Supercapacitor Energy Storage System for Fault

Ride Through in Grid-Connected PV Array

Muhammed Y. Worku, KFUPM, Saudi Arabia and M.A.Abido, KFUPM, Saudi Arabia

Abstract—A fault ride through, power management and control

strategy for grid integrated photovoltaic (PV) system with

supercapacitor energy storage system (SCESS) is presented in

this paper. During normal operation the SCESS will be used to

minimize the short term fluctuation as it has high power density

and during fault at the grid side it will be used to store the

generated power from the PV array for later use and for fault

ride through. To capture the maximum available solar power,

Incremental Conductance (IC) method is used for maximum

power point tracking (MPPT). An independent P-Q control is

implemented to transfer the generated power to the grid using a

Voltage source inverter (VSI). The SCESS is connected to the

system using a bi-directional buck boost converter. The system

model has been developed that consists of PV module, buck

converter for MPPT, buck-boost converter to connect the

SCESS to the DC link. Three independent controllers are

implemented for each power electronics block. The effectiveness

of the proposed controller is examined on Real Time Digital

Simulator (RTDS) and the results verify the superiority of the

proposed approach.

Index Terms-- Active and reactive power control, fault ride

through, MPPT, Photovoltaic system, RTDS Supercapacitor

Energy storage

I. INTRODUCTION

Power generation using renewable sources is promising to tackle problems associated with price volatility and carbon impact of fossil fuels. The development of renewable energy based Distributed Generation (DG) is moving fast to meet the worldwide urgent needs of utilizing clean energy sources to create a clean energy future and minimizing costs. Among the renewable energy sources, solar energy is promising and photovoltaic (PV) system provides the most direct method to convert solar energy into electrical energy without environmental contamination. The solar power converted to electrical power by photovoltaic (PV) system can be integrated to the grid if it meets the grid code [1]. Thus controlling the power electronics blocks that are used for grid integration is vital in order to get the best out of the solar energy.

The output voltage and current of a PV cell are dependent

on the irradiation and temperature received at any instant of

time. Because of these variations in the input parameters and

the non-linear relation between voltage and current, there is

always one point that gives the maximum power and these

days PV panels are accompanied by maximum power point

tracking (MPPT) controllers. Different MPPT design methods

have been presented over the years [2]-[5]. These include

Perturb and observe (P&O), Incremental Conductance (IC),

artificial intelligence techniques such as Artificial Neural

Networks (ANNs) and Fuzzy Logic control. The widely used

methods by many researchers are IC and P&O due to their

simplicity.

One disadvantage of using PV as a power generation unit

is that its output power is unpredictable as a result of varying

irradiation and temperature. To overcome this fluctuation

different energy storage devices are integrated to the PV

system. Among these, battery and supercapacitor energy

storage system (SCESS) are used in the literature because of

their high energy and power densities respectively [6]-[9].

Some work has been done to integrate SCESS with

STATCOM for low voltage and fault ride through as well as

smoothing power fluctuation of wind energy system [10]-

[12]. Low voltage ride through and fault response of grid

connected solar inverters without energy storage is discussed

in [13]-[16]. Different linear and nonlinear control techniques

have been proposed recently on isolated and grid connected

PV with energy storage system [17]-[21]. However, most of

them address on how to use the energy storage for other

issues and control techniques. Issues related to fault ride

through using the energy storage has not been discussed.

In this paper, the power generated from PV arrays is

integrated to the grid using a buck converter, a bi-directional

buck boost converter and a VSI. To minimize the fluctuation

and store energy during grid fault, supercapacitor energy

storage system (SCESS) is tapped on the DC link of the VSI.

The energy storage is connected through a bidirectional buck

boost converter and this converter controls the DC link

voltage to a constant value for power delivery to the grid.

Controlling the buck converter’s duty cycle takes care of the Muhammed Y. Worku is with the King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia (e-mail:

muhammedw@kfupm.edu.sa).

M. A. Abido is with the King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia (phone: +966-508-757-838; fax:

+966138603535; e-mail: mabido@kfupm.edu.sa).

maximum output voltage for MPPT. The implemented P-Q

vector control independently controls the active and reactive

power for grid integration depending on the power demand

from the power management control (PMC). Real Time

Digital Simulator (RTDS) is used to test the whole system

and results are provided for verification. The rest of the paper

is organized as follows. The proposed system structure and

PV array modeling is described in section II. The proposed

controller is described in section III. RTDS simulation results

are presented in section IV and section V concludes the

paper.

