-
Performance of DFIG during symmetrical and
asymmetrical grid faults with damping controller
based SSSC
D.V.N.Ananth1, G.V.Nagesh Kumar2, D.Deepak Chowdary3,
K.Appala Naidu2 1DADI Institute of Engg. & Technology,
Anakapalli,
Visakhapatnam, Andhra Pradesh, INDIA, [email protected],
ph: +91-8500265310 2Vignan’s Institute of Information
Technology, Visakhapatnam,
Andhra Pradesh, INDIA, [email protected] 3Dr. L.
Bullayya Engg. College for Women, Visakhapatnam, Andhra
Pradesh, INDIA
[email protected]
Abstract
The renewable energy resources like wind with doubly fed
induction generator (DFIG) is
playing a vital role in meeting the ever-increasing load demand.
Most of the industrial and
commercial loads are sensitive to fault, as surge current
damages the system. The DFIG
wind turbine set is very sensitive when grid fault occurs, which
damages the stator and
rotor winding and also the converters and the capacitor. To
overcome these effects, DFIG
grid connected system is equipped with damping controller based
static synchronous series
compensator (SSSC) based series FACTS device with a new control
scheme for oscillations
damping and quicker voltage injection technique. Voltage damping
circuit is provided in
the outer control loop of SSSC for improving voltage profile of
stator and rotor. The inputs
for the damping circuit are rotor speed and stator real power
and controller is designed
with cascaded 2nd order lead-lag compensator. The results are
presented for single line,
two lines and three lines to ground faults and system behavior
is examined.
Keywords: DFIG, static synchronous series compensator (SSSC),
damping lead
lag compensator, symmetrical and asymmetrical faults
1. Introduction With the latest trends in renewable energy
resources, wind turbine based power
generation is getting importance as conventional synchronous
generator based
power plants are not alone sufficient to convene with the ever
growing load
demand. The DFIG based wind generators are getting popular as
real and reactive
power sharing, load withstanding capability, low cost converters
are better than
other wind generators. Faults are inevitable for any power
system and are very
dangerous for DFIG wind turbine system [1]. Based on this paper,
if fault current
is not controlled, the inrush current will damage the
converters, dc capacitor, stator
and rotor winding. Crowbar type of protection is used to divert
the fault inrush
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current, thereby damage can be prevented [2-4]. This type of
protection has a major
drawback that, it will draw huge reactive power from the grid as
the DFIG now
runs like squirrel cage induction generator. Therefore many
recent works deal with
new control strategies to overcome different types of faults.
Improved
demagnetizing, feed-forward transient current control,
current-reversely-tracking
control (CRTC) etc [5, 6] are recently proposed techniques for
DFIG to improve
fault ride through and the authors are almost successful.
New controllers like PI + resonant (PIR) or P+ Resonant (PR),
internal model
control (IMC) [7], sliding mode control (SMC), fuzzy and many
are used to improve
the speed of operation instead of conventional PI controller.
Metaheuristic control
techniques like bacterial forging, particle swarm optimization
etc techniques are
also used for tuning of PI parameters under different fault
conditions. Feed forward
regulator, magnetization current compensation, LQR, impedance
based high
frequency resonance etc are new techniques to improve
performance when DFIG
tied to a week grid.
The FACTS devices like dynamic voltage restorer (DVR), STATCOM
[8, 11],
UPFC, dual STATCOM [9], fault current limiters (FCL) [10],
energy storage
devices like SMES are extensively used in the literature to
overcome any type of
faults and to have better performance of DFIG. Among all these
devices, DVR,
STATCOM and FCL are more promising and help in maintaining
nearly flat
profile during severe faults. In this paper, static synchronous
series compensator
(SSSC) is used to overcome different types of fault occurring
near the grid. In this a
24 unit DFIG equivalent grid connected system is considered. The
performance of
the DFIG under single line, two lines and three lines to ground
faults are observed
with SSSC using MATLAB/ SIMULINK.
