Research on model predictive control for multilevel converters

Cáckết quả trong luận án là trung thực, chưa từng được công bố và sử dụng để bảo vệ trong bất NGHIÊN CỨU PHƯƠNG PHÁP ĐIỀU KHIỂN DỰ BÁO CHO CÁC BỘ NGHỊCH LƯU ĐA MỨC RESEARCH ON MODEL PR

LỜI CAM ĐOAN

Tôi xin cam đoan đây là công trình của tôi Tất cả các ấn phẩm được

công bố chung với các cán bộ hướng dẫn khoa học và các đồng nghiệp đã

được sự đồng ý của các tác giả trước khi đưa vào luận án Cáckết quả trong

luận án là trung thực, chưa từng được công bố và sử dụng để bảo vệ trong bất

NGHIÊN CỨU PHƯƠNG PHÁP ĐIỀU KHIỂN DỰ BÁO

CHO CÁC BỘ NGHỊCH LƯU ĐA MỨC

RESEARCH ON MODEL PREDICTIVE CONTROL FOR

MULTILEVEL CONVERTERS

LUẬN VĂN THẠC SĨ KHOA HỌC ĐIỂU KHIỂN VÀ TỰ ĐỘNG HÓA

HÀ NỘI-2018

LỜI CAM ĐOAN

Tôi xin cam đoan đây là công trình của tôi Tất cả các ấn phẩm được

công bố chung với các cán bộ hướng dẫn khoa học và các đồng nghiệp đã

được sự đồng ý của các tác giả trước khi đưa vào luận án Cáckết quả trong

luận án là trung thực, chưa từng được công bố và sử dụng để bảo vệ trong bất

NGHIÊN CỨU PHƯƠNG PHÁP ĐIỀU KHIỂN DỰ BÁO

CHO CÁC BỘ NGHỊCH LƯU ĐA MỨC

RESEARCH ON MODEL PREDICTIVE CONTROL FOR

LỜI CAM ĐOAN

Tôi xin cam đoan bản luận này là công trình của riêng tôi, do tôi tự thiết kế dưới

sự hướng dẫn của thầy giáo PGS TS Trần Trọng Minh Các số liệu và kết quả là hoàn toàn trung thực

Để hoàn thành luận văn này tôi chỉ sử dụng những tài liệu được ghi trong danh mục tài liệu tham khảo và không sao chép hay sử dụng bất kỳ tài liệu nào khác Nếu phát hiện có sự sao chép tôi xin chịu hoàn toàn trách nhiệm

Hà Nội, ngày 10 tháng 10 năm 2018

Tác giả luận văn

TỔNG QUAN VỀ ĐỀ TÀI

1 Lý do chọn đề tài

Điều khiển các bộ biến đổi đa mức như cầu H nối tầng đặt ra nhiều vấn đề do số lượng các module tăng nên nhiều theo số mức Bằng các cấu trúc điều khiển thông thường thì các mạch vòng điều khiển sẽ rất phức tạp Phương pháp điều khiển dự báo FCS-MPC dựa trên tính toán tối ưu hàm mục tiêu (cost funcion) trong không gian hữu hạn các trạng thái làm việc có thể cho phép xây dựng nên một hệ thống điều khiển có cấu trúc đơn giản hơn, lược bỏ khâu điều chế PWM, có thể đưa đến những ứng dụng thực tế

2 Đối tượng nghiên cứu

Nghiên cứu phương pháp điều khiển dự báo dựa trên không gian hữu hạn các trạng thái làm việc của sơ đồ nghịch lưu đa mức cấu trúc cầu H nối tầng Sau đó áp dụng thuật toán điều khiển này cho ứng dụng nghịch lưu nối lưới và điều khiển động cơ không đồng bộ Trong khuân khổ cuốn luận văn này, tính đúng đắn của thuật toán điều khiển dự báo FCS-MPC sẽ được kiểm chứng thông qua mô hình mô phỏng trên phần mềm Matlab-Simulink

3 Đóng góp khoa học trong luận văn

Đưa ra thuật toán điều khiển dự báo FCS-MPC cho bộ biến đổi 7 mức cấu trúc cầu

H nối tầng với số bước tính là hai bước, giúp bù thời gian trễ trong quá trình tính toán, đo lường, deadtime, v.v… khi triển khai thực nghiệm Thuật toán lựa chọn tập hợp các vector liền kề giúp giảm đáng kể khối lượng tính toán khi tối ưu hóa hàm mục tiêu

