Examples And Tips; Missing Data Or Other Line Supply Topologies; How Can I Estimate The Rsc If The Line Supply Connection Point Is Unknown; How Can I Make Rough Estimates Only Using The Transformer Data - Siemens SINAMICS S120 Engineering Manual

2 Examples and tips

Examples and tips
2
2.1

Missing data or other line supply topologies

How can I estimate the RSC if the line supply connection point is unknown?

For various applications, it is possible that the data of the line connection point for end
users is not precisely known. This is especially the case for the line short-circuit power
or various cable lengths, which are frequently not known. For the following simplified
calculations, in some cases the determined RSC can be significantly higher than the
real value. As a consequence, an appropriate reserve must always be taken into
consideration. Exceptions are appropriately marked.
Additional possibilities for making an estimate exist:
1. The line short-circuit power is not known.
In the power range of SINAMICS S120 Line Modules Booksize, in European line
supplies/grids, the line short-circuit power is typically very high when compared to
the apparent power of the Line Module; this means the influence of this value on the
relative short-circuit power is low.
Assume a typical value of S''
2. The cable length is unknown.
Unknown cable lengths, especially in the low-voltage line supply, can have a
significant impact on the relative short-circuit power. As a consequence, in these
cases, for the final installation location, a maximum length of the supply cable must
be defined, and this value should be used for the calculation.
If this is not possible, then cable lengths are specified as being 0 m. This means
that the impedances are no longer taken into consideration in the calculations.
3. The transfer point of the power utility company is at the low-voltage level and
therefore there is no data available about the medium-voltage grid. In this case,
insert the short-circuit power in the formulas as before. The length of the medium-
voltage cable should then be inserted as l
voltage remains at the preassigned value, or is simply set higher than zero, as it is
no longer incorporated in the calculation without the cable length. In this case, this
does not result in any inaccuracy in the calculation.
4. There is no additional line transformer after the transfer point in the low-voltage
supply. In this case, enter u
case, this does not result in any inaccuracy in the calculation.

How can I make rough estimates only using the transformer data?

If, in addition to the transformer data, no additional data for determining the line
impedance is available, then the relative line short-circuit power can only be very
roughly estimated.
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