4th Power Electronics,Drive Systems & Technologies Conference (PEDSTC2013),Feb 13-14,2013,Tehran,Iran
A Control Method for Integrating Hybrid Power Source into an Islanded Microgrid through CHB Multilevel Inverter A. Ghazanfari, M. Hamzeh, H. Mokhtari The Center of Excellence in Power System Management & Control Sharif University of Technology, Tehran, Iran
[email protected],
[email protected],
[email protected]
Abstract-This paper proposes a control strategy for an is
higher voltage levels with negligible distortion, cascaded H
landed microgrid to effectively coordinate hybrid power source
bridge (CHB) multilevel inverters are preferred for high power
(HPS) units and to robustly control individual interfaced invert ers under unbalanced and nonlinear load conditions. Cascaded H-bridge (CHB) multilevel inverters are flexibly deployed in
applications among other topologies [4]. A hybrid power source (HPS) offers superb scalability and
order to enhance the power quality and redundancy. The HPS
more flexibility for power management capability. Due to
employs fuel cell (FC) as the main and supercapacitors (SC)
the clean and environment-friendly specifications, fuel cells
as complementary power sources. Fast transient response; high performance; and high power density are the main characteristics of the proposed HPS system. The presented control strategy consists of a power management strategy for the HPS units and
(FC) are considered as the main power source in this paper. The slow dynamic response of the FC stack and quick load changes demand the usage of supercapacitor (SC) as a storage
a voltage control strategy for the CHB multilevel inverter. A
system with high power density [5]. Therefore, parallel hybrid
multi proportional resonant (multi-PR) controller is employed to
FC/SC power source and CHB multilevel inverter are used for
regulate the load voltage at unbalanced and nonlinear load condi
each DG unit to assure enhancement of service redundancy,
tions. The proposed multi-PR controller includes a fundamental voltage controller with harmonic compensators. Digital time domain simulation studies in the PSCADIEMTDC environment
modularity and power quality in a microgrid, particularly in the presence of non-linear and unbalanced loads. In this paper, a control strategy for voltage control of a
are given to verify the overall proposed system performance.
Index Terms-Hybrid power source, fuel cell, supercapacitor, CHB multilevel inverter, multi-PRo
CHB multilevel inverter is proposed to enhance the dynamic response and power quality of the microgrid in the presence of unbalanced and non-linear loads. Based on the decentralized control strategy, each DG subsystem individually controls
I. INTRODUCTION
its own output and the overall microgrid system. A multi Microgrids aim to provide a solution to reform the current
proportional resonant controller (multi-PR) is proposed for
power system toward a new concept for future energy dis
regulating the load voltage. The use of a multi-PR controller in
tribution systems. It plays a key role for renewable energy
nonlinear load conditions is more advantageous as compared
integration and energy management capability improvement.
to conventional PR controllers [6].
The increasing interest in microgrids demands more reliable components and advanced control strategies [1].
The presented multi-PR controller effectively compensates the harmonic and negative-sequence currents of nonlinear and
A microgrid may inherently be subjected to significant
unbalanced loads to achieve sinusoidal balanced voltages at
degrees of unbalanced conditions due to the presence of single
all buses of the microgrid. In addition, the proposed HPS
phase loads and/or DG units. Moreover, abundance of non
configuration and the power management scheme can actively
linear loads in distribution networks can cause problems in
distribute the power demand among the main and auxiliary
the voltage control of an islanded microgrid. Nevertheless, a
power sources of the DG unit even in unbalanced load
microgrid should be able to operate under unbalanced and non
switchings. The effectiveness of the proposed control strategy
linear load conditions without any performance degradations.
is demonstrated through simulation studies conducted in the
Based on the IEEE standards [2], [3], the voltage unbalance
PSCAD/EMTDC environment.
factor (VUF) and the voltage total harmonic distortion (THD)
II. DESCRIPTION OF THE DG UNITS COMPONENTS
should be maintained within 2% and 5%, respectively in a distribution network.
