劉 慧，馬 柯

（1.丹麥奧爾堡大學(xué)能源系，奧爾堡9220，丹麥；2.上海交通大學(xué)電子信息與電氣工程學(xué)院風(fēng)力發(fā)電研究中心，上海200240）

模塊化多電平變流器（MMC）在電網(wǎng)故障下功率器件應(yīng)力分析

劉 慧1，馬 柯2

（1.丹麥奧爾堡大學(xué)能源系，奧爾堡9220，丹麥；2.上海交通大學(xué)電子信息與電氣工程學(xué)院風(fēng)力發(fā)電研究中心，上海200240）

模塊化多電平變流器MMC（modular multilevel converter）因具有很多的優(yōu)越特性，因而在過去十年中得到了快速的應(yīng)用與發(fā)展。但是在一些典型的應(yīng)用場合，如海上風(fēng)電傳輸，MMC變流器的功率器件有著非常嚴(yán)酷的運(yùn)行環(huán)境以及工況，這些因素都將會轉(zhuǎn)化為電力電子器件的電氣及熱力學(xué)應(yīng)力，加速器件的疲勞與故障，從而對變流器系統(tǒng)的可靠性造成不利影響。首先分析了海上風(fēng)電傳輸應(yīng)用領(lǐng)域的MMC變流器系統(tǒng)在電網(wǎng)故障時功率半導(dǎo)體器件應(yīng)力特性，并對MMC系統(tǒng)的運(yùn)行工況以及拓?fù)浣Y(jié)構(gòu)做了簡單介紹，最后對在各種單相電網(wǎng)電壓跌落下的器件損耗及熱應(yīng)力特性做了研究與分析。

模塊化多電平變流器；可靠性；任務(wù)剖面；低電壓穿越

Introduction

Offshore wind power have many advantages such as more stable wind speed,fewer disturbances with larger power capacity,all these features make the expansion of wind power plant from onshore to offshore attractive.modular multilevel converter-high voltage direct current（MMC-HVDC）systems［1-6］,which offer a series of merits over other existing wind power transmission systems such as higher DC voltage, modular structure,and improved redundancy,are gradually becoming an interesting solution for connecting offshore wind farms with the transmission grids.However,the semiconductors in the MMCHVDC system for offshore wind power plant may be more inclined to subject to failures,especially the power converters［7］,due to the harsh operating condi-tions and high number of component counts.Therefore,research on the failure of semiconductors in the MMC-HVDC system is of importance,not only for cost reduction but also for the improvement of reliability performance.

There are various types of failures in power semiconductor and two of the most commonly observed failure mechanismsare die-attach solder fatigue and bond-wire lift-off.Both of the failures are caused by thermal expansion coefficient（TEC）mismatch between adjacent layers in the power semiconductor modules.Power loss variation of MMC will lead to temperature cycling in the device,and the temperature cycling may affect the connections of the die-attach solder or bond wires.When the power cycling and temperature cycling come to a certain number,the die-attach solder or the bond wire will wear out［8-12］.Unlike other types of converters,the semiconductors in MMC bear different stress and correspondingly suffer various lifetime which is quite meaningful for the guidance of heatsink design and maintenance.In this paper,the thermal loading of the different semiconductors in the same module in MMC are investigated under different mission profiles.

The paper is organized as follows:The working region and the mission profile of the MMC for wind power application are first analyzed in the paper.Then the topology of MMC is introduced based on which the dynamic power losses under various mission profiles are calculated.Afterwards the corresponding thermal profile in the power semiconductor devices is studied with the variation of the power losses and the datasheet based electro-thermal model.Then the conclusion is given as last.

圖1 應(yīng)用于海上風(fēng)電傳輸?shù)腗MC變流器系統(tǒng)Fig.1 Wind power transmission system with MMC

圖2 MMC變流器系統(tǒng)子模塊與結(jié)構(gòu)Fig.2 Structure of modular multilevel converter（MMC）and sub-module

1 Power Loss Analysis

To illustrate the lifetime estimation procedure, an offshore wind power generation HVDC transmission system shown in Fig.1 is chosen as a study case, and 9 wind turbines with 2 MW rated power each are composed as an offshore wind farm.Both the generator-side converter and the grid-side converter employ the three-phase MMC,which is detailed in Fig.2,it can be seen that each phase of the MMC has twoarms being composed of N sub-modules and a series connected inductor.These sub-modules are identical and generally configured in half-bridge.Each submodule consists of two insulated-gate bipolar transistors（IGBTs）T1,T2and two antiparallel connected diodes D1,D2,as well as an energy storage capacitor C.The Infineon IGBT module FZ750R65KE3（Tj=150℃）is chosen as power semiconductor devices.The system specifications are basically designed according to Tab.1.

