modulation and control of modular power converters for

Modulation and control of modular power converters for high-power applicationsProf. Marco Liserre

Outline Modular power converters in high power applications

Parallel power converters for low-voltage, high-current and high-bandwidth services

Modular Multilevel Converters for management of high-voltage dc-grid

The Reliability Challenge addressed through Power Routing

Conclusions

Marco Liserre [email protected]

Modulation and control of modular power converters for high-power applications

High-power applications

Source: Interreg Project PE-Region

• FACTS and HVDC

• Power Generation

• Transportation

• Data Centers

Growing power demand

More Controllability



High-power applicationsLarge Data Center


AC-Architecture

• Data Center traffic triples from 2015 to 2020

• Consuming around 2 percent of the worlds electrical energy

DC-Architecture

• Direct integration of DC back up power• Less conversion stages• Higher efficiency• Reduced installation and maintenance costs• Smaller footprint

• DC back up power of the UPS needs to beconverted to AC power

• Bypassing of UPS improves efficiency but introduces reduced system reliability

Source: Emerson Network Power

Source: Emerson Network Power

Source: Cisco


High-power applicationsEV Charging stations

Source: Bloomberg

Source: Bloomberg

High expected growth of EV

Decreasing battery costs will result in high market penetration

High demands forcharging infrastructure



High-power applicationsfrom HVDC to Supergrid

Losses

AC 6 % per 100 km (110 kV)AC 5 % per 1000 km (800 kV)DC 2,8 % per 1000 km (800 kV)



High-power applicationsSmart Transformer for meshed and hybrid grids

Source: LV-Engine project

Significant reduction in 11kV/LV network reinforcement caused by the uptake of LCTs & electrification of heat

& transport sectors.



• Lower voltage/current rated devices• Scalability in voltage/power• Reduced dv/dt and/or di/dt• Advantageous for high step up/down ratios

• Fewer number of components• High Voltage devices• Simple control/communication system

Non Modular vs. Modular



Handling faults in Modular topologies







Conclusions



Parallel Inverters

• Parallel inverters are widely utilized in high-power applications

• Interleaved operation of parallel inverters

Multi-MW WES STATCOM ST APF ED

Use of low-voltage devices

with trafo

Improved quality of the

output current

Decreased filter size

Circulating currents

High-bandwidth

control

MV bus

MV grid20 kV

20 kVuSC=2.5%

0.510 kV

Lf,1

Lf,2

dc

ac

dc

ac

dc

ac

dc

ac

dc

ac

dc

ac

Lf,3

Lf,4

Lf,5

Lf,6

T1

T2

3

3

3

3

3

3

3

uSC=2.5%20 kV0.510 kV

Parallel InvertersNPC-3L-Topologydc: 1500 Vac: 920 V, 300 A

Filters

MV impedance measurement system

MV transformers

imeas

Zgrid



Fieldmeasurements in Vollstedt (Nordfriesland)


Circulating Current

• CMV of the paralleled system

• Circulating current

• In 3-level inverters, the CMV can be reduced by choosing optimal carrier arrangement

• Reduced CMV leads to diminished circulating current

circulating current increases with smaller filters



Modulation Methods for parallel NPC-converters

• Sine Pulse-Width Modulation (SPWM)• Maximum modulation index (MI) - 1.0

• Space-Vector Pulse-Width Modulation (SVPWM)• Overmodulation up to MI – 1.15• Lower CMV compared to that of the SPWM

• Discontinuous Pulse-Width Modulation (DPWM)• Overmodulation up to MI – 1.15 • Reduced number of switching operations of each device• Diminished stress of power semiconductors• Decreased power losses of converters• DPWM with 60° phase shift (DPWM60) has the best

performance in systems with unity Power Factor (PF)• DPWM30 is considered as a tradeoff for wide PF range

Conventional carrier-based modulation methods (a) SPWM, (b) SVPWM, (c) DPWM60 and (d) DPWM30

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1 1

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

(a)

(b)

c)(

(d)



• Switching waveform during one sampling period:• CMV

dc

da

db

vaz1

vbz1

vcz1

vaz2

vbz2

vcz2

O

N

O

O

Vd c/3

-Vd c/3

0

O

P

O

P

N

N

O

Ts Ts

N

1 1 1 12 2 2 22 2 23 12 2 2 22 2 21 1 1 11 1 123 3 3 33 3 323 3 3 33 3 344 4 4 44 4 434 4 4 44 4 45

