for Distributed Power Generation

Dmitri Vinnikov, Indrek Roasto, Tanel Jalakas Department of Electrical Drives and Power Electronics

Tallinn University of Technology Tallinn, Estonia dm.vin@mail.ee

Abstract— This paper presents a new step-up DC/DC converter topology intended for the distributed power generation systems. The topology utilizes the voltage fed quasi Z-source inverter with continuous input current on the primary side, a single-phase isolation transformer and a voltage doubler rectifier. To increase the power density of the converter, implementation of the three-phase auxiliary AC-link (three-phase inverter and three-phase isolation transformer) and the three-phase voltage doubler rectifier are proposed by the authors. The paper describes the operation principles of the proposed topologies and analyzes the theoretical and experimental results.

Keywords- fuel cell powered systems, distributed power generation, DC/DC converters, control algorithms

I. INTRODUCTION Distributed power generation, when fully implemented, can

provide reliable, high-quality, and low-cost electric power. As a modular electric power generation close to the end user, it offers savings in the cost of grid expansion and line losses. If connected to the power grid, the bi-directional transactions between the grid and the local generation result in grid capacity enhancement, virtually uninterrupted power supply, and optimum energy cost due to the availability of use/buy/sell options [1].

Distributed power is a concept that covers a wide spectrum of schemes used for local electric power generation from renewable and non-renewable sources of energy in an environmentally responsible way. Basic schemes are mainly based on solar energy, wind energy, fuel cells, and microturbine engines.

A fuel cell (FC) is potentially the most efficient and modern approach to distributed power generation. The efficiency of the conversion, i.e., the ratio of the electrical output to the heat content of the fuel, could be as high as 65-70% [1]. In fact, its electrical efficiency could be greater than 70% in theory. The current technology has only been capable of reaching

efficiencies of around 45%. Combined cycles are envisaged to bring the electrical efficiency of plants based on high temperature cells up to around 60% [2].

For the interconnection of a low DC voltage producing fuel cell (40...80 V DC) to the residential loads (typically, 230 V AC single phase or 3x400 V AC), a special voltage matching converter is required. A typical structure of the two-stage interface converter is presented in Fig. 1. Due to safety and dynamic performance requirements, the interface converter should be realized within the DC/DC/AC concept. This means that low voltage from the fuel-cell first passes through the front-end step-up DC/DC converter with the galvanic isolation; afterwards the output DC voltage is inverted in the three-phase inverter and filtered to comply with the imposed standards and requirements (second DC/AC stage).

=

=

=

~

STEP-UPISOLATED DC/DC

CONVERTER

THREE-PHASEFOUR-WIREINVERTER

600VDC

RESIDENTIALLOAD

(230 VAC 1 Ph.or

400 VAC 3 Ph.)

FUEL CELL(40...80 VDC)

TWO-STAGE INTERFACE CONVERTER

Figure 1. Typical structure of the interface converter for the residential

fuel cell powered systems.

The given paper is devoted to the new power circuit topology to be implemented in the front-end DC/DC converter for the distributed power generation. The topology proposed (Fig. 2) utilizes the voltage-fed quasi-Z-source (qZSI) inverter with the continuous input current at the converter primary, high-frequency step-up isolation transformer, bridge rectifier and output filter assembly. In contrast to earlier presented topologies [3-5], the novel converter provides such advantages as increased reliability, isolation transformer with drastically reduced turns ratio, reduced impact on the fuel cell and simple control strategy. Finally, to improve the power density of the

978-1-4244-5172-2/09/$26.00 ©2009 IEEE

An Improved High-Power DC/DC Converter

converter, the topology with a three-phase intermediate AC-link is discussed in the final section of the paper.

II. DESCRIPTION OF PROPOSED TOPOLOGY

The qZSI with continuous input current implemented at the converter input side (Fig. 2) has a unique feature: it can boost the input voltage by introducing a shoot-through operation mode, which is forbidden in traditional voltage source inverters (VSI). Thus, the varying output voltage of the fuel cell is first preregulated by adjusting the shoot-through duty cycle; afterwards the isolation transformer is being supplied with the voltage with a constant amplitude value. Although the control principle of the ZSI is more complicated than the traditional VSI, it provides a potentially cheaper, more powerful, reliable and efficient approach to be used for fuel cell powered systems.

Figure 2. Simplified power circuit diagram of the . topology proposed with the voltage-fed quasi-Z-source (qZSI) inverter with the continuous input

current.