The structure of the hybrid FC/SC power source, which is
Microgrids usually consist of multiple DG units integrated
connected to each CHB multilevel inverter cell, is presented
via power electronic inverters to form an enhanced voltage
in Fig. 1. The presented HPS consists of proton exchange
source. In order to improve power quality and redundancy,
membrane FC stacks which provide the main power, and
introducing multilevel inverters into the microgrid catches
the SC modules to supply power transient demands. The
growing interest. Due to modularity and ability to operate at
SC modules have to be incorporated to guarantee the power
978-1-4673-4484-5/13/$31.00 ©2013 IEEE
495
Hybrid
-
-
FC/SC - -
-
-
- -
-
-
-
-
-
-
-
- -
-
-
-
-
-
(a)
-
I 1
r--I I I I I I I I I I
Fig. I.
Proposed structure of the hybrid FC/SC power source.
.......----- Ua ----
-----------------
�
�
ffa
L,.
ffb
Lfb
f,c
L,c
•
•
�
•
(b)
iDa
i'a
if!>
iOb
quality and reliability of the HPS and proper functionality of the corresponding microsource.
+
the main and auxiliary sources in each HPS are connected
V""
to the dc-link of a H-bridge cell through a unidirectional
-
output voltage to the desired dc-link value and smooth the
+
+
T' JC y Cfa
Vb"
VO" C,.
-
-
C'c
n
and bidirectional full-bridge dc/dc converter, respectively. In the FC and SC units, full-bridge converters adapt the units
iOe
ire
In order to achieve high efficiency and galvanic isolation,
Fig. 2. (a) One-phase structure of a CHB multilevel inverter. (b) Circuit diagram of a three-phase, three-wire DG unit.
output current. Moreover, the power flow directions of the SC module is controlled allowing for bidirectional power flow and flexibility of the power management. One-phase structure of a CHB multilevel inverter is shown in Fig. 2(a), where n H-bridge inverter units are connected in series to provide an output voltage waveform of 2n + 1 steps.
The inverter phase output voltage is the sum of the output voltages of the cells. Fig. 2(b) shows the circuit diagram of a
three-wire DG subsystem whose detailed model is described
in [5].
III. OPERATION PRINCIPLES OF THE PROPOSED CONTROL STRATEGY
The proposed control strategy comprises power manage ment of each HPS system and voltage control of the CHB multilevel inverter.
Fig. 3.
Proposed control strategy of hybrid FC/SC power source.
A. Control Strategy of the HPS
The proposed control strategy for the hybrid FC/SC power source is depicted in Fig. 3. The HPS employs the FC as the
main and the SC as the complementary power source. The SC
modules support the FC to meet the transients and the grid
converter determine the parameters of controllers by choosing the appropriate bandwidth and phase margin.
power demand. Having two separate full-bridge converters in
In the FC reference generator block, the dc-link power
parallel simplifies power management capability and increases
current is shared equally among the HPS units and is divided
the overall performance and flexibility of the hybrid power
by the FC voltage of each HPS unit to generate the FC current
source. The HPS is designed such that the SC converter control
reference. The FC reference generator block determines the
system regulates the dc-link voltage, and the FC converter
reference current of the FC which is limited to the maximum
control system provides the dc-link power demand.
and minimum currents that the FC can generate. Furthermore,
The unidirectional power flow of the FC converter leads to
its slope is limited to avoid the fuel starvation phenomena and
decoupled dynamics of the FC and SC converters. Therefore,
guarantee the safe operation of the FC stack. A PI controller
controller design for each converter can be done separately.
determines the duty cycle of the FC converter. For control
The dc-link voltage and FC current controllers are the key
design purposes, dc/dc converters are modeled by state-space
components in this control strategy. The voltage-loop response
averaging technique. The FC control-to-input current transfer
of the SC converter and the current-loop response of the FC
function which is derived from the average model of the full-
496
TABLE I PARAMETERS OF CONTROLLERS.
