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D e sign and control of Modular Multilevel Converter as an Active Front End

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Design and control of Modular Multilevel Converter as an Active Front End

PanagiotisAsimakopoulos

Technical Student, TE-EPC-MPC

Supervisor: Examiner:

KonstantinosPapastergiou Massimo Bongiorno

Researcher Professor

TE-EPC-MPC, CERN Chalmers University of Technology

Project description

Aim of part A:

- To examine the state of the art in the MMC design and control
- To compile a design guideline for the sizing of the MMC as an AFE
- To evaluate a voltage balancing control method
- To evaluate the overall performance of the topology in an AFE application

Aim of part B:

- Obtain practical experience with a small-scale prototype in the design of the converter as an AFE and as full-bridge for magnet supply.

Contents

- Modular Multilevel Converter (MMC) operation/circuit analysis:
- MMC sizing approach
- Module capacitor
- Arm inductor
- Semiconductors ratings
- Number of modules per arm decision
- Control strategy of the MMC
- Voltage balancing control
- Active Front End control
- Actual sizing of the system
- Simulation results
- Conclusions
- Current work

MMC operation and analysis

arm

1-phase inverter mode, N=2 modules per arm

leg

2N+1 levels in output voltage

- Main idea: Building of the output voltage stepwise
- For each module the average capacitor’s voltage
- is . The module capacitor is inserted or bypassed.

MMC operation and analysis

Module’s mode of operation

- To apply to the load:
- Bypass both the upper modules
- Insert both the lower modules
- For the opposite is valid.
- To apply 0 V to the load:
- Insert one module from each arm

MMC operation and analysisLoad voltage

Circulating current

MMC operation and analysisCirculating current

The arm current consists mainly of 3 components:

- The fundamental frequency (load) component
- The dc component transferring the real power
- A second harmonic component
- The dc and the second harmonic component
- form the circulating current the part of the
- current that does not flow to the load.

MMC operation and analysisSecond harmonic component

Difference between the sum of the upper caps voltages

and the sum of the lower caps-mainly first harmonic

Sum of all capacitors’ voltages in a leg-mainly second harmonic

Capacitor voltage

- The capacitor voltage consists mainly of:
- an average dc value of .
- first and second harmonic component.

MMC overview

Main advantages:

- Effective switching frequency=actual switching frequency *number of modules per phase-leg significant reduction of switching losses
- No need for bulk filters at the ac side
- Modularity-Redundancy due to the extra modules that can be inserted to substitute the failed ones
- Simple mechanical construction
- Low voltage ratings for the semiconductors
- Scalability: No DC-link voltage limitation
- Possibility to by-pass a faulted module and not trip the converter
- Stepwise change in the output voltage reducing the Electromagnetic Interference (EMI)

Disadvantages:

- Extra control for the voltage balancing of the modules’ capacitors is needed
- Need for monitoring all the capacitors’ voltage values
- More difficult power stage dimensioning due to the capacitors, the arm inductors and the circulating current
- Circulating current in the phase legs mainly consisting of second harmonic, which increases the losses at the components

Arm inductance dimensioning

The main reasons for the utilization of the inductance are summarized in the following bullet points:

- Limitation of the current at the voltage steps
- Replacement of a filter at the ac side
- Limitation of the current in the case of a fault in the dc-link.
- Limitation of the current circulating among the legs consisting mainly of second harmonic.

The arm inductor is rated for the arm current:

For the 1-phase example:

For the 3-phase converter

Arm inductance dimensioning

Arm current filtering

- The increase of modules per arm filters the output voltage, the arm and load current.

Arm inductance dimensioning

2nd harmonic suppression

The 2nd harmonic increases the losses in the system. It does not really affect the components ratings.

To avoid a significant increase of the arm inductance, two solutions are proposed:

Filter [5] or dedicated second harmonic controller [4]

Arm inductance dimensioningFault in dc-link – Half-bridge module

The main reason for the arm inductance is the fault current limitation in the case of a dc fault. The ac voltage source feeds the fault via the antiparallel diodes in the modules.

Arm inductance dimensioningFault in dc-link – Full bridge module

The modules capacitors are connected in series during the fault via the antiparallel diodes. No need for a high value of the arm inductance. The capacitors should be dimensioned to handle the voltage fluctuation during the fault.

Module capacitance dimensioning

Calculation of the charge lost during the capacitor’s discharge phase

,

where the instants when the arm current crosses the x-axis. The instants depend on the fundamental frequency value.

For a specified percentage p of voltage ripple:

To avoid the resonant peak by and in 100 Hz:

Semiconductors ratings

- Voltage ratings

The average capacitor voltage with an extra margin depending on the maximum allowed ripple is used.

- Current ratings

The current load of the components inside the module is not equally distributed. The arm current for a 3-phase MMC is equal to:

The direction of the power is defined by the sign of the dc component of arm the current.

For AFE operation has a negative sign giving a negative dc-offset to the arm current.

For inverter operation has a positive sign giving a positive dc-offset to the arm current.

Semiconductors ratings

Upper switch current Lower diode current

Lower switch current Upper diode current

Upper arm current Upper arm current

- The current rating for the half bridge module is calculated based on the highest rms value. For the AFE the maximum ratings are for the lower diode of the half-bridge.
- The rms value of the components’ current is much lower than the peak value.
- The arm current ratings can be used for the module ratings.

