5.1.3.Supercapacitor energy storage systems

The supercapacitor ESS is typically connected to the DC link of the most popular consumer AC drive power stage (Fig. 2). As an example of very perspective supercapacitor application the supercapacitor ESS for traction purposes will be reviewed. The power stage consists of two main parts – a traction consumer AC drive connected through the overhead network to a traction substation and a supercapacitor ESS. The single direction substation equipment without regenerative braking and any type of energy storage is common for urban transport and includes an uncontrollable diode rectifier. The same ESS connection scheme is used for industrial consumer appliances, e.g. for elevators, lifting devices and other drives with regenerative braking [13]. Due to the short charge-discharge cycle lifetime accumulator battery must be used as primary energy source in the case of unavailable network voltage and never used in parallel to networ for peak shaving operation, therefore the battery is connected instead of network substation rectifier. Similarly the supercapacitor ESS is connected to DC link of small and unpredictable renewable and alternative energy sources such as wind, solar, internal combustion engine and fuell cell. All these sources are connected instead of substation rectifier.

The traction vehicle power scheme layout with simplified generalized substitution scheme of typical traction and industrial consumer power circuit is shown in Fig. 3. Independently from drive type – DC or AC, motor converter is replaced by current source I1. The positive direction of the source current corresponds to the braking mode; in the traction mode the current direction is opposite.

Fig. 2. The power stage of vehicle AC traction drive with supercapacitor ESS

Fig. 3. A tramcar traction drive with hybrid energy storage system (a) and its simplified power circuit substitution scheme (b)

In the braking mode the current flows to the DC power line, for example overhead line in the case of traction application. If it is impossible for some reason or the overhead current is not large enough, the braking IGBT VT_br, which is controlled by a pulse width modulator with 1000 Hz frequency, begins to operate. The control system works as voltage V_Cf stabilizer with a 780 V setting for 600 V catenary voltage. The energy is dissipated in a brake rheostat R_br. The pulse currents are closed through capacitor C_f, whereas the overhead line current is smoothed by choke L_f. Between capacitor C_f and choke L_f a radio-frequency filter L_rf, C_rf is also connected.

While connecting such a supercapacitor to the existing vehicle power scheme it is necessary to take into account that a current controller must provide:

• two-way voltage boost/buck energy interchange between the traction power circuit and the supercapacitor;

• smoothed charge/discharge current of the supercapacitor;

• controllable initial charge of the supercapacitor;

• protection of the supercapacitor against overcurrents caused by overhead short circuits.

If the supercapacitor current controller is connected to point A, the above requirements could be satisfied by the full controller scheme only [7,12]. Fig. 4 shows its connection to the consumer power circuit.

Fig. 4. Connection of the full scheme of supercapacitor’s C current controller to point A of consumer power circuit

The connection in Fig. 4 satisfies all the imposed requirements, being however too complicated. Moreover, a couple of weakly damped oscillation circuits are met in this scheme, e.g. C_f, L_f, L1, C1 when IGBT VT1 is switched on, which causes parasitic oscillations in a power circuit and complicates considerably the development of the control system.

The scheme could be significantly simplified by the following measures:

• Connection of the supercapacitor’s current controller to point B. Then the controller-induced pulse currents could be filtered in the overhead line by elements L_rf, C_rf, L_f already installed in the tramcar.

• Choice of the supercapacitor’s operating voltage, which should be higher than the highest overhead voltage or lower than the lowest overhead voltage. Then a boost converter will be needed in one direction and a buck converter in the other, which could be achieved with two switching elements.

Since the overhead voltage may be varied between 450 V and 780 V, it would be economically justified to select the supercapacitor voltage up to 450 V. Then the scheme can be simplified as shown in Fig. 3. For the basic operation of the scheme it is enough to use the IGBT switches VT1 and VT3. Switch VT1 and diode D3 work as a voltage buck converter in the charging mode of supercapacitor C, whereas VT3 and D1 (at VT2 switched on) - as a voltage boost converter in the discharging mode. The IGBT VT2 is needed for protection of supercapacitor C in the cases when a short circuit or an undervoltage occurs in the overhead or traction converter scheme. When supercapacitor C is discharging the VT2 switch is continuo­usly on, but it is being switched off in the case of supercapacitor overcurrent. For dissipating the energy stored by choke L1 it is necessary to install additional snubber circuits. Capacitor C_f is used as the basic element for the buck/boost converter. Capacitor C1 only compensates the connecting cable’s inductances and must be placed as close as possible to elements VT1, VT2, VT3 and C; its capacity is considerably lower than that of C_f.

If the supercapacitor’s voltage is selected higher than 780 V, then, in the same manner, three IGBT switches are needed to provide that the initial charging of capacitor C is controlled, but in this case one choke and one capacitor are additionally required. Therefore the scheme shown in Fig. 3 could be considered the optimal in view of the required number of elements.

At charging the capacitor from the initial voltage V_Cmin to voltage V_Cmax, the energy stored by the capacitor is

. (5)

It is obvious from Eq. (5) that the energy capability of the capacitor strongly depends on the minimum voltage V_Cmin to which it is discharged. The lower V_Cmin is the more energy could be stored in the capacitor. The manufacturers of the capacitors recommend that V_Cmin ≥ 0.5V_Cmax, which allows up to 75% of the energy stored by capacitor to be utilized. However, the lower V_Cmin is, the higher the necessary capacitor current at the same braking power P_br. At the beginning of the process the braking power is maximal but the voltage of the capacitor is minimal, therefore

. (6)

As the semiconductor elements must be chosen corresponding to current I_Cmax which must not exceed the level allowable for the capacitor, both the aspects should be taken into account for a proper choice of the capacity C and the voltage range. If the total braking energy W_br is stored, from expressions (5) and (6) at W_C = W_br we can find that

(7)

and

. (8)

In general terms, the braking mode of the vehicle could be characterized by the initial braking power P_brmax = 300 kW which linearly decreases to zero during braking [7]. The calculated braking energy is W_br = 900 kJ. Substituting these values into expressions (8) we obtain V_Cmin and I_Cmax curves in dependence on capacity C at V_Cmax = 450 V (Fig. 5).

The supercapacitor contains a great number of series-connected single capa­citors. Currently, the capacitors with the allowable voltage of 2.7 V manufactured by the Maxwell

1

2

2

Capacity C, F

Fig. 5. Voltage V_Cmin (curve 1) and current I_Cmax (curve 2) vs. capacity C at P_brmax = 300 kW, W_br = 900 kJ and V_Cmax = 450 V

Technologies, Inc. have the largest available capacity of 3000 F (of these capacitors 16 V, 48 V and 125 V modules are made). They also have the optimal ratio between energy density and price. Taking into account a possible voltage disbalance between series-connected capacitors it is desirable that the maximum voltage on a capacitor is not exceeding 2.5 V. This means that at least 180 series-connected single capacitors are needed to achieve the 450V level, with the total capacity of the module equal to 16.66 F. It is clear that the capacity choice is not free; this could be doubled or multiplied by parallel connection of two and more such modules. From Fig. 5, at C = 17 F we have V_Cmin = 310.8 V, which is permissible, but I_Cmax = 965 A, which is too high from the viewpoint of the choice of capacitors and IGBT switches. As the optimal value C=33F could be considered, which corresponds to the parallel connection of two 180-element capacitor mo­dules. At C = 33 F voltage V_Cmin = 384.6 V and current I_Cmax = 780 A. This is the total current at the parallel connection of supercapacitor modules, while for each supercapacitor branch it is twice smaller, i.e., 390 A, which is allowable.