Cascaded H-Bridge Multilevel Converter Topology and ... - IEEE Xplore

3rd IEEE International Symposium on Power Electronics for Distributed Generation Systems (PEDG) 2012

Cascaded H-Bridge Multilevel Converter Topology and Three-phase Balance Control for Large Scale Photovoltaic Systems Sebastian Rivera and Bin Wu

Samir Kouro

Department of Electrical and Computer Engineering Ryerson University Toronto, Ontario, Canada Email: [email protected]

Departamento de Electrónica Universidad Técnica Federico Santa Mar´ıa Valpara´ıso, Chile Email: [email protected]

Abstract—The increase in the power levels of photovoltaic (PV) energy conversion systems has resulted in new large-scale grid connected configurations that have reached the megawatt level. This substantial increment in the power levels imposes new challenges to the grid interfacing converter, and therefore results in new opportunities to be explored. This work introduces a new medium voltage multilevel scheme based on a three-phase cascaded H-bridge (CHB) converter and multiple PV strings. The proposed configuration enables a large increase of the total power capacity of the PV system, while the introduction of a multilevel converter helps to improve both power quality and efficiency and medium voltage operation at the grid side. The main challenge of the proposed configuration is to handle the inherent power imbalances that occur not only between the different cells of one phase of the converter but also between the three phases. Simulation results of a 7-level CHB PV system are presented to validate the proposed topology and control method.

I. I NTRODUCTION Grid connected solar photovoltaic energy conversion systems have substantially increased their installed capacity over the last years. This is mainly influenced by the continuous reduction of the PV modules costs, the rising conversion efficiency and the sustained increase in the prices of fossil fuels [1]. Also, in terms of power ratings there is a clear trend of moving into wider ranges, considering that only in 2010, power plants with power ratings over 500 kW were connected to the grid with a cumulative power of 3.5 GW, increasing the worldwide installed capacity to 40 GW [2]. Furthermore, large scale PV plants have greatly contributed to this increase with more than 150 plants over 10MW, mainly concentrated in western European countries. The largest PV power plant currently in operation is located in Perovo, Ukraine with a nominal power capacity of 100 MW [3]. Currently large-scale PV plants are interfaced by two type of power converter configurations: the centralized topology and the multistring topology [4]. The centralized topology is characterized by a large amount of PV modules in series to reach the desired PV string voltage. Several of these strings are then paralleled to reach the total power level of the PV system.

978-1-4673-2023-8/12/$31.00/ ©2012 IEEE

Hong Wang and Donglai Zhang Shenzhen Academy of Aerospace Technology Shenzhen City, Guangdong Province of China, China

The dc power is interfaced to the utility by a centralized gridtied inverter, commonly a three-phase 2-level voltage source inverter (2L-VSC). Isolation, if required is usually provided by a low frequency transformer at the ac side. The advantage of this configuration is the simplicity of the structure and control (only one converter) and reduced cost. The main disadvantage is the lower power output due to a single maximum power point tracking (MPPT) for the whole plant, which is affected by module mismatch and partial shading. On the other hand, the multistring concept [5] also uses a centralized grid tied inverter, but has a distributed dc-bus in which each string is connected through a dc-dc converter. Commonly these are boost converters. If necessary, isolation can be included at the ac side with low frequency transformer, or with medium frequency transformers within the dc-dc stage (using flyback or push-pull converters) [6]. The main advantage of the multistring concept is the increased modularity, allowing to combine different types of modules and even dc-dc string converters. It also decouples the grid converter control from the PV string control, which allows independent MPPT tracking of each string, increasing the power output. The main disadvantage is the higher cost and topology complexity of having additional power converters, sensors, and control systems. Nevertheless, the higher conversion efficiency has proven to be a superior advantage in long term, hence it is considered the state of the art configuration today. These two configurations are usually connected to the grid using the conventional 2L-VSC, but the sustained increase in the power ratings of the PV plants will put higher demands on the power quality and efficiency of the conversion system and this topology will not be able to fulfill such requirements. Furthermore, taking into account the wind energy conversion systems scenario, where demanding grid codes have been developed in order to regulate its operation [7], these energy conversion systems could be regulated using a similar code, pushing further the limits of the 2L-VSC This has motivated the use of multilevel converters in grid tied PV energy conversion systems [8]-[16], being the 3-level

