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Voltage source inverters for high-power,variable-voltageDC power sourcesZ.Chen and E.Spooner

Abstract: The paper discusses the applications of voltage source inverter (VSI) based powerelectronic systems for interfacing variable-voltage DC sources to the grid. A variable-speed windpower conversion system is used for illustration, where the VSI-based interface needs to convert avariable DC voltage to a nearly constant AC voltage with high-quality power. The power controlprinciples of VSI are described. Various system configurations and switching strategies are examinedby analysis, simulation and experimental methods. It is shown that better utilisation ofsemiconductors and more flexible control may be achieved by using a separately controlled DC link,rather than a directly connected VSI that has to operate at a lower modulation ratio at higher power.In some cases, multipulse inverter structures may be preferred, despite higher component count,because of reduced switching losses, fault tolerance and the absence of filters. The solutions developedin the study could be applied at a different scale to other renewable energy sources, such as wave orsolar photovoltaic devices.~

List of principal symbols and abbreviations= DC link capacitance= DCDC convertor switch duty ratio, D = t,,,f,,t,= fundamental component of AC current= DC link current= current distortion component= current nth-order harmonic component= inductance between AC grid and VSI= modulation ratio of SPWM inverter= voltage ratio of D CDC convertor= real power= reactive power= inverter voltage fundamental component= DC-link voltage= voltage distortion component \,= voltage nth-order harmonic component=AC system voltage= power angle between VSI output voltage V&)and grid voltage V,= phase angle between V, and L(,)= displacement power factor, DPF =cos qb,= electromotive force= power factor

0 EE, 2001IEE Proceedgs online no. 20010405DOI: 10.1W9hpgtd20010405Paper fmt received 4th May 2000 and in revised fomi 15th January 2001Z. Chen is with the Department of Engineering and Technology,De MontfortUniversity, Queens Building, The Gateway. Leicester, LE1 9BH, UKE. Spooner is with the School of Engineering, University of Durham, SclenceLaboratories,South Rd, Durham, DH1 3LE, UK

~

PM = permanent magnetPWM = pulse-width modulationSHE = selective harmonic eliminationSPWM = sinusoidal pulse-width modulationSVPWM = space vector pulse-width modulationSUR = switch utilisation ratioTCHD = total current harmonic distortionTVHD = total voltage harmonic distortionVSI = voltage source inverter1 IntroductionVoltage source inverters (VSI) are commonly used to trans-fer real power from a DC power source to an AC load,such as an AC motor. Usually, the DC source voltage isnearly constant and the amplitude of the AC output volt-age is controlled by adjusting the PWM ratio of the VSI.The PWM ratio often varies with the delivered power, witha higher ratio corresponding to higher power. VSIs are alsobecoming widely adopted for other applications, such asgrid connection for renewable energy sources, where a vari-able-voltage DC power source supplies power to an ACsystem with a nearly constant voltage.Renewable energy sources are increasingly contributingto our electrical power needs, and this trend is acceleratingas fossil fuel sources are depleted and their combustionproducts pollute the environment. Renewable energysources are dispersed in nature, and grid connection willeffectively increase the energy capture so the future electric-ity supply system will have many more renewable powersources than todays power stations. With the notableexceptions of biomass combustion and waste incineration,renewable sources are often variable in time and not com-pletely predictable. Wind energy is a prominent example ofsuch renewable sources. Various wind energy conversionsystems have been studied [1-6]. The advent of high-powerand high-frequency semiconductors with fast gate turn-off

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capability, such as integrated gate commutated thyristors(IGCTs), will extend the applications of the self-commu-tated VSI into high-power areas. Connecting the renewableenergy sources into the grid will be a major applicationarea for VSIs.This work is derived from attempts to develop an inter-face for a modular, permanent magnet (PM), variable-speed gear-less wind turbine generator [7]. The modularPM generator has rnany discrete coils, whose outputs canbe rectified and combined to produce a smooth DC output[8]. Diode rectifiers can be used for simplicity and econ-omy. The limitation on the power transfer due to the inter-nal inductance of the generator can be overcome by shunt-connected AC capacitors [9]. For high-efficiency operationof the wind turbine, the shaft speed should be varied inproportion to wind speed. The voltage and current at thegenerator-rectifier terminal should follow the optimalcurves, as shown in Fig. 1 [lo]. The ideal operating speedcan be realised by adjusting the electrical load of the sys-tem, and consequenlly the torque on the machine shaft.

