How to Determine Short Circuit Power of a Utility Point

Er, how about the square of the voltage divided by the loop impedance?

no I am asking if there can be an impedance measurement in the coupling point ? I mean if there can be an alive measurement for determining the impedance of the utility in that point?

Of course. It will be determined by testing.

so I can take a multimeter and measure the impedance seen from the High v voltage transformer secondary ? So I can calculate the short circuit power upstream and then short circuit current downstream ?

Get a qualified Electrician to measure it using some form of loop tester. It should form part of the documentation before the system is energised and at appropriate maintenance intervals thereafter.

Guidance may be found in good ol' British Standard 7671.

The short circuit current at any point in a utility's network is generally known from the load flow and short circuit studies that are performed by that utility. The more complex the grid is near the point in question, the more conservative will be the number given. The engineer looks at the studies and picks the operating conditions that provide the worst case (highest possible) sc current at that point.

The quick assumption is that there is a 1 pu voltage source feeding the combined impedance(s) of the transmission line(s) to the customer connection. This will give the maximum sc contribution (from the utility) that could ever be developed at that point, or 1/Z. However, usually just X is used since R is small and lowers the sc current, R is important for voltage regulation and power factor studies though.

The number you receive may never be reached during normal operating conditions, and if you try to calculate it in real time you will find that it is rare that you will approach it. To ensure that you get the best number from the utility be prepared to provide as much information as possible about the type of loading/equipment that will be on the customer side.

The local utility board will give you "dynamic fault MVA" at their power bus feeding your installation. From there it is just matter of including the loop impedance of your feeding cable and conversion of values to base MVA calculation to get the symmetrical estimated fault level at the point of supply to your installation.

You can takes the transformer nameplate data and assume the utility SC as infinite bus then you have a SC values required to select switchgears and protective devices or else ask to utility or distribution companies for such SC details

Bye

As far I understand, your question is about how the upstream network take a contribution to short circuit on the downstream, am I correct?

Normally, you can got utility contribution from the utitility engineer. As far as I know, it ussually in MVAsc. So, you dont have to measure the utility impedance beacuse it all already include in MVAsc (if you understand with that). If you dont get the MVAsc from the utility engineer, please refer to IEC 60076-5. After you got this MVAsc value, you only have to include that value in your calculation or in your power system study software.

I think each country has it own standard for MVAsc on the network, it depend on grid area in your country. So, it will be more accurate if you ask to the utility engineer.

dynamic fault MVA is important I see. But when you calculate the short circuit downstream you should assume that there is a national grid upstream is not it? So there should be an approximately zero impedance approach till you are not able to predict the grid behind up to some point. But I wonder if it is practical conclusions drawn from the experiences at that region that utility engineers declare this fault MVA or is this source impedance (I refer to thevenin theorem here:)) which is measured by measurement devices whatever, which makes us able to calculate the s.c level by just using this value in the voltage levels relevant to us.? I thank you in advance...

In practise you will not realise infinite sources. Even in networked system, islanding facilities, bus sectionalisers, etc are provided to keep the grid stable. In standard operating conditions, few elements of in feed and out feed are kept on the same bus and this configuration will remain unchanged except under extraordinary conditions like collapse of certain sections of grid etc. Based on set configuration, it is easy to see that source is not limitless and easily convertible to base MVA calculation.This is the basis of dynamic fault MVA figures. It is dynamic value as it will change under different sets of loading and addition / disconnection of active sources. Now why would you really like to know the exact value of fault MVA? At best, the value you get from the utility engineer should be used as a reference and other parameters of the specific installation should also be included along with application of safety factor to select KA rating of your switching and active equipments. In theory I think it should be possible to measure the total utility circuit open loop impedance from your point of supply using a high powered meggar and keeping all the elements of the circuit in place.( experts - please correct me if I am wrong ). But I would not like to be the man holding the probes or operating the test equipment lest some fool in utility chooses the exact moment to feed the grid juice.

Say I have a transformer substation built for a factory , and I have no necessary data given to me to be able to calculate s.c levels when I connect my factory substation to the national grid. What is the proceeding to be followed technically,How can I estimate the system fault power at the point I connect my factory substation to the national grid. am I clear now?

Ahhh....

Now we are confronting the bane of electrical engineering design, where no credible design details are provided but yet a safe and more importantly, very economical installation is expected. .

In typical installations, a 25 KA, 3 Second rated, 11 / 33 KV breaker will be a safe bet. Downstream of the main breaker, the KA rating of other equipments can be determined based on transformer impedence etc. However installations located very close to high capacity power sources, will require a detailed study of system parameters to select or validate higher KA rating of incoming breaker.

OK, you have a substation, I assume with metering. While normally the utility can and will supply the data on short circuit impedance, the smaller countries and even parts of the United States with really small Coop's or utilities don't have the data and the SC impedance can be quite high in many of these cases. Impedance is impedance and can be inferred by measuring the voltage at different loads. That will give you the voltage drop for a given change in amp loading as long as the utility doesn't have a nearby generation, a voltage regulator, or LTC transformer in the circuit. Then calculate back towards infinity for the time at which the voltage drops to zero at the load. That gives an estimate of the short circuit impedance or maximum amps at short circuit. I prefer to think of Short circuit available amps rather than impedance although you get to both at the same time during calculations. I have found that it is easy to get data from a customer on the same power line near the point of a new substation when we were designing substations. They frequently were happy to talk about problems on the power system and many times they also became customers of ours. Sometimes the utility was just plain stubborn and didn't want to give out facts, especially when it divulged how serious the regulation was at that point in the system. We found that many times when we were designing substation and had trouble getting data that we had to put in an LTC transformer to regulate voltage during varied loading. In some cases where the voltage regulation on the incoming line was excessive it was found to violate the requirements of the state regulators. The utility was not about to divulge facts in writing to support that, although we did occasionally get utility personnel to tell us verbally but letting us know they were not to be quoted.

. "That will give you the voltage drop for a given change in amp loading as long as the utility doesn't have a nearby generation, a voltage regulator, or LTC transformer in the circuit. Then calculate back towards infinity for the time at which the voltage drops to zero at the load. That gives an estimate of the short circuit impedance or maximum amps at short circuit"

I have been impressed by this nice explanation . However I could not understand the method you apply here? What do u mean with bolded part? A little bit more explanatory of this bolded part pls....