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NPC and the single-phase cascaded H-bridge converters the most used topologies. In the first case, the NPC increases the power rating of the system up to 40 MW (the largest commercially found NPC [9]). However too many modules must be connected in series first, to reach medium voltage operation, and then paralleled to reasonably meet the rated power of the converter, leading back to the low conversion efficiency exhibited in the centralized converter. To overcome this situation [10] has included a dc-dc stage to boost the voltage and reduce the number of series connected modules, and also features independent MPPT for each string. On the other hand, the CHB has been widely proposed for PV conversion systems [11] - [16], mainly because it easily reaches medium voltage range but also by exploiting a feature that was considered a drawback when operating as inverter, which was the need of several isolated dc sources connected to each cell. For PV systems the CHB allows the connection on several strings to the grid, allowing independent MPPT for each one improving the conversion efficiency of the system. Despite this converter has been widely proposed for PV generation, these approaches are mostly for single phase systems. The main reason for this is the inherent imbalance in the power processed by each phase, due to partial shading, module mismatch among other reasons. If this is not properly addressed it could lead to unbalanced grid currents, which is not allowed by the grid code. A three-phase application is suggested in [15] for a 5 level CHB, featuring independent MPPT algorithms based on the regulation of the slopes of the modules currents and voltages through hysteresis controllers. Nevertheless, the imbalance scenarios are not analyzed with the exception of [16]. This work proposes a new hybrid centralized multistring scheme, based on [16], removing the dc-dc stage and connecting directly the PV strings to the H-bridge power cells, in order to improve the system reliability, by reducing the number of component of the system, the complexity of the control and also the cost of the energy conversion system. In addition, the utilization of a CHB allows to have independent MPPT algorithms in each one of the power cells of the converter. Simulation results are presented, under the two imbalance scenarios: the per-cell imbalance and the per-phase imbalance. II. T OPOLOGY D ESCRIPTION The proposed three-phase CHB multilevel PV system configuration is illustrated in Fig. 1. From the power circuit three main parts can be identified: the H-bridge power cells of the CHB (k per phase), the PV strings, and the optional isolation transformer. The proposal is based on the system presented in [16], but the dc-dc stage has been eliminated to reduce both the cost and the control complexity of the system. Despite the removal of this stage makes the grounding of the modules more complex, this proposal is aimed for smaller scale system or for locations were the dc grounding is not mandatory, as in some European countries [17]. In addition the reliability has been increased due the reduction on the number of components in the system.

Multistring bus a1

Phase a

H-Bridge Cell dc ac

a1 v dc

Multistring bus a2

Multistring bus ak

Utility Grid

a isa Rs Ls b c

dc ac

vca

ac

N

n Transformer (Optional)

dc Phase b Phase c

Cascaded H-Bridge Multilevel Converter

Fig. 1.

H-Bridge Cell

Proposed cascaded H-bridge PV system configuration.

One of the main features of the CHB is its modularity; it can easily reach medium voltage by adding more power cells to each phase. This particular topology is commercially available to reach different voltage levels: 3.3 [kV] (three cells per phase), 6.6 [kV] (six cells per phase) and up to 11 [kV] (eleven cells per phase). Another interesting feature of the CHB for PV applications is the need of several isolated dc sources connected to each power cell, thus allowing independent MPPT algorithm to each string. The voltage synthesized by the converter (vca , vcb and vcc ) are the result of the sum of the ac output all the cells on one phase. The CHB is usually modulated using Phase-Shifted PWM (PS-PWM). Considering that this converter can reach medium voltage, the low frequency transformer is optional and mainly for isolation if needed. For sake of simplicity, this proposal will consider a 3.3 [kV] rated CHB with k = 3 cells per phase. The PV strings are composed of the series connection of several PV modules, to reach the voltage levels for the dcij , where i = a, b, c stands for link of the power cells (vdc the phase, and j = 1, 2, · · · , k determines the cell number). These strings are then connected in parallel in order to meet the power ratings of the converter and total power of the PV system. Therefore, the proposed topology is highly modular and flexible. Even more, the proposed technique allows the proper operation of the system under uneven power distribution between the power cells and the phases, and theoretically allows to have different kind of PV modules, or even installed capacity on each power cell. Nevertheless, to reduce the uneven power distribution by design, and leave the imbalances exclusively to mismatch between the modules and partial shading the system should be designed as balanced as possible. Therefore, the installed capacity on each power cell is the same for all. III. P ROPOSED C ONTROL M ETHOD The following sections will describe the independent control loops which compose the proposed control scheme: one for the grid connection tasks and other to perform the PV strings voltage regulation, MPPT, and the compensation mechanism

691


ijk PV

{ kj=a,b,c =1,2,3 9

MPPT Algorithms 9

Pa Pb Pc 3 * vcd

* isd

eT PI

PI

* isq

* vca

dq

* vcq

* vcb

abc

* vcc

jk vdc

voltages. Finally the loop is closed using the current measurements and the dc-link voltage errors.

PVMultistring Arrays dc-buses

{ kj=a,b,c =1,2,3

PS-PWM + Phase & Cell Imbalance Compensation

Sk

B. Power Imbalance Problem Description 7L-CHB

PI

PLL

μ

isq

dq

isd

abc

vsa vsb vsc

isa isb isc Transformer Utility Grid

Fig. 2. Voltage oriented control diagram with imbalance compensation for proposed topology.

for the imbalance scenarios. A. Voltage Oriented Control One of the main-stream control schemes used for grid tied converters will be used to regulate the interaction between the CHB and the utility grid: Voltage Oriented Control (VOC). The selection of this scheme is mainly because it includes a modulation stage and, as it will be shown in the following sections, will greatly simplify dealing with the power imbalance scenarios. As it is established in [18], this scheme is based on a cascaded control loop and a transformation between the three-phase coordinates to a synchronous frame oriented with the grid voltage. The CHB modified VOC block diagram is presented in Fig. 2. It can be seen that the outer control loop regulates the sum of the dc-link voltages, in order to determine the overall active power P needed to control the system. The reference for the voltage loop is obtained from the MPPT algorithms, which delivers the reference for each dcbus voltage level. Then the total voltage error eT is computed and controlled by the PI. It is important to highlight that this scheme will not perform any power distribution between the cells of the converter and if not taking into consideration the dc voltages of the system will drift. In this work, the active distribution among the cells will be done by the modulation stage. The MPPT algorithm considered is the well known perturb and observe (P&O) algorithm [19]. The active power reference is indirectly given by the outer voltage linear Proportional Integrative (PI) controller in terms of i∗sd , which is proportional to the active power. Finally the reference for isq is chosen depending on the system requirements, typically set to zero to inject the energy to the grid with unity power factor. This current component indirectly sets the reference for reactive power Q. Both current components are also controlled by PI controllers which deliver the converter reference voltage in the synchronous frame, which are transformed back to the three-phase coordinates. The synchronization of the dq transformation is performed using a Phase Locked Loop (PLL) with the measured grid

The power generated by the PV array depends strongly on two factors: solar irradiance and operating temperature of the panel. These two conditions will affect the maximum power point of the PV module, which may vary slightly between adjacent modules, but can be quite different between several sectors of a large PV plant due to partial shading and module mismatch. Because of this it is very unlikely to have identical power delivered by each PV array in the system, resulting in different power processed by both the power cells of a particular phase, and between the phases of the converter. This situation originates two types of power imbalances: per-cell imbalance and per-phase imbalance. As their names suggest, the first is when the power delivered to each power cell of the phase i are not equal (Pi1 = Pi2 = Pi3 ), and the second is when the total power delivered to each phase are different (Pa = Pb = Pc ). These two imbalances affect the performance of the CHB in two different ways: the per-cell imbalance will make the dc-voltages drift from its reference value, distorting the signals generated by the converter and potentially damage it; while the per-phase imbalance affects the grid currents making them unbalanced, which is not allowed by the grid code and also originates pulsation in the power signals. C. Power Imbalance Compensation Method The two types of imbalances will be addressed sequentially: first the per-phase imbalanced is compensated through the modification of the phase voltage references, and later the per-cell imbalance is compensated generating new reference voltages for each cell. The reason for this is that, the per-cell imbalance implies a per phase imbalance, because the affected phase will process different power than the unaffected ones. 1) Per-phase Imbalance Compensation: The idea of this strategy is to share equally the power at the ac side, despite the power generated at the dc side of each phase is different under unbalanced situations. Traditional VOC generates balanced three phase reference voltages as output of the current control loops. Since the power generated by the different h bridges can be different due to MPPT, balanced phase voltages will thus imply unbalanced currents. In order to solve this problem, this scheme will induce an inverse unbalance in the phase voltages, proportional to the power imbalance, through the modification of the converter neutral point voltage. Consider the phase voltage equations from the power circuit shown in Fig. 1 at the grid side given by

