8/2/2019 Enhancement of Power Quality Using Dstatcom
1/51
CHAPTER-1
INTRODUCTIONINTRODUCTION
OVERVIEW:
The electric power system is considered to be composed of three
functional blocks - generation, transmission and distribution. For a reliable
power system, the generation unit must produce adequate power to meet
customers demand, transmission systems must transport bulk produce adequate
power to meet power over long distances without overloading system stability
and distribution systems must deliver electric power to each customers
premises from bulk power systems
Distribution system locates the end of power system and is connected to
the customer directly, so the power quality mainly depends on distribution
system. The reason behind this is that the electrical distribution network failures
account for about 90% of the average customer interruptions. In the earlier days,
the major focus for power system reliability was on generation and transmissiononly as these more capital cost is involved in these. In addition their
insufficiency can cause widespread catastrophic consequences for both society
and its environment.
But now a days distribution systems have begun to receive more
attention for reliability assessment. Initially for the improvement of power
quality or reliability of the system FACTS devices like static synchronous
compensator (STATCOM), static synchronous series compensator SSSC),
interline power flow controller (IPFC), and unified power flow controller
(UPFC) etc are introduced.
These FACTS devices are designed for the transmission system. But
now a days more attention is on the distribution system for the improvement of
power quality, these devices are modified and known as custom power devices.
The main custom power devices which are used in distribution system for
1
8/2/2019 Enhancement of Power Quality Using Dstatcom
2/51
power quality improvement are distribution static synchronous compensator
(DSTATCOM), dynamic voltage Restorer (DVR), active filter (AF), unified
power quality conditioner (UPQC) etc.
In this from the above custom power devices, D-Statcom and
D-statcom with LCL passive filter is used to reduce voltage sag and total
harmonic distortions.
2.1..LITERATURE SURVEY:
Power Quality in electric networks is one of today's most concerned
areas of electric power system. The power quality has serious economic
implications for consumers, utilities and electrical equipment manufacturers.
The impact of power quality problems is increasingly felt by customers -
industrial, commercial and even residential. Some of the main power quality
problems are sag, swell, transients, harmonic, and flickers etc
By custom power devices, we refer to power electronic static controllers
used for power quality improvement on distribution systems rated from 1 to 38
kV . This interest in the practice of power quality devices (PQDs) arises from
the need of growing power quality levels to meet the everyday growing
sensitivity of customer needs and expectations. One of those devices is the
Distribution Static Compensator (DSTATCOM), which is the most efficient and
effective modern custom power device used in power distribution networks. Its
application includes lower cost, smaller size, and its fast dynamic response to
the disturbance.
Several research papers and reports addressed the subject of improving
power quality in distribution system by the use of custom power devices. The
followings present a brief review of the work undertaken so far
N.G. Hingorani, [1] presents the concept of custom power is now becoming
familiar. The term describes the value-added power that electric utilities and
other service providers will offer their customers in the future. The enhanced
2
8/2/2019 Enhancement of Power Quality Using Dstatcom
3/51
level of reliability of this power, in terms of reduced interruptions and less
variation, will stem from an integrated solution to present problems, of which a
prominent feature will be the application of power electronic controllers to
utility distribution systems and/or at the supply end of many industrial and
commercial customers and industrial parks.
Yash Pal, A. Swarup, et al. [2] presents a comprehensive review of
compensating custom power devices mainly DSTATCOM (distribution static
compensator), DVR (dynamic voltage restorer) and UPQC (unified power
quality compensator). It is aimed at providing a broad viewpoint on the status of
compensating devices in electric power distribution system to researchers and
application engineers dealing with power quality problems. is highly required to
increase the reliability of the distribution system
2.2. SCOPE OF WORK :
From the literature review, it is observed that the work on the
investigation on power with compensating devices is very much diversified.
However it is observed that there is a scope to investigate the effectiveness of
compensating devices for different loads and with different depends on
distribution system. As the customers demand for the reliability of power
supply is increasing day by day, so the reliability of the distribution system has
to be increased. Electrical distribution network failures account for about 90%
of the average customer interruptions. So it is highly required to increase the
reliability of the distribution system.
The objective of the proposed work is to improve the power quality or
reliability in the distribution system with the use of custom power device.
Different conditions are considered to analyze the operation of D-Statcom for
the improvement the power quality in distribution system.
3
8/2/2019 Enhancement of Power Quality Using Dstatcom
4/51
CHAPTER-2
POWER QUALITY
3.1.INTRODUCTION
Power quality is The provision of voltages and system design so that
the user of electric power can utilize electric energy from the distribution
system successfully without interference or interruption. A broad definition of
power quality borders on system reliability, dielectric selection on equipment
and conductors, long-term outages, voltage unbalance in three-phase systems,power electronics and their interface with the electric power supply and many
other areas.
3.2 POWER QUALITY- A BIG ISSUE
Power quality in electric networks is one of today's most concerned
areas of electric power system. The power quality has serious economic
implications for consumers, utilities and electrical equipment manufacturers.
Modernization and automation of industry involves increasing use of
computers, microprocessors and power electronic systems such as adjustable
speed drives. Integration of non-conventional generation technologies such as
fuel cells, wind turbines and photo-voltaic with utility grids often requires
power electronic interfaces.
The power electronic systems also contribute to power quality problems
(generating harmonics). Under the deregulated environment, in which electric
utilities are expected to compete with each other, the customer satisfaction
becomes very important. The impact of power quality problems is increasingly
felt by customers - industrial, commercial and even residential.
4
8/2/2019 Enhancement of Power Quality Using Dstatcom
5/51
3.3 PROBLEMS ASSOCIATED WITH POWER QUALITY
3.3.1 MOMENTARY PHENOMENA
3.3.1.1 Transients:
Transients are unwanted decay with time and hence not a steady state
problem. A broad definition is that a transient is that part of the change in a
variable that disappears during transition from one steady state operating
situation to the other". Another synonymous term which can be used is surge.
3.3.1.2 Long Duration Voltage Variations:
When rms (root mean square) deviations at power frequency last longer
than one minute, then we say they are long duration voltage variations. They
can be either over voltages which is greater than 1.1p.u or under voltages which
is less than 0.9p.u.
