Control of Ultracapacitors Energy Exchange

Indrek Roasto, Tonu Lehtla, Taavi Moller and Argo Rosin

Tallinn University of Technology/ Department of Electrical Drives and Power Electronics, Tallinn, Estonia

Abstract-Ultracapacitors can store and also give out largeenergy pulses. That makes them perfectly suitable as shortterm energy storage devices in electrical vehicles. The usageof ultracapacitors prevents over sizing of the battery packdue to peak power demands. This paper introduces thebasic concept of how ultracapacitors could be used inTramways. The main purpose of the paper is to introduceone possible control-strategy of ultracapacitors energyexchange. With the energy exchange is meant the movementof energy form the ultracapacitors into distribution networkand vice versa. Three different regulators, proportional (P),integration (I), and the combination of both (PI) are testedand compared to each other. Practical results are analyzedand compared with the simulation results.

I. INTRODUCTIONAll new tramways today have a modern voltage

converter on board to control speed and acceleration.These converters are many times more effective, than theold resistive control. One more advantage of voltageconverters is the possibility of regenerative breaking i.e.they can break electrically by leading the energy back intothe network. However this is only reachable in situations,where another tramway in the near is simultaneouslytaking up energy form the network. Since these situationsoccur seldom, the regenerative breaking can often not beused. The breaking energy is turned into heat on breakingresistor. The solution would be to use some kind of energystorage device like batteries. Batteries however have ahigh energy density but quite low power density, whichmeans they cannot store or give out large power pulses.That makes batteries unsuitable for breaking energystorage device. The according battery pack would beexcessive over sized. Good alternative to batteries areultracapacitors. They have higher power density thanbatteries and very long life expectancy up to 500 000cycles. Ultracapacitors suit best for Power sources, wheredischarge is needed in approximately 1...60 seconds [1].

II. APPLYING ULTRACAPACITORS IN TRAMWAY

For Railway applications there are basically twopossibilities for applying ultracapacitors for energystorage. One is to recycle energy in the vehicle andanother is at power supply substations. This paper isconsidering the first option [2].

In Fig. 1 a simplified power electronics circuit of aTramway with ultracapacitors is considered. The tramwayis supplied form the medium-voltage (Ulm,e= 600V) DCdistribution network. LI and Cd, create a DC filter. Rb. is abreaking resistor. The DC motor (M) is connected to theDC link through bidirectional traction converter.Ultracapacitors are connected to the DC link through asingle step up/down converter. This solution allows thevoltage adaptation of the ultracapacitor tank during itscharge or discharge.

Fig. 1. Basic schematic of a Tramway (DC traction drive)

The main advantages of this solution are its low priceand simple construction.

There is also a disadvantage. The discharge currentthrough the upper diode (D3) cannot be controlled incertain situations. Despite of the disadvantage thissolution was also used in the test prototype of theultracapacitor energy storage device [3].

III. CONTROL-STRATEGY OF THE ULTRACAPACITORS

The most important factor is the line voltage (Uline).Line voltage fluctuation is related to the energyconsumption from the distribution network. If a tramwayis accelerating, it causes a voltage drop in the network.Other way if the tramway is in regenerative breakingmode, the line voltage goes up. Therefore according to theline voltage you can decide whether there is lack ofenergy or energy overflow in the network and whether tocharge or discharge ultracapacitors. The main idea is tostabilize voltage fluctuations in the net and hold the linevoltage inside certain boundaries. To control energyexchange, three voltage values need to be defined asshown in Fig. 2. In addition ultracapacitors charge statemust be estimated. Since the energy of a capacitor isproportional to the terminal voltage as shown in the ( 1),the charge state can be estimated according to terminalvoltage.

540V 600V 660V

Fig. 2. Predefined Line voltage values; Umin- minimum allowed line

voltage, Un- nominal voltage, Umax- maximal accepted line voltage

1-4244-0121-6/06/$20.00 C 2006 IEEE

- - 11I I I

1401 EPE-PEMC 2006, Portoro2, Slovenia

BEGIN

Uline <Um

UI<lne >Umax/

yes

Lack of Energy

\UC charged

Nominal area

yes Energy overflow

UC charged

yes yes

are not already discharged. If the line voltage Uline raisesUmax, ultracapacitors will be charged unless they are notcharged yet. Charging and discharging will stabilize theline voltage so that it stays between values U1i1 and Umax.

Charge and discharge processes can be controlled inno various ways. In this paper mainly three controllertypes P,

I, PI are considered. The controller controls line voltageby affecting charge or discharge current of theultracapacitors. That is done by pulse width modulation(PWM), where pulse width (PW) is the control variable.The regulator calculates PW according to the line voltage.

