SUBTRANSIENT REACTANCE - GENERATORS 2

[basiliou]

Hi to all

I'm kind of involved in power generation , and i have a question to make about the X"d characteristic of a generator. Reference generator is 3 phase - 50 Hz output - 400 Volts AC , Star connection with grounded neutral.

I'm lloking for more information about the X"d value.
manufacturer indicates that this value in mu generator is 0.14 (in p.u system)
That means that the Subtransient current in case of a 3phase shortcircuit will be 1/0.14 = 7.1 times (p.u) of the nominal current.
BUT , my customer is asking for a X"d value lower than 0.12
That means a bigger subtransient curren will occur in case of a failure.
I cannot understand finally if the X"d value of a generator should be small enough or big enough.

I suppose that in case of a failure (short circuit) we would like to absorb subtransient current quickly as well as to limit this current as low as possible.
Why then they ask for lower X"d values ?

Best Regards

George Vasiliou

 

[rbulsara]

Your understanding is correct about calculating the short circuit currents.

Some generators with winding pitch desinged to lessen or absorb harmonics end up with lower X" and it does increaese SCC. Only your customer can answer as to why he/she wants lower X".

Only thing you can do is calculate SCC and see it its within the rating of the switchgear etc and inform the customer. Account for parallel sources.

Some customers may want to match the data of an existing generator is they are trying to match the unit for parallel operation.

I do not see any benifit of lower than nominal X" which is somewhere between 0.13 to 0.20.

 

[mpparent]

Lower Xd'' would make the generator source stiffer. As rbulsara said, make sure you check your bracing down stream, and take into account whether or not the system is a "closed" transition system or an "open" one.

Mike

 

[basiliou]

Only one thing i have to add here , the specific customer is a telecomunication company , and they pay huge attention to the harmonic levels of the generator.
By the way the winding pitch of the generatos is 3/4 .

Is actually X"d affecting directly the harmonics level produced by my generator ?

I'm trying to figure out a reasonable answer for why they need lower X"d values , taking as a fact that only telecomunication companies have this low x" requirement (their loads are inverters , converters , and other similar telecomunication equipment)

Regards


George

 

[AusLee]

Hi,

I read in a Schneider Electric guide the following:

1. Installation:
One relatively small generator
One Transformer
One ATS between the two feeding the input breaker in the MDB

2. The problem:
They say that if a short circuit appears on the LV side while supplied from the TR, then the 250 or 500MVA of the MV ring will incurr so much current that the input breaker in the MDB will trip instantaneously.

However, if the short circuit happen when the load is supplied from the generator, a large, but not sufficiently large current will flow in the the breaker and sollicitate the equipment causing a lot of strain in the MDB before the breaker trips.

The solution in this case is to have a generator supply immediately a large short circuit current for setting of the MDB input breaker to work the same way when fed either from the TR or the generator.

"Immediately" means a low x"d. The short-circuit after 100ms is governed by x'd and the permanent short-circuit is gien by the synchroneous reactance.

I might have got what i read worng, so please correct me to understand it better. But if that was correct, i think this problem can also be solved by an electronic trip unit in the incoming breaker, no?

 

[jghrist]

Most nonlinear loads can be treated as constant current sources for harmonics. The harmonic voltage that results from serving a constant current harmonic source is the current times the source impedance. The impedance to harmonic currents for a generator is X"d. Thus, reducing X"d will reduce harmonic voltages for a given nonlinear load.