DFIG Based Wind Turbine Contribution to System Frequency Control
Two area load frequency control for DFIG based wind turbine system using modern energy … ·...
Transcript of Two area load frequency control for DFIG based wind turbine system using modern energy … ·...
Two area load frequency control for DFIG based
wind turbine system using modern energy storage
devices
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
Abstract
In this paper, energy storage devices like super conductor magnetic energy
storage system (SMES) and thyristor controlled capacitor storage phase
shifters (TCPS) and FACTS device like static synchronous series
compensator (SSSC) are used to damp oscillations in a power system. In
general, with increase in number of wind generators connected to grid,
penetration issues slowly increases. Due to this, if there is a sudden change
in load in one area, frequency deviation in all areas takes place which leads
to electro-mechanical oscillations in the system. To damp out these
oscillations tie-line based frequency controllers (TLFC) were generally used
for DFIG systems. Hence for effective damping of oscillations, SMES, TCPS
and SSSC are chosen for DFIG based wind turbine system. It is to find a
suitable device among TLFC, SMES, TCPS, SSSC to work in coordination
to control frequency regulation and tie-line power for area DFIG based
systems. Simulation results prove that oscillations damping can be effective
if coordinated SMES and TCPS or with SSSC installation in both areas are
better options.
Keywords: SMES, TCPS, SSSC, inter-area oscillations and damping, tie-
line power, TLFC, DFIG, wind generator penetrations, frequency control,
and automatic generation control.
1. Introduction
Now days, renewable energy resources like wind are getting importance as
conventional power plants are not alone sufficient to convene the rising 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
International Journal of Pure and Applied MathematicsVolume 114 No. 9 2017, 113-123ISSN: 1311-8080 (printed version); ISSN: 1314-3395 (on-line version)url: http://www.ijpam.euSpecial Issue ijpam.eu
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other wind generators [1]. The understanding of DFIG during load changes is very
important for improving the stability of the system [2-5]. These papers describe
that, due to sudden load changes, tie-line frequency and real power from DFIG and
conventional plants deviates from normal value. During this process, weak system
starts to deviates and will trip. This makes the next weaker generator to trip and
soon and finally leads to frequency collapse. Hence, load frequency control for DFIG
wind generator set is necessary similar to conventional power plants. In
synchronous generator system, governor control helps in frequency regulation. In
DFIG based system rotor and grid side controller plays a vital role in regulating
frequency and real power output from the stator.
Apart from load deviations, wind speed variations also affect the frequency
output of DFIG. For this, primary and secondary frequency regulators are adopted
to restore to normal values during perturbations [6-10]. Automatic generation
control (AGC) is recently demonstrated on different wind generators with droop,
deloading or inertia based as primary frequency control. Auxiliary power system
based control as secondary frequency control to achieve quicker response during
load deviations. Since decoupled active power control for DFIG is adopted
generally, electromechanical dynamics is very difficult during large frequency
deviations. This makes, DFIGs are very difficult to participate in frequency
regulation. The above primary and secondary frequency regulations are alone not
sufficient to restrict frequency regulation. Hence FACTS devices are helpful in
achieving quicker and better frequency stability and better load delivery to tie-
lines.
To overcome the effect of load variations, FACTS devices [11-15] like
thyristor based phase shift regulators (TCPAR) or static synchronous series
compensators (SSSC) or energy storage devices like battery or superconducting
magnetic energy storage system (SMES) are used. Coordination of FACTS and
energy storage devices are used recently for conventional power plants to reach
better frequency stability margin. These devices help in sharing real power
between the tie-lines and generators by controlling the generator phase angle
jumps during load deviations. Hence better results of frequency regulation,
maximum power transfer and enhanced reactive power support with power system
stability is achieved with coordinated FACTS and energy devices.
Hence in this paper, coordinated FACTS- FACTS and FACTS- SMES are
checked using MATLAB simulation to find the better device and coordinated device
for frequency regulation for DFIG based two area systems. The devices are
designed to provide negative oscillations during disturbance and thereby frequency
and real power oscillations are mitigated.
