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2 Medium Frequency-Link Power Converion for High Power enity Renewable Energy Sytem Md. Rabiul lam, Youguang Guo, Jianguo Zhu, Haiyan Lu Faculty of Engineering and nformation Technology Univerity of Technology, Sydney, utralia btract Recent advance in olid-tate emiconductor and magnetic material have provided the impetu for medium frequency-link baed medium voltage power converion ytem, which would be a poible olution to reducing the weight and volume of renewable power generation ytem. To verify thi new concept, in thi paper, a laboratory prototype of.6 k medium frequency-link power converion ytem i developed for a caled down k grid application. The deign and implementation of the prototyping, tet platform, and the experimental reult are analyzed and dicued. t i expected that the propoed new technology would have a great potential for future renewable and mart grid application. Keyword-amorphou alloy; medium frequency-link; medium voltage converter; renewable energy ytem; direct grid integration. NTROUCTON Currently, there are over 8 GW of wind power and over 69 GW photovoltaic (P) generation capacity intalled worldwide and more than MW renewable power plant have now become a reality. The renewable energy ource ha variable daily and eaonal pattern and conumer power demand requirement alo have different characteritic. Therefore, it i difficult to operate a tand-alone power ytem upplied from only one type of renewable energy reource unle there are appropriate energy torage facilitie. f enough energy torage capacity i not available epecially in medium/large cale ytem a grid connected renewable power generation may be the only practical olution. For grid connection, a power frequency (5 or 6 Hz) tep-up tranformer i uually ued in the renewable generation ytem to feed the renewable energy into a medium voltage grid (e.g. 6 6 k). For example, the weight and volume of a.69/ k,.6 M tranformer are typically in the range of 6 8 ton and 5 9 m, repectively []. liquid-filled M tep-up tranformer ue about 9 kg of liquid a the coolant and inulator, which require regular monitoring and replacement. Thi heavy and large ize power frequency tranformer ignificantly increae the weight and volume of the ytem a well a the running and maintenance cot of the renewable generation ytem. Thee drawback are critical in offhore and remote area application, where the cot of intallation and regular maintenance are extremely high. uring the lat few year, cientit and reearcher have been trying to find out effective technologie for the compact and lightweight renewable generation ytem. Elimination of tep-up tranformer by medium voltage converter ha attracted Jianxun Jin Center of pplied Superconductivity and Electrical Engineering Univerity of Electronic Science and Technology of China Chengdu, China great attention recently. Conidering the recent advance in olid-tate emiconductor and magnetic material, the modular multilevel cacaded converter topology may be the natural choice for the development of medium voltage converter [, ]. multi-coil modular permanent magnet generator [4] and a few 6-phae generator [5] were propoed to eliminate the tepup tranformer of the wind turbine power generation ytem. However, thee approache require pecial modular generator or multiple traditional generator to generate iolated multiple C upplie for the modular multilevel cacaded converter, and introduce electrical iolation problem between generator and grid. n order to overcome thee drawback, a multiphae iolated C/C converter baed medium voltage P inverter topology wa propoed in [6]. n thi ytem, the voltage balancing i the challenging iue, ince each module i connected to a P array through a C/C converter. an improvement, a ingle C-link baed P inverter ha been preented in [7]. lthough thi deign may reduce the voltage balancing problem on the grid ide, the generation of common C-link voltage from different P array make the inverter operation complex and limit the range of operation of the maximum power point tracker (MPPT). an alternative approach, a common medium frequencylink baed medium voltage converter wa reported in [8, 9]. The electromagnetic deign and analyi of the