line to line single phase transformer connection 1

[protoslash]

Hi

I have two identical distribution single phase transformers. If the first one HV bushings are connected to phase A and B, and the second one is connected to phase B and A. Then does that mean the loading current flow on the feeder phase A and B are 0 (cancel out)?

thanks

 

[jghrist]

Please explain the situation better. How is on HV bushing connected to Phase A and B? What's the difference between "connected to Phase A and B" and "connected to Phase B and A"? How are the loads connected?

 

[protoslash]

well transformers have two primary bushing (H1 and H2), secondary is just single phase to ground, assume X1 is the high side and X2 is connected to ground, load will be connected from X1 to ground(X2)

when i say connected to phase A and B, i meant the H1 connected is to A phase and H2 connection is B phase. B A connection means the other way around

 

[bacon4life]

The loads from both transformers add together.

In a normal installation with both H1 bushings attached to the same phase, the voltage between the two X1 bushings would be close to zero volts. With your reverse polarity setup, if you measured between X1 bushings on the two transformers you would measured approximately twice rated voltage.

 

[protoslash]

thanks for the reply, but the two transformer secondary are not tied together in any way. treat them as independent locations.

if these two transformers are supplying the same amount of load each, i was wondering how does the current flow on the primary phase conductors assuming the system contains nothing else.

 

[xnuke]

Loading is power consumption, protoslash. Why would you think it can "cancel out?"

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[protoslash]

well, if i have only 1 transformer on the system connected between A and B, the current will flow from A to B. If i connect a transformer between B and A, theoretically the current for this connection is from B to A? Now if we have both connection on the same feeder, what happens to the current flow in the system? I don't think it's zero either, since that imply no power transfer. But then, what is the current on the primary phase conductors?

 

[che12345]

Dear Mr. protoslash
1. I take it as: 1. Primary Phase A to H1 and Phase B to H2 of Transformer T1, and
1.1. Phase A to H2 and Phase B to H1 of T2, and
2. Secondary X1 and X2 of T1 is connected to a load R1/current I1, and
2.1 Secondary X1 and X 2 of T2 is connected to another load R2/ current I2 respectively, and
2.2. R1 and R2 are respective loads NOT connected together.
3. Assuming that both trafo T1 and T2 are with 1:1 ratio:
3.1. T1 loaded with load R1 current = I1 , and
3.2. T2 loaded with load R2 current = I2 , respectively.
3.3. Primary Phase A = Phase B read current (I1+I2) NOT 0.
Che Kuan Yau (Singapore)

 

[protoslash]

thanks che

what about return current of I1? let's remove the second transformer for now. the system is just 1 transformer connected between A and B. phase A will have I1 like you said, but the current on phase B will be negative I1 or I1 angle 180.

now add the second transformer. I don't believe phase A = phase B = I1 = I2

 

[davidbeach]

Draw it out. Two phases, two single phase transformers. Leave off trivial details like bushing designations but include the secondary load. Work out the current flows (hint, this can be done perfectly well without knowing bushings). Now add bushing designations. What changes?

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[protoslash]

yeah I figured it out with vector diagrams. turned out it doesn't matter which bushing goes on which phase,

if one transformer is connected AB, primary feeder phase A current is I, B would be -I.

now add a second transformer, regardless of connection order AB or BA, the primary feeder phase A current is 2I and B current is -2I

so on three transformers between AB result in 3I regardless bushing to phase relationship.

 

[che12345]

Dear Mr.protoslash (Electrical)(OP)30 Nov 22 22:58
"...#1. what about return current of I1? let's remove the second transformer for now. the system is just 1 transformer connected between A and B. phase A will have I1 like you said, but the current on phase B will be negative I1 or I1 angle 180."
1. NO. Phase AB of a 3-phase A,B,C source is an ac single vector. NOT 180 or 120 deg apart.
1.1. T1 supplies the load R1 with current I1 , and T2 supplies the load R2 with current I2.
Note: That is load 1 and load 2 are separate respective loads, not connected together.
".... #2. now add the second transformer. I don't believe phase A = phase B = I1 = I2 "
2. The power source phase AB (is a single vector) supplying T1 load I1 and T2 load I2 respectively. Therefore, the current on [phase A=phase B] = [I1+I2]. NOT phase A = phase B = I1 = I2
Che Kuan Yau (Singapore)

 

[cranky108]

Sounds to me like you would have a short circuit. You should blow the primary fuses.
Besides that, many distribution transformers have more than one secondary bushing, with the center connected to ground.
That's how you get 120/240. The only transformers normally with only two secondary bushings are for voltages higher than 240 (277 or 480).
At least in the US.

 

[protoslash]

@che12345
"2. The power source phase AB (is a single vector) supplying T1 load I1 and T2 load I2 respectively"
this is not true, when you flip the H1H2 for the second transformer, the voltage vector BA is supplying this transformer, Vab amd Vba is 180 out of phase. Also Phase A = Phase B is incorrect, they are only equal in magnitude, but the phase angle has to be 180 degrees apart.

@cranky108
can you explain how to get a short circuit here? the transformer secondaries are not connected in anyway. Also, for simplicity, let's pretend these are simple two winding transformer, secondary is grounded on 1 end(say X2), no center tap/split phase. i guess voltage doesn't matter either, you can assume 1:1 turn ratio.

 

[xnuke]

Stop concerning yourself with current and voltage phase/direction and what cancels. The concept is simple - power is power - the only thing that cancels out load power is source power. You have two transformers delivering power to load, not one transformer serving load power and one connecting a power source.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[che12345]

Dear Mr. protoslash (Electrical)(OP)1 Dec 22 15:03
"2. The power source phase AB (is a single vector) supplying T1 load I1 and T2 load I2 respectively"
"....this is not true, when you flip the H1H2 for the second transformer, the voltage vector BA is supplying this transformer, Vab amd Vba is 180 out of phase. Also Phase A = Phase B is incorrect, they are only equal in magnitude, but the phase angle has to be 180 degrees apar."
With due respect, you are totally WRONG!"
1. In a 3-phase system phase A,B,C ; phase AN, BN, CN are three vectors with 120 deg apart.
Attention: 1. Phase AB, BC, CA vectors (also 120 deg apart between them). It is = 1.732 of AN, BN, CN in magnitude.
2. In your case, taking say only AB phase:
2.1. Phase AB , It is a single vector (NOTHING to do with 180 or 120 deg apart).
2.2. Phase AB supplies T1 with current I1 and T2 with current I2. Therefore, [Current A = Current B] = [I1+I2].
3. It DOEN'T matter whether phase A is connected to T1 H1 and T2 H2.
3.1. The source phase AB supplies load [I1 + I2].
3.2. Yes, the secondary currents are NOT in phase. It doesn't matter as they are different loads (NOT connected to each other)
Che Kuan Yau (Singapore)

 

[LionelHutz]

Reversing the connections on a passive load won't cause the current drawn from the source to change. That is Engineering 101 stuff.

So no, you didn't figure out a way to make an over unity machine.