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Voltage-clamping components are indispensable for both solid-state circuit breakers (SSCBs) and hybrid circuit breakers (HCBs) to protect the solid-state switch from overvoltage damage and absorb the remnant energy in the system loop inductances.
This article compares various voltage-clamping components (e.g., metal-oxide varistors [MOVs], transient-voltage-suppression [TVS] diodes, capacitor-based snubber circuits, etc.) in terms of operating voltage ranges, surge-current capability, energy-absorbing capability, cost and more. The presentation this article is based on can be found here.
SSCBs and HCBs
SSCBs offer the benefits of extremely rapid fault isolation and the ability to interrupt current without producing arcs. They are gaining popularity due to advancements in power semiconductor devices, such as silicon carbide (SiC) MOSFETs, which greatly reduce conduction losses. HCBs, by the integration of a mechanical switch and a solid-state switch, offer the benefits of minimal conduction losses and relatively rapid current interruption.
The voltage-clamping component (Figure 1) is essential for both SSCBs and HCBs. It serves two purposes:
- To limit the peak voltage across the power semiconductor device and prevent overvoltage damage
- To dissipate the remaining energy in the system’s parasitic inductances after the solid-state switches are turned off
The Vpk/Vop ratio is utilized to compare various voltage-clamping components and assess their performance.
MOVs
MOVs are the predominant voltage-clamping elements employed in SSCBs and HCBs. These components are constructed using various materials, such as zinc oxide and silicon oxide. At low applied voltage levels (below the clamping voltage), the MOVs exhibit high impedance properties. As the voltage increases to reach the clamping voltage, the impedance of the MOVs decreases fast, enabling the flow of current.
Market-available MOVs exhibit a range of package configurations, spanning from small surface-mount to large screw-mount. These MOVs also offer vast voltage ranges, with some devices capable of functioning at up to 3.5 kVDC. Additionally, they possess significant surge-current and energy-absorbing capabilities. MOVs can also conduct current and voltage in both directions and are very inexpensive when compared with alternative voltage-clamping devices like TVS diodes.
TVS diodes
Transient-voltage-suppression (TVS) diodes are frequently employed as voltage-clamping components in SSCBs or HCBs. These diodes function similarly to avalanche diodes but can withstand high peak current and energy.
The TVS diodes can exhibit either unidirectional or bidirectional behavior and possess a rapid response time, similar to MOVs. In contrast with MOVs, TVS diodes have a restricted voltage range (less than 530 V for a single device) and a limited ability to handle high peak currents, as they are accessible only in small surface-mount and through-hole packaging devices. To attain a higher voltage rating or absorb a greater amount of energy, it is necessary to either join TVS diodes in series or in parallel.
One further disadvantage of the TVS diode is its relatively high cost in comparison with MOVs. The TVS diode can be several times more expensive than MOVs, even when they have identical energy-absorption and voltage needs.
Capacitor-based voltage-clamping circuits
The capacitor is a commonly utilized energy storage component in power electronics. It can also be employed in a voltage-clamping circuit to absorb any remaining energy stored in the parasitic inductances of the system. Capacitor-based snubber circuits are commonly employed in power electronics converters to restrict the rate of change of voltage during the turn-off of the solid-state device and mitigate voltage spikes.
In the case of SSCBs, the snubber circuit, which relies on capacitors, can effectively restrict the rate of change of voltage (dV/dt) during the turn-off process of the semiconductor switch. This capability is particularly important for SSCBs that utilize thyristors. Various types of snubber circuits exist, including those composed just of capacitors, the RC snubber and the RCD snubber, which incorporates a resistor, a diode and capacitors.
Experimental results
MOV devices have several benefits, including a broad range of operating voltages (up to 3.5 kVDC per device), high-surge-current and energy-absorption capacities, and a moderate cost compared with alternative voltage-clamping components.
Nevertheless, the ratio of peak clamping voltage to maximum operating voltage (Vpk/Vop) for this component is substantially elevated (>1.63) in comparison with that of the TVS diodes (>1.59). Several techniques are suggested to decrease the peak-voltage-to-output-voltage ratio (Vpk/Vop) of the MOV. These include implementing an active adjustable switch in series with the MOV or connecting the MOV in a free-wheeling configuration with the solid-state device.
TVS diodes have a lower peak clamping voltage in comparison with MOVs. However, they have a restricted voltage range (less than 530 V for a single device) and a limited capacity for handling peak current (only small surface-mount and through-hole package devices are available). To attain a greater voltage rating or absorb a larger amount of energy, it is necessary to link more TVS diodes in either a series or parallel configuration.
One further disadvantage of the TVS diode is its significantly greater cost in comparison with MOVs. TVS diodes can be several times more expensive than MOVs, even when they have equal energy absorption and voltage needs. Capacitor-based snubber circuits can serve as voltage-clamping components for SSCBs. One advantageous characteristic of these circuits is their ability to control turn-off dV/dt, thereby reducing the energy stresses on the solid-state switches during fault-current interruption.
Various types of capacitor-based snubber circuits exist, such as the RC snubber and RCD snubber. An inherent concern with the capacitor-based snubber circuit is the occurrence of current oscillations following the shutdown of the solid-state device. To resolve this problem, the literature suggests using a snubber circuit consisting of a series connection of an MOV and a capacitor. This circuit effectively suppresses the oscillation by carefully selecting MOVs with the appropriate voltage level.
References
1Park et al. (2019). “Overvoltage Suppressing Snubber Circuit for Solid State Circuit Breaker considering System Inductances.” 10th International Conference on Power Electronics and ECCE Asia (ICPE 2019–ECCE Asia), pp. 2647–2652.
2Yi et al. (2021). “Snubber and Metal Oxide Varistor Optimization Design of Modular IGCT Switch for Overvoltage Suppression in Hybrid DC Circuit Breaker.” IEEE Journal of Emerging and Selected Topics in Power Electronics, 9(4), pp. 4126–4136.
3Magnusson et al. (2013). “On the use of metal oxide varistors as a snubber circuit in solid-state breakers.” IEEE PES ISGT Europe 2013, pp. 1–4.
4Zhu et al. (2017). “Performance analysis of RCD and MOV snubber circuits in low-voltage DC microgrid system.” IEEE Applied Power Electronics Conference and Exposition (APEC), pp. 1518–1521.
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