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A thyristor is primarily a very fast "switch-on" device and is not designed to be used as a "switch-off" device.
A circuit breaker is primarily a "switch-off" device which is used to automatically break the flow of current from a source to a load whenever the amount of current exceeds the rated current value of the breaker.
So, because a thyristor does not, all by itself, "open" a circuit, a thyristor cannot be used to work as a circuit breaker.
Notes
Current flow through a thyristor can only stop if - external to it - the current flowing through it gets reduced to below its minimum holding current or, of course, to zero, as would happen either:
i) if the load was disconnected from it by a separate circuit breaker that is wired into the circuit between the thyristor and the load;
or
ii) if the source was disconnected from it by a separate circuit breaker that is wired into the circuit between the supply and the thyristor.
Further notes
A thyristor is designed to close or "make" a circuit whenever:
a) an appropriate value of trigger voltage is put onto its gate
and then only if:
b) the current from the source to the load can flow through it in its "forward" direction.
One way of ensuring it automatically disconnects the load from the source is to use an alternating current supply and always "trigger" the thyristor so that it turns on during each positive-going part of each ac cycle.
Then, whenever the ac supply voltage returns to zero at the end of each positive half-cycle (after which it goes into the negative half-cycle) the thyristor will automatically break any further flow of current.
By using other electronic circuitry to determine exactly when the gate is triggered (earlier or later in each positive-going part of each cycle) the amount of power supplied to the load can be controlled very accurately.
That is why the main application for thyristors is to provide a precisely controlled way too use alternating currents to allow varying amounts of power to be supplied to electrical loads.
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What does forced commutation mean?
The term "Forced commutation" is used when a thyristor is turned off using external components. If a thyristor is used in a DC circuit, when first turned on, it will stay on until the current goes to zero. To turn off the thyristor it is possible to use a Forced commutation circuit. The circuit creates a reverse voltage over the thyristor (and a small reverse current) for a short time, but long enough to turn off the thyristor. A simple circuit consist of a precharged capacitor and a switch (e.g. another thyristor) parallel to the thyristor. When the switch is closed, the current is supplied by the capacitor for a short while. This cause a reversed voltage over the thyristor, and the thyristor is turned off.
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