CHAPTER 2 OVERVIEW OF GRID CONNECTED …shodhganga.inflibnet.ac.in/bitstream/10603/49374/7/07...16 CHAPTER 2 OVERVIEW OF GRID CONNECTED SOLAR PHOTOVOLTAIC SYSTEMS 2.1 INTRODUCTION

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CHAPTER 2

OVERVIEW OF GRID CONNECTED SOLAR

PHOTOVOLTAIC SYSTEMS

2.1 INTRODUCTION

The PV Power Conversion System (PCS) plays an important role in

renewable energy source based power generation. The PCS converts the

available DC supply from the source into required AC voltage (415 V, 50 Hz)

as specified by the utility grid by means of power electronic converter and is

feed into the grid. This chapter focuses on the components of a PV grid

connected system and its characteristics. The various power conversion

systems are studied in literature and compared based on efficiency, cost and

volume from which a suitable power converter for PV power conversion

system is suggested to obtain high reliability and high efficiency with low

cost. Then a brief literature review of transformerless grid connected system

is carried out to identify the setbacks of conventional transformerless inverter.

Finally synchronization methods of grid connected system are discussed.

2.2 SYSTEM DESCRIPTION

Figure 2.1 shows the block diagram of a grid connected PV system.

It consists of PV plants, MPPT controller, PWM controller, power conditioner

(inverter), and filter. PV plant converts the sunlight into DC power, and a

power conditioning unit that converts the DC power to AC power. The

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generated AC power is injected into the grid and/or utilized by the local loads

through the filter. In some cases, the PV system is combined with storage

devices which improve the availability of the power. The subsequent section

provides more details about various components of the PV system.

MPPT Controller

PWMController

DC to AC

Inverter

3ØGridFilterPV

Array

PLL

Figure 2.1 Block diagram of PV grid connected inverter

2.3 PV ARRAY MODEL

The PV plant consists of PV cells and it is arranged in series and

parallel combination to supply the desired DC voltage and current. Normally

PV cell is made up of silicon semiconductor and each silicon cell generates

0.6V. The commercially available PV module consists of 36 or 72 cells

connected in series to form a PV plant. The typical PV module used for

simulation parameters are listed in Table 2.1.

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Table 2.1 Electrical parameters of BP 3235T PV panel at standard test condition G=1000W/m2 C

PV Parameters Ratings

Maximum output Power (PMax) 235 W

Power Tolerance (PT) [0 to 5] W

Percentage Efficiency of Module ( ) 15.0%

Voltage at maximum power point (VMPP) 30.0 V

Current at maximum power point (IMPP) 7.84A

Open circuit voltage (Voc) 37.44 V

Short circuit current (Isc) 8.83 A

The simple equivalent circuit of the PV cell model is shown in

Figure 2.2. It consists of ideal current source in parallel with the diode. The

practical model of PV cells consists of series resistance (Rs) and parallel

resistance (Rp) and is shown Figure.2.3.

+

DVD

IScVpv

ID

I+

- -

Figure 2.2 Basic equivalent circuit of PV cell

From the basic model, output current (I) is represented in

Equation (2.1) and diode current is given in Equation (2.2).

sc DI = I - I (2.1)

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qvdkT

d 0I =I e -1 (2.2)

+

-

DVD

Iph Vpv

ID

I+

-

Rs

RP

Figure 2.3 Practical equivalent circuit model of PV cell

The voltage current (Vg Ig) relation of the PV plant is given in

the literature

Rauschenbach (1980).

s sph 0

s t sh

V+IR V+IRI=I -I exp -1 -n V R

(2.3)

where

V - Module voltage

I - Module current

Iph - Photon generated current

I0 - Dark saturation current respectively

Vt - Junction thermal voltage, Rs is the series cell resistance,

and Rsh cell shunt resistance

ns - number of cells connected in series

Is - saturation current,

K - Boltzmann constant 1.38×10-23 J / K

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A - solar cell ideal factor of the diode,

q - electron charge 1.6×10-19 C

t

KTAV =q

The photo current mainly depends on the solar irradiance and cell

temperature (Tsai et al 2012).

ph sc i c refI =(I +k (T -T ))G

where

Isc - Shortcircuit current of PV cell at 25°C

G - Solar insolation (kW/m2). = (1 kW/m2),

ki - Temperature coefficients

T-ref- Reference temperature of the cell.

