Space Vector Modulated Cascaded H-BridgeMultilevel Converter for Grid Integration Of Large

Scale Photovoltaic Power Plants

Ajay Kumar MoryaIIT Bombay,India

ajaymorya@ee.iitb.ac.in

Prof. Anshuman ShuklaIIT Bombay,India

ashukla@ee.iitb.ac.in

Abstract— The increasing power rating of large scale gridconnected photovoltaic (PV) power plants has opened newopportunities for research in the suitable converter and controltechniques considering the existing grid codes. In this paper,a medium voltage multilevel converter is considered based ona three-phase cascaded H-bridge (CHB) multilevel converterand multiple string dc-dc converters. Multilevel converters havereduced switching frequency, good power quality with less THDthus reducing filter requirements. The need of several sourceson the dc side of the CHB makes this multilevel technologyattractive for photovoltaic applications. Moreover,the converterstructure is very flexible and modular . The main challenge ofthis configuration is to handle the power imbalances amongthe different cells of one phase of the converter as well asamong the three phases. A control strategy based on space vectormodulation(SVM) to deal with these imbalances is proposed inthis paper. Simulation results of a 5-level CHB for a multistringPV system in MATLAB/Simulink are presented to prove theeffectiveness of the proposed control method.

Index terms-Cascaded H-bridge inverter ,Grid-connected PVsystems,Maximum power point tracking (MPPT),Multilevel con-verter,Photovoltaic systems,Renewable energy,Space vector pulsewidth modulation.

I. INTRODUCTION

In recent years, interest in electrical power generation

from renewable energy sources, such as photovoltaic (PV)

or wind power systems is increasing because these sources

are environment-friendly and abundant. Grid connected solar

photovoltaic energy conversion system is a leading renewable

energy source considering the installed capacity in the last

10 years [1]. Presently,the centralized topology and the mul-

tistring topology are popular power converter configurations

for high power PV plants[2]. String means a number of PV

modules connected in series.The centralized topology has a

large number of PV modules in series to reach the desired PV

string voltage and then a number of them are connected in

parallel to reach the total power level of the PV system. The

available dc power is interfaced to the grid by a centralized

grid connected inverter. The advantage of this configuration

is that the structure and control are very simple as there is

only one converter so cost is also reduced. But it has some

severe limitations, such as high voltage dc cables are required

between the PV modules and the inverter, power losses due

to a centralized maximum power point tracking(MPPT) and

losses due to mismatch between the PV modules.On the

other hand, the multistring concept [2] ,which is becoming

the state of the art configuration today, has several strings,

each connected to a dedicated dc-dc converter and then to a

common dc-ac inverter. The main advantage of the multistring

concept is modularity which allows combination of different

types of modules. This is advantageous, compared with the

centralized system, since control of each string is independent.

Recently, medium voltage converters have been proposed

for grid connected PV systems [3][4].A three phase CHB

has been presented in [4].A single phase CHB with dc link

balancing control has been presented in [5].In [6], a single

phase CHB with energy balance control for grid connected PV

applications has been presented.A CHB converter based PV-

battery hybrid system for better electromagnetic compatibility

has been presented in [7].The CHB has main advantages for

PV systems: it provides several dc-links to which several PV

strings, each one with independent MPPT, can be connected to

take it to medium voltage level.But there is an inherent power

imbalance among the cells because each H-bridge cell has its

independent PV system whose power points may be different.

This imbalance can cause the dc-link voltages to drift. The

dc-link voltage imbalance deteriorates the power quality as

it causes voltage distortion at the converter output.The most

drastic outcome can be converter failure if voltage limits of

the capacitors are exceeded causing large revenue loss. In three

phase CHB , an additional challenge is to handle imbalance

between the three phases, since each cell has its own MPPT.

This imbalance will lead to unbalanced currents, which is not

allowed by grid codes.

