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NEW ENERGY MANAGEMENT AND HYBRID ENERGY STORAGE IN METRO RAILCAR

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Istvan Szenasy

Szechenyi University, Dept. of Automation

Hungary

- Our approach uses modeling and simulation to evaluate the potential of capacitive energy storage, an emerging technology for renewable energy.
- Supercapacitors and other battery technologies can contribute to rapid energy recovery.
- Fast energy recovery is especially important in electric vehicles, in which powerful batteries enable regenerative braking.

- Renewing braking energy can significantly reduce the total energy consumed in short-distance passenger traffic using electrified lines or urban vehicles.
- Using Matlab-Simulink to model an urban-metro railcar of the Budapest Metro Railway, we have demonstrated that reducing the minimal capacitance value needed can make supercapacitor-based energy storage more viable.

Mass without load 34 t, fully loaded 44 t.

The total rated motor power is 200 kW,

the nominal speed is 75 km/h,

the maximum acceleration is greater than 1 m/s2,

the average distances are approximately 800 m between stations,

the overhead line voltage is 750V DC nominally.

The weakening of the DC motor fieldsbegins over the speed of 36 km/h.

Charging-discharging of the SCAP ( abbreviated C) is executed by its bidirectional DC-DC converter.

Figure2. Aim and direction of simulations and calculations in modelling

Our objective wasto determine the lowest necessary capacitance value for a supercapacitor (C) for the storage of all regenerative brakingenergy underdifferentconditions (mass, speeds, grades, stopping distances.)

We set the optimum initial supercapacitor voltageto 840 V DC before starting the railcar.

The lowest voltage of C was 400 V at the end of driving/start of braking.

We achieved this under all conditions by:

- tuning the variation of C, the capacitance value,

- applyingthe ‘beforehand charged energy to C’ ECo

applyingthe ‘constant charging power Pct’from the overhead line,

throughexecution of a Matlab-Simulink simulation.

The benefits of applying a constant charging-power Pct are:

- a system operating in a cycle in which capacitance charging is adequate,
- more equal grid,
- lower grid losses in the motoring operation mode.
In this investigation, the value of the factor d

d=(Ucmin /Ucmax) (1)

is about 48 % if the Ucmin is 400 V. In a real-world application, this value of d is acceptable.

Our simulation considered the two distancesbetween stations.

When varying the mass for a railcar from 30 to 55 t, the maximum motor current is a function of mass to achieve similar acceleration and speed

Figure 7. The speed, the covered distance, the motor currents

and powers between two stations800m apart

Energety characteristics (for the same case in figure 7)

Figure8. Mass, m is varied from 35-55 t. Energy consumption of motors Emot,

energy of C Ec, voltage of C Uc and current of overhead lineIlinev. time. Due to constantly

charging power,line current remains constant.

Improving the charging method

- If the charging power is not only a constant value but is varied by some function of the total motor current or motor power over the time of traction, then
- Line energy consumed is higher, energy derived from C is lower, and the necessary value of C will be less.
- Consequently, the charging power has two components,
- - one as a function of motor power adjusted by a “correction factor”
- - another a much lower “constant charging power”, Pct.
- (In investigating other functions as well, the correction factor consistently performed best, proportional to motor power.)

Varied the set of energy management by a range of correction factor values, from 0 to0.4

Figure 9 The energy consumption Eused does not depend on the correction factor. An increase in the current from the line decreases the needed value of C.

Calculating the actual energy Ech to charge into C by charging power Pch may be realized with two components:

At correction factor 0.4 the energy by motors flowsin rate of 60 % from the C, and 40 % from overhead line.

These task is solvable by the adequate voltage-control of DC-DC converters.

Energy management is executable with the controller, measuring

- motor current and voltage,

- speed,

- and line voltage

and calculating the motor power.

Application of the correction factor

Figure 11. Overhead line current is NOT constant, varying in proportion to the grade at corrfact=0.4

The decrease of the needed capacitance by applied correct factor 0.4

Figure 14. The decreasingof the possible need minimum values of the capacitance

between cases corrfact=0 and corrfact=0.4. (In the case of corrfact=0, C increases by 5.5 F)

The energy saving vs. the speed and the distance

Figure12. Theratedenergy saving vs. the speed (km/h) and the distance (m) between stations.

(The value of 0.55 is 55 %.)Energy saveddoes not depend onthe correction factor.

The actually needed minimum capacitance vs. the speed and mass

Figure 13. The actually needed minimumvalues of the capacitance C needed vs. the speed (km/h) and mass (t) at corrfact=0.4,

by its two-varied function

Hybrid energy storage by Li-ion batteries and by supercapacitor

Fig. 10: The model of hybrid energy storage system on a railcar: the battery is parallel switched with scap and both controlled by energy-management through own DC-DC converter. We sold aseparately variable method for handle the control of capacitive energy storage and one of the battery.

The curves of the Matlab’2009-modelled Li-ion battery (750 V, 30 Ah)

Fig. .The curves of the Li-ion battery. If the discharge current is low as like 40 A the discharging time is 2,25 hour and this time decreases to 8,2 minutes if the current set to 180 A.

- Searching of a suitable control method
- We set a model according to Fig. 11, and we solved the separately variable method for handle the control of capacitive energy storage and one of the battery.
- For managing all these tasks we investigated the behaviour of two control for the two energy-storage. In this model we applicate a current-limit method instead of a current control: we had searched and set the suitable upper and lower current values of the battery.
- We could show that the values of battery current are suitable all operation cycle.

- The limitations of battery current are suitable all operation cycle.
- When the limitation operates the need current flows to or from capacitor only.
- These peaks of current are proved by the capacitor in both direction.
- In this solution achived an aime that the energy storage is firstly proved by battery, but for giving or receiving the peak-current there is a little supercapacitor.

The system features in grade + 40 %o in 5 distances :

Figure 15: Battery voltage, S.O.C., current battery, current SCAP, voltage SCAP according to Fig. 14. PCt=124 kW, cf=0.271, SOCo=66 %, current limits +172, - 180 A.

Figure 14: Speed, distance, motor current, motor power and line current. Grade = + 40%o.

The system features in grade - 30 %o :

Fig. 26. Thegrade is - 30 %o. The speed of energy consumption (‘dch’) is high, and under regenerativ braking at charging (‘ch’) is longer and moderate.

PCt=0 kW, cf=0, SOCo=66 %, current limits +180, - 217 A

The needed capacitance: 1 to 1.6 F, the decreasing is very significant (for a hybrid energy storage,with cooperation a Li-ion battery 750V, 30 Ah)

- The available decreasing ratio of the needed hybrid energy storage system at case SCAP is 30 % to 60 % - with this improved hybrid energy control method.
- These are significant values as decreasing in volume, mass and price.
- This novel process and its results are practically independent of the type of the traction motor.
- For these tasks the mass of SCAP is about 1500 kg. The mass of 800 kg about with presented Li-ion battery + SCAP hybrid storage-system, without converters.
- Mass reduction of this hybrid storage system is significant, about 50 %rated to supercapacitor type energy storage.

The correction factor must be varied, 0.1 to 0.4, instead of a constant value 0.4

- The change of Pct, constant power from line is larger:

Thanks for your attantion