Controller
Parameters Value
FC Converter
kp-0.5 , kI-O.Ol
SC Converter
kp-2 , kI-0.002
Current controller
kc=0.9
Lead compensator (C(s)
kFl , k2=67 , k3=11700
Gl(S)
k4-5.8 , k5-5170 , k6-5.4e6
G5(S)
k7-7.9 , ks-2860 , kg-4.5e6
G7(S)
klO=8.3 , kll=3420 , k12=le7
Fig. 4.
bridge converter is defined as:
d
1
+
RC 2h (1 S) (1- D) + 2 2 2 nFc L S nFc LCS2 2 R(l D) + (1 D)2 _
(1)
_
To balance out the current ripple rejection of the down stream inverter and slow dynamics of the FC stack, the parameters of the controller are modified to set the current loop bandwidth at least half decade higher than 628.3 rad/s [7]. Therefore, the current-loop bandwidth of the FC converter is set at 3374 rad/s to obtain a phase margin of 82°. For the dc-link voltage control, the SC voltage control loop uses the dc-link voltage measurement as a state variable to perfectly follow its reference, i.e., Vdc-linkref. To regulate the dc-link voltage, a PI controller determines the duty cycle of the SC converter. It means that if the SC is charging or discharging, the duty cycle of the SC converter will decrease or increase respectively to maintain the dc-link voltage regula tion. The SC control-to-output voltage transfer function which is derived from the average model of the full-bridge converter can be written as:
d
1
+
2 2 nsc L S nsc LCS2 R(l - D)2 + (1 - D)2
(2)
The frequency response of the compensated system indi cates that the voltage-loop bandwidth for the SC converter is 113.7 rad/s. The parameters of the designed controllers are listed in Table I. B.
Control Strategy of the Inverter
To obtain a dynamic model for the DG units, the circuit diagram representation for each CHB-based DG subsystem is given in Fig. 2 (b). Using KVL and KCL, the state space equations of the DG subsystem of Fig. 2 in the abc frame are simply derived. Since, there is no zero-sequence current, the abc-frame equations can be transferred to the stationary reference (0(3) frame using Clark's transformation. Since the matrix transfer function of the DG subsystem in the o(3-frame is diagonal, two identical SISO controllers can be independently designed for the quadrature axes 0 and (3 [8]. Fig. 4 shows the block diagram of the proposed voltage con troller in o(3-frame. To increase the overall performance and
Block diagram of the proposed multi-PR controller.
internal stability of the voltage control loop, an inner current loop is incorporated. The outputs of the voltage controllers are considered as the reference signals for the current controllers. These signals are compared with the currents of the series filter and are added to the feedforward signals ioa and io{3. The resultant signals are then applied to the current controllers to generate the control signals Ua and U{3. Finally, the control signals in o(3-frame are transformed to abc-frame and then applied to the modulation unit. In the modulation unit, phase-shifted PWM is applied as the modulation strategy which provides an even power distribution among the units. Moreover, it naturally balances the dc capacitor-voltages and mitigates the CHB multilevel inverter input current harmonics [9]. The current controller is a simple gain, kc, whose value is calculated such that the damping factor of the dominant poles of the inner loop system becomes 0.7. To eliminate the impact of load dynamics, the output current is added as a feedforward term to the output of the voltage control loop. According to the internal model principle, a reference (dis turbance) can be asymptotically tracked (rejected) if and only if the controller contains the Laplace transform of the reference signal in its transfer function. Therefore, the zero steady state tracking error for a sinusoidal reference signal with frequency w is achieved if and only if the controller contains the transfer function
2
1
� ��
2' The output currents (Ioa and 10{3), which
can be c n d red as disturbances in the control system, contains fundamental and higher order harmonics in nonlinear load conditions. It should be noted that the loads are connected to the microgrid via Y/ � transformers, and therefore, in the presence of unbalanced and nonlinear loads, not zero-sequence nor third-order harmonic currents exist at the DG side of the microgrid. To achieve zero steady state error of the closed loop system in the presence of harmonic currents, a multi-PR controller is proposed as:
where C, G1, G5, and G7 are formulated as:
497
C(8)
=
k1
8 k2 + -' 8 k3
-
+
(4)
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lD:� 'tJ ,;, ta
:;
20 0 -20 -40
MV Microgrid
(a)
80�
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6 Y
1
LV Distribution t rk
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(b)
Fig. 6.
