Alternate Arm Converter Operation of the Modular Multilevel Converter

8/17/2019 Alternate Arm Converter Operation of the Modular Multilevel Converter

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978-1-4799-5776-7/14/$31.00 (c)2014 IEEE

Alternate Arm Converter Operationof the Modular Multilevel Converter

M.M.C. Merlin, P.D. Judge, T.C. Green, P.D. MitchesonImperial College London

London, [email protected]

F. Moreno, K. DykeAlstom GridStafford, UK

[email protected]

Abstract — A new operating mode of the Modular Multilevel

Converter (MMC) using modified arm current waveforms

inspired from the working principle of the Alternate Arm

Converter (AAC) is presented in this paper. A reduction in the

cell voltage deviation is observed at power factors close to unity

at the cost of an increase in power losses, especially when

reactive power is required. This gain in voltage margin is then

used in further optimizations of the MMC performance, mainly

focusing on either increasing the number of redundant cells or

improving the overall power efficiency of the converter.

I. I NTRODUCTION

The Modular Multilevel Converter (MMC) [1], illustrated inFig. 1, was the first modular Voltage Source Converter (VSC)to provide both low switching losses and low AC currentdistortion; these features contribute to the higher powerefficiency and the significant reduction in the size of the ACfilters of recent VSC power stations [2]. A number of MMCconverter stations are now in operation and more are plannedin the next couple of years with power ratings ever increasing.Over the last few years, a new family of hybrid topologies [3-6], such as the Alternate Arm Converter [4,5] (AAC), drawn

in Fig. 2, have emerged. These topologies essentiallycombine both the 2-level VSC and the MMC together inorder to improve on some characteristics such as smalleroverall volume, better power efficiency and, in sometopologies, DC-side fault blocking capability. It has also beenshown [7] that the AAC has a better temperature distribution between the IGBT modules of a cell in comparison to theMMC. Extensive studies have also presented differentmodelling approaches of the electical dynamics of thesemultilevel converters and it has been shown that reduceddynamic models have an acceptable level of accuracy whileoffering an appreciable boost in computing speed [8-11]. TheAAC has a relatively similar electrical topology to the MMCwith the main difference being the presence of directorswitches, i.e. series-IGBTs, in its arms in series with the stackof cells. The switching state of these director switchesdetermines which arms are both generating the convertervoltage waveform and carrying the AC current to therespective DC terminal. This approach has proven [4,5] to be

both power and volume effective, resulting in (i) the numberof cells be significantly reduced, e.g. up to half the number ofcells compared to the MMC, (ii) the average cell voltagedeviation is lower allowing smaller capacitors in the cells and(iii) the use of full H-bridge cells instead of half-bridge cells,thus the AAC is able to block DC-side faults withoutcompromising its overall power efficiency. In the case of theMMC, it was observed rapidly after the topology was

proposed that unplanned circulating currents (mainly secondharmonics) were running between the legs [12]. This issuewas later solved by implementing additional feedback loopsin charge of monitoring the arm currents [13]. Besides, it hasalso been observed that this circulating current can be used toreduce the cell voltage deviation of the cell capacitors [12] or by adding a 2nd harmonic filter at the midpoint of the arminductors [14].This paper presents an original arm current waveform for thein the MMC, inspired by the working mechanism of theAAC. This study focuses on the effects of changing the armcurrent waveforms which only implies an update of thecontrol system (software) but not the converter itself(hardware) which thus stays the same. This implies that (i) both the AC and DC quantities (e.g. power, voltage andcurrent magnitudes) remain unchanged, (ii) the cells are stillof the half-bridge type and (iii) the passive components (e.g.cell capacitors and arm inductors) keep the same values.Furthermore only the top level part of the controller isaffected (e.g. current control) but the low-level control staysthe same thus this does not require changes in either thewiring or the communication with the cells. Finally the benefits of this update both in terms of cell capacitor voltagedeviation and power efficiency will be explored and marginsfor further optimizations are discussed. Finally, modularVSC topologies such as the MMC and the AAC can also beconnected in front-to-front arrangements (meaning that the

AC sides are facing each others) in order to form a DC/DCconverter such as in [15]. Therefore the current modulationconcept introduced in this paper can potentially also be usedin such DC/DC topologies with equivalent pros and cons asdiscussed in this paper.

This study has been financed by the EPSRC under the UK PowerElectronics Centre – Converter Theme grant (EP/K035096/1)

mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


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Figure 1 - Half-bridge MMC topology

Figure 2 - AAC topology

II. ARM CURRENT WAVEFORMS

In its classic operating mode, the MMC distributes equally(i) the AC current between its upper and lower arms and (ii)the DC current between the three different legs. The completeset of equations describing the arm currents is given in (1)-(6).The signs present in these equations depends on the directionof the currents, using what is illustrated in Fig 1 and 2.

