a novel three phase multi-string multilevel inverter with high dc-dc closed operation for...

A Novel Three Phase Multi-string Multilevel Inverter with High DC-DC

Closed operation for Photovoltaic System

Abstract

This paper presents a novel three phase multi-

string multilevel inverter; this inverter reduces

number power devices and high performances.

Before this inverter provide a high step up DC-DC

converter with PI controller for better conversion

efficiency and to improve the output dc voltage of

varies renewable energy sources. This multi-string

multilevel inverter consists of six switches only

instead of eight switches in cascaded H-bridge

multilevel inverter in order to reduce conversion

losses. The main objective of this paper is to save

cost and size by removing any kind of transformer

as well as reducing the power devices .This multi-

string inverter topology have more advantages

such as better output waveforms ,lower

electromagnetic interference and low THD. Finally

this inverter connects to three phase induction

machine for analysis. Simulation and experimental

results show the effectiveness of proposed solution.

1. Introduction

In recent year’s electrical energy requirement very

high, because of different factors like raises

population, industries, colleges and hospitals, etc.,

conventional energy sources based on oil, coal and

natural gas have proven to be highly effectives

drives of economic progress, but at the same time

damaging to the environment and to human health.

Therefore the traditional fossil fuel based energy

sources are facing increasing pressure on a host of

environmental fronts, with perhaps the most serious

challenge confronting the future use of coal being

the greenhouse gas reduction targets. The potential

of renewable energy sources (RES) is enormous as

they can in principle meet many times the world’s

energy demand. Renewable energy sources such as

solar systems, fuel cells, micro-turbines and wind

has become a more issues for delivering premium

power

to loads with power quality, reliability and high

efficiency in converters of RES. In such systems,

RES usually supply a dc voltage that varies in a

wide range according to varies load conditions.

Thus, a dc/ac power converting processing

interface is required and is compliable with

residential, industrial, and utility grid standards [1]-

[2]. Various converter topologies have been

developed for RESs that demonstrate effective

power flow control performance whether in grid-

connected or stand alone operation. Among them,

solutions that employ high frequency transformers

or make no use of transformers at all have been

investigated to reduce size, weight, and expense.

For low-medium power applications, international

standards allow the use of grid-connected power

converters without galvanic isolation, thus allowing

so called “transformer less” architectures.

Furthermore, as the output voltage level increases,

the output harmonic content of such inverters

decreases, allowing the use of smaller and less

expensive output filters. As a result, various

multilevel topologies are usually characterized by a

strong reduction in switching voltages across

power switches, allowing the reduction of

switching power losses and electromagnetic

interference (EMI). A three-phase multi-string five-

level inverter integrated with an auxiliary circuit

was recently proposed for dc/ac power conversion.

This topology used in the power stage offers an

important improvement in terms of lower

component count and reduced output harmonics.

Unfortunately, high switching losses in the

additional auxiliary circuit caused the efficiency of

the multi-string five-level inverter to be

approximately 4% less than that of the

conventional multi-string three-level inverter. In

[3], a novel isolated single phase inverter with

generalized zero vectors (GZV) modulation scheme

was first presented to simplify the configuration.

However, this circuit can still only operate in a

limited voltage range for practical applications and

suffer degradation in the overall efficiency as the

duty cycle of the dc-side switch of the front-end

conventional boost converter approaches unity.

Furthermore, the use of isolated transformer with

Koppineni R N V Subbarao1

Asst.Prof in GIET Polyt College

Rajahmundry, AP, India

Atti V V Srinivas 3

Asst.Prof in GIET College


D.Vani 2

Asst.Prof in GIET Polyt College


39

International Journal of Electrical Engineering Research & Applications (IJEERA)

Vol. 1 Issue 3, August - 2013

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multi windings of the GZV based inverter results in

the larger size, weight, and additional expense [3].

