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Chapter I INTRODUCTION
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Chapter I
INTRODUCTION
1. Introduction
1.1. Introduction
The modern power distribution network is constantly being faced with an
ever-growing load demand. Distribution networks experience distinct change from a low
to high load level everyday. Electric load growth and higher regional power transfers in a
largely interconnected network becoming more complex and less secure power system
operation. Power generation and transmission facilities are unable to meet these new
demands.
Many loads at various distribution ends like domestic utilities ,computers,
process industries, adjustable speed drives, printers microprocessor based equipments etc.
have become intolerant to voltage fluctuations, harmonic content and interruptions.
Growth of electronic loads has made the quality of power supply a critical issue. There
fore numerous problems have to be attended in monitoring the operation of such a
system, like voltage fluctuations, power losses, etc. Power system engineers facing these
challenges to operate the system in more a flexible.
Electrical power losses in distribution systems correspond to about 70% of
total losses in electric power systems. These electrical losses can be considerably reduced
through the installation and control of reactive support equipments, such as capacitor
banks, reducing reactive currents in distribution feeders and so on.
Conventional solutions for solving distribution network problems, like
tap-changing transformers to control the voltage along feeders are no longer viable,
because the distribution network will be changed from a passive network into an active
network and thus the voltage profile is not predictable any more. One of the most severe
problems faced by distribution networks operators is voltage drop along distribution
feeders, which is caused by real and reactive power flow. Voltage control is a difficult
task because voltages are strongly influenced by random load fluctuations.
Voltage profile can be improved and power losses can be considerably
reduced by installing Custom Power Devices or Controllers at suitable location. These
controllers which are also named Distribution Flexible AC Transmission System (D-
2
FACTS) [1] are a new generation of power electronics-based equipment aimed at
enhancing the reliability and quality of power flows in low-voltage distribution networks.
Custom power is formally defined as the employment of power electronic
or static controllers in distribution systems rated up to 38 kV for the purpose of supplying
a level of reliability or PQ that is needed by electric power customers who are sensitive to
power variations. Custom power devices or controllers [2-3] include static switches,
inverters, converters, injection transformers, master-control modules and energy-storage
modules that have the ability to perform current-interruption and voltage-regulation
functions with in a distribution system.
Custom Power Devices is classified into three categories by their structures such as
Dynamic Voltage Restorer (DVR), Distribution STATCOM (DSTATCOM) and Solid-
State Breaker (SSB). In the present paper D-STATCOM, a member of Custom power
controllers family, is considered.
The D-STATCOM is a shunt-connected, solid-state switching power
converter that provides flexible voltage control at the point of connection to the utility
distribution feeder for power quality (PQ) improvements and also exchanges both active
and reactive power (current) [4] with the distribution system by varying the amplitude
and phase angle of the converter.
Since this device is utilized in steady-state condition for long term,
because of limited capacity of energy storage system, it cannot inject active power to the
system for long term. Therefore, a suitable model for D-STATCOM has been proposed
in load flow program, which is applicable in large distribution systems.
The effects of D-STATCOM on voltage improvement at other nodes are considered and
the optimum location of D-STATCOM in the distribution network is determined.
In the proposed method D-STATCOM is considered in modified load flow
computations. Further the optimal location is identified to place D-STATCOM for the
purpose of loss reduction and voltage improvement.
Load flow is an important method for analysis, operation and planning
studies of any power system in a steady-state condition. In this paper an efficient method
for node and line identification utilized in load flow has been proposed.
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The load flow method [5] is modified by considering all the line
parameters and load flow solution is obtained. By considering these modifications the
line losses are still reduced and voltage profile is also improved. D-STATCOM is then
modeled [6] and incorporated in the system under consideration. The results obtained are
highly satisfactory and hence the method can be applied a system of any size.
1.2. Distribution System
The electrical system between the substation fed by transmission system
and consumers meters is called distribution system. It is the final stage in the transfer of
power to the individual customers. It generally consists of feeders, distributors and the
service mains.
(a) Feeders: A feeder is a conductor which connects the substation to the area where
power is to be distributed .Generally tappings are taken from the feeders, so that current
remains same through out .The main consideration in the design of feeder is the current
carrying capacity.
(b) Distributor: A distributor is a conductor from which tappings are taken from supply to
the consumers. While designing the distributor, voltage drop along its length is the main
consideration.
(c) Service main: A service main is generally a small cable which connects the distributor
to the consumers terminals.
The a.c.distribution system is classified in to Primary distribution system,
Secondary distribution system.
Distribution substation:
The distribution system is fed through distribution substation. Each
substation normally serves its own load area, which is a subdivision of the area served by
the distribution system. At the distribution substation the sub transmission voltage is
reduced for general distribution throughout the area. The substation consists of one or
more power transformers together with the necessary voltage regulating equipments,
buses and switchgear. The substation designs are based on the consideration such as load
density, high side voltage, low side voltage, reliability, voltage drop, cost and losses.
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Primary distribution system
It is the part of a.c.distribution system which operates at voltages some
what higher than general utilization and handles large blocks of electrical energy than the
average low voltage consumer uses. The voltage used for primary distribution depends
upon the amount of power to be conveyed and the distance of the substation to be used.
The most commonly used primary distribution voltages are 11kV, 6.6kV and 3.3kV.
Primary feeder supply small industrial consumers. Due to economic considerations
primary distribution is carried out by 3-phase, 3-wire system.
Secondary distribution system
It is that part of a.c.distribution system which includes the range of
voltages at which the ultimate consumer utilizes the electrical energy delivered. The part
of the electric utility system which is between the primary system and the consumers
property is called the 'secondary system'. Here distribution transformers are connected to
primary feeder and its sub feeders and laterals. These transformers serve to step down the
voltage from distribution voltage to the utilization voltage. Each transformers supplies a
consumer or group of consumers over its secondary circuit through his service leads and
meter. The secondary distribution employs 400/230v, 3-phase, and 4-wire system.
Utilization:-
Loads of power systems are divided into industrial, commercial and
residential. Large industrial loads are served directly from the sub transmission network
.Small industrial loads are served from the primary distribution network.
Commercial and residential loads consist largely of lighting, heating and
cooling .These loads are independent of frequency and consume negligibly small amount
of reactive power.
The ratio of power utilized by the consumers of electric power to the
power produced at generation stations must be high as possible. In other words the losses
occurring in carrying electric power from the generator to the consumers must be kept at
the minimum. These losses are called “line losses” or I2R losses in the line.
5
Distribution losses
It has been established that 70% of the total losses are occurring in the
primary and secondary distribution system, while transmission and sub transmission lines
account for only 30% of the total losses. There fore the primary and secondary
distribution system must be properly planned to ensure losses within the acceptability
limits.
(a) Factors effecting distribution system losses
Factors contributing to the increase in the line losses in the primary and
secondary distribution system are Feeder length, Inadequate size of conductor, Location
of distribution transformer, Use of over rated distribution transformers, Low voltages,
Low power factor and Poor workman ship in fittings.
(b) Methods for reduction of line losses
The following methods are adopted for reduction of distribution system
losses are Constriction of new substation, Reinforcement of the feeder, Reactive power
compensation, HV distribution system, Grading of conductor, Using shunt compensation
techniques, Feeder reconfiguration and DG unit placement. Distribution power losses can be considerably reduced by installing
Custom Power Devices or Controllers at suitable location. These controllers which are
also named Distribution Flexible AC Transmission System (D-FACTS) are a new
generation of power electronics-based equipment aimed at enhancing the reliability and
quality of power flows in low-voltage distribution networks. ). D-FACTS mean FACTS
(Flexible AC Transmission Systems) (Hingorani, 1993) that are diverted to distribution
systems.
1.3. Voltage Improvement Techniques
To improve the power quality some devices need to be installed at a
suitable locations. These devices are called custom power devices, which make sure that
customers get pre specified quality and reliability of supply the compensating devices
compensate a load, i.e., its power factor, unbalance conditions or improve the power
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quality of supplied voltage, etc. some of power quality improvement techniques are given
as below.
1.3.1. Shunt Capacitors
Regulation of the power factor to increase the transmission capability and reduce
transmission losses. Shunt capacitors are primarily used to improve the power factor in
transmission and distribution network, resulting in improved voltage regulation, reduced
network losses, and efficient capacity utilization. .Improved transmission voltage
regulation can be obtained during heavy power transfer conditions when the system
consumes a large amount of reactive power that must be replaced by compensation.