IA+ =

2

3 (1)

IA− =

2

3 (2)

IB+ =

2

3 (3)

IB− =

2

3 (4)

IC+ =

2

3 (5)

IC− =

2

3 (6)

The AAC distributes its AC currents in a different patternas only one arm in each leg carries the full AC current whilethe other has its director switch opened, blocking any currentfrom flowing through the arm. Using the variablesαA,B,C ∈ {0,1} which represent whether the AC current of a particular leg is passing either through the top or bottom arm,the set of equations describing the arm currents in AAC mode

are given in (7)-(12):

IA+ =

3 (7)

IA− = (1 )

3 (8)

IB+ =

3 (9)

IB− = (1 )

3 (10)

IC+ =

3 (11)

IC− = (1 )

3 (12)

However the AAC inherently generates a 6-pulse ripple ontop of the DC current waveform as a consequence of thealternating nature of the arm conduction periods. Since theobjective of this study is to migrate an already existing MMCconverter to an AAC mode of operation without having tochange the hardware but only by updating the control system(i.e. software), installing a DC filter is out of question. Toresolve this issue, an additional active filtering current isadded to the arm currents in order to keep the DC currentsmooth. This filtering current IF (13) is obtained by calculatingthe ripple component of the DC current waveform resultingfrom AAC operation in order to suppress it by adding acirculating current of opposing sign through all three legs.

= +

+ +

= −

− −

= (13)

The addition of this active filtering current alters theoriginal nature of the AAC mode as the arms will nowcontinuously run a current through them as opposed to foronly one half of the cycle and no current during the other halfof the cycle because the director switches would be closed.However the magnitudes of the arm currents will be small fora large proportion of the time, while the arm is not carryingthe main AC current, compared to the MMC mode ofoperation, as shown in the simulation section below. Finally,only the arm current waveform will be different because thecurrents seen at both the AC and DC terminals of the converterwill be the same as under the normal MMC mode of operation.

III.

SIMULATION A.

Simulation Model

In order to assess the benefits of the AAC mode ofoperation, a 120 MW MMC converter has been simulatedusing Simulink® and the Artemis® toolbox in order tosimulate a reasonably large number of half-bridge cells. Thecharacteristics of this simulation model are listed in Table 1. In order to better match realistic MMC converter, triplenharmonic voltage injection has been used (about 15% of the


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fundamental magnitude) in order to shape the convertervoltage waveform into an almost trapezoidal waveform and to push the power rating to its theoretical maximum while stillusing only half-bridge cells. Furthermore, a power lossanalysis has also been performed by post-processing thevoltage and current waveforms in each cells using the powerloss data from the datasheet of the 3.3 kV 1.2 kA HiPak IGBT

device 5SNA 1200E330100 [16] using the method describedin [7, 17] . In the later part of the simulation section, the steadystate junction temperature of the different semiconductordevices in a cell (e.g. top and bottom IGBT and diodemodules) by applying the individual power loss figure of eachdevice into a power-thermal model derived in ANSIS andverified in [17] and assuming a coolant temperature of 60ºCshared by all the semiconductor devices.

Table 1 - Characteristics of the simulated MMC model

Rated power 120 MW

DC bus ±50 kV

AC line 55 kV

AC frequency 50 Hz

Triplen harmonic voltage 15%

Number of cells per stack 56

Cell capacitor 8 mF

Phase inductor 8 mH

Arm inductor 6 mH

B.

Arm current waveforms

The arm current waveforms resulting from this mode ofoperation are observed in this section. Fig. 3 shows the toparm current waveform in both MMC and AAC operating

modes (respectively the red and green curves) under unity power factor rectifier mode. The third curve (blue) is theresult of the difference between the two waveforms and can be interpreted as the equivalent circulating current which can be injected in the converter to move from the MMC mode tothe AAC mode of operation. It can be observed that at unity power factor a large amount of the fundamental cycle is spentwith a small amount of current magnitude in AAC mode asopposed to the MMC mode. As more reactive power isinvolved in the conversion process as shown in Fig 4 (sameamount of active and reactive power) and in Fig. 5 (reactive power only), the arm current waveform becomes more andmore distorted in AAC mode with still a significant portionof the cycle used by low magnitude current but at the expenseof an increasing peak values to attain twice the value in theMMC mode. The power losses have computed for differentoperating points and the results listed in Table 2 with the total power loss values plotted in accordance to the angular valueof their respective operating points in Fig. 6.

