Siemens SENTRON 3VA1, Руководство пользователя

Страница: 174 / 676

Applications
3.5 DC network applications of 3VA molded case circuit breakers
3VA molded case circuit breakers with IEC certificate
172
Manual, 03/2019, A5E03603177010-03
3.5 DC network applications of 3VA molded case circuit breakers
The main applications for 3VA circuit breakers in DC installations or networks are:
● Public transport systems, e.g. electric cars, underground rail networks and streetcars
● UPS systems
● Photovoltaic systems
● Wind power plants
3VA1 molded case circuit breakers as line protection devices with thermal overload releases
and magnetic trip units with short-circuit protection are suitable for use in DC networks.
Features specific to direct current
Short-circuits are interrupted in DC networks by a method similar to the one applied to high
alternating currents, i.e. by current limiting. A high arc voltage develops in molded case
circuit breakers, which forces a current zero when the arc voltage exceeds the value of the
recovery voltage. The current rise, however, is determined by the time constant and not the
make time or power factor.
Overloads are interrupted by a completely different principle to the method applied in AC
networks. With direct currents, there is no periodic zero crossing and thus also no period of
time without magnetic energy in the circuit during which the conditions for extinguishing the
arc are favorable. The arc is not extinguished until the arc voltage exceeds the line voltage
and the current reduces to zero. In other words, a high arc voltage must be developed in
order to interrupt the direct current.
The rated operational current values are generally the same for DC and AC applications.
With DC applications, the breaking capacity depends only on:
● The arc extinction method used
● The line voltage