Martina Calais Vassilios G. Agelidis Centre for Renewable Energy Systems Technology Australia (CRESTA)

Curtin University of Technology GPO Box U1987, Perth 6845, Western Australia

pcalaism9cc.curtin.edu.au

Abstract - Multilevel voltage source inverters offer several ad- vantages compared to their conventional counterparts. By syn- thesising the AC output terminal voltage from several levels of voltages, staircase waveforms can be produced, which ap- proach the sinusoidal waveform with low harmonic distortion, thus reducing filter requirements. The need of several sources on the DC side of the converter makes multilevel technology attractive for photovoltaic applications. This paper provides an overview on different multilevel

topologies and investigates their suitability for single-phase grid connected photovoltaic systems. Several transformer- less photovoltaic systems incorporating multilevel converters are compared regarding issues such as component count and stress, system power rating and the influence of the photo- voltaic array earth capacitance.

I. INTRODUCTION

Grid connected photovoltaic (PV) systems, in particular low power, mostly single-phase PV “rooftop” systems and their contribution to clean power generation is recognised more and more worldwide. Grid connected PV rooftop systems are generally privately owned, single-phase sys- tems in a power range of up to 10 kW. The main aim of a private operator who owns such a system is to max- imise its energy yield. Issues such as long life time (20 years and longer), high (part-load-) efficiency and good environmental conditions (availability of solar radiation) are hence of importance to the private operator. Other important requirements for these PV systems (see Fig. 1) are the fulfillment of standards concerning power quality, electromagnetic compatability, acoustic noise limitations as well as safety and protection requirements.

The most important issues, however, for PV grid con- nected systems to gain wide acceptance are reliability and low cost. Figures from 1995 show that the operating inavail- ability of inverters for low power PV systems due to defects is 6 to 7 days per year [l], which compares unfavourably with household appliances such as refrigerators or washing machines. And today’s costs for commercially available low power sine-wave inverters for PV applications range from 0.9 to 3 USSNp where as drive converters in the same power range are available for 0.25-0.5 US$/Wp [2].

First commercially available grid connected PV inverters were line commutated inverters, followed by self commu- tated, puls width modulation inverters including either line or high frequency transformers, often incorporating several

VAgelidis 9 curtin. edu. au

Fig. 1. Issues regarding grid connected PV systems for the low power range.

stages of power conversion [3]. Newest trends in this field are string based units with a power rating around 1 kW [4], [5] and transformerless concepts [6], [7], [8]. For larger systems the overall efficiency can be increased through application of several, small, string inverters replacing a single unit which avoids losses through module mismatch and decreases the DC wiring effort. Transformerless concepts (in particular inverters with high input voltages) are advantageous regarding their high efficiencies. Their peak efficiencies of up to 97% are equivalent to efficiencies reached in drives applications [5]. Avoiding the transformer has the additional benefits of reducing cost, size, weight and complexity of the inverter. However, the removal of the transformer and hence its isolation capability has to be considered carefully.

Multilevel converter technology is based on the synthesis of the AC voltage from several different voltage levels on the DC bus. As the number of voltage levels on the DC side increases, the synthesised output waveform adds more steps, producing a staircase wave which approaches the sinusoidal wave with minimum harmonic distortion [9]. Multilevel converters are particularly interesting for high power applications such as FACTS since the need of filters is reduced and the efficiency is high because all devices switch at fundamental frequency [ 101, [ 111. In low power applications where switching frequencies are not as restricted as in high power applications various control methods such as multicarrier pulse width modulation or multiple hysteresis band control methods can be used to further reduce harmonics in the stepped waveforms [12], [ 133. Multilevel converter topologies are especially suitable for PV applications since due to the modular structure of PV arrays different DC voltage levels can easily be provided.

This paper provides an overview on various multilevel

0-7803-4756-0/98/$10.00 1998 IEEE 224

topologies which have been suggested or are considered for (transformerless), single-phase grid connected systems. Each topology is briefly described, listing advantages and disadvantages regarding issues such as component count and stress, system power rating and the influence of the photovoltaic array earth capacitance. Due to quick voltage and current transitions most power electronic equipment emits disturbances which propagate either by conduction or radiation. In transformerless systems additionally leakage currents due to the photovoltaic array earth capacitance can occur and increase electromagnetic emissions (both conducted and radiated). Since the paper focuses on transformerless systems the issue of leakage currents in transformerless photovoltaic systems will be discussed first.