692

disa − vsa dt disb − vsb vcb + vN n − Rs isb − Ls dt disc − vsc vcc + vN n − Rs isc − Ls dt

vca + vN n − Rs isa − Ls

= 0,

(1)

= 0,

(2)

= 0.

(3)


* vca

Pav Pa

.

* v~ ca

ra

.

* vcb

.

Pb

rb

.

* vcc

.

Pc

rc

min-max sequence

v0

* v~ cb

(a)

r r

r

* v~ cc

.

(b)

Imbalance Ratios

Fig. 3. Proposed weighted min-max injection to overcome phase power imbalance. (c)

This implies that the currents depend in part on the common mode voltage vN n . This is an important issue because it shows that is possible to redistribute the power generated at the dc side in order to keep balanced the currents at the input side, by moving the neutral point of the converter in a way that the exceeding power of the other phases can be used to increase the lower one. Similar methods have been developed for the CHB operation under fault condition, in which the neutral point is moved by changing the phase between the converter voltages, in order to balance the output currents and maximize the utilization of the remaining operative cells [20], [21], [22]. The proposed scheme takes this idea, but instead of modifying the amplitude or phase of the voltages, the neutral point will be moved through the injection of a zero sequence in the modulation stage, because of its simplicity and intuitive implementation. Also, as the injected voltage is a zero sequence it does not alter the line to line voltages, allowing the symmetrical operation of the converter even under unbalanced situations. This can be easily achieved by computing an imbalance ratio ri (i = a, b, c) for each phase given by Pav , where i = a, b, c, (4) Pi where, Pi i = (a, b, c) stands for the dc power delivered by each phase and Pav is the average power given by Pa + Pb + Pc (5) Pav = 3 After obtaining the factors the min-max sequence v0 is calculated according to =

ri

∗ ∗ ∗ max {ra vca , rb vcb , rc vcc } + (6) 2 ∗ ∗ ∗ min {ra vca , rb vcb , rc vcc } . 2 This originates the compensated phase reference voltages as shown in Fig. 3 and are defined by

v0

=

(d)

Fig. 4. Example of phase a imbalance reference compensation: a) three-phase converter reference voltages, b) weighted reference voltages, c) particular case with no phase imbalance (traditional min-max), d) imbalance compensated reference voltages with weighted min-max sequence injection.

∗ ∗ ∗ ∗ ∗ ∗ v˜ca = vca − v0 , v˜cb = vcb − v0 , v˜cc = vcc − v0 .

(7)

The advantage of balancing the currents this way is that there is no need to implement more sensors or estimators, because the powers are already available as they are needed for the MPPT algorithm. Furthermore, the computational cost is really low, as the only thing that has to be done is to obtain the sequence and pass it to the modulation stage. Also it presents a good dynamic behavior as the factors are corrected as soon as the generated power in the phase changes. Moreover, regardless the unbalanced situation, the mechanism is always the same and it will lead the system back to balance. The original and modified injected sequence are presented in the figure 4, and it can be seen that in the proposed scenario this signal has larger values at the instants that the affected phase reaches its peak value, which helps this phase to increase its current without interfering with the per cell balancing mechanism. 2) Per-cell Imbalance Compensation: Once the per-phase imbalance has been compensated, there is still the chance that the power delivered to the different cells of one phase of the converter is also unbalanced. The converter is modulated using phase-shifted PWM and therefore the compensated voltage reference given by the VOC loop is the same for all the cells. The carrier signals are shifted to produce the multilevel stepped

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TABLE I S IMULATION PARAMETERS * vca

* vca1

Parameter Grid voltage Grid frequency Input filter inductance Input filter resistance DC-link capacitance DC-link voltage per cell Total DC-link voltage per phase Device average switching frequency Equivalent output frequency per phase Base power Base voltage Base current Base frequency No. of strings connected to each dc-bus No. of series connected PV modules per string Open circuit voltage of module Maximum power voltage of module Short circuit current of module Maximum power current of module

1

a1 vdc

Fig. 5.

