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MITIGATION OF VOLTAGE SAGS AND SWELLS BY USING NEW CASCADED Z-SOURCE MULTI-LEVEL INVERTER BASED DVR USING FUZZY
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ABSTRACT
The power quality requirement is one of the major issues for power companies at
Their customers. The analysis of power disturbance characteristics and finding
solution to the power quality problems have resulted in an increased interest for
power quality. The most concerning disturbances affecting the quality of the
power in the distribution system are voltage sag and swell. The DVR is used
to mitigate the voltage sag and swell on sensitive load. In this paper new
cascaded multi-level inverter based DVR is proposed to enhance the voltage
restoration property of the system. The ZSI uses an LC impedance grid to couple
power source to inverter circuit and prepares the possibility of voltage buck
and boost by short circuiting the inverter legs. Additionally a fuzzy logic
control scheme for cascaded multi-level inverter based DVR is proposed to
obtain Voltage sag s well, flicker, harmonics distortion, impulse transients and
interruptions are the various power quality problems addressed in the
distribution system. Of the above power quality problems, a voltage sag and
swell disturbance poses a series threat to the industries. It can occur more
frequently than any other power quality phenomenon. Dynamic voltage restorer
(DVR) is one of the power electronic devices connected in series to the
distribution system . It compensates the voltage disturbances by injecting the
voltage of suitable magnitude and phase in series with the line. The DVR, with its
excellent capabilities , when installed between the supply and the sensitive load, can
compensate voltage sag and swells.
INTRODUCTION
1. INTRODUCTION
Power Quality problems encompass a wide range of disturbances such as voltage sags
swells, flicker, harmonics distortion, impulse transient, and interruptions. Voltage sags can occur
at any instant of time. Voltage swell, is defined as an increase in rms voltage or current at the
power frequency for durations from 0.5 cycles to 1 min. typical magnitudes are between 1.1 and
1.8 up. Swell magnitude is also described by its remaining voltage, in this case, always greater
than 1.0. The main objective of this project is to improve the voltage quality in distribution
system.
Voltage sag and swell can cause sensitive equipment (such as found in semiconductor or
chemical plants) to fail, or shutdown, as well as create a large current unbalance that could blow
fuses or trip the breakers. These effects can be very expensive for the customer, ranging from
minor quality variations to production downtime and equipment damage. 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. Switching off a large inductive load or Energizing a
large capacitor bank is a typical system event that causes swells. Dynamic voltage restorer
(DVR) is one of the power electronic devices connected in series to the distribution system. It
compensates the voltage disturbances by injecting the voltage of suitable magnitude and phase in
series with the line. The DVR, with its excellent capabilities, when installed between the supply
and the sensitive load, can compensate voltage sag and swell.
In this project Z-source multi level inverter based DVR is proposed. It employs a unique
X-shaped impedance network on its DC side for achieving both voltage buck and boost
capabilities This unique features of ZSI cannot be obtained in the traditional voltage source and current source inverters.
Here the control scheme used employed in Z-source inverter based DVR is fuzzy
controller. The most common choice controller of the DVR is the PI controller since it has
simple structure and it can offer relatively satisfactory performance over a wide range of
operation. But by using fixed gains, the controller may not provide the required control
performance, when there are variations in the system parameters and operating conditions. It
appears that the non linear controllers are more suitable than the linear type since the DVR is
truly a non linear system. The proposed fuzzy controller will provide the desired injecting
voltage.
1.2 OBJECTIVE OF THESIS
The main objective of this project is to improve the voltage quality in distribution system.
The Z-source multi level inverter based DVR is used to mitigate the voltage sag and swell the
compensation is further improved by using fuzzy controller. At the end, MATLAB SIMULINK
model based simulated results were presented to validate the effectiveness of the proposed control method.
CHAPTER 2
N.H. Woodley, Senior Member, IEEE, Pittsburgh, PA 15235 USA, lEEE
Transactions on Power Delivery, Vol. 14, No. 3, July 1999.
