phase is faulted and the transformer is not<br />
loaded. The transformer is a three-phase, delta–wye-connected distribution transformer; the CT connections<br />
are wye at both sides of the transformer.<br />
Figure 3.8.19 shows the differential element 1 inrush current; this element uses I AB current. This signal<br />
looks like a typical inrush current. The current signal has low second-harmonic content and high dc<br />
content compared with the fundamental. Another interesting fact is that this signal also has high thirdharmonic<br />
content. Figure 3.8.20 shows the second, third, and fourth harmonic as percentages of fundamental.<br />
Notice that the second harmonic drops below 5%. Figure 3.8.21 shows the dc content as a<br />
percentage of fundamental of the inrush current. The dc content is high during the event; this is useful<br />
information for adding security to the differential relay.<br />
The differential elements operate as follows:<br />
3.8.7.1.1 Second- and Fourth-Harmonic Blocking<br />
The low second- and fourth-harmonic content produces misoperation of the differential element that<br />
uses independent harmonic blocking.<br />
3.8.7.1.2 All-Harmonic Restraint<br />
The harmonic-restraint relay that uses all harmonics maintains its security because of the high thirdharmonic<br />
content of the inrush current.<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC