Abstract Key words: Introduction · Also, Pulse width modulation technique is used in grid connected inverters control technique . The important function is a specific value stabilization

Investigation on Voltage stability of the DC bus in a Single phase

Grid connected PV system C. Benin Pratap

1, Geevin S R

1, G. Shine Let

2

1EEE-Department of Electrical Sciences,

2ECE-Department of Electrical Sciences,

Karunya Institute of Technology and Sciences

Coimbatore, India

[email protected]

Abstract

This paper presents an investigation on the voltage stability of the DC bus

when subjected to continuous change in irradiance and temperature. It is

important to have a stable voltage at the input of the Voltage source inverter for

grid integration. A single phase Grid connected PV system is used for this

analysis. Perturb & Observe technique is used for Maximum power point tracking.

The output voltage and power at the grid is also taken for analysis .The bus

voltage is maintained by an MPPT algorithm which controls the switching pulses

of a boot converter which is connected to the dc bus. The voltage output of the

DC bus is maintained by minimum response time and complexity.

Keywords: MPPT, Voltage stability, Grid, Boost Converter

Introduction

The most popular and abundant available renewable energy is solar

energy. This is used as an alternative energy source in most of the commercial and

industrial power generation. Solar energy is an economical, alternate energy

resource. Using solar energy, electricity is generated in a clean, reliable and quiet

manner. Advancement in electronics and various control techniques made it use

for many applications. Solar energy is converted into electric energy by using

photovoltaic (PV) cell. It acts as an direct DC generator [1]. PV cells can be

connected in series and/or parallel form to obtain the required output voltage and

output current. Many PV cells together form a solar array [2].

The sunshine about 3000hour/year produces solar radiation of 5.4 kWh/m2

per day in an annual average shows the immense potential of solar energy [3].

Solar energy supply systems called as PV systems, supply power in stand-alone

mode or grid-connected mode. In stand-alone mode, the generated supply power

from solar energy is directly given to any electrical gazette [4]. The photovoltaic

technology has an installed capacity of 2000MW in 2006 and increased to 300

GW by the end of 2016[5]. A large number of PV modules are connected to form

large standalone and grid connected power generation plant [6]. As years go by

International Journal of Pure and Applied MathematicsVolume 119 No. 16 2018, 1659-1669ISSN: 1314-3395 (on-line version)url: http://www.acadpubl.eu/hub/Special Issue http://www.acadpubl.eu/hub/

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the installation cost of PV systems are dropped to 50%[7]. To obtain maximum

power from solar energy maximum power point tracking (MPPT) is necessary [8].

Based on the revolution of earth around the sun, in PV array a nonlinear i–v

characteristics is obtained. To track the maximum power MPPT is used. Based on

the factors, illumination level and temperature [9]. The output power of PV array

varies. It is challenging to distinguish the global maximum power point (MPP)

because of completely change in environment and shadowing conditions [10].

Line commutated inverter is the first grid connected inverter [11]. International

standard (IEC61727) and the present standards EN61000-3-2, IEEE1547 and the

U.S. National Electrical Code (NEC)690 are considered while using inverter for

grid connections [12].

In Grid connected PV systems, Gird-connected voltage source inverters

(VSIs) are generally applied. Also, Pulse width modulation technique is used in

grid connected inverters control technique. The important function is a specific

value stabilization of dc-bus voltage of the inverter because the PV modules

output voltage varies with temperature, irradiance, and the effect of maximum

power-point tracking (MPPT). Also, the energy should be fed from the PV

modules into the utility grid by inverting the dc current into a sinusoidal

waveform synchronized with utility grid [13]. Fig.1 shows the complete blocks

from PV system to grid connection [2].

Fig.1 Block diagram

PV Modelling

A PV cell model is shown in fig.2. The operational characteristics is

similar to pn diode. The PV cell working depends on surface temperature and

solar radiations. The solar cell equivalent circuit can be represented using single

diode or double diode. But single diode equivalent gives required accuracy and

can be used for the development of explicit models [4].

International Journal of Pure and Applied Mathematics Special Issue

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Fig.2 Single-diode equivalent solar cell

In fig.2 Iph is photocurrent, Rsh is shunt resistance in PV cell, Rs is series resistance

in PV cell, V and I are the output voltage and output current of a PV cell

respectively. The photocurrent is temperature dependent and is given in equation

(1)

𝐼𝑝ℎ = (𝐼𝑝ℎ)𝑇1+ 𝐾0(𝑇 − 𝑇1) (1)

where, T1 represents normal standard temperature.

