Short circuit power

The short-circuit power is a term from electrical power engineering and is particularly relevant for power grids and their short-circuit treatment. It is a rating and is expressed as an apparent power .

General

Expressed in formulas, the short-circuit power S _{k is} the product of the short-circuit current I _k , the nominal voltage U _n and the linkage factor : ${\displaystyle \textstyle {\sqrt {3}}}$

${\displaystyle S_{k}=U_{n}\cdot I_{k}\cdot {\sqrt {3}}}$

In physical terms, the short-circuit power is not an actual power, as S _k values ​​are linked which do not occur simultaneously. This follows from the fact that the voltage in the event of a short circuit and at the short circuit point is practically 0 V and the nominal voltage U _n is fixed in the case of no short circuit current I _k .

The short-circuit power is a rating used to quantify the stress on an electrical system and, in particular, the switching capacity of circuit breakers . A circuit breaker must have a breaking capacity greater than the short-circuit power in order to be able to switch the current flow safely and without damage to the switch in the event of a short circuit. In high-voltage networks, the short-circuit power, depending on the voltage level, network topology and location of the short-circuit point, is in the range of a few MVA up to 1000 MVA. The highest short-circuit power occurs with busbars when they are fed by several sources with low impedance .

The short-circuit current I _k is not apparent from the resistance of the load, but from the network impedance Z according to the Ohm's law to

${\displaystyle I_{k}={\frac {c\cdot U_{n}}{{\sqrt {3}}\cdot Z}}}$

According to the standard, the factor c takes into account the tolerance of the mains voltage (c = 0.95… 1.1). This means that the short circuit power can also be in the form

${\displaystyle S_{k}={\frac {c\cdot U_{n}^{2}}{Z}}}$

represent.

With a fixed nominal voltage, the short-circuit power of a network can also be expressed by the network impedance. A high short-circuit power is a measure of the voltage quality and interference immunity of a power grid. The short-circuit power can be influenced by expanding a power grid. Technically, this can also be achieved through a decentralized network structure.

A distinction must be made between the short-circuit power and the power primarily converted into heat at the short-circuit point, which occurs as a physically relevant variable. Typically, in the absence of a full short circuit, the heat is converted at the short circuit point in the form of an arc , which increases the arc power

${\displaystyle P_{l}=U_{l}\cdot I_{k}}$

with the arc voltage U _l .

Current limitation

With the subsequent expansion of power grids and the associated increase in short-circuit power, it can happen that existing circuit breakers are not designed for the then higher short-circuit power. In order to avoid the costly replacement of the switches, so-called short-circuit current limiters can be used. Typical areas of application are busbar sections fed several times in parallel at the medium-voltage level , sometimes also at the maximum voltage level, which are connected to a busbar via short-circuit current limiters during normal operation. In normal operation, the parallel connection results in a low network impedance in the area of ​​the busbar. In the event of a short circuit, the current limiting unit splits the busbar into several separate sections, which limits the short circuit current and the subsequent disconnection process can take place without damage to the circuit breaker.

Common current limiters at medium voltage level are pyrotechnic current limiters , which in the event of a short circuit break the individual busbar sections by means of an explosive charge in a tubular connector before the circuit breaker is triggered. The ignition is carried out by an electronic circuit which must quickly and reliably detect the short circuit before the circuit breaker is switched off. Disadvantages are the maintenance effort and the technically complex detection of a short circuit for triggering the explosive charge, since false triggering, for example due to high inrush currents or lightning discharges, must be avoided.

There are also other methods, such as superconducting current limiters, which are part of the resistive current limiters. In normal operation, the busbar sections are connected via superconducting couplings, which in the event of a short circuit become highly resistive and thus limit the current. The cooling devices required for superconductors are disadvantageous.

literature

• Rene Flosdorff, Günther Hilgarth: Electrical energy distribution . Teubner, 2003, ISBN 3-519-26424-2 .
• Adolf J. Schwab: electrical energy systems . Springer, 2009, ISBN 978-3-540-92226-1 (Chapter 11).

swell

1. ↑ http://www.competence-site.de/energie.nsf/0348E3D7E357630BC125703600443BA4/$File/das_konzept_der_elektromagnetischen_vertraeglichkeit.pdf
2. ↑ Superconducting current limiters made from YBCO tape conductors ( memento of the original from March 4, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.