Figure 3 - Arm A+ current waveform in MMC (red), AAC (green)modes and the resulting circulating current (blue) with active poweronly

Figure 4 - Arm A+ current waveform in MMC (red), AAC (green)modes and the resulting circulating current (blue) with both activeand reactive powers

Figure 5 - Arm A+ current waveform in MMC (red), AAC (green)modes and the resulting circulating current (blue) with reactive

power only


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Table 2 – Power losses of the MMC model depending on the mode of operation and operating point relative to the apparent power

Active Power

Reactive Power

1 pu

0 pu

0.7 pu

0.7 pu

0 pu

1 pu

-0.7 pu

0.7 pu

-1 pu

0 pu

-0.7 pu

-0.7 pu

0 pu

-1 pu

0.7 pu

-0.7 pu

MMCmode

Conduction

losses0.50% 0.44% 0.36% 0.37% 0.38% 0.37% 0.35% 0.45%

Switchinglosses

0.15% 0.14% 0.13% 0.13% 0.15% 0.15% 0.15% 0.16%

Total

losses0.65% 0.58% 0.49% 0.51% 0.53% 0.52% 0.50% 0.60%

AAC

mode

Conduction

losses0.53% 0.56% 0.51% 0.43% 0.38% 0.40% 0.49% 0.53%

Switching

losses0.16% 0.22% 0.24% 0.20% 0.17% 0.26% 0.30% 0.32%

Total

losses0.69% 0.78% 0.75% 0.63% 0.55% 0.66% 0.79% 0.85%

Figure 6 - Power losses as percent of the total apparent power in MMC and AAC modes for different operating points

C. Observation at unity power factor

The previous set of results indicates that this new AAC modeis only potentially attractive for unity power factor only as both the arm current peak values and the power losses areincrease dramatically when reactive power is involved in theconversion process. The next part of this paper assumes thatonly these two operating points (inverter and rectifier active power only) are considered with the results focusing on therectifier mode mainly since the inverter mode is merely anopposite phase angle version of the former.

Figure 7 - Arm current waveforms in MMC mode

Figure 8 - Arm current waveforms in AAC mode

Fig. 7 and 8 show the arm current waveforms respectivelyin MMC and AAC modes of operation. On one hand, theMMC mode results in the arms continuously conducting alarge amount of current as opposed to the AAC mode where asignificant part of the cycle is spent with only a small amount


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of current (around -150 A). This remaining low magnitudecurrent mainly consists of the filtering current IF (13). On theother hand, the peak value of the arm current currents is lowerin the MMC mode compared to the AAC mode (respectively1.3 kA and 1.6 kA thus 23% higher in the AAC case).

Figure 9 – Upper cell voltage waveforms in MMC mode at normal

PWM frequency (Fpwm = 169 Hz)

Figure 10 – Lower cell voltage waveforms in MMC mode at normal PWM frequency (Fpwm = 169 Hz)

The other observable difference between these two modesof operation is the voltage deviation of the cell capacitors. Fig9 and 11 present both the average and individual cell voltagesin one arm for respectively the MMC and AAC modes at thesame phase-shifted PWM [18] at the same frequency(169 Hz). The AAC mode exhibits a lower voltage deviation, both in average and individual voltages, due to the way thevoltage and current waveforms of its arms combine to create

the energy exchange. Furthermore, the waveforms of theupper and lower stack voltages are similar in each modes ofoperation (Fig. 9 and 10 for the MMC mode and Fig. 11and 12 for the AAC mode), with the lower stack voltagewaveform being essentially a time-shifted version of the upperstack voltage waveform. The individual cell voltages are moreevenly distributed around the average value in the AAC modeof operation compared to the MMC mode. In the next sectionof this paper, only the upper stack voltage will considered. Interms of power efficiency, the two modes of operation

generate approximately the same amount of power losses atunity power factor as noted previously with the MMC mode performing slightly better in switching losses while the AACmode wins on conduction losses. The simulation results aresummarized in Table 3.

Figure 11 – Upper cell voltage waveforms in AAC mode at normal

PWM frequency (Fpwm = 169 Hz)

Figure 12 – Lower cell voltage waveforms in AAC mode at normal PWM frequency (Fpwm = 169 Hz)

IV. OPTIMIZATION SCENARIOS

A. Reduced PWM frequency

Since the AAC mode of operation improves the cellvoltage deviation, two possible optimizations can be done.The first possible optimization consists of reducing the PWMfrequency. This will result in (i) higher differences betweenthe cell voltages but (ii) cell voltages still approximatelywithin the same envelope as in the MMC mode while (iii) theswitching losses will decrease. Fig. 13 and Table 3 (second“AAC” column) show the results for a reduced PWMfrequency at 119 Hz. As predicted, the average cell voltage isunaffected but the individual cell voltages are furtherdistributed around the average cell value but stillapproximately within the limits of the MMC mode ofoperation. The conduction losses are unchanged because thecurrent waveforms are unchanged but the switching lossesdecreased because the IGBTs are not switching as often,leading to a marginally better power efficiency figure.


