Behavior of Photo voltaic system during grid disturbances

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Behaviour of Photovoltaic Systems

During Grid Disturbances

A Project Report

Submitted in Partial Fulfilment of

Requirement for the Degree of

Master of Engineeringin

Electrical Engineering

By

Vijayakumar S.

Department of Electrical Engineering

Indian Institute of ScienceBangalore - 560 012

India

June 2012


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Acknowledgements

I am grateful to Dr. U.J. Shenoy, for offering me this exciting project. I sincerely thank him

for his patience, motivation and lectures.I would like to thank Prof. G. Narayanan for his courses and the challenging assignments

right from Electronics Circuits lab to PWM course.

I thank Prof.Udaya kumar, Prof.M.K.Gunasekaran, Prof.L.Umanand and Dr.Vinod John

for their exceptional lectures.

I specially thank Arun Karuppaswamy, Senthilkumar(Sun Edison Ltd), Amit Kumar and

Sachin Srivastava (ABB) for their suggestions and help during the project. I also thank all

my classmates and friends for their kind co-operation and help that makes me IISc a homely

experienceI thank Mr. H.N.Purushothama, Mr. K.Jagannath Kini, Mr D.M.Channegowda at the

department office for their good and kind administrative activities.

I also admire the help of Mr.C.S.Kore, DGM-Larsen & Toubro Ltd, for granting me a

study leave in this great organization.

I also thank Indian Institute of Science for giving me this opportunity to pursue my higher

education in this mighty institution. I sincerely thank hostel and administrative section for

their kind help during my tenure in IISc.

i


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Abstract

Use of Renewable energy is highly motivated around the world due to various environmental

issues. Many countries planning for modest addition of green energy to their grid, both ontransmission and distribution level. These Distributed generations improves power system

reliability and power quality, but it leads to a problem related to power system stability

and security. So addition of these distribution resources in the existing network open up

new challenges in power system protection due to its unpredictable nature and multi feed

capability.

This project deals with the transient behaviour of grid connected photovoltaic sys-

tem(PV) under various grid and climatic conditions. A study is carried out with detailed

modelling of the system considering both dynamic and steady state responses. Impact ofvarious PV system dynamic elements, during abnormal conditions was analysed. PV system

along with distribution systems are modelled by using MATLAB/Simulink, such that faster

simulation and better response is obtained.

Overall System is divided into nonlinear photovoltaic system and linear distribution sys-

tem. To improve the simulation performance, non linear Photovoltaic system is modelled in

detail, but distribution system is modelled as a state space equation. Analysis carried out

by using photovoltaic plant with a capacity of (2 500kW) 1MW interfaced with the 33kVdistribution system.

Overall, the project work involves, modelling of full PV system (includes modelling of

photovoltaic panel, controllers, MPPT, PLL), distribution network and system interface. By

using the PV system model, behaviour of photovoltaic system during abnormal conditions,

such as loss of grid (unintentional island), system fault, disturbances in the grid voltage (at

PCC), frequency and irradiance are studied and documented.

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Contents

Acknowledgements i

Abstract ii

List of Tables vii

List of Figures viii

Nomenclature xi

1 PV Generation - Current Scenario 11.1 Global PV Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 PV Installation in India . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Solar Mission JNNSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Photovoltaic System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4.1 PV System Grid interface . . . . . . . . . . . . . . . . . . . . . . . . 4

1.5 Project Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.5.1 Activities in the Project . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.6 Organization of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 PV Panel Modelling 7

2.1 PV Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Important Terms in PV Panel . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.1 Irradiance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.2 Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.2.3 NOCT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

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iv Contents

2.2.4 Standard Test Conditions(STC) . . . . . . . . . . . . . . . . . . . . . 12

2.2.5 Open Circuit VoltageVoc . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.6 Short Circuit CurrentIsc . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.2.7 Maximum Power Point MPP. . . . . . . . . . . . . . . . . . . . . . . 12

2.2.8 Maximum Power Voltage and Current (Vm&Im) . . . . . . . . . . . . 13

2.2.9 Fill Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2.10 Temperature Co-efficient v &i . . . . . . . . . . . . . . . . . . . . 13

2.3 Manufacturer Data Sheet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.4 PV Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.4.1 Single Diode Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.4.1.1 Series ResistanceRs . . . . . . . . . . . . . . . . . . . . . . 15

2.4.1.2 Shunt ResistanceRsh . . . . . . . . . . . . . . . . . . . . . . 16

2.4.2 Double Diode Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.4.3 Sandstrom Equation . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.4.4 A.J. Anderson Translation equation . . . . . . . . . . . . . . . . . . 18

2.4.5 Bleasser Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.4.6 Michigan University Model. . . . . . . . . . . . . . . . . . . . . . . . 19

2.4.6.1 Calculation of b (fit variable) . . . . . . . . . . . . . . . . . 202.5 PV Model Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.6 PV System Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.6.1 Selection of PV Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.6.2 Inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.6.3 Maximum Power Point Tracking (MPPT) . . . . . . . . . . . . . . . 24

3 PV System Control 26

3.1 Synchronous Reference Frame Strategy . . . . . . . . . . . . . . . . . . . . . 263.1.1 Synchronous Frame Transformation for Three Phase Balanced System 27

3.1.2 Synchronous Frame Transformation for Three Phase Unbalanced System 28

3.2 Phase Locked Loops (PLL). . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.3 Current Reference Generation . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.4 Filter Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.5 Current Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.5.1 Current Equations in d-q Domain . . . . . . . . . . . . . . . . . . . . 33


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Contents v

3.5.2 Current Controller Design . . . . . . . . . . . . . . . . . . . . . . . . 35

3.5.3 PV System Current Calculation . . . . . . . . . . . . . . . . . . . . . 36

3.6 DC bus Capacitor Voltage Controller . . . . . . . . . . . . . . . . . . . . . . 38

3.6.1 Voltage Controller Design . . . . . . . . . . . . . . . . . . . . . . . . 39

3.6.2 Dynamic Simulation of DC bus Capacitor . . . . . . . . . . . . . . . 40

3.7 Simulation of PV System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4 Distribution System 44

4.1 Important terms in Distribution System . . . . . . . . . . . . . . . . . . . . 44

4.2 System Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

4.3 Distribution System Parameters . . . . . . . . . . . . . . . . . . . . . . . . . 46

4.3.1 Modelling of Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

4.3.2 Infinite Grid Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.3.3 Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.3.4 Distribution Line Model . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.4 Distribution System Simulation . . . . . . . . . . . . . . . . . . . . . . . . . 48

5 System Simulation Method 50

5.1 Issues in System Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.2 Methods to Improve Simulation Performance . . . . . . . . . . . . . . . . . . 51

5.3 System Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.4 Simulation of Line faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.5 Simulation of Loss of Grid (Islanding). . . . . . . . . . . . . . . . . . . . . . 54

6 Impact of DG on Area EPS 56

6.1 Tripping Parameters for Distribution System . . . . . . . . . . . . . . . . . . 57

6.1.1 Thumb Rule for Islanding . . . . . . . . . . . . . . . . . . . . . . . . 57

7 Simulation Results 58

7.1 Case 1: Normal Operation of Full PV System . . . . . . . . . . . . . . . . . 58

7.2 Case 2: Change in Irradiance from 1000W/m2 to 500W/m2 . . . . . . . . . . 60

7.2.1 MPPT =0.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

7.2.2 MPPT=0.01. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

7.3 Case 3: Grid Voltage Disturbance . . . . . . . . . . . . . . . . . . . . . . . . 63


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vi Contents

7.3.1 Grid Voltage Variation from 1pu to 0.85pu at t=0.25s. . . . . . . . . 63

7.3.2 Grid Voltage Variation from 1pu to 1.1pu at t=0.25s . . . . . . . . . 66

7.4 Case 3 : Change in Grid Frequency . . . . . . . . . . . . . . . . . . . . . . . 66

7.5 Case 4: Three Phase Fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

7.5.1 Three Phase Fault in Line-1 . . . . . . . . . . . . . . . . . . . . . . . 68

7.5.2 Three Phase Fault at Line-1 with Inverter . . . . . . . . . . . . . . . 71

7.6 Case-5: Loss of Grid - Islanding . . . . . . . . . . . . . . . . . . . . . . . . . 71

7.6.1 Loss of Grid,Load1 = 750kW, Qf= 1.7Load2 = 70kW at t= 0.25s 727.6.2 Loss of Grid,Load1 = 500kW, Qf= 1.7Load2 = 70kW at t= 0.25s 747.6.3 Loss of Grid, Load