The voltage-fed qZSI with continuous input current was first presented in [6] as a modification of a currently popular voltage-fed Z-source inverter (ZSI) [7]. The ZSI has such a significant drawback as discontinuous input current during the boost mode, which could have a negative influence on the fuel cell. The discussed qZSI shown in Fig. 2 features continuous current drawn from the fuel cell as well as lower operating voltage of the capacitor CZ2, as compared to the ZSI topology. The operating dc voltages of the capacitors CZ1 and CZ2 could be estimated as:

INZ

ZCZ U

DDU

211

1 , (1)

INZ

ZCZ U

DDU212 , (2)

where Dz is the duty cycle of the shoot-through state:

TtD Z

Z , (3)

where tZ is the duration of the shoot-through state and T is the operation period. The shoot-though state here is the simultaneous turning on of both switches of the same phase leg of the inverter (Fig. 3). This operation state is forbidden for the traditional VSIs because it causes the short circuit of the dc link capacitors. In the discussed qZSI, the shoot-through state is used to boost the magnetic energy stored in the dc side inductors without short-circuiting the dc capacitors. This increase in inductive energy in turn provides the boost of voltage seen on the transformer primary winding during the traditional operating states of the inverter. Thus, coupled with

the qZSI network, the traditional VSI could realize both the voltage buck and the boost functions without any additional switches using special control algorithms only.

Figure 3. Equivalent model of the inverter during the shoot-through state.

The simplified equivalent model of the VSI during the shoot-through state is presented in Fig. 3. During this operation mode the peak current through the inverter switches reaches its maximum. In the presented scheme the switches are represented as resistors. The short current is distributed between both inverter arms. During the shoot-through state, the voltage of the isolation transformer drops to zero, thus changing the operation duty cycle of the transformer (Fig. 4c).

A. Control Principle of the qZSI-Based Single-Phase DC/DC Converter

As was discussed earlier, compared with a traditional VSI, the quasi Z-source inverter has an extra switching state: shoot-through, when the top and bottom switches of the inverter legs are simultaneously turned on. Fig. 4 presents the control principle of the single-phase qZSI in the shoot-through state. Fig. 4a shows the theoretical switching pattern of the traditional single-phase VSI based on the triangular carrier. To control the shoot-through states, the two straight lines (Up and Un) were introduced (Fig. 4b). When the triangular waveform is greater than the upper envelope Up or lower than the bottom envelope Un, the inverter switches turn into the shoot-through state (Fig. 4b). The resulting voltage waveform of the primary winding of the isolation transformer during the shoot-through states is presented in Fig. 4c.

Figure 4. Operating principle of the proposed DC/DC converter in the shoot-through state.

Each operating period of the proposed topology consists of a shoot-through state tZ and an active state tA: AZ ttT . (4)

In the active states, one and only one switch in each phase leg conducts. The (4) could also be represented as

1AZAZ DD

Tt

Tt , (5)

where DZ is the duty cycle of the shoot-through state and DAis the duty cycle of the active state. The duty cycle of the shoot-through state could never exceed 0.5.

It should be noted here that in the presented control scheme the switching frequency of the transistors as well as the operation frequency of the quasi Z-source network will be four times higher than the frequency of the fundamental harmonic of the isolation transformer’s primary voltage (Fig. 4c). That fact is always to be considered during selection of proper components and operating frequencies of the proposed converter.

B. Operation of the of the Single-Phase qZSI This section discusses the simulation and experimental

results of the 500 W qZSI-based single-phase DC/DC converter. The desired operating parameters are presented in Table I.

TABLE I. Desired Operating Parameters of the Investigated Converter

Parameter Value Minimal input voltage, UIN,min 40 V Minimal input voltage UIN,max 80 V Desired DC-link voltage amplitude UDC 80 V Output power P 500 W Switching frequency, fsw 20 kHz Fundamental frequency of the isolation transformer’s primary voltage, fTR

5 kHz

Capacitance of Cz1 and Cz2 330 uF Inductance of Lz1 and Lz2 50 uH Type of primary switches IGBT

In the given application, the desired value the DC-link voltage UDC was selected to be 80 V. It is assumed that the converter is always operating with the rated load and between two boundary operating points, which correspond to the minimal and maximal input voltages. In the first case, the shoot-through state should be used to boost the input voltage to the predefined DC-link voltage level. In the second case, when the input voltage is equal to the desired DC-link voltage, no shoot-through is applied and the qZSI operates as a conventional VSI.

