International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
A Control Scheme to Enhance Power Productivity
Using Static Synchronous Condenser for Wind
Energy System
PREMDEEP A1, RASOOL AHEMMED2
1
EEE KL UNIVERSITY, INDIA
2
AsstProfessor, EEE KL University, INDIA
Abstract—Power we gain from wind energy system is not
constant because of the fluctuations of wind and it won’t be in
phase with the grid load power. This may affect the power
productivity. The power quality and wind turbine performance
are measured on the basis of measurements and norms being
specified in international-technical Commission standard.Injection of the active power, reactive power, variation of voltage,
flicker, harmonics, and electrical behavior of switching
operations that are measured as per the national/international
guidelines. The paper mainly concerns about the power stability
problem due to wind turbine installation with the grid. In this
scheme a static synchronous compensator is connected at a
common coupling with a battery energy storage system (BESS) to
lessen the power productivity issues. The BESS is integrated to
sustain the real power source under fluctuating wind power. The
static synchronous compensator control scheme for wind energy
system for power quality improvement is simulated using
MATLAB/SIMULINK in power system block set. The proposed
scheme lessens the effect of reactive power demand of the load on
the main supply source and on the induction generator. The
control scheme for power productivity improvement and grid coordination rule been presented are as per IEC-standard.
Index Terms—International electro-technical commission (IEC),
power quality, wind generating system (WGS).
I. INTRODUCTION
In order to enhance the utilization of conventional renewable
Energy resources like wind, biomass, hydro, co-generation,
etc. In sustainable energy system, energy conservation and the
use of renewable source are the key paradigm. The need to
integrate the renewable energy like wind energy into power
system is to make it possible to minimize the environmental
impact on conventional plant. The integration of wind energy
into existing power system presents a technical challenges and
that requires consideration of voltage regulation, stability,
power quality problems. The power quality is an essential customer-focused measure and is greatly affected by the operation of a distribution and transmission network. The issue of
power quality is of great importance to the wind turbine.
The main criteria that being concerned is the power factor we
need the unity power factor in order to maintain the synchronization along source and load side for constant power flow.
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The individual units can be of large capacity up to 3 MW,
feeding into distribution network, particularly with customers
connected in close proximity. In the fixed-speed wind turbine
operation, all the fluctuation in the wind speed are transmitted
as fluctuations in the mechanical torque, electrical power on
the grid and leads to large voltage fluctuations. During the
normal operation, wind turbine produces a continuous variable output power. These power variations are mainly caused
by the effect of turbulence, wind shear, and tower-shadow and
of control system in the power system. Thus, the network needs
to manage for such fluctuations. The power quality issues can
be viewed with respect to the wind generation, transmission
and distribution network, such as voltage sag, swells, flickers,
harmonics etc. However the wind generator introduces disturbances into the distribution network. One of the simple methods
of running a wind generating system is to use the induction generator connected directly to the grid system.However; induction
generators require reactive power for magnetization. When the
generated active power of an induction generator is varied due
to wind, absorbed reactive power and terminal voltage of an
induction generator can be significantly affected. A proper
control scheme in wind energy generation system is required
under normal operating condition to allow the proper control
over the active power production. In the event of increasing
grid disturbance, a battery energy storage system for wind
energy generating system is generally required to compen- sate
the fluctuation generated by wind turbine. A STATCOMbased control technology has been proposed for improving the
power quality which can technically manages the power level
associates with the commercial wind turbines. The proposed
STATCOM control scheme for grid connected wind energy generation for power quality improvement has following objectives.
The paper is organized as follows. The Section II introduces
the power quality standards, issues and its consequences of wind
turbine. The Section III introduces the grid coordination rule
for grid quality limits. The Section IV describes the topology
for power quality improvement. The Sections V, VI, VII describes the control scheme, system performance and conclusion
respectively.
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II. P O W E R QUALITY STANDARDS,
ISSUES AND ITS CONSEQUENCES
A. International Electro Technical Commission Guidelines
The guidelines are provided for measurement of power
quality of wind turbine. The International standards are developed by the International Electro-technical Commission
(IEC), IEC standard 61400-21, describes the procedure for
determining the power quality characteristics of the wind
turbine [4].
