VARIABLE SPEED CONSTANT FREQUENCY

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A Flexible Active and Reactive Power Control Strategyfor a Variable Speed Constant Frequency Generating System

Yifan Tang, Student Member IEEE Longya Xu, Senior M ember IEEEThe Ohio State University

Department of Electrical Engineering2015 Neil AvenueColumbus, OH 43210

Abstract- Variable speed constant frequency generatingsystems are used in wind power, hydro power, aerospaceand naval power generations to enhance efficiency andreduce friction. In these applications, an attractivecandidate is the slip power recovery system comprisingof doubly-excited induction machine or doubly-excitedbrushless reluctance machine and PWM inverters withdc link. In this paper, a flexible active and reactivepower control strategy is developed, such that the op-timal torque-speed profile of the turbine can be followedand overall reactive power can be controlled, while themachine copper losses has been minimized. At the sametime, harmonics injected into the power network has alsobeen minimized. In this manner, the system can functionas both a high efficient power generator and a flexiblereactive power compensator.

I. Introduction

Variable speed constant frequency (VSCF) powergenerating is desirable in many situations. A salient ex-ample is the wind power generation, where turbine speedshould be able to vary according to t he wind speed, suchthat energy efficiency can be achieved with a reduced tor-sional stress and windage friction on the wind mill blades.While in variable speed, the system output voltage shouldbe maintained at a constant frequency to interface withthe power system. Other applications of VSCF includehydro power generation, aerospace and naval power gen-eration.

A promising VSCF generating concept is the slippower recovery system composing of a doubly-excited in-duction machine and power converters. In recent years,many researchers have made contributions to the progressof this concept. Among them, [l] proposed a decoupledactive and reactive power control strategy for doubly-fed

induction machines. A cycloconverter was used in the ro-tor circuit, resulting in control simplicity while restrictingcontrol flexibility. The overall system power flow problemwas not studied, as only the output power from the sta-tor side is controlled. [2] studied the overall power flowof a self-cascaded induction generator, for which a syn-chronous condenser is needed t o provide the necessary re-active power for field excitation. Since only simple thyris-tor inverters were employed in the system, the system'sability to handle reactive power is disabled. [3] studiedpower regeneration of a typical singly-fed adjustable speeddrive by simple thyristor rectifier using an innovative dcreactor circuit. It is a good example of cost-reduction forpower regeneration of high-power drives. For a generatingsystem, however, this scheme would not be a wise choice ifreactive power control is required. In our previous work,we had proposed a stator field oriented control of doubly-fed induction machine, in which both active and reactivepower of the stator are controlled by a PWM regulatedcurrent in the rotor circuit. We have also shown that theconcept in [4] is applicable to the doubly-excited brushlessreluctance machine [ 5 ] .

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Nevertheless, many issues in slip power recoveryVSCF system have not been fully addressed, such as theflexible control of both active and reactive power of theoverall system, stability problem as especially associatedwith the dc link voltage, control coordination between thetwo inverters, etc. In this paper, power control of VSCFslip power recovery generating system are discussed. Aclosed-loop control strategy is developed to coordinatethe dual PWM inverters in the rotor circuit. Flexibleand stable control of overall active and reactive power isobtained, while the machine coppor losses are minimized.It is shown that with the control strategy proposed, theVSCF system can actually function both as a power gen-erating system and as a reactive power compensator. Ap-plication in wind power generation is simulated to verifythe proposed control strategy.


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11. Control Strategy Developm ent

A schematic slip power recovery system is shownin Fig. 1, with the reference directions of active andreactive power as indicated. As a generating system,obviously in most situations P, < 0. When the ma-chine in variable speed operates below synchronous speedslip power P, > 0; when the machine operates abovesynchronous speed, P, < 0. Note that doubly-excitedgenerators are inherently capable of super-synchronousspeed operation. To ensure sub-synchronous and super-synchronous speed range operation, the requirement liesin the configuration of the power converter.