This per cell and per phase imbalance problem has been

tackled in [4] using phase shifted sine triangle PWM. Grid

current balance is achieved by moving the neutral point of

the converter using zero sequence injection in a way that the

phase voltages are unbalanced inversely proportional to the

power unbalance of the converter.For per cell imbalance com-

pensation, the dc-link voltage error V ∗dc- V a1

dc is regulated with

a PI controller, whose output is used to adjust the amplitude of

the per unit reference signal.Here one PI controller is required

for each cell.

In this paper, a novel method based on space vector

modulation has been proposed to solve the three phase un-

balance and cell unbalance problem in CHB, which is also

suitable for digital implementation. The configuration and

proposed control system is simulated for a three phase 5 level

CHB.Multilevel converters have many degrees of freedom,

compared with two-level converters: more voltage levels, zero

common-mode voltage vectors and redundant switching states.

Carrier based PWM schemes can not always fully exploit

these advantages.So, significant research is going on in new

modulation strategies .One of the modulation methods that has

the potential to use these degrees of freedom more effectively

is space vector modulation (SVM)[8].

II. TOPOLOGY DESCRIPTION

A. Working Principle

Fig. 2 shows one phase of 5 level cascaded H-Bridge

configuration.If V is the voltage of dc-link capacitor Cdc of

each H-Bridge cell, the output of one cell will have three levels

namely +V, 0 and -V. The switch position , the output voltage

and the states of the H-Bridge are given in Table I.Note that S1

and S4 are complementary and S2 and S3 are complementary.

A three level inverter can be realized Using one H-Bridge.

These H-Bridge cells can be connected in cascade to obtain

multilevel cascaded H-Bridge inverter. In a five level inverter

as shown in Fig. 2, phase voltage will have five levels, namely

+2V, + V,0, - V and -2 V.

B. Topology

The three phase CHB multilevel PV system configuration is

illustrated in Fig. 1. The power circuit consists of three parts:

the PV strings,the dc-dc converters and the H-bridge cells of

the CHB. Usually commercial CHBs have 3 cells per phase

to reach 3.3kV or six cells to reach 6.6kV medium voltage

levels. Phase-Shifted PWM (PS-PWM) is usually used for

modulation of CHB as it ensure uniform cell usage . Multilevel

converters have low switching frequency due to high power

rating of the system , limits of switching devices, efficiency

and heat dissipation constraint. The PV string consists of

series connection of several PV modules, to reach a voltage

level close to the dc link voltage of one cell of the CHB

converter. The number of modules per string can be different

and interfaced through different dc-dc converters. Several

strings can be connected to each dc-bus up to the power rating

allowed by each cell.In multistring configuration ,it is possible

to use strings of different power ratings . Nevertheless, system

should be designed with same installed capacity for each dc-

bus so that imbalances occur only due to mismatch among

modules, partial shading and possible disconnection of one or

more strings due to some fault.In this way ,the overall system

has more fault tolerance and will be able to operate properly

with good performance over a wider range of disturbances

[4]. In this work, the main contribution is the analysis and

control of the power imbalances of the grid tied converter

using space vector modulation(SVM), and therefore less detail

will be given on the dc-dc stage control and topology.Boost

dc-dc converter is among the most commonly used converter

where systems without isolation are allowed or isolation has

been used on ac side. In case of the CHB topology it is

Fig. 1. Cascaded H Bridge PV system configuration

Fig. 2. 5 level Cascaded H Bridge- ’a’ phase

recommended to have isolation on dc side, so each string can

be grounded for protection purpose as parasitic capacitance of

the PV modules causes high leakage currents.For simplicity,

boost dc-dc converter has been used in this work.60 degree

coordinate system has been used to implement SVM [9] .

III. PROPOSED CONTROL METHOD

The controller has two types of independent control loops

in this topology: one for the inverter which controls the grid

currents and dc-link voltages and the other for the dc-dc

converters which is used to control the PV string voltage

to the reference given by the MPPT algorithm.Incremental

conductance method has been used for MPPT [10]. The

following subsections will elaborate on the control method

and power imbalance compensation technique for the CHB

converter.

