Energy storage system using a series connection of supercapacitors, with an active device for equalising the voltages Philippe Barrade, Serge Pittet, and Alfred Rufer Laboratoire d’Electronique Inductrielle Swiss Federal Institute of Technology Lausanne CH–1015 Lausanne EPFL, Switzerland Phone: +41–21–693–2628, Fax: +41–21-693–2600 E-Mail:
[email protected]
Abstract An energy storage system based on battery and supercapacitors is presented. It allows bigger amount of intantaneous power. The properties of the proposed system are oriented in high efficiency, in a special topology with parallel channels. The paper presents also an active sharing device, for equalizing the voltages across a series connection of supercapacitor. Based on a buck–boost topology, this device ensures an optimal value for the stored energy, with a high efficiency. Key words: energy storage, supercapacitors, active voltage sharing
1
Introduction
Supercapacitors represent one of the most interesting new developments in the field of energy storage [1], especially concerning the awayability of high instantaneous power, to be combined with a classical electrochemical battery. This paper shows a combined storage device, with the needed static converters for a supply operation with constant output voltage. A parallel structure with individual channels operating in the discontinuous mode and time delayed pulses allows supercapacitor current with poor ripple, even with the use of small coupling inductors. Reduced weight and reduced commutation losses are the interesting properties of the presented device. Using supercapacitor enforces a series connection of them in order to reduce the related power losses in the associated power electronic converter. Due to difference of values of each supercapacitor, the total voltage will not be equally distributed between the different capacitors. A local over-voltage could appear over one or several supercapacitor, and the stored energy wouldn’t be optimized. This paper shows also an active device for voltage sharing on each supercapacitor, witch ensures no over-voltage, a maximum energy stored with a high efficiency.
2
Static converter for combined storage device
2.1
The power interface
For a supercapacitor, the amount of stored energy is bound to the square of its voltage magnitude. Also for electrochemical batteries, the output voltage varies with the status of charge. Power supply at constant voltage level is the outgoing situation and imposes the presence of a double conversion structure, each based on associations of reversible DC-DC-Converters as shown Fig. 1. Each converter is composed of various legs. By shifted pulsing of the converters brings, a significant reduction in the current ripple of the energy sources is obtained [2]. This allows simultaneously to design the needed inductors at low values. With a strong increased number of legs, each commutation cell can be operated in so called discontinuous conduction mode. Designing the system for a discontinuous current mode in each individual channels brings the first advantage of the use of very small inductors. The combination of eight paralleled channels, each operating with intermittent current, results in a global current with very poor ripple [3]. The current turn-on conditions of the transistors are privilegious, due to the zero-value of the associated diode current. T L
D 1
L
C
super
U
B a tt
U
T 1
3
D 1
3
C 2
T
T 2
D
su p e r
T L
T D
3
L
D 2
5
b a tt
U d
4 4
7
D 5
7
4
T
T 6
D 6
8
D 8
Figure 1. Association of reversible DC–DC converters
Very low commutation losses caracterize this topology. For the intermittent mode of the reversible DC-DC converter, a special logic is needed, with separated activation of upper or lower devices with a set-value current criteria and time delay.
2.2
The share of the two complementary current sources
The output voltage of the complementary storage device is influencable by each current sources, this means by the converter-interface of the battery and also by the converter-interface of the supercapacitor. The control of this common quantity, Uout , must done by only one superposed voltage controller, which is connected to one only of the two current sources at the same time. This demands the use of a switching device, controlled by current-level detection devices. A possible control scheme is given Fig. 2. One of the studied control strategies is based on two specified limitations of the battery current, a lower and an upper limit. A simulation of a strongly varying load current is given in Fig. 3. The output voltage is kept constant, and the current is shared according the lower and upper limits. In Fig. 3, both the battery and the supercapacitor currents are shown. The most interesting quantity is the supercapacitor voltage, illustrating the alternating unloading and loading phenomena.
7 ?
4
4
E?
4
E>
K
Figure 2. Control scheme
80
60
50
40 30
20 10
0 0
5
10
15
20
25
Capacitors current (A)
Load current (A)
2.3
The practical realization
In order to verify the results, a practical realization has been made. A complete system with control has been mounted in a rack [4]. In Fig. 4, the verification system of the storage device is shown. A 12V /10Ah lead battery is used, together with a series connection of 6 supercapacitors of each 1000 Farad. The supercapacitors allow a rated current up to more than 180 amps. With that combination, a momentaneous power demand of more than 1500W is possible. At the right side of the rack, the supercapacitor stack is visible. In the middle, the battery is also represented. Two DC-DC converters each in a double buck-and-boost topology are mounted at the left side of the battery, together with the control card. This card is a specially developed control card for power electronic experimentation, and is based on a powerful DSP with floating point instructions. It includes a large periferical FPGA. In that device, functions as modulators can be freely programmed. The fast AD converters are also on the same card, and operates in paralleled mode. Acquisition time of 600ns is so possible. For the current control, and in order to avoid filtering of the actual value, the sampling is triggered with signals activated inside the numeric modulator.