2...
1 1 I
______
Single-line diagram of the microgrid. TABLE II
PARAMETERS OF THE MICROGRID'S SUBSYSTEMS. Parameter
Frequency(Hz)
Fig.
5.
Output impedance of the open-loop
DG
PFC
unit.
It is worth mentioning that the orders of the harmonic current for compensation are determined according to the bandwidth of the voltage control system. In this case, harmonic compensators are considered for the fifth- and seventh-order based on the required bandwidth of the voltage control system. In (4), ki(i 1, ..., 12) coefficients are the unknown param eters of the multi-PR controller that should be determined. To obtain these coefficients, the following requirements are to be met: 1) the system phase margin should be more than 30°; 2) the system gain margin should be more than 3; 3) the bandwidth of the open-loop system should be less than 10 percent of the switching frequency; 4) the disturbance (harmonic currents) should be rejected. Considering the aforementioned performance indices and using MATL AB SISO tools, the coefficients of the designed controller can be found. These parameters are listed in Table L The frequency response of the open-loop controlled system considering the inverter model and the designed controller is shown in Fig. 5. It is clear that the desired phase margin, gain margin, and bandwidth of the system are achieved. The phase margin is almost 30 deg, gain margin is infinite, and the bandwidth of the system is close to 400 Hz. In addition, the gain of the system at 50 Hz and other harmonic frequencies is high enough to significantly reject disturbances. RESULTS
Fig. 6 shows a single-line diagram of a microgrid which is implemented in the PSCADIEM TDC environment to in vestigate the effectiveness of the proposed control strategy. The testbed is composed of a one-feeder distribution system and a DG unit which is connected to the feeder by CHB multilevel inverters to supply a combination of unbalanced and nonlinear loads. The load is connected to the feeder via Y/ b. transformer. It is assumed that the microgrid system operates in an islanded mode. The CHB multilevel inverter is controlled in a decentralized control manner and equipped with the proposed multi-PR, presented HPS power management.
Rated power
fsw
Switching frequency
nFC nSC LFc Lsc Csc CFC
FC converter winding ratio
Pac Vac Cdc-link rf Lf Ct
=
IV. S IMULATION
Representation
Value
HPS Parameters 200 kW
I
kHz
3
SC converter winding ratio
2
FC Inductor
l.6 mH
SC Inductor
2.4 mH
SC converter capacitor
1.8
FC converter capacitor
2.4
mF mF
Inverter Parameters Rated power
1200 kVA
Nominal ac voltage
2.4 kV
dc-link capacitance
2000/.iF
Resistance
l.5
Inductance
0.5 mH
Capacitance
100 /.iF
mSl
In each HPS system, the FC current absolute slope is limited to 0.0625 p.u.s-1 to prevent the fuel starvation phenomenon. Maxwell Technologies Boostcap BMOD0165-type SC is used as an energy storage. The dc-link voltage of each HPS system is regulated at 1 kV. The microgrid parameters are given in Table II. Initially, the microgrid is operating under balanced and linear load condition. At t=6 s, a six-pulse diode rectifier with 300 kV A and PF=0.95 is connected to the LV side of the feeder. Subsequent to the first load change, at t=12 s, a single phase RL load with 260 kV A and PF=0.98 is disconnected from phase c of the LV side of the feeder. The instantaneous real and reactive powers of the load with respect to these changes are shown in Fig. 7 (a). Due to the presence of unbalanced and nonlinear loads, 100 Hz-frequency and high frequency ripples are introduced into the power components of feeder respectively. Fig. 7 (b) illustrates the positive-, and negative-sequence currents at the fundamental frequency, and the harmonic currents in one phase at the LV-side. Due to the non-linear load connection at t=6 s, the positive-sequence component and the harmonic current of load are increased. At t=12 s, when the single-phase load is disconnected, the positive-sequence current is decreased and the negative-sequence current is increased. Fig. 8 illustrates the instantaneous currents and voltages of the DG unit terminal prior and subsequent to the nonlinear load connection at t=6 s. The switching patterns of the voltage is
498
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(b)
(b)
1
>' " ,,0
(c)
6
Fig. 7.