A. voltage oriented control

The modified voltage oriented control(VOC) scheme block

diagram is illustrated in Fig. 3. As with classic VOC, there

is an outer voltage control loop and an inner current control

loop. The outer loop controls the dc-link voltage, and since the

CHB has several dc-links, their average is controlled. Thus,the

total active power needed to control all the dc-link voltages

is obtained. The active power reference given by the voltage

Fig. 3. Voltage Oriented Control diagram with imbalance compensation

loop, is proportional to the id current component, while the

reactive power is proportional to the iq component.The system

model in d-q variables can be written as :

diddt

= −Rs

Lsid + wiq +

(ed − vd)

Ls(1)

diqdt

= −Rs

Lsiq − wid +

eqLs

(2)

where Rs and Ls are grid interconnection resistance and

inductance respectively,w is grid frequency in rad/s , ed and

eq are inverter voltage d and q components and vd and vq are

grid voltage d and q components.Here, there is cross coupling

between two control variables id and iq . For easy and effective

control,control action for id and iq has been decoupled by

defining two variables x1 and x2 :

x1 = +wiq +(ed − vd)

Ls(3)

x2 = −wid +eqLs

(4)

Now,the modified system equations are :

diddt

= −Rs

Lsid + x1 (5)

diqdt

= −Rs

Lsiq + x2 (6)

In decoupled control , variable x1 is controlling only id and the

control variable x2 is controlling only iq . The forcing functions

ed and eq can be obtained as:

ed = vd + Ls(x1 − wiq) (7)

eq = Ls(x2 + wid) (8)

The reactive power reference is usually set to zero for unity

power factor operation. Although ,it can be controlled at

different values for reactive power compensation if demanded

by utility as an ancillary service. Both currents are regulated

using PI controllers that give the converter reference voltage,

which is then converted from d-q reference frame to three

phase voltage references. To ensure unity power factor op-

eration,synchronization is performed using a PLL with the

phase voltages of the grid. The three phase grid currents

are measured and fed back for the current control loop. The

voltage reference given by the current loop is then modulated

using space vector modulation. Note that in order to properly

control the dc-link voltages of each cell and compensate the

inherent power imbalances(as discussed earlier), the SVM

switching needs to be modified. The problem description and

solution for the power imbalance is discussed in the next

sections.

B. Space Vector Modulation

In space vector modulation,we represent the converter de-

mand voltage as a rotating reference on the α-β plane, on

which the converter switching states occupy discrete points.

The reference vector is synthesized by selecting converter

states adjacent to the reference for time periods such that the

time averages of the reference and the converter voltages are

equal during each sampling period. Many switching states are

redundant as they produce identical vectors. The vertex of each

triangle represents a space vector, which is defined by

V = Vaej0 + Vbe

j2π/3 + Vcej4π/3 (9)

where Va , Vb and Vc are the phase voltages of the inverter.

The diagram can be divided into six major triangular sectors

(I to VI), and the details of sector I is given in Fig. 4.In this

work ,60◦ coordinate system is used:

Vm = V cos(θ)− V sin(θ)√(3) (10)

Vn = V cos(60◦ − θ)− V sin(60◦ − θ)/√(3) (11)

where Vm and Vn are the coordinates of a space vector V

in the 60◦ coordinate system, and V and θ are its amplitude

(length) and phase angle respectively. The advantage of using

this coordinate system is easy dwell time calculation and easy

identification of triangle in which the reference vector lies.

C. Power Imbalance Problem Description

The power generated by a PV module depends heavily on

solar irradiance and operating temperature of the module. The

maximum power point will depend on these two operating

conditions which can be quite different among various parts

of large PV plants due to partial shading and module mis-

match. Because of this, power delivered by each PV string

to the power cells of the CHB will not be identical causing

power imbalance among them. Two types of power imbalance

may exist: power imbalance among the cells of a phase

and imbalance among three phase powers . The first means

for example that the power handled by each H-bridge of a

particular phase are not equal (Pa1 �= Pa2 for a two-cell CHB).

