Research on a novel buck boost converter for wind turbine systems

ICSET 2008Abstract—For converter-based wind turbine systems in grid-connected applications as distributed generators DG, variable sources often cause wide changes in the converter inpu

ICSET 2008

Abstract—For converter-based wind turbine systems in

grid-connected applications as distributed generators (DG),

variable sources often cause wide changes in the converter input

voltage above and below the output ac voltage, thus demanding

a buck-boost operation of converters Many traditional

full-bridge buck converters, two-stage converters and

single-stage buck-boost converters either have complex

structure or have limited range of input dc voltage The authors

have proposed and developed an innovative single-phase,

single-stage, flyback-based, buck-boost converter for renewable

energy systems, especially for wind turbine systems in

grid-connected applications This paper focuses on the analysis

of the working principles, computer simulation of the operation,

and design consideration of the converter for grid-connected

applications

I. INTRODUCTION For inverter-based wind turbine systems in grid-connected

applications as distributed generators (DG), resources often

cause wide variations in the input voltage to inverters above

and below the output ac voltage This is particularly true for

PV and wind systems This then demands the buck-boost (i.e.,

step-down and step-up) operation of inverters

Traditional full-bridge buck inverters do not have the

flexibility of handling a wide range of input dc voltage, and

require heavy line-frequency step-up transformers [1]

Although this topology currently has the largest market share

of the commercial DG system market due mainly to its

simplicity and electrical isolation, it is gradually replaced by

advanced topologies using “more silicon and less iron” This

leads to the pursuance of compact designs with wide input

voltage ranges and improved efficiency

Two-stage inverters normally accomplish dc voltage boost

in the first stage, and achieve buck dc-ac conversion in the

second stage, with a typical high-frequency transformer to

accomplish the voltage boost [1] Although they can

accommodate a wide range of input voltage, the complicated

structure makes them costly, particularly for small wind

turbine systems

A single-stage inverter is an inverter with only one stage of

conversion for both stepping-up and stepping-down the dc

voltage from wind turbine sources and modulating the

sinusoidal output current or voltage Single-stage buck-boost

inverters, have a simple circuit topology and low component

count, leading to low cost and high efficiency Previously

Bing Hu is with University of New Brunswick, NB, Canada (e-mail:

hubing_1977@ hotmail.com)

Liuchen Chang is with University of New Brunswick, NB, Canada

(e-mail: lchang@ unb.ca)

Yaosuo Xue is with University of New Brunswick, NB, Canada (e-mail:

y.x@ unb.ca)

available single-stage buck-boost inverters either need more than 4 power switching devices or have a limited range of input dc voltage Most of them have two symmetrical dc-dc converters operating in the opposite phase angle in order to generate a sinusoidal current waveform feeding to a single-phase grid

For small grid-connected wind turbine systems, inverters should be small, inexpensive and reliable Further efforts have been directed to innovative inverters and controls

II. A NEW BUCK-BOOST INVERTER WITH 4 SWITCHING

DEVICES The Authors have firstly proposed and developed an innovative single-phase, single-stage, flyback-based, buck-boost inverter for renewable energy conversion systems, especially for wind turbine systems in both grid-connected and standalone applications As shown in Figure 1, this buck-boost inverter has 4 switching devices [2]

The operating principles of this buck-boost inverter can be described by three operation modes

Mode 1: Charge mode

In this mode, switches T1 and T4 are turned on and switches T2 and T3 are turned off From an energy point of view, during Mode 1, inductor L1 is charged to store energy and the output current is provided by the discharge of capacitor C

Mode 2: Positive half cycle (PHC) discharge mode

Research on a Novel Buck-Boost Converter for Wind Turbine Systems

Bing Hu, Liuchen Chang, Yaosuo Xue

Fig 1 A new buck-boost single-stage inverter with

4 switching devices

In this mode, switch T4 is turned off and T3 is turned on,

while T1 is keeping on and T2 off.In this mode, the dc source

is disconnected temporarily from the output

Two current conduction modes can be defined here If the

time of Mode 2 is so short that the inductor current is not

decreasing to zero when the next charge cycle Mode 1 starts,

the current of energy-storage inductor is continuous, and we

define this operation the continuous conduction mode

(CCM) On the contrary, if the inductor current drops zero in

Mode 2 and probably sustains zero for certain time, the

operation is defined the discontinuous conduction mode

(DCM)

