battery super capacitor hess

8/12/2019 Battery Super Capacitor HESS

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A Power Electronic Interface for a BatterySupercapacitor Hybrid Energy Storage

System for Wind Applications

Wei Li and Gza JosMcGill University, Department of Electrical and Computer Engineering,

Room 633, McConnell Engineering Building, 3480 University, Montreal, Quebec, Canada, H3A 2A7

E-mail: [email protected], [email protected].

Abstract-An energy storage system (ESS) in a wind farm is

required to be able to absorb wind power surges during gusts,

and have sufficient energy storage capacity to level wind

fluctuations lasting for longer periods. ESS using a single

technology, such as batteries, or supercapacitors, will havedifficulties providing both large power and energy capacities.

This paper proposes a flow-battery supercapacitor hybrid ESS,

which takes advantage of the two complementary technologies to

provide large power and energy capacities. The flow-battery is

directly coupled to the WTG dc bus while the supercapacitor has

a dc/dc IGBT converter interface. The dc bus voltage varies

within a certain limit determined by the variable battery

terminal voltage. With the supercapacitor absorbing high

frequency power surges, the battery power rating, degree of

discharge, and power losses are all reduced. Therefore the

battery in the hybrid ESS has low cost and high longevity; and

the system overall efficiency is improved.

Key words- wind energy, wind turbine generator (WTG), energy

storage system (ESS), battery energy storage system (BESS), flowbattery, supercapacitor.

I. INTRODUCTION

Wind power fluctuates and therefore adversely impacts

power systems. Integration of energy storage systems (ESS)

into wind farms is a promising solution to manage wind power.

For near-to-midterm (seconds to minutes) power management,

the most commonly implemented storage technologies in a

wind farm are battery energy storage system (BESS),

advanced capacitors and supercapacitors, flywheel energy

storage (FES), and superconducting magnetic energy storage

(SMES).

In wind power applications, an ESS is required to have alarge power capacity to absorb power surges during wind

gusts, and also a large energy capacity for deep wind

fluctuations lasting for minutes or longer. The power and

energy ranges of the four technologies are projected in Figure

1, [1]. The battery technology provides a high energy capacity

but low power, while on the other end the supercapacitor has a

high power capacity but is short in duration. The flywheel and

SMES fall somewhere in the middle. None of the four single

ESS technologies fit themselves well into the wind application

requirement: having both high energy and power capacities.

A battery-supercapacitor hybrid ESS will take advantages

of both technologies and provide high power and energy

capacities. The power and energy ranges of a hybrid ESS in

Figure 1 will have a larger area spanned by the capacitor and

batteries, covering the ranges of all four single technologies.

The longevity of batteries is significantly affected by the

depth of discharge (DOD), a percentage of battery energyhaving been withdrawn. High DOD will decrease the battery

life cycle number and shorten its lifetime. In a hybrid ESS, the

supercapacitor will absorb power surges and decrease the

DOD of the battery.

Flow battery can provide large energy storage. Its energy

and power capacities are independent: the energy capacity

depends on the tank size, while the power rating depends on

the cell stack size and flow rating for pumps and piping. Flow

battery alone is not cost-effective for high power applications.

With assistance of the supercapacitor, the battery power rating

in the hybrid ESS can be reduced. And therefore the battery

cost is reduced.Furthermore, the supercapacitor can improve the overall

efficiency of the storage system.

ESS of battery and supercapacitor technologies for wind

applications are studied in the following literatures. Modelling

and design of a Vanadium-Redox flow battery (VRB) in wind

farm is given in [2] and [3]. A state-of-charge feed back

control system for VRB storage in a wind farm is proposed in

[4]. In [5]-[7], BESS in a wind farm is used to improve power

quality and wind farm stability. Studies on supercapacitor

modelling and efficiency are provided in [8]. The applications

of supercapacitor storage in wind farms are given in [9]-[11].

Figure 1. Specific power versus specific energy ranges for ESS technologies.

101102103104105106107108

1 10 100 1000

Energy Wh/kg

PowerW/kg Advanced

capacitors

SMES Flywheel

Batteries

978-1-4244-1668-4/08/$25.00 2008 IEEE


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II. MODELLING OF FLOW BATTERY AND SUPERCAPACITOR

The characteristics of the main battery storage and

supercapacitor technologies are shown in Table 1. Compared

to other batteries, the flow battery is a better candidate for

wind applications due to its many advantages, including high

lifecycle, high storage efficiency, a wide range of poweroutputs, and low maintenance cost. However its life cycle and

efficiency are still low compared to the supercapacitor

technology.

TABLE ICHARACTERISTICS OF BATTERY AND SUPERCAPACITOR TECHNOLOGIES

STORAGETECHNOLOGY

LIFE CYCLEDC-DC

EFF.TIME SCALE

Flow Battery 10,000 cycles 75~80% Minutes- Hours

Lithium-Ion 3000 cycles 97%Seconds-

Minutes

Sodium-Sulfur (NaS) 2250 cycles 89% Minutes-Hours

supercapacitor 106~108cycles 86~ 98%Seconds -

Minutes

A. Vanadium Redox Battery Modelling

A Vanadium-Redox flow battery (VRB) model is given in

Figure 2. In this model, Rreaction and Rresistive are internal

resistance, which account for the losses due to reaction

kinetics, mass transport resistance, membrane resistance, and

other losses inside the cell stack. The parasitic losses are used

to account for the recirculation pumps and the cell-stack by-

pass currents. The state of charge (SOC), the percentage of

available energy remaining in a battery, is modelled as theintegral of the battery internal stack power. The battery

internal stack voltage and the recirculation pump current are

directly related to the battery SOC.

In Japan, VRB storage systems with various capacities,

from 179kW*8h to 1.5MW*1h, are installed to test for a

variety of applications [12]. In this paper, the rated current of

the VRB model is 800A. The VRB has 500 series cell stacks,

with a nominal output voltage of 700V (50% SOC and zero

current) and a minimum voltage of 525V. The VRB power

rating is 420kW, which is the current rating multiplied by the

minimum voltage. The VRB has an approximate 79%

efficiency, which is 15% internal losses and 6% parasitic, at

the operating point of 20% SOC and rated discharging current.

The model parameters are list in the Appendix.

In reality the VRB voltage varies linearly in the 20% to

80% SOC region. The voltage-current (VI) transfer

characteristic of this VRB model is given in Figure 3(a). The

VRB voltage increases as SOC increases and as the charging

current increases. When the VRB works in the 20% to 80%

SOC linear region, it has a maximum 19% boost, or 835V at

80% SOC and 800A charging current, and a maximum 19%

dip, or 565V at 20% SOC and 800A discharging current,

compared to its nominal voltage (700V at 50% SOC and zero

current).

Vba

ttery

Figure 2. VRB battery model.

The VRB efficiency is given in Figure 3(b). The battery

charge efficiency is defined as the ratio of the stored

electrochemistry power over the input power, while the

discharge efficiency is the ratio of the output power over the

released electrochemistry power. The efficiency is around

80% in most operating conditions; and drops as the battery

current is lower where the parasitic losses become a

significant portion.

The dynamic response of the VRB is verified at the

transitions when the VRB current changes from rated charging

current (800A) to rated discharging current (-800A) and the

vice versa, Figure 4. The VRB output voltage takes 6ms to

reach the steady state. In wind power applications, the wind

turbine has an inertial constant on the order of seconds, which

filters out wind fluctuations at higher frequencies. Therefore

the VRB response time is more than sufficient to manage the

wind power.

-800 -400 0 400 800500

600

700

800

900

Current A

Voltage(V)

SOC=80%

SOC=50%

SOC=20%

Chargedischarge

Idle Battery

(a) VI transfer characteristic

-800 -600 -400 -200 0 200 400 600 80050

60

70

80

90

100

Charge Current (A)

Efficiency(%)

SOC=80%

SOC=50%

SOC=20%

discharge charge

(b) Battery efficiency

Figure 3. VRB battery characteristics.


