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 https://electricaltechnologyrishi.blogspot.com

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 https://electricaltechnologyrishi.blogspot.com

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......

CT burden calculation. How to calculate Current Transformer burden

Before starting the discussion about CT or current transformer burden calculation we should know what is a burden!!!!!!

Burden is something which irritates?!!!! Actually burden is some additional load, which you need to bear.

Just like that the current transformer also need to drive some load which is called burden of CT or current transformer. Now what are the burdens?

For knowing that we need to understand what are the elements that are connected with the current Transformer. A protection class CT or current transformer has relays connected with it. And a metering class CT has mainly ammeter and multifunction meter connected with it. All this instruments has a small amount of power demand which is imposed on the current Transformer as burden of CT.

So basically sum of power demand of the instruments connected with the CT is called burden of CT. 

Now for outdoor switchyard what happened the current Transformer is installed on field. But the relay, MFM, ammeter etc are installed a certain distance away from the CT location, inside a panel. And this distance is connected via cable. So now there is a power loss in the cable, so that loss will also be included in CT burden calculation.

This is called lead burden. However lead burden js applicable mostly for CT installed at out door. Now,

Single line diagram of metering and protection https://electricaltechnologyrishi.blogspot.com

Let us consider this picture. Here we can see a an Ammeter, a Tri vector meter and an MFM is connected in series with the current Transformer. So their VA rating will be added to the burden of CT.

Now from a typical catalogue of MFM and ammeter we have found that

Ammeter has a burden of around 0.9 VA.

MFM has burden of 0.5 VA (MFM  has two burden, one is on CT and the other one is on PT)

And TVM has a burden of say 0.6 VA.

So their total VA demand=2 VA.

Now we will calculate the lead burden. It is assumed that CTs are outdoor type, and they are connected with the meters with cable. Now we will find out the power loss in the cable.

Three line diagram of metering connection to CT https://electricaltechnologyrishi.blogspot.com

So from the above diagram we can see that each CT is connected to the meter. So for completing the operating circuit(i.e. you can see that as per the ammeter selector switch position two leads will be connected between CT and meters, so we need to consider two cable) two nos. of cable runs are required, so we need to calculate the power loss for two runs. Generally 2.5 sq. mm Cu cables are used for such connection, so we will also consider the same.

2.5 sq mm Cu cable has resistance of 9.8 Ohm/ km.

Let's say the metering panels and CTs are 100 m away so we need to calculate resistance of 200 m cable. 200 is considered for to and fro run.

So total resistance will be 9.8*200/1000=1.96 Ohm

Say CT secondary rating is 1A, so total power loss=I^2*R=1*1*1.96=1.96 VA.

So, total burden will be 2+1.96=3.96 VA

So CT selected VA should be minimum 5 VA, however considerable margins are kept to burden selection.

Say the burden is selected as 15 VA.

Now let us see the condition for protection CT.

Relay connection to the current Transformer https://electricaltechnologyrishi.blogspot.com

From the diagram we can see the connection of the Relay with the CT. Here also for calculating the lead burden our consideration will similar as that for protection.

Since the relay, meter all are housed in a same panel so lead resistance will be same for both the case.

Numerical relay has burden of around 0.2 VA. So total burden for protection CT is 1.96+0.2= 2.16 VA.

Let us select 15 VA CT. Now Selection of such higher value of CT burden will give us another advantage. We will see that.

Suppose our selected CT is 5P20 class CT, i.e. its Accuracy limit factor is 20. Now if the VA is higher the actual ALF will increase.

The corrected or actual accuracy limit factor or ALF can be found as following

ALF'=ALF*(Pi+Pn)/(Pi+Pr)

Where 

ALF'= The actual accuracy limit factor

ALF= Rated accuracy limit factor

Pi=CT internal I^2*R loss

Pn=Selected CT burden

Pr=Actual burden on CT.

Out CT is 1A secondary rated, and CT resistance shall be maximum 5 ohm (Rct<=5 ohm).

So, Pi=5VA

Pn= say 15

Pr= 2.16 VA (burden of numerical relay and cable)

And the CT is a 5P20 CT so ALF=20.

So, ALF'= 55.86

So our 5P20 CT will act like 5P56.

Its accuracy limit factor will increase. So thus the actual ALF of CT increases with higher burden selection.

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Thank you......