New cycloconvertor for power-systemhigh-frequency links

M.K. Forster, B.E., Mem.I.E.E.E. and J.T. Boys, M.E., Ph.D.

Indexing terms: Convenors, Power transmission and distribution, Cycloconvertors, High-frequency links

Abstract: The recently developed high-frequency link or power-system intertie incorporates two naturallycommutated cycloconvertors (NCCs) with a high-frequency passive resonating tank circuit. A new circuitconfiguration of thyristors for the NCC has been developed which achieves improved operating character-istics compared to the conventional NCC. The theoretical and practical results of the new NCC showimproved output voltage distortion and reduced input reactive loading compared to the standard NCC, bothof which are advantageous when used in high-frequency links. Implementation of the new NCC is discussedtogether with a critical comparison between the two types.

List of symbols and abbreviations

± V

1 phase

tphase harm

'phase react

I phase real

iRMS

m,n,p,q,uNCCr

SCRtVfund

a

etc. = coefficients of Fourier series usedfor spectrum of input current tonew cycloconvertor

= coefficients of Fourier series used forspectrum of output voltage from newcycloconvertor

= average DC output voltage of the 'fly-wheel' SCR NCC for the particular'firing' angle a, depending on the out-put voltage polarity V, and outputcurrent polarity /

= assumed peak sinusoidal output phasecurrent of 'flywheel SCR' NCC

= instantaneous input current for asingle-phase output circulating current-free 'flywheel SCR' NCC

= unwanted frequency components in1 phase

= fundamental reactive component of* phase

= fundamental real component of iphase= RMS input current of 'flywheel SCR'

NCC= integers= naturally commutated cycloconvertor= output voltage ratio between the maxi-

mum fundamental sine-wave output tothe required fundamental sine-waveoutput (i.e. 0 < r < 1)

= silicon-controlled rectifier= time= required fundamental component of

output phase-to-neutral voltage wave-form

= instantaneous output phase voltagewith required fundamental componentof Vfund

= peak input phase voltage to NCC= 'firing' angle, in radians, for particular

phase delayed from the line voltagezero crossover (i.e. blue-red linevoltage for red phase etc.)

= cos"1 [1 —\r sin

Paper 2470C (P6, P9), received 4th January 1983The authors are with the Electrical Engineering Department, School ofEngineering, University of Auckland, Private Bag, Auckland, NewZealand

output load displacement angle, i.e.fundamental power-factor angle (note:lagging angle if positive)input angular frequency to NCCoutput angular frequency of NCC

Introduction

A more recent development in power-system interties, com-pared to high-voltage direct-current (HVDC) transmission, isthe high-frequency AC (HFAC) link or intertie which has beenadvanced by Gyugyi and Pelly [1], and Espelage and Bose [2]for control and conditioning of high powers (i.e. 500kW andabove). The link enables power transfer between two variable-frequency (asynchronous), variable-voltage polyphase AC(or DC) sources with full four-quadrant operation, whilecontrolling reactive power flow at each end of the link, andmaintaining ohmic isolation between the two interconnectedsystems at the intermediary high frequency. These character-istics are not attainable with conventional power systeminterties such as HVDC links. The applications of the HFAClink [3, 4] are almost limitless, and include large AC motorspeed control, power-system interconnection, and reactive,real and harmonic power control and compensation.

7.1 Operation of HFAC LinkThe HFAC link consists of two naturally commutated cyclo-convertors (NCCs) connected back-to-back with a high-fre-quency single-phase or polyphase passive resonant tank circuit(or high-frequency base) as illustrated in Fig. 1. The input ofeach NCC is connected to the high-frequency (HF) base,while each output is connected to a low-frequency powersource or load.

The NCC is a static frequency changer with bidirectional

polyphaseinput

polyphaseHFbase

polyphaseoutput

out in-o-NCC

in outrPrn

NCC

0HF passive resonanttank circuit

Fig. 1 General form of an HFAC link

IEEPROC, Vol. 130, Pt. C, No. 3, MA Y 1983 111

power capability. It contains thyristors or SCRs as the staticswitching elements. The NCC commutates (or switches) onthe high-frequency input (thus consuming reactive power) toproduce a lower frequency output. A practical limitation ofthe NCC is that the output/input frequency ratio must be lessthan unity, and the smaller the ratio the smaller the outputvoltage distortion.

The two NCCs in the HFAC link are commutated by theHF base and thus reactively load the tank circuit, increasing itsoscillatory frequency. By controlling the input/output powerbalance of the link, a small amount of power is inserted intothe tank circuit to maintain oscillation and to regulate thetank voltage. As long as the tank circuit losses are supplied andoscillation maintained, almost any amount of power can betransferred through the HFAC link from one NCC to the other.

1.2 New modified NCCThe simplest NCC to use in an HFAC link is a midpoint (orhalfwave) cycloconvertor discussed by Pelly [5]. Hence fora three-phase HF base a standard three-pulse NCC would beused. (The simplest form of NCC is used to reduce the numberof thyristors in the HFAC link configuration.) To improvethe operating characteristics of the HFAC link, the outputvoltage distortion of the two NCCs must be reduced and thisnecessitates either increasing the HF base frequency (i.e.improving the input/output frequency ratio of the NCCs),increasing the pulse number of the NCCs, or increasing thenumber of HF base phases. These improvements have associ-ated difficulties and drawbacks.