20
70
60
0
3
-20
40
30
20
10
0 0
5
10
t (s)
15
20
25
Active sharing device
-40
-60
3.1
-80
Preamble
-100 0
5
10
15
20
25
10
50
Capacitors voltage (V)
Batteries current (A)
Figure 4. Practical realization
9.5
9
8.5
8
7.5
7
6.5 0
5
10
t (s)
15
Figure 3. Simulation results
20
25
Because of the technology limits, the maximum voltage of a supercapacitor when charging is low, near 2.5V . In order to reduce the power losses in the associated power electronic when charging and discharging, a series connection of several supercapacitors is needed to increase the operating voltage. Due to difference in the values of each supercapacitor, the total voltage over a series connection
+
Table 1. Voltage sharing and stored energy 7
?
d = 0% 1 7
+
7
Uf (V) Uc1f (V) Uc2f (V) E (J) ?
5 2.5 2.5 6250
d = −20% no sharing 4.5 2 2.5 4500
d = −20% sharing 5 2.5 2.5 5625
Figure 5. Series connection will not be equally distributed between the different supercapacitors. It can lead to an unsymmetrical voltage share between the capacitors. If this effect is not compensated for a local over-voltage could appear over one of several supercapacitors with a risk of destruction of this component. We can consider e.g. a series connection of two supercapacitors Fig. 5, associated with a current source in order to charge and discharge this system. Introducing d the relative difference between C1 and C2 values, express in percent, we define the values of C1 and C2 : C1 = C
and C2 = C
d + 100 100
(1)
Where C is the reference value for the capacitors. Because the voltages across the supercapacitors must be limited to 2.5V , the series connection is considered charged when the total voltage is 5V . Under this condition and if the initial voltage is zero, we can establish :
Uc1f =
d + 100 Uf d + 200
and Uc2f =
100 Uf d + 200
(2) Where Uc1f and Uc2f are the voltages across C1 and C2 at the end of charging and Uf is the total voltage (5V ). According to those relations, we can consider two extreme cases for C = 1000F . The first one is related to ideal supercapacitors (d = 0%). The second case is related to a bad supercapacitor C2 (d = −20%, C1 = 1000F , C2 = 800F ). The results are synthetized in Table 3.1. In the case of same values for each supercapacitor (d = 0%), the stored energy is 6.25kJ. When d is −20%, the voltages across each capacitor are not equally distributed. In order to avoid any over-voltage across a supercapacitor, the charging process is ended as soon as one capacitor reaches is maximum voltage (2.5V ). In that case, the charging process is ended when C2 is 2.5V . The voltage across C1 is 2V , and the total voltage is only 4.5V instead of 5V . As a result, the stored energy is 28% lower than the ideal case (d = 0%).
The third case take into account a device, connected across each supercapacitor, which allows the sharing of the two voltages Uc1f and Uc2f . Even if the stored energy is still lower than 6.25kJ, this energy is 20% bigger than the case where d = −20% without any voltage sharing device. The second main advantage of a voltage sharing device is to keep the voltages across each supercapacitor on their nominal values with no over-voltage.
3.2
Voltage sharing devices
A common way to equalize the voltages in a series association of capacitors is to connect across each of them a resistor. The values of the resistors are fixed by various criteria. The main criteria to consider is to fix a dynamic of the voltage sharing equal to the dynamic of the charge process of the supercapacitors. This allows to avoid any overshoot of the individual voltages. The main disadvantage of this solution is the power dissipated in each resistor. A second solution consists in the use of zener diodes, connected across each supercapacitors. Those zener diodes have to limit the voltages of the capacitors, in order to fix the maximum value to 2.5V . There is no power dissipation as far as all the voltages are less than the limit voltage (2.5V ). But the power dissipation can be important if many supercapacitors reach their limit voltages. As an illustration, we propose Fig. 6 some simulation results dealing with a series connection of five supercapacitors : four of them are 1000F , the fifth is a 800F supercapacitor. We propose the balance of the energy at the end of a charging process from 0V to 12.5V , using a voltage sharing device. In all the considered cases, the stored energy in the supercapacitors is 15kJ. We will compare this reference energy to the energy given by the charging device. The solution which use some resistors generates too much losses. The values of the resistors (0.1Ω) have been chosen to have a good dynamic of voltage sharing. But this dynamic is not quick enough : the charging process is ended in 40s, but it takes 400s
18000
1
?