The second is the difference among total power handled by

each phase of the converter (Pa �= Pb �= Pc). These two

type of imbalances affect the control of the CHB converter.

The per-cell imbalance leads to dc-link voltages from that

phase drifting from their reference value, which causes inverter

Fig. 4. 5 level Space Vector diagram with only first sector shown (Due tospace limitation)

TABLE ICELL STATES & OUTPUT VOLTAGE

Switch status Cell State Outputvoltage

S1,S2 On(S3,S4 Off) 1 VS3,S4 On(S1,S2 Off) -1 -VS1,S3 On(S2,S4 off) or S2,S4On(S1,S3 Off)

0 0

output voltage distortion. The per phase imbalance affects the

current control loop due to which the grid current will be

unbalanced . The fact that VOC returns a single balanced

voltage reference, means that if the power is different among

the cells, the currents must be unbalanced. This is not allowed

by existing grid codes.

D. Power Imbalance Compensation Method

Each cell can have 3 states as shown in Table I.During cell

state 1 and -1,capacitor Cdc will discharge and during cell

state 0 ,it will charge .In SVM redundant states are available as

shown in Fig 4.These states can be used for dc link balancing.

Following steps are taken to select the switching state of

converter :

TABLE IIPHASE STATUS

Voltage Phase StatusVdc,ph<396 1

396<=Vdc,ph<=404 2Vdc,ph>404 3

TABLE IIICELL STATUS

Voltage Cell StatusVdc,cell<196 1

196<=Vdc,cell<=204 2Vdc,cell>204 3

TABLE IVCELL BALANCE STATUS

Cell 1 status Cell 2 status Cell balance status2 2 13 3 21 1 31 3 43 1 53 2 61 2 72 3 82 1 9

TABLE VPHASE STATUS AND ALLOWED STATES

Phase Status Cell 1 stateconstraint

Cell 2 stateconstraint

States allowed

1 0 0 02 No No All3 1,-1 1,-1 2-2,0

Fig. 5. Capability to start up from zero initial dc link voltages : Vdc,ref

and Vdc [refer to Fig. 3]

Fig. 6. Capability to start up from zero initial dc link voltages (a)Phase adc link voltages (b)Phase b dc link voltages (c)Phase c dc link voltages

(1) Find phase status for all phases and cell status for all

cells as given in Table II and Table III.

(2) If phase status for all phases and cell status for all cells

is 2 ,then use LSA method [9] to select the switching state

TABLE VICELL BALANCE STATUS AND ALLOWED STATES

Cell balanceStatus

Cell 1 stateConstraint

Cell 2 stateConstraint

States allowed

1 No No All2 1,-1 1,-1 2,-2,03 0 0 04 0 1,-1 1,-15 1,-1 0 1,-16 1,-1 No All7 0 No 1,-1,08 No 1,-1 All9 No 0 1,-1,0

Fig. 7. Performance with imbalance caused by step change in solar radiation at t=0.6 s (a)Line-line Converter output voltage(Vab,Vbc,Vca) (b)Grid currents(ia,ib,ic) with scaled down grid voltage Va to show synchronism (c)Power handled by cells of phase a showing cell power imbalance(Pa1,Pa2) (d)Powerhandled by 3 phases showing phase power imbalance(Pa,Pb,Pc)

Fig. 8. DC-link voltages with cell and phase imbalance at t=0.6 s (a)Phasea (b)Phase b (c)Phase c

Fig. 9. Line to neutral voltages of converter

for the converter and go back to step (1) otherwise go to step

(3).

(3) Find cell balance status for each phase as given in

Table IV.

(4) Find the desired states for each phase depending upon

phase status and cell balance status as given in Table V and

Table VI respectively.

