OK, my question is about sizing the transformers for Building or retrofitting an electric railroad museum substation. How big should they be? I'm talking kw... 120kw, 200kw, 400kw etc. and I'm talking total for all transformers.

Is it possible to significantly overload the transformers for the short times a streetcar is applying power, like you can overload traction motors?

I'm not a power engineer, but it seems your question is backwards. Your rectifier should be rated for its anticipated load, and then you'd have the input transformers with a matching power rating. As for figuring the anticipated load, that's a function of how many cars would be on line at the same time, how often they start and stop, how hilly the terrain is, distance from the substation, feeder lines, etc. Here at TMNY we'll be installing trolley power within a dense utility setting, so we'll be relying on consultant engineers to figure this all out!

And about your overload question, yes, generally power equipment will carry nominal (able to carry this load indefinitely) and surge (short bursts of higher demand) ratings.

It seems that my PM to Robert McDowell is not getting through.

Robert, if this is the case, please e-mail me at <artschwartz@sprintmail.com>

Loaded question.....and completely dependent on many variables.

Consider KVA rather than KW given you will have losses in translation. There will be line losses, leakage......I wouldn't know where to begin apart from the very good advice about starting with the anticipated load and working out from there.

Best.

dave

Yes, I got your message, Art and replied. What you spoke of was extremely informative and it's a pity it wasn't posted public. I got yet another PM as well. As for the other answers, the way I wrote my question seems to have invited a lot of wild arm-waving. I should have asked simply: what is the nameplate capacity of the transformers at your trolley museum?

I'll put up one example, main transformers of 150KVA, and a recently added second sub at 200KVA.

The Pennsylvania Trolley Museum has two nominal 500 kw rectifiers each with its own 500 kva dry transformer (by the rules 475 kw DC output). Normally, one operates at a time but both are able to operate together. The PLCs on each one and the Red Lion keeps lots of data. The units are designed to handle peaks of almost 200%. They have been operating for 4.5 years and we have had something like 35 seconds (except during faults) of time over 100% with the max at 115%. Not much RMSing here. The 500 kva dry transformers were purchased used for $3500 each in 2005, the going rate then.

Conservative design would mean the transformers are only overloaded somewhat as an exception.

We also have a small "booster" unit designed to automatically steal demand from our west site when the east site (where the large rectifiers are) is having a "super-peak" . It has a 30 kva transformer (we are RMSing it but it has proper PLC protection). In 4.5 years it has been available, it has been used 42 hours (I just connected in to get the number). Little W1 has been very profitable to PTM. We also have some moderate load control of facility AC, electric heat, and hot water tanks. We are presently giving W1 some steroids to prepare for this year's Washington County Fair in August. The used 30 kva transformer cost $150 in 2005.

I can public post the previous private email if anyone is interested. I can be contacted at my home email also: dcbockus@columbus.rr.com

Dennis Bockus, BSEE
Electrical Engineer for the Pennsylvania Trolley Museum

Generally when someone sends PM instead of posting I assume there's a professional reason, and I wouldn't want to lay any pressure on anyone to post beyond their comfort level. But I do think would find it interesting and some would find it valuable. It's just the kind of quality stuff from expert posters that could make RyPN special. I find the usual armchair foamer fare to be tiring. Is there another board somewhere which is actually for railway preservation?

Ok, I have a couple of emails requesting this:

The question was asked how to size a transformer for use with a trolley rectifier? How much can this transformer be overloaded in a cyclic basis?

As in most electrical equipment, the answer is the RMS (Root Mean Square) load subject to refinement for factors such as ambient temperature, altitude, correct voltage application, and a few other items. In a highly fluctuating load (trolley operation definitely qualifies), RMS capacity does not anywhere near equal the demand which is a linear average over a period of time such as 15 or 30 minutes used by many utilities for billing purposes. It is higher.

The NEC, ANSI, and others attempt to linearize this by not going hugely over the nameplate capacity of standard transformer. Of course, this same NEC side steps jurisdiction over the electric railway power supply. Do check your state and local codes however.

On to the RMS issue. For simplicity, I will base this discussion to the three phase full-wave bridge (six diode legs) units used by most museums.
Also for talking purposes, we will use a standard 300 kva transformer. The first item is the “form factor” of this bridge gives a power factor in round numbers of 0.95 which reduces the output capacity to 285 kva. Now, if we had a perfectly flat load, think 25 cars with their heaters blazing away, we could get a demand of 285 kw. (In the real world we would have to add in the transformer and rectifier losses).

Now comes the RMS of the trolley load itself and this is where the fun begins. Calculating the RMS, if you have data collection at short intervals such as every two seconds, it is very easy, couple of minutes with a spreadsheet. The readings must be at regular intervals. Each reading is squared and this is performed for at least 30 minutes or more. Then the average of all the squares added is found. Next, take the square root of this average and you have the RMS value.

The Pennsylvania Trolley Museum has such 2 second data logging and gathering in the small PLCs that run their rectifiers. A test was run last month to simulate the demand necessary for the 2012 Washington County Fair service this August. With a test, we could stop it to avoid exporting large demand dollars to the power company. The cars used were three PCC cars plus PST 14 (for power purposes, a super PCC) effective headway was 6.5 minutes and the duration of the test was 96 minutes. We passed the demand test and the RMS was calculated just to know it. It was 150% of the demand. Since PTM has plenty of transformer capacity, the RMS was of no concern.

So for our example, to get 285 kw of demand, about 450 kva of transformer would be required. If it were a 300 kva transformer, it would be gone. Of course, RMS will vary in other situations. There are ways to get it from existing situations.

To answer the second question, the writer’s 40 years of steel plant experience will be used. First, anything over 300% is not reasonable, you are piling up the RMS so quick that any other load except near zero will not work. Frequent loads over 200% will start seeing squirrely things start to happen. Most involve other parts of the circuit. Mechanical lugs start having problems by overheating and coming loose. The fix here is cheaper and less labor, use compression lugs. Fasteners will thermally and mechanically ratchet (always in the wrong direction of course) which can be handled with proper locking and in some cases double locking. Switches, fuses, and breakers can have heating problems (see the NEC for 80% and 100% rated).

If one is close on RMS capacity, a fault or two can take you over depending on how quick it is detected, cleared, and reclosed. Finally, frequent and repeated overloads and faults can mechanically damage the transformer as they generate significant stress in the windings which will shorten the life of the unit.

One last note, liquid cooled transformers as compared to dry, have larger thermal time constants so they take longer to heat up and longer to cool down. Single phase oil filed distribution pole transformers tend to be more rugged as they have a shell-type core, better for cooling and mechanical strength.