International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 4, April 2014)
Challenges of Integration of Wind Power on Power System
Grid : A Review
Anil Gupta1, Dr. Arun Shandilya2
1
Assistant Professor, Department of Electrical Engineering, LNCT, Bhopal, MP, India
2
Professor, Department of Electrical Engineering, MANIT, Bhopal, MP, India
Abstract— The wind energy generation, utilization and its
grid penetration in electrical grid are increasing worldwide.
The wind generated power is always fluctuating due to its
time varying nature and causing stability problems. This
weak interconnection of wind generating source in the
electrical network affects the power quality and reliability.
The influence of the wind turbine in the grid system
concerning the power quality measurements are the variation
of voltage, flicker, harmonics.This paper provides a brief
overview of the technical and operational issues related to
integration.Paper also considers grid codes that make wind
energy more grid-compatible to ensure further growth of this
promising renewable source of energy. Finally, potential
technical challenges to the integration of large-scale wind
energy into the power grid are reviewed with their available
mitigation techniques.
These fluctuations have a negative impact on stability
and power quality in electric power systems[2].Large scale
Integration of DG units in the distribution grid not only
affects the grid planning but also has an impact on the
operation of the distribution grid[3]. Aspects which are
influenced by the connection of DG units are 1) power
quality 2) voltage control 3) grid losses 4) fault level 5)
protection system. The increase in number of incoming
induction generators to the grid, causes the power quality
problems mainly in current Harmonics, reactive power and
power factor. These problems will be more severe in weak
grids. The simultaneous switching operation of Induction
Generators results into excessive inrush of reactive power
from grid, which is undesirable. The next sections describe
the power quality issues and their mitigation techniques.
Keywords—Electrical grid, Harmonics, Power quality,
Flicker, Wind turbines, Grid codes.
II. POWER QUALITY ISSUES
Power Quality has become very important issue over the
last decade. A key reason for the increasing importance is
the rapid spread of the use of equipments sensitive to
power system disturbances and the widespread use of nonlinearly behaving power electronic converters. The addition
of windTurbines can have a significant effect and increases
the complexity of this problem. Depending on the grid
configuration and the type of wind turbine used, different
power quality problems may arise.[4] The random nature of
wind resources, the wind farm generates fluctuating electric
power. These fluctuations have a negative impact on
stability and power quality in electric power systems.Large
scale Integration of DG units in the distribution grid not
only affects the grid planning but also has an impact on the
operation of the distribution grid. Aspects which are
influenced by the connection of DG units are 1) power
quality 2) voltage control 3) grid losses4) fault level 5)
protection system.The increase in number of incoming
induction generators to the grid, causes the power quality
problems mainly in current Harmonics, reactive power and
power factor. These problems will be more severe in weak
grids[5-8]. The simultaneous switching operation of
Induction Generators results into excessive inrush of
reactive power from grid, which is undesirable.
I. INTRODUCTION
Electricity is produced in bulk by using power plants
with natural gas, coal or oil as primary sources.
Combustion of such fuels results in emission of pollutants
like oxides of Carbon, Nitrogen and Sulfur. Due to shortage
of energy resources, the energy crisis and greenhouse
emission problems have been gradually increases day by
day. Greenhouse gas emissions particularly carbon dioxide
is the main cause of global warming. In recent years, a lot
of energy policies and research projects focused on the
energy-saving[1]. Large wind and solar farms have been
installed in power systems around the world due to
environmental problems caused by using fossil energy
resources. The increasing environmental awareness
requires that the system operator should supply electricity
to consumers with minimum emissions. Environmental
protection has been raised recently due to the concerns
regarding global weather and air pollution. Renewable
energy sources like wind energy, are widely applied to
reach emission reduction with the increasing concern of
environmental protection. Wind power generation does not
produce harmful emissions. But with random nature of
wind resources, the wind farm generates fluctuating electric
power.
880
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 4, April 2014)
In this case an attempt is made to investigate the causes
of poor power Quality in grid connected renewable Wind
energy system. The voltage fluctuations, reactive power
compensation, poor power factor and harmonics distortion
are the main aspects of power quality problems in
integrating wind energy with the smart power grid due to
the inherent characteristics of these resources as shown in
Fig. 1.