1 = 1300kW, Qf= 1.7Load

2 = 70kW at t=

0.25s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

7.6.4 Loss of Grid,Load1 = 900kW, Qf= 1.7Load2 = 70kW at t= 0.25s 757.6.5 Summary of Voltage & Current Data . . . . . . . . . . . . . . . . . . 76

7.7 Case 6: Study of Nuisance Trip for Fault Ride Through Enabled PV System 77

8 Conclusions 80

A MATLAB Simulation 83

A.1 Over all system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83A.2 Distribution Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

A.3 Distribution System - Phase a . . . . . . . . . . . . . . . . . . . . . . . . . 85

A.4 PV System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

References 87


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List of Tables

4.1 Line Parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

6.1 System response to abnormal voltages. . . . . . . . . . . . . . . . . . . . . . 57

7.1 Voltage at PCC & PV System (500kW) Output Current . . . . . . . . . . . 76

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List of Figures

1.1 Schematic diagram PV system. . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1 PV Operating Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 V-I & P-V Characteristics of Sanyo 215W PV Panel. . . . . . . . . . . . . . 9

2.3 Concept of Air Mass Spectrum[7] . . . . . . . . . . . . . . . . . . . . . . . . 10

2.4 Solar Light Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.5 Manufacturer Data Sheet. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.6 Single Diode Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.7 Double Diode Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.8 PV Panel Model implementation . . . . . . . . . . . . . . . . . . . . . . . . 20

2.9 PV Panel Model in MATLAB/SIMULINK . . . . . . . . . . . . . . . . . . . 21

2.10 PV System Single line diagram . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.11 Structure of PV Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.12 Flowchart of P&O MPPT Algorithm . . . . . . . . . . . . . . . . . . . . . . 25

3.1 Frame Transformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.2 Sequence Component of Unbalanced System . . . . . . . . . . . . . . . . . . 28

3.3 Sequence Component for L-G Fault at Phase C . . . . . . . . . . . . . . . . 293.4 Sequence component for L-G fault at phase C . . . . . . . . . . . . . . . . . 30

3.5 Simplified structure of SRF PLL with filter. . . . . . . . . . . . . . . . . . . 30

3.6 Performance of PLL during fault at t=0.5s . . . . . . . . . . . . . . . . . . . 31

3.7 Inverter - Filter - Grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.8 Current controller block diagram . . . . . . . . . . . . . . . . . . . . . . . . 35

3.9 Current Controller Response . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.10 Overall Current Controller Block Diagram . . . . . . . . . . . . . . . . . . . 37

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List of Figures ix

3.11 Calculation of PV System Current . . . . . . . . . . . . . . . . . . . . . . . 37

3.12 Voltage Controller Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . 38

3.13 Voltage Controller Response . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.14 Voltage Controller with Power Balance . . . . . . . . . . . . . . . . . . . . . 41

3.15 Simulation of PV system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

4.1 DG interconnection Terms IEEE std. 1547 . . . . . . . . . . . . . . . . . . . 45

4.2 Single Line Schematic Diagram of the Photovoltaic System Interfaced with

Distribution Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.1 System Implementation in MATLAB/SIMULINK . . . . . . . . . . . . . . . 535.2 Simulation of Line Fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5.3 Simulation of Loss of Grid(Islanding) . . . . . . . . . . . . . . . . . . . . . . 55

7.1 PV System Response for MPPT perturbation =0.1, E= 1000W/m2,T = 25oC 59

7.2 Distribution system Line Currents ig1, ig2, ig3, & is1 for MPPT=0.1, E =

1000W/m2, T= 25oC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

7.3 Grid Voltage at PCC, Phase a Current & Voltage at PCC & iga1, iga2 for

MPPT=0.1, E= 1000W/m2, T = 25oC . . . . . . . . . . . . . . . . . . . . . 60

7.4 (a)Change in Irradance, (b) Three Phase Output Current of the PV System

MPPT=0.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

7.5 PV System Response for Irradiance Change (1000W/m2 to500W/m2) MPPT=0.1 61

7.6 (a) Transient Portion of PV Current , Power & System Output Current

MPPT=0.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

7.7 (a) Transient Portion of PV Current ,Voltage, Power & System Output Cur-

rent MPPT=0.01 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

7.8 Transient Response of the PV system for Grid Voltage Variation 1pu to 0.85pu 647.9 Grid Voltage (varying from 1pu to 0.85pu)at PCC and PV System Current . 64

7.10 Transient Portion of PV system Elements for Grid Voltage Variation of 1pu

to 0.85pu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

7.11 PV Voltage Transient for Different MPPT Perturbation during Grid Voltage

Variation of 1pu to 0.85pu . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

7.12 Transient Response of the PV system for Grid Voltage Variation 1pu to 1.1pu 66

7.13 Grid Voltage (varying from 1pu to 1.1pu)at PCC and PV System Current . 67


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x List of Figures

7.14 PLL Dynamics for Grid Frequency Variation from 50Hz to 45Hz . . . . . . . 67

7.15 Single Line Schematic of Distribution System . . . . . . . . . . . . . . . . . 68

7.16 Transient Response of the PV system During Three Phase Fault at Line -1 . 68

7.17 Distribution System Line Currents During Fault at Line1 . . . . . . . . . . 69

7.18 PV System Fault Current & Grid Voltage at PCC During Fault at Line-1 . . 69

7.19 Transient Portion of PV System Current & DC Bus Voltage During fault at

Line-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

7.20 PV System Fault Current & Grid Voltage at PCC During Fault at Line-1

with Inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

7.21 Transient Response of the PV system During Loss of Grid . . . . . . . . . . 72

7.22 Distribution System Line Currents During Loss of Grid . . . . . . . . . . . 72

7.23 Grid Voltage at PCC & PV System Current During Loss of Grid. . . . . . . 73

7.24 Grid Voltage Vector & Grid Frequency During Loss of Grid. . . . . . . . . . 73

7.25 Grid Voltage at PCC & PV System Current During Loss of Grid for Load-

1=500kW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

7.26 Grid Voltage at PCC & PV System Current During Loss of Grid for Load-

1=1300kW. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

7.27 Grid Voltage at PCC & PV System Current During Loss of Grid for Load-1=900kW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

7.28 Grid Voltage Vector & Grid Frequency During Loss of Grid with Different

Connected Load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

7.29 Transient Response of the PV system During Fault at Middle of Line-3 . . . 78

7.30 Distribution System Line Currents During Fault at Middle of Line-3 . . . . 78

7.31 Grid Voltage at PCC & PV System Current During Fault at Middle of Line-3 79

A.1 Over all System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

A.2 Distribution Network Implementation . . . . . . . . . . . . . . . . . . . . . . 84

A.3 Distribution System - Phase a implementation . . . . . . . . . . . . . . . . 85

A.4 PV system Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . 86


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Nomenclature

Symbols : Definitions

Vdc : Voltage of the dc bus

M P P : Maximum power point

P LL : Phase locked loops

P V : Photo voltaic

Isc : Short circuit current

Voc : Open circuit voltage

tauv : Voltage temperature co-efficient

taui : Current temperature co-efficientIm : Maximum power current

Vm : Maximum power voltage

Iph : Photon Current

Io : Diode reverse saturation current

VT : Diode thermal voltage

K : Boltzmann Constant

Ipv : Photovoltaic cell current

Vpv : Photovoltaic cell voltagePpv : Power generated by Photovoltaic arrays

n : Ideality factor

E : Solar irradiance

: Shadowing factor

Lf : Filter inductor

Cf : Filter Capacitor

isg : Current injected into infinite grid

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xii Nomenclature

isa : PV system phase a output current

vi : Inverter output voltagevg : Infinite grid voltage

Ns : Number of PV panel connected in series to form a PV string

Np : Number PV string connected in parallel

Vom : Zero sequence peak voltage

V1m : Positive sequence peak voltage

V1m : Negative sequence peak voltage

Idcap : d-axis component of the current require to regulate the DC bus voltage

Ts : Sensor time constantKs : Sensor gain

Tf : Filter time constant

Td : Inverter time constant

Vsd : d-axis component of grid space voltage vector at PCC

Vsq : d-axis component of grid space voltage vector at PCC

: Grid frequency in rad/s

via, vib&vic : Three phase output voltage of the inverter

vsa, vsb&vsc : Three phase grid voltage at PCC(Bus-1)vga, vgb&vgc : Three phase substation(infinite grid) voltage

v1La, v1Lb&v1Lc: Three phase voltage at Bus-2

v2La, v2Lb&v2Lc: Three phase voltage at Bus-3


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Chapter 1

PV Generation - Current Scenario

Increasing fear about global warming leads to better international awareness in renewable

energy sources. Many agreements have been signed by countries (i.e Kyoto, Copenhagen

and Durban) to reduce the global warming. IPCC reports on climate change (i.e arctic

snow melting) warned the global community on carbon foot print. Many countries agreed to

decrease their carbon foot print. In Copenhagen[1], India has made a commitment to reduce

its emissions per unit of GDP 20% to 25% below 2005 levels by 2020. Studies shows that

38% of carbon is coming out from electricity generation.