Increasing the HF base frequency increases standing lossesof the tank circuit, and the cycloconvertors owing to theincreased switching rate. Available turn-off times for thethyristors are also reduced which necessitate the use of invertergrade thyristors instead of line commutated devices. Employingsix- or twelve-pulse NCCs, instead of three-pulse, requires theuse of either input or output transformers for each NCC toproduce the necessary phase shift and isolation. Input trans-formers would be preferable to output transformers because,at the high frequency operation, they would be smaller in sizeand more economical. Increasing the number of phases in theHF base increases the complexity of the HF passive tank circuit,possibly reducing its stability while greatly increasing thenumber of thyristors in each NCC. Thus, reducing the outputdistortion of the NCCs in the HFAC link, in comparison tothe three-phase three-pulse NCC system, requires a considerableincrease in complexity and expenditure.

A modified version of the three-pulse NCC has been devel-oped to improve the output voltage distortion of the HFAClink, while maintaining a three-phase tank circuit withoutinput transformers for the NCCs. The new NCC uses a 'fly-wheel SCR' concept requiring a neutral connection from theHF base and two extra thyristors for each NCC output phase.The modified NCC also has reduced input reactive loading;thereby decreasing the frequency variation of the tank circuitto within tighter limits.

In this paper, the operation and method of control of themodified NCC are described, followed by a theoretical com-parison between the new NCC and the conventional three-pulse NCC. Test measurements are used to verify theoreticalpredictions and to establish the operational limits and penal-ties for using the new NCC compared to the standard three-pulse NCC.

2 Operation and control of 'flywheel SCR' NCC

The idea of 'flywheeling' in a thyristor convertor is not a newidea. Pelly [5] gives the characteristics of half-controlledthyristor rectifiers, using a flywheel diode to obtain one quad-

112

rant operation. The advantages of the flywheel diode arereduced output-voltage distortion, reduced input reactivecurrent, and reduced RMS input current. Farrer and Andrew[6] expanded the 'flywheel' idea to a single-phase bridgefully controlled convertor, where each leg of the bridge (twothyristors in series) are used for flywheeling load currentwhile the supply is effectively disconnected. This configurationenables two quadrant operation with 'flywheeling'. Drury,Farrer and Jones [7] compared 'flywheeling' for both single-phase and three-phase fully controlled bridge convertors tonormal, and asymmetric (or sequence) control of the thyristors.Stefanovic [8] proposed the insertion of two extra thyristorsin the three-phase bridge convertor, together with a neutralconnection from the supply for 'flywheeling'. Finally,Takahashi, Akagi and Miyairi [9] proposed four-quadrantcycloconvertor operation using a single or three-phase bridgeconfiguration similar to that of Farrer and Andrew [6].However, as mentioned previously, the cycloconvertor orNCC used for HFAC links should be a simple three-pulse orequivalent type NCC which does not require input transformers.Hence a different 'flywheeling' cycloconvertor or NCC wasproposed by Forster, Boys and Woodward [10], which is athree-phase three-pulse NCC (see Fig. 2a) with two extra'flywheeling' thyristors as shown in Fig. 2b.

3-phaseinput

No-

singlephaseoutput

Fig. 2 Circuit configurations of a standard three-phase to single-phase, three-pulse NCC (a), and a 'flywheel SCR' NCC (b), with optionalcirculating current reactors

2.1 Control strategy of 'flywheel SCR' NCCThe 'flywheel SCR' NCC modifies the basic NCC by includingthe neutral input to the cycloconvertor as a 'fourth phase'.The neutral thyristors act as flywheel thyristors because theyconnect directly across the single-phase load output, as seen inFig. 2b, ignoring the optional circulating current reactor. Thecontrol strategy for the 'flywheel SCR' NCC achieves both asinusoidal output-voltage waveform and a noncirculatingcurrent mode of operation. Both of these characteristics arepreferred for large power NCCs to reduce distortion and toeliminate the circulating current reactor thereby reducing thesize of the thyristors needed for a given output rating.

The control strategy used with the 'flywheel SCR' NCCallows for natural commutation of the thyristors from thephase to neutral, and subsequently neutral to phase. Theneutral thyristor is turned 'on' or 'fired', whenever the instan-taneous output-voltage polarity becomes opposite to that ofthe required average output voltage. Hence, during a positivehalfcycle of the NCC output-voltage sinusoidal waveform, ifthe instantaneous output voltage becomes negative the neutralthyristor is fired, resulting in the instantaneous output voltagebecoming zero, not negative. This process can be readily seenin Fig. 3b, which shows a computer simulated sinusoidal

IEE PROC, Vol. 130, Pt. C, No. 3, MA Y 1983

output waveform of the 'flywheel SCR' NCC. The instan-taneous output voltage is never negative during the positivesinusoidal output voltage halfcycle, and never positive duringthe negative halfcycle.

T

Fig. 3 Theoretical phase-to-neutral output voltage waveforms of thenoncirculating current standard three-pulse NCC (a), and 'flywheelSCR'NCC (b)

The 'operating' conditions of both NCCs are 50 Hz input frequency,8 Hz output frequency with 30° lagging phase angle at full outputvoltage ratio r of l .0.

The linearisation of the cycloconvertor process is achievedin the standard NCC by use of the 'cosine wave crossing'control method, as discussed by Pelly [5]. For the new NCC,the switching process is different and new linearisation ormodulating functions are required, as shown in Appendix 7.1.The modulating functions, described by eqns. 5 and 7, areplotted in Fig. 4 for one input phase. These functions deter-mine the various phase thyristor firing points when comparedwith the required output-voltage waveform.

