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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013 5701505
Design and Control of a Photovoltaic Energy andSMES Hybrid System With Current-Source
Grid InverterZheng Wang, Zhixiang Zou, and Yang Zheng
AbstractThis paper proposes a novel photovoltaic (PV) energyand superconducting magnetic energy system (SMES) hybrid sys-tem based on the current-source grid inverter (CSGI). The key isto integrate the SMES coil into the dc link of CSGI for PV energyand battery systems. Thus, the SMES and PV energy systemcan share the CSGI, and the hybrid system offers more straightforward control on thegrid side. The battery is added to the systemfor increase of storage capacity and effective operation underquenching condition. The dc choppers are applied to exchange the
power between the PV, battery, and SMES. The battery-side dcconverter and the PV-side boost converters are utilized to delivertheir power to the voltage bus of dc choppers. The dc choke isproposed to take place of the SMES coil while quenching conditionoccurs. The control schemes for the proposed hybrid system forboth normal SMES and quenching conditions are presented. Theoperation of the hybrid system under faulty grid conditions is alsogiven. The simulation is developed to verify the validity of theproposed system and control schemes.
Index TermsBattery, current-source grid inverter (CSGI),faulty grid condition, photovoltaic (PV) energy, superconductingmagnetic energy system (SMES).
I. INTRODUCTION
BECAUSE of the merits of low emission and little me-
chanical parts, the photovoltaic (PV) power generation
is drawing more and more attention today. However, the PV
power suffers from the irregular solar radiation, and exhibits
the unstable behavior. To make better use of the PV power, the
energy storage could be applied to compensate the fluctuating
PV power. Besides, when the faults occur in the main grid,
the distributed generation (DG) units and the local loads will
be disconnected from the main grid and become islanding.
Thus, the energy storage system is required to support the bus
voltage and match the power for the local loads as well as the
DG units in the islanding area. As a high efficiency energystorage, the superconducting magnetic energy storage system
Manuscript received October 7, 2012; accepted December 2, 2012. Date ofpublication March 7, 2013; date of current version March 29, 2013. This workwas supported in part by the National Natural Science Foundation of China(51007008 and 51137001), by the Doctoral Fund of Ministry of Education ofChina (No. 20100092120043), by the Scientific Research Foundation for theReturned Overseas Chinese Scholars of State Education Ministry, and by theTeaching and Research Funding for Outstanding Young Teacher of SoutheastUniversity.
The authors are with the School of Electrical Engineering, Southeast Univer-sity, Xuanwu District, Nanjing 210096, China (e-mail: [email protected]).
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TASC.2013.2250172
(SMES) has been used to stabilize the power, enhance the low
voltage ride through capability, and improve the power quality
for renewable energy systems [1], [2]. So, the SMES could act
as the energy storage for improving the operating performance
of PV systems.
However, most previous research are focused on the separate
grid inverters based SMES systems and renewable energy sys-
tems. Recently, the SMES has been proposed to be integratedin the DC link of voltage source converter (VSC) on the rotor
side of a doubly-fed induction generator (DFIG) wind energy
system to improve the operating performance [3]. The SMES
and the wind energy system share the common grid inverter.
On the other hand, the current source converter (CSC) has been
proposed for the wind energy system because of the features of
simple configuration, small device number, low dv/dt, simplePWM strategy, reliable current protection, and inherent four
quadrant operation ability [4]. Meanwhile, the CSC is verified
suitable for the SMES since it can deliver the energy to the grid
directly without DC chopper, and it can provide more reactive
power to the grid than the VSC under the same active power
rating [5]. Actually, the CSC based SMES and wind energyhybrid system has been proposed by integrating the SMES coil
into the DC link of CSC fed wind energy system in [6]. The
CSC based hybrid system offers the compact configuration and
more straightforward control on the grid side.