Since GDP growth is linearly related with electricity production, each year India hasto increase the generation by 10% (current installation of 190 GW). Currently India is the

third biggest consumer of coal and its growing deficit and increasing export tax in coal

rich countries (i.e., Indonesia) leads to shut down of many thermal stations. Also 1unit of

electricity produces 1kg of carbon, it is difficult to go forward with coal based thermal plant.

There is another statistics[2] says that, if Indian car owning capability (1% now) increase to

25% (1 in 4), then India have to import 20 times more than current oil import, it is not at

all feasible. So hybrid or e-car will be the only option. This leads to increased dependency

of electricity in the future year. Mean time India is trying to improve its per capita energyconsumption at least to 1000 units/annum.

Indian government is looking forward on renewable energy sources, especially in solar.

But disadvantage with solar energy is its cost, because it is more expensive than other

conventional and renewable energy sources (Rs. 10/unit for solar thermal, Rs. 10/unit for

solar PV, Rs.10.49 for diesel, and Rs.2.50 for coal and Rs.4.20 for wind). Due to increment

in the oil price, solar PV generation cost went below conventional diesel and gas based plant

(by considering cumulative unit cost). During last year, cost of the PV was decreased from

1


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2 Chapter 1. PV Generation - Current Scenario

40 to 50%, due to increased capacity of panel and other auxiliary equipments production,

further cost reduction in the generations are expected in the near future.

1.1 Global PV Installation

Installation of solar PV has been growing rapidly world wide. Installation of PV generations

(Maximum Peak at STC) is increased from[4]7.5GW at 2007 to 70GW at the end of 2011.

Many countries in the world trying for modest addition of the PV generation. Out of the

total installation, Europe itself accounts for around 52GW PV generation, German leading

is with 27GW of PV installation, followed by USA, Japan, Spain and China. Many large

scale PV plant installations are in progress (E.g 550MW Topaz Plant and 500MW Blythe

Solar Plant both in California etc). Greece plans to add 10000MW before 2015 andIsrael looking for major addition of PV generation.

Studies show that world-wide installation of PV generation will reach above 1.8TW

(which is approximately equal to present power installation of whole India). As of now

Golmud solar park (200MW) is the biggest individual solar plant in the world, but largest

solar park is Charanka Solar Park (214MW) Gujarat India, installation in this park expected

to increase upto to 500MW

1.2 PV Installation in India

India is one of the high solar insolation receiving country around 4 7unit/m2 with 250-330solar days, where western Rajasthan receive high irradiation and north east on the lower

side. So the high solar irradiance with higher population density makes India as a perfect

choice for solar PV. Demo project constructed by MNRE around India shows that, country

has highest capacity utilization factor[3] (CUF around 15%-19% ) only after US Mojavedesert. Gujarat and Rajasthan found to be a better choice due to its land availability (14.4

million acre in Gujarat, 51 million acre in Rajasthan) and high solar radiation. Around 5

acre is required for 1MW generation. Solar can supply power to entire country. Studies

shows that Indias available non conventional energy potential -solar energy accounting to 40

%. So India cannot avoid solar for its future progress.

PV generations in India is experiencing more than 100% growth over last two years, In

November 2011 India had 146MW PV installation, but as on MAY 2012 India has 979.4MW


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1.3. Solar Mission JNNSM 3

installed PV generation. Gujarat is leading the race with 654MW PV installation, Rajasthan

has around 200MW installed PV generation. Gujarat government is planning for very high

PV installation. Patan district (in Gujarat) have highest PV installation in Asia. Charanka

Solar Park (in Gujarat)is the biggest PV plant in Asia with planned capacity of 500MW,

in this 214MW which is already installed. RIL 40MW thin film (500000 Panels) solar plant

and Adani 40MW plant are the largest individual PV plants in India. Gujarat government

proposed multi purpose Narmada Canal PV plant, which is first of its kind in the world.

Already 1MW PV plant is installed on the Narmada canal.

1.3 Solar Mission JNNSM

Indian government (MNRE) proposed a big project called Jawaharlal Nehru national solar

mission (JNNSM) at a cost of 19bn US$. To generate 20000 MW of grid connected solar

energy before 2022, 1000 MW before 2013, and additional 3000 MW before 2017, using solar

photovoltaic and solar thermal technique. It is planned to add power in 33 KV line and

above (which will be purchased by NVVN), also to add generation in 11 kV line (roof top

PV) with the subsidy by MNRE Rs. 12 for solar PV and Rs.10 for solar thermal (excluding

utility PPA).

1.4 Photovoltaic System Overview

PV cell converts light energy into electrical energy. Many PV cells connected in series will

form PV cell string to achieve high voltage, and many strings are connected in parallel to

increase the current capability will form a PV module or PV panel. PV cells generate DC

voltage and this DC voltage is converted into AC by using power electronics inverter. PVoutput is proportional to the light energy (irradiance). So there is a need for storage battery,

to ensure supply during less sun irradiance (in night) hours. But this leads to a high cost

and environmental pollution because of chemical substance in the battery. Also batteries

have less life time. So grid connected PV systems as shown in the Fig.1.1 have been very

popular, PV system will inject all its power to the electrical grid, so PV system is fully

utilized. Grid integration is must for MW size PV plants. Proper scheduling between PV

and other conventional power plant can ensure full uninterruptible power supply.


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1.4.1 PV System Grid interface

As shown in the Fig.1.1 .The main building blocks of the grid connected PV system arePV panel arrays, a Voltage Source inverter(VSI), MPPT, PLL, Filter & Transformer. PV

panel and its characteristic are explained in chapter 3. Different control strategies are used

to control & interface PV system to the grid. Popular control strategies are

Figure 1.1: Schematic diagram PV system

Synchronous reference frame control (SRF)

Stationary reference frame control

Based on the control parameter, many interface control algorithms are proposed in the

literatures. Important interface control techniques[5] are

Constant P-Q control

P-V control

Constant Current control

The studies carried out in the project uses the constant current control interface, due to itssimplicity in UPF based power injection to the grid.

1.5 Project Work

Main aim of the project is to study the behaviour of the grid connected PV system during

various grid disturbances such as three phase fault, voltage variation, frequency variation,

sudden change in the irradiance and loss of grid (Islanding).


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1.6. Organization of the Thesis 5

1.5.1 Activities in the Project

To carry out the proposed study, requires modelling of both power system and power elec-tronic elements. List of project tasks are,

Modelling of PV panel using only manufacturer data sheet

Implementation of MPPT

Controller & PLL Design

Modelling of PV system Interface

Distribution system study and parameter selection

MATLAB/SIMULINK implementation of full system (including Distribution system)

Exploring faster and user friendly simulation method

Using the PV system model, studying the system behaviour for various grid distur-bances

1.6 Organization of the Thesis

Chapter 2 discusses about implementation of manufacturer data sheet based photo-voltaic panel model, selection of PV panel, implementation of MPPT and specification

of PV system. Also important terms used in PV system are explained.

Chapter 3discusses about PLL design with moving average filter, current and voltagecontroller design, implementation of (UPF)constant current based interface, DC bus

capacitor dynamic simulation, simulation of full PV system.

Chapter 4 discusses about overview of distribution system, parameter selection of givendistribution system and dynamic equations of distribution system

Chapter 5, discusses about, issues in the system simulation, solution to the problemand exploring the option of faster & user friendly simulation method.

Chapter 6, discusses about IEEE 1547 protection data and overview of impact ofdistribution system on grid.


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In Chapter 7, Simulation results for various grid disturbances are documented

Chapter 8, concludes the project

View of system simulation blocks are given in the Appendix.


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Chapter 2

PV Panel Modelling

Photovoltaics is the direct conversion[6] of light into electricity at the atomic level. Some

materials exhibit a property known as the photoelectric effect that causes them to absorb

photons of light and release electrons. When these free electrons are captured, an electric

current results that can be used as electricity.