PI

ua1 Modulation index compensator

Per cell power imbalance compensation for cell a1.

waveform, but essentially will impose the same average usage of the cells drawing similar average current from each one. In presence of unbalance this will cause voltage drift of the dclinks causing distortion in the converter voltage and possible damage of the power cells. This type of imbalance has been dealt with in the past for single phase CHB PV systems. The same approach used in [14] is considered in this work. The idea behind the balancing mechanism is to distribute the usage of the cells of one phase in the same proportion of the imbalance by redistributing the ON and OFF times given by the PWM strategy. In this way, more or less active power is drawn by the cell according to the imbalance experimented by the system. The cell balancing scheme is presented in Fig 5. ∗ a1 − vdc ) is regulated with a The dc-link voltage error (vdc PI controller, whose output is used to adjust the amplitude of the per unit reference signal. In this way the amplitude of the references used for each cell are modified proportionally to the error of their respective dc-link voltages. This results in a feedforward correction that redistributes the ON and OFF times so that balance is achieved. Considering the dynamics introduced by this scheme, it is expected to have a slower response compared to the phase balancing mechanism. To validate the proposed control scheme a three-phase seven level CHB (three cells per phase) rated at 3.3 kV is considered as the grid tied inverter. As for the PV module used to generate the DC power, the model is based on the Sharp/NUU235F1 module, which has a rated power output of 235 W and 30 V under nominal conditions of temperature and solar radiation. Considering this, to allow the converter to control properly the ac side currents a total of 35 PV modules in series have been connected. In addition, to meet the converter rated power, 20 strings have been paralleled per power cells. This results in a total of a PV array composed by 6300 modules reaching an installed capacity of 1.48 MW. Note that the size of the PV system can be further increased by adding more strings in parallel. Commercial cascaded H-bridges can easily reach several tens of MW. The system was simulated using Matlab/Simulink and the parameters presented in Table I. To evaluate the performance under the mentioned disturbances the system is forced to the imbalance scenarios, therefore the radiation levels will be changed twice. The system

Value 1 pu 1 pu 0.085 pu 1.36 · 10−5 pu 8.55 pu 0.32 pu 0.95 pu 10 pu 60 pu 1.48 MVA 3.3 kVRMS 259 ARMS 50 Hz 20 35 37 V 30 V 8.6 A 7.84 A

starts with all the power cells under the rated solar radiation (1 kW/m2 ) and temperature (25o C). Then at t = 1 [s] a per cell imbalance is forced in phase a, reducing the radiation of the strings connected to cell a1 and a2 to 0.8 [pu] and 0.6 [pu] respectively. As explained in the previous sections, this situation also implies a per phase imbalance since, due to these reductions the total power processed by phase a is lower than the rest of the phases. However, the system is further challenged by forcing a per phase imbalance at t = 1.1, through the reduction on the radiation of all the power cells connected to phase c to 0.65 [pu]. This will validate the range in which the proposed feedforward technique allows to bring the system to balance. Note that the radiation of the power cells of the phase b were not disturbed throughout the experiment.

Voltage [pu] Voltage [pu]

* vdc

Symbol vâb fs Ls Rs C vdc 3vdc fsw fsw SB vB iB fB Np Ns Voc Vpm Vsc Ipm

1

* v~ca

* v~cb

* v~cb

v0

0

(a)

−1

1

* vca1

* vca2

* vca3

0

(b)

−1 0.95

1

1.05

1.1

1.15

Time [s] ∗ , v ∗ , Fig. 6. Converter references voltages: a) Compensated references (˜ vca ˜cb ∗ ) and weighted min-max sequence (v ) for a phase imbalance in phase v˜cc 0 ∗ , c at t = 1.1[s], b) Compensated references for each cell of phase a (vca1 ∗ ∗ vca2 , vca3 with cell imbalance at t = 1[s]).