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Table 3 - Simulation results in rectifier mode at unity power factor

Mode of operation MMC AAC AAC AAC

Average cell voltage 1.800 kV 1.800 kV 1.800 kV 1.880 kV

PWM carrier frequency 169 Hz 169 Hz 119 Hz 169 Hz

Average peak-to-peak cell voltage

deviation 252 V 172 V 172 V 166 V

Maximum individual cell voltage 1,980 V 1,954 V 1,942 V 2,003 V

Minimum individual cell voltage 1,660 V 1,696 V 1,673 V 1,767 V

Arm current (Maximum value) 1,323 A 1,643 A 1,643 A 1,643 A

Arm current (Minimum value) -503 A -244 A -244 A -244 A

Arm current (Average value) 402 A 402 A 402 A 402 A

Arm current (RMS value) 753 A 803 A 803 A 803 A

Conduction losses 468 kW 459 kW 459 kW 459 kW

Switching losses 180 kW 202 kW 156 kW 209 kW

Total power losses 648 kW 661 kW 615 kW 668 kW

Power efficiency (semiconductor only) 99.46% 99.45% 99.49% 99.44%Temperature IGBT 1 (steady state) 75ºC 74ºC 72ºC 73ºC

Temperature Diode 1 (steady state) 74ºC 72ºC 70ºC 72ºC

Temperature IGBT 2 (steady state) 74ºC 75ºC 75ºC 76ºC

Temperature Diode 2 (steady state) 100ºC 104ºC 103ºC 104ºC

Number of extra redundant cells 0 0 0 2.8 (5%)

Figure 13 – Upper cell voltage waveforms in AAC mode with

reduced PWM frequency (Fpwm = 119 Hz)

B. Increased average cell voltage

The second possible optimization is to use the maximumcell voltage headroom made available by the reduced cellvoltage deviation. This optimization will result in a smallnumber of cells (around 5% in this case) being maderedundant, thus increasing the overall reliability of theconverter station in case of cell failures. Fig. 14 shows that thecells can be charged up to the new nominal value of 1,880 Vwith the highest peak value for the individual cell voltages being not much higher than in the MMC case.

Figure 14 – Upper cell voltage waveforms in AAC mode with higher

nominal cell voltages (Vcell = 1,880 V)

Table 3 shows that the switching losses went up

proportionally to the higher nominal cell voltage. However, itcould be possible to reduce further the power losses, e.g. by bypassing electrically the now-redundant cells. The peak-to- peak voltage deviation is lower than in the other AAC modecases because of the quadratic nature of the relationship between the cell voltage deviation and the energy deviation.The latter is still unchanged but the higher nominal cellvoltage means that the energy deviation will happen at ahigher part of the quadratic curve, hence the slightly smallerresulting voltage deviation.


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V.

CONCLUSION

The AAC mode of operation of an existing MMC stationhas been presented in this paper. This new mode of operationdoes not require any hardware modification of the converterstation but rather an update of the control system, hencemainly software. Generally the power losses increase in theAAC mode of operation compared to the MMC mode but areapproximately the same when operating close to unity power.Given this fact, the study focused only on active powerconversion operating points. In this condition, the AAC-modestill exhibits higher peak arm current combined with asignificant part of the cycle at low current magnitudes but,most importantly, smaller cell voltage deviations (e.g. 30%lower) compared to the classic MMC mode are observed.

This last fact can be used to further optimize the performance of the MMC converter station in two differentways. First, reducing the switching frequency of the PWMsignal will result in a substantial decrease in the switchinglosses, hence a higher power efficiency figure for theconverter station. This leads also to an increase of the

individual cell voltage deviations but those are still containedwithin the same limits set during the previous MMC mode.Second, increasing slightly the nominal voltage of the cellsresults in having some cells being made redundant(approximately 5% of the cells in each stack) since the stacksare able to produce more voltage than normally required.Despite the higher nominal voltage, the maximum peak valuehit by the cell voltages is still the same maximum value setduring the previous MMC mode of operation. From here,either (i) these redundant cells are electrically bypassed inorder to further improve the power efficiency of the converterstation, or (ii) they can be used as back-up cells in case of othercell failures, thus further improving the overall reliability ofthe MMC.