However, the research on integration of PV energy systems
and SMES based on current-source grid inverter (CSGI) is
still absent now. Different from the traditional configuration
where the PV and the energy storage systems are connected
with separate grid inverters on grid side, this paper proposes a
novel hybrid system by sharing the common CSGI between the
PV and SMES systems. The proposed system not only has the
inherent merits of CSGI, but also provides more straight for-ward control and improves the fault tolerant capability. The key
technique is to connect the SMES coil and the PV converters in
cascade at their DC links. Thus, the generated PV energy can
be stored effectively in the SMES when the proposed hybrid
system is disconnected from the point of common coupling
when the grid faults occur. The boost converter is designed to
implement the maximum power point tracking (MPPT) for PV
arrays, and the DC chopper acts as the power interface between
the CSGI and the PV-side boost converter. To improve the en-
ergy storage capacity and provide the reliable operation under
quenching condition of SMES, the battery system is connected
to the voltage bus of DC choppers through a bidirectional DC
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Fig. 1. Configuration of the CSGI based PV and SMES hybrid system.
converter. The control schemes of the proposed hybrid system
are designed for both normal SMES condition and quenching
condition. The operation strategies for the system under grid
fault conditions are also developed.
II. SYSTEMC ONFIGURATION
Fig. 1 shows the configuration of the proposed CSGI based
PV and SMES hybrid system. The SMES coil is integrated
in the DC link of the CSGI. When the SMES is in normal
state, the switchesK1 andK2 are kept on. The DC chopper 1is used to exchange the energy between the SMES coil and
the DC link capacitor C1. The PV array 1 is connected to theDC link capacitor C1 through a boost converter. The similarconfiguration is adopted for the PV array 2. The output sides
of the DC chopper 1 for PV array 1 and DC chopper 2 for
PV array 2 are connected to the SMES coil in series, in such
a way that the SMES could deliver the energy from the PV to
the grid.
When the quenching condition occurs, the energy in SMES
must be dumped by the metal oxide varistor (MOV), which is
paralleled to the SMES coil. The switchesK1andK2are turnedoff under this condition. To keep the system exchanging the
energy with the grid, an additional DC chokeLdcis connectedto the DC link of CSGI by turning onK3andK4. The battery isadded to the system for increase of the energy storage capacity.
Also, the battery can store the energy from PV effectively when
the quenching condition of SMES and the grid fault occur.
III. CONTROL OF
SYSTEM
UNDER
NORMAL
GRI D
CONDITION
A. Normal SMES Condition
Fig. 2 shows the control diagram of the SMES under the
normal grid condition. The closed-loop active power controller
and reactive power controller generate the d-axis and q-axiscurrent references, respectively. The PI control is used for the
power controller.P andQ are the output active and reactivepower references, and P and Q are the real active and reac-tive power. The feedforward current referencesP/1.5vd andQ/1.5vdare added to the d-axis andq-axis current reference,respectively. vdis the d-axis voltage in grid, and vldand vlq are
the d-axis and q-axis voltages on the grid-side capacitors Crin Fig. 1. The low-bandwidth capacitor current compensation
Fig. 2. Control diagram of CSGI for SMES.
Fig. 3. Control diagram of proposed hybrid system under normal SMES andgrid condition.
terms sCrvlq andsCrvld are added to reduce the steady-state deviation of grid currents, and the high-bandwidth damp-
ing components rivldhand rivlqh are used to damp the possibleLC resonance. s is the grid frequency, and ri is a virtualresistor implemented by control approach. vldh and vlqh arethe high-frequency d-axis and q-axis capacitor voltages afterusing high-passing filters. The space vector modulation (SVM)
is used to modulate the output current of CSGI.The control strategy for the hybrid system under normal
SMES and grid condition is given in Fig. 3. For the DC
chopper 1, the DC link voltage v1 is controlled to track aconstant reference v1 by switches S1 and S2. S1 and S2 actsynchronously, and the duty ratioD1is determined by:
D1 = Kp1(v
1 v1) + Ki1
(v
1 v1) dt (1)
where Kp1 and Ki1 are the proportional and integral itemsfor voltage controller of DC chopper 1. The similar control
strategy is adopted for the DC chopper 2 to regulate the DC link
voltagev2 to track the reference v2. Thus, the duty ratio D2is generated for the switches S3 and S4. The boost converterof PV 1 functions to regulate the output voltage of PV 1 to
implement MPPT. So, the DC link voltage v3 is controlled totrack its referencev3 by tuning the duty ratioD3 of switchS5.Also the PI controller is used:
D3 = Kp2(v
3 v3) + Ki2
(v3 v3) dt. (2)
The control strategy for the boost converter of PV 2 is similar,
and the DC link voltage v4 is controlled to track the referencev4 with the duty ratioD4. For the battery system, the charging
or discharging current ibattery is controlled with the switchesS7 or S8. To charge the battery, S8 is kept off and S7 is
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Fig. 4. Control diagram of CSGI for dc choke.