At the line surge impedance loading level, the shunt capacitor would decrease the
line losses by more than 30%.In distribution and industrial systems, it is common to use
shunt capacitors to compensate for the highly inductive loads, thus achieving reduced
delivery system losses and network voltage drop.
Benefits
Improved power factor
Reduced transmission losses
Increased transmission capability
.Improved voltage control
Improved power quality
Other Application
Harmonic filters
1.3.2. Shunt Reactors
The primary purpose of the shunt reactor is to compensate for capacitive charging
voltage, a phenomenon getting prominent for increasing line voltage. Long high voltage
transmission lines and relatively short cable lines (since a power cable high capacitance
to earth) generate a large amount of reactive power during light power transfer conditions
which must be absorbed by compensation. Otherwise, the receiving terminals of the
transmission lines will exhibit a voltage rise voltages .A better fine tuning of the reactive
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power can be made by the use of a tap changer in the shunt reactor .It can be possible to
vary the reactive power between 50 to 100% of the needed power.
Benefits
Simple and robust customer solution with low installation costs and minimum
maintenance
No losses from an intermediate transformer when feeding reactive
compensation from a over voltage level
No harmonics created which may require filter banks
1.3.3. Series Capacitor
Voltage flickers can become a significant problem for power distributor
when large motor loads are introduced remote locations. Installation of a series capacitor
in the feeder strengthens the network and allows such load to connected to existing lines,
avoiding more significant investment in new substations or new distribution lines.
The use of mini cap on long distribution feeders provides self regulated
reactive power compensation that efficiently reduces voltage variations during large
motor starting.
Benefits
Reduce voltage fluctuations(flicker)
Improved voltage profile along the line
Easier starting of large motors
Self regulation
1.3.4. Filters
Harmonic filters may be used to mitigate, and in some cases, eliminate problems
created power system harmonics. Non linear loads such as rectifiers, converters, home
electronic appliances, and electric arc furnaces cause harmonics giving rise to extra losses
in power equipment such as transformers, motors and capacitors. They can also causes
other, probably more serious problems, when interfering with control systems and
electronic devices. Installing filters near the harmonic sources can effectively reduce
harmonics.
8
These filters consist of capacitor banks with suitable tuning reactors and damping
resistors. For small and medium size loads, active filters, based on power electronic
converters with high switching frequency, may be a more attractive solution.
Benefits
Eliminates harmonics
Improved power factor
Reduce transmission losses
Increased transmission capability
Improved voltage control
Improved power factor
Other applications
Shunt capacitors
1.3.5. Static Var Compensator
Static var compensators are used in transmission and distribution network mainly
providing dynamic voltage support in response to systems disturbances and balancing the
reactive power demand of large and fluctuating industrial loads. A static var compensator
is capable of both generating and absorbing variable reactive power continuously as
opposed to discrete values of fixed and switched shunt capacitors or reactors.
With continuously variable reactive power supply, the voltage at the svc bus may be
maintained smoothly over a wide range of active power transfer or system loading
conditions. This entails the reduction of network losses and prevention of adequate power
quality to the electric energy end users.
Static var compensators are mainly used to perform voltage and reactive power
regulation. However, when properly placed and controlled, Static Var Compensators can
also effectively counteract system oscillations. A Static Var Compensator, in effect, has
the ability to increase the damping factor (typically by 1-2 MW per Mvar installed) on a
bulky power system witch is experiencing power oscillators.
Static Var Compensator (SVC) is used most frequently for compensation of
disturbances generated by the Electrical Arc Furnaces (EAF) with a well-designed Static
Var Compensator (SVC), disturbances such as flicker from the EAF are mitigated
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Flicker, the random. The random voltage variations can also be disturbing to other
process equipment fed from the same grid. The proper mitigation of flicker is therefore a
matter of power quality improvement as well as an improvement to human environment.
Benefits:
Increased power transfer capability
Additional flexibility in grid operation
Improved grid voltage stability
Improved grid voltage control
Improved power factor
Other applications:
Power oscillation damping
Power quality (Flicker Mitigation, Voltage, Balancing)
Grid voltage support
1.3.6. STATCOM
Static Compensator, when connected to the grid, can provide dynamic voltage
support in response to system disturbances and balance the reactive power demand of
large and fluctuating industrial loads. A Static Compensator is capable of both generating
and absorbing variable reactive power continuously as opposed to discrete values of fixed
and switched shunt capacitors or reactors. With continuously variable reactive power
supply, the voltage at the Static Compensator bus may be maintained smoothly over a
wide range of system operation conditions .This entails reduction of network losses and
provision of sufficient power quality to the electric energy end- users.
Static Compensator uses voltage source converters to improve furnace
productivity similar to a traditional Static Var Compensator while offering superior
voltage flicker mitigation due to fast response time. Similar to Static Var Compensator,
the Static Compensator can elegantly be used to restore voltage and current balance in the
grid, and to mitigate voltage fluctuations generated by the traction loads.
Benefits Increased power transfer capability Additional flexibility in grid operation Improved grid voltage stability
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Improved grid voltage control Improved power factor Eliminated flicker Harmonic filtering Voltage balancing Power factor correction Furnace/mill process productivity improvement
Other Applications Power quality(Flicker mitigation, Voltage balancing) Grid voltage support
1.3.7. Dynamic voltage Restorer
It is a series compensating device. It is used for protecting a sensitive load that is connected downstream from sag/swell etc. It can also regulate the bus voltage at the load terminal.
1.3.8. Static Current Limiter (SCL) It limits a fault current by quickly inserting a series inductance in the fault path.
1.3.9. Static Circuit Breaker It breaks a faulted circuit much faster than a mechanical circuit breaker.
1.3.10. Static Transfer Switch (STS)
It is connected in the bus tie position when a sensitive load is supplied between two feeders. It protects the load by quickly transferring it from the faulty feeder from the healthy feeder.
1.3.11. Unified Power Quality Conditioner (UPQC) This device, like the Unified Power Flow Controller (UPFC) consists of two
voltage inverters. The capabilities of this device are still unexplored .However it can simultaneously perform the tasks of Distribution Static Compensator and Dynamic Voltage Restorer.
11
1.4. Literature Survey
From literature there exist several control strategies which are usually
based on mathematical approach. Plenty of work has been dedicated to applying the
mathematical optimization techniques for system planning. Before the emergence of
FACTS devices, early research on planning reactive power compensation has employed
linear programming [9], discrete programming [10], parameter sensitivity [11], nonlinear
programming [12], etc. So far, with the development of computer technology and
optimization theory, more and more sophisticated models recently have been established
for FACTS devices allocation problems.
A method of applying shunt capacitors for voltage control and peak loss
reduction is discussed [13]. The concept is extended to the optimization of total monetary
savings due to both peak loss and energy loss reductions. A computer program is
developed to aid engineers in the application of such a method. In [14], a successful
attempt was made to solve the problem using the dynamic programming approach. This
optimization technique has eliminated the previously mentioned problems of optimum
number and standard bank size. The method, however, was capable of dealing with the
fixed type of capacitors only.
In [15], the location of SVC is determined by modal analysis of reduced
power flow Jacobian matrix. The critical mode is found by stressing the system to the
vicinity of the saddle node of P–V curve, so that the voltage stability issue can be
addressed. This method is verified via a 1380 bus system. Three types of FACTS
controllers are considered, SVC, TCSC and UPFC. The location with the best average
controllability index of FACTS controllers is selected. In addition, an extended voltage
phasers approach (EVPA) [16] is established for SVC, TCSC and STATCOM allocation
to enhance loadability and voltage stability respectively. The authors in [17] propose a
scheme for TCSC and TCPST planning via linear optimal power flow (OPF) method.
The location is selected via implementing OPF to optimise the cost of installation and
size. In addition, market consideration is also included in TCSC placement.
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The D-STATCOM (distribution static compensator) with fast response is
an effective solution for improving the power quality of distribution systems. The
dynamic compensation of D-STATCOM in 10/0.4 kV distribution system is simulated
with Matlab, which proves the superiority and feasibility of D-STATCOM.
