Transformer Impedance Computations

[tersh]

Here is the exact connection to make it clearer.

In the main panel is 240v AC power source protected by the 2 pole Siemens First Surge 140,000A SPD type 2.

Then 10 meters away to the load there is this 240-120v step down Hammond 500va isolation transformer:

The 240-120v step down transformer is driving a 120v Leviton SPD type 3 surge protector:

First understand that an SPD works by having significant source impedance so that by divider action, the MOV would have less current and voltage. If one doesn't understand this. Then pls. understand this first because my question is related to this (it's brought up by Golddigger but then he didn't follow with the question what would happen if the impedance at source and equipment is equal hence confusing me for days)..

So what happens when you have say equal impedance in the source and near equipment. What would happen to the SPD type 2 (the Siemens First Surge) at main panel. It's seeing 2 both equal impedance at source and near equipment.

 

[topgone]

Tersh
Your drawing is incomplete, when it comes to a pulse of energy (high frequency) it different theory and different solution, simple 60hz formula doesn’t work. It is not easy to solve and get exact numbers.
Where is an impedance of your wiring?
What is a frequency of voltage surge?
It easy to understand what is happening here by thinking in energy dimensions and wave theory, see below picture.

Do you know why it's recommend to have additional SPD’s close to equipment?

Yep. I can remember being flabbergasted to find an expensive data transmitter electronics damaged just because out of the 20 SPDs, all were shot from previous strikes. Manager was informed in advance the need for additional SPDs but didn't bother buying some!

 

[tersh]

Here is the exact connection to make it clearer.

In the main panel is 240v AC power source protected by the 2 pole Siemens First Surge 140,000A SPD type 2.

Then 10 meters away to the load there is this 240-120v step down Hammond 500va isolation transformer:

The 240-120v step down transformer is driving a 120v Leviton SPD type 3 surge protector:

First understand that an SPD works by having significant source impedance so that by divider action, the MOV would have less current and voltage. If one doesn't understand this. Then pls. understand this first because my question is related to this (it's brought up by Golddigger but then he didn't follow with the question what would happen if the impedance at source and equipment is equal hence confusing me for days)..

So what happens when you have say equal impedance in the source and near equipment. What would happen to the SPD type 2 (the Siemens First Surge) at main panel. It's seeing 2 both equal impedance at source and near equipment.

No one knows the answer to the above? I know SPD was not being taught in electrical engineering in the past. But is it being taught now?

 

[Russs57]

SPD’s and RCD’s have been used for a long time. The way they work and the names they have been called are all that has changed.

Given that you have a very small/compact system all you need is a single type one on the line side of main panel. It is vital to place it as close as possible to source and keep all impedance as low as possible. After all you are looking for that SPD to pass thousands of volts and amps. I wouldn’t waste my time with downstream type 2 and 3 unless I had hundreds of feet of wire from source to load.

You might want to look into isolated power systems and line isolation monitors. I would consider them the ultimate in safety. You just might find you are safer than you think and the improvements you want will make it less safe. I can’t say without a better understanding of current installation.

 

[topgone]

SPD’s and RCD’s have been used for a long time. The way they work and the names they have been called are all that has changed.

Given that you have a very small/compact system all you need is a single type one on the line side of main panel. It is vital to place it as close as possible to source and keep all impedance as low as possible. After all you are looking for that SPD to pass thousands of volts and amps. I wouldn’t waste my time with downstream type 2 and 3 unless I had hundreds of feet of wire from source to load.

You might want to look into isolated power systems and line isolation monitors. I would consider them the ultimate in safety. You just might find you are safer than you think and the improvements you want will make it less safe. I can’t say without a better understanding of current installation.

For small voltage aberrations, use surge protection devices! For larger and more destructive strikes, use lightning arresters! It's not that something just came out of the blue and people don't know it.

 

[tersh]

SPD’s and RCD’s have been used for a long time. The way they work and the names they have been called are all that has changed.

Given that you have a very small/compact system all you need is a single type one on the line side of main panel. It is vital to place it as close as possible to source and keep all impedance as low as possible. After all you are looking for that SPD to pass thousands of volts and amps. I wouldn’t waste my time with downstream type 2 and 3 unless I had hundreds of feet of wire from source to load.

I'd just like to know what would happen if the impedance of the source is the same as the impedance near load (for sake of discussion and understanding it only). Wouldn't the SPD function at all or would it just take more effort. I just couldn't find any reference anywhere how the SPD work with impedance. There is totally nonexistence information about this anywhere. No textbooks mentioned this.

