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ANALYSIS OF GRID INTEGRATED SYSTEM
T.R.Sumithira 1 ,R.Rameshkumar2
1.Lecturer,Department of Electrical and Electronics Engineering,
Vivekananda College of Engineering for Women,Tiruchengodu.
2.Lecturer,Department of Mechanical Engineering ,S.S.M. Engineering
College,Komarapalayam.
Email:sujayra_6@yahoo.com,ramesh_rac@yahoo.co.in
Abstract
Embedded Generation (EG) is predicted to play a prominent role in the
electric power systems of the future. The term “embedded generation”
refers to electricity generation connected at distribution level rather
than transmission level. The insertion of EGs presents a new set of
conditions to distribution networks. The aim of this paper is to conduct
a voltage stability analysis using an iterative power system simulation
package, PSCAD/EMTDC Simulator, to evaluate the impact of strategically
placed EG on distribution systems when subjected to a fault, and with
that respect the critical voltage variations and collapse marginsare
studied. This paper concludes with the discussion of EGs’ excellent
options for system reactive power compensation and voltage stability.
1. INTRODUCTION
In the last decade, environmental issues and concerns have increasingly
come to the forefront. One area that attracts greatest environmental
concern is energy use. Energy conservation policies in several countries
encourage the use of renewable energy or so called "green energy"
sources such as wind, hydro, solar and biomass.
Embedded generation (EG) has the potential to promote the extensive use
of renewable sources. The term "embedded generation" refers to
electricity generation connected at distribution level rather than
transmission levels.EG can reduce the effect of losses while providing
reactive power and contingency reserves to the network. It can also
reduce the need for new transmission and distribution facilities
consequently reducing overall infrastructure costs. However, connection
of EGs to the distribution networks is not easy due to commercial and
technical considerations. This is because planned radial distribution
systems were designed without EG in mind.
The objective of this work was to investigate network faults and
stability issues that need to be taken into account in order for high
penetrations of wind farms to be connected to the network. These must
be investigated and resolved in order to build the required confidence
that a high penetration of wind generators connected to the network is
both feasible and safe.
The aim of this paper is to conduct a power system analysis using an
iterative power system simulation package,to evaluate the impact of
strategically placed EG on distribution systems with respect to the
networks' losses,Power flows, steady-state voltage variations, collapse
margins, and fault level contributions. The methods used in this study
are based on modelling of a simple network and the dynamic stability of a
typical small fixed speed wind generator.
In the first phase the simulations have been performed to study the
impact of network faults on the voltage stability of wind generators.
Results are presented for balanced 3-phase faults applied on the 11 kV
distribution system. The studies indicate that faults on the
distribution systems (close to the wind farm) may cause instability of
the wind farms. The voltage drop investigations show that for a 100%
voltage drop at a 11 kV connection point a very fast clearance time
(less than 90 ms) is required to maintain stable operation of a 2MW wind
generators. Possible remedial measures include the use of fast acting
reactive power support, e.g. a Static Reactive Power Compensator
(STATCOM).
This paper commences with an overview of renewable energy and the
important role of EG to promote the greater use of renewable sources.
This is followed by a comprehensive description of the adopted
methodology and the test systems used for the analysis. The results from
the performed studies and simulations are discussed in detail. Finally,
the paper will conclude with the summary of findings and provide
relevant recommendations for future development in this area.
2. BACKGROUND
The generation of electrical power using sustainable sources of energy
is developing rapidly with the worldwide installed capacity of wind
generation now exceeding 25GW. the government set an ambitious target of
installing 3,000 MW generating capacity by 2003.[1]
Tamil Nadu has the distinction of 719 MW (75% of total) wind farms at
the end of September 1998 (2). Andra Pradesh has 58 MW (6%) and Gujarat
has 168.64 MW, or 16% of the total capacity installed. See the map
below for more details on wind-farm distribution.[2]
Until recently, wind farms connected within the India network had been
limited to small sized installations, connected at distribution voltage
levels.
The connection standards do not currently require wind farms to
support the power system during a network disturbance. During a network
fault the wind turbines were disconnected from the system and then
subsequently reconnected when the fault has been cleared.
However, the network design grid codes are now being revised for the
increased penetration of wind generators. The wind farms will now have
to continue to operate during system disturbances.
3.GENERATION TECHNOLOGIES
3.1 Induction Generators as Embedded Generators
Induction machines are use extensively in the power system as induction
motors but are not widely used as generators. Despite their simplicity
in construction , they are not preferred as much as synchronous
generators. This is mainly due to the defined relationship between the
export of P and absorption of Q. However, induction generators have the
benefits of providing large damping torque in the prime mover,which
makes it suitable for the application in fixed speed wind turbines [3].
The fixed speed wind turbine uses a squirrel cage induction generator
that is coupled to the power system through a connecting transformer as
shown in Figure 1. Due to different operating speeds of the wind turbine
rotor and generator, a gearbox is used to match these speeds. The
generator slip slightly varies with the amount of generated power and is
therefore not entirely constant.
