Momentary Paralleling of transformers or sources|Part 1|Operation sequence of momentary Paralleling|How momentary Paralleling is implemented

What is Momentary Paralleling of Transformer or incomers? Momentary Paralleling

In books you will mostly find parallel operation of transformers. In practice you will find its momentary operation. That is momentary Paralleling. Now there will come several questions.

1. What is momentary Paralleling?

2. Why we use momentary Paralleling?

3. Why continuous Paralleling of Transformer isn't used for distribution level?

Now let us answer one by one.

We all know that parallel operation of transformers means when two or more nos. of transformers are connected together in a same bus bar.

Main advantage of Paralleling is that addition of transformer can take up additional loads. But it has a major disadvantage. The main disadvantage of continuous parallel operation is that the fault level becomes double. So for distribution level parallel operation is hardly used. 

Suppose you need to deliver say 2 MVA power. Now if you select 2 nos of 1.25 MVA transformers with 6% percentage impedance and run them in parallel so their equivalent percentage impedance will be 6/2=3%. So total fault MVA will be 1.25/0.03=41.67 MVA

But if we select 2.5 MVA trafo with 9% impedance then our fault MVA will be 2.5/0.09=27.77 MVA.

Now one may say what if we consider 12% percentage impedance for 1.25 MVA trafo, but that is not feasible as that will cause huge voltage drop.

Moreover if one transformer fails you will loose 50% of source.

So it is often practiced to use the source with additional 100% capacity so that redundancy is maintained

Now you have understood why Paralleling is not used in distribution level.

Now comes the Momentary Paralleling. Momentary Paralleling is two transformers or two sources connected in parallel for a short duration, then either one of the source is disconnected or the Paralleling is discontinued.

Momentary paralleling of two sources
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Suppose in the above picture there are two incomers. But one is in circuit or switched on and the other is off. Now you need to maintain source 1, so you want to off source 1 and want to feed the loads from source 2. If you switch off source 1 breaker, and then close source 2 breaker then all the loads will be shut down. And you will need to restart the entire system. So to avoid this interruption we will go for momentary Paralleling. So for a brief period of time both thr sources will connected i.e. paralleled. After a pre set time one source (Source 1) be tripped. So you need a trip selector switch or TSS to select which source will be tripped. But if the selected source fails to trip the incoming source (here source 2) will trip to avoid sustain Paralleling.

Now let us see another case where the system bus has two sources from two transformers and they are isolated by a bus coupler.

Momentary paralleling of two sources with bus coupler
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Normal operating condition is both the incomers i.e. incomer 1 and incomer 2 is closed and delivering loads, and the bus coupler is open, so that there is no paralleling between the two sources. So here bus coupler is Paralleling components.

Case-1

Now as we mentioned that we select transformer with 100% backup so we can switch off one source. Suppose we want to take transformer 2 in maintanence, so we need to open the incomer 2 breaker, and for feeding loads of that bus section we need to close the buscoupler. Now we will do it by momentary Paralleling so that loads are not interrupted.

So our target is closing of bus coupler and

Opening of incomer 2 breaker. 

Remember that momentary Paralleling is a manual process, so we need to put the selection at manual mode. Only that will allow closure of all three breakers after synchro checking. Otherwise these three breakers will be interlocked, so you will be able to close only two breaker at a time.

Hence, Trip Selector switch i.e. TSS shall be at Incomer 2 position, i.e. incomer 2 will trip.

Now we will give close command to buscoupler. Synchro check relay will check the incoming source voltage details with the running system voltage details i.e. magnitude, phase sequence, frequency etc.

Incoming power source is determined by the trip Selector switch position.

After the synchro check relay compares the two circuits the bus coupler get closed if permissive is given by synchro check relay.

So now all the three sources are closed. And a timer get energized. After certain time incomer 2 will open and deactivate the timer.

Now here is an interesting thing, if the incomer 2 doesn't trip, the timer will remain energized, and then it will send trip signal to bus coupler so that sustain Paralleling doesn't occurs.

Case 2:

In case 1 we have seen that we have taken transformer 2 out of service for maintenance with closing the buscoupler, and shifting the loads to transformer 1 by momentary Paralleling. So suppose our transformer 2 is inspected and ready to be in operation, so we want to back in our normal operation i.e. two incomers are running and bus coupler is open. So here also we will do the same thing with momentary Paralleling.

Now our trip Selector switch or TSS will be at bus coupler, because we want to trip the bus coupler after successful momentary Paralleling.

And auto-manual selection shall be at manual mode operation. Often this is given as Auto-Independent-Manual selector switch.

Now we will give close command to incomer 2. Synchro check will be between bus Pt at bus B and line PT for incomer 2. If there is no variation or variations are with in tolerable limit incomer 2 will get closed, and after some time bus coupler will open.

Case 3

Scenario may me something else. Our switchboard incomers and bus coupler often has auto change over scheme. Suppose two incomers ar closed and bus coupler is opened. Now there is a problem at transformer 2, so incomer 2 will trip due to under voltage. Due to the auto change over scheme bus coupler will automatically close after incomer is tripped (only condition is lock out relay of incomer 2 has not operated i.e. no fault at incomer 2). 

Now we have cleared the fault and want to bring back incomer 2 into circuit, with out interruption of the power supply to the loads. So we will use momentary Paralleling. Our Trip Selector switch shall be at bus coupler, and incomer 2 shall be closed. Bus coupler will automatically open.

So this how momentary Paralleling works and applied. 

Hope you have enjoyed the article. In the second part of the article we will be discussing about the control schematic of momentary Paralleling.

Thank you......