8
10
12 Time(sec)
14
16
18
Microgrid response to unbalanced and nonlinear load changes;
(a) Instantaneous real and reactive power. (b) Positive-sequence, negative sequence, and harmonic components of load. 11.98
Fig. 9.
11.99
12
12.01
12.02
Time(see)
12.03
12.04
12.05
12.06
(a) Instantaneous current waveforms, (b) switching patterns of the
output voltage, and (c) voltage waveforms of each phase of the DG unit's
CHB inverter due to the single-phase load disconnection.
..
l"� ....
(a)
� 1.2 ..... .... 1 .....:...
>' " ° "
(b)
� I. �"'F
-1 -2 -3
� 0.1
..
(c)
:
1 I� )l :i
:
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>' Fig. 10.
" >0
5.98
Fig. 8.
(a) voltage THD, and (b) voltage unbalance factor at DG unit
terminal.
5.99
6
6.01
6.02
Time(see)
6.03
6.04
6.05
6.06
Fig. LO(a) depicts the voltage THD of the DG unit during
(a) Instantaneous current waveforms, (b) switching patterns of the
and subsequent to load switchings. After the nonlinear load
output voltage, and (c) voltage waveforms of each phase of the DG unit's
connection, the voltage THD is increased from 1.05% to
CHB inverter due to the nonlinear load connection.
almost 1.4%, which is within the permissible range. Fig. LO(b) shows the VUF for the DG unit during the load changes. The VUF of the DG unit is below 2% while supplying 48 A
shown in Fig. 8(b) which demonstrates 5-level phase voltage
negative-sequence current to load. The results show that the
of the CHB multilevel inverter. As shown in Fig. 8 (c),
proposed voltage control strategy can effectively compensate
the proposed multi-PR controller provides a set of regulated
the harmonic and negative-sequence currents.
sinusoidal voltages at the DG unit terminals.
Fig. 11 shows the dc-link voltage of each HPS of DG unit
Fig. 9 shows the instantaneous currents and voltages of
during and subsequent to the load changes. The controllers
the DG unit terminal prior and subsequent to the single
maintain the dc-link voltages of each HPS within 5% of its
phase load disconnection at t=12 s. Fig. 9(b) illustrates the
rated value under sudden load changes. Moreover, the power
switching patterns of the voltage which provide a set of
oscillation of each CHB multilevel inverter cell leads to the
regulated balanced voltages at the DG unit terminals as shown
LOO Hz-ripple on the dc-link voltage.
in Fig. 9(c).
Fig. 12 shows the currents of the FC stacks and SC units
499
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(b)
1.2
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O lL....i....
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:� 1 �________1IIIj 5 Fig. 11.
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..... c ...
0.8
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Time(see)
;:
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15
5
10
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Time(see)
: ;�·�'f'·��·T·�· "-."1'��··��·�·���...r----.,
15
l'e1
--lso1
'��--"'1 ,-,-
T he dc-link voltage waveforms to the unbalanced and nonlinear _
load changes.
in each HPS of the DG unit. At t=6 s, the FC stacks in all phases of the CHB inverter increase their output power to
l'c2
--1'02
O lL-'-__"'-_---'__....i.__ .. ...i._ ... ---!==:=J .
