# Dual-source inverter for hybrid PV–FC application

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## Abstract

Multi input–multi output Power electronic interface for hybrid energy resources has gathered much of interest. In this paper, a new configuration for cascade connection of two Z-Impedance networks is presented which is able to extract power from two separate low voltage energy sources such as photovoltaic panels and fuel cell stacks. The required load power can be shared between two sources which may have different low values. The power sharing is controlled by adjusting the shoot through duration and time shifting. The proposed converter includes the features of conventional Z-source converter such as voltage boost and shoot through immunity of the inverter. Steady state operation principles, switching methods and the main relations for power flow from two inputs to the output are derived. A prototype of the converter is built and tested for different operation modes. The results are compared with simulations and theoretical analysis which verify well performance of the system.

## Keywords

Partial power processing Isolated Z-source converters High step up converters DC/AC converter Power sharing## 1 Introduction

Power generation by green sources such as wind and solar energy are growing day to day because of continuously growing energy demand and destructive the effects of fossil fuels on the future of the planet. Free and abundant solar energy has proven to be a challenging source of energy in the most parts of the world. Because of its nature of intermittency to supply power continually, another supplemental power source such as batteries or fuel cells is required. Individual energy provision from these resources is not cost-effective owing to low reliability due to the dependence of produced energy to different atmospheric conditions and high initial costs [1].

In order to develop the concept of hybrid energy systems and required power electronic interface, many researches are conducted [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. Some of them have introduced on new structure and the others studied switching methods. Some studies have added energy storage devices to the system by using auxiliary circuits for better utilization from renewable energies.

Low count switching devices.

High efficiency.

Low emission and conduction noise.

Ability to boost the DC-link voltage.

Eliminating short circuit condition creating by turning on two switch of one leg of the VSI.

Continuous input current by some topology.

Based on the operation principles of the basic topology (ZSI), many new topologies are proposed to improve the voltage gain, capacitor voltage, low semiconductors count and etc. [21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33].

Using hybrid systems can be implemented via two separated ZSI which sink energy from separated sources and conduct them to grid or the stand alone load. But it increases system weight, size, cost and decrease efficiency. A type of multi input ZSI is introduced in [19]. In that, several voltage or current sources can share their power into one or several ZSI but the input values must be equal in each of any pair placed in symmetric site. It is a practical constraint.

Multi-input Z-source inverters get a number of sources as input. The sources may be voltage or current sources and have different output levels from power, voltage or current viewpoint. Energy storage devices can be used in these systems when some of the inputs are renewable energy kind. It is clear that these systems have special switching methods for charge and discharge of energy storage devices. These switching methods must consider SOC (state of charge) of batteries. The literatures studied hybrid source systems generally employ the theory of multiport converters [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19].

Some literatures devoted to introduce new topologies of multi-input inverters [34, 35, 36, 37, 38, 39]. Some of these introduced a multi-input inverter which is suitable for single-phase applications [36, 37, 38, 39] and the others are suitable for three-phase applications [34, 35]. Liu et al. [34] has employed double-stage energy conversion method for producing a three-phase output from two DC inputs. This method is simple for implementation but the total system have more cost, size, weight, loss and etc. Singh et al. [35] has used single-stage energy conversion method. This method is better than double-stage energy conversion method but complexity increase is an issue in this situation. Singh et al. [35] employed a good topology by use of eight extra switches which can result lower efficiency.

In this paper, a dual-input ZSI is proposed which extracts energy from two separate low voltage renewable energy sources such as FCs and PVs and conducts it to an AC output load by employing single-stage energy conversion method. Higher voltage of the dual operation, sharing power between two inputs by controlling the shoot through duty cycles of two Z-Impedance networks, and feasibility of implementation the concept for other impedance network structures are the main advantages of the proposed structure. Two switching methods used for power management are described. For each switching method, the relations governing on amount of power sharing by each source and the value of output voltage are explained.

The paper is organized as follows: in Sect. 2, first the proposed converter is introduced and then two switching methods with all important relations and curves are explained. Section 3 is devoted to present simulation and experimental results. In Sect. 4 total conclusions are expressed.

