A Novel Control Strategy for Hybrid Energy Storage System to Relieve Battery Stress

Fangcheng. Liu, Jinjun. Liu and Linyuan. Zhou School of Electrical Engineering, Xi’an Jiaotong University

28 West Xianning Road, Xi’an, Shaanxi 710049 China

Abstract- Energy storage system in wind power system is required to deal with the difference of power between generator side and load side. However, single storage element can not fulfill this goal ideally. Then, Hybrid energy storage system (HESS) is proposed that which combines two (or more) different kinds of storage elements has more superior performance than single storage element. In this paper, a topology of hybrid energy storage system which consists of a battery and a super capacitor is used. Besides, a corresponding control strategy is proposed, which use moving average filter (MAF) to substitute traditional low pass filter to distinguish the DC and AC component of the imbalance power and deliver the DC part to the battery and absorb the AC part to the super capacitor. It makes full use of the energy storage elements and the battery life is extended.

Index Terms- Hybrid energy storage system;

battery; super capacitor; control strategy;

I. INTRODUCTION Among all the renewable resources, wind power have

being widespread concerned from all countries for renewable energy projects. But it also has serious problem because wind speed is intermittent and fluctuates frequently, and that would influence the system stability significantly. Integrating energy storage system to the wind power system can enhance the stability and reliability of the whole power system. Many storage technologies have been proposed and testified for the validity. Such as battery [1], super capacitor [2], flywheel and superconducting magnetic energy storage (SMES) [3]. However, only one storage element can’t enhance the system stability ideally.

Several topologies and control strategies have been proposed that combine super capacitor to improve the condition of battery storage system, and they have already proved its validation. But there still have some points need to be improved. For example, the DC side voltage is out of control that would affect normal operation both generator side converter and grid side converter [4], and, the battery current reference would contain certain AC component that would increase the charging/discharging cycles [5], the current flow through

This paper and its related research are supported by grants from the Power Electronics Science and Education Development Program of Delta Environmental & Educational Foundation and National Key Basic Research Program of China (2009CB219705).

battery still fluctuate quickly when used in high power situation [6] that would influence the battery life dramatically.

The fluctuate power can be distinguished into DC component and AC component with the help of filter. The DC component indicates the energy requirement of the whole system. If super capacitor is used to compensate a large part of the energy, the total number of super capacitor is increased enormously.

Comparing to super capacitor, battery is more suitable to afford the energy for quite a long time due to its large capacity. But battery is failed to provide the AC component, one reason is the battery usually response very slowly, so the whole performance is effected. Another reason is that the high frequency part of power would increase the cycle number of charge and discharge to battery. However, super capacitor could response very fast, and have infinite cycle times comparing to battery. Therefore, the super capacitor is suitable to afford the AC component.

From the point of view of the system performance, the basic idea is delivering the DC part to battery and delivering the AC part to super capacitor.

In this paper, a topology of hybrid energy storage system is proposed. To prolong the battery life besides deal with the excessive energy, a moving average filter (MAF) is used to calculate the DC component of the fluctuate power then make it as the reference of battery side converter in stead of the traditional LPF. Both the battery and the super capacitor are connected to the DC side through bidirectional Buck/Boost converters. Concerned about that current flow through battery should be smoothed as possible to prolong the battery life and the terminal voltage is brought down to reduce the cost, the inductors are connected to the storage elements. The super capacitor is controlled to maintain the DC bus voltage. Section III presents the structure of hybrid energy storage system and control strategy. In section IV, simulation results are presented to testify the validity of the proposed system.

II. CONSIDERATION ABOUT HYBRID SYSTEM

A. Storage density The characteristics between energy storage elements

are quite different. From the angle of cost and the maturity of technology, two major elements are presented in Ragone chart with ordinary electrolytic capacitor in Fig.1. It is shown that the battery can store larger energy than supercapacitor if the weights of them are the same.

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And also, the supercapacitor can deal with more power fluctuation than the battery if the weights are the same.

Combining different storage elements together can enhance the whole storage density. Both the ability of absorbing power and storing energy can be maximized when referring to some very strict constraint in volume and weight.

Ener

gy D

ensit

y (W

h/kg

)

Fig.1. Ragone Chart

B. Battery life and efficiency The battery life can be influenced by quite a lot factors.

The more energy discharged by the battery, the less cycle times available, as shown in Fig.2. When a supercapacitor is added to the whole storage system, the peak power can be handled by the supercapacitor as specific control purpose, and then the power provided by the battery is decreased, so the depth of discharge (DOD) is also decreased and more cycle times are available.

