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# Re: [femm] Calculation of screen current

- To: femm@yahoogroups.com
- Subject: Re: [femm] Calculation of screen current
- From: David Meeker <dmeeker@foster-miller.com>
- Date: Sat, 04 Oct 2003 10:56:59 -0400
- In-reply-to: <blkcgs+nea2@eGroups.com>
- References: <blkcgs+nea2@eGroups.com>
- User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.3.1; MultiZilla v1.4.0.3J) Gecko/20030425

Winfried Graupner wrote:

> `Dear All,
> First of all many thanks to David Meeker for this great program.
> I use this tool for EMC related issues.
> I would like to address a problem in conjunction to calculation of
> currents in screened cables.
> The arrangement (see file: Screen current.zip in FEMM file directory)
> consists of three single screened cables of a three-phase system.
> The cables are laid in parallel at a certain distance. All screens
> are connected to each other but they are isolated from the domain
> boundary.
> The magnitude of the screen current depends on the mutual coupling,
> the frequency and the impedance of the representative screen loop
> (see file: Screen current.zip in FEMM file directory).
> For low frequencies the resistor in this loop cannot be neglected
> against the reactance of this loop. This disruptive factor is the
> cause, why a cable with e.g. copper-braid screen is not suited at low
> frequencies.
> For frequencies, approx. four times higher than the corner frequency
> of the cable the magnitude of the current in the screen reaches the
> excitation current and becomes independent on frequency.
> In principle the calculation with FEMM confirms this mechanism as
> shown in the diagram (see pdf-file).
> For frequencies in this intermediate section below 10 kHz the current
> in the centre screen is lower than in the right and left screen due
> to its lower self inductance (higher corner frequency).
> However, I have no explanation for the current distribution in the
> right screen. The magnitude versus frequency of this current must be
> the same as for the current in the left screen due to quasi-
> symmetrical conditions. But the trace does not follow this rule.
> I could establish the following details:
> - increasing of solver precision or refinement of the mesh has no
> noticeable influence on this phenomenon
> - the current density in all three screens is not only composed of
> eddy currents but also of a certain share of source current in spite
> of their isolation from the domain boundary
> - the phenomenon changes from the right to the left screen if the
> complex space vector rotates in the opposite direction (e.g. exchange
> of phase currents in two conductors)
> - this phenomenon completely disappears if all three cables are
> arranged in a triangle with perfect symmetry, in this case all three
> screen currents have the same amount versus frequency.
> What is wrong? Do I violate the scope of FEMM?
> I would be pleased for any help!
> Best regards
> Winfried
> `

If you have three-phase conductors arranged in a line, the problem is fundamentally just not symmetric--it sounds like you didn't do anything wrong in setting up and solving the problem. This observation comes up periodically on the mailing list. See, for example: http://groups.yahoo.com/group/femm/message/1672
Dave.

    --
    David Meeker
    Senior Engineer
    Foster-Miller, Inc.
    350 Second Avenue
    Waltham, MA 02451-1196
    781-684-4070
    781-890-3489 (fax)
    dmeeker@foster-miller.com

- References:
  - Calculation of screen current
    - From: "Winfried Graupner" <winfried.graupner@gmx.de>

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