1.1 Introduction

Today photovoltaic (PV) power systems are becoming more and more

popular, with the increase of energy demand and the concern of environmental

pollution around the world. Four different system configurations are widely developed

in grid-connected PV power applications: the centralized inverter system, the string

inverter system, the multi string inverter system and the module-integrated inverter

system. Generally three types of inverter systems except the centralized inverter

system can be employed as small-scale distributed generation (DG) systems, such as

residential power applications. The most important design constraint of the PV DG

system is to obtain a high voltage gain. For a typical PV module, the open-circuit

voltage is about 21 V and the maximum power point (MPP) voltage is about 16 V.

And the utility grid is 220 or 110 Vac. Therefore, the high voltage amplification is

obligatory to realize the grid-connected function and achieve the low total harmonic

distortion (THD). The conventional system requires large numbers of PV modules in

series, and the normal PV array voltage is between 150 and 450 V, and the system

power is more than 500 W. This system is not applicable to the module-integrated

inverters, because the typical power rating of the module-integrated inverter system is

below 500 W, and the modules with power ratings between 100 and 200 W are also

quite common. The other method is to use a line frequency step-up transformer, and

the normal PV array voltage is between 30 and 150 V. But the line frequency

transformer has the disadvantages of larger size and weight. In the grid-connected PV

system, power electronic inverters are needed to realize the power conversion, grid

interconnection, and control optimization. Generally, gird-connected pulse width

modulation (PWM) voltage source inverters (VSIs) are widely applied in PV systems,

which have two functions at least because of the unique features of PV modules. First,

the dc-bus voltage of the inverter should be stabilized to a specific value because the

output voltage of the PV modules varies with temperature, irradiance, and the effect

of maximum power-point tracking (MPPT). Second, 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. Therefore, it is clear that for the inverter-

based PV system, the conversion power quality including the low THD, high power

factor, and fast dynamic response, largely depends on the control strategy adopted by

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the grid-connected inverters. In this paper, a grid-connected PV power system with

high voltage gain is proposed. The steady-state model analysis and the control strategy

of the system are presented. The grid connected PV system includes two power-

processing stages:

a high step-up ZVT-interleaved boost converter for boosting a low voltage of PV

array up to the high dc-bus voltage, which is not less than grid voltage level; and a

full-bridge inverter for inverting the dc current into a sinusoidal waveform

synchronized with the utility grid. Furthermore, the dc–dc converter is responsible for

the MPPT and the dc–ac inverter has the capability of stabilizing the dc-bus voltage to

a specific value. The grid-connected PV power system can offer a high voltage gain

and guarantee the used PV array voltage is less than 50 V, while the power system

interfaces the utility grid. On the one hand, the required quantity of PV modules in

series is greatly reduced. And the system power can be controlled in a wide range

from several hundred to thousand watts only by changing the quantity of PV module

branches in parallel. Therefore, the proposed system can not only be applied to the

string or multi string inverter system, but also to the module-integrated inverter

system in low power applications. On PV systems employing neutral-point-clamped

(NPC) topology, highly efficient reliable inverter concept (HERIC) topology, H5

topology, etc., have been widely used especially in Europe. Although the transformer

less system having a floating and non earth-connected PV dc bus requires more

protection, it has several advantages such as high efficiency, lightweight, etc.

Therefore, the non isolation scheme in this paper is quite applicable by employing the

high step-up ZVT-interleaved boost converter, because high voltage gain of the

converter ensures that the PV array voltage is below50Vand benefits the personal

safety even if in high-power application. Fig. 1 shows the proposed grid-connected PV

power system.

1.1.1 PV System

A photovoltaic system (or PV system) is a system which uses one or more solar

panels to convert sunlight into electricity. It consists of multiple components,

including the photovoltaic modules, mechanical and electrical connections and

mountings and means of regulating and/or modifying the electrical output

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Fig 1.1: PV System

1.1.2 Photovoltaic modules

Due to the low voltage of an individual solar cell (typically ca. 0.5V), several

cells are wired in series in the manufacture of a "laminate". The laminate is assembled

into a protective weatherproof enclosure, thus making a photovoltaic module or solar

panel. Modules may then be strung together into a photovoltaic array. The electricity

generated can be either stored, used directly (island/standalone plant)or fed into a

large electricity grid powered by central generation plants (grid-connected/grid-tied

plant) or combined with one or many domestic electricity generators to feed into a

small grid (hybrid plant). Depending on the type of application, the rest of the system

("balance of system" or "BOS") consists of different components. The BOS depends

on the load profile and the system type. Systems are generally designed in order to

ensure the highest energy yield for a given investment.

1.1.3 Photovoltaic arrays

The power that one module can produce is seldom enough to meet

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requirements of a home or a business, so the modules are linked together to form an

array. Most PV arrays use an inverter to convert the DC power produced by the

modules into alternating current that can power lights, motors, and other loads. The

modules in a PV array are usually first connected in series to obtain the desired

voltage; the individual strings are then connected in parallel to allow the system to

produce more current. Solar arrays are typically measured under STC (Standard Test

Conditions) or PTC(PVUSA Test Conditions), in watts, kilowatts, or even megawatts.

Costs of production have been reduced in recent years for more widespread use

through production and technological advances. One source claims the cost in

February 2006 ranged $3–10/watt while a similar size is said to have cost $8–10/watt

in February 1996, depending on type.[2] For example, crystal silicon solar cells have

largely been replaced by less expensive multicrystalline silicon solar cells, and thin

film silicon solar cells have also been developed recently at lower costs of production.

Although they are reduced in energy conversion efficiency from single crystalline

"siwafers", they are also much easier to produce at comparably lower costs.

1.1.3.1 Applications

Standalone systems

Fig 1.2: Solar powered parking meter.

A standalone system does not have a connection to the electricity "mains" (aka

"grid"). Standalone systems vary widely in size and application from wristwatches or

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calculators to remote buildings or spacecraft. If the load is to be supplied

independently of solar insolation, the generated power is stored and buffered with a

battery. In non-portable applications where weight is not an issue, such as in

buildings, lead acid batteries are most commonly used for their low cost. A charge

controller may be incorporated in the system to: a) avoid battery damage by excessive

charging or discharging and, b) optimizing the production of the cells or modules by

maximum power point tracking (MPPT).However, in simple PV systems where the

PV module voltage is matched to the battery voltage, the use of MPPT electronics is

generally considered unnecessary, since the battery voltage is stable enough to provide

near-maximum power collection from the PV module. In small devices (e.g.

calculators, parking meters) only direct current (DC) is consumed. In larger systems

(e.g. buildings, remote water pumps) AC is usually required. To convert the DC from

the modules or batteries into AC, an inverter is used.

Fig 1.3: Solar Inverter

A schematic of a bare-bones off-grid system, consisting (from left to right) of

photovoltaic module, a blocking-diode to prevent battery drain during low-insolation,

a battery, an inverter, and an AC load such as a fluorescent lamp off-grid PV system

with battery charger.

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Fig 1.4: Bare-bones off-grid system

1.1.3.2 Solar vehicles

Ground, water, air or space vehicles may obtain some or all of the energy

required for their operation from the sun. Surface vehicles generally require higher

power levels than can be sustained by a practically-sized solar array, so a battery is

used to meet peak power demand, and the solar array recharges it. Space vehicles have

successfully used solar photovoltaic systems for years of operation, eliminating the

weight of fuel or primary batteries.