694



1 0

(a)

ca cb cc

−1 2

(b)

0 ab 1

1 0

isa isb isc

−1

0

−1

Voltage [pu]

Current [pu]

−2

(c)

0.4

Pa Pb Pc

(d)

Pa1 Pa2 Pa3

(e)

0.3 0.2 0.1 0.17

Power [pu]

Power [pu]

0.5

0.13 0.10 0.07 0.03 0.95

1

1.05

1.1

1.15

Time [s] Fig. 7. Dynamic performance with a step cell imbalance in phase a at t = 1[s] and a step phase imbalance in phase c at t = 1.1[s]: a) Converter phase voltages (vca , vcb , vcc ), b) Line to line converter voltage (vab ), c) Grid currents (isa , isb , isc ) with grid voltage vsa shown scaled to highlight synchronism, d) Power processed by each phase (Pa , Pb , Pc ), e) Power processed by each cell of phase a (Pa1 , Pa2 , Pa3 ).

IV. S IMULATION R ESULTS

voltage and regain the balance in the dc buses.

The simulation results for the mentioned scenarios are presented in Figures 6-8. Before analyzing the voltage and current waveforms obtained it is better to study the voltage reference signals provided in Fig. 6, this will help to understand the effects of the power imbalances and how the proposed scheme acts in order to overcome their appearance. As mentioned before, the cell imbalance also causes a phase imbalance, and it is clearly appreciated in Fig. 6a that after t = 1 [s], the traditional min-max sequence is changed in order to compensate the difference between the power processed by the phases and the average power. As phase a is generating less power, the zero sequence v0 is in phase with its voltage. This becomes obvious for the phases b and c, which are modified to redistribute the power and increase the current isa . Now, in relation with the difference in the power delivered among the cells of phase a, Fig. 6b clearly shows how the amplitude of each cell voltage reference is modified in order to redistribute the ON/OFF times of the cell accordingly with the imbalances. As it is shown in Fig. 6b, the reference for the cell a2 has dropped to its half approximately, because its radiation has dropped to 0.6 [pu]. This reduction in the modulation index allows to reduce the cell ON times, helping to increase the

Then, at t = 1.1 [s] the phase imbalance becomes stronger due the drop of 35% of the radiation of all the PV strings connected to phase c. It can be seen that despite both imbalance scenarios take place simultaneously the system is able to overcome and compensate them, bringing the system back to balance in both ac-side currents and also dc-link voltages. Finally, the overall performance of the proposed scheme can be observed in Fig. 7. The change of the neutral point of the converter is clearly shown in Fig. 7a, as the inverter phase voltage appears to be unbalanced and distorted. However this allows to reduce the effect of the differences in the power processed by phase and cells and deliver balanced currents to the grid. This is because the weighted min-max is a zero sequence injection, and as the system is three phase balanced, does not appear in the line-to-line voltages, as it is presented in Fig. 7b, and maintains its sinusoidal waveform. The performance of the method is confirmed in Fig. 7c, where the three-phase currents are completely balanced, despite the converter phases are processing different power levels. Also it can be seen that the feedforward mechanisms does not alter the synchronism as the system maintains the unity power factor operation. To confirm the effect of the changes in the

695

a1 vdc

a2 vdc

Balanced scenario

a3 vdc

0.3 0.2

Current [pu]

0.4

(a)

Cell imbalance

0.1 0 0.4

b1 vdc

b2 vdc

b3 vdc

0.3

Current [pu]


Voltage [pu]


(b)

0.2 0.1 0 0.4

c1 vdc

c2 vdc

c3 vdc

(c)

Phase imbalance

0.1 1

0

(a) isd isq

−1

1 0

(b) isd isq

−1 1

1.05

1.1

1.15

Time [s]

0.2

0.9

Phase imbalance

1

0.95

0.3

0 0.8

Cell imbalance

1.1

1.2

1.3

Fig. 9. Power delivered to the grid: a) synchronous frame currents no compensation method, b) synchronous frame currents proposed scheme.

