| 7.4 |
Cautions in CT Selection
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While choosing a CT for a particular application, it is necessary to observe following precautions. |
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The CT rating and continuous load current should match. For example, if maximum load current is 90A, a 100:5 CT may |
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be acceptable but 50:5 is not acceptable. |
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The maximum fault current should be less than 20 times the CT rated current. For example, 100:5 CT can be used, so |
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long as burden on the CT is within the rated values and maximum primary fault current is below 2000A. |
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The voltage rating of CT should be compatible. For example, 100:5 C100 would give linear response, upto 20 times |
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rated current provided CT burden is kept below(100/20 x 5 = 1 ). With 2 burden, this CT can be used only if maximum current is limited to 1000A. |
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Paralleling of CT's e.g. in differential protection, or with SLG fault can create significant errors in CT performance. One |
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should generally ascertain that magnetizing current is kept much below the pick up value. |
| 7.4.1 |
Exercise Problems |
| 1. |
If the current ratio is adequate for a protection, but CT burden is high; then the performance of CT may deteriorate due to |
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large magnetizing current and/or saturation problem (see fig 7.9). The CT performance can be improved by connecting the CT's in series (see fig 7.10). |
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Show the dotted terminals for correct secondary series connection in fig 7.10. |
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What is the VA of CT in fig 7.9 and 7.10 respectively? |
| 2. |
Mark the following statements as true or false: |
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Electromechanical relays tend to saturate at high currents. This reduces the relay burden on CT, and so that the CT |
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performance at moderately high currents may be considered better than at relay's rated burden at 5A. |
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Use of instantaneous over current relays has the potential to overcome this problem of saturation of CT's. |
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Differential protection can operate on external faults due to the unequal saturation of CT's. |
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