Distribution Problems that result in costly loss of production to critical processes create a
dilemma for both the serving utility and the energy consuming customer. Since the problem of
production losses occurs on the customer's side of the meter, it is also on the customer's side
where the success of any problem mitigation effort must be measured. However, the cause of the
problem can originate on either side. The optimum solution may be the result of the application
of system solutions and power quality improvement equipment on either or both side.
Traditionally, for problems originating on the utility side, the approach has been to work on the
customer side to desensitize critical loads while "cleaning up" the circuits on the utility side.
However, once these efforts have been accomplished, there are still many situations where it is
not possible to provide sufficient improvement. In these cases, the customer-side solutions
usually become very expensive, e.g. large uninterruptible power supplies (UPS), while on the
utility side options to build new dedicated circuits or substations may be very difficult and still
not provide the needed degree of improvement (isolation). A survey of utility industrial and
commercial customers was conducted during May-June, 1992. The purpose of the survey was to
determine the criteria used by these customers in making decisions resulting in the purchase,
installation and operation of power conditioning equipment to assure acceptable disturbance-free
electrical service. In general, customers understand that outages cannot be completely eliminated
on the power system. However, they are much less forgiving when their processes are upset by
momentary disturbances which are usually more frequent than complete outages. The majority of
customers questioned stated that they would prefer a utility-provided solution with the cost
included in their power bill as an alternative to the purchase, installation, and operation of their
own (customer-owned) on-site equipment for the mitigation of power line
disturbances. The Dynamic Voltage Restorer (DVR) has been developed as part of the utility
Custom Power Program to provide just such a solution. The prototype DVR in a portable trailer enclosure.
Chellali BENACHAIBA, Brahim FERDI Be char University Center, Algeria,
Electrical Power Quality and Utilization, Journal Vol. XIV, No.1,
2008.
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 systems, 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. Apart from nonlinear loads, some system events, both usual (e.g.
capacitor switching, motor starling) and unusual (e.g. faults) could also inflict power quality
problems . 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
common place in both the industrial sectors and the domestic environment. Because of this a
heightened awareness of power quality is developing amongst electricity users. There are many
types of Custom Power devices. Some of these devices include: Active Power Filters (APF),
Battery Energy Storage Systems (BESS), Distribution Statics synchronous compensators
(DSTATCOM), Distribution Series Capacitors (DSC), Dynamic Voltage Restorer (DVR), Surge
Arresters (SA), Super conducting Magnetic Energy Systems (SMES), Static Electronic Tap
Changers (SETC), Solid-State Transfer Switches (SSTS), Solid State Fault Current Limiter
(SSFCL), Static Var Compensator (SVC), Thyristor Switched Capacitors (TSC), and
Uninterruptible Power Supplies (UPS) . Each of Custom Power devices has its own benefits and
limitations. The most effective type of these devices is considered to be the Dynamic Voltage
Restorer (DVR). There are numerous reasons why the DVR is preferred over the others. A few
of these reasons are presented as follows. The SVC pre-dates the DVR, but the DVR is still
preferred because the SVC has no ability to control active power flow . Another reason is that the
DVR costs less compared to the UPS . Not only the UPS is costly, it also requires a high level of
maintenance because batteries leak and have to be replaced as often as every five years . Other
reasons include that the DVR has a higher energy capacity and lower costs compared to the
SMES device . Furthermore, the DVR is smaller in size and costs less compared to the
DSTATCOM . Based on these reasons, it is no surprise that the DVR is widely considered as an
effective custom power device in mitigating voltage sags . In addition to voltage sags and swells
compensation, DVR can also added other features such as harmonics and Power Factor
correction. Compared to the other devices, the DVR clearly provides the best economic solution
for its size and capabilities.
Takushi Jimi chi, Student Member, IEEE, Hideaki Fujita, Member, IEEE,
and Hirofumi Akagi, Fellow, IEEE TRANSACTIONS ON INDUSTRY
APPLICATIONS, VOL. 44, NO. 3, MAY/JUNE 2008.