Fig.1 Power Flow of Slip Power Recovery SystemBoth cycloconverters and thyristor inverters can

be used in this situation. Harmonic distortion and poorpower factor are the major shortcomings, along with lim-ited control flexibility. To realize field oriented control ofthe variable speed generator, and to achieve overall activeand reactive power control and harmonic reduction, thedual PWM inverter structure with dc link is an attractivecandidate. As a result, many new control issues arise andof most importance is the coordination between the twoPWM inverters. For convenience, in the following anal-ysis the two inverters are termed rotor side inverter andnetwork side inverter respectively.

A . Field Oriented Control hrough Rotor SideInvert er

In a VSCF generating system, control schemesfor the doubly-excited induction machine are expectedto achieve the following objectives: 1) The inductiongenerator is required to track a prescribed torque-speedcurve, for maximum power capturing; 2 The stator out-put voltage frequency must be constant; 3 ) Flexible reac-tive power control is achievable. Of course, these controlobjectives must be achieved with the system stability.

The stator field orientation control is based onthe stator d-q model, where the reference frame rotatessynchronously with respect to the sta tor flux, with thed-axis of the reference frame instantaneously overlaps the

axis of the stator winding flux. In shor t, w = w e and Aqs= 0. For such a reference frame selection, the machinedynamical equations can be written as 141

Since the d-axis of the reference frame is the in-stant axis of the stator winding flux, the phase angle ofthe stator voltage is generally not a constant in the ref-erence frame, although its frequency and magnitude areconstants constrained by the power system. The electro-magnetic torque and stator active power can be derivedas

In the doubly-excited induction machine, thelevel of the stator flux remains approximately unchanged,restricted by the constant magnitude and frequency ofthe stator voltage. Therefore, as can be observed from6), the torque control can be achieved by controlling therotor current component orthogonal to t he stator windingflux. Then from 7), sta tor active power is subsequentelycontrolled.

The reactive power at the terminal of the statorwinding can be derived as

or, from (1) and 2 ) , with the stator flux remains un-changed,

9)3 PQs = 5 2 W e d s i d sAs 3 ) indicates, i d s is controllable by i d , , with

Ads unchanged. Therefore, the d-axis component of therotor current, i d , can be controlled to regulate the statorreactive power.

Since the control of stator active power P viai and the control of stator reactive power Qs ia i d ,are essentially decoupled, a decoupler is not necessary toimplement field orientation control for the slip power re-covery system. The flux control is generally unnecessarysince it maintains a constant level, while the control ofreactive power becomes possible.

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B. Minimizat ion of Machine Coppor LossesIt is well known that slip power recovery configu-

ration has improved energy efficiency. Field oriented con-trol described above enhances this improvement by per-mitting variable speed opera tion with reactive power con-trol. There is, however, still another improvement possi-ble. By controlling the reactive power circulation of thesystem, the coppor losses can be minimized. This will beanalyzed in this subsection.

The machine overall coppor losses can be writtenas

By using 1) to 4), 10) can be derived as

In l l ) , i has been used to control torque oractive power, and A d s remains approximately unchangedas described above, then the machine coppor losses is afunction of idd. It can be shown that for P,, to achievethe minimum, it is necessary t hat

However, in the field oriented control, as alreadydiscussed, ids controls the stator reactive power. There-fore, it is sufficient to conclude that stator reactive powerflow determines the level of coppor losses. From 9) and12), the optimal stator reactive power flow can be shownas

C Control of Network Side InverterThrough field oriented control of the rotor side

inverter, the optimal torque-speed profile can be trackedand stator output reactive power can be separately con-trolled. The dc link capacitor provides dc voltage to therotor side inverter and any at tempt t o store active powerin the capacitor would raise its voltage level. Thus to

ensure stability of the system, power flow of the invertershould guarantee the following control objective:

The dc link dynamical equation can be writtenas

in which v d is the dc bus voltage and C s the capacitance,and assuming no power losses for the inverters, then thedc link currents l and i 2 as indicated in Fig. 1 can bederived as

Then from 15) through 17), as long as 14) issatisfied, dc link voltage maintains stable, though smallripples might be present due to the instantaneous inequal-ity between PI and Pr and a small variation may occurduring transient as a result of energy tranferring. As canbe seen from Fig. 1, another result of (14) is that the over-all generated active power equals to the electromagneticactive power, i.e.