The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in

1839, who found that certain materials would produce small amounts of electric current

when exposed to light. In 1905, Albert Einstein described the nature of light and the

photoelectric effect on which photovoltaic technology is based, for which he later won aNobel prize in physics. The first photovoltaic module was built by Bell Laboratories in 1954.

It was billed as a solar battery and it was mostly just a curiosity as it was too expensive

to gain widespread use. In the 1960s, the space industry began to make the first serious

use of the technology to provide power aboard spacecraft. Through the space programs,

the technology was advanced, its reliability was established, and the cost began to decline.

During the energy crisis in the 1970s, photovoltaic technology gained recognition as a source

of power for non-space applications

2.1 PV Principle

Solar cells are made out of a semiconductor material. Since light is the collection of small

packets called quanta, and it is a electromagnetic wave. Light Energy contains many photons,

which move in different frequencies, each frequency in the lights spectrum, contains a specific

energy E = h. When PV material is exposed to light, some photons are reflected on the

PV panel surface, remaining photons are absorbed by the PV material, depending on the on

7


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8 Chapter 2. PV Panel Modelling

material property, the absorbed photon, which have energy greater than band gap energy of

the semiconductor material (for Si energy gap is 1.12eV) generate free charge carriers in the

semiconductor bulk. Due to band bending in p-n junction semiconductor, electrons try to

move to lower energy level (n region) and holes try to move higher energy level (p region),

so it creates a potential difference between p & n region. From the above we can say that

voltage across the cell is dependent on the energy gap. Due to the generated voltage (across

p-n junction) there will be current circulating inside the cell (diode forward bias) called dark

current (since it is not directly related to irradiance). Fig.2.1shows the operating region of

PV p-n junction semiconductor devices.

Figure 2.1: PV Operating Region

From the Fig.2.1it can be observed that PV Cell has a non-linear V-I characteristics.Also

it is depends on irradiance, Temperature and other climatic condition as shown in the Fig.2.2.

2.2 Important Terms in PV Panel

PV output voltage and current depends on temperature and irradiance. Power generated

by PV panel is proportional to irradiance. Also there are other important parameters which

are considered for the design of PV panel are explained in this section


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2.2. Important Terms in PV Panel 9

Figure 2.2: V-I & P-V Characteristics of Sanyo 215W PV Panel

2.2.1 Irradiance

The radiation of the sun reaching the earth is distributed over a range of wavelengths from

300 nm to 4 micron approximately, which is partly reflected by the atmosphere and partly

transmitted to the earths surface. Photovoltaic applications used for space, such as satellites

or spacecraft, have sun radiation availability different from that of PV applications at the

earths surface. The radiation outside the atmosphere is distributed along the different

wavelengths in a similar fashion to the radiation of a black body at temperature 5762K

following Plancks law, whereas at the surface of the earth, the atmosphere (e.g. ozone layer

which filter UV light) selectively absorbs the radiation at certain wavelengths. It is common

practice to distinguish two different sun spectral distributions

Air Mass Spectrum- AM0Spectrum outside the earths atmosphere on a plane is perpendicular to the sun at the

mean earth-sun distance. The power density outside the earths atmosphere is 1367W/m2 and this is known as the solar constant

Air Mass Spectrum - AM1.5Air mass refers to the relative path length of the direct solar beam through the atmo-

sphere. The path of the light through the atmosphere is shortest when the sun is at

its zenith(perpendicular to the earths surface), the path length is 1.0 (AM 1.0) and

this gives rise to the AM1 spectrum. Obviously, the sun is not always at the zenith.


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When the angle of the sun from zenith increases, the air mass increases so that at an

angle of 48.2o the air mass is 1.5(refer F.ig2.3). This has been adopted as the standard

sunlight spectrum for terrestrial arrays(refer Fig.2.4) for actual spectrum.

Figure 2.3: Concept of Air Mass Spectrum[7]

Figure 2.4: Solar Light Spectrum

Important terminology to express magnitude of solar light:

Spectral irradiance IPower received by a unit surface area in a wavelength differential d, the units are

W/m2m


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2.2. Important Terms in PV Panel 11

Irradiance

The integral of the spectral irradiance extended to all wavelengths of interest andthe units are W/m2

Radiation/Insolation

The time integral of the irradiance extended over a given period of time, therefore

radiation units are units of energy is given inkWh/m2day, or simply electrical unitsper day.

2.2.2 Temperature

PV output will change with respect to temperature[8], because band gap, carrier concentra-

tion are dependent on temperature. For maximum power output of the module, it is needed

to lower operating temperatures but increase the irradiance. The typical temperature coef-

ficient of power is 0.5%/oC for mono or polycrystalline silicon cells. The cell temperature

of open rack modules (e.g. a-Si), however, is governed by several external factors such as

ambient temperature, irradiance level, wind speed, wind direction, and tilt angle of the mod-

ule in the array. Temperature considered is actually the PV cell temperature and not its

temperature of the rack or atmospheric temperature. As per standard PV cell outputs, are

given in cell temperature of 25o.

2.2.3 NOCT

Nominal operating cell temperature (NOCT) is the cell temperature, when open circuited

panel subjected to a irradiance of 800W/m2, ambient temperature of 20oCand wind speed

of 1m/sat a module tilt angle 45o. This will give the idea of cell temperature rise in the PV

panel. Nominal value of NOCT will be around 43o 50oC. this information is available inthe manufacturing data sheet.

NOCT temperature given in data sheet will be useful to find approximate PV panel

temperature using known ambient temperature [2]

Tc=(TNOCT Ta) E

800 (2.1)


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2.2.4 Standard Test Conditions(STC)

The STC (also known as SRC Standard Reporting Conditions) is defined with nominal celltemperature 25oC, nominal irradiance level 1000W/m2 at spectral distribution of Air Mass

1.5 solar spectral content. Many of the important parameter given in the data sheet is based

on this condition.

2.2.5 Open Circuit VoltageVoc

Voltage across the PV panel, when it is open circuited (IPV = 0). This voltage will change

with respect to irradiance and temperature. Voc

will increase with respect to irradiance,

decrease with respect to temperature(-ve coefficient v). This is the important parameter of

PV panel, which is used to calculate maximum PV voltage and important for system design.

BothVoc andv are given in the manufacturer data sheet at STC. All points lie in the x-axis

as shown in Fig.2.2are open circuit voltages with respect to different irradiance.

2.2.6 Short Circuit Current Isc

Current supplied by PV Panel when its terminals are shorted. This current will change

with respect to irradiance and temperature. Isc will proportionally increase with respect to

irradiance, logarithmically increase with respect to temperature(+ve coefficient i). This is

the important parameter of PV panel and is used to calculate maximum fault current, it is

also important for system design. Both Isc and i are given in the manufacturer data sheet

at STC.All points lie in the y-axis as shown in Fig.2.2are short circuit currents with respect

to different irradiance.

2.2.7 Maximum Power Point MPP

As shown in the Fig.2.2PV voltage and currents are non linearly related, so at a particular

voltage, PV panel supplies maximum power, that in-turn changes with respect to climatic

conditions (e.g. irradiance, Temperature, . . . etc.). The point(voltage, current) in Fig.2.2at

which PV panel supplies maximum power is called maximum power point (MPP). So to oper-

ate PV at MPP requires separate controller called MPPT (maximum power point tracker).

MPPT will play an important role in PV system dynamics. Maximum power at STC is

considered as a power rating of the PV panel.


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2.3. Manufacturer Data Sheet 13

2.2.8 Maximum Power Voltage and Current (Vm&Im)

Voltage and current of the PV panel, when it is operating in maximum power point arecalled maximum power voltage and current. Values ofVm&Im are given in the data sheet at

STC.

2.2.9 Fill Factor

Every cell has a life expectancy. As time progresses, the quality of cell goes down. Hence, it is

essential to check the quality[9], periodically so that it can be discarded once the quality falls

below certain level. The quality of the cell called Fill Factor (FF) which can be calculated

as

FillFactor= Im VmVoc Isc (2.2)

Ideally, the Fill Factor should be 1 or 100%. However, the actual value of FF is about 0.8

or 80%. A good panel has fill factor in the range of 0.7 to 0.8. For a bad panel it may be as

low as 0.4.

2.2.10 Temperature Co-efficient v &i

Temperature co-efficient (vV /oC & iA/

oC) given in the PV panel data sheet is used to

quantify change in the PV panel voltage and current with respect to temperature. This is

very important to model a PV panel.