IV. SIMULATION

Discharging UC Standby Standby Charging UC Standby

Fig. 3. General algorithm of the control strategy ofthe ultracapacitors

E = CU2 ( 1)2

Where the U is the capacitor terminal voltage and C isthe capacitance.The whole control process can be divided into three steps:

1. line voltage measurement and estimation,2. ultracapacitors voltage measurement and

estimation3. charging, discharging or standby of the

ultracapacitorsThe general algorithm of ultracapacitors control

strategy is shown in Fig. 3. If the line voltage Ulime sinksunder Urin, ultracapacitors will be discharged unless they

All models are based on the test prototype. So thesimulation results can be compared with realmeasurements on the prototype. The simulation is dividedinto two parts: charging ultracapacitors, dischargingultracapacitors. In both cases three different controllers (P,I, PI) are compared.

Since this whole model is based on a scaled testprototype, the voltages and currents are scaled down aswell. The used values are as follows:Uln = 50V (nominal line voltage)Umax = 60V (maximal allowed line voltage)Um11 = 40V (minimal allowed line voltage)

A. Charging UltracapacitorsSimplified charge model of ultracapacitors is shown in

Fig. 4. The model is divided into three parts. The first partis a simplified model of the distribution network. Thesecond part presents a tramway and in the third partcontrol blocks are showed. Although the model is not veryprecise, considering the real tramway, it still can be usedto research control principles.DC motors of the tramway are replaced with a resistor

Rload as shown in Fig. 4. That means the load is fixed.

Distribution networkRine

El

I-I.VMI VM2

I1Ct

Tramway

C2

1500u R218k

0.47u-_

C3 _ <, R31500u 18k

Control-block

Linevoltage

Um ax Negator GAIN

INTG

pw m 1

PW

TUcl

PWI\/Si

'-/

4 Ducl

1~~~~4 1- i~~~~ 4--

TPH1

Vuc

Fig. 4. Electric schematic of charge model of ultracapacitors

1402

CTRL := 1 %LI AM1

lom , Ruc

\ Duc2 00- Cuc

VM3J~ 20

I DIl

AM2

- RloadD12 .Ma

Ij

i .I i v

I I I I I I I I I

-no

no

-q___

Voltage characteristic with P controller0.12k

80

60

40

20

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Fig. 5. Charging ultracapacitors with P controller (Simulo

Voltage characteristic with controller0. 12k

0.1k

80

60

20

:00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Fig. 6. Charging ultracapacitors with I controller (Simula

Change characteristic of line voltage0. 12k

0.1k

80

60

40X

20

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Fig. 7. Charging ultracapacitors with PI controller (Simul

The transistor Tll is constantly opened. UltracapacitorsVM21V [A are charged through a step-down converter. The transistor

Tucl is controlled with PWM. Substation voltage El isvariable. It is measured by voltmeter VMI. Thecharacteristic of the substation voltage (VMI.V) is shownin Fig. 5, Fig. 6 and Fig. 7.The control process begins with line voltage

measurement. That is done by voltmeter VM2 as shown int [s] Fig. 4. Secondly the terminal voltage of Ultracapacitors is

measured. If the voltage exceeds the maximal allowedation) value i.e. capacitors are full charged, the switch S1 will

open and interrupt the charge process. The capacitorvoltage is controlled by a two-point element with

VM1.V [] hysteresis (TPHI).VM2.V[

Firstly a pure proportional (P) controller was tested i.e.the integral (INTG) part was temporary denied. The resultis presented in Fig. 5. It is clear that a pure P controller isnot suitable for this kind of process. It does not stabilizethe line voltage near Umax (60 V). A steady-state error(about 20 V) remains.

t [s] A pure integration (I) controller gives much betterresult as shown in Fig. 6. The line voltage is stabilized and

tion) kept under Umax (60 V). But the disadvantage of Icontroller is its slow reaction. It is noticeable in Fig. 6, theline voltage (VM2.V) is raising over 80 V before the

VM1 [V regulators influence will be remarkable.VM2[V

Combining P and I controllers we get PI controller. Theresult of PI controller is shown in Fig. 7. Compared to theother controllers it is undoubted the best solution. It actsquicker as a pure I regulator, because of the P part andthere is no steady-state error because of the I part.

B. Discharging Ultracapacitorst [s] Simplified discharge model of ultracapacitors is shown

Lation) in Fig. 8. Similar to the charge model the discharge modelconsists of three parts- distribution network, tramway andControl-block.

