Multiple transformers in series

[Opie11]

Texie, bingo on your entire post. I am new at my current position and that is exactly the mentality I walked into when inquiring about certain setups we install and maintain.

I have worked with transformers in the past but nothing like i currently am. I actually do have questions regarding grounding and bonding but not prepared enough to ask them quite yet.

I am trying to be convincing in stating that the right way of doing things is the safe way of doing things. An uphill battle for sure.

 

[electrofelon]

I have to add a couple of questions.

If 2 transformers are within 5 feet of each other and 1 feeds the other how would you treat them as separate? I am confused when a transformer secondary becomes a transformer primary in such a short distance.

Also, are transformers able to be used as a bidirectional device? Is the expectation there that they can be used this way?

An ocpd can meet multiple requirements. So if you have one between two transformers, where the first one requires secondary protection and the second one requires primary protection, and it is sized appropriately it can meet both requirements. It can also meet a third requirement for the transformer secondary conductors of the first one.

Yes transformers are bi directional but energizing current might be significantly more for a transformer used in reverse and could cause tripping upon energizing if the ocpd isn't big enough. Note also there is a newish NEC requirement that a transformer must be labeled as suitable for reverse feed.

 

[winnie]

Code doesn't explicitly describe protection of 'cascaded' transformers, but one can find guidance in the situation where code permits primary protection to protect secondary conductors.

When you have a single phase transformer with a two wire primary and secondary, there is a strict current ratio between primary and secondary. Code permits the primary OCPD, reflected through this current ratio, to protect the secondary conductors.

For example a 50A breaker on the primary of 480:240 single phase 2 wire transformer provides the same protection of the downstream conductors as a 100A breaker on the secondary side.

A similar allowance applies to 3 phase delta:delta transformers.

Importantly this downstream protection does not apply to single phase transformers with a neutral, nor to delta:wye transformers.

I think one might infer that in cascade arrangements where current values are strictly related at each stage, that the protection 'flows' from stage to stage. But this is not in code.

Voltage drop in transformers is a separate issue.

Jon

 

[gar]

221020-1713 edt

From a Webster dictionary ---
7: an arrangement of the parts of or elements in an electric circuit whereby the whole current passes through each part or element without branching

So if I connect the output of one transformer to the input of a next transformer, and nothing else, then I have a series connection. Continue to another in the same fashion, then I still have a continuing series connection. Do as many as you want, and it is still a series connection.

If at various points along the way you branch off some of the power, then you still have the original series circuit, just some of its input power does not go to the end of the original path.

On current protection of transformers you need an understanding of a transformer.

Most power transformers are built with wire wound on a magnetic core material' Because of the magnetic characteristics of the core very tight magnetic coupling can be achieved between any primaries, and secondaries. This achieves efficiency of power transfer. Yes, power can flow in either direction thru a transformer. And simultaneously power can flow in both directions.

For the moment I won't go into the problem of circuit protection.

.

 

[TwoBlocked]

I have to add a couple of questions.

If 2 transformers are within 5 feet of each other and 1 feeds the other how would you treat them as separate? I am confused when a transformer secondary becomes a transformer primary in such a short distance.

Also, are transformers able to be used as a bidirectional device? Is the expectation there that they can be used this way?

Can't see what difference the distance makes, 2 transformers are 2 transformers. Think about this, the only time a transformer is not fed from another transformer is when it is fed from a power source (generator, etc.)

Yes, transformers are "bidirectional". If it is a 240V/120V transformer, you can either have the 240V as primary, with a 120V secondary, or the other way around. BUT! If the chosen secondary is Delta or not split-phase, some decisions have to be made on how to either ground a phase or alarm in case of a grounded phase. I thin this was texie's concern.

 

[TwoBlocked]

221020-1713 edt

From a Webster dictionary ---
7: an arrangement of the parts of or elements in an electric circuit whereby the whole current passes through each part or element without branching

So if I connect the output of one transformer to the input of a next transformer, and nothing else, then I have a series connection. Continue to another in the same fashion, then I still have a continuing series connection. Do as many as you want, and it is still a series connection.

...

If we stick to the definition, no, it wouldn't be a series circuit. No current flows from the primary to the secondary.

 

[gar]

2210120-2233 EDT

TwoBlocked:

I provided one definition from the Internet that was a reasonably useful definition. I have looked for some other sources on the Internet, and did not find a good one.

Because one electron does not flow from a primary winding to a secondary winding does not mean that what secondary winding electrons do is not a direct effector of the electrons on the primary winding. Actually it is not so much what one one electron does, but what many electrons on the secondary do that affect electrons on the primary side.

From an electrical theory perspective your comment just muddies a good understanding of electrical circuit theory.

Have you studied in any EE courses?

 

[Jpflex]

What is the purpose of hooking the transformers in series….didn’t know you could do that. I am aware of in parallel!

Seems like a series connection would just be repetitive coupling either increasing or decreasing voltage to a desired amount same affect as increasing turns ratio? My guess any way. But how is parallel connection, what happens here?

I have a calculation for voltage drop will try to get it from book

 

[TwoBlocked]

2210120-2233 EDT

TwoBlocked:

I provided one definition from the Internet that was a reasonably useful definition. I have looked for some other sources on the Internet, and did not find a good one.

Because one electron does not flow from a primary winding to a secondary winding does not mean that what secondary winding electrons do is not a direct effector of the electrons on the primary winding. Actually it is not so much what one one electron does, but what many electrons on the secondary do that affect electrons on the primary side.

From an electrical theory perspective your comment just muddies a good understanding of electrical circuit theory.

Have you studied in any EE courses?

