Modelling and Performance Analysis of Synchronous Reference Frame Phase-Locked Loop for Three-phase Grid-connected PV Generation System

Due to the introduction of the feed-in tariff policy mechanism in many countries, the grid-connected Photovoltaic (PV) generation system becomes new attractions as it encourages the energy producers to generate and supplies electricity to the grid. The characteristic of the inverter’s voltage waveform, its amplitude, phase-angle and phase sequence are vital parameters for effective PV-grid synchronization. This paper presents a double stage converter, DC-DC boost conversion included to maximum power point tracking (MPPT) is achieved. Second stage DC-AC conversion is achieved using voltage source inverter (VSI). Grid connected synchronization is achieved by using synchronous reference (SRF) base phase locked loop (PLL). A details design and model for each section of the proposed system are also presented. The result shows that the implementation of synchronous reference frame (SRF) with PLL able to good achieved.


Introduction
The photovoltaic (PV) generation system becomes a focal interest nowadays due to the fact that it is one of promising renewable energy resources to be integrated with the utility grid. By a general definition, a grid-connected PV generation system is an electrical circuitry generating power from photovoltaic (PV) modules or arrays and delivered it to the utility grid. On the other hand, a stand-alone or off-grid PV system is designed to operate independently from the utility grid such as the situation in remote communities where the utility grid is unreachable. In numerous existing stand-alone PV systems, a battery bank is used for energy storage. There are advantages and setbacks to each type of PV connection on which largely depends on the design specifications and criteria of the connected loads. The gridconnected PV generation system poses some notable challenges to researchers. The increasing penetrations level of PV systems into the utility grid have risen several potential problems relating to power quality together with safety issues [1]. Due to these concerns, IEEE standard 929-2000 which intends for the utility interface of PV systems becomes a reference source for energy producers in integrating PV systems with the utility grid [2]. The modelling and performance analysis of synchronous reference frame (SRF) with phase-locked loop (PLL) for a three-phase grid-connected photovoltaic (PV) generation system is presented.

Characteristic PV Module
The electrical characteristics of the photovoltaic (PV) module are influenced by the effect of the intensity of irradiation, and surface temperature, of PV cell. Both parameters will alter the maximum power point or MPP of the PV module accordingly [3]. Generally, the equivalent circuit of a PV cell is represented by a single diode model. The output current of the PV module can be modelled mathematically as given by Eq. (1), where and are the number of PV cells in parallel and series respectively, is the light current, is a reverse saturation current of the diode, is the magnitude of the charge of the electron, is a Boltzmann constant, is ideality factor of the junction, is the output voltage of the PV cell, is temperature in Kelvin, and are a series and shunt resistance and is stand for a total generated output current of PV modules.

Maximum Power Point Tracking (MPPT) Technique
Perturbation and observation (PO) maximum power point tracking technique are implemented to relocate the exact value of maximum power point (MPP) [4]. As illustrated by flowchart diagram as in Fig. 2, the value of PV modules output power at is been compared with value of previous output power at − 1 . Then the differential between both values is been checked weather it is higher or lower than zero and vice versa. Furthermore, the next cycle of this algorithm will check the differential value of the measured output voltage. Based on this judgment, the PO algorithm will generate a proportional step size duty cycle to power switches accordingly. This sequential will be repeated until it finds a true maximum power point at certain level of irradiation, and surface temperature, of PV modules.

DC-DC Converter
DC-DC boost converter is required in most PV system applications. It perform to operate the PV panel at maximum power point and boost the DC voltage level to the appropriate level so that it is easily converted into the desire AC voltage. A boost converter provides the control of the PV module output voltage so that the PV module operate at maximum output operating point and maintain the voltage input to the inverter. The Fig.3 shows the circuit diagram of the boost converter. The DC-DC boost's output voltage can be calculated as follows: where , and ! are the input voltage, the output voltage and the value of duty cycle respectively.  the desired pulse train to the power switches. The 180° degree conduction mode is a preferred method as it has better utilization of the switches. The RMS line voltage, % of the three-phase VSI in corresponds to the output voltage, of the DC-DC boost converter is given: where & ' is the modulation index. The instantaneous line-to-line voltage +, , ,and -+ be expressed in Fourier series as: where is the output voltage value of the DC-DC boost converter, => is the phase angle in radian and is the number of harmonics.

Control Strategy of Synchronous Reference Frame -Phase Locked Loop
Designing a control mechanism for the three-phase grid-connected PV generation system employs two control loops which are an external voltage and an internal current control loop respectively. The voltage control loop is implemented to regulate the output power from PV modules to the grid as well as to balance the power flow whereas the current control loop is used to regulate the injected current to the grid and keep it in phase with grid voltage to achieve unity power factor. Phase-locked loop, (PLL) control technique is a vital component in grid synchronization and it is implemented in D synchronous reference frame (SRF). PLL able to provide fast and accurate grid information with a high degree of immunity against disturbances, harmonics, unbalances, sags/swells, notches and other types of distortions in the input signal. The basic function of the PLL [5], [6] is to measure Phase angle θ from the angular frequency ω of the grid Voltage. In the SRF-PLL [7] the voltages of the three phases of the Grid E, F, G which are separated in phase by an angle of 120° from each other are converted into D reference frame in a two stage transformation process i.e EFG frame to HI reference frame to D reference frame [8]. The transformation block diagram is shown in fig 4(a).
Where S is transfer function of the PI controller.
Hence by designing the proper controller can be successfully track the grid frequency = K and phase N K . The PPL control structure as given in Fig. 4(b), where = T is the fundamental frequency of the grid and O * is the reference set point voltage of O .

Grid -Connected PV System and Simulation Results
Based on the block diagram of the grid-connected PV system Fig. 1, the simulation of the designed models is carried out in Matlab/Simulink. The summary of the detailed specifications and parameters of the proposed system are given in Table 1.
The PV system can be connected to the three-phase grid system via a DC-DC Boost converter and a voltage control VSI. In VSI the size of filter or decoupling inductor or transformer is a matter of concern as it is an essential part of system to makes the power flow possible with low THD. The simulation process is started with the generation of − and V − characteristics of the PV modules as illustrated in Fig. 5. The generated shape should able to gives a proper information regarding the capacity of the proposed system.   The PWM inverter pulse being feed is by using simple SPWM. The output of the PWM controller and there by changing the inverter switching sequence and thereby controlling the output of inverter. The PWM pulses being generated is given in Fig.7.  The simulation results for phase to phase voltage of three-phase inverter, output voltage before filtered as shows in fig. 9 below. And the 415 V, 50 Hz phase to phase output voltage +, , ,and -+ of the three-phase transformer (the voltage after filtered) are given in Fig.10. These voltage are synchronous voltage for connected to grid.

Conclusion
The modelling and performance analysis of synchronous reference frame (SRF) phase-locked loop (PLL) for three-phase grid-connected PV generation system has been presented. In Grid connected PV generation system one the main important issue is grid synchronization which is well addressed here in this paper by the use of SRF PLL. The switching of the three-phase inverter is performed here by using SPWM.