However, because these speed variations are in the order of 1
per cent this wind turbine is normally referred to as constant speed.
Nowadays, this type of wind turbine is nearly always combined with stall
control of the aerodynamic power, although pitch-controlled constant
speed wind turbine types have been built in the past. Induction machines
consume reactive power and consequently, it is present practice to
provide power factor correction capacitors at each wind turbine. These
are typically rated at around 30 per cent of the wind farmcapacity.[4].
As the stator voltage of most wind turbine electrical generators is
690V, the connecting transformer of the wind turbine is essential for
connection to the distribution network and should be considered when
modeling the electrical interaction with the power system.
3.2 Effects Of Embedded Generations
Impacts of EG
Connecting a generation scheme to a distribution network will affect the
operation and performance of the network depending on the scheme and
rating of the generator itself [5]. The
impacts are as follows:
Power Flows
Voltage stability
Fault Analysis
Impact of EG on the Networks
3.2.1.Powerqualityproblems
Although the main issues of power quality are common to distribution
networks, whether active or passive the addition of wind generation can
have a significant impact on power quality. Individual units can be very
large (2.5MW), and are often connected to distribution circuits with
high source impedance.
The connection of fixed-speed turbines to the network needs to be
managed carefully if excessive transients are to be avoided. However,
fixed speed operation using a low-slip induction generator, will lead to
cyclic variations in output power and hence network voltage.
A voltage dip is a sudden reduction in the network voltage to a value
between 100 per cent and 0 per cent followed by a voltage recovery after
a short period of time, conventionally 1ms to 1 min. Dips between 10
per cent and 15 per cent of the terminal voltage are commonly due to
switching of loads, whereas larger dips may be caused by faults.
Start-up of a wind turbine may cause a sudden reduction in the voltage
followed by a voltage recovery after a few seconds.
Voltageflicker
Voltage flicker describes dynamic variations in the network voltage.
Traditionally it was of concern when the connection of large fluctuating
loads (e.g. arc furnaces, rock crushing machinery, sawmills, etc.) was
under consideration. However, it is of considerable significance for
windfarms, which: (i) often use relatively large individual items of
plant compared to load equipment; (ii) may start and stop frequently;
(iii) may be subject to continuous variations in input power.
Harmonics
A wind turbine with an induction generator directly connected to the
grid without an intervening power electronic converter is not expected
to distort the voltage waveform. Power electronics applied for soft
start may generate short-duration high-order current harmonics but their
duration and magnitude are usually small. Hence for a system with
fixed-speed wind turbines emission limits for harmonics are not a
constraint.
FACTSsolutions
FACTS technology allows greater flexibility for voltage and power flow
control in power systems offering a number of unique features that
makes them effective to handle power quality issues introduced by the
connection of large windfarms. Two FACTS power electronic device
solutions such as SVC (Static VAR Compensator) and STATCOM (Static
Synchronous Compensator) are described as follows.
The STATCOM belongs to a family of power electronics controllers that
base their operation on the Voltage Source Converter principle. The most
basic configuration of the STATCOM consists of a two-level VSC with a
dc energy storage device; a coupling transformer connected in shunt with
the ac system, and associated control circuits. The dc energy storage
device may be a battery, whose output voltage remains constant or it may
be a capacitor, whose terminal voltage can be raised or lowered by
means of suitable converter control.
3.2.2Voltage Stability
A system experiences a state of voltage instability when there is a
progressive or uncontrollable drop in voltage magnitude after a
disturbance, increase in load demand or change in operating condition
[6]. The main factor, which causes these unacceptable voltage profiles,
is the inability of the distribution system to meet the demand for
reactive power.
Under normal operating conditions, the bus voltage magnitude (V)
increases as Q injected at the same bus is increased. However,when V of
any one of the system’s buses decreases with the increase in Q for that
same bus, the system is said to be unstable [5].
Although the voltage instability is a localised problem, its impact on
the system can be wide spread as it depends on the relationship between
transmitted P, injected Q and receiving
end V. These relationships play an important role in the stability
analysis and can be displayed graphically.
PV Curves
When considering voltage stability, the relationship between transmitted
P and receiving end V is of interest. The voltage stability analysis
process involves the transfer of P from one region of a system to
another, and monitoring the effects to the system voltages, V. This type
of analysis is commonly referred to as a PV study [5].
The Figure 2 shows a typical PV curve. It represents the variation in
voltage at a particular bus as a function of the total active power
supplied to loads or sinking areas. It can be seen that at the “knee” of
the PV curve, the voltage drops rapidly when there is an increase in
the load demand. Load-flow solutions do not converge beyond this point,
which indicates that the system has become unstable. This point is
called the Critical point. Hence, the curve can be used to determine the
system’s critical operating voltage and collapse margin. Generally,
operating points above the critical point signifies a stable system. If
the operating points are below the critical point, the system is
diagnosed to be in an unstable condition [5].
