Distributed Control of Hybrid AC-DC Microgrid with Solar Energy, Energy Storage and Critical Load Tan Ma, Student Member, IEEE, Brandy Serrano Student Member, IEEE, and Osama Mohammed, Fellow, IEEE Abstract-In
this paper, a novel power flow control method for a
hybrid AC-DC microgrid with solar energy, and energy storage is proposed for the integration of a pulse load. This micro grid works in islanding mode with a synchronous generator and PV farm
supplying
power
to
the
system's
AC and DC sides,
respectively. A bidirectional AC-DC inverter is used to link the
AC and DC sides by controlling the active and reactive power flow between them. The PV farm is connected to the DC bus through a DC-DC boost converter with maximum power point tracking
(MPPT)
functionality. A Battery bank is connected to
the DC bus through a bidirectional DC-DC converter. The system is tested with a pulse load connected to the AC side. Simulation
results
verify
that
the
proposed
topology
is
coordinated for power management in both the AC and DC sides under
critical
loads
with
high
efficiency ,
reliability
and
robustness in islanding modes.
Keywords-Energy storage, grid control, hybrid micro grid, system, frequency and voltage amplitude regulation.
I.
PV
INTRODUCTION
Hybrid power systems are growing in popularity due to the increase in renewable power conversion systems connected to low voltage AC distribution systems that are used as distributed generators and implemented in micro grids. Their growth has also been attributed to the environmental issues caused by conventional fossil fueled power plants [1]-[3]. Furthermore, DC grids are resurging due to the development of new semiconductor techniques and sustainable DC power sources such as solar energy. There has also been an increase in DC loads, such as plug-in electric vehicles (PEVs) and light emitting diodes (LEDs), connected to the grid to save energy and decrease greenhouse gas emissions. The PEVs can be viewed as energy storage devices when they are parked in the garage, allowing them to increase the stability and efficiency of the micro grid they are connected to. One of the major technical challenges in micro grids is the interconnection of a pulse load which can cause voltage collapse, oscillation of the angular velocity in the generators, and degradation of the overall system performance. Researchers have proposed several ideas and models relating to renewable energy sources and storage, ranging from their scheduling and PEV charging optimizations to the feasibility of PEV vehicle-to-grid (V2G) services[4]-[6]. The authors are with the Energy Systems Research Laboratory, Department of Electrical and Computer Engineering,Florida International University,Miami,FL 33174 USA (e-mail:
[email protected]).
However, these models only propose the idea without thorough analysis of the of the energy conversion between the AC and DC sides. Researchers have also proposed several ideas and models of AC-DC micro grids but their systems operate without the influence of a pulse load. System stability and coordination control of power electronics devices during islanding mode with the influence of a pulse load is still an open issue. At the same time, several researchers have already proposed ideals and models of hybrid AC/DC micro grids[7] [9], but the systems are either working in grid connected mode or islanding mode without the influence of a pulse load. Hybrid power systems face far more challenges when operating in islanding mode than they do when operating in grid connected mode. During islanding mode, the AC side can no longer be viewed as an infinite bus, which results in load variations adversely affecting the frequency and voltage of the system. Power flow should be controlled between the AC and DC sides to maintain stability on both sides of the grid. Accurate energy availability estimation is also needed for prolonged periods of operation. Therefore, the coordination control of the power devices to help the system remain stable under the influence of a pulse load is still an open issue. In this paper, a hybrid AC-DC micro grid with solar energy, energy storage, and a pulse load is proposed. This micro grid can be viewed as a PEV parking garage power system or a ship's power system that utilizes sustainable energy and is influenced by a pulse load. The battery banks inject or absorb energy on the DC bus to regulate the DC side voltage. The frequency and voltage of the AC side are regulated by a bidirectional AC-DC inverter. The power flow control of these devices serves to increase the system's stability and robustness. This paper is organized as follows; system configuration and modeling of the PV farm and battery banks are presented in Section II. Coordinated control of the converters in islanding mode is presented in Section III. Section IV demonstrates the simulation results that verify the proposed topology and control method that increases system stability and efficiency under the influence of a pulse load. Finally, conclusions are drawn in Section V. II. A.
