1. Performance Analysis of Wind - Fuel Cell
Hybrid System linked with BESS-VF Controller
Authored By
Narayan P. Gupta, Sushma Gupta, Anil Kumar
Presented By
Narayan P. Gupta
Department of Electrical Engineering
Oriental Institute of Science & Technology, Bhopal, India
2. Contents
Introduction
Wind Turbine Systems
Vector Control Scheme of GSC and RSC
Solid Oxide Fuel Cell Energy Conversion System
Modeling of D-STATCOM
Simulation Results
Conclusion
Appendix
References
2
3. Introduction
Wind energy is most promising renewable energy source & its share is increasing w.r.t.
to installed capacity, worldwide. India has fifth largest installed capacity in the world.
Due to the intermittent characteristics of wind resources, it has been a challenge to
generate a highly reliable power with wind turbines.
To overcome this limitation, a solid oxide fuel cell (SOFC) is used in conjunction with
wind generating plant.
A multi-source hybrid power system increases energy availability significantly, it
becomes advantageous for practical applications that need highly reliable power
regardless of location
The DFIG based variable speed wind turbine (type C) is the most popular in the
growing wind market.
This system uses two back-to-back PWM voltage source converters in the rotor circuit
designed as grid side converter (GSC) and rotor side converter (RSC).
The vector control strategy is used for both converters
3
4. Contd…
These high frequency switching converters of DFIG will inject additional harmonics in
the system, which are increasing with wind speed variations.
Load unbalancing, Poor power factor, Harmonics, Voltage drop at PCC, Reactive
Power are some of important power quality issues related with nonlinear load.
D-STATCOM is used to mitigate the voltage dip, compensate the reactive power,
flicker, harmonic mitigations, load balancing and neutral current compensation.
4
5. Power Quality
Electric Power Quality basically refers to maintaining a near sinusoidal power distribution bus
voltage at rated magnitude and frequency.
Variation in voltage, current or frequency is generally termed as PQ problems
Good power quality means
Constant voltage level
Low harmonic distortion
Less transient events
Good power factor
POWER QUALITY
FLICKER
SYSTEM
TRANSIENTS
ELECTROMAGNATCINTE
RFERANCE
SYSTEM
HARMONICS
VOLTAGE SAG
& SWELL
NOISE
LOW
POWER FACTOR
6. Sources Of Harmonics
Contd…
Modern (Power-Electronic)
Types
Traditional (Classical) Types
Transformers
Rotating
Machines
Arc
Furnaces
Rectifiers
Contributors
Reactive power
Harmonic pollution
Load imbalance
Fast voltage variations
Controlled
Devices
Inverters
SMPS
Cycloconverter
Fluorescent
Lamps
HVDC
Transmission
Consequences
Unexpected power supply failures
Equipment overheating and failure
Electro magnetic interferences and noise in power system
Increase of system losses
oversize installations to cope with additional electrical stress
Malfunction of protective relaying and voltage sensitive devices
Mains voltage flickering
6
7. Simulink Test System
A_SOFC
A
a
B_SOFC
B
b
Discrete,
T s = 5e-006 s.
Wind
C
C_SOFC
SOFC
c
Wind Speed m/s
Breaker
Wind Turbine
Tm
m
A_grid
A
a
aA
A
a
B_grid
B
b
bB
B
b
C_grid
C
c
cC
C
[output]
c
+
15 KW DFIG
PCC
A
B
C
Transformer
33KV/440V
Grid
A
B
C
A
B
C
-
Pulse_RSC
Pulse_GSC
Controller
g
g
Pulses
Non Linear Load
C
H
O
K
E
+
+
DST AT COM Controller
A
A
B
-
+
B
Grid Side
Converter
C
B
b
C
c
C
C
A
a
Cdc
B
g
A
Rotor Side
Converter
Transformer
BESS DSTATCOM
Fig. Simulink configuration of test system
7
8. Vector control Scheme of Grid Side Converter
Grid Side Converter is used to maintain the dc-link voltage constant
The voltage oriented vector control technique is used to control the GSC
The PWM converter is current regulated with the direct axis current is used to regulate the DC link
voltage where as the quadrature axis current component is used to regulate the reactive power
The reactive power demand is set to zero to ensure the unit power factor operation
The voltage balance across the line is given by
Fig. – Schematic diagram of GSC
Using the abc-to-dq transformation, the corresponding equation in the dq-reference frame rotating at ωe is
8
9. Contd…
The control scheme utilizes current control loops for id and iq
The id demand being derived from the dc-link voltage error, through a standard PI controller
The iq demand determines the reactive power flow between the grid and grid side converter
The iq demand is set to zero to ensure unit power factor operation.
