Design & Synchronization of three phase grid connected PV

simulation model of solar PV grid-connected system using voltage source inverter with sinusoidal pulse width modulation has been developed.

“DESIGN AND SYNCHRONIZATION OF THREE PHASE GRID CONNECTED SOLAR PHOTOVOLTAIC SYSTEM”

ABSTRACT

Three phase 10.44 kW grid-connected solar energy system as a feasible power generation is designed and simulated using MATLAB SIMULINK software and analysis of PV is performed. To obtain the fast and accurate response of photovoltaic (PV) system maximum power point tracking techniques like Perturb and Observe algorithm are used. The DC-DC converter is designed which will boost the low DC-voltage of the photovoltaic (PV) system to the high DC-voltage required for grid synchronization. Design of 10.44 kW photovoltaic systems consists of 24 PV panels (SPR-435NE-WHT-D) of 435 W each is used to generate power for a maximum three phase 5 kW load. Inverter with bidirectional power flow is connected to a photovoltaic array which consists of six parallel strings and each string consists of four series-connected solar panels. The designed photovoltaic system is a type of hybrid system so to charge the battery bank either in bulk or float mode for eight series of 12V and 200Ah battery two stages most suitable for lead acid battery charging method that is constant voltage and trickle charging is adopted and phase lock loop (PLL) feedback control system is used to generate a signal and this signal has been used as a reference signal for the synchronization of the inverter voltage with that of grid. Finally, a simulation model of solar PV grid connected system using voltage source inverter with sinusoidal pulse width modulation strategy has been developed in MATLAB.

Objective

1. To design a three-phase grid-connected photovoltaic system with phase locked loop control strategie.
2. To Design of battery charge controller alone with bidirectional DC-DC converter.
3. To design inverter control loop which will produce a comtrolled PWM signal which will control the switching on and off of igbt switches in inverter.

Inverter control : Inputs to the control loop are the three-phase grid voltage and currents. The Gate signals at the inverter controls the switches on and off period. These signals are generated by inverter control loop. The output from the inverter control loop is the controlled PWM signals. These signals control the switching on and off of IGBT switches in inverter. Inverter generates three phase sinusoidal voltage and currents.

Phase locked loop (PLL) and dq0 transformer This section in the inverter control converts the voltage and currents to per unit values. PLL takes the grid voltage and finds its angle and frequency. This plays an important role in making inverter output and grid angles equal. dq0 transformer converts three phase voltages and currents from abc to dq0 reference frame. Conversion into dq0 frame provides accurate control for the signals

The inverter control loop consist of following units which is to be designed that takes grid voltage adn grid current.

• To Design of DC voltage regulator
• Design of PLL and dq0 transformation control unit
• Design of current regulator
• Design of reference voltage generation unit
• Analysis of grid connected pv- system

DC voltage control loop : This loop regulates the DC voltage at the input of the inverter. PI controller gains were selected using the formulas 𝐾𝑝 = 3𝐶 20𝑇𝑠 𝐾𝐼 = 𝐾𝑝 20𝑇𝑠 Where, K_p and K_I are the proportional and integral gains of the PI controller, C is the boost converter capacitor value and T_s is the switch period of the inverter switches.

Current control loop: This loop regulates current in dq0 reference frame.

Reference voltage and PWM generator This section converts the voltage back into abc reference frame from dq0 frame. Pulse width modulator (PWM generator generates PWM signals by the comparison of the three phase reference signals and the triangular wave with the particular frequency.

Synchronization Control:

Phase Locked Loop (PLL) is a control system that produces an output signal in phase/reference with that of the input signal. PLL when used in Grid interfacing matches the Grid parameters like voltage, current, frequency with that of the Inverter. Only when the Voltage, Current and that of the frequency and phase of the sources are maintained the same, will there be synchronism maintained. The phase locked loop maintains the voltage constant throughout the system. The reference value of voltage is compared with inverter voltage and the error is computed. Similarly, the current value is determined. The quantities of stationary reference frame is converted to rotating reference frame using Park’s transformation and vice-versa using Inverse Park’s Transformation. The PLL helps to maintain the inverter and the grid in synchronism and during any faulty condition it brings it back to synchronism.

Grid synchronization

There are five conditions that must be met for the synchronization process to take place. They are the source (generator or Sub-network) must have

1. The same Line voltage,
2. Frequency,
3. Phase sequence,
4. Phase angle and
5. Waveform to which the system is being synchronized

Concept of Conventional P&O Algorithm

P&O algorithm is the simplest, cheapest, most popular used in practice. The basic P&O scans the PV curve of PV module in search for the MPP by changing the operating point which is known as perturbation step, and then measuring the change in P (ΔP), known as observation step. If ∆P/∆V is greater than zero, the perturbation of voltage should be increased from point "A" towards MPP as shown at the left side of Fig. 3. If ∆P/∆V is lower than zero, the perturbation of voltage should be decreased from point "B" towards MPP as shown at the right side of Fig. 3. The P&O keeps searching for the MPP until it has found an operating point such that ∆P/∆V is closely to zero in any direction; this condition is called steady-state. At steady state, the operating point oscillates around the MPP giving rise to the wastage of some amount of available energy. These oscillations can be minimized by reducing the fixed step size, but it takes relatively more time to reach MPP. The P&O keeps perturbing the system in order to detect a change in the MPP (caused by a change in the environmental conditions), which triggers a new scan. The flowchart of the basic P&O MPPT algorithm is presented in Fig. 4.

Design parameters calculations:

The two main factors for the boost converter design are the inductor and capacitor selection.

Equation1: L=(D(1-D) V_DC)/(f_SW ∆I_L )

•D is the duty cycle: D=1-Vin/Vout

•Vdc is the DC-link voltage

•fsw is the switching frequency = 20 kHz

•∆I_L is the inductor current (30%)

•The D value turns out to be 0.4168 for the input voltage of 300 V

•Output voltage of 500V

The two main factors for the boost converter design are the inductor and capacitor selection.

Equation2: C_dc=P_in/(ωV_dc ∆V_dc )

•P_in is the power injected into the grid

•∆V_dcis the voltage ripple = 0.5

•ω is the switching frequency in rad/sec

•D is the duty cycle = 0.4168

Maximum power = 10.4 kW is considered

Methodology

Work remaining

•Design of battery charge controller

•Design of DC voltage regulator

•Design of PLL and dq0 transformation control unit

•Design of current regulator

•Design of reference voltage generation unit

•Synchronization of grid connected pv- system