SYSTEM CONFIGURATION AND MODELING
Grid Configuration
978-1-47 99-3960-2114/$3l.00 ©2014 IEEE
I
ACGrid ACBus synchronous
AC/DC Power
�
DC Miero-grid
.------,--.--"M� -
PY panel
" iI
ii Bidirectional
---.J
:
AClDe
L- converter
Vp'�RI=1
D
H
._____
,
+1
I I L...-__...I._ ...-'--____ I
HI-----I I: Ii!I
Fig. 3 Equivalent circuit of a PV panel
Fig. I Hybrid AC-DC microgrid power system.
Fig. 1 shows the hybrid micro grid configuration where the synchronous generator, PV farm, and loads are connected to its corresponding AC and DC sides. The AC and DC sides are linked through a bidirectional three phase AC-DC inverter and transfonner. This paper focuses on islanding mode operation so the coordination control of power electronics devices operating in grid connected mode are not discussed. The proposed system works in islanding mode but can operate in grid connected mode if the AC bus is tied to the utility grid. The Matlab Simulink model shown in Fig. 2 is used to simulate the system operations under different circumstances. A 10 kW PV farm is connected to the DC bus as the DC source through a DC-DC boost converter with MPPT functionality. A 50 Ah lithium-ion battery with 108V tenninal voltage is connected to the DC bus through a bidirectional DC-DC boost converter to regulate the DC bus voltage. A synchronous three phase generator with 13.8 kVA and 208V phase to phase nns terminal voltage is connected to the AC side. The rated voltages for DC and AC sides are 340 V and 208V phase to phase rms, respectively. A 15 kW pulse load is connected to AC and DC sides, respectively. B.
Modeling ofPV panel
Fig. 3 shows the equivalent circuit of a PV panel with a load. Equations (1)-(3) show the mathematical model of the PV panel and its output current [10]. All the parameters are shown in Table I:
q Vpv 1pv nplph -nplsat· [exp« -- )(- + 1pvRs)) -1] (1) AkT ns =
1ph Isa/
Ipv
=
Lpv Rpv
-
STpv
vpv
Irr(
(1sso + ki(T - Tr))·
:0
(2)
0
1
qEgap T 3 1 1 ) exp«IZ1)'( - )) T Tr �.
Description
Symbol Vae
iph ism q A
k
Rs Rp
lsso k, T, lrr
Egap np ns
S T
C.
Vd +
Vb Battery bank
-
ib Lb
Rb STd
STC
ST4 ST6 ST2
ia ib ic
Transformer
G
Cac
Fig. 2 The compact Matlab Simulink model of the proposed micro grid
1.1753x10.8 1.602xl0·'9 C 1.50
1.38xI0·23 JK I
Q Q
0.037998 993.51
5.96 A 1.7xl0·3 301.18K
2.0793x10.6 A 1.leV 528 480 0-1000
Wlm2
350K
An accurate battery cell model is needed to regulate the DC bus voltage in islanding mode. The battery terminal voltage and SOC need to be estimated during operation. A high fidelity electrical model of lithium-ion battery model with thermal dependence is used [11 ]. The equivalent circuit of the battery model is shown in Fig. 4. The instantaneous response is modeled by a resistor Ro and the hysteresis response is modeled by a non-linear RC circuit R/ and C/. Emf represents the internal voltage of the battery. All four parameters are varying with different sacs and temperatures, so four lookup tables are established by using the parameter estimation toolbox in Simulink Design Optimization for these four parameters under different sacs and temperatures. The flow diagram of the parameter estimation procedure is shown in
rl�}=';;:." R1
I
Generator
AC filter Lac Rac
64.2 V 5.9602 A
Modeling ofLithium-ion battery bank
(3)
ST1 ST3 ST5
idc
Cd
Vaule
Rated open circuit voltage Photocurrent Module reverse satuation current Electron charge Ideality factor Boltzman constant Series resistance of a PV cell Parallel resistance of a PVcell Short-circuit current SC current temperature coefficient Reference temperature Reverse saturation current at T, Energy of the band gap for silicon Number of cells in parallel Number of cells inseries Solar radiation level Surface temperature of the PV
D DC LOAD
PV panel
+
=
TABLE I
PARAMETERS FOR PHOTOVOLATIC PANEL
emf
.:L T
L. -l l-
".