16000
1
14000
+ 1
Energy (J)
12000
1
10000
7
A G
A G
) 7 1
8000
1
6000
?
+
A G
?
7 ?
4000
2000
0
0
5
10
15
20
25
30
35
40
t (s)
Figure 7. Principle of the active sharing
Fig. 6(a)Zener Diodes 4
14
x 10
discharging process. To respect this design criteria, we can establish :
12
Energy (J)
10
Ieq = I
8
6
4
2
0
0
50
100
150
200
250
300
350
400
450
500
t (s)
Fig. 6(b)Resistors Figure 6. Energy cost to equalize the voltages of all the capacitors. As a consequence, the charging device has to provide nearly 120kJ to store 15kJ in the capacitors. The global efficiency is too low : 12.5%. The use of zener diodes offers a better efficiency. The charging device provides 16.3kJ, for 15kJ finally stored in the supercapacitors, with a good dynamic of voltage sharing. The efficiency is nearly 90%, but the main disadvantage of that solution is to use the zener diodes to dissipate power when maximum local voltages are reached.
3.3
Active device for equalizing the voltages
In order to obtain a sharing of each voltages, with the best efficiency, we propose a new topology of device, based on the topology given Fig. 7 for two supercapacitors. The principle of this solution consists in a current deviation of the main charging current I, by means of two auxiliary current sources. Depending on the difference of voltages between the two supercapacitors during the charging or discharging process, the value and the sign of the sharing current Ieq should be chosen in order to obtain a dynamic of voltage sharing identical to the dynamic of the charging or
d d + 200
(3)
This relation shows that for a current of charging I = 100A , if C1 = 1000F and C2 = 800F (d = −20%), the sharing current should be Ieq = −11.1A. The sharing current sources can be synthetized as proposed Fig. 8. The proposed topology is a current reversible buck-boost converter. Compared to the previous topology, the two current sources have been replaced by two transistors/diodes. But the principle stay identical if the current in the inductor is two times bigger than the current Ieq previously established. In order to optimize the efficiency of such a device, it is recommended to drive this converter to obtain a discontinuous conduction mode working. Two different cases should appear. When a positive current 2Ileq is needed (Uc1 > Uc2 ), the transistor T2 is turned off, and T1 is turned on and off with a given frequency. At the opposite, when a negative current 2Ileq is needed (Uc1 < Uc2 ), the transistor T1 is turned off, and T2 is turned on and off with the same switching frequency. The switching process is started as soon as a significant difference of voltage has been detected between C1 and C2 . The switching process is stopped when the two voltages become the same. I T
D 1
C 1
L
e q
I
c 1
U 1
2 I
e q
A U I
T 2
D 2
C 2
c 1
c 2
U
c 2
Figure 8. Active sharing device
D
A soon as the difference of voltages is less than the forward voltage of the diodes, and if the switching of T1 and T2 is defined by a 50% duty cycle, the buckboost converter works in discontinuous conduction mode. Under those conditions, we can establish the main design criteria of such a topology, taking into account the relationship which gives the value of the needed sharing current Ieq as a function of the main current I :
1
C
L
T
C
3
C 2
L 2
D 3
T 2
1
1
D 2
D
L
T
4
4
3
C 3
C 4
L 2
'
D
T 2'
3
D '
T 3'
4
5
4
'
D
T 4'
5
'
T 5'
Figure 9. Active sharing device 2
(4)
Inductor Voltage (V)
2.5
2
Voltages (V)
Uc1 1 d + 200 Uc1 1 + Leq f = 16 I d Uc2 + Ud
T
D 1
1.5
1 0.5 0 −0.5 −1 −1.5
6
6.2
6.4
6.6
6.8
7
7.2
7.4
7.6
7.8
t (s)
8 −4
x 10
10 1
Inductor Current (A)
Where f is the switching frequency, Leq the value of the inductor , and Ud is the forward voltage of the diodes. This relationship ensures the choice of the inductor and the switching frequency values for a main current I given, and for a relative difference d of C1 and C2 given. The defined topology and the established design criteria are valid for a two supercapacitors series connection, but can be extended for a n supercapacitors series connection, as shown Fig. 9 for a 5 supercapacitors series connection. The series connection is divided in associations of two supercapacitors. A buck-boost converter is associated for each pair of capacitor. The converter T1 D1 T2 D2 allows the voltage sharing of C1 and C2 . The converter T2 D2 T30 D30 allows the voltage sharing of C2 and C3 , etc. The same design criteria is applied for each converter. If the main current I is 100A, for d = 20%, the values of all the inductors must be 1mH if the switching frequency is 10kHz. As an illustration, we propose simulation results, dealing with a 5 supercapacitors series connection. Each of them is a 1000F capacitors, except one which is 800F . The first result shows the voltages across each capacitor during the charging process. Even if the sharing is not instantaneous, all the voltages are equal at the end of the process, with no over-voltage on the supercapacitors. The second result shows the voltage applied to an inductor, and the current in that inductor once a process of voltage sharing is decided. The considered converter works in discontinuous conduction mode. That means that the current peak should be important to have a mean value as fixed by the used design criteria. Finally, we can compare values of the energy given by the charging source and the energy finally stored in the supercapacitors, taking into account power losses in the semiconductors. This show a 97% efficiency, witch is the best obtained compared to other solutions.