The use of a dynamic voltage restorer (DVR), or a voltage sag compensator, is one of the
most effective solutions to “restoring” the quality of voltage at its load-side terminals when the
quality of voltage at its source-side terminals is disturbed. A traditional DVR mainly consists of
series and shunt converters connected back-to-back and a common dc capacitor used as an
energy-storage element. The DVR injects three-phase compensating voltages in series to the
power lines through a three-phase series transformer or three single-phase series transformers.
The energy required for the compensation of voltage sags is taken from the dc capacitor or
another energy-storage element such as a double-layer capacitor, a superconducting magnet, or a
lead-acid battery. The DVR is superior to the uninterruptible power supply (UPS) in cost and
efficiency because the DVR has a smaller converter rating than the UPS. However, the need for
reductions in cost, weight, and physical size has inhibited the DVR from greater penetration and
wider acceptance in spite of the superior performance that is possible with its use. The authors of
have described a DVR that is characterized by connecting the series converter to the source side
and the shunt converter to the load side. This system configuration allows the use of an
extremely small dc capacitor because the dc capacitor does not play any role in feeding electric
dc energy to the series converter during voltage-sag compensation, but acts as a dc capacitor for
smoothing the common dc-link voltage. This means that the dc capacitor is no longer an energy
source for the compensation of voltage sags.
Fang Zheng Pang, Senior Member, IEEE TRANSACTIONS ON INDUSTRY
APPLICATIONS, VOL. 39, NO. 2, MARCH/APRIL 2003
An impedance-source power converter for implementing dc-to-ac, ac-to-dc, ac-to-ac, and
dc-to-dc power conversion. The Z-source converter employs a unique impedance network (or
circuit) to couple the converter main circuit to the power source, thus providing unique features
that cannot be observed in the traditional voltage-source and current-source converters where a capacitor and inductor are used, respectively. The Z-source converter overcomes the conceptual
and theoretical barriers and limitations of the traditional voltage-source converter and currentsource
converter and provides a novel power conversion concept. The Z-source concept can be
applied to almost all dc-to-ac, ac-to-dc, ac-to-ac, and dc-to-dc power conversion. the Z-source
concept can be applied to the entire spectrum of power conversion. Based on the concept, it is
apparent that many Z-source conversion circuits can be derived. As another example, the Zsource
concept can be easily applied to adjustable-speed drive (ASD) systems . The Z-source
rectifier inverter system can produce an output voltage greater than the ac input voltage by
controlling the boost factor, which is impossible for the traditional ASD systems.
Farad Aslam Department of Electrical & Instrumentation Engineering
Tapper University, Patiala, Punjab, India International Journal of Computer
Applications (0975 – 8887) Volume 17– No.6, March 2011
Fuzzy systems are universal approximates. Fuzzy controlled systems models do not
require any certain model for implementation of system under consideration. These proofs stem
isomorphism between an abstract algebra and linear algebra and the structure of a Fuzzy system,
which comprised of an implication between actions and conclusion as antecedents and
consequents. Abstract algebra incorporates systems or models dealing with groups, fields and
rings. Linear algebra incorporates system models dealing with vector spaces, state vector and
transition matrices. The primary benefit of fuzzy system theory is to approximate system
behavior where numerical functions or analytical functions do not exist. Hence, Fuzzy systems
have high potential to understand the very systems that are devoid of analytical formulations in a
complex System. Complex systems can be new systems that have not been tested, they can
involve with the human conditions such as biological or medical systems. The ultimate goal of
the fuzzy logic is to form the theoretical foundation for reasoning about the imprecise reasoning,
such reasoning is known as approximate reasoning. A proportional controller could lead to offset
between the desired set point and the actual output. This is because the process input which is
controller output and the process output come to new equilibrium values before error goes down
to zero. Now to make the controller output proportional to the integral of the error desired
compensation is to be provided. This is known as the proportional integral control. As long as
there continuous to be an error signal to the controller, the controller output will continue to
change. Therefore, the integral of error forces the error signal to zero.