1.1.3.4 Small scale DIY solar systems

With a growing DIY-community and an increasing interest in environmentally

friendly "green energy", some hobbyists have endeavored to build their own PV solar

systems from kits or partly diy. Usually, the DIY-community uses inexpensive

and/or high efficiency systems (such as those with solar tracking) to generate their

own power. As a result, the DIY-systems often end up cheaper than their commercial

counterparts. Often, the system is also hooked up unto the regular power grid to repay

part of the investment via net metering. These systems usually generate power amount

of ~2 kW or less. Through the internet, the community is now able to obtain plans to

construct the system (at least partly DIY) and there is a growing trend toward building

them for domestic requirements. The DIY-PV solar systems are now also being used

both in developed countries and in developing countries, to power residences and

small businesses.

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1.1.3.5 Grid-connected system

Fig 1.5: Diagram of a residential grid-connected PV system

A grid connected system is connected to a large independent grid (typically the

public electricity grid) and feeds power into the grid. Grid connected systems vary in

size from residential (2-10kWp) to solar power stations (up to 10s of GWp). This is a

form of decentralized electricity generation. In the case of residential or building

mounted grid connected PV systems, the electricity demand of the building is met by

the PV system. Only the excess is fed into the grid when there is an excess. The

feeding of electricity into the grid requires the transformation of DC into AC by a

special, grid-controlled inverter.

In kW sized installations the DC side system voltage is as high as permitted (typically

1000V except US residential 600V) to limit ohmic losses. Most modules (72

crystalline silicon cells) generate about 160W at 36 volts. It is sometimes necessary or

desirable to connect the modules partially in parallel rather than all in series. One set

of modules connected in series is known as a 'string'.

1.1.3.6 Building systems

In urban and suburban areas, photovoltaic arrays are commonly used on

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rooftops to supplement power use; often the building will have a connection to the

power grid, in which case the energy produced by the PV array can be sold back to the

utility in some sort of net metering agreement. Solar trees are arrays that, as the name

implies, mimic the look of trees, provide shade, and at night can function as street

lights. In agricultural settings, the array may be used to directly power DC pumps,

without the need for an inverter. In remote settings such as mountainous areas,

islands, or other places where a power grid is unavailable, solar arrays can be used as

the sole source of electricity, usually by charging a storage battery. There is financial

support available for people wishing to install PV arrays. In the UK, households are

paid a 'Feedback Fee' to buy excess electricity at a flat rate per kWh. This is up to

44.3p/kWh which can allow a home to earn double their usual annual domestic

electricity bill. The current UK feed-in tariff system is due for review on 31 March

2012, after which the current scheme may no longer be available.

1.1.3.7 Power plants

Fig 1.6: Waldpolenz Solar Park, Germany

A photovoltaic power station is a power station using photovoltaic modules

and inverters for utility scale electricity generation, connected to an electricity

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transmission grid. Some large photovoltaic power stations like Waldpolenz Solar Park

cover a significant area and have a maximum power output of 40-60 MW.

1.1.3.8 System performance

At high noon on a cloudless day at the equator, the power of the sun is about

1 kW/m², on the Earth's surface, to a plane that is perpendicular to the sun's rays. As

such, PV arrays can track the sun through each day to greatly enhance energy

collection. However, tracking devices add cost, and require maintenance, so it is more

common for PV arrays to have fixed mounts that tilt the array and face due South in

the Northern Hemisphere (in the Southern Hemisphere, they should point due North).

The tilt angle, from horizontal, can be varied for season, but if fixed, should be set to

give optimal array output during the peak electrical demand portion of a typical year.

For the weather and latitudes of the United States and Europe, typical insolation

ranges from 4 kWh/m²/day in northern climes to 6.5 kWh/m²/day in the sunniest

regions. Typical solar panels have an average efficiency of 12%, with the best

commercially available panels at 20%. Thus, a photovoltaic installation in the

southern latitudes of Europe or the United States may expect to produce 1

kWh/m²/day. A typical "150 watt" solar panel is about a square meter in size. Such a

panel may be expected to produce 1 kWh every day, on average, after taking into

account the weather and the latitude. In the Sahara desert, with less cloud cover and a

better solar angle, one could ideally obtain closer to 8.3 kWh/m²/day provided the

nearly ever present wind would not blow sand on the units. The unpopulated area of

the Sahara desert is over 9 million km², which if covered with solar panels would

provide 630 terawatts total power. The Earth's current energy consumption rate is

around 13.5 TW at any given moment (including oil, gas, coal, nuclear, and

hydroelectric).

1.1.3.9 Tracking the sun

Trackers and sensors to optimize the performance are often seen as optional,

but tracking systems can increase viable output by up to 100%. PV arrays that

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approach or exceed one megawatt often use solar trackers. Accounting for clouds, and

the fact that most of the world is not on the equator, and that the sun sets in the

evening, the correct measure of solar power is insolation – the average number of

kilowatt-hours per square meter per day. For the weather and latitudes of the United

States and Europe, typical insolation ranges from 4kWh/m²/day in northern climes to

6.5 kWh/m²/day in the sunniest regions. For large systems, the energy gained by using

tracking systems outweighs the added complexity (trackers can increase efficiency by

30% or more).

1.1.3.10 Shading and dirt

Photovoltaic cell electrical output is extremely sensitive to shading. When

even a small portion of a cell, module, or array is shaded, while the remainder is in

sunlight, the output falls dramatically due to internal 'short-circuiting' (the electrons

reversing course through the shaded portion of the p-n junction). If the current drawn

from the series string of cells is no greater than the current that can be produced by the

shaded cell, the current (and so power) developed by the string is limited. If enough

voltage is available from the rest of the cells in a string, current will be forced through

the cell by breaking down the junction in the shaded portion. This breakdown voltage

in common cells is between 10 and 30 volts. Instead of adding to the power produced

by the panel, the shaded cell absorbs power, turning it into heat. Since the reverse

voltage of a shaded cell is much greater than the forward voltage of an illuminated

cell, one shaded cell can absorb the power of many other cells in the string,

disproportionately affecting panel output. For example, a shaded cell may drop 8

volts, instead of adding 0.5 volts, at a particular current level, thereby absorbing the

power produced by 16 other cells. Therefore it is extremely important that a PV

installation is not shaded at all by trees, architectural features, flag poles, or other

obstructions. Most modules have bypass diodes between each cell or string of cells

that minimize the effects of shading and only lose the power of the shaded portion of

the array (The main job of the bypass diode is to eliminate hot spots that form on cells

that can cause further damage to the array, and cause fires.). Sunlight can be absorbed

by dust, snow, or other impurities at the surface of the module. This can cut down the

amount of light that actually strikes the cells by as much as half. Maintaining a clean

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module surface will increase output performance over the life of the module.

· Temperature

Module output and life are also degraded by increased temperature.

Allowing ambient air to flow over, and if possible behind, PV modules

reduces this problem.

· Module efficiency

In 2010, solar panels available for consumers can have a yield

of up to 19%, while commercially available panels can go as far as

27%. Thus, a photovoltaic installation in the southern latitudes of

Europe or the United States may expect to produce 1 kWh/m²/day. A

typical "150 watt" solar panel is about a square meter in size. Such a

panel may be expected to produce 1 kWh every day, on average, after

taking into account the weather and the latitude

· Module life

Effective module lives are typically 25 years or more.

