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Technologies > Circuit Breaker Basics
Circuit breakers will be positioned on DC grids and act when a fault
occurs. Breakers would have to fulfill some basic requirements. Current zero crossing
should be created to interrupt the current once a fault occurs. At the same time
the energy that is stored in the system's inductance should be dissipated and the
breaker should withstand the voltage response of the network.
There are two types of HVDC circuit breakers:
electromechanical and solid-state. Electromechanical can be grouped into three
categories: inverse voltage generating method, divergent current oscillating method, and inverse
current injecting method. Only the inverse current injecting method can be used
in high voltage and current ratings. In this type of breaker, current zero can be
created by superimposing an inverse current (of high frequency) on the input current
by dis-charging a capacitor (that was pre-charged) through an inductor. (Explained
on next section) The cost of components required for an electromechanical DC circuit
breaker would not be significantly higher than that of an AC circuit breaker. Electromechanical HVDC circuit breakers are available up to 500 kV, 5 kA and have a fault-clearing time of the order of 100 ms.
Solid-state circuit breakers are the second type of HVDC breakers.
These breakers can interrupt current much faster (which is required in some cases)
than electromechanical circuit breakers, having an interruption time of a few milliseconds.
They are based on Integrated Gate Commutated Thyristors (IGCT), which compared to
IGBT (bipolar thyristors) have lower on-state losses. Current flows through the
IGCT and in order to interrupt, the IGCT is turned off. Once that happens, voltage
quickly increases until a varistor (that is in parallel to the thyristor) starts
to conduct. The varistor is designed to block voltages above the voltage level of
the system. The main disadvantages of these types of circuit breakers are the high
on-state losses and the capital costs.
Typical ratings of solid-state circuit breakers in operation are 4 kV, 2 kA, although in ratings of up to 150 kV, 2 kA were considered.
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