2.3.1 Characteristics of PV Module

The voltage-current (V-I) characteristics and power-voltage

characteristics (P-V) of the PV plant are illustrated in Figures 2.4 (a) and (b)

respectively and it shows that both V-I and P-V curves are nonlinear because

PV panel is made of semiconductor material. The Maximum Power Point

(MPP) curve is shown in Figure 2.4 (c). The output power of PV plant is

maximum at certain value of voltage (VMPP) and this point is called as

maximum power point. The corresponding voltage (VMPP) and current (IMPP)

at Maximum Power Point (MPP) are denoted in Figure 2.4 (c).

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5 10 15 20 25 30 350

50

100

150

200

250

300

Voltage(V)

(a)

5 10 15 20 25 30 350

2

4

6

8

10

Voltage (V)

(b)

VPV (V)

IMPP

ISc

VMPP Voc

PMPP

(c)

Figure 2.4 Characteristics of a single PV plant (a) P-V curve (b) I-V curve (c) Maximum power curve

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2.3.2 Effect of Irradiance Change

9 18 27 36 450

50

100

150

200

250

300

Voltage (V)

Irradiation 1200 w / m2Irradiation 1000 w / m2Irradiation 800 w / m2Irradiation 600 w / m2Irradiation 400 w / m2

(a)

9 18 27 36 450

4

8

12

Voltage (V)

Irradiation 1200 w / m2Irradiation 1000 w / m2Irradiation 800 w / m2Irradiation 600 w /m2Irradiation 400 w / m2

(b)

Figure 2.5 Characteristics of the PV cell at constant temperature and variable irradiance (a) I-V curves (b) P-V curves

The output power of PV plant is the function of voltage a

C) and variable irradiance, the current voltage

characteristics are plotted as shown Figure 2.5. It is observed that as

irradiance is increased from 400 W/m2 to 1000 W/m2, the generated voltage

and hence the output power of PV plant is also increased.

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2.3.3 Effect of Temperature Change

5 10 15 20 25 30 35 40 45 500

50

100

150

200

250

Open crcuit Voltage(V)

Temp 15 CTemp 20 CTemp 30 CTemp 40 CTemp 50 C

(a)

5 10 15 20 25 30 35 40 450

2

4

6

8

10

PV Voltage (V)

Temp 50 CTemp 40 CTemp 30 CTemp 20 CTemp 15 C

(b)

Figure 2.6 Characteristics of the PV cell at variable temperature and constant irradiance (a) I-V curves (b) P-V curves

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The Figure 2.6 shows current-voltage characteristics and voltage

power characteristics of PV plant at irradiance of 1000W/m2 and different

temperature condition (Chatterjee et al 2011). From the Figure it is evident

that increasing temperatu C results in reduction of

voltage from 42V to 36V and there is slight increase in current from 8.8A to

8.9A. Hence output power is reduced to 180 W from 230W.

2.4 POWER CONDITIONING SYSTEM

This is a power converter which interfaces the PV to utility grid and

converts the DC supply from the PV plant to AC supply as requirement by the

utility grid. Based on the galvanic connection between PV plant and grid, the

power conditioning system can be broadly classified into two types such as

isolated power conditioning system and non isolated power conditioning

system (Yaosuo Xue & Liuchen Chang 2004).

2.4.1 Isolated PV Power Conditioning System

In isolated type PV system the isolation between PV plant and grid

is achieved by using a line frequency (LF) transformer at the output of the

inverter (AC side) or by using high frequency (HF) transformer DC-DC

converter at the input side of the inverter. Figures 2.7 (a) and (b) show the

grid connected PV system with galvanic isolation at the DC side and the AC

side. In low frequency (power frequency) transformer system involves huge

size, increasing magnetic loss and low efficiency than high frequency

transformer based DC-DC converter system. This high frequency transformer

involves complex control resonant problems and which increase the cost of

the PV system.