In case of single phase to overcome the

aforementioned problem, the objective of this paper

is to study a newly constructed transformerless five

level multi-string inverter topology for RESs. In

this paper, the aforesaid GZV-based inverter is

reduced to a multi-string multilevel inverter

topology that requires only six active switches

instead of the eight required in the conventional

cascaded H bridge (CCHB) multilevel inverter. In

addition, among them, two active switches are

operated at the line frequency. In order to improve

the conversion efficiency of conventional boost

converters, a high step-up converter is also

introduced as a front-end stage to stabilize the

output dc voltage of each RES modules for use

with the simplified multilevel inverter. The newly

constructed inverter topology offer strong

advantages such as improved output waveforms,

smaller filter size, and lower EMI and total

harmonics distortion (THD). In this letter, the

operating principle of the developed system is

described, and a prototype is constructed for

verifying the effectiveness of the topology.

2. Photovoltaic System

A Photovoltaic (PV) system directly converts

sunlight into electricity. The basic device of a PV

system is the PV cell. Cells may be grouped to

form panels or arrays. The voltage and current

available at the terminals of a PV device may

directly feed small loads such as lighting systems

and DC motors. A photovoltaic cell is basically a

semiconductor diode whose pn junction is exposed

to light. Photovoltaic cells are made of several

types of semiconductors using different

manufacturing processes. The incidence of light on

the cell generates charge carriers that originate an

electric current if the cell is short circuited.

Figure1: Equivalent circuit of a PV device

The equivalent circuit of a PV cell is shown in

figure1. In the above diagram the PV cell is

represented by a current source in parallel with

diode, RS and RP represents series and parallel

resistance respectively. The output current and

voltages from PV cell are represented by I and V.

The V-I characteristic of PV cell is shown in

figure2. The net cell current I is composed of the

light-generated current Ipv and the diode current

Id.

Figure2: Characteristic V-I curve of the PV cell

I=Ipv – Id (1)

Where

Id = Io exp (qV/akT)

Io = leakage current of the diode

q = electron charge

k = Boltzmann constant

T = temperature of pn junction

a = diode ideality constant

The basic equation (1) of the PV cell does not

represent the V-I characteristic of a practical PV

array. The basic equation of PV array requires the

additional parameters as shown in figure.

I = IPV – [exp (V+RS/Vta) – 1] – (V+RS/RP) (2)

Where Vt = NSkT/q is the thermal voltage of the

array with NS cells connected in series.

3. Proposed Concept This topology configuration for single phase

consists of two high steps up dc/dc converters

connected to their individual dc-bus capacitor and a

simplified multilevel inverter. Input sources, PV

module 1, and PV module 2 are connected to the

inverter followed a linear resistive load through the

high step-up dc/dc converters. For three phases

consists of same as of single phase connection but

each phase connected by 1200 phase

difference.

The studied simplified five-level inverter is used

instead of a conventional cascaded pulse width-

modulated (PWM) inverter because it offers strong

advantages such as improved output waveforms,

smaller filter size, lower THD and EMI.

High step up converter introduced the output

voltage is compared with the reference value. The

error is given to the PI controller and the driving

pulses for the converter are generated. The

converter output voltage meets the reference value.

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The boosted DC voltage to Multi-string multilevel

inverter is shown in figure 3.

Figure 3: single phase multi-string five level inverter

3.1. High step-up converter stage

In this study, high step-up converter topology is

introduced to boost and stabilize the output dc

voltage of various RESs such as PV and fuel cell

modules for employment of the proposed

simplified multilevel inverter. The coupled

inductor of the high step-up converter in Fig. 4 can

be modeled as an ideal transformer, a magnetizing

inductor, and a leakage inductor. According to the

voltage–seconds balance condition of the

magnetizing inductor, the voltage of the primary

winding can be derived as

Vpri = Vin * (D/1-D)

Hence, the voltage conversion ratio of the high

step-up converter, named input voltage to bus

voltage ratio, can be derived as

𝑉𝑠𝑖

𝑉𝑝𝑟𝑖=

2 + 𝑁𝑠𝑁𝑝 ∗ 𝐷

(1 − 𝐷)

3.2. Multi-string Multilevel Inverter

This paper reports a new single-phase and block

diagram of three phase multi-string topology,

presented as a new basic circuitry in Fig. 3

Figure4. Basic Single Phase Multi-string Five level

inverter

Figure 5. Block diagram of Three Phase Multi-string

Multi Level Inverter

This three phase inverter consists of three (R, Y

and B) phase conductors connect to load and return

conductors connected to ground. In this three phase

inverter each phase conducts with 1200

difference.