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0.01 0.015 0.02 0.025 0.03

500

600

700

800

Time (s)

BatteryVoltage(V)

Max charge to

max discharge

response time

Max discharge

to max charge

response time

Figure 4. VRB battery dynamic response.

B. Supercapacitor Modelling

The supercapacitor is modeled by an ideal capacitance and

an equivalent series resistance (ESR), Figure 5. The ESR

accounts for resistive losses in the dielectric, plate material,

and electrolytic solution. The actual capacitance and ESR are

dependent on terminal voltage, voltage charge rate, current,and temperature. However within the capacitors working

region, fixed value capacitance and ESR can accurately model

a real supercapacitor.

Figure 5. Supercapacitor model.

-30 -20 -10 0 10 20 300

100

200

300

400

Current (A)

Efficiency(%)

SOC=100%

SOC=70%

SOC=40%

SOC=10%

ChargeDischarge

(a) VI transfer characteristic

-30 -20 -10 0 10 20 300.8

0.85

0.9

0.95

1

Current (A)

Efficiency(%

)

SOC=100%

SOC=70%

SOC=40%

SOC=10%chargedischarge

(b) Supercapacitor charge and discharge efficiency

Figure 6. Supercapacitor characteristics.

In this paper, the model is based on a 0.58F, 400V

supercapacitor. The ESR provided by the manufacturer is

0.6, which is measured and verified in [8]. The maximum

charging or discharging current is 33.3A. The supercapacitor

works in the 10% to 100% SOC region. The voltage-current

(VI) transfer characteristic and the efficiency of thesupercapacitor are derived from the model, Figure 6.

Compared to the VRB battery, the terminal voltage of the

supercapacitor has smaller variation, 5%, during charge and

discharge at a fixed SOC. However the supercapacitor has a

much larger voltage variation when the SOC varies. The

supercapacitor has a higher efficiency than the flow-battery,

generally above 92% if the SOC is higher than 40%.

The supercapacitor has a very rapid dynamic response. As it

is switched from the maximum charging current (33.3A) to the

maximum discharging current (-33.3A), or the vice versa, the

output voltage simultaneously reaches its steady state.

III. WIND TURBINE AND ESS INTEGRATION

A. ESS Topologies

The supercapacitor and battery are voltage sources, and thus

are usually connected to the WTG dc bus through dc/dc

converters. A typical topology of a VRB BESS and WTG is

given in Figure 7 (a). A permanent magnet synchronous

machine (PMSM) is connected to the grid through a full rated

ac/dc/ac converter. The VRB storage is installed in the dc bus

with a bidirectional dc/dc IGBT chopper interface. The VRB

storage power and energy is controlled by the dc/dc chopper.

To have a hybrid ESS in the WTG, a straightforward way is

to have two dc/dc choppers, which connect the supercapacitor

and the battery storage respectively to the dc bus, Figure 7 (b).

The powers of the supercapacitor and the battery are

controlled separately by the two choppers, and their references

are managed and coordinated by an ESS management

algorithm.

In this paper, an alternative topology is proposed, where the

battery is directly coupled to the WTG dc bus, Figure 7 (c). In

this topology, the power of the battery storage cannot be

directly controlled, but is rather determined by the

combination of the powers generated from the generator,

supplied to the grid, contributed from the supercapacitor

storage device, and power losses. The dc bus voltage varieswithin a certain limit due to the varying VRB voltage as a

function of its SOC and current. However the apparent

advantage of this topology is that it saves one dc/dc chopper

and its associated losses, which is a key parameter in ESS.


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(a) A VRB ESS integrated in a PMSM WTG

(b) A two-chopper Hybrid ESS in a PMWM WTG

(c) The hybrid ESS with VRB directly coupled to dc bus.