The resulting idealised operation of the 'flywheel SCR'NCC producing a single-phase sinusoidal output is illustratedin Fig. 5. Fig. 5a shows the three-phase input sine waves,with the corresponding modulating waves shown as brokenlines (as illustrated in Fig. 4) for the 'flywheel SCR' NCC.Each input phase has its respective positive or negative output-current modulating wave. The intersections of the modulatingwaves with the required low:frequency output-voltage wave-form in Fig. 5b give the various turn-on and turn-off times ofthe phase SCRs, indicated by vertical broken lines. The assumed

Fig. 4 Positive and negative output-current modulating functions fora particular input phase voltage

positive output-current modulating function9/4 n

negative output-current modulating function I —\ 9 / 4 7T

output-current waveform is shown as a broken line in Fig. 5b.Figs. 5c—e show the 'on' and 'off times of the red, yellow andblue phase SCRs, respectively. (The 'on' time is represented asa high level, and the 'off time as a low level.) Fig. 5/givesthe 'on' and 'off times of the neutral SCRs. (The neutral SCRis 'on' whenever all phase SCRs are 'off.) The resulting single-phase output-voltage waveform of the 'flywheel SCR' NCCis shown in Fig. 5g. The time average between each phase-to-phase SCR conduction of the output-voltage waveform is therequired sinusoidal output waveform in Fig. 5 b.

Fig. 3 compares the output-voltage waveforms of the stan-dard three-pulse NCC to the 'flywheel SCR' NCC for a single-phase sinusoidal output.

\J\i\jFig. 5 Production of single-phase output voltage waveform (g) of thecirculating current-free 'flywheel SCR' NCC from three-phase inputsine waves (a)

2.2 Practical implementation of the 'flywheel SCR' NCCTo implement the 'flywheel SCR' NCC in Fig. 2b with thy-ristors, while operating with no circulating current requiresnatural commutation of the thyristors, together with a stra-tegy for obtaining bank selection (i.e. current crossoverdetection) which is hindered by the extra neutral SCRs.

2.2.1 Natural commutation in 'flywheel SCR' NCC: Naturalcommutation of the SCRs from phase-to-neutral, neutral-to-phase and phase-to-phase (for the particular case of a = n/6or 3TT/2) requires a slight modification to the strategy given inSection 2.1. However, the modification does not require alter-ation of the modulating function already derived.

A study of Figs. 5g and 3b shows that, for the positive-current bank of thyristors, natural commutation does notoccur when the required output voltage is negative and commu-tation from neutral to a phase SCR is required. For the negative-current thyristor bank natural commutation does not occurfrom the neutral to phase SCRs when the required outputvoltage is positive. In both these cases, natural commutationcan be achieved by prefiring the phase thyristor by 10—20° toenable commutation to occur, and allow sufficient time for

IEEPROC, Vol. 130, Pt. C, No. 3, MAY 1983 113

phase

'phase

Fig. 6 Oscilloscope trace photographs of instantaneous DC outputvoltage produced by 'flywheel SCR' NCC

In each photograph the top two traces are the output DC voltage andone input phase voltage of the NCC, while the lower trace is the inputcurrent. Photographs a, c and e illustrate positive output current, whileb, d and / illustrate negative output current. Photographs a and b showpositive output-voltage production, c and d show negative voltageproduction, and e and / show zero voltage production. (The prefiringangle is approximately 20°.)

the thyristor to regain its forward blocking capability. Tosimplify control of the 'flywheel SCR' NCC and to yield asymmetrical transfer function between positive and negativeoutput voltage for both positive and negative output current,all phase thyristors are prefired when the required outputvoltage and current are of opposite polarity; and the neutralthyristors are delayed from firing by the same interval whenthe required output voltage and current are of the same polarity.This commutation process is illustrated in Fig. 6 by actualwaveforms taken from a 'flywheel SCR' NCC producing DC ofpositive and negative output voltage for positive and negativeoutput current.

The change in the required output voltage due to this 'pre-firing' is almost insignificant, and hence modification of themodulating function is unnecessary. However, the volt-secondintegral of the prefiring must be sufficient to allow commu-tation of the load current to occur with a supply of finiteimpedance.

2.2.2 Zero-current detection for bank selection in 'flywheelSCR' NCC: To achieve bank selection for the circulatingcurrent-free mode of operation of the 'flywheel SCR' NCC, asuitable method of zero-current detection must be used. Threebasic methods of current-zero detection have been described inthe literature:

(a) sensing the voltage across all convertor thyristors (lowthyristor voltage indicates thyristor conduction) [11—15].

(b) sensing current flow through an auxilliary thyristor ordiode in series with the output [15, 16]

(c) sensing the gate-cathode voltage for each convertorthyristor (positive bias indicates thyristor conduction) [17].

The zero-current detector for the 'flywheel SCR' NCC usesboth methods a and b above. Initial detection of zero outputcurrent is obtained by monitoring the current through a series

O(L-L)

Fig. 7 Typical AC load output voltage and current oscilloscope wave-forms for the new 'flywheel SCR' NCC

Photographs a and b illustrate output voltage (top trace) and outputcurrent (lower trace) for purely resistive, and resistive-inductive single-phase loads, respectively. Photographs c—f illustrate phase-to-neutraloutput voltage (top trace), line-to-line output voltage (middle trace),and output line current (lower trace) for a three-phase delta-connectedinduction machine load running at full load torque at 5, 10, 15 and20 Hz, respectively, for 50 Hz input into the cycloconvertor.

diode in the output of each thyristor bank. Once zero currentis detected, the voltage across each phase SCR (except theneutral SCR) is monitored. A true zero-current condition isindicated by zero-current detection in the phase thyristors andin the series diode.

Bank selection and current crossover methods from thepositive to negative current banks have been explored by Pelly[5], Tso and Law [18], and Hamblin and Barton [11], forstandard cycloconvertors. The methods include bank cross-over at the first current zero, crossover at the current zeronearest the fundamental load current crossover, and crossoverat the current zero nearest the calculated crossover pointknowing the load power factor. For resistive/inductive cyclo-convertor loads (such as induction motors), the simplestcurrent crossover method is to change banks at the first currentzero after the required output-voltage polarity has changedsign. This method is equally applicable to the 'flywheel SCR'NCC with resistive/inductive loads. However, for power-systemcycloconvertor loads, the power factor of the output may belagging or leading, so that the previous method is unsuitable.Provided that the frequency is constant, it is a simple matterto measure the AC output current and pass it through a phase-lead filter to obtain an advanced fundamental current wave-form. Current crossover occurs when a true current zero isdetected after the advanced fundamental filtered currentcrosses through zero.