Fig. 8. Electric schematic of discharge model of ultracapacitors

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-1-1

A ., ,. . .. 6 Lk,l,.JAL..Vfvfhqm

I

Capacitor discharge with P controllerVM1.V [V]Vw.V [V]

20

08t[st

Fig. 9. Ultracapacitor discharge with P controller (Simulation)

Capacitor discharge with controller

0. 12k

0.1k

VM1.V [V]VM2.V [V\

80

60~

40< _

20

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 t [s]

Fig. 10. Ultracapacitor discharge with I controller (Simulation)

Capacitor discharge with Pi controller0. 12k

0.1k

VM1.V [V]VM2.V []

80

60 t\ --

20 ~ ~

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 t [s]

Fig. 11. Ultracapacitor discharge with PI controller (Simulation)

Ultracapacitors are discharged through a step-upconverter. The voltage of the substation El is measured byvoltmeter VMI. The change characteristic of thesubstation voltage (VMI.V) is shown in Fig. 9, Fig. 10and Fig. 11.

The line voltage is measured by the voltmeter VM2.The measured value is subtracted from the setpoint Umin.The subtraction gives system error. According to the errorpulse width will be changed. By opening the transistorTuc2 we actually create a short circuit. Therefore maximalpulse width cannot be allowed and it is limited to 70 %.

Secondly the discharge depth of the ultracapacitors isevaluated. In case the terminal voltage of ultracapacitorssinks under allowed level, the switch S 1 will be opened,interrupting the step-up conversion.

Three controller types were tested. A pure P controllercreates a steady-state error and strong voltage ripple asshown in Fig. 9.A pure I controller gives us desired results, no

steady-state error and no voltage ripple. The onlydisadvantage is its inertia. Integration takes time and thatmakes the control slow. The voltage stabilizes within 400ms as presented in Fig. 10.

Fig. 12. Scaled prototype of the ultracapacitor energy storage device

PI controller achieved best results. The proportionalpart speeds up the responds time the integral parteliminates the steady-state error of the P controller. In Fig.Ithe voltage progression is shown. The line voltage will

be stabilized and hold near Urin (40 V).

V. PRACTICAL TESTSA scaled prototype was build to observe the behavior of

such system and compare the measurements with thesimulation results. The scaled prototype is presented in thepicture of Fig. 12. The prototype has followingparameters: maximal input voltage U1 put=380V DC,maximal input current Imax= 50 A DC, ultracapacitystorage device C = 20 F, Uuc max = 112 (8 capacitors in therow, 160 F and 14 V each), voltage converters are realizedwith a IGBT block. As control unit a Siemensmicrocontroller C167 is used.

The electrical schematic of the prototype is shown inFig. 13. Voltage is measured with two voltmeters VMIand VM2. In addition current is measured with threeammeters AMI, AM2 and AM3.

The system was programmed in C language. Threedifferent controllers (P, I and PI) were implemented usingfollowing mathematical algorithms [4], [5]:

PW = KP' IAUline

PW = KI AU1i, At

PW = KP *AUline +KI* AUine -At

(2)

(3)

(4)

1404

0. 12k

0.1k

80

60 <

40 r ....UJMW N

Rline AM1

El + ~ jvml ciiE1(A) °V 0. 47U

C03 R31500u 18k TuC2]

Fig. 13. Electrical sc]

Where PW is pulse width, Kp is proportional constant,Ki is integration constant, AUiine is system error, At is thetime step.

The equation ( 2) is the algorithm of the proportionalcontroller. System error is proportional to the controlvariable PW. The Equation ( 3) is presenting the algorithmof the integration controller. The change speed of thesystem error AUiine is proportional to the control variablePW. By adding ( 2) and ( 3) we get the algorithm of the PIregulator i.e. equation ( 4).

A. Charging UltracapacitorsThe charge process starts, when the line voltage VM1

reaches Umax (60 V). The purpose is to hold line voltagebelow 60 V. The energy overflow will be stored in theultracapacitors. The substation voltage El is changed thesame way as in the Simulation shown in Fig. 5, Fig. 6 orFig. 7.

Using P controller, the voltage cannot be held in desiredlimits. It rises up to the 80 V as shown in Fig. 14. Thesteady-state error is approximately 80-60 = 20 V. Theresult is almost the same as in the simulation. ComparingFig. 5 with Fig. 14 the only difference is higher voltageripple in Fig. 14. That probably means, that the realregulator needs more time to react on voltage fluctuations.In the simulation the regulator reacted without any delay.

