S
AC S
BGate Drive
Gate Drive
(a) Switching cell components: flying capacitor, two switches and corresponding gate drive circuits.
(b) The series reliability structure of each switching cell simplified as a single failure rate.
Figure 5.13: Switching cell reliability structure for analyzing cell-level failures and redundancy.
Prior work in [228] has shown that redundancy through spare switching cells can improve system-level reliability more rapidly than module redundancy as cell (and component count) increases. Some multilevel topologies provide inherent cell-level redundancy; in the case of a faulty component, the affected cell may be bypassed – allowing continued operation of the inverter phase-leg, albeit in a degraded state (e.g, reduced level count, decreased output power, etc.). For example, given a failure in one switch of the cascaded H-bridge inverter submodule, the cell may be bypassed by closing both of the top or bottom switches of the H-bridge. The half-bridge variant of the modular multilevel converter also supports this functionality with the addition of a single extra bypass switch [229]. Other variants of submodule for modular multilevel converters support can add this functionality with further additions to the component count [230].
However, the flying capacitor multilevel converter does not generally have this cell-level redundancy, except under certain faults and with appropriate component voltage and current ratings [231]. Therefore, this section discusses if and how failures of a specific component in a given switching cell can be mitigated. Referring to Fig. 5.13a, the following component failures are considered:
Switch Sk: short-circuit failure
Switch Sk: open-circuit failure
Switch Sk: loss of controllability
Gate drive: loss of controllability
Flying capacitor Ck: short-circuit failure
Flying capacitor Sk: open-circuit failure
Note that a failure in any of these components essentially leads to a failure of the switching cell, as indicated by the series structure in Fig. 5.13b.
5.2.1 Switch S
k: short-circuit failure
Although a short-circuit fault is not guaranteed to manifest as an ideal, low-resistance path, consideration of such a condition provides a useful starting point to analyze both the fault dynamics as well as the steady-state, post-fault condition. The notable result from this fault, where the mitigating action is to close the surviving complementary switch, is the loss of a discrete voltage level. This is apparent for faults in intermediate cells, where switches Sk and Sk closed and adjacent flying capacitors Ck and Ck+1 (or the DC capacitance) are in parallel. In the case of a fault in cell 1 (at the ac outlet), C1 simply becomes shorted.
For an incipient fault, transient stresses may be avoided through a graceful transition from N to N − 1 levels [71]. However, in an acute fault, the stresses associated with the discharge of the adjacent capacitors must be tolerated by the failed and surviving devices for the converter to remain operational. Prior work in [231] presented an energy stress associated with the re-balancing of charge that silicon IGBT dies must tolerate. However, for discrete MOSFET devices, the pulsed current limit is the most appropriate constraint.
While desaturation circuits have also been demonstrated to protect individual GaN devices from an overcurrent condition [232], they do not represent a path to mitigate the device once it has failed short. Nonetheless, it may be possible to leverage the same type of circuit to rapidly detect a short circuit failure and implement the dynamic level change described above.
5.2.2 Switch S
k: open-circuit failure
A switch failing as an open-circuit is most likely caused by the mechanical disconnection of the device terminals, or from failure of the gate drive circuitry. In the case of the former, cell redundancy may be achieved through the use of parallel switches; these devices would continue normal operation in parallel with the open circuit switch. Indeed, paralleling devices already provides an appealing strategy to reduce the overall on-resistance of the switching cells or increase the overall power handling capability of the inverter. The surviving device then either must carry the full current (potentially increasing likelihood of subsequent failures due to increased current stress), or the overall inverter power must be decreased.
If the failure is not an ideal open circuit but instead an intermediate state (i.e., a loss of controllability), a redundant switch could still allow the circuit to be reconfigured for operation at N − 1 levels. Here, voltages on Ck and Ck−1 would be equalized and both the redundant switch in Sk and Sk would be closed. The converter would then continue to operate in an N − 1 state, removing the failed switch from operation. Otherwise, in the absence of a redundant switch or in the case where the gate drive circuitry failed in an off or uncontrollable state, cell functionality cannot be recovered. As shown in [231], charge balance on the capacitors is no longer possible and continued operation would overcharge capacitors while simultaneously failing to deliver a sinusoidal output.
5.2.3 Gate drive failure
If the gate drive circuitry is compromised, there is little recourse in terms of circuit redun-dancy – especially if all redundant switches share the same gate driver. However, if an independent gate driver is provided per device, it is possible that the redundant driver could provide the same recourse as the open-circuit case above: bypass the affected gate driver and switch. If redundant gate drivers is not an option (as this would be a significant component increase), a similar recourse could be to add a fail-bypass, where the affected switch is forced into a bypass state if the gate driver can no longer provide the appropriate switching signal.
This would be similar to the way in which redundant actuators on aircraft are designed to overpower a failed or stuck actuator for a given control surface [174]. The containment action is then identical to the open-circuit switch failure above that addressed loss of controllability.
5.2.4 Flying capacitor C
k: short-circuit failure
In order to survive an acute short-circuit fault of the flying capacitance Ck, all switches Sx and Sx for 1 ≤ x ≤ k need to withstand energy from the lower voltage flying capacitors discharging through switch reverse conduction. Furthermore, the switches Sk+1| and Sk+1|
need to withstand the full, pre-fault voltage across Ck+1. Post fault operation at N −k levels is then dependent on whether the capacitors and switches k +1 and above can tolerate the new, elevated voltages. Indeed, failure of higher voltage flying capacitors (closer to the dc side) requires switches to withstand greater reverse conduction from lower voltage capacitors and surviving switching devices to be significantly overrated in voltage than a failure in a lower voltage switch; it may be possible to provide partial coverage from appropriate component derating to allow for some level of fault tolerance.
5.2.5 Flying capacitor C
k: open-circuit failure
An open-circuit failure of a flying capacitor Ck requires no additional components to allow for N − 1 operation. However surviving capacitors will need to be rebalanced, while all surviving capacitors and switches will need to be rated to withstand the new, elevated
voltages. Unlike the tandem switches adjacent to a bypassed level in [71], either Sk/Sk or Sk+1/Sk+1 may remain closed to reduce switching losses.