The standard norms are specified.
1) IEC 61400-23: Wind turbine generating system, part-21.
Measurement and Assessment of power quality characteristic of grid connected wind turbine
2) IEC 61400-15: Wind Turbine—measuring procedure in
determining the power behavior.
3) IEC 61400-3-5: Assessment of emission limit for fluctuating load IEC 61400-12: Wind Turbine performance.
The data sheet with electrical characteristic of wind turbine
provides the base for the utility assessment regarding a grid
connection.
B. Voltage Variation
The voltage variation issue results from the wind velocity and
generator torque. The voltage variation is directly related to
real and reactive power variations. The voltage variation is
commonly classified as under:
• Voltage Sag/Voltage Dips.
• Voltage Swells.
• Short Interruptions.
• Long duration voltage variation.
The voltage flicker issue describes dynamic variations in the
network caused by wind turbine or by varying loads. Thus the
power fluctuation from wind turbine occurs during continuous
operation. The amplitude of voltage fluctuation depends on grid
strength, network impedance, and phase-angle and power factor
of the wind turbines. It is defined as a fluctuation of voltage in
a frequency 10–35 Hz.
C. Harmonics
The harmonic results due to the operation of power electronic
converters. The harmonic voltage and current should be limited
to the acceptable level at the point of wind turbine connection to
the network. To ensure the harmonic voltage within limit, each
source of harmonic current can allow only a limited contribution, as per the IEC-61400-35 guideline. The rapid switching
gives a large reduction in lower order harmonic current compared to the line commutated converter, but the output current
will have high frequency current and can be easily filter-out.
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D. Self-Excitation of Wind Turbine Generating System
The self-excitation of wind turbine generating system
(WTGS) with an asynchronous generator takes place after
disconnection of wind turbine generating system (WTGS)
with local load. The risk of self-excitation arises especially
when WTGS is equipped with compensating capacitor. The
capacitor connected to induction generator provides reactive
power compensation.
E. Consequences of the Issues
The voltage variation, flicker, harmonics causes the malfunction of equipment’s namely microprocessor based control
system, adjustable speed drives. It may leads to tripping of
protection devices and may stop the process and even can
damage of sensitive equipment. Thus it degrade the power
productivity in the grid.
III. GRID COORDINATION RULE
The American Wind Energy Association (AWEA) led the effort for adoption of the grid code for the interconnection of the
wind plants to the generating system. The first grid code was
focused on the distribution level, later it took a stand in
developing its own grid code for contributing to a stable grid
operation. The rules for realization of grid operation of wind
generating system at the distribution net- work are defined
According to Energy-Economic Law, the operator of
transmission grid is responsible for the organization and
operation of interconnected system.
1) Voltage Rise (u): The voltage rise at the point of common
coupling can be approximated as a function of
maximum ap- parent power S m a x of the turbine, the
grid impedances R and X at the point of common
coupling and the phase angle Φ, given in (1)
Δu = Smax (Rcos
sin ) /U2 ……. (1)
Where Δ u —voltage rise, S m a x … Apparent power,
Φ—phase difference, U—is t h e n om i n a l vol t a ge o f
g r i d. The Limiting voltage rise value is ≤ 2.
2) Voltage Dips (d): The voltage dips is due to startup of
wind turbine and it causes a sudden reduction of
voltage. It is the relative % voltage change due to
switching operation of wind turbine. The decrease of
nominal voltage change is given in (2).
d = Ku . Sn/ Sk …. (2)
Where d is relative voltage change, Sn rated apparent
power, Sk short circuit apparent power, and Ku sudden
voltage reduction factor. The acceptable voltage dips
limiting value is ≤ 3%.
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3) Flicker: The measurements are made for the maximum
no.of specified switching operation of wind turbine
with 10 min period and 2-h period are specified in (3).
PLT = C (ΨK) …..(3)
Where PLT is long term flicker, C (ΨK) is flicker coefficient
calculated from Rayleigh distribution of the wind speed. The
boundary for flicker coefficient is < 0.4 for avg time of 2h.