2. Rotor and grid side converter design The direct and
quadrature (d and q) axis DFIG rotor voltage equations and the
rotor and stator windings double d and q axis fluxes are given
by
{
( )
( )
( )
( ) ( ) ( )
(1)
From the basic equations of DFIG [9], the rotor direct and
quadrature axis
voltages are expressed as
(
)
(2a)
(
)
( ) (2b)
where is synchronous speed and is rotor speed.
The block diagram of RSC in Fig.1a is based control circuit for
better
performance under LVRT problem. The sub-circuit is EFOC
technique using the
equations 2a and 2b. These equations if rewritten as decoupled
parameters as in
equations 3a and 3b are developed for RSC controller.
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( ) (3a)
( ) (3b)
The rotor speed in general is and the stator synchronous speed.
However, this is varied from to a new synchronous speed during
abnormal
conditions is or simply At ideal situations,
which is reference stator d-
axis vector flux is zero in magnitude and q-axis flux . These
holds good for stator
flux magnitude at a given back emf and the rotor speed. The
rotor dq axis
transient current are represented in the equations 4a and 4b
as
(4a)
(
)
(4b)
The rotor reference voltages in two axis Park’s dq
transformation is rewritten
with the help of equations 3a and 3b are given below. This is
the output rotor
windings voltage during normal and transient conditions are
(
(
) ) (5A)
(
(
) ) (5B)
The direct (d-axis) and quadrature (q-axis) axis are two axis
rotating frame of
reference. Now again, the stator d axis and q axis currents in
equations are written
in terms of stator voltage and rotor currents
dr
s
mdr
s
m
s
dsds i
X
X
Xs
Vsi
L
L
Li
(6a)
,qrs
mqs i
X
Xi
(6b)
The grid two axis voltage in GSC current, voltage, grid
resistance and
inductance forms as
1dgqgs
dg
dggdg VLidt
diLiRV
(7a)
1qgdgs
qg
qggqg VLidt
diLiRV
(7b)
The dynamic dc voltage across the dc link capacitor is given in
equation (8) as a
function of grid side converter (GSC) and rotor side converter
(RSC) and dc link
voltage.
dc
rgdc
CV
PP
dt
dV
(8)
So, the change in DC link voltage across the capacitor depends
on rotor voltage
and stator voltage. Hence based on the above discussion, the GSC
and RSC control
schemes are developed which is explained in the next section. It
is observed that if
rotor voltages are controlled effectively using proper control
technique and PWM
operation, the rotor current flow is controlled. When the stator
voltage magnitude
increase or decrease from normal value, consequently the rotor
currents will be
affected and vice-versa. This means, if there is a decrease or
increase in the stator
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voltage, if rotor currents are adjusted, the stator voltage
magnitude can be
improved. In the similar way, with d and q axis control of rotor
voltages, the
electromagnetic torque (EMT) can be controlled. Also, it is
observed that, the stator
voltage is like a quadratic function with square of voltage term
and a single stator
voltage term. If stator voltage increase or decrease suddenly,
it leads to an
oscillations and change in magnitude respectively in the EMT.
The leakage factor
can be stated as .
3. RSC and GSC design and operation The RSC refers to rotor side
converter and GSC is for grid side converter. The
RSC operation is to manage and preserve the speed of rotor
during abnormal
conditions like faults thereby stator frequency connected to
grid must not deviate
by using d-axis current control scheme. It also helps in
regulating the grid reactive
power using q-axis current control method as in Fig. 1a. The
outer loop of RSC
consists of speed and reactive power control loops and inner
control loops has d and
q two axis current control loops. The reference speed of rotor
is derived using
lookup table method from the wind turbine optimal power output
PmOpt and grid
power demand. Based on the value of Pm,gOpt, rotor is rotated at
optimal speed to
draw optimal (maximum) power from DFIG WECS set. The Speed error
is
minimized with PI controller to zero value and the output is the
product of stator
flux (Fs) and ratio of stator and rotor inductances (Ls and Lr)
to obtain reference
rotor q-axis current (Iqr*). The output from each PI controller
is controlled with a
disturbance voltage VdqR to get reference pulse generation
voltage. It is to note
that, the pulses are synchronized at slip frequency of RSC
rather at fundamental
nominal grid frequency. The synchronizing slip frequency is
converted back using
inverse Park’s transformation to get abc stationary rotor
reference PWM voltage
parameters as in the figure.
