Author: Ing . Radutiu Dan-Comes Project advisor: Sef.Lucr.Dr . Ing . Ioan Serban

1. TRANSILVANIA UNIVERSITY OF BRAŞOV  FACULTY OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCEStudy Program: Advanced Electrical Systems Dissertation ThesisControl of a small photovoltaic system for integration in an autonomous microgrid Author:Ing.Radutiu Dan-Comes Project advisor:Sef.Lucr.Dr.Ing.IoanSerban

2. Table of contents • Motivation • Introduction in Photovoltaic Systems • Project objectives • PV system description • PV system modeling • Results and discussions • Conclusions

3. 1.Motivation • Due to the energy demand growth, the high price of oil and the concern for the environment, renewable energy is in the headlines. • One type of renewable source is the photovoltaic cell, which converts sunlight to electrical current. • Solar PVs are among the fastest growing energy sources in the world. • A diagram of global cumulative installed capacity evolution is presented.

4. 2.Introduction in PVSystems • The basic elements of a PV system are the modules that are usually series-connected. A series of PV modules is called a string. • The inverter is the key element of the grid-connected PV power systems. • The main function is to convert the DC power generated by PV panels into grid-synchronized AC power. • A block diagram of a typically Photovoltaic Power System is presented:

5. 3.Project Objectives • The goal of this project is to analyze and simulate a small photovoltaic system of 4.5kW, connected to a single-phase microgrid. • The converter topology used is the multi-string . • The main points that are presented, are the MPPT control and the control of the inverter in order to connect and synchronize with the microgrid. • Moreover, a specific control function is added for opertion in weak grids, which contributes to the faster stabilization of the microgrid frequency during the periods with energy excess. The multi-string topology of the 4.5kW PV System

6. 4.PV system description • In order to create theoretical models to simulate the behavior of a specific solar panel, in the literature have been made different mathematical models. • These mathematical models will only be approximations to the behavior of the panels and the accuracy of the models depend on how many internal phenomena are considered. • The characteristic equation for the PV cell and the simplified equivalent model are given by: The equivalent model of photovoltaic cell The ideal model of a PV celll

7. Schematic of the boost DC-DC converter as a part of the microgrid-connected PV system The steady-state voltage and current relations of the boost converter are : η = Efficiency of the boost converter D = Duty cycle IPV = PV array output current VPV = PV array output voltage ID= Dc bus current from inverter side VD = Dc bus voltage from inverter side

8. MPPT algorithm • In P&Q algorithm, a slight perturbation, ΔD, is introduced in the system. • This perturbation causes the power of the solar module to change. • If the power increases due to the perturbation, then the perturbation is continued (D+ΔD) in that direction. • After the peak power is achieved the power at the next instant decreases and after that the perturbation reverses (D-ΔD). • The flow chart of MPPT algorithm is presented.

9. Inverter description and control • Almost all the commercial single-phase inverters for DG systems inject only active power to the grid, i.e. the reference current is computed from the reference active power P*. • For the grid-connected PV inverters in the range of 1-5 kW, the most common control structure for the DC-AC grid converter is current-controlled H-bridge PWM inverter . A schematic of the inverter as a part of the grid-connected PV system • The current control strategy using Proportional –Sinusoidal Signal Integrators is adopted. A general control block is presented: Current control scheme

10. P-SSI single-phase inverter control block diagram • The measured voltage, Vg from the AC side of the inverter is passed by the quadrature signal generator which provides two output components, Vgα and Vgβ in α-β reference frame. The two output signals from the quadrature signal generator, Vgα and Vgβ are implemented in the current reference block for achieving the reference current I*grid and filtered by a SSI filter. • A PI controller is used to regulate the DC voltage to a preset value of 400V control inputs.This PI regulator provides the reference active power, P*. • The reference reactive power Q* is set to zero, because no voltage support is required. • The result will provide the Pulse Width Modulation signal that is applied on the four IGBTs from the inverter.

11. The PV System Configuration • The Multi String Inverter Concept has been developed to combine the advantage of higher energy yield of a string inverter. Each string has its own MPP tracker, which independently optimizes the energy output from each PV string . The two DC-DC converters are connected via a DC bus through a central inverter to the grid.