THESIS OVERVIEW

1 Problem statement

Control multilevel converters such as cascaded H-Bridge multilevel converters pose many problems as the number of module increases By the conventional control strategies, the control loops will be very complex The finite control set model predictive control (FCS-MPC) control strategies is based on cost function optimization in the finite number of switch states This could allow the control system to be simpler structure, the system does not need a modulator, can be led

to practical applications

2 Object of the study

The FCS-MPC control strategy for three-phase CHB multilevel converter is studied in this thesis It is applied in grid-connected CHB as DC-AC converter for isolated DC sources such as PV panels generating power to gird and an IM driver application Within the framework of the thesis, the correctness of the MPC algorithm will be verified through Matlab-Simulink software

3 My contributions

Proposal FCS-MPC control strategy for three-phase CHB seven level, predictive horizon at two-steps compensate delay time The subset of adjacent vector state (SAVS) method is proposed to reduce computational when optimizing cost function

Acknowledgments

First of all, I would like to express sincere thanks to my supervisor : Assos Prof Tran Trong Minh for his constant encouragement and guidance He has walked me through all the stages of the work of my Master of Science project The work in this thesis is based on research carried out at the Institute for Control Engineering and Automation (ICEA), Hanoi University of Science and Technology (HUST)

I would like gratitude ICEA as well as the financial support provided by the National project number: KC.05.03/16-20, “Nghiên cứu, thiết kế và chế tạo hệ thống khắc phục nhanh sự cố tăng/giảm điện áp ngắn hạn cho phụ tải” and: ĐTĐLCN.44/16, “Nghiên cứu thiết kế và chế tạo hệ truyền động servo xoay chiều

ba pha”

