A Multi-functional Utility Interface of BIPV Systems Based on Cascade Multilevel Converter Academic research paper on "Electrical engineering, electronic engineering, information engineering"

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Energy Procedia 17 (2012) 356 - 365

2012 International Conference on Future Electrical Power and Energy Systems

A Multi-functional Utility Interface of BIPV Systems Based on Cascade Multilevel Converter1

Guohui Zeng, Minnian Cao and Yuchen Chen

College of Electrical and Electronic Engineering Shanghai University of Engineering Science

Shanghai, P. R. China zengcoffee@gmail.com

Abstract

Building integrated photovoltaic (BIPV) systems bring forward many requirements to grid-tied inverters in these aspects such as generating efficiency, shading effects, harmonic & reactive current compensation and quick economical payback, etc.. This paper presents a multi-functional utility interface of BIPV systems based on cascade multilevel converter. It has two operating modes: one is operated as grid-tied inverter at normal insolation level and the other is operated as active power filter (APF) at very low insolation level or at nighttime. Compared with traditional inverters, cascade multilevel converter is much more efficient and suitable for utility interface of BIPV system because it has the merits of high efficiency, minimum switching frequency, low switching stress and EMI, approximate sinusoidal waveform voltage output with low total harmonic distortion (THD), modular structure and good dynamic stability. Simulation and experimental results have verified its feasibility and advantage.

© 2012 Publishied by Els evier Ltd. Selection and/or peer-review under responsibility of Hainan University.

Keywords: Multi-functional. Cascade multilevel converter. Grid-tied inverter. Active power filter. BIPV.

l.Introduction

Recently, photovoltaic energy as an alternative energy resource has been widely discussed and investigated due to the merits of pollution-free, noise-free, abundant and broadly available. Significant progress in photovoltaic application has been made around the world in these years, especially in building-integrated photovoltaic (BIPV) systems. However, there exist some technical and economical issues that make a negative effect on the commercialization of BIPV systems. First of all, the overall generating efficiency of existing BIPV system is low. Most surveys carried on about BIPV systems performance show average losses up to 25% in electricity production for the main causes of mismatching

1 This work is supported by SUES DSF Project A24000821 to G. Zeng and SUES SF Project 2008XZ02 to M. Cao.

1876-6102 © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Hainan University. doi:10.1016/j.egypro.2012.02.106

losses, partial shadows and loss in PV inverter [1]. Secondly, it is apparent that BIPV system is idle at very low insolation level or at nighttime. The components of the system have not been used for most time during a day. Thirdly, the initial cost of photovoltaic generating system is still very high and the economical payback is a long time, up to 20 years. The commercialization of BIPV systems brings forward many requirements to grid-tied inverters.

At the same time, most electric devices used in offices, homes or intelligent buildings are single phase loads or non-linear electronic equipments. They work largely at night and produce a great deal harmonic and reactive current that depress the power factor of the utility grid. Survey results show that harmonics pollution from civil buildings accounts for 40.6% total harmonics and that these power factors are almost below 0.8, even less than 0.5 [2]. As we know, most electric appliances of intelligent buildings, OA and communications are very sensitive to harmonics. Up to now, power quality has been hot topics in distribution network because of the increasing power demand and the higher power reliability requirements of sensitive loads, especially in BIPV systems. Therefore, to provide high power quality in a distribution system, APF and reactive power/harmonics compensation, is an indispensably necessary technology.

Cascade multilevel converters are increasingly developed in high power application, especially in medium voltage AC drive, static Var compensators and APF due to its many advantages [3]-[6]:

• Generate sinusoidal wave voltage with low THD.

• The circuit structure of each converter unit is uniform, which makes it easy to be designed,

assembled and maintained.

• High efficiency, low loss and low switching stress & EMI.

• The cascade converter has less component count and is more suitable for utility applications than

other multilevel inverters.

• A cascade M-level inverter consists of (M-1)/2 H-bridges in which each bridge has its own separate

dc source. It can reduce the initial and running costs tremendously compared with the traditional

PWM inverter without bulky transformers.

This paper presents a multi-functional utility interface of BIPV systems based on cascade multilevel converter. This presented BIPV system has two operation modes: one is operated as grid-tied inverter at normal insolation level with independent maximum power point tracking (MPPT) for each PV module, and the other is as shunt APF at very low insolation level or at nighttime to improve the system power factor by detecting and compensating for reactive current. The modular structure of cascade multilevel converter decreases mismatching loss and shading effect. The overall generating efficiency of BIPV system is maximized by independent MPPT for each PV module. Moreover, the economical payback of system is speed up based on two operation modes. This new system, however, poses challenging problems such as harmonics eliminating with different DC voltage sources, MPPT for each PV module in grid-tied inverter operation mode, and voltage control and balance of each DC capacitor in the APF operation mode. A prototype of a multi-functional utility interface of BIPV system using a 7-level cascade inverter has been built. This paper will summarize features, feasibility, and control schemes of the utility interface of BIPV systems. Analytical, simulated, and experimental results demonstrate the feasibility of the presented system.