medium frequency-link wa reported in [, ]. Thi paper preent the overall deign and implementation of medium frequency inverter, and fat recovery diode baed rectifier for each module, and tet reult of the medium frequency-link power converion ytem. The medium frequency-link baed medium voltage power electronic converter will have the following advantage: (i) having no requirement of pecial or multiple generator for wind turbine generator ytem, (ii) having a wide range of MPPT operation for P ytem, (iii) inheriting the C-link voltage balance due to ingle C upply, (iv) allowing a direct gird connection without uing tranformer, (v) being an overall compact and lightweight ytem, and (vi) minimizing inherently the grid iolation problem through medium frequency-link. The electromagnetic deign and analyi of the medium frequency-link wa reported in [, ]. Thi paper preent the overall deign and implementation of medium frequency inverter, and fat recovery diode baed rectifier for each module, and tet reult of the medium frequency-link power converion ytem. The medium frequency-link baed medium voltage power electronic converter will have the following advantage: (i) no requirement of pecial or multiple generator

3 for wind turbine generator ytem, (ii) wide range of MPPT operation for P ytem, (iii) inherent C-link voltage balance due to ingle C upply, (iv) direct gird connection without uing tranformer, (v) overall compact and lightweight ytem, and (vi) inherently minimizing the grid iolation problem through medium frequency-link. current to the timing capacitor via an internal current mirror with high accuracy. Maximum ignal high time i determined by the riing capacitor voltage wherea dead time i determined by the timing capacitor dicharge. CC Ω Source Rectifier and (or) MPPT Rectifier (MF) C nverter (MF) H- bridge k k k Medium voltage grid 4.kΩ.μF RT 6 5 RF N N nf E/.kΩ NC 4 Clk CC UC85N RT 5 Gnd C 4 9 Ramp 7 CT 6 PGnd SS 8.μF Out Out CT Medium frequency-link Modular multilevel cacaded converter.μf Figure. lock diagram of medium frequency-link baed medium voltage converter ytem for tep-up tranformer-le direct grid connection of renewable power ource.. ESGN N MPLMENTTON OF MEUM FREQUENCY NERTER full-bridge medium frequency inverter i developed by uing Semikron SKGH compact inulated gate bipolar tranitor (GT) module. Thi inverter can generate a quare wave primary excitation voltage of the medium-frequency-link with a medium frequency. The Semikron SKH op with SKH PS i ued a an iolated driver for SKGH module. Fig. how a photograph of the medium-frequency inverter. high peed PWM controller UC85N i ued to generate the witching ignal. implified verion of the witching ignal generator circuit i hown in Fig.. UC85N i a high performance PWM controller with low tart up current, accurate ocillator frequency, leading edge blanking, latched fault logic, full-cycle oft tart, retart delay after fault and many more. Filtering capacitor SK GH GT baed H-bridge inverter (mounted on the top of the heatink) Heatink Figure. photo of medium-frequency inverter GT driver circuit Switching ignal The ocillator of the UC85N i a aw-tooth which utilize two pin; one for the timing reitor R T and the other for timing capacitor C T. The reitor program the charging Figure. Simplified circuit of the witching ignal generator. aed on the deired maximum duty cycle, max, the timing reitor can be calculated a R T m ( max ) () k. m (.9) Two alternative ignal are generated through a T flip-flop; output ignal frequency i the half of ocillator frequency a configured internally. Therefore, twice ocillator frequency (compare to output) i required to deign the timing capacitor. aed on the calculated value of timing reitor and deired maximum duty cycle the timing capacitor can be calculated a.6 max CT () Rt f F 4nF. The complete witching ignal generator circuit with UC85N C i fabricated and teted in the Laboratory. The gate pule were meaured and compared with the imulation reult a hown in Fig. 4. The meaured pule were found highly conitent with the imulation reult. Pule amplitude.5 x -4 Figure 4. Simulated gate pule to drive the medium frequency inverter. n Matlab imulation environment gate pule are capable to drive the GT.

