A Hybrid Cascaded Multi-level Converter for Power Storage System

Zedong Zheng1, Kui Wang1, Ling Peng2, Yongdong Li1, Lie Xu1 1. Tsinghua University,

Haidian district, Beijing, China Tel.: +86 / (10) – 62785481 Fax: +86 / (10) – 62772450

E-Mail: zzd@tsinghua.edu.cn URL: http://www.tsinghua.edu.cn

2. China ship development and design center, Wuhan, Hubei province, China

Acknowledgements This work is funded by the national nature science foundation of China (NSFC) 51107066.

Keywords « energy storage », « voltage equalization », « cascaded multilevel converter », «battery charger», «battery management systems »

Abstract Power storage system using battery and super-capacitor are more and more popular in renewable energy, smart grid and electrical vehicles. But the voltage and current of the basic battery cells and super-capacitor are limited. So in practical applications, a large number of cells are connected in seriously or parallel to improve the output voltage and current ability. Due to the differences between the cells, voltage and current equalizations are necessary to prevent overcharge or over-discharge on some cells. In this paper, a novel hybrid cascaded multi-level converter is proposed to solve the voltage equalization problems. The battery cells are connected by the cascaded half-bridges instead of being connected directly. The charge and discharge of the battery cells can be controlled separately and then the voltage equalization can be realized. Some experimental results will be given to verify the effects of the proposed converter.

Introduction As the development of the renewable energy generation, power storage systems are more and more used to smooth the power fluctuations produced by the random of the wind or solar energy[1-2]. Power storage system is also a very important part in the future smart grid and electric vehicles [3]. The power storage system used in these systems should have large power volume, high voltage and large current. But the output voltage and current of single battery and super-capacitor cells are limited. So in practical applications, a large number of battery cells are usually connected in serious and parallel to form a large power storage system. Sine there are more or less some differences between the cells, for example, the variability of manufacturing, the difference on cell architecture , the voltage and current equalization method must be used in practical applications. Even if the battery cells used are screened to be as consistent as possible before use, some differences will appear with the use of battery cells. In the application of battery cells in serious, the summarized terminal voltage is usually used to determine the end of charge and discharge. If there are some unbalances between the terminal voltages, even if the summarized terminal voltage is still in normal range, the terminal voltage on some cells may exceed of its limitation and these cells will be damaged by overcharge or over-discharge. So the voltage equalization circuit is necessary in seriously connected system. Some voltage equalization circuits are proposed in [4-6]. The first kinds use the resistance in parallel with the battery cells. The cell with higher terminal voltage will be discharged by the parallel resistance until its voltage is equal to the others. So the over-charge and over-discharge of battery cell

can be avoided. But the efficiency of this kind circuit is very low as the energy loss on the resistance. The other kind circuit can realize terminal voltage and state-of-charge (SOC) balance by transferring energy among the battery cells by some power electronics converters shown in Fig.1. But the circuits and their control are very complex. When used in power grid or electric vehicles, another inverter is also needed to connect with the AC loads or AC sources as shown in Fig. 2. A two-level inverter is used to convert the DC voltage to AC. Compared to the two-level inverter shown in Fig.2, the multi-level converters are more and more used which can output a higher voltage with less dv/dt as the increase of the voltage steps. In the popular multi-level converters, the voltage steps are produced by the flying capacitors. If these flying capacitors are replaced by battery cells or super-capacitors, the voltage or state-of-charge (SOC) balance can be realized easily which is similar with the voltage balance control in multi-level converters. Furthermore, the output voltage is multi-level, so the dv/dt is reduced which can reduce the harmonics greatly. When used for motor drives, the control performance can be improved. When used for power grid, the filter inductance can be reduced. The cascaded converters, such as cascaded H-bridge and modular multi-level converter (MMC) are more suitable for the power storage systems because of the modular structure and expansibility. [7-9] In this paper, a new hybrid cascaded multi-level converter is proposed which can realize both the voltage equalization and the charge-discharge control at the same time. Each battery cell can be controlled separately to equalize the terminal voltage and SOC. So the functions of battery charger and discharger with battery management system (BMS) are realized by the single converter. The AC output is a multi-level voltage with fewer harmonics than the traditional two-level circuit. There are two kinds of power electronics devices in the proposed circuit. One is the low voltage devices used for the control of battery cells which work in higher switching frequency. The other is the higher voltage devices used for the output control of the whole converter. They can just switch in base frequency to reduce the switching loss. Some experimental results have been carried out to verify the effects of the proposed circuit.