Vd
D- axis grid voltage
Vq
Q- axis grid voltage
Vd1
D- axis grid side converter voltage
Vq1
Q- axis grid side converter voltage
id
D- axis grid side converter current
iq
Q- axis grid side converter current
Fig. – Vector Control Scheme of GSC
9
10. Vector control Scheme of Rotor Side Converter
The main purpose of the machine side converter is to maintain the rotor speed constant irrespective
of the wind speed
Vector control strategy has been implemented to control the active power and reactive power flow of
the machine using the rotor current components
The active power flow is controlled through idr
The reactive power flow is controlled through iqr
To ensure unit power factor operation like grid side converter the reactive power demand is also set
to zero (Qref = 0)
The currents iq and id can be controlled using Vq and Vd respectively
The control scheme utilizes cascade control
The inner current control loops are used for controlling the d-axis and q-axis rotor currents
The outer power control loops are used to control the active and reactive power on the stator
10
11. Contd…
The d-q axis rotor voltage equation is given by,
Vdr* and Vqr* equation is given by,
Where V'dr and V'qr are found from the current errors processing through standard PI controllers
The q-axis reference current i*qr is found from the reactive power errors
The reference current i*dr can be found either from the reference torque or form the speed errors (for the
purpose of speed control) through standard PI controllers
Where Te*=(Pmech-Ploss)/ωr
Ploss = Mechanical Losses + Electrical Losses
11
13. Fuel Cell Energy Conversion System
Fuel cell converts chemical energy of a reaction into electricity with byproduct of water and heat .
Fuel cell consists of an electrolyte layer in contact with two electrodes on either side. Hydrogen fuel is fed to
anode and oxygen from air is fed to cathode.
At anode Hydrogen is decomposed into positive and negative ions
Only positive ions flow from anode to cathode .
Recombination of positive and negative ions with oxidant takes place at cathode to form depleted oxidant (or
pure water).
Anode Reaction:
Cathode Reaction:
Overall Reaction:
The dc Voltage across FC stack is given by nernest’s equation
Where
Vfc – Operating dc voltage (V),E0 – Standard reversible cell
potential (V), pi – Partial pressure of species i (Pa), r – Internal
resistance of stack (S), I – Stack current (A), N0 – Number of
cells in stack, R – Universal gas constant (J/ mol K), T – Stack
temperature (K), F – Faraday’s constant (C/mol)
13
14. Contd…
Proton exchange membrane fuel Cell (PEMFC)
Alkaline Fuel cell (AFC)
Phosphoric acid fuel cell (PAFC)
Molten Carbonate fuel cell (MCDC)
Solid oxide fuel cell (SOFC)
Direct Methanol fuel cell (DMFC)
High-temperature operation of SOFC-1800oF removes the need for a precious-metal catalyst, thereby
reducing the cost.
It also allows SOFCs to reform fuels internally, which enables the use of a variety of fuels (the input to
the anode can be hydrogen, carbon monoxide or methane)and reduces the cost associated with adding a
reformer to the system.
The electrolyte used is a ceramic oxide which increases the cost of SOFCs.
At the cathode, electrochemical reduction takes place to obtain oxide ions. These ions pass through
the electrolyte layer to the anode where hydrogen is oxidized to obtain water.
In case of carbon monoxide, it is oxidized to carbon dioxide.