Over voltage is due to switching off a load or energizing a capacitor
bank. Also incorrect tap settings on transformers can result in over voltages.
Under voltage are the results of actions which are the reverse of events that
cause over voltages i.e. switching in a load or switching off a capacitor bank.
3.3.1.3 Sustained Interruptions:
If the supply voltage becomes zero for a period of time which is greater
than one minute, then we can say that it is a sustained interruption. Normally,
voltage interruption lasting for more than one minute is often unending and
requires human intervention to restore the supply. The term outage is also
used for long interruption. However it does not bring out the true impact of the
power interruption. Even an interruption of half a cycle can be disastrous for a
customer with a sensitive load.
3.3.1.4 SHORT DURATION VOLTAGE VARIATIONS:
The short duration voltage variations are generally caused by fault
conditions like single line to ground or double line to ground and starting of
large loads such as induction motors. The voltage variations can be temporary
5
8/2/2019 Enhancement of Power Quality Using Dstatcom
6/51
voltage dips i.e. sag or temporary voltage rise i.e. swells or a absolute loss of
voltage which is known as interruptions .
Voltage Sags:
Voltage sag is defined as the reduction of rms voltage to a value
between 0.1 and 0.9p.u and lasting for duration between 0.5 cycle to 1 minute.
Voltage sags are mostly caused by system faults and last for durations ranging
from 3 cycles to 30 cycles depending on the fault clearing time. It is to be noted
that under-voltages (lasting over a minute) can be handled by voltage
regulation equipment. Starting of large induction motors can result in voltage
dip as the motor draws a current up to 10 times the full load current during the
starting. Also, the power factor of the starting current is generally poor.
VOLTAGE SWELLS:
A voltage swell is defined as a raise in rms voltage which is between 1.1
and 1.8p.u for time duration between 0.5 cycles to 1 minute. A voltage swell is
characterized by its magnitude (rms) and duration. As with sag, swell is
associated with system faults. A SLG (single line to ground) fault can result in a
voltage swell in the healthy phases. Swell can also result from energizing a
large capacitor bank. On an ungrounded system, the line to ground voltages on
the ungrounded phases is 1.73p.u during a SLG fault. However in a grounded
system, there will be negligible voltage rise on the un faulted phases close to a
substation where the delta connected windings of the transformer provide low
impedance paths for the zero sequence current during the SLG fault.
INTERRUPTION:
If the supply voltage or load current decreases to less than 0.1 p.u for a
period of time not more than one minute is known as interruption. Interruption
can be caused either by system faults, equipment failures or control
malfunctions.
The interruptions are measured by their duration alone. The duration
due to a fault is determined by the operating time of the protective devices.
Duration of an interruption due to equipment malfunction can be irregular.
6
8/2/2019 Enhancement of Power Quality Using Dstatcom
7/51
Some interruptions may also be caused by voltage sag conditions when there are
faults on the source side.
3.3.2 STEADY STATE PHENOMENA
3.3.2.1 Waveform Distortion
This is defined as a steady-state deviation from an ideal sine wave of
power frequency.
There are five types of waveform distortion:
(a) DC offset
(b) Harmonics
(c) Inter harmonics
(d) Notching
(e) Noise
3.3.2.2 Voltage Imbalance:
Voltage imbalance can be defined using symmetrical components. The
ratio of the negative sequence or zero sequence component to the positive
sequence component is a measure of unbalance. The main cause of voltage
unbalance is single phase loads on a three phase circuit which resulting in load
imbalance. Severe imbalance can be caused by single-phasing conditions in the
system.
3.3.3 VOLTAGE FLUCTUATIONS AND FLICKER:
Voltage fluctuations are systematic variations of the voltage or a series
of random changes in the voltage magnitude which lies in the range of 0.9 to
1.1p.u. High power loads that draw fluctuating current, such as large motor
drives and arc furnaces, cause low frequency cyclic voltage variations that result
in flickering of light sources like incandescent and fluorescent lamps which can
cause significant physiological discomfort or irritation in human beings.
The voltage flicker can also affect stable operation of electrical and
electronic devices such as motors and CRT devices. The typical frequency
spectrum of voltage flicker lies in the range from 1 Hz to 30 Hz.
7
8/2/2019 Enhancement of Power Quality Using Dstatcom
8/51
3.3.4 POWER FREQUENCY VARIATIONS:
Power frequency variations are defined as the deviations of the system
frequency from its particular value of 50 or 60 Hz. The variations in the
frequency begin from the changes in the load and the response of the generators
to meet the load. Thus the load characteristics which dependence on the
frequency and the control characteristics of the generators change the shift in
the frequency.
In current interconnected power systems, frequency variations are
insignificant most of the time unless governor and load frequency controls are
disabled under a system of power shortages and a lack of grid discipline.
Profitable incentives or disincentives that ensure balance between existing
generation and load may help control over frequency variations under normal
operating conditions.
3.4 SOLUTION OF POWER QUALITY PROBLEMS:
For the improvement of power quality there are two approaches.
According to first approach the solution to the power quality problems can be
done from the utility side. The first approach is called load conditioning, which
ensures that the equipment is less sensitive to power disturbances, allowing the
operation even under significant voltage distortion. The other solution is to
install line conditioning systems that suppress the power system disturbances.
In this approach the compensating device is connect to low and medium
voltage distribution system in shunt or in series. 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 source having a reactive element such
as a capacitor.
However, with the restructuring of power sector and with shifting trend
towards distributed and dispersed generation, the line conditioning systems or
utility side solutions will play a major role in improving the inherent supply
quality; some of the effective and economic measures can be identified as
following:
8
8/2/2019 Enhancement of Power Quality Using Dstatcom
9/51
3.4.1 THYRISTOR BASED STATIC SWITCHES:
The static switch is a versatile device for switching a novel element into
the circuit when the voltage support is desired. It has a dynamic response time
of about one cycle. To correct rapidly for voltage spikes, sags or interruptions,
such 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.
3.4.2 ENERGY STORAGE SYSTEMS:
Storage systems can be used to protect sensitive production equipments
from shutdown which is caused by voltage sag or temporary interruptions.