PROBLEM OF EXISTING SYSTEM AND PROPOSED
CONCEPT
3.1 PROBLEM OF EXISTING SYSTEM
The most common distribution problems are sags and swells ,transient voltages these
disturbances can cause the system faults or damages .these damages can leads every
expansive to avoid those type of problems this project can helps.
DVR is the PI controller since it has simple structure and it can offer relatively
satisfactory performance over a wide range of operation. But by using fixed gains, the
controller may not provide the required control performance, when there are variations in the
system parameters and operating conditions. It appears that the non linear controllers are
more suitable than the linear type since the DVR is truly a non linear system.
3.2 PROPOSED CONCEPT
Dynamic voltage restorer (DVR) is one of the power electronic devices
connected in series to the distribution system. It compensates the voltage disturbances by
injecting the voltage of suitable magnitude and phase in series with the line. The DVR, with
its excellent capabilities, when installed between the supply and the sensitive load, can
compensate voltage sag and swell. Z-source multi level inverter based DVR is proposed. It
employs a unique X-shaped impedance network on its DC side for achieving both voltage
buck and boost capabilities This unique features of ZSI cannot be obtained in the traditional
voltage source and current source inverters.
The control scheme used employed in Z-source inverter based DVR is fuzzy
controller. The proposed fuzzy controller will provide the desired injecting voltage into the distribution system in order to mitigate voltage sag and swell.
DYNAMIC VOLTAGE RESTORER (DVR)
Dynamic Voltage Restorer (DVR) is one of the custom power devices that are used as
an effective solution for the protection of sensitive loads against voltage disturbances in
power distribution system. The efficiency of the DVR depends on the performance of the
efficiency control technique involved in switching the inverters.
INTROCUCTION
Among these problems (sags, swells, harmonics…) voltage sags are the most severe
disturbances. In order to overcome these problems the concept of custom power devices is
introduced recently. One of those devices is the Dynamic Voltage Restorer (DVR), which is
the most efficient and effective modern custom power device used in power distribution
networks. DVR is a recently proposed series connected solid state device that injects voltage
into the system in order to regulate the load side voltage. It is normally 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, DVR can also added other
features like: line voltage harmonics compensation, reduction of transients in voltage and fault current limitations.
OPERATION
In normal conditions, the dynamic voltage restorer operates in stand-by mode.
However, during disturbances, nominal system voltage will be compared to the voltage
variation. This is to get the differential voltage that should be injected by the DVR in order to
maintain supply voltage to the load within limits.
The amplitude and phase angle of the injected voltages are variable, thereby allowing
control of the real and reactive power exchange between the dynamic voltage restorer and the
distribution system. The DC input terminal of a DVR is connected to an energy storage
device of appropriate capacity. As mentioned, the reactive power exchange between the DVR
and the distribution system is internally generated by the DVR without AC passive reactive
components. The real power exchanged at the DVR output AC terminals is provided by the
DVR input DC terminal by an external energy source or energy storage system.
BUILDING BLOCKS OF DVR
i. Detection and control block
ii. Voltage source inverter
iii. Filter components
iv. Isolation transformers
BASIC CONFIGERATION OF DVR
The general configuration of the DVR consists of:
I. An Injection/ Booster transformer
ii. A Harmonic filter
Iii. Storage Devices
Iv. A Voltage Source Converter (VSC)
v. DC charging circuit
INJECTION OR BOOSTER TRANSFORMER
The Injection or Booster transformer is a specially designed transformer that attempts
to limit the coupling of noise and transient energy from the primary side to the secondary
side. Its main tasks are:
It connects the DVR to the distribution network via the HV-windings and
transforms and couples the injected compensating voltages generated by the voltage
source converters to the incoming supply voltage.
