Isolines calculation

Isolines calculations electrical equipment::

Transformer stations

isolinien3D representation of the isolines
The calculations are carried out using the software EFC-400 or Copperfield. This allows the exact determination of three-dimensional electric and magnetic fields in the vicinity of electrical systems and is characterized by high calculation accuracy, as well as an excellent three-dimensional visualization of the calculation results. The digital simulation and reproduction of the system corresponds to the specific technical data.
  • Isolines calculation of transformer stations
  • Develop magnetic field shielding concepts for NISV-conform transformer stations
  • Project shielding for rooms to protect people and equipment
  • Advice for existing systems regarding restructuring possibilities

Cable trays

KabeltrassePresentation of magnetic field spread of a cable tray with isolines
The calculation software WinField allows the calculation of electric and magnetic fields. The calculation of the magnetic fields is done according to the law of Biot-Savart. The software enables the arrangement of the conductor and the field calculation in a three-dimensional space. Prerequisite for a correct calculation is the absence of ferro- and diamagnetic substances and the neglect of the induction of eddy currents.
When the field generating conductor is approximable by filamentary sub-conductors, the WinField software allows the calculation of the quasi-static magnetic fields of different electrical power supply systems . It is therefore used for the field calculation of overhead lines, cables, network stations and switchboards. The result output is possible graphically as well as in the form of calculation values.

Free lines

FreileitungRepresentation of magnetic field propagation of a free line

The strength and the distribution of electric magnetic fields in the vicinity of a free line depend on many factors. The main factors, which determine the strength and distribution of the electric and magnetic fields, are voltage, current, form of the mast and arrangement of the conductor cables, number of conductor cables and sag of conductor cables.

The sag of the cables, at a defined mast as well as voltage and current, determines the field strengths on the ground. The sag of the cables depends on the temperature of the cables and therefore increases with increasing transmission power (current) and the air temperature. Only momentary field strengths of free lines can be measured. This is because of the dependence of the sag of the cables on the weather and the changing current flow. For this reason and for the assessment of worst-case scenarios, field calculation programs for the identification of the electrical field and the magnetic flux density of free lines come into use.

Free air systems

FreiluftanlageRepresentation of magnetic field propagation of a free air system of a 110kv substation
The calculation of the magnetic field propagation of free air systems/substations is extremely difficult and requires a close cooperation with the operators of the system.
In addition to the exact replica of all parts of the systems, the switching states are to be defined, because the current distribution varies depending on the condition. This leads to large differences in the magnetic field propagation. 

Railway systems

BahnRepresentation of magnetic field propagation of a 2-track route
In the transfer of energy in railway systems (SBB = 16.7 Hz), the currents flow from substation through contact wire, suspension ropes and gain cable, to the traction unit.
To be taken into special consideration is the return flow, which is distributed onto the rails, the soil, as well as the return conductor/ground conductor. How the current is divided, is of great importance for the result of the calculation.

Example of transformer station calculations::


What is needed for a calculation ?

•  Disposition drawing, plans
- ground plan and cut of transformer station and the floors with OMEN
- plans in standard formats (dxf, pdf, paper)

•  Used components
- Medium-voltage system (disposition, principle scheme)
- Transformers (technical data sheet, dimension drawing)
- Low voltage distribution (disposition, position of the busbars)
- Cable types

•  Technical data
- operating voltage, operating currents
- operating state

• Description of the OMEN
- marking and description of the OMEN


The transformer station is drawn 1:1 in the 3D-CAD program EFC400 or Copperfield®. The provided documents serve as a basis.fingerdok_GR

Isolines calculation

The system is virtually passed with current and based on that the isolines are calculated and displayed.
As a result, the customer receives a report with location data and isolines images of the magnetic flux density. The isolines are represented below: XY (view from above), XZ (view from the front), YZ (view from the right) and documented as 3D display.


Example of substation calculation::


ansicht unterwerk110kv
What is needed for a calculation?
•  Disposition drawing, plans
- Layout and cuts of the system
- Plans in standard formats (DXF, pdf, paper)
•  Used components
- Transformers (technical data sheet, dimension drewing)
- Number of fields MS-system (disposition, position of busbars)
•  Technical data
- Operating voltage, operating currents, operating state
View of the substation


The entire system is drawn in 3D. The documents provided serve as basis. The compliance with the 26. BlmSchV. is checked in the example. The customer also wanted to know the distance between interference immunity norm EN 61000-4-8, for the future placement of electronic devices in new constructions of substations. The dimensions of the calculated substation are 200m x 80m.
Key figures: 11 cells free air system 11okV:
  • 2 three-winding transformers 20MVA (2x10KVA, parallel)
  • 11 fields MS-System 20kV Duplex busbar
  • 2 auxiliary transformers 400kVA, 20kV
  • 2 earth fault extinguishing coils (Petersen coils)
unterwerk110kv 20mva transformatoren unterwerk schema ausschnitt
110kV free air system Extinguishing coils,
auxiliary transformers
Disposition of the substation

Isolines calculation

The system is virtually passed with current and based on that the isolines are calculated and displayed. The isolines are represented below: XY (view from above), XZ (view from the front), YZ (view from the right) and documented as 3D display.

b-feld unterwek b-feld schnitt
Isolines calculation substation
Cut through system

Comparison EFC400 with Copperfield®:


We use the simulation programs EFC400 (Winfield) and Copperfield® for the calculation of the spread of electrical and magnetic fields. It was interesting to check how the results of the two programs match each other.