These are generally DC storage systems such as UPS, batteries,
superconducting magnet energy storage (SMES), storage capacitors or even fly
wheels driving DC generators are used. The output of these devices can be
supplied to the system through an inverter on a momentary basis by a fast
performing electronic switch like GTO or IGBT etc. Sufficient energy is fed to
the system to compensate for the energy that would be lost by the fault
conditions like voltage sag or interruption.
However there are many different methods to mitigate voltage sags and
swells, but the use of a custom Power device is considered to be the most
efficient method. Flexible AC Transmission Systems (FACTS) for transmission
systems, the term custom power pertains to the use of power electronics
controllers in a distribution system, particularly, to deal with a variety of power
quality problems. Just as FACTS improves the power transfer capabilities and
stability limits, custom power makes sure customers get pre-specified quality
and reliability of supply.
There are many types of Custom Power devices like Active Power
Filters (APF), Battery Energy Storage Systems (BESS), Distribution static
synchronous compensators (DSTATCOM), Dynamic Voltage Restorer (DVR),
Surge Arresters (SA), Super conducting Magnetic Energy Systems (SMES),
Static Electronic Tap Changers (SETC), Solid-State Transfer Switches (SSTS),
9
8/2/2019 Enhancement of Power Quality Using Dstatcom
10/51
Solid State Fault Current Limiter (SSFCL), and unified power quality
conditioner (UPQC).
CHAPTER-4CHAPTER-4
FACTS
4.1 INTRODUCTION:
10
8/2/2019 Enhancement of Power Quality Using Dstatcom
11/51
4.2 NEED OF CUSTOM POWER DEVICES:
Power quality is one of major concerns in the present era. Distribution
system locates the end of power system and is connected to the customer
directly, so the reliability of power supply mainly depends on distribution
system. It has become important, especially, with the introduction of
sophisticated devices, whose performance is very sensitive to the quality of
power supply.
Power quality problem is an occurrence manifested as a nonstandard
voltage, current or frequency that results in a failure of end use equipments. The
electrical distribution network failures account for about 90% of the average
customer interruptions. As the customers demand for the reliability of power
supply is increasing day by day, so the reliability of the distribution system has
to be increased. One of the major problems dealt here is the power sag.
Power distribution systems, ideally, should provide their customers with
an uninterrupted flow of energy at smooth sinusoidal voltage at the contracted
magnitude level and frequency. However, in practice, power systems, especially
the distribution system, have numerous nonlinear loads,which significantly
affect the quality of power supplies.
As a result of the nonlinear loads, the purity of the waveform of supplies
is lost. This ends up producing many power quality problems. While power
disturbances occur on all electrical systems, the sensitivity of todays
sophisticated electronic devices makes them more disposed to the quality of
power supply.
For some sensitive devices, a temporary disturbance can cause
scrambled data, interrupted communications, a frozen mouse, system crashes
and equipment failure etc. A power voltage spike can damage valuable
components.
11
8/2/2019 Enhancement of Power Quality Using Dstatcom
12/51
To solve this problem, custom power devices are used. One of those
devices is the Distribution Static Compensator (DSTATCOM), which is the
most efficient and effective modern custom power device used in power
distribution networks. Its appeal includes lower cost, smaller size, and its fast
dynamic response to the disturbance.
4.3 CONFIGURATIONS:
The compensating type custom power devices can be classified on the
basis of different topologies and the number of phases. For power quality
improvement the voltage source inverter (VSI) bridge structure is generally
used for the development of custom power devices, while the use of current
source inverter (CSI) is less reported. The topology can be shunt
(DSTATCOM), series (DVR), or a combination of both (UPQC).
CHAPTER-5
D-STATCOM
5.1 INTRODUCTION:
12
8/2/2019 Enhancement of Power Quality Using Dstatcom
13/51
Among the power quality problems like sag, swell, harmonic etc,
voltage sag is the most severe disturbances in the distribution system. To
overcome these problems the concept of custom power devices is introduced
lately. One of those devices is the DSATCOM, which is the most efficient and
effective modern custom power device used in power distribution networks.
DSTATCOM is a recently proposed shunt connected solid state device
that injects voltage into the system in order to regulate the load side voltage. It
is generally installed in a distribution system between the supply and the critical
load feeder at the point of common coupling (PCC).Other than voltage sags and
swells compensation, DSTATCOM is used to reduce the Total harmonic
distortions.
5.2 PRINCIPLE OF D-STATCOM:
FIG: DSTATCOM
A DSATCOM is a solid state power electronics switching device
consisting of either GTO or IGBT, a capacitor bank as an energy storage device.
It is linked in shunt between a distribution system and a load that shown in
Figure.
13
8/2/2019 Enhancement of Power Quality Using Dstatcom
14/51
However, when voltage sag occurs in the distribution system, the
DSTATCOM control system calculates and synthesizes the voltage required to
preserve output voltage to the load by injecting a controlled voltage with a
certain magnitude and phase angle into the distribution system to the load. Here
a LCL passive filter is used in order to reduce the total harmonic distortions and
PWM , PI controller are being used.
5.3 BASIC ARRANGEMENT OF DSTATCOM:
Voltage Source Converter
Controller
Energy Storage Device
LCL Passive Filter
FIG: Schematic diagram of a D-STATCOM
EQUATIONS OF D-STATCOM:
14
8/2/2019 Enhancement of Power Quality Using Dstatcom
15/51
Where,
5.3.1 VOLTAGE SOURCE CONVERTER:
A voltage-source converter is a power electronic device that connected
in shunt or parallel to the system. It can generate a sinusoidal voltage with any
required magnitude, frequency and phase angle. The VSC used to either
completely replace the voltage or to inject the missing voltage. The missing
voltage is the difference between the nominal voltage and the actual.
It also converts the DC voltage across storage devices into a set of three
phase AC output voltages. It could be a 3 phase - 3 wire VSC or 3 phase - 4
wire VSC. Either a conventional two level converter or a three level converter is
used. For DSTATCOM application, the VSC is used to momentarily replace the
supply voltage or to generate the part of the supply voltage which is absent. The
VSC here is a two level i.e two phase.
5.3.2 ENERGY STORAGE DEVICE:
15
8/2/2019 Enhancement of Power Quality Using Dstatcom
16/51
The function of storage devices is to supply the required energy to the
VSC via a dc link for the generation of injected voltages. DC source is
connected in parallel with the DC capacitor. It carries the input ripple current of
the converter and it is the main reactive energy storage element. This DC
capacitor could be charged by a battery source or could be recharged by the
converter itself.