R EFERENCES

[1] A. Lesnicar and R. Marquardt, “An innovative modularmultilevel converter topology suitable for a wide power range,” inPowerTech Conference Proceedings, 2003 IEEE Bologna, vol. 3, jun 2003

[2] Sellick, R. L., and M. Åkerberg. "Comparison of HVDC Light(VSC) and HVDC Classic (LCC) Site Aspects, for a 500MW 400kV HVDCTransmission Scheme." ACDC 2012, Birmingham UK.

[3] D. R. Trainer, C. C. Davidson, C. D. M. Oates, N. M. MacLeod,D. R. Critchley, and R. W. Crookes, “A new hybrid voltage-sourcedconverter for HVDC power transmission,” CIGRE Paris Sess. 2010, 2010.

[4] M. Merlin, T. Green, P. Mitcheson, D. Trainer, D. Critchley, andR. Crookes, “A new hybrid multi-level voltage source converter with dc fault

blocking capability,” in AC and DC Power Transmission, 2010. ACDC. 9thIET International Conference on, Oct 2010.

[5] Merlin, M.M.C.; Green, T.C.; Mitcheson, P.D.; Trainer, D.R.;Critchley, R.; Crookes, W.; Hassan, F., "The Alternate Arm Converter: A New Hybrid Multilevel Converter With DC-Fault Blocking Capability,"Power Delivery, IEEE Transactions on , vol.29, no.1, pp.310-317, Feb. 2014.

[6] Adam, Grain Philip, et al. "New breed of network fault-tolerantvoltage-source-converter HVDC transmission system." Power Systems,IEEE Transactions on 28.1 (2013): 335-346.

[7] Judge, P.D.; Merlin, M.M.C.; Mitcheson, P.D.; Green, T.C.,"Power loss and thermal characterization of IGBT modules in the AlternateArm converter," Energy Conversion Congress and Exposition (ECCE), 2013IEEE , vol., no., pp.1725-1731, 15-19 Sept. 2013

[8] Saad, H.; Dennetiere, S.; Mahseredjian, J.; Delarue, P.; Guillaud,X.; Peralta, J.; Nguefeu, S., "Modular Multilevel Converter Models forElectromagnetic Transients," Power Delivery, IEEE Transactions on ,vol.29, no.3, pp.1481-1489, June 2014.

[9] Peralta, Jaime, et al. "Detailed and averaged models for a 401-level mmc – hvdc system." Power Delivery, IEEE Transactions on 27.3(2012): 1501-1508.

[10] Beddard, A.; Barnes, M.; Preece, R., "Comparison of DetailedModeling Techniques for MMC Employed on VSC-HVDC Schemes,"

Power Delivery, IEEE Transactions on , vol.PP, no.99, pp.1,11[11] Caitríona E. Sheridan; Michaël M. C. Merlin; Timothy C. Green,“Reduced Dynamic Model of the Alternate Arm Converter”, COMPEL2014, Santander, June 2014

[12] Wang, Kui, et al. "Voltage balancing and fluctuation-suppressionmethods of floating capacitors in a new modular multilevel converter."Industrial Electronics, IEEE Transactions on 60.5 (2013): 1943-1954.

[13] Antonopoulos, Antonios, Lennart Angquist, and H-P. Nee. "Ondynamics and voltage control of the modular multilevel converter." PowerElectronics and Applications, 2009. EPE'09. 13th European Conference on.IEEE, 2009.

[14] Jacobson, Bjorn, et al. "VSC-HVDC transmission with cascadedtwo-level converters." Cigré session. 2010.

[15] Luth, T.; Merlin, M.M.C.; Green, T.C.; Hassan, F.; Barker, C.D.,"High-Frequency Operation of a DC/AC/DC System for HVDC

Applications," Power Electronics, IEEE Transactions on , vol.29, no.8, pp.4107-4115, Aug. 2014

[16] ABB HiPak IGBT Module 5SNA 1200E330100. Datasheetavailable online. [Online]. Available: http://www.abb.com/semiconductors

[17] U. Drofenik, D. Cottet, A. Müsing, J. M. Meyer, and J. W. Kolar,“Modelling the thermal coupling between internal power semiconductor diesof a water-cooled 3300V/1200A HiPak IGBT module,” in Proceedings ofPower Conversion and Intelligent Motion Conference, 2007

[18] Tu, Qingrui, Zheng Xu, and Lie Xu. "Reduced switching-frequency modulation and circulating current suppression for modularmultilevel converters." Power Delivery, IEEE Transactions on 26.3 (2011):2009-2017.

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Alternate Arm Converter Operation of the Modular Multilevel Converter