2.3 Manufacturer Data Sheet

To model the practical PV panel, its required to use only the data given in the manufacturer

data sheet. Refer Fig.2.5for the sample manufacturer data sheet. Clear understanding ofthe data given in the data sheet is must for modelling of the PV panel. All the quantities

are already explained above.

2.4 PV Modelling

PV panel modelling is very important for dynamic analysis of PV system. Many models are

proposed in the literature.


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SANYO North America

Energy System Solutions Division

550 S. Winchester Blvd., Suite 510San Jose, CA 95128, U.S.A.

www.sanyo.com/solar

[email protected]

SANYO North America. All Rights Reserved. 1/6/2010

Model HIT Power 215A or HIT-N215A01

Rated Power (Pmax)1 215 W

Maximum Power Voltage (Vpm) 42.0 V

Maximum Power Current (Ipm) 5.13 A

Open Circuit Voltage (Voc) 51.6 V

Short Circuit Current (Isc) 5.61 A

-0.336%/ C

-0.143 V/ C

1.96 mA/ C

NOCT 114.8F (46C)

CEC PTC Rating 199.6 W

19.3%

17.1%

Watts per Ft.2 15.85 W

600 V

Series Fuse Rating 15 A

Warranted Tolerance (-/+) -0% / +10%

Module Area 13.56 Ft2(1.26m2)

Weight 35.3 Lbs. (16kg)

62.2x31.4x1.8 in. (1580x798x46 mm)

Cable Length +Male/-Female 46.45/40.55 in. (1180/1030 mm)

No. 12 AWG / MC4TMLocking Connectors

Static Wind / Snow Load 60PSF (2880Pa) / 39PSF (1867Pa)

63.2x32x72.8 in. (1607x815x1850 mm)

34 pcs./1234.5 Lbs (560 kg)

952 pcs.

Ambient Operating Temperature -4F to 115F (-20C to 46C)2

1 hailstone (25mm) at 52 mph (23m/s)

Class C

UL 1703, cUL, CEC

5 Years Workmanship, 20 Years Power Output

1STC: Cell temp. 25C, AM1.5, 1000W/m22

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0 10 20 30 40 50 60

0

25

50

75

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0 10 20 30 40 50 60

1000W/m2

800W/m2

600W/m2

400W/m2

200W/m2

Unit: inches (mm)

HIT Power 215A

31.4 (798)

Front Side Back

62.

2(

1580)

1.8 (46) 1.5 (37)

13

(330)

15.47 (393)

Ground (4 places)

0.165 (4.2) 0.205 (5.2)

0.165 (4.2) 0.205 (5.2)

-

+

40.

55(

1030

)

46.4

5(

1180)

4x mounting holes 0.276 (7)

Section A-A

1.

8

(46)

1.5 (37)

HITis a registered trademark of SANYO Electric Co.,LTd..The name HIT comes from Heterojunction with

intrinsic Thin-layer which is an original technology ofSANYO Electric Co.,Ltd..

Figure 2.5: Manufacturer Data Sheet


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2.4. PV Modelling 15

2.4.1 Single Diode Model

Photovoltaic cell is made of simple p-n junction semiconductor[8]. But instead of externalvoltage excitation, light (photons) is used to excite the electrons and holes.and the voltage

across the cell is depends on the energy gap. Due to the voltage across p-n junction, which

forward biases the junction, there will be current circulating inside the cell called dark current

(independent of irradiance). Hence PV is modelled as a current source parallel with a diode

as shown in Fig.2.6and it is represented as single diode model.

Figure 2.6: Single Diode Model

Ipv = Iph Io eV+IpvRS

VT

1

V + IpvRSRSh (2.3)

VT = nKT

qby neglecting Rsand Rsh

Ipv = ISC IO

eVVT 1

(2.4)

W hen Ipv = 0then Voc = VT ln

1 +

ISCIO

(2.5)

When Vpv = 0then Isc = Iph

From the Eq.2.5, open circuit voltage is logarithmically related with irradiance (PV panel

photon current)

2.4.1.1 Series ResistanceRs

The series resistive losses are present in practical solar cells. In fact, the current generated in

the solar cell bulk travels towards the contacts through resistive semiconductor material, both

in the base region( normally P type-not heavily doped) and in the narrow emitter region(N

type), which are normally heavily doped. Besides these two components, the resistance of the


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metal grid, contacts and current collecting bus also contribute to the total series resistive

losses. It is common practice to assume that these series losses can be represented by a

lumped resistor Rs which is dependent on temperature is called the series resistance of the

solar cell.

2.4.1.2 Shunt Resistance Rsh

A number of shunt resistive losses are identified, such as localized shorts at the emitter layer

or perimeter shunts along cell borders are among the most common. This is represented

generally by a lumped resistor, Rsh, in parallel with the intrinsic device.

2.4.2 Double Diode Model

In the above model, effect of non ohmic losses, due to recombination in the space charge

region of the solar cell is not considered. This is relevant at low voltage bias and can be

represented in an equivalent circuit by a second diode term with a saturation current I02,

which is different from the saturation current (I0) of the ideal solar cell diode, and given

ideality diode factor different from 1, normally diode with ideality factor 2 is used for this

purpose.

In practice, only few devices exhibit a totally ideal I(V) characteristic with ideality coef-

ficient equal to unity, so it is common practice to also add a parameter n to account for non

idealities in the dark current diode and the single diode model can be modified to take this

effect into account.

Figure 2.7: Double Diode Model

Ipv =Iph I01

e

Vpv+IpvRs

nVT

1

I02

e

Vpv+IpvRs

2VT

1

Vpv+ IpvRsRsh

(2.6)


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In Fig.2.7, first diode(I0)with ideality factor n accounts for dark current (independent of

irradiance) and second diode with a ideality factor of 2 accounts for non ohmic shunt losses

in the solar cell.

The above models have some disadvantages,

Many parameters (Rs, Rsh, I01, I02, Iph, VT) are not available, because it is depended onproperty of the material. Also there are not given by manufacturer.

Semiconductor equations are derived from five carrier transport differential equations,which assume uniform doping and crystalline material. But this is not true in the case

with practical module and thin film PV (which have p-i-n junction). So there is a

need to model PV Cell that uses only the information given by the manufacturer data

sheet. Also it must take care of change due to climatic condition (normally irradiance,

Temperature and wind speed). Translation equations are proposed to translate voltage

and current from one condition to another condition.

forth coming models will use only the data given in the data sheet as shown in the

Fig.2.5.

2.4.3 Sandstrom EquationIn the past, there is a need for translating PV current and voltage from one environmen-

tal condition to another environmental condition[10]. First accepted translation equation is

proposed by an IEEE paper by J. D. Sandstrom of the Jet Propulsion Laboratory (JPL)

published in 1967. The paper presents some very good correlative results between experi-

mental measurements and the resultant analytical predictions over a cell temperature range

from 20oCto 130oC, and over an irradiance range of 500W/m to 3000W/m. The equations

were widely used and standardized as IEC 891 model[11] correction procedure 1.

Isc = Isc1

E2E1

1

+ i(T2 T1) (2.7)I2 = I1+ Isc (2.8)

V2 = V1 v(T2 T1) IscRs k(T2 T1)I2 (2.9)P2 = V2 I2

the above equations Eq. 2.15& Eq. 2.16are used to translate PV current and voltage from

one (I1 & V1) irradiance, tempertaure (E1 & T1 normally in STC) to an another (I2 & V2)


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irradance and temperature (E2 & T2)

2.4.4 A.J. Anderson Translation equation

The above equations Eq.2.15 and Eq.2.16 suggest that the temperature effect on current

is independent of irradiance, if i assumed as a constant temperature co-efficient. The

irradiance effect on voltage (Voc) as in Eq. 2.16 is accounted for by the term Isc Rs,but by experiment, it is found (by Anderson[10]) that variation in voltage due to change

in irradiance found very little effect on PV voltage. So for different RS values Sandstrom

voltage equation found incorrect. Corrected Sandstrom equations proposed by Anderson are

given as below;

ISC2 = ISC1

[1 + (T1 T2)] [E1/E2] (2.10)

VOC2 = VOC1

[1 +(T1 T2)]

1 +lnE1E2

(2.11)I2 = I1

ISC2ISC1

(2.12)

V2 = V1VOC2

VOC1 (2.13)

P2 = I2 V2

Where & are neutered temperature co-efficients for avoiding the error due to scaling

= Isc2 Isc1Isc2(T2 T1) & =

Voc2 Voc1Voc2(T2 T1)

the above equations would result inIscltranslating toIsc2 along the lineR = 0 ohms,and

Voc1 would translate to Voc2 along the line R =. The I-V point pairs in between wouldsimilarly translate along lines of constant resistance with values ofR = V1

I1and value of

ca be found by using experiment data, otherwise = kTq

. The above equations can be used

to find all PV parameters by using manufacturer data in STC, but it is not providing the

relation between PV voltage and current.