PMgeneratorrectifier

@

I0.4 0.5 0. 6 0.7 0. 8 0.9 1.0generator speed, p.u.Fig.1_ _ optimal Pd curve.......... optimal Vdcurve~ _ _ _ ptimal Id curve

re al DC-lutkpower, voltage and current againrt windspeed

4 v ~ , d l ~ ~ ~ d._ ...............................

Analysis, simulation and experimental work were carriedout to study the power conversion system. The simulationmodels were developed and the power electronic systemswere simulated with PSPICE. The system dynamic per-formance and the effects on power systems have also beenstudied. Detailed information can be found elsewhere[ll-131. In this paper, we examine the features of VSI tech-niques for connecting variable DC sources into AC sys-tems, in particular, the interface for the modular PMgenerator multi-diode rectifier system.2Fig. 2 shows a PRlI generator wind energy conversionsystem consisting of a basic VSI, where Ls represents theinductance between the inverter AC voltage (VJ(kl)nd theAC system voltage (V,), and is is the current injected intothe grid. The output waveform of a VSI can be decom-posed into a fundamental sinusoidal component and a setof harmonics. For the analysis of power transfer, theharmonics and the resistance of the circuit are ignored. Thefundamental voltage and current components can be repre-sented by the phasor diagrams in Fig. 3. Figs. 3a and bcorrespond to the hndamental voltage control (voltagecontrol mode) and the fundamental current-control(current-control mode) methods, respectively [14, 151.

Power control of grid connected VSI

440

Fig.2 Directly connected VS1

a

t ' s ibFig.3a Voltage-controlledVSI6 Current-controlled VSI

Phasor dizgrmof gridconnected voltage source inverter

2.I Power control of voltage-controlled VSIThe phasor diagram of Fig. 3a is similar to that of a syn-chronous generator. The real power and reactive powersent into the grid by a voltage-controlled VSI are expressedin p.u. values asT T T T

where X , = 2@,LS, and ji is the grid frequency. Eqn. 1shows that, for the given grid voltage V ,and inductanceL,,the real power P, and reactive power Q, can be controlledby regulating the magnitude of and the power angle 6.According to eqn. 1, therefore, the ideal real power Pidealand the desired reactive power QideOran be obtained bysetting the following ideal power angle aides, and inverterAC terminal voltage V1(,)illEai

The power angle 6 can be controlled by timing the semi-conductor switching, and the voltage magnitude can becontrolled by PWM switching or by means of a separatelycontrolled DC link.2.2 Power control of current-controlledVSIA VSI can also operate in current-control mode; by switch-ing the semiconductors, the AC current waveform may becontrolled to follow a desired reference sinusoidal wave-form, so the magnitude and phase angle are controlled. Togenerate the desired current, the current- controlled VSIshould be operated in closed-loop mode [15]. The powercontrol of a current-controlledVSI can be explained via thephasor diagram shown in Fig. 3b , where I ,] represents thefundamental component of the inverter AC current, V, is

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the AC system voltage and is the phase angle between V ,and IS1. he power injected into the grid can be expressedin p.u. values asP, =PI =v, I,1 cos $1Qs =VsIs1 sin 41 (3 )The desired output real power and reactive power can thenbe controlled by regulating the magnitude I,, and the anglewith respect to the gnd voltage:

1o0.90.80.70.60.50.4

._10

3

--0.3

(4)

---~

----

The above-mentioned two types of operation mode (volt-age-controlled and current-controlled VSIs) can be imple-mented with various circuits and switching strategies. Twogroups of circuit configurations are discussed in this paper(i) the VS I directly connected between the diode rectifierand the grid; (ii) the VSI connected to the rectifier via aseparately controlled DC link.power angle for unity power factor

12r

1a

/1.0201.018

i1.016

inverterAC voltage for unity power factor

z1.014-0,g1.012-m'1.010 -921.008-I 1.006-.E1.004-91.0021 ooo0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00generator speed, p.u.bFig.4 VSIpower mgle (U ) cmd voltuge ( h ) ugccinst windspeedDPF =1.0, vs=1.0p U,, X, 0.2 p.u.