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PVArray

HF DC / AC Inverter

AC to DC Rectifier

DC to AC

Inverter

3ØGrid

(a)

PVArray

DC-DC Converter

DC to AC

Inverter

LFTransfor

mer

3ØGrid

(b)

Figure 2.7 Grid connected PV system with transformer galvanic isolation (a) High frequency (HF) DC-DC converter transformer system (b) Low frequency (LF) transformer system

2.4.2 Non Isolated PV Power Conditioning System

The non isolated grid connected PV system is again classified in to

single-stage and multistage power conditioning systems.

In single-stage, only one power processing stage is available to

convert the PV power to AC supply. The single stage power conditioning

system is depicted in Figure 2.8 (a). Nowadays, single stage power converters

are most widely used in PV applications. The single- stage inverter can

perform the buck, boost, and both buck- boost input voltage, inversion and

maximum power point. The single-stage inverter has the advantages of

improved efficiency, low cost, more reliability, modularity, and compact size

than multistage power conversion systems (Prasad et al 2008). The main

drawback of transformerless single-stage system is that it injects DC current

in the AC side (grid) which saturates the core of the magnetic component

present in the power system.

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In a multistage power conversion system more than one power

processing stages are involved as shown in Figure 2.8 (b). The non isolated

multistage system consists of front end DC-DC converter which is used to

buck, boost or buck-boosts the PV output voltage. In this method maximum

power is tracked by using maximum power point tracker in the DC-DC

converter itself. The multistage non isolated system involves more number of

switching devices than single-stage system; this causes reduction in efficiency

of PV system.

PVArray

DC to AC

InverterFilter

3ØGrid

(a)

PVArray

Non isolatedDC-DC

Converter

DC to AC

Inverter

Filter3Ø

Grid

(b)

Figure 2.8 Grid connected PV system with transformerless inverter (a) Single-stage system (b) Multi-stage system

2.5 CONNECTION TOPOLOGIES OF PV SYSTEMS

PV modules and power conditioning systems are connected in

different combinations in a solar PV plant. Based on the connection between

PV module and inverter the PV system is classified as follows

(Kjaer & Blaabjerg 2005);

a. Centralized inverter topologies

b. Master slave inverter topologies

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c. String inverter topology

d. Multi-string inverter topologies and

e. Modular topology.

2.5.1 Centralized Inverter Topology

This topology is used for large PV systems like 710kW to several

megawatts. A single inverter is used as a centralized inverter and it is shown

in Figure 2.9. To avoid further amplification the PV modules were divided

into series of connections (string), each string generates sufficiently high

voltage. The main drawbacks of this system are:

i) In case of inverter failure whole PV power production is stopped.

ii) There is loss of power due to partial shading of the PV

plants.

= AC bus

Central Inverter

PV

strings

PVPV

Figure 2.9 Centralized inverter topologies

2.5.2 Master Slave Inverter Topologies

In this topology, the inverter is connected in parallel configuration

with AC bus as shown in Figure 2.10. If any one of the inverter fails, another

inverter can supply power to the grid so that reliability is improved. The

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inverter is designed to operate based on the irradiance of the sun. So this

scheme will improve the life span of the inverter and its efficiency. This

topology is used for high power ratings. However it suffers from high cost,

power loss due to PV plant mismatch, and partial shading (Pregelj et al 2002),

(IEA International Energy Agency Innovative Electrical concepts, Report IEA

PVPS T7-7: 2001).

=

=

AC bus

InvertersPV Strings

Figure 2.10 Master Slave inverter topologies

2.5.3 String Inverter Topology

In this method, PV strings are connected with an individual inverter

as shown in Figure 2.11. Each string inverter is operated at its own maximum

power point (MPP) which leads to reduced power loss due to plant mismatch

and partial shading. The main drawback of this method is high cost due to

more number of inverter. This type of inverters is used for rating of (2-3kW).

=

=

AC bus

String InvertersPV Strings

Figure 2.11 String inverter topology

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2.5.4 Multi-String Inverter Topology

=

=

=

=

DC bus

AC bus

PV Strings

Multistring Inverter

=

=

=

DC-DC Converters

Figure 2.12 Multi string inverter topologies

Figure 2.12 shows the multi-string inverter topology which consists

of two-stage power conversion processes. In the first-stage, strings are

connected to its own DC-DC converter for tracking MPP and boost up the

voltage. Then in the second stage, the inverter converts the DC supply to AC.