This problem is considered in the power quality and
wind turbine generating system operation and computed
according to the rule given in IEC 61400-3-7 standard,
―Assessment of emission limit for fluctuating load‖. The
relative % voltage change due to switching operation of
wind turbine is calculated as
d = 100Ku(k)Sn/S* k
(1)
Where d - Relative voltage change,
Ku(k) - Voltage change factor,
Sn - Rated apparent power of wind turbine and S* k short
circuit apparent power of grid. The voltage dips of 3% in
most of the cases are acceptable.
V. HARMONICS
Fig. 1. Major potential technical impacts of integrating wind energy
into the grid.
Harmonics can be injected both at the generation and the
consumer end. At the consumer end, harmonics are caused
by non linear loads such as television, personal computers,
compact fluorescent lamps, and so forth.The harmonics
distortion caused by non-linear load such as electric arc
furnaces, variable speed drives, large concentrations of arc
discharge lamps, saturation of magnetization of transformer
and a distorted line current. The current generated by such
load interact with power system impedance and gives rise
to harmonics. The effect of harmonics in the power system
can lead to degradation of power quality at the consumer’s
terminal, increase of power losses, and malfunction in
communication system[12]. The degree of variation is
assessed at the point of common connection, where
consumer and supplier area of responsibility meet. The
harmonics voltage and current should be limited to
acceptable level at the point of wind turbine connection in
the system. According to standard IEC61400-21 guideline,
harmonic measurements are not required for fixed speed
wind turbines where the induction generator is directly
connected to grid. Harmonic measurements are required
only for variable speed turbines equipped with electronic
power converters. In general the power converters of wind
turbines are pulse-width modulated inverters, which have
carrier frequencies in the range of 2-3 kHz and produce
mainly inter harmonic currents. The harmonic
measurement at the wind turbine is problem due to the
influence of the already existing harmonic voltage in the
grid. The wave shape of the grid voltage is not sinusoidal.
There are always harmonics voltages in the grid such as
integer harmonic of 5th and 7th order which affect the
measurements.
III. VOLTAGE VARIATION
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. If a large proportion of the
grid load is supplied by wind turbines, due to output
variations wind speed changes which can cause voltage
variation and this affects the normal operation of system[9].
The voltage variation can occur in specific situation mainly
as a result of load change and these canbe expected
particularly in the case of generator connected to the grid at
fixed speed. The large turbine can achieve significantly
better output using variable speed operation, particularly in
the short time range.
IV. VOLTAGE DIP
The Voltage dip is a very common and serious type of
power quality disturbance due to its effects on sensitive
equipment and industrial processes[10] . Voltage dip could
occur when there is a large load such as motor start up,
transformer energising, capacitor energising, switching of
electronic load, momentary overload or a fault in the
system network. It can cause the disconnection of wind
generators, which could have a negative impact on the
stability of the network due to loss of generation[11].It is a
sudden reduction in the voltage to a value between 1% to
90 % of the nominal value after a short period of time,
conventionally 1ms to 1 min.
881
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 4, April 2014)
Today’s variable speed turbines are equipped with self
commutated PWM inverter system. This type of inverter
system has advantage that both the active and reactive
power can be controlled, but it also produced a harmonic
current. Therefore filters are necessary to reduce the
harmonics. The harmonic distortion is assessed for variable
speed turbine with a electronic power converter at the point
of common connection. The total harmonic voltage
distortion of voltage is given as in (2).
40
VTHD =√[∑(V2 h/V1)*100]
h=2
The characteristics of the load and level of power system
significantly decides the effects of harmonics. IEEE
standards are adapted in most of the countries. The
recommended practice helps designer to limit current and
voltage distortion to acceptable limits at point of common
coupling (PCC) between supply and the consumer.
1. IEEE standard 519 issued in 1981, recommends
voltage distortion less than 5% on power lines below
69 kV.
2. IEEE standard 519 was revised in 1992, and impose
5% voltage distortion limit.
(2)
VI. FLICKERS
Vh - hth harmonic voltage and V1 –fundamental
frequency 50 Hz. The THD limit for various level of
system voltages are given in the table 1.0
The wind generators sometimes produce oscillatory
output power, which could cause flickers in the power
system network. Flicker is the one of the important power
quality aspects in wind turbine generating system. Flicker
has widely been considered as a serious drawback and may
limit for the maximum amount of wind power generation
that can be connected to the grid. Flicker is induced by
voltage fluctuations, which are caused by load flow
changes in the grid. The flicker is mainly produced by
fluctuations in the output power due to wind speed
variations. There are many factors that affect flicker
emission of grid connected wind turbines during
continuous operation, such as wind characteristics and grid
conditions[12]. Variable-speed wind turbines have shown
better performance related to flicker emission in
comparison with fixed-speed wind turbines. The flicker
study becomes necessary and important as the wind power
penetration level increases quickly. Owing to smoothing
effect, large wind turbine produced lower flicker than small
wind turbines, in relation to their size.