If the input voltage is below the predefined DC link voltage level, the converter utilizes the shoot-through states, thus ensuring the corresponding boost ratio B of the input voltage:

IN

DC

UUB (6)

For the current application, the maximal boost ratio could be achieved at the minimal input voltage and rated load:

.24080

min,max

IN

DC

UUB (7)

The duty cycle of the shoot-through state could be found by

2

1 1BDZ . (8)

The maximum shoot-through duty cycle in the discussed application is

25.02

1 1max

maxBDZ . (9)

To confirm the above presented assumptions a series of simulations were performed. The operation of the single-phase qZSI was verified in the boundary operating points UIN,min and UIN,max. To provide a better appearance, simulation waveforms are shown with the timescale corresponding to one full period of the operating voltage of the isolation transformer. The inverter is operated with no dead-time. The operating frequency seen on the isolation transformer windings is 5 kHz, while the inverter switching frequency as well as the frequency seen by the quasi impedance network is 20 kHz.

The first simulations (Figs. 5-7) were performed with an input voltage UIN,min = 40 V and with the shoot-through mode (DZ=0.25). The operating duty cycle of the isolation transformer is DA=0.75.

Figure 5. Input voltage (VIN) and DC link voltage (VDC) during the minimal input voltage and maximal boost ratio (Dz=0.25).

Figure 6. Voltage profiles of the capacitors Cz1 and Cz2.

Figure 7. Primary voltage of the isolation transformer (DA=0.75).

The results of simulations were verified on the experimental setup of the qZSI-based single-phase DC/DC converter. As the experiments show (Figs. 8 and 9), the simulated and measured

results are in full agreement for the operating point with minimal input voltage and rated load when the shoot-through duty cycle reaches its maximum. Moreover, it was stated that the discussed topology ensures continuous input current during the voltage boost mode (Fig. 8b), thus reducing negative impact on the proton exchange membrane of the fuel cell module. The peak-to-peak current ripple could be further decreased by the implementation of qZ-inductors with increased inductance values.

(a) (b) Figure 8. Input and DC link voltages (a) and input current (b) with

Dz=0.25.

(a) (b) Figure 9. Voltage profiles of dc capacitors (a) and primary voltage of the

isolation transformer (b) with Dz=0.25.

The second group of simulations (Figs. 10-12) was performed with an input voltage UIN,max = 80 V. In that operating point, the shoot-through mode was eliminated(DZ=0). The operating duty cycle of the isolation transformer is DA=1. As can be seen from Fig. 11a, without the shoot-through state, the voltage of capacitor Cz1 is equal to the input voltage and the voltage of the capacitor Cz2 is zero. This demonstrates that a single-phase qZSI can be operated as a traditional PWM inverter without any modifications in hardware.

Figure 10. Input voltage (VIN) and DC link voltage (VDC) during the maximal input voltage and ”no-boost” mode (DZ=0).

Figure 11. Voltage profiles of the capacitors Cz1 and Cz2.

Figure 12. Primary voltage of the isolation transformer (DA=1).

The results of simulations were verified on the experimental setup of the qZSI-based single-phase DC/DC converter. As the experiments show (Figs. 13 and 14), the simulated and measured results are all in agreement for the operating point with maximal input voltage and rated load, when the shoot-through operating mode is eliminated.

(a) (b)

Figure 13. Input and DC link voltages (a) and input current (b) with Dz=0.

(a) (b)

Figure 14. Voltage profiles of dc capacitors (a) and primary voltage of the isolation transformer (b) with Dz=0.

III. DESIGN OF THE OUTPUT STAGE AND STEP-UPISOLATION TRANSFORMER

A specific feature of the proposed topology is that it utilizes the qZSI as the primary side inverter of the isolated DC/DC converter. Earlier applications of the ZSI and the qZSI were

UIN IIN

UDC

UCZ2

UCZ1

UTR,pr

UIN IIN

UDC

UCZ2

UCZ1

UTR,pr

only connected with the single-stage DC/AC applications (motor drives). Since there is a need of the isolation transformer in the proposed topology, certain adjustments are to be made in the design of the power circuit. It is well-known that the transformer is the main contributor to the size of any switching-mode power supply since it contributes about 25…30% of the overall volume and more than 30% of the overall weight [8]. In order to design a small and compact converter, it is essential to reduce the size of the isolation transformer (as well as output filter components). Moreover, the output rectifier should have reduced power dissipation to ensure better efficiency of the converter. Thus, the design of the output stage and the isolation transformer implies a great challenge for the designer.