C1 Hysteresis response
I
Fig. 4 Lithium-ion battery equivalent circuit.
Ro
for
T
T
T
discharge current profile
"(, ��Rl' Cl� t Simscape model
i
simulated
battery
�
experimental
voltage
voltage
Fig. 7 The control block diagram for bi-directional DC-DC converter
power is larger than the total load in the hybrid microgrid, the PV should be turned to off-MPPT to help the system balance the power flow.
no
modify parameters
end
Em
(SoC,T),
Ro
(SoC,T),
R,
(SoC,T), C, (SoC,T)
Fig. 5 Flow diagram of the parameter estimation procedure.
Fig. 5. The SOC of each single battery cell can be calculated by equation (4). SOC
III.
=
100(1+
f
ibdt
--
Q
)
(4)
COORDINATED CONTROL OF THE CONVERTERS
Three types of converters are utilized in this proposed hybrid micro grid. These converters must be actively controlled in order to supply uninterrupted power with high efficiency and quality to critical loads on the AC and DC sides during islanding mode. The control method for the converters is discussed in this section. A.
Boost converter control with MPPT
In islanding mode, the boost converter of the PV farm operates in on-MPPT or off-MPPT which is based on the system's power balance and the SOCs of the battery banks. In most situations, this boost converter can operate in the on MPPT mode since the variation of the solar irradiance is much slower compared with the power adjustment ability of the AC generator. Therefore, for a given load either on the AC or DC side, the PV should supply as much power as possible to maximize its utilization. However, if the battery banks' SOCs are high (near fully charged) and the PV's maximum output tart
In this paper, the perturbation and observe (P&O) method is used to track the maximum power point. The algorithm utilizes the PV farm output current and voltage to calculate the power. The values of the voltage and power at the k'h iteration (Pk) are stored, then the same values are measured and calculated for the (k+ J)'h iteration (Pk+,). The power difference between the two iterations (,dP) is calculated. The converter should increase the PV panel output voltage if tJP is positive and decrease the output voltage if tJP is negative, which [mally will adjust the duty cycle. The PV panel reaches the maximum power point when tJP is approximately zero. The flow chart of the P&Q MPPT algorithm is given is Fig. 6. B.
The bi-directional converters of the batteries play an important role in islanding mode to regulate the DC bus voltage. A two closed-loops controller is used to regulate the DC bus voltage. The control scheme for the bi-directional DC DC converter is shown in Fig. 7. The outer voltage controlled loop is used to generate a reference charging current for the inner current controlled loop. The error between the measured DC bus voltage and the system reference DC bus voltage is set as the input of the PI controller, and the output is the reference current. The inner current control loop will compare the reference current signal with the measured current flow through the converter and finally generate a PWM signal to drive the IGBT STd or STc to regulate the current flow in the converter. For example, when the DC bus voltage is higher than the reference voltage, the outer voltage controller will generate a negative current reference signal, and the inner current control loop will adjust the duty cycle to force the current flow from the DC bus to the battery, which results in charging of the battery. The energy transfers from DC bus to the battery, and the DC bus voltage will decrease to the normal value. If the DC bus voltage is lower than the normal value, the outer voltage control loop will generated a positive current reference signal, which will regulate the current flow from the battery to the DC bus, and because of the extra energy injected from the batteries, the DC bus voltage will increase to the normal value. C.