1.1.4 Components

1.1.4.1 Trackers

A solar tracker tilts a solar panel throughout the day. Depending on the type of

tracking system, the panel is either aimed directly at the sun or the brightest area of a

partly clouded sky. Trackers greatly enhance early morning and late afternoon

performance, substantially increasing the total amount of power produced by a system.

Trackers are effective in regions that receive a large portion of sunlight directly. In

diffuse light (i.e. under cloud or fog), tracking has little or no value. Because most

concentrated photovoltaics systems are very sensitive to the sunlight's angle, tracking

systems allow them to produce useful power for more than a brief period each day.

Tracking systems improve performance for two main reasons. First, when a solar

panel is perpendicular to the sunlight, the light it receives is more intense than it

would be if angled. Second, direct light is used more efficiently than angled light.

Special Anti-reflective coatings can improve solar panel efficiency for direct and

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angled light, somewhat reducing the benefit of tracking.

1.1.4.2 Inverters

Fig 1.7:Inverter for grid connected PV

On the AC side, these inverters must supply electricity in sinusoidal form,

synchronized to the grid frequency, limit feed in voltage to no higher than the grid

voltage including disconnecting from the grid if the grid voltage is turned off.

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Fig 1.8: Maximum power point tracking

On the DC side, the power output of a module varies as a function of the

voltage in a way that power generation can be optimized by varying the system

voltage to find the 'maximum power point'. Most inverters therefore incorporate

'maximum power point tracking'. A solar inverter may connect to a string of solar

panels. In small installations a solar micro-inverter is connected at each solar panel.

For safety reasons a circuit breaker is provided both on the AC and DC side to enable

maintenance. AC output may be connected through an electricity meter into the public

grid. The meter must be able to run in both directions. In some countries, for

installations over 30kWp a frequency and a voltage monitor with disconnection of all

phases is required.

1.1.4.3 Mounting systems

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Fig 1.9: Ground mounted system

Modules are assembled into arrays on some kind of mounting system. For

solar parks a large rack is mounted on the ground, and the modules mounted on the

rack. For buildings, many different racks have been devised for pitched roofs. For flat

roofs, racks, bins and building integrated solutions are used.

1.1.4.4 Connection to a DC grid

DC grids are only to be found in electric powered transport: railways trams

and trolleybuses. A few pilot plants for such applications have been built, such as the

tram depots in Hannover Leinhausen and Geneva (Bachet de Pesay). The 150 kWp

Geneva site feeds 600V DC directly into the tram/trolleybus electricity network

provided about 15% of the electricity at its opening in 1999.

1.1.4.5 Hybrid systems

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Fig 1.10: Hybrid Power System

A hybrid system combines PV with other forms of generation, usually a diesel

generator. Biogas is also used. The other form of generation may be a type able to

modulate power output as a function of demand. However more than one renewable

form of energy may be used e.g. wind. The photovoltaic power generation serves to

reduce the consumption of non renewable fuel. Hybrid systems are most often found

on islands. Pellworm island in Germany and Kythnos island in Greece are notable

examples (both are combined with wind). The Kythnos plant has diocane diesel

consumption by 11.2% [20] .There has also been recent work showing that the PV

penetration limit can be increased by deploying a distributed network of PV+CHP

hybrid systems in the U.S. The temporal distribution of solar flux, electrical and

heating requirements for representative U.S. single family residences were analyzed

and the results clearly show that hybridizing CHP with PV can enable additional PV

deployment above what is possible with a conventional centralized electric generation

system. This theory was reconfirmed with numerical simulations using per second

solar flux data to determine that the necessary battery backup to provide for such a

hybrid system is possible with relatively small and inexpensive battery systems. In

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addition, large PV+CHP systems are possible for institutional buildings, which again

provide back up for intermittent PV and reduce CHP runtime.

1.1.5 Power quality

The power quality of power supply of an ideal power system means to supply

electric energy with perfect sinusoidal waveform at a constant frequency of a specified

voltage with least amount of disturbances. Power quality is an issue that is becoming

increasingly important to electricity consumers at all levels of usage. Sensitive

equipment and non-linear loads are now more commonplace in both the industrial

commercial sectors and the domestic environment. Because of this a heightened

awareness of power quality is developing amongst electricity users. Occurrences

affecting the electricity supply that were once considered acceptable by electricity

companies and users are now often considered a problem to the users of everyday

equipment. How ever the harmonic is one of the major factor due to which none of

condition is fulfilled in practice. The presence of harmonics, disturbs the waveform

shape of voltage and current, and increases the current level and changes the power

factor of supply and which in turn creates so many problems.

In this part we introduces the commonly accepted definitions used in the field

of power quality and discusses some of the most pertinent issues affecting end-users,

equipment manufacturers and electricity suppliers relating to the field. This Special

Feature contains a range of articles balanced to give the reader an overview of the

current situation with representation from the electricity industry, monitoring

equipment manufacturers, solution equipment manufacturers, specialist consultants

and government research establishments. The term ‘power quality’ has come into the

vocabulary of many industrial and commercial electricity end-users in recent years.

Previously equipment was generally simpler and therefore more robust and insensitive

to minor variations in supply voltage. Voltage fluctuations coming from the public

supply network were therefore not even noticed. Now equipment is used which

depends on a higher level of power quality and consumers expect disruption-free

operation. Wide diversity of solutions to power quality problems is available to both

the distribution network operator and the end-user.

More sophisticated monitoring equipment is readily affordable to end-users,

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who empower themselves with information related to the level of power quality they

receive. the following paragraphs introduce the definitions of power quality

measurable quantities or occurrences. A voltage dip is a reduction in the RMS voltage

in the range of 0.1 to 0.9 p.u. (retained) for duration greater than half a mains cycle

and less than 1 minute. Often referred to as a ‘sag’. Caused by faults, increased load

demand and transitional events such as large motor starting . A voltage swell is an

increase in the RMS voltage in the range of 1.1 to 1.8 p.u. for a duration greater than

half a mains cycle and less than 1 minute. Caused by system faults, load switching and

capacitor switching. A transient is an undesirable momentary deviation of the supply

voltage or load current. Transients are generally classified into two categories:

impulsive and oscillatory.

1.2 Harmonics

Harmonics are periodic sinusoidal distortions of the supply voltage or load

current caused by non-linear loads. Harmonics are measured in integer multiples of

the fundamental supply frequency. Using Fourier series analysis the individual

frequency components of the distorted waveform can be described in terms of the

harmonic order, magnitude and phase of each component. The electricity is produced

and distributed in its fundamental form as 50hz in India.

A harmonics is defined as the content of signal who’s frequency is integer

multiple of the system fundamental frequency. Due to harmonic effect the sinusoidal

waveform is no longer have stand and it become non-sinusoidal or complex

waveform. The complex waveform consists of a fundamental wave of 50 Hz and a

number of other sinusoidal waves whose frequencies are integral multiple of

fundamental wave like 2f(100hz). 3f (150 Hz), 4f (200 Hz) etc. Wave having

frequency of 2f, 4f, 6f etc are called the even harmonics and those having frequency of

3f, 5f, 7f etc are called as odd harmonics. When fundamental frequency is super

imposed with high-level harmonics, it results into complex wave and which is non

sinusoidal. When non-linear load draws current, that current passes through all of the

impedance that is between the load and the system source (See Figure 4). As a result

of the current flow, harmonic voltages are produced by impedance in the system for

each harmonic. These voltages sum and when added to the nominal voltage produce

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voltage distortion. The magnitude of the voltage distortion depends on the source

impedance and the harmonic voltages produced. If the source impedance is low then

the voltage distortion will be low. If a significant portion of the load becomes non-

linear (harmonic currents increase) and/or when a resonant condition prevails (system

impedance increases), the voltage can increase dramatically.