Fig. 5. Control diagram of proposed hybrid system under quenching of SMESand normal grid condition.
controlled to regulate ibattery . The duty ratio ofS7, namely D5is given by:
D5 =Kp3ibattery ibattery
+Ki3
ibattery ibattery
dt.
(3)
To discharge the battery, S7 is turned off and S8 is controlledwith the duty ratioD6, which can be determined similarly.
B. Quenching Condition
When the quenching condition occurs, the SMES is discon-
nected from the DC link of the CSGI, and the energy is dumped
by the MOV. The common dc chokeLdc is connected withK3and K4to act as the DC link. Different from the control of CSGIunder normal SMES condition, the DC choke current should
be regulated by the CSGI in the quenching condition. Fig. 4
shows the corresponding control diagram of the CSGI, and
Fig. 5 shows the control of the whole system under quenching
condition. To make the DC choke current idctrack its referencevalue
i
dc, the closed-loop DC link current controller generates
the d-axis current reference id. The feedforward of PV and bat-tery power is added to the d-axis current reference to improvethe system dynamics. The closed-loop reactive power controller
generates the q-axis current reference iq . The low-bandwidthcapacitor current compensation and high-bandwidth damping
components are added to the current references, and the SVM
is applied to modulate the CSGI.
IV. CONTROL OFS YSTEMU NDERG RI DFAULTS
When the faults occur in the grid, the proposed hybrid system
can be disconnected from the grid and the bypass operation
works for the CSGI system. The upper and the lower switchesare conducted at the same time during the bypass operation.
Fig. 6. Control diagram of proposed hybrid system under faulty grid condi-tion: (a) normal SMES and (b) quenching of SMES.
Thus, the CSGI is short circuited on grid side, and the hybrid
system becomes stand-alone. Due to the existence of SMES
and battery systems, the PV can keep working properly and
the PV energy can be stored in SMES or battery. Fig. 6
shows the control diagram of the hybrid system under faulty
grid conditions. For the normal SMES condition as shown in
Fig. 6(a), the CSGI is short circuited on grid side and the
SMES current is adjusted by the input power on PV and battery
sides. The control schemes of PV and battery systems are
same as those under normal grid condition. For the quenching
condition of SMES as shown in Fig. 6(b), the DC choke current
is controlled by the DC chopper 1. The duty ratio D7 of theswitches S1 and S2 are given by the PI controller, and theproportional and integral parameters areKp4and Ki4. The DClink voltage of DC chopper 1, namely v1 is controlled by thebattery-side DC converter instead. The operation strategies of
PV-side converters systems are similar to those in last section.
V. SIMULATIONV ERIFICATION
The Matlab/Simulink is used to simulate the proposed system
in Fig. 1. The fixed-step Runge-Kutta solver is used, and the
step size is 10 s. The switching frequency of the CSGI is2.16 kHz, and the switching frequencies for the DC choppers,
the battery-side DC converter, and the PV-side boost converters
are 5 kHz in simulation. The rated phase-to-phase voltage of
grid is 2.3 kV. The inductance of SMES coil is 10 H, and its
rated current is 800 A. The grid-side inductance Lsis 1.2 mH,the grid-side capacitorCr is 178F, and the DC choke induc-
tance is 40 mH for the CSGI. The DC link capacitance forthe DC choppers C1 and C2 are both 6000 F and the DClink capacitance for the PV-side boost converters C3 and C4are 3000F. The inductance for the battery-side converter andPV-side converters are all 3 mH.
A. Case1. Normal SMES and Normal Grid Condition
Fig. 7 shows the results when both the SMES and grid
conditions are normal. As shown in Fig. 7(a), the DC link
voltage of DC chopper is controlled as 1200 V. The output
voltages of PV1 and PV2 change irregularly, which are related
to the effect of MPPT under random solar radiation. Hence,
the irregular output power are generated by PV1 and PV2, asshown in Fig. 7(b). With the control scheme of SMES in Fig. 2,
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Fig. 7. Simulated results under normal SMES and normal grid condition:(a) voltages, (b) output power, and (c) currents.
