AC Electric Field vs DC Electric Field?

[js2020]

TL;DR Summary

I am trying to understand the parameters used for calculating DC and AC electric fields.

I have an object that will be under DC excitation in operation but will be qualified using 60 Hz AC. Because of this, I am interested in 2 simulations.

1) I would like to simulate E-field intensity representing a 60 Hz excitation. Do I need to do a transient simulation to truly get this value at the AC-peak, do I set the excitation to the AC-peak value, or do I need to set the excitation as the AC-RMS value to simulate the e-field intensity for 60 Hz sinusoidal excitation?

2) I would like to consider only a static (DC) electric field. My understanding is that should actually be the electrostatic simulation; however, electrostatic only considers permittivity and not bulk conductivity. In Ansys Maxwell, to consider bulk conductivity, I have to select DC conduction and check "include insulator fields'. In this case, I think it will consider permittivity AND bulk conductivity. Can anyone confirm if this is this is correct? For a true DC electric field, would bulk conductivity be the only parameter of interest and not the permittivity?

 

[Baluncore]

Welcome to PF.

For a true DC electric field, would bulk conductivity be the only parameter of interest and not the permittivity?

A DC field will have a gradient determined by the resistance of the materials.

For AC models of well insulated devices, conductors will become equipotentials, while the permittivity will determine the local gradient of the field. You might model dielectric layers as capacitors in series.

If the AC field is specified as Vrms then the peak field will vary between Vrms * ±√2 ;
You may need to simulate an open circuit to identify and be sure of the calibration of AC stimulation.

It really will depend on the device you are testing, the reason for testing and the effects you may expect to see. Can you please better describe the device you are modelling in the simulation.

 

[js2020]

Hi,
Thanks for your reply. What I'm trying to model is the electric field between two connectors. I've attached a screenshot of the 3D models. Basically, I would like to determine the what spacing I need between the two. First, I would like to model them in air. Next, I would like to include the enclosure which will be let's just say nylon for now. At the end of the day, I am trying to answer the following questions.

1) What equations should I use to estimate the E-field intensity between the two in air.
a) what equation should I use for electrostatic assuming AC? I was thinking of using an equation that considered electric field between two cylinders which would be a large simplification for this since it would probably be true only for a long cylinder.
b) What equation should I use for electrostatic assuming DC? I know that this would be considering the volume resistivity in air

2) What equations should I use to estimate the same points as in 1 a,b but with a piece of solid dielectric between the two. I know this will depend on the volume resistivity of the solid, as well as air. I think air can be tricky since it changes depending on temperature, humidity, elevation, etc. I also know this will be a lot more complicated since it will be a mixed insulation system but will hopefully be manageable.

3) I would like to simulate the points from 1a,b and 2a,b to see how close it got. I hope that explains what I'm wanting to do clear enough. I've attached two pictures to describe 1 and 2.

 

[Baluncore]

Simulation of a safe separation is unwise since the breakdown of electric fields is partly environmentally determined, so somewhat unpredictable.
A simulation will give you false confidence.

The alternative is the routine calculation of all possible leakage paths when subjected to contamination and ageing of materials. The calculation required will depend on the application.

The advantage of a dielectric wall is that the surface leakage path is lengthened. But the joint between that wall and the bulkhead may become a weak point. High DC voltages attract contaminants to surfaces. Sometimes a coaxial cable is more reliable than a single conductor because the electric field is fully contained inside the cable and connector.

Questions that determine the computational approach are ...
Is the voltage between the conductors AC, DC or both ?
What is the peak voltage between the two electrodes ?
Where is the nearest chassis or protective ground ?
Is this a low impedance milliamp power supply, or very high impedance microamp circuit ?
Do you have a data sheet with voltage ratings for the connectors?
Will the equipment be subject to lightning or electrostatic discharge ?