3DC sourceVSI direct connection bet ween grid and variable3.I VSIoperation requirementsFig. 4 shows the required variation of Vlcl) o transfer realpower to an infinite busbar at a unit displacement powerfactor (DPF) via an inductor with 0.2 p.u. reactance. It canbe seen that the required AC voltage variation of a voltage-controlled VSI is much less than the required voltage varia-tion at the rectifier terminal; as shown in Fig. 1, where thevoltage at the generator-rectifier terminal varies about 2.5

IE E Proc -Gener Transm Distrib , Vo l 148. No 5, September 2001

times for wind speed between 0.5 p.u. to 1.0 P.u., whereasthe variation of the inverter output voltage is less than 2%.Obviously, the power electronic interface needs to satisfythese voltage requirements.3.2 Modulation ratio of directly connected PWMVSIThe directly connected VSI shown in Fig. 2 has a simplecircuit configuration. Power angle and voltage magnitudecontrols can be performed with various PWM switchingtechniques, such as SPWM, SHE and SVPWM etc. Thedirectly connected VSI has to accommodate the differentvoltage requirements at the rectifier side and inverter sideby varying the PWM ratio. A turbine speed range of 2:l(whch may be needed in practice for effectively extractingthe energy from the wind) corresponds to the rectifier volt-age range of 2.5:1, as shown in Fig. 1. Here SPWM is usedas an illustration, and the linear modulation region (Ma51 O) is preserved for minimum voltage waveform distortion.For maximum utilisation of the devices, the modulationratio is set to 1.0 for the minimum DC-link voltage at therectifier terminal. The ratio is then reduced with the DCvoltage increasing, as shown in Fig. Sa. A rather low M ,(about 0.4) has to be used for the rated wind speed.Figs. Sb and 6a show the simulated waveforms at 0.5 and1O p.u. wind speeds, respectively. Moreover, the range ofmodulation may be extended further if the normal vana-tions in grid voltage are also to be accommodated.

SPWM modulation ratio for unity power factor

:::I , , , , , , , , , ,00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0generator speed, p.u.a

SPWMVSI waveforms at 0.5 p.u. wind speed2000~

1oooc

-20001 I I I I I I I I0 0.005 0.010 0.015 0.020 0.025 0.030 0.0350.040time, sbFig 5 SPWM-VSIin sirple system0 SPWM-VSI modulation ratio against wind speedb waveforms of SPWM-VSI; M a =1.0, Vd =241V, 6 = 1.4" at 0.5 p.u. wind speed(0 V V S I , v(ii) vac line),v(iii) VAC phase),V(iv) 4*IA0 A

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3.3 Switch utilisation of directly connected VSIThe switch utilisation ratio (SUR) is the ratio of volt-ampere (VA) capacity delivered by the converter to the VArating of the semiconductors. It may be used to describethe effectiveness of the semiconductor utilisation. For athree-phase six-pulse inverter, the SUR is [161(5)

Assuming a ripple-free DC-link voltage and a sinusoidalAC current waveform, the following relations hold:Is peak = \/2Isl,mrte and Vd.peak =V d , r a t e

where Zsl,,.utrnd V,,,,, are the rated AC current (rms) andDC-link voltage, respectively. For the linear modulationregion, substitute eqn. 6 into eqn. 5 to obtain(7)

where IX1ollows the same trajectory as the real power ofFig. 1 for unity DE'F operation. Using the Mu curve inFig. 5a and the V, curve in Fig. 1can obtain SUR charac-teristics, as plotted in Fig. 66.SPWbl VSI waveforms at 1O p.u. wind speed

2500r20001500

-2000- I I0 0.005 0.010 0.015 0.020 0.025 0.030 0.0350.040time, sa

0.10q0.08

' 0:3 0:4 015 016 0:7 0:8 0:9 1.6generator speed, p.u.bFig.6

u Waveforms of SPWM-VSI; MO=0.41, V,r =600V, 6 = 11.3" at 1.0 p.u. windspeedb SPWM-VSI SU R against wind speed(ii) Va c (line), V(iii) Vac (phase), V

