HVDC Circuit Breakers

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Technologies > Solid State Circuit Breakers The second type of circuit breaker we will be analyzing is the solid-state circuit breaker. In the following figure we can see that a solid-state circuit breaker uses gate-commuted thyristors instead of integrated gate-commuted thyristors for semiconductor devices, this is due to the fact that in this topology our immediate concern is lowering the on-state losses. When there is no circuit failure detected current flows through the GCTs. Once it is detected, the semiconductors are switched-off. This leads to the rapid increase of the voltage until the varistor begins to conduct. Any voltage higher than the grid voltage is blocked due to the design of the varistor. This in turn, leads to the demagnetization of the line inductance. In this topology, parallel connections are unnecessary; hence we have a total of fourteen devices. In the above diagrams as we can see the maximum current in solid-state circuit breaker has almost no dependence on the grid voltage and at the same time the higher the voltage the lower the turn-off time. Figure 3: Solid State Circuit Breaker                                                 Figure 4: Grid Voltage vs. Time                                                     Figure 5: Grid Voltage vs. Maximum Current   Voltage Solid-State Mechanical with Snubber circuit Conventional Hybrid Forced Commutation 6kV 6000 5400 8100 11400 12kV 9000 8800 27500 21200 20kV 21000 18000 47500 34800 150kV 300000 4450000 54544000 2602000         Figure 6: Cost comparison for four different circuit breaker models The solid-state circuit breaker model has many advantages compared to the rest of the circuit breaker topologies. First of all, because of the lack of any mechanical components the solid-state circuit breaker is much more responsive, which leads to a reduced turn-off time. Furthermore, the higher the voltage across the inductor the higher the overvoltage which allows the demagnetization process to be performed faster as we can see on figure 4. This fast turn-off process limits the peak current to low levels compared to the rest of the circuit breaker topologies. Hence, for the same grid voltage the power losses per circuit breaker are very low in the solid-state case. With all this in mind, one would assume that the solid-state circuit breaker is the better choice due to the low component cost, low turn off time and low peak current regardless of grid voltage or cable length. This however is not the case because of one significant disadvantage; the high on-state losses. As we can conclude from figures 4, 5 for low to medium grid voltages the low turn-off time and low peak voltage are not low enough to counter the power losses during the on-state. In high voltage grids however, the solid-state topology has an advantage since the power saved per circuit break is sufficient to make them more economic compared to the other circuit breakers, even when the on-state losses are taken into consideration.   <<Prev  Back  Next>>