Various AC single-phase output-voltage and current wave-forms, for passive and active loads using the methods ofcommutation and bank selection for the 'flywheel SCR'NCC outlined in the preceding text, are given in Figs. 7 and 8.These photographs show the bank selection and changeoverfor passive resistive and inductive loads, together with leadingand lagging active power-system loads. Note also the 'pre-firing' of the thyristors for natural commutation.

114 IEE PROC, Vol. 130, Pt. C, No. 3, MA Y1983

I phase real is:

hhase real =

Fig. 8 Oscilloscope waveforms of voltage and current with the outputof the 'flywheel SCR' NCC (input frequency is 400 Hz) connected to anactive 50Hz power system

The top trace of each photograph is the 50 Hz phase-to-neutral outputvoltage waveform produced by one phase of the NCC, the middletrace is the 50 Hz phase-to-neutral voltage of the same power-systemphase, and the lower trace is the current flowing from the power systeminto the NCC for the particular phase shown. Photographs a and b showpower (at unity power factor) flowing out of and into the powersystem, respectively. Photographs c and d show reactive power (at zeropower factor) flowing into and out of the power system, respectively.

2.3 Theoretical relations for 'flywheel SCR' NCCUsing the circulating current-free mode of operation for the'flywheel SCR' NCC, the expression for the single-phase outputvoltage VQ is given in Appendix 7.2. In this expression (i.e.eqn. 8), the required output fundamental-frequency com-ponent Vfund is:

fund4TT -rsm (0

The rest of eqn. 8 represents the unwanted output frequencycomponents or distortion components, which can be groupedinto the following frequency spectra:

3(2« - 1)01 ± 2mdo

and

6«0,±(2m + 1)0O

where n = 1,2, 3 , . . .ra = 0, 1 , 2 , 3 , . . .

The amplitudes of these components are independent of theinput and output frequencies 0f and 0O, but depend on theoutput-voltage ratio r and output displacement angle 0. If theoutput of the NCC is three-phase with no neutral connection,some of the above frequency components cancel (i.e. zerosequence components) yielding the following reduced set ofoutput line voltage frequency components:

3 ( 2 w - l ) 0 , ± ( 3 ( m + 1)± 1)0O

and

6«0 f ±(6m± 1)0O

where « = 1, 2, 3 , . . .m = 0, 1 ,2,3, . . .

The expression for the NCC input current with a single-phasesinusoidal output current is also given in Appendix 7.2. Theresulting real component of the fundamental input current

IEEPROC, Vol. 130, Pt. C, No. 3, MAY 1983

3/or—

47T

COS 0 Sin 0,f (2)

Also, the reactive component of the fundamental inputcurrent iphase react is:

2 / r °° i 1hhase react = ~Y COS 0(t • X asl2u 2 _ ' C°S 2u0

(3)where flsi2u is defined in eqn. 9.

The remaining terms in eqn. 9 are the frequency distortioncomponents which can be grouped into the following spectra:

et ± 20O

(2/i + 1)0O

{2n+\)di±2m6o

2(n+ l)0i±(2m+ 1)0 o

where n = 0, 1, 2, 3, . . ./w=0, 1,2,3, . . .

The special distortion components 0,- ± 20o are necessary forany three-phase/single-phase frequency changer or cyclo-convertor to produce the oscillatory power component of20o in any single-phase load.

For an NCC with a three-phase output, some of the aboveinput frequency components cancel, yielding the resultingspectra:

3(2/1+1)0*

(2/i+ l)6t±6m6o

2(n+ l)di±3(2m+ 1)6 o

where « = 0 ,1 ,2 ,3 , . . .m = 0 , 1,2,3, . . .

As for the output-voltage frequency components the input-current component amplitudes are independent of the inputand output frequencies 0,- and 0O, but depend on the output-voltage ratio r and output displacement angle 0. Note that theamplitudes of the three-phase output unwanted frequencycomponents compared to the single-phase output componentsare three times larger for the input currents and V3 timeslarger for the output line voltages.

2.4 Comparison and results of 'flywheel SCR' NCC com-pared to standard NCC

Initial comparison between the output-voltage distortion ofthe 'flywheel SCR' NCC and the standard NCC, by consider-ation of Fig. 3, shows the 'flywheel SCR' NCC has less dis-tortion. Fig. 3 shows that the ripple at the fundamental zero-voltage crossover is negligible for the new NCC compared tothe very large ripple for the standard NCC. The required sinu-soidal output voltage for the new NCC is not distorted bydiscontinuous load currents (caused by such loads as resistiveloads), from which the standard NCC suffers, owing to fre-quent polarity reversal of the instantaneous output voltage.

