Fault Current on Generators

[AppleJaxJap]

I have a generator for a project. in this example it is a 2000KW/2500KVA .8 PF and 480V so the full load amps @ 80% PF is 3007A

However, if this same generator was running at unity power factor then the most amps that I could get from it is 2406A.

The X”d for this unit is let’s say 10%

This unit has an HRG and there are only 3 wires no neutral. so only LLL and LL faults need to be considered That gives us a 3P fault current of 30,070A

My question:
A bolted fault is just a low resistive path why do I need to consider power factor on the faulted path?
In my way of thinking the bolted fault should happen at unity power factor and the available FC should be 24,060A and not 30,070A.
No literature shows this so I realize I must be missing something. What is it?

 

[dogzzz]

What you have told may be true for LV system (480 V) , but certainly not the case with MV .

Who trips the breaker in first half cycle (10 milli seconds) , or in one cycle 20ms (50 Hz).
Also it is mechcanically not possible because the breaker takes about 50 to 60ms to open.
Thus a breaker with breaking capacity = AC asymmetrical peak current is not required ( please read IEC 62271-100)

So , the question is how much breaking current should be considered ?
As per IEC 62271-100 , mechanical time delay for breaker to open (in milli second) + 1/2 cycle( in milli seconds) is calculated .
For ex. breaker opening time is 60 ms and then half cycle shall be 10ms , thus time delay comes out to be 70ms .
At 70ms lets say the DC component became 61% of the maximum value , thus accordingly the breaking capacity of the breaker will change .

Now talking about Making current .

As I told you that , breaker in most of the cases have to withstand the first Short Circuit cycle .
Thus , in this case , this short circuit forces will produce repulsive forces on the breaker parts/contacts ,
which for sure will reduce the life of the breaker and may be possible breaker may get damage .
Thus , breaker should not get deformed/buckled ( dont know correct english) till the fault gets cleared .
This capacity of breaker to withstand the repulsive forces is called Making Capacity and is highest at highest current which is peak asymmetric current .

Thus , while buying a breaker the breaking and making capacity should be taken into account.

 

[waross]

The original question concerned a 480 Volt generator.
Once more:
The rated symmetrical breaking capacity of a circuit breaker is not the actual capability of the breaker. The breaker is tested with a current that includes the asymmetrical component of the symmetrical current.
You are making something quite complicated that is very simple.
Step one: Determine the CORRECT rated current. In the original example 3007 Amps. Nameplate KVA over Nameplate voltage.
Step two: Determine the Available Short Circuit Current. In the original example 30070 Amps. Current from step 1 over nameplate PU impedance.
Step three: Select a breaker rated to interrupt the calculated Available Short Circuit Current. This breaker has been tested and rated for the worst case fault current associated with the rated symmetrical current at normal X/R ratios.
Reality check; If you breaker rating is close to the Available Short Circuit Current and your X/R ratio is high,
Then either use a breaker with the next highest rating,
or
Determine the X/R ratio that breakers are tested at and the actual test current and re-evaluate your choice of interrupting rating.
Hint; Search this site. The topic of breaker testing and the X/R ratio used in testing has been discussed before. The information is here somewhere.
The short circuit mechanical forces on internal breaker parts are present whether the breaker is making or breaking fault current.
Issues of making capacity versus breaking capacity and limits on the number of time that a given breaker may safely interrupt a fault current are issues for Medium voltages and High voltages.
The original question concerned a low voltage generator.
If you want to discuss breaker ratings at medium voltage and high voltage, We have no problem with that.
However it is getting off topic.
Feel free to start another thread on the topic of medium voltage making capacity of circuit breakers.
Hint; You will get better responses if your questions are related to the thread title.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dogzzz]

I see you point , thanks and lets close this !!