表1 本文MMC網(wǎng)測變流器系統(tǒng)主要參數(shù)Tab.1 Specifications of the MMC system

As the generator-side converter can share a similar analysis approach with the grid-side converter,for simplicity only the latter is analyzed in this paper.A phase shift carrier pulse width modulation（PSCPWM）scheme［13］is applied and its output line-to-line voltages vab,vbc,vcaare shown in Fig.3.The distortion of the output voltage is caused by the charging and discharging of the capacitors in sub-modules.The output current iaand arm currents ia1,ia2in phase A are displayed in Fig.4.The distortion of the arm currents is caused by the dc components and ac components in the zero sequence current.

圖3 網(wǎng)側(cè)MMC變流器的參考輸出電壓Fig.3 Output voltage of the grid-side converter with MMC

圖4 網(wǎng)側(cè)MMC變流器A相輸出電流以及橋臂電流Fig.4 Output current and arm currents in phase A of the grid-side converter with MMC

There are several types of grid faults in such system and three typical grid faults are defined and explained in［14］,single-phase ground fault,two-phase connected fault and three-phase ground fault.However, no matter which type it is,when a short-circuit fault happens in the power grid system,normally,the voltage dips will be detected by the grid-connected converter through the ac bus it is connected（PCC）,and then the corresponding LVRT control algorithm of the WTS is activated,making the converter shift from the normal operation mode to the LVRT mode.In this paper,only single-phase ground fault in phase A is focused on［15,16］.

Different from the traditional two-level or three-level converters,the power calculation of MMC is much more complicated.Therefore,the MMC working principle along with its power loss calculation is introduced.

The working principle of the sub-module in MMC can be divided into four stages depending on the gate signal of each device g（gT1,gT2,gD1,gD2denoting the gate signal of upper IGBT,lower IGBT,upper Diode and lower Diode,respectively）and the current direction ikp（n）（k=a,b,c,p=upper arm,n=lower arm）.As shown in Fig.5,the following can be obtained for the operation:

圖5 A相子模塊的PWM調(diào)制信號Fig.5 PWM pattern for sub-modules in phase A

（1）From t1to t2,the arm current ikpis positive,if T1is turned on,T2is turned off,the current flows through D1and C,the capacitor is charged as illustrated in Fig.6（a）and PSM=PD1;otherwise,T1is turned off,T2is turned on,the current will go through T2（Fig.6（b））and PSM=PT2;

（2）In the next stage from t2to t3,the arm current ikpis negative,when T1is turned off,T2is turned on,the current flows through D2（Fig.6（c））,PSM=PD2; oppositely,when T1is on,T2is off,the current will go through T1and C,the capacitor is discharged as shown in Fig.6（d）,PSM=PT1.

圖6 MMC變流器系統(tǒng)子模塊的4個工作模式Fig.6 Four working stages of the sub-module in the MMC

Where PSMis the power loss of the sub-module,is the loss of D1,T2,D2,T1respectively.This can be represented as shown in Tab.2 and the simulation results are shown in Fig.7 which is obtained under the rated wind speed.

表2 MMC變流器系統(tǒng)子模塊的4個工作模式Tab.2 Four working regions of sub-module in MMC

In Fig.7,the upper arm current（Iap）,the gating signal for the upper IGBT（Gap）,the capacitor voltage and current（Vcand Ic）,the currents flowing through the four semiconductors in the first module in upper armis shown.As can be seen in Fig.7,when the arm current iapis positive,the current go through D1,T2and when the arm current iapis negative,the current flow through,D2,T1depending onthe status of the gating signal Gap.The results agree with the definition in Tab.2.

圖7 MMC變流器系統(tǒng)額定工作狀態(tài)下的器件電流應(yīng)力Fig.7 Simulation results of working principle of MMC

The power losses of the semiconductors in one sub-module in phase A under the rated wind speed are given in Fig.8 by employing the power loss model which shares the same idea as that in［17］.From Fig.8, the semiconductors in the same module consume different power losses and the T2occupies the most,followed by T1and D1orderly and D2has the smallest power loss.The unbalanced power loss is caused by the distortion of the arm current due to the zero sequence component in the arm current,especially the dc component which makes the positive arm current lasting much longer time than the negative arm current in each fundamental period.Therefore,according to the above analysis,the D1and T2work longer time than D2and T1and thereby consume more energy.

