Single-Stage Boost Inverter Reliability in Solar Photovoltaic Applications

Eric Hofreiter and Ali M. Bazzi, Member, IEEE

Abstract—The boost inverter presents an interesting topology for solar photovoltaic (PV) applications as a single stage that provides regulated, boosted, and inverted output voltage from a nonlinear dc input. While the boost inverter has been previously addressed from design, implementation, and control perspectives, this paper analyzes its fault modes, failures, and reliability. The inverter is first experimentally tested to validate a simulation model used in the reliability analysis. The simulation model is then used to analyze the effect of various faults on the system performance. Results of this analysis yield a Markov reliability model from which the mean-time-to-failure (MTTF) of the inverter is found. Results show that the boost inverter can be a very reliable topology with low component count for PV applications.

Keywords— microinverter, solar photovoltaic inverter, boost inverter.

I. INTRODUCTION With the worldwide push towards renewable energy

systems, solar photovoltaic (PV) energy has witnessed increasing interest, especially from the power electronics and control perspectives. Several power electronics converters have been used to interface PV panels, which are nonlinear dc sources, with the grid or ac loads. Two-stage topologies have a dc-dc converter connected to a dc-ac inverter, where the first dc-dc stage is used for maximum power point tracking (MPPT), and the second dc-ac stage is used for inversion. Such topologies are very common with different dc-dc and dc-ac combinations; the dc-ac stage can be central or local as shown in Fig. 1 (a, b). With more components in two-stage topologies, single-stage topologies shown in Fig. 1(c) are attractive from a component count perspective [1], excluding redundancy in all topologies. The boost inverter [2] and buck-boost inverter [3] are popular single-stage topologies which can provide MPPT and voltage boost, regulation, and inversion.

A Markov reliability model of a boost inverter is briefly presented in this paper. The boost inverter is selected due to its simple structure, ability to boost and invert voltage, and low component count that simplifies its failure analysis. Even though the methodology presented here has been presented in [8] for a boost converter, it is of interest to compare this single-stage topology to the boost converter reliability shown in [8]. Results show that the single-stage inverter is as reliable as dc-dc stages reported in the literature. With the appropriate choice of components, this can result in the whole dc-ac

inversion process to be more reliable in the single-stage case compared to that of the two-stage case.

(a) (b)

(c)

Fig. 1. Two-stage (a, b) vs. single-stage (c) topologies

II. LITERATURE REVIEW The boost inverter was proposed in 1999 [2] and is shown

in Fig. 2 with a PV panel input. What makes this topology more attractive now is its simplicity along with the push towards micro-inverters in solar photovoltaic applications as shown in [1].

Fig. 2. Boost inverter with PV panel

Lower component count can impact cost and reliability. But, having less components does not guarantee higher reliability;

978-1-4577-1683-6/12/$26.00 ©2012 IEEE

in fact, performance of systems with fewer components can degrade under faults—less control flexibility with fewer states to control outputs. The boost inverter operates as two dc-dc boost converters: The left dc-dc converter consists of L1, C1, (Q1, D1), and (Q3, D3), while the right dc-dc converter consists of L2, C2, (Q2, D2), and (Q4, D4). The PV panel provides a common dc input to the converters, and their output is taken differentially across the load. The output of each dc-dc converter alone is dc, and the switching signals are chosen so that the ac components of both converters are out-of-phase by 180o. When the differential voltage is taken across the load, the dc biases cancel and the ac components add, producing a boosted ac load voltage with zero dc offset. The output is then filtered to produce a smooth sinusoid. More details about boost inverter operation are shown in [2].

Failure analysis and reliability evaluation of PV systems has been under study for over two decades, with significant work in 1982 [4]. Most of the published work has addressed PV panels and dc-dc converters or two-stage topologies, e.g., [5, 6]. The main attempt to design boost inverters for maximum reliability is shown in [7], but it only provides mathematical descriptions and lacks experimental validation of the models used. Markov reliability modeling provides a flexible tool to estimate the mean time to failure (MTTF) of a system with predefined fault modes and failure rates. The procedure used here is similar to that in [8] and is used to compare the single-stage dc-ac inverter shown here to a dc-dc boost converter shown in [8].

III. MODEL VALIDATION Before performing any reliability and failure analyses on the

boost inverter, it is essential to validate a simulation model using which analyses can be performed, as in [8]. The simulation model is used to avoid experimental faults that can cause permanent damage to the system, and to automate the fault injection and performance evaluation process. The experimental setup of this inverter was built and compared to a Simulink model. In order to verify the boosting capability of the inverter, nominal operation was tested at a switching frequency of 2 kHz, fundamental frequency of 60 Hz, dc input voltage at 10V, and an R-L-C load which includes the L-C low-pass filter and a resistive load in parallel. Fig. 3 shows the voltage boost to be around 18 V peak.

It is important to note that running the setup at high voltages and frequencies while injecting faults can cause irreversible damage to the inverter. Thus, for model validation, only basic operation is studied using at 10 V input supply and 60 Hz square wave switching with the same R-L-C load used earlier. Even though this operation does not yield voltage boost due to the low switching frequency, it validates the simulation model under similar operating conditions to that of the experimental setup.

Several faults were tested in both simulations and experiments with a dc power supply input, and the list of faults is shown in Table 1. Inductors are assumed to be very rugged and reliable, but their faults can be easily integrated to the model.