Fig. 8. Simulated results under normal SMES and faulty grid condition:(a) output power and (b) currents.
the system can absorb the constant active power of 200 kW
from the grid. The discharging power of battery is constant,
which is due to the constant discharging current of 600 A in
Fig. 7(c). The increase of SMES current in Fig. 7(c) is caused
by the power difference between the grid side and the PV
side of CSGI. The amplitude of grid current is constant, since
the power on grid side is constant. During the simulation, the
control parameters areKp1 =Ki1 = 0.05 for DC choppers,Kp2 =Ki2 = 0.05 for PV-side boost converters, and Kp3 =Ki3 = 0.05 for the battery-side converter. The proportional andintegral parameters in the PI controllers for grid-side power in
Fig. 2 are 0.001 and 0.01, respectively. The virtual resistor riis 0.2.
B. Case2. Normal SMES and Faulty Grid Condition
When the fault occurs in the grid, the system performance
with normal SMES is verified in Fig. 8. The grid side of SMES
is short circuited, so no output power is generated on the grid
side. In the simulation, the operation of PV arrays is same as
that in last case. By tuning the charging current of battery,the irregular power generated by PV arrays is absorbed by the
Fig. 9. Simulated results under quenching of SMES and normal grid condi-tion: (a) voltages, (b) output power, and (c) currents.
battery. Consequently, the SMES current does not change since
no energy is exchanged with the SMES coil.
C. Case3. Quenching of SMES and Normal Grid Condition
When quenching of SMES occurs, it is disconnected from
the system, and taken place by a common DC choke. Fig. 9
shows the simulated results under such condition, where the
DC choke current of CSGI is controlled as 800 A with the
scheme in Fig. 4. The DC link voltage of DC chopper is also
controlled as 1200 V. The output voltage of PV1 is changedfrom 600 V to 900 V at t= 3.5 s, and the output voltage ofPV2 is changed from 750 V to 1000 V with MPPT as shown
in Fig. 9(a). Accordingly, the output power from the PV array
becomes less in Fig. 9(b). The output power of the system
to grid becomes from negative to positive, which means the
system injects power to the grid at first and then absorbs power
from the grid. The battery-side DC converter keeps the constant
charging current and power. In the simulation, the proportional
and integral parameters in the PI controllers for DC choke
current are 2 and 5, respectively. Other control parameters are
same as those in case 1.
D. Case4. Quenching of SMES and Faulty Grid Condition
When quenching of SMES and grid faults occur at the same
time, the grid side of CSGI is short circuited, and the DC choke
takes place of the SMES coil in the DC link. Fig. 10 shows the
results under this case. Since the CSGI is short circuited, the
DC link current is controlled to be 800 A by the DC chopper 1
instead as shown in Fig. 10(c). The DC link voltage of DC
chopper 1 is controlled as 1200 V by the battery-side converter
in Fig. 10(a). Similar to other cases, the PV output power
changes irregularly in Fig. 10(b). The battery compensates the
fluctuation of PV power. So the charging current of battery
varies in Fig. 10(c). The proportional and integral parameters insimulation are 0.1 and 0.1 for the PI controller in DC chopper 1
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Fig. 10. Simulated results under quenching of SMES and faulty grid condi-tion: (a) voltages, (b) output power, and (c) currents.
to regulate the DC link current. The corresponding parameters
are 0.05 and 0.05 for the PI controller of the battery-side
converter to provide the constant output DC voltage of 1200 V.
VI. CONCLUSION
In this paper, a novel PV and SMES hybrid system is
proposed based on the CSGI. The SMES is integrated into
the DC link of CSGI to provide the power buffer between the
PV energy systems and the grid. The battery is added to the
system to increase the storage capacity and improve operating
performance under quenching condition and grid faults. The
DC choppers are used to link the voltage buses of battery-side
and PV-side DC converters to the SMES coil. A DC choke is
designed to take place of the SMES coil in the DC link of CSGI
when the quenching condition occurs. The control schemes ofsuch hybrid system are proposed for different working condi-
tions, including the normal and the faulty grid conditions, as
well as the normal SMES condition and quenching conditions.
The simulation results have been given to verify the validity of
the proposed system and operating strategies.
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