Capacitors of today have very small losses and are therefore not subject to overload due to heating caused by overcurrent in the circuit. Overload of capacitors are today mainly caused by overvoltages. It is the total peak voltage, the fundamental and the harmonic voltages together, that can cause overload of the capacitors.
Based on simulation results above, the proposed over-current protection method could eliminate the pumping-up voltage for small DC-link capacitor VSI. As shown in Fig. 11, the experimental platform has been set up to verify the proposed over-current protection method.
After 1.67 ms, protection switch Q1 is turned off. Compared with the results of ‘all-turn-off’, the proposed method can guarantee the capacitor voltage little change. The rapid current change will be caused by position fault for field-oriented control (FOC). Figs. 13a and b show the experimental results of over-current caused by position fault.
For VSI employing small DC-link capacitor, the situation of over-load using the ‘all-turn-off’ protection is selected to analyse in this part. The three-phase currents are approximately sine waveform at the moment of over-current, which is beneficial to analysing the over-current process.
Each capacitor unit consist of a number of elements protected by internal fuses. Faulty elements in a capacitor unit are disconnected by the internal fuses. This causes overvoltages across the healthy capacitor units. The capacitor units are designed to withstand 110% of the rated voltage continuously.
In addition to the relay functions described above the capacitor banks needs to be protected against short circuits and earth faults. This is done with an ordinary two- or three-phase short circuit protection combined with an earth overcurrent relay. Reference // Protection Application Handbook by ABB
Capacitors
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