Fig. 1a Rotor side converter (RSC) control scheme for DFIG,
rs
m
LL
L21
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Fig. 1b Grid side converter (GSC) control scheme for DFIG
The GSC basic block diagram is depicted in Fig.1b. For a given
wind speed, the
turbine reference control power is predictable with a lookup
table. The stator real
power (Pstator) is calculated and the power error is the
difference between two
powers (dP) which are preserved near the zero value by using PI
controller. The PI
controller output is multiplied with constant (Kp) called real
power constant gives
actual convenient power after interruption. The difference in
square of reference
capacitor voltage across dc link (Vdc*) and square of actual dc
link voltage (Vdc) is
[Vdc*-Vdc] controlled using PI controller to get reference
controllable real power.
The error in the reference and actual controllable power is
divided by using 2/3Vsd
to get direct axis (d-axis) reference current near grid terminal
(Igdref). Difference in
Igdref and actual d-axis grid current is controlled by PI
controller to get d-axis
voltage. But to achieve better response to transient conditions,
decoupling d-axis
voltage is added as in case of separately excited DC motor. This
decoupling term
helps in controlling steady state error and fastens transient
response from DFIG
during low voltage ride through (LVRT) or during sudden changes
in real or
reactive powers from/ to the system.
The block diagram of GSC control circuit is shown in Fig. 1b and
RSC is
designed to get better performance for LVRT issues at (PCC)
point of common
coupling. During normal conditions like steady average wind
speed and good
ambient temperature, the reactive power will be zero or very low
and hence stator
power pumped to the grid will be high. This power control can do
use the outer
control loop of GSC. The reference power is obtained from the
characteristic lookup
table with characteristics of DFIG adopted for desired
operation. This reference
power is compared to actual power and is maintained using the PI
control of GSC
as shown in Fig.1. During faults, the stator power varies based
on the reactive
power demand, which will is supplied by GSC through the
capacitor at the back to
back converters. As reactive power demand increases, stator
power changes
accordingly, and hence the terminal voltage at GSC change
respectively and
thereby direct axis current injecting at PCC changes. During
steady sate, stator
rms voltage and reactive power are constant. But when the fault
occurs, the stator
voltage changes, hence reference rms stator voltage changes.
This will make the
quadrature component of GSC current to vary. This total
mechanism is fast and
can work for symmetrical as well as asymmetrical faults.
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4. Design of FACTS and energy storage devices
for LFC The FACTS and energy storage devices are being used in
power system for
many applications like voltage mitigation, power quality
improvement, power
transfer capability improvement, power oscillations damping,
frequency regulation
etc. Among many FACTS devices SSSC is an excellent series FACTS
device used
for real and reactive power control. Voltage stability will be
improved with reactive
control and frequency control is with real power control. The
block diagram of
SSSC is shown in Fig.2a and with transfer function based control
design in Fig. 2b.
The SSSC produces three phases voltage in quadrature with the
line current follow
an inductive or capacitive reactance based on the current flow
in the transmission
line. The magnitude and polarity of Vq decides the compensation
to be inductive or
capacitive to stabilize the frequency and real power deviations
during wind speed
or load change.
Fig. 2a Block diagram of DFIG with SSSC device
Fig. 2b transfer function based control of SSSC
5. Result Analysis For the DFIG network in Fig.2a, the
simulation results are presented in this
section. A fault is situated near the PCC at 0.3s and cleared at
0.5s. The behavior of
DFIG system and the compensation of SSSC for different faults
under cases three,
double and single phases to ground are analyzed.