Fig. 15 represents experimental result of the integrationcontroller. Integration controller creates no steady-stateerror. However in the beginning a voltage peak occurs dueto slowness of the integration control. The test result is inaccordance with the simulation. There are two littledifferences in the test result, higher voltage ripple andvoltage peak in the beginning (over 100 V). This impliesto the fact that the real regulator works slower than thecomputer simulation.

UVM1[V]

Rload33

;hematic of the prototype

Fig. 15. Charging ultracapacitors with I controller

..[[V ] .:.:

t [ms]

Fig. 16. Charging ultracapacitors with PI controller

Again the PI controller is giving the best results aspresented in Fig. 16. The controller has no steady-stateerror and responds quickly to voltage change. If youcompare the test results with the results of the simulation,you can notice no difference. The computer model isaccurate enough.B. DischargingUltracapacitors

The discharge process starts, when the line voltageVM1 sinks below Umin (40 V). The purpose is to hold linevoltage above 40 V. The lack of energy in the net will becovered with ultracapacitors energy. The substationvoltage El is changed as shown in Fig. 9, Fig. 10 and Fig.11.A pure proportional controller creates a steady-state

error (about 10 V) and a strong voltage ripple as showedin Fig. 17. The test result is in accordance with thesimulation. No essential difference can be seen.The integration controller creates no ripple or steady-

state error, but is quite slow. It needs some time torespond to the system error (AUiine). Therefore in the

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Fig. 14. Charging ultracapacitors with P controller

beginning a voltage peak accrues as shown in Fig. 18. Theline voltage stabilization takes about 1 s. In the simulationit took only 400 ms. Apparently real regulator worksslower than the computer simulation.The best choice is PI controller. It has no steady-state

error and it also responds quicker than integrationregulator as it can be seen in Fig. 19. Comparing Fig. 19with Fig. 11 no substantial differences can be seen.

UVM1l[V]:::

50

..

30

20

-..Pb C hI: 10Volt 500rmst [ms]

Fig. 17. Discharging ultracapacitors with P controller

UVM1l[V]-

50

40~:30....

20

) Ch.l: 1 0 Volt 500 mst [ms]

Fig. 18. Discharging ultracapacitors with I controller

UVM1[V]:

50

40

30

20

t [ms]

Fig. 19. Discharging ultracapacitors with PI controller

VI. CONCLUSIONUltracapacitors can be used as short-term energy

storage devices in the tramway. Instead of heatingbreaking energy in the resistor, the energy can be stored inthe super capacitor tank and used later in accelerationprocess. The control-strategy presented in this paper wasmainly based on the line voltage. The line voltagefluctuations give information, whether there is energyoverflow or lack of energy in the distribution network andaccording to that ultracapacitors can be charged ordischarged.

There was a good harmony between test and simulationresults. Since the real regulator doesn't react instantly,little time delays occurred by some tests compared to thesimulation. But in principle the computer models workedfine.The advantage of the control strategy described in this

paper is its simple algorithm. The disadvantage is that linevoltage measurement does not give information aboutvoltage breakdowns i.e. if the line voltage breaks down forsome reason, then all the tramways will discharge theirultracapacitors into the network. The voltage breakdowncould be detected using an additional current control.However the purpose of this paper was to simulate andresearch the line voltage stabilization with ultracapacitors.Simulation and tests gave the desired results. The linevoltage was kept inside allowed limits and voltage peakswere smoothed. Which means that this control strategy ofenergy exchange works fine. Three types of controllerswere tested to control ultracapacitors energy exchange inthe tram. The best results were received by using the PIcontroller. The proportional part rises the controllerresponse time and integration part eliminates thesteady-state error. Although the PI controller worked fine,as next step also PID controller should be tested andsimulated. It is however more complicated but also moreadvanced and can give even better results than jus PIcontroller.

REFERENCES[1] S.R.Holm, H. Polinder, J.A. Ferreira, P. van Gelder and R. Dill,

"A Comparison of Energy Storage Technologies as EnergyBuffer in Renewable Energy Sources with respect to PowerCapability"

[2] E. Takahara, J. Yamada, "A Study for Electric Double LayerCapacitor Application to Railway Traction Circuits for EnergySaving"

[3] Prof. A.Rufer, D. Hotellier, and P. Barrade, "Power-ElectronicInterface for a Supercapacitor-Based Energy-StorageSubstation in DC-Transportation Networks", IEEETransactions on power delivery, vol. 19, no. 2, April 2004

[4]

[5]

C. T. Kilian, "Modern Control Technology: Components andSystems"K. Beuth, and W. Schmusch, "Grundschaltungen",Elektronik 3, 13th ed. Wiirtzburg: Vogel, 1997, pp. 381-387

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