FIG: ENERGY STORAGE DEVICE
5.3.3 LCL PASSIVE FILTER:
LCL Passive filter is more effective on reducing harmonic distortion.
The line-filter between the converter and the grid can be reduced by using an
LCL-filter instead of an L-filter. The main drawback with this is that the LCL-
filter will introduce a resonance frequency into the system. Harmonic
components in the output voltage can lead to resonance oscillations and
instability problems unless they are properly handled.
One way of reducing the resonance current is by adding a passive
damping circuit to the filter. This damping circuit can be purely resistive,
causing relatively high losses, or a more complex solutions consisting of a
combination of capacitors and inductors.
16
8/2/2019 Enhancement of Power Quality Using Dstatcom
17/51
A typical LCL-filter is shown in Figure where V1 is the grid side
voltage, Vc is the voltage across the filter capacitor and V2 is the converter
output voltage.
FIG: LCL PASSIVE FILTER
EQUATIONS:
To design an efficient LCL Passive filters make sure that,
5.3.4 CONTROLLER :
In this project we use a Proportional plus integral controller.
Proportional- integral controller (PI Controller) is a feedback controller which
drives the system to be controlled with a weighted sum of the error signal
17
8/2/2019 Enhancement of Power Quality Using Dstatcom
18/51
(difference between the output and desired set point) and the integral of that
value.
FIG: PI CONTROLLER
In this case, PI controller will process the error signal to zero. The load
r.m.s voltage is brought back to the reference voltage by comparing the
reference voltage with the r.m.s voltages that had been measured at the load
point. It also is used to control the flow of reactive power from the DC capacitorstorage circuit.
PWM generator is the device that generates the Sinusoidal PWM
waveform or signal. To operate PWM generator, the angle is summed with the
phase angle of the balance supply voltages equally at 120 degrees. Therefore, it
can produce the desired synchronizing signal that required.
PWM generator also received the error signal angle from PI controller.
The modulated signal is compared against a triangle signal in order to generate
the switching signals for VSC valves.
18
8/2/2019 Enhancement of Power Quality Using Dstatcom
19/51
CHAPTER-6
POWER QUALITY PROBLEMS IN DSTATCOM
The major power quality problems here are voltage sag and harmonic
distortion.
6.1 VOLTAGE SAG
Voltage sags and momentary power interruptions are probably the
most important PQ problem affecting industrial and large commercial
customers. These events are usually associated with a fault at some location in
the supplying power system. Interruptions occur when the fault is on thecircuit supplying the customer. But voltage sags occur even if the faults
happen to be far away from the customer's site.
Voltage sags lasting only 4-5 cycles can cause a wide range of sensitive
customer equipment to drop out. To industrial customers, voltage sag and a
momentary interruption are equivalent if both shut their process down. A
typical example of voltage sag is shown in fig . The susceptibility of utilization
equipment to voltage sag is dependent upon duration and magnitude of voltage
sags and can be defined.
FIG : VOLTAGE SAG
19
8/2/2019 Enhancement of Power Quality Using Dstatcom
20/51
6.1.1 Characteristics of Voltage Sags:
Voltage sags which can cause equipment impacts are caused by faults on
the power system.Motor starting also results in voltage sags but the magnitudes
are usually not severe enough to cause equipment mis operation
How a fault results in voltage sag at a customer facility?
The one line diagram given below in fig. 3 can be used to explain this
phenomenon.
Consider a customer on the feeder controlled by breaker 1. In the case
of a fault on this feeder, the customer will experience voltage sag during the
fault and an interruption when the breaker opens to clear the fault. For
temporary fault, enclosure may be successful.
Anyway, sensitive equipment will almost surely trip during this
interruption. Another kind of likely event would be a fault on one of the
feeders from the substation or a fault somewhere on the transmission
system, In either of these cases, the customer will experience a voltage sag
during the actual period of fault. As soon as breakers open to clear the fault,
normal voltage will be restarted at the customer's end. Fig is a plot of rms
voltage versus time and the waveform characteristics at the customer's location
for one of these fault conditions.
20
8/2/2019 Enhancement of Power Quality Using Dstatcom
21/51
This waveform is typical of the customer voltage during a fault on a
parallel feeder circuit that is cleared quickly by the substation breaker. The total
duration of fault is 150m sec. The voltage during a fault on a parallel feeder
will depend on the distance from the substation to fault point. A fault close to
substation will result in much more significant sag than a fault near the end of
feeder. Fig 5 shows the voltage sag magnitude at the plant bus as a function of
fault location for an example system.
FIG: VOLTAGE SAG CHARACTERISTIC DURING FAULT
A single line to ground fault condition results in a much less severe
voltage sag than 3-phase fault Condition due to a delta--star transformer
connection at the plant. Transmission related voltage sags are normally much
more consistent than those related to distribution. Because of large amounts of
energy associated with transmission faults, they are cleared as soon as possible.
This normally corresponds to 3-6 cycles, which is the total time for fault
detection and breaker operation Normally customers do not experience an
interruption for transmission fault. Transmission systems are looped or
networked, as distinct from radial distribution systems. If a fault occurs
as shown on the 115KV system, the protective relaying will sense the fault
and breakers A and B will open to clear the fault.
21
8/2/2019 Enhancement of Power Quality Using Dstatcom
22/51
FIG: MAGNITUDE OF VOLTAGE SAG
While the fault is on the transmission system, the entire power system,
including the distribution system will experience Voltage sag. Fig shown the
magnitude of measured voltage sags at an industrial plant supplied from a 115
kV system .
FIG: VOLTAGE SAG OF MAGNITUDE AND DURATION
22
8/2/2019 Enhancement of Power Quality Using Dstatcom
23/51
Most of the voltages were 10-30% below nominal voltage, and no
momentary interrupts were measured at the plant during the monitoring period
(about a year). Fig given a three-dimensional plot illustrating the number of
sags experienced as a function of both the voltage sag magnitude and the
duration.
