Hierarchical Optimization of an On-Board Supercapacitor Energy Storage … · 2020. 9. 4. · ZHONG...

2576 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 69, NO. 3, MARCH 2020

Hierarchical Optimization of an On-BoardSupercapacitor Energy Storage System

Considering Train Electric BrakingCharacteristics and System Loss

Zhihong Zhong , Student Member, IEEE, Zhongping Yang , Member, IEEE, Xiaochun Fang , Member, IEEE,Fei Lin , Member, IEEE, and Zhongbei Tian , Member, IEEE

Abstract—In order to absorb the regenerative braking energyof trains, supercapacitor energy storage systems (ESS) are widelyused in subways. Although wayside ESS are widely used, becauseof the influence of no-load voltage, and so on, a wayside ESS cannotabsorb all the regenerative braking energy in some special cases,and the brake resistor is still activated, which leading to the wast-ing of energy. In order to completely replace the on-board brakeresistor, this paper configures a certain on-board super-capacitor,and based on a DC-side series super-capacitor topology, proposesa hierarchical optimization energy management strategy (EMS).The EMS is divided into three layers: Firstly, the strategy canincrease the inverter-side voltage in a short time without changingthe traction network voltage, and improve the train braking char-acteristic curve by utilizing the short-time overvoltage capabilityof the inverter and the motor; Secondly, by coordinated controlwith a wayside supercapacitor, the residual regenerative brakingenergy can be absorbed even in special cases; Finally, based onloss calculation and current prediction, this strategy can effectivelyreduce the system loss by adjusting the DC voltage on the inverter-side. The proposed control strategy is validated through RT-LABexperiment, and the experimental results agree well with the theory.

Index Terms—Braking characteristic curve, energymanagement strategy, on-board supercapacitor, regenerativebrake failure, system loss, urban rail.

NOMENCLATURE

UDC DC voltage on inverter side.UFC Supporting capacitor terminal voltage.Uc On-board supercapacitor terminal voltage.VCE/VFO Turn-on voltage drop of IGBT/diode.Vn IGBT rated voltage.Usmax Maximum voltage amplitude of motor.usd/usq Voltage of d/q axis in motor.idc Traction network current.

Manuscript received January 10, 2019; revised October 21, 2019, December1, 2019, and January 13, 2020; accepted January 14, 2020. Date of publicationJanuary 17, 2020; date of current version March 12, 2020. This work wassupported in part by the Fundamental Research Funds for the Central Universitiesunder Grant 2019JBM061. The review of this article was coordinated by Dr. B.Akin. (Corresponding author: Zhihong Zhong.)

Z. Zhong, Z. Yang, X. Fang, and F. Lin are with the Beijing Jiao-tong University, Beijing, 100044 China (e-mail: [email protected];[email protected]; [email protected]; [email protected]).

Z. Tian is with the University of Birmingham, Birmingham, West Midlands,U.K. (e-mail: [email protected]).

Digital Object Identifier 10.1109/TVT.2020.2967467

isd/isq Current of d/q axis in motor.Im Current flowing through the switch tube.In IGBT rated current.Ismax Maximum current amplitude of motor.Eline Line loss.Esc Internal resistance loss of supercapacitor.Econ_igbt IGBT on state loss.Esw_igbt IGBT switching loss.Emotor Motor loss.Esystem System loss.Ebrake Total regenerative braking energy.Eon(t)/Eoff (t) Turn-on/off loss under rated conditions.ωe Stator angular frequency.ωbase/ω1 Critical frequency of weak magnetic region

1/2.Rline Line resistance.rCE/rFO Turn-on resistor of IGBT/diode.Vout Inverter output voltage amplitude.fout Output frequency.N Carrier ratio fs/fout.m Modulation ratio.cosϕ Power factor.Lr/Lm/Ls Rotor/magnetizing/stator inductance.Te/σ Motor torque/ leakage inductance coeffi-

cient.np Pole pairs.

I. INTRODUCTION

W ITH the rapid development and the continuous maturityof the rail transit industry, energy consumption have be-come the focus of attention. In order to effectively utilize regen-erative braking energy, train timetable optimization, reversiblesubstation technology and energy storage technology are gettingmore and more attention. Train timetable optimization studiesthe synchronization problem of multi-train operation. By adjust-ing the traction and braking characteristics of the train, when onetrain brakes and regenerative energy is fed back to the third rail,the other train simultaneously accelerates and absorbs energyfrom the third rail [1]–[3]. Reversible substation provides a pathfor braking energy to enable it to flow in the opposite direction

