Inrush Current Issue

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From your experience, if inrush current occur how big is the current effect on system and set ? and how to solve ?

Why you want to connect 4 sets of 1250kvA transformers in parallel to one 5000kVA transformer? Is that your homework or imagination?

How you can start a project where you cannot use correctly the transformer terminologies?

Hi Hien,

4 x 1250 sets are parallel with others and main outgoing sets breaker connected to MV trafo (detail above), in case of customer out of production they just need to run 1 set to cover light and data center. With do respect on you, I did not imagine or doing home work. If you have an experience or deep analyze you could share. Im only technician, have not good knowledge as you and other. But I'd like to learn from you the expert. OK

Thanks 4 ur respond anyway

Sorry, I still cannot understand this application.

"4 x 1250 sets are parallel with others and main outgoing sets breaker connected to MV trafo." Does trafo mean transformer? I still don't understand why you connect 4 in parallel then their outgoing is connected to one 5000 kVA transformer? It means you have 5 transformers (1 connects in series with 4)?

So, you will have 4 separated incoming breakers for those transformer?

Thank you if you could enlighten me.

The purpose of transformer parallel connection, is to increase the loading capacity while replacing the existing transformer is impossible.

Operating-in-parallel means that these transformer shall operate at the same time. With one breaker at the outgoing, you will not be able to eliminate one or two transformer to save the operation cost? You kow why?

Sorry this application is very clear: 4no. 1250 KVA generators with individual outputs of 380 volts are connected in parallel and the output from the load transfer breaker (switch) is fed to a 5MVA MV step up trafo (transformer) with primary input of 380V and secondary output of 4.1KV .As you can see, there is only one trafo and four gen-sets NOT 5 transformers!

Hi Roty.

If you stop and look at the diagram you will understand it is not clear at all. For example there are 3 gensets, not 4.

The secondary output is 20kV not 4.1kV.

The ATS is across the 20kV side thus RAM, Hien and I suspect this is where the issue is.

Are the "4 sets 4 x 1250 kVA/380VAC" in parallel transformers or generators?

At what point do you want to know the inrush current?

Thank you for your cue, Ram. This now comes out as an interesting question.

So, the customer wants to save fuel of gensets when there are only light loads switched on.

I don't know much about how the inrush current will affect to genset in this application but to mitigate the inrush current in this case, I think, we should switch on one breaker to this 5MVA transformer prior to starting up genset. This operation will reduce dramatically the inrush current since the genset takes time to build up the voltage.

The nature of the loads creating the inrush current is the real problem. Not only does the type of motor (synchronous, induction) matter so does the nature of the load being driven (high torque, low torque, pulsing, non-linear, etc.) along with any speed control, starting control, etc. Closing the breaker and waiting for the generator to build up voltage can cause problems like motors going subsynchronous and never reaching full speed and/or drawing excessive currents during speed ramp up.

Best bet would be to bring the diesel up to speed and voltage then close the breaker, letting the governor and AVR correct for the inrush.

Hi RAM,

Yes, the load will be randomly. I ve experience like this case too but haven't solve yet and now facing same project. From last of my experience, we've manage the load, so when black start happen, the sets not suddenly take all the load but arrange sequentially but then this way wasn't solve the problem, the main breaker at outgoing trafo keep tripping

Hi All,

Thanks for your responds, here I provide you snapshot of single diagram.

Thanks! I assume that you have some form of synchronization and interconnected speed and voltage control to ensure proper load balancing and var control as well.

As far as the breaker tripping I suspect one of your loads is an across the line synchronous motor driving a high starting torque load whose running kw is more than 25-50% of a single diesel's rating. Please let us know.

Ram, it should be asynchronous rather then synchronous, am I correct?

For the main outgoing breaker tripping, when you bring the generator online, it should be sure that the phase is synchronized with each other in term of frequency and phase and the main incoming switchgear to tranfo shall be the appropriate one which can handle this out-of-phase current.

For the SLD, looking for the genset side, there is small issue that the configuration should be 0.4/29kV instead.

Hien, We're talking two different things (types of motors vs. how to connect to a grid) but you bring up an interesting point which OP didn't mention, that there is an external connection on the other side of the transformer. Unfortunately I could not read the fine print on the SLD. Using the recent comments from others it appears that there is a connection to another source of generation. Judging from what I've read so far it appears that there is no attempt at synchronizing the local generation to the utility, just close the generator bus breaker and of course watch the transformer trip out on the huge currents from out of phase/mismatched voltage synchronization. Op is one lucky guy that the only thing that happened was the breaker tripped.

Hi Loadshare,

So we all understand this better, can you show us the diagram of the ATS please.

It looks like the ATS has PLN (Indonesian Power) incoming to ATS @ 20kV then you have another 20kV travo on the client side. Is this correct?

Hi Tamu,

Correct, another trafo at distribution.

Can we please see the diagram of the complete ATS system?

Hi,

RAM, we already try to isolated the load. Vendor of trafo said cause of tripping breaker is inrush current present on system, solution is the sets all run together till main generator breaker closed and load, then one by one manually switch off the set. here is good link for calculating IC http://www.bambangdjaja.com/index.php?option=com_content&task=view&id=22&Itemid=34 , but again im still curious. Does IC happen only on power source trafo or sum with distribution trafo ?

Hi, Please read my earlier post to Hien, where I explained that I'm getting the feeling that you're trying to simply close the generator breaker into the transformer when the high side is energized by another source. Please tell me I'm wrong and that if you are doing that you are synchronizing properly.

Assuming I'm wrong above and that you are indeed operating an isolated system, then the strategy of having all generation online and operating through some form of load-sharing/VAR-sharing master controller should allow you to randomly apply load, but, and it's a big but, if too many really big high inertia loads just happen to start simultaneously, you are going to trip the breaker because your prime movers lack sufficient inertia to overcome the loads and/or your excitation system lacks sufficient field forcing capability to maintain the terminal voltage at the motors, either of which has the potential to lead to high currents through the breaker as the motors start too slowly and/or fail to come up to speed before the current dies down below the breaker trip level.

One more thing, you still haven't revealed the nature of your loads, that would help everybody tremendously...

Loadshare,

Another thought to add to this... at your ATS you can add a second or two of delay to transfer to the gen bus. This will allow the gens to take the inrush hit of that 5MVA transformer, delay for few cycles to let them to settle in and then roll over and energize the downstream loads.

Actually, this application is clear in term of concept. But the OP, same as many others, still has not described enough what he has been looking for and dealing with.

All the thing here is just to focus on inrush current and it seems that this is the only challenge from the consultant.

I think that, one again, this application is popular and the best way is to consult the supplier of genset. Until now, we also don't know why his consultants said about the inrush current?

The problem, for nothing, possibly, may be not just inrush current.

This is just my curiosity, what does Loadshare mean?

Hi All,

Thank for you respond and explanation, I do appreciate.

On start discussion, I made a simple example which similar to the actual in order to make easier to understand.

@Hien: Purpose I ask about inrush current, because the distributor of transformer said no inrush current then I remember on my previous project, it exist even at no load. And I'm looking for advise from this forum regarding this. Also this is AMF/TS

@RAM: Yes, sets synchronizing and close its CB and step up to the transformer, its simple

@Switchman: you mentioned to make sure the breaker doesnt use instantenous trip. So in my head there will be a LBS but I think its dangerous because no protection or could you please advise me what kind of breaker that doesnt use IT ?.

@Tamu: OK, I will provide you the missing part of diagram

So, at temperory result, if Im not wrong:

Transformer step up 0,4/20 kV 5000 kVA In primary = 7217 amps (possibility I inrush = 10 x 7217 amps)

The sets 400 VAC, 3 x 1406 kVA Ig Total = 6088 amps, Protection over current at set 125 % (t 1 sec) = 1,25 x 6088

Main sets CB In = 6300 Instatenous trip can be set to = 86.000 amp (t 80 ms) and 100 kA for ST

Please, correct me

Thanks