Purely Inductive Ripple Power Storage for Improved Lifetime in

Solar Photovoltaic Micro-inverter Topology Ramprakash Kathiresan1,2, Pritam Das1,2, Thomas Reindl1 and Sanjib Kumar Panda1,2

1Solar Energy Research Institute of Singapore, Singapore - 117574 2Electrical and Computer Engineering Department, National University of Singapore, Singapore - 117583

Abstract — In a solar photovoltaic (PV) system, decoupling of power ripple is necessary in order to maximize the energy harvest because the power flowing to the grid is time varying while the power extracted from the photovoltaic panel needs to be maintained constant. This necessitates the requirement of energy storage elements between the input and output to decouple the unbalance of power. The location of the decoupling element can be either at the PV-side or the AC-side. The traditionally used energy storage element is the electrolytic capacitor. Due to its fast degradation, it is often replaced by film capacitors with active decoupling schemes that perform better at higher temperatures with less degradation, however at significantly higher cost. From an economic perspective, both electrolytic and film capacitors have similar lifetimes, which are substantially lower than the typical warranty of the solar module (20-25 years) though. An inductor, in contrast, is an energy storage element with aging of less than 0.5 percentage over a 20-year lifecycle. Further, the reduced temperature drift compared to a capacitor makes it a highly suitable candidate for storage of power ripple in solar photovoltaic power converters. This paper demonstrates a proof-of-concept decoupling scheme using an inductor as the energy storage element at the PV-side of a DC-AC single stage power converter.

Index Terms — Power decoupling, Inductive ripple storage, Micro-inverter, Lifetime.

I. INTRODUCTION

The numbers of solar photovoltaic (PV) system installations

are growing exponentially and are adding GWs of renewable

power to the global energy mix [1]. The main reason for the

tremendous growth is due to the so-called "grid-parity" being

achieved in several countries. Also, the drastic reduction in PV

module prices (partially also due to over-capacity in the

market), has led to a wide-spread use of PV for a variety of

applications. The most common form of installation is grid-

connected PV systems, which require an inverter to convert

the available DC output of the PV generator into a grid-

compatible AC form. PV inverters are available in different

forms depending on the capacity and functionalities.

Generally, there are: central inverters (typically > 100 kWp),

string inverters (typically 1- 100 kWp) and micro-inverters

(typically < 1 kWp).

This work is related to micro-inverters. Like in other

inverter types, the power flow to the grid is time-varying while

the power extracted from the solar-PV panel needs to be

maintained constant for maximizing the harvested energy. This

requires an energy storage element to store the power ripple

and hence decouple the unbalance of power.

Fig.1. Description of decoupling of DC and AC power [2]

Mathematically, the instantaneous output power ‘po’ (see

Fig. 1) of the inverter in a single phase grid connected system

consists of:

an average output power ‘Pdc’ and

a time varying pulsating power ‘pac’ which oscillates

at twice the line frequency.

(1)

(2)

where ‘U’ and ‘I’ are the amplitudes of the grid voltage and

grid current respectively. When Pdc > pac, the surplus energy is

stored into the energy storage element; and is later released

when pac > Pdc.

To serve the purpose of decoupling, a capacitor is

commonly used as the energy storage element. Presently,

electrolytic capacitors are the most widely used means of

decoupling and are placed across the PV panel terminals. This

will result in a very large capacitance so as to restrict the

voltage ripple to low values. Also, they have a lifetime of

around 7000 hours only at a temperature of 105°C [3].

Alternatively used film capacitors have less degradation and

higher ripple current density and are employs a power

decoupling circuit. Several topologies employing a power

decoupling circuit with capacitive energy storage elements are

suggested as in [4]-[6]. However, film capacitors have an

increased cost of nearly 3 to 4 times that of electrolytic

capacitors [7]. Batarseh et.al. have extensively reviewed the

decoupling techniques for micro-inverters [2] with different

capacitive methods presently available, which showcases that

the lifetime of the capacitor is the limiting factor. Hence, a

means of decoupling with an increased cycle life while still

UIPdc2

1

1cos(2 )

2acp UI wt

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being easily available and cost-effective would be desirable.

The solution proposed in this work addresses this lifetime

issue for PV inverters by using an inductor to eliminate the

second harmonic ripple power. It is to note that the inductor

has an aging factor of less than 0.5% over a 20-year life cycle

and hence the lifetime is significantly higher than both

electrolytic and film capacitors.