QV Curves
Voltage stability depends on how the variations in Q and P affect the
voltages at the load buses. The influence of reactive power
characteristics of devices at the receiving end (loads or compensating
devices) is more apparent in a QV relationship.It shows the sensitivity
and variation of bus voltages with respect to reactive power injections
or absorptions [6]. Figure 2 shows a typical QV curve, which is usually
generated by a series of load-flow solutions.
Figure 3 shows a voltage stability limit at the point where the
derivative dQ/dV is zero. This point also defines the minimum reactive
power requirement for a stable operation [6].
An increase in Q will result an increase in voltage during normal
operating conditions. Hence, if the operating point is on the right side
of the curve, the system is said to b e stable.Conversely, operating
points in the left side of the graph are deemed to be unstable.
4. TEST SYSTEMS
4.1 Assumptions Of The Dynamic Stability Calculations
The study cases were based on the network shown in Figures 1.The system
represented for the dynamic stability simulation was a 13.8 kV voltage
source in series with an impedance. The voltage of the source was 1 p.u.
The wind turbines is of 2 MW capacity. These is represented by a single
equivalent coherent fixed-speed induction generator.[6].A lumped
11kV/.69kV wind turbine terminal transformer with 5% impedance was used
to connect the wind turbine to the 33kV/11kV.The computer program,
PSCAD/EMTDC, was used to simulate dynamic stability[7].
4.2 Simulation
Many studies have been conducted on EG connected to 11kV networks and
have published several results. However, very little studies have been
conducted on the reticulation regions.The 5 Bus system was adopted It
was used to demonstrate the effects of EG and to understand the concept
of embedded generation.
The STATCOM is a two-level VSC with voltage-space-vector PWM control. A
conventional PI controller has been used to control the reactive power
output of the STATCOM. For the simulation results it was
ed that the 11kV network was subjected to a single-phase fault along one
of the parallel circuits, of 150ms duration at 2 seconds. The faulty
circuit is disconnected after the fault clearance. The main simulation
results produced by using PSCAD/EMTDC are shown in Figure 4 and Figure
5.
The voltage response at the 11kV point of connection of the windfarm
(busbar B4) without the STATCOM in operation is shown in Figure 5 . As
shown in Figure 5 the voltage at the high voltage point of connection of
the windfarm (B4) does not recover the prefault voltage value after the
clearance of the fault. That is, the windfarm does not have the
capability to ride through the fault. However, when the STATCOM is set
in operation the windfarm is able to ride through thefault.
Fig 4.
Fig.5
The voltage recovery of the windgenerator due to the voltage support
and reactive power compensation provided by the STATCOM.
It can be observed that the STATCOM supplies some reactive power to
the wind generator under normal operation. During the fault the reactive
power supplied by the STATCOM is decreased due to the voltage drop.
After the fault, the STATCOM supplies an amount of reactive power to the
wind generator and compensates its requirements for reactive power in
order to ride through the fault.
5. CONCLUSIONS
Simulations have been performed to study the impact of network faults on
the stability of small wind generators.Results are presented for single
line to ground faults applied on the 11kV system. The most common fault
is the single line to earth fault which accounts for 75-85 % of all
faults [9]. The impact of 1-phase faults upon the stability of fixed
speed wind farms will be much less severe.The studies indicate that
faults on the distribution system (close to the windfarm) may cause
instability. The voltage drop results show that for a 100% voltage drop
at the 11kV connection point, a very fast clearance time (less than 90
ms) is required to maintain stable operation of a 2MW wind
generator.Possible remedial measures are included with the use of fast
acting reactive power support. The first phase of the desired work is
performed and further in the second phase the Impact of FACTS on 11kv
distribution system is studied.
6. REFERENCES
1] B Rajsekhar, F Van Hulle, J.C Hansen, “Indian Wind Energy Programme;
performance
and future directions” Energy Policy 27 1999 669-678.
2]A Jagdeesh, “Wind energy development in Tamil Nadu and Andhra Pradesh,
India
Institutional Dynamics and barriers”, Energy Policy 28 2000 157-168.
3]Thomas Ackermann, Lennart Soeder, “Wind Energy Technology and current
status: a
review”, Renewable and Sustainable Energy Review 4 2000 315-374.
[4] Dr Olimpo Anaya-Lara, Prof Nicholas Jenkins of the Manchester Centre
for Electrical Energy, University of Manchester, UK and Dr Phillip
Cartwright, head of engineering at Areva T&D Technology Centre, UK,
[5] K. Pandiaraj, G. Hodgkirson, B. Fox, “Use of Embedded Generators for
voltage support in rural distribution networks”, UPEC 2002 35th
Universities Power Engineering Conference, 2000
[6] P. Kundur, “Power System Stability and Control”,McGraw-Hill Inc.,
New York, USA, 1994
[7] PSCAD/EMTDC Simulator Version 4.2 User’s Guide.
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