1.4

Time [s] Fig. 8. DC-link voltages with cell imbalance in phase a at t = 1[s] and a1 , v a2 , v a3 ), with phase imbalance in phase c at t = 1.1[s]: a) phase a (vdc dc dc b1 b2 b3 c1 c2 c3 b) phase b (vdc , vdc , vdc ), c) phase c (vdc , vdc , vdc )

nents is directly reflected in the power signals. On the other hand, Fig. 9b shows the currents components obtained using the proposed scheme, at it clearly shows how the oscillations are mitigated for the exact same imbalance scenario. V. C ONCLUSION

radiation levels, Fig. 7d shows the total power processed by each phase, and despite the difference, its clear how each phase keeps tracking its independent maximum power point. Finally, Fig. 7e presents the power managed by each one of the cells connected to phase a. It should be noted that the phase b is not altered by any of the changes induced in the other phases, and continues tracking its optimal power point, confirming that the proposed method allows to perform independent MPPT algorithms even without the removal of the dc-dc stages. To complete the verification of the proposal, Fig. 8 presents the dynamic response of the dc link voltages of the system. In the case of phase a, despite de initial drift from the reference value, the system is able to return to the balanced point. Another remarkable aspect is that the effect of the corrections taken by the balancing mechanisms is negligible in the unaffected phase. To emphasize the performance of the control scheme, Fig. 9 provides important information. In the previous sections it was stated that if these unbalances are not taken into consideration, it will result in unbalanced currents at the acside, and therefore the powers injected to the grid will have a pulsating component. This figure shows the dq synchronous frame components of the grid current, it can be seen that both isd and isq components present oscillation if there is no compensation for the imbalances. Furthermore, as these components are respectively proportional to P and Q (assuming the grid voltage does not present important harmonic distortion), according to 3 vsd isd (8) 2 3 Q = − vsd isq , (9) 2 therefore, any oscillation in the currents synchronous compoP

=

A new medium voltage converter interface for large scale PV energy conversion systems is presented. It is based on a three-phase CHB centralized-multistring hybrid topology. It is proven that the elimination of the dc-dc stage does not alter the performance of the scheme proposed in [16], the MPPT algorithms are independent between the different cells despite the imbalance scenarios. The limit of unbalance that can be handled without affecting the controllabiltiy of the converter is subject of further research. The scheme is able to deal the simultaneous presence of the two power imbalances, keeping both the dc-link voltage with its references and the grid currents balanced. This disturbances were overcomed by including two simple feedforward compensations in the modulation stage: the injection of a weighted zero sequence and the modification of the modulation index according to the dc-link voltage deviation. The simplicity of the balancing mechanisms allows to implement this scheme in the existing hardware of the commercially available products. The injection of a zero sequence does not affect the line to line voltage, and despite the apparent distortion of the converter phase voltages, they keep their sinusoidal waveform and the currents are also highly sinusoidal. Both the proposed topology and control method enables the theoretical concentration of a large scale PV plant of up to 120MW into a single grid tied medium voltage converter. This also allows to use the attractive features of this kind of converter, such as the superior power quality, low switching frequency, and even possible fault tolerant operation. Furthermore, the use of a multilevel converter allows to comply with the challenges imposed by the current grid codes. ACKNOWLEDGMENT The authors would like to thank the financial support from Shenzhen Academy of Aerospace Technology (SAAS)