Figure 7. Topologies of VRB ESS and Hybrid ESS in a PMWM WTG

B. ESS Controls

The ESS power and energy is regulated by the ESS

management algorithm through the control of the power

passing through the power electronic interfaces of the ESS and

WTG. The major task of the ESS management algorithm is to

manage the output wind power according to certain

specifications. The ESS management algorithm can be either a

simple one based on a high pass filter, or a complicated one

using fussy logic method.

In this paper, a simple ESS management algorithm is used

for the demonstration purpose. The ESS power, PESS, is

regulated as the result of the wind power, Pw, passing through

a high pass filter, as in (1) . Therefore the majority of the wind

power fluctuations with the frequencies higher than 0.02Hz

are absorbed by the ESS, leaving a smoother output power

only containing lower frequency fluctuations.

wESS Ps

sP

501

50

+= , (1)

In the hybrid ESS, the higher frequency part of the power

fluctuations are absorbed by the supercapacitor. The

supercapacitor power, PESS_cap, is controlled as in (2). The

VRB takes care of the rest part of power fluctuations being

absorbed by the ESS.

wcapESS Ps

sP

101

10_

+= , (2)

This ESS management algorithm does not account for

power losses in the energy storage. With power losses, theSOC of the ESS gradually decreases over time. In practice, the

algorithm has to compensate for the power losses to prevent

complete discharge of the energy in the storage. The control

schemes of the ESS and WTG power electronic interface for

the three ESSs are shown in Figure 8.

(a) Control of the reference VRB ESS

(b) Control of the two-chopper Hybrid ESS.

(c) Control of the hybrid ESS with VRB directly coupled to dc bus.

Figure 8. Schemes of the ESS management algorithm.


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C. WTG and ESS Parameters

The PMSM WTG power rating is 2MW, the dc bus is

1400V, and the grid side ac voltage is 690V. The VRB and

supercapacitor storages are scaled up using the basic units

modeled in the previous section.

The VRB storage has two 420kW VRB units. The ratedpower is 840kW, and the full power duration time is 60s, or

the energy capacity is 50.4MJ (=840kW*60s). The two VRB

units are installed in parallel when they are connected to the

dc bus through a chopper, and in series when they are directly

coupled to the dc bus.

The supercapacitor storage has 250 0.58F supercapacitor

units, two units in one stack with 125 parallel stacks. For each

unit, the voltage works between 0.3pu to 1pu (400V) and the

maximum current is 33.3A. Each unit has an effective energy

capacity of 42.2kJ, and a maximum power of 4kW at 0.3pu

(120V) voltage. The ratings of the total supercapacitor storage

are 1MW, 10.56MJ, 800V.

D. Operation Constraints for Directly-coupled VRB ESS

In the directly-coupled VRB ESS, the dc bus voltage equals

the VRB voltage and therefore varies. The variations of the dc

bus voltage have to be constrained within certain limits. The

upper dc voltage limit has to respect the ratings of the ac/dc

converters: a higher maximum dc voltage will result in

increased rating of the power electronics in the converters. A

lower dc voltage will increase the PWM modulation index ma,

given in (3) where Vacis the phase-phase rms ac voltage and

Vdc is the dc voltage. The PWM is usually working in the

linear modulation region, where the modulation index ma is

less than one.

dc

ac

aV

Vm

3

8= , (3)

In this paper, the dc voltage variation is limited between

15% and +10%. When the dc voltage assumes its minimum

value and the ac voltage is 1.05pu (725.5V), the modulation

index mais 0.99, less than 1.

-800 -400 0 400 8001100

1200

1300

1400

1500

1600

1700

Current (A)

Voltage(V)

SOC=80%

SOC=50%

SOC=20% Vmax

Vmin

Rateddischarging

power

Rated

charging

power

A B

C

D

E

F

ChargeDischarge

Figure 9. Constrained operation region for VRB directly coupled to dc bus.

The operation region of the VRB has to be constrained to

limit its voltage variation between -15% and +10%. The VRB

VI characteristic with constraints is given in Figure 9. The

lines AB and DE are the upper and lower voltage constraints,

the curves BC and EF are the charging and discharging power

rating, and the curves FA and CD are the maximum andminimum operating VRB SOC. The VRB has to work within

the region ABCDEFA.

