Increase of resistance after heating at earth-faults

In order to consider the increase of temperature when there is a short circuit occurs between phase and neutral/ground there are two methods defined in dpPower Analyzer.

Note! To invoke this type of calculation select the check box Resistance increase calculation LV in the Other parameters form of Results/Administration.

 

1.Template method via the increase of resistance factors.

 

2.Calculation of the increase of resistance via material constants and faulty/final temperatures.

 

The Code list to be used is  “Line type”

 

Method 1 – Template method

If the template method is used, then the factors for the increase of resistance of the feeder and service conductors must be set in line type data.

Code-list values:

Feeder phase resistance increase

Feeder neutral resistance increase

Service phase resistance increase

Service neutral resistance increase

 

Method 2

If the calculation of the increase of resistance is done according to method 2 the following must be stated.

Note! If this method is used, then the template values should not be set (or  = 1).

Code-list values:

Phase temperature coefficient

The inverted temperature-coefficient of the phase conductor at 0°C.

E.g. 234.5 for copper

Max temp phase

The highest allowed temperature in °C for the phase-conductor.

Start temp phase

The temperature in °C of the phase conductor when fault occurs.

Neutral temperature coefficient

The inverted temperature-coefficient of the neutral conductor at 0°C.

Max temp neutral

The highest allowed temperature in °C for the neutral conductor.

Start temp neutral

The temperature in °C of the neutral conductor when fault occurs.

The increased resistance is only used for calculating earth fault currents and not for three phase short circuit currents.

C-Factors

Regarding voltage factors (C-Factors).

Sometimes to be on the safe side when calculating tripping times a voltage security factor c must correct the earth-fault current when controlling. These c-factors are set in the calculation parameters for fault current calculation and according to the rated c-factors on fuses respectively or template values in the parameters.

There are three different C-factors in the fuse codelist, and parameters.

•C-factor for OH free line.

•C-factor for conductor if protecting device acts as both overload and shortcircuit protection.

•C-factor for conductor if protecting device acts as shortcircuit protection only.

The C-factor used depends on the settings in the parameters (radio-buttons).

Feeder cable should be tested with c-factor from:

•Over-load and short-circuit or

•Only short-circuit.

Service cable should only be tested with c-factor from:

•Over-load and short-circuit or

•Only short-circuit.

Coding of conductor-types

Currently, the following coding of line types (objecttypes) are valid:

[700100] Feeder line insulated free line, interpreted as s free cable

[700101] Feeder cable ug, interpreted as cable

[700102] Feeder cable oh, interpreted as free-cable

[700103] Feeder line free line, interpreted as free-cable

[700104] Feeder cable sea, interpreted as cable

[700110] Service line insulated free line, interpreted as free-cable

[700111] Service cable ug, interpreted as free-cable

[700112] Service cable oh, interpreted as free-cable

[700113] Service line free line, interpreted as free-cable

[700114] Service cable sea, interpreted as cable

The use in the calculation program

When controlling the earth fault current the c-factor value of the protected fuse will first of all be used.

If it is not possible to obtain a value through the fuse code list, then a template value will be used instead.

For reduced network parts, the lowest c-factor for the merged sections will be used for the resulting section.

The C-factor in use is accessible as a general result term and it’s also stated in one of the fixed reports Combined report for LV networks.