Owing to separate MPPT of the PV plant string, output energy is increased,

but losses and cost will be increased due to two processing stages. This

topology is used up to 10 kW.

2.5.5 Modular Inverter Topology

Modular inverter topology is used for micro inverter technology.

Each and every module has its own inverter, mounted on the panel itself and

it is shown in Figure 2.13. This topology has the advantages of reduced losses

due to partial shading problem and flexible plant design. The main drawback

is the high cost of the system and increased thermal stress. This topology is

used up to 500 W.

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=

=

=

AC bus

Module InvertersPV Module

Figure 2.13 Modular topology

2.6 COMPARISION OF TRANSFORMER AND

TRANSFORMERLESS INVERTER TOPOLOGIES

A comparison of transformerless inverter topology with high

frequency transformer inverter topology and low frequency transformer

inverter topology for PV grid connected inverter rated up to 6.6kW is

presented based on literature (PHOTON inverter data base). The system

efficiency, cost, and volume of the PV system are considered for comparison.

In Transformerless inverter topology, PV inverter is directly connected to the

grid and there is no galvanic isolation between the PV plant and the utility

grid. Whereas in high frequency transformer inverter, high frequency DC-DC

isolated converter gives the electrical isolation (galvanic isolation) between

the PV plant and grid and in the low frequency transformer inverter, the low

frequency transformer (power frequency transformer) is connected at the

output of the inverter and grid, which provides the isolation between grid and

PV plant.

The PV power conversion system has two types of efficiency, one

is conversion efficiency: called as maximum efficiency and the other being

European efficiency which is calculated at different levels of irradiance by

using the formula given in Equation (2.5) (Haberlin et al 1997).

20% 30% 100%5% 10% 30%0.03% 0.06 0.13 0.1 0.48 0.2Eu (2.5)

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(a)

(b)

(c)

Figure 2.14 Comparison PV Grid connected inverter topology (Photon inverter database 1992) (a) Efficiency Vs Inverter rating (b) Weight Vs Inverter rating (c) Volume Vs Inverter rating

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Figure 2.14 (a) shows the comparison of inverter efficiency and

rating. It is observed that the transformerless inverter has a maximum

efficiency of 98% while the inverters with galvanic isolation have a maximum

efficiency of 96.5% only. Similarly the weight and volume of transformerless

topology and transformer topology are compared and shown in

Figures 2.14 (b) and Figure 2.14 (c) respectively. It is clear that

transformerless inverter topology has maximum efficiency, less weight, and

volume when compared to the other two galvanic separation topologies. From

the above comparison it is revealed that transformerless inverter PV system

has 1.5 % higher efficiency than the inverter with transformer topology.

2.7 REVIEW OF TRANSFORMERLESS INVERTER FOR GRID

CONNECTED PV SYSTEM

In 1980, the first line commutated inverter topology was used for

PV based electric drive application and it was used up to 10 kW (Bulawka et

al 1987) as shown in Figure 2.15. It has the advantage of high efficiency, low

cost, and robustness. However, it suffered from a low power factor of 0.6 to

0.7 only.

PV Array

ACFilter

L

Figure 2.15 Line commutated PV inverter

Nowadays, forced commutated inverters are mostly used for a

power rating of more than1.5kW as shown in Figure 2.16. This inverter is

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operated at higher switching frequency to avoid the acoustic noise

(Bulawka et al 1987). This results in high switching loss and lower efficiency.

The forced commutated inverters (voltage source inverter) are operated in a

buck mode and hence require an input voltage of double times the output

voltage, which results in higher rating of power electronic device. Also it is

suffered from short circuit problem.

S1 S3

S6S4

+

-

S5

S2

Cpv

Vpv3

phasegrid

PV

=

Filter

Figure 2.16 Forced commutated PV inverter

To boost the input voltage of PV plant, a boost type inverter called

current source inverter (CSI) based grid connected inverter is proposed

(Chen & Smedley 2008) and is depicted in Figure 2.17.