SPWM

5 5 5 55 5 545 66 5

dc

da

db

vaz1

vbz1

vcz1

vaz2

vbz2

vcz2

O

N

O

O

Vd c/3

-Vd c/3

0

O

P

O

P

N

N

O

Ts Ts

N

SVPWM

dc

da

db

vaz1

vbz1

vcz1

vaz2

vbz2

vcz2

O

N

O

O

Vd c/3

-Vd c/3

0

O

P

O

P

N

N

O

Ts Ts

N

dc

da

db

vaz1

vbz1

vcz1

vaz2

vbz2

vcz2

O

N

O

O

Vd c/3

-Vd c/3

0

O

P

O

P

N

N

O

Ts Ts

N

DPWM30DPWM60

icir

vcm icir

vcm icir

vcm icir

vcm

CMV amplitude is higher




Comparion with a 0.9 modulation index

• Comparison of the circulating current• DPWM30 has the lowest circulating current at the MI higher than 0.6• SPWM and SVPWM have lower circulating current at the MI lower than 0.6• DPWM60 and DPWM30 have better current quality compared to SPWM and SVPWM




Laboratory Results

• Experimental setup• Medium Voltage Analyzer system consisting of 6 paralleled NPC-inverters

(2 NPC used for the experiment)• Operated in Open-Loop condition• System parameters

Symbol Description Value

Lf AC Inductor filter 25 uH

Rf AC Resistive Load 20 Ω

VDC DC-Link voltage 600 V

CDC DC-link capacitance 2 mF

fsw Switching frequency 30 kHz



• Circulating current is significantly higher than the output current

• SVPWM and DPWM30 have the lowest circulating current

• DPWM60 and DPWM30 have the best current quality

• DPWM30 has the best overall performance SPWM SVPWM DPWM60 DPWM30




Conclusions

• The modulations SPWM, SVPWM, DPWM60 and DPWM30 have been compared for the case of parallel and interleaved operation of NPC Converters, with the following conclusions:

• DPWM30 has the most significant effect on the system performance

-Lowest circulating current at the MI higher than 0.6

-Improved current quality

-Lowest amount of power losses in the whole MI range

• SPWM can be considered as the worst modulation method

-Highest circulating current at the MI higher than 0.6

-Worst current quality

-More power losses at higher MI

• DPWM60 and DPWM30 have better current quality compared to SPWM and SVPWM




Grid Identification

More Electric Aircraft

Active Power Filter

High-speed Machine

High-frequency current control is required in several applications

Resonant-based current control loop Track sinusoidal waveforms with zero steady-

state error without additional reference frame transformation

Accuracy and control degradation at high-frequencies due to digital implementation

PWM and computational delay compensationis necessary to increase the stability margins and the robustness of the system to plant parameter variations.

High-bandwidth control of parallel NPC-converters



Internal Model Principle Control• a harmonic model of the reference needs to be added in the direct branch of the loop

P+RES Controller

Z P+RES Controller

( ) ( ) ( ), 2 2

cos sinres res resP RES p i

res

sG s K K

sϕ

ϕ ω ϕω+

−= +

+

( ) ( ) ( )( ), 2

cos cos2 cos 1

i s res res res sP RES p

res s

K T z z TG z K

z z Tϕ

ϕ ϕ ωω+

− − = +− +

1

f fL s R++

-

( )*g si kT

PWMgi

RESω

convv( )*conv sv kT

( )RESG z

pK +

+

( )g si kTDigital Controller

skT

Filter

iK

Plant model in Z – time domain

( )( )( )1 f S f

f S f

R T Ln

T R T Lf

z eG z

R z e

−−

−

−=

−

Impulse invariant discretization Z




Kp tradeoff between high bandwidth and optimal damping Kp =Lf /3Ts damping factor ξ=0.7

PM = 60° crossover frequency fco=1/6πTs.

System delay causes instability when the resonant frequency is over the bandwidth of the system!φres should compensate the delay introduced by the plant GT(z)

φres = -∠GT(z)2-sample phase lead

( )( ) ( ) ( )

( )

3 2

,2 2

4cos 3cos 1 2cos2 cos 1

s res s res s s res sRES Ts

res s

T z T T T TG z

z z Tω ω ω

ω

− + − =− +

32 2res res sTπϕ ω= +

Linear phase compensation of ∠GT(z)

moving the zero of the transfer function with a two-step prediction

for frequencies higher than f90 = 5Rf /πLf

High-bandwidth control of parallel NPC-converters Modulation and control of modular power converters for high-power applications


Open-loop Bode diagrams with zone-phase delay compensation

-20

0

20

40

60

80

100

120

Mag

nitu

de (d

B)

10 1 10 2 10 3 10 4

Frequency (Hz)

-540

-360

-180

0

Phas

e (d

eg)

fr e s

= 50Hz

fc o

750Hz 1.5kHz

2.5kHz 5kHz

φ = π/2 + 3/2ω r e sT

sφ = 2ω

r e sT

s

-20

-10

0

10

20

GM

(dB)

