Transformer Impedance Computations

[tersh]

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.

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Oh, but the rule of SPDs/MOV is that there must be minimum impedance between SPD and load (or equipment). So you mean it's not really true?

 

[tersh]

Oh, but the rule of SPDs/MOV is that there must be minimum impedance between SPD and load (or equipment). So you mean it's not really true?

Or let's use the words directly from Golddigger who is a moderator so his words carry a lot of weight. He said there must be minimum impedance between the SPD and Load, what exact scenerio was he referring to when he mentioned in message #29 in http://forums.mikeholt.com/showthread.php?t=195234&page=3

"

One other side track from your overall situation:
The operation of a surge protective device depends on its being able to shunt away any voltage spikes above a certain level with a very low resistance, making a voltage divider with the source impedance of the fault that is causing the voltage excursion. That means that the SPD needs to have the absolute minimum impedance between it and the protected circuit conductors.
A transformer, even an autotransformer, will represent a relatively large impedance on the scale of an operating SPD.
With only one nominal 120V SPD to work with you will not be able to protect your 240V wiring and its connected loads.
If you had a way to run a solid metal path between the service transformer secondary at the pole and the location of your SPD(s) you would be able to use 120V SPDs in a symmetric circuit on either side to the line to that neutral/ground wire.
Just a ground electrode will give you little or no effective SPD operation unless you run two 120V SPDs symmetrically to that ground electrode or a single 240V SPD between the hot wires with no neutral/ground connection. "
​

 

[gar]

181218-0704 EST

tersh:

Suppose you have one single load to protect that has two terminals as the only points that you have access to?

You want your MOV as close as possible to those two terminals, and the MOV leads as short as possible connecting to said terminals. Then in series with the power leads feeding the terminals you want as much series impedance as feasible.

This produces the greatest voltage divider action, and keeps the clamped voltage as close as possible to the actual internal clamping voltage in the MOV.

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[tersh]

181218-0704 EST

tersh:

Suppose you have one single load to protect that has two terminals as the only points that you have access to?

You want your MOV as close as possible to those two terminals, and the MOV leads as short as possible connecting to said terminals. Then in series with the power leads feeding the terminals you want as much series impedance as feasible.

This produces the greatest voltage divider action, and keeps the clamped voltage as close as possible to the actual internal clamping voltage in the MOV.

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So the reason to make the MOV as close as possible to the load is to avoid voltage rise for every meter increased (which occurs when longer lines have greater impedance that can increase the voltage from the injecting surge current) and not because of divider action, right? I tried to apply the voltage divider action to the impedance near the load too.

 

[gar]

181218-2049 EST

tersh:

Consider a battery with internal impedance Zi and a load of Zload. For a fixed battery voltage and a constant Zload the greater Zi is the lower will be the Vload.

So we want a lot of Zi before the MOV and very little internal impedance in the MOV and wires to the MOV. The amount of wire or impedance from where the MOV wires connect to Zi to the load is not of great importance.

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[tersh]

181218-2049 EST

tersh:

Consider a battery with internal impedance Zi and a load of Zload. For a fixed battery voltage and a constant Zload the greater Zi is the lower will be the Vload.

So we want a lot of Zi before the MOV and very little internal impedance in the MOV and wires to the MOV. The amount of wire or impedance from where the MOV wires connect to Zi to the load is not of great importance.

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Is the above a typo.. did you mean "wires to the MOV" or "wires to the load"?

 

[gar]

181218-1940 EST

tersh:

That was not a typo.

An MOV is basically a two terminal device. If I connect a wire to one terminal, and go to the far end of that wire, then the far end of that wire and the other MOV terminal constitute a new two terminal network. This new network has a higher internal impedance than the original MOV alone. This new two terminal network is what becomes the effective transient limiter in any circuit it is connected to. This is what your green shunt device is.

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[tersh]

181218-1940 EST

tersh:

That was not a typo.

An MOV is basically a two terminal device. If I connect a wire to one terminal, and go to the far end of that wire, then the far end of that wire and the other MOV terminal constitute a new two terminal network. This new network has a higher internal impedance than the original MOV alone. This new two terminal network is what becomes the effective transient limiter in any circuit it is connected to. This is what your green shunt device is.