Fig: 1.11: Distorted-current induced voltage distortion

1.2.1 Total harmonic Distortion (Distortion factor)

The THD is defined as the ratio of the rms value of the harmonic components

to the rms value of the fundamental component and usually expressed in percent. This

index is used to measure the deviation of a periodic wave form containing harmonics

from a perfect sine wave. For a perfect sine wave at fundamental frequency, the THD

is zero.

1.3 Active power filter:

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Active power filters are powerful tools for compensating for not only the

current harmonics produced by non-linear loads, but also the reactive power and

unbalance of non-linear and fluctuating loads. The shunt active power filter operates

as a controlled current source connected in parallel to the non-linear loads for

injecting current harmonics into the ac source. The injected current harmonics are

equal in magnitude but opposite to the load current harmonics

1.3.1 Shunt active power filter:

Along with increasing demand on improving power quality, the most popular

technique that has been used is Active Power Filter (APF); this is because APF can

easily eliminate unwanted harmonics, improve power factor and overcome voltage

sags.

Harmonic is defined as “a sinusoidal component of a periodic wave or quantity

having a frequency that is an integral multiple of the fundamental frequency”.

Harmonic is turnout of several of frequency current or voltage multiply by the

fundamental voltage or current in the system. Previous technique used to compensate

load current harmonics is L-C passive filter; as a result the filter cannot adapt for

various range of load current and sometimes produce undesired resonance. In

electrical power supply there are many nonlinear power loads drawing non-sinusoidal

current. Non sinusoidal current will pass through the different kind of impendence in

the power system and produce voltage harmonics. This will affect to the power system

components especially sensitive equipment.

1.4 Conclusion

The increasing use of power electronics-based loads (adjustable speed drives,

switch mode power supplies, etc.) to improve system efficiency and controllability is

increasing the concern for harmonic distortion levels in end user facilities and on the

overall power system.

1.5 Summary:

· The most important design constraint of the PV DG system is to obtain a high

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voltage gain.

· Most PV arrays use an inverter to convert the DC power produced by the

modules into alternating current that can power lights, motors, and other loads.

· Harmonics are measured in integer multiples of the fundamental supply

frequency.

· For a perfect sine wave at fundamental frequency, the THD is zero.

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· Introduction

Power quality, or more specifically, a power quality disturbance, is generally

defined as any change in power (voltage, current, or frequency) that interferes with the

normal operation of electrical equipment. The study of power quality, and ways to

control it, is a concern for electric utilities, large industrial companies, businesses, and

even home users. The study has intensified as equipment has become increasingly

sensitive to even minute changes in the power supply voltage, current, and frequency.

Unfortunately, different terminology has been used to describe many of the existing

power disturbances, which creates confusion and makes it more difficult to effectively

discuss, study, and make changes to today’s power quality problems. The Institute of

Electrical and Electronics Engineers (IEEE) has attempted to address this problem by

developing a standard that includes definitions of power disturbances. The standard

(IEEE Standard 1159 1995, "IEEE Recommended Practice for Monitoring Electrical

Power Quality") describes many power quality problems, of which this paper will

discuss the most common. Commercial ac power appears as a smooth, symmetrical

sine wave, varying at either 50 cycles every second (Hertz – Hz). The sinusoidal wave

shape, voltage changes from a positive value to a negative value, 50 times per second.

When this flowing wave shape changes size, shape, symmetry, frequency, or develops

notches, impulses, ringing, or drops to zero (however briefly), there is a power

disturbance. As stated, there has been some ambiguity throughout the electrical

industry and businesses community in the use of terminology to describe various

power disturbances. For example, the term “surge” is seen by one sector of the

industry to mean a momentary increase in voltage as would be typically caused by a

large load being switched off. On the other hand, usage of the term “surge” can also

be seen as a transient voltage lasting from microseconds to only a few milliseconds

with very high peak values. These latter are usually associated with lightning strikes

and switching events creating sparks or arcing between contacts. A communication

mistake can have expensive consequences, which includes downtime, or even

equipment damage.

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· Classification Of Power Quality

This IEEE defined power quality disturbances shown in this paper have been

organized into seven categories based on wave shape:

· Transients.

· Interruptions.

· Sag / under voltage.

· Swell / Overvoltage.

· Waveform distortion.

· Voltage fluctuations.

· Frequency variations.

1. Potentially the most damaging type of power disturbance, transients fall into two

subcategories:

· Impulsive

· Oscillatory

Impulsive transients are sudden high peak events that raise the voltage and/or current

levels in either a positive or a negative direction. Causes of impulsive transients

include lightning, poor grounding, the switching of inductive loads, utility fault

clearing, and ESD (Electrostatic Discharge). The results can range from the loss (or

corruption) of data, to physical damage of equipment. Of these causes, lightning is

probably the most damaging.

An oscillatory transient is a sudden change in the steady-state condition of a

signal's voltage, current, or both, at both the positive and negative signal limits,

oscillating at the natural system frequency. In simple terms, the transient causes the

power signal to alternately swell and then shrink, very rapidly. Oscillatory transients

usually decay to zero within a cycle (a decaying oscillation). These transients occur

when you turn off an inductive or capacitive load, such as a motor or capacitor bank.

An oscillatory transient results because the load resists the change.

An interruption is defined as the complete loss of supply voltage or load current.

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Depending on its duration, an interruption is categorized as instantaneous,

momentary, temporary, or sustained.

Types

· Instantaneous 0.5 to 30 cycles.

· Momentary 30 cycles to 2 seconds.

· Temporary 2 seconds to 2 minutes.

· Sustained greater than 2 minutes.

Solutions to help against interruptions vary, both in effectiveness and cost. The first

effort should go into eliminating or reducing the likelihood of potential problems.

Good design and maintenance of utility systems are, of course, essential. This also

applies to the industrial customer's system design, which is often as extensive and

vulnerable as the utility system.

Sag is a reduction of AC voltage at a given frequency for the duration of 0.5 cycles to

1 minute’s time. Sags are usually caused by system faults, and are also often the result

of switching on loads with heavy start-up currents. Some of the same techniques that

were used to address interruptions can be utilized to address voltage sags: UPS

equipment, motor generators, and system design techniques. However, sometimes the

damage being caused by sags is not apparent until the results are seen over time.

Under voltages are the results of long-term problems that create sags. The term

“brownout” has been commonly used to describe this problem, and has been super

ceded by the term under voltage. Under-voltages can create overheating in motors,

and can lead to the failure of nonlinear loads such as computer power supplies. The

solution for sags also applies to under-voltages. More importantly, if an under voltage

remains constant, it may be a sign of a serious equipment fault, configuration

problem, or that the utility supply needs to be addressed.

· Swell / Overvoltage.

23

A swell is the reverse form of sag, having an increase in AC voltage for

duration of 0.5 cycles to 1 minute’s time. For swells, high-impedance neutral

connections, sudden (especially large) load reductions, and a single-phase fault on

a three-phase system are common sources. The result can be data errors, flickering

of lights, degradation of electrical contacts, semiconductor damage in electronics,

and insulation degradation. Power line conditioners, UPS systems, and Ferro

resonant "control" transformers are common solutions.

