Page 1
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
1
Page 2
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
2
Page 3
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
3
Page 4
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
4
Page 5
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.
5
Page 6
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.
6
Page 7
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
7
Page 8
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
8
Page 9
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
9
Page 10
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
10
Page 11
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
11
Page 12
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.
12
Page 13
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
13
Page 14
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
14
Page 15
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
15
Page 16
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,
16
Page 17
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
17
Page 18
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:
18
Page 19
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
19
Page 20
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.
20
Page 21
· 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.
21
Page 22
· 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.
22
Page 23
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
Page 24
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
Page 25
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
Page 26
· 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
Page 27
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
Page 28
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
Page 29
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
Page 30
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
Page 31
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
Page 32
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
Page 33
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
Page 34
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
Page 35
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
Page 36
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
Page 37
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
Page 38
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
Page 39
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
Page 40
· The dc-link plays an important role in transferring this variable power from
renewable energy source to the grid.
40
Page 41
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
Page 42
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
Page 43
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
Page 44
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
Page 45
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
Page 46
4.3 MATLAB/Simulink Results:
46
Page 47
Fig 4.6: Schematic Diagram
47
Page 49
Fig 4.7: 3-phase Source Active and Re-active powers
49
Page 52
Fig 4.8: 3-phase Source Active and Load Re-active powers
52
Page 53
Fig 4.9: 3-Phase Source active and Inverter Re-active powers
53
Page 56
Fig 4.10: Measurments of Source currents,Load current,Inverter
currents,Source voltage
56
Page 57
Fig 4.11: Harmonics in the Load Currents
57
Page 59
Fig 4.12: Power Factor
59
Page 60
Fig 4.13: Source Currents
60
Page 61
Fig 4.14: Total Harmonic Distortion of Source currents
61
Page 62
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
Page 63
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
Page 64
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.
64