You might want to look into isolated power systems and line isolation monitors. I would consider them the ultimate in safety. You just might find you are safer than you think and the improvements you want will make it less safe. I can’t say without a better understanding of current installation.

 

[gar]

181216-2352 EST

tersh:

Use your basic electrical theory knowledge to study what happens in the circuits you present.

An MOV on the output side of your small control transformer will be very useful. There is a high internal impedance in the control transformer compared to the impedance of stuff up to the main panel. The MOV at the main panel is useful because it lowers the peak voltage into the control transformer.

You do not need to have been taught about MOVs, you can study about them on your own.

Get a datasheet on an MOV and see how its V-I curve looks. Then apply theory of circuits.

The way to produce good lightning protection is to place a large copper sheet a the entry to the building. All wiring passes thru capacitors in this sheet. The sheet is earthed by a wide conductor, lower AC impedance. High current arc discharge elements are placed from the incoming lines to the conductive plated. Probably place some inductors in series before the capacitors. Inside place the first MOVs. These terminate their ground to the conductive sheet. Near loads needing protection place MOVs and possibly series inductance before the MOV.

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[tersh]

181216-2352 EST

tersh:

Use your basic electrical theory knowledge to study what happens in the circuits you present.

An MOV on the output side of your small control transformer will be very useful. There is a high internal impedance in the control transformer compared to the impedance of stuff up to the main panel. The MOV at the main panel is useful because it lowers the peak voltage into the control transformer.

You do not need to have been taught about MOVs, you can study about them on your own.

Get a datasheet on an MOV and see how its V-I curve looks. Then apply theory of circuits.

The way to produce good lightning protection is to place a large copper sheet a the entry to the building. All wiring passes thru capacitors in this sheet. The sheet is earthed by a wide conductor, lower AC impedance. High current arc discharge elements are placed from the incoming lines to the conductive plated. Probably place some inductors in series before the capacitors. Inside place the first MOVs. These terminate their ground to the conductive sheet. Near loads needing protection place MOVs and possibly series inductance before the MOV.

.



.

I need to see some sample computations what would occur to the MOV when it's between both high impedance source and high impedance load. You see. My last subjects of electrical theory was in 1991. My course was electronics and communications engineering. We had not many subjects about AC. And I never applied them afterwards. So I need to review now. We focused on DC before, and not much on AC. Remember it's not a university at Caltech but in the asia where we only analyzed up to Z80 microprocessor.

About the MOV between say two equal impedance source and load. How does it behave? Thanks.

 

[tersh]

I need to see some sample computations what would occur to the MOV when it's between both high impedance source and high impedance load. You see. My last subjects of electrical theory was in 1991. My course was electronics and communications engineering. We had not many subjects about AC. And I never applied them afterwards. So I need to review now. We focused on DC before, and not much on AC. Remember it's not a university at Caltech but in the asia where we only analyzed up to Z80 microprocessor.

About the MOV between say two equal impedance source and load. How does it behave? Thanks.

Here is a thing. Even a major distributor of SPDs in the Philippines who is a licensed electrical engineer doesn't know how MOV works with impedance. He just memorized the supplier instruction to put MOV with respect to ground. And that's it. Even new graduates of electrical engineering don't know the electrical theory of SPDs/MOVs. It's not in their textbook. Yes we may be more dumb about it. But is there at least a book or a chapter of a textbook devoted to circuit theory of SPDs/MOV?

For now. I just want to understand say about the MOV between say two equal impedance source and load. How does it really behave? By voltage divider action only? what else?​

 

[tersh]

Here is a thing. Even a major distributor of SPDs in the Philippines who is a licensed electrical engineer doesn't know how MOV works with impedance. He just memorized the supplier instruction to put MOV with respect to ground. And that's it. Even new graduates of electrical engineering don't know the electrical theory of SPDs/MOVs. It's not in their textbook. Yes we may be more dumb about it. But is there at least a book or a chapter of a textbook devoted to circuit theory of SPDs/MOV?

For now. I just want to understand say about the MOV between say two equal impedance source and load. How does it really behave? By voltage divider action only? what else?​

Another thing. Licensed electrical engineers in my country are good in designing proper sizing of wires and breakers based on the loads. This is what they mostly do. Many don't or ignore analyzing unique circuits. So they forgot about MOV and Impedance. The major local SPD distributor who is licensed electrical engineer personally brought his electrician to install some 240v SPD, but he used 320v MCOV for the 120v connection line to ground (in the office building with neutral and 120v availability). He said he never connect line to line because that was not how the china supplier taught him. Later when I consulted with a US based electrical engineer, the latter mentioned 150v MCOV must be used for 120v line to ground and not 320v MCOV. The local EE didn't get it and reasoned 320v MCOV was the VPR rating. I corrected him by telling him VPR is separate from MCOV. I didn't use the 320v MCOV SPD at main panel and instead use the Siemens First Surge after a lot of technical discussions with the US based EE.

