Parallelling Substation Transformers 5

[hmchi]

I have heard from a Japanese customer of mine that their utilities would run their distribution substations with two or more step-down transformers in parallell [put it another way, with the bus tie breakers normall closed]. He did not see any reason for concern, and yet could not name any advantage of such practice, other than the fact that if one transformer fails, and taken out by the differential relays, the loads would not see an immediate loss of power.

But, after all, transformers do not fail very often --- why such 'theatrics' for such a low risk ?

It seems to me that the disadvantages are :

[1] Increased short circuit current level, either putting system at risk, or necessitate more expensive and more capable fault interrupters.

[2] The risk of circulating currents running from one transformer to another, due to the slight differences in the secondary windings, even if the primary source is identical.

[3] Difficult and exacting differential protection required --- not only expensive, but one false move, you're dead ...

I hope to hear from utility engineers using this practice to enlighten me --- there must be something I missed in our imperfect info exchange --- one of us not communicating in his mother tongue.

I would like specifically to hear about the circulating current issue --- most utilities would not run closed loops at distribution except from the same transformer --- due to the concern of circulating currents. Wouldn't parallelling substation transformers create the same concern ? when the transformers are not perfectly matched ? Can someone cite IEC or IEEE standards on this issue ?

Thanks

 

[RajT]

There are no regulatory constraints which prohibits running a closed bus tie systems and its all a matter of choice noting the risks and disadvantages (and some advantages too) as you state. The circulating current issue should not be of a great concern as small amount of circulating currents would have negligible effect on transformer life.

Utilities often do run a closed bus tie system, though I would think only in the circumstances whereby both transformers are needed to cope with the demand.

 

[hmchi]

What are the advantages ?

[whereby both transformers are needed to cope with the demand]

How does closing the bus tie help meeting the demand ? I can see if transformers are evenly loaded, it might help somewhat. But wouldn't it be better to try to even the loads and staying separate ?

 

[rbulsara]

This is called 'Spot Network' of transformers. While the secondaries are closed, they are closed through a 'Network Protector" which is a breaker with reverse power sensing.

Please read up on Spot Networking on any electrical tehchnical handbook or NEC. It is typically employed by utilities in urban downdown areas to feed large low voltge loads. Main advantage is that loss of one of the transformers or a primary side fault (on one of the feeders) will not disrupt the service. The network protector senses fault on the primary side (line side of the breaker) and isolates it from the common bus. Spot Networking is expensive and requires careful engineering and hence not popular in commercial applications.

 

[hmchi]

I am familiar with spot network or secondary network, but this is clearly not their intent, nor are there network protectors used.

What they seemed to be doing is to run 3 40MVA transformers in parallel in a substation.

I guess I was just trying to see what make them go this route and what we are missing by not doing the same ..

 

[Siddhuca]

I speaking from practical experience.It is very usefull for even distribution of load among transformers.Some feeders may be loaded higher than others.Paralleling helps distribute the load and better volatge regulation on the overloaded feeder

 

[spc300]

One advantage is that if the transformers are on a transmission line that has local breakers, such that a fault on the transmission line would remove one of the transformers from service, the others running in parallel would carry the load. This involves the use of a load side breaker on the LV side of the transformer which trips along with the t-line breaker. On has to load the transformers such that N-1 condition does not cause "loss of life" loading of the remaining units. By doing such, the load only "sees" a momentary dip during the fault clearing and not an "outage" due to the t-line fault.

 

[hmchi]

Yes, the transformer differential protection usually trips both primary and secondary breaker at the same time via the lockout relay.

Siddhuca, you actually do this in actual practice. Did you follow any IEEE guideline or look up any standards or literatures about doing so ?

 

[Shortstub]

Ignoring the fact that SC level is almost doubled, success is also dependent on ground-fault relaying strategy, i.e., location, residual or zero-sequence, and delayed or not!

 

[reactive]

Running 3 x 40 MVA units in parallel will send the fault level through the roof. Add the motor fault contribution to that to make matters worse.

You don't give voltages or transformer impedances (we could guess them to be about 12%) so we can't estimate the prospective fault current accurately, although it would probably be in the region of 800 MVA. What is the switchgear rated at?

If the switchgear is rated for the fault level, technically I cannot see any reason not to run them in parallel if the load configuration makes this desirable.

Here in South Africa as an example, on mining 6,6 kV systems we find that 2 x 20 MVAs in parallel would normally cause 31,5kA (symmetrical break)/78kA (asymmetrical make) switchgear to be marginal when motor contribution is added to the prospective fault level.

Motor contribution is often a forgotten factor and we find many clients working on steady state fault levels. In a recent study we had as much as 8 kA (6,6 kV) added to the fault level due to the motor contribution.

 

[hmchi]

Redwing, Thanks for your input.

But, why are you running them in parallel ? What do you want to get from it that you do not get running them separately ? The even distribution of loads is not that big a deal and can always been done by adjusting feeders, right ?

The 31.5KA switchgears are a lot more expensive than the 16KA switchgears. Additionally, unless you use current limiting fuses, the increased fault level causes a lot more damage to the circuit [like cables, etc.] not immediately in the vicinity of fault.

Is circulating current an issue with your parallelling the 2 x 20MVA ?

To answer your question, I think that the secondary voltage of the substation is 6.6kv, as someone the Japanese electricity laws designated that all distribution system must use 6.6kv.

The Japanese gentleman is looking for suitable current limiting fuses to limit the let-thru current so that he could use lower rated switchgears. Sort of Rube Goldberg to me ...

 

[reactive]

I agree, if the load configuration does not require paralleled transformers then there is no reason to run in parallel.