S1 S3

S6S4

S5

S2

3phasegrid

+

Vpv

-

PV

VL

=S

Figure 2.17 Boost type PV inverter

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The current source inverter (CSI) inverters have the common mode

voltage problem due to parasitic capacitance effect. To overcome the short

circuit problem in the conventional voltage source inverter, the dual buck half

bridge inverter (DBHBI) was introduced (Zhilei Yao et al 2009a) as shown in

Figure 2.18. However, DBHBIs have the drawback of high switching stress

and low input utilization than the bridge type inverters.

C1

C2

S1

S2D1

D2

VdcAC

=

Figure 2.18 Dual buck half bridge PV inverter

S1

S2D1

D2

Vdc

AC

S3

S4D3

D4

Figure 2.19 Dual buck full bridge PV inverter

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Dual Buck Full Bridge Inverter (DBFBI) is presented by

Zhilei Yao et al (2009b) and it is illustrated in Figure 2.19. It has high input

utilization factor. There is no short circuit problem and voltage stress

problem. However, it requires more power switches which lead to complexity

of control. Owing to low voltage gain and dead time problems in the

conventional inverter, Peng et al (2003) have proposed the impedance source

inverter (Z-Source inverter) for fuel cell applications. It has high voltage gain,

high immunity to electromagnetic interference and can perform buck and

boost operation in single stage. Figure 2.20 shows circuit diagram of three-

phase Z-source inverter. Gajanayake et al (2009) have utilized the ZSI for

distributed generation application. The main feature of ZSI is that it handles

the shoot through state effectively to boost the input voltage in single stage.

The arrangement of ZSI is depicted in Figure 2.20. However, it requires larger

values of passive element and also it suffers from high input ripple current

and resonant problems.

C1

L1D

S1 S3

S6S4

+

-

S5

S2

CpvVpv 3

phasegrid

PV C2

=L2

Figure 2.20 Z-Source inverter fed PV system

The main drawbacks of transformerless grid connected inverter are

common mode voltage and current problems due to the presence of galvanic

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connection between PV plant and grid. This introduces additional leakage

current through parasitic capacitance of the ground. It creates electromagnetic

interference, harmonic injection into the grid and results in power loss

(Lopez et al 2010). To eliminate the ground current, Neutral Point Clamped

(NPC) PV inverter technology is introduced by Huafeng Xiao &

Shaojun Xie (2013). It enjoys the advantages of low current ripple and low

value of filter requirement; it is shown in Figure 2.21.

DC

DC

AC

Lf Li L/2 R/2

R/2L/2

Cp

Cp Cf

Rf

Figure 2.21 Neutral point clamped PV inverter

This NPC inverter requires two PV strings of the same rating. Each

string conducts for only one half cycle. This results in high volume of

decoupling capacitor which increases the total cost and size of the

system. Optimized transformerless inverter topology proposed by

Huafeng Xiao et al (2011) is based on Heric topology and H6 topology.

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ACL1

L2

Cpv

Cf

PV

Cpv

+

-

S1

S2

S3

S4

S5

S6

Cdc2

Cdc1

Figure 2.22 Optimized transformerless inverter

Figure 2.22 shows the optimized transformerless PV grid connected

inverter which consists of six switches for single-phase operation and it leads

to more power loss and cost. Also the amount of input voltage supplied by the

PV plant is higher than the output voltage.

The inverter reliability is mainly affected due to the shoot through

problem and is overcome by using Z-source inverter (ZSI). The resonant

problem and common mode problem of ZSI are controlled by proper

modulation techniques and inverter topologies. The modified modulation

scheme is proposed for the improved ZSI based PV grid connected system by

Bradaschia et al (2011). The improved ZSI fed PV system is depicted in

Figure 2.23.

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C1

L1D

S1 S3

S6S4

+

-

S5

S2

CpN

Vpv 3phasegrid

PV C2

=L2

P

N

Figure 2.23 Modified ZSI fed transformerless PV grid connected inverter

An improved transformerless inverter with common mode leakage

current elimination is presented by Bo Yang et al (2012) as shown in

Figure 2.24. The presence of two decoupling switches in the DC side results

in reduced harmonics and elimination of common mode leakage current. But

it has poor reliability due to the additional switches in the inverter.