## 2 Steady state operation principles and topology analysis of the proposed converter

*D*

_{st1}and

*D*

_{st2}).

Ability to control the power sharing between two sources by adjusting the switching mode, the amount and phase shift of the shoot through times.

High DC-link voltage applied to the inverter, not requiring extreme values of

*D*_{st}.Power sharing by two Impedance networks without any extra elements.

Simple switching methods.

### 2.1 Circuit description

As shown in Fig. 1, instead of two inductors, *Z*-*Source 1* contains two transformers in the structure, which the secondary voltages (*V*_{sec1} and *V*_{sec2}) are in series with *Z*-*Source 2*, and therefore DC-link voltage of the inverter is equals to *V*_{X}+ *V*_{sec1}+ *V*_{sec2}. The required inductors of *Z*-*Source 1* are the magnetizing inductances of the transformers. Two switching methods which will be explained later, shows expect in optimum switching method. The output are three level signal and it is required that an small *LC* filter be placed in the output for bypassing the high frequency harmonics.

### 2.2 The steady state operation analysis

Due to the circuit symmetry, circuit analysis is performed for a part of the circuit. Obviously, the other part have the same behavior.

*Z*-

*Source 1*works similar to the traditional Z-source converter [20] and therefore the relations for the capacitor voltages (

*V*

_{C1}and

*V*

_{C2}) and the output voltage (

*V*

_{O1}) are the same:

*Z*-

*Source 1.*Considering the voltage drops across

*L*

_{1}, secondary voltage of the transformer

*V*

_{sec1}is positive while

*S*

_{1}is ON (shoot-through state for

*Z*-

*Source 1*) as

*V*

_{C1}is applied to the primary winding of the transformer (similar for

*V*

_{sec2}and

*V*

_{C2}). During this time interval (equals to

*D*

_{st1}

*T*),

*V*

_{sec1}is:

*S*

_{1}is off), the secondary voltage is negative (because of volt-second law for the magnetizing inductors of the transformers) and is equal to:

*D*

_{st1}and

*D*

_{st2}and time position relative to each other, two operation modes is defined as shown in Fig. 2. In the following, the operation principles of the converter in both modes are described.

#### 2.2.1 Mode I

*D*

_{2}may be conducting or not, which depends on the inputs voltage level, the output current level and the duty cycles values (

*D*

_{st1}and

*D*

_{st2}). In the next, it is supposed that all symmetric elements have same values and behavior (such as transformers turn ratio (n) or \(L_{3}\) and \(L_{4}\) or …). Figure 3 Shows the equivalent circuits for all three states of the converter in

*Mode I*and actual current directions (red dashed lines) in all the circuit branches.

According to Fig. 3a, in \(D_{st2} T\) (subinterval 1), because both Z-sources are in shoot through state, so \(V_{L3} = V_{C3} + 2V_{sec1} = V_{C3} + 2nV_{C1}\), \(V_{O2}^{1} = 0\). It is clear that \(L_{3}\) charges with the value is larger than \(V_{C1}\) (in compared to classic Z-source) and so when *Z*-*source 2* inters in non-shoot through state it produces the value larger than the inductor voltage in classic Z-source.

*Z*-

*source 2*is in shoot through state (subinterval 1) and have non-zero values in other states (subinterval 2 and 3). Figure 4 illustrates \(V_{O2}\) with the related shoot through duty cycles for the converter. In order to eliminate flowing the high order sinusoidal voltage harmonics, an

*LC*filter is required in the output of the inverter.

*L*3 yields:

*kvl*for the circuits result:

When the proposed topology operates in *Mode I,* via controlling \(D_{st1}\) and \(D_{st2}\), \(V_{O2}\) and the values of the active power flowed to the load can be controlled. Now, for evaluation the operation of the converter from power sharing viewpoint, the curve showing \(\frac{{P_{z1} }}{{P_{out} }}\) for \(V_{i1} = V_{i2}\) for different values of \(D_{st1}\) and \(D_{st2}\) is drawn.