11. LEAKAGE CURRENTS IN TRANSFORMERLESS PV SYSTEMS

Avoiding the transformer in PV inverter topologies results in a galvanic connection of the grid and the PV array. Due to the capacitance between the PV array and earth, potential differences imposed on the capacitance through switching actions of the inverter inject a capacitive earth current. The PV array earth capacitance is then part of a resonant circuit consisting of the PV array, DC and AC filter elements and the grid impedance. Due to necessary efficiency optimisa- tion of PV systems the damping of this resonant circuit can be very small so that the earth current can reach amplitudes well above permissible levels. Also, the resonant frequency is not fixed due to the varying, on environmental conditions dependent PV array earth capacitance. Depending on the topology, switch states and environmental conditions the ca- pacitive earth current can cause more or less severe (con- ducted and radiated) electromagnetic interference, distortion of the grid current and additional losses in the system. Mea- sures to minimise this current are mentioned in [7] and [ 141 and include e.g. adding passive components to dampen the resonant circuit.

The magnitude of the PV array earth capacitance depends on weather conditions and physical structure of the array. It can be estimated according to the physical dimensions of the PV array and its grounded frame area. One electrode of the capacitance is formed by the photovoltaic cells, the other by the grounded frame (see Fig. 2(b)). In the worst case the complete surface of the PV array is covered by a conducting layer (e.g. formed through humidity or dirt) increasing the area of the grounded electrode of the array (see Fig. 2 (a)).

Table I summarises estimations and measurement results of PV module earth capacitances for mono-crystalline mod- ules with the following specifications and dimensions:

Ppeak 55 w Cells in series 36 V M P P ( 2 5 "C) 17V I M P P ( 2 5 "C> 3.23 A

Length x width x depth 1004 mm x 448 mm x 38.5 mm

For arrays consisting of several modules the capacitances add independent of series or parallel connection.

V O G P ~ "C) 21.2 v

Module Frame /

Fig. 2. (a) Maximum and (b) minimum PV module earth capacitance [14].

TABLE I PV MODULE IEARTH CAPACITANCES

Measured 1 l l00F I 4.2 DF I

111. MULTILEVEL INVERTER TOPOLOGIES

A. HalfBridge Diode Clamped

Fig. 3(a) shows a half-bridge diode clamped three- level inverter (HBDC) [15] as part of a single-phase transformerless grid connected PV system as suggested in [ 161. With simultaneously switching on the switches S1 and 5 2 a positive voltage can be created at the inverter output terminal. A zero output voltage is created by switching on S2 and S3 and a negative voltage is created by switching on S3 and S4 respectively. In order to allow power transfer into the grid the DC bus voltages VPVAI and VPVAZ have to be always higher than the grid voltage amplitude 6grid. Since currently avatilable PV modules have operating voltages around 17 V a large number of modules is required resulting in a minimum system size of approximately 3 kW. An advantage of this system is that the midpoint of the PV array is grounded which eliminates capacitive earth currents and their negative influence on the electromagnetic compatability of the circuit.

The half-bridge diode clamped inverter can be expanded from three-levels to five:-levels as shown in Fig. 3(b). Five switch combinations where always four switches are switched simultaneously generate five different voltage levels at the AC output of the inverter, e.g. switching on S1, S2, S3 and S4 at the same time generates V P V A l + VPVA2 at the AC output, switching on S2, S3, S4 and S5 generates VPVA2 at the AC output and so forth:In [I71 a three phase grid connected PV systenn using a diode clamped five-level inverter is discussed. By adding more levels on the DC bus the number of levels of the voltage at the inverter output terminals are also increased. This allows for reduced distor- tion of the output waveform. To further reduce harmonics an extra degree of freedom is given through choosing the number of cells in series (and thus the voltages) of the outer PV sub arrays (1 and 4) differently to those of the middle PV sub arrays (2 and 3 ). Drawbacks of this topology,

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,........... ..... ( j ! j j s t i DI s2: ....... IIW.*l

1 ............

..................

(4 (b)

Fig. 3. Grid connected PV systems with (a) half-bridge diode clamped three-level inverter (HBDC) [16] and (b) half-bridge diode clamped

five-level inverter.

however, are the high number of semiconductor devices required and since the loading of the outer PV sub arrays (1 and 4) is different to that of the middle PV sub arrays (2 and 3) careful sizing of each PV sub array is neces- sary to ensure maximum power transfer from each sub array.

B. Full Bridge Single Leg Clamped

In [18] and [12] a full-bridge single leg switch clamped inverter (SLSC) is described and suggested for residential PV systems. The topology (see Fig. 4(a)) comprises of a conventional full-bridge (switches S a l , Sa29 sbl and s b 2 ) where a bi-directional switch (realised with Sa3, Sa4, Dal and Da2) is added which controls current flow to and from the midpoint of the DC bus. When applied in a transformerless PV system the minimum system size with this topology is approximately 1.5 kW.