The flicker level depends on the amplitude, shape and
repetition frequency of the fluctuated voltage waveform.
Evaluating the flicker level is based on the flicker meter
described in IEC 61000-4-15. Two indices are typically
used as a scale for flicker emission, short-term flicker
index, Pst and long-term flicker index, Plt. Plt is estimated
by certain process of the Pst values. It is assumed that wind
turbines under study is running at normal operation; hence,
the long-term flicker index (Plt), which is based on a 120min time interval, is equal to Pst and, therefore, Pst is only
considered in this work. The normalized response of the
flicker meter described in Figure 2.
TABLE I
VOLTAGE HARMONICS LIMIT
System Voltage (kV)
Total Harmonic Distortion (%)
400
2.0
220
2.5
132
3.0
THD of current ITHD is give as in (3)
40
ITHD =√[∑(I2 h/I1)*100]
h=2
(3)
Where Ih - hth harmonic current and I1 –fundamental
frequency (50) Hz. The acceptable level of THD in the
current is given in table 2.
TABLE II
CURRENT HARMONIC LIMIT
Voltage level
ITHD
66 kV
5.0
132kV
2.5
Various standards are also recommended for individual
consumer and utility system for helping to design the
system to improve the power quality.
882
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 4, April 2014)
Fig. 3. Low voltage ride through (LVRT) capability
Many countries in Europe and other parts of the world
are developing or modifying interconnection rules and
processes for wind power through a grid code. The grid
codes have identified many potential adverse impacts of
large scale integration of wind resources. The low voltage
ride through (LVRT) capability, which is one of the most
demanding requirement that have been included in the grid
codes and shown in Fig. 3. It defines the operational
boundary of a wind turbine connected to the network in
terms of frequency, voltage tolerance, power factor, fault
ride through is regarded as the main challenges to the wind
turbine manufactures. The wind turbine should remain
stable and connected during the fault while voltage at the
PCC drop to 15% of the nominal value i.e. drops of 85%
for the part of 150 msec. Only when the grid voltage fall
below the curve, the turbine is allowed to disconnected
from the grid. Wind farms using squirrel cage induction
generators directly connected to the network will suffer
from the new demands, since they have no direct electrical
control of torque or speed, and would usually disconnect
from the power system when the voltage drops more than
10–20% below the rated value. The LVRT basically
demands that the wind farm remains connected to the grid
for voltage dips as low as 5%.
Fig. 2. Influence of frequency on the perceptibility of sinusoidal
voltage change
The flicker level (Pst ≤ 1) is a threshold level for
connecting wind turbines to low voltage. The
measurements are made for maximum number of specified
switching operation of wind turbine with 10-minutes period
and 2-hour period are specified, as given in (4)
Plt = C()Sn/SK
(4)
Where Plt - Long term flicker. C() - Flicker
coefficient calculated from Rayleigh distribution of the
wind speed. The Limiting Value for flicker coefficient is
about 0.4, for average time of 2 hours.
VII. WIND TURBINE LOW VOLTAGE RIDE THROUGH
CAPABILITY
Fault ride through has come to play a role in
strengthening power system security due to the increase in
the integration of wind power in recent times. It requires
the generators to remain connected in the likelihood of a
disturbance on the network. A severe disturbance such as a
fault could lead to a voltage dip and if the generators are
unable to remain connected it could lead to an excessive
loss of generation[5]. This could cause stability problems
and may eventually lead to cascaded tripping of other
generators. The impact of the wind generation on the
power system will no longer be negligible if high
penetration levels are going to be reached. The extent to
which wind power can be integrated into the power system
without affecting the overall stable operation depends on
the technology available to mitigate the possible negative
impacts such as loss of generation for frequency support,
voltage flicker, voltage and power variation due to the
variable speed of the wind and the risk of instability due to
lower degree of controllability.
VIII. POWER QUALITY ISSUES MITIGATION TECHNIQUES
Advanced custom power devices with adequate
converter and control systems such as SVCs and
STATCOMs can mitigate voltage instability, reactive
power problems and harmonic distortion as well as
improve the PQ of the network. In order to enhance the
terminal voltage quality, SVCs were used for reactive
power compensation of wind power induction generators
but STATCOMs are superior compared to other flicker
mitigation methods such as SVCs.