Fig. 6 Flow chart of P&O MPPT method
Bi-directional DC-DC converter control
Bi-directional AC-DC inverter control
The frequency and voltage amplitude of the three phase AC side is not fixed during islanding operation so a device is needed to regulate these variables. A bi-directional AC-DC
TABLE II HYBRID MICROGRID SYSTEM PARAMETERS
RJ_
Symbol
Description
Vaule
Cp,' Lp,' Cd
Solar panel capacitor Inductor for solar Panel boost converter DC bus capacitor AC filter inductor Inverter equivalent resistance Battery converter inductor Resistance of Lb Rated AC grid frequency Rated DC bus voltage Rated AC bus p-p voltage (rms) Transformer ratio
/OOuF
Lac Rae Lb Rb
f
Vd Vm n/ln2
5mH 6000uF /,2mH 0,30hm 3.3mH 0,5
Q
60Hz 300V 208V /: /
between the battery and the DC bus. In the end, the energy is transferred between the battery and the AC side to balance the power flow in the system. IV. Fig, 8 The control block diagram for bi-directional AC-DC converter
inverter is used with the active and reactive power decoupling technique to keep the AC side stable. The Control scheme for the bi-direction� l AC-DC inverter is shown in Fig. 8. In d-q . coordmates, Id IS controlled to regulate the active power flow through the inverter to regulate the AC side frequency, and Iq is controlled to regulate the reactive power flow through the inverter to regulate the AC side voltage amplitude. Multi-loop control is applied for both frequency and voltage regulation. For frequency control, the error between measured frequency and reference frequency is sent to a PI controller which generates the id reference. To control the voltage amplitude, the error between the measured voltage amplitude and the reference voltage amplitude is sent to a PI controller to generate iq reference. Equations (5) and (6 ) show the AC side voltage equations of the bi-directional AC-DC inverter in ABC and d-q coordinates respectively. Where are AC side voltages of the inverter, and are the voltages of the AC bus. are the adjusting signals after the PI controller in the current control loop.
Vb, Vc)
(Lla, LIb, LIe)
(Ea, Eb, Ee)
(Va,
(5)
[][
Lac!!..�d dt q
=
][ ] [ ] [ ] [ ]
V E a Lac _-roRLace �Rae �dlq + Vqd _ Eqd + !1!1dq
The operations of the hybrid micro grid utilizing a 10.07 kW PV farm under the influence of a 10 kW pulse load is simulated to verify the proposed control algorithms. The rated output power of the synchronous generator is 13.8 kW, and a 4 kW constant load is connected in the AC side. Five 51.8V 21Ah Lithium-ion battery banks are connected individually to the DC bus through bidirectional DC-DC converters. The system parameters for the hybrid micro grid are listed in Table II. The MPPT of the boost converter is enabled at O.4s. The output power, the terminal voltage of the PV panel, the duty cycle of the boost converter and the solar irradiance are shown in Fig. 9, For general study, two kinds of solar irradiance variances with different charging rates are used in this study. Before O.4s, the duty cycle is set at 0.5, the terminal voltage of � he PV panel is 149V and the output power from the PV panel IS only 9.56 kW. After the MPPT is enabled, the duty cycle is decreased to 0.45. The terminal voltage is increased to 165V. In this way, the PV panel reaches the maximum power output of 10.07 kW. The simulation results show that the boost converter with MPPT functionality can track the maximum
r![ : i�E : : : : : 1 l::t; ; � n : s=;z, ';d 1 o
(6)
When the pulse load is connected or disconnected to the AC side, the frequency or the voltage amplitude will be altered. After detecting the variance from the phase lock loop (PLL) or voltage transducer, Id and Iq reference signals will be adjusted to regulate power flow through the bi-directional AC-DC inverter. Because of the power flow variances, the DC bus voltage will also be influenced. The DC bus voltage transistor will sense the voltage variance in the DC bus, and the bi directional DC-DC converter will regulate the current flow
SYSTEM SIMULATION RESULTS
�
o ----�O5.� -----� , --�15. �--�2--�=2�5. �--�3
o
05.