· Over voltages

Over voltages can be the result of long-term problems that create swells. An

overvoltage can be thought of as an extended swell. Over voltages are also

common in areas where supply transformer tap settings are set incorrectly and

loads have been reduced.

Waveform Distortion

There are five primary types of waveform distortion:

· DC offset

· Harmonics

· Inter harmonics

· Notching

· Noise

· DC offset

Direct current (dc) can be induced into an ac distribution system, often due to

failure of rectifiers within the many ac to dc conversion technologies that have

proliferated modern equipment. DC can traverse the ac power system and add

unwanted current to devices already operating at their rated level. When a transformer

saturates, it not only gets hot, but also is unable to deliver full power to the load, and

24

the subsequent waveform distortion can create further instability.

· Harmonics

Harmonic distortion is the corruption of the fundamental sine wave at

frequencies that are multiples of the fundamental. Symptoms of harmonic problems

include overheated transformers, neutral conductors, and other electrical distribution

equipment, as well as the tripping of circuit breakers and loss of synchronization on

timing circuits that are dependent upon a clean sine wave trigger at the zero crossover

point.

Harmonic distortion has been a significant problem with IT equipment in the

past, due to the nature of switch-mode power supplies (SMPS). These non-linear

loads, and many other capacitive designs, instead of drawing current over each full

half cycle, “sip” power at each positive and negative peak of the voltage wave. The

return current, because it is only short-term, (approximately 1/3 of a cycle) combines

on the neutral with all other returns from SMPS using each of the three phases in the

typical distribution system. Instead of subtracting, the pulsed neutral currents add

together, creating very high neutral currents, at a theoretical maximum of 1.73 times

the maximum phase current. An overloaded neutral can lead to extremely high

voltages on the legs of the distribution power, leading to heavy damage to attached

equipment. At the same time, the load for these multiple SMPS is drawn at the very

peaks of each voltage half-cycle, which has often led to transformer saturation and

consequent overheating. Other loads contributing to this problem are variable speed

motor drives, lighting ballasts and large legacy UPS systems. Methods used to

mitigate this problem have included over-sizing the neutral conductors, installing K-

rated transformers, and harmonic filters.

Spurred on by the remarkable expansion of the IT industry over the last decade,

power supply design for IT equipment has been upgraded via international standards.

One major change compensates for electrical infrastructure stresses caused, in the

recent past, by large clusters of IT equipment power supplies contributing to excessive

harmonic currents within a facility. Many new IT equipment power supplies have

been designed with power-factor corrected power supplies operating as linear, non-

harmonic loads. These power supplies do not produce the waste current of harmonics.

25

· Harmonics Mitigation Techniques

The generation of harmonics, whenever an adjustable speed drive is used, is

inevitable. The order and magnitude of these harmonics will greatly depend on the

drive configuration and system impedance. The various harmonic mitigation

techniques available are as follows:

2.3.1 Phase Multiplication: whether the drive is AC or DC, the common means of

reducing harmonics generation while in the design process is by phase multiplication

or harmonic cancellation. It is effective in reducing low order harmonics as long as the

load is balanced.

2.3.2 Passive filters: Improved power factor reduces high frequency harmonics. Large

tuning reactors are not used as instability may occur due to parallel resonance with the

source impedance. Performance depends upon source impedance; it cannot be

measured accurately and can vary with system changes. Hence, passive filters are not

appropriate for cycloconverters.

2.3.3 Active filters: With improved power factor, the output current can be

controlled. Active filters provide stable operation against AC source impedance

variation, and fast responsive irrespective of the order and magnitude of harmonics.

These filters are appropriate for cycloconverters. The initial and running costs are

usually higher than passive filters. The injection may flow into other components.

2.3.4 Harmonic injection: Harmonic injection takes care of uncharacteristic

harmonics. System impedance is not a part of the design criteria as it may give rise to

low order harmonics.

2.3.5 Harmonic mitigation techniques with PWM: harmonics can be reduced to

less than one per cent of the fundamental with the help of PWM; it is programmable

to eliminate specific harmonics. In addition to the above techniques, harmonics can be

reduced by a number of circuit techniques.

26

2.3.5.1 Inter harmonics

Inter harmonics are a type of waveform distortion that are usually the result of

a signal imposed on the supply voltage by electrical equipment such as static

frequency converters, induction motors and arcing devices. Cycloconverters (which

control large linear motors used in rolling mill, cement, and mining equipment), create

some of the most significant interharmonic supply power problems. These devices

transform the supply voltage into an AC voltage of a frequency lower or higher than

that of the supply frequency.

The most noticeable effect of interharmonics is visual flickering of displays and

incandescent lights, as well as causing possible heat and communication interference.

Solutions to interharmonics include filters, UPS systems, and line conditioners.

2.3.5.2 Notching

Notching is a periodic voltage disturbance caused by electronic devices, such as

variable speed drives, light dimmers and arc welders under normal operation. This

problem could be described as a transient impulse problem, but because the notches

are periodic over each ½ cycle, notching is considered a waveform distortion problem.

The usual consequences of notching are system halts, data loss, and data transmission

problems.

One solution to notching is to move the load away from the equipment causing the

problem (if possible). UPSs and filter equipment are also viable solutions to notching

if equipment cannot be relocated.

2.3.5.3 Noise

Noise is unwanted voltage or current superimposed on the power system

voltage or current waveform. Noise can be generated by power electronic devices,

control circuits, arc welders, switching power supplies, radio transmitters and so on.

Poorly grounded sites make the system more susceptible to noise. Noise can cause

27

technical equipment problems such as data errors, equipment malfunction, long term

component failure, hard disk failure, and distorted video displays.

There are many different approaches to controlling noise and sometimes it is

necessary to use several different techniques together to achieve the required result.

Some methods are:

· Isolate the load via a UPS.

· Install a grounded, shielded isolation transformer.

· Relocate the load away from the interference source.

· Install noise filters.

· Cable shielding.

2.3.5.4Voltage Fluctuations

Since voltage fluctuations are fundamentally different from the rest of the

waveform anomalies, they are placed in their own category. A Voltage fluctuation is a

systematic variation of the voltage waveform or a series of random voltage changes,

of small dimensions, namely 95 to 105% of nominal at a low frequency, generally

below 25 Hz.

Any load exhibiting significant current variations can cause voltage fluctuations.

Arc furnaces are the most common cause of voltage fluctuation on the transmission

and distribution system. One symptom of this problem is flickering of incandescent

lamps. Removing the offending load, relocating the sensitive equipment, or installing

power line conditioning or UPS devices, are methods to resolve this problem.

2.3.5.5 Frequency Variations

Frequency variation (Figure 18) is extremely rare in stable utility power systems,

especially systems interconnected via a power grid. Where sites have dedicated

standby generators or poor power infrastructure, frequency variation is more common

especially if the generator is heavily loaded. IT equipment is frequency tolerant, and

28

generally not affected by minor shifts in local generator frequency. What would be

affected would be any motor device or sensitive device that relies on steady regular

cycling of power over time. Frequency variations may cause a motor to run faster or

slower to match the frequency of the input power. This would cause the motor to run

inefficiently and/or lead to added heat and degradation of the motor through increased

motor speed and/or additional current draw.

To correct this problem, all generated power sources and other power sources causing

the frequency variation should be assessed, then repaired, corrected, or replaced.