For convenient illustration, the switching function

of the switch in Fig. 4 is defined as follows:

𝑆𝑎𝑗 = 1, 𝑆𝑎𝑗 𝑂𝑁

0, 𝑆𝑎𝑗 𝑂𝐹𝐹 , 𝑗 = 1,2,3

𝑆𝑏𝑗 = 1, 𝑆𝑏𝑗 𝑂𝑁

0, 𝑆𝑏𝑗 𝑂𝐹𝐹 , 𝑗 = 1,2,3

Table I lists switching combinations that generate

the required five output levels. The corresponding

operation modes of the multilevel inverter stage are

described clearly as follows.

1. Maximum positive output, 2VS: Active

switches Sa 2, Sb 1, and Sb 3 are ON; the

voltage applied to the LC output filter is

2VS.

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2. Half-level positive output, +Vs: This

output condition can be induced by two

different switching combinations. One

switching combination is such that active

switches Sa 2, Sb 1, and Sa 3 are ON; the

other is such that active switches Sa 2, Sa

1, and Sb 3 are ON. During this operating

stage, the voltage applied to the LC output

filter is +Vs.

3. Zero output, 0: This output condition can

be formed by either of the two switching

structures. Once the left or right switching

leg is ON, the load will be short-circuited,

and the voltage applied to the load

terminals is zero

4. Half-level negative output, −Vs: This

output condition can be induced by either

of the two different switching

combinations. One switching combination

is such that active switches Sa 1, Sb 2, and

Sb 3 are ON; the other is such that active

switches Sa 3, Sb 1, and Sb 2 are ON.

5. Maximum negative output, −2Vs: During

this stage, active switches Sa 1, Sa 3, and

Sb 2 are ON, and the voltage applied to the

LC output filter is −2Vs.

Figure 6. Modulation strategy for reference signal

Table1

Switching combination

In these operations, it can be observed that the open

voltage stress of the active power switches Sa 1 , Sa

3, Sb 1, and Sb 3 is equal to input voltage VS ;

moreover, the main active switches Sa 2 and Sb 2

are operated at the line frequency. Hence, the

resulting switching losses of the new topology are

reduced naturally, and the overall conversion

efficiency is improved.

The two input voltage sources feeding from the

high step up converter is controlled at 100V, i.e.

Vs1 = Vs2 = 100V. The switch voltages of Sa1, Sa2,

Sa3, Sb1, Sb2, and Sb3 are all shown in Fig. 6. It is

evident that the voltage stresses of the switches

Sa1, Sa3, Sb1, and Sb3 are all equal to 100V, and

only the other two switches Sa2, Sb2 must be 200V

voltage stress. For three phase inverter as similarly

as single phase inverter but each phase operates

with 1200 phase difference

3.3. Comparison with CCHB inverter

The average switching power loss Ps in the switch

caused by these transitions can be defined as

𝑃𝑠 = 0.5 𝑉𝑑𝑠 𝐼𝑜 𝑓𝑠[𝑡𝑐 𝑜𝑛 + 𝑡𝑐(𝑜𝑓𝑓)]

Where tc(on) and tc(off) are the turn-on and turn-

off crossover intervals, respectively; Vds is the

voltage across the switch; and Io is the entire

current which flows through the switch.

Figure 7. Five level inverter of CCHB

For simplification, both the proposed circuit and

CCHB inverter are operated at the same turn-on

and turn-off crossover intervals and at the same

load Io. Then, the average switching power loss Ps

is proportional to Vds and fs as shown in table2.