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A 1 meter wire has more internal impedance than a MOV? When you mentioned "The amount of wire or impedance from where the MOV wires connect to Zi to the load is not of great importance.". Zi or internal impedance refers to the wire? Why did you say it is not of great importance when Zi of one meter wire is more than the MOV impedance itself?

 

[gar]

181218-2124 EST

tersh:

Consider one idea at a time. Do you understand what I said in post numbered #47?

Any particular wire length that I connect to an MOV has no particular relation to the actual internal impedance of the MOV alone. However, adding an external wire to an MOV makes that serial combination look like an MOV with a higher internal impedance than of the MOV alone.

Do this: plot the i v curve for an MOV. Assume some added external linear resistance in series with the MOV. For various i points on the MOV curve calculate the i*R drop and add it to the MOV curve. These new points constitute a new curve that represents the i v curve of the combination of the MOV and the added external resistance. This should be self evident.

The new curve is the limiting characteristic of the combination of the original MOV plus the external series resistance. Clearly this combination does not provide a limiting function as low as the MOV alone.

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[tersh]

181218-2124 EST

tersh:

Consider one idea at a time. Do you understand what I said in post numbered #47?

Any particular wire length that I connect to an MOV has no particular relation to the actual internal impedance of the MOV alone. However, adding an external wire to an MOV makes that serial combination look like an MOV with a higher internal impedance than of the MOV alone.

Do this: plot the i v curve for an MOV. Assume some added external linear resistance in series with the MOV. For various i points on the MOV curve calculate the i*R drop and add it to the MOV curve. These new points constitute a new curve that represents the i v curve of the combination of the MOV and the added external resistance. This should be self evident.

The new curve is the limiting characteristic of the combination of the original MOV plus the external series resistance. Clearly this combination does not provide a limiting function as low as the MOV alone.

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Thanks I understood it. But kindly rephrase this confusing sentence.. "The amount of wire or impedance from where the MOV wires connect to Zi to the load is not of great importance."

If Zi is internal impedance. Then the sentence is like "The amount of wire or impedance from where the MOV wires connect to internal impedance to the load is not of great importance."

It's confusing since both MOV and wire has internal impedance. So were you referring to the internal impedance of the wire above when you mentioned Zi? But then MOV is directly connected to the wire (Zi), so what 2nd order wires were you referring to that connects between the MOV and the wires?


​

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[gar]

181218-2205 EST

tersh:

I believe the Zi we are talking about is the internal impedance of your voltage source (power source) which includes all wires and transformers from the source voltage to the point where the MOV is connected. Assume for the present there is no additional load other than the MOV. This series combination of voltage source, Zi, and the MOV determines the voltage across the MOV.

That combination can now be replaced by some non-linear equivalent voltage source and internal impedance.

For many loads, including wires, that connect across the new equivalent circuit the voltage to the final load will be not greater than the open circuit voltage across the MOV before connecting the load. Thus, the length of wires from the MOV to the actual load does not create any significant problems. Greater voltage could occur at the final load if resonance in that combination occurred. But with moderate length wires and typical loads this is not likely.

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[tersh]

181218-2205 EST

tersh:

I believe the Zi we are talking about is the internal impedance of your voltage source (power source) which includes all wires and transformers from the source voltage to the point where the MOV is connected. Assume for the present there is no additional load other than the MOV. This series combination of voltage source, Zi, and the MOV determines the voltage across the MOV.

That combination can now be replaced by some non-linear equivalent voltage source and internal impedance.

For many loads, including wires, that connect across the new equivalent circuit the voltage to the final load will be not greater than the open circuit voltage across the MOV before connecting the load. Thus, the length of wires from the MOV to the actual load does not create any significant problems. Greater voltage could occur at the final load if resonance in that combination occurred. But with moderate length wires and typical loads this is not likely.

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Ah. so the dreaded 1000 volts per meter of wire length doesn't apply after the MOV. The reasoning is this:​

"L1 represents the inductance of the long wire. I1 is the 8kA 80/20 waveform. The longer the wire, the larger L1 will be, and the larger the voltage across I1 will be when the pulse waveform is applied. That is all the article is trying to say. Yes, a longer wire has more inductance, and consequently, it will have more voltage develop AT THE POINT OF CURRENT INJECTION. Not down at the end of the line away from the current injection point."

