A Robust Control Approach to Smooth Output Power of Wind Units Using Hybrid Storage

Systems Considering State of Charge

Arash Jamehbozorg California State University, Los Angeles

Los Angeles, CA ajamehb@calstatela.edu

Yanzhu Ye, Ratnesh Sharma NEC Labs America, Inc.

Cupertino, CA yanzhuye, ratnesh@nec-labs.com

1

Integration of Wind Units

• Main Challenges:

– Intermittent nature of wind speed

– Output power fluctuation of wind units

– Sound pollution

– Aesthetic issue

– Etc.

2

Output power smoothing of wind units using storage systems

Hybrid Storage Systems

• Types of storage units

– High power, low energy: Supercapacitor (EDLC), SMES, Flywheel

– High Energy, low power: Batteries

• One of the decisive factors affecting the lifetime of storage units is State of Charge (SoC).

• It has been shown that when a storage unit operates at SoC between 30%-90%, its lifetime increases.

3

Modeling

• A SCIG wind turbine parallel to a hybrid storage unit is connected to infinite bus

4

Modeling

• Modeling of the battery

𝑣𝑏𝑒 = 𝑉𝑜 − 𝑅𝑜𝑖𝑏𝑒 − 𝐾𝑄

𝑄 − 𝑖𝑏𝑒𝑑𝑡+ 𝐴. exp −𝐵 𝑖𝑏𝑒𝑑𝑡

𝑆𝑜𝐶𝑏𝑒 = 100 1 − 𝑖𝑏𝑒𝑑𝑡

𝑄%

5

Modeling

• Modeling of EDLC

𝑆𝑜𝐶𝑠𝑐 = 100𝑉𝑠𝑐𝑉𝑠𝑐,𝑛

2

%

6

Modeling

• DC/AC Converter

7

Proposed Control Strategy

Mode I (Normal Mode)

• When SoC of the battery and EDLC are both inside the acceptable range, this mode is used. In this mode, the control strategy of each converter is as follows:

1. DC/DC converter of the EDLC regulates the DC-link voltage of HSS (𝑉𝑑𝑐).

2. DC/DC converter of the battery continuously keeps SoC of the EDLC (𝑆𝑜𝐶𝑠𝑐) in range.

3. DC/AC converter controls the output active power of HSS (𝑃ℎ𝑠𝑠) to smooth the output power of wind unit and reactive power of HSS (𝑄ℎ𝑠𝑠).

8

Proposed Control Strategy

Mode I (Normal Mode)

𝐼𝑞,𝑟𝑒𝑓 =2

3.𝑃ℎ𝑠𝑠,𝑟𝑒𝑓

𝑉𝑞𝑔 𝐼𝑑,𝑟𝑒𝑓 = −

2

3.𝑄ℎ𝑠𝑠,𝑟𝑒𝑓

𝑉𝑞𝑔

9

10 20 30 40 50 60 70 80 90 100-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

SOCsc

[%]

I be

,re

f [pu]

Proposed Control Strategy

Mode II (𝑆𝑜𝐶𝑠𝑐 is out of range)

• When SoC of EDLC is out of range, this mode is activated. In this mode the control strategies are as follows:

1. DC/DC converter of EDLC charges/discharges the EDLC based on the SoC of EDLC in order to bring SoC of EDLC back to normal range. A constant current equal to 60% of nominal current is used as EDLC reference current (Isc,ref) .

2. DC/DC converter of battery regulates DC-link voltage.

3. DC/AC converter controls the output active power of HSS to smooth the output power of wind unit and reactive power of HSS (same as Mode I).

10

Proposed Control Strategy

Mode III (𝑺𝒐𝑪𝒃𝒆 is out of range)

• When SoC of battery is out of range Mode III is activated. In this mode control strategies are as follows:

1. DC/DC converter of the EDLC regulates the DC-link voltage of HSS (𝑉𝑑𝑐) (Same as Mode I).

2. DC/DC converter of battery charges/discharges the battery based on the SoC of battery in order to bring the SoC of battery back to normal range. A constant current is used as battery reference current (𝐼𝑏𝑒,𝑟𝑒𝑓).

3. DC/AC converter controls the output active power of HSS to smooth the output power of wind unit and reactive power of HSS (same as Mode I).

11

Proposed Control Strategy

Mode IV (𝑆𝑜𝐶𝑠𝑐 and 𝑆𝑜𝐶𝑏𝑒 are out of range)

• When SoC of both EDLC and battery are out of range this mode is activated. This mode is the only one in which HSS cannot smooth the output power of the wind unit. Control strategies of HSS in this mode are:

1. DC/DC converter of EDLC charges/discharges the EDLC based on the SoC of EDLC to bring the SoC of EDLC back to normal range. A constant current is used as EDLC reference current (Isc,ref).

2. DC/DC converter of battery charges/discharges the battery based on the SoC of battery to bring the SoC of battery back to normal range. A constant current is used as battery reference current (𝐼𝑏𝑒,𝑟𝑒𝑓).

3. DC/AC converter regulates the DC-link voltage for safe operation of HSS, and controls the output reactive power of HSS.

12

Proposed Control Strategy

• State diagram of control system of HSS

13

Simulation Results

• Acceptable range of SoC is defined to be 35% < 𝑆𝑜𝐶 < 95% for both EDLC and battery.

14

0 20 40 60 80 1007

8

9

10

11

12

13

t [sec]

Vw

[m/s

]

Simulation Results

15

0 20 40 60 80 1000.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

2.4

t [sec]

P [p

u]

Pwind

Ptot

0 20 40 60 80 100-1

-0.5

0

0.5

1

t [sec]

PS

C [p

u]

0 10 20 30 40 50 60 70 80 90 100-0.5

0

0.5

1

t [sec]

PB

at [p

u]

0 10 20 30 40 50 60 70 80 90 1001

2

3

4

t [sec]

Con

trol M

ode

Simulation Results

16

0 20 40 60 80 10020

40

60

80

100

t [sec]

SoC

SC

(%)

0 10 20 30 40 50 60 70 80 90 10040

60

80

100

t [sec]

SO

CB

at (%

)

Simulation Results

Control Mode Activation Time (%)

Mode I 96.20

Mode II 2.40

Mode III 0.79

Mode IV 0.61

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Conclusions

• A new algorithm is presented to control hybrid storage system

• State of charges of both battery and supercapacitor are considered

• Results show that the proposed algorithm can successfully smooth the output power of wind unit in more than 99% of the times

• Using this algorithm, a smaller size of hybrid storage system can be used

18