1. ENERGY MANAGEMENT AND HYBRID ENERGY STORAGE IN METRO RAILCAR
ICRERA 2012 Nagasaki
November 2012
ENERGY MANAGEMENT AND HYBRID ENERGY STORAGE IN METRO RAILCAR
Istvan Szenasy
Szechenyi University
Dept. of Automation
Hungary

2. ICRERA 2012 Nagasaki
Renewing braking energy can significantly reduce the energy consumed in short-distance traffic
Using Matlab-Simulink to model an urban-metro railcar of the Budapest Metro Railway as a theoretical case,
we have investigate the possibilities reducing the needed minimal capacitance value

3. Railcar model with capacitive energy storage
ICRERA 2012 Nagasaki
Railcar model with capacitive energy storage
Mass without load 34 t,
fully loaded 44 t.
The total rated 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
The weakening of the DC motor fields begins over the speed of 36 km/h.
Charging-discharging of the SCAP
is executed by its bidirectional DC-DC converter.
Fig. 10: The railcar model

4. ICRERA 2012 Nagasaki
Figure 2. Aim and direction of simulations and calculations in modelling
Our objective was to determine the lowest necessary capacitance value for a supercapacitor (C) under different conditions (mass, speeds, grades, stopping distances.)
We’d achieved this under all conditions by:
- by varied of the capacitance value C,
- applying the ‘beforehand charged energy to C’ Eco, the initial energy level in capacitor
applying the ‘constant charging power Pct’, a constant power from the overhead line
The object is to storage all braking energy

5. ICRERA 2012 Nagasaki
Varied the mass from 30 to 55 t, the maximum motor current is a function of mass to achieve near the same acceleration and speed
Figure 3. The speed, the covered distance, the motor currents
and powers between two stations 800 m apart

6. ICRERA 2012 Nagasaki Energetics for previous case
Figure 4. Mass is varied from t. Energy consumed: Eused, energy charged to C: Ect,
energy of C supcap: Ec, voltage of C: Uc and current of overhead line: Ilinev. time. Due to constant charging power Pct line current remains constant

7. ICRERA 2012 Nagasaki For improving the charging method
we change the energetics conception:
if the charging power will not be a constant value, but is varied by function of the motor power, then
taken energy from line will be higher, the energy derived from C will be lower, and the necessary value of C will be less.
Consequently the charging power has two components,
- one, as function of motor’s power multiplied by a “correction factor”,
- another, a “constant charging power”, Pct, which will be lower now

8. ICRERA 2012 Nagasaki
The appliable range of correction factor is 0 to 0.4. Its effects are presented here:
The current - time area correspond to an electrical charge in the capacitor, that decreases the needed capacitance
Figure 5. The energy consumption, Eused depends not on the correction factor. An increase in the current from the line decreases the needed value of C.

9. ICRERA 2012 Nagasaki
The actual energy Ech to charging into C by power Pch may be realized by two components: the constant charger power and an ratio of motor’s power
At correction factor 0.4 the energy by motors flows in 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

10. ICRERA 2012 Nagasaki Application of the correction factor
Figure 6. Line current will not be constant, but varying in proportion to the corrfact=0.4

11. Energy saving depends not on the correction factor
ICRERA 2012 Nagasaki
The theoretically available energy saving vs. the speed and the distance
Figure 7. The rated energy saving vs. the speed and the distance between stations.
Energy saving depends not on the correction factor

12. The actually need minimum capacitance vs. the speed and the mass
ICRERA 2012 Nagasaki
The actually need minimum capacitance vs. the speed and the mass
Figure 8. The actually needed minimum values of the capacitance C needed vs. the speed and mass, at corrfact=0.4
by its two-varied function

13. ICRERA 2012 Nagasaki
The decreasing of the needed capacitance by correction factor 0.4
Figure 9. The decreasing of the possible needed minimum capacitance, if correction factor changes from 0 to 0.4

14. ICRERA 2012 Nagasaki
Hybrid energy storage by Li-ion batteries and by supercapacitor
Fig. 10: The model of hybrid energy storage: the battery is parallel switched with scap. Both are controlled by energy-management through own DC-DC converter.

15. ICRERA 2012 Nagasaki
The curves of the Matlab-modeled Li-ion battery (Matlab’2009)
Fig. 11.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.

16. ICRERA 2012 Nagasaki
Searching a suitable control method
We set a model according to Fig. 10, and we solved a method for control the capacitive storage and the battery
For managing all these tasks we investigated the behaviour of controls for the two energy-storages.
In this model there is a current-limit method instead of a current control: we’d searched and set the suitable upper and lower current limits of the battery.

17. ICRERA 2012 Nagasaki
These limits are suitable all operation section.
When the limits operate, the current flows to, or from the capacitor only
This solution is achieved an aime: the energy storage should be improved and its volume be reduced by battery.
For giving or receiving the peak-currents there will be sufficient a little supercapacitor

18. ICRERA 2012 Nagasaki
The system features in grade %o through 5 distances :
Figure 12: Speed, distance, motor current, motor power and line current. Grade = + 40%o.
Figure 13: 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, A.

19. ICRERA 2012 Nagasaki The system features in grade - 30 %o :
Fig. 14. The slope is - 30 %o. Energy consumption is at ‘dch’, and charging by regenerativ braking is at ‘ch’.
Here the capacitor gives or receives the surplus only, in a triangle shape .
Pct=0 kW, cf=0, SOCo=66 %, current limits are +180, A

20. The applied current limits vs. grade
ICRERA 2012 Nagasaki
The applied current limits vs. grade
Fig. 15: the function of applied upper current limits vs. grade
Fig. 16: the applied lower current limits vs. grade

21. ICRERA 2012 Nagasaki The main result:
the need smallest capacitance is 1 to 1.6 F,
the decreasing is very significant (for a hybrid energy storage, with cooperation a Li-ion battery of 750V, 30 Ah)
Fig. 17: The possible minimum needed capacitance for Scap at hybrid storage is decreased to F,

22. Here the grade is varied
ICRERA 2012 Nagasaki
The correction factor now must be varied, from 0.1 to 0.4, instead of its constant value of 0.4
Fig. 18: In hybrid system the correction factor must be varied from 0.1 to 0.4 to achieving lower capacitance, in function of grades, mass and speeds.
Here the grade is varied

23. Here the grade is varied
ICRERA 2012 Nagasaki
The changement of constant power from line, Pct is larger:
Fig. 19: The constant power from line must be set that change of S.O.C. should be adequate
Here the grade is varied

24. ICRERA 2012 Nagasaki
The available decreasing ratio of the needed hybrid energy storage system at case SCAP is 30 % to 60 %
These are significant 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 scap type energy storage.

25. Thanks for your attantion
ICRERA 2012 Nagasaki
Thanks for your attantion