1
1.5
o
05.
1
1.5
0
05.
1
1.5
lime
(5)
Fig. 9 PV farm output power control with MPPT.
25.
3
2
25.
3
2
25.
3
DC bus vohage with influence of PC
320
side pulse h'ld
I
315
Reference �Itage - Measured voltage
310
(b) Generator output activc and rcactivcpowcr
18000 16000
305 300
A
r
12000
295
10000
290
8000 -
285
6000 -
280
-Acti\, powcr � -Reactlvcpower
L
14000
"
l
0
0.5
1.5
Tilllc(s)
2.5
3.5
4000 2000
Fig.IO DC bus voltage with the influence of solar irradiance variation and pulse load
0
0
0.5
1.5
power point with fast response.
Time(s)
2.5
3.5
(c)
Fig. 10 shows the DC bus voltage with the influence of solar irradiance variation on the PV panels and the pulse load connection in the AC side. The bidirectional DC-DC converter was enabled at t=O.ls, and the DC bus voltage entered the steady state in less than 0.3 seconds. The AC side pulse load connection and disconnection did not greatly impact the DC bus voltage since the battery banks have enough energy to support and balance the power flow with a quick response . When the 10 kW resistive pulse load was connected to the AC bus, the total load in the AC side was 14 kW which exceeded the generator's output limitation by 0.2 kW. Fig. 11 (a), (b) and (c) shows the AC side voltage generator's output
Fig. II Microgrid AC side pulse load reponse without DC support
power without DC side aid by the AC-DC inverter. At t = 2.2s, the system collapsed, and both the frequency and voltage dropped considerably. The system couldn't recover even after the pulse load was disconnected after t=3s. The batteries are able to support the AC side by injecting or absorbing power to the AC bus through the bidirectional inverter that links the AC and DC side. The frequency and voltage amplitude on the AC side also remain stable due to the separate control of the active and reactive power flow control. Fig. 12. (a) and (b). show the AC bus current and voltage. ACside current with AC plusc bad influence
� ith P"=IS C=0I 0= d inn"= ,=ncc A_C_b" "�OI= agC� � w� __ �__ � � ,
__�__�
2�.
-
-� -100 -1� -200 0.5 0.5
2.5
1.5
3.5
1.5
Time(s)
2.5
3.5
(a)
(a)
ACbus current with ACpluse bad influence
100 50
-so -100
0.5
1.5
Timc(s)
2.5
3.5
Time(s)
(b) Fig. 12 Microgrid AC side voltage and current response with DC support.
AC side frequency response
62�--�--�----�����----�--�--�
61. 5 61
60. 5
V.
58.5 58 57.5 57L---�--�----�--�--�----�--�--� o 0.5 1.5 2.5 3.5 Time(s)
(a)
5
Gcncrator output active and re.1ctivc power
I
2
Active powcr - Re.1ctivc powcr
5
�
1 5 0 5
second. When the pulse load was disconnected from the AC side, the frequency increased to 62 Hz and returned to steady state in less than 0.5s. The power flow through the AC bus and the power generated from the generator is shown in Fig.13 (b) and (c). The hybrid microgrid is stable in both its AC and DC side.
Jv-. -,.. 0. 5
...
�
A -
1. 5
REFERENCES
..