4) Harmonics: The harmonic distortion is assessed for
variable speed turbine with an electronic power
converter at the point of common connection [9]. The
total harmonic voltage distortion of voltage is given as
in (4):
VTHD
=
∑
100……… (4)
Fig. 1. Grid connected system for power quality improvement.
Where Vn is the nth harmonic voltage and
is the
fundamental frequency (50) Hz. The THD limit for 132 KV
is < 3%.
THD of current ITHD is given as in (5)
ITHD= ∑ 100……. (5)
Where IN is the nth harmonic current and I1 is the fundamental
frequency (50) Hz. The THD of current and limit for 132 KV is
< 2.5%.
IV. TOPOLOGY FOR POWER QUALITY IMPROVEMENT
The STATCOM based current control voltage source inverter
injects the current into the grid in such a way that the source
current are harmonic free and their phase-angle with respect to
source voltage has a desired value. Thus it improves the
power factor and the power quality. To accomplish these goals,
the grid voltages are sensed and are synchronized in
generating the current command for the inverter. The
proposed grid connected system is implemented for power
quality improvement at point of common coupling (PCC), as
shown in Fig. 1.
The grid connected system in Fig. 1, consists of wind energy generation system and battery energy storage system with
STATCOM.
A. Wind Energy Generating System
Wind generations are based on constant speed topologies
with pitch control turbine. The induction generator is used
in the proposed scheme because of its simplicity, it does
not require a separate field circuit, it can accept constant
and variable loads, and has natural protection against
short circuit. The available power of wind energy system
is presented as PWIND = ƿ
.
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Where ƿ (kg/m3) is the air density and A (m2) is the area swept
out by turbine blade, Vwind is the wind speed in mtr/s. It is not
possible to extract all kinetic energy of wind, thus it extract a
fraction of power in wind, called power coefficient Cp of the
wind turbine, and is given in (7).
Pmech = Cp .Pwind……… (7)
Where Cp is the power coefficient, depends on type and operating condition of wind turbine. This coefficient can be express
as a function of tip speed ratio λ and pitch angle ϴ. The mechanical power produce by wind turbine is given in (8)
Pmech = ½ Ƿ II R2 V3 wind Cp …. (8)
Where R is the radius of the blade in meters.
B. BESS-STATCOM
The battery energy storage system (BESS) is used as an energy storage element for the purpose of voltage regulation.
Since it rapidly injects or absorbed reactive power to stabilize
the grid system. It also control the distribution and
transmission system in a very fast rate. When power
fluctuation occurs in the system, the BESS can be used to
level the power fluctuation by charging and discharging operation. The battery is connected in parallel to the dc capacitor
of STATCOM [10]–[14].
The STATCOM is a three-phase voltage source inverter
having the capacitance on its DC link and connected at the
point of common coupling. The STATCOM injects a compensating current of variable magnitude and frequency component
at the bus of common coupling.
C. System Operation
The shunt connected STATCOM with battery energy storage is
connected with the interface of the induction generator and
non-linear load at the PCC in the grid system. The STATCOM
compensator ou t p ut i s va r i e d a c c or d i n g t o t h e
c on t r ol l e d
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as sample tem- plate Vsm, sampled peak voltage, as in (9).
1/2
Vsm = { ( +
+ )} ………. (9)
Voltage and the RMS value of unit vector usa,usb,usc as shown in
(10).
usa =
usb =
usc =
……. (10)
In-phase unit voltage template as, in (11)
i*Sa= I.uSa
Fig. 2. System operational scheme in grid system.
i*Sb= I.uSb
i*Sc= I.uSc ……. (11)
Where I is proportional to magnitude of filtered source voltage
for respective phases. This ensures that the source current is
controlled to be sinusoidal. The unit vectors implement the important function in the grid connection for the synchronization
for STATCOM. This method is simple, robust and favorable as
compared with other methods [18].
B. Bang-Bang Controller
Bang-Bang current controller is implemented in the current
control scheme. The reference current is generated as in (10) and
actual current are detected by current sensors and are subtracted
for obtaining a current error for a hysteresis based bang-bang
controller. Thus the ON/OFF switching signals for IGBT of
STATCOM are derived from hysteresis controller [19].