A transformerless PV system with similar characteristics can be realised with a full-bridge single leg diode clamped inverter (SLDC) as shown in Fig. 4(b) [18]. With the single leg diode clamped configuration the devices Da1, Da2,

Sal, Sa2, Sa3 and Sa4 all can be rated for half the blocking voltage of switches sbl and S b 2 , whereas with the single leg switch clamped configuration this only applies to the devices Dal , Da2, Sa3 and S a d , not Sal and Sa2. In both systems both PV sub arrays are symmetrically loaded.

D. Step

C. Cascaded (CC)

Fig. 5 shows a transformerless grid connected PV system where a cascaded inverter [19] is used for DC to AC power conversion. The topology comprises of two full-bridges with their AC outputs connected in series. Each bridge can create three different voltage levels at its AC output allowing for an overall five-level AC output voltage. The advantage of this topology is the modular character. In [6] the concept is suggested for transformerless PV systems using more than two full-bridges connected in series on the

IrWlbl - ............... ..........

............. la)

................ PI

Fig. 4. Grid connected PV systems with (a) full-bridge single leg switch clamped inverter (SLSC) and (b) full-bridge single leg diode clamped

inverter (SLDC) [18] .

AC side with small DC bus voltages of e.g. 40 V each. High power applications using cascaded inverters are described in [l l l , and I201.

The step converter [21] switches PV sub arrays of different voltages to the AC output. In [22] a topology using five arrays with nominal voltages of 1 1 V, 22 V, 44 V, 88 V and 176V is suggested for a grid connected PV system as shown in Fig. 6. A first conversion stage generates a rectified AC voltage waveform with 32 different voltage levels, a second conversion stage switches the polarity of every second half-wave generating an AC voltage with 63 different voltage levels. The energy delivered from each of the PV sub arrays increases with increasing voltage. Each PV sub array has different sizing requirements in order to ensure maximum power extraction of each individual PV array during operation.

E. Magnetic Coupled

Fig. 7 shows a single-phase PV system with a mag- netic coupled inverter as described in [23]. The inverter consists of three full-bridges each with their midpoints connected to a primary winding of a transformer. The sec- ondary windings of the transformers are connected in series. Due to different turns ratios of each of the transformers and the ability of each full-bridge to create three different voltages across the primary winding (+VPVA, -VPVA

Inverter ...............

-... : Fig. 5. Grid connected PV system with a cascaded inverter (CC) [ l I ]

226

. . . . . . . . . .................. InwIlU

Fig. 6. Grid connected PV system with a step inverter 1221.

and 0) the voltage at the AC terminals can be comprised of 27 levels. The advantage of this circuit is the relatively accurate replica of a sine wave accomplished with low switching frequencies. A major drawback of the circuit, however, is the need for three transformers.

E Flying Capacitor (FC)

In Fig. 8 a half-bridge three-level flying capacitor inverter is suggested for a transformerless grid connected PV system. Flying capacitor converters (which are also referred to as floating capacitor or imbricated cell multilevel converters) are described in [24] and [lo]. The features of this topology are similar to the diode clamped topology. Important for the operation of this converter is a stable voltage ratio of V P V A l / V C 2 = VPVA2/VC2 = 1. There- fore control methods are required which ensure that the average current flowing in the capacitor C2 is zero. This complicates the control of the inverter and excludes solution with varying duty-cycles (e.g. hysteresis control).

IV. DISCUSSION

The following system comparison does not include all described topologies. It excludes the magnetic coupled topology since it focuses on transformerless systems. Considered are also only those topologies, where the amounts of energy extracted from each PV sub array are equal, which simplifies the design of the systems, hence the step and the half-bridge diode clamped five-level topology are not included in the comparison. Table I1 compares the remaining topologies regarding minimum rated power, Pr,min, number of DC bus capacitors, number of semicon- ductor devices and their ratings, possible levels of the AC voltage at the inverter output terminals and the negative influence of the PV array earth capacitance. Additionally, transformerless systems incorporating full-bridge (FB) and half-bridge (HB) topologies are included.

The determination of the minimum rated power of

I c

I- Fig. 7. Grid connected PV system with magnetic coupled inverter.

