A Simple and Reliable Rectifier forPMSG Wind Turbines by Using

Series Reactive Compensator Named MERSTakanori Isobe, Takayuki Kawaguchi, Tsukasa Sakazaki and Ryuichi Shimada

Research Laboratory for Nuclear Reactors, Tokyo Institute of TechnologyTokyo, Japan 152–8550

Abstract—This paper proposes a series reactive powercompensator named MERS to be applied to PMSG windturbine. PMSG usually requires reactive power control toachieve full use of the generator voltage rating; therefore,a diode rectifier cannot be used and more complex volt-age source converter is usually used as a generator sideconverter. The series compensator enables use of the dioderectifier with PMSG; therefore, relatively simple generatorside converter can be achieved. This paper proposes thepower conversion configuration using the series compensateddiode rectifier with dc current link topology, and describesoperation principle and compensation strategy to achieve fulluse of the generator capability at each rotation speed. Smallscale experiments were conducted with 1.5 kW rated PMSG,and the results confirms the principles and advantages.

I. INTRODUCTION

Permanent magnet synchronous generator (PMSG) is apromising candidate for large scale offshore wind turbinesbecause of its simple structure and possibility of highpole number and gear-less configuration. These featuresresult in maintenance efforts reduction and they becomemuch more important for future wind power generationespecially in offshore wind farm [1]. On the other hands,using PMSG has challenges in rare earth materials andcomplexity of power-electronics stage. Usually, reactivepower controllability is needed to the generator sideconverter to control the terminal voltage of the generatorand maximize the generator ratings utilization; on theother hands, a simple diode rectifier can be used for dc-excited synchronous generators.

Usually a voltage source type active rectifier withhigh frequency switching is used for the generator sideconverter of large scale PMSG; however, the complexityof the converter results in reduced reliability, increasedloss and cost. High frequency switching converter alsointroduces ground leakage current problems and generatorside filter needed. This paper proposes a series reactivepower compensator named MERS to be applied to PMSGand achieve relatively simple rectifier with reactive powercontrollability. The series compensator enables use ofdiode rectifier with PMSG and the compensator itselfis also simple. Especially the MERS is controlled withline frequency switching; therefore, the additional partto the diode rectifier can be implemented with relativelylow effort compared to other possible reactive powercompensators.

(a) (b)

Fig. 1. Circuit configuration of MERS. (a)Full-bridge. (b)2-switchconfiguration.

II. MAGNETIC ENERGY RECOVERY SWITCH (MERS)

A. Configuration and Characteristics

Series compensator named magnetic energy recoveryswitch (MERS) [2], [3] and its application for wind tur-bine has been proposed [4]. The MERS works as a seriesvariable capacitor by line frequency switching and phaseangle control. From a certain aspect, the MERS is oneof FACTS (flexible ac transmission system) device worksas a series compensator [5] like GCSC (gate commutatedseries capacitor) [6], [7] and SSSC (static synchronousseries compensator) [8], but has different operation rangeand implementation characteristics. GCSC and SSSC havebeen studied for power transmission applications well;however, not introduced to other indutrial applications.

Fig. 1(a) shows the basic configuration of the MERS.The MERS consists of four semi-conductor switches anda capacitor. Semi-conductor switches are required to havereverse conductivity (or free-wheeling diode is needed)and turn-off capability. At this moment, IGBTs and MOS-FETs are candidates. In the full-bridge configuration, thecapacitor is charged with unipolar voltage and electrolyticcapacitors can be used. However, almost all current flowsinto the capacitor; therefore, electrolytic capacitors areusually not adequate due to their current ripple ratings.

The MERS works as an active series compensator, inother words, adjustable series capacitor. In comparison toconventional converters, using the MERS has followingadvantages:

1) Line frequency switching, comparatively lowswitching frequency in usual system frequency of50 / 50Hz. Additionally low conduction loss semi-conductor device can be used, if a special design isapplied [10].

(a) (b) (c)

(d) (e) (f)

Fig. 2. Possible current paths of MERS. The flowing current is positivedirection (left to right in the diagram) in (a), (b) and (c), and negativedirection (right to left) in (d), (e) and (f). The capacitor is dischargedin (a) and (d), and charged in (c) and (f). When the capacitor voltageis zero, the current flows in two parallel paths as shown in (b) and (e),which are referred as ’parallel bypass mode’.

(a) (b)

(c) (d)

Fig. 3. Waveform modes of MERS. (a)No MERS. (b)Discontinuousmode. (c)Balance mode. (d)Dc-offset mode.