It is also quite interesting to note that the Bharat Heavy Electric Limited
(BHEL), India was successful in developing distribution scale STATCOM also known as
D-STATCOM which has successfully been installed in industry. The worlds first
commercial STATCOM (±80 MVA, 154 kV) was developed by Mitsubishi Electric
Power Products, Inc. and installed at Inuyama substation in Japan in 1991. STATCOM
also finds its application in industries for flicker reduction.
Generally, distribution networks are radial and the R/X ratio is very high.
For this reason, conventional Newton- Raphson (NR) [18] and fast decoupled load-flow
[19] methods do not converge. Goswami and Basu [20] have presented a direct method
for solving radial and meshed distribution networks. However, the main limitation of
their method is that no node in the network is the junction of more than three branches,
i.e. one incoming and two outgoing branches. Jasmon and Lee [21-22] have proposed a
new load-flow method for obtaining the solution of radial distribution networks. They
have used the three fundamental equations representing real power, reactive power and
voltage magnitude. They have solved the radial distribution network using these three
equations by reducing the whole network into a single h e equivalent. Das et al. [23] have
proposed a load-flow technique for solving radial distribution networks by calculating the
total real and reactive power fed through any node. They have proposed a unique node,
branch and lateral numbering scheme which helps to evaluate exact real- and reactive
power loads fed through any node and receiving-end voltages.
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1.5. Scope of the Project
In this project, the structure and principle of operation, implementation of
Distribution Static Synchronous Compensator are discussed. And the Proposed method
for modeling D-STATCOM is considered in modified load flow computations. Further
the optimal location is identified to place D-STATCOM for the purpose of loss reduction
and voltage improvement and program is done.
Such device is employed to provide continuous voltage regulation using controlled
converter. The advantage of this type of compensator over conventional SVC’s is the
improved speed of response. This speed of response means that such a device is ideally
suited to application with a rapidly varying load.
Two standard distribution systems consisting of IEEE-15 and IEEE-29
buses are considered and the D-STATCOM model is applied to load flow and
corresponding results are also presented and are compared.
.
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Chapter II
CUSTOM POWER DEVICES
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2. Custom Power Devices
2.1. Introduction
Modern power systems are complex networks where hundreds of
generating stations and thousands of load centers are interconnected through long power
transmission and distribution networks. The main concern of consumers is the quality and
reliability of power supplies at various load centers where they are located at .Even
though the power generation in most well developed countries is fairly reliable.
Power distribution systems ideally, should provide their customers with an
uninterrupted flow of energy at smooth sinusoidal voltage at the constant magnitude level
and frequency. However in practice power systems especially the distribution systems
have numerous non linear loads, which significantly effect the quality of power supplies.
As a result of the non linear loads, the purity of the waveform of supplies is lost. This
ends up producing many power quality problems. Apart from non linear loads some
system events both usual (e.g. capacitor switching, motor starting) and unusual (e.g.
faults) could also inflict power quality problems. The consequence of power quality
problems could range from a simple nuisance flicker in the electrical lamps to loss
thousands of dollars due to production shutdown.
A power quality problem is defined as any manifested problem in voltage
or current or leading to frequency deviations that result in failure or misoperation of
customer equipment .
Voltage sag is defined as the sudden reduction of supply voltage down
90% to 10% of nominal, followed by a recovery after a short period of time. A typical
duration of sag is, according to the standard 10 ms to one minute .Voltage sag can cause
loss of production in automated process since voltage sag can trip a motor or cause its
controller to malfunction.
Voltage swell, on the other hand, is defined as a sudden increasing of
supply voltage up 110% t0 180% in rms voltage at the network fundamental frequency
duration 10ms to one minute. Switching off a large inductive load or energizing a large
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capacitor bank in a typical system event that causes swells. To compensate the voltage
sag or swell in a power distribution system, appropriate devices need to be installed at
suitable location. These devices are typically placed at the point of common coupling
(PCC) which is defined as the point where the ownership of the network changes.
2.2. Custom Power Technology
The concept of custom power was introduced by N.G .Hingorani[2] in
1995.Like for transmission systems , the term custom power pertains to the use of power
electronic controllers in a distribution system , especially, to deal with various power
quality problems. Just as FACTS improves the power transfer capabilities and stability
margins, custom power makes sure customers get pre-specified quality and reliability of
supply. This pre-specified quality may contain a combination of specifications of the
following.
1. Low phase unbalance
2. No power interruptions
3. Low flicker at the load voltage
4. Low harmonic distortion in load voltage
5. Magnitude and duration of over voltages or under voltages with in specified limits
6. Acceptance of fluctuations
7. Nonlinear and poor power factor loads without significant effect on terminal voltage
8. "tight" voltage regulation including short duration sags or swells
These can be done on the basis of an individual, large customer, industrial
or commercial parts or a supply for a high community on wide area basis. Custom power
technology is a general term for equipment capable of mitigating numerous power quality
problems Basic functions are fast switching and current or voltage injection for
correcting anomalies in supply voltage or load current, by injecting or absorbing reactive
and active power respectively.
The concept of Flexible Alternating Current Transmission Systems
(FACTS) and Custom Power is widely studied by the researcher. FACTS use Power
electronic devices and methods to control the high-voltage side of the network for
improving the power flow. Custom Power is for low-voltage distribution, and improving
17
the poor power quality [24] and reliability of supply affecting factories, offices and
homes. Power quality and Reliability are becoming important issues for critical and
sensitive loads after introducing the term of Custom Power by Hingorani in early 1980s.
Custom power is formally defined as the employment of power electronic
or static controllers in distribution systems rated up to 38 kV for the purpose of supplying
a level of reliability or PQ that is needed by electric power customers who are sensitive to
power variations. Custom power devices or controllers [6] include static switches,
inverters, converters, injection transformers, master-control modules and energy-storage
modules that have the ability to perform current-interruption and voltage-regulation
functions with in a distribution system.
The power electronic controllers that are used in the custom power
solution can be a network reconfiguring type or a compensating type. The network
reconfigurating devices are usually called switchgears which include current limiting,
current breaking and current transferring devices. The solid state or static versions of the
devices are called: solid state current limiter (SSCL), solid state breaker (SSB), and solid
state transfer switch (SSTS). The compensating devices compensate a load, i.e. its power
factor, unbalance conditions or improve the power quality of supplied voltage, etc. These
devices are either connected in shunt or in series or a combination of both. This class of
devices includes the distribution static compensator (D-STATCOM), dynamic voltage
restorer (DVR), and unified power quality conditioner (UPQC) [2]. Among compensating
devices, a Dynamic Voltage Restorer can deal with voltage sags and swells which are
considered to have a severe impact on manufacturing places such as semiconductors and
plastic products, food processing places and paper mills.
Custom Power Devices is classified into three categories by their
structures such as Dynamic Voltage Restorer (DVR), Distribution STATCOM
(DSTATCOM) and Solid-State Breaker (SSB). Two of the devices DSTATCOM and the
DVR share a similar architecture. Both are based on the voltage source converter. DVR is
connected in series with the line where as DSTATCOM is in shunt with the line across
the load. Among these devices, the main purpose of DVR that injects voltage in series
with a distribution feeder is reducing the effect of short-term voltage sags, dips, swells
and momentary interruptions.
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The proposed system has a function of generating and absorbing voltage
by self-charging control technique. This system has three states: 1) normal operation, 2)
charging operation and 3) recharging operation. The paper discusses control issues and
the proposed control algorithm. The proposed control technique is applied to
DSTATCOM for protecting voltage sags, swell and momentary interruption.
2.3. Family Custom Power Devices: -
The family of emerging power electronic devices being offered to achieve
these Custom Power [2-3] objectives includes:
(a) Distribution Static Compensator (D-STATCOM) to protect the distribution system
from the effects of a polluting, e.g. fluctuating, voltage sags, swells, transients or
harmonics non-linear (harmonics producing), and load.
(b) Dynamic Voltage Restorer (DVR) to protect a critical load from disturbances, e.g.
sags, swells, transients or harmonics, originating on the interconnected transmission or
distribution system.
(c) Solid-State Breaker (SSB) to provide power quality improvement through
instantaneous current interruption thereby protecting sensitive loads from disturbances
that conventional electromechanical breaker cannot eliminate.
(d) Solid-State Transfer Switch (SSTS) to instantaneously transfer sensitive loads from
a disturbance on the normal feed to the undisturbed alternate feed.