2.4.5 Bleasser Equations

It is also a corrected form of Sandstrom equation, In current equation[12] effect of irradiance

during temperature change is included. Also, instead of transforming short circuit current


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Isc as in Eq.2.14, actual current (PV current for the present load)is transformed directly.

Voltage equation is mostly same as Sandstrom equation. This equation is standardized by

IEC 891[11]as a correction procedure 2.

Isc = Isc1

E2E1

1

+ i(T2 T1) (2.14)I2 = I1+ Isc (2.15)

V2 = V1 v(T2 T1) IscRs k(T2 T1)I2 (2.16)P2 = V2 I2

The above equations also does not providing the relation between PV voltage and current. So

there is a need for an appropriate model which uses only the data given in the manufacturer

data sheet, and it has to provide the V-I characteristic equation.

2.4.6 Michigan University Model

This model[13]is based on single diode model where Rsh is neglected, and material property

parameters are assumed as variable and its value found by using known parameter, which is

given in the data sheet.

Ipv(Vpv) = Imax i Imax i e

Vpvb(+1)(Vmax+v)

1b

(2.17)

= E2

E1N

Imax = ISC

1 exp(1/b)

= 1 VminVmax+v

i = 1 +T Ci

100 (T2 T1N)

v = T CV (T2 T1N)PowerPpv = Vpv Ipv (2.18)

Vmin is the open-circuit voltage rating of the solar panel array for an effective intensity of

light less than 20% over the solar panels, T1N refers to a STC temperature T1N = 20oC,

E1Nrefers to a STC temperature, T1N= 1000W/m2, Vmax is open circuit voltage at STC.

T Ci & T CV refers to the temperature co-efficient of panel given in the data sheet. Fit

variable b can be found by using available information in the data sheet.


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2.4.6.1 Calculation of b (fit variable)

Maximum PV power can be calculated[14] by Eq.2.18, Vm & Im is the PV current andvoltage at maximum power point, this information is given in the data sheet for STC. At

the maximum Power

dPpvdVpv

= 0 differentiate Eq.2.18

0 = 1

Vmb ( + 1 ) (Vmax+ v)

exp(

Vmb(+1)(Vmax+v)

1b) (2.19)

Using a guess value b = 0.09 calculate Vm(1000W/m2, 25oC) by solving Eq.2.19. Equation

Eq.2.17and Eq.2.18are used to calculate Im(1000W/m2, 25oC) andPm(1000W/m

2, 25oC).

These calculated values have to be compared with values from a module data sheet. Now the

guess value of fit parameter b can adjusted iteratively to minimise the error ofPm(1000W/m2, 25oC)

at STC. Fit variable b for SANYO 215A panel is calculated as b=0.079.

2.5 PV Model Implementation

MATLAB/Simulink is used to implement PV model. Both Anderson equation and Michiganuniversity models are implemented in the single block as shown in the Fig2.9. Here all the

required data sheet parameters, environmental (E & T), design datas are fed into to the

model as shown in Fig.2.8, by using given parameters. PV panel model calculates current

Ipv based on the PV panel voltage Vpv as per the Eq.2.17. Implemented model is simulated

Figure 2.8: PV Panel Model implementation


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2.6. PV System Design 21

for different environmental conditions and results are matching with the given manufacturer

data sheet curve(refer Fig.2.2for data sheet curve and Fig.2.5for simulated results).

Figure 2.9: PV Panel Model in MATLAB/SIMULINK

2.6 PV System Design

Two PV plant with the capacity of 1MW (2 500kW) is modelled for the analysis as shownin the Fig.2.10. Here SANYO 215A (215W) PV panel as in Fig.2.5is used for simulation.

Many panels connected in series and parallel as shown in the Fig .2.11 for achieving high

voltage and power.


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2.6.1 Selection of PV Arrays

Number of panels required are selected based on the calculation given below; From the

Figure 2.10: PV System Single line diagram

Fig.2.10the expression for inverter output voltage is given as

vi = Lfdis

dt + vgrid (2.20)

If vgird = Vgmsin(t)

F or U P F is = Ismsin(t)

So vi = LfIsmcos(t) + Vmsin(t) (2.21)

vi = jXLis+ vgrid (2.22)

Required inverter voltage is decided by grid voltage and voltage across inductor as in Eq. 2.22.

Also inverter voltage is depends on DC bus voltage (PV Voltage) and method of switching.

maximum inverter voltage possible for Sine Triangle PWM is Vi = Vpv/2. Maximum inverteroutput voltage required is calculated by assuming inductor voltage of 0.1pu, when rated

current flow through the inductor, and 10% increase in grid voltage is considered

Required max Vimax = 0.1Vgrid+ 1.1 Vgrid (2.23)Vpvmin = 2Vimax (2.24)

It is required to connect many PV panels in series to achieve the voltage requirement of

the inverter (as referred in Eq.2.23). To supply maximum possible power to the grid at all


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environmental condition, minimum PV voltage (less irradiance and high temperature) must

be equal to two times of maximum required inverter voltage(as given in Eq.2.24).

For PCC line voltage of 415V, minimum of 20 SANYO 215A panels (Ns = 20) have to

be connected in series as shown in the Fig.2.11, So maximum possible voltage is equal to

Ns Voc= 51.3 20 = 1026Vand minimum PV voltage is equal to N s Vmin= 820V.

Figure 2.11: Structure of PV Array

To achieve the required power rating (normally represented in STC), many PV string

must be connected in parallel as given in Eq.2.25, which are called as PV arrays.

Np= Ppv

Ns Vm Vmis at ST C (2.25)

For 500kW PV system Np = 117 PV string must be connected in parallel. So 500kW PV

system need 2340 SANYO 215A panels. Actual rating of PV system is equal to 504.18kW.

Ppv = (Ns Np) Ppanel

2.6.2 Inverter

Since, main focus is on studying transient response for large signal disturbances, so harmonics

generated by the inverter is not of much interest. So average model of inverter is sufficient

for the analysis, but simulation carried out for both systems with and without inverter shows

no significant difference found.

Inverter Design Details

Two level three phase inverter with the rating of 500kW


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Sine triangle PWM with a switching frequency equal to 5kHz

LC filter is used

Inverter is modelled as a first order system with the time constant equal to the half ofswitching time (bandwidth is equal to twice that of switching frequency)

Constant current mode control is used to inject UPF power to the grid

Voltage controller is used to regulate the DC bus voltage.

2.6.3 Maximum Power Point Tracking (MPPT)

As observed fro the earlier studies we seen earlier due to the non linear property of PV panel,

at a particular operating point (Vpv, Ipv) PV generate maximum power is called maximum

power point, Hence it required to have an reliable technique to track the maximum power

point irrespective of the environmental conditions. A MPPT system is therefore always

implemented to optimize the efficiency of the photovoltaic energy conversion. This tracking

system adjusts the inverter voltage reference signal as shown in the Fig.2.12and hence, the

dc voltage at the output of the solar array.Many MPPT techniques are proposed[15]in the literature, most popular being incremen-

tal conductance method and P&O (Perturbation & Observation method). P&O method is

used due to its simplicity and easy implementation. the P&O method works by perturbing

Vpv and observing the impact of this change on the output power of the PV array as shown

in the Fig2.12.

At each cycle, Vpv and Ipv are measured to calculate P(k) . This value of P(k) is

compared to the value P(k 1) calculated at the previous cycle. If the output power has

increased, Vpv is adjusted further in the same direction as in the previous cycle. If theoutput power has decreased, Vpv is perturbed in the opposite direction as in the previous

cycle. is thus perturbed at every MPPT cycle. When the maximum power point is reached,

oscillates around the optimal value . This causes a power loss that increases with the step

size of the perturbation. If this step width is large, the MPPT algorithm responds quickly

to sudden changes in operating conditions. On the other hand, if the step size is small

the losses under stable or slowly changing conditions will be lower but the system will not

respond quickly to rapid changes in temperature or irradiance. There is always trade off


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Figure 2.12: Flowchart of P&O MPPT Algorithm

between speed and steady state loss. Here 0.1V perturbation with the time 2ms is used

to accelerate the simulation speed. MPPT tracking range is set between 700 to 1026 V.