2. Area generation control modeling A. Two area power system model
The simulation diagram using transfer function of interconnected two area
power system is shown in Fig.1. It has a conventional steam based synchronous
generator and DFIG based wind energy conversion system in both areas. The
behavior of DFIG and a conventional system with sudden change in load in area is
investigated and frequency regulation plots are analyzed. PNC is added to the
system, as shown in Eq. (1):
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, (1)
where
,
Fig.1 MATLAB based DFIG system with basic primary, secondary and tertiary
control
B. DFIG Modeling
For wind generators based AGC with frequency regulation, the fixed-speed wind
turbines (WT) like PMSG and asynchronous generators circumvent from getting
maximum available power to preserve a reserve margin for regulation of frequency.
With the sophisticated control, stored kinetic energy (KE) in the wind turbine (WT)
mechanical system is extracted with DFIG using primary and secondary frequency
regulation techniques. The DFIG system produces power for irregular mechanical
WT speed and extracts KE for the primary frequency control. The power output
from DFIG is restricted as chosen by the operator. The factors affect the active
power sharing with the grid by DFIG are the wind speed, dynamic mechanical
power output control to a definite level with hoard mechanical energy like flywheel
etc. When the system frequency decreases, the torque set-point are increased, the
rotor speed decreases, and KE is unrestricted. PNC has two components: power
and frequency deviation using conventional inertial control and ΔPref* is
dependent on optimal turbine speed using wind speed is:
(2)
where Kdf is a frequency deviation derivative constant, and Kpf is the frequency
deviation constant, Δf is the frequency deviation after a high-pass filter. The
equivalent DFIG set improves the optimal speed formerly the transient in
frequency is ended. A power reference point Pω, making the speed to track a
preferred speed reference, is given by equation (2) as
(
) ∫( ) (3)
where Kωp and Kωi are the PI controller constants to get quick transient speed
variation and better speed recovery. This makes DFIG system to provide the
desired active power to fewer divergences. The total DFIG power injection to tie-
line is given by ΔPNC as under equation (4):
(4)
The DFIG inertia interprets into H as in SG based inertia control when this DFIG
unit also supplies to inertia of the system. This inertia is restricted by Kdf, whereas
Kpf presents the system damp, given by equation (5a) as:
(
)
( ) (5a)
where (
)
and ( ) The DFIG to system inertia part with
improved inertial control is given in (5b) equation as
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(
)
( ) (5b)
The equation (5b) is based on the deviation in the frequency using a washout filter
time constant Tω, which depends on a SG based primary regulation behavior
during transient. The reference point is given as
( ) (6)
Here ΔX2 is the frequency change measured and R is the conventional droop
constant, for the wind turbine generator coupled to the grid. The injected active
power by the DFIG is PNC. The injected power is compared with PNCref to obtain
the optimal power output for getting rotor reference speed, where optimal power is
obtained as in Fig. 1. The mechanical power captured by the wind turbine is given
by Eq. (7):
h (
)
(7)
3. Frequency Regulation Frequency control or regulation has three levels of control like primary, secondary
and tertiary control. Primary control refers to an AGC, governors and turbine, with
operation depends on respective loads switching. This combined effect of above
three on frequency is said to be natural frequency. Area Control Error (ACE)
minimization helps in balancing area’s generation for inter-tie systems which
promises the frequency regulation and real power are distributed in each area so
that coordinate control of frequency deviation is possible. Area tie control has three
major functions like frequency control, local load variation deviation control, and
supervision with natural frequency retort to the variation in remote load. For areas
A and B of a two-area system, ACE in each area is generally represented as in
equations (8a and 8b) as 010A Aa As AACE T T B f f (8a) 010B Ba Bs BACE T T B f f (8b)
The summation of all available tie line deviation terms in a AGC system must be
ideally zero, like, TAa-TAs=-(TBa-TBs). Now summing and manipulating
equations (8a) and (8b), we have
010
A B
A B
ACE ACEf f
B B
(9)
The equation (9) says that the error in system frequency is proportional to the sum
of every area ACEs and inversely proportional to area frequency balancing. If any
power deviation in load (ΔPL) in area A takes place, then ACE in A (ACEA) under
AGC strategy is expressed as:
0
0
10
( ) 10
10
10
A a s A
L GA LA A
L A A
L A A
ACE T T B f f
P P P B f f
P f B f
P B f
(10)
and
1,GA
A
P fR
LA AP D f is the increase in DFIG generation and decrease in load
parameters respectively in area A with primary frequency regulation to steady-
state frequency deviation ∆f given by ∆f=-∆PL/βS and
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101A A
A L
s
BACE P
(11a)
similarly, 10B B
B L
s
BACE P
(11b)
where
1A A A
A
B DR
,
1B B B
B
B DR
,
1 1s A B A B
A B
D DR R
.