In this paper, an inductive means of ripple power storage is

proposed and implemented at the DC side of the DC-AC

converter. While compared to capacitive means of decoupling,

inductive method shall be economical and also practical at

high temperature range due to its reduced temperature drift.

Also, there will be significant cost savings as elucidated in

Table I.

TABLE I

COMPARISON OF COST AND LIFE OF DECOUPLING ELEMENTS

Decoupling

component (for

250W)

Cost

(USD)

Lifetime

(operation hrs @ 85°C)

Electrolytic

capacitor 4 10,000

Film capacitor 12 30,000

Proposed

inductor 4.5

0.5% degradation over

20 years

The following sections describe the system description and

proof-of-concept demonstration of the proposed decoupling

technique.

II. SYSTEM DESCRIPTION

The proposed purely inductive decoupling scheme can be

applied to any DC to AC converter including those with high

frequency galvanic isolation and require ripple mitigation at

the DC side. Typically all micro-inverters have galvanic

isolation between input and output for safety purposes.

A micro-inverter based on a simple fly-back type DC-AC

converter with galvanic isolation and output unfolding circuit

and related filter is demonstrated here as shown in Fig. 2(a).

The micro-inverter output is connected to the utility grid.

Now, the DC link between the solar panel and the inverter

experiences a second harmonic ripple (i.e, ripple at twice the

frequency of the AC grid), which needs to be eliminated. It is

suggested here to add an inductor and an auxiliary circuit

along with a suitable controller to enable the ripple power

storage mechanism at the PV side. When the ripple mitigation

technique is exercised, high MPP efficiency operation could

be achieved.

The proposed technique could be implemented in all types of

DC to AC converters as generalized in Fig. 2(b) and is

especially applicable for single-stage micro-inverter topologies

with high frequency galvanic isolation. This is because in

single stage micro-inverter, the power converter needs to

accomplish both the tasks namely: Maximum Power Point

Tracking (MPPT) operation along with rectified sine

waveform modulation followed by unfolding; so the simple

means of power ripple decoupling is to place the decoupling

element at the PV or DC side. Decoupling at the AC output

side will require several active switches and additional

complexity in the control method. Also, in the case of AC-side

decoupling, a third phase shall be necessary to implement the

power decoupling and the control complexity is increased

dramatically [2].

Unfolder

circuit

L

C AC utility

grid

Flyback

transformer Output filter

Solar

Panel

Auxiliary

circuit

IPV

IGRID

VGRIDVPV

Sfly

Inductor as energy storage element

Fig. 2 (a). Proposed Inductive ripple storage mechanism implementation in a DC-AC converter employing Flyback topology

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Solar panel

Vaux+

DC/ACAC

utility

grid

Iref

Vaux-

Fig. 2 (b). Block diagram representation of the generalised system.

A. Auxiliary circuit description

The proposed auxiliary circuit schematic as shown in Fig. 3

consists of two switches S1 & S2, two diodes D1 & D2 and an

inductor as the energy storage element. The inductor is

represented with a series resistance ‘RL’ and inductive

reactance ‘XL’.

MOSFET switches and diodes are connected in an H-bridge

fashion where one phase is controlled by the switches S1 & S2

and the other phase by the diodes D1 & D2 only. This

auxiliary circuit is connected at the points Vaux+ and Vaux- of the

DC link to handle the second harmonic ripple power. Current

sensors are used for measuring the currents ‘Iref’ and ‘Iaux’ as

shown in Fig. 2 and Fig. 3 respectively.

Fig.3. Schematic representation of the auxiliary circuit.

The operation of the auxiliary circuit (see Fig. 4) can be

divided into the following zones when ripple energy has to be

stored inside the inductor, denoted in the paper as BUCK

mode, in which S2 will remain ON:

1) Zone-1 (t0< t <t1): At time t0, the auxiliary switch S1 is

turned ON and switch S2 remains ON during the energy

storage mode. By doing so, a positive voltage of ‘VDC’ at

the output of the PV panel is reflected across the inductor

so that the current through the inductor will rise, thus

“pumping” energy into the inductor.

2) Zone-2 (t1< t <t2): At time t1, the auxiliary switch S1 is

turned OFF but switch S2 remains ON. The current in the

inductor freewheels through D1 and S2 with ideally zero

volt across the inductor. The current through the inductor

will remain constant in this zone. This mode ends with

turning switch S1 ON again, which signifies the completion

of one switching cycle in the buck mode.