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https://orcid.org/0000-0003-4274-6604https://orcid.org/0000-0002-7639-8217https://orcid.org/0000-0002-6773-9171https://orcid.org/0000-0001-9052-7233https://orcid.org/0000-0001-7295-3327mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]

ZHONG et al.: HIERARCHICAL OPTIMIZATION OF AN ON-BOARD SUPERCAPACITOR ENERGY STORAGE SYSTEM 2577

and feed power back to the main AC grid [4]–[6]. An energystorage system (ESS) that stores regenerative braking energy inan electrical storage medium, such as a supercapacitor [7], abattery [8], and a flywheel [9], and releases to the traction netor the third rail when needed. Storage media can be placed onthe vehicle [10] or on the ground [11]. Compared with the firsttwo methods, the advantage of energy storage is that it endowsregenerative braking energy with a time attribute [12]. Andcompared with other forms of energy storage, supercapacitors(SC) have higher power density, longer service life and can beused in a wider range of temperature, making them more suitablefor application in urban rail trains [13]–[15].

Wayside supercapacitors are usually installed in existing sub-stations or in specific places where the voltage of the contactnetwork varies greatly. The wayside SC has been widely studied,because the volume and weight limitation of installing a waysideSC is not very strict. Based on different optimization objects,different energy management strategies (EMS) are proposed.Reference [16] effectively solves the problem of voltage sag byadjusting the relationship between traction or braking energyand charging and discharging speed. Reference [17] proposesa strategy for dynamically adjusting the threshold, which caneffectively reduce the peak power of traction and braking. Thecharging and discharging strategy proposed in reference [18]can increase the frequency and number of vehicles withoutupgrading the substation. According to the reference [19], whenthe brake train and the energy storage device are too far apart,directly controlling the SOC of the super-capacitor can achievebetter results. Reference [20] considers the minimum energyconsumption and the maximum energy interaction betweentrains. It is pointed out that the best effect is obtained whenthe brake car voltage is equal to the braking resistor startingvoltage, and the strategy of charging and discharging thresh-old follow-up is proposed. Reference [21] points out that theoptimization of super-capacitor energy management strategycan be regarded as an isoperimetric problem. Based on theprinciple of Lagrange extreme value, the energy managementstrategies of minimizing loss, optimizing voltage stabilizationand minimizing substation output are proposed respectively. Inreference [22], the energy management strategy with minimumloss is also analyzed by using the principle of Lagrange extremevalue, and the optimization method of relevant parameters isproposed. The wayside supercapacitor has many advantages,but there are three unavoidable disadvantages: (a) it cannot becombined with train braking characteristics to increase brakingenergy; (b) when the braking train is far away from the energystorage device, the energy storage device cannot fully absorb thebraking energy, which will trigger the braking resistor; (c) thetransmission line loss is too high.

In onboard ESS, the storage medium is placed on the vehicle.It can be placed on the roof or under the floor of the vehicle. Theresearch of on-board supercapacitor, on the one hand, similarto the research of the wayside supercapacitor, the energy man-agement strategies such as the minimum loss, the best voltagestabilizing effect and the reduction of peak power are put forwardaccording to different optimization objectives. [10], [23]–[25].On the other hand, a series of energy management strategies

are proposed by combining the energy management strategy ofon-board super-capacitor with motor control, or motor tractionand braking characteristics. Reference [26] uses the real-timespeed of the motor to adjust the charge and discharge threshold ofthe supercapacitor, improving the overall charge and dischargeefficiency. Based on the series supercapacitor topology, refer-ence [27] proposes an energy management strategy that works inconjunction with the motor’s operating state, which can steadilyincrease the DC link voltage and reduce the peak input powerwithout degrading the motor drive performance. Reference [28]improves the series topology of reference [27] and improvesthe efficiency of charging and discharging. In reference [29],based on the series topology proposed in reference [28], theswitching devices of the series topology and the inverter areconsidered as a whole, and a new modulation strategy whichcan improve the energy utilization and reduce the harmonicof the motor output current is proposed. Although reference[30] suggests that the volume and important proportion of theon-board super-capacitor designed in this paper are very smalland can absorb all the residual regenerative braking energy, it isundeniable that the on-board super-capacitor is still limited bythe overall weight and residual volume of the vehicle in mostcases. In addition, the current research is lack of considerationof motor working conditions.

In order to give full play to the advantages of on-boardsuper-capacitor and wayside super-capacitor, a coordinated con-trol strategy of on-board super-capacitor and wayside super-capacitor is proposed based on the serial topology of reference[28]. Compared with the existing strategies, the strategy of thispaper can achieve the following three innovations on the premiseof ensuring the maximum interaction between trains and theoptimal use of energy: (1) It can improve the train braking char-acteristic curve, so as to increase the total regenerative brakingenergy. (2) Under the coordinated control of the on-board andwayside ESS, the residual regenerative braking energy of thetrain can be completely stored, and the brake resistor can becompletely replaced. (3) Based on the prediction of currentand the calculation of traction drive-system loss, the tractiondrive-system loss can be minimized by adjusting the DC voltage.