As I look back through this thread, seems everyone got muddled with the idea of transformers in series. It's just not how it is explained. A transformer fed from another is a "Separately Derived System." Or to take it a step further, would someone say that a boiler that runs a generator, that feeds a transformer, that feeds a motor-generator set, that feeds a gyroscope, makes the boiler in series with the gyroscope? (OK, I spent some time at sea...)

As far as what I have studied, let's keep this about what's right, not who's right.

 

[gar]

221021-0516 EDT

TwoBlocked:

My view is that you do not know how to do circuit analysis.

If I have a cascaded sequence of one transformer connected to others in a series arrangement, then one simple equivalent circuit of that combination in one form is simply one series inductor and resistor connected in series with the load and power source.

.

.

 

[TwoBlocked]

221021-0516 EDT

TwoBlocked:

My view is that you do not know how to do circuit analysis.

...

Play the ball, not the man.

 

[ggunn]

Play the ball, not the man.

That's why over the years I have become more of a baseball fan than a football fan. Let's Go, Astros!

Whether it rigorously qualifies as a series circuit or not is irrelevant; the important thing is that the OP's description accurately conveys to the rest of us the nature of his question.

 

[electrofelon]

That's why over the years I have become more of a baseball fan than a football fan. Let's Go, Astros!

Whether it rigorously qualifies as a series circuit or not is irrelevant; the important thing is that the OP's description accurately conveys to the rest of us the nature of his question.

Did anyone ACTUALLY think the OP was wanting to connect transformer windings in series?? Come on guys, I know this is the MH forum but that's a bit much

 

[gar]

221021-1043 EDT

I have now reread the original question, and I don't really know what the question is.

I originally thought we had one transformer fed from a power source, and the output ( its secondary ) was then connected to the primary of a different transformer. Next the process was repeated thru several more transformers.

But now I think the questions is not clear enough such that we could also read the question to mean that several transformer primaries were in parallel, and that their secondaries were series connected. But i really don't think this was the question.

.

 

[ggunn]

221021-1043 EDT

I have now reread the original question, and I don't really know what the question is.

I originally thought we had one transformer fed from a power source, and the output ( its secondary ) was then connected to the primary of a different transformer. Next the process was repeated thru several more transformers.

That is how I read it.

 

[Opie11]

Hopefully to come back on track, no pun intended, I will restate my original question/ concern.

First a little context. Our application/loads require different voltages from 120 up to 550volts. This is achieved by an engineer spec'ing multiple transformers usually one after the other. Yes, the secondary side will go to a load that requires whatever the output voltage is being supplied but then will also feed into the primary side of another transformer. Sometimes the transformers are literally right next to each other and sometimes they are over a 1/4 mile or more away.

Regardless if there are 2 or 7 transformers being used this way I just want to properly and safely protect the wire and components. I am unsure of how to do this with multiple transformers set up like this. I feel like it is not worth it or very difficult in trying to figure out the voltage loss.

 

[wwhitney]

I think this thread would be helped if you posted a sample one line diagram with more specific questions.

Cheers, Wayne

 

[TwoBlocked]

I think it is VERY important to calculate voltage drop in this situation. If a multi-tap transformer is chosen halfway through the system you should be able to overcome the drop at the end. A boost transformer is another possibility.

 

[winnie]

Voltage drop calculations are conceptually simple, but one can quickly get lost in the details. Generally we make approximations to keep things simple.

Voltage drop is what we call the voltage 'lost' in the supply elements, differentiated from the voltage 'dropped' across the load. But it is all the same thing: when current flows through something, some amount of voltage is needed to push that current through the device.

For the simplest resistive voltage drop, you simply have to remember that your supply wires are actually very low value resistors in series with your load. All of the current passes from the source, through one supply wire, through your load, and through another supply wire. To calculate 'voltage drop' you first determine the load current, next determine the supply wire resistance, multiply current * resistance, and get the voltage lost in the supply wires.

With transformers it is conceptually the same, but rendered more complex because the transformer has reactive impedance. Now you have to consider not only the voltage drop, but also the phase angle of that voltage drop. It is still simply Ohm's law, but generalized because instead of resistance you need 'impedance'.

Each transformer can be considered a _perfect_ transformer in series with a bit of resistance and a bit of inductive impedance. The perfect (or 'ideal') transformer trades voltage for current by the transformer ratio, with no voltage drop at all. Then the series impedance adds the realistic voltage drop. If you knew the transformer resistance and inductive impedance, you can simply incorporate it into your voltage drop equations, alongside the resistance (and inductance) of the supply wires.

Unfortunately transformer specifications don't do the simple thing and state the resistance and inductance. Instead the transformer will have its '% impedance' which is the total resistance and inductive reactance, but expressed as a % voltage drop at full load. A transformer with a 480V secondary and 5% impedance will have 24V voltage drop at full load. Before you way 'whoah that is a huge voltage drop', remember that % impedance includes both the _inductance_ and the _resistance_ of the transformer. Most of the % impedance is reactive, meaning that it is causing the voltage phase angle to shift but the magnitude doesn't actually drop.

To correctly calculate the voltage drop of cascaded transformers, the simplest approach is to draw a schematic where each transformer is replaced by an 'equivalent' resistance and inductance, and then calculate the voltage drop of this equivalent circuit.

-Jon

 

[synchro]

Unfortunately transformer specifications don't do the simple thing and state the resistance and inductance. Instead the transformer will have its '% impedance' which is the total resistance and inductive reactance, but expressed as a % voltage drop at full load. A transformer with a 480V secondary and 5% impedance will have 24V voltage drop at full load. Before you way 'whoah that is a huge voltage drop', remember that % impedance includes both the _inductance_ and the _resistance_ of the transformer. Most of the % impedance is reactive, meaning that it is causing the voltage phase angle to shift but the magnitude doesn't actually drop.

Very true for resistive loads. With inductive loads there will be some voltage drop across the "inductive divider" that is formed.