By controlling the duration of zone-1 and zone-2 of Buck

mode, the net energy stored in the inductor can be controlled.

Again, the operation of the auxiliary circuit can be divided

into the following zones when ripple energy has to be

extracted from the inductor, denoted in the paper as BOOST

mode, in which S2 will always remain OFF:

1) Zone-1 (t’0< t <t’1): At time t’0, the auxiliary switch S1 is

turned ON and switch S2 always remains OFF. The current

in the inductor will freewheel through D2 and S1. By doing

so, zero volt is incident across the inductor.

2) Zone-2 (t’1< t <t’2): At time t’1, the auxiliary switch S1 is

turned OFF. The current in the inductor flows through D2

and D1 so that a voltage of ‘-VDC’ is incident across the

inductor so that the current through the inductor will ramp

down in this zone and the stored energy in the inductor is

released. End of this mode signifies the completion of one

switching cycle in the boost mode.

By controlling the duration of zone-1 and zone-2 of Boost

mode, the net energy released from the inductor can be

controlled.

Fig.4. Ideal waveforms of both BUCK and BOOST modes of

operation.

B. Control method

The control method for the proposed inductive energy

storage scheme is explained in Fig. 5 and Fig. 6. The control

system has to ensure that the second harmonic current required

by the DC-AC converter is completely provided by the

discharging and charging of the auxiliary inductor.

To do so, band-pass filters with center frequency ‘fc’

matching the 2nd harmonic frequency of the utility AC grid are

used to extract the second harmonic components ‘Iaux2’ and

‘Iref2’ of currents ‘Iaux’ and ‘Iref’ respectively from the DC-AC

converter and from the positive rail of the auxiliary circuit. A

RLXL

D1

D2

VGS1

S1

VGS2

S2

Vau

x+

Vau

x-

Iaux

Inductor

current, IL

Inductor

voltage, VL

Gate VGS1

Gate VGS2

t

BUCK mode BOOST mode

t0 t1 t2

S1

S2

D1

S2

D2

S1

D2

D1

t'0 t'1 t'2

VDC

-VDC

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proportional-integral (PI) controller is used to minimize the

error between the peak values ‘Iaux2p’ and ‘Iref

2p’ of currents

‘Iaux2’ and ‘Iref

2’ that are already obtained. The signal from the

PI controller is multiplied with the modulus of the grid voltage

‘Vgrid’ phase shifted through ‘-45°’ to obtain the control signal

‘Ctrl’ for modulation of the auxiliary circuit as shown in Fig.5.

Fig.5. PI controller implementation for proposed auxiliary circuit

Fig.6. Modulation logic (ST: saw-tooth carrier)

The detection of the afore-mentioned boost and buck modes

are done using a phase detector circuit. When pac > Pdc, i.e. the

AC load requires more energy than what is provided by the

DC source, the inductor is discharged and helps in meeting the

load demand – hence the term ‘BOOST’. When pac < Pdc, i.e.

the AC load requires less energy than what is provided by the

DC source, the inductor is charged from this excess amount of

energy - hence the term ‘BUCK’.

C. Inductor Design

The 3-dimensional design of the proposed inductor used as

the energy storage element in this paper is as shown in Fig. 7.

The inductor core consists of a lamination stack made up of

0.35mm cold-rolled grain-oriented electrical (Fe-Si) steel as in

[8]. The winding consists of several turns of copper foil. A

planar geometry of the core is considered for improved

packaging of the inductor. The fundamental considerations and

calculations are followed as described in [9].

Fig.7. 3-D representation of the proposed Inductor.

III. SIMULATION & EXPERIMENTAL RESULTS

The proposed technique is simulated on a 250W micro-

inverter powered by a PV panel with 45V, 6.0A as shown in

Fig.8, at the MPPT voltage and current, using the simulation

software PSIM Ver.9.3.1. The value of the inductor used is

6mH.

Fig.8. Simulated Output Current and Voltage of PV Panel (V: Volts,

I: Amps)

Both the buck and boost mode respectively are dependent

on the status of the ripple. According to the status of ripple

inherited by the ‘Ctrl’ signal, the switch S1 is modulated with a

saw-tooth carrier for controlled charging and discharging of

the inductor. The inductor current and related waveforms in

the buck and boost mode of operation are shown in Fig. 9.