Contents

Contents

Acknowledgments 1

Contents 2

List of figures 4

List of tables 5

List of abbreviations 6

1 Overview FCS-MPC for CHB multilevel converter 7

1.1 Three-phase CHB multilevel converter 7

1.1.1 Structure of a three-phase CHB multilevel converter 7

1.1.2 Modulation techniques 9

1.2 Modeling of three-phase CHB multilevel converter 11

1.3 FCS-MPC control strategy 14

2 FCS-MPC for gird-connected three-phase CHB 17

2.1 FCS-MPC for grid-connected formulation 17

2.1.1 Discrete-time model predictive current control 18

2.1.2 Cost funcion optimization and vector state selection 19

2.1.3 Subset of adjacent vector state 20

2.2 Current reference generation 21

2.3 Simulation results .22

2.4 Conclusion 24

3 FCS-MPC based current control of an IM 25

3.1 Mathematical model of an IM 25

3.2 FCS-MPC for IM formulation 25

3.2.1 The required signal estimation 27

3.2.2 Discrete-time model predictive current .27

3.2.3 Cost funcion optimization and vector state selection 28

3.3 Simulation results .28

3.4 Conclusion 31

4 Summary and future works 32

References 33

Appendix A Simulation FCS-MPC for a gird-connected details 35

A.1 Simulation model 35

A.2 MPC algorithm function 36

Appendix B Simulation FCS-MPC for an IM details 37

B.1 Simulation model 37

B.2 MPC algorithm function 38

Appendix C List of publications 40

List of figures

List of figures

Figure 1.1 H-Bridge switch state 7

Figure 1.2 Three-phase CHB seven level converter 8

Figure 1.3 SPWM multicarrier strategy 9

Figure 1.4 Space vector for three-phase CHB three level .10

Figure 1.5 H-Bridge converter 11

Figure 1.6 Vector state in CHB seven level converter 13

Figure 1.7 Classification of MPC strategies applied to power converter 14

Figure 1.8 FCS-MPC block diagram 15

Figure 2.1 Block diagram of FCS-MPC gird-connected 17

Figure 2.2 Vector state for CHB seven level three-phase 20

Figure 2.3 Simulation results of the proposed FCS-MPC 23

Figure 2.4 FFT analysis output current (phase A) .24

Figure 3.1 Block diagram of FCS-MPC for IM 26

Figure 3.2 Simulation results of output current and voltage 29

Figure 3.3 Simulation results of the proposed FCS-MPC 30

Figure 3.4 FFT analysis output current (phase A) .31

Figure A.1 Simulation overview of FCS-MPC for a grid-connected 35

Figure A.2 FCS-MPC controller in subsystem 36

Figure B.1 Simulation overview of FCS-MPC for an IM 37

Figure B.2 FCS-MPC in subsystem 38

List of tables

Table 1.1 Switch state H-Bridge converter 11

Table 1.2 Level state CHB seven level converter 12

Table 2.1 Simulation FCS-MPC for grid connected parameters 22

Table 3.1 Simulation FCS-MPC for IM parameters 28

List of abbreviations

List of abbreviations

NPC Neutral diode clamped multilevel converters

FC Flying capacitor multilevel converters

MMC Modular multilevel converters

CHB Cascaded H-Bridge multilevel conveters

IGBT Insulated Gate Bipolar Transistors

APOD Alternative phase opposite disposition

POD Phase opposite disposition

SVM Space vector modulation

MPC Model predictive control

FCS-MPC Finite control set model predictive control

CCS-MPC Continuous control set model predictive control OSV-MPC Optimal switching vector model predictive control OSS-MPC Optimal switching sequence model predictive control

SAVS Subset of adjacent vector state

RMS Root mean square

FFT Fast Fourier transform

THD Total harmonic distortion

FOC Field oriented control

Chapter 1

Overview FCS-MPC for CHB multilevel converter

Multilevel converters include: Neutral diode clamped (NPC), flying capacitor (FC), modular multilevel converters (MMC) and cascaded H-Bridge (CHB) However, technology of CHB is one of the well known, most advantageous and basic method

Control CHB multilevel converters will be complex when number of cells increase The FCS-MPC control strategy can be considered as a solution simply handles this problem

1.1 Three-phase CHB multilevel converter

1.1.1 Structure of a three-phase CHB multilevel converter

The Figure 1.1 shows three switch state of H-Bridge (as named is cell), each cell

make three level voltage: -1; 0 and 1

Chapter 1 Overview FCS-MPC for CHB multilevel converter

In CHB multilevel converter, number of cells are connected in series Each cell has separate DC source which is obtained from fuel cells, batteries, capacitors, transformers,…

Activity of m cells in each phase will make 2m+1 voltage level Figure 1.2

is example of CHB three-phase seven level Three-phase CHB multilevel converter is simply like three single-phase converter connected in wye configuration

• It doesn’t need capacitors or diodes for clamping

• Entire IGBT switching in basic fundamental frequency (or near this frequency), so that reduce power lose switch

• The harmonics reduce because IGBT switching small frequency

• The wave is quite sinusoidal in nature

Disadvantages:

• CHB needs separate DC sources for each leg

• Controller will be complex if number of cells increase

Additional detail can be found in Appendix C [2], [3] and [4]

1.1.2 Modulation techniques

a Sin-PWM (SPWM) multicarrier strategy

In the SPWM, each phase uses single sinusoidal reference For m cells need 2m

triangular carriers The carriers have the same frequency, the same peak to peak amplitude Sinusoidal reference is compared with each carrier to determine the switching output voltages for the power converter

Figure 1.3 SPWM multicarrier strategy

There are four strategies of multicarrier PWM Figure 1.3 is showed

multicarrier PWM strategy for single-phase CHB five level It requests four triangle carriers and only one sinusoidal reference

Chapter 1 Overview FCS-MPC for CHB multilevel converter

• Phase-shift (PS) carrier PWM strategy Each carrier is phase-shift by 360°/4=90° from it’s adjacent carrier

• Phase disposition (PD), all carriers are in phase 0°

• Alternative phase opposite disposition (APOD), all carriers are alternatively

b Space vector modulation (SVM)

SVM technique reduces the influence of common-mode voltages and this avoids the use of any triangular carriers SVM conveniently provides more flexibility such

as redundant switching sequences, adjustable duty cycles; and it is more suited to digital implementations

V 1

(1,1,0) (0,0,-1)

V2

(-1,0,-1) (0,1,0)

V3

(0,1,1) (-1,0,0)

V 4

(-1,-1,0) (0,0,1)

V5

(1,0,1) (0,-1,0)

V0

2 1

1 3 4

2

Figure 1.4 Space vector for three-phase CHB three level

These advantages of SVM can lead to a significantly improved performance

of multilevel converters, especially when the level number of the converter is large

The space vector of a three-phase CHB three level shows in Figure 1.4

However, SVM for higher level converter is difficult There generally are 6(n-1) 2

triangles in the space vector diagram of a three-phase n level converter, reference

vector can be located within any triangle SVM selects suitable switch states of the located triangle and apply them for corresponding need duty cycles in an switching sequence

