Improved Y-Source Inverter for Distributed Power ... - IEEE Xplore

2015 23rd Iranian Conference on Electrical Engineering (ICEE)

Improved Y-Source Inverter for Distributed Power Generation Mojtaba Forouzesh, Alfred Baghramian

Nastaran Salavati

Department of Electrical Engineering University of Guilan Rasht, Iran [email protected], [email protected]

Department of Electrical Engineering Islamic Azad University, Rasht Branch Rasht, Iran [email protected]

Abstract—In this paper, an improved Y-source inverter with continuous input current has been proposed for distributed power generation. Continuous input current is a promising feature in converters working with renewable energy sources. Low current ripple drawn from renewable energies like fuel cell and photovoltaic systems are always desirable. Furthermore, the need for large input filters in the proposed Y-source inverter has been eliminated. Principals and analysis of the improved Y-source inverter have been discussed and corresponding equations have been derived. Computer simulations using PSIM software have been done to validate performance of the proposed inverter in photovoltaic systems. It is shown that all theoretical expressions are in accordance with the simulation results. Keywords—Y-source inverter; dc-ac conversion; photovoltaic system, distributed power generation.

I. INTRODUCTION In recent years, electricity generation is moving toward distributed power generation and demand for clean and efficient electric power generations has been increased due to air pollution concerning and high costs of fossil energy sources. Hence, the need for a power conditioning system to achieve reliable electric power from distributed energy sources like fuel cell (FC) and photovoltaic (PV) system is undeniable [1]. In 2002, Z-source impedance network proposed for dc-ac conversion. The Z-source inverter has interesting features that makes it suitable for renewable energy sources like FC and PV systems. The most promising feature of the Z-network for an inverter is the advantage of simultaneously buck/boost behavior. The voltage gain of the inverter determines by the amount of intentionally shoot-through applied to the Z-source inverter’s upper and lower bridge switches. The Z-source impedance network also increases reliability of the inverter, because short circuits that happens by electromagnetic interference (EMI) noise cannot destroy the device anymore. This is a unique feature of the Z-source inverter [2]. Since then, many articles have been published about impedance source inverters in distributed power generations [3-7]. In most of these articles, the quasi-Z-source inverter has been investigated in PV or FC systems, which is because of its inherent continuous input current feature [6]. Voltage boost of the Z-source and quasi-Zsource inverters theoretically can be infinite, but in order to achieve high voltage gain, large shoot-through should be utilize. Therefore, lower modulation index has to be used in the Zsource inverters. Among impedance sources, magnetically

c 978-1-4799-1972-7/15/$31.00 2015 IEEE

coupled impedance sources show promising due to their high voltage gain and design freedoms. Major topologies in this category are T-source [8], trans-Z-source [9], ī-source [10], and the latest in this type is Y-source [5]. As mentioned before, another prominent feature for inverters operating in distributed power generation is continuous input current. For instance, in PV systems input current should be continuous for better tracking of the maximum power point (MPP). In addition, continuous input current drawn from FC system would improve its efficiency and lifetime. The input current of mentioned magnetically coupled inverters is pulsating and is not continuous. In this paper, in order to overcome this problem and to mitigate impact on the dc energy source, an improved Y-source inverter with continuous input current is proposed for distributed energy applications. The whole paper is organized as follows. In section II. Principles of the improved Y-source inverter described and its analytical equations have been derived. Implementation of the proposed inverter on a PV system has been demonstrated in section III. In section IV, computer simulations have been done to validate equations and principles noted in the previous sections. Finally, conclusion is bring in the last section. II. PRINCIPLES AND ANALYSIS OF THE IMPROVED Y-SOURCE INVERTER WITH CONTINUOUS INPUT CURRENT Fig. 1 shows the proposed Y-source inverter in which the improved Y-source network couples distributed energy sources to the inverter bridge. The proposed Y-source inverter has all the benefits related to the Y-source impedance network besides it has continuous input current. Like Z-source inverter, the Ysource inverter has one more state named shoot-through zero state besides to its six active states and two zero states that related to traditional inverters. As shown in Fig. 2 the proposed

