130Design and Control of a Photovoltaic Energy And

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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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5701505 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013

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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5701505 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013

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.

REFERENCES

[1] Z. Wang, K. T. Chau, B. Yuwen, and F. Li, Power compensation and powerquality improvement based on multiple-channel current source converterfed HT SMES,IEEE Trans. Appl. Supercond., vol. 22, no. 3, p. 5701204,Jun. 2012.

[2] S. Dechanupaprittha, K. Hongesombut, M. Watanabe, Y. Mitani, and

I. Ngamroo, Stabilization of tie-line power flow by robust SMES con-troller for interconnected power system with wind farms, IEEE Trans.

Appl. Sup ercond., vol. 17, no. 2, pp. 23652368, Jun. 2007.[3] J. Shi, Y. Tang, Y. Xia, L. Ren, and J. Li, SMES based excitation system

for double-fedinduction generator in windpower application,IEEE Trans.Appl. Sup ercond., vol. 21, no. 3, pp. 11051108, Jun. 2011.

[4] J. Dai, D. Xu, and B. Wu, A novel control scheme for current-sourceconverter-based PMSG wind energy conversion system, IEEE Trans.Power Electron., vol. 24, no. 4, pp. 963972, Apr. 2009.

[5] I. J. Igelesias and J. Acero, Comparative study and simulation of optimalconverter topologies for SMES systems, IEEE Trans. Appl. Supercond.,vol. 5, no. 2, pp. 254257, Jun. 1995.

[6] Z. Wang, Y. Zheng, M. Cheng, and S. Fan, Unified control for a windturbine-superconducting magnetic energy storage hybrid system basedon current source converters, IEEE Trans. Magn., vol. 48, no. 11,pp. 39733976, Nov. 2012.

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130Design and Control of a Photovoltaic Energy And