1.2 Modeling of three-phase CHB multilevel converter

Each cell of converter is described in Figure 1.5

Figure 1.5 H-Bridge converter

Sign IGBT switch state: “0” corresponding IGBT is off and “1”

corresponding IGBT is on Table 1.1 shows switch state each cell Output voltage

obtained are 0; V dc and –V dc corresponding switch state is 0; 1 and -1

Table 1.1 Switch state H-Bridge converter Gate state

Chapter 1 Overview FCS-MPC for CHB multilevel converter

common-mode voltagev ZN

Table 1.2 Level state CHB seven level converter

1, 0, -1, -2, -3}*V dc, this is called level state {3, 2, 1, 0, -1, -2, -3}

Level state phase A, B and C are grand total 127 reasonable different vector

state v

Output voltage each cell:

11

V2 V3

V4

V5 V6

V7 V8

V9 V10

V22 V23 V24

V37 V38 V39 V40

V41 V42 V43

V44 V45

V46

V47

V48

V50 V49

V51

V52

V53 V54 V55 V56 V57

V58 V59 V60

V62 V63 V64 V65

V66 V67 V68 V69

V70 V71

V91 V92 V93 V94 V95 V96

V97 V98

V99 V100

V101 V102 V103

β

Figure 1.6 Vector state in CHB seven level converter

Because of v AZ+v BZ +v CZ =0, so common-mode v ZN as express:

13

The vector state v can be expressed in terms of complex coordinate by

using the Clarke transformation:

Chapter 1 Overview FCS-MPC for CHB multilevel converter

future behavior of the controlled variables over a predictive horizon, n-steps The

information is used by the MPC control strategy to provide the control action sequence for the system by optimizing a user-defined cost function It should be noted that the algorithm is executed again every sampling period and only the first

value of the optimal sequence is applied to the system at instant k

Model predictive control

Figure 1.7 Classification of MPC strategies applied to power converter

Classification of MPC strategy applied to power converter is showed in

Figure 1.7, [2] MPC strategy can be divided into two types: continuous control

set MPC (CCS-MPC) and discrete of the power converters finite control set MPC (FCS-MPC)

The CCS-MPC computes a continuous control signal and then uses a modulator to generate output voltage in the power converter The main advantage

of CCS-MPC when applied to power converter is that it produces a fixed switching frequency The main disadvantage of CCS-MPC is present a complex formulation

of the MPC problem

The FCS-MPC based on finite number of switching state to formulate the MPC algorithm and does not need a modulator FCS-MPC can be divided into two types: optimal switching vector MPC (OSV-MPC) and optimal switching sequence MPC (OSS-MPC) OSV-MPC is the most popular MPC control strategy for power converter It uses the output vector state of the power converter as the control set The main advantage of OSV-MPC: it only calculates prediction for this control set, therefore it reduces the optimal problem to an enumerated search algorithm This makes the MPC strategy formulation very intuitive The disadvantage of OSV-MPC is that only one output optimal vector state is applied during the complete sampling time period, lead to uncontrolled switching frequency

In FCS-MPC, the prediction model of the system needs to be discretized Therefore, the MPC algorithms are usually implemented in digital hardware like

as DSP or FPGA The common of FCS-MPC regularly uses Euler approximation

to discretize a one-step or multiple-step

Optimizaton Predictive

Load

Measurement Estimation FCS-MPC

Figure 1.8 FCS-MPC block diagram

Figure 1.8 shows FCS-MPC block diagram Assume, control variable x

follow the reference variable x *, procedure design FCS-MPC following basic steps:

• Measurement, estimation the control variable in the sampling time instant

• For every switch states of the converters, predictive (using the mathematical

model) the behavior of variable x in the n-steps time

• Evaluate the cost function for each prediction

Chapter 1 Overview FCS-MPC for CHB multilevel converter

• Select the switch states that minimize the cost function, S opt applied to the converters

In the experiment, driver, measurements and IGBT exist delay time The computational time is needed in the predictive control algorithm to predict the variables, and processor delay deteriorates the performance of the predictive control at the experimental investigation To solve this problem, it can be

considered the predictive horizon at (k+n)th sampling time to predict the variables which are compared with the references, and determine the cost functions The

optimum S opt is selected corresponding to the minimum cost function, and applied

it in the power converter

Chapter 2

FCS-MPC for gird-connected three-phase CHB

2.1 FCS-MPC for grid-connected formulation

The FCS-MPC control strategy predicts behavior of the load current for each

possible vector state v generated by the power converter The prediction of the

current is based on discretized model of system

Cost function optimization

Prediction (k+2)th

Figure 2.1 Block diagram of FCS-MPC gird-connected

In abc coordinate, a block diagram of predictive current control is described

in Figure 2.1 The procedure designs FCS-MPC for grid-connected included

mainly three steps [5]: