Transformer Power Not Enough for Load

Dear Iukawa,

You should not conduct a calculation like what you have done.

I suggest you to provide load list of connected loads to this transformer. Then you can use diversity factor, such as continuous load (d.f = 1), standby load d.f = 0%), and intermittent load (50%). Then you get the total estimated load kVA (total continues, intermittent and stand by loads).

Then you have to add one of the highest load connected load to the total kVA above, to get peak load demand.

This method is only one of several method in sizing the tranformer. I believe your transformer become less than 1 MVA.

Regards,

Dear Sir, Your suggestion is right. I had also the same doubt what Mr.Iukawa had.

The real challenge here is.. how do we calculate the Transformer, Breaker and associated cable ratings during the pre-execution stage. I have worked with consultants and i found out what was the criteria :

1) Understand the type of load and electrical nature of load : whether resistive/Inductive(motors)/ Harmonic load(UPS)

2) Understand the quantity of such above loads

3) Whether the loads above are continous operation or Intermittent (i.e duty cycle of Loads and derive the Load factor/ Diversity factor)

4) Understand the overload cycles : This applies to AC/DC motors where the nameplate rating shows (x) KW but it is designed for overloads,say 150% for 2 hours,
125% for 2 hours. this will be important for switchgear rating considerations.

5) Once we understand the above, then we have to calculate the vector sum of all the powers. ( please don't calculate power in scalar method because current is vector form and has direction).

6) once we get the actual power required, we may need to add another 20-25% margin for the transformer rating.( for further minor expansion)

7) it is advisable not to consider a single transformer for a huge KVA rating. instead we can split the KVA into 2 or 3 transformer as per the load requirement. This will ensure the full load efficiency of transformers as well preventing complete black-out in case a single transformer fails. Further, the nominal rating of transformer can be further brought down if we consider capacitor power factor correction panel along with series inductors to bring down the harmonic level. This will prevent overheating of transformer, cables and loads.

8) When the number of transformer are decided, then we have to decide the configuration of breakers and their ratings. Here, care should be taken to coordinate the relay settings in upstream and down stream breakers. it is preferred that the breakers close to transformer trip only for dead short circuits and down stream breakers trip for overloads as per the load conditions. this will prevent entire tripping of system.

9) We also need to see that we don't select transformer rating which is 50 % ( as a example) more than actual load (KW) requirement. The disadvantages are : a)efficiency of transformer will be less since magnetic losses are constant b) poor load factor c) risk of high short-circuit feeding capacity to the fault from the source( limited by its internal impedance).

additional margin of 20 % KVA is ok( my opinion.. sorry about that).

I hope i get a further feed backs form all.

Regards

What is your Generator MVA rating? If your generator's out put is only to the Transformer, then definitely the MVA ratings has to match.

The connected load Vs the actual maximum load mismatches are there and is expected, but in your case it is only 1/3rd, that is just a bit too much, but then the ACBs are kept a bit over rated (eg we have a 600KVA load with 800KVA ACB).

Also may be the customer has kept the ACBs based on future load- just a bit too dangerous, since in case of faults, these will not sense.

Finally you must know the connected load (not from breakers, from actual loads), pattern of loading etc and of course the Generator capacity.

May be the loads are cyclic- the ACBs are not all energised together. We have this type of scenario in factories as well as we have ACBs stand bye - unenergised, part of panel for future expansion.

The generator MVA rating is at 5.875 MVA.

The generator is not only supplying power to this transformer. In fact, the whole system consists of 6 generator operating in parallel distributing power to 20 substations.

Hmm. Seems like i overlook an important factor about the nature of the load. Upon closer review of the drawing, i realise there are some loads which are at standby mode and some loads are only needed to start the Engine to drive the generator .

Only a few loads are continuous to ensure soomth operation of the Diesel Genset.

No wonder the Transformer is rated at such a rating.

Thanks guys for your respective inputs in helping me clear my doubts !

Really appreciate it !

you have given the breaker ratings only.the transformer sizing will be done on load running continuously not on breaker ratings.firstly you should have total load connected,then diversity.Latly you will get m.d .which is continuous load.then you can decide trafo rating.

The number and size of the breakers has nothing to do with the transformer load. Use a clip on amp meter and measure the load or use a recording device to get an actuall measurement.

hi, u just make a list of connected load & find out its cycle duty & multiply with cycle duty or load factor & get the sze of requirment of continuous load.

Your system is undersized by appx 91%

I p = 115A, V p = 6.6 KV S = 1.3MVA; (1.3/.68) x 100 = 191%

Quote "Your system is undersized by appx 91%

I p = 115A, V p = 6.6 KV S = 1.3MVA; (1.3/.68) x 100 = 191%"

you want to explain to everyone on this forum how you came to this conclusion when you have no idea what the load is.

LOL. that guest is not going to explain anything

Sometimes do we drag a post without contribution ?

- The OP is studying customer's single line diagram - so the question of putting the clamp on meter and measuring current - does it arise? Even the generator is not supplied yet to charge the loads.

- The OP has based on the info given already realised the things amiss and has explained in #5. The thread could have been closed at that one can see that after the first couple of posts, no new point has come out. In fact the #1 only could have been enough (even #2 was just an explanation of #1).

Come on guys, for seeing the name in a post, we always have the BBT of Kris going strong.

They could be applying diversity factor

Patrick Whowha

Note that the breaker rating is not the full load amperage of the particular load it is protecting.Even as you have sum them up,the total amperage you got is not the total operating full load amperage of the plant.So it is not the best method to calculate the plant MVA.

The breakers are meant to handle fault currents which are usually higher currents than their full load current.

You should ask your client or customer if they intend to bring all the load ON at a time and probably get the full load current rating of all their load(protected by the breakers) from the data plate of the loads,surely they would provide you with useful information that would enable you to solve their problem.But they only requested a 6.6kv generator from you and nothing is wrong helping them a little further too.

Patrick Whowha

who knows?.. may be the plan is correct, but not the size of the generator as well as the transformer is not .. or... you may have to consider the demand factor of the power house. if it is still fully loaded then the best thing to do is not to overload the transformer. you have monitor its power delivered as you switch on your loads. just sharing my opinion.

If you try to calculate you may fall into deep trouble as you don't know load variation over time or diversity. The correct way is to connect a data analyzer/power logger at the secondary of transformer and record V,I,kW,pf,Hz,harmonics,kVar,kVA,maximum demand etc over a period(8 or 24hrs) and get the maximum load from the graph of max demand(kVA). If you want more detailed study connect a datalogger at each feeder and take the readings from which you can study the loading pattern(simultaneous loads) and by rescheduling the loads you can reduce the max demand.