Dimensioning of Power Distribution Systems (2)

Continued from the first part: Dimensioning of Power Distribution Systems (1)

Circuit types

Distribution circuit

Dimensioning of cable routes and devices follows the maximum load currents to be expected at this distribution level.

As a rule:

I_{b max} = ∑ installed capacity x simultaneity factor

Switching/protective device and connecting line are to be matched with regard to overload and short-circuit protection.

In order to ensure overload protection, the standardized conventional (non-)tripping currents referring to the devices in application have to be observed. A verification based merely on the rated device current or the setting value I_r would be insufficient.

Basic rules for ensuring overload protection:

Rated current rule

Non-adjustable protective equipment

I_b ≤ I_n ≤ I_z

The rated current I_n of the selected device must be between the calculated maximum load current I_b and the maximum permissible load current I_z of the selected transmission medium (cable or busbar).

Adjustable protective equipment

I_b ≤ I_r ≤ I_z

The rated current I_r of the overload release must be between the calculated maximum load current I_b and the maximum permissible load current I_z of the selected transmission medium (cable or busbar).

Tripping current rule

I₂ ≤ 1.45 x I_z

The maximum permissible load current I_z of the selected transmission medium (cable or busbar) must be above the conventional tripping current I₂-/1.45 of the selected device.

The test value I₂ is standardized and varies according to the type and characteristics of the protective equipment applied.

Basic rules for ensuring short-circuit protection:

Short-circuit energy

K²S² ≥ I²t

(K = Material coefficient; S = Cross-section)

The amount of energy that is set free when a short-circuit occurs – and up to the moment it is cleared automatically – must be less than the energy that the transmission medium can carry as a maximum or there will be irreparable damage. As a standard, this basic rule applies in the time range up to max. 5 s.

When devices with a tripping unit are used, observance of this rule across the entire characteristic device curve must be verified. A mere verification in the range of the maximum short-circuit current applied (I_{k max}) is not always sufficient, in particular when time-delayed releases are used.

Short-circuit time

t_a (I_{k min}) ≤ 5 s

The resulting current-breaking time of the selected protective equipment must ensure that the calculated minimum short-circuit current I_{k min} at the end of the transmission line or protected line is automatically cleared within 5 s at the most.

Overload and short-circuit protection need not necessarily be provided by one and the same device. If required, these two protection targets may be realized by a device combination. The use of separate switching protective devices could also be considered, i.e., at the start and end of a cable route. As a rule, devices applied at the end of a cable route can ensure overload protection for that line only.

Final circuits

The method for coordinating overload and short-circuit protection is practically identical for distribution and final circuits. Besides overload and short-circuit protection, the protection of human life is also important for all circuits.

Protection against electric shock

t_a (I_{k1 min}) ≤ t_{a perm}

If a 1-phase fault to earth (I_{k1 min}) occurs, the resulting current breaking time t_a for the selected protective equipment must be shorter than the maximum permissible breaking time t_{a perm} that is required for this circuit according to IEC 60364-4-41/DIN VDE 0100-410 to ensure the protection of persons.

Because the required maximum current breaking time varies according to the rated system voltage and the type of load connected (stationary and non-stationary loads), protection requirements regarding minimum breaking times t_a perm may be transferred from one load circuit to other circuits. Alternatively, this protection target may also be achieved by observing a maximum touch voltage.

As far as the choice of switching protective devices is concerned, it is important to bear in mind that long connecting lines are characterized by high impedances, and thus strong attenuation of the calculated short-circuit currents.

Depending on the system operating mode (coupling open, coupling closed) and the medium of supply (transformer or generator), the protective equipment and its settings must be configured for the worst-case scenario for short-circuit currents.

In contrast to supply or distribution circuits, where the choice of a high-quality tripping unit is considered very important, there are no special requirements on the protective equipment of final circuits regarding the degree of selectivity to be achieved.

The use of a tripping unit with LI characteristics is normally sufficient.

Reference: Siemens – Power Engineering Guide Edition 7.0 (chap. Switchgear and Substations)