For three phases five level inverter of CCHB

compare with proposed topology required number

switches are reduced six switches and switching

loss is nearly half that of CCHB inverter.

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Table 2

Comparison of two multi level inverter for single phase

3.4. DC–DC Closed loop with PI

Controller

In the closed loop model, the simulation is carried

out to meet the reference value. The closed loop

model of the DC- DC step up converter is shown in

Figure 7.

Figure 7. Block diagram of DC-DC closed loop with PI

controller

The output voltage is compared with the reference

value. The error is given to the PI controller and the

driving pulses for the converter are generated.

Block diagram of PI controller as shown in figure

8. U* signal given to high step up converter switch,

the converter output voltage meets the reference

value. It can be seen that the output remains

constant. This constant voltage given to five level

inverter and load voltage is directly proportional to

inverter input.

Figure 8. Block diagram of PI controller

4. Simulation Results

4.1. Three Phase Resistor Load

Simulations were performed by using MATLAB/

SIMULINK to verify that the proposed inverter

topology. Three phase five level inverter with

resistor load is shown in figure 9. Each phase

voltage fed from two PV panels via high dc-dc

converter, this dc-dc converter operates with PI

controller.

Figure 9. MATLAB/SIMULINK model of Three Phase

Five Level Inverter with R-Load

A prototype system with a high step-up dc/dc

converter stage and the simplified multilevel dc/ac

stage are built with the specifications of the two

preceding high step-up dc/dc converters are 1)

input voltage 30V; 2) controlled output voltage

100V; and 3) switching frequency 2 kHz; 4) three

phase output voltage 200V.Simulation results of

three phase load voltage, single phase voltage

waveform and dc-dc output voltage are shown in

figure 10, figure 11 and figure 12.

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Figure 10. Simulation result of three phase load Voltage

waveform

Figure 11. Simulation result of single phase waveform

(200V Peak-Peak)

Figure 12. DC-DC output voltage waveform (100V).

This high DC-DC converter stage provide every

time 100V is shown in figure 12, the output voltage

of this converter compare with reference voltage

(100V) which gives error signal every time and this

error given to PI controller ( Kp = 0.001 and Ki =

0.01). After that driving pulses are generated by

triangular waveform with high frequency.

4.2. Three Phase Induction Motor Load

Figure 13. MATLAB/SIMULINK model of three phase

five level inverter with induction motor

For better analysis this new three phase multi-string

five level inverter connected to induction motor is

shown in figure 13. The analysis of three phase

induction motor results is shown in figure 14,

figure 15 and figure 16.

Figure 14. Simulation result of induction motor stator

current and voltage/phase

Figure 15. Simulation result of a induction motor voltage

and current

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Figure 16. Rotor speed and electromagnetic emf of

induction motor

The conversion efficiency of the implemented

inverter and THD of the output voltage measured

in this case are approximately 96% and 3%,

respectively. The studied multilevel inverter has

lower THD than the CCHB multilevel inverter.

5. Conclusion

In this paper modeling and simulation of a novel

three phase multi-string multi level inverter with

high dc-dc closed loop topology that produces a

significant reduction in the number of power

devices required to implement multilevel inverter.

This inverter topology has more advantages such as

improved output waveforms, and lower EMI and

THD. The proposed topology has minimum

number of switches compare than other

configuration.

6. Reference

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andD.Martin, “Design of parallel inverters

for smooth mode transfer micro-grid

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[2] C. T. Pan, C. M. Lai, and M. C. Cheng,

“A novel integrated single phase inverter

with an auxiliary step-up circuit for low-

voltage alternative energy source

application,” IEEE Trans. Power

Electron,

[3] C. T. Pan, W. C. Tu, and C. H. Chen, “A

novel GZV-based multilevel single phase

inverter,” in Proc. Taiwan Power

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[4] “A novel high step-up ratio inverter for

distributed energy resources (DERs),” in

Proc. IEEE Int. Power Electron.

[5] L. M. Tolbert and T. G. Habetler, “Novel

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[6] R. Gonzalez, E. Gubia, J. Lopez, and

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