5.1 Case A: Three phases to ground or symmetrical fault
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For the symmetrical fault at PCC with 0.002Ω resistance between
0.3 and 0.5,
the waveforms are given in Fig.3. The voltage at grid is
decreased from 1pu to
0.5pu (per unit) and the current shrink from 1pu to 0.4pu as
shown in Fig. 3a.
When the fault is suppressed at 0.5s, the voltage of grid raised
to 1pu slowly than
the current with surge of 1.1pu. It is due to the fact that
sudden change in load
impedance leads to current increase in the network. The similar
behavior is
observed with stator as it is also linked to the same phases
directly. The rotor
voltage and current are shown in the figure 3b. The rotor
voltage is almost constant
and there is a small dip in the current from 0.55pu to 0.4pu.
The RSC and GSC
helps in maintaining the voltage and SSSC helps in still
maintain at a better
voltage profile by mitigating the surge currents. So, voltage
and current profiles are
maintained with converters and SSSC.
Cur
rent
(p.u
.)
V
olta
ge (p
.u.)
Time (S)Time (s)
Cur
rent
(p.u
.)
V
olta
ge (p
.u.)
Fig. 3a Grid voltage (top) and current (bottom) in pu Fig. 3b
rotor voltage (top)
and current (bottom) in pu for three phases to ground fault
Time (S)
Torqu
e (p.u
.) ro
tor sp
eed (
p.u.)
Cap
. Volt
(p.u.
)
Time (S)
Vol
tage
(p.
u.)
Fig. 3c dc caapcitor voltage, rotor speed and electromagnetic
torque (EMT) in pu Fig. 3d SSSC injecting voltagein pu for three
phases to ground fault
From the Fig 3(c), the capacitor voltage is nearly constant dc
voltage magnitude
during and after the fault. The ripples decreased due to
reversal of current in the
RSC and GSC towards rotor and grid PCC. The dynamics of these
are given by the
equations (3a) to (8) as given in section 2. The rotor speed is
almost constant at
1.2pu i.e., during the fault from 0.3 to 0.5s and maintained
constant then at 1.22pu.
The electromagnetic torque (EMT) is initially at 0.6pu at 14m/s
wind speed and
reached to 0.05pu during fault without ripples and when fault is
cleared, the EMT
attained its pre-fault value. The oscillations in torque are
damped because of the
RSC control scheme with better control strategy and with SSSC
damping control
nature proposed in the paper. The SSSC injecting voltage is
shown in Fig. 3(d). At
the instant of fault, the voltage dip will be very high and
surge currents increases.
The dc capacitor voltage between the RSC and GSC of DFIG is
increased. To
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mitigate them all, voltage injection of SSSC has to be high as
shown in this figure
3d. Then during the fault, because of voltage injection, the
voltage profile of SSSC
is maintained at 0.28pu for compensation and then decreased to a
smaller value
after the fault is cleared. The fifth order lead lag compensator
helps to maintain
generator speed and active powers from oscillations. This is
done when the inputs
are given command to the outer control loop of SSC. The inner
control loops helps
in controlling the current parameters, thereby with proper
tuning of PI controllers,
the voltage injection will be quicker and accurate. So, system
performance during
three phase to ground fault is improved with proposed SSSC
control scheme and
RSC and GSC control, schemes of DFIG.
5.2 Case B: Two phases to ground fault
The same system behavior with two lines to ground asymmetrical
fault in
phases A and B is shown in Fig.4. It is observed that A and B
phases voltage
magnitude decreased while phase C magnitude is having better
voltage profile. But
voltage surges are produced in the faulty phases during the
fault. The grid current
in two faulty phases increased and the healthy phase decreased
to a smaller value
as in Fig.4a. such type of fault is less severe than symmetrical
fault for
conventional synchronous generator plant. But, it is very
dangerous for wind
generator and too for DFIG as it has low rating converter.