14
15. qH2
Pref
current
equation
Vfc
I
Flow Rate
equation
Partial Pressure
Equations
qO2
pH2O
pO2
pH2
r
Voltage Equations
N
Fig. Block diagram for dynamic model of SOFC
Where qH2 – Fuel flow (mol/s), qO2 – Oxygen flow (mol/s),
KH2 – Valve molar constant for hydrogen (kmol/s atm),
KO2 – Valve molar constant for oxygen (kmol/s atm),
KH2O – Valve molar constant for water (kmol/s atm), τH2
– Response time for hydrogen (s), τO2 – Response time
for oxygen (s), τH2O – Response time for water (s), τe –
Electrical response time (s), τf– Fuel response time (s),
Uopt – Optimum fuel utilization, rHO – Ratio of hydrogen to
oxygen, Kr – Constant (kmol/s A), Pref – Reference power
(kW)
15
16. Power conditioning for fuel cell connected to Grid
DC/DC
Converter
DC/AC
Inverter
Coupling
inductor &
Transmission
line
AC
BUS
DC BUS
DC
DC
Fuel Cell Power Plant
LC Filter
DC
Utility Grid
AC
Control
Signal
Vabc
Control
Signal
Iabc
Pulses
VDCref
PQ
DC-DC Controller
dq/abc
PI
- +
Idref
PI
-
Id
Iqref
+
Qref
-
Pref
Q
+
P
-+
VDC
PI
PI
Idref
Iqref
Iq
abc/dq
angle
PLL
Iabc
Fig. Power conditioning for fuel cell connected to Grid
16
17. Performance of SOFC
Fuel Cell Voltage
450
The number of400
cells connected in series is taken as 450. Initially for a 50 kW of load the calculated SOFC
voltage is 403 volt.
The output voltage of the DC/DC converter is maintained almost constant at 700V throughout the loading
350
conditions, by using a PI controller along with the DC/DC converter.
300
Fuel Cell Voltage
450
250
400
200
350
output voltage of DC-DC converter
150
1000
300
900
100
800
250
50
700
600
Voltage (V)
2000
500
150
400
0
0.1
0.2
0.3
0.4
0.5
Time
0.6
0.7
0.8
0.9
1
Fig. Output Voltage waveform of SOFC and DC-DC Converter waveform
17
300
18. Control Scheme of D-STATCOM
Va
Vrated
Unit Voltage
Template Generator
Uc
Ua Ub
Vt
Vtref
_
In phase
component
Refrence current
*
*
*
*
Iscd
Isad
Quadrature Voltage
Templete Generator
Wa
Wb
Wc
+
AC Voltage
PI
controller
Quadrature
Component Refrence
current
*
Ismq
-
*
Isaq
*
Isbq
+
Vabc
Ig
*
Iamd
+
Frequency
PI controller
+
*
Iscq
Isa
Isb
Prated
Compensation of
amplitude of active
power component
of current
*
Ismd
Isbd
D-STATCOM is used to mitigate
the voltage dip & compensate the
reactive power.
It is also capable of flicker &
harmonic mitigation’s, load balancing
and neutral current compensation.
STATCOM consists of DC voltage
_
source behind self commutated
inverters using IGBT & coupling
transformer. It is connected in shunt
with distribution feeder.
It generates a current injection,
which is added to non sinusoidal load
current. Thus phase current taken
from grid will be nearly sinusoidal
With voltage/frequency controller
output voltage and frequency can be
kept constant
Vc
Vb
+
0
* *
*
Isa Isb Isc
Frequency
measurement
-
Fref
*
Isn
Hysteresis Current controller
Isc
Battery
S1
S3
S5
S7
Cbattery
R1
Ila
Ilb
Ilc
Iln
Lf,Rf
Lf,Rf
Vdc
Cdc
Lf,Rf
Lf,Rf
S4
S6
S8
+
-
Ro
S2
Voc
+
18
19. Contd…
Prated = 15KW Vrated = 415 volt
The amplitude of active component of current is i*smd = igen(n) - ismd(n)
In phase component of reference source current are estimated as:
i*sad = i*smd Ua, i*sbd = i*smd Ub , i*scd = i*smd Uc
Where Ua , Ub and Uc are in-phase unit current vectors , given by
Ua = Va / Vt , Ub = Vb / Vt , Uc = Vc / Vt
Where Vt is the amplitude of supply voltage
Vt = { 2/3(Va2 + Vb2 + Vc2)}1/2
instantaneous quadrature component of reference source current is estimated as
i*saq = i*smqwa, i*sbq = i*smqwb , i*scq = i*smqwc
The unity amplitude template having instantaneous value in quadrature with instantaneous
voltage Va, Vb, Vc are derived as
Wa = -Ua /√3 + Uc /√3
Wb= √3Ua/2 + ( Ub- Uc )/2√3
Wc= -√3 Ua/2+ (Ub-Uc)/2√3
19
20. Contd…
The total reference source current is the sum of in-phase & quadrature component of reference
source current.