883
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 4, April 2014)
STATCOMs is faster, smaller and having better
performance at low voltage conditions . A STATCOM
based control mechanism is very useful to reduce the power
quality problems on integrating wind energy into the grid.
Simulation results show that an optimised STATCOM can
cancel out the harmonic parts of the load current. Here the
system canbe capable of meeting the reactive power
demand from the wind generator and the load at the PCC to
the grid. This scheme improved the quality of power
significantly and fulfilled the PQ requirement. It is
observed that the STATCOM can considerably improve the
voltage profile at the PCC by regulating the reactive power
of the grid during faults and maintaining an appropriate
level of voltage sag on the grid and prevents the turbine
from being disconnected from the grid during certain levels
of voltage sag on the grid side. Efficient design of power
electronic
converters,
adequate
reactive
power
compensation and optimum design of wind turbines and the
grid connection all play a key role in minimising the
observed potential challenges and increases the efficiency
of the system.
REFERENCES
Harry Davitian, ―Wind Power and Electric Utilities: A Review of the
Problems and Prospects‖, Wind Engineering vol. 2,no. 4,pp. 128,1978.
[2] John K. Kaldellis, D. Zafirakis The wind energy evolution: A short
review of a long history. Renewable Energy vol.36,no.11,pp. 18871901,2011.
[3] Z. Chen, ―Issues of Connecting Wind Farms into Power Systems‖,
IEEE/PES Transmission and Distribution Conference & Exhibition:
Asia and Pacific Dalian, China,vol.II, 2005.
[4] Stefan Marko, Ivan Darul’a, ―Large Scale Integration Of Renewable
Electricity Production Into the Grids‖, Journal of Electrical Engg,
vol. 58,no.1 pp.58–60,2007.
[5] Inigo Martinez de Alegrıaa, Jon Andreua, Jose Luis Martına, Pedro
Iban ezb, Jose Luis Villateb, Haritza Camblongc, ―Connection
requirements for wind farms: A survey on technical requierements
and regulation‖ , Renewable and Sustainable Energy Reviews,
vol.11, no.6,pp.1858–1872,2007
[6] J. Charles Smith, Michael R. Milligan, Edgar A. DeMeo, and Brian
Parsons, ―Utility Wind Integration and Operating Impact State of the
Art‖ IEEE Transaction On Power Systems vol.22,no.3,pp.900908,2007.
[7] Doina Dragomir,‖The Aspects Related To The Grid Connection Of
The Wind Farms Into National Power System‖,U.P.B. Sci. Bull.
Vol.73,no.1,pp.273-279,2011.
[8] S.K. Khadem,M. Basu,M.F.Conlon, ―Power Quality In Grid
Connected Renewable Energy Systems: Role Of Custom Power
Devices‖ International Conference On Renewable Energies and
Power Quality, Spain,vol.III,pp.23-25 March 2010.
[9] Pavlos S. Georgilakis Technical challenges associated with the
integration of wind power into power systems Renewable and
Sustainable Energy Reviews vol.12,no.14, pp.852–863,2008.
[10] G.M. Shafiullah , Amanullah,Shawkat Ali, PeterWolfs, ―Potential
challenges of integrating large-scale wind energy into the power
grid–A review‖ Renewable and Sustainable Energy Reviews
vol.20,no.8, pp.306–321,2013.
[11] I.WASIAK and Z. HANZELKA Integration of distributed energy
sources with electrical power grid Bulletin Of The Polish Academy
Of Sciences TECHNICAL SCIENCES vol.57,no.4,pp.297309,2009.
[12] Sharad W., Mohod,Mohan, V. Aware, ―Power Quality Issues and Its
Improvement in Wind Energy Generation Interface to Grid System‖,
MIT International Journal of Electrical and Instrumentation Engg.,
vol.1,no.2,pp.116-122,2011.
[1]
IX. CONCLUSION
In response to the energy needs and environmental
concerns, electricity from wind generators is considered as
one of the future solutions. However, the variability and the
diffuse nature of the wind power can be challenging to the
operation of a power system.The wind turbines connected
to weak grids have an important influence on power
system. The weak grid is characterized by large voltage and
frequency variations, which affects wind turbines regarding
their power performance, safety and allied electrical
components. This paper gives a comprehensive literature
review to explore these potential impacts and their
available mitigation techniques.
884