2.Utility interface of Bipv System Based on Cascade Multilevel Converter

The cascade H-bridges converter is a cascade of H-bridges in a series configuration, as shown in Fig. 1. Each H-bridge uses a separate DC source. The number of output phase voltage levels m in a cascade converter with 5 separate DC sources is m = 2s +1 possible levels. The duty cycle for each voltage level can be rotated so that each DC source and bridge shares the same load [6].

Fig. 1 Cascade H-bridge m-level multilevel inverter.

A system configuration of the proposed multi-functional utility interface of BIPV system is shown in Fig. 2. This system has two operation modes: one is operated as grid-tied inverter under normal insolation, and the other is as APF under very low insolation level or at nighttime. The operating principle and control of each operation mode is described in the next section.

One of the great advantages of the utility interface with cascade inverter is that it eliminates the bulky transformers required by static Var compensators that employ the multi-pulse inverter and can respond much faster. This inverter generates almost sinusoidal staircase voltage with only one time switching per line cycle that the initial and running costs and the EMI will be dramatically reduced below that of the traditional PWM inverter. The output voltage and harmonic spectra of 3,7,15 levels converter with step modulation and fundamental frequency switching is shown in Fig.3.

Fig.2. a system configuration of the proposed multi-functional BIPV system based on cascade multilevel converter One of the great advantages of the utility interface with cascade inverter is that it eliminates the bulky transformers required by static Var compensators that employ the multi-pulse inverter and can respond much faster. This inverter generates almost sinusoidal staircase voltage with only one time switching per line cycle that the initial and running costs and the EMI will be dramatically reduced below that of the

traditional PWM inverter. The output voltage and harmonic spectra of 3,7,15 levels converter with step modulation and fundamental frequency switching is shown in Fig.3.

% CC № JO! 4(41 0.H t№ 3 J i Ï It I! » i- » 3! +)

f>m fïl 'jV".fc"'M.r' drier

Fig. 3 Output voltage and harmonic spectra of 3, 7, 15 levels converter

3. Analysis of the Proposed Utility Interface of Bipv Systems

As shown in Fig. 2, the proposed system has two operating modes, which poses some challenges on its control algorithm.

3.1.Grid-tied Inverter Mode

The proposed system is operated in this mode during daytime and with enough insolation, shown in Fig. 4. In this mode, PV arrays can either feed power to utility line synchronously, or can supply power to loads exclusively with sinusoidal voltage when the utility power fails.

Some papers have demonstrated the superiority of cascade converter as grid-tied inverter [4] [5] [7]. Compared to other conventional inverters, it can improve the performance of the PV system mainly by three ways of reducing mismatch losses, partial shadows of the array, and modular structure, etc. However, the efficient use of the available solar energy of each PV modules requires the implementation of an independent MPPT for each PV module of these groups. Although these multilevel converters are initially prepared for applications where each one of the modules delivers the same power, their optimal use in photovoltaic generators require an independent delivering of each one of these modules [7].

Fig. 4 The system configuration of grid-tied inverter mode.

• MPPT of each PV module:

To track the maximum power point of PV modules, an intelligent fuzzy logic controller with an adaptive fuzzy inference engine is adopted, as shown in Fig. 5, which has the merits of simplicity, fast response, low over-tuning, high control precision, and easy implementation [8].

Fig.5 The intelligent adaptive fuzzy inference engine

• Harmonic eliminating

Since in real applications it is impossible that the DC sources are equal, the traditional algorithm to get the switch angles for the H-bridges in a cascade converter by assuming the DC sources are equal can't lead to the perfect result. To ensure that the THD in the voltage output waveform is small enough, a new algorithm is applied in this paper to determine the switching angles so as to not produce harmonics and a fast real-time computing & implement [6].

The Fourier series expansion of the output voltage waveform of the multilevel inverter with nonequal dc sources is

V (at) =

4Vd » 1 (1)

—dL X —(V cos(n^) + --- + VS cos(n<9s ))sin(n©t)

n n=1,3,5,- n

where s is the number of dc sources, and the product VdcVi is the value of the dc source. The objective here is to choose the switching angles so as to make the first harmonic equal to the desired fundamental voltage and specific higher harmonics of equal to zero.