1677

D

Fuel Cell or PV array

L1 C1

S1 S3 S 5

L3 L2

C2

с

Vi S2 S4 S 6

To AC Load

Fig. 1. Illustration of the proposed Y-source inverter


L1

-

L3 L2

C1 Vdc

In (5), ݀௦௛ is the shoot-through zero state of the inverter. From (1) that is dominant in both the shoot-through and nonshoot-through modes, the voltage ܸ஼ଶ across capacitor ‫ܥ‬ଶ can be written as

VL

D

+ C2

ܸ஼ଶ ൌ

+

-

(a)

L1

-

Vdc

L3 L2

C1 +

C2

ܸ෠௜ ൌ Vi

ͳ ܸ ͳ ൅ ݊ଵଷ ௗ௖ ͳെቀ ቁ ͳ െ ݊ଶଷ

-

The peak value of ac voltage per phase can be derived as ܸ෠௔௖ ൌ

Fig. 2. The proposed Y-source inverter equivalent circuit view from the dc source, (a) the shoot-through state and (b) the non-shoot-through state

Y-source inverter has two working modes. The equivalent circuit of the shoot-through mode is depicted in fig .2(a) where at least one leg of the inverter are turned ON, causing diode D to be in reverse-bias. In this state, capacitors are discharging in the magnetizing inductance of the coupled inductors. By adding capacitor C1 to the circuit, a current flow path will be exists at the input port in shoot-through mode. By assuming that the voltage across L1 is ܸ௅ then the following equation can be written, where ݊ଵଶ ൌ ܰଵ Τܰଶ and ݊ଵଷ ൌ ܰଵ Τܰଷ are turn ratios of the coupled inductors. ܸ஼ଶ ൌ ܸௗ௖ ൅ ܸ஼ଵ

ܸ௅ ൌ

ܸ௅ ܸ௅ െ ൌ Ͳ ՜ ݊ଵଶ ݊ଵଷ

݊ଵଶ ݊ଵଷ ሺܸ ൅ ܸ஼ଵ ሻ ݊ଵଶ െ ݊ଵଷ ௗ௖

ܸ௅ െ݊ଵଶ ൅ ܸ஼ଵ ൌ Ͳ ՜ ܸ௅ ൌ ܸ ݊ଵଶ ͳ ൅ ݊ଵଶ ஼ଵ

ܸௗ௖ െ ܸ௅ െ

ܸ௅ െ ܸ௜ ൌ Ͳ ݊ଵଶ

ܸ஼ଵ

(8)

ேయ ିேమ

ܸ෠௜ ൌ

ͳ ܸ ൌ ‫ ܤ‬ή ܸௗ௖ ͳ െ ‫݀ܭ‬௦௛ ௗ௖

‫ܤ‬ൌ

ͳ ͳ െ ‫݀ܭ‬௦௛

ܸ෠௔௖ ൌ

ͳ ܸௗ௖ ܸௗ௖ ‫ܯ‬ ൌ‫ܯ‬ή‫ܤ‬ή ͳ െ ‫݀ܭ‬௦௛ ʹ ʹ

‫ ܩ‬ൌ‫ܯ‬ή‫ܤ‬

(9) (10) (11) (12)

(2)

(3) (4)

In (4), ܸ௜ is the dc link voltage of the inverter bridge. By applying the volt-second balance principle to the inductor‫ܮ‬ଵ , the voltage ܸ஼ଵ across capacitor C1 can be obtained. ͳ ൅ ݊ଵଶ ቀ ቁ݀ ݊ଷଶ െ ͳ ௦௛ ൌ ͳ ൅ ݊ଵଷ ͳെቀ ቁ݀ ͳ െ ݊ଶଷ ௦௛

ͳ ܸௗ௖ ή‫ܯ‬ή ͳ ൅ ݊ଵଷ ʹ ͳെቀ ቁ݀ ͳ െ ݊ଶଷ ௦௛

In (8), M is the modulation index of the inverter. By introducing a winding factor for the Y-source coupled inductors ே ାே as‫ܭ‬௒ ൌ య భ , (7) and (8) can be rewritten.

