Download power quality improvement in grid connected system using

POWER QUALITY IMPROVEMENT IN GRID CONNECTED SYSTEM
USING STATCOM
T.Rajithavani
M.tech student scholar
Department of Electrical&Electronics Engineering
Shri Vishnu Engineering College for Women
Bhimavaram
Abstract:
- The main objective of this paper is to
improve the power quality and transient stability
of grid connected wind farm using a Static
synchronous compensator(STATCOM).when the
grid is affected by voltage fluctuations, faults and
harmonics which are occurred at point of common
coupling(PCC) are reduced by the static
synchronous
compensator
(STATCOM)
connected to grid system. Here a novel control
strategy is applied on the STATCOM to improve
the reliability and to run the STATCOM in safe
limit. The Applied control strategy is proportional
resonant control strategy (PR) which reduces the
steady state error between reference and
measured signals and the generated signals from
PR controller are given to STATCOM to
effectively suppress the STATCOM over current.
Here PLL is used for grid synchronization.PR
controllers are working in stationary reference
frame in which transformations are reduced and
simpler.
Index terms power quality, transient stability,
grid system, point of common coupling (PCC)
wind, STATCOM, phase Locked loop (PLL)
[6].PR controller, stationary reference frame
1. INTRODUCTION
An increasing demand for more electric power
coupled with depleting natural resources has led to an
increased need for energy production from renewable
energy sources such as wind and solar. Now a day the
usage of renewable energy sources is increased
widely to improve the short comings of conventional
energy resources. The latest technological
advancements in wind energy conversion and an
increased support from governmental and private
institutions have led to increased wind power
generation in recent years Wind power is the fastest
growing renewable source of electrical energy.By
using wind fed to the wind turbine generators
mechanical energy fed to the Induction Generator
through the transmission line connected to the grid.
Fluctuations in wind energy [4], voltage fluctuations
and harmonic compensation due to nonlinear loads
S.Dileeep Kumar Varma
Associate professor
Department of Electrical & Electronics Engineering
Shri Vishnu Engineering College for Women
Bhimavaram
are reduced by using a static synchronous
compensator. In the past decade, researchers have
explored use of proportional-resonant (PR) regulators
for control ac-dc converters using the stationary α-β
frame. PR regulator can offer fast dynamic response
and zero steady state error .PR controllers are carried
out in stationary reference frames with phase locked
loop .in stationary reference frames three phase two
phase transformations are required .these three phase
equations are converted into two phase by Clarke
transformation.
PCC
Grid
system
RL load, non
linear load, step
load, faulty
load
Control output
Inverter
control
Switchi
-ng
control
inputs
STATCOM
Wind
energy
system
Fig.1: Basic block diagram
When variable loads, non linear loads and faulty
loads are connected to the PCC they cause voltage
drop, harmonics, and transient instability in the grid
system. To overcome this faults STATCOM is a
FACTS device to control reactive power
dynamically, utilizing a voltage source converter
connected in shunt to the power system which main
purpose is to perform reactive power compensation
and voltage control at the PCC. A proper control
scheme in wind energy generation system is required
to mitigate these power quality problems. Efficient
control scheme of STATCOM based on PR
(proportional resonant) control strategy with PLL
control strategy is presented to control the grid
currents. Here the Application of a PR control
strategy on STATCOM is for controlling the
STATCOM over current and PCC voltage. It is also
concluded that the proposed control strategy can
effectively reduce the effect of abnormal conditions.
II PROPOSED CONTROL SYSTEM
Wind
System
Line inductance
Utility distribution grid
Non linear load
R1
RL
Load
R2Rrrr L1
R2r
PR current
control strategy
L2
L2
R3L3
L3
STATCOM
Step load
Fig.2: System configuration
Fig 2 shows the grid connected wind energy system
with and without STATCOM is analyzed under RL
load, nonlinear load, step change in load and faulty
loads which are connected at PCC. When RL load is
connected voltage is dropped at PCC and effect the
grid voltage. When a nonlinear load is connected to
the PCC the harmonics in the load will affect the
grid. When step change in load voltage effect the
PCC voltage. By the STATCOM performance with
PR controller plays predominant role and THD also
reduced effectively
+
+ +……….
X100
(1)
V1
STATCOM is connected in shunt between the source
and load via a coupling inductance Lc, so that it
provides the necessary reactive current for the
respective functions of the STATCOM such as power
factor correction, harmonic elimination, etc. The
controller of the STATCOM in this work is based on
proportional resonant control theory.
%THD =
III.PROPOSED CONTROL STRATEGY
Here PR controller is achieved in stationary
reference frame. The transformation is done from
ABC to αβ reference frame. Here three phase
sinusoidal variables are controlled instead of constant
variables. Three phase sinusoidal quantities produces
two phase quantities in a fixed reference frame vary
sinusoidal with time. Where as in DQ rotating frame
they are stay constant. Fig 3 shows the Grid system
converter control using PR controller which is carried
out in stationary reference frame.
Fig.3 Block diagram of STATCOM control
From the above block diagram three phase voltage
signals from the grid are synchronized with system
using PLL .As well as three phase current signals
from grid are converted to two phase stationary
signals by Clarke transformation .Then this signals
are compared with the two phase stationary signals
which are generated from the PLL. These signals are
compared error signal is given to PR controller. The
generated two phase signals are converted to three
phase by inverse Clarke transformation those are
given to PWM then pulse from PWM given to
STATCOM to operating in safe manner.