 

[js2020]

Thank you for your suggestions. Unfortunately, I can't really predict breakdown voltage through simulation because of varying air conditions as you mentioned, but I can add the parameters for worst case air conditions and at least get an idea of what to expect for air conditions. I found a paper from a while back that measured leakage current for various relative humidity levels. Using the leakage current and and applied voltage, I can get the volume resistivity at that given air condition. I plan to use these values for simulation. From there, I will set an arbitrary threshold of something like <2 kV/mm. If I can ensure that the peak efield intensity in the worst case air conditions that I will likely never see is low enough, I should be able to ensure I have a partial discharge free design in normal operating conditions. The efield intensity in the solid dielectric will be relatively low since it's mainly being used to help reduce the field intensity at the connector interface. Cutouts to ensure enough creepage can then be added.

To answer the questions though...
The voltage will be DC voltage. Therefore, I plan to do do DC conduction simulations in Maxwell 3D for the design. Initial tests will be using an AC source so I plan to do AC simulations as well just to get a feel for what I may expect.

The peak voltage between electrodes is 24 kV nominal. The design parameters are accounting for 30 kV though. That is 24 kV nominal + 5% operating tolerance + 5% ripple + 10% design margin. That would put me at about 29.1 kV so I'm designing for 30 kV. The operating tolerance and ripple are hand waving but should be fair assumptions.

This is a low impedance power supply

The connectors are only rated for 10 kV. The thing is, they're rated for 10 kV using a coax cable so it assumes it will be holding off the full 10 kV. Once this design is complete, the spacing should be adequate so that each connector can hold off 30 kV since the opposite electrode is far away.

The equipment will not be subject to lightning. I'm trying to prevent ESD in this design. For that, I may use an ESD material such as carbon filled nylon or carbon filled ABS. Volume restivity and permittivity for different materials will be included in simulation determine how peak field intensity changes with different materials though.

 

[js2020]

Ok so I just thought that I'd reply to this since I found something out. I know that my questions were kind of all over the place but I found out that Ansys DC conduction simulations are truly DC electric field only. One thing to pay attention to is your materials conductivity. By default, if it is less that 1 S/m, Ansys will see the conductivity as 0 and there will be no results...in other words, your E field intensity will show 0 V/m everywhere. Change the tolerance through the Maxwell settings to less than your lowest conductivity in order to get the results you are looking for?

 

[Baluncore]

I see no advantage in the two insulated connectors.
The 10 kV rated connectors are part of a leakage path, a series voltage divider.
If the gap between the connector shells is more conductive than the internal connector insulation, then the connectors will be operating at more than 10 kV. That will degrade their insulation.
It is difficult to know without data on the construction of the connectors.

 

[js2020]

I see no advantage in the two insulated connectors.
The 10 kV rated connectors are part of a leakage path, a series voltage divider.
If the gap between the connector shells is more conductive than the internal connector insulation, then the connectors will be operating at more than 10 kV. That will degrade their insulation.
It is difficult to know without data on the construction of the connectors.

Yes you are correct. Let me clarify what I meant with my last message.

The material conductivity threshold settings should be adjusted. If you have a conductor that is 5e-9 S/m, ansys will treat this as a perfect insulator. In that case, there will be 0 current in the DC field analysis which leads to 0 V/m electric field intensity. If you lower the threshold below 5e-9 S/m, it will treat it as an ideal insulator. Which is what you want, and you will see your fields.

In other words, make sure that your threshold is set lower than your best conductor or it will ignore it all together.

 

[Baluncore]

In that case, there will be 0 current in the DC field analysis which leads to 0 V/m electric field intensity.

I can understand the simulation current being assumed zero. I do not see why the electric field intensity should also be zero without a short circuit.

I think you may have missed my point about reality. The three layers of insulation will not share the electric field equally, 10kV + 10kV + 10kV. When the space between the connectors gets damp, one connector will begin to fail. 13kV + 5kV + 12kV. As leakage increases it will pass field to the other connector, or the gap between the connector shells. The progressive failure of all insulation follows.