圖8 MMC變流器系統(tǒng)A相子模塊額定工作狀態(tài)下的器件實(shí)時損耗Fig.8 Power loss of the devices in the first SM in phase A

Fig.9 gives the operation profile in German grid codesforthe MMC outputsundersingle-phase ground fault in phase A with the grid voltage dip severity denoted as Dip=0.5 p.u..The output voltages, load currents,and the instantaneous active/reactive power values are shown,respectively.As it is shown in Fig.9,the output voltages under the given grid fault condition are consistent with the fault definition, and the currents are symmetrical among the three phases,which means that only positive sequence currents are injected according to the low-voltage ridethrough（LVRT）codes for wind turbine systems（WTSs）issued by several countries［18］.Due to the existence of negative sequence voltage,there is a second order frequency oscillation in the delivered active and reactive power values under the conditions of one-phase and two-phase unbalanced grid faults.

圖9 在A相電網(wǎng)電壓跌落時MMC變流器輸出波形（電壓跌落0.5 p.u.,風(fēng)速vw=11 m/s）Fig.9 Outputs of the grid-side MMC under Singlephase ground fault in phase A（Dip=0.5 p.u.,vw=11 m/s,German grid code[18]）

The averaged active power and reactive power delivered by MMC versus voltage dips under the single-phase ground fault in phase A are shown in Fig.10.It can be seen that the reactive power is significantly different,especially when Dip＜0.5 p.u..

圖10 A相電網(wǎng)電壓跌落幅值下MMC變流器有功和無功（風(fēng)速vw=11 m/s）Fig.10 Active and reactive power values versus dip severity on the PCC under single-phase ground fault in phase A（vw=11 m/s,German grid codes[18]）

The loss distributions of the four power switching devices in the same module in each phase at different grid voltage dip severity values under the single-phase ground fault in phase A are indicated in Fig.11,in which the AT1,BT1,CT1mean T1in phaseare D1in phaseare T2in phase A,B,C andare D2in phase A,B,C.All of them are obtained under the rated wind speed vw=11 m/s.It can be seen that the loss distribution among the three phases of the converter are asymmetrical under the one-phase unbalanced grid fault.Moreover,the most stress is imposed to T2,which is different from the normal condition.

圖11 各種單相電網(wǎng)電壓跌落幅值下MMC變流器器件損耗（風(fēng)速vw=11 m/s）Fig.11 Power loss versus grid voltage dip severity under single-phase ground fault in phase A（vw=11 m/s,German grid codes[18]）

2 Thermal Distribution of Power Device

The thermal distribution of the MMC can be obtained based on the power loss calculated above and the dynamic electro-thermal model denoted by the thermal equivalent circuit diagrams in Fig.12.

In Fig.12,TJIGBTand TJDiodeare junction temperatures of IGBT and Diode,respectively;TCcase temperature;THheatsink temperature;TAambient temperature;Zththe impedances from junction to ambient;ZthJC,the impedances from junction to case;ZthCHthe impedances from case to heatsink;ZthHAthe impedances from heatsink to ambient.Both ZthTJCand ZthDJCare composed of the equivalent thermal resistance and capacitance networks,generally described by Foster and Cauer［19］.Equivalent impedance can be obtained from the datasheet.Thermal resistances Rican be directly read,whereas thermal capacitances Cican be computed from τi=RiCiwhich is given in datasheet.PLIGBTmeans the power loss in the UGBT, PLDiodemeans the power loss in the freewhecling diode.The junction temperature calculation can be expressed as

圖12 功率半導(dǎo)體器件熱力學(xué)模型Fig.12 Electro-thermal network of power semiconductor devices

Here,a fourth-order Cauer model is employed as described in Tab.3 and RTH=0.009 K/W and RDH= 0.014 K/W.

表3 功率器件4階Cauer型熱力學(xué)模型參數(shù)Tab.3 Specification of the fourth-order cauer model

Based on the previous loss results and the above thermal model,the junction temperature of the power devices under various grid faults can be investigated by PLECS blockset in Simulink.The thermal profiles under normal condition single-phase ground fault in phase A are shown in Fig.13 and Fig.14,respectively.