1u

2u

3u

1Q

2Q3Q

4Q

nu

1c

2c

3c

nc

1L

2L

1nL −

Fig.1. voltage equalization circuit

1u

2u

3u

1Q

2Q3Q

4Q

nu

1c

2c

3c

nc

1L

2L

1nL −

Fig. 2 Traditional power storage system with

voltage equalization circuit and inverter

Topology of the hybrid cascaded multi-level converter The proposed hybrid multi-level converter is shown in Fig. 3. There are two parts, one is the cascaded half-bridge converters with battery cells which are shown on the left, and another one is H-bridge converters in each phase shown on the right. Each cascaded half-bridge can be controlled to output a battery voltage or zero, the battery cell is correspondingly connected or bypassed from the circuit. So a variable voltage is produced by the cascaded half-bridges on the DC bus which is also the input of the H-bridge. The amplitude voltage of the DC bus is determined by the number of cascaded half-bridge converters and the voltage of the battery cell. As the battery cell voltage is limited, the semiconductor

devices in half-bridges can use the low-voltage and low on-resistance devices, such as MOSFETs, then the conduction loss is very small. Compared to the traditional two-level converter shown in Fig. 2, the voltage step in a single switch course is much smaller, so the switching loss can be greatly reduced which is similar with multi-level converter. The H-bridge is used to change the direction of the DC bus voltage to produce AC voltage, so the reference value of the DC bus voltage is the absolute value of the AC voltage. The switching frequency of devices in the H-bridge can be just the same as base frequency and the switching loss can be reduced. Furthermore, the H-bridge always switches at zero DC bus voltage, so the switching loss is near zero. Since all the cascaded half-bridges can be controlled individually, a multi-level voltage can be formed at the output. On the other hand, the more of the cascaded level, the more voltage levels there are in the output, and the output voltage is more approach to the ideal sinusoidal. So it is very suitable for the large power and higher voltage storage system, such as the electric vehicles and the storage systems in power grid. Compared to the voltage equalization circuit in Fig.2, the number of the power electronics devices and sensors is not increased significantly. By the individually control of the cascaded half-bridges, the batter cell can be connected or bypassed from the circuit, so the charge or discharge speed of each battery cell can be adjusted according to its terminal voltage or state of charge (SOC) which will be explained in the next section. When the converter is used as the energy storage in a power system, the converter is connected with the power grid directly by some filter inductances as shown in Fig. 4(a). The AC reference voltages are given by the closed-loop control of active-power and reactive-power control which is similar with the PWM rectifiers and normal power storage system [10]. When the converter is used for power source to drive some load, for example, in electric vehicle (EV), the AC motor is connected at the output side of the converter as shown in Fig. 4(b). The AC reference voltages are given by the motor control system. When the battery cells need to be recharged, the converter can be connected directly with the three-phase power source as shown in Fig. 4(a). The charging control method is similar with normal on-line energy storage system. If the traditional constant-current or constant-voltage recharge process are needed, an additional DC source is necessary which will be explained in next sections.

11K

12K

21K

22K

1nK

2nK

1S

2S

3S

4S

Fig.3. Hybrid cascaded multi-level converter

Cascaded Cells

au

bu

cu

O

Cascaded Cells

Cascaded Cells

Power Grid

(a) Converter connected with power grid

au

bu

cu

O

(b) Converter connected with AC motor

Fig. 4 Three-phase hybrid cascaded multi-level converter

Voltage balance control method As explained in former section, the reference AC voltages are obtained according to the requirement of the power grid or the load. At normal AC voltage output, not all the battery cells are need to be connected in the circuit. So the terminal voltage or SOC can be balanced by the rotational use of the battery cells. In the proposed converter, each battery cell is controlled separately by a half-bridge converter. Taking the discharge as example, the voltage or SOC balance method is studied here. For the cascade half-bridge converters, define the switching state as follows:

1 upper switch is conducted0 lower switch is conductedxS⎧

= ⎨⎩

The modulation ratio xm is defined as the average value of xS in a PWM period. In the converter shown in Fig. 4, when 1xS = , the battery is connected in the circuit, the battery is charged or discharged. When 0xS = , the battery is bypassed from the circuit, the battery is neither discharged nor charged.