2.3.1. Distribution Static Compensator (D-STATCOM)
The D-STATCOM is a solid-state dc to ac switching power converter that
consists of a three-phase, voltage-source forced air-cooled inverter. In its basic form, the
D-STATCOM injects a voltage in phase with the system voltage, thus providing voltage
support and regulation of VAR flow.
The D-STATCOM can also be used to reduce the level of harmonics on a
line. Because the D-STATCOM continuously checks the line waveform with respect to a
reference AC signal, it always provides the correct amount of harmonic compensation.
By a similar argument, the D-STATCOM is also suitable for reducing the impact of
voltage transients. The amount of load that can be supported is determined by the MVA
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rating of the inverters, and the length of time that the load can be maintained by the
amount of energy storage provided.
The D-STATCOM is available in ratings from 2 to 10 MVA in modular 2-
MVA increments. These are similar in performance to SVC. Using only capacitors or
inductors or batteries, these devices can draw / supply both leading and lagging currents.
They have a very good response time and are more suitable for special industrial loads
like arc furnaces.
2.3.2. The Dynamic Voltage Restorer (DVR)
The DVR is a solid-state dc to ac switching power converter that injects a
set of three single-phase ac output voltages in series with the distribution feeder and in
synchronism with the voltages of the distribution system. By injecting voltages of
controllable amplitude, phase angle and frequency (harmonic) into the distribution feeder
in instantaneous real time via a series injection transformer, the DVR can "restore" the
utility of voltage at its load-side terminals when the quality of the source-side terminal
voltage is significantly out of specification for sensitive load equipment. The reactive
power exchanged between the DVR and the distribution system is internally generated by
the DVR without any ac passive reactive components, i.e. reactors and capacitors. For
large variations (deep sags) in the source voltage, the DVR supplies partial power to the
load from a rechargeable energy source attached to the DVR dc terminal. The DVR is
available in ratings from 2 to 10 MVA in modular 2-MVA increments.
The DVR is capable of generating and absorbing the voltage
independently controllable real and reactive power. It consists of three-phase voltage
source inverter, injection transformer, DC LINK and Rectifier for charging the DC LINK
or Battery. As you know, Rectifier is generating the harmonic problem in distribution
lines. Rectifier or devices for charging DC LINK is useless in this proposed system by
their structure.
2.3.3. Solid-State "Instantaneous" Current Interruption
Current interruption technology, utilizing high power Solid-State
Breakers (SSB), to solve most of the distribution system problems that result in voltage
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sags, swells, and power outages. When combined with a current limiting reactor or
resistor, the SSB can rapidly insert the current limiting device into the distribution line to
prevent excessive fault current from developing from sources of high short circuit
capacity, e.g. multi-sourced distribution substations. At the power levels associated with
15-kV and higher voltage class systems, commercially available Gate Turn-Off (GTO)
thyristors and conventional Thyristors (SCRs) can be used for the AC switch.
The SSB consists of two parallel-connected circuit branches: a solid-state
switch composed of GTOs and a solid-state switch using SCRs in series with a current
limiting reactor or resistor. The GTO switch is the main circuit breaker used to clear
source-side faults. It is rated for the maximum normal line current, but not rated for fault
currents. It is normally closed and conducts current uninhibited until the magnitude of the
current reaches a pre-set level at which point it opens rapidly interrupting the current
flow.
2.3.4. Solid-State "Instantaneous" Load Transfer
Introducing a line of Solid-State Transfer Switches capable of providing
uninterruptible power to critical distribution-served customers. Solid-state, fast acting
(sub-cycle) breakers can instantaneously transfer sensitive loads from a normal supply
that experiences a disturbance to an alternate supply that is unaffected by the disturbance.
The alternate supply may be another utility primary distribution feeder or a standby
power supply operated from an integral energy storage system. In this application, the
SSB acts as an extremely fast conventional transfer switch that allows the restoration of
power of specified quality to the load within 1/4 cycle.
The SSTS consists of two three-phase SSB's, each with independent
control. The status of the three individual phase switches in each SSB will be individually
monitored, evaluated, and reported by continuous real-time switch control and
protections circuits. The operation of the two SSB's will be co-ordinates by the transfer
switch control circuit that monitors the line conditions of the normal and alternate power
sources and initiates the load transfer in accordance with operator selectable criteria.
The SSTS can be provided with either SCR or GTO switches depending
upon the specific load transfer speed requirements. SSTS voltage and current ratings are
21
being developed for 4.16 to 34.5 kV and 300 to 1200 System protection practices are
accommodated in the SSTS available control modes depending upon the critical load
requirements and utility preferences/practices.
In this project report D-STATCOM, a member of Custom power
controllers family, is considered.
2.4 Distribution STATCOM 2.4.1. Introduction
Distribution STATCOM (D-STATCOM) is utilized to compensate power
quality problems and also it can quickly regulate its susceptance to provide dynamic
reactive compensation and regulate the bus voltages in the power system.
The D-STATCOM is a shunt-connected, solid-state switching power
converter that provides flexible voltage control at the point of connection to the utility
distribution feeder for power quality (PQ) improvements such as unbalanced load,
voltage sag, voltage fluctuation and voltage unbalance and also exchanges both active
and reactive power (current) [6] with the distribution system by varying the amplitude
and phase angle of the converter.
2.4.2. Voltage source converters (VSC)
A voltage-source converter is a power electronic device, which can
generate a sinusoidal voltage with any required magnitude, frequency and phase angle.
Voltage source converters are widely used in adjustable-speed drives, but can also be
used to mitigate voltage dips. The VSC is 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. The converter is normally based on some kind of energy
storage, which will supply the converter with a DC voltage. The solid-state electronics in
the converter is then switched to get the desired output voltage. Normally the VSC is not
only used for voltage dip mitigation, but also for other power quality issues, e.g. flicker
and harmonics.
22
Voltage source converters are of two type’s viz. series voltage controller and
shunt voltage controller. However D-STATCOM belongs to the shunt voltage controller.
In this project, the D-STATCOM is used to regulate voltage at the connecting bus.
2.4.3. Structure and Principle of Operation
(a) Structure
General structure of D-STATCOM is similar to STATCOM, which is
schematically shown in fig.1, consists of energy storage device, voltage source converter,
a coupling transformer connected in shunt to the distribution network through a coupling
transformer.
Fig 2.1: Schematic diagram of a D-STATCOM
Using a converter, the devices appear as fully synchronous sources which
are capable of absorbing and injecting reactive power on an electricity system at
distribution voltages.
In this model, D-STATCOM is capable of injecting active power in
addition to reactive power. Since this device is utilized in steady-state condition for long
term, because of limited capacity of energy storage system, it cannot inject active power
to the system for long term for voltage regulation purpose.
23
Therefore, for the steady-state application, D-STATCOM consists of a
small DC capacitor and a voltage source converter and the steady-state power exchange
between D-STATCOM and the ac system is reactive power.
But, there are several factors that must be considered when designing the
D-STATCOM and associated control circuits. In relation to the power circuit the
following issues are of major importance:
• DC link capacitor size
• Coupling transformer reactance and transformation ratio
• Output filters equipment
Fig 2.2: Schematic diagram of a D-STATCOM, only reactive power exchange.
The VSC connected in shunt with the ac system provides a multifunctional
topology which can be used for up to three quite distinct purposes:
1. Voltage regulation and compensation of reactive power;
2. Correction of power factor; and
3. Elimination of current harmonics.
24
(b)Principle of Operation
D-STATCOM is to suppress voltage variation and control reactive power
in phase with system voltage. It can compensate for inductive and capacitive currents
linearly and continuously.
The VSC converts the dc voltage across the storage device into a set of
three-phase ac output voltages. These voltages are in phase and coupled with the ac
system through the reactance of the coupling transformer. Suitable adjustment of the
phase and magnitude of the D-STATCOM output voltages allows effective control of
active and reactive power exchanges between the D-STATCOM and the ac system. Such
configuration allows the device to absorb or generate controllable active and reactive
power.