Experiments demonstrated that the maximum power point tracking (MPPT) dictates the

dynamic behavior of the PV generator


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Chapter 3

PV System Control

Synchronizing the PV system to the grid needs information about certain grid parameters,

these informations are obtained from PLL. Here synchronous reference frame (d&q axis)

based control, where three phase quantity is translated into a rotating space vector, which

is used to control the power flow. In this study, grid voltage vector is aligned towards d-

axis. So aligning of current vector to the grid d axis ensures a unity power factor operation.

Separate controllers are used to control active (Id) , reactive (Iq) power, and separate voltage

controller is used to control the DC bus voltage. PLL is used to track the gird frequency

and voltage.

3.1 Synchronous Reference Frame Strategy

Synchronous reference frame (SRF) strategy is a popular method used in the generation of

the current reference and constant current control interface uses this SRF based. The syn-chronous reference frame strategy uses co-ordinate transformations to generate the current

reference. It employs the well known Clarkes Transformation and Parks Transformation for

this purpose. uses the primitive machine model, where balanced current flow in the in-

duction machine (R-Y-B) static three phase winding (R-B-Y) result into rotating flux space

vector. Synchronous reference is given as reference to the controller and the controller output

is obtained, reverse transformations are employed to transform the quantities back to the

actual three-phase system.

26


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3.1. Synchronous Reference Frame Strategy 27

3.1.1 Synchronous Frame Transformation for Three Phase Bal-

anced System

va = Vmsin(t ) (3.1)vb = Vmsin(t 120) (3.2)vc = Vmsin(t + 120) (3.3)

Three phase balanced system can be easily represented by two variables(phase quantities).

Since third being found by using (va + vb + vc= 0), it is possible to transform the three phase

balanced system (a, b, c) to a stationary two phase system ( ) as shown in Fig.3.1By

Figure 3.1: Frame Transformation

using orthogonal decomposition of vector,

v

v

=

1 1

2 1

2

032

32

va

vb

vc

(3.4)

for balanced system, where va+ vb+ vc= 0

v = 3

2va (3.5)

v =

3

2 (vb vc) (3.6)

v and v vary sinusoidally with respect to time, this stationary reference frame can be

translated[16] to synchronously rotating reference frame by using Parks Transformation.

Space vector V = v +jv , rotating in a synchronous speed (same like flux vector of


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3.1. Synchronous Reference Frame Strategy 29

After substituting Eq.3.10,3.11&3.12in Eq.3.4, zero sequence component will cancel out

each other and only positive and negative sequence component will be present.

v = 3

2(V1msin(t + 1) +V2msin(t +2)) (3.13)

v =

3

2 (V1m(sin(t +1 120) sin(t +1+ 120))) +

3

2 (V2m(sin(t +2+ 120) sin(t + 2 120))) (3.14)

By substituting Eq.3.13,3.14in Eq.3.7, assuming d axis is aligned towards positive sequence

voltage.

= t 2

+ 1 (3.15)

Vd = 3

2(V1m V2mcos(2t + 1+ 2)) (3.16)

Vq = 3

2(V2msin(2t +1+ 2)) (3.17)

From the above equations, due to the unbalance in the three phase quantity, d axis component

of the three phase quantity will have effect of second harmonics negative sequence component.

This is because negative sequence space vector will move in opposite direction with respect

to positive sequence vector as shown in the Fig.3.3. For bolted L-G fault on C-phase,

(Vc = 0 Va = 239V(rms phase voltage), and V b = 239V(rms phase voltage)). The

sequence components are shown in the Fig.3.3, equivalent d & q axis components are shown

in the Fig.3.4.

Figure 3.3: Sequence Component for L-G Fault at Phase C


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30 Chapter 3. PV System Control

Figure 3.4: Sequence component for L-G fault at phase C

3.2 Phase Locked Loops (PLL)

To supply unity power factor current and to maintain synchronism with the grid, PV sys-

tem needs grid angle,voltage and frequency, Synchronous reference frame PLL with moving

average filter[17] is implemented as shown in the Fig.3.5. Moving average filter is used

to eliminate 2nd

harmonic negative sequence component as in Eq.3.16. Dynamics of PLLhave significant role in transient behaviour of PV system. Because important parameters

(Vsd, Vsq, sinj(),cos(), ) are obtained from PLL, which have significant influence in

the modulation index. The parameter of the controller chosen using symmetrical optimum

Figure 3.5: Simplified structure of SRF PLL with filter

method[18]. value ofkp = 0.5 and T = 0.08. Performance of PLL shown in the Fig3.6. in

this case L-G fault is created at time t=0.25s.


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3.3. Current Reference Generation 31

Figure 3.6: Performance of PLL during fault at t=0.5s

3.3 Current Reference Generation

All the power generated by the PV must be inject to the grid, at the same DC bus voltage

have to maintained constant. Power injected by the inverter to the gird in d-q domain is

given as,

S = 3

2VmIm

= 2

3(Vd+ jVq)(Id jIq)

= 2

3(VdId+VqIq+ j(VqId VdIq))

So active power is P = 2

3(VdId+VqIq)

Vq = 0 space vector aligned towards daxis

P = 2

3(VdId) (3.18)

Power Generated by PVPpv

= Vpv

Ipv

(3.19)

Equating Eq.3.18and Eq.3.19, current reference set by the PV system is

Id = 3VpvIpv

2Vd(3.20)

Idref = Id Idcap (3.21)

Idrefis the current reference to the current controller, Idcap- Current required by the capacitor

to maintain constant voltage. To protect and increase the life time of the inverter, current


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reference is limited to its rated current or 1.5 to 2 times of the rated current. In this study

current reference is limited to 1.5 times of the rated current.

3.4 Filter Design

LC filter is used to filter out the harmonics as specified by standard IEEE 1527, for grid

connected application. For LC filter, value of the inductor is selected as same like L filter.

value of C must be chosen[19], such that resonant frequency of the filter is higher than the

bandwidth of the closed current loop. Normally maximum inductor voltage is limited to

0.1pu of the rated voltage. Maximum possible inductor can selected as shown below,

VLf = 0.1 VsphVsph =

4153

= 239.6V

LfIs = 239.6V where Isrms = 701A for500kW PV Plant

Lf = 0.1088mH

Cf = 470F

Resonant Frequency Fs =

1

2LCFs = 545.57Hz (3.22)

For ease of simulation, filter capacitance is considered in distribution system side (33kV)

along with shunt capacitance of the system, so transformation of capacitance from one

voltage level to another voltage done by using energy balance.

1

2Cf1Vs1

2 = 1

2Cf2Vs2

2

HereVs1 = 415V;

V(s2) = 33kV

C1f = 470F

So C2f = 470

415

33000

2C2f = 0.0743F (3.23)

Inductor is designed using area product method and its resistance Rf= 1.02m


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3.5. Current Controller 33

3.5 Current Controller

Controls the current injected to the grid and convert the current reference Idref (as in

Eq.3.21), Iqref(equal to zero for UPF) into required inverter voltage by controlling the mod-

ulation index. Decoupled Id & Iq are used to simplify the MIMO[16] system into two SISO

system. From the Fig.3.7,

Figure 3.7: Inverter - Filter - Grid

via = Lfdisadt

+isaRf+ vsa (3.24)

vib = Lfdisbdt

+ isbRf+ vsb (3.25)

vic = Lfdiscdt

+ iscRf+vsc (3.26)

if vsa = Vsmsin(t)

for UPF isa = Ismsin(t)

So via = LfIsmcos(t) + Vsmsin(t) (3.27)

Current injected to the grid (at PCC)isacan be controlled by controlling the inverter voltage

via as given in the Eq.3.27. Inverter voltage can be controlled by switching the inverter in a

different modulating signal.

3.5.1 Current Equations in d-q DomainHere synchronous reference frame basedconstant current controlinterface is used to connect

the PV system to the grid. All current equations must be transferred to d-q domain. For

balanced system, current injected to the distribution system (at PCC) is given by,

isa = Ismsin(t) (3.28)

isb = Ismsin(t 120) (3.29)isc = Ismsin(t + 120) (3.30)


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Multiplying 32 Eq.3.24 as per Eq.3.5

vi= Lfdis

dt + isRf+ vs (3.31)

Multiplying32 (Eq.3.25- Eq.3.26) as per Eq.3.6

vi =Lfdisdt

+ isRf+vs (3.32)

So space vector Is= is+jis and Vs= vs+jvs from Eq.3.31& Eq.3.32

Vi=LfdIsdt +I

sRf+ Vs (3.33)

As we know

+j= (d+jq)e where = t 2

(3.34)

(Eq.3.31+j Eq.3.32)e

Vid = LfdIsddt

+ IsdRf LfIsq+ Vsd (3.35)

Viq = LfdIsqdt +IsqRf+ LfIsd+ Vsq (3.36)

Since space vector Vsis aligned with d-axis, so Vs= Vsd& Vsq = 0. From the above equations

Eq.3.35& Eq.3.36,d-axis and q-axis components are coupled to each other, so its an MIMO

control, to convert this into SISO, feed forward term(as given in Eq.3.37), this will eliminate

the decoupling term as given in the equation.