A further comparison between the output-voltage distor-tion caused by the two types of NCC is achieved by calculatingthe amplitudes of the unwanted frequency components ineqn. 8, and comparing the values with the correspondingcomponents for the standard three-pulse NCC, both with nocirculating current. This comparison is difficult as the possible

115

variables are so numerous; but, in general, at high values ofthe output-voltage ratio r (i.e. 1.0—0.6) the unwanted com-ponents are approximately equivalent, whereas for small rvalues (i.e. 0.5-0.1) the 'flywheel SCR' NCChas significantlysmaller unwanted frequency components. In particular, thecomponents are generally smaller than the fundamental com-ponents for the new NCC, whereas the components in thestandard NCC can be larger than the required output wave-form. An example showing the significant unwanted frequencycomponent magnitudes for various r values for the two NCCs,operating with a 10 : 1 frequency transformation, is shown inFig. 9. The heavy lines indicate the theoretical values and thepoints in Fig. 9b represent measured values. The agreementbetween measurement and theory is veiy good, consideringthat theory did not take into account 'prefiring' of the thy-ristors for natural commutation, input source impedance, ora finite time for current crossover to occur. It must be notedthat r — 1.0 in the new NCC produces a lower output voltage,compared with the standard NCC, owing to the firing-angleminimum being set at a = n/6, instead of a = 0 as in thestandard NCC. For this reason, the new NCC is particularlysuited for production of low output-voltage ratios r with lowdistortion, whereas the standard NCC would be better suitedfor production of large r values.

Slow, variable-speed drives using induction or synchronousmachines would be particularly suited for the use of the'flywheel SCR' NCC. Low output distortion at low voltages isessential for smooth operation of machines at low speeds,otherwise torque pulsations occur. These conditions are easily

0.8- fundamental (50Hz)

0.2 0.4 0.6output voltage ratio, r

0.8 1.0

—1500Hz— 1400,1600Hz

1300,1700Hz2750,3250Hz3150,2850Hz3050,2950Hz

a. oE a.

0.5

£0.4d:-• 0.3

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fundamental (50Hz)

1500Hz

'— 1400,1600Hz1300,1700Hz2950.3050Hz2750.3250 Hz

!— 2850,3150Hz

0.2 0.4 0.6 0.8output voltage rat io, r

1.0

Fig. 9 Theoretical significant unwanted single-phase output voltagefrequency components of standard circulating current-free three-pulseNCC (a), and the 'flywheel SCR' NCC (b) as a function of the outputvoltage ratio r

Measured results are plotted in b for the new NCC to show agreementwith theory. (The prefiring angle for the measured results was 6°.)The operating conditions of the NCCs are SOOHz input frequency,50 Hz output frequency with a 0° load displacement angle, whileassuming continuous load current for the standard three-pulse NCC.

satisfied with a 'flywheel SCR' NCC, whereas the conventionalthree-pulse NCC produces significant distortion at low outputvoltages necessary for slow-speed operation. A six-pulse NCCwould be better suited for machine drives using standardNCCs, however, the new 'flywheel SCR' NCC is a moreeconomic solution. Fig. 7 shows typical current and voltagewaveforms with the new NCC driving a fully loaded inductionmachine at various speeds.

Considering the comparison between the two NCCs for theinput current characteristics, only the input reactive loadingand input harmonic distortion need be considered. The inputreal current component is simply determined by the load onthe cycloconvertor output and does not depend on the func-tion of the cycloconvertor 'switches'. The quadrature inputcomponent of an NCC does depend, to some extent, on theoutput reactive current requirement, and also on the switchingprocess employed in the NCC. The difference in the quadraturecomponent of current between the 'flywheel SCR' NCC(calculated from eqn. 3) and the standard NCC, as a functionof the r ratio and the load displacement factor, is plotted inFig. 10. The load displacement factor is the cosine of the anglebetween the fundamental input current and the fundamentalinput real component (i.e. cos 0). As can be seen in Fig. 10,the major advantage of the 'flywheel SCR' NCC is that itconsumes substantially less reactive input current than thestandard NCC. It is worth noting that the standard NCCs reac-tive input loading is independent of the pulse number; i.e.the reactive current requirement of a standard NCC is a funda-mental requirement of all standard NCCs. Thus, the new NCChas improved input reactive loading compared to all the variouspulse number standard NCCs.

To obtain the measured reactive input component, forcomparison with theoretical predictions, the output currentmust approach a sinusoidal waveform, as assumed for thetheoretical input-current calculations. The results plotted inFig. 10c were obtained by using an HF supply as the input ofthe new NCC, and the output (at 50 Hz) was connected to the50 Hz mains supply via a 50 Hz tuned series resonant LC filter.This achieved a sinusoidal load current with a variable loaddisplacement factor obtained by simply changing the phaseand amplitude of the NCC output voltage with respect to theAC mains supply. Typical output current waveforms, usingthis configuration, are shown in Fig. 8. (These are typicalinput characteristics achievable with the HFAC link connectedto a 50 Hz power system.)

Finally the input current distortion terms need to be com-pared for the two types of NCC. Fig. 11 shows the significantinput current unwanted frequency components for a 10 : 1frequency ratio between input and output, assuming a single-phase output only. The frequency components with anasterisk (*) are not present if the output is three-phase. Meas-ured results for the new NCC are also plotted in Fig. 1 \b.As can be seen in Fig. 11, the distortion terms are generallylarger for the new NCC than the standard NCC. However, theRMS input current iRMs> defined by eqn. 4 below, plottedin Fig. 12 for both NCCs shows that the new NCC has reducedinput RMS current.

*RMS — real ' 'phase react "" 'phase harm ( v

where iphase harm are the unwanted frequency components inthe input current. The above results imply that the new NCChas greatly reduced fundamental input current due to areduced reactive component, with slightly higher input distor-tion components than the standard NCC.

In summary the inclusion of two 'flywheel SCRs' into thestandard three-pulse, three-phase/single-phase NCC givesgreatly improved output-voltage distortion, and reduced input

116 /EE PROC, Vol. 130, Pt. C, No. 3, MAY 1983

reactive loading at the expense of increased input-currentdistortion. The 'flywheel SCR' NCC does not suffer fromdiscontinuous load currents, as does the standard NCC, whichimproves the operational characteristics of the NCC, especiallyfor machine drives. The new NCC also has reduced input RMScurrent compared to the standard NCC.

3 Advantages of using 'flywheel SCR' NCCs in HFAC links

The relative advantages and disadvantages of the 'flywheelSCR' NCC compared to the standard NCC have been given inthe preceding Section. However, when implementing the 'fly-wheel SCR' NCC into an HFAC link the above mentionedadvantages are particularly relevant to the link, whereas thedisadvantages are somewhat negated by the link's operationand characteristics.

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factor

Fig. 10 Theoretical reactive input-current component requirement ofthe standard NCC (a), and the 'flywheel SCR' NCC (b), together withthe measured results for the 'flywheel SCR'NCC (c)

The input reactive current is plotted as a function of the output voltageratio r, and the load displacement factor (cos 0) for positive and nega-tive throughput power. The measured results were taken with a pre-firing angle of 16°. The tolerance of the results plotted in (c) enablethe dashed lines to be drawn to indicate the trends.

The 'flywheel SCR' NCC requires a neutral input connec-tion, whereas the standard NCC does not necessarily requireone. The HF base, which would normally consist of a three-phase resonant tank containing inductors and capacitors inparallel resonance, would contain a star point or neutral. Thedisadvantage of the 'flywheel SCR' NCC requiring a neutralconnection is therefore irrelevant for the HFAC link, becausethe tank circuit would contain one, whether or not the NCCsrequire a neutral.

Two extra thyristors are required for each three-phase/single-phase 'flywheel SCR' NCC compared to the standardthree-pulse NCC. However, the standard three-pulse NCC hasgreater output-voltage distortion than the 'flywheel SCR'NCC.To improve the output distortion using standard NCCs necessi-tates using a higher pulse number NCC: e.g a six-pulse bridgeNCC, containing six more thyristors than the three-pulse NCCand requiring input or output transformers for isolation. Thetwo extra thyristors required for the 'flywheel SCR' NCC toobtain improved output distortion are inexpensive comparedto the higher cost of a conventional six-pulse NCC.

The input-current distortion of the new NCC is unimportantwhen considering its use in the HFAC link. The injected har-monic components only affect the passive resonating tankcircuit, but do not affect the input or output currents of theHFAC link. (The input or output currents of the link can benearly sinusoidal as indicated by the photographs in Fig. 8.)The tank circuit filters the NCC input current harmonics. As

1.0

§ §0.8

Q.

itude

ampi

0

3

DO.6Q.C

2 0.4ca>E

1 5 0.2E c

o ^1£. O

fundamental-(500Hz)

400,600Hz

850,1150Hz1950,2050Hz2500 Hz

0.2 0.4 0.6output voltage ratio , r

0.8 1.0

I.ir

I^\fundamental*)\(500Hz)

50 Hz

950.1050Hz

25OOHz1500Hz1950,2050Hz

,1600Hz0.2 0.4 0.6 0.8

output voltage ratio, r

Fig. 11 Theoretical significant unwanted input-current frequencycomponents for single-phase output of standard circulating current-free three-pulse NCC (a) and 'flywheel SCR' NCC (b) as a function ofoutput voltage ratio r

Measured results for the 'flywheel SCR' NCC are plotted in b, togetherwith measured result trends, shown dashed. The operating conditions ofthe NCCs are 500 Hz input frequency, 50 Hz output frequency with 0°load displacement factor (assuming continuous load current for thethree-pulse NCC), and a prefiring angle of 16° for the measured results.(The results marked with * are not present if the output of the NCC isbalanced three phase.)

IEEPROC, Vol. 130, Pt. C, No. 3, MAY 1983 117

long as the current distortions are not too significant, the tankcircuit oscillation will be unaffected, and thus the performanceof the HFAC link is not degraded by the new NCC's inputcurrent distortion.

As can be seen in Fig. 10 the major advantage of the'flywheel SCR' NCC is that it requires substantially less reac-tive input current than the standard NCC (independent ofpulse number). As a consequence, the frequency variation ofthe HFAC link using the new NCC is drastically reduced,because the reactive loading on the passive resonant tankcircuit is less than for the standard NCC. (Reactive loading onthe HF passive resonating tank will increase its oscillatoryfrequency.) This enables the HF tank resonating inductors tobe reduced in size for the new NCC, while the tank frequencystill lies within the set maximum and minimum operatingfrequencies for adequate performance of the HFAC link andits NCCs. (The tank resonating capacitors are fixed in value bythe required loaded quality factor of the tank for maintenanceof tank oscillation during changes in the link's throughputpower.) In fact, it is possible with the new NCC to reduce the

1.15r

balancedthree phaseoutput

single phaseoutput

output loaddisplacementfactor

. "0 i0.20 (leading)

0.9

0.8

10.6 ±1.0 ±0.6 ±0.2 0 output load(lagging) d j S p ( a c e m e n t

factor

0 i0.2 ±0.6 HO ±0.6 ±0.2 0 o u l p u t , o a d

(leading) (lagging) displacementfactor

Fig. 12 Theoretical RMS input current per unit of RMS outputcurrent for standard three-pulse NCC (a), the 'flywheel SCR'NCC withsingle-phase output (b), and three-phase output (c) as a functionof output load displacement factor and output voltage ratio r

tank inductors, while also increasing the minimum tank fre-quency (for improved HFAC link output distortion), andreducing the maximum frequency (for reduced thyristorswitching losses and longer thyristor recovery times).

The 'flywheel SCR' NCC is particularly suited for produc-tion of low output distortion single phase, whereas the stan-dard NCC single-phase output contains large harmonic com-ponents, often larger than the fundamental (see Fig. 9). OneHFAC link application is production of single phase fromthree phase for traction; hence the new NCC is best suited foruse in this HFAC link. For the three-phase standard NCCoutput, the above mentioned large harmonic componentscancel in the line-to-line output voltages. In a delta (or 3-wire)system this is advantageous, however, in a star (or 4-wire)system these components are present as a zero-sequence orcommon-mode component. The 'flywheel SCR' NCC hasgreatly reduced output harmonic components and is thereforesuited for HFAC links with single- or three-phase outputscompared to the standard NCC. In fact, as the 'flywheelSCR' NCC is able to support single-phase loads with low outputdistortion, it is able to support weak, unbalanced powersystems when used in a HFAC link.

A disadvantage of the 'flywheel SCR' NCC is the availableturn-off times for the thyristors; these times are governed bythe 'prefiring' angle. This 'prefiring' interval allows bothcommutation of the thyristors and the regaining of the thy-ristor forward blocking capability. In the HFAC link, thethyristors commutate on the HF tank which has a very lowtransient source impedance for the NCCs. Therefore, commu-tation time for the thyristors in an HFAC link is minimal andthe 'prefiring' time becomes the effective allowable thyristorrecovery time. For a maximum HF tank frequency of 500 Hzand a 'prefiring' of 20°, the allowable thyristor recovery timeis approximately 100jus. The 'flywheel SCR' NCC wouldprobably require inverter grade thyristors for the HFAC link,however, with the rapid technological advance with thyristors,high-power inverter grade thyristors would not pose a problem.It must be noted that standard NCCs would not requireinverter grade thyristors owing to the large commutationmargin allowable compared to the new NCC.

4 Conclusions

To improve the performance characteristics of the HFAC linkpower-system intertie, without reverting to higher pulse num-ber configuration NCCs and input phase shifting transformers,the 'flywheel SCR' NCC is proposed.

The new 'flywheel SCR' NCC is shown to have reducedoutput-voltage distortion and considerably less reactive powerinput than the standard NCC. It also has no tendency to dis-tort the output-voltage waveform with discontinuous loadcurrents — as does the standard NCC. However, the 'flywheelSCR' suffers from increased input-current distortion which isunimportant when considered for application in HFAC links.

The advantages of the new NCC have to be evaluatedagainst the disadvantages of increased cost and increased con-trol complexity. The 'flywheel SCR' NCC requires two extrathyristors per output phase than the standard three-pulse NCC;but this is a small cost increase compared with using six-pulseNCCs for the HFAC link. However, the new NCC requiresinverter grade thyristors, whereas the standard NCC can useline commutated devices. Present thyristor technology isreducing the price gap between inverter grade and standardthyristors. The increased control complexity is due to thedifferent modulating waveform necessary for control linearis-ation, together with the different bank selection and currentcrossover required.

118 IEE PROC, Vol. 130, Pt. C, No. 3, MA Y 1983

5 Acknowledgments

M.K. Forster gratefully acknowledges the support of the NewZealand Electricity Division of the Ministry of Energy whilepursuing postgraduate research at Auckland University.

6 References

1 GYUGYI, L., and PELLY, B.R.: 'Static power frequency changers- Theory, performance and application' (John Wiley & Sons, NewYork, 1976)

2 ESPELAGE, P.M., and BOSE, B.K.: 'High-frequency link powerconversion', IEEE Trans., 1977, IA-13,pp. 387-394

3 GYUGYI, L., and CIBULKA, F.: 'The high-frequency base con-verter — A new approach to static high-power conversion', ibid.,1979, IA-15, pp. 420-429

4 FORSTER, M.K., and BOYS, J.T.: 'Applications of new high powerelectric convenors' (to be published) (only when paper has beenaccepted)

5 PELLY, B.R.: 'Thyristor phase-controlled converters and cyclo-converters — Operation, control and performance' (Wiley & Sons,New York, 1971)

6 FARRER, W., and ANDREW, D.F.: 'Fully controlled regenerativebridges with half-controlled characteristics', Proc. IEE, 1978, 125,(2), pp. 109-112

7 DRURY, W., FARRER, W., and JONES, B.L.: 'Performance ofthyristor bridge convertors employing flywheeling', IEE Proc. B,Electr. Power Appi, 1980, 127, (4), pp. 268-276

8 STEFANOVIC, V.R.: 'Power factor improvement with a modifiedphase-controlled converter',IEEE Trans., 1979, IA-15, pp. 193-201

9 TAKAHASHI, I., AKAGI, H., and MIYAIRI, S.: 'Improvement ofcycloconverter power factor via unsymmetric triggering method',Electr. Eng. Jpn., 1976, 96, pp. 88-94

10 FORSTER, M.K., BOYS, J.T., and WOODWARD, J.L.: 'A thy-ristor controlled high frequency link power system intertie', J.Electr. & Electron. Eng. Aust., 1981, l,pp. 177-183

11 HAMBLIN, T.M., and BARTON, T.H.: 'Cycloconverter controlcircuits',IEEE Trans., 1972, IA-8, pp. 443-453

12 FORD, J.S.: 'Current-zero detection in thyristor convertors',Electron. Lett., 1973, 9, (4), p. 73

13 DAVIS, R.M., and DOWNING, B.R.: 'Zero-voltage detection inthyristor and triac convertors', ibid., 1973, 9, (12), pp. 267-268

14 ALI, A.M.: 'Fast changing four quadrant convertor', Proc. IEE,1977, 124, (10), pp. 883-884

15 MACPHERSON, D.E., and WHITTINGTON, H.W.: 'Zero currentdetection in thyristor circuits'. IEE Conf. Publ 154, 1977, pp.26-28

16 DANIELS, A.R., and SLATTERY, D.T.: 'Zero-current detection inpower convenors', Proc. IEE, 1976, 123,(12), p. 1372

17 ARRILLAGA, J., and HISHA, H.: 'Fast on/off detection of silicon-controlled rectifiers and its influence on convertor controllability',IEEE Trans., 1979, IECI-26, pp. 22-26

18 TSO, S.K., and LAW, H.Y.: 'Converter blanking methods for triaccycloconverters', Int. J. Electr. Eng. Educ, 1980, 17, pp. 155-162

7 Appendixes

7.1 Derivation of 'flywheel SCR' NCC modulating functionNeglecting commutation voltage drop and thyristor turn-off

time, the average positive voltage Ea produced by the positiveoutput current thyristors in Fig. 2b is calculated using thedefinitions given in Fig. 13. The result is

In/3 d{dit)

2iT

•n 5TTassuming — < a < —

6 6

(5)

where a is termed the firing angle, Vn is the peak input phasevoltage, Of is the input angular frequency and t is time.