10

100

1000

0% 25% 50% 75% 100%Depth of discharge

Fig.2. Relationship between DOD and available cycle times

There is strict relationship between discharging time and discharging current of battery [7] [8]. That’s the very famous Peukert’s law, as follows:

N

pC I t= (1)

Where, Cp is the nominal capacity of battery, expressed in A·h,

I is the discharging current, expressed in A; N is Peukert constant, dimensionless; t is discharging time, expressed in h. Only for an ideal battery, the constant N would equal

one. In this case the actual capacity would be independent of the current. But for actual battery, the constant N is

always larger than 1, So if the battery is discharged at high current rate, it can be concluded that the actual capacity discharged is less than the nominal capacity obviously , there is certain power loss in battery, and it would influence the efficiency of the whole system.

When very large charging current is applied to the battery, there would be some hydrogen and oxygen gas given off due to that the intrinsic electrochemistry reaction could not response fast enough. And the generated gas would damage the active material in the plate, so that the battery would lose effectiveness [9].

It can be concluded from the above analysis that the main control principle is to control the current flow through battery as smooth as possible.

III. CONFIGURATION AND CONTROL STRUCTURE

A. System configuration The system under investigate is shown in Fig.3. The

generator side converter is usually controlled according to MPPT (Maximum Power Point Tracking) and the grid side converter is controlled to supply energy to main grid or micro grid. It’s not concerned how these two converters are controlled in this design. So a current source is used to imitate the imbalanced power between generator and load. The detailed topology is shown in Fig.4.

Figure.3. Wind power system with HESS

Figure. 4. Configuration of the proposed system

B. Moving average filter As traditional design, a LPF is used to extract the DC

component of the imbalanced power between generator and load, which equals the DC component of the current source if the DC side voltage keeps constant in this design.

But single LPF would bring down the bandwidth of whole control loop, which would response very slowly. The dynamic performance would not achieve the design aim. So a MAF (moving average filter) is considered that would improve the filter performance.

The basic principle of a MAF is shown in Fig.5. When

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the filter works, the algorithm would set to calculate the average value of the data in buffer, as a new sampling datum is sent to the buffer, the earliest datum is overwritten by the latest one, then the filter output the new calculated average value of the buffer.

Providing that the time span of the sliding window is T, the sampling number during this span is N, and a buffer that could store data of N points. The data array x(n) (n=0,1,2…N-1) is stored in the buffer, then the sliding window reaches the point x=l, the data before point x=l-N are overwritten, new average value is output. The mathematic algorithm is :

1( ) [ * ( 1) ( ) ( )]X l N X l x l N x lN

= − − − + (2)

n

n

x(n)

N

N

Fig.5. Moving average filter principle

In order to comparison the MAF with the LPF, a simulation test is carried out. A mixed signal combined a constant magnitude with different order harmonics is sent to MAF and LPF separately, and the constant value of the signal is change from 15 to -15 at t=25s. The LPF is 5 orders Butterworth filter and the cut-off frequency is 0.25Hz, and the time span of the sliding window in MAF is set as 1s. The result is shown in Fig.6.

Fig.6. Output of MAF and LPF with mixed signal

The top figure in Fig.6 is the mixed signal, the fluctuating value is 40sin 2 30sin10 30sin14t t tπ π π+ + , and the constant is changed. Besides, the middle figure is the output of LPF, and the bottom is the output of MAF. It

can be concluded that the MAF calculate the DC component faster than the LPF. This could prove that the MAF have better dynamic performance than LPF.

C. Storage elements converter The DC side current of Buck/Boost converter is

discontinuous, so integral blocks are used to calculate the average value of the reference and the controlled current. It is also concerned that current flow through battery is smoothed as possible to prolong the battery life and as small as possible when concerning the efficiency of battery [7] [8].The control block is shown in Fig.7.

Figure.7. Control diagram of battery side converter

The Buck/Boost converter connected to the super

capacitor is used to maintain the DC bus voltage. The basic principle is that if the DC component is absorbed by the battery, then the AC component would cause the DC bus voltage change. If the super capacitor is used to maintain the DC voltage, it is equal to use super capacitor to deal with the AC component. The main benefit is that the terminal voltage of super capacitor stack is half of DC bus voltage when the bidirectional energy transfer operation is ensured, so less capacitors will be installed, the whole cost is saved. The control block is shown in Fig.8.

Figure.8. Control diagram of SC side converter

IV. SIMULATION AND RESULTS In order to verify the analysis and specification, the

simulation investigation is carried out. The DC side equivalent capacitor is set to 6.6mF. Besides, 200F super capacitor and 100Ah battery are selected to verify this control strategy. In addition, the DC side voltage is required to maintain at 300V. The terminal voltage of super capacitor is 150V and battery terminal voltage is also 150V.

Fig.9. Single battery storage system

At first, a single battery storage system without

supercapacitor would be simulated to make comparison between the proposed hybrid energy storage systems, the configuration is shown in Fig.9 and the control strategy is

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shown in Fig.10. The imbalanced current source is set as 15 40sin 2 30sin10 30sin14imbalanceI t t tπ π π= − + + + (A).The

simulation results are shown in Fig.11.