(1)

In (2), ܸௗ௖ is the voltage of input power source. In the nonshoot-through mode when inverter is operating in its normal condition, the stored energy releases to power the load. The equivalent circuit of this mode is depicted in fig. 2(b) where the following equations can be written. ܸ௅ ൅

(7)

+

(b)

ܸௗ௖ ൅ ܸ஼ଵ ൅

(6)

From (4) and considering (5) and (6), the peak value of dc link voltage of the inverter bridge can be obtained as

VL

D

ͳ െ ݀௦௛ ܸௗ௖ ͳ ൅ ݊ଵଷ ͳെቀ ቁ ݀௦௛ ͳ െ ݊ଶଷ

(5)

In (9) and (12), B is boost factor of the impedance network and G is gain of the proposed inverter, respectively. Because of the Y-source inverter uses narrower shoot-through duty cycle than the other impedance sources, it can operate with larger modulation index. Moreover, larger modulation index for the Y-source inverter causes lower total harmonic distortion (THD) of the output voltages and currents. In the improved Y-source in order to achieve continuous input current with less ripple, the value of two capacitors C1 and C2 should be selected carefully. The proper capacitance ratio for no current jump in transition between two modes should obey from (13). ‫ܥ‬ଶ ܰଵ ൅ ܰଶ ൌ ൌ‫ܭ‬െͳ ‫ܥ‬ଵ ܰଷ െ ܰଶ

(13)

In other words, for the specific winding factor (K), the capacitor C2 should has (K-1) times larger capacitance value than capacitor C1.

1678


As it is known, for the converters working with renewable sources like fuel cell and photovoltaic systems, continuous input current is a promising feature. Continuous input current drawn from a fuel cell can improve its efficiency and lifetime. Furthermore, when input current ripple minimized in PV systems, the MPP can be tracked more accurately. Therefore, the PV system can operate more efficiently with decreased input current ripple. In the next section, the proposed inverter is implemented with a PV array.

III. IMPLEMENTATION OF THE PROPOSED INVERTER IN PV POWER GENERATION SYSTEM Fig. 3 illustrates configuration of the proposed Y-source inverter in a PV generation system. It will be shown that by the help of improved Y-source inverter there is no need to bulky input filters. In order to control the proposed Y-source inverter in a PV system, all boost control methods that are related to the Z-source inverter can be used. Besides, for better utilization of the PV system, shoot-through and modulation index should be controlled separately. Usually control of M is used to regulate the output voltage of inverter and control of ݀ௌ் is used to achieve maximum power point track (MPPT). Therefore, in this article simple boost control (SBC) is used for regulating the output voltage of inverter. It is assumed that the system is operating in the MPP mode. Fig. 4 shows the control strategy and its corresponding switching sequences, in which two envelope voltage line Vp and Vn are employed to generate required shoot-through. In SBC, in order not to affect the output voltage, the maximum of shoot-through can be expressed as ݀ௌ் ൌ ͳ െ ‫ܯ‬

(14)

To achieve maximum boost from the proposed inverter, substituting (14) in (11) the maximum peak of ac output voltage in terms of modulation index can be written. ܸ෠௔௖ ൌ

ܸ௣௩ ‫ܯ‬ ή ‫ ܯܭ‬െ ‫ ܭ‬൅ ͳ ʹ

(15)

For 220 V phase output RMS voltage, PV array with maximum output voltage of 900 V can be used. Furthermore, for output voltage above 625 V, the improved Y-source inverter can work in buck mode and below that voltage; it would work in boost mode. Because in the Y-source inverter there is no need to

D

Vpv

L1 C1

Fig. 4. Simple boost control with sinusoidal reference voltage and its related switching sequence

use large shoot-through duty cycles, the proposed inverter would operate with high modulation indexes in both buck and boost modes even with SBC. IV. SIMULATION RESULTS Simulations are carried out in Power Sim (PSIM) software to validate the theoretical equations and to verify the performance of proposed inverter. Improved Y-source inverter that was implemented in the Simulink software is shown in Fig. 3. Typical I-V and P-V characteristics for a PV array is illustrated in Fig. 5(a) and 5(b) for two different environmental conditions. Hence, simulations were done for two case conditions. Case 1: at irradiation 250 W/m2 and ambient temperature 0Û&, maximum output power 8.2 kW andܸ௠௣௣ ൌ ͹ͳͲ. Case 2: at irradiation 1000 W/m2 and ambient temperature 75Û&, maximum output power 18.25 kW andܸ௠௣௣ ൌ ͶͲͲ. Simulation parameters for both cases are as follows. C1=100µF, C2=300µF, Lm=1mH and coupled inductors turns ratio is 1:2:3 (ܰଵ ǣ ܰଶ ǣ ܰଷ ). Output filter parameters are Lf=2mH and Cf=80µF for each phase. Frequency of carrier voltage is 10kHz and output frequency of the inverter is 50 Hz.