If in the converter \(D_{st1} = D_{st2}\), subinterval 2 will be eliminated, the output value will be a two level signal as in classic Z-source inverter and the non-zero level of it is calculated by (8) for \(D_{st1} = D_{st2}\). This is named *Optimum Switching Method* which is very useful because in the output, the *LC* filter isn’t required. In Fig. 5, this mode is achieved for \(D_{st1} = D_{st2}\) and it is seen that controller can simply control the \(P_{z1}\) only by one duty cycle value.

#### 2.2.2 Mode II

*Mode Ӏ*is ability to control the contribution of

*Z*-

*source 1*to handle the load. If the system wants to supply the load so that the

*Z*-

*source 2*should have the more contribution,

*Mode ӀӀ*will be better than

*Mode Ӏ*. As

*Mode I*, in one period, when

*Z*-

*source 1*is in shoot through state and

*Z*-

*source 2*isn’t in shoot through state, \(V_{O2}\) has its maximum value. Also, \(V_{O2}\) is a three-level signal. Figure 7 shows shoot through signals and DC-link voltage for this Mode.

*Mode Ӏ*so that subintervals 2, 3 and 4 in this Mode is equal to subintervals 1, 2 and 3 in

*Mode Ӏ*, respectively. Only subinterval 1 is different from them and is according to Fig. 8:

*Mode I*, it can be said:

*Mode I*, it is necessary to extract the active power values produced by

*Z*-

*source 1*and

*2*, simultaneously. The calculation and simplification of the relations governing on the system in this Mode is deduced:

Figure 9 shows an increase in the values of \(D_{st}\) or \(D_{ov}\), increases \(\frac{{P_{z1} }}{{P_{out} }}\). So with controlling the values of \(D_{st}\), \(D_{ov}\) the contribition of *Z*-*source 1* to handle the load and so sharing the power will be controlled.

## 3 Simulation and experimental results

Main parameters of converter prototype

Symbols | Definitions | Values |
---|---|---|

| Z-source 1 input voltage | 20 V |

| Z-source 2 input voltage | 24 V |

| The resistance of load (delta connection) | 33 Ω |

| The forward voltage of input diodes | 0.38 V |

| The on state voltage of IGBTs | 1.5 V |

| The transformers turn ratio | 1 |

| The transformers primary winding resistance | 0.1 Ω |

| The transformers secondary winding resistance | 0.1 Ω |

| The transformers primary winding leakage inductance | 1.5 µH |

| The transformers secondary winding leakage inductance | 1.5 µH |

| The transformers magnetizing inductance | 272 µH |

| The inductances of Z-source 2 | 380 µH |

| Four capacitors of the converter | 1000 µF |

| Switching frequency | 20 kHz |

First, the converter is simulated and implemented in *Mode I.* The duty cycles in the subintervals are \(D_{st1} = 0.3, D_{st2} = 0.2\). The results of simulation are as follow.

*Z*-

*source 1*. So the primary current is sum of magnetizing current (which is almost high value) and the secondary current referred to the primary side. Therefore, the curves of Fig. 13a and b aren’t the same from the value and the shape viewpoint. As seen in Fig. 14a and b, it is clear that simulations and experimental results for the transformers windings current verify each other.

According to Fig. 14c and d, when the system is in subinterval 2, it is clear that the power is shared between two Z-source networks because the current value of input source 2 has been lower than the current in subinterval 3.

## 4 Conclusion

This paper introduced a new topology of two Z-source inverter, which can share the power between two sources with low voltages. The converter can be used in hybrid renewable energy resources. The topology have further advantages such as high voltage gain. Two switching methods with their relation and operation analysis were explained, which showed that by adjusting the shoot through values for two Z-impedance networks, power flow can be controlled. DC-link voltage and the value of shared power can be controlled via choosing switching methods and its duty cycles in each subintervals. The simulation and experimental tests proved the theoretical analysis.

## Notes

### Compliance with ethical standards

### Conflict of interest

The authors declare that they have no conflict interest.

### Research involving human participants and/or animals

Not applicable.

### Informed consent

All authors confirm the content.

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