So your argument is that at the MOV, there is no further injection of current to increase the voltage. The following is then false or just a myth?

http://www2.schneider-electric.com/...277471/en_US/Surge protection devices SPD.pdf

​

 

[gar]

181218-2329 EST

tersh:

You need to stick with one basic idea, concept, circuit, or whatever and get a fundamental understanding of that basic item. Then expand to other changes. You have to compare comparable things. It is not feasible to get an understanding if you jump all over the place. So far you do not seem to understand how to effectively apply an MOV in a circuit, or how to pick apart the particular circuit to study its operation.

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[tersh]

181218-2329 EST

tersh:

You need to stick with one basic idea, concept, circuit, or whatever and get a fundamental understanding of that basic item. Then expand to other changes. You have to compare comparable things. It is not feasible to get an understanding if you jump all over the place. So far you do not seem to understand how to effectively apply an MOV in a circuit, or how to pick apart the particular circuit to study its operation.

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I understood what you were saying already. Idea of MOV is simple. It's to cause voltage divider action by initiating low impedance clamping in the load after voltage threshold is reached.

About the 1000v per meter increase article. If the point of injection of the lightning is one mile away. During the point of injection it can already sense whether there is very long length between the source impedance to MOV to the load. Isn't it. So the more impedance the greater the current the lightning will strike or induce. I think this is what the article means, agree?

 

[gar]

181218-2411 EST

tersh:

First, the article assumes a current source, not a voltage source, and that is perfectly valid to do so, but at some point this may not be valid. But that is really not the point of the article.

The article is trying to point out that the lead length attached to an MOV can make the voltage much higher at the point where the MOV plus its leads is the shunt. Thus, you want short leads from the MOV to where the shunt is connected to limit voltage.

Further it is pointing out that the very rapid rise of current, high frequencies, is why the inductance of a short piece of wire, although small, can cause the limiting voltage to be higher than might be initially expected based on wire resistance.

The article is not about the leads from the shunting point to the load.

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[tersh]

181218-2411 EST

tersh:

First, the article assumes a current source, not a voltage source, and that is perfectly valid to do so, but at some point this may not be valid. But that is really not the point of the article.

The article is trying to point out that the lead length attached to an MOV can make the voltage much higher at the point where the MOV plus its leads is the shunt. Thus, you want short leads from the MOV to where the shunt is connected to limit voltage.

Further it is pointing out that the very rapid rise of current, high frequencies, is why the inductance of a short piece of wire, although small, can cause the limiting voltage to be higher than might be initially expected based on wire resistance.

The article is not about the leads from the shunting point to the load.

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Why didn't they include the leads from the shunting point to the load? (shown in pink) I've been wondering about this for 3 months.

 

[gar]

181219-0819 EST

tersh:

They show the leads to the MOV, a series string of wires including the MOV in the series string, and the voltages across all the series segments with the MOV. U appears to be their label for voltage. So then in their drawing is shown a voltage Vload = U1+Up+U2 across the load.

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[winnie]

Something to keep in mind: you have different sources of 'surge' to protect against.

If you are protecting from 'excessive voltage between your supply conductors', then you need impedance in your supply circuit 'upstream' of the SPD. The SPD acts as a voltage divider with that supply impedance to reduce the peak voltage at the terminals of the SPD. Any 'downstream' impedance between the SPD and the load will act to reduce the voltage further, so for this particular type of surge impedance between the SPD and the load is not an important factor.

Another source of 'surge' is common mode voltage between different lines connected to your equipment. Say for example that you have a computer with a power supply connection and a phone line connection. If you are trying to protect from surges _between_ these different lines, then you somehow need SPD between them. The most common approach here would be SPD connected between lines and the local 'ground' (meaning the chassis of the equipment, not necessarily true earth ground.) In this case you are probably thinking of the power supply SPD and the phone line SPD as separate devices, and clearly you want to minimize the impedance between these units.

Yet another source of 'surge' is induced voltage caused by magnetic fields from nearby events. Here you can get large induced voltages, and impedance between the SPD and the load means that the SPD doesn't protect from these induced voltages.

As gar says, you need to understand one concept, and then think about the entire circuit and see how this concept applies from different directions.