This is a convenient way to completely characterize the actual or
expected voltage sag conditions at a site. Evaluating the impact of voltage sags
at a customer plant involves estimating the member of voltage sags that can be
expected as a function of the voltage sag magnitude and then comparing
this with equipment sensitivity.
The estimate of voltage sag performance are developed by performing
short-circuit simulations to determine the plant voltage as a function of fault
location throughout the power system. Total circuit miles of line exposure that
can affect the plant (area of vulnerability) are determined for a particular sag
level.
Historical fault performance (fault per year per 100 miles) can, then
be used to estimate the number of sags per year that can be expected below the
magnitude. A chart such as the one in fig 8. Can be drawn in splitting the
expected number of voltage sags by magnitude. This information can be used
directly by the customers to determine the need for power conditioning
equipment at sensitive loads in the plant.
6.1.2Voltage-Sag Analysis- Methodology
The methodology is outlined is (proposed) of IEEE Gold book (IEEE
standard 493, Recommended practice for the design of reliable industrial and
commercial power system) The methodology basically consists of the following
four steps:
23
8/2/2019 Enhancement of Power Quality Using Dstatcom
24/51
6.1.2.1Voltage Sag Calculation
Sliding faults which include line-line, line to ground, line to line- to
ground and three phase are applied to all the lines in the load flow. Each line is
divided into equal sections and each section is faulted as shown in fig 9.
6.1.2.2 Sag Occurrence Calculation:
Based upon the utilities reliability data (the number of times each line
section will experience a fault) and the results of load flow and voltage sag
calculations, the number of voltage sags at the customer site due to remote
faults can be calculated.
Depending upon the equipment connection, the voltage sag occurrence
rate may be calculated in terms of either phase or line voltages dependent upon
the load connection. For some facilities, both line and phase voltages may be
required. The data thus obtained from load flow, Voltage sag calculation,
and voltage sag occurrence calculation can be sorted and tabulated by sag
magnitude, fault type, location of fault and nominal system voltage at the fault
location
6.2 Study of Results of Sag- Analysis:
The results can be tabulated and displayed in many different ways to
recognize difficult aspects. Area of vulnerability can be plotted on a
geographical map or one - line diagram (fig above). These plots can be used to
target transmission and distribution lines for enhancements in reliability.
Further bar charts, and pie-charts showing the total number of voltage sags
with reference to voltage level at fault point, area/zone of fault, or the fault
type can be developed to help utilities focus on their system improvements
(figs.) To examining the existing system, system modifications aimed at
mitigating or reducing voltage sags can also be identified, thus enabling cost
benefits analysis. Possible such system structural changes that can be identified
include.
24
8/2/2019 Enhancement of Power Quality Using Dstatcom
25/51
Reconnection of a customer from one voltage level to another,
Installation of Ferro-resonant transformers or time delayed under voltage, drop
out relay to facilitate easy ride - through the sag Application of static transfer
switch and energy storage system., Application of fast acting synchronous
condensers, Neighborhood generation capacity addition , Increase service
voltage addition through transformer tap changing, By enhancement of system
reliability.
6.3 Solutions to Voltage Sag Problems:
Efforts by utilities and customers can reduce the number and severity of
sags.
A. Utility solutions:
Utilities can take two main steps to reduce the detrimental effects of
sags
(1) Prevent fault
(2) Improve fault clearing methods
Fault prevention methods include activities like tree trimming, adding
line arrests, washing insulators and installing animal guards. Improved fault
clearing practices include activities like adding line recloses, eliminating fast
tripping, adding loop schemes and modifying feeder design. These may reduce
the number and /or duration of momentary interruptions and voltage sags but
faults cannot be eliminated completely.
B. Customer solutions:
Power conditioning is the general concept behind these methods. Fig 12
is a schematic f the general approach used.
Power conditioning helps to
1. Isolate equipment from high frequency noise and transients.
2. Provide voltage sag ride through capability
25
8/2/2019 Enhancement of Power Quality Using Dstatcom
26/51
6.4 TOTAL HARMONIC DISTORTION:
Harmonics
Introduction:
The typical definition for a harmonic is a sinusoidal component of a
periodic wave or\quantity having a frequency that is an integral multiple of the
fundamental frequency. Some references refer to clean or pure power as
those without any harmonics. But such clean waveforms typically only exist in a
laboratory. Harmonics have been around for a long time and will continue to do
so. In fact, musicians have been aware of such since the invention of the first
string or wood wind instrument. Harmonics (called overtones in music) are
responsible for what makes a trumpet sound like a trumpet, and a clarinet like a
clarinet.
Electrical generators try to produce electric power where
voltageone frequency associated with it, the fundamental frequency. In the
North America, this frequency is 60 Hz, or cycles per second. In European
countries and other parts of the world, this frequency is usually 50 Hz. Aircraft
often uses 400 Hz as the fundamental frequency. At 60 Hz, this means that sixty
times a second, the voltage waveform increases to a maximum positive value,
then decreases to zero, further decreasing to a maximum negative value, and
then back to zero. The rate at which these changes occur is the trigometric
function called a sine wave, as shown in figure . This function occurs in many
natural phenomena, such as the speed of a pendulum as it swings back
and forth, or the way a string on a voilin vibrates when plucked. Fig . Sine wave
The frequency of the harmonics is different, depending on the fundamentalfrequency. For example, the 2nd harmonic on a 60 Hz system is 2*60 or 120
Hz. At 50Hz, the second harmonic is 2* 50 or 100Hz.300Hz is the 5th harmonic
in a 60 Hz system, or the 6th harmonic in a 50 Hz system. Figure shows how a
signal with two harmonics would appear on an oscilloscope-type display, which
some power quality analyzers provide.
In order to be able to analyze complex signals that have many different
frequencies present, a number of mathematical methods were developed. One of
the more popular is called the Fourier Transform. However, duplicating the
26
8/2/2019 Enhancement of Power Quality Using Dstatcom
27/51
mathematical steps required in a microprocessor or computer-based
instrument is quite difficult. So more compatible processes, called the FFT for
Fast Fourier transform, or DFT for Discrete Fourier Transform, are used. These
methods only work properly if the signal is composed of only the fundamental
and harmonic frequencies in a certain frequency range.
The frequency values must not change during the measurement period.