The discharging power from this cell is:

x xP S u i= ⋅ ⋅ (1)

Here xu is the battery cell voltage. i is the charging current on the DC bus. So the modulation ratio can be used for the power flow control from each cell. Then the voltage balance or SOC balance can be realized. To reduce the switching loss of the converter, only one half-bridge converter is permitted to work in switching state, the others keep their state unchangeable with 1xS = or 0xS = . The battery with higher terminal voltage can be controlled to discharge during the whole switch period with 0xS = . The battery with lower terminal voltage will be controlled in switch state with 0 1xm< < or disconnected with 1xS = . In traditional multi-level converters with flying capacitor, the phase disposition (PD) carrier-based PWM is very suitable for the voltage balance control. In the circuit proposed in this paper, the similar PWM method is adopted to realize the terminal-voltage or SOC balance control of the battery cells which is shown in Fig. 5[11]. The position of each battery voltage in the carrier wave is determined by the comparison of their terminal voltages. The battery with higher voltage is placed in the bottom layer while the battery with lower voltage is placed in the top layer. Then the battery cell with lower voltage will just work in switch state or be bypassed and less energy is discharged from these cells. Similarly,

the cell with higher voltage will be discharged more. If the carrier-wave is arranged by the SOC, the SOC balance control can be realized instead of the voltage balance. As the SOC of the battery cells are very difficult to be estimated on time, the terminal voltage balance control is usually used in practice. The position of each battery voltage in the carrier wave is re-arranged only once in every modulation cycle to form the staircase shape voltage with less switching times. During normal use, the battery terminal voltages and SOC change very slowly, so the carrier wave updated by base frequency is enough for the voltage and SOC balance. The direction of the output AC voltage is controlled by the H-bridge, So the control of H-bridge is just determined by the sign of the voltage reference. The amplitude of the reference voltage is controlled by the cascaded converter arms. The reference values of the DC bus voltage output by the cascaded half-bridges are the absolute value of the AC reference voltage as shown in Fig. 5. The control method during charging state can be also got by the similar method as shown in Fig.6 where the arrange method of the carrier-wave is opposite with the discharging state.

1xu2xu3xu

1 2 3x x xu u u> >

Fig.5. Carrier wave during discharge

1 2 3x x xu u u> >

1xu2xu3xu

Fig.6. Carrier wave during charge

In the converters with more levels, all the bridge arms can work in on-off state and the PWM switching can be eliminated. This is very suitable for the applications in power gird with medium or high voltage, where several hundreds of levels are usually needed. The reduced switching frequency can greatly improve the efficiency and power density of the converter.

Constant current charging method When the converter is connected with power grid for battery recharging, the output of the converter is also AC voltages which means all the battery cells are recharged intermittently. If the traditional constant-current or constant-voltage recharge method are needed by the battery cell, the recharge method shown in Fig. 4(a) is not suitable any more. In this case, a DC voltage source is need for the battery charging. The charging current and voltage can be controlled by the proposed converter itself according to the necessary of the battery cells. The charging circuit is shown in Fig. 7. A circuit breaker is used to switch the DC bus from the H-bridge to the DC voltage source. Furthermore, a filter inductor is connected in series with the DC source to realize the current control. The DC voltage can also be realized by the H-bridge and a capacitor. The H-bridge worked as a rectifier by the diodes and a steady DC voltage is produce in help of the capacitors.