2
Time (s)
2. 5
[1]
c. K. Sao and P. W. Lehn, "Control and power management of converterfed MicroGrids," IEEE Trans. Power Syst., vol. 23, no. 3, pp. 1088-1098,Aug. 2008
[2]
A. Mohamed, F. Carlos, T. Ma, M. Farhadi, O. Mohammed, "Operation and protection of photovoltaic systems in hybrid AC/DC smart grids," IECON 2012 - 38th Annual Conference on IEEE Industrial Electronics Society,pp.1104-1109,25-28 Oct. 2012
[3]
R. H. Lasseter and P. Paigi,"Microgrid: A conceptual solution," in Proc. IEEE 35th PESC,Jun. 2004,vol. 6,pp. 4285--4290.
[4]
A. Mohamed, V. Salehi,T. Ma and O. Mohammed, "Real-Time Energy Management Algorithm for Plug-In Hybrid Electric Vehicle Charging Parks Involving Sustainable Energy," submitted to IEEE Transactions on Sustainable Energy. Manuscript ID: TSTE-00207-2012.
[5]
T. Ma, and O. Mohammed. "Optimal charging of plug-in electric vehicles for a car park infrastructure," to be published on IEEE Trans. industry Applications, July. 2014.
[6]
T. Ma, O. Mohammed, "Economic Analysis of Real-time Large Scale PEVs Network Power Flow Control Algorithm with the Consideration of V2G Services ",accepted by lAS annual meeting,2013
[7]
X. Liu; P. Wang; P. C. Loh, "A Hybrid AC/DC Microgrid and Its
3. 5
(b)
-2000 ::-�--7- ---;0'-;c 3. . 5-----O 2. 5 5------;:---- --o!; 0 ---;:'0.50- ------5-------:,"-;c Timc(s)
(c)
Coordination Control," Smart Grid, IEEE Transactions on , vol.2, no.2, pp.278,286,June 2011.
Fig.13 Microgrid AC side pulse load response with DC support.
When the pulse load is connected to the AC side, the current flow through the AC bus increased immediately, and after the pulse load disconnected from the AC side, the current slightly decreased to keep the system in balanced. The AC bus voltage transient response during the pulse load variation is shown in Fig. 12 (b). The AC voltage amplitude returned to its normal value in less than three cycles. Fig. 13.(a) shows the AC side frequency variation. The AC bi-directional inverter was enabled at t=O.4s, and the AC side frequency was stable at 60 Hz in less than O.4s. When the resistive pulse load is connected at t=l.3s, the frequency dropped to 57 Hz and returned to 60 Hz in less than one
DC
CONCLUSION
In this paper, a coordination power flow control method of multi power electronic devices is proposed for a hybrid AC-DC microgrid operated in islanding model. The microgrid has a PV farm and a synchronous generator supply energy to its DC and AC side. Battery banks are connected to the DC bus through bi-directional DC-DC converter. The AC side voltage amplitude and frequency are regulated by the bi-directional AC-DC inverter. The system topology together with the control algorithm are tested with the influence of pulse load. The simulation results show that the proposed microgrid with the control algorithm can greatly increase the system efficiency, stability, and robustness.
[8]
A. Mazloomzadah, M. Farhadi, O.A Mohammed "Hardware implementation of Hybrid AC-Dc power system Laboratory Involving Renewable energy sources " Accepted to be Presented in the American society for Engineering Education annual Conference,ASEE 2013
[9]
A. Mohamed, V. Salehi and Osama Mohammed, "Reactive Power Compensation in Hybrid AC/DC Networks for Smart Grid Applications," in Proc. of Innovative Smart Grid Technologies Conf., ISGT Europe 2012,Berlin,Germany,October 14-17,2012
[10] M. E. Ropp and S. Gonzalez, "Development of a MATLAB/simulink model of a single-phase grid-connected photovoltaic system," IEEE Trans. Energy Conv.,vol. 24,no. I,pp. 195-202,Mar. 2009. [11] T. Huria, M. Ceraolo, J. Gazzarri, R. Jackey, "High fidelity electrical model with thermal dependence for characterization and simulation of high power lithium battery cells," Electric Vehicle Conference (IEVC), 2012 IEEE International pp.I,8,4-8 March 2012