One of the most widespread topologies for the output stage of a single-phase DC/DC converter is a full-bridge rectifier (FBR) with the LC-filter (Fig. 16a). In that case the output voltage of the converter UOUT depends on the operating duty cycle DA, which, in turn, depends on the duty cycle of the shoot-through state DZ :

Z,secTRA,secTROUT DUDUU 1 , (10)

where UTR,sec is the amplitude voltage value of the secondary winding of the isolation transformer It means that in the conditions of varying input voltage, the output voltage of the converter will also vary with the discussed control methodology. The nominal output voltage will be achieved only at the minimal input voltage, when the shoot-through duty cycle is maximal. In all other operating points it will exceed the predefined nominal value of the output voltage (Fig. 15).

1

1,05

1,1

1,15

1,2

1,25

1,3

1,35

Input voltage

Out

put v

olta

ge o

f con

verte

r with

fu

ll-br

idge

rect

ifier

(p.u

.)

Uin,min Uin,max

Figure 15. Demonstration of the output voltage rise with the rise of the input voltage of the investigated converter with a full-bridge rectifier (caused

by the rise of the DA).

To resolve that problem, some modifications should be done in the control algorithm of the converter, thus increasing its complexity. Namely, the conventional zero states (when the primary winding of the isolation transformer is shorted through either the top or bottom inverter switches) should be added to the control algorithm for controlling of the magnitude of the output voltage.

The simplest method to cope with the above-discussed problem will be to implement the voltage doubler rectifier (VDR) on the output stage of the converter (Fig. 16b). In that case the output voltage of the converter is

2sec,TROUT UU . (11)

From (11) it is seen that the output voltage of the converter with the VDR does not depend on the duty cycle of the active state anymore. It improves the control flexibility of the

proposed converter with no modifications in the control algorithm.

(a) (b)

Figure 16. Full-bridge rectifier with LC-filter assembly (a) and voltage doubler rectifier (b).

Fig. 17 shows the experimental results of the VDR. The converter was tested in two boundary operating points: with input voltages 40 and 80 V and at the rated load. The shoot-through duty cycle was set for each case study personally to ensure 80 V on the DC link. The isolation transformer has the turns ratio 1:1. As seen from the diagrams, despite the variation of the shoot-throug duty cyle (and, in turn, duty cycle of the active state), the voltage on the converter output remains unchanged for any operating point. Thus, it is concluded that the VDR proposed for the current application could provide an output voltage without intolerable ripple and with a magnitude twice the amplitude voltage value of the secondary winding of the isolation transformer.

(a) (b) Figure 17. Operation waveforms of the voltage doubler rectifier at

different input voltage conditions: input voltage UIN=40 V, DZ=0.25 (a) and input voltage UIN=80 V, DZ=0 (b).

Other benefits of the VDR rectifier implemented on the secondary stage of the proposed converter could be listed as follows:

1) improved rectification efficiency due to reduced voltage drop (twice reduced number of rectifying diodes and full elimination of the smoothing inductor);

2) turns number of the secondary winding of the isolation transformer could be reduced by 62% (turns ratio 1:3.75 in the case of VDR instead of 1:10 of FBR) due to the voltage doubling effect available with the VDR.

Finally, it could be stated that the proposed qZSI-based single-phase DC/DC converter with a high-frequency step-up transformer and a voltage doubler rectifier could be positioned as a new alternative for the front-end DC/DC converter for residential power systems with the operating power up to 10 kW.

IV. QZSI-BASED DC/DC CONVERTER WITH THREE-PHASE INTERMEDIATE AC-LINK AND VOLTAGE DOUBLER RECTIFIER

Modern trends in residential power systems are aimed to increase the efficiency and power density of electronic

UOUT

UTR,sec

UOUT

UTR,sec

80 V

160 V

converters to enhance the feasibility of the whole system. For the discussed application, an increase in power density (more power for the same volumetric space of the converter) could be achieved by the implementation of the three-phase intermediate AC-link instead of the single-phase one. The new topology for the front-end DC/DC converter proposed by the authors for the high-power applications is presented in Fig. 18. It utilizes the three-phase qZSI with continuous input current, a three-phase isolation transformer stack (winding could be connected in different configurations to obtain the desired output voltage) and a three-phase voltage doubler rectifier.

Figure 18. Proposed power circuit layout of the three-phase isolated DC/DC converter with a qZ-source inverter and paralleled voltage doubler rectifiers.