-Jon

 

[RumRunner]

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.

A good question is half answered but your question is convoluted.

Allow me to ask a question which you might think being vituperative. I don't want to sound like someone riding on a high horse—since no one has brought this thing up or even considered important.

What is Impedance Z2 doing in your MOV path to ground? If you are confused—then it is OK we all get confused once in a while.

“What is going to happen to the MOV” you ask.

Different effects happen depending on where the MOV is deployed in a circuit. The first thing to consider in selecting the rating of MOV is the voltage where you intend to use it in order to get the optimum benefit.

In your schematic, you did not specify what voltage your circuit will be operating on. If you are running DC, you can't be referring to typical wave form.

If you are running this circuit on 220 volt AC, the upper tolerance limit of the voltage source is important. . . all the more important to state the voltage that you either intentionally omitted or simply missed in your schematic.

The first junction between the two resistors in red would have a difference in voltage compared to the second one because of the voltage drop offered by resistance of the second resistor.
So, if you locate the MOV closer to the load (indicated electronics) you would need a different rating.

Selecting the correct rating is crucial. Too high will allow more transients and clamping capability is compromised. Too low will cause the MOV to be super-sensitive.

MOVs are dispensible commodities—more like fuses. The difference is they can work repeatedly until the semiconductor can no longer divert spikes that they are designed to do.
Fuse on the other hand is thrown out the first time after doing its job.

MOVs are rated on how much energy is dissipated in joules—not in amps. So, your 3000 Amps in your post is inconsequential.


For example-- as stated above-- if running at 220 volts, you apply a 10% a high line condition which is 220 + 10% = 242.
This would be the continuous voltage that you expect to encounter due to several conditions that may affect voltage stability. Higher voltage could be experienced in low demand hours.


After determining this “upper voltage”, you go to the table below. You select the closest continuous voltage –you don't have to be precise-- just close enough. For example if your voltage is 240 you can select 260 and follow that column to find the correct rating. Note that it will clamp at 450 volts. It will even tell you what catalog number it is. How cool is that?

ICs are made this way having been through rigorous testing.

The 10 ohm resistor or the length of wire would have minimal impact but still important to be aware of.


https://sciencing.com/size-mov-surge-protection-8708803.html

Not all machines may have access to tables perhaps due ownership of documents. I'm running Linux.

 

[tersh]

Something to keep in mind: you have different sources of 'surge' to protect against.

If you are protecting from 'excessive voltage between your supply conductors', then you need impedance in your supply circuit 'upstream' of the SPD. The SPD acts as a voltage divider with that supply impedance to reduce the peak voltage at the terminals of the SPD. Any 'downstream' impedance between the SPD and the load will act to reduce the voltage further, so for this particular type of surge impedance between the SPD and the load is not an important factor.

Another source of 'surge' is common mode voltage between different lines connected to your equipment. Say for example that you have a computer with a power supply connection and a phone line connection. If you are trying to protect from surges _between_ these different lines, then you somehow need SPD between them. The most common approach here would be SPD connected between lines and the local 'ground' (meaning the chassis of the equipment, not necessarily true earth ground.) In this case you are probably thinking of the power supply SPD and the phone line SPD as separate devices, and clearly you want to minimize the impedance between these units.

Yet another

source of 'surge' is induced voltage caused by magnetic fields from nearby events.​

Here you can get large induced voltages, and impedance between the SPD and the load means that the SPD doesn't protect from these induced voltages.

Winnie, about this "

source of 'surge' is induced voltage caused by magnetic fields from nearby events.". Were you referring to the typical surge caused by lightning, for example the 6000v, 3000A, 8/20 usec waveform surge? gar just mentioned impedance between the SPD and load was not significant yet you mentioned impedance between the SPD

means that the SPD doesn't protect from these induced voltages. This was also what Golddigger has mentioned but didn't elaborate. Can you please elaborate how it could occur? Remember gar shows us impedance between SPD and load is not significant. Again refer to this diagram.

​

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Btw.. I'm familiar with MOV specs like MCOV, VPR, etc. so didn't give the line voltage because I just want to know specifically what impedance between MOV and load can do. Thanks.

As gar says, you need to understand one concept, and then think about the entire circuit and see how this concept applies from different directions.

-Jon