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A cable is a three-phase cable connection between two nodes, whose type data are known. The electrical properties are usually copied from the type file.

 

In the type file, data for circuits with three and single conductor cables are included. The single conductor cables are always treated per circuit of three parallel cables and therefore do not need to be entered separately.

 

A cable can only be installed between two nodes with the same nominal voltage.

 

 

PARAMETERS

 

General

 

In this screen, the positions of the switches on both sides of the cable can be changed.

 

 

The cable runs from the From-node to the To-node. The From-node and To-node can be interchanged using the <> button.

 

Subnet border

An open switch in such a cable is displayed as a double flag, provided that a flag for an open switch is selected for the symbols in the editor options.

 

Cable parts in series

A cable consists of one or more cable parts in series. The cable is configured or modified as follows:

With the green buttons + cable parts can be added to a cable. With this, a cable part can be placed in front of or behind another cable part. With the red button X a cable part is removed from the list.

In some calculations, such as fault analysis and protection, the specific sequence of the cable parts in a cable connection is taken into account. This is particularly important when alternating strong and weak cable pieces are present in one branch.

With the <> button the order of the cable parts can be reversed.

 

With the button ... the parameters of the cable type of a cable section can be viewed and changed.

 

The length, load capacity and load factor can be specified for all cable parts.

 

 

Parallel cable parts

For each cable part, the number of parallel cable circuits for that cable part exists. The advantage is that for a double circuit connection, of which both circuits are protected with a common protection relay, no auxiliary node needs to be made. Moreover, the network diagram becomes less cluttered.

The equivalent electrical data is calculated for all parallel circuits jointly in the relevant cable section.

For a cable part that consists of two parallel three-conductor cables, there are two parallel circuits. The #Parallel attribute is then 2.

In the case of a cable component consisting of two parallel circuits of single-core cables, in reality six single-core cables are next to each other. Since each circuit consists of 3 cables, the #Parallel attribute is also equal to 2.

A short circuit in one of the parallel cables can only be calculated correctly if the specific cable is modelled separately.

 

Ampacity factor

With this factor, Inom can be reduced if several cables are next to each other. This can occur with cables that are routed at substations.

 

Cable type

The form that is called from the General tab with the button ... gives the following information about the selected cable section: type, shortened type name, maximum ampacity and maximum short-circuit current.

 

 

Type

In the list of applicable types are all the cables from the cable database of which Unom equals 75 - 350% of the Unom of the two nodes.

The type name consists of a maximum of 40 characters.

After selecting the desired cable type from the cable list, all parameters are copied.

The Types.xlsx type file supplied with Vision contains data from many commonly applied cable types.

See also: Type

 

TR

Specification of the temperature for which the specified resistance applies. This makes it possible to calculate the approximate behaviour of the grid at a different temperature (Tact) on the basis of the correction factor:

(1+0.004(Tact-20)) / (1+0.004(TR-20)).

This formula assumes that the skin effect and proximity effect are constant and therefore has a somewhat greater tolerance for very thick cables (cross-sections of 1000-3000 mm2). The calculation of the maximum short-circuit current according to IEC 60909 is based on a conductor temperature of 20° C.

 

R0 and X0

For the calculation of short circuits with earth contact, the following is tested: R0>R1 and X0>0. If not, a warning is generated.

 

Inom

The nominal current that a cable may carry depends on the permissible conductor and jacket temperature and is partly determined by the specific thermal resistance of the soil G. For each cable type, an Inom can be specified for three different values ​​of G. A nominal current can also be specified for overground cables (free in air).

The stated values ​​for G are used in the calculations to select the appropriate maximum current loading. With several cable parts, the weakest part is driving.

 

TIk(1s)

The conductor temperature at Ik(1s) is used for calculating the minimum short-circuit current according to IEC 60909.

 

Frequency

The business reactance is based on the nominal frequency, which is taken from the type data: X = ωL. With a deviating operating frequency (specified in the Options, at Calculation) the operating reactance is corrected.