 

[js2020]

O ok I see what you mean. I found a paper that tested the volume sensitivity of air at different relative humidity ranging from like 60-100%. By looking at the leakage current. I found the volume resistivity and I'm doing the simulations under this condition. The efield intensity in the solid compared to its data sheet rating is far below what it's actually subject to, based on simulation. The efield intensity in air is something like 7 kV/mm so I expect to definitely see that as my problem area. So I'm trying to get the field in air to be >2kv/mm and keep it the same order of magnitude-ish in the solid since it's already so much below the data sheet rating. I do understand the datasheet rating is under a specific test condition and thickness but the difference is so large I don't think it's a concern.**edit**
As far as why the efield intensity is zero with no current...I think it's because I am doing DC conduction analysis. If there is 0 current, there will be zero voltage dropped through it, right? That's the way it was explained to me anyway. I'm not doing electrostatic so permittivity is not considered. With a perfect insulator, there is no conduction path so no voltage drops...no efield distribution. Is that right?

 

[Baluncore]

I found a paper that tested the volume sensitivity of air at different relative humidity ranging from like 60-100%.

The surfaces, especially tight corners attract dust and moisture at 100% humidity. At 30 kV, volume conductivity is not the problem. Insulation will fail due to surface conduction that will pyrolise the dust to carbon.

 

[js2020]

The surfaces, especially tight corners attract dust and moisture at 100% humidity. At 30 kV, volume conductivity is not the problem. Insulation will fail due to surface conduction that will pyrolise the dust to carbon.

Does it matter that its 30 kV or is it the efield intensity that matters? If I had a tiny board with 3 kV and max field intensity if 3kV/mm, would it be the field intensity that matters or the applied voltage? In other words, if the spacing/design is adequate, why is it the applied voltage that matters and not the field intensity?

This may be a topic for a different discussion and if so just let me know and I'll post it there. Its directly related to this conversation though. I thought I could use a let's just say 10 kV rated cable at 30 kV as long as it was far enough away from other ground potentials. I figured the insulation if 10 kV would be the rating assuming I have ground right in it or near by?**sorry for the "kg" and not "kV" in the original post. Autocorrect got me.

 

[Baluncore]

why is it the applied voltage that matters and not the field intensity?

Because you can get corona discharge from dust attracted to EHV.

High voltage insulated cables have layers of conductive metal foil, called chroming. They are designed to balance the voltage drop across the deep thickness of insulation. Leakage through the layers forms a resistive potential divider that evens out the electric field.

EHV insulators on transmission lines are designed so the surface path is very long, and the rain can wash the surfaces.

 

[js2020]

Yes, I understand that EHV cables can get dust and get corona. From my understanding, this is because the dust is a protrusion and creates a field enhancement which will cause localized corona at a lower line voltage.

What I'm wondering is the effect of using an 8 kV rated cable, with no shielding, at 10 or even 30 kV as long as ground potentials are far enough away. So if I took all that 8 kV cable and placed it at 8 kV, it should ideally be able to have a ground conductor right next to it and be fine because that's what its rated for. If I looked at the efield intensity at the surface of the insulator and let's say its 2kV/mm...if I move the ground conductor 1 meter away, can I increase the voltage on the cable to a much higher voltage, until the surface efield intensity is again 2 kV/mm and expect similar operation?

Even in terms of dust and whatnot, it's the e field that attracts the particles, wouldn't particles be just as attracted regardless of the differential voltage as long as the efield intensity was the same?

 

[Baluncore]

I cannot see what advantage insufficient insulation might be. You would do better with none, since all the potential would then appear across the one gap material. You could use bare Cu or Al tubes as conductors, a larger diameter will reduce the corona. Either way you will need a cage with an interlock switch on the access panel to keep people safe.

Do you normally assume insulated cables are safe to touch? Would you knowingly touch an insufficiently insulated cable? Who are you trying to kill?
If it is you, then what do you think the Coroner's report will say?

 

[js2020]

I wouldn't even touch l a cable that had a sufficient insulation rating if it was at 30 kV. I practice strict safety procedures to avoid that trip to the coroner.