Number of modules per arm

The decision of N is based on the criteria:

- The voltage ratings of the semiconductors; the module semiconductor rating is related to the equation
- The same ratings are valid for the module capacitors. If N is doubled the capacitance is doubled according to the equation but the energy stored in every capacitor becomes finally the half.
- The grid current ripple; For a large N the voltage step between the modules insertions/bypasses is small the equivalent switching frequency is high the current ripple is small The arm inductance value can be reduced.

It is a techno-economical decision…

Trade off among:

- cost of the extra modules
- the size of the capacitors
- the size of the arm inductance
- the size of the external grid side filters

Control of MMC as an AFE

Equivalent circuit

Control of MMC as an AFE

General control strategy of an AFE converter

- ac current controller for grid current quality
- dc-link voltage controller

Control of MMCVoltage balancing

- Voltage balancing is required internally in the converter due to the module capacitors.

- Two parts [2]:
- Averaging control: Keeps the average capacitor voltage in the leg to the specified reference + provides circulating current control (the 2nd harmonic controller is based on this)
- Individual balancing control: Keeps every capacitor’s voltage individually to the specified reference

Control of MMC as an AFE

- In the MMC for the specific application where the load cycle is well-specified, the dc-link voltage controller can be assigned to the averaging controller.
- The current controller can receive a reference based on the application.

General current controller scheme in dq coordinates

Actual sizing of the systemMethodology

Approach simulated: Load oriented sizing of the total system

- The magnet ratings for the specific application.
- A model to emulate the magnet and the H-bridge.
- The dc-link capacitor for the reactive power supply to the magnet
- The MMC operating as an AFE for the ohmic losses supply of the magnet. No reactive power is drawn from the grid.
- The transformer ratings are based on the converter ratings.

Actual sizing of the systemLoad specifications

- Magnet characteristics: Proton Synchrotron Booster total load used:

Actual sizing of the systemLoad model

The load is finally modeled as a controlled current source.

Actual sizing of the systemDC-link sizing

- Maximum voltage drop across the magnet:

For a maximum modulation index below unity a margin is provided to the H-bridge input voltage (dc-link capacitor nominal voltage):

6.5 kV

- For a given maximum capacitor ripple of 20% the initial capacitor voltage is:
- The capacitor provides the reactive power losses of the magnet:

The capacitance of the dc-link capacitor is:

F

Actual sizing of the systemMMC power ratings

- The MMC supplies the power losses of the magnet.

The maximum output voltage of the converter for a sinusoidal PWM is

The ac grid phase voltage has the amplitude:

The factor 0.8 is applied in order to take into consideration the maximum modulation index and to provide a margin for the current controller.

Actual sizing of the systemMMC components ratings

- The arm inductance’s ratings for rms arm current .
- Three models were built with N=4, 6, 10. The module capacitance is:
- The semiconductors rms current values were calculated with the simulation:

The rms ratings are much lower than the peak values of the current. The repetitive peak current of the selected device must be also taken into account.

- The transformer ratings are: 18 kV/3.8 kV, 2.72 kA. 0.14 mH.

Conclusions

- MMC sizing process
- An investigation about the arm inductance sizing criteria was made:
- The most important criterion is the dc-link fault tolerance A full-bridge module can limit the fault current.
- A proposal for the module capacitor sizing was described:
- It is based on the ac and dc side currents.
- It is independent of the switching frequency, depends on the grid frequency and can be generally adjusted to the application.
- An approach for the semiconductors ratings was attempted:
- The load of the semiconductors in the half-bridge depend on the mode of operation.
- The rms value of the current has a large deviation among the semiconductors.
- The peak currents of the semiconductors have a significant difference from the rms values.
- System sizing process
- The first approach used for the simulation is to provide the reactive power with the dc-link capacitor and to draw the real power from the grid.
- Control
- The control of the grid current is assigned to a current controller.
- The control of the dc-link voltage is assigned to the average controller of the voltage balancing control.

More challenges coming

- Design and construction of a small scale prototype to:
- validate the conclusions and perform improvements
- Implement and test the control strategy with a constant power flowing from the grid side to avoid stressing the grid with periodical power peaks

Planning for the prototype tests:

- 1-phase rectifier
- 3-phase rectifier
- Full-bridge converter based on the Modular Multilevel concept as a magnet supply
- System of a 1-phase rectifier and full-bridge magnet supply both based on the modular multilevel structure

References

- S. Rohner, S. Bernet, M. Hiller and R. Sommer, “Modelling, Simulation and Analysis of a Modular Multilevel Converter for Medium Voltage Applications”, IEEE International Conference on Industrial Technology, 14-17 March, 2010
- M. Hagiwara, H. Akagi, “Control and Experiment of Pulsewidth - Modulated Converters”, IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 25, NO. 7, July, 2009
- A. Antonopoulos, L. Angquist, H.-P. Nee, “On Dynamics and Voltage Control of the Modular Multilevel Converter”, European Power Electronics Conference (EPE), Barcelona, Spain, September 8-10, 2009.
- X. She, A. Huang, X. Ni and R. Burgos, “AC Circulating Currents Suppression in Modular Multilevel Converter”, 38th Annual Conference on IEEE Industrial Electronics Society, Montreal, Canada, 2012.
- K. Ilves, S. Norrga, L. Harnefors and H.-P. Nee, “Analysis of Arm Current Harmonics in Modular Multilevel Converters with Main-Circuit Filters”, 9th International Multi-Conference on Systems, Signals and Devices.

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