AC

L1

L2

PV

-S2

S3

S4

S5

S6

Cdc

S1

Figure 2.24 H6 PV inverter topology

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The improved inversion of Z source inverter called as voltage

fed Trans Z-source inverter (T-source inverter) is proposed by

Wei Qian et al (2011). It has a low component count and can buck and boost

the input voltage to required level by allowing shoot through period. This is

compact and has high efficiency. In order to overcome the drawbacks of the

conventional system VSI and ZSI, the T-source inverter is suggested for grid

connected PV system. The diagram of the proposed T-source grid connected

PV inverter system is shown in Figure 2.25.

C

L1

D

S1 S3

S6S4

+

-

S5

S2

CpvVpv 3

phasegrid

PV

L2

1:n

Vc

P

N

=

Figure 2.25 T-source inverter for PV grid connected system

2.8 GRID SYNCHRONIZATION TECHNIQUES

Synchronization is the most important factor in a grid connected

PV system in which the following conditions should be satisfied. The voltage

magnitude; frequency and phase angle of PV system must be the same as that

of utility grid. The power factor of power supplied to the grid must be within

the specified limits with proper reactive power compensation. There are

several methods followed in grid connected PV system synchronization such

as zero crossing detection (ZCD), filtering of grid voltages, Phase Locked

Loop (PLL), and dark lamp method. In this research work, synchronous

reference frame phase locked loop method is used.

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In conventional method, grid voltage is duplicated to synchronize

the PV inverter with grid. The output current of the inverter is in phase with

the grid voltage (De Souza et al 2002). It is a simple method but produces

distortions, transients in the output current and no control for reactive power

flow.

The phase locked loop (PLL) method is commonly used in

synchronization of inverter with grid. It consists of a sinusoidal multiplier

Phase Detector (PD), a loop Filter (LF) and a voltage controlled oscillator

(VCO). To speed up the synchronization method, the modified stationary

frame PLLs is proposed (Thacker et al 2009). It improves the immunity to

input noises, disturbances, and removes the double-line frequency ripple

generated by PD during grid voltage/frequency variations.

However in this research work, the standard existing SRF PLL is

used and explained in this section. The structure of SRF PLL as shown in

Figure 2.26. The PLL converts the oscillating grid voltage (orthogonal

-q). Then a PI regulator is used to

regulate Vq so that the phase of the q component can be locked. A low pass

filter is used to fit high frequency harmonics. This effectively allows the

inverter to have the ability to control reactive power flow by Kaura Vikram

(1997) and & Se-Kyo & Blasko (2000).

dq

Orthogonal signal

generation

PI

IC

1---S

Mod(2 )

1----2

LPFRMS

calculation LPF

V

V

V

Vq

Vd

f

V

Figure 2.26 Structure of synchronous frame (dq) PLL

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2.9 GRID STANDARD

The designed system needs to comply with the strict utility grid standards. Then it must follow certain standards specified by the utility authority of every country. The grid connected system must obey the following norms.

1) Total harmonic distortion (THD) should be below 5%. The individual harmonic levels are specified in the standard IEEE 1547.

2) Power factor should be maintained at nearly unity.

3) Less amount (less than 0.5% of rated output current is allowable) of DC current is injected to the grid.

4) Automatic reclosing and synchronizing should be taking place in PV generations and utility grid.

5) Grounding of the system (leakage current and grid current should be maintained)

6) Voltage and frequency should be within permissible range.

2.10 SUMMARY

The comprehensive overview of grid connected PV system is presented in this chapter. Various types of grid connected inverters used in PV system have been discussed. The choice of inverter for PV grid connected system has been analyzed based on efficiency, volume, and weight. It is observed that transformerless inverter is the better choice for low voltage grid connected PV systems. Further the recent developments in transformerless power conversion system have been explored. Different types of transformerless PV system have been compared based on power conversion stage, input utilization factor, and reliability. Finally, the TSI is identified to overcome the drawbacks of conventional VSI and ZSI. The consecutive chapter analyzes the modeling, and the control methods of TSI for PV grid connected system are described.