Vid = Vid+ LfIsq+ |Vs| (3.37)

Vid = Viq LfIsd (3.38)By substituting Eq.3.37into Eq.3.35and Eq.3.38into Eq.3.36

Lf

dIsddtdIsqdt

=

R 0

0 R

Id

Iq

+

Vsd

Vsq

(3.39)

From the Eq.3.39, it is possible to control Isd &Isq independently. Input to the controller is

current error (Iderror&Iqerror) and output of the controller is voltage (Vid&V

iq).


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Figure 3.8: Current controller block diagram

3.5.2 Current Controller Design

PI controller is used for control, parameter of the controller is selected based on[20]. Here

filter inductor acts as a plant (where Tf = LfRf

), and sampling time (Ts/2) is taken as a

sensor time constant, is shown in Fig.3.8. Zero of the PI controller is used to remove the

pole of the plant where Tc=Tf.

Tc= L

R= 0.1057s

After eliminating system pole, the closed loop transfer function[21](same for both d and q

current controller) is equal to

Vid(s)

Isd(s) =

KG

TTfRf

(1 +sTs)

s2 + sT

+ GKKsRfTTf

T = Td+Ts=Tswitching

n =

GK KsTTfRf

2n = 1

Tif = 0.707

So K = TfRf2GTKs

(3.40)

Here Tf = 0.1057s Rf = 1.02m G = 1 T = 0.0002, Ks = 0.0001 and by substituting in

Eq.3.40

Calculated value Controller gain K = 0.272, Fig3.9 shows the response of the controller.

The Fig.3.10shows the overall current controller with added decoupling terms.


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Figure 3.9: Current Controller Response

3.5.3 PV System Current Calculation

Grid voltage at PCC is sensed and as per Fig.3.10current controller sets the inverter output

voltage, so PV system current is given by,

isan(s) = (vian vsan)

sLf+ Rfn phase quantity

where vian = 1

3(vab vca) (3.41)

The above Eq.3.41 valid only if system is balanced, but unbalance is very common during

fault. Also transformer is Y ad there is no neutral terminal available to measure gridphase voltages at PCC. So current must be derived using only line voltages, by applying

KVL in the below circuit.


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Figure 3.10: Overall Current Controller Block Diagram

As shown in the Fig.3.11. Voltage across the filter (inductor with its resistance) can be

Figure 3.11: Calculation of PV System Current

obtained by using the Eq.3.42

VLfa

VLfb

VLfc

=

0.6667 0.3333 0.3333

0.3333 0.3333 0.33330.3333 0.6667 0.3333

x1

x2

0

(3.42)


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Where

x1 = viab vsabx2 = vibc vsbc

isan(s) =VLfa

sLf+ Rf

isbn(s) =VLfb

sLf+ Rf

iscn(s) =VLfc

sLf+ Rf

3.6 DC bus Capacitor Voltage Controller

PV is directly connected across DC bus capacitor, So Voltage across capacitor must be

maintain constant to harvest maximum power from the PV, also to reduce output ripple

and to ensure balanced inverter output. Voltage is controlled by controlling the output

current as given in the Eq.3.43.

Icap=Ipv SaIsa+ SbIsb+ScIsc (3.43)

From the above equation, by controlling the current injected to the gird, DC bus capacitor

voltage can be controlled. PI voltage controller is used to regulate the dc bus voltage. This

controller converts voltage error to current required by the capacitor as shown in the Fig.3.12

Voltage PI controller gives d-axis currentIdcap, which must be injected to the capacitor (to

Figure 3.12: Voltage Controller Block Diagram

regulate the voltage at the MPP level). This current is given as input to the current controller


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3.6. DC bus Capacitor Voltage Controller 39

and converted into Idc (actual current through capacitor) and given to the plant.

Idc = K2 Idcapwhere K2 =

2

3

Vrms(LL)

Vdc

3.6.1 Voltage Controller Design

From the Fig.3.12, system have two poles at origin, also open loop must maintain -20dB

gain zero crossing for achieving better phase margin. Symmetrical optimum method is

used to design the controller, in this method, PI controller zero is selected such that, gain

zero crossing(at (c)) of the open loop transfer function placed symmetrically between PIcontroller zero (1/Tv) and open loop system dominant pole (1/T). Current controller is

simplified as a first order system as shown in the Fig.3.12.

Open loop transfer function is given by

Vdc(s)

Vdcref(s) =

Ke(1 +sTv)

S2Tv(1 +sT)

whereT = 2T+Ts

Ke = KsK2KvKct

open loop phase margin is equal to

Phase margin = tan1a tan1

1

a

if a = 2

Phase margin = 36.87o

c = 1

aT

Tv = a

c(3.44)

Kv = cC

KctK2Ks(3.45)

Here Ks= 1 , c= 1000rad/s C= 10mF & Kct= 1

so Kv = 10

K2Tv = 0.002s

Voltage reference is generated by the MPPT, MPPT will always perturbate the capacitor

voltage reference (of the controller) to track the maximum power. in this current control


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interface, current loop act as a inner loop, voltage loop act as a outer loop and MPPT act

as a outer loop for voltage controller.

So Voltage controller must be slower than current controller and MPPT perturbation time

must be set slower than the voltage controller.

Figure 3.13: Voltage Controller Response

3.6.2 Dynamic Simulation of DC bus Capacitor

Since full simulation is done by mathematical equations using Simulink blocks, all the possible

voltage variation must be modelled. It is found that, capacitor voltage will vary due to

Change in PV maximum power point Ipv &Vpv

Output Current (isa, isb, Isc)

Change in Maximum power point, taken into account by the voltage controller as shown in

the Fig.3.12and later modelled either by injecting the capacitor current[22] or using power

balance in the voltage controller block diagram as shown in the fig. 3.14. Since average


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3.7. Simulation of PV System 41

inverter model is used for simulation, so power balance is sufficient to account for PV system

output current variations Here only active power balance is considered, since reactive power

Figure 3.14: Voltage Controller with Power Balance

wont affect capacitor voltage significantly. Effect of active power balance on the capacitor

voltage is given by Eq.3.46

Pcap = VpvIpv VsdIsd+VsqIsqVD =

VpvIpv VsdIsd+ VsqIsqVpvsC

(3.46)

by using the above equations, dynamics of the capacitor is successfully implemented, response

found satisfactory

3.7 Simulation of PV System

PV system is simulated by considering all the its dynamic elements, which already discussed

in the previous sections. PV system have nonlinear elements as shown below

CdVpv

dt =Ipv (Saisa+Sbisb+ ScIsc) (3.47)

from the above Eq.3.47, Ipv is non linear function ofVpv. PV system have set of non linear

dynamics equations, solving those equations using small signal analysis is not valid for large

disturbances like faults. So all PV system elements are fully modelled by using its charac-

teristic equations in MATLAB/SIMULINK as shown in the Fig.3.15.

Working of the PV System model is given below

PV panel supplies the current (Ipv) based on the DC bus capacitor voltage (Vdc= Vpv)


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MPPT sensesIpv &Vpv , and set the voltage reference for the voltage controller. MPPTalways perturbate the DC bus voltage towards maximum power point.

Based on the power generated (Vpv Ipv) by the PV panel, synchronous current ref-erence (Idpv) is generated and fed to the current controller to inject all possible PV

power to the grid.

Voltage controller try to regulate the DC bus voltage as dictated by the MPPT andgenerate the capacitor synchronous current reference (must be injected to the capaci-

tor) to regulate the voltage. It fails during power imbalance (e.g. fault)

Capacitor current reference is subtracted (Idpv Idcap) from the PV current referenceand given to the current controller as the reference current (Idref).

Current controller sets the inverter pole voltages based on the current reference andcontrol the power flow to the grid, for maintaining UPF Isq = 0.

Based on the grid voltage (at PCC) space vector magnitude ( Vsd&Vsq) with its angle(), current c

Behavior of Photo voltaic system during grid disturbances

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