3 Pp = Pc = - -wrA iqr2Reactive power flow constitutes another control

objective:

QI= Q' - Q a 19)where Q is the overall reactive power command requiredby the power network.

In the stator flux d-q reference frame,

Since Vd s M 0 v q s M vm , PI and QI can be con-trolled by i l and id1 respectively. In the same referenceframe as determined by the machine stator flux i,l and id1are also field oriented currents, produced by the networkside current regulated PWM inverter.

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111. Implementationand Simulations B Dynamic Speed Tracking Response

A . Implementation Scheme of Closed Loop Sys-t e m

Based on the control strategy discussed above,Fig. 2 shows an implementation of the overall controlsystem, which enables the slip power recovery system tofunction as both a VSCF generating system and a reac-tive power compensator. Individual control of the rotorside inverter and of the network side inverter and relatedfeedback between the two inverters are shown.

Fig.: Control Implementation

A current-regulated pulse width modulation (CR-PWM) voltage source inverter provides field oriented cur-rents i and idr to the rotor circuit, controlling electro-magnetic torque and stator reactive power, respectively.Torque command is given by the turbine optimal torque-speed profile and reactive power command is calculated tominimize the machine copper losses. Overall active powergenerated is directly related to the torque, as indicated by6) and 18).

Another CRPWM voltage source inverter is usedto interface with the power network. In the same d-qreference frameas determined by the machine stato r flux,its currents i l and id1 are also field oriented, controllingPI and I respectively. Therefore, as discussed earlier, PJis controlled through i l to stablize the dc bus voltage and

I is controlled through id1 to meet the overall reactivepower command.

Wind power generation is a typical applicationwhere VSCF generating system is becoming very attrac-tive. For a maximum wind power capturing, the genera-tor turbine is required t o track a prescribed torque-speedprofile. This can be readily achieved by the proposedcontrol strategy, as simulated in this section. Figs. 3 to5 show speed-tracking response of the system when thewind speed increases linearly in 4 seconds. Ideal inverteroutput currents are assumed in the simulation.

16000 1 2 3 416000 1 2 3 4

Fig.3 Turbine Speed Tracking

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-500 0.5 1 15 2 7.3 3 3.5 4timc (=a

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Fig.4 Rotor Current Waveform during Speed Track-ing

As shown in Fig. 3(a), through appropri-ate gearing between the wind-mill and th e generatorshaf t, the corresponding speed reference covers both sub-synchronous and super-synchronous speeds. Optimalwind turbine torque-speed curve has been assumed to bea square profile. As shown in Fig. 3(c) , electromagnetic

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. . . . .Oo o 0.01 o m s am am om ams OM abu OM

tfmc =e)

~ , ....I '. q... ..................... .......... ........o as I 15 z 3 3.5 4

tfmc =cl

Fig.5 Power Flow of the System during Speed Track-ing

torque is controlled in such a way that the net torque ac-celerates the turbine speed linearly, following speed com-mand instantaneously.

As shown in Fig. 4(a), i controls the torque,and idr remains unchanged since the s tator reactive powermaintains its optimal value. As can be seen in Fig. 4(b),rotor current is controlled by the rotor side inverter tohave correct slip frequencies.

As explained earlier, to maintain dc bus voltagelevel, active power flow of the two inverters should equal.This is shown in Figs. 5 (a,b). Fig. 5(c) shows the reactivepower flow of the system. Stator reactive power flowQ scontrolled to minimize the coppor losses, while the overallreactive power command Q is satisfied by controlling theslip reactive power QI.