2.3.5.6 Voltage Imbalance

A voltage imbalance is not a type of waveform distortion. a voltage imbalance

(as the name implies) is when supplied voltages are not equal. While these problems

can be caused by external utility supply, the common source of voltage imbalances is

internal, and caused by facility loads. More specifically, this is known to occur in

three phase power distribution system where one of the legs is supplying power to

single phase equipment, while the system is also supplying power to three phase

loads.

A quick way to assess the state of voltage imbalance is to take the difference

between the highest and the lowest voltages of the three supply voltages. This number

should not exceed 4% of the lowest supply voltage. Below is an example of this quick

way to get a simple assessment of the voltage imbalance in a system.

· Solutions to Power Quality Problems

There are two approaches to the mitigation of power quality problems. The

solution to the power quality can be done from customer side or from utility side. First

approach is called load conditioning, which ensures that the equipment is less

sensitive to power disturbances, allowing the operation even under significant voltage

29

distortion. The other solution is to install line conditioning systems that suppress or

counteracts the power system disturbances. A flexible and versatile solution to voltage

quality problems is offered by active power filters. Currently they are based on PWM

converters and connect to low and medium voltage distribution system in shunt or in

series. Series active power filters must operate in conjunction with shunt passive

filters in ord er to compensate load current harmonics. Shunt active power filters

operate as a controllable current source and series active power filters operates as a

controllable voltage source. Both schemes are implemented preferable with voltage

source PWM inverters, with a dc bus having a reactive element such as a capacitor.

Active power filters can perform one or more of the functions required to compensate

power systems and improving power quality. Their performance also depends on the

power rating and the speed of response.

Solutions will play a major role in improving the inherent supply quality; some of the

effective and economic measures can be identified as following:

· Lightening and Surge Arresters:

Arresters are designed for lightening protection of transformers, but are not

sufficiently voltage limiting for protecting sensitive electronic con trol circuits from

voltage surges.

· Thyristor Based Static Switches:

The static switch is a versatile device for switching a new element into the circuit

when the voltage support is needed. It has a dynamic response time of about one

cycle. To correct quickly for voltage spikes, sags or interruptions, the static switch can

used to switch one or more of devices such as capacitor, filter, alternate power line,

energy storage systems etc. The static switch can be used in the alternate power line

applications. This scheme requires two independent power lines from the utility or

could be from utility and localized power generation like those in case of distributed

generating systems. Such a scheme can protect up to about 85 % of interruptions and

voltage sags.

Energy Storage Systems:

Storage systems can be used to protect sensitive production equipments from

30

shutdowns caused by voltage sags or momentary interruptions. These are usually DC

storage systems such as UPS, batteries, superconducting magnet energy storage

(SMES), storage capacitors or even fly wheels driving DC generators. The output of

these devices can be supplied to the system through an inverter on a momentary basis

by a fast acting electronic switch. Enough energy is fed to the system to compensate

for the energy that would be lost by the voltage sag or interruption. In case of utility

supply backed by a localized generation this can be even better accomplished.

· Electronic tap changing transformer:

A voltage-regulating transformer with an electronic load tap changer can be used

with a single line from the utility. It can regulate the voltage drops up to 50% and

requires a stiff system (short circuit power to load ratio of 10:1 or better). It can have

the provision of coarse or smooth steps intended for occasional voltage variations.

· Harmonic Filters

Filters are used in some instances to effectively reduce or eliminate certain

harmonics. If possible, it is always preferable to use a 12-pluse or higher transformer

connection, rather than a fil ter. Tuned harmonic filters should be used with caution

and avoided when possible. Usually, multiple filters are needed, each tuned to a

separate harmonic. Each filter causes a parallel resonance as well as a series

resonance, and each filter slightly changes the resonances of other filters.

· Constant-Voltage Transformers:

For many power quality studies, it is possible to greatly improve the sag and

momentary interruption tolerance of a facility by protecting control circuits. Constant

voltage transformer (CVTs) can be used on control circuits to provide constant

voltage with three cycle ride through, or relays and ac contactors can be provided with

electronic coil hold-in devices to prevent mis-operation from either low or interrupted

voltage.

· Digital-Electronic and Intelligent Controllers for Load-Frequency Control:

Frequency of the supply power is one of the major determinants of power quality,

which affects the equipment performance very drastically. Even the major system

31

components such as Turbine life and interconnected-grid control are directly affected

by power frequency. Load frequency controller used specifically for governing power

frequency under varying loads must be fast enough to make adjustments against any

deviation. In countries like India and other countries of developing world, still use the

controllers which are based either or mechanical or electrical devices with inherent

dead time and delays and at times also suffer from ageing and associated effects. In

future perspective, such cont rollers can be replaced by their Digital -electronic

counterparts.

· Use of Custom Power Devices to Improve Power Quality

In order to overcome the problems such as the ones mentioned above, the

concept of custom power devices is introduced recently; custom power is a strategy,

which is designed primarily to meet the requirements of industrial and commercial

customer. The concept of custom power is to use power electronic or static controllers

in the medium voltage distribution system aiming to supply reliable and high quality

power to sensitive users. Power electronic valves are the basis of those custom power

devices such as the static transfer switch, active filters and converter-based devices.

Converter based power electronics devices can be divided in to two groups: shunt-

connected and series-connected devices. The shunt connected devices is known as the

DSTATCOM and the series device is known as the Static Series Compensator (SSC),

commercially known as DVR. It has also been reported in literature that both the SSC

and DSTATCOM have been used to mitigate the majority the power system

disturbances such as voltage dips, sags, flicker unbalance and harmonics.

32

2.6 Conclusion

For lower voltage sags, the load voltage magnitude can be corrected by

injecting only reactive power into the system. However, for higher voltage sags,

injection of active power, in addition to reactive power, is essential to correct the

voltage magnitude. Both DVR and DSTATCOM are capable of generating or

absorbing reactive power but the active power injection of the device must be

provided by an external energy source or energy storage system. The response time of

both DVR and DSTATCOM is very short and is limited by the power electronics

devices. The expected response time is about 25 ms, and which is much less than

some of the traditional methods of voltage correction such as tap - changing

transformers.

2.7 Summary:

· The IEEE defined power quality disturbances shown in this paper have been

organized into seven categories based on wave shape.

· A flexible and versatile solution to voltage quality problems is offered by

active power filters.

· Solutions will play a major role in improving the inherent supply quality; some

of the effective and economic measures can be identified in chapter.

· The concept of custom power is to use power electronic or static controllers in

the medium voltage distribution system aiming to supply reliable and high

quality power to sensitive users.

·

33

3.1 Introduction

ELECTRIC utilities and end users of electric power are becoming increasingly

concerned about meeting the growing energy demand. Seventy five percent of total

global energy demand is supplied by the burning of fossil fuels. But increasing air

pollution, global warming concerns, diminishing fossil fuels and their increasing cost

have made it necessary to look towards renewable sources as a future energy solution.