This showed many local EE doesn't really study every circuit as their task is just sizing right wire and breakers for load.

My questions about impedance and the MOV is not asking how best to connect it. But how to compute for it. So I can review my electronics engineering lessons back in the days of Operation Desert Storm. MOV/SPD should be my domain better than the local EE.

So when your source impedance and near load impedance is identical. The MOV current won't be low and voltage wouldn't be low, right? I know the MOV needs the source impedance to lower the current by divider action (that's why needing low load impedance).

But how bad is it? Would the MOV be totally ineffective then when source and load impedance is close in values?

I was not asking for advice whether to retain the Siemens First Surge at main panel or remove the transformer/type 3 spd thing. I just want to learn how to compute it. In fact, I let electrician installed it that way to understand it and not for really practical purpose (which is just using the Siemens First Surge type 2 SPD and nothing else).

Winnie, are you familiar with MOV/SPD, what do you think? (and others familiar with MOV and impedance thing.. many thanks).

 

[mivey]

Sorry I haven't been following but I noticed gar's post. Where is this equal source and load impedance coming from?

If it is what I think tersh proposes, a practical source won't function well like that. The supply would not be stiff enough in a power application. You would like the available fault current to be at least 10 times the load current. Even stiffer is better.

A source as weak as tersh proposes would not drive the load. A simple voltage divider calc would show that.

 

[tersh]

Sorry I haven't been following but I noticed gar's post. Where is this equal source and load impedance coming from?

If it is what I think tersh proposes, a practical source won't function well like that. The supply would not be stiff enough in a power application. You would like the available fault current to be at least 10 times the load current. Even stiffer is better.

A source as weak as tersh proposes would not drive the load. A simple voltage divider calc would show that.

Fault current? We were describing surge current.. not GFCI ground fault current.

I was just asking what would happen to the MOV (shown in green) when it's between the source having great impedance and say a transformer near the load with great impedance. The value is just for sake of computations and not signifying anything. Assume a surge of typical 6000v, 3000A, typical surge waveform hits it. It's just an example of the surge current suppressed at the MOV.

 

[gar]

181217-0758 EST

tersh:

Your last several posts are a big help. Electrical engineer in your country does not have the same meaning as in the US and Western Europe.

An MOV is a non-linear resistor. In this case meaning its resistance changes as voltage or current to the MOV changes. A linear resistor would have a resistance value that was constant independent of voltage or current up to some power dissipation level.

For a good discussion on MOVs see
https://www.vishay.com/docs/29079/varintro.pdf

Study this, and then return with questions.

I studied EE from 1948 thru 1961 with a 2 year interruption from 1951 thru 1952 for Naval active duty. Never had a class that specifically covered MOVs. But basic EE provides the background. I started to use MOVs by 1960 and studied their characteristics from some GE databooks. But this is no different than the study of vacuum tubes and later semiconductors. I first learned about non-linear resistors via the study of vacuum tube characteristics in 1941.

You need to view the MOV from a series circuit perspective. A constant impedance in series with an MOV fed from a variable voltage source.

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[tersh]

181217-0758 EST

tersh:

Your last several posts are a big help. Electrical engineer in your country does not have the same meaning as in the US and Western Europe.

An MOV is a non-linear resistor. In this case meaning its resistance changes as voltage or current to the MOV changes. A linear resistor would have a resistance value that was constant independent of voltage or current up to some power dissipation level.

For a good discussion on MOVs see
https://www.vishay.com/docs/29079/varintro.pdf

Study this, and then return with questions.

I studied EE from 1948 thru 1961 with a 2 year interruption from 1951 thru 1952 for Naval active duty. Never had a class that specifically covered MOVs. But basic EE provides the background. I started to use MOVs by 1960 and studied their characteristics from some GE databooks. But this is no different than the study of vacuum tubes and later semiconductors. I first learned about non-linear resistors via the study of vacuum tube characteristics in 1941.

You need to view the MOV from a series circuit perspective. A constant impedance in series with an MOV fed from a variable voltage source.

.

.

It's rare for me to meet someone who has actually gone through world war II. My peers are in the 1970s.

Thanks for the reference. I'll read it. If others have textbook chapters on it, please share too. I just want to learn how to compute so my 5 years of college is not wasted. And from the computation. I may even get to become an SPD vendor in the country. Most vendors install the 320v MCOV in 120v line to ground. It really needs 150v MCOVs.

