In this article, I have presented the effects of utility fault level on low voltage equipment. Assuming an infinite bus as the utility fault level will have a significant cost implications particularly on the power transformer.
To resolve this issue, IEEE (C57.12.00 – 2015) and IEC (60076-5:2006) have presented typical values of short-circuit apparent power of the system to be used in design if no data is available. Please note that IEEE values are higher than the IEC values despite IEC presenting current North American practice, which should have covered IEEE. However, the IEEE standard is more current than its IEC counterpart.
Which value to use? It will all be up to the Electrical Engineer to decide which value of short-circuit apparent power will be used for the analysis.
IEEE Std C57.12.00-2015
Table 14 on short-circuit apparent power of the system to be used unless otherwise specified has been completely revised.
In establishing the fault values shown in Table 14, consideration was given to currently available maximum circuit breaker ratings and existing fault levels determined from user surveys. It is recognized that some systems will present fault levels that exceed these levels. If additional margin is desired, users may specify pre-fault voltage conditions (e.g., 1.05 p.u.) or an infinite bus. For low impedance transformers, specifying an infinite bus may increase the cost of the transformer significantly.
| Maximum system voltage (ANSI C84.1) (kV rms) | Nominal system voltage (kV rms) | System fault capacity (based on maximum system voltage) | |
|---|---|---|---|
| (kA rms) | (MVA) | ||
| Below 48.3 | Below 46 | 63 | – |
| 48.3 | 46 | 63 | 5,270 |
| 72.5 | 69 | 63 | 7,910 |
| 121.0 | 115 | 80 | 16,770 |
| 145.0 | 138 | 80 | 20,090 |
| 169.0 | 161 | 80 | 23,420 |
| 242.0 | 230 | 80 | 33,530 |
| 362.0 | 345 | 80 | 50,160 |
| 550.0 | 500 | 80 | 76,210 |
| 765.0 | 735 | 80 | 106,000 |
| 800.0 | 765 | 80 | 110,850 |
IEC 60076-5:2006
3.2.2.4 The short-circuit apparent power of the system at the transformer location should be specified by the purchaser in his enquiry in order to obtain the value of the symmetrical short-circuit current to be used for the design and tests.
If the short-circuit apparent power of the system is not specified, the values given in Table 2 shall be used.
| Highest voltage for equipment, Um kV | Short-circuit apparent power MVA |
|
|---|---|---|
| Current European practice | Current North American practice | |
| 7.2, 12, 17.5 and 24 | 500 | 500 |
| 36 | 1,000 | 1,500 |
| 52 and 72.5 | 3,000 | 5,000 |
| 100 and 123 | 6,000 | 15,000 |
| 145 and 170 | 10,000 | 15,000 |
| 245 | 20,000 | 25,000 |
| 300 | 30,000 | 30,000 |
| 362 | 35,000 | 35,000 |
| 420 | 40,000 | 40,000 |
| 525 | 60,000 | 60,000 |
| 765 | 83,500 | 83,500 |
| NOTE If not specified, a value between 1 and 3 should be considered for the ratio of zero-sequence to positive-sequence impedance of the system. | ||