If 10 A AB , 10 B BB , then A LACE P and 0BACE which states the setting
frequency prejudice to equal frequency response coefficient. In other words, AGC in
area A will take care of its own power disparity while AGC in area B is liable in
allocating the system frequency control through LFC and governor response.
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 is improved with reactive control
and frequency control is with real power control. The SSSC block diagram is shown
in Fig.2a in red color box. 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 stabilizes the frequency and real
power deviations during wind speed or load change. Similarly, the thyristor control
phase angle stabilizer (TCPS) is a real power device for damping power and
frequency oscillations like SSSC. Here the speed deviation is sensed and gives
command to Δω1 control signal to TCPS. Then there will be shift in phase angle
produced by TCPS which controls the real power flow.
For SMES based energy storage system, with abrupt rise in load demand, the
stored energy is nearly instantly released during DFIG WECS to the grid. The coil
instantaneously gets charged to its full value based on the design on converter 1
and 2. Thus absorbing portion of surplus energy in the system and released and the
coil current gets its regular value. The SMES is also a second order lead/ lag
cmpensator like SSSC in frequency regulation mode.
Fig. 2a (i) Block diagram of SSSC Fig.2a (ii) Block diagram representation
of TCPS
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Fig. 2b (i) transfer function based control of SSSC Fig.2b (ii) tf control of SMES
Fig.2b (iii) transfer function based control of TCPS The TCPS structure as a frequency controller function is shown in Fig. 4. The per
unit rotor speed deviation (Dxi, i = 1, 2), which gives the details of each mode of
concern, is used as the input controller signal. There are two parameters called
stabilization gain Ku and TCPS time constant TPS to be optimized for the better
operation of the TCPS frequency controller.
)(11
1
1
11
4
3
2
1 ssT
K
sT
sT
sT
sTP
SSSC
SSSCSSSC
(12a)
)(1
112 ssT
KKTP
TCPS
fTCPS
(12b)
)(11
1
1
11
4
3
2
1 ssT
K
sT
sT
sT
sTKP
SMES
SMESfSMES
(12c)
The blocks design of the SSSC, TCPS and SMES shown in Fig.2 is designed based
on the equations 12a, 12b and 12c. The choice of time constants and controller
constants helps to damp the oscillations and improve the sustainability during load
disturbances or wind perturbations.
5. Improving LFC offered by WG using SMES,
TCPS and SSSC The complete block diagram of two area load frequency control for DFIG based WT
system using different FACTS devices like SSSC is shown in Fig. 2a (i) and with
TCPS is shown in Fig.2a (ii). The internal control diagram of SSSC, SMES and
TCPS are shown in Fig. 2b (i), (ii) and (iii). The parameters of the LFC DFIG
system with different FACTS and SMES devices is given in the appendix.
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Fig. 3a MATLAB diagram of FACTS and SMES representation Fig. 3b
MATLAB diagram of coordinated FACTS- SMES
The Area 1 with 1800MW is in the top and the Area 2 with 1200MW is represented
in the bottom of the above figures with conventional and DFIG based wind farms.
The DFIG wind turbine is controlled with pitch angle control and generator is
having primary, secondary and tertiary frequency regulators as in Apart from
these, the DFIG system is provided with FACTS devices alone as shown in Fig. 3a
and with coordination as FACTS- FACTS or FACTS-SMES is shown in Fig. 3b.