This paper is organized as follows: Section II is the intro-duction and explanation of onboard-wayside SC hybrid ESSand series SC topology. Section III is the introduction to thehierarchical optimization strategy of the on-board SC, whichintroduces the three optimization objects of the ESS. The lastpart is verification of the energy management strategy of theRT-LAB experimental device.

II. INTRODUCTION OF HYBRID ENERGY STORAGE SYSTEMAND ON-BOARD SUPERCAPACITOR TOPOLOGY

The DC power supply network of an urban rail transit systemwith wayside and on-board ESS is shown in Fig. 1. It can beseen that the on-board SC is connected in series between theinverter and the filter capacitor (FC). Compared with traditionaltopology [7], it has two advantages:

1) The topology does not need charging inductance, whichreduces the volume and weight of the system. In the series



Fig. 1. Onboard-wayside supercapacitor hybrid energy storage system.

Fig. 2. Series supercapacitor topology.

topology of this paper, the charge and discharge circuitof super capacitor is in series with the motor load. Whilethe motor load is an inductive load, it can play the roleof charge inductance. The experiment of [29] shows thatthe charge and discharge current of the circuit is smoothbefore and after the switch;

2) The topology can improve the braking force withoutchanging the filter capacitor voltage. The reasons are: a)The high-speed braking time of the train is short, generallywithin 10 s, it is acceptable to increase the inverter-sidevoltage in the short time; b) The supercapacitor in thetopology of this paper is installed on the right side of thefilter capacitor. Therefore, by applying the topology of thispaper, the voltage on the inverter side can be increasedwithout affecting the voltage of the filter capacitor andtraction network; c) The voltage on the DC side of theinverter directly affects the maximum braking force of themotor.

The series topology used in this paper is shown in Fig. 2, andits six working states are shown in Fig. 3.

For example, Fig. 3(a) corresponds to the traction charge state,in which the traction network current flows to the load motor,and the current flows through the switch tube D3 and D4 tocharge the on-board supercapacitor. At this time, the voltage atthe inverter side is lower than that at the filter capacitor side, andthe voltage relationship is: UDC = UFC − Uc. The completecorrespondence between the switching state and the workingstate is shown in Table I.

1) In state (a) and (d), UDC = UFC − Uc, inverter operatesin buck mode.

2) In state (b) and (c), UDC = UFC + Uc, inverter operatesin boost mode.

3) In state (e) and (f), UDC = UFC , inverter operates instandby mode.

Where, HB is a high-speed circuit breaker, FC is a filtercapacitor, Q1, Q2, Q3 and Q4 are IGBTs that control the charge

Fig. 3. Six working states of series topology in this paper. (a) Traction-charge.(b) Traction-discharge. (c) Brake-charge. (d) Brake-discharge. (e) Traction-idle.(f) Brake-idle.

TABLE IWORKING STATES OF THE SUPERCAPACITOR TOPOLOGY

and discharge of SC, D1, D2, D3 and D4 are their reverseparallel diodes, EDLC is an SC, and M is the load of the tractionmotor. The state of “Traction charge” means: when the train is intraction, the supercapacitor is in the charging condition. T1–T4represent the switch states of IGBT tubes Q1–Q4, 1 representson, and 0 represents off.

III. HIERARCHICAL OPTIMIZATION STRATEGY

In this section, the hierarchical optimization strategy of theon-board SC is introduced in detail. The section is divided intofour parts, the first three introducing three different optimizationobjects which are optimized from different aspects to absorbmore regenerative braking energy. There is a priority relationshipbetween the three optimization objects. The specific priorityrelationship and control process are described in the fourth part.

A. Improvement of Train Braking Characteristic Curve

The main idea of this part is to enhance the braking force ofthe motor in the high-speed zone, so as to increase the electricbraking energy. The relationship between DC voltage and motorpower, and the control strategy to enhance braking force in high-speed zone are introduced.

1) Relationship Between Voltage and Maximum BrakingForce: In a synchronous rotating coordinate system, accordingto rotor field orientation, the steady-state formulas of IM statorvoltage are given below [32]:{

usd = Rsisd + ωeσLsisq

usq = Rsisq + ωeLsisd(1)



Fig. 4. Two regions of weak magnetism are illustrated.