II. SYSTEM MODELLING

Fig. 1 depicts the configuration of grid integrated PV

system with energy storage connected to the DC link. The PV

array which is formed from a number of modules converts the

solar irradiation and temperature into DC current and voltage.

These values are varying throughout the day depending on

the irradiation and temperature at any particular time. To

make the DC power generated from the PV array suitable for

the inverter and implement MPPT, a buck converter is used.

The duty cycle of the buck converter is continuously adjusted

under varying irradiation and temperature to instantly locate

the maximum voltage or current to obtain the maximum

power output from the PV arrays. The energy storage is

connected at the DC link between the buck converter and the

inverter using a bidirectional buck boost converter. The DC

link voltage is kept constant by this converter for real power

delivery by absorbing any mismatch between the generated

power and the power transferred to the grid. A P-Q control is

implemented for the inverter to transfer the available power

of the DC link to either an AC load or to the main grid during

normal operation and during fault at the grid side the DC link

power will be stored in the SCESS.

DC-DC

Converter

(BUCK)

DC/AC

(Inverter)

Duty PWM

PV Array

pvVpvIGridDCV

DCI

gVLITr

BUCK-

BOOST

CONVERTER

SCESS

SCESS Control

SCP

PVP gPDC Link

Fault

Fig.1. Grid connected PV system with energy storage

III. THE PROPOSED CONTROL STRATEGY

Three power electronics blocks namely buck converter,

bidirectional buck-boost converter and VSI are used to

integrate the PV system to the energy storage and to the grid

and hence three control blocks are proposed. The first control

block uses IC to control the duty of the buck converter and

the MPPT. The second control block controls the DC link

voltage to a constant value for real power delivery and the

third control block is a P-Q control for the VSI. This P-Q

control can be set by the PMC depending on the AC load

need or the grid power demand. Each control block is

discussed in the next section.

A. Buck Converter Controller

The function of this converter is to force the PV array

change its operating point under varying irradiation and

temperature to locate the MPPT. IC method is used because

of its simplicity in this paper to locate the MPP by calculating

the PV power at each instant and changes PV array’s

operation point to capture the maximum available power. The

PV output power is given by P=VI and from the rule of the

IC method, the derivative of the PV power will be zero at the

maximum operating point. Applying the chain rule of

derivative yields

V

VI

V

P

)(=0 (7)

Rearranging the terms, equation (7) could be written as

V

I

V

I

(8)

The buck converter’s duty cycle is controlled by the IC based

MPPT until equation (8) is satisfied. Input of the IC based

MPPT is the PV array’s output voltage VPV and current IPV

and its output is the reference voltage, Vref, as shown in Fig.2.

This voltage will be compared with VPV and the difference

will be processed by a PI regulator. After comparison of a

high frequency carrier signal with the output of the PI (which

is the Duty), firing pulse will be generated for the buck

converter. The DC link voltage VDC will be the Duty

multiplied by the PV array output voltage, VPV. The DC link

capacitor CDC removes the offset of this voltage.

+

-

LIDI

SHRD

SHISR

PVI

PVV

To

Inverter

Incremental

Conductance

refV

+

-

PI

Duty

DCVDCC

PVI

PVV1C

Buck ConverterPV Array

To

SCESS

PVI

PVV

Fig.2. IC Based MPPT and buck converter controller

A. Bidirectional Buck-Boost Converter Controller

The topology of the bi-directional buck-boost converter is

shown in Fig. 3. The bidirectional converter acts as a buck

converter to recharge the supercapacitors in one direction and

as a boost converter to transfer energy to the link capacitor in

the other direction. IGBTs are used as the switching devices

in the circuit. The operation of the converter is controlled by

the DC link voltage and the voltage of the supercapacitors.

The main purpose of the bidirectional buck-boost converter is

to maintain the voltage of the DC link relatively constant at a

reference value. To make this buck-boost converter controller

stable, a lower limit is placed on the supercapacitor voltage

which is 50% of the maximum value. The state of charge

(SoC) controls the supercapacitor voltage to be between

0.5VSCmax and VSCmax so that 75% of the energy stored is

utilized. The inductor L is designed from the boost mode

using a duty cycle of about 0.5 [22].

S1

S2

SCESS

L

+

-

SCI

DCV+

-

SCV

Fig.3. Buck boost converter to integrate the SCESS

Two cascaded loops are implemented for the buck-boost

converter as shown in Fig.4. The outer loop is a voltage

control that controls the DC link voltage by comparing the

measured DC link voltage with the reference and generates

the reference supercapacitor current ISCref. This reference

current is compared with the actual inductor current ISC to

generate the gating signals for the IGBTs. The reference

current ISCref in Laplace transform is given as:

)(*)(s

kkVVI IO

PODCDCrefSCref (13)

The control signal Vs in Laplace transform is given by

)(*)(s

kkIIV II

PISCSCrefS (14)

Where, kP’s are the proportional and kI’s are the integral

constants for the buck boost converter given in Table I.

DCrefV

DCV

PI+

-

SCrefI +

-SCI

PI

S2

S1Not

+

-

Vs

Fig.4. Controller for the buck boost converter

The size of the supercapacitor is chosen depending on the

amount of energy required to minimize the fluctuation from

the PV source. From Fig.1,

SCpvg PPP (15)

Where, Pg is the grid demand power,

Ppv the maximum power generated from PV array

Psc the supercapacitor power

If the power demand from the grid or load connected to the

AC side is less than the power generated from the PV array,

the excess energy will be stored in the supercapacitor for later

use when the power demand is high from the PMC. During

fault on the grid side, the generated power from the PV array

will be stored in the SCESS and it will be used to ride

through the fault by supporting active and reactive power to

the grid.

A. P-Q Controller for the Inverter

The available DC link power has to be converted to three

phase AC power to supply either AC loads or for grid

integration using an inverter shown in Fig.5. Depending on

the grid power demand or AC load, P-Q controller is

implemented for the inverter.

C

S3 S5S1

S4 S6 S2

aV

bVcV

AC

AC

AC

cgV

agV

bgV

R L

VDCaI

bI

cI

Fig.5. Two level three phase inverter

Where, R and L are resistance and inductance of the

distribution line respectively. Ia, Ib and Ic are the distribution

line currents; Va,Vb, Vc are the inverter output voltages; Vag,

Vbg, Vcg are the grid voltages. Using synchronous rotating

reference frame (D-Q axis), decoupled active and reactive

current control technique is implemented using a standard PI

controller. In the current control technique, the active current

component ID controls the active power and reactive power

flow is regulated by controlling IQ. The PI controllers force

these currents to track certain reference commands IDREF and

IQREF, respectively. Utilizing the instantaneous power theory

[23]:

)(2

3

)(2

3

QLDDLQg

QLQDLDg

gdc

IVIVQ

IVIVP

PP

(16)

Where Pdc is the DC link capacitor power, Pg and Qg are the

grid side active and reactive power respectively. The power

reference signals for the active and reactive power controllers

are obtained from the PMC. If the reference active and

reactive power is known, the respective IDREF and IQREF

currents are determined from equation (18) as:

)(3

222

LQLD

LQREFLDREF

DREFVV

VQVPI

(17)

)(3

222

LQLD

LDREFLQREF

QREFVV

VQVPI

(18)

The control scheme for the P-Q controller is shown in Fig.

6. Since the D and Q components are coupled, cross-

coupling term and a feed forward voltage are used to improve

the performance of the PI current controllers.

The D-axis component control signal in the Laplace frame is:

DLDDDREFDI

DPD LIVIIs

kkV )(*)( 1

1 (19)

And Q-axis component control signal in the Laplace frame is:

QLQQQREF

QI

QPQ LIVIIs

kkV )(*)(

2

2 (20)

DCV

DCREFV

PI+

-

DREFI

LDI

+

-

PI

DREFv

0

LQI

PI+

-

QREFv

LDV

Dv

+

-+

LQV

Qv+

++

L

L

DQ/

ABC

aV

SVMbV

cV

Firing

Pulse

to

Invert

er

Fig. 6. Decoupled P-Q inverter control

IV. RTDS BASED RESULTS AND DISCUSSION

A PV array with the specification given in the Appendix is

developed on RTDS. It consists of 50 series connected and 20

parallel connected modules. Fig.7 shows the applied three

phase fault and Fig.8 shows the complete RTDS model of the

grid connected PV system used in this paper. For the

reference solar intensity of 1000 W/m2 and 25

oc, the

operating voltage VMP and current IMP at the MPPT will be

50×17.4 V=870 V, and 20×3.05 A=61A, respectively. The

expected maximum output power at this operating point from

this PV array is 53 kW (870V×61 A). The P-Q controller

from the PMC is set to 50kW and if the irradiation or

temperature varies this set point will be varied accordingly.