S PWM-VSI in Junple system

(9 VVSI,V(iv) lac.A

A small improvement can be achieved by using altema-tive switching schernes such as space vector pulse width442

modulation (SVPWM) or a harmonic injection techniquethat produce slightly higher output AC voltage with thesame level of input DC voltage. However, if a VSI isrequired to convert a wide range of DC voltage to a nearlyconstant AC voltage and deliver a higher power at a lowmodulation ratio, the drawback of low switch utilisationratio exists. This leads to a design of higher peak currentwith higher conduction power losses and higher semicon-ductor cost.A current-controlled VSI connected directly to the varia-ble DC link would suffer the same poor switch utilisationratio as a voltage-controlled VSI. Moreover, an appropri-ate DC voltage level is required to drive the output currentto follow the specified reference waveform.44. I Circuit configuration and operating principleThe above-mentioned voltage-matching requirements canalso be accommodated using a controllable DC link, forexample, incorporating a D C D C converter between thegenerator-rectifier and the VSI, as shown in Fig. 7. TheD C D C converter creates two voltage levels, one at the rec-tifier terminal to provide the optimal generator-rectifierloading condition shown in Fig. 1, and the other at theinverter terminal to follow the voltage curve shown inFig. 4b or simply a constant DC voltage. The modulationratio of the VSI can then be fured or vaned over a greatlyreduced range.

Separately cont rolled DC link -VSI system

PM generator DC/DCrectifier DC link id convertor gridv41~

. . .,.,,..... .........

Fig.7 Suriulutionmodel of DUDC convcrior VS I4.2 DC/DC converter controlled DC linkThe average input and output voltages of a DC DC con-verter are related by the on and off duration (tonand torr>ofthe semiconductor S, i.e. the switch duty ratio D, where D=to,&,, andf,,, is the switching frequency. If the VSI oper-ates at a fmed switching pattern (a constant modulationratio), the required voltage ratio Nk of the DCDC con-verter will be

(8)d,17Nk = vd,R , e awhere V , , cleuland Vd,lid,ul re the ideal average voltages atthe rectifier and inverter DC terminals (Figs. 1 and 46),respectively. Assume that the system may have Nk close tounit at the rated condition, then the resultant Nk with vary-ing speed has the characteristic illustrated in Fig. Sa. Therequired ratio D of a step-up converter is shown in Fig. 86.With the ideal voltage presented at the VSI DC terminal,the SUR curve of a six-pulse square wave switching VSI isshown in Fig. 9.It can be seen that the SUR of the VSI isimproved by using the separately controlled DC link. Ifhigh-frequency switching schemes are used, the AC funda-mental voltage would be slightly less, giving a slightlyreduced SUR.The main components of the DC D C converter VSI sim-ulation model are -shown in Fig. 7, where a smoothingcapacitor is placed at the converter input terminal to main-tain a smooth DC-link voltage for the rectifier operation.

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The DC capacitor at the inverter DC terminal absorbs cur-rent ripples from both the D C D C converter and theinverter. The results for a typical condition are shown inFig. 10, where (i) V, (inverter) is the inverter DC terminalvoltage; (ii) V, (rectifier) is the rectifier DC terminal volt-age; (iii) I (inductor) is the DC link inductor current; and(iv) 5* Vg s the D C D C converter switching signal. 500U)Ep 400-C/DC convertor voltage ratio at unity power factor

4'0[.

-(ii)

'"1.02.52.01.5-

1.0

--

-.o 10.5- o%9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

\\01 1 , , I , I t %0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00generator speed, p.u.

bFig.8 D U D C Convertor (U ) voltage UIUA (h) switch rutios ugunizct wwulspeedVSI switch utilisation ratio at unity power factor

0._c0.10-

p 0.08 -0.-c30.06

I0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00generator speed, p.u.SUR clumcteristic,br squure-wuve switchingig.9

I EE Prw- G en er . Traium. Dbrrib., Vol. 148, No . 5, September 2001

DC/DCconvertor waveforms at 8kHz D=0.288ool700t

$ 100. .P n n n n n n n n n ~ n n n n n n n n D n n0 n n n n n f l n f l '0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

time, s X 0 3Fig.10(i) Vd (inverter). V(ii) Vd (rectifier),V(iii) I (inductor), A(iv) 5* Vu , V

DUDC convertor simulutionresultsP =0.5 P.u.,6 = 570, D =0.28, V c , , ~ 0.71 P.U.