So far, in the PHC of ac output, the energy is transferred

from dc source (i.e Wind Turbine) to ac grid through the

alternations of Mode 1 and Mode 2

Mode 3: Negative half cycle (NHC) discharge mode

This mode is combined with Mode 1 to provide ac NHC

output when switch T1 is tuned off and T2 is turned on

Through a flyback operation, the current of primary side of

the coupled inductor L drops to zero suddenly and the current

of secondary side reaches to the initial current of primary

side, if the inductances and turns of both sides of the coupled

inductor are identical and there is no magnetic leakage

The only differences between Mode 3 and Mode 2 are that

in Mode 3, the grid is in the negative half cycle and the

discharging current has an opposite direction Then similar

arguments regarding energy exchange and transfer in Mode 2

can be also applied to Mode 3 As a result, in the NHC of ac

output, the energy is transferred from dc source to ac grid

through L1, L2 and C by the alternations of Mode 1 and

Mode 3

In summary, during each switching interval, the

energy-storage inductor is charged from a dc source (i.e

Wind Turbine) and discharged to a grid through a low pass

filter The inductor current can be discontinuous as shown

in Figure 2, and continuous as shown in Figure 3

The simulation waveforms for the buck-boost inverter

subject to a variable dc voltage sources, controlled by an open

loop feedforward compensation [3], are shown in Figure 4

The current total harmonic distortion (THD) of the 120V grid

side is 2% for a switching frequency of 9.6 kHz

The simulation waveforms for the buck-boost inverter

subject to a variable dc voltage sources, controlled by a

closed-loop sinusoidal PWM modulation [3], are shown in

Figure 5 The current total harmonic distortion (THD) of the

120V grid side is 3.4% for a switching frequency of 9.6 kHz

It is noted that the grid voltage has been assumed containing

significant harmonic contents

Fig 2 Buck-boost inverter operation in the discontinuous current mode

Fig 3 Buck-boost inverter operation in the continuous current mode

Fig 4 Simulated waveform of the buck-boost inverter under the

open loop feedforward compensation control

III. A NEW BUCK-BOOST INVERTER WITH 3 SWITCHING

DEVICES Based on the buck-boost inverter with 4 switching devices

as developed by the Authors for small distributed generators,

further improvements have been proposed, which leads to a

new buck-boost inverter with 3 switching devices [4] This

inverter is shown in Figure 6 The simple circuit topology of

this invention provides the possibility for a low cost and high

efficiency dc-ac converter appropriate for small wind turbine

applications The inverter has a low component count with

only 3 power semiconductor switches to accomplish dc-ac

conversion

The inverters can accommodate a wide range of input dc

voltage for an improved energy output from variable wind

turbine resources The input source and the output grid are

separated based on flyback operation principles As

compared to traditional buck inverters with line-frequency

transformers, two-stage buck-boost inverters, and other

single-stage buck-boost inverters, both the component count,

cost and size of the newly proposed buck-boost inverter are

reduced, thereby presenting a more reliable and economical

design for wind turbine systems and other distributed generators

The two coupled inductors L1 and L2 have the same inductance L Since only one switch is turned on in each operation mode and an inductor is always connected in the charge/discharge circuit, the dead time for preventing two switches from shoot-through can be eliminated The inverter operation can be divided into charge and discharge operation working in the positive half cycle and in negative half cycle, similar to the buck-boost inverter with 4 switching devices, as presented in the previous section

Mode 1: Charge mode

During Mode 1, switches Q2 and Q3 are turned off, and switch Q1 is turned on to charge inductor L1 from the dc source through diode D1 Capacitor C provides the continuous current for the grid in Mode 1 The governing equations are the same as in Mode 1 of the 4-switch buck-boost inverter

Mode 2: Positive half cycle (PHC) discharge mode

Mode 2 is the discharge mode in positive half cycle During Mode 2, switches Q1 and Q3 are turned off, and switch Q2

is turned on to discharge the energy, which was stored in inductor L1, to the grid through diodeD2 Figure 7 is the operation waveforms in a positive half cycle

I(Q1) I(Q2)

Iout1

Ich+

Idisch+

Ich+

Idisch+

Idisch+

Q2 Q1

D1 D2

ON

ON

ON

ON

t1 t0

I(L1)

Mode 3: Negative half cycle (NHC) discharge mode

Mode 3 follows Mode 1 in a negative half cycle of the grid voltage During Mode 3, switches Q1 and Q2 are turned off, and switch Q3 is turned on The energy which is stored in the coupled inductor L1 will transfer to the coupled inductor L2

and then discharges to the load through switch Q3 and diodeD3 Figure 8 is the operation waveforms in a negative half cycle

Assume that the resistance of the switches, diodes, and coupled inductors are negligible; two coupled inductors are perfectly coupled; the inverter works in discontinue current mode (DCM); the averaged current of Mode 2 is the average output current of the inverter and can be expressed as,

Fig 5 Simulated waveform of the buck-boost inverter under the

closed loop sinusoidal PWM control

Fig 6 Newly proposed buck-boost inverter with 3 switching

devices

Fig 7 Operation waveforms in a positive half cycle

³ = =

=

2 grid s

2 2 dc 1

LI V LT T V dt i T

where,

grid

1

LI

T = , and TS is the switching period

It is assumed that the utility line voltage Vgrid is expressed

as a sinusoidal waveform:

) sin(

2

V = ω (2)

One of the algorithms for sinusoidal PWM is to control the

turn-on time of switch Q1 in proportion to the utility voltage

Vgrid

) sin(

s

T = ω (3)

where k is the coefficient factor Substituting (2) and (3)

into (1), the ac output current iac is expressed as,

) sin(

2 2

2 s 2 dc

L k T V

i = ω (4)

In practical implementation of an inverter control, a

sinusoidal reference wave, serving as the modulating signal,

is compared with a triangular wave, serving as the carrier

signal The intersection points determine the switching angles

and pulse widths as in Figure 9 [4]