The short circuit withstand of cables depends on the amount of thermal energy (I^2t) that is produced by the fault and is a function of the duration of the fault. Obviously, the larger the cable the higher it's thermal withstand.

Current limiting fuses are clumsy as you lose all your protection coordination. Much better to reconfigure the loads so that the transformers run independently.

 

[rbulsara]

I was waiting for some utility person to respond...

But running substation transfomers in parallel is very common at utiltiy subs and even at many industrial locations. The trasfomers breaker invariably will have a directional relay on it, no utiltiy co is that foolish to paraller 40 mva units without a reverse power trip. No one wants to backfeed a fault on primary side.

Plus this is no theatrics! Adavange of enhancing availability /continuity of service followed by load sharing capablity and evenly loading the transformers by far outweighs any perceived disadvantages you are mentioning. More so in urban areas (or developed countries)than rural areas.

Increased SCC is only relative and not necessarily bad this level. Firstly the HV equipment and bussing are normally rated for high enough SCC regardless. More SCC also (not that it is the reason) may provide enough fault current for downstream devices to operate to achevie coordination.

Differential protection is a norm , whether or not in parallel. This insallation are run under duly supervised conditons. You are comparing it to a non attended commercial distribution system, and unnecessarily getting wound up. Relax!

In fact at one of our client's commercial facility two 13.8 kV utility service feeders are closed together and they have 51 and 67 devices. This is not for capacity but to enhance availability of service.

 

[hmchi]

Exactly the kind of input I want to hear ... Someone who considered all the pros and cons and spell out why going one way instead of the other ... Thanks.

Any other UTILITY person(s) wish to comment ? Most of all, I want to know how widespread this practice is ...

 

[ohhn]

Mr./Mrs hmchi:
our stepdown transformers are connected in parallel (Honduras Interconnected System, Central America)and they normally have On Load Tap Changers (OLTC). You can have two kind of controls for the OLTC, one that tries to have the same tap (no more than one tap of difference) on both transformers or one that measures the circulating current and change the taps of both transformers in order to minimize it.
As to why have transformers in parallel, it's easy to answer:
First, it may be demanded by contract or the load grew and instead of buying a bigger transformer and discarding the one installed, you buy another equal to the one installed.
When you do this, you get the advantage that you don't loose all the load when you loose a transformer.
So, think about having one transformer and then as the load grows you install more transformers in parallel which is cheaper and with abigger reliability

 

[jbartos]

Suggestion: Two transformers in parallel create the more complex power distribution. However, as stated in above posting, the higher availability of the power supply is there, even though some load shedding may be need if the two transformers in parallel transform more than one transformer rated MVA or kVA. Therefore, some customers may eventually be without electricity. However, it is obvious that it is better to have some power supply than none.
Paralleling transformers reduces the transformer impedance significantly, i.e. the parallel transformers may have very low impedance, which creates the high short circuit currents.
Therefore, some current limiters are needed, e.g. reactors, fuses, high impedance buses, etc.
Nowadays, the harmonics are more present in the power distribution systems due to nonlinear loads. The low transformer impedance reduces the voltage Total Harmonic Distortion (THD). Therefore, there are some potential savings possible on the harmonic filters.

 

[CKent]

I agree with hmchi. I am a utility engineer and our practice avoids paralleling substation transformers both at transmission and distribution levels. There are a number of conditions that must be satisfied for parallel operation of transformers. More often, not satisfying some of these conditions results to circulating current which effectively reduces the capacity of one of the transformers. Thus, the 'limiting' total combined capacity is lower than the actual capacity of the trasnformers combined. Additionally, as mentioned, this results to increase in SCC which would require larger rated CBs (which is substantially expensive than two CBs for the separate operation of PXFs).

Load management, as again mentioned, I think is better. This means shifting some of the loads to the new circuit that will be serve by the new PXF. The only application I could think of is if the load to be served have grown but could not be split, which I think is very remote case. The increase in security or reliability could be a big factor, but at such a higher cost. This must therefore be weighed. And besides, if one of the PXFs in the parallel connection would bog-down, more or less the other would be affected, especially if the load to be carried is higher than the remaining PXF (which would be the case that's why paralleling was initiated), thus interruption would also occur.

 

[hmchi]

How big a problem can the circulating current be ? Are there documentation or Standards guidances on that ? It seems to me that if nothing else, it should be looked upon as a loss. whose magnitude could be calculated, I assume. Does anyone has a guesstimate formula on this ?

In Singapore and Hong Kong, where the city utilities use 11kv and 22kv 'closed loops' and pilot wire relays such that a fault on the main cables would not cause any load loss, they always have the closed loop going back to the same transformer, presumably to avoid circulating currents. However, if circulating current loss is really small like some previous posts suggest, one might think they would have desired to have the loop going to another transformer in a distant substation, so that a transmission fault would not kill the closed loop like the present design.

A friend of mine opines that if they order PXFs with matching characteristics, this circulating currents can be alleviated. Any comment on how practical is this opinion ?

 

[Siddhuca]

I would say most of the utilities in the Southern States of India were I worked have Transformers in parallel. Only when their impedences did not match, or the OLTC was not the Master slave type(to prevent transformers go out of step)the transformers were individually operated. We had differential protection.For two 10 MVA transformers the Breaking capacity of the Circuit breakers was 500MVA ie 21KA for 22KV voltage. Considering the cost of construction of these substations the breakers costing more is not a big factor considering the advantage these stations provide.

 

[hmchi]

10MVA X 2 is quite comfortable for parallelling, especially if you have OLTC. Would you still do the same when your load grows to our Japanese friend's --- 3 x 40MVA ?