4 The ignal high time (witch on) and dead time (witch off) are meaured uing curor of the ocillocope a hown in Fig. 5 and 6 and duty cycle i alo calculated. T high T high_ be compared with C/C chopper circuit output. The XYS SEE5-CC uper fat recovery (5 n) dual diode module i conidered for the development of module rectifier of medium voltage converter. f we aume the capacitor dicharge rate to remain contant at the C level, the peak to peak ripple voltage, r(p-p) can be approximated by a waveform a hown in Fig. 7, which ha a peak to peak value of o and a time period of T r and centered around the C level. o m Figure 5. Meaured witching ignal; on-time calculation o T rc T r T t r(p-p) T high_ t T dead Figure 6. Meaured witching ignal; dead-time calculation The percentage of duty cycle, % can be calculated a Thigh % () T T high dead % The expected time period, T and frequency, f H of the medium frequency inverter can alo be calculated a T T T T T (4). high_ dead_ high_ dead_ f H. 8kHz 6 T 99.. ESGN N MPLMENTTON OF MEUM FREQUENCY RECTFERS Single phae diode rectifier i conidered to convert medium frequency alternating quantity to C quantity with fat recovery diode. Each module i aociated with a eparate rectifier. The available average voltage i about 67. Table depict the technical information of the module rectifier. TLE. POWER COMPONENTS N THE NERTER SE RECTFER Technical data Unit voltage (rm) verage output voltage P of the diode alue The input voltage waveform of thi rectifier i like alternating train of pule. The rectified output waveform can Figure 7. Capacitor voltage of the module rectifier f i the output rectifier average current, f i the ource voltage frequency, i the output rectifier C output voltage, off i the diode off time or capacitor dicharge time factor, and γ i the ripple factor, then the peak to peak and rm output voltage can be deduced a off r off r( p p) C Cfr T (5) and r( p p) r( rm) 4 Cf off Cf Hence, the ripple factor can be deduced a r ( rm) off 4 Cf (7) From (7), minimum value of the capacitor can be deduced a C off F 4 f (8) f i, f i khz, i 67, off i % and ripple factor i %, the minimum value of capacitor, C for the each module rectifier can be calculated a. C nF The 88EE5T4P aluminum electrolytic capacitor i ued for the module rectifier circuit. The maximum power handled by a power diode and the temperature of the diode junction are related ince the power diipated by the device caue an increae in temperature at the junction of the device. The junction temperature (T J ), cae temperature (T C ), and ambient (air) temperature (T ) are related by the device heat-handling capacity can be preented in term of thermal-electric analogy a θ J = (θ JC + θ CS + θ S ) (9) off (6) 4

5 and T J P T, () J where θ J i the total thermal reitance (junction to ambient), θ JC i the tranitor thermal reitance (junction to cae), θ CS i the inulator thermal reitance (cae to heat ink), θ S i the heat-ink thermal reitance (heat ink to ambient), and P i the power diipation. Uing (9) and () the heat-ink thermal reitance can be deduced a TJ T JC CS S P TJ T ( ) S JC CS. () P From the data heet of SEE5-CC, θ JC i.6 o C/W, θ CS i.5 o C/W, T J i 7 C and P i about W. Therefore, the heat-ink thermal reitance can be calculated a. 7 4 o S (.6.5) 6.9 C / W. EXPERMENTL TESTNG N RESULTS NLYSS caled down prototype of.6 k medium frequencylink with ingle primary and ix econdary winding i developed, a hown in Fig. 8 to generate the balanced iolated ix C upplie for a three-phae five-level modular multilevel cacaded converter. Fig. 9 how the voltage waveform of primary and econdary ide of the medium frequency-link. n gilent Technologie S64 ocillocope with P5 high voltage differential probe wa ued to oberve the voltage waveform. The meaured waveform were found highly conitent with the theoritical and imulation reult. The imulated primary and econdary ide voltage waveform are hown in Fig. and. Secondary-5 Secondary-6 Secondary-4 Secondary- Primary-P Secondary- Secondary- Figure 8. photo of the medium frequency-link; ingle primary and 6 econdary winding; Metgla 65S heet of μm thickne and 5 mm width wa glued with raldite on the urface of each layer to develop the core. To minimize the proximity effect, Litz wire are ued for winding with ingle layer placement. The C-link voltage were meaured and found approximately equal at about 67. Fig. how the meaured voltage waveform of the medium frequency rectifier, which i found highly conitent with imulation reult. The Fig. how the imulated voltage waveform of rectifier circuit. The -H curve of the medium frequency-link were analyzed at different magnitude of excitation current