An example of a three dc source case is now considered so that the switching angles are chosen so as to not generate the fifth- and seventh-order harmonics while achieving the desired fundamental voltage. The mathematical statement of these conditions is then

—^ (V cos(^1) + V2 cos($2) + V3 cos(6>)) = Vf

V cos(5^1) + V2 cos(56>2) + V3 cos(56>) = 0 (2)

V cos(7^1) + V2 cos(76>2) + V3 cos(7#3) = 0

This is a system of three transcendental equations in the unknowns 61 , d2 and 63. The solution methods have been presented in many papers [6] [9] [10].

3.2.APF mode

When the insolation is very low or it is at night time, the utility interface operates as an active power filter that can be controlled to block harmonic or reactive current flowing from the nonlinear loads. So the system can provide pure, constant sine-wave voltage to the loads that are sensitive to harmonics, and correct power factor to unity. Fig. 6 shows the system configuration for harmonic and reactive current detection and control. The cascade converter is connected in parallel with the system and can be employed to compensate for reactive current and regulate the terminal voltage.

To compensate for harmonic and reactive current, the load current iL is sensed, and its harmonic

and reactive components are extracted. The current reference, fc, of the compensator can be the load

reactive current component, harmonic component, or both depending upon compensation objectives.

1) Harmonic and Reactive Current Detection:

To ensure the compensation current of APF to trace the reference harmonic and reactive current, an adaptive PI controller is applied in this paper. First of all, an adaptive closed loop is adapted to detect harmonic and reactive current quickly and robustly, shown in Fig. 7 [11]. In Fig.7, LPF is a low pass filter, C3s/2r is the transform matrix from three phase stationary coordinate system to two phase rotary coordinate system, and C2r/3s is the inverse matrix of C3s/2r.

Fig. 6 The system configuration of shunt APF mode.

sin(©t ) sin(©t - sin(©t + cos(®t ) cos(®t - cos(®t +

sin(©t )

cos(®t )

sin(©t - cos(®t -sin(©t + cos(®t +

Fig. 7 Harmonic and reactive current detection circuit.

• DC Side Balance Control

Since each H-bridge of cascade multilevel converter has separate DC side, it is easy to implement DC voltage balance control. An example of a five-level cascade converter is shown in Fig.8.

Fig. 8 An example of DC side balance control

• AC Side Decoupling Control

The AC side control should ensure the compensation current of APF to trace the reference harmonic and reactive current. The low frequency model of three phase voltage source converter can be expressed

'a -R 0 0 'a Ua ea

b = 0 -R 0 'b - ub + eb

0 0 -R 'c Uc ec

Equation (5) can be expressed as the following (6) at two phase rotary coordinate system.

''d ' " -R a)L 'd Ud + ed

L'q J -rnL - R \ 'q Uq eq

id " UU ed a

iq _ ~ C3sf2r h .Uq _ ~ C3s/2r ub _eq _ ~ C,i2r %

}c _ Uc ec

Obviously, 'd and lq are coupling. The AC side decoupling control is shown as Fig.9. To improve the dynamic response rate, adaptive control and variable structure control can be introduced.

Fig. 9 AC side decoupling control.

4.Conclusions

This system can optimize the use of the existing components in a typical grid-tied PV system. A simulation system of the proposed multi-functional utility interface based on seven-level cascade converters is established in this paper. Simulation results with the software of Matlab have verified the feasibility of the proposed system in both inverter mode and APF mode. The simulation parameters are: AC source voltage 220V, AC source frequency 50Hz, AC side inductor 0.5mH, AC side resistance 0.5 Q and DC link capacitor 6000 ¡uF .

The simulated waveforms are shown in Fig.10, where Fig.10a shows the simulated output voltage and current waveforms of the proposed multi-functional utility interface operated as a grid-tied inverter. It can be seen that the output current is sinusoidal and in phase with line voltage, which means feeding only real power to the utility grid. Fig. 10b shows the simulated waveforms of source voltage, load current, and compensation current of the proposed interface operated as a shunt APF. It can be seen that the APF system have a good compensation effect at low device switching frequency.

5.Conclusions

A multi-functional utility interface of BIPV system based on cascade multilevel converters has been presented in this paper. It can optimize the use of the existing components in a typical grid-tied PV system. Besides acting as an attractive grid-tied inverter for photovoltaic application, it can also operate as an active power filter to improve the system power factor by detecting and compensating for inactive power current, when the system is at nighttime or without insolation. This utility interface based on cascade multilevel converter is especially suitable for BIPV system applications with these merits of less mismatch losses, higher generating efficiency, better power quality, and easy installation and service because of its modular and simple structure. With this system, the main issues will get a good solution and the commercialization of BIPV will be accelerated. Simulation and experimental results have verified the feasibility and superiority of the proposed multi-functional BIPV utility interface.

(a) (b)

Fig. 10 Simulation results of the proposed system: (a) in grid-tied inverter operation mode, and (b) in shunt APF operation mode

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