φ = π/2 + 3/2ω r e sT

sφ = 2ω

r e sT

s φ = zone-phase

250 500 1000 10000Resonant Frequency (Hz)

0

10

20

30

40

50

60

70

PM (d

eg)

fc o

2 23 2

2 2

res res s res co

res res s res co

T f

T f

ϕ ω ω ππϕ ω ω π

= <

= + ≥High PMs provided by the linear compensation in high frequencies

Stability provided by the 2-sample delay method in low frequencies

fco = 1/6πTs (crossover frequency)

+

Stability margins at different fres in case of linear, 2-sample and zone-phase delay compensation




Zone-phase delay compensation

Symbol Description Value

LV (rms) LV grid side 510 V / 50Hz

MV (rms) MV grid side 20 kV

Lf AC Inductor filter 25 uH

Rf AC Resistive filter 2.6mΩ

vDC DC-Link voltage 1200 V

fsw Switching frequency 30 kHz

Kp Proportional gain 0.25

Ki,50 Integral gain at 50Hz 62.5

Ki,res Integral gain at fres 62.5 / 125 / 250 / 375

NPC 1-4

Control Cabinet 1, 2

Control Cabinet 3

NPC 5,6

500A/div 4 ms/div

200A/div 500Hz/div

fres = 250Hz

iabc,1

iabc,2

iabc,tot

fres = 5kHz

10A/div 500Hz/div

20A/div 1 ms/div

Single NPC-converter: 30Apeakat 5kHz grid current injection

2-parallel NPC converters: 800Apeakat 250Hz grid current injection.

Table: power stage and current controller parameters used in simulations and experiments.

Experimental setup: 1.6 MVA MV grid impedance analyzer based on 6-parallel 3-phase-NPC converters.

High-bandwidth control of parallel NPC-converters Modulation and control of modular power converters for high-power applications


100A/div 20 ms/div

32 2res res sTπϕ ω= +2res res sTϕ ω=

0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2

Time (s)

-800

-600

-400

-200

0

200

400

600

800

Grid

Cur

rent

ig

(A)

iA

iB

iCf

r e s = 250Hz

φr e s

= 2ωr e s

Ts

φr e s = π/2 + 3/2ω r e s

Ts

0.1 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2Time (s)

-50

-40

-30

-20

-10

0

10

20

30

40

50

Grid

Cur

rent

ig (A

)

iA

iB

iC

0.199 0.1995 0.2-30

-20

-10

0

10

20

30

fr e s

= 5kHz

φr e s

= 2ωr e s

Ts

φr e s = π/2 + 3/2ω r e s

Ts

50A/div 1 ms/div

32 2res res sTπϕ ω= +2res res sTϕ ω=

300Apeak /250Hz grid current injection when switching the delay compensation from the 2-sample based method to the linear formulation

30Apeak /5kHz grid current injection when switching the delay compensation from the 2-sample based method to the linear formulation.




• Phase compensation is mandatory in high frequency resonant controllers to compensate the phase lag introduced by PWM/computational delay and to improve system stability.

• A zone-phase compensation based on the combination of the 2-sample delay compensation and linear method has been proposed leading to robust stability with high PMs, within the entire frequency range (250Hz-10kHz).

• Simulation and experimental results, performed on a developed MV grid impedance analyzer with 1.6MVA rated power, confirm the effectiveness of the proposed phase compensation.

High-bandwidth control of parallel NPC-converters Conclusions







Conclusions



Modular Multilevel Converters

• Modular Multilevel Converters are emerging technologies in

• Management of HV- or MV-dc-grid

HVDC STATCOM/WES

Hybrid grids APF ED

Use of low-voltage deviceswithout trafo

Decreased overall filter

size

Enabling Hybrid grid at

MV and HV

Optimal designDC-grid fault

handling capability



HVDC-fault handling and new services

Frequency Support

New protections concepts



MVDC fault Management with TSEP

Full Bridge SM

Partners:

MVDC grids could be used for EV-charging stations

MVDC breakers can be omitted if the MMC has fault-handling functionalities

Thermo-sensitive Electro Parameter (TSEP) help in using optimally the HW capability



Goal: development of MMC topology with reliable fault clearing capability and increased efficiency through reduced number of bipolar cells.

How can we increase the efficiency and reduce the cost of MMC, while keeping its fault clearing capability?What is the minimum required number of bipolar cells to achieve reliable fault clearing of

the MMC?How does this new MMC topology behave in normal and fault operation?What are the challenges related to the reduced number of bipolar cells?