i*sa = i*saq + i*sad
i*sb = i*sbq + i*sbd
i*sc = i*scq + i*scd
The reference source current (i*sa,i*sb,i*sc) are compared with measured source current .The error of
current are computed as:
isa_error = isa - i*sa
isb_error = isb - i*sb
isc_error = isc - i*sc
These error signals are the drive of hysteresis current controller to generate six firing pulses for VSC
Design of BESS Controller
BESS consist of a CC-VSC with the battery at DC link. The terminal voltage of battery is given
by Vbattery = (2√2/√3) VL; where VL is line rms
The equivalent capacitance can be determined from
Cbattery = (kwh * 3600 * 103) / 0.5 (V2ocmax – V2ocmin)
The parallel circuit of R1 & Cbattery is used to describe the energy & voltage during charging &
discharging
20
21. -1
1.3
1.4
1.5
1.6
1.7
1.8
Simulation for Unbalanced Reactive Load
supply current
Iabc
20
0
Three single phase reactive load are applied between each phase and neutral at t=1.0 sec. At t=1.4sec
one phase is -20
removed and another at t=1.5 sec making load unbalanced
load current
50
1.3
1.4
1.5
1.6
1.7
1.8
capacitor current
Ilabc
20
Icap
10
0
0
-10
-50
1.3
-20
1.3
1.4
1.4
1.5
1.5
40
1.6
1.7
1.8
1.6
controller current
Time
1.7
1.8
load neutral current
1.6
1.7
1.8
Icabc
20
0
-20
-40
1.3
1.4
1.5
load neutral current
20
20
source neutral current
Iln
Isn Iln
20
0
10
0
0
-20
-20
1.3
-10
1.3
-20
Icn
Icn
50
50 1.3
1.4
1.4
1.5
1.5
1.6
1.6
1.7
1.7
1.8
1.8
compensator neutral current
compensator neutral
1.4
1.5
1.6
Time
1.7
1.8
0
0
-50
-50
1.3
1.3
1.4
1.4
1.5
1.5
1.6
1.6
1.7
1.7
1.8
1.8
21
22. Simulation for Unbalanced Non Linear Load
A diode rectifier with resistive load and L-C filter at its DC side is considered. At t=1.0sec balanced non
linear load is inserted at t=1.4 sec one phase is removed and another at t=1.5 sec making load
unbalanced.
It seems that controller current becomes nonlinear for eliminating the harmonic current.
supply voltage
Vabc(pu)
1
0
-1
1.3
1.4
1.5
1.6
1.7
1.8
1.7
1.8
1.7
1.8
supply current
Iabc(A)
20
0
-20
1.3
1.4
1.5
1.6
capacitor current
20
Icap(A)
10
0
-10
-20
1.3
1.4
1.5
1.6
Time
22
23. Is
0
-20
0
-50
1.3
1000
Contd…
0.4
1.4
0.6
0.8
00
dc link1.6
voltage
1.5
600
0
1000
4001.3
1.3
1000
1.4
1.4
Vdc_V
1.6
Vdc_V
1.6
frequency
Vdc_V
frequency
1.5
1.5
1.5
1.5
1.6
1.6
1.7
1.7
1.8
1.8
500
1000
60
20
20
40 500
40
500 0
20 1.3
0
20
1.3
0
0
0 1.3
01.3 5
1.3 5
Icabc
Icabc
controller current
controller current
0
1.4
1.5
1.4
1.4
1.5
1.5
1.4
1.4
1.5
1.5
50
1.4
1.4
load neutral current
1.6
1.5
1.5
1.6
1.7
1.7
1.8
1.7
1.8
1.7
1.7
1.8
1.8
1.7
2
0
1.6
1.8
1.6
Te
1.6 Te
1.6
1.6
Te
Time
Time
1.7
1.7
1.7
1.7
1.8
1.8
1.8
1.8
0
-5-5
-5 1.3
1.3
1.3
1.4
1.4
1.4
1.5
1.5
1.5
1.6
1.6
1.6
1.7
1.7
1.8
1.8
1.7
1.8
P(KW)
P(KW)
P(KW)
20 2020
0
0
0
0
P(KW)
P(KW)
P(KW)
Iln
10
10
10 10
1.3
1.3
1.3
5050
10
1.4
1.4
1.4
1.4
1.5
1.5
1.5
1.5
1.6
1.6
1.7
1.7
1.6
compensator neutral current 1.7
1.6
compensator neutral current 1.7
Time
Time
1.8
1.8
1.8
1.8
0
1.3
5
1.5
1.5
1.6
1.6
1.7
1.7
1.8
1.8
1.5 1.5
1.6 1.6
1.71.7
1.81.8
1.4
1.5
1.6
Q(KVAr)
Q(KVAr)
1.7
1.8
Q(KVAr)
5
0
0
-5
1.3 -5
1.4
1.4
1.4 1.4
5
0
00
-50
-50
1.3
1.3
0 0
1.3 1.3
Q(KVAr)
Q(KVAr)
Q(KVAr)
-10 -50
-50
-10 1.3
Isn
Isn
1.6
0
1.8