In the charging course of the battery, the charging current should be controlled. The current state equation is as follows:

charge-f f dcdiR i L u udt

+ = (2)

Here dcu is the DC bus voltage output by the cascaded half-bridges. chargeu is the voltage of the DC source. ,f fR L are the resistance and inductance of the inductive filter between the cascaded half-bridges and the DC source. During the charging cycle, the voltages of the battery cells and the DC voltage source might be variable, so the switching states of the cascaded half-bridges will be changed to make the charging current constant. The charging current control scheme is shown in Fig. 8. A PI regulator is used to make the current be constant by changing the output DC voltages of the cascaded cells. The voltage of the DC source is used as a feed-forward compensation at the output of the PI controller. In practical applications, the value of the DC source’s voltage is almost constant, so the feed-forward compensation can also be removed and the variation of the DC source voltage can be compensated by the feedback of the PI controller.

UL+

-

S1 S3

S2 S4

K

Kcharge

Kdischargei

K11

K12

K21

K22

Kn1

Kn2

+ -

Loaddcu argch eu

,f fR L

Fig. 7. Charging circuit of battery with DC source

fbI

dcVrefI

Fig. 8. Current control scheme for the battery charging

Experimental results A three-phase four-level experimental platform as shown in Fig. 9 has been built up to verify the performance of the proposed converter and the control scheme. A control board with DSP TMS320F28335 is used as the controller. A CPLD chip is used for the PWM expansion to control all the half-bridges and H-bridges. The lead-acid battery modules of 5Ah and 12V are used as the battery cell. The battery modules are monitored by the LEM Sentinel which can measure the voltage, temperature and resistance of the battery modules. The measured information is transferred to the DSP controller by RS-232 communication. An induction motor (IM) was driven with the variable-voltage-variable-frequency (VVVF) control method. The parameters of the motor are shown in Table I.

Fig.9 Experimental platform

TABLE I. Parameters of the induction motor

Paramter Value Rated power 0.55kW Rated line voltage 380V Rated line current 1.5A Rated frequency 50Hz Number of pole paires 2 Rated speed 1390rpm When the induction motor is used as the load of the converter, the output voltages and output current during motor acceleration are shown in Fig. 10. The first curve is the DC bus voltage, the second is AC output voltage,. It can be found that the DC bus voltage is the absolute value of the AC voltage. The third curve is AC output current. It can be seen that during the acceleration of the motor, the voltage levels will increase as the increase of the reference voltage. The multi-level voltage is very approach to the ideal sinusoidal wave, so the stator current of the motor is very sinusoidal although the AC voltage amplitude is still very low compared to the rated voltage. The three-phase voltage during steady state is shown in Fig. 11. The DC bus current is shown in Fig. 12. Since single phase is fed by each cascaded half-bridges, there is power pulsation of twice frequency in each phase. That means there is circulating power on the DC bus which will cause reversed current on the DC bus. So there are negative values on the DC bus current while the DC bus voltage is always positive as shown in Fig. 11. This is the natural characters in all the single-phase voltage sources.

Fig.10. Output voltage and current during motor acceleration

Bridge arms and battery cell

Control BoardDSP+CPLD

H bridges

Voltage measure

Phase A Phase B Phase C

Current Sensor

Fig. 11. Three-phase output voltage of multi-level

Fig. 12. DC bus current with some loads

The terminal voltages of the battery cells during a long time discharge are shown in Fig. 13. The terminal voltages are measured every 10 minutes. It can be seen that by the proper PWM method as shown in Fig. 5, the terminal voltages will be balanced finally by the proposed voltage balance method.

Fig. 13. Voltage balance during discharge

When the constant-current charging method shown in Fig. 9 is used, the current control results is shown in Fig. 14 and the DC bus voltage produced by the cascaded half-bridges are also shown in the same figure. It can be found that when the current command is given, the DC bus current can track the reference value quickly. Then the constant current charge of the battery cells can be realized.

Fig. 14 DC bus voltage and charging current when a charging current reference was given

Conclusion The proposed hybrid cascaded multi-level converter can control the charge and discharge of the battery cells separately to realize the voltage balance, while the desired AC voltage can be obtained directly at the output side. By the proper PWM selection, the terminal voltage or SOC of the battery cells can be balanced automatically. The output AC voltage is a multi-level voltage where the number of levels is proportional to the number of cascaded battery cells. So the output AC voltage is very near to the ideal AC voltage, then the harmonics can be greatly reduced. A constant-current charge method is proposed where the additional recharger is not needed any more. The proposed converter topology is very suitable for large power storage system.

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DC bus current

DC bus voltage

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