Since the past decade, there has been an enormous interest in many countries on

renewable energy for power generation. The market liberalization and government’s

incentives have further accelerated the renewable energy sector growth. Renewable

energy source (RES) integrated at distribution level is termed as distributed generation

(DG). The utility is concerned due to the high penetration level of intermittent RES

in distribution systems as it may pose a threat to network in terms of stability, voltage

regulation and power-quality (PQ) issues. Therefore, the DG systems are required to

comply with strict technical and regulatory frameworks to ensure safe, reliable and

efficient operation of overall network. With the advancement in power electronics and

digital control technology, the DG systems can now be actively controlled to enhance

the system operation with improved PQ at PCC. However, the extensive use of power

electronics based equipment and non-linear loads at PCC generate harmonic currents,

which may deteriorate the quality of power. Generally, current controlled voltage

source inverters are used to interface the intermittent RES in distributed system.

Recently, a few control strategies for grid connected inverters incorporating PQ

solution have been proposed. In an inverter operates as active inductor at a certain

frequency to absorb the harmonic current. But the exact calculation of network

inductance in real-time is difficult and may deteriorate the control performance. A

similar approach in which a shunt active filter acts as active conductance to damp out

the harmonics in distribution network is proposed in a control strategy for renewable

interfacing inverter based on – theory is proposed. In this strategy both load and

inverter current sensing is required to compensate the load current harmonics. The

non-linear load current harmonics may result in voltage harmonics and can create a

serious PQ problem in the power system network. Active power filters (APF) are

extensively used to compensate the load current harmonics and load unbalance at

34

distribution level. This results in an additional hardware cost.

However, in this paper authors have incorporated the features of APF in the,

conventional inverter interfacing renewable with the grid, without any additional

hardware cost. Here, the main idea is the maximum utilization of inverter rating which

is most of the time underutilized due to intermittent nature of RES. It is shown in this

paper that the grid-interfacing inverter can effectively be utilized to perform following

important functions: 1) transfer of active power harvested from the renewable

resources (wind, solar, etc.); 2) load reactive power demand support; 3) current

harmonics compensation at PCC; and 4) current unbalance and neutral current

compensation in case of 3-phase 4-wire system. Moreover, with adequate control of

grid-interfacing inverter, all the four objectives can be accomplished either

individually or simultaneously. The PQ constraints at the PCC can therefore be strictly

maintained within the utility standards without additional hardware cost.

Fig. 3.1: Schematic of proposed renewable based distributed generation system.

35

3.2 SYSTEM DESCRIPTION

The proposed system consists of RES connected to the dc-link of a grid-

interfacing inverter as shown in Fig. 1. The voltage source inverter is a key element of

a DG system as it interfaces the renewable energy source to the grid and delivers the

generated power. The RES may be a DC source or an AC source with rectifier

coupled to dc-link. Usually, the fuel cell and photovoltaic energy sources generate

power at variable low dc voltage, while the variable speed wind turbines generate

power at variable ac voltage. Thus, the power generated from these renewable sources

needs power conditioning (i.e., dc/dc or ac/dc) before connecting on dc-link. The dc-

capacitor decouples the RES from grid and also allows independent control of

converters on either side of dc-link.

3.2.1 DC-Link Voltage and Power Control Operation

Due to the intermittent nature of RES, the generated power is of variable

nature. The dc-link plays an important role in transferring this variable power from

renewable energy source to the grid. RES are represented as current sources connected

to the dc-link of a grid-interfacing inverter. Fig. 3.2 shows the systematic

representation of power transfer from the renewable energy resources to the grid via

the dc-link. The current injected by renewable into dc-link at voltage level Vdc can be

given as

Where PRES is the power generated from RES.

36

Fig. 3.2: DC-Link equivalent diagram

The current flow on the other side of dc-link can be represented

as,

where P inv, PG, and Ploss are total power available at grid-interfacing inverter side,

active power supplied to the grid and inverter losses, respectively. If inverter losses

are negligible then P RES = PG.

3.2.2 Control of Grid Interfacing Inverter

The control diagram of grid- interfacing inverter for a 3-phase 4-wire system is shown

in Fig. 3. The fourth leg of inverter is used to compensate the neutral current of load.

The main aim of proposed approach is to regulate the power at PCC during: 1) P RES =

0 ; 2) P RES < total load power (PL); and 3) P RES> PL

While performing the power management operation, the inverter is actively controlled

in such a way that it always draws/ supplies fundamental active power from/ to the

grid. If the load connected to the PCC is non-linear or unbalanced or the combination

37

of both, the given control approach also compensates the harmonics, unbalance, and

neutral current.

The duty ratio of inverter switches are varied in a power cycle such that the

combination of load and inverter injected power

38

Fig. 3.3: Block diagram representation of grid-interfacing inverter control.

appears as balanced resistive load to the grid. The regulation of dc-link voltage carries

the information regarding the exchange of active power in between renewable source

and grid. Thus the output of dc-link voltage regulator results in an active current (Im).

The multiplication of active current component (Im) with unity grid voltage vector

templates Ua, Ub, Uc generates the reference grid currents(Ia*, Ib

*, Ic*). The reference

grid neutral current (In*)is set to zero, being the instantaneous sum of balanced grid

currents. The grid synchronizing angle obtained from phase locked loop (PLL) is used

to generate unity vector template as

The difference of this filtered dc-link voltage and reference dc-link voltage is given to

a discrete.PI regulator to maintain a constant dc-link voltage under varying generation

and load conditions. The dc-link voltage error at th sampling instant is given as:

3.3 Conclusion

The actual dc-link voltage is sensed and passed through a first-order low pass filter

(LPF) to eliminate the presence of switching ripples on the dc-link voltage and in the

generated

reference current signals.

3.4 Summary

· Diminishing fossil fuels and their increasing cost have made it necessary to

look towards renewable sources as a future energy solution.

39

· The dc-link plays an important role in transferring this variable power from

renewable energy source to the grid.

40

4.1 Introduction

MATLAB is a high-performance language for technical computing. It

integrates computation, visualization, and programming in an easy-to-use

environment where problems and solutions are expressed in familiar mathematical

notation. Typical uses include-

· Math and computation

· Algorithm development

· Data acquisition

· Modeling, simulation, and prototyping

· Data analysis, exploration, and visualization

· Scientific and engineering graphics

MATLAB is an interactive system whose basic data element is an array that

does not require dimensioning. This allows solving many technical computing

problems, especially those with matrix and vector formulations, in a fraction of the

time it would take to write a program in a scalar non-interactive language such as C or

FORTRAN.

4.2 Main parts of MATLAB

The MATLAB system consists of six main parts:

4.2.1 Development Environment

This is the set of tools and facilities that help to use MATLAB functions and

files. Many of these tools are graphical user interfaces. It includes the MATLAB

desktop and Command Window, a command history, an editor and debugger, and

browsers for viewing help, the workspace, files, and the search path.

41

4.2.2 The MATLAB Mathematical Function Library

This is a vast collection of computational algorithms ranging from elementary

functions, like sum, sine, cosine, and complex arithmetic, to more sophisticated

functions like matrix inverse, matrix Eigen values, Bessel functions, and fast Fourier

transforms.

4.2.3 The MATLAB Language

This is a high-level matrix/array language with control flow statements,

functions, data structures, input/output, and object-oriented programming features. It

allows both "programming in the small" to rapidly create quick and dirty throw-away

programs, and "programming in the large" to create large and complex application

programs.

4.2.4 Graphics

MATLAB has extensive facilities for displaying vectors and matrices as

graphs, as well as annotating and printing these graphs. It includes high-level

functions for two-dimensional and three-dimensional data visualization, image

processing, animation, and presentation graphics. It also includes low-level functions

that allow to fully customize the appearance of graphics as well as to build complete

graphical user interfaces on MATLAB applications.