As constrained by the voltage limits, the maximum charging

current (power) decreases as SOC increases above 50%, and

the maximum discharging current (power) decreases as SOC

decreases below 35%. The maximum charging power is

50.6% of the rated power when the SOC is 80%, and the

maximum discharging power is 79.0% when the SOC is 20%.

The VRB management algorithm has to regulate the VRB

SOC within its working region by decreasing the charging

power in the high SOC region and decreasing the discharging

power in the low SOC region. Therefore the dc voltage

variation limits does not greatly restrict the operation of the

VRB, considering that the control will generally reduce the

charging and discharging currents at these extremes anyways.

IV. ESS PERFORMANCE IN WIND APPLICATIONS

A reference VRB ESSFigure 7(a) and a hybrid ESS

with VRB directly coupled to the dc busFigure 7(c) are

tested in a benchmark of the same PMSM WTG and ESS

management algorithm introduced in the previous section.

The pre-filtered WTG power is given in Figure 10(a). The

average WTG power is 1.72MW. The ESS filtered WTG

power is superimposed in Figure 10(a) for a clear comparison.The wind power fluctuations with frequencies above 0.02Hz

are suppressed by the ESS.

The power, SOC, terminal voltage, and power losses of the

VRB in the two ESSs are listed in Figure 10(b-e). The power,

SOC, power losses of the supercapacitor in the hybrid ESS is

shown in Figure 10(f-h). The corresponding data are

summarized in Table II.

The two-chopper hybrid ESSFigure 7(b), is tested using

the same benchmark. The achieved results of power flowing

and storage efficiency are very close to that of the directly-

coupled-VRB hybrid ESS and therefore not repeated in this

paper. The two-chopper hybrid ESS has a constant dc busvoltage and one more dc/dc chopper compared to the directly-

coupled-VRB hybrid ESS.


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0 100 200 300 400 500 6000.8

1

1.2

1.4

1.6

1.8

2x 10

6

Time (s)

Win

dPower(W)

Pre-filtered

Filtered

(a) Pre-filtered (dotted) and post-filtered (solid) WTG powers.

0 100 200 300 400 500 600-8

-6

-4

-2

0

2

4

6x 10

5

Time (s)

VRBPower(W)

VRB ESS

Hybrid ESS

(b) VRB powers in the VRB ESS (dotted) and the Hybrid ESS (solid).

0 100 200 300 400 500 600

30

40

50

60

70

80

Time (s)

VRBSOC(%)

VRB ESS

Hybrid ESS

(c) VRB SOC in the VRB ESS (dotted) and the Hybrid ESS (solid).

0 100 200 300 400 500 6000.85

0.9

0.95

1

1.05

1.1

Time (s)

VRBvoltage(pu)

VRB ESS

Hybrid ESS

(d) VRB voltage in the VRB ESS (dotted) and the Hybrid ESS (solid).

0 100 200 300 400 500 6000

2

4

6

8

10

12

14x 10

4

Time (s)

VRBPowerLoss(w)

VRB ESS

Hybrid ESS

(e) VRB Power losses in the VRB ESS (dotted) and the Hybrid ESS (solid).

0 100 200 300 400 500 600-8

-6

-4

-2

0

2

4

6

8x 10

5

Time (s)

SupercapacitorPower(W)

(f) Supercapacitor power in the Hybrid ESS.

0 100 200 300 400 500 6000.2

0.4

0.6

0.8

1

Time (s)

Sup

ercapacitorSOC(%)

(g) Supercapacitor SOC in the Hybrid ESS..

0 100 200 300 400 500 6000

5000

10000

15000

Time (s)

SupercapacitorPowerLoss(W)

(h) Supercapacitor power losses in the Hybrid ESS.Figure 10. Performance characteristics of the VRB ESS and Hybrid ESS.