2) Simple gate angle control like thyristor.3) Soft-switching operation within a certain operating

range.4) The full-bridge configuration has a wide operating

range compared to other series compensation tech-nology.

B. Operation Principles

Fig. 2 shows possible current paths of the full-bridgeMERS. Two switches are turned on and off in pair, U-Y are always opposite to V-X. By controlling two pairsof switches, the capacitor can be connected to ac circuitin series alternately with different polarity, or shorted.Remarkable paths are (b) and (e), which can be achievedwhen the voltage of the capacitor is zero, and in these

Fig. 4. Normalized equivalent reactance of MERS, Xmers/Xc, asfunction of gate control angle, δ, where Xmers is the fundamentalequivalent reactance of the MERS and Xc is the reactance of theequipped capacitor inside of the MERS. (a) to (d) are correspondingto waveform modes in Fig. 3.

paths, the capacitor is shorted and does not inject anyvoltage to the ac circuit. By line frequency switching andcontrolling current paths in a cycle, this circuit can workas an adjustable series capacitor.

Schematic waveforms are shown in Fig. 3. The phasedifference between the flowing current and the switching,δ, can be controlled. When δ = 0 as shown in Fig. 3(a),the flowing current does not charge the capacitor; there-fore, the MERS does not inject any voltage to the circuit.By increasing δ, generated voltage in the capacitor isincreasing and this waveform mode is referred as dis-continuous mode. Finally when δ = 90, the injectedvoltage waveform becomes pure sinusoidal as shown inFig. 3(c), and has the same amplitude as the case of usinga fixed capacitor whose capacitance is equal to one ofthe MERS capacitor. This waveform mode is referred asBalance mode. Additionally, the full-bridge configurationcan inject higher voltage by dc-offset mode as shown inFig. 3(d). In all waveform modes, fundamental componentof the injected voltage has 90 degree phase differenceto one of the flowing current, and amplitude can becontrolled. This clearly means that the MERS works as anadjustable capacitor about their fundamental components.The equivalent reactance of the MERS, Xmers, can varyfrom 0 to ∞.

Normalized equivalent reactance of the MERS is shownin Fig. 4. Xmers can be controlled by δ in discontinuousmode. In dc-offset mode, δ is always 90 and Xmers

cannot be determined by only δ. Practically, control isimplemented by PLL (phase lock loop) based on gener-ator voltage or by using rotary encoder, and Xmers canbe easily controlled by simple phase angle control withreference of generator voltage or rotor position.

C. 2-switch Configuration

Fig. 1(b) shows 2-switch configuration which also canwork as a series compensator, but with reduced numberof semi-conductor switches. In this configuration, the ca-pacitor is charged and discharged without semi-conductorconduction, and only when the capacitor voltage is zero,current flows into semi-conductor switches. This can be

(Current source Inverter)

Fig. 5. Proposed configuration using MERS for single wind turbine.

an attractive advantage from points of semi-conductorrating and conduction loss. If the operation is in dis-continuous mode and very near to balance mode, semi-conductor conduction duration becomes very small. Thiswill result in low conduction losses and small currentratings required as well. Additionally, two semi-conductorswitches share their emitter (or source) terminals; there-fore, gate drive circuit can be simple.

On the other hands, as a drawback, the 2-switchconfiguration can not achieve dc-offset mode, since aremaining voltage in the capacitor will cause shortingcharged capacitor and very high current flowing into semi-conductor switches. Therefore, the operating range of thisconfiguration is limited as 0 < Xmers < Xc, where Xc

is the reactance of the equipped capacitor.

III. PROPOSED CONVERTER FOR SINGLE WINDTURBINE USING PMSG

A. Configuration

The MERS can compensate for synchronous reactanceof the generator; therefore, generator terminal voltagecan be controlled and achieve good utilization of gen-erator ratings. In other words, reactive power supply tothe PMSG can be controlled by more simple method,that is usually implemented by high switching frequencyconverters.

The MERS applied for wind turbine using PMSG hasbeen proposed in [9]; however, the proposed configurationconsists of a diode rectifier and a voltage source type grid-side converter, connected via dc voltage link; therefore,additional step-up chopper is actually needed for lowturbine speed operation since the diode rectifier withthe MERS does not have voltage step-up capability byitself. This paper proposes the same concept but withcurrent link topology as shown in Fig. 5. By using currentlink topology and current source converter as grid-sideconverter, the power from low speed operated generatorcan be transferred to the grid, whose voltage is constantand higher than the generator voltage, without additionalstep-up converter.