4.2.5 The MATLAB Application Program Interface (API)

This is a library that allows writing in C and FORTRAN programs that interact

with MATLAB. It includes facilities for calling routines from MATLAB (dynamic

linking), calling MATLAB as a computational engine, and for reading and writing

MAT-files.

4.2.6 MATLAB Documentation

42

MATLAB provides extensive documentation, in both printed and online

format, to help to learn about and use all of its features. It covers all the primary

MATLAB features at a high level, including many examples. The MATLAB online

help provides task-oriented and reference information about MATLAB features.

MATLAB documentation is also available in printed form and in PDF format.

4.2.7 Mat lab tools

4.2.7.1 Three phase source block

Fig 4.1: Three phase source

The Three-Phase Source block implements a balanced three-phase voltage

source with internal R-L impedance. The three voltage sources are connected in Y

with a neutral connection that can be internally ground.

4.2.7.2 VI measurement block

The Three-Phase V-I Measurement block is used to measure three-phase

voltages and currents in a circuit. When connected in series with three-phase

elements, it returns the three phase-to-ground or phase-to-phase voltages and the three

line currents

Fig 4.2: VI measurement block

43

4.2.7.3 Scope

Display signals generated during a simulation. The Scope block displays its

input with respect to simulation time. The Scope block can have multiple axes (one

per port); all axes have a common time range with independent y-axes. The Scope

allows you to adjust the amount of time and the range of input values displayed. You

can move and resize the Scope window and you can modify the Scope's parameter

values during the simulation

4.2.7.4 Three-Phase Series RLC Load

The Three-Phase Series RLC Load block implements a three-phase

balanced load as a series combination of RLC elements. At the specified frequency,

the load exhibits constant impedance. The active and reactive powers absorbed by the

load are proportional to the square of the applied voltage.

Fig 4.3: Three-Phase Series RLC Load

44

4.2.7.5 Three-Phase Breaker block

The Three-Phase Breaker block implements a three-phase circuit breaker

where the opening and closing times can be controlled either from an external

Simulink signal or from an internal control signal.

Fig 4.4: Three-Phase Breaker block

4.2.7.6 Gain block

Fig 4.5: Gain block

The Gain block multiplies the input by a constant value (gain). The input and the gain

can each be a scalar, vector, or matrix.

45

4.3 MATLAB/Simulink Results:

46

Fig 4.6: Schematic Diagram

47

48

Fig 4.7: 3-phase Source Active and Re-active powers

49

50

51

Fig 4.8: 3-phase Source Active and Load Re-active powers

52

Fig 4.9: 3-Phase Source active and Inverter Re-active powers

53

54

55

Fig 4.10: Measurments of Source currents,Load current,Inverter

currents,Source voltage

56

Fig 4.11: Harmonics in the Load Currents

57

58

Fig 4.12: Power Factor

59

Fig 4.13: Source Currents

60

Fig 4.14: Total Harmonic Distortion of Source currents

61

Conclusion

This paper has presented a novel control of an existing grid interfacing inverter to

improve the quality of power at PCC for a 3-phase 4-wire DG system. It has been

shown that the grid-interfacing inverter can be effectively utilized for power

conditioning without affecting its normal operation of real power transfer. The grid-

interfacing inverter with the proposed approach can be utilized to:

· Inject real power generated from RES to the grid, and/or,

· Operate as a shunt Active Power Filter (APF).

This approach thus eliminates the need for additional power conditioning

equipment to improve the quality of power at PCC. Extensive MATLAB/Simulink

simulation as well as the DSP based experimental results have validated the proposed

approach and have shown that the grid-interfacing inverter can be utilized as a multi-

function device.

The current unbalance, current harmonics and load reactive power, due to

unbalanced and non-linear load connected to the PCC, are compensated effectively

such that the grid side currents are always maintained as balanced and sinusoidal at

unity power factor. Moreover, the load neutral current is prevented from flowing into

the grid side by compensating it locally from the fourth leg of inverter. When the

power generated from RES is more than the total load power demand, the grid-

interfacing inverter with the proposed control approach not only fulfills the total load

active and reactive power demand (with harmonic compensation) but also delivers the

excess generated sinusoidal active power to the grid at unity power factor.

62

REFERENCES:

[1] J. M. Guerrero, L. G. de Vicuna, J. Matas, M. Castilla, and J. Miret, “A wireless

controller to enhance dynamic performance of parallel inverters in distributed

generation systems,” IEEE Trans. Power Electron., vol. 19, no. 5, pp. 1205–1213,

Sep. 2004.

[2] J. H. R. Enslin and P. J. M. Heskes, “Harmonic interaction between a large

number of distributed power inverters and the distribution network,” IEEE Trans.

Power Electron., vol. 19, no. 6, pp. 1586–1593, Nov. 2004.

[3] U. Borup, F. Blaabjerg, and P. N. Enjeti, “Sharing of nonlinear load in parallel-

connected three-phase converters,” IEEE Trans. Ind. Appl., vol. 37, no. 6, pp. 1817–

1823, Nov./Dec. 2001.

[4] P. Jintakosonwit, H. Fujita, H. Akagi, and S. Ogasawara, “Implementation and

performance of cooperative control of shunt active filters for harmonic damping

throughout a power distribution system,” IEEE Trans. Ind. Appl., vol. 39, no. 2, pp.

556–564, Mar./Apr. 2003.

[5] J. P. Pinto, R. Pregitzer, L. F. C. Monteiro, and J. L. Afonso, “3-phase 4-wire

shunt active power filter with renewable energy interface,” presented at the Conf.

IEEE Rnewable Energy & Power Quality, Seville, Spain, 2007.

[6] F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control

and grid synchronization for distributed power generation systems,” IEEE Trans. Ind.

Electron., vol. 53, no. 5, pp. 1398–1409, Oct. 2006.

[7] J. M. Carrasco, L. G. Franquelo, J. T. Bialasiewicz, E. Galván, R. C. P. Guisado,

M. Á. M. Prats, J. I. León, and N. M. Alfonso, “Powerelectronic systems for the grid

integration of renewable energy sources: A survey,” IEEE Trans. Ind. Electron., vol.

53, no. 4, pp. 1002–1016,

Aug. 2006.

[8] B. Renders, K. De Gusseme, W. R. Ryckaert, K. Stockman, L. Vandevelde, and

M. H. J. Bollen, “Distributed generation for mitigating voltage dips in low-voltage

distribution grids,” IEEE Trans. Power. Del., vol. 23, no. 3, pp. 1581–1588, Jul. 2008.

[9] V. Khadkikar, A. Chandra, A. O. Barry, and T. D. Nguyen, “Application of UPQC

to protect a sensitive load on a polluted distribution network,” in Proc. Annu. Conf.

IEEE Power Eng. Soc. Gen. Meeting, 2006, pp. 867–872.

[10] M. Singh and A. Chandra, “Power maximization and voltage sag/swell ride-

63

through capability of PMSG based variable speed wind energy conversion system,” in

Proc. IEEE 34th Annu. Conf. Indus. Electron. Soc., 2008, pp. 2206–2211.

[11] P. Rodríguez, J. Pou, J. Bergas, J. I. Candela, R. P. Burgos, and D. Boroyevich,

“Decoupled double synchronous reference frame PLL for power converters control,”

IEEE Trans. Power Electron, vol. 22, no. 2, pp. 584–592, Mar. 2007.

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