(𝐼𝑝ℎ)𝑇1= (𝐼𝑠𝑐)𝑇1

𝐺

𝐺𝑆𝑇𝐶 (2)

𝐾0 =(𝐼𝑠𝑐)𝑇2−(𝐼𝑠𝑐)𝑇1

𝑇2−𝑇1 (3)

The PV current is directly proportional to irradiance. The saturation current in PV

cell is given by equation (4),

𝐼0 = (𝐼0)𝑇1∗ (

𝑇

𝑇1)

3𝑛⁄

∗ 𝑒𝑞(𝐸𝑔)𝑇1

𝑛𝑘(

1

𝑇−

1

𝑇1) (4)

(𝐼0)𝑇1=

(𝐼𝑠𝑐)𝑇1

𝑒𝑞(𝑉𝑜𝑐)𝑇1

𝑛𝑘𝑇1 −1

(5)

Boost Converter

Boost converter is used for DC to DC converter. For many applications,

the available DC voltage from a battery is not sufficient to drive much powered

devices. To boost the available DC voltage to the required voltage, boost

converter circuit is used. Fig. 3 shows the boost converter basic circuit. Switching

transistors are the main components. Switching transistors can be power

MOSFET or bipolar power transistors. The selection of switching transistor is

based on switching speed, cost, required voltage and current level. Based on the

magnetic field around the inductor, the transistor switching operation happens.


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Fig.3 Boost Converter Circuit

Maximum Power Point Tracker (MPPT)

Fig.4 MPPT Flow Diagram

In grid-connected PV systems, MPPT is now popularly used. For a set of

environmental conditions, MPPT maximize the power output and intern the array

efficiency is maximized. For maximum power tracking, many researchers have

come with different algorithms. Among them perturb and observe (P&O) method

tops in the literature. Fig.4 shows the MPPT approach based on P& O algorithm

[15]. PV cell output current and voltage is given as input for the algorithm. This

algorithm follows many iterations. After every iteration, the system compares the


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current and previous power value with some constant. Based on the comparison,

the algorithm adjusts the duty cycle either negative or positive to the current duty

cycle.

Voltage Source Inverter

The inverter in power electronics represents power conversion from dc to

ac. The input to the inverter can be dc current or voltage source. If the input is a

dc voltage source, then it is voltage source inverter (VSI). Fig. 5 shows the three-

phase voltage source inverter [16]. The DC bus supplies current to the inverter.

The inverter switches on and off based on the step change in the input dc current.

If the power flow is from dc bus to ac load, the average magnitude of dc current

remains positive,

Fig.5 Voltage Source Inverter

Grid Integration

Fig. 6 Simulink Model of 600V Grid

Fig. 6 shows the matlab simulink model of a 600V grid which is supplied with a

voltage source inverter. Fig.7 shows the integration of voltage source inverter to

the utility grid.


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Fig. 7 Integration of VSI to utility grid

Result and Discussions

The simulation model considered is the 100kW PV array connected with a

25KV grid through a DC-DC boost converter with MPPT tracking and a three-

phase voltage source converter using Matlab Simulink. Fig. 8a shows the input

irradiation and temperature at which the test results are taken. Analysis is done by

varying the temperature and irradiation (from 1000 to 500), and the voltage

stability of the DC bus plotted.

Fig.8a Input irradiation and Temperature

Fig.8b Voltage stability @changing irradiance


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Fig.9 VSC Control Parameters

Fig.10 PV array Characteristics

Fig 8b shows the voltage stability of the dc bus voltage at reduced irradiation

level. This affects the power delivered to the Grid however the grid integration

parameters are maintained constant. Fig. 9 VSC control parameters Id ref, Id, Iq

variation with time. Fig.10 shows the characteristics of PV array such as

irradiation, temperature, voltage. Fig 11 gives the duty cycle.Fig12 and 13 shows

the output voltage of Grid and voltage source inverter respectively and they both

provide a constant output voltage.


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Fig.11 PV array Duty cycle Fig.12 Va(Grid)

Fig 13.Output voltage at VSI

Conclusion

In this work, a simple single phase grid connected photovoltaic array is

analysed for obtaining an efficient synchronisation and power. The basic P&O

method is used for Maximum power point tacking which makes the response of

the boost converter fast and effective. The stability of the DC output voltage is the

most important factor for the steady Grid supply that is achieved by the MPPT

algorithm. The output voltage of the VSI is controlled for effective Grid

integration. The irradiation fluctuation can be adapted easily by this proposed

system and Grid connection of low power sources are made easy and effective.

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Abstract Key words: Introduction · Also, Pulse width modulation technique is used in grid connected inverters control technique . The important function is a specific value stabilization

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