Both line commutated converter and self-commutatedconverter with a high frequency switching can be usedfor the grid-side converter. The selection depends on ratedpower and requirements of harmonics improvement andreactive power controllability.

Fig. 6. Equivalent circuit in single phase diagram. PMSG is representedby nominal induced emf, E, and synchronous reactance, Xs. Dioderectifier connected to dc current source can be represented by a resistiveload, which draws a constant ac current.

(a) (b)

Fig. 7. Phasor diagrams. (a)No compensation (Xmers = 0). (b)Halfcompensation (Xmers = 1/2 · Xs).

B. Compensation Strategy

Fig. 6 shows an equivalent circuit in single phase dia-gram. The PMSG is modeled as a voltage source, whichrepresents nominal induced emf, and an inductance, whichrepresents synchronous reactance. The MERS is modeledas variable capacitor. The diode rectifier with dc currentlink can be modeled as a resistive load, which draws aconstant ac current, Igen, which can be controlled by thedc-side current, Idc, as Igen =

√2/3Idc. Consequently,

the following discussion focuses on only fundamentalcomponents.

Fig. 7 shows phasor diagrams when the generatorspeed and current are fixed. Therefore | ˙Igen| and |E| arefixed and |Vs| is consequently fixed. Vs and ˙Vmers have90 degree phase angle difference to ˙Igen, and Vd has thesame phase angle with ˙Igen. Without compensation, | ˙Vgen|is lower than |E| due to Vs. With compensation, | ˙Vmers|can be controlled since the equivalent reactance of theMERS, Xmers, can be controlled; by doing that, the phaseangle of ˙Igen and | ˙Vgen| can be controlled. Fig. 7(b) showsthe phasor diagram when Xmers is controlled to be a halfof Xs. In this condition, | ˙Vgen| becomes equal to |E|.

Fig. 8 shows Vgen, generator active power, P , andgenerator power factor, cos ϕ, as function of Xmers, whichare obtained from the phasor diagram. The graph indicatesthat increasing Xmers results in increase of Vgen, and Vgen

becomes equal to E when Xmers is equal to a half ofXs. This operating point is referred as half compensationin the following discussion. ϕ is defined as the phaseangle difference between ˙Vgen and ˙Igen; therefore, cos ϕindicates the power factor at the generator terminal. The

Fig. 8. Generator voltage, Vgen, active power, P , and generator powerfactor (in lead), cos ϕ, as function of Xmers when Xs is 0.6 p.u.

graph indicates that the power factor of the generatorbecomes lead by increasing Xmers, and reactive power issupplied to the generator. P is defined as Vgen ·Igen ·cos ϕ,normalized by that achieved when Vgen and Igen becomeunity. P achieves the maximum when Xmers is equalto Xs. This point is referred as full compensation. Atthe point, the phase angle of ˙Igen is equal to that of E;therefore, this power is the maximum power which canbe drawn from this machine with the rated current andspeed, if there is enough voltage rating of the stator.

Fig. 9 shows Vgen and P as function of Igen withvarious Xmers. Without compensation, Vgen decreasesaccording to increase of Igen. By using half compensation,Vgen can be maintained at E. Full compensation can ob-tain more power; however, Vgen is increased and becomeshigher than E. The generator voltage rating should beequal to or higher than E since Vgen becomes E in noload condition, since the proposed rectifier configurationhas no reactive power and terminal voltage controllabilitywhen the active power is zero. The full compensationrequires a higher generator voltage rating than E. Thiscompensation strategy can be selected by designing thestator voltage rating to accept this voltage; however, theincrease in the stator capacity cannot be compensatedby the increase in maximum power. From the pointof voltage rating utilization in whole operating range,the half compensation is most attractive compensationstrategy.

C. MERS Capacitor Selection

For a constant speed operation, Xmers can be fixed toachieve a good compensation; therefore, a fixed capacitorcan be used. However, for variable speed and frequencyoperation, variable Xmers should be used to achieveoptimum compensation in whole operating speed.

Reduced frequency results in decrease of Xs and re-quired Xmers. Additionally, it also results in increaseof Xc with the same capacitance. Therefore, if thecapacitance is selected to achieve that Xc is equal torequired Xmers at the maximum frequency, Xmers isalways lower than Xc. This means that only discontin-uous mode operation is needed for the MERS, and the2-switch configuration, which has some advantages in

Fig. 9. Generator voltage, Vgen, and active power, P , as function ofIgen when Xs is 0.6 p.u. Solid lines show those with half compensation(Xmers = 0.3), and dotted lines show those without compensation(Xmers = 0) and with full compensation (Xmers = 0.6).

