MPPT VOLTAGE REGULATING IN THREE-PHASE GRID-CONNECTED PHOTOVOLTAIC SYSTEM

st , 2013) ABSTRACT : The paper presents the method of harmonic attenuation with better dynamic response in the three-phase grid-connected photovoltaic system by using Maximum Power Point Tracking (MPPT) voltage regulator with varied increment. Since electrical power of PV array depends on the climatic condition. Maximum Power Point Trackers are used to extract maximum energy from photovoltaic generators. In this paper, every time the amount of electrical power generated by solar arrays changes continuously with weather conditions, MPPT voltage is regulated quickly with large increment to decrease dynamic response time, and then with small increment to get lower current THD. The simulation results in Simulink/Matlab show that the proposed MPPT voltage regulator with varied increment is better than with fixed.


INTRODUCTION
The demand of renewable energy sources such as solar energy is becoming more popular for sustainability and environment with enormous potentials [1]. In order to convert solar DC source to three-phase AC power needs to have 3-phase inverters that have been well researched in recent years [2]. Objectives of three-phase grid-connected inverters are low THD to meet grid-connected standard IEEE Std 929-2000 [3] and good dynamic response to get high efficiency [4]. Many MPPT techniques have been proposed in the literature [5], [6], [7], [8] to determine MPPT voltage. In these methods, dynamic power responses are often slow (more than 0.1s) and THD values at the output of inverter haven't been evaluated.
The Perturb and Observe (P&O) method with constant increment in Fig. 1 is an example. If small increment is in large change irradiance condition, dynamic response time is long but output THD is lower. On the contrary, with large increment, dynamic response time is short but THD is higher. In this paper, the proposed diagram of grid-connected photovoltaic system is shown as Fig. 7 Vs(t) > Vs(t-1) Vs(t) > Vs(t-1) yes yes

SYSTEM DESCRIPTION
The proposed control scheme of three-phase grid-connected PV system that has been simulated includes 2 parallel rows, each row consists of 34 PV modules (RS-P618-85W) in series connected through a DC bus to a threephase inverter that is connected to an ideal 380V grid through a LCL filter. In this system, the DC link voltage is controlled accordingly to the necessary output power. The dq control structure is normally associated with PI controllers since they have a satisfactory response when regulating DC variables. Since the controlled current has to be in phase with the grid voltage, the phase angle used by the abc to dq transformation block is extracted from the grid voltage throughout the PLL.

PV array model
The PV array is simulated using a model of moderated complexity based on [9]. In this model, a PV cell is represented by a current source in parallel with a diode, and a series resistance as shown in Fig. 2

The proposed MPPT scheme
One of the most important objectives of any grid-connected PV converter is to extract as much power as possible from the PV array.
There are various methods to perform MPPT [10], (P&O) [11] and fuzzy logic [12] methods.  (2) and (3) Where V gd and V gq are grid voltages respectively in dq frame. i d and i q , V id and V iq are inverter currents and voltages respectively in dq frame. L g is inductance between inverter and grid voltage.
The output voltage of inverter is determined by the output of current regulator. In equation (2) and (3), the inverter voltage is the sum of grid voltage components and voltage drop components at the inductor L g . In order to compensate the voltage deviation caused by approximate parameters and differential voltage components, PI regulator is usually used in most current regulators. Equation (2) and (3) can be represented by the equation of inverter voltage like equation (4) and (5).

Simulation parameters
The system scheme shown as Fig .7 has been simulated on Matlab/Simulink and Table 1 is the summary of the system parameter values.

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The simulation results in Fig. 8 to Fig. 11 show that dynamic responses of PV array power are very fast but current THD is higher than 5%.  The simulation results in Fig. 12 to Fig. 15 show that dynamic responses of PV array power are very slowly but current THD is lower than 5%. The system responses to an irradiance step is shown in Fig. 16 to Fig. 20

CONCLUSIONS
A three-phase grid-connected photovoltaic inverter system has been presented. In this