In my country. People who take Electrical Engineering ends up in different paths.

1. Some take it just to have a course in college then go to other business afterwards
2. Some just signs and seals building plans
3. Some end up as electrical contractors which install wires, breakers, feeds, in building and they earn big money
4. Some end up selling electrical items either wholesale or retails
5. Some work at the power industries

Only the last profession above needs the EE knowledge to compute or analyze circuits. This is why most doesn't do it and forget. We don't have any high tech facilities like nuclear power plants or design firm to require EE that can actually analyze unique circuits and compute for them. But they all know how to compute for load and wire sizes and breaker, bus bar sizes and it's all that matters.

 

[gar]

1812117-2137 EST

tersh:

I was typing my post at the time you submitted the post with the diagram. Very nicely drawn diagram.

I assume what you have in red is the MOV. Until a threshold voltage is reached this is a very high resistance. Above the threshold the red device can be approximated as a constant voltage (battery of correct polarity, 400 V) with a low internal resistance (1 ohm). Thus, this device serves as a clamp to that voltage plus an I*R drop.

The large transient voltage would originate at the left side of your drawing. Suppose that was 10,000 V, the MOV clamp voltage is 400 V, then the red device would see 400 V (the threshold voltage) + 10,000 - (10,000-400)*1/11 = 400 + 873 = 1273 . Now you divide by two because you have 10 ohms in series with the 10 ohm load, and the load voltage is then 873/2 = 436 V. Put another MOV across the load, possibly with a threshold of 300 V, and you calculate what the load voltage will be. It will be toward the 300 V.

Check my calculations for any errors.

.

 

[tersh]

1812117-2137 EST

tersh:

I was typing my post at the time you submitted the post with the diagram. Very nicely drawn diagram.

I assume what you have in red is the MOV. Until a threshold voltage is reached this is a very high resistance. Above the threshold the red device can be approximated as a constant voltage (battery of correct polarity, 400 V) with a low internal resistance (1 ohm). Thus, this device serves as a clamp to that voltage plus an I*R drop.

The large transient voltage would originate at the left side of your drawing. Suppose that was 10,000 V, the MOV clamp voltage is 400 V, then the red device would see 400 V (the threshold voltage) + 10,000 - (10,000-400)*1/11 = 400 + 873 = 1273 . Now you divide by two because you have 10 ohms in series with the 10 ohm load, and the load voltage is then 873/2 = 436 V. Put another MOV across the load, possibly with a threshold of 300 V, and you calculate what the load voltage will be. It will be toward the 300 V.

Check my calculations for any errors.

.

No. I should have labeled them clearly. The red is simply the impedance of the wires. The green one is the MOV

 

[gar]

181217-2240 EST

tersh:

Where I said red should have been green. My mistake. You were clear I made a translation mistake.

.

 

[tersh]

1812117-2137 EST

tersh:

I was typing my post at the time you submitted the post with the diagram. Very nicely drawn diagram.

I assume what you have in red is the MOV. Until a threshold voltage is reached this is a very high resistance. Above the threshold the red device can be approximated as a constant voltage (battery of correct polarity, 400 V) with a low internal resistance (1 ohm). Thus, this device serves as a clamp to that voltage plus an I*R drop.

The large transient voltage would originate at the left side of your drawing. Suppose that was 10,000 V, the MOV clamp voltage is 400 V, then the red device would see 400 V (the threshold voltage) + 10,000 - (10,000-400)*1/11 = 400 + 873 = 1273 . Now you divide by two because you have 10 ohms in series with the 10 ohm load, and the load voltage is then 873/2 = 436 V. Put another MOV across the load, possibly with a threshold of 300 V, and you calculate what the load voltage will be. It will be toward the 300 V.

Check my calculations for any errors.

.

So the red is green as you corrected above. Green is the MOV.

Some warned never to have high impedance between the MOV and the equipment (or load). So there is no problem even if there is high impedance between them.. isn't it?

 

[tersh]

So the red is green as you corrected above. Green is the MOV.

Some warned never to have high impedance between the MOV and the equipment (or load). So there is no problem even if there is high impedance between them.. isn't it?

Oh, the diagram is a mistake. I was modelling the impedance of a transformer put before the load. So the transformer impedance must be modeled as impedance parallel to the load.

How do you think it would change it? Note green is the MOV. Red is the impedance of source and transformer. Many thanks.

 

[gar]

181217-2329 EST

tersh:

Your original model was correct. The right hand 10 ohm impedance is the series impedance of the isolation transformer. The shunt impedance of the isolation transformer is relatively high compared to its series internal impedance.

.