Table 1. Fault modes of the boost inverter and PV panel Component Faults

MOSFET Open circuit (OC), Short circuit (SC)

Panel filter capacitor Capacitance drops by 25%, OC, or SC

Link capacitors Capacitance drops by 25%, OC, or SC

Panel

OC voltage drops by 50% from nominal SC current drops by 50% from nominal

or, OC voltage & SC current drop by 25% from nominal

The voltage across the resistive load excluding the L-C filter

(top trace, 10 V/div) and input current from the PV panel (bottom trance, 1 A/div) were monitored in simulations and experiments, and sample results are shown in Figs. 4-9. Under nominal operation before t=0, the voltage is sinusoidal with 10 V peak. When a fault is injected, both voltage and current waveforms are affected. For example, in Figs. 4 and 5 CF is shorted causing the input supply to be shorted. This leads to zero V across the input, and thus the output voltage drops to zero. In Figs. 6 and 7, C1 suffers an OC condition, thus leading to more harmonics in the load voltage. In Figs. 8 and 9, the capacitor reduction leads to an offset in the output voltage as the voltage levels are different on either side of the load. This yields to current distortion in every other cycle. The current waveforms show slight differences in amplitude, but the waveform shape is similar. It is clear from the results shown in Figs. 4-9 that the model captures most essential dynamics of the experimental setup and can be used to approximate the performance under faults.

Fig. 3.Boosted ac voltage across the resistive load and input current

Fig. 4 Experimental result for SC of CF

Fig. 5. Simulation result for SC of CF

Fig. 6 Experimental result for OC of C1

Fig. 7. Simulation result for OC of C1

Fig. 8. Experimental result for C1 reduction by 25%

Fig. 9. Simulation result for C1 reduction by 25%

IV. RELIABILITY MODELING In reliability simulations, PWM switching at 10 kHz was

employed to achieve the desired voltage boost at 60 Hz fundamental frequency. MPPT of the PV panel was achieved using a perturb-and-observe algorithm which varies the modulation index of the PWM switching.

All faults were injected into the system simulation model, one fault at a time, after the system response reached steady state in 150 ms. When the system survives the first fault,

a second fault is injected in another component after 250 ms. Only two consecutive faults are addressed as the probability of the system surviving three consecutive faults is very low. The condition for system survival is that the load should receive at least 47.5 W, otherwise, the system would be considered in failure mode. Fault injections were carried out under different irradiance values to cover a wide operating range of the PV panel. Failure rates are shown in Table 2, and extracted from [9].

0.05 0.1 0.15 0.2 0.25-20

-10

0

10

20

Time (s)

Load

vol

tage

(V

)

0.05 0.1 0.15 0.2 0.25-2

-1

0

1

2

Time (s)

Inpu

t cu

rren

t (A

)

0.05 0.1 0.15 0.2 0.25-20

-10

0

10

20

Time (s)

Load

vol

tage

(V

)

0.05 0.1 0.15 0.2 0.25-2

-1

0

1

2

Time (s)

Inpu

t cu

rren

t (A

)

Table 2. Fault modes of the boost inverter and PV panel Component Failure Rate (per 109 hours)

MOSFET 7.2

Panel filter capacitor 5.67

Link capacitors 4.56

Panel 920

Note that the failure rate of the PV panel is large due to having 6x12 cells in series, where each cell is modeled as a photo-diode. Given that this number can significantly affect the final MTTF calculations, reliability was evaluated with and without the PV panel faults. Also, note that the failure rate used in [8] is different due to a different series/parallel arrangement of the cells in the panel. Except for the PV panel failure rate, all other failure rates are close to those used in [8], and this is essential to compare the reliability of single-stage and two –stage micro-inverters.

After the fault injection process is complete, the most severe faults were found to be on the input side, which are related to the PV panel or the filter capacitor. This is expected since the input power drives the rest of the system. The system “states” are determined as either failed or survived after injecting every fault, and a Markov chain of all these states is built. The Markov chain is then used to construct the state transition matrix (Φ), which is used with the Chapman-Kolmogorov equation (eq. 1) to solve for the probability (P) of being at a surviving state (eq. 2). The reliability of the system (R(t)) is then found as the sum of the elements in P. For further information regarding the

( ) ( )T

T T Tdt tdt

= =PP PΦ (1)

( ) (0)TT t Tt e=P PΦ

(2)

The MTTF is found as the integral of R(t),

0MTTF ( )R dτ τ

∞= ∫ , (3)

and the mathematical details can be found in [10].

The MTTF of the boost inverter without PV panel faults is found to be 53 years, and 17 years with the PV panel faults. The significant drop of the MTTF in the PV panel case is due to the high failure rate assumed for the panel. Other PV reliability models, such as a single high power photo-diode, can be used instead.

The boost inverter is very attractive from a reliability perspective when compared to the boost converter from [8], whose MTTF is 74 years without considering PV panel faults. But, the boost converter is only one of two stages used with both local and central inverters:

• The converter from [8] with a local H-bridge is expected to have an MTTF that is lower than the boost inverter. This is true since the H-bridge alone is expected to have an MTTF that is very similar to that of the boost inverter given that no faults were injected in L1 and L2 in the analysis presented here.

• The central inverter failure can sabotage the whole system compared to both single- and two-stage micro-inverters.

The boost inverter MTTF (53 years) is higher than other results reported from mathematical models, e.g., [7]. This implies that the boost inverter can be very reliable as a subsystem that interfaces PV panels to an ac grid or load, but further research should follow with realistic PV panel failure rates and a more comprehensive list of faults.

V. CONCLUSIONS AND FUTURE WORK This paper presented analyses of faults, failures, and

reliability of a single-stage boost inverter in PV applications. A simulation model was validated for reliability simulations, where faults and failures did not cause permanent system damage. A Markov reliability model was developed for the inverter with and without a PV panel, and the MTTF of the boost invert was found to exceed 50 years. This result shows a promising future of the boost inverter in PV applications, and is more optimistic than purely mathematical results that were previously reported in the literature. Future work related to this topic includes further analysis regarding the efficiency and transient performance of the boost inverter.

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