Failure of these rules to be maintained can result in mis-information. For
example, if a voltage waveform is comprised of 60 Hz and 200 Hz signals, the
FFT cannot directly see the 200Hz. It only knows 60, 120, 180, 240,..., which
are often called bins. The result would be that the energy of the 200 Hz signal
would appear partially in the 180Hz bin, and partially in the 240Hz bin.
An FFT-based processer could show a voltage value of 115V at 60 Hz,
18 V at the 3rdharmonic, and 12 V at the 4th harmonic, when it really should
havebeen30Vat200Hz.
These in between frequencies are called inter harmonics. There is
also a special category of inter harmonics, which are frequency values less than
the fundamental frequency value, called sub-harmonics.
For example, the process of melting metal in an electric arc furnace can
result large currents that are comprised of the
fundamental , inter harmonic, and sub harmonic frequencies being drawn from
the electric power grid. These levels can be quite high during the melt-down
phase, and usually effect the voltage waveform.
Effects of harmonics:
The presence of harmonics does not mean that the factory or office
cannot run properly .Like other power quality phenomena, it depends on the
stiffness of the power distribution system and the susceptibility of the
equipment. As shown below, there are a number of different types of equipment
that can have mis-operations or failures due to high harmonic voltage
and/or current levels. In addition, one factory may be the source of high
harmonics but able to run properly. This harmonic pollution is often carried
back onto the electric utility distribution system, and may effect facilities on the
same system which are more susceptible.
27
8/2/2019 Enhancement of Power Quality Using Dstatcom
28/51
Some typical types of equipment susceptible to harmonic pollution
include: - Excessive neutral current, resulting in overheated neutrals. The odd
triple n harmonics in three phase wave circuits are actually additive in the
neutral. This is because the harmonic number multiplied by the 120 degree
phase shift between phases is a integer multiple of 360 degrees. This puts the
harmonics from each of the three phase legs in-phase with each other in the
neutral, as shown in figure
Harmonic problems are almost always introduced by the consumers
equipment and installation practices. Harmonic distortion is caused by the high
use of non-linear load equipment such as computer power supplies, electronic
ballasts, compact fluorescent lamps and variable speed drives etc, which create
high current flow with harmonic frequency components.
The limiting rating for most electrical circuit elements is determined by
the amount of heat that can be dissipated to avoid overheating of bus bars,
circuit breakers, neutral conductors, transformer windings or generatoralternators.
28
8/2/2019 Enhancement of Power Quality Using Dstatcom
29/51
TOTAL HARMONIC DISTORTION:
THD is defined as the RMS value of the waveform remaining when the
fundamental is removed. A perfect sine wave is 100%, the fundamental is the
system frequency of 50 or 60Hz.
Harmonic distortion is caused by the introduction of waveforms at
frequencies in multiplies of the fundamental ie: 3rd harmonic is 3x the
fundamental frequency / 150Hz. Total harmonic distortion is a measurement of
the sum value of the waveform that is distorted.
FIG: HARMONIC DISTORTION
Power Measurement:
Despite the use of good quality test meter instrumentation, high current
flow can often remain undetected or under estimated by as much 40%. Thissevere underestimation causes overly high running temperatures of equipment
and nuisance tripping. This is simply because the average reading test meters
commonly used by maintenance technicians, is not designed to accurately
measure distorted currents, and can only provide indication of the condition of
the supply at the time of checking.
Power quality conditions change continuously, and only instruments
offering true RMS measurement of distorted waveforms and neutral currents
29
8/2/2019 Enhancement of Power Quality Using Dstatcom
30/51
can provide the correct measurements to accurately determine the ratings of
cables, bus bars and circuit breakers.
Neutral Currents
High harmonic environments can produce unexpected and dangerous
neutral currents. In a balanced system, the fundamental currents will cancel out,
but, triple- Ns will add, so harmonic currents at the 3rd, 9th, 15th etc. will flow
in the neutral. Traditional 3 phase system meters are only able to calculate the
vector of line to neutral current measurements, which may not register the true
reading. Integral 1530, 1560 and 1580 offer a 3 phase 4 wire versions with a
neutral 4th CT allowing true neutral current measurement and protection in high
harmonic environments.
Harmonic Profiles
There is much discussion over the practical harmonic range of a
measurement instrument; however study of the harmonic profiles of typically
installed equipment can guide the system designer to the practical solution. A
typical harmonic profile graph will show a logarithmic decay as the harmonic
frequency increases. It is necessary to establish the upper level at which the
harmonic content is negligible.
For Example:
A laptop switch mode power supply causes approximately 25% of 3rd
harmonic, 19% of 5th harmonic, 10% of 7th harmonic and 5% of 9th harmonic
etc. Therefore it can be seen that almost all the harmonic content in an IT
dominated load will be below the 15th harmonic. In a 3 phase load
incorporating 6 pulse bridge technology as is common in many variable speed
drives, UPS systems and DC converters, similar profiles will be observed but
extending to the 25th and 27th harmonic. It can therefore be deduced that in the
majority of industrial and commercial applications an instrument measuring up
to the 31st harmonic is ideal.
30
8/2/2019 Enhancement of Power Quality Using Dstatcom
31/51
6.5 MINIMIZATION OF HARMONICS:
Care should be undertaken to make sure that the corrective action taken
to minimize the harmonic problems dont actually make the system worse. This
can be the result of resonance between harmonic filters, PF correcting
capacitors and the system impedance. Isolating harmonic pollution devices on
separate circuits with or without the use of harmonic filters are typical ways of
mitigating the effects of such. Loads can be relocated to try to balance the
system better. according to the latest NEC-1996 requirements covering such.
Whereas the neutral may have been undersized in the past, it may now be
necessary to run a second neutral wire that is the same size as the phase
conductors. This is particularly important with some modular office partition-
type walls, which can exhibit high impedance values. Use of higher pulse
converters, such as 24-pulse rectifiers, can eliminate lower harmonic values.