(a)

(b)

Fig. 10. Experimental setup. (a)Schematic diagram. (b)Overview oftested generator and induction motor as prime mover.

power-electronics implementation, can be used with somedesign margin to avoid dc-offset mode operation.

IV. EXPERIMENTAL VERIFICATION

A. Setup

Small scale experiments were conducted with 1.5 kWrated PMSG. Fig. 10(a) shows schematic diagram of theexperimental setup and Table I shows circuit parameters.The gate signals for MERS compensators were generatedbased on the rotor position by using a rotary encoder.In the dc-side of a diode rectifier, a smoothing inductor

(a) (b) (c)

Fig. 11. Experimentally measured waveforms of generator terminal voltages, current and the MERS voltage with rated speed and current. (a)Xmers

= 7.08(Ω). Discontinuous mode was observed. (b)Xmers = 14.9(Ω). Balance mode was observed. (c)Xmers = 19.1(Ω). Dc-offset mode was observed.

TABLE IPARAMETERS OF EXPERIMENTAL CIRCUIT

Smoothing inductor of dc-link 101 mHCapacitance of MERS capacitors 120 µF (15.2 Ω at 87.5 Hz)

TABLE IIPARAMETERS OF TESTED PMSG

Rated power Pn 1.5 kWRated voltage Vn 188 VRated current In 5.5 ARated rotation speed nrated 1750 min−1

Rated frequency frated 87.5 HzNo-load induced voltage E 163 V at 1750 min−1

Synchronous reactance Xs 9.07 Ω (0.53 p.u.∗)Stator resistance rs 1.55 Ω (0.09 p.u.∗)

* Based on the no-load induced voltage.

and an electronic load with current control operation wereconnected in series.

Table II shows parameters of the tested generator.The generator was actually an interior permanent magnettype synchronous motor and was used as generator inthe experiments. The machine had higher rated voltagethan the no-load induced voltage; moreover, the ratedvoltage was higher than the expected maximum terminalvoltage which can be achieved by full compensation.Therefore, the no-load induced voltage was used as thebasis of calculation in the following discussion. Availablemaximum power as generator, which was expected to beachieved by full compensation, was approximately equalto the rated power as motor.

B. Results with Constant Speed and Current

Fig. 11 shows waveforms with the rated generatorcurrent (5.5 A) and speed (1750 min−1). Gate angle forthe MERS was controlled, and as a result, Xmers waschanged. When Xmers is lower than the reactance ofthe actual capacitor, Xc, the discontinuous mode was

Fig. 12. Experimentally measured terminal voltage, Vgen, power factor,cos ϕ, and output active power, P , of the generator as function ofXmers. The no-load induced voltage at this rotation speed, E, is alsoshown. Theoretical curves which come from the fundamental analysisincluding consideration of stator resistance are also shown as solid lines.

observed as shown in Fig. 11(a). When Xmers is ap-proximately equal to Xc, the balance mode was observedas shown in Fig. 11(b). When Xmers is higher than Xc,the dc-offset mode was observed as shown in Fig. 11(c).The generator voltage waveforms were much distorted;however, the current waveforms had comparatively goodharmonic characteristics.

Fig. 13. Experimentally measured output active power with the ratedgenerator current (5.5 A) as function of rotation speed. The maximumpower obtained by adjusting Xmers in each rotation speed are plottedas maximum power with compensation.

Fig. 12 shows generater terminal voltage, power factorand output active power as function of Xmers with a con-stant dc-link current which achieves the rated generatorcurrent. Theoretical curves which can be available fromthe analysis of fundamental components in the previoussection are also shown in the same figure. The statorresistance of the tested generator cannot be neglected;therefore, the analysis including the stator resistance wasused. The experimentally measured values were wellagreed to the theoretical curves. The maximum outputpower was achieved when Xmers was approximately equalto Xs. By using higher compensation area, the outputpower, P , can be widely controlled. This result indicatesthe MERS can control power without dc-link currentcontrol. However, the terminal voltage, Vgen, becomeshigher than the no-load induced voltage, E, and actualrated voltage as motor, Vn, in this case.

C. Variable Speed Operation

For wind turbine application, characteristics with vari-able speed operation should be considered. The sameexperiments were conducted with several rotation speeds.Fig. 13 shows maximum generator output power with therated current with compensation as function of speed. Inthe experiments, constraint of the generator terminal volt-age was not considered; therefore, the achieved maximumpower from all possible operating points about Xmers areplotted. The output power without compensation is alsoshown. By controlling generator current within the rating,the power can be controlled in the all below area of theplotted points.