4.3 VSI switching strategiesFreeing the VST from the task of controlling the voltageover a wide range makes alternative VSI circuit configura-tions and switching schemes possible, leading to furtherimprovement of the system performance. For example,PWM switching techniques or multiple pulse inverter cir-cuits can be efficiently used with the separately controlledDC link for harmonic minimisation. The following circuitconfigurations and switching strategies are discussed in t hspaper:(i) voltage-controlled VSI with fixed modulation ratio.(ii) voltage-controlled VSI with variable PWM modulationratio.(iii) current-controlled PWM VSI.4.3.1 Voltage controlled VSI with fixed modula-tion ratio: In this case, the VSI needs only regulate thepower angle with a fixed modulation ratio since the DCvoltage at the inverter terminal is controlled to follow theideal voltage profile. Two types of method can be used:square wave switching with multi-pulse inverter configura-tion and fixed pattern PWM switching.4.3. ?. ? Square-wave switching and multi-pulseVSI: In the square-wave switching scheme, the upper andlower switches in one inverter leg conduct 180" alternativelyto generate square wave line-to-line voltage waveforms.The ratio of the fundamental AC voltage to the DC volt-age is fixed. The switching frequency and power losses ofthe VSI are relatively low, but the harmonic contents in theoutput waveforms are high. Therefore, the method maynot be used in a simple six-pulse form, but it can be used ina multi-pulse inverter system consisting of several (n)bridges of square-wave switching six-pulse inverters.A 24-pulse inverter is illustrated in Fig. 11, where theoutput of each bridge is phase-shifted from its partners,and the separate outputs are combined in a phase-shiftingtransformer to create a 6n-pulse overall output. This typeof system can be designed to produce hgh-quality output.The rating of each six-pulse bridge is a fraction of the total,so that standard bridges may be assembled to form a mod-ular system. The separate bridges have good SUR (Fig. 9)and the whole inverter system assembled with the bridge

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modules will have the same SUR. In a 6n-pulse invertersystem, the orders of the remaining harmonics are 6n x k f1 where k is any integer, and the magnitudes of the remain-ing harmonics are tlne same as that in a six-pulse system.The simulated waveforms of a 24-pulse VSI system areshown in Fig. 12a, where the traces proceed downwards atdifferent vertical axes (same scale) in the followingsequence: (i) Vvsl is, the inverter AC terminal line-to-linevoltage; (ii) VAC(line) is the grid line-to line-voltage; (iii)Vac (phase) is the grid phase to neutral voltage; (iv) 4*IAcis the AC current. The harmonic spectra of the VSI ACoutput voltage are shown in Fig. 12b.

10030 80 -EcQ0C.-

60.iii.c

Fig.1 1 Multi-puke V S I ~ircuit24pulse

7

24-pulse VSI waveforms,50r

I . I , 1 1 1 , : I I , I ,I

v, xsm

50 100 150order of harmonicsb

OOL l kFig.12D =0.28, 6=5.7", 24-pulse invertera Waveforms (i) Vvs,, V (ii) Va c (line), V (E) V ac (phase), V (iv) 4*1~c,b Inverter AC voltage harmonics

Multi-puke VSdr w u v e f o m simulution results

4.3.1.2Fixed-pafitern PWM switching: All thePWM schemes, such as SVPWM or SPWM, can be effi-444

ciently used with a fmed modulation ratio. However, SHEswitchmg offers better harmonic performance than SPWMor equivalent harmonic performance at a lower switchngfrequency. The switching instants can be pre-determinedand stored within the controller. Fig. 13 shows the simu-lated voltage waveform and harmonics of eliminating 5th,7th, llth, 13th, 17th, 19th, 23rd and 25th harmonics in asix-pulse VSI; the waveforms are as in Fig. 12. 571113 1719 23 25SHE waveforms2500r

200015001000

W

-1500-20000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040time, s

a571113 171923 25SHE SI voltage (simulation)2Or

orderof harmonicsbFig.13D =0.28, 6=5.7"a Waveforms (i) Vvs,, V (ii) V A C line), V (iii) VAC phase), V (iv) 4 * I ~ c ,b Inverter AC voltage harmonicsSHE-VSI simulution results

It can be seen that the selected lower order harmonics areeliminated and the higher order voltage harmonics areincreased, but the total current harmonic distortion can bereduced. If more switching angles are adopted, the har-monic performance will be further improved at the cost ofswitching power losses. If necessary, the fundamental com-ponent of the output voltage can also be controlled byincreasing the number of switchings per cycle, which hasthe drawback of greater power losses.4.3.2 Voltage controlled VSI with variable modu-lation ratio: In thls case, the DC/DC converter simplykeeps a constant voltage at the VSI terminal. Therefore, themodulation ratio of the PWM inverter should be variable,but only withm a range of 5% (Fig. 4). (The harmonic per-formance of natural sampling SPWM is given in Section5.3.6.) Other PWM methods have similar characteristics,