CHAPTER-7
D-STATCOM TEST MODELS
DSTATCOM is a device, which is used to
31
8/2/2019 Enhancement of Power Quality Using Dstatcom
32/51
Improve the Voltage Sag
Reduce the Total Harmonic Distortion
7.1 D-STATCOM TEST SYSTEM:
FIG: SINGLE LINE DIAGRAM OF THE TEST SYSTEM
The above figure comprises of a 230kV, 50Hz transmission system,
which is represented by a Thevenin equivalent, feeding into the primary side of
a 3-winding transformer connected in Y/Y/Y, 230/11/11 kV. A varying load is
connected to the 11 kV, secondary side of the transformer. A two-level D-
STATCOM is connected to the 11kV tertiary winding to provide instantaneous
voltage support at the load point.
A 750 F capacitor on the dc side provides the D-STATCOM energy
storage capabilities. Breaker 1 is used to control the period of operation of theD-STATCOM and breaker 2 is used to control the connection of load 1 to the
system.
7.2 METHODOLOGY:
STEP-1:
Start the program.
Design a distribution system using Matlab version R2009A.
32
8/2/2019 Enhancement of Power Quality Using Dstatcom
33/51
Create distortions by inserting different types of faults such as
Three phase to ground Fault(TPG)
Single Line to ground Fault (SLG)
Double Line to ground Fault (DLG)Line to Line (LL) into the distribution system.
STEP-2:
Run the simulation.
Vary the values of the fault resistances for different types of faults.
If voltage sag >0.9p.u , it means the condition satisfies go to STEP-3
Otherwise i.e., if voltage sag
8/2/2019 Enhancement of Power Quality Using Dstatcom
34/51
(breaker-2) to the load. Another winding is connected to the breaker (breaker-
1) to DSTATCOM with LCL Filter.
FIG: SIMULINK TEST MODELOF DSTATCOM WITH
LCL PASSIVE FILTER
7.4 RESULTS WITHOUT DSTATCOM:
The scope-2 gives the output wave forms of simulink model without
DSTATCOM. For different types of fault resistances we get different
wave forms.
6.4.1 OUTPUT WAVEFORMS:
34
8/2/2019 Enhancement of Power Quality Using Dstatcom
35/51
FIG : VOLTAGE SAG AT 0.2 *10^-6 for SLG FAULT
35
8/2/2019 Enhancement of Power Quality Using Dstatcom
36/51
FIG: VOLTAGE SAG AT 6.2*10^-7 FOR DLG FAULT
FIG: VOLTAGE SAG AT 4.99*10^-7 FOR TPG FAULT
36
8/2/2019 Enhancement of Power Quality Using Dstatcom
37/51
FIG: VOLTAGE SAG AT 8.1*10 -7 FOR LL FAULT
From the figure we can see the voltage sag i.e., the reduction in voltage.
For different values of fault resistances we get different types of waveforms.
Here we can see the waveforms for different types of faults. The wave forms are
not greater than 0.9 p.u which shows that there is a voltage reduction or voltage
sag.
37
8/2/2019 Enhancement of Power Quality Using Dstatcom
38/51
7.5 OUTPUT WAVEFORMS WITH DSTATCOM:
The scope 1 gives the output waveforms of simulink model with
DSTATCOM . The following are the figures of voltage sags
FIG: VOLTAGE SAG AT 1.3p.u FOR SLG FAULT
38
8/2/2019 Enhancement of Power Quality Using Dstatcom
39/51
FIG: VOLTAGE SAG AT 1.3p.u FOR DLG FAULT
FIG: VOLTAGE SAG AT 1.4p.u FOR TPG FAULT
39
8/2/2019 Enhancement of Power Quality Using Dstatcom
40/51
FIG: VOLTAGE SAG AT 1.41p.u FOR LL FAULT
From the figure we can see the voltage sag i.e., the reduction in voltage.
For different values of fault resistances we get different types of waveforms.
Here we can see the waveforms for different types of faults. The wave forms are
greater than 0.9 p.u which shows that there is a improvement in voltage sag.
40
8/2/2019 Enhancement of Power Quality Using Dstatcom
41/51
7.6 OUTPUT WAVE FORMS OF DSTATCOM WITH LCL PASSIVE
FILTER:
The power gui opens an FFT analysis which gives the output wave
forms
0 0 . 0 5 0 . 1 0 . 1 5 0 . 2 0 . 2 5 0 . 3
- 0 . 5
0
0 . 5
S e l e c t e d s i g n a l : 1 5 c y c l e s . F F T w i
T i m e ( s )
0 5 1 0 1 5 2 00
0 . 2
0 . 4
0 . 6
0 . 8
H a r m o n i c o r d e r
F u n d a m e n t a l ( 5 0 H z ) = 1 , T H D .
M
ag
(%
ofFundam
ental)
FIG: OUTPUT WAVEFORMS OF DSTATCOM WITH
LCL PASSIVE FILTER
41
8/2/2019 Enhancement of Power Quality Using Dstatcom
42/51
7.7 SIMULINK MODEL OF DSTATCOM WITHOUT LCL PASSIVE
FILTER:
42
8/2/2019 Enhancement of Power Quality Using Dstatcom
43/51
FIG: SIMULINK TEST MODEL FOR DSTATCOM WITHOUT
LCL PASSIVE FILTER
RESULTS:
7.7.1 OUTPUT WAVEFORMS OF DSTATCOM WITHOUT LCL PASSIVE
FILTER:
0 0 . 0 5 0 . 1 0 . 1 5 0 . 2 0 . 2 5 0 . 3
- 0 . 0 5
0
0 . 0 5
S e l e c t e d s i g n a l : 1 5 c y c l e s . F :
T i m e ( s )
0 5 1 0 1 5 2 00
1
2
3
4
5
6
H a r m o n i c o r d e r
F u n d a m e n t a l ( 5 0 H z ) = 8 . 8 2
M
ag
(%
ofFundam
ental)
FIG: OUTPUT WAVEFORM OF DSTATCOM WITHOUT
LCL PASSIVE FILTER:
43
8/2/2019 Enhancement of Power Quality Using Dstatcom
44/51
CHAPTER-8
CONCLUSION
Power quality in electric networks is one of today's most concerned
areas of electric power system. The power quality has serious economic
implications for consumers, utilities and electrical equipment manufacturers.
Most occurring power quality problems are voltage sag and total
harmonic distortion. These are due to faults such as increase in load while
starting a motor, transformer energizing etc., The impact of power qualityproblems is increasingly felt by customers - industrial, commercial and even
residential.