For every operating point within the predefined boundaries [UIN,min; UIN,max], the output voltage of the converter at the shoot-through mode could be estimated as

Z

ININOUT Dn

Un

BUU21122 , (12)

where n is the turns ratio of the three-phase isolation transformer.

(a) (b) Figure 19. Experimental waveforms of the proposed converter operating at

minimal input voltage (UIN=40V) and maximal shoot-through duty cycle (DZ=0.25): input voltage, current and DC-link voltage (a) and line voltages of

the primary winding of the three-phase transformer stack.

(a) (b) Figure 20. Operation waveforms of the voltage doubler rectifier at different

input voltage conditions: input voltage UIN=40 V, DZ=0.25 (a) and input voltage UIN=80 V, DZ=0 (b).

Fig. 19 presents the experimental waveforms of the proposed three-phase converter. As seen from Fig. 19a, the three-phase qZSI operates with continuous input current. The operating frequency of the quasi Z-network is six times higher

than the fundamental harmonic frequency of the isolations transformer.

As in the case of the single phase VDR discussed earlier, the proposed three-phase VDR could provide output voltage without intolerable ripple and with the magnitude twice the amplitude voltage value of the secondary winding of the isolation transformer in all operating points of the converter (Fig. 20).

V. CONCLUSIONS

This paper has presented two new isolated step-up DC/DC converter topologies with quasi-impedance-source inverters. The topology is intended for applications with widely varying input voltage and stabilized output voltage and when the galvanic separation of the input and output sides is required. The high-frequency transformer stack is responsible for providing the input/output galvanic isolation demanded in many applications. The paper is focused on an example of the step-up DC/DC converter with high-frequency isolation for the distributed power generation systems. The operating principle, converter design methodology, simulation and experimental results are presented and analyzed. Moreover, to improve the power density and reliability, the updated converter topology with the three-phase auxiliary AC link and the three-phase voltage doubler rectifier was proposed and verified.

ACKNOWLEDGMENT

Authors thank Estonian Science Foundation (Grant ETF8020 “Research of Advanced Control and Diagnostics Systems for the High-Power IGBT Converters”) for their financial support.

REFERENCES

[1] Zobaa, A.F.; Cecati, C., "A comprehensive review on distributed power generation," International Symposium on Power Electronics, Electrical Drives, Automation and Motion, SPEEDAM 2006, pp.514-518.

[2] J. Padulles, G. W. Ault, J. R. McDonald, “An Approach to the Dynamic Modelling of Fuel Cell Characteristics For Distributed Generation Operation”, IEEE Power Engineering Society Winter Meeting 2000, Vol. 1, 2000, pp. 134-138.

[3] Yu Jin Song; Enjeti, P.N., "A high frequency link direct dc-ac converter for residential fuel cell power systems", IEEE 35th Annual Power Electronics Specialists Conference, 2004. PESC 04. 2004, vol.6, pp. 4755-4761, 20-25 June 2004.

[4] Mazumder, S.K.; Burra, R.K.; Huang, R.; Tahir, M.; Acharya, K.; Garcia, G.; Pro, S.; Rodrigues, O.; Stasinopoulos, M., "Single-stage low-cost and energy-efficient isolated phase-shifted high-frequency inverter followed by a forced cycloconverter for universal residential fuel cell power system," IEEE International Conference on Electro/Information Technology, 2008. EIT 2008., pp.408-413, 18-20 May 2008.

[5] Andersen, G.K.; Klumpner, C.; Kjaer, S.B.; Blaabjerg, F., "A new green power inverter for fuel cells", IEEE 33rd Annual Power Electronics Specialists Conference, 2002. PESC’02. 2002, vol.2, pp. 727-733, 2002.

[6] Anderson, J.; Peng, F.Z., "Four quasi-Z-Source inverters," Power Electronics Specialists Conference, 2008. PESC 2008. IEEE , vol., no., pp.2743-2749, 15-19 June 2008

[7] Fang Zheng Peng, "Z-source inverter," IEEE Transactions on Industry Applications, vol.39, no.2, pp. 504-510, Mar/Apr 2003.

[8] Petkov, R.: Design issues of the high-power high- frequency transformer. Proceedings of International Conference on Power Electronics and Drive Systems, 1995, (pp. 401-410, vol.1).

UOUT

Uab80 V

160 V

UOUT

Uab

UAB

UAC

UBC

IIN

UIN

UDC