The current ratings of the power semiconductor switches of

the 3-switch buck-boost inverter are the same as those of the

4-switch buck-boost inverter presented in the previous

section The voltage stresses of the power semiconductor

devices in the charging control circuits are the same for the

4-switch buck-boost inverter (T1 and T4) and for the 3-switch

buck-boost inverter (Q1), and are equal to the Vdc+vc, where

vc is the capacitor voltage and is in the same order as the grid voltage The voltage stresses of the power semiconductor devices in the discharging circuits of the 3-switch buck-boost inverter and the 4-switch buck-boost inverter are somewhat different The blocking diodes in the discharging circuits of the two inverters have the same reverse voltage of Vdc+vc However, the switching devices (IGBTs or MOSFETs) in the discharging circuits of the 3-switch inverter have a reverse voltage of 2vc, which is twice the reverse voltage of the 4-switch inverter of vc For a 120V/60Hz single-phase grid, the peak value of vc is in the level of 200V In summary, the voltage stress of some switching devices of the 3-switch buck-boost inverter is twice that of the 4-swich inverter, but still in the range readily available in commercial IGBT or MOSFET devices

IV. SIMULATION RESULTS OF THE 3-SWITCH BUCK-BOOST

INVERTER

In designing the parameters of the inverter, a consideration for overall inverter operation under various input voltage is required Through simulation studies, the required modulation index and operation region under variable dc input voltages are presented in Figure 10

Fig 8 Operation waveforms on a negative half cycle

Fig 10 Control-to-output curves of SPWM control

(L = 150 × 10-6H, V = 120V)

Based on SPWM strategy of Figure 9, the inverter gating

signals are shown in Figure 11 The operation of the inverter

is simulated for different dc source voltages from 50V to

300V, as if it were from a wind turbine source The inverter is

designed for a rated power of 1 kW The grid voltage is fixed

at 120V/60Hz The switching frequency is set at 5 kHz,

considering a compromise between reducing switching losses

and ensuring output current quality Figures 12-15 present the

simulated output current waveforms Table I summarizes the

simulation parameters and output current performance

From the figures, it has been seen that the newly proposed flyback single-stage

single-phase buck-boost inverter can accomplish both buck

and boost operation, feeding power to a grid with a reasonable

power quality from a widely variable dc source

Fig 11 Gating signals of the 3-device buck-boost inverter

TABLE I

SUMMARY OF SIMULATION RESULTS

Fig 12 Output current waveform when the dc voltage is 50V

Fig 13 Output current waveform when the dc voltage is 100V

Fig 14 Output current waveform when the dc voltage is 200V

Fig.15 Output current waveform when the dc voltage is 300V

The implementation of the 3-switch buck-boost inverter is still yet to be done The output current waveforms are to be improved, possibly using a close-loop sinusoidal PWM as presented

V. CONCLUSION

The Authors have proposed an innovative single-phase, single-stage, flyback-based, buck-boost inverter for renewable energy conversion systems, based on a previously developed 4-switch buck-boost inverter The simple circuit topology of this inverter provides the possibility for a low cost and high efficiency dc-ac converter The inverters have a low component count with only 3 power semiconductor switches to accomplish dc-ac conversion with a high output power quality The inverter can accommodate a wide range of input dc voltage for an improved energy output from variable

PV resources The inverter separates the input source from the output grid through a flyback operation As compared to traditional buck inverters with line-frequency transformers, two-stage buck-boost inverters, and previous single-stage buck-boost inverters, both the cost and size of the newly proposed inverter are reduced, thereby presenting a more reliable and economical design for wind turbine systems The

DC

voltage

Power

factor

Output current

Output power

THD (%)

50 V 0.30 1.24 A 44.6W 2.05

100 V 0.50 1.52 A 91.2W 2.00

200 V 0.95 5.66 A 645.2W 4.50

300 V 0.98 8.50 A 999.6W 4.90

Fig 9 Sinusoidal pulse-width modulation

analysis of the working principles, and computer simulation

of the operation for this inverter have proved its feasibility for dc-ac conversion in wind turbine applications

The authors wish to thank Natural Sciences and Engineering Research Council of Canada (NSERC) for the financial support to this research project

[1] Xue, Y., Chang, L., Baekhj Kjaer, S., Bordonau, J and Shimizu, T.,

“Topologies of single-phase inverters for small distributed power generators: an overview,” IEEE Trans Power Electronics, vol 19, pp 1305-1314, Sept 2004

[2] Liu, Z., Study Of Single-Phase Single-Stage Buck-Boost Inverters, University of New Brunswick M.Sc Thesis, Aug 2004

[3] Xue, Y., Study Of Single-Phase Single-Stage Buck-Boost Inverters, University of New Brunswick M.Sc Thesis, Jan 2004

[4] Xue, Y., Chang, L., "Closed-Loop SPWM Control for Grid-Connected Buck-Boost Inverters,” IEEE Power Electronics Specialists Conference

2004, Aachen, Germany, Vol 5, pp.3366-3371, June 2004