The controller of the D-STATCOM is used to operate the inverter in such a way that
the phase angle between the inverter voltage and the line voltage is dynamically adjusted
so that the D-STATCOM generates or absorbs the desired VAR at the point of
connection. By varying the amplitude of D-STATCOM output voltage can control the
reactive power exchange between the inverter and the AC system. If the amplitude of the
output voltage is increased above that of AC system voltage, the inverter generates
reactive power for the AC system. If the amplitude of the output voltage is decreased
below that of the AC system, the inverter absorbs the reactive power. If the output
voltage is equal to the AC system voltage, the reactive power exchange is zero, but
actually they have a little phase difference to compensate the loss of transformer winding
and inverter switching, so absorbs some real power from system.
The real power exchanges between the inverter and the AC system can be
controlled by altering the phase angles between the inverter output and the AC system
voltages. The inverter supplies real power to the AC system if the inverter output voltage
is made to lead the corresponding AC system voltage. Conversely, the inverter absorbs
real power from the AC system, if the inverter output voltage is made to lag the AC
system voltage.
There are two techniques for controlling the STATCOM. The first
technique, referred to as phase control, is to control the phase shift to control the
STATCOM output voltage magnitude. The other technique referred to as Pulse Width
25
Modulation (PWM) on the other hand allow for independent control of output voltage
magnitude and phase shift (phase angle of the output voltage); in this case, the DC
voltage is controlled separately from the AC output voltage.
The name is an indication that STATCOM has a characteristic similar to
the synchronous condenser, but as an electronic device it has no inertia and is superior to
the synchronous condenser in several ways, such as better dynamics, a lower investment
cost and lower operating and maintenance costs.
With the advent of D-STATCOM, better performance can be reached in areas such as:
• Dynamic voltage control in distribution systems;
• Power oscillation damping;
• Transient stability improvement;
• Ability to control not only reactive power but, if needed, also active power (with a DC
energy source available).
Such device is employed to provide continuous voltage regulation using
controlled converter. The advantage of this type of compensator has over conventional
SVC’s is the improved speed of response. This speed of response means that such a
device is ideally suited to application with a rapidly varying load.
26
Chapter III
PROPOSED METHOD
27
3. PROPOSED METHOD
3.1. Radial Distribution System Load Flow Method
3.1.1. Introduction
The most frequent study of an electrical power system, whether a transmission
or a distribution systems, corresponds to the analysis of the operating steady state
conditions. Efficient computer load flow methods have been developed based on busbar
admittance matrix and busbar impedance matrix. But these are designed thinking on
transmission systems, so that their application to distribution systems usually does not
provide good results.
Load flow is a very important and fundamental tool for the analysis of any
power system and is used in operational as well as planning stages.
Load flow analysis of distribution system has not received much attention unlike
load flow analysis of transmission systems. However some work has been carried out on
the load flow analysis of distribution network. Distribution Power Flow is an important
tool for the analysis of distribution system and it is used in the operational as well as in
planning stages.
Such a load flow method must be able to model the special features of distribution
systems in sufficient detail. The well-known characteristics of an electric distribution
system are
1) Radial or weakly meshed structure,
2) Multiphase and unbalanced operation,
3) Unbalanced distributed load,
4) Extremely large number of branches and nodes and
5) Wide-ranging resistance and reactance values.
Generally, distribution networks are radial and the R/X ratio is very high compared
to a transmission system. This makes the distribution system ill-conditioned. That is why
the conventional load flow method such as Newton-Raphson,(NR)[18] and the Fast
Decoupled Load Flow(FDLF)[19] method and their modification are not suitable for
solving the load flow problem of such an ill-conditioned system. For most of the cases
28
NR and FDLF methods failed to converge in solving the load flow problem of
distribution system or converged in high iterations.
In this paper, a modified load-flow technique is considered for solving
radial distribution networks. The proposed method involves only the evaluation of a
simple algebraic expression of receiving-end voltages. The proposed method is very
efficient. It is also observed that the proposed method has good and fast convergence
characteristics.
In this paper, a modified load-flow technique is considered for solving
radial distribution networks. The proposed method involves only the evaluation of a
simple algebraic expression of receiving-end voltages also node and line identification
[25] utilized in load flow has been proposed. The proposed method is very efficient. It is
also observed that the proposed method has good and fast convergence characteristics.
Two standard radial distribution systems consisting of IEEE-15 and IEEE-
29 nodes are considered for solving radial distribution system.
3.1.2. Mathematical formulation
3.1.2.1. Assumption
We assumed that the three-phase radial distribution networks are balanced and can be
represented by their equivalent single-line diagrams. This assumption is valid for 11kV
rural distribution network in India and elsewhere. Distribution lines have small line shunt
capacitance (different from the shunt capacitor banks that are considered as loads) is
considered.
3.1.2.2. Solution Methodology
The load flow method of radial distribution network can be solved in three sets of
equations.
1. Identification of the nodes beyond all the branches.
2. Determination of branch currents.
3. Determine the nodal voltages.
Procedure to determine the voltage at each bus
The distribution load flow method is used to calculate the voltage at each bus and
total real and reactive power losses.
29
Before proceeding to the fundamentals of power system control and
stability limits, some factors influencing active and reactive power flows on the power
system are needed to be discussed. The power transfer between two buses is related to
some parameters:
• Sending and receiving bus voltages
• Power angles between two buses
• Series impedances of the transmission line connecting the two buses.
Consider a single line diagram of two buses of a radial distribution system
as shown in Fig.3.1, the number of branches nb and the number of buses t are related
through t = nb+1.
Fig 3.1: Sing
Where R and X are
active and reactive powers o
PLS and QLS refers to the load
Initially, a flat volta
charging currents of all the lo
The load current of node k is
( )LkI k =
Where PLk(k) and QLk(k) ar
respectively.
The charging current at node
Vk VS
ILiPLS+jQLS
PLk+jQLk
le line diagram of two buses of a distribution system.
resistance and reactance of the branch. PLk and QLk are the
f node k. ILi is the current flowing in the line. Subscript ‘L’ in
connected at Sth bus.
ge (1 p.u) of all the nodes is assumed and load currents and
ads are computed using Eqs. (3.1) and (3.2) respectively.
( ) ( )*( )
Lk LkP k jQ kV k
−,
for k = 2, 3,……. nb (3.1)
e active and reactive power of load connected to node k,
k is
30
, for k = 2, 3… nb (3.2) 0( ) ( )* ( )CkI k y k V k= Here shunt admittance yo is considered as small.
Branch current
Branch Current I(n) is equal to the sum of the load currents of all the nodes beyond
that branch n plus the sum of the charging currents of all the nodes beyond that branch n
i.e.,
(3.3) 1 1
( ) ( ) ( )nb nb
Lk Ckk n i n
I n I k I k= + = +
= +∑ ∑Where branch impedance is Z = R + j X
Therefore, if it is possible to identify the nodes beyond all the branches, it is
possible to compute all the branch currents.
Voltage at buses
A generalized equation of receiving-end voltage, sending-end voltage, branch
current and branch impedance is
V (a2) = V (a1) - I (i) * Z (i) (3.4) Where i is the branch number and a1 and a2 are
a1 = RE (i)
a2 = SE (i)
Where RE (i) is the receiving end and SE (i) is the sending end of branch i.
Power losses
The real and reactive power loss of branch i are given
Lreal (i) = |I (i) |2 * R (i) (3.5)
Lreactive (i) = |I (i) |2 * X (i) (3.6) Where Lreal (i) and Lreactive (i) are the active and reactive power losses at branch i.
At first identification of the nodes beyond all the branches is realized through an
algorithm
31
3.2. Identification of the Nodes
For identification of nodes, consider the single line diagram of radial distribution system
feeder in fig.3.2.
Fig 3.2: Single-line diagram of radial distribution network
The branch number, sending-end and receiving-end node of this feeder are given
in table 1.
Table: 3.1 Given the line data. ---------------------------------------------------------- Branch Sending end Receiving end ---------------------------------------------------------- 1 1.000 2.000 2 2.000 3.000 3 3.000 4.000 4 4.000 5.000 5 3.000 6.000 6 6.000 7.000 7 4.000 8.000 8 8.000 9.000 ----------------------------------------------------------- First we define the variables:
i = 1, 2, 3, ..., nb ( i indicates branch );
ip is the node count (identifies the number of nodes beyond a particular branch);
IN (ip) is the node identifier (helping to identify nodes beyond all the branches);
N (i) is the total number of nodes beyond branch i; and
ie(i, ip + 1) is the receiving-end node.