L3 L2

RL

Lf Vi

C2

Cf

Fig. 3. Improved Y-source inverter configuration in the PV power generation system.

1679


voltages across capacitor C1 and C2 and the input current are depicted in Fig. 6(b). As mentioned before, in the buck mode the dc-link voltage is equal to the input voltage, as it is obvious in Fig. 6(b) they overlaps each other.

Typical I-V and P-V characteristics at 250 (W/m2 ) & 0 (deg.C) 15 8 12

In the second case, the inverter works in its boost mode. In order to achieve 220 V phase RMS voltage, the required modulation index can be calculated using (16).

4 6

PV power (kW)

PV current (A)

6 9

‫ܯ‬ൌ 2

3

0

0

100

200

300

400 500 600 PV voltage (V)

700

800

900

‫ܤ‬ൌ

(a) Typical I-V and P-V characteristics at 1000 (W/m 2 ) & 75 (deg.C) 20

45

15

30

10

15

5

ͺ ൈ ξʹ ൈ ʹʹͲ െ ͶͲͲ

ͳ ൌ ͳǤ͹ʹ Ͷ‫ ܯ‬െ ͵ Va

Vb

(19)

Vc

400 200 0 -200 -400

PV power (kW)

PV current (A)

(18)

ൌ ͲǤͺͻͷ

The boost factor of the inverter in this case can be calculated using (17).

0 1000

60

͸ ൈ ξʹ ൈ ʹʹͲ

Iin

0

0

100

200

300 PV voltage (V)

400

500

80 60 40 20 0

0 600

0

(b) Fig. 5. Typical I-V and P-V characteristics for a PV array, (a) at 250 W/m2 & 0Û& and (b) at of 1000 W/m2 & 75Û&

ܸ௣௩ ‫ܯ‬ (16) ή Ͷ‫ ܯ‬െ ͵ ʹ In the first case, the inverter works in buck mode, in which ݀ௌ் ൌ Ͳ and B=1. To achieve 220 V phase RMS voltage, the modulation index can be calculated as ܸ෠௔௖ ൌ

ʹ ൈ ξʹ ൈ ʹʹͲ ൌ ͲǤͺ͹͸Ͷ ͹ͳͲ

0.08 Time (s)

0.12

(a)

In order to operate in the MPP in both cases, 17.7ȍ and 7.95ȍresistive loads for each phase are used in case 1 and 2, respectively. With the mentioned turns ratio, the winding factor can be calculated as k=4. Therefore, (15) can rewritten as

‫ܯ‬ൌ

0.04

(17)

Since, in this case the inverter operates as a normal voltage source inverter and only a pure dc current flows through the inductors. Hence, the input voltage is equal to the dc-link voltage. Furthermore, the voltage across capacitor C1 is zero and the voltage across capacitor C2 is equal to the input voltage. Simulation results for the first case are depicted in Fig. 6. Transient response from start to steady state for three phase voltages of the inverter and input current are shown in Fig. 6(a). Steady state waveforms for the input and dc-link voltages, the

1680

Vin

Vi

Vc1

Vc2

800 600 400 200 0 800 600 400 200 0

Iin 40 30 20 10 0 0.1398

0.13985

0.1399 Time (s)

0.13995

0.14

(b) Fig. 6. Simulation results for buck mode, (a) transient waveforms of the phase voltages and input current, (b) steady state waveforms of the input and dc-link voltages, capacitors C1 and C2 voltages and the input current.


Va

Vb

Hence, the proposed inverter is suitable for working with PV and FC systems that demand for continuous input current.

Vc

400 200 0 -200 -400

V. CONCLUSION In this paper, a novel Y-source inverter with continuous input current for distributed power generation has been presented. The continuous input current feature is always desirable because of reducing stress on the input energy source. Moreover, inverters with continuous input current can works more efficiently in PV and FC systems. In this paper, it is shown that the proposed inverter can operates with PV systems in different environmental conditions. Mathematical equations and analyses of the proposed inverter have been demonstrated. Simulation Results for two cases with buck and boost characteristics for the proposed inverter validated trueness of the derived equations and verified the performance of the proposed inverter. Furthermore, the simulation results of the boost mode operation showed that the input current of proposed inverter is continuous. Hence, presented inverter is promising for operating with low voltage renewable energy sources.