Therefore, we have reproduced a variety of identical resources in both programs, set the currents identically and then compared the results.

Conlustion: The results of the two programs are quite close, however, the Coppefield® calculates somewhat more conservative than the EFC400. The differences are in the range of 3-5%.

Comparison MS-system

Comparison of a MS-system

Basis for the calculation:
MS-system type Unisec
Field 1: Load disconnector input 630A
Field 2: Load disconnector outlet 630A

EFC400 view from above   EFC400 view from the front
Copperfield view from above   Copperfield view from the front

Comparison NS-distribution

Comparison of NS-distribution

Basis for the calculation:
Field 1: 1200mm Sefag outlet strips
Field 2: 500mm Sirco connected from below
Field 3: 1200mm Sefac outlet strips
In:       833 A

View from above 1m from floor TS EFC-400   View from the front EFC-400
View from above 1m from floor TS Copperfield   View from the front Copperfield

What should it be?::

The isoline calculation tools have a variety of class libraries implemented. What to do if a required component is missing? Redraw or simply use a similar component? The following example shows calculated isolines with a similar component:


Available/similar: 20kV transformer: Required: 21.6kV Transformator: Formula:
Power 1000kVA
Voltage OS 20 kV
Voltage US 0.40 kV
Uk 6%
In(os)  28,857 A  (28.87)
In(us)  1442,856 A (1443.38)
In(inductor_seg) 72,698 A  (86.603)
(calculated values are in red)
Power 1000kVA
Voltage OS 21.6 kV
Voltage US 0.42 kV
Uk 6%
In(os)  26,73 A
In(us)  1374,64 A
In(inductor_seg) 82,478 A
I(us) = S / √3x U (us)
I(os) = S / √3x U (os)
In(inductor) = In*uk /s
In(inductor_seg) = In(spule) x 2.5

(s = Shielding effect boiler 2.5)


The comparison of the already in the library  available/similar  and the precisely required transformer.
Left: The vavailable/similar transformer Right: The exact required transformer

Conclusion: The calculation error is approximately 5%. On a first glance, the difference is not overly large. However, if only similar types are being used for all components, or the integration of switches and crossings is waived, the result can be heavily distorted, in or against the favor of the operator of the station. For this reason we suggest that only library components are being used, which comply with the manufacturer. Otherwise, the components have to be reproduced.

Caution: The components available in the libraries often don’t comply with the actual information of the manufacturers and thus lead to incorrect results.

Worst case or best case?::

The following example is a comparison of a low-voltage distribution with different connection of the outflows. Here we have a clear difference between the "worst-case" and "best-case":

worst best case
Left: “Best case”: Total power is evenly distributed on all outflows Right: “Worst case”: Total power is used at the end of the busbar

Conclusion: The deviation of the isolines calculation is ~15%. It is essential to reproduce the outflows in the simulation according to how they are actually arranged. Otherwise the results of the calculation are inaccurate

Unbalance and magnetic fields::

How does an unbalanced load of the rail of a NS-distribution affect the magnetic field strengths? Here are the results of calculations with different unbalanced loads on a low-voltage distribution:

I: L1=866A,  L2=866A,  L3=866A
5% Asymmetrical
I: L1=866A,  L2=844A,  L3=822A
10% Asymmetrical
I: L1=866A,  L2=822A,  L3=779A
15% Asymetrical
I: L1=866A,  L2=801A,  L3=736A
unsym NSV 0 unsym NSV 5 unsym NSV 10 unsym NSV 15

Conclustion: Asymmetries have a very negative effect on the field strengths. The 1µT line spreads, in the calculated example with an unbalanced load, from 15% around 1.2m of just 6m to over 7m.
If the neutral conductor current at an unbalanced, similar load is considered too (112A at 15% asymmetry), the field strengths are even further negatively influenced..

unsym NSV 15 Neutral

Medium voltage switchgear systems in accordance with IEC 62271-200::

IEC 62271-200Clearance space over MS-System
Since February 2007 the new standard IEC 62271-200 / VDE 0671 part 200 is the measure of all things, when it comes to type tests for air and gas insulated medium voltage switchgear systems. After a transitional period of three
years, which ended on February 1st 2007, these standards are fully valid and replace the EN 60298 or. VDE 0670 part 6 completely.
What has this standard to do with shields ?
According to standard IEC 62271-200 / VDE 0671 part 200, each manufacturer of medium-voltage systems has to supply information for its establishment, therefore no one can be harmed by an arc fault in case of malfunction.
The manufacturer will guarantee this through baffles, pressure channels or precisely defined clearance space to the ceiling. These technical precautions and applicable clearance spaces must be observed and may under no circumstances be changed. It is generally not allowed to make any changes to the switchgear system. In particular, specified clearance spaces, with e.g. shields that are hung from the ceiling, provided those do not comply with the applicable clearance spaces, are not to be changed. However, ceiling-mounted “Ceiling shields” are permitted, as long as they do not violate the clearance space specification.
In case of trouble, downward hung shielding parts can be ejected by the pressure wave and thus lead to serious accidents. In addition, hot gases are redirected to the front, reinforced by the shielding, which puts the operating personnel at risk.

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