The performance of the DFIG system is tested without FACTS, but with basic
protection and with FACTS and coordinated devices. The damping and settling
behaviour is tested for frequency deviation and power regaining capability which is
discussed in the next section.
6. Simulation results The simulation results for the test system with FACTS alone is shown in Fig. 3a
and with coordinated FACTS and SMES is in Fig. 3b. A 10% change in load
command is given in area 1, frequency deviation at 5s is observed in both areas 1
and 2 with FACTS or SMES alone and with coordination is shown. The blue color
line is with DFIG basic protection without FACTS. It is observed that, the
frequency deviation reaches -0.26 Hz and slowly settles after 30s in area 1, reaches
-0.22Hz and settles nearly in the same time. With SSSC, frequency deviation in
area 1 reached -0.15Hz and settles at 35s and reached -0.21Hz at area 2 and settled
at 35s. With SMES shown with red has a deviation of -0.05 Hz and settles in 15s in
area 1 and with deviation of -0.1Hz and settled within 10s in arae 2. The deviation
with TCPS is nearly flat without much deviation and is having ideal behaviour in
both the areas 1 and 2. Hence, among all the FACTS and SMES devices, TCPS is
abetter device for DFIG based system.
Now considering the same system with coordinated FACTS-FACTS or FACTS-
SMES devices in area 1 and area 2 with same disturbance in area 1 at 5s. The
coordinated FACTS-FACTS or SMES refers to application of two or more in same
area in coordination to damp oscillations. It is observed that without coordinated
FACTS, the frequency deviation is same without FACTS devices. With coordinated
FACTS, it is observed that, the frequency deviation in both areas 1 and 2 are
nearly constant at zero value with 10% increase in the load. With SMES-SMES,
the deviation is high as with pink color markings, with SMES - TCPS coordination
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as wit red color data, the deviation is better than SMES-SMES. with SMES –
TECPS (with green color marking), the deviation is very small and system is
completely stable and is better than other twio cases or with TCPS alone.
Fig 4a(i) with FACTS device, change in frequency in area 1, Fig. 4a (ii) area 2
Fig 4b(i) coordinated FACTS-SMES, change in frequency in area 1, Fig. 4b (ii) area 2
7. Conclusions The load frequency control (LFC) with DFIG based wind turbine system with
FCATS and SMES is studied in this paper. The LFC and area control error (ACE)
behavior of DFIG system is performing in the similar way as with a conventional
system. The paper studied the behavior of DFIG frequency regulation system with
load deviation in area 1 of the two areas using different FACTS devices like SSSC
and TCPS and energy storage device like SMES in one case and with coordinated
FACTS and SMES in other case. With basic primary, secondary and tertiary
frequency control mechanism, the DFIG frequency reached to normal value after
few oscillations. With SSSC, frequency settling is better than with FACTS. But
TCPS is better than SMES and is better than SSSC in controlling real power and
frequency deviation. With coordinated FACTS- SMES, SMES and TCPS behavior
is best, SMES-SSSC is better and SMES-SMES is good than without FACTS or
SMES. Hence, along with basic frequency regulation, application of full rated
TCPS or with half rated each with coordinated SMES-TCPS is a better option for
LFC.
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Appendix He1=3.5; He2=3.5; Kagc1=0.05; Kagc2=0.05; Ta1=0.2; Ta2=0.2; Kp1=12; Kp2=12;
Kwi1=0.1;Kwp1=1.58; Kwi2=0.1; Kwp2=1.61; R1=3; R2=3; Th1=0.1; Th2=0.1;
Tr1=0.1; Tr2=0.1; Tw1=6; Tw2=6; Tp1=10; Tp2=15; Tt1=1; Tt2=1; Wmax=1.4;
Wmin=0.0; T0=0.07; B1=1.1; Pmax=3; Pmin=0; K=20.1378; T1=1.5025; T2=0.5386;
T3=0.06268; T4=0.05075; Tsm=10.50; Tsm=10.50; K1=20.2188; T11=0.587;
T21=0.158; T31=0.0575; T41=0.2316; Tsm1=0.2151; Tsm1=10.2151; Kfi=1.5;
Tps=0.1; T12=0.866;
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