c-factor as per IEC 60909 standard for a large industrial power network

[krisys]

[COLOR=]It is a large industrial power network which has a total power generation capacity of about 140 MW and connected to Utility through two nos. of 132/34.5 kV, 105/140 MVA transforms. These transformers have on load tap changers (OLTC).

Within the power network of the industrial complex, there are many 33/11.5 kV power transformers. All are having the on load tap changer to maintain the transformer secondary side voltage at 11 kV.

Presently the network is getting slightly modified (upgraded) and as part of this project a power system study is being carried out by the contractor. The short circuit study is being done using IEC 60909.
As per the standard it is recommended to apply the c-factor for the pre-fault voltage in order to obviate the higher network voltage, unknown tap position etc.

But the contractor is using the latest power system study software for the study. He has modeled the system and run the load flow study and set the transformer tap position in order to have the more realistic operating scenario. Now the question is, in spite of setting the transformer tap for the full load condition, (OLTCs are available to maintain the correct bus voltage at 11 kV and 33 kV), do we still need to apply the c-factor for the short circuit calculation?

In my view, the IEC 60909 is a more generic standard, which assumes the general power distribution network, ignoring the system loading conditions, transformer tap position etc. Where as in our case, the network modelling is very specific. Hence the use of c-factor may not be necessary in our case. Any view?

Note:
In our case, the evaluated fault level at one of the 11 kV switchgears is marginally exceeding the rated value.

 

[7anoter4]

In my opinion, you may use the actual equipment-generators, transformers, cables and overhead lines-data, but you don't know the System Network still. So you have to use-according to table 1 IEC 60909-0 c=1.1 for maximum short-circuit current and c=1 for minimum.
IEC 60909-0 ch.3.2 Network feeders states:
"If a three-phase short circuit in accordance with figure 6a is fed from a network in which only the initial symmetrical short-circuit current l”kQ at the feeder connection point Q is known, then the equivalent impedance ZQ of the network (positive-sequence short-circuit impedance) at the feeder connection point Q should be determined by:"
ZQ=c*UnQ/sqrt(3)/I"kQ
If the utility could state for the network system the maximum and the minimum supply voltage and the system short-circuit impedance
in the point Q then you would not need to use c.

 

[ppedUK]

If you are assessing switchboard fault ratings, I would always err on the side of caution and apply the Cmax factor. If you are in trouble with one of your switchboard fault ratings using the voltages from the load flow study, then you would more than likely fail using the C factor method. How can you be sure that the utility won't increase their fault levels by different system configs if you are that close? Can you claim to be calculating to IEC60909 if you don't use the C factors??