If a < —, then6

3\/3Ea = — — Vn cos a

z(6)

This is the same linearisation function as obtained for thestandard NCC depicted in Fig. 2a. Thus, for our purposes, andto simplify control of the new NCC, the firing angle a is neverless than n/6, so that two modulating functions are not requiredfor linear operation.

V|

_v ^ - ^ _

Fig. 14 Average negative DC output voltage Ea \+Y produced bypositive output-current thyristors in 'flywheel SCR' NCC (in Fig. 2b)for firing angle a

The instantaneous output voltage is shown as the shaded area.

The similar expression for negative output voltage, positiveoutput current, using Fig. 14, is

9it

Fig. 13 Average positive DC output voltage Ea \++Y produced by

positive output-current thyristors in 'flywheel SCR' NCC (in Fig. 2b)for firing angle a

The instantaneous output voltage is shown as the shaded area.

IEE PROC, Vol. 130, Pt. C, No. 3, MA Y 1983

-v2n

(7)

assuming — ^ a ^ —6 2

7.2 Output voltage and input current expressions for'flywheel SCR' NCC

The equation for the output phase voltage of the 'flywheelSCR' NCC can be developed in a similar way to that of theexpression for the standard NCC by Pelly [5]. Referring toFig. 5, the output voltage is expressed as the product of theexistence functions, as defined by Figs. 5c—/(where 'on' = 1,and 'off' = 0), with the respective phase input voltages andneutral. (This expression neglects input source impedance andassumes continuous output current with instantaneous bankcrossover.)

119

The instantaneous single-phase output voltage Vo, relativeto the neutral, is expressed as:

9VVo = —^ {rsindnt4rr

£ bs6o cos 2pdot = % sin 5/3 - } sin 7j3

oo

I bc3 sm(2q+\)6ot =q = 0

sin dot|sin0or|

sin(30,-2p0o)f]

+, [sin (60, + (2q+ l)0o)f

-s in (60,-(2^7+ l)0o)f]

^cos2]3+ k cos 40

_2_5.7

-sin (120,- -(2<?+ 1)0O)*] +

- cos [(30£ + (2q + 2n + 2)6o)t - (2/i

- cos [(30, - (2q + 2n + 2)6o)t + (2/i + 1)0]

+ cos [(361 - (2q ~ 2n)6o)t - (In + 1)0] ]

OO OO

-si 16.6n[cos[(60,

+ (2p - 2« - l)6o)t + (2n + 1)0]

- cos [(60,- + (2p + 2n+ 1)6o)t - (2n + 1)0]

+ cos [(60,- - (2p + 2n+ 1)6o)t + (2n + 1)0]

- cos [(60,- - (2p ~2n- \)6o)t

(8)

where r is the output voltage ratio, i.e. the ratio of the funda-mental output-voltage amplitude to the maximum fundamentalamplitude achievable, 0O and 0,- are the output and inputangular frequencies of the NCC, respectively, 0 is the outputdisplacement angle, i.e. the lagging power-factor angle betweenthe output fundamental current and voltage, and the '&' coef-ficients are the coefficients of the Fourier series defined by:

bs3 cos2p60t = \ sin 2j3 — \ sin 4/3p

where

0 = cos"1 [1 —\r\ sin0of |]

The instantaneous input phase current iphase f°r a single-phaseoutput current (assumed to be sinusoidal for simplicity) canalso be derived for the 'flywheel SCR' NCC in a similar way tothat performed by Pelly [5]. The result is:

3Ior 31 riPhase = —— COS 0 Sin 6 (t ~ -~~ X

47T O7T

[sin [(0f + 26o)t - 0] + sin [(0, - 20o)f + 0] ]

P = 0

-sin [(2p-\)6ot

tr S «

-sin

s in[ (20 i - (2< 7 - l )0 o ) f -0] ]

oo

+ I «c32n+1[sin[(30,--2«0o)r-0]

sin

sin

- sin [(30, - (2/i + 2)0o)r + 0] ] + . . .

120 IEEPROC, Vol. 130, Pt. C, No. 3, MAY 1983

cos[(di-2qd0)t]]

+ I «C22n+1[cos[(20,.-(2n+l)0o)f]n = 0

t + (2n+l)0o)t]]

_ ]a s i 2 g

T [

+ cos [(dt + (2q - 2u)6o)t + 2w0]

+ cos [(di-(2q-2u)6o)t-2u(p]

+ cos [(Pi ~{2q

[cos

+ cos

- cos [(261 + (2/i

- cos [(20, + (2/1 - 2M 4- i)0o)r + 2M0] ] + . . .

(9)

where Io is the peak current of the assumed sinusoidal funda-mental output current of frequency 6O, and the 'a' coefficients.are the coefficients of the Fourier series defined by:

X a2p cos 2pdot = |3

and

asl2qcos2qd0t = sinj3

as22q cos 2<?(9of = i sin 2/3

and

I ac22n+lsin(2n+\)6ot =n = 0

- c o s 2/3)

IEEPROC, Vol. 130, Pt. C, No. 3, MAY 1983 121