Note here that before identification of nodes and branches, ip has to be reset to
zero.
ie (i, ip + 1) will now be explained. Consider the first branch in Fig. 3.1, i.e. i =
1; the receiving-end node of branch 1 is 2, i.e. RE (i) = RE (1) = 2. Therefore ie(i, ip +
1 2 3 4 5 (1) (2) (3) (4)
8
(5) (7)
6
7
(6) (8)
9
32
1) = ie(1, ip + 1) will help to identify all the nodes beyond branch 1. This will help to find
the exact current flowing through branch 1. Similarly, consider branch 2, i.e. i = 2; the
receiving-end node of branch 2 is 3, i.e. RE (i) = RE (2) =3.
Therefore, ie(i, ip + 1) = ie(2, ip + 1) will identify all the nodes beyond branch 2.
No node will be repeated while identifying nodes [25] beyond a particular branch.
Identification of nodes beyond all the branches, which helps in computing the exact
current flowing through all the branches, has been explained using an algorithm.
3.2.1. Algorithm 1: Identification of nodes beyond a branch Step 1.read the system data.
Step 2. i =1
Step 3. k = i + 1, set ip = 0
Step 4. nc = 0
if {RE (i) = SE(k)} and {ip = 0} go to step 10
Otherwise go to step 12
Step 5. if {ip = 0} go to step 10
Otherwise go to step 6
Step 6. it = 1
Step 7. if {RE(i) = ie(i, ip+1)}then nc = 1
Otherwise go to step 8
Step 8. it = it +1
If {it ≤ ip} go to step 7
Otherwise go to step 9
Step 9. if {nc = 1} go to step 12
Otherwise go to step 11
Step 10. ie (i, ip + 1) = RE(i)
Step 11. ip = ip +1
IN(ip) =1
ie(i, ip + 1) = RE(i)
N(i) = ip + 1,
Step 12. s = s + 1
If {s ≤ nb} go to step 6
Otherwise go to step 13
33
Step 13. if {iP =0} go to step 14
Otherwise go to step 15
Step 14. ie(i, ip + 1) = RE(i)
N(i) = ip + 1, go to step 15
Step 15. i = i + 1
If {i ≤ nb-1}go to step 3
Otherwise go to step 16
Step 16. ie(nb, 1) = RE(nb)
N(nb) = 1
Step 17. Stop
By using this algorithm we can find the identification of nodes beyond all branches.
Table: 3.2 Identification of nodes beyond all branches for fig.3.2 ------------------------------------------------- br nodes beyond branch ------------------------------------------------- 1 2 3 4 6 5 8 7 9 2 3 4 6 5 8 7 9 3 4 5 8 9 4 5 5 6 7 6 7 7 8 9 8 9 -------------------------------------------------- The load current and charging current of each node are calculated by using
Eqs. (3.1) and (3.2) and identification of nodes are determined. Then it is easy to
calculate the branch current is given by
(3.7) 1 1
( ) { ( , )} { ( , )}nb nb
k kI i IL ie i k IC ie i k
= =
= +∑ ∑ The voltage of each node is then calculated by using Eqn. (3.4). Real and
reactive power loss of each branch is calculated by using Eqs. (3.5) and (3.6),
respectively.
The convergence of the proposed method is that if, in successive iterations
the difference between the real and reactive power delivered from the substation is less
than 0.1kW and 0.1kVAr, then it has converged.
34
3.3. LOAD FLOW ALGORITHM
To determine the voltage at each node in radial distribution network, the
modified load flow method is used and the algorithm is as follows.
Step 1: Read the line and load data.
Step 2: Determine the nodes beyond each branch and their total number.
Step 3: Initialize the voltage of all nodes to 1p.u and phase angle to zero.
Step 4: Find all load currents and charging currents of each nodes using Eqn. (3.1) and
Eqn.(3.2) by using these branch currents are determined given in Eqn.(3.3)
Step 5: Calculate the voltages and phase angles at each node by using Eqn. (3.4).
Step 6: If the voltage at each node for two successive iteration is within a certain
tolerance (10-4p.u) the solution is reached go to step 8 else, repeat step 5 to 7 until
convergence is reached.
Step 7: Read the results
In this modified load flow method line charging capacitance is also taken
into account in step 4 in the above algorithm which is actually neglected in the load flow
method presented in reference 5.
The proposed method can be used to find out the voltages of nodes, D-
Statcom current, phase angle and injected reactive power by D-STATCOM. These are
used to determine the load currents in the proposed load flow method, and power losses.
Here the bus voltage magnitude in the node where D-STATCOM is located is set to a
nominal value of 1p.u.
3.4. Modeling of D-STATCOM 3.4.1. Assumptions
It is assumed that the source is a balance, sinusoidal three-phase voltage supply. .
However the following assumptions are considered.
• The three AC mains voltages are balanced
• The three - phase load is balanced and linear
• The inverter switches are ideal
• DC link output is ripple free
• The filter components are reactive and linear
35
3.4.2. Mathematical formulation
Consider a single line diagram of two buses of a distribution system shown in
Fig.3.3. and its branch currents and voltages are calculated by using phasor diagram
method.
Fig 3.3: Singl
Subscript ‘L’ in PLk an
Where R and
active and reactive pow
3.4.3. Phasor diagram
The phasor dia
Fig 3.4: P
The mathematical equa
SV
Vk VS
ILi PLS+jQLS
PLk+jQLk
e line diagram of two bus system of a distribution system
d QLk refers to the load connected to bus k.
X are resistance and reactance of the branch. PLk and QLk are the
ers of node k. ILi is the current flowing in the line
gram of the two buses of the distribution system shown in Fig.3.4.
hasor d
tion for
µ∠ =
βζ
VK
i
s -RILi
I
iagram of
two buse
KV β∠ −
Li
V
volta
s of t
LiZI
3
µ
-jXIL
ges and current of the system
he distribution system is given by
ζ∠ (3.8)
6
This gives relation between voltage and current.
Where SV µ∠ and KV β∠ are the voltage of buses K and S before compensation
respectively, Z = R + j X is the impedance between buses K and S, LiI ζ∠ is the current
flow in line. Voltage KV β∠ and LiI ζ∠ current are derived from the load flow
calculations.
3.4.4. Installing Distribution-STATCOM
A D-STATCOM consists of a two-level Voltage Source Converter
(VSC), a DC energy storage device, a coupling transformer connected in shunt to the
distribution network through a coupling transformer. The VSC converts the DC voltage
across the storage device into a set of three-phase AC output voltages. These voltages are
in phase and coupled with the AC system through the reactance of the coupling
transformer. Suitable adjustment of the phase and magnitude of the D-STATCOM output
voltages allows effective control of active and reactive power exchanges between the D-
STATCOM and the AC system. Such configuration allows the device to absorb or
generate controllable active and reactive power. A single-phase equivalent circuit of two buses of a distribution system and
its phasor diagram after installing D-STATCOM are shown in Fig.5 and Fig.6,
respectively. Generally, voltage of buses in the system is less than 1 p.u. and it is desired
to compensate voltage of interested bus (Vs) to 1 p.u. by using D-STATCOM.
By installing D-STATCOM in distribution system, all nodes voltage,
especially the neighboring nodes of D-STATCOM location, and branches current of the
network, change in the steady-state condition. D-Statcom is used to regulate voltage
variations and control reactive power in phase with system voltage. It can compensate for
inductive and capacitive currents linearly and continuously.
The schematic diagram of buses K and S of the distribution systems,
when D-STATCOM is installed for voltage regulation in bus S, is shown in Fig.3.5.
37
Fig 3.5: Single l
In this d
adjusting the voltage d
controlled by adjusting t
3.4.5. Phasor diagram m
The phasor diag
D-STATCOM at Sth nod
Fig 3.6: Phasor
Vk VS
k PLS+jQLS
PLk+jQL
ine diagram
STA
iagram, the
rop across
he output v
ethod:-
ram of the
e as shown
diagram of
ILi
ζ
ID-stat
ILi
of two buses of a distribution system with D-
TCOM consideration.
shunt injected current Istat corrects the voltage sag by
the system impedance. The value of Istat can be
oltage of the converter.
two buses of the distribution system after installing
in Fig.3.6.