1.5
0.5
0
0
5
10
15
20
t (s)
25
30
35
40
0 −10 −20 −30 −40
6
6.2
6.4
6.6
6.8
7
t (s)
7.2
7.4
7.6
7.8
8 −4
x 10
Figure 10. Simulation results
3.4
The practical realization
A 5 supercapacitors 12V stack has been designed to verify theoretical studies. Four of them are 800F supercapacitors and one is 1000F to test the principle with a dispersion d = 25%. The maximum voltage for each supercapacitor has been fixed to 2.3V . As 12V cannot be reached without equalizing (some capacitor voltage would exceed 2.3V ) a lower voltage charge watchword has been used for measurements. In order to obtain the best efficiency, the transistors of the active device should be chosen with a Rdson as low as possible. We have chosen a 4mΩ Rdson , 185A n-MOS transistor. For significantly lower switching losses, ultra-fast and soft recovery diodes are needed. To keep a high current with low frequency the inductor must be very low (about 8mH). A short 4mm2 flex with one turn over a ferrite will provide such a low inductance value with a low parasitic serial resistor. As described in the theoretical study, the transistors are controlled by pairs to share the voltages on each associated pairs of supercapacitors. The control structure for two transistors is given Fig. 11. This structure provides a good precision ( about ±20mV ) and is not noise-sensitive while the voltage information on the supercapacitor is taken before that the transistor turns on. The schematic is suited for bipolar voltage source. The electronic circuitry has been matched to unipolar source with auto supply, in order to make the cells fully independent. The supercapacitor’s voltage drop influences the equalizing current value. For that reason, it’s not
C
U
re f
U c
-
+ 1 V 1 0 0 H z
k
D -la tc h
N a n d
d riv e r M O S
G a te T 1
D -la tc h
N a n d
d riv e r M O S
G a te T 2
x 2 0 -1 V
C k
Figure 11. Control structure useful to equalize at the beginning of the charge. When the full cell voltage rises 3V (nearly 600mV on each supercapacitor), an integrated boost converter ensures a 15V source for control circuitry, which can start equalizing. A photography of that device is shown Fig. 12. The performances on such a device are shown Fig. 13, on witch are represented the voltages across three of the five supercapacitors, without and with voltage sharing. The results for the voltage sharing present also the current in one equalizing device inductor. The average value is as expected proportional to the voltage drop on the supercapacitors. This ensures a correct voltage sharing across each supercapacitor.
Figure 13(a) Voltage sharing
Figure 13(b) Current across an inductor Figure 13. Experimental results
4
Conclusion
The use of modern supercapacitors allows a complementation of normal batteries, in order to increase the maximum instantaneous output power. We have defined the converters used as interface, designed with reduced switching losses, also with limited volume and weight of passive components. In order to increase the voltage across a supercapacitor device, a series connection is needed. We have defined an active voltage sharing device, witch ensures an optimal stored energy value with no over-voltage over any supercapacitor and an optimal efficiency.
A prototype has been realized , for witch a request for a patent has been suggested (Number of registration SE9903153 − 6).
References [1] X. Andrieu. Supercapacitors, CEC workshop on solid lythium batteries. Brussels, January 1991. [2] S.M. Halpinet Al. Application of double-layer capacitor technology to static condenser for distribution system voltage control. Trans. on Power Systems, 11(4), Nov. 1996. [3] J.M. Meyer A. Rufer. A high current low ripple, low weight PFC rectifier using a standart power module. PCIM 98 Power Quality, 1998. N¨ urnberg. [4] H. Ravokatrasolofo A. Rufer. Static converter for complementary energy storage with battery and supercapacitor. PCIM 99 Power Quality, 1999.
Figure 12. Practical realization