and at different excitation frequencie. The plotted -H curve have been compared with the material manufacturer data and found highly conitent. Fig. 4 how the -H curve at excitation current of khz. The developed medium frequency-link power converion ytem i teted with a threephae five-level k modular multilevel cacaded converter and found highly atifactory performance. The line voltage of the modular multilevel cacaded converter after filter circuit are hown in Fig. 5. Excitation at khz Primary voltage ( ) Secondary voltage (76 ) Figure 9. Meaured primary and econdary ide voltage of the prototype medium frequency-link. oltage () x -4 Figure. Simulated primary ide voltage of the medium frequency-link; at khz excitation current. oltage () x -4 Figure. Simulated econdary ide voltage of the medium frequency-link; at khz excitation current. khz excitation voltage Rectified voltage (67 ) Figure. Meaured rectified output voltage of the prototype medium frequency rectifier. 5

6 The meaured line voltage of the propoed ytem were alo compared with the imulation reult, and they were found highly conitent with the theoretical and imulation reult. Fig. 6 how the imulated line voltage of the k converter ytem after line filter. oltage () Figure. Simulated output voltage of the medium frequency rectifier. t 5 and 45 o C khz excitation current t and 45 o C Figure 4. Meaured -H curve at excitation current of and 5. The link i excited by khz quare wave primary voltage. The magnetic field intenity and magnetic flux denity are calculated by meauring the primarycoil excitation current and open circuit terminal voltage of each of the ix econdary coil.. CONCLUSON The propoed medium frequency-link can be a good olution to provide balanced iolated multiple C upplie for modular multilevel cacaded converter to develop medium voltage converter. Thi medium voltage converter baed direct grid connection approach eliminate not only the grid ide heavy tranformer but alo reduce the ize of line filter. C C Line voltage with LC filter. k (rm) 5 Hz output Figure 5. Meaured line voltage of the prototype k converter. oltage (k) Figure 6. Simulated line voltage of the propoed k converter. REFERENCES [] M. R. lam, Y. G. Guo, and J. G. Zhu, tranformer-le compact and light wind turbine generating ytem for offhore wind farm, Proc. EEE nternational Conference on Power and Energy, Kota Kinabalu, Malayia, ec. 5,, pp [] M. R. lam, Y. G. Guo, J. G. Zhu, and. G. orrell, eign and comparion of k multilevel voltage ource converter for local grid baed renewable energy ytem, Proc. 7th nnual Conference of the EEE ndutrial Electronic Society, 7 Nov., Melbourne, utralia, pp [] M. R. lam, Y. G. Guo, and J. G. Zhu, Performance and cot comparion of NPC, FC and SCH multilevel converter topologie for high-voltage application, Prof. nt. Conference on Electrical Machine and Sytem, ug., eijing, China, pp. 6. [4] X. Yuan, J. Chai, and Y. Li, tranformer-le high-power converter for large permanent magnet wind generator ytem, EEE Tran. Sutainable Energy, vol., no., pp. 8 9, July. [5] X. Yuan, Y. Li, J. Chai, and M. Ma, modular direct-drive permanent magnet wind generator ytem eliminating the grid-ide tranformer, Proc. the th European Conf. on Power Electronic and ppl., arcelona, pp. 7, Sep. 9. [6] H. Choi, W. Zhao, M. Ciobotaru,. G. gelidi, Large-cale P ytem baed on the multiphae iolated C/C converter, Proc. EEE nternational Sympoium on Power Electronic for itributed Generation Sytem, alborg, enmark, 5-8 June, pp [7] S. Kouro, C. Fuente, M. Perez, and J. Rodriguez, Single C-link cacaded H-bridge multilevel multitring photovoltaic energy converion ytem with inherent balanced operation, Proc. 8th nnual Conference on EEE ndutrial Electronic Society, Montreal, QC, Canada, 5 8 Oct., pp [8] M. R. lam, Y. G. Guo, and J. G. Zhu, novel medium-voltage converter ytem for compact and light wind turbine generator, Proc. utralaian Univeritie Power Engineering Conference, ali, ndoneia, September 6 9,. pp. 6. [9] M. R. lam, Y. G. Guo, and J. G. Zhu, H-bridge multilevel voltage ource converter for direct grid connection of renewable energy ytem, Proc. EEE Power & Energy Society nnovative Smart Grid Technology Conf., 6 Nov., Perth, utralia, pp. 7. [] M. R. lam, Y. G. Guo, and J. G. Zhu, medium-frequency tranformer with multiple econdary winding for grid connection through H-bridge voltage ource converter, Proc. nt. Conf. Ele. Machine and Sytem, Sapporo, Japan, Oct. 4,, pp. 6. [] M. R. lam, Y. G. Guo, and J. G. Zhu, medium-frequency tranformer with multiple econdary winding for medium-voltage converter baed wind turbine generating ytem, Journal of pplied Phyic, vol., no. 7, pp , May. 6