µ
-jXILi
VK
s-RILi
voltages an
µnew
38
V
d currents of the system shown in Fig.3.5
-jX ID-stat
-RID-stat
Vsnew
Consequently, ID-Stat must be kept in quadrature with voltage of the
system. By installing D-STATCOM in distribution system, all nodes voltage, especially
the neighboring nodes of D-STATCOM location, and branches current of the network,
change in the steady-state condition.
3.4.6. Mathematical equations:-
From the phasor diagram we can see that
( )new K LiSnewV R jX IV µ β ζ∠ = ∠ − + ∠
( )2D Stat newR jX I π µ−− + ∠ + (3.9)
And the phase angle of injected D-STATCOM current (ID-stat) from phasor diagram is
2D stat newI π µ−∠ = + (3.10)
Where 2D Stat newI π µ− ∠ + is the injected current by D-STATCOM, newSnewV µ∠
is the voltage of bus S after compensation by D-STATCOM, KV β∠ is the voltage of
bus k after D-STATCOM installation, LiI ζ∠ is the current flow in line after D-
STATCOM installation. Voltage KV β∠ and current LiI ζ∠ are derived from the load
flow calculations.
Separating the real and imaginary parts of Eqn.(3.9) yields:
Using the notations below:
u1 = Re ( KV β∠ ) – Re (Z LiI ζ∠ ),
u2 = Im ( KV β∠ ) – Im (Z LiI ζ∠ ),
b = VSnew, c1 = -R, c2 = -X,
1λ = ID-Stat, 2λ = newµ .
Separating the real and imaginary parts of Eqn.(3.9) and using these notations
we get
bcos 2λ = u1 – c1 1λ sin 2λ – c2 1λ cos 2λ (3.11)
bsin 2λ = u1 – c2 1λ sin 2λ + c1 1λ cos 2λ (3.12)
39
Where a1, a2, c1 and c2 are constants, b is the magnitude of
compensated voltage (e.g. 1 p.u.), 1λ , 2λ are variables to be determined.
Rearranging Eqs. (3.11) and (3.12) for 1λ we get:
2 11
1 2 2
sinsin cosb u
c c 2
λλλ λ
−=
− − (3.13)
and
2 21
2 2 1
sinsin cosb u
c c 2
λλλ λ
−=
− + (3.14)
Where 1λ = ID-Stat,
By equating Eqs. (3.13) and (3.14), it can be shown that
(u1c2 – u2c1) sin 2λ + (- u1c1 – u2c2) cos 2λ + bc1=0, (3.15)
Considering δ= sin 2λ in Eqn.(3.15), we will get a quadratic equation in δ.
By solving the quadratic equation we get the value of δ.
(P12+P2
2) δ2 + (2P1bc1) δ +(b2c12
–P12) = 0, (3.16)
Where
P1 = (u1c2 – u2c1), P2 = u1c1 + u2c2
There are two roots for δ and therefore, two values are calculated for 2λ and 1λ , but
only one is acceptable. To determine the correct answer, these roots are examined under
boundary conditions in the load flow results:
If b = VSnew = VS, then 1λ = ID-Stat = 0 and
2λ = newµ = µ
After testing these conditions on load flow, correct answer is selected.
And then we know that 2λ = newµ and also we assume δ = sin 2λ , then
2λ = arc sin (δ). (3.17)
Then the injected reactive power by D-Statcom can be written as
jQD-Stat = VSnew I*D-Stat. (3.18)
and * denotes conjugate of complex variable.
40
Where
= SnewV Snew newV µ∠ (3.19)
ID-Stat = 2D Stat newI π µ− ∠ + (3.20)
After finding reactive power, current and voltage, load flow is run using Matlab code.
The real power injected by the D-STATCOM is equal to zero (i.e.,
Pinj=0) and the bus voltage magnitude in the node where D-STATCOM is located is set
to a nominal value of 1p.u.
The phase angle at the compensated node and the reactive power
injection of D-STATCOM are calculated by Eqs. (3.17) and (3.18), respectively. If the
reactive power generated (or absorbed) by the D-STATCOM, Qinj, exceeds the rating of
the D-STATCOM device, it is fixed at this value (i.e., Qinj=Srat).
3.5. PROPOSED ALGORITHM The algorithm proposed in this paper was developed in MATLAB. The
following section describes the control algorithm implemented for modeling D-
STATCOM in modified load flow method
To determine the voltages at each node, real and reactive power losses in
radial distribution network, the proposed method can be summarized in the following
algorithm.
Step 1: Read the line and load data.
Step 2: Determine the voltages and losses by using modified load flow method without
installing D-STATCOM.
Step 3: Install D-STATCOM and it is desired to compensate voltage of interested bus
(VS) to 1 p.u. and find out injected reactive power, phase angle and D-STATCOM
current (ID-stat) by using Phasor diagram method given in eqn. (3.18), eqn. (3.17)
and eqn. (3.20) respectively.
Step 4: Determine voltages by using Eqn. (3.9) and run modified load flow method
again.
41
Step 5: Then repeat step.2 and again place D-STATCOM at another location and repeat
step.3 and 4 to find out Voltages and losses at all nodes.
Step 6: Read the results and find out the node at which minimum losses is obtained and
it is selected for optimum location for D-STATCOM.
In this modified load flow method line charging capacitance is also taken
into account in the above algorithm which is actually neglected in the load flow method
presented in reference [5].
42
Chapter IV
RESULTS & DISCUSSION
43
4. RESULTS & DISCUSSION
7
8
1 32
9
9
10
14
15
54
8 11 12 13
6 11 12
10 7
41 2 3
13 5
146
In this project, software program has been developed in Matlab for simple
and modified load flow method and optimal location of D-STATCOM on radial
distribution system.
This Matlab program is tested for two systems viz., IEEE-15 bus and
IEEE-29 bus systems. The 15-bus and 29-bus data has been reported in tables. The
converged voltages on this system after doing load flow method are presented in table.
Then after installing D-STATCOM, at each node and the proposed method is applied.
The results obtained for these systems are briefly summarized in this section. 4.1. IEEE 15 bus system: 4.1.1. One line diagram of IEEE-15 bus radial distribution system
Fig 4.1: One line diagram of IEEE-15 bus radial distribution system
44
4.1.2. Line data and load data of 15-bus distribution system
The line data and load data of 15-bus distribution system are as follows.
Table 4.1: Line data for 15- bus radial distribution system
No of buses: 15, No of lines: 14, Base Voltage: 11KV, Base KVA: 100KVA
Branch Sending end
bus
Receiving end
bus
Resistance
R(ohms)
Reactance
X(ohms)
1 1 2 1.3531 1.3235
2 2 3 1.1702 1.1446
3 3 4 0.8411 0.82271
4 4 5 1.5235 1.0276
5 2 9 2.5573 1.7249
6 9 10 1.0882 0.734
7 2 6 1.2514 0.8441
8 6 7 2.0132 1.3579
9 6 8 1.6867 1.1377
10 3 11 1.7955 1.2111
11 11 12 2.4484 1.6515
12 12 13 2.0132 1.3579
13 4 14 2.2308 1.5047
14 4 15 1.1970 0.8074
45
Table 4.2: Load data for 15 bus radial distribution system
Pl Ql
0 0
44.0999 44.0999
70 71.4142
140 142.8285
44.9909 44.9909
140 142.8285
140 142.8285
70 71.4142
70 71.4142
44.9909 44.9909
140 142.8285
70 71.4142
44.9909 44.9909
70 71.4142
140 142.8285
No of buses: 15, No of lines: 14, Base Voltage: 11KV, Base KVA: 100KVA
46
4.1.3. Load Flow Results of 15-bus system The voltages for this system after running load flow are shown in table 4.3. Table 4.3: load flow solution with out D-STATCOM
Node no: Voltage Magnitudes
in p.u
1 1.0000
2 0.9720
3 0.9574
4 0.9516
5 0.9506
6 0.9589
7 0.9567
8 0.9576
9 0.9685
10 0.9673
11 0.9506
12 0.9463
13 0.9449
14 0.9493
15 0.9492
Without D-STATCOM, and running load flow method the total real and
reactive power loss of this system are 60.0628 kW and 55.5749 kVAr, respectively.
47
4.1.4. Results of 15-bus system after installing D-STATCOM
After installing D-STATCOM, running the load flow method, the Voltage
Magnitudes after compensation in p.u at 10th-node is given by table 4.4.