TABLE IIESS PERFORMANCE COMPARISON*

Reference VRB ESS Hybrid ESS

Maximum VRB Power

(output or input)0.333pu (666.0kW) 0.150pu (301.1kW)

Maximum depth of

discharge (DOD) of VRB67.2% 59.5%

Maximum voltagedeviation of VRB

-12.6% to 7.7% -4.8% to 5.5%

Average power losses

in VRB0.0259pu (51.8kW) 0.0215pu (42.9kW)

Maximum supercapacitor

power (output or input)- 0.326pu (651kW)

Maximum DOD of

supercapacitor- 73.4%

Average power losses in

supercapacitor- 0.0005pu (1.0kW)

Total average power

losses in ESS0.0259pu (51.8kW) 0.0220pu (43.9kW)

*Powers are per-unitized on the WTG rating, 2MW.


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The following differences between the two ESSs are

observed.

(a) The maximum VRB power in the hybrid ESS (301.1kW)

is less than half of that in the reference VRB ESS

(666.0kW) because the high frequency part of the power

fluctuations are absorbed by the supercapacitor. Thatmeans the VRB power rating in the hybrid ESS can be

significantly reduced, which leads to a reduction in the

VRB cost.

(b) The VRB depth of discharge (DOD) in the hybrid ESS is

generally 5% to 8% less than that in the reference VRB

ESS over the testing period, Figure 10(c). Therefore the

VRB in the hybrid ESS will have an increased life cycle.

(c) For the hybrid ESS, the VRB is directly coupled to the

dc bus. Therefore the dc bus will have the same voltage

variation as the VRB terminal voltage, which is between

-4.8% and 5.5%. In the reference VRB ESS, the dc bus

voltage remains at 1 pu although the VRB terminal has a

much larger variation, -12.6% to 7.7%.

(d) The total power losses in the hybrid ESS are reduced.

Considering the power losses in the energy storage only,

the total average power losses of the hybrid ESS is

43.9kW (42.9kW in the VRB and 1.0kW in the

supercapacitor), only 85% of the losses in the reference

VRB ESS (51.8kW).

V. CONCLUSIONS

A battery supercapacitor hybrid ESS can provide both high

power rating and high energy capacities and therefore is well

suited to wind energy applications.A topology of the hybrid ESS is proposed where the battery

is directly coupled to the WTG dc bus. The dc voltage varies

due to the variable battery terminal voltage. The dc voltage

variation limits introduce constraints, which restrict the range

of operation of the VRB.

The hybrid ESS requires an additional energy storage

device, in the form of supercapacitor. However compared to

the reference flow battery storage, which uses only a single

technology, it has the following merits:

Lower battery power rating (45% of the reference), and

thus lower battery cost,

Lower battery degree of discharge (5% to 8% less than

the reference), and thus longer battery longevity,

Lower total power losses in the storage (85% of the

reference), and thus higher overall system efficiency.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the contributions of C.

Abbey, from the McGill University Electric Energy Systems

Research Laboratory, to this work. They also acknowledge the

financial support of the Natural Sciences and Engineering

Research Council (NSERC) of Canada.

APPENDIX

Controllable variables and parameters of the Vanadium-Redox battery (VRB) model in Figure 2 are listed as below.

SOCt+1= SOCt+Vstack*Istack*t/Wbase

)1

ln0.051364.1(

+=

SOC

SOCnVstack

=

SOC

II stackpump

||0126.1

n(no. of cells) Rfixed Rreaction Rresistive Celectrodes

500 25.92 0.07476 0.04984 0.012F

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[3] M. -H. Li, T. Funaki and T. Hikihara, A Study of Output TerminalVoltage Modeling for Redox Flow Battery Based on Charge andDischarge Experiments, Fourth Power Conversion Conference, April,2007, pp.221-225.

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Volume 43, Issue 3, May-june 2007, pp. 769 776.[10] T.A. Smith, J.P. Mars, G.A. Turner, Using supercapacitors to improvebattery performance, IEEE 33rd Annual Power Electronics SpecialistsConference, PESC 02, June 2002 pp. 124 128, vol.1.

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