Literature Review Of PV Panel Emulator

INTRODUCTION

Due to power prevailing power crisis people tend to use renewable energy sources. Among those

sources photovoltaic cells play a major role. Despite of their high cost people are motivated to

use them because of their high reliability and high efficiency. Therefore it is important to predict

PV panel’s behavior under various conditions such as irradiance temperature and load

conditions. There is various methods describe in literature to emulate PV panel. In this project

we are going to design and analyze the performance a PV panel emulator considering various

load conditions such as nonlinear linear loads. Although PV panel emulators are available in the

market they are very expensive. So building a low cost PV emulator is another objective of this

project.

A photovoltaic emulator is a power supply that can show similar current and voltage

characteristics as a PV panel. This method we are going to design a power supply with given PV

panel parameters. The power supply will be controlled by stm32 development board. The

emulator calculates a current-voltage (I-V) curve based according to the user given PV panel

parameters (irradiance and temperature). The power supply track the calculated I-V curve when a

load change occurs to mimic a PV panel. The PV emulator provide a controlled environment that

is not affected by external factors such as weather.

PV EFFECT

Sunlight is converted in to electrical energy from the photovoltaic cell. Photovoltaic panel (array)

is a series combination of such PV cells to generate increased voltage. Solar cell is a special

diode that is composed of a p-type and an n-type semiconductor sandwiched together. The

doping method determines the efficiency of converting light in to electrical energy of the PV

cell. And it also determines the price of the PV panel.

Figure 1 PV cell cross section

When the sunlight hit the PV panel. Photons will get sufficient energy, they cause electrons to

move (from N to P only) causing excess electrons in the N-layer and a shortage in the P layer.

That will create a voltage difference between the layers.

MODEL OF PV PANEL

There are several model to model a PV panel. The models are vary with the complexity. They

are as follow

I. Ideal Model – comprises only with a current source and a diode

II. Simplified Model – comprises with a current source, diode and a series resistance

III. Practical Model – comprises with a current source, diode, series resistance (Rs)

and a shunt resistance (Rsh)

Figure 2 Models of PV cell

PV panel parameters are stated for Standard Test Conditions (STC). The STC is defined as

follow

“The test conditions to measure photovoltaic cells or modules nominal output power. Irradiance

level is 1000W/ , with the reference air mass 1.5 and solar spectral irradiance distribution and

cell or module junction temperature of 25 C”

PV PANEL CHARCTERISTICS

Figure 3 PV cell model

For the above model

Where

Rs - series resistance (Ω)

Rsh - shunt resistance (Ω)

Vt - thermal voltage (V)

k -Boltzmann constant (J/K)

q - electron charge (C)

T:- cell temperature (°C)

TSTC- cell temperature at standard test conditions(°C)

n -number of series cells in the module

A- ideality factor of the diode

Iph - photo generated current (A)

PV PANEL CHARACTERISTIC CURVES

PV panel I-V curve is plotted as follow

Figure 4 I-V curve of the PV cell

Here Isc is the short circuit current of the panel where the voltage is zero. Voc is the open circuit

voltage of the panel where the panel output current is zero. Open circuit voltage is the maximum

panel output voltage. Short circuit current is the maximum output current of the panel.

Pmax is the maximum power point of the panel. Imp and Vmp are current and voltage at the

maximum power point respectively.

Figure 5 : I-V curve under different irradiance levels

Figure 6 :PV panel I-V curve at different temperature levels

PV POWER CURVES

PV panel power can be obtained by multiplying the output current and the output voltage. Power curve can be obtained by plotting the calculated power versus the voltage. Following figure shows both the I-V curve and the typical power curve. In that figure the maximum power point is important. PV panel generally want to generate power at its maximum power point because at that point the panel harvest the maximum power from the sunlight. So it is important to track the maximum power point in the operation of PV panel. Maximum Power Point (MPP) tracking algorithms are there to track the maximum power point

Figure 7: power curves of PV panel

Output power is depend on the irradiance and the temperature. Following figures show the effect of temperature and irradiance on power. As the irradiance reduced the power curve will scale proportionally and the maximum power point will also change.

Figure 8 : PV power under different irradiance levels

Vo

Io

Figure 9 : PV panel power under different temperatures

MODEL OF PV EMULATOR

PV emulator consists of a DC-DC power converter and a controller to control the switching mode of the power supply according to the variations of PV panel parameters and I-V curve of the PV panel.

The block diagram of the emulator shown in the following figure

RL

G

T

Ipv

Figure 10: PV emulator block diagram

Buck Convertor

PV Model

Voltage controller

SELECTION OF POWER SUPPLY

A power supply will be selected so that it will be able to emulate most solar panels and it should be inexpensive that have a capability of control the output from an external feedback controller. The power supply’s power ratings should be matched or higher than the PV panel parameters. The power supply’s maximum output voltage and maximum current should be higher than the PV panel’s open circuit voltage and short circuit current respectively.

According to above specifications following switch mode power supplies were suggested

1. Buck converter

2. Boost converter

3. Buck-Boost converter

Since PV panel produce small voltages it is better to use a step down DC-DC converter.

Therefore Step up Boost converter is not suitable for this purpose and Buck Boost converter is also not suitable. Therefore we selected Buck converter as the power converter.

Figure 11: Schematic of a buck converter

INDUCTOR SELECTION

Calculating inductor value of the inductor is most crucial in designing the Buck converter. It is assumed that the converter is in Common Conduction Mode. Therefore the inductor does not discharge completely during the switch off time. The following equations assume an ideal switch and ideal diode.

Where

fsw – buck converter switching frequency

LIR- inductor current ratio as a percentage of IOUT

An LIR of 0.3 represents a good tradeoff between efficiency and load transient response.

More inductor current will be there with an increased LIR constant.

Inductor peak operating current can be calculated from the following equation

OUTPUT CAPACITOR SELECTION

Output capacitor is need to minimize the voltage overshoot and ripple present at the output. Large overshoots are caused by insufficient capacitance.

From the following equation output capacitance can be found

Where ∆V is the maximum output-voltage overshoot

INPUT CAPACITOR SELECTION

Input capacitor must be able to handle the input current ripple. The amount of ripple current capacitor must be handled can be taken as this

The capacitor current rating should withstand this current. The capacitor value depends on the input power source’s impedance

DIODE SELECTION

For a diode power dissipation is the limiting factor. Worst case power dissipation can be calculated as follow

Where VD is the voltage drop across the diode at the given current Iout MAX .

REFERENCES

1) Microsource Interface For A Microgrid – Dr P.J Binduhewa

2) Photovoltaic Emulator Adapatable To Irradiance, Temperature And Panel-Specific I-V Curves -Joseph Durago

3) First Course On Power Electronics And Drives –Ned Mohan

4) Power Electronics Technology June 2006