Network impedance
Network impedance
(OP)
During short circuit current calculations, we have to estimate the network impedance.Normally it is given as say 40 KA or 30GVA for 400 kV system.But feeder voltage can vary 440 to 360 KV on max and min taps.Should we consider the network impedance as calculated at nominal voltage for calculations at other voltages also?or should we alter it taking in to actual feeder voltage .As per IEC 60909-0,it seems we have to consider only nominal value.( formula no6 in above IEC )






RE: Network impedance
RE: Network impedance
The actual values are oscillating from a minimum to a maximum every hour and from location to location of the system. The rated short circuit current [or power] it is the maximum possible for rated voltage and at a certain point]. According to IEC 60909 Unq =Nominal system voltage. Then for 400 kV system Unq=400 kV.
RE: Network impedance
I would agree with you if the system is at normal tap (1pu) whether there is fluctuations or not. However at 1.1pu or 0.9pu the SC values at the bus will be different. At 1.1pu the values will be greater than that at nominal and the converse holds true for the system at 0.9pu.
RE: Network impedance
RE: Network impedance
RE: Network impedance
if you set your bus voltage at 1.1pu for what ever the reason this becomes your new nominal voltage value. If it is a situation where the bus voltage is set at 1.1pu and due to loading it falls to say 1.05pu then in presenting your SC values you would use the value at which the bus voltage was set at, which is 1.1pu not 1.05pu. Note if you can raise the bus voltage to 1.1pu, then this should be used to derive the max. SC value at that bus.
RE: Network impedance
opmgr1.My application is estimation of Sc current in trfs.With tapping position in transformer,should I change the reference voltage for calculating the system impedance? esp after referring above IEC?
RE: Network impedance
The term initial symmetrical short-circuit power, S′′k, i.e. the fault level, stands for a fictitious value determined as the product of the initial symmetrical sc current I′′k and the nominal system voltage Un at the short-circuit location.
S"k=sqrt(3)*I"k*Un
The calculation of the short-circuit impedance is in general based on the rated data of the electrical equipment and the topological arrangement of the system and has the advantage of being possible both for existing systems and for systems at the planning stage.
For system in-feed ZQ=c*UnQ/sqrt(3)/I"kQ according to IEC 60909.
We take c=cmax=1.1 for ZQ calculation.
In general, two short-circuit currents, which differ in their magnitude, are to be calculated:
The maximum short-circuit current which determines the capacity or rating of electrical equipment [cmax=1.1 for Un>1 kV]; and
the minimum short-circuit current which can be a basis, for example, for the selection of fuses, for the setting of protective devices, and for checking the run-up of motors[cmin=1 for Un>1 kV].
RE: Network impedance
RE: Network impedance
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Or "Switchgear Manual –ABB" Chapter 3 and other.
RE: Network impedance
RE: Network impedance
RE: Network impedance
RE: Network impedance
RE: Network impedance
RE: Network impedance
I hope this helps.