Curre
nt (p
.u.)
Vol
tage
(p.u
.)
Time (S)
Curre
nt (p
.u.)
Volt
age
(p.u
.)
Time (S)
Fig. 4a Grid voltage (top) and current (bottom) in pu Fig. 4b
rotor voltage (top) and current (bottom) in pu for two phases to
ground fault
Time (S)
Torq
ue (p
.u.)
rot
or s
peed
(p.u
.) C
ap. V
olt (
p.u.
)
Time (S)
Vol
tage
(p.
u.)
Fig. 4c dc caapcitor voltage, rotor speed and electromagnetic
torque (EMT) in pu Fig. 4d SSSC injecting voltagein pu for three
phases to ground fault
The rotor voltage and current with SSSC control scheme proposed
is shown in
Fig. 4b. It is observed that voltage and current profile of SSSC
are improved. The
voltage of rotor is almost constant and the current is having
ripples without surges
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and rise in magnitude and has harmonic nature during the fault.
the sudden
change in flux do not decay instantly in the stator. The rotor
mutual flux exchange
with oscillating and damping nature of RSC makes it to have the
waveform like
this. If current limiting control wit faster decay control helps
in improving the
profile but the control scheme becomes little complicated.
However, the behavior is
better than the earlier system behavior without two times surge.
In fig. 4c, the dc
voltage across the capacitor between the back to back converters
is having small
oscillations between 1 and 0.9pu. The rotor speed is nearly
constant at 1.2pu
during and after the fault. The EMT is having oscillatory nature
with the two
phases to ground fault. It is not decaying to zero or reversing
the polarity as with
conventional systems. The SSSC voltage injection is shown in
Fig. 4d, the faulty A
and B phase voltages increased while the healthy phase voltage
injection is like
normal value during and before the fault. Hence, voltage
injection of SSSC is better
with the proposed control scheme.
6. Conclusion In this paper, DFIG grid connected system is
analyzed with two and three
phases to ground fault in two cases. The performance is better
with proposed RSC
and GSC control scheme and further improved with proposed SSSC
control scheme
than with the literature survey. The voltage and current profile
of rotor is improved
and dc voltage across the back to back converters is nearly
constant without swell
during the fault. The EMT is having lesser swings than
conventional system with
symmetrical or asymmetrical fault without changing the polarity
of torque to
negative. The rotor speed is almost constant. The SSSC injecting
voltage is quick
and accurate as damping controller based fifth order transfer
function is newly
proposed in this paper. The RSC and GSC control schemes are
proposed with
lookup table based technique to have better reactive power to
rotor speed profile.
hence our proposed scheme is better than conventional control
schemes with DVR
or SSSC arrangement with easier control scheme, faster and
accurate and holds
good for symmetrical or asymmetrical fault with any decrease in
the grid voltage.
Appendix The simulation parameters of DFIG used are, Rated
Voltage = 690V, Rated
Power = 1.5MW, Stator Resistance Rs = 0.0049pu, , Stator Leakage
Inductance Lls
= 0.093pu, rotor Resistance Rrӏ = 0.0049pu, Rotor Leakage
inductance Llr1 =
0.1pu, Number of poles = 4, Mutual Inductance Lm = 3.39 pu,
Inertia constant =
4.54pu, DC link Voltage = 415V, DC link capacitance = 0.2F, Grid
Voltage = 25 KV,
Grid requency = 60 Hz. nominal wind speed = 14 m/sec. Grid side
Filter: Lfg =
0.6nH, Rfg = 0.3Ω, Rotor side filter: Lfr = 0.6nH, Rfr = 0.3mΩ,
wind speed
variations considered here in seconds: 8, 15, 20 and 10 at 15,
25 and 35s. variation
in grid voltage: 0.8 to 1 and to 1.2pu at 20 and 30s, Reactive
power variation: -0.6
to 0 and +0.6pu at 20 and 30s.
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