Fig.10. Control diagram of Single battery storage system

The DC bus voltage is maintained around 300V, that’s to say the single battery storage system operates well,

Little surplus power is delivered to the DC bus equivalent capacitor. But the battery current fluctuates as the imbalance current changes. It can be known that the batteries charge and discharge a lot, and the magnitude of battery current is very large, the whole system efficiency decreases and the battery life is shorten.

A model of the proposed hybrid energy storage system as shown in Fig.4 is also established to testify the validity. A same current source as the single system is also used in order to check whether the whole system operates well or not. The simulation results are shown in Fig.12.

(a). Current source waveform (b). DC bus voltage (c). Battery current waveform

Fig.11. Simulation results of single battery energy storage system with specific current source

(a). Current source waveform (b). DC component of current source (c). Actual battery current

(d). DC bus voltage (e). Controlled current (f). Enlargement of controlled current

Fig.12. Simulation results of proposed hybrid energy storage system with specific current source

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(a). Current source waveform (b). DC component of current source (c). Actual battery current

(d). DC bus voltage (e). Controlled current (f). Enlargement of controlled current

Fig.13. Simulation results of proposed hybrid energy storage system with random current source

It can be seen the DC component of the fluctuant current is extracted by LPF very well, and the average value of controlled current almost equal to the DC component value well as intended. Meanwhile, the DC side component is maintained at 300V as designed, although there is little ripple. The battery current is smoothed, and then the cycle times decrease. The value of battery current is almost double as the DC component of source, that’s because the battery terminal voltage is half as the DC bus voltage in this design, the total power is equal. So the whole system is operating well as designed.

At last, a random current source is used to testify the variation between different modes of battery operation. It is proved that the circuit is worked as intended. The simulation results are presented in Fig.13.

V. CONCLUSION In this paper a hybrid energy storage system in wind

power system and corresponding control strategy to relieve battery stress is proposed. The battery operation condition is improved a lot in this system. As the MAF is put into use to calculate the DC component of fluctuate power, the battery just need to compensate energy regardless of power fluctuation and the current is smoothed. So the cycle times of battery decrease and the efficiency of battery is enhanced. That also means the battery stress is relieved a lot. The DC bus voltage is maintained by super capacitor to make sure there would be no damage to control of the grid and turbine side converters. The simulation has been done to confirm the validity. In addition, this system can be used in

distributed generation systems not only wind power system and this would be concluded through detailed analysis.

REFERENCES [1] Divya. K.C ,Ostergaard.Jacob, “Battery energy storage

technology for power systems—An overview,” Electric Power Systems Research , 2009, 79 (4) page(s) : 511-520.

[2] Juanhua Wang, Jiancheng Zhang, Yun Zhong, “Study on a Super Capacitor Energy Storage System for Improving the Operating Stability of Distributed Generation System,” Electric Utility Deregulation and Restructuring and Power Technologies, 2008. DRPT 2008. Third International Conference on, 6-9 April 2008, page(s): 2702-2706.

.[3] Changjin Liu, Changsheng Hu, Xiao Li, Yi Chen, Min Chen, Dehong Xu, “ Applying SMES to smooth short-term power fluctuations in wind farms” , Industrial Electronics, 2008. IECON 2008. 34th Annual Conference of IEEE, 10-13 Nov. 2008, page(s): 3352-3357.

[4] Y.Jia, R.Shibata, N.Yamamura, M.Ishida, “Smoothed-Power Output Supply System for Battery of Stand-alone Renewable Power System Using EDLC,” Power Electronics and Motion Control Conference, 2006. IPEMC 2006, vol 3, Aug. 2006,page(s): 1-5.

[5] Garcia, F.S. Ferreira, A.A. Pomilio, J.A. “Control Strategy for Battery-Ultracapacitor Hybrid Energy Storage System,” Applied Power Electronics Conference and Exposition, 2009. APEC 2009. Twenty-Fourth Annual IEEE, 15-19 Feb. 2009 ,page(s): 826 - 832

[6] Yu Zhang, Zhenhua Jiang ,Xunwei Yu, “Control Strategies for Battery-Supercapacitor Hybrid Energy Storage Systems,” Energy 2030 Conference, 2008. ENERGY 2008. IEEE, 17-18 Nov. 2008, page(s): 1-6.

[7] A. M. van Voorden, Laura M.Ramirez.Elizondo,

933

Gerard.C.Paap, Jody.Verboomen, Lou.van.der.Sluis, “The Application of Super Capacitors to relieve Battery-storage systems in Autonomous Renewable Energy Systems,” Power Tech, 2007 IEEE Lausanne, 1-5 July 2007 ,page(s): 479 – 484.

[8] D. Doerffel, S.A. Sharkh, “A critical review of using the Peukert equation for determining the remaining capacity of lead-acid and lithium-ion batteries”, Journal of Power Sources, Vol.155(No.2) ,2006, page(s):395–400.

[9] Xin Zheng, “Research on Charging and Regenerative Technology of Lead-Acid Battery” [D], Chengdu: Southwest Jiaotong University, 2008.

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