 

Copy and paste type data

The type data of a cable type can be copied to a special clipboard by clicking Copy type data in the Cable type form at the bottom left. The data can be pasted into another new or existing cable with Paste type data.

 

Reliability

The reliability data applies to the entire cable and does not have to be specified separately for each cable part

 

 

Localization

 

Distance determination via pulse reflection

Using the attribute pulse velocity for the cable type, the distance from a node to the fault location (short circuit) can be determined. The distance is calculated from the entered runtime and the pulse velocity of each cable type. This function is particularly useful if many different cable types are used in a cable connection.

Via cable trav. time of a possible non-disturbed conductor, the exact measured running time of the entire cable can optionally be specified, for better localization.

The two distances indicate the resulting range of the location, using pre-programmed error margins.

 

Location by distance(s)

The calculated distances, or own distance(s), can be entered here to determine the geographical location(s).

 

 

Connection

 

 

 

 

MODELLING

 

Pi model

The pi model is used for modelling lines and cables. For R, L, Rshunt and C holds:

 

 R        resistance

 L        depending of the inductive reactance

 C        capacitive reactance

 Rshunt        based on tangens delta

 

 

 

The shunt resistance is calculated from the tan(δ), the dielectric loss angle δ is measured to determine the degree of ageing of a cable connection. The tan(δ) of a cable depends, among other things, on the type of insulation, the ageing and the temperature of the cable. A cable using XLPE as an insulator has a tan(δ) typically between the 0.35E-3 at 20 ° C and 0.61E-3 at 90° C, in Vision this temperature dependence is neglected. Below the derivation of Rshunt from the dielectric loss factor tan(δ):

       

 

 

 

The pi-model is valid for cables up to approximately 50 km and for lines up to approximately 200 km. Longer lines can be modelled by applying fictitious nodes, as a result of which several part lines arise. For example, a three-section Pi model provides an accuracy to 1.2 % for a quarter wavelength line (a quarter wavelength corresponds with 1500 and 1250 km at 50 and 60 Hz respectively). Source: J. Arillaga, D.A. Bradley, P.S. Bodger: "Power System Harmonics".

 

Inom 'and Snom' for branches

For branches the terms Inom' and Snom' have been introduced for signalling an overload in the load flow. These values have been introduced to obtain clarity about the maximum load capacity of a branch. Snom' is used for a transformer. For a cable, the value of Inom' is determined from the input data of the weakest cable section. For the cable Inom' is specified as follows:

•Inom' is the weakest cable type.Inom (G) x ampacity factor over the cable parts.

•G refers to the chosen heat resistance of the ground.

 

IEC (60) 909

The zero sequence data are not relevant for the calculation of a symmetrical closure (FFF) or a two-phase closure without earth contact (FF).

For the calculation of short circuits with earth contact, the following is tested: R0> R1 and X0> 0. If not, a warning is generated.

Zero sequence impedances of cable connections are difficult to determine and depend, among other things, on:

•one three-phase or three one-phase cable

•distance, plane or triangle

•Grounding of the earth screen (one-sided, two-sided, cross bonding)

•other conductive objects such as other cables or pipelines

 

The maximum permissible short-circuit time tmax is calculated on the basis of Ik(1s).

 

The calculation of the maximum short-circuit current according to IEC 60909 is based on a conductor temperature of 20° C. The conductor temperature at Ik(1s) is used for calculating the minimum short-circuit current according to IEC 60909.

The conductor resistance (R) is specified for reference temperature (TR). The resistance for the actual temperature (Tact) is calculated on the basis of the correction factor:

(1+0.004(Tact-20)) / (1+0.004(TR-20)).

The zero sequence resistance (R0) is not corrected for temperatures that deviate from 20 ° C.

 

Harmonics

The distributed parameter model is used for calculating harmonics. See: Harmonics: Model