(5) At any particular instant,select the desired switching state

of the converter.It should satisfy the need of at least one

phase state .

(6) If it does not satisfy the need of even a single phase

state,select the switching vector calculated by LSA method.

IV. SIMULATION RESULTS

A three phase 5 level CHB with two cells per phase is

considered for simulation results. Each dc-link is controlled to

200V. The system has capability to start up on its own from

zero initial dc link voltages.It can be clearly seen in Fig. 6 that

initial voltage of all six dc link capacitors is zero.The mean

dc link voltage reference (refer to Fig. 3) is ramped up to 200

V from zero as shown in Fig. 5.In steady state, all dc links

are balanced as shown in Fig. 6a,Fig. 6b,Fig. 6c.

The PV module modelled and simulated has rated power

output of 224 W and 28V rated voltage at 25◦C with a solar

radiation of 1kW/m2. Considering the dc-link voltage and the

module rated output voltage a total of 3 PV modules are

connected in series. Four such cells are connected in parallel

to reach a total power rating of 2688 W per cell. This makes a

total of 72 modules rated at total of 16.1 KW. Matlab/simulink

has been used as simulation tool. To test the performance of

the proposed imbalance control method, a step change has

been performed to the radiation level. All power cells of

the converter start at rated temperature (25◦C) and radiation

1kW/m2(1 pu). At t=0.6 s , a step change to the radiation of

the strings connected to cell a1 and b1 is performed to force

a per cell as well as per phase imbalance. Solar radiation is

lowered to 0.8pu for both cells.

The resulting modulated converter output voltage wave-

forms are shown in Fig. 7a. The performance of the imbalance

compensation is obvious from Fig. 7b, where the three phase

TABLE VIISIMULATION PARAMETERS

Parameter ValueGrid Voltage(Line-Line) 400V RMS

Grid Frequency 50 HzRated Power 16.1 KW

Grid interconnection inductance 3 mHGrid interconnection resistance .09 Ω

DC-link capacitance 10 mFPer cell DC-link voltage 200 V

Per phase DC-link voltage 400 VEquivalent output frequency of phase voltage 5800 Hz

grid currents appear completely balanced in steady state,

despite the converter phases are operating at different power

levels. The grid phase ’a’ voltage has been scaled down

and plotted together with the currents to show the proper

synchronism . The power handled by each cell of phase ’a’ of

the converter is illustrated in Fig. 7c. Here the effect of the

step change in the solar radiation given to cell a1 is clearly

visible. Finally, the total power of each phase of the converter

are given in Fig. 7d. The system is proved to be working with

good performance and power quality in spite of significant

imbalance introduced to different cells and phases.

To fully validate the imbalance compensation method the

dc-link voltages of the both cells of each phase are given

in Fig. 8. The dynamic behavior of the per cell imbalance

compensation can be appreciated in the dc-link voltages of

all three phases as shown in Fig. 8, from t=0.6s.The line to

neutral voltage of the converter is shown in Fig. 9.

The simulation results show that the proposed method can

achieve the control goals meeting the grid code requirements

with good power quality and the Photovoltaic system maxi-

mum power output for the given radiation.

V. CONCLUSION

Control strategy based on SVM has been developed for

medium voltage converter interface based on a three phase

CHB multilevel topology under cell as well as phase power im-

balance condition . The multistring converter structure consists

of dc-dc and a grid tied dc-ac converter. The grid side control

and the PV strings control are decoupled in this converter. The

prominent challenges related to CHB multilevel configuration

are the possible existence of two types of power imbalance:

among the power cell of one phase of the converter and among

the phases of the converter. These problems have been solved

by using novel space vector modulation. The three phase CHB

multistring topology with the proposed control and imbalance

compensation methods can be used for PV power plants.

Additional advantages are the inherent superior power quality

of the CHB , low switching frequency (higher efficiency) and

possible fault tolerant operation.

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