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although some schemes may offer slightly increased outputvoltage.4.3.3 Current controlled PWM VSI: For a current-controlled VSI the inverter DC terminal voltage may bekept at a constant value like the VSI above. The controllerreference current varies with wind speed. To keep a unitpower factor for all wind speed, the reference current needsto follow a curve similar to the optimal power curve givenin Fig. 1.The current-controlled PWM VSI can offer bet-ter dynamic performance due to the closed-loop control. Itcan be seen that the current-controlled PWM VSI presentsa wider current regulation range than the power angle inthe voltage-controlled VSI; hence better stability of the con-trol system is expected.Fig. 14 shows the simulation results of a current-control-led VSI. The sequence of waveforms is grid AC voltage,reference current and grid current. It can be seen that thesystem can provide controllable reactive power with welldefined harmonic spectra.

CC-VSIwaveforms at unit power factor

80706oE 50403020

--L

-80010 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040time, sa100 CC-VSIcurrent harmonic spectra at unit power factor (simulation)ni;5050 100order of harmonicsbOOFig.14

Unity power factor, 3. I5 kHzU Waveforms (i) AC phase),V (ii) IA C (reference),A (iii) IA C (phase),Ab Grid current harmonic spectraCurrent-controlledVS I ~~av~$oomvinulation remlts

4.3.4 Comparison of harmonic reduction meth-ods:The harmonic performances of the discussed schemes(Table 1) are shown in Table 2. The worst-case operatingcondition (purely reactive power generation) was taken.The conditions are 0" power angle, 1.2 p.u. inverter ACoutput voltage fundamental component and 1.0 p.u. gridIE E Proc -Gener TrunJm Dtstrrb , Vu1 148. N o 5 , September 2001

voltage. Harmonics up to the lOlst harmonic wereincluded for total harmonic distortion calculation.The switchmg power loss is proportional to the switchingfrequency, and the number of transitions is given inTable 2. If the discussed current-controlled VSI has thesame switchmg frequency as the voltage-controlled SPWMVSI, a similar harmonic pattern and power losses can beexpected.Table 1: Inverter harmonic reduction options

~

Identifier ParticularsSS6SS12

SS24

SHE1

SHE2

SHE3

SPWM

square-wave switching six-pulse inverter systemsquare-wave switching 12-pulse nverter 5 stem(2x 6-pulse bridges)square-wave switching 24-pulse nverter system(4x 6-pulse bridges)selective harmonic elimination strategy to eliminate5th. 7th, 1 th, and 13th harmonicsselective harmonic elimination strategy to eliminate5th, 7th. I l th , 13th. 17th and 19th harmonicsselective harmonic elimination strategy to eliminate5th. 7th. I lth , 13th. 17th, 19th. 23rd and 25th harmonicsnatural sample sine PWM with a frequency index(ftrjfc,,tro,)of 21 and modulation index (vcontr,,'vtrJf 1

Table2: Comparisonof harmonic reduction strategiesIdentifier TCHD %x,=0.2 p.u.

Number of DC link voltage/transitions/device/ inverter outputcycle voltage (line-line)

SS6ss12SS24SHE1SHE2SHE3SPWM

28.82766.33181.545113.346210.28648.340113.841

2 1.28262 1.28262 1.282618 1.395226 1.403234 1.407042 1.6340

It can be seen that multi-pulse inverter schemes havelower switching power losses than other schemes and cansignificantly reduce the total current harmonic distortion,but the penalty is the increased equipment investment cost.On the other hand, high-frequency switching techniquescould reduce the harmonic distortion in a certain rangewithout requiring more equipment, but the power lossestend to increase rapidly as more low-order harmonics areeliminated.5 Experimental resultsThe low-power laboratory models of the discussed VSIsystems have been designed and built for demonstration.IGBTs are used to construct the power converters. Theblock diagram of the control circuits is shown in Fig. 15.A transformer with four windings on the inverter side isused in conjunction with multi-pulse inverters; 6-pulse, 12-pulse and 24-pulse circuit configurations can be arranged.Fig. 11 illustrates the 24-pulse case, where four 6-pulseinverters are connected in parallel to a common D C source,but in the modular generator system it would be feasible toprovide separate DC supplies, leading to a high degree ofredundancy and fault tolerance.Figs. 16-19 show some test results. It can be clearly seenthat the harmonic distortion is significantly reduced in a 24-pulse inverter system and harmonic fdters may not be