1.8
load neutral current
Source neutral current
Source neutral current
50
Isn
Isn
Iln
Te
-20
-20
Te
Te
5
1.3
1.3
1.4
DC link voltage
60
1.4
1.4
1.2
500
800
Vdc V
f f
Vdc V
Vdc V
Vdc
Vdc
Load current
Load current
-50
-50
1.3
1.3
1
1000
50
50
Ilabc
Ilabc
0.2
1.3
1.4
1.5
1.4
1.5
1.6
Time1.6
1.7
1.8
1.7
1.8
23
26. SOFC Power (KW)
Power allocation between Grid, SOFC and Wind - DFIG
SOFC Power (KW)
80
60
40
20
0
Load Power (KW)
Load Power (KW)
100
50
0
DFIG Power (KW)
DFIG Power (KW)
100
50
0
-50
-100
NLL Power (KW)
NLL Power (KW)
100
50
0
-50
Grid Power (KW)
Grid Power (KW)
200
100
0
-100
-200
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
27. Contd…
Percentage THD of Supply voltage and Supply current under balanced / unbalanced linear,
nonlinear load
Sr no
Type of Load
% Total Harmonic
Distortion
1
2
Balanced Resistive load
Va
0.33
Unbalanced Resistive load
0.33
3.23
3
Balanced reactive load
0.33
3.32
0.36
3.35
4
Unbalanced Reactive load
ia
3.23
5
Balanced non linear load
0.34
3.53
6
7
Unbalanced nonlinear load
0.37
5.45
3.43
49.74
Balanced nonlinear load without
STATCOM
27
28. Conclusions
Being the wind energy is intermittent in nature; hybrid system constituting fuel cell; operated in
synchronism with wind generator and grid will be the feasible solution.
The interfacing of hybrid system with grid should comply the grid code requirements and power quality
standards
Vector control scheme is very useful for controlling the grid side converter and rotor side converter; the
system can work for any change in wind speed.
The dc link voltage of converters, rotor speed, and active power and reactive power exchange between
machine and grid is almost constant during all operations.
The DFIG machine is perfectly synchronised with the SOFC -Grid and any sudden change in the load
demand is met by sharing of power as per their rating.
With non linear load at PCC, the THD of supply current becomes more than IEEE-519-1992 limit (i.e.
THD > 5%), hence D-STATCOM is used to restrict the total harmonic distortion caused by load.
Indirect current control scheme using hysteresis controller is useful for controlling of D-STATCOM .
D-STATCOM improves the THD spectrum of source current and voltages; even in the case of any
sudden change in nonlinear load.
D-STATCOM can be used as a solution to compensate reactive power, neutral current compensation,
load balancing and harmonic elimination.
Power Factor correction has been done with help of D-STATCOM
28
29. Appendix
Parameters of 15 KW, 440 V, 50Hz, DFIG
Rs = 0.023 pu, Rr = 0.016 pu, Ls = 0.018 pu, Lr = 0.16 pu,
P = 6, J = 0.385 pu
Parameters of GSC and RSC
Vdc = 830 volt, Cdc = 9200 µF, Kp and Ki (GSC) = 0.83 and 5, Kp and Ki (RSC) =
0.6 and 8, Frequency of the grid-side and rotor-side PWM carrier
= 2250 HZ and 1350 Hz
SOFC Parameter
T = 1273K, Eo = 1.18 Volt, N= 450, Kr = .996*10-6Kmol/(s atm), Uopt = 0.85, KH2 =
8.43*10-4 Kmol/(s atm), KO2 = 2.81*10-4 Kmol/(s atm), KH2O = 2.52*10-3 Kmol/(s
atm), ),
=26.1s,
= 78.3 s,
= 2.91 s, R = 0.16Ω, rHO = 1.145
29
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2.
3.
4.
5.
6.
7.
8.
9.
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30