12. 5.PV system modeling The overall photovoltaic system in Simulink

13. The PV module based on the equivalent model of a PV cell in Simulink.

14. The inverter control method, implemented in Simulink

15. In order to study the accuracy of the control method adopted it will be considered only the inverter side connected to the microgrid and the PV generator is replaced with an equivalent DC voltage source of 400V. • The inverter side interconnected with the microgrid

16. 6.Results and discussions • For the overall PV System simulation , this figures illustrates the voltage , the power and the duty cycle from the two PV arrays, for the irradiance level of 1000W/ m2

17. To see the efficiency of the MPPT method adopted, a set of measurements for different irradiance levels from 200 W/m2 up to 1000 W/m2 with a step of 50W/m2 are performed. • The results are presented below: • It can be observed that the Maximum Power Point Tracker efficiency is increasing with the increase of the power, reaching at the maximum measured power, an efficiency of 99.199 %. • The average efficiency of the Perturb and Observe algorithm used is 96.284 %.

18. Related the second simulation, measurements for different values of the reference active power were performed. • The values of this power are given with a step of 500 W in a range of 500 W to 4500 W (the reactive power is considered zero). • Two simulation cases are considered, one without voltage harmonics introduced in the microgrid block and the other one, with voltage harmonics. • Harmonics of 3rd (5%), 5th (4%), 7th (3%), 9th (2%) and 11th (1%) order were included in the Simulinkmicrogrid block, thus testing the system operation under adverse operating conditions. Reference power vs. THDI without voltage harmonics included into microgrid Reference power vs. THDI with voltage harmonics included into microgrid THDv=7.5%

19. The current harmonic analysis with harmonics introduced in the system for 4500W peak power. The voltage harmonic analysis with harmonics introduced in the system for 4500W peak power.

20. In the figure below are illustrated the active and the reactive power waveforms at peak power of 4500W. • The control is enabled at 0.2s. • It can be observed that the reactive power starts varying a short period of time and it can be noticed that the active and reactive powers start to be stable after 0.3 s and steady-state is achieved

21. The output voltage and current waveforms from the inverter at the peak power of 4500W are presented

22. Frequency stability is a very important element in the safe operation of an autonomous microgrid. • In case of frequency deviations, the requirements specify that the PV Power System has to adopt strategies to support the frequency, otherwise the disconnection is necessary and may create instability. • The Grid Code states that during frequency increase the generating PV plant has to reduce its active power because frequency control is directly related to the active power balance in the system. • So in order to decrease the active power of a PV System, the MPPT is derated with a certain degree depending of the frequency deviation. • A third simulation model is presented in order to study the MPPT derating and a new microgrid model of 20 kW is implemented. The model of the microgrid is performing similar like a small conventional power system that includes generators, battery energy storage systems and loads.

23. MPPT derating MPPT derating implemented in Simulink

24. To analyze the functionality of the MPPT derating loop and to compare the results, it will be considered two cases, the first one when the MPPT derating is disabled and the second one when the MPPT deratings is enabled. • The diagrams of the two cases will be compared on the same graphic. MPPT derating coefficient for the two cases Microgrid frequency for the two cases Active and reactive powers for the two cases

25. Conclusions • This work presents modeling and control of a small photovoltaic system for integration in a microgrid. Matlab/Simulink was chosen as a software platform to analyze the system performance. • The Perturb and Observe method is used to maximize the output power and the flow chart has been presented. • Today topologies with classic single-stage inverter appear replaced by multi-string configurations with dual-stage inverter. • This paper deals with a single-phase inverter for microgrid-connection operation. • The adopted control method employs a current control scheme accomplished in the stationary reference frame based on sinusoidal signal integrators. • This study shows that the proposed control scheme offers a simple way to study the performance for utility interface applications. It is simple to implement and capable of producing satisfactory output power quality. • Having a steady frequency means having a balanced operation between generation and consummation. • During frequency increase, the PV Power Plant has to reduce its active power without the disconnection from the low voltage network. • So a MPPT derating function has been introduced and analyzed.

26. Thank you!