In addition, the Injection / Booster transformer serves the purpose of isolating the
load from the system (VSC and control mechanism).
HARMONIC FILTER
The main task of harmonic filter is to keep the harmonic voltage content generated by
the VSC to the permissible level. The filtering scheme in the dynamic voltage restorer can be
placed either on the high voltage-side or the inverter side of the series injection transformer.
The advantage of the inverter-side filter is that it is on the low-voltage side of the series
transformer and is closer to the harmonic source. Using this scheme, the high-order harmonic
currents will be prevented from penetrating into the series transformer thus reducing the
voltage stress on the transformer. However, when the DVR acts as a source the introduction
of the filter inductor L causes voltage drop and a phase-angle shift in the fundamental
component of the inverter output. This can affect the control scheme of the DVR. As the
filter is located on the high-voltage side of the series transformer, high-order harmonic
currents will penetrate into the series transformer, thus necessitating a higher rating of the
transformer. However, the leakage reactance of the transformer can be used to aid the
filtering characteristic. A common problem facing these two filtering schemes is that the
filter capacitor will cause an increase in the inverter rating.
STORAGE DEVICE
Energy storage is required to provide real power to the load when large voltage sags
take place. Examples of energy storages are lead-acid batteries, flywheel, SMES, etc.
The energy storage device used for this study is lead-acid batteries. Energy storage batteries
are devices in which electric energy is stored in electrochemical form by creating electrically
charged ions. Batteries provide rapid response for either charge or discharge, but the
discharge rate is limited by chemical reaction rates so that the available energy depends on
the discharge rate. Lead-acid battery technology is also a mature technology.
VOLTAGE SOURCE CONVERTER
A VSC is a power electronic system consists of a storage device and switching
devices, which can generate a sinusoidal voltage at any required frequency, magnitude, and phase angle. In the DVR application, the VSC is used to temporarily replace the supply
voltage or to generate the part of the supply voltage which is missing.
There are four main types of switching devices: Metal Oxide Semiconductor Field
Effect Transistors (MOSFET), Gate Turn-Off Thyristors (GTO), Insulated Gate Bipolar
Transistors (IGBT), and Integrated Gate Commutated Thyristors (IGCT). Each type has its
own benefits and drawbacks. The IGCT is a recent compact device with enhanced
performance and reliability that allows building VSC with very large power ratings. Because
of the highly sophisticated converter design with IGCTs, the DVR can compensate dips
which are beyond the capability of the past DVRs using conventional devices. The purpose of
storage devices is to supply the necessary energy to the VSC via a dc link for the generation
of injected voltages. The different kinds of energy storage devices are Superconductive
magnetic energy storage (SMES), batteries and capacitance.
DC CHARGING CIRCUIT
The dc charging circuit has two main tasks.
The first task is to charge the energy source after a sag compensation event.
The second task is to maintain dc link voltage at the nominal dc link voltage.
CONTROL AND PROTECTION
The control mechanism of the general configuration typically consists of hardware
with programmable logic. All protective functions of the DVR should be implemented in the
software. Differential current protection of the transformer, or short circuit current on the
customer load side are only two examples of many protection functions possibility
The main considerations for the control system of a DVR include: detection of the start and
finish of the sag, voltage reference generation, transient and steady-state control of the
injected voltage, and protection of the system. The control system was used to control the
DVR with a sampling and switching frequency. It requires measurement of four parameter
groups.
Three phase-voltages before the DVR to detect voltage sag and for feed forward
control of the output voltage.
Three-phase voltages after the DVR for feed-back control of the output voltage.
Three currents in the converter to protect the DVR by both saturation control and over
current.
The dc-link voltage for dc voltage compensation (to decouple the controller from
variations in the dc-link voltage), for converter protection, and to provide energy
storage information.
With the grid voltage in its normal level the DVR is held in a null state to keep the losses to a
minimum. Once voltage sag is detected the DVR converts into active mode to react as fast as
possible and inject the required ac voltage to the grid.