NEW ENERGY MANAGEMENT AND HYBRID ENERGY STORAGE IN METRO RAILCAR. Istvan Szenasy Szechenyi University, Dept. of Automation Hungary. NEW ENERGY MANAGEMENT AND HYBRID ENERGY STORAGE IN METRO RAILCAR.
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Szechenyi University, Dept. of Automation
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:
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
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,
- 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.
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 correction factor must be varied, 0.1 to 0.4, instead of a constant value 0.4
Thanks for your attantion