Fig. 12.
6
8
10
12 Time(sec)
14
16
18
Dynamic response of the DG unit to load changes: currents of FC
stacks and SC units of each HPS, (a) phase a, (b) phase
b,
and (c) phase c.
reach the reference power which is determined by the HPS controller. The SC modules compensate the shortage power
•
of the FC stacks while the FC stacks increase their output
•
power at the limited response rate. The SC modules will be •
to the load demand. a
and
c
b
REFERE NCE S
of the CHB inverter are decreased, remains unchanged. As shown in
Fig. 12, the FC stacks current of phases
a
and
c
[l]
V.
and
c
[2] IEEE Recommended Practice for Electric Power Distribution for Indus trial Plants. ANSIIIEEE Std. 141, 1993. [3] IEEE Recommended Practices and Requirements for Harmonic Control
of the CHB inverter.
in Electrical Power System. IEEE Std. 519, 1992. [4]
CONCLUSION
tonomous microgrid considering the HPS and CHB multilevel
vol. 61, pp. 1043-1051, Mar. 2012. [5]
proposed strategy includes power management of the hybrid FC/SC power source and the CHB multilevel inverter voltage
system in a medium voltage microgrid," IEEE Trans. Smart Grid, vol. 3, pp. 1903-1910, Dec. 2012. [6]
software. The results show that the proposed strategy:
Li, and M. S. Munir, "A flexible harmonic control approach
A. Ghazanfari, M. Hamzeh, and H. Mokhtari,
"Power management
strategy for a multi-hybrid fuel cell/energy storage power generation systems," in Proc. IEEE PEDSTC, pp. 348-353, 2012. [8] M. Hamzeh, H. Karimi, and H. Mokhtari, "A new control strategy for a multi-bus mv microgrid under unbalanced conditions," IEEE Trans.
unbalanced and nonlinear loads. The performance of the pro posed control strategy is investigated using PSCADIEMTDC
Y. W.
Ind. Electron., vol. 59, pp. 444-455, Jan. 2012. [7]
Furthermore, a multi-PR controller is presented to regulate the voltage of the CHB multilevel inverter in the presence of
J. He,
through voltage-controUed dggrid interfacing converters," IEEE Trans.
control. The main characteristics of the proposed HPS are high performance; high power density; fast transient response.
A. Ghazanfari, M. Hamzeh, H. Mokhtari, and H. Karimi, "Active power management of multihybrid fuel celIlsupercapacitor power conversion
inverter under unbalanced and nonlinear load conditions. The
Power Syst., vol. 27, pp. 2225-2232, Nov. 2012. [9]
J. Chavarria, D. Biel, F. Guinjoan, C. Meza, and J. J. Negroni, "Energy balance control of pv cascaded multilevel grid-connected inverters under
robustly regulates the voltage of the microgrid under unbalanced and nonlinear load conditions;
•
J. Pereda and J. Dixon, "23-level inverter for electric vehicles using a single battery pack and series active filters," IEEE Trans. Veh. Techno!.,
This paper presents an effective control strategy for an au
•
Liu, J. F. Chen, T. Liang, and R. Lin, "Multicascoded sources for a
IEEE Trans. Power Electron., vol. 26, pp. 931-942, Mar. 2011.
difference between the FC stacks and the load demand charges a
W.
high-efficiency fuel-ceU hybrid power system in high-voltage application,"
are decreased
to meet the load demand in these phases. Moreover, the power the SC modules in phases
effectively manages the power among the power sources in the HPS system.
After the unbalanced load change at t=12 s, the output but the current of phase
accurately balances the dc-link voltage of each H-bridge cell; and
in standby state when the FC stacks power capacity is equal
currents of phases
enhances the dynamic response of the microgrid;
reduces THD and improves power quality by using CHB multilevel inverters;
500
level-shifted and phase-shifted pwm modulations," IEEE Trans. Ind. Electron., vol. 60, pp. 98-111, Jan. 2013.