In this project we discuss about the improvement of voltage sag in
distribution system using DSTATCOM and reduction of total harmonic
distortion by using LCL passive filter with DSTATCOM in Distribution system.
Atlast we have seen the output waveforms regarding them using
Matlab/simulink.
44
8/2/2019 Enhancement of Power Quality Using Dstatcom
45/51
45
8/2/2019 Enhancement of Power Quality Using Dstatcom
46/51
ENHANCEMENT OF POWER QUALITY
DISTRIBUTION SYSTEM USING D-STATCOM
INDEX
Abstract
List of Figures:
List of Tables:
CHAPTER-1: MATLAB INTRODUCTION 1-30
CHAPTER-2: INTRODUCTION 31-33
2.1LITERATURE SURVEY
2.2SCOPE OF WORK
CHAPTER-3: POWER QUALITY 34-40
3.1 INTRODUCTION
3.2 POWER QUALITY-A BIG ISSUE
3.3 PROBLEMS ASSOCIATED WITH POWER QUALITY
3.3.1 MOMENTARY PHENAMINA
3.3.1.1 TRANSIENTS
3.3.1.2 LONG DURATION VOLTAGE VARIATIONS
3.3.1.3 SUSTAINED INTURRUPTION
3.3.1.4 SHORT DURATION VOLTAGE VARIATIONS
3.3.2 STEADY STATE PHENMINA
3.3.2.1 WAVE FORM DISTORTION
3.3.2.2 VOLTAGE IMBALANCE
3.3.3 VOLTAGE FLUCTUVATIONS AND FLICKERS
3.3.4 POWER FREQUANCY VARIATIONS
46
8/2/2019 Enhancement of Power Quality Using Dstatcom
47/51
3.4 SOLUTIONS OF POWER PROBLEMS
3.4.1 THYRISTER BASED STATIC SWITCHES
3.4.2 ENERGY STORAGE SYSTEM
CHAPTER-4: CUSTOM POWER DEVICES 41-47
4.1 INTRODUCTION
4.2 NEED OF CUSTOM POWER DEVICES
4.3 CONFIGURATIONS
4.3.1 CONVERTER BASED CLASIFICATION
4.3.2 TOPOLOGY BASED CLASIFICATION
4.4 BENEFITS WITH THE APPLICATION OF CUSTOM
POWER DEVICES
CHAPTER-5: D-STATCOM 48-53
5.1 INTRODUCTION
5.2 PRINCIPLE OF D-STATCOM
5.3 BASIC ARRANGEMENTS OF D-STATCOM
5.3.1 VOLTAGE SOURCE CONVERTER
5.3.2 ENERGY STORAGE DEVICE
5.3.3 LCL PASSIVE FILTER
5.3.4 CONTROLLER
CHAPTER-6: POWER PROBLEMS IN D-STATCOM 54-66
6.1 VOLTAGE SAG
6.1.1 CHARACTERSTICS OF VOLTAGE SAG
47
8/2/2019 Enhancement of Power Quality Using Dstatcom
48/51
6.1.2 VOLTAGE GENERATING ANALYSIS-
METHODOLOGY
6.1.2.1 VOLTAGE SAG CALCULATIONS
6.2. STEADY OF RESULT SAG CALCULATIONS
6.3 SOLUTION OF VOLTAGE SAG PROBLEMS
6.4 TOTAL HORMONIC DISTORTION
6.5 MINIMISATION OF HORMONICS
CHAPTER-7: D-STATCOM TEST MODELS 67-79
7.1 D-STATCOM TEST SYSTEM
7.2 METHODOLOGY
7.2.1 PROCEDURE
7.3 SIMULINK MODEL FOR THE TEST SYSTEM
7.4 RESULTS WITHOUT D-STATCOM
7.5 RESULTS WITH D-STATCOM
7.6 OUTPUT WAVE FORM OF D-STATCOM WITH LCL
PASSIVE FILTER
7.7 SIMULINK MODEL OF D-STATCOM WITH OUT LCL
PASSIVE FILTER
7.7.1 OUTPUT WAVE FORM OF D-STATCOM WITHOUT
LCL PASSIVE FILTER
CHAPTER-8: CONCLUSION 80
48
8/2/2019 Enhancement of Power Quality Using Dstatcom
49/51
LIST OF FIGURES:
FIGURE NUMBER NAME OF THE
FIGURE
PAGENO
5.2.2 45
5.2.3(a) 48
5.2.3(b) 49
50
LIST OF TABLES:
TABLE NUMBER NAME OF THE
TABLE
PAGE NO
2.2 Types of HT Motors 23
ABSTRACT
49
8/2/2019 Enhancement of Power Quality Using Dstatcom
50/51
This project presents the enhancement of voltage sags, harmonic
distortion using Distribution Static Compensator (D-STATCOM) with LCL
Passive Filter in distribution system. The model is based on the Voltage Source
Converter (VSC) principle. The D-STATCOM injects a current into the system
to mitigate the voltage sags. LCL Passive Filter was then added to D-
STATCOM to improve harmonic distortion and low power factor.
A new PWM-based control scheme has been implemented to control the
electronic valves in the D-STATCOM. The D-STATCOM has an additional
capability to sustain reactive current at low voltage, and can be developed as a
voltage and frequency support by replacing capacitors. Voltage sag is a short
time event during which a reduction in r.m.s voltage magnitude occurs. Voltage
sags are improved with insertion of D-STATCOM. When the value of fault
resistance is increased, the voltage sags will also increase for different types of
fault.
Suitable adjustment of the phase and magnitude of the D-STATCOM
output voltages allows effective control of active and reactive power exchanges
between D-STATCOM and AC system. The PI controller will process the
error signal to zero. The load r.m.s voltage is brought back to the reference
voltage by comparing the reference voltage with the r.m.s voltages that had
been measured at the load point. It also is used to control the flow of reactive
power from the DC capacitor storage circuit. The PWM generator can produce
the desired synchronizing signal that is required. PWM generator also receives
the error signal angle from PI controller.
The modulated signal is compared against a signal in order to generate
the switching signals for VSC valves. To enhance the performance of
distribution system, D-STATCOM was connected to the distribution system.
50
8/2/2019 Enhancement of Power Quality Using Dstatcom
51/51