Iin 100 80 60 40 20 0

0

0.04

0.08 Time (s)

0.12

(a) Vin

Vi

Vc1

Vc2

800 600 400 200 0 Iin

REFERENCES

48

[1]

47 46 45 44 0.1398

0.13985

0.1399 Time (s)

0.13995

0.14

(b) Fig. 7. Simulation results for boost mode, (a) transient waveforms of the phase voltages and input current, (b) enlarged waveforms of the input, dc-link, capacitors C1 and C2 voltages and the input current.

From (9) and (19), the dc-link voltage of inverter can be calculated asܸ෠௜ ൌ ͸ͺͻ. Furthermore, from (16) the peak of output phase voltage can be calculated asܸ෠௔௖ ൌ ͵ͳͳ. Considering (10), the shoot-through duty cycle can be calculated as݀ௌ் ൌ ͲǤͳͲͷ. Consequently, from (5) and (6), the voltages across capacitor C1 and C2 can be calculated as ܸ஼ଵ ൌ ʹͳ͹ andܸ஼ଶ ൌ ͸ͳ͹, respectively. Fig. 7 illustrates the simulation results for boost mode. The phase voltages and the input current are shown in Fig. 7(a). Enlarged waveforms for the voltages of input source, dc-link, capacitors C1 and C2 are shown in first part of Fig. 7(b) and the input current is shown in the second part. All calculated parameters are in consistent with the simulation results appears in Fig. 6 and Fig. 7. It has been shown that the proposed inverter can operates with low modulation index in a PV system for both buck and boost modes. Moreover, it is clear that in the improved Y-source inverter, input current is continuous and the need for large input filters are eliminated.

F. Blaabjerg, Y. Yang, and K. Ma, "Power electronics-Key technology for renewable energy systems-Status and future," in Electric Power and Energy Conversion Systems (EPECS), 2013 3rd International Conference on, 2013, pp. 1-6. [2] F. Z. Peng, "Z-source inverter," Industry Applications, IEEE Transactions on, vol. 39, pp. 504-510, 2003. [3] Y. Huang, M. Shen, F. Z. Peng, and J. Wang, "Z-source inverter for residential photovoltaic systems," IEEE Trans. Power Electron, vol. 21, pp. 1776-1782, 2006. [4] J.-W. Jung and A. Keyhani, "Control of a fuel cell based Z-source converter," Energy Conversion, IEEE Transactions on, vol. 22, pp. 467476, 2007. [5] Y. P. Siwakoti, P. C. Loh, F. Blaabjerg, S. J. Andreasen, and G. E. Town, "Y-Source Boost DC/DC Converter for Distributed Generation," Industrial Electronics, IEEE Transactions on, 2014. [6] Y. Li, J. Anderson, F. Z. Peng, and D. Liu, "Quasi-Z-source inverter for photovoltaic power generation systems," in Applied Power Electronics Conference and Exposition, 2009. APEC 2009. Twenty-Fourth Annual IEEE, 2009, pp. 918-924. [7] J.-H. Park, H.-G. Kim, E.-C. Nho, T.-W. Chun, and J. Choi, "Gridconnected PV system using a quasi-Z-source inverter," in Applied Power Electronics Conference and Exposition, 2009. APEC 2009. TwentyFourth Annual IEEE, 2009, pp. 925-929. [8] R. Strzelecki, M. Adamowicz, N. Strzelecka, and W. Bury, "New type Tsource inverter," in Compatibility and Power Electronics, 2009. CPE'09., 2009, pp. 191-195. [9] W. Qian, F. Z. Peng, and H. Cha, "Trans-Z-source inverters," Power Electronics, IEEE Transactions on, vol. 26, pp. 3453-3463, 2011. [10] 3 & /RK ' /L DQG ) %ODDEMHUJ ī-Z-source inverters," IEEE transactions on power electronics, vol. 28, pp. 4880-4884, 2013.

1681