C Steady State Operation With PWM Regu-lated Currents

With regulated currents produced by the twoPWM inverters, Figs. 6 to 8 simulate the operation of theslip power recovery system. Note that the overall reactivepower command is zero, proving that the system is capa-ble of operating at unity power factor. Hence with theproposed control strategy, the slip power recovery systemis also attractive for adjustable speed drive applications.

D. Dynamic Active and Reactive Power StepResponse

Dynamic performance of the system for a stepchange in active power command is simulated in Fig. 9and dynamic performance for a step change in reactive

time =c)

Fig.6 Controlled Torque and DC Bus Voltage

o oms aoi o m am 0.025 om ams OM aws 0.0stfmc =e)

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Fig.7 Power Flow with PWM Regulated Currents

o aoi oms am am om 0 O M 0.045 0 05tfmc (=e)

.lo01o oms 0.01 0.01s am am om o m s O.M aws 0.05tbne =cl

Fig.8 PWM Regulated Currentspower command is simulated in Fig. 10. Active powercontrol and reactive power control is essentially decou-

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pled. Note th at in wind power generation, it is moredesirable to capture maximum wind energy by control-ling electromagnetic torque, than meeting active powerrequirements. Th e step change in active power in Fig.9 actually further demonstrates speed tracking capabilityas simulated in Fig. 3.

Its noticeable in Fig. 9(a) and Fig. lO(a) tha tdynamic overall active power is decreasing slightly afterthe simulation start -up and the step transition. This isdue to the decrease in slip active power from the networkside inverter, Pl, to restore the dc bus voltage to the nom-inal value. When a steady active power is preferred overa nominal dc bus voltage, slip active power can be main-tained while the dc bus voltage is having small variationsfrom its nominal value.

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0 -

c o u tevnenf mlbusvdtl,.--..2 50 =Y.

. a.L n=375 os(377t) v1000 0.005 0.01 0.015 0 02 P99.5

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Fig.9 Active Power Control Dynamics

0 0.005 0.01 0.015 0.02 9 0.005 0.01 0.015 0 02lh (ICC) -3

Fig. 10 Reactive Power Control Dynamics

IV. Conclusions

In this paper , an overall control strategy has beenproposed for a VSCF power generating system, compris-ing of a doubly-excited induction machine and dual PWMinverters. Stable and decoupled active and reactive powercontrol is achieved through field oriented current regula-tion. In addi tion, machine coppor losses are minimizedthrough controlling reactive power circulation in the sys-tem. The cost of the dual PWM inverter system is morethan justified by the reduction of the rat ing of the powerconverter, the flexible controllability of active and reactivepower and the satisfactory current harmonic spectrum, inaddition to substantial improvement in efficiency.

Acknowledgment

Th e work in this paper is supported by a ResearchInitiation Grant ESC9111256 from the National ScienceFoundation and a Seed Grant from The Ohio State Uni-versity.

References

[l] M. Yamamoto and 0 Motoyoshi, Active and Re-active Power Control for Doubly-Fed Wound RotorInduction Generator, IEEE Trans. Power Electron-ics, Vol. 6, No. 4 , October 1991, pp. 624-629

[2] F. Shibata and K. Taka, A Self-cascaded Induc-tion Generator Combined with a Separately Con-trolled Inverter and a Synchronous Condenser,IEEE Trans. Industry Applications, Vol. 28, No. 4,July/August 1992, pp. 797-807

[3] A. Matsui, K. Tsuboi and S . Muto, Power Regen-erative Controls by Utilizing Thyristor Rectifier ofVoltage Source Inverter, IEEE Trans. Industry Ap-plications, Vol. 28, No. 4, July/August 1992, pp. 816-823

[4] Y. Tang and L. Xu, Stator Field Oriented Control ofDoubly-Excited Induction Machine in Wind PowerGenerating System, IEEE 35th Midwest Sympo-sium on Circuits and Systems, Washington, DC, Au-gust 1992

[5] L. Xu and Y. Tang, A Novel Wind-Power Gen-erating System Using Field Orientation ControlledDoubly-Excited Brushless Reluctance Machine,IEEE Industry Application Society Annual Meeting,Houston, TX, October 1992

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