The insulation is used due to nearby parts. For instance, if I have a 30 kV wire near a 24 kV part, the differential voltage there is only 6 kV as long as it is far from my 0 V Earth ground parts. The goal here is to reliably increase power density. That's why I'm questioning using a cable rated for a lower voltage at a much higher voltage as long as it's far from ground and the efield intensity along the surface is the same, or lower, than if it was near a ground conductor or part. I don't see a reason why that wouldn't be acceptable since at the end of the day the insulation rating is based on efield intensity and not actual applied voltage...so I think anyway.

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

@js2020 -- Please check your PMs (private messages). I sent you a PM asking for more information about your project. Thank you.

 

[berkeman]

Thread is re-opened. This turns out to be for a university project, and the OP has a supervisor for the project and is aware of OSHA and UL regulations about HV safety. Thanks.

 

[berkeman]

The insulation is used due to nearby parts. For instance, if I have a 30 kV wire near a 24 kV part, the differential voltage there is only 6 kV as long as it is far from my 0 V Earth ground parts. The goal here is to reliably increase power density. That's why I'm questioning using a cable rated for a lower voltage at a much higher voltage as long as it's far from ground and the efield intensity along the surface is the same, or lower, than if it was near a ground conductor or part. I don't see a reason why that wouldn't be acceptable since at the end of the day the insulation rating is based on efield intensity and not actual applied voltage...so I think anyway.

I think that one of the key safety considerations is what can happen under a single fault condition. That's a big part of UL testing -- a single fault should not cause a shock or fire hazard.

So what could happen if there were a single fault somewhere in your HV circuit? If one of the HVs were shorted to ground, does that trip the whole set of voltages into a fault condition? Or could one of the conductors be de-energized and shorted to ground while the others are still at HV?

That kind of consideration is why UL creepage and clearance distances are what they are for AC Mains primary PCB circuit traces -- they are all at HV together, but you still need to space them away from each other, much like you need to isolate Primary and Secondary traces.

 

[js2020]

I think that one of the key safety considerations is what can happen under a single fault condition. That's a big part of UL testing -- a single fault should not cause a shock or fire hazard.

So what could happen if there were a single fault somewhere in your HV circuit? If one of the HVs were shorted to ground, does that trip the whole set of voltages into a fault condition? Or could one of the conductors be de-energized and shorted to ground while the others are still at HV?

That kind of consideration is why UL creepage and clearance distances are what they are for AC Mains primary PCB circuit traces -- they are all at HV together, but you still need to space them away from each other, much like you need to isolate Primary and Secondary traces.

There would be no shock or fire hazard. All users will be in a separate room and isolated from HV. The main power supply is current limited with current protection. The power converter also has its own short circuit protection with soft shutdown protocol. Additionally, per OSHA, each converter is discharged with a capacitive discharge stick, then shorted with a grounding stick. Each set of capacitors is then connected to ground with a strap to ensure they do not increase in voltage due to dielectric absorption. However, this is not being done per UL or IEC standards specifically as the purpose of this is to increase the power density beyond current standards. That does not mean safety is not paramount though.

That is what leads into my question about using an 8 kV rated cable at 30 kV near a 24 kV part and far from ground. At the end of the day, the insulation is designed to withstand a specific electric field intensity (kV/mm). From the insulation perspective, I don't think it matters the actual voltage of the conductor as long as the differential voltage from nearby parts are in the range of the insulation rating or 8 kV in this example. I understand that other nearby parts contribute to the overall efield distribution and that is being investigated. I was more so questioning if I may be overlooking something from the insulation perspective where this has been tried and failed or if this is common practice. I understand if there is no good answer for this here as I've been searching for an answer myself and haven't found much. That's part of what brings me here.
**edit**
I should also add that partial discharge measurements before, during, and after testing will be performed so I will see the severity of insulation degradation if it occurs. This is just an attempt to find a hole in my logic if there are any.