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grid- multi-pulseEHEsignalgenerator VS I gatedrivingvoltage zero phaseshifter ----c-controller ratio DCIDC convertorregulator gate driving

Qr,I T 1 '

("ref) Id 'd vsFig.15 Control circuit block diugrmTIME:07:56:12I TIME:00:34:24I

Fig.16 DUDC converto wuvefinru(i) gate voltage, chl 1Vi1 V, 5V, loops, inductor(ii) current, ch2 1AiIOOmV, IOOmV, 100psTIME:OI :19:37

Fig.17Line-to-line(i) chl 500Vil V, 100mV. 5mj ,(ii) inverter output current, ch2 I AiIOOmV, IOOmV,Sms, experimental, 5, 7, I I , 13,harmonic elimination. 6 =5.7"Grrd voltage of'SHEPWM-VSI

needed since the TCHD can be within the normal statutorylimit. The multi-pulse inverter connection is more effectivein reducing harmonic distortion than high-frequencyswitching techniques. A disadvantage is that a phase shifttransformer is required. However, if the implementationcan be carried out for a complete wind farm, rather thanseparately for each machine, then the cost may be nogreater than normall transformers for separate high-fre-quency inverters of each turbine.6 VSI interfaced wind power in isolated syst emsIn an isolated system, the generated power always has tobe balanced with the consumed power; the optimal446

Fig.18Line-to-line(i) chl 500Vil V, 100mV, 5m s(ii) inverter output current, ch2 1AiIOOmV, 50mV. Sms, experimental, 6 =5.7"Grid voltage of24-puLse VSI

experimentCC-VSI (unity power factor)0.4r

-0.4I0 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040time,sFig.19 wavcjhm of'curren~-contr~lle~iS IUnit power factor; waveform sequence: grid phase voltage 50Vi0.1, reference current0.5Ai0.1 and grid current 0.5Ai0.1operation of energy source and the reactive power controlare restricted although the VS I type interface can stilltransfer power to the consumer. The power angle or thepower factor angle are no longer useable as controlvariables, and the power factor will depend on the natureof the load, although the harmonic reduction techniquesdiscussed above are still valid. Fig. 20 shows simulationand experimental waveforms for such a system with anSPWM-VSI.In an isolated system, it could be difficult to meet theconsumers' power demand without some auxiliary control-lable energy sources, such as diesel generators or energystorage systems. Therefore, grid connection, wheneverpossible, is an effective way to utilise the renewable energysources.

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waveforms ofSPWM VS I inan isolatedsystem

E0 0 --200-

-400 -,-6000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040,-6000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040time, s

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bS P WM-VS I in isoluted systemig .20a Simulated voltage and current waveforms for isolated system(9 V V SJ .V(ii) IO*IA,-, Ah Mcasurcd voltage and current waveforms for isolated system(i) chl inverter AC voltage IOOVil V, 500mV. 5m s(ii) ch2 current I AiIOOmV, 200mV, 5m s

9 ConclusionsIn the paper, we have discussed the applications of VSIconnected to variable DC power sources. A modular PMgenerator wind power system (where the power electronicsystem has to convert a widely varying DC voltage to anearly constant voltage) was taken as an example to illus-trate the effectiveness of various circuit configurations andcontrol strategies. The results of simulation and laboratoryexperimental studies confirm the expected performance ofthe discussed system.Both voltage- and current-controlled VSIs can transferthe optimal power into the grid with controllable reactivepower and low harmonic pollution. Current-controlled VSIhas a wider current regulation range, and hence betterstability of the control system is expected.

The direct connection of VSI to a varying DC voltagesource has the simplest circuit configuration, but poordevice utilisation and high power loss because a lowermodulation ratio has to be used for hgher power opera-tion; it is therefore an unattractive system for the discussedapplications.A separately controlled DC link can free the VSI fromwide-range modulation ratio control duty and provide theflexibility for design and operation of the inverter, so as toallow the implementation of more efficient harmonic mini-misation methods, including selective harmonic eliminationand multi-pulse inverter techniques.Under the discussed DCDC converter-controlled DClink schemes, various optimal power control, reactivepower regulation and harmonic reduction methods havebeen examined. In the aspect of harmonic reduction, amulti-pulse inverter configuration is sometimes preferred,despite higher component count, due to reduced switchinglosses, fault tolerance and the absence of filters.812

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