During a fault at the grid side, the generated power from the

PV array will be stored in the SCESS. The SCESS helps the

system ride through by providing active and reactive power.

The carrier frequency used by the VSI is 1620 Hz (27×60

Hz). To demonstrate the effectiveness of the proposed

controller for fault ride through and power smoothing using

energy storage for grid connected PV system, three phase

fault is applied at the grid side.

Fig.7.Applied three phase to ground fault controller

Fig.8. RSCAD model of grid connected PV system with the applied fault

A. Three Phase Grid Fault is Applied

A three phase fault of three cycles (50msec) is applied on the

grid side and the fault is cleared after another three cycles.

For comparison, two systems have been developed one with

SCESS and the other one without energy storage in RTDS.

The response of the system for the applied fault is depicted

from Figs 9-17. The grid voltage after the described fault is

shown in Fig.9. Since power is being exchanged between the

PV array and the SCESS, the power generated from the PV

array, PPV is unaffected because of the fault at the grid side

and is stored in the SCESS PSC as shown in Fig.10. The

oscillation of the grid power because of the fault is reduced

for a system having a SCESS than without energy storage as

shown in Fig.11. Fig.12 shows the power stored in the

SCESS which is generated by the PV array during the fault.

The reactive power set point prior to the fault is zero but

during the fault, the SCESS participates in riding the fault by

increasing the reactive power Qg as shown in Fig.13. The DC

link voltage is controlled by the buck boost converter for the

system having the SCESS but the inverter controls this

voltage if there is no energy storage. As can be seen from

Fig.14 the DC link voltage is kept constant to its reference

value of 650 V for a system equipped with SCESS. The

operation of the PV array is unaffected if the system has

energy storage as shown in Fig.15. The PV array MPPT

tracks the maximum voltage in spite of the fault as shown in

Fig.16. The actual and reference charging current of the

SCESS is shown in Fig. 17.

Fig.9. Grid voltage after three phase fault is applied

Fig.10. PV array power PPV with SCESS and with no energy storage

Fig.11. Grid active power Pg for a three phase fault with and without energy

storage

Fig.12.SCESS power PSC for the applied fault on the grid side

Fig.13. Grid reactive power Qg during three phase fault

Fig.14. DC link voltage for the applied fault

Fig.15. PV array voltage VPV during three phase fault

Fig.16. MPPT output voltage Vref for the applied fault

Fig.17.Actual and reference SCESS current during three phase fault

V. CONCLUSION

This paper presents grid connected PV system with

supercapacitor energy storage system (SCESS) for fault ride

through and to minimize the power fluctuation. Incremental

conductance based MPPT is implemented to track the

maximum power from the PV array. The generated DC

power is connected to the grid using a buck converter, VSI,

buck-boost converter with SCESS. The SCESS which is

connected to the DC link controls the DC link voltage by

charging and discharging process. A P-Q controller is

implemented to transfer the DC link power to the grid.

During normal operation the SCESS minimizes the

fluctuation caused by change in irradiation and temperature.

During a grid fault the power generated from the PV array

will be stored in the SCESS. The SCESS supplies both active

and reactive power to ride through the fault. RTDS based

results have shown the validity of the proposed controller.

ACKNOWLEDGMENT

The authors would like to acknowledge the support

provided by King Abdulaziz City for Science and

Technology (KACST) through the Science and Technology

Unit at King Fahd University of Petroleum and Minerals

(KFUPM) for funding this work through project No. 09-ENE-

773-04 as a part of the National Science, Technology and

Innovation Plan (NSTIP).

Appendix Parameter of a PV panel

Parameter Value

Open Circuit Voltage 21.7 V

Short circuit current 3.35 A

Voltage at PMAX 17.4 V

Current at PMAX 3.05 A

Series connected modules 50

Parallel connected modules 20

Number of PV cells in each model 36

Ideality factor of PV diode 1.5

Temperature Dependancy factor 3

Reference Temperature 25oc

Temperature Coefficient of ISC 0.065

Reference solar intensity 1000 W/m2

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