The magnetic flux density of the proposed inductor design is

investigated using COMSOL Multiphysics Ver.4.4. The core

parameters from [8] are incorporated. The magnetic flux

density norm attains 1.89 T at the maximum while injected

with a DC current of 15A. The distribution of the magnetic

flux density in the inductor core is as shown in Fig. 10 which

proves the sufficient and appropriate sizing of the inductor.

Fig. 10. Magnetic flux density of the proposed inductor design

validated in multi-physics tool.

Iref2

Iaux2

Ctrl

+-

|Vgrid Ð-45°|

K + ʃ Peak current conversion

Peak current conversion

Iref2p

Iaux2p

ST

Ctrl

Ctrl>ST

Ctrl<ST

Buck

Boost

VGS1

+

-

+

-

VGS2Buck

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For experimental purpose, a modular solar array simulator -

Agilent E4360 is used to mimic a solar panel input. The

voltage and current values at MPPT were maintained at 45V

and 6.0A respectively, same as in simulation condition. The

auxiliary circuit is introduced at the DC side of the fly-back

type DC to AC converter. The charging and discharging action

is observed in the current in the ripple energy storage inductor

is as shown in Fig. 11. The output current of the flyback

transformer at the secondary side is as shown in Fig. 12. The

MPPT voltage, grid current and grid voltage respectively are

shown in Fig. 13.

Fig. 11. Auxiliary inductor current waveform

From the simulation and experimental results, it is proven

that the proposed power ripple decoupling circuit is able to

store the ripple power effectively which leads to operation of

the micro-inverter with MPPT. Moreover the output current

and voltage of the micro-inverter are in phase with minimal

harmonic distortion which can only be realized by proper

MPPT operation of the overall converter wherein the output

voltage of the emulated PV panel is always stabilized at its

MPPT value with negligible amount of low frequency ripple.

Fig. 12. Flyback transformer output current waveform

Fig.13. Experimental waveforms showing MPPT voltage, grid

voltage and grid current respectively.

Fig. 9. Simulated Auxiliary Inductor Current, 2nd harmonic ac current of dc-ac converter, boost, buck mode detection (I: Amps)

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IV. CONCLUSION

Long lifetime of the power electronics is the primary

concern in any solar PV installation that is supposed to last for

20-25 years. By implementing a purely inductive decoupling

scheme, the reliability of the DC-AC inverter which is the key

electronic component of a PV system is improved without

significant cost and complexity compared to other means of

active power decoupling schemes provided in the literature.

Based on the successful simulation results, a patented

hardware prototype is developed to implement this scheme in

an “AC module” (i.e. PV module with a micro-inverter) rated

at 250W and the proof-of-concept results are discussed.

REFERENCES

[1] International Energy Agency, “Energy technology

perspectives2008- scenarios and strategies to 2050” [Online]. Available: http://www.eia.org

[2] Haibing Hu; Harb, S.; Kutkut, N.; Batarseh, I.; Shen, Z.J., "A Review of Power Decoupling Techniques for Microinverters With Three Different Decoupling Capacitor Locations in PV Systems," Power Electronics, IEEE Transactions on , vol.28, no.6, pp.2711,2726, June 2013

[3] C. C. Dubilier, “Type 381EL 105 ◦C ultra-long life snap-in, aluminum.”

[4] T. Shimizu,K.Wada, andN.Nakamura, “Flyback-type single-phase utility interactive inverter with power pulsation decoupling on the dc input for an ac photovoltaic module system,” IEEE Trans. Power Electron., vol. 21, no. 5, pp. 1264–1272, Sep. 2006.

[5] S. B. Kjaer and F. Blaabjerg, “Design optimization of a single phase inverter for photovoltaic applications,” in Proc. IEEE 34th Annu. Power Electron. Spec. Conf., Jun. 2003, vol. 3, pp. 1183–1190.

[6] H. Hu, S. Harb, X. Fang, D. Zhang, Q. Zhang, Z. J. Shen, and I. Batarseh, “A three-port flyback for PV micro-inverter applications with power pulsation decoupling capability,” IEEE Trans. Power Electron., vol. 27, no. 9, pp. 3953–3964, Sep. 2012.

[7] “Advances in Capacitors and Ultracapacitor for power electronics”, IEEE Applied Power Electronics Conference, Industrial Session 1.3, March 2013.

[8] Material specification of electrical steel : TEMPEL International grade GS0140 066 C02.

[9] Colonel Wm. T. McLyman, “Transformer and Inductor Design Handbook,” Kg Magnetics, Inc.

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