Table 4.4: load flow solution with D-STATCOM
Node no:
Voltage Magnitudes after compensation in p.u at 10th-node
1 1.0000
2 0.9780
3 0.9677
4 0.9623
5 0.9614
6 0.9655
7 0.9634
8 0.9643
9 0.9750
10 1.0000
11 0.9506
12 0.9468
13 0.9457
14 0.9601
48
Real and Reactive power losses of 15 – bus distribution system after
installing the D-STATCOM at each nodes is given by table 4.5.
Table 4.5: Real and Reactive power losses of 15 – bus distribution system after installing
the D-STATCOM at nodes are specified.
Nodes Real power losses Reactive power losses
2 48.2240 45.0755
3 55.6690 51.6311
4 48.5986 44.7036
5 41.0954 38.6371
6 46.2655 43.5056
7 47.2820 44.0329
8 54.9474 50.8193
9 50.2550 46.4612
10 37.9444 34.8113
11 44.3756 41.1492
12 48.9887 45.4502
13 44.9764 41.1875
14 48.8523 45.3901
The minimum losses are obtained i.e., 37.9444kW and 34.8113kVAr of
real and reactive power losses respectively, when the device is placed at node -10 shown
in table 4.5.
49
7
9 8
6 7
4 1
2 3
9 10
8
6 5
4 3
2 1
10
11 11
12
12
13
14
13
18
17
17
16
16
15
15
14
20 20
21 21
22
25
26
27
26
28
27
23
24 5
25
24
23
22 19
1
8
Fig
4.2:
Sin
gle
line
diag
ram
of I
EEE-
29 b
us D
istri
butio
n sy
stem
4.2.
IEE
E-2
9 bu
s sys
tem
:
19
50
4.2. IEEE 29-bus system: 4.2.1. Line data and load data for 29- bus system
The line data and load data of 29-bus distribution system are as follows.
Table 4.6: Line data for 29- bus system
Branch sending receiving Resistance Reactance 1 1 2 1.8216 0.7580 2 2 3 2.2270 0.9475 3 3 4 1.3662 0.5685 4 4 5 0.9180 0.3790 5 5 6 3.6432 1.5160 6 6 7 2.7324 1.1370 7 7 8 1.4573 0.6064 8 8 9 2.7324 1.1370 9 9 10 3.6432 1.5160 10 10 11 2.7520 0.7780 11 11 12 1.3760 0.3890 12 12 13 4.1280 1.1670 13 13 14 4.1280 0.8558 14 14 15 3.0272 0.7780 15 15 16 2.7520 1.1670 16 16 17 4.1280 0.7780 17 17 18 2.7520 0.7780 18 2 19 3.4400 0.9725 19 19 20 1.3760 0.3890 20 20 21 2.7520 0.7780 21 21 22 4.9536 1.4004 22 3 23 3.5776 1.0114 23 23 24 3.0272 0.8558 24 24 25 5.5040 1.5560 25 6 26 2.7520 0.7780 26 26 27 1.3760 0.3890 27 27 28 1.3760 0.3890
No of buses: 29, No of lines: 27, Base Voltage: 11KV, Base KVA: 100KVA
51
Table 4.7: Load data for 29- bus system
Node no Pl Ql 1 140.00 90 2 80 50 3 80 60 4 100 60 5 80 50 6 90 40 7 90 40 8 80 50 9 90 50 10 80 50 11 80 40 12 90 50 13 70 40 14 70 40 15 70 40 16 60 30 17 60 30 18 70 40 19 50 30 20 50 30 21 40 20 22 50 30 23 50 20 24 60 30 25 40 20 26 40 20 27 40 20
No of buses: 15, No of lines: 14, Base Voltage: 11KV, Base KVA: 100KVA
52
4.2.3. Load flow solution for 29-bus system
The voltages in per unit are obtained for this system after running load
flow method is shown in table 4.8.
Table 4.8: load flow solution without D-STATCOM
Node no Voltages in p.u Node no Voltages in p.u
1 1.0000 17 0.8477
2 0.9797 18 0.8470
3 0.9603 19 0.9603
4 0.9505 20 0.9592
5 0.9444 21 0.9577
6 0.9228 22 0.9565
7 0.9100 23 0.9505
8 0.9041 24 0.9484
9 0.8946 25 0.9464
10 0.8837 26 0.9100
11 0.8755 27 0.9094
12 0.8720 28 0.9091
13 0.8635
14 0.8563
15 0.8524
16 0.8504
When the proposed load flow method is executed without D-STATCOM,
the total real and reactive power loss of this system are 303.927kW and 122.3292 kVAr
respectively.
53
4.2.4. Results of 29-bus system after installing D-STATCOM
After installing D-STATCOM, running the load flow method for 29-bus
system, the Voltage Magnitudes after compensation in p.u at 13th-node is given by
table 4.9.
Table 4.9: load flow solution with D-STATCOM
Node no Voltages in p.u Node no Voltages in p.u
1 1.0000 17 0.8490
2 0.9973 18 0.8472
3 0.9954 19 0.9949
4 0.9942 20 0.9942
5 0.9932 21 0.9928
6 0.9901 22 0.9909
7 0.9875 23 0.9937
8 0.9861 24 0.9922
9 0.9837 25 0.9890
10 0.9801 26 0.9890
11 0.9779 27 0.9885
12 0.9768 28 0.9879
13 1.0000
14 0.8563
15 0.8540
16 0.8516
54
Table 4.10: Real and Reactive power losses of 15 – bus distribution system after
installing the D-STATCOM at each node is given.
Nodes Real power losses Reactive power losses
2 222.9651 89.3485
3 230.7026 92.6054
4 223.2182 89.4897
5 224.9190 90.1879
6 228.1189 91.5345
7 215.7298 86.3536
8 216.4618 86.6710
9 208.2909 83.2839
10 198.3834 79.4189
11 438.0603 176.1914
12 227.4905 91.9262
13 194.3258 78.7030
14 238.3141 95.6078
15 261.5701 104.4960
16 200.5714 81.1681
17 210.3367 84.9354
18 232.4672 93.3656
19 233.6183 93.8158
20 232.2015 93.2887
21 233.7131 93.9616
22 225.6816 90.5424
23 232.0193 93.1783
24 233.6178 93.8242
25 232.6227 93.4192
26 218.8908 87.7012
27 231.1892 92.8270
28 232.8760 93.5262
55
The minimum losses are obtained i.e., 194.3258 kW and 78.7030 kVAr
of real and reactive power losses respectively, when the device is placed at node -13
shown in table 4.10.
Thus D-STATCOM improves the voltage of both nearby downstream
nodes and nearby upstream nodes, especially the nodes located between D-STATCOM
and the source.
Therefore, in the two examples i.e., a IEEE 15-bus and IEEE 29-bus
distribution systems minimum loss occurs when D-STATCOM is placed at nodes 10th
and 13th respectively.
56
Chapter V
CONCLUSION
57
5. CONCLUSION
A simple and modified load-flow technique has been proposed for solving
radial distribution networks. The method has good and fast convergence characteristics
compared with some other existing methods. Later D-STATCOM is applied to proposed
load flow calculations in 15- and 29-bus IEEE test systems. The optimum location for D-
STATCOM is identified based on minimum losses. The results indicated that the
proposed model can be applied for large distribution systems. The computer program is
developed using the Matlab.
Scope for Future Work
The three-level inverter requires more numbers of switches and diodes
compared to two-level inverter. But it has following significant advantage. It can be used
for high power applications as the semiconductor devices are subjected to less voltage
and current stresses.
A new PWM-based control scheme has been implemented to control the
electronic valves in the two-level VSC used in the D-STATCOM and DVR. As opposed
to fundamental frequency switching schemes already available in the MATLAB/
SIMULINK, this PWM control scheme requires only voltage measurements. This
characteristic makes it ideally suitable for low-voltage custom power applications only.
58
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59
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PAPER PRESENTED IN NATIONAL CONFERENCE
N.Suresh and Dr.T.Gowri Manohar, “Optimal Citing of Custom Power
Controller in Distribution System for loss reduction”, paper presented in National
Conference on Oct 6-7, 2009, at GSSSETW, Mysore, Karnataka.
63
