Interruption of DC current differs significantly from AC current interruption, because a current zero crossing must be created in the DC case, whereas it comes 100 times every second in AC. Moreover, after zero creation, a voltage must be generated to counteract the DC voltage and the huge energy, stored in the DC system, must be handled by the circuit breaker.
In testing, all the stages of the interruption process must be represented:
- the adequate rate of rise of fault current
- fast current zero creation by an auxiliary interrupter (power electronics or mechanical)
- generation of excess voltage that counteracts the system voltage
- dissipation of magnetic energy
- withstand of DC voltage after interruption
KEMA Laboratories is designing test-circuits for DC circuit breakers supplied by AC generators running at low power-frequency. It this way, during the interruption process, the DC voltage can be kept sufficiently “constant” to match the actual DC fault conditions.
In the initial phase of the process, tests are performed with low frequency current, supplied by voltages up to 200 kV and currents up to 40 kApk.
In the left figure, a 18 Hz AC current is simulating a DC fault. In the right figure, a model of a proto-type 80 kV hybrid DC circuit breaker was inserted in the (modelled) test-circuit, limiting the fault current to 10 kA and interrupting it 6 ms later. The numbers refer to the stages of the interruption process explained above.
Key point of the test is that during the decay of the fault current from 10 kA to zero (stage 4), sufficient voltage remains available to represent the DC conditions adequately.
This test was performed with 4 generators in parallel and 2 step-up transformer banks in series each with three in parallel. In a later phase of the project, 6 generators and 10 step-up transformers will be used for maximum direct power.