Why do we need a new MMC topology with fault clearing capability?



MMC optimum design

(MMC)Optimum converter design

Optimization Routine

Pos arm volt Neg arm volt

Output volt Output cur



MMC optimum design Modulation and control of modular power converters for high-power applications

HEART-Plattform Labor: Medium Voltage LaboratoryMögliche Tests im Labor 3 test fields (10.4 m² each)

Autotransformer with 10,8,6,4,2 kV up to 1 MW circulating power cooling system (60 kW liquid, 10 kW air) maximum current 1600 A Availability of asynchronous MV Source controlled by a

Digital Real time Simulation System (RTDS or OPAL-RT)



Experimental investigations

• Power Snom: 100 kVA• DC voltage Udc: initially 4kV• AC voltage Uac,LL: initially 2kV• N° of SM/arm: 4, in expansion to 8• N° of SM: 24, in expansion to 48!

Fig.: Medium voltage demonstrator

Upper arm SM voltage

Lower arm SM voltage

Arm voltage

AC side currrent

Fig.: Single phase initial validation waveforms

Fig.: FB cell



MMC-based DC Fault Management

Minimum number ofbipolar (FB)-cells

HB-MMC FB-MMC HA-MMCMarco Liserre [email protected]



DC current

Upper arm currents

Lower arm currents

STATCOM

Blocking instant




Low voltage prototype of HA-MMC with dc fault blockingcapability

Short clearing time

SM over voltage withinacceptable range



• Recent MMC topologies with fault clearing capability suffer from high cost and losses.• MMC-based fault clearing is fast but does not offer selectivity.

• The proposed MMC topology offers sufficient fault clearing capability in low-inductive HVdcsystems. The advantages are: Reduced losses Reduced costs

ConclusionsMMC-based DC Fault Management







Conclusions



Power Routing to handle Reliability in Modular topologies

Power Routing !

• Scalable• Fault-tolerant

• More components -> more failures ?



HEART Project: power routing for active management of the reliability

Design for reliability

Hosting capacity

Graph theory

Modular design

sensing and communication



Reliability Enhancement by Uneven Loading of Cells



Power routing concept

Improve the efficiency: activate/de-activate parallel power paths to work on the maximum efficiency point,mainly in light power.Only the components in the activated power paths are stressed, while the power quality is affected

On/off control for parallel power converters (State of the art)



Balanced power

Activating/deactivating

Power routing concept

Control the lifetime: Identify aged IGBTs and reduce the power processed by them, until the repair orreplacement of the module. Consequently, optimize remaining useful lifetime and efficiency

Power routing for parallel power converters (Innovation)



Series connected building blocks can share the power unequally:

Unequal power Pa ,Pb and Pc is processed

The devices connected to the cells suffer different stresses

The concept requires a sufficient margin of Vgrid/Vdc

The potential of the algorithm is mission profile dependent Concept of (multi-frequency) power routing for a

seven level CHB-converter

Power Routing in cascaded H-bridges



Power Routing in cascaded H-bridges

Control variables of the power routing for series-

connected building blocks.

Demonstration of the concept for a highly varying mission profile with the resulting junction temperatures and accumulated damage for the power

semiconductors in the cells.



Maintenance-driven control



Maintenance-driven control

Power is routed in the cells to achieve thermal stress based RUL control

Possibility to reduce and control maintenance instances



Experimental validation

Experimental setup with 5-level CHB and DABs with Open Module and IR Camera

Without

Power Routing

Parameters of prototype

PDAB1 = 1.7 PDAB2

PDAB1 = PDAB2

Balanced Cell

Overloaded Cell Unloaded Cell



With

Power Routing

Monte-Carlo simulation results

Slightly different junction temperatures of the devices result in different lifetimes

With similar loading, the lifetime will have a high variance

With power routing the variance of the lifetime is significantly reduced

The time to a 5% failure probability is increased and mean lifetime is also slightly increased



• Modular structures allows an active management of the lifetimes of the different cells

• Use of “power routing” can lead to consider different challenges (unequal aging, consequences of substitution of faulty cells)

• Even a maintenance-driven control is possible to improve the maintenance scheduling

Power Routing to handle Reliability in Modular topologiesConclusions



• High-power inverters modular approach:• parallel (and interleaved) low-voltage inverters with transformer

• Low circulating current with suited modulation• Delay compensation for high-bandwidth control specially demanding for small

filters• Modular Multilevel Converters without transformer:

• optimal design of the cell and temperature monitoring• dc-fault handling capability

Overall Conclusions

• Active thermal control by means of Power Routing:• Effective way to handle difference in aging of cells• Maintenance-driven control



modulation and control of modular power converters for

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