The switching function SA for phase ‘a’ is expressed as (12).
Fig. 3. Control system scheme
iSa ˂ ( isa* - HB) → SA=0
Strategy, so as to maintain the power quality norms in the grid
iSa > ( isa* - HB) → SA=1………. (12)
system. The current control strategy is included in the control
scheme that defines the functional operation of the STATCOM Where HB is a hysteresis current-band, similarly the switching
compensator in the power system to the nonlinear load in the function SB, SC can be derived for phases “b” and “c”.
VI. SYSTEM P ERFORMANCE
grid system. The main block diagram of the system operational
scheme is shown in Fig. 2.
The proposed control scheme is simulated using SIMULINK
in power system block set. The system parameter for given
V. CONTROL SCHEME
system is given Table I.
The system performance of proposed system under dynamic
The control scheme approach is based on injecting the cur- condition is also presented.
rents into the grid using “bang-bang controller.” The controller
uses a hysteresis current controlled technique. Using such tech- A. Voltage Source Current Control—Inverter Operation
nique, the controller keeps the control system variable between
The three phase injected current into the grid from
boundaries of hysteresis area and gives correct switching sig- STATCOM will cancel out the distortion caused by the nonnals for STATCOM operation as shown in Fig.3.
linear load and wind generator. The IGBT based three-phase
The control algorithm needs the measurements of several inverter is connected to grid through the transformer. The genvariables such as three-phase source current isabc.. DC voltage eration of switching signals from reference current is simulated
Vdc, inverter current iiabc with the help of sensor. The current within hysteresis band of 0.08. The choice of narrow hysteresis
control block, receives an input of reference current isabc* and band switching in the system improves the current quality. The
actual current iSabc are subtracted so as to activate the operation control signal of switching frequency within its operating band,
as shown in Fig. 4.
of STATCOM in current control mode [16]–[18].
The choice of the current band depends on the operating
voltage and the interfacing transformer impedance. The comA. Grid Synchronization.
In three-phase balance system, the RMS voltage source am- pensated current for the nonlinear load and demanded reactive
plitude is calculated at the sampling frequency from the power is provided by the inverter. The real power transfer from
source Phase voltage (V sa, Vsb , Vsc) and is expressed,
TABLE 1 SYSTEM PARAMETERS
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue5- May 2013
TABLE 1 SYSTEM PARAMETERS
S.No.
Parameters
Ratings
1.
Grid Voltage
3-phase,415V,50 Hz
2.
Induction
Motor/Generator
3.35 kVA,415V,50 Hz, P=4,
Speed=1440rpm,Rs=0.01Ω,
Rr=0.015Ω,Ls=0.06H,Lr=0.06H
3.
Load Parameter
Non-linear Load 25kw
4.
Inverter Parameters
DC Link Voltage = 800V,
DC link Capacitance=100µF,
Switching frequency = 2 kHz.
5.
IGBT Rating
Collector Voltage=1200v, Forward
Current=50A, Gate voltage=20v.
Fig. 4. Switching signal within a control hysteresis band
The batteries is also supported by the controller of this
inverter,
C. STATCOM—Performance Under Load Variations
The wind energy generating system is connected with grid
having the nonlinear load. The performance of the system is
measured by switching the STATCOM at time t = 0.7s in the
system and how the STATCOM responds to the step change
command for increase in additional load at 1.0 s is shown
in the simulation. When STATCOM controller is made ON,
without change in any other load condition parameters, it starts
to mitigate for reactive demand as well as harmonic current.
The dynamic performance is also carried out by step change in a
load, when applied at 1.0 s. This additional demand is fulfill by
STATCOM compensator. Thus, STATCOM can regulate the
available real power from source. The result of source current,
load current are shown in Fig. 5(a) and (b) respectively. While
the result of injected current from STATCOM are shown in
Fig. 5(c) and the generated current from wind generator at PCC
are depicted in Fig. 5(d).
Fig. 5. (a) Source Current. (b) Load Current. (c) Inverter
Injected Current. (d) Wind generator (Induction generator) current.