Both the maximum power and the power withoutcompensation are approximately linear to the speed. Theresult confirms that the current source topology enablespower conversion even in low induced voltage operation.The generator power was increased by compensation ineach speed. In the experiments, there was no generatorvoltage limitation because of enough voltage rating ofthe tested machine. If the stator voltage has some limi-tation, the maximum power will be less but reasonablefor the ratings. These experimental results indicate that

the proposed configuration can work in variable speedoperation and enables full use of the generator capabilityat each rotation speed; therefore, is valid for wind turbineapplication.

V. CONCLUSIONS

The MERS offers a simple reactive power compensa-tion method for PMSG and this paper proposed the powerconverter using the MERS and a diode rectifier withcurrent link topology for wind turbine using PMSG. Theproposed method can be a simple rectifier solution due tothe use of diode rectifier and simplicity of the MERSitself. Additionally, only a partial capacity is neededfor compensator; concretely, the voltage ratings requiredfor semi-conductor devices and the capacitor for theMERS corresponds to the voltage across the synchronousreactance (or half). This can be advantageous especiallyin low synchronous reactance generator case.

The MERS offers comparatively simple rectifier withreactive power controllability; therefore, has advantagesalso in high synchronous reactance case. Allowing highsynchronous reactance in generator design has possibilityto achieve much compact generator. Reducing size andweight of generator on top of the tower brings manyadvantages to whole system design of wind turbine.

REFERENCES

[1] J. Ribrant and L. M. Bertling, “Survey of failures in wind powersystems with focus on swedish wind power plants during 1997-2005,” IEEE Transactions on Energy Conversion, Vol. 22, No. 1,pp.167–173, (2007).

[2] T. Takaku, T. Isobe, J. Narushima, H. Tsutsui, and R. Shimada,“Power factor correction using magnetic energy recovery currentswitches”, T. IEE Japan, Vol.125-D, No.4, pp.372–377 (2005) (inJapanese)

[3] T. Takaku, T. Isobe, J. Narushima, H. Tsutsui and R. Shimada,“Power factor correction using magnetic energy recovery currentswitches,” Electrical Engineering in Japan, vol. 160, No. 3, pp.56–62, (2007)

[4] J. A. Wiik, A. Kulka, T. Isobe, K. Usuki, M. Molinas, T. Takaku,T. Undeland, and R. Shimada:, “Loss and rating considerations ofa wind energy conversion system with reactive compensation bymagnetic energy recovery switch (MERS)”, EPE Journal, Vol.18,No.3, pp.25–30 (2008)

[5] J. A. Wiik, F. D. Wijaya, and R. Shimada:, “Characteristics ofthe magnetic energy recovery switch (MERS) as a series FACTScontroller”, IEEE Trans. on Power Delivery, Vol.24, No.2, pp.828–836 (2009)

[6] G. G. Karady, T. H. Ortmeyer, B. R. Pilvelait and D. Maratukulam,“Continuously regurated series capacitor,” IEEE Transactions onPower Deliverly, vol. 8, No. 3, pp. 1348–1355, (1993)

[7] E. H. Watanabe, L. F. W. de Souza, F. D. de Jesus, J. E. R. Alvesand A. Bianco, “GCSC - gate controlled series capacitor: anew facts device for series compensation of transmission lines,”2004 IEEE/PES Transmission and distribution conference andexposition: Latin america, pp. 981–986, (2004)

[8] N. G. Hingorani and L. Gyugyi, “Understanding FACTS, Conceptsand Technology of Flexible AC Transmission Systems,” IEEEPress, December, (1999)

[9] J. A. Wiik, A. Kulka, T. Isobe, K. Usuki, M. Molinas, T. Takaku,T. Undeland, and R. Shimada, “Loss and rating considerations ofa wind energy conversion system with reactive compensation bymagnetic energy recovery switch (MERS)”, EPE Journal, Vol.18,No.3, pp.25–30 (2008)

[10] R. Shimada, J. A. Wiik, T. Isobe, T. Takaku, N. Iwamuro,Y. Uchida, M. Molinas and T. M. Undeland, “A new AC currentswitch called MERS with low on-state voltage IGBTs (1.54 V)for renewable energy and power saving applications,” Proceedingsof the 20th International Symposium on Power SemiconductorDevices & ICs, pp. 4–11, (2008)