圖13 MMC網(wǎng)測變流器在正常電網(wǎng)電壓下子模塊器件熱應(yīng)力（風(fēng)速vw=11 m/s）Fig.13 Thermal distribution of the grid-side MMC under normal condition（vw=11 m/s）

圖14 在單相電網(wǎng)電壓跌落下MMC網(wǎng)測變流器子模塊器件熱應(yīng)力（跌落幅值0 p.u.，風(fēng)速vw=11 m/s）Fig.14 Thermal distribution of the grid-side MMC under single-phase ground fault（Dip=0 p.u.,vw=11 m/s）

Fig.13 displays the thermal distribution under normal condition in phase A,B and C,in which it can be seen that the operation is balanced among the three phases under normal condition.Moreover,no matter which phase,T2and D1are the most stressed in this situation.

The junction temperatures for the devices in the same module in the three phases of MMC undergoing single-phase grid fault are shown in Fig.14（a）～（c）, respectively.It is shown that the thermal loading behaviors in the three phases of the converter are totally different from each other under this fault condition.In phase A,T1and T2are the most stressed,while in Phase B,T1and D2are more stressed and in phase C, T2and D1are the most stressed as compared to normal condition.

The junction temperatures of the switches and the diodes under different dip severity values of the one-phase grid fault are shown in Fig.15.It is noted that when Dip is below 0.5 p.u.,T1and T2have the higher temperatures in phase A,while in phase B,T1and D2have the higher temperatures and in phase C, the temperatures of T2and D1are higher than the other two.When Dip is above 0.5 p.u.,the temperatures in all three phases trend to the normal condition.

圖15 在各種單相電網(wǎng)電壓跌落下MMC網(wǎng)測變流器，子模塊器件熱應(yīng)力（風(fēng)速vw=11 m/s）Fig.15 Thermal distribution of the grid-side MMC under single-phase ground fault（vw=11 m/s）

4CONCLUSION

Because the loading conditions of power devices in the MMC under grid faults are still not yet clarified,this paper has focused on the investigation of influence of different mission profile and various grid voltage dips on the operating and loading behaviors of MMC.The analyzing methods and results are of importance for understanding and improving the thermal performance of the MMC under grid fault condition.

Depend to the types and severity values of grid faults,as well as LVRT control strategies,the operation status of the grid-connected MMC such as the output current,and the delivered power are significantly different.It is noticed that the operation conditions of the MMC are unsymmetrical among the three phases when unbalanced grid faults are present.

Both the grid faults and severity values of voltage dips will lead to unbalanced device loading among the three phases of MMC.It should be notedthat some power devices under the LVRT operation may have even higher junction temperature than the normal full load operation condition.This overloading should be taken into account when choosing the power devices and designing the heat sink system for the wind power converter.

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Loading Analysis of Power Devcie in Modular Multilevel Converter Under Grid Faults

LIU Hui1,MA Ke2
（1.Department of Energy Technology,Aalborg University,Aalborg 9220,Denmark；2.school of Electronic Information and Electrical Engineering,Shanghai Jiao Tong University,Shanghai 200240,China）

Due to a series of merits,Modular Multilevel Converters（MMCs）have been developing dramatically in the last decade.However in some applications like offshore power transmission the MMC modules are subjected to a variety of tough mission profiles with adverse working conditions,all of these could lead to thermo-mechanical fatigues in the components and joints of the power modules and thereby causing reliability challenge to this converter.This study focuses on the loadin ganalysis of MMC considering the grid faults in the High Voltage Direct Current（HVDC）application for offshore wind power.The faulty conditions as well as the topology of the MMC are analyzed in order to better understand the loading characteristics of MMC.Then the power loss calculation along with the electro-thermal simulation is conducted to uncover the thermal loading behaviorsof MMC under various single-phase grid fault conditions.

modular multilevel converter（MMC）;reliability;mission profile;low-voltage ride-through（LVRT）

劉慧

劉慧（1986-），女，博士，研究方向：電力電子可靠性，E-mail：hui@et.aau.dk。

馬柯（1985-），男，博士，特別研究員，研究方向：電力電子可靠性提高及設(shè)計，可再生能源發(fā)電，E-mail：kema@sjtu.edu.cn。

10.13234/j.issn.2095-2805.2016.6.1

TM464

A