Thus the DC link along with the static synchronous
condenser is provided for the supporting purpose in order to
act in the both ways as to relive the main supply from the
reactive power demand from the non-linear loads and also to
support the back up power supply to the static synchronous
condenser and also the main functionality of it is it injects the
currents in to the point of common coupling in order to cancel
out the harmonics that are being introduced by the non-linear
loads in to the source at the point of common coupling as the
control is being carried out the mechanism is been maintained
by a microcontroller through its pulses in order to elimianate
the harmonics the switching operation should be in a position
to withstand the heavy dips and swells and also it provides the
respective reactive power for the wind energy systen for its
operation.
Fig. 6. STATCOM output voltage.
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REFERENCES
[1] F. Zhou, G. Joes, and C. Abbey, “Voltage stability in weak connection
wind farm,” in IEEE PES Gen. Meeting, 2005, vol. 2, pp. 1483–1488.
[2] T . Kinjo and T. Senjyu, “Output leveling of renewable energy by electric double layer capacitor applied for energy storage system,” IEEE
Trans. Energy Conv., vol. 21, no. 1, Mar. 2006.
[3] R. S. Bhatia, S. P. Jain, D. K. Jain, and B. Singh, “Battery energy
storage system for power conditioning of renewable energy sources,”
In Proc. Int. Conf. Power Electron Drives System, Jan. 2006, vol. 1, pp.
501–506.
Fig. 7. Supply Voltage and Current at PCC.
C. Power Quality Improvement
It is observed that the source current on the grid is affected
due to the effects of nonlinear load and wind generator, thus
purity of waveform may be lost on both sides in the system.
The inverter output voltage under STATCOM operation with
load variation is shown in Fig. 8. The dynamic load does affect
the inverter output voltage. The source current with and without
STATCOM operation is shown in Fig. 9. This shows that the
unity power factor is maintained for the source power when the
STATCOM is in operation. The current waveform before and
after the STATCOM operation is analyzed. The Fourier analysis
of this waveform is expressed and the THD of this source current
at PCC without STATCOM is 4.71%.
The power quality improvement is observed at point of
common coupling, when the controller is in ON condition. The
STATCOM is placed in the operation at 0.7 s and source current
waveform is shown in Fig. 11 with its FFT. It is shown that the
THD has been improved considerably and within the norms of
the standard.
The above tests with proposed scheme has not only power
quality improvement feature but it also has sustain capability to
support the load with the energy storage through the batteries.
[4] S. W. Mohod and M. V. Aware, “Grid power quality with variable
speed wind energy conversion,” in Proc. IEEE Int. Conf. Power Electronic Drives and Energy System (PEDES), Delhi, Dec. 2006.
[5] F u . S. Pai and S.-I. Hung, “Design and operation of power converter for
micro turbine powered distributed generator with capacity expansion
capability,” IEEE Trans. Energy Conv., vol. 3, no. 1, pp. 110–116, Mar.
2008.
[6] J. Zeng, C. Yu, Q. Qi, and Z. Yan, “A novel hysteresis current
control for active power filter with constant frequency,” Elect. Power
Syst. Res., vol. 68, pp. 75–82, 2004.
[7] M. I. Milands, E. R. Cadavai, and F. B. Gonzalez, “Comparison
of control strategies for shunt active power filters in three phase four
wire system,” IEEE Trans. Power Electron., vol. 22, no. 1, pp. 229–236,
Jan.2007.
VII. CONCLUSION
This paper introduces the enhancement of power
productivity using a static synchronous condenser based control
scheme in wind energy system with nonlinear load. The
operation of the control system acquired for wind energy
system with static synchronous condenser for maintaining
power quality is simulated in MATLAB/SIMULINK. It has an
ability to set off the harmonic parts of the load current. It
ensures the source voltage and load current are in-phase and
aids the reactive power demand for the wind generator and load
at PCC. Thus it gives a chance to improve the utilization factor
of transmission lines. The integrated wind generation and static
synchronous condenser with BESS have shown excellent
performance. Thus the scheme proposed accomplishes the
power productivity.
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