GREAT MINDS THINK ELECTRIC / NEW ENERGY MANAGEMENT AND HYBRID ENERGY STORAGE IN METRO RAILCAR...

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NEW ENERGY MANAGEMENT AND

HYBRID ENERGY STORAGE IN METRO RAILCAR

Istvan SzenasySzechenyi University, Dept. of Automation

Hungary


NEW ENERGY MANAGEMENT AND HYBRID ENERGY STORAGE

IN METRO RAILCAR

• 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.


IN METRO RAILCAR



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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 fields begins over the speed of 36 km/h. Charging-discharging of the SCAP ( abbreviated C) is executed by its bidirectional DC-DC

converter.



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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) for the storage of all regenerative braking energy under different conditions (mass, speeds, grades, stopping distances.)

We set the optimum initial supercapacitor voltage to 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, - applying the ‘beforehand charged energy to C’ ECo applying the ‘constant charging power Pct’ from the overhead line,through execution of a Matlab-Simulink simulation.



IN METRO RAILCAR

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 distances between stations.



IN METRO RAILCARWhen 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 stations 800 m apart



IN METRO RAILCAREnergety characteristics (for the same case in figure 7)

Figure 8. 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 line Ilinev. time. Due to constantly

charging power, line current remains constant.



IN METRO RAILCAR

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.)



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Varied the set of energy management by a range of correction factor values, from 0 to 0.4

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.



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t t t

dtPmotcorrfactdtPctdtPchEch0 0 0

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 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, measuring - motor current and voltage, - speed, - and line voltageand calculating the motor power.



IN METRO RAILCARApplication of the correction factor

0 50 100 150 200 s0

20

40

60

grade= -20,-10,0,10,20 %o, corrfact=0.4, d=800m, m=40 t, Imaxmot=f(grade)

v (

km

/h)

0 50 100 150 200 s0

1000

s (

m)

0 50 100 150 200 s-400

-200

0

200

400

Imot

(A)

0 50 100 150 200 s

-200

0

200

400

Pm

ot

(kW

)

0 50 100 150 200 s0

200

400

I line (

A)

grade=20%o

grade=20%o

grade=20%o

grade=20%o, C=11.6, Pct=48

grade= -20%o, C=15,Pct=0

grad=10%o, C=10.4, Pct=25

grade=0%o, C=10, Pct=2

grade= -10%o, C=15, Pct=0

Figure 11. Overhead line current is NOT constant, varying in

proportion to the grade at corrfact=0.4



IN METRO RAILCAR

The decrease of the needed capacitance by applied correct factor 0.4

Figure 14. The decreasing of 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)



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The energy saving vs. the speed and the distance

Figure12. The rated energy saving vs. the speed (km/h) and the distance (m) between

stations.(The value of 0.55 is 55 %.) Energy saved does not depend on the correction factor.



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The actually needed minimum capacitance vs. the speed and mass

Figure 13. The actually needed minimum values of the capacitance C needed vs. the speed (km/h) and mass (t) at corrfact=0.4,by its two-varied function



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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 a separately variable method for handle the control of capacitive energy storage and one of the battery.



IN METRO RAILCAR

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



IN METRO RAILCAR

• 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.



IN METRO RAILCAR

• 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.



IN METRO RAILCAR

0 100 200 300 400 500 560s

0204060

v, km

/h

grade = 40% o, d = 5*800 m, I max mot = 300 A, m = 40 t

0 100 200 300 400 500 560s0

2000

4000

s, m

0 100 200 300 400 500 560s

-200

0

200

400

Imot,

A

0 100 200 300 400 500 560s-200

0

200

400

Pmot,

kW

0 100 200 300 400 500 560s

200

300

I line

, A

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

0 100 200 300 400 500 560s700

800

900

Uba

tt, V

grade = 40% o, d = 5*800 m, I max mot = 300 A, m = 40 t

0 100 200 300 400 500 560s586062646668

SOC

bat

t, %

0 100 200 300 400 500 560s-200

0

200

Ibat

t, A

0 100 200 300 400 500 560s

-100

0

100Ic

ap, A

0 100 200 300 400 500 560s

600

800

Uca

p, V

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.

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



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The system features in grade - 30 %o :

Fig. 26. The grade 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



IN METRO RAILCAR

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.


IN METRO RAILCAR


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


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


IN METRO RAILCAR


Thanks for your attantion


IN METRO RAILCAR

GREAT MINDS THINK ELECTRIC / NEW ENERGY MANAGEMENT AND HYBRID ENERGY STORAGE IN METRO RAILCAR...

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