Fig. 8. Grid connected PV system with a half-bridge three-level flying capacitor inverter (FC).

the systems, Pr,min, and respectively the number of re- quired PV modules is biased on a maximum grid voltage amplitude of Ggrid,max = 1.1 4 * 240 V. All listed topologies have step-down characteristics. Therefore, for the “half-bridge” topologies (HBDC, FC, HB) the DC bus voltages VPVA~L = VpVAZ have to be always higher than ijgrid,max. For the “full-bridge” topologies V P v A i = VPVA2 > Cgrid,max/2 applies. Since the operating voltage of silicon cells reduces with increasing temperature, the DC bus voltage is lowest on hot summer days. This lowest operating voltage determines the mini- mum number of cells which have to be connected in series to ensure energy transfer from the PV array to the grid at all times. Then, based on the minimum number of cells connected in series, the highest possible voltage, the open circuit voltage on the coldest day has to be calculated since it determines the voltage rating of the DC bus capacitors as well as those of the semiconductor devices. For silicon solar cells the temperature behaviour and hence the voltage variations can be estimated according to [25]. The system sizes listed in Table I1 have been calculated for environ- mental conditions for Perth, Western Australia (maximum ambient temperature in summer: 45” C, minimum ambient temperature in winter: OOC) and for typical, available PV modules as specified in section 11. Major drawbacks regarding the minimum size of all discussed systems is the lack of flexibility and the relatively high number of modules required (for half-bridge f.opology systems twice as many as for full-bridge topology systems). By adding additional step up conversion stages, sizing flexibility can be enhanced, however, overall system e:fficiencies will decrease. Modules with higher operating voltages are favourable for the discussed systems since installation costs can be reduced. Today’s availability of high voltage modules with operating

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TABLE I1 SYSTEM COMPARISON

I I FB I SLSC I SLDCl CC I FC I HBDC I HB]

modules No. of Ca-

Vblock,max 0.7 0.7 0.7 0.35 0.7 0.7 /kV or or

0.35 0.35 Imax/A 9 9 9 9 18 18 AC volt- 3 5 5 5 3 3

1.4

18 2

I aee levels I I I I l l I I

Ilblock,max = 700 v T O ~ O ~ O ~ Y Imax = 9 A

SLSC 4 SLDC 2 cc 0

1 Capacitive 1 yes 1 yes 1 yes 1 yes 1 no I no 1 yes 1 Earth Currents

Vblock,max = 350 v Imax = 9 A

2 4 8

TABLE I11 NUMBER OF SWITCHES

voltages above 30 V, however, is still limited. Advantageous in respect to minimising leakage currents

are t o p ~ ~ ~ g i e s where the PV array can be grounded. Here the HBDC topology is more favourable than the FC topol- ogy, where there is need to control the floating capacitor voltage.

The HBDC compares favourably with the conventional HB topology in several ways. Firstly in respect to avoid- ing the influence of capacitive earth currents due to the grounded midpoint of the array, secondly in the ability of creating a three-level instead of two-level voltage at the inverter output and thirdly in respect to lower voltage switch ratings. When compared with the FB topology the HBDC i s still preferable due to the grounded midpoint of the PV array.

SLSC, SLDC, and CC can create five-level inverter output voltage waveforms and consequently demand less filter effort on the AC side. In all three topologies, however, PV array earth leakage currents can have negative impact and measures to decrease these are required. For the three topologies the required numbers of semiconductor switching devices and their ratings are specified in Table 111. Considering costs, the CC topology promises to be cheaper to produce than the SLSC and SLDC due to its modular nature.

V. CONCLUSION

In this paper several single-phase, multilevel topologies suggested for PV grid connected systems have been re- viewed. Amongst the topologies for transformerless sys- tems the HBDC and CC have been identified as the most

promising topologies. However, with the CC topology (when applied in a transformerless system) measures are necessary to decrease the capacitive earth currents which are caused by potential differences imposed on the PV ar- ray earth capacitance. Also further research is required in order to evaluate, whether the advantages of multilevel con- version justify higher cost due to higher component count.

The step-down nature of all topologies requires system sizes of 1.5 kW upwards in transformerless applications due to the relatively low operating voltage of most currently available PV modules. Availability of PV modules with higher operating voltages is desirable since this would re- duce system cost. An additional step-up conversion stage between PV array and inverter can increases the flexibility regarding the system size, but will reduce the systems’ over- all efficiency.

VI. ACKNOWLEDGEMENTS

The authors wish to acknowledge the valuable discussions with Mr Michael Dymond and Mr Andrew Ruscoe, Power Search Ltd, Perth, Western Australia, with Dr. Mike Mein- hardt from PEI Technologies, National Microelectronics Re- search Centre, Cork, Ireland and with Mrs Johanna Myrzik, ISET, University of Kassel, Germany.

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