HPR(s) = KP+KI S/
(2)
Transfer function from the block diagram shows
proportional plus resonant gains. Here the
compensation terms are no need because they are not
cross coupling current terms. Here the transformation
is
implemented
in
stationary
(Clarke)
transformation.[2]The transfer function of single and
three phase PR controllers and filters are derived
using internal model control, modified state
transformation [13]
Single phase PR controller in stationary reference
frame and their equivalence in synchronous reference
frame.
Fig.5.Equivalent representations of PR and
synchronous PI controllers
In single phase PI control, PI control is approached
by closest equivalence is developed to multiply the
feedback error (t), by sine and cosine functions. The
functions are usually synchronized with grid voltage
using a phase-locked loop (PLL).This achieves the
same effect of transforming the component at the
chosen frequency to DC ,leaving all other
components as AC quantities. In case of non-linear
loads connected to the grid thus the harmonic
component is taken as the error signal.
PR CONTROLLER
e(t)=E1 COS(ωt+θ1)+E3cos(3ωt+θ3)
The objective of current controller s is to have zero
phase and zero magnitude error .The principle is to
find an equivalence ac compensation network with
same frequency response characteristic to the
synchronous frame controller.
Instead of transforming the feedback error to the
equivalent synchronous frame for processing, an
alternative approach of transforming the controller
GDC(s) from the synchronous to the stationary frame
is also possible.
GAC(S)= GDC(s-jω)+ GDC(s+jω)
(3)
(4)
Where GAC(S) represents the equivalent stationary
frame transfer function. Therefore, for ideal and non
ideal integrators. The ideal and non ideal integrators
are
Fig.4: Block diagram for PR controller
=
and
= (1+(
)) (5)
ω represents controller gain and ωc is cut off
frequency where ki and ωc <<ω).the derived ac
integrators GAC(S) are expressed
=
(6)
=
= cut
(7)
off frequency ω=controller gain
=
this control structure is used
according to the internal model principle; in
introduce a mathematical model that can generate the
required sinusoidal reference along the open loop
control path. And therefore can ensure overall zero
steady state error [12]
connecting nonlinear load, and increasing step
change in the load voltage sag is created in PCC
voltage, harmonics in source voltage and current.
when step change in load at PCC then STATCOM
absorb the injected voltage.
V.SIMULINK RESULTS AND DISCUSSIONS
The control scheme has been
MATLAB/ SIMULINK model.
simulated
on
A. voltage regulation in the grid under
variable load
By connecting the RL load at PCC the voltage
dropped from the rms voltage 240v to 224v.
For three phase systems the transfer function is
follows
(8)
Fig7. VOLTAGE SAG WITHOUT STATCOM
The above Equation have no cross coupling nondiagonal terms, implying that each of α and β
stationary axes can be treated as a single-phase
system. The transfer function of pr controller and
filter is shown in figure [5].It is derived from the
closed loop control which having a proportional plus
resonant gains.
From fig.3 voltage sag in grid is reduced after
t=0.15” because the STATCOM come into action
By comparing the reference and measured signals error sign
IV. SIMULINK MODEL
Fig.8. Voltage sag with STATCOM
After the transition time voltage is in phase with the
current
Fig.6.Simulink model with PR control strategy
The control circuit is achieved by the basic block
diagram and after going all conversions at PCC by
connecting RL load voltage is dropped, and by
Fig9. Power factor
B. Suppressing harmonics in the grid under
nonlinear load currents.
When a nonlinear load is connected to the PCC
harmonics in the load will affect the grid. From fig.10
the harmonics in grid is reduced after t=0.05”
because the STATCOM come into action along with
PR controller.
Fig12.Powerfacotr of source voltage and current
D. Transient stability under step change in load
When the load is step change at PCC will effect the
grid voltage. Change in load voltage is controlled by
STATCOM. After transition time 0.05” there is no
change in the source voltage. The increased amount
is carried out by STATCOM.
Fig 10.Three phase source currents
After transition time STATCOM is come into contact
with system then the harminc currents are injected to
STATCOM.SO after 0.05s STATCOM current have
ripples.
Fig 13.Step change in load voltage
when load is step changed STATCOM absorb the
load voltage fig 14.shows the STATCOM current
after transition time.
Fig 11.STATCOM current
C. Power factor correction under nonlinear
load.
From Fig.12 the power factor becomes unity when
STATCOM connected to PCC after t=0.05”.
Simulation waveforms of the power factor are
compared with and without STATCOM.
Fig.14. STATCOM current
E. Reduction of fault voltage
When a faulty load is connected to PCC there is fault
voltage or current in that phase cause voltage sag at
PCC effects the load.fig 15 shows the phase A fault
voltage.
Fig.15: Phase a fault voltage
By connecting STATCOM at PCC after 0.1s
STATCOM is come into contact with system then
fault in that load is minimized then voltage is raised
after 0.15s.
Fig.16: Phase A Voltage wave with STATCOM
When non linear load is connected at the point of
common coupling (PCC) THD with and without
STACOM is compared.
the PR current controller for harmonic reduction
place a predominant role.
Fig18. THD with STATCOM
VI CONCLUSION
This paper provides the grid model based PR control
strategy when the grid is effected by non linear load,
RL load and step load. The effect of these loads on
the grid is reduced by specially designed stationary
reference frame with PLL control strategy. Control
strategy proposed has lesser computational burdens
and has less steady state error between actual and
measured signals. Simulation results shown that the
PCC voltage has been effectively regulated and THDi
indicate that PR control improves it effectively.PR
control is a good alternative to implement an inverter
system control with reduced harmonic content
injected into the grid. Simulation results have been
compared with and without STATCOM. The
STATCOM based PR current controller with PLL
control strategy guarantees fast transient response
and high static performance via an internal current
control loops.
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