A single phase five-level inverter with single and multiple switch fault tolerance capabilities

Abstract

The reliability of multilevel inverters (MLIs) is of great importance, when they are employed for applications such as aircrafts, electric vehicles, standalone, and grid connected photovoltaic (PV) system. To achieve required output voltage during post-fault scenarios, several fault tolerant topologies of multilevel inverter have been proposed. However, the primary constraints which add on a limitation to their implementation are increased device count and inability to tolerate single and multiple switch faults with the conduction of lesser or equal switches as in pre-fault case. The inability to tolerate various types of faults, such as open circuit or short circuit, cause severe impact on the system and further increase the cost. In this regard, a fault tolerant (FT) multilevel inverter topology which tolerates aforementioned faults is proposed. The proposed topology is able to withstand both single switch and multiple switch faults with equal or lesser blocking voltage. The proposed topology employs only unidirectional switches, thus increasing the reliability and reducing the cost of inverter. The proposed work is validated experimentally.

Appendix A

1.1 Evaluation of reliability

Reliability estimation of the proposed topology and other works based on Military Hand book MIL-HDBK-217F [29, 30] is presented.

The probability of having no failure for duration t is defined by function R_s(t) as

$$ R_{s} (t) = e^{{ - \left( {\lambda_{sys} \times t} \right)}} $$

(A1)

where λ_sys is the failure rate of system.

The mean time to failure (MTTF) can be calculated from R_s(t) as

$$ {\text{MTTF}} = \int\limits_{0}^{\infty } {R_{s} \left( t \right){\text{d}}t} = \frac{1}{{\lambda_{{{\text{sys}}}} }} $$

(A2)

The failure rate of MOSFET (failures per million hours) is given by

$$ \lambda_{{\text{S}}} = \lambda_{{\text{b}}} \pi_{{\text{T}}} \pi_{{\text{A}}} \pi_{{\text{Q}}} \pi_{{\text{E}}} $$

(A3)

where λ_b is the base failure rate, $\pi_{{\text{A}}}$ is application factor which depends on the rated output power, $\pi_{{\text{Q}}}$ is the quality factor and $\pi_{{\text{E}}}$ is the environmental factor. The temperature factor $\pi_{{\text{T}}}$ depends on the junction temperature T_J as given by

$$ \pi_{{\text{T}}} = \exp \left[ { - 1925\left( {\frac{1}{{T_{{\text{J}}} + 273}} - \frac{1}{298}} \right)} \right] $$

(A4)

where T_J is a function of the ambient temperature T_a (25 °C), the junction to thermal ambient resistance θ_JA, and the total power dissipation (conduction loss and switching loss together) of the switching device P_S, given as

$$ T_{{\text{J}}} = T_{{\text{a}}} + \left( {\theta_{{{\text{JA}}}} } \right)P_{{\text{S}}} $$

(A5)

Similarly the failure rate of diode (λ_diode failures/million hours) is calculated as

$$ \lambda_{{{\text{diode}}}} = \lambda_{{\text{b}}} \pi_{{\text{T}}} \pi_{{\text{S}}} \pi_{{\text{C}}} \pi_{{\text{Q}}} \pi_{{\text{E}}} $$

(A6)

where $\pi_{{\text{S}}}$ is the electrical stress factor given by $V_{{\text{s}}}^{2.43}$, where V_s is the ratio of applied voltage to rated voltage, $\pi_{{\text{C}}}$ is the contact construction factor. The temperature factor $\pi_{{\text{T}}}$ is given by

$$ \pi_{{\text{T}}} = \exp \left[ { - 2100\left( {\frac{1}{{T_{{\text{J}}} + 273}} - \frac{1}{298}} \right)} \right] $$

(A7)

where

$$ T_{{\text{J}}} = T_{{\text{a}}} + \left( {\theta_{{{\text{JA}}}} } \right)P_{{\text{D}}} $$

(A8)

where P_D, is the power dissipated across diode.

The failure rate of capacitor (λ_cap failures/million hours) is given by

$$ \lambda_{{{\text{cap}}}} = \lambda_{{\text{b}}} \pi_{{\text{T}}} \pi_{{\text{C}}} \pi_{{\text{V}}} \pi_{{{\text{SR}}}} \pi_{{\text{Q}}} \pi_{{\text{E}}} $$

(A9)

where $\pi_{{\text{C}}}$ is the capacitance factor given by (C in µF)^0.23, $\pi_{{{\text{SR}}}}$ is the series resistance factor and $\pi_{{\text{V}}}$ is the voltage stress factor given by

$$ \pi_{{\text{V}}} = \left( {\frac{S}{0.6}} \right)^{17} + 1 $$

(A10)

where S is ratio of the operating voltage to the rated voltage.

The temperature factor $\pi_{{\text{T}}}$ is given by

$$ \pi_{{\text{T}}} = \exp \left[ {\frac{{ - E_{{\text{a}}} }}{{8.617 \times 10^{ - 5} }}\left( {\frac{1}{T + 273} - \frac{1}{298}} \right)} \right] $$

(A11)

The failure rate of the system with N number of inputs is given by

$$ \lambda_{{{\text{sys}}}} = \lambda_{{\text{S}}} + \lambda_{{\text{D}}} + \lambda_{{{\text{cap}}}} $$

(A12)

where suffix i is the ith input and suffix b indicates the boost element in the converter. The parameters used for the calculation of reliability are listed in Table

Table 6 List of the parameter values employed for reliability evaluation

6.