Network impedance 1

[prc]

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 )

 

[opmgr1]

No you should adjust base on the existing Voltage. These SC values are on a particular voltage base "nominal" and as such the values must be adjusted to reflect the new voltage value.

 

[7anoter4]

As you know, of course, the rated values are almost never the actual values.
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.

 

[opmgr1]

7anoter4,
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.

 

[prc]

omgr1, clause 6 of IEC is taking factor of 1.1,probably for the highest voltage.But to get realistic values is it not necessary to take actual voltages instead of nominal voltages?

 

[jghrist]

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.

40 kA or 30 GVA are not measurements of impedance. If you calculate the system impedance from the given fault current kA or fault GVA, I think you should use whatever voltage base was used to calculate these values. Then apply a multiplier to the driving voltage used in the fault current analysis as appropriate for the purpose of the analysis.

 

[opmgr1]

prc,
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.

 

[prc]

jghrist,utilities instead of giving network impedance in ohms,normally express the quantities as I mentioned and we have to calculate the impedance from it.That is what I meant.I am also of your view taht we should consider the actual impedance of the grid.But standards mention something different.So I seek expert opinion.
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?

 

[7anoter4]

The calculation method of IEC 60909 determines the sc currents at fault location using the equivalent voltage source, cUn/sqrt(3).
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].

 

[opmgr1]

7another4 hit the nail on the heaad in that in presenting max SC values the max bus voltage must be used, in this case 1.1pu. You can google "Calculation Of The Fault Level Contribution Of Distributed Generation According To IEC Standard 60909" By TH. Boutsika, S. Papathanassiou, N. Drossos. for additional information and explanations.

 

[7anoter4]

This one of a lot as:

http://home.zcu.cz/~tesarova/IP/Proceedings/Proc_2006/Files/54-59 Mester.pdf

Or "Switchgear Manual –ABB" Chapter 3 and other.

 

[prc]

Thank you,7anoter4 &opmgr1.In transformers, we have to calculate current flow at max and min taps to check short circuit withstand capacity. SC current is system voltage at that tap divided by trf impedance plus system impedance. In case of 400 kv trf at 440 Kv and 380 kv.Transformer impedance will be different at max and min taps.But it seems as per IEC, we need not change the network impedance and can take the nominal impedance for calculations.

 

[opmgr1]

No you would not neeed to change the network impedance on the primary side of the transformer. However any SC values at or downstream from the 400kV bus (secondary side of the transformer) must reflect the voltage at which bus is set.

 

[prc]

opmgr1, the tap changer is on 400 Kv for 400 kv variation with constant secondary voltage.ie when system voltage changes to say 440 KV tap changer shifted to tap no1 where transformer impedances are differentfrom nominal tap.As per IEC, it seems, the same net work impedance in ohms shall be used at all taps.To me it is slightly strange.

 

[opmgr1]

Let me see if I get what you are saying. The tap changer is an on-load tap changer with a regulator to maintain the secondary bus voltage at 400kV. If this is the case then the bus voltage will always be close to 1pu and so there would not be a need to change the impedance. In this case the nominal impedance should be used. The previous posts assumed that the tap was an off-load tap changer and that the secondary bus voltage would be manually raised to 1.1pu which in this case the SC values would need to reflect this.

 

[prc]

In this case primary bus voltage is shifting from from 440 to 380 kv and to keep secondary voltage of 220 kV at constant level,OLTC will be operated.Utility spec says primary bus fault level as 40 kA.Then should we use different ohms as network impedance based on bus voltage or go with the nominal value corresponding to rated volatge of 400 kV at max and min taps also.

 

[opmgr1]

Use the nominal ohmic/pu value for the network impedance. The voltage factor c which can be considered as a sort of safety factor should then be applied to compensate for Voltage variations, changing of the transformer taps etc. For the system you described (>35kV) the voltage factor c=1.10 should be used for max. SC current and if you are interested in min. SC current then c=1.00 (nominal) should be used.

I hope this helps.