**7. Reliability Assessment**

Reliability assessment is the process of estimating a device's lifespan and chance of failure. Reliability is vital to a system's seamless operation. Manufacturing companies work with reliability analyses to build durable, high-performing, and low-maintenance goods. This idea of "reliability" includes various aspects for assessing a device's reliability. Figure 28 shows reliability categories and how to calculate system reliability [103,144,206,207].

**Figure 28.** Reliability classifications.

Lifespan estimation is crucial, and a device's or part's lifespan can be estimated by calculating the mean time to failure. A high MTTF suggests reliability. The MTTF can be calculated using MIL-HDBK 217E. These standard handbooks will help calculate a device's failure rate (FR) and mean time between failures (MTTF). Reliability depends on several aspects. Figure 29 shows a system's reliability influence factors.

**Figure 29.** Reliability influence factors.

*(a)* Reliability

"Reliability" can be defined as the ability of an object to perform its intended function within specified conditions and time frames. This attribute is commonly assessed by quantifying the probability or frequency of failures.

*(b)* Failure

The system fails when it stops doing the requested task. Thus, the time it takes something to function without breaking down is frequently unpredictable. Failure can be quick or delayed. A sudden failure is called cataleptic failure.

*(c)* Failure Rate (FR)

The "failure rate" is a crucial aspect in the assessment of system reliability. The chance of failure at a specific moment can be determined by utilizing the "failure rate" function.

*(d)* Mean Time to Failure (MTTF)

The MTTF measures how long an item or system lasts, on average, before breaking down. This malfunction has rendered the device useless. The MTTF is often provided among components with hourly or thousand-hour service life requirements.

*(e)* Mean Time to Repair (MTTR)

The MTTR is the typical amount of time needed to repair broken equipment, and its value is directly proportional to the quantity of care it receives [103].

*(f)* Availability and Average Availability

Availability is the probability that a system will be functional at a particular moment. The FR and MTTF are the most crucial metrics for this reliability analysis. As the FR is time invariant, it can be used to describe D(t) [103]. The FR is a statistical measure of the frequency with which a failure happens within a certain time frame. Combining the above failure rates, the exponential distribution is utilized to obtain the probability distribution function. The proportion of attempts that fail is also represented by "λ" as follows.

$$\mathbf{P}\left(\mathbf{t},\lambda\right) = \lambda\left(\mathbf{e}^{-\lambda\mathbf{t}}\right) \tag{14}$$

The reliability function can be obtained from Equation (15):

$$\mathbf{D}\left(\mathbf{t},\lambda\right) = \mathbf{e}^{-\lambda\mathbf{t}}\tag{15}$$

The failure in time (FIT) is a metric for estimating the "failure rate" which is defined as the average number of failures per time interval:

$$1\text{ FIT} = 10^{-9}\text{ failure/hour} \tag{16}$$

$$\text{MTTF} = \int\_0^{+\infty} D(t)dt. \tag{17}$$

$$MTTF = \frac{1}{\lambda} \tag{18}$$

Using MIL-HDBK-217E specifications, Table 14 calculates FR [103]. Based on device counts, power electronic circuits can determine their MTTFT [103]. The MTTFT decreases as device numbers increase. The MTTFT increases with the component count. The inverter topologies are evaluated by the number of components needed. The reliability features (FR and MTTF) are calculated using the approximation technique [103] and summarized in Tables 14 and 15, as well as graphically represented in Figure 30.


**Table 14.** Failure rates of each component by using the approximation method [103].

**Table 15.** Expected mean failure time for three standard inverters.


**7.2 7.5**

**NPC FC CHB**

**MLI Topologies**

**Failure Rate (failures / h)**

(**e**) Failure rate (**f**) Mean time to failure

**Figure 30.** List of comparisons among three basic MLI topologies [103].

**3.6**

Calculation of the overall *MTTF* for power electronic circuits involves estimation of the cumulative failure rate of the constituent circuit parts. In order to obtain the total failure rate, denoted as *λTotal*, it is necessary to multiply the number of components such as switches, diodes, and capacitors by their respective FR values, as specified in Equation (19):

$$
\lambda\_{\text{Total}} = \left(\lambda\_S \times \text{N}\_{\text{SWT}}\right) + \left(\lambda\_D \times \text{N}\_{\text{DIO}}\right) + \left(\lambda\_C \times \text{N}\_{\text{CAP}}\right) \tag{19}
$$

The total *MTTF* of the circuit can be calculated by Equation (20):

$$MTTF\_{Total} = \frac{1}{\lambda\_{Total}}\tag{20}$$

The *MTTF* of power electronic circuits can be determined by considering the number of device counts. When there are a large number of device counts, the related total mean time to failure (*MTTFTotal*) is reduced. A higher *MTTFTotal* is observed when the number of components is lower. In this study, the main aim is to evaluate the average duration of inverter topologies by considering the number of components needed for every individual topology.

## **8. Challenges and Recommendations**

The utilization of renewable energy systems in power grids has been enhanced due to advancements in power electronics devices and related technologies. However, challenges remain pertaining to electricity quality, grid reliability, and security. In order to ensure the quality of grid power, a multitude of standards and guidelines have been established for grid-connected RES. Based on the reviewed literature, it is understood that additional research is required in the following areas:

Challenges:


Recommendations:


• In grid-connected solar PV systems, safe and reliable operation of the multilevel inverter depends on the use of suitable safety mechanisms and control strategies, which are listed in Table 16.

**Table 16.** Grid-fault challenges and recommendations for multilevel inverters in grid-connected solar PV systems.


**Figure 31.** Perception evaluation and future development of SC-MLIs.

**Figure 32.** Monte Carlo-based reliability study of PV inverters [103].

**Figure 33.** PV system mission profile translation diagram by PV array size ratio Rs consideration [103].