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and National Science Foundation of China (NSFC) under project #2010DFB63050. Also the financial support provided by the Chilean National Fund of Scientific and Technological Development (FONDECYT), under grant No. 1110783, and by Centro Cient´ıfico-Tecnológico de Valpara´ıso (CCTVal) No. FB021 of Universidad Técnica Federico Santa Mar´ıa, R EFERENCES [1] Renewable Insight - Energy Industry Guides, “PV power plants 2011, industry guide,” available at http://www.pv-power-plants.com/, 2011. [2] Renewable Energy Policy Network for the 21st Century, “Renewables 2011 global status report,” available at http://www.ren21.net/REN21Activities/Publications/GlobalStatusReport/ tabid/5434/Default.aspx, 2011. [3] “http://www.pvresources.com/pvpowerplants/top50.aspx.” [4] F. Blaabjerg, Z. Chen, and S. B. Kjaer, “Power electronics as efficient interface in dispersed power generation systems,” vol. 19, no. 5, pp. 1184–1194, Sep. 2004. [5] M. Meinhardt and G. Cramer, “Multi-string-converter: The next step in evolution of string-converter technology,” in in Proc. 9th Eur. Power Electronics and Applications Conf., 2001. [6] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, “A review of single-phase grid-connected inverters for photovoltaic modules,” vol. 41, no. 5, pp. 1292–1306, Sep./Oct. 2005. [7] “Grid code - high and extra high voltage,” E.ON Netz Gmbh, April 2006. [8] M. Calais and V. G. Agelidis, “Multilevel converters for single-phase grid connected photovoltaic systems-an overview,” in Industrial Electronics, 1998. Proceedings. ISIE ’98. IEEE International Symposium on, vol. 1, Jul. 7–10, 1998, pp. 224–229. [9] S. Kouro, M. Malinowski, K. Gopakumar, J. Pou, L. G. Franquelo, B. Wu, J. Rodriguez, M. A. Perez, and J. I. Leon, “Recent advances and industrial applications of multilevel converters,” Industrial Electronics, IEEE Transactions on, vol. 57, no. 8, pp. 2553 –2580, aug. 2010. [10] S. Kouro, K. Asfaw, R. Goldman, R. Snow, B. Wu, and J. Rodriguez, “Npc multilevel multistring topology for large scale grid connected photovoltaic systems,” in Power Electronics for Distributed Generation Systems (PEDG), 2010 2nd IEEE International Symposium on, june 2010, pp. 400 –405. [11] O. Alonso, P. Sanchis, E. Gubia, and L. Marroyo, “Cascaded h-bridge multilevel converter for grid connected photovoltaic generators with independent maximum power point tracking of each solar array,” in Power Electronics Specialist Conference, 2003. PESC ’03. 2003 IEEE 34th Annual, vol. 2, June 2003, pp. 731–735 vol.2. [12] J. Negroni, F. Guinjoan, C. Meza, D. Biel, and P. Sanchis, “Energysampled data modeling of a cascade h-bridge multilevel converter for grid-connected pv systems,” in International Power Electronics Congress, 10th IEEE, oct. 2006, pp. 1 –6. [13] E. Villanueva, P. Correa, J. Rodriguez, and M. Pacas, “Control of a single-phase cascaded h-bridge multilevel inverter for grid-connected photovoltaic systems,” Industrial Electronics, IEEE Transactions on, vol. 56, no. 11, pp. 4399–4406, Nov. 2009. [14] S. Kouro, B. Wu, A. Moya, E. Villanueva, P. Correa, and J. Rodriguez, “Control of a cascaded h-bridge multilevel converter for grid connection of photovoltaic systems,” in Industrial Electronics, 2009. IECON ’09. 35th Annual Conference of IEEE, nov. 2009, pp. 3976 –3982. [15] G. Brando, A. Dannier, and R. Rizzo, “A sensorless control of hbridge multilevel converter for maximum power point tracking in grid connected photovoltaic systems,” in Clean Electrical Power, 2007. ICCEP ’07. International Conference on, may 2007, pp. 789 –794. [16] S. Rivera, S. Kouro, B. Wu, J. Leon, J. Rodriguez, and L. Franquelo, “Cascaded h-bridge multilevel converter multistring topology for large scale photovoltaic systems,” in Industrial Electronics (ISIE), 2011 IEEE International Symposium on, june 2011, pp. 1837 –1844. [17] Z. Liang, R. Guo, G. Wang, and A. Huang, “A new wide input range high efficiency photovoltaic inverter,” in Energy Conversion Congress and Exposition (ECCE), 2010 IEEE, sept. 2010, pp. 2937 –2943. [18] M. Malinowski, M. P. Kazmierkowski, and A. M. Trzynadlowski, “A comparative study of control techniques for pwm rectifiers in ac adjustable speed drives,” IEEE Transactions on Power Electronics, vol. 18, no. 6, pp. 1390–1396, November 2003.

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