The steady-state formula of torque is given below [31]:

Te = npL2mLr

isdisq (2)

The motor operation is limited by stator current and voltage,as shown in (3). [32]{

usd2 + usq

2 ≤ Usmax2isd

2 + isq2 ≤ Ismax2

(3)

As shown in Fig. 4 [32], in low-speed domain, constant torquecontrol is adopted, and the torque is determined according toactual acceleration needs. In this region, the motor voltage hasnot yet reached the maximum voltage limit, and only the currentlimit need be satisfied. With an increase in motor speed, thevoltage constraint ellipse becomes smaller and smaller. Whenthe voltage constraint ellipse and the current constraint circlecoincide with point B, the motor is out of the constant torqueregion, and the critical speed is ωbase [31].

When the motor speed exceeds ωbase, the output voltage ofinverter reaches the maximum voltage, and the motor enters thesquare-wave working condition. This area is also called flux-weakening field. In this area, the maximum torque output can beobtained when the motor runs at the intersection of the voltage-constrained ellipse and the current-constrained circle. Bycombining (1) and (3), the d-q axis current can be calculatedas formula (4) shown. Where Usmax is 0.866 times as much asUDC . ⎧⎪⎨

⎪⎩i∗sd =

√(Usmaxωe )

2−(σLsIsmax)2L2s−(σLs)2

i∗sq =√

I2smax − (i∗sd)2(4)

By combining (2) and (4), the relationship between voltageand torque can be obtained:

Te = 1.5npLm

2

Lr

√√√√√(

UDCωe

)2− (σLsIsmax)2

L2s − (σLs)2

×

√√√√√−(

UDCωe

)2+ (LsIsmax)

2

L2s − (σLs)2(5)

Fig. 5. Control strategy for improving electric power in the high-speed zone.

When the motor speed equals ω1, the motor works at point D,and the torque hyperbola is at a tangent to the voltage limitellipse. When the motor speed continues to rise, the motorenters the second region of flux weakening. In this region,the maximum output torque of the motor is the tangent pointof the torque curve (2) and the voltage constraint ellipse (3),and the current vector trajectory of the maximum output torqueis the DF of the middle line of Fig. 3.

By combining (2) and (3), the relationship between d-q axiscurrent and voltage in flux-weakening region 2 is obtained:⎧⎨

⎩i∗sd =

Usmax√2ωeLs

i∗sq =Usmax√2ωeσLs

(6)

By combining (2) and (6), the relationship between voltageand torque can be obtained:

Te = 1.5npLm

2

Lr

UDC2

2σωe2Ls2 (7)

2) Control Strategy for Improving Braking Characteristics:The relationship between motor output torque and DC-sidevoltage is analysed in detail in the previous section. It can beseen that the maximum output torque of the motor is limited bythe DC voltage on the inverter side, whether in flux-weakeningregion 1 or flux-weakening region 2. That is to say, the maximumoutput torque of the motor can be increased by increasing theDC voltage on the inverter side, and the braking characteristiccurve of the train can be improved. On the basis of theory, theshort-time overvoltage capability of inverter and motor can beused to improve the high-speed braking performance of the trainand increase its regenerative braking energy without changingthe traction grid voltage. The specific control flow chart forimproving the train braking characteristic curve is shown in thefollowing figure.

In Fig. 5, the boost mode means the brake-charge mode, corre-sponding to state c in Table I. The switchable mode means that inthe current state of SC, more than one charging and dischargingmode can be selected. And it is necessary to correspond betweenFig. 9 and 10 to determine which ones can be selected.

B. Coordinated Control Strategy

As mentioned above, a wayside SC cannot absorb all regener-ative braking energy in some special cases due to the influenceof no-load voltage and train location. On the other hand, thecharging power of the on-board SC under series topology islimited by line current. If the on-board SC is required to absorba large amount of braking energy, its charging time must bepredicted.



Fig. 6. Schematic diagram of coordinated control.

1) Coordinated Control Considering No-Load Voltage: Inthe control strategy for the wayside SC, the dynamic adjustmentof charge and discharge threshold is important [20]. If thecharging threshold is too high, the brake resistor of the trainwill be activated. If the charging threshold is too low, lowerthan the no-load voltage, the substation will charge to the SC,reducing the use efficiency of the SC.

Reference 20 pointed out that setting an appropriate chargingthreshold makes the traction converter voltage of the brake trainequal to the brake resistor operating voltage, which can achievethe best control effect. The calculation of charging threshold isshown below, where uch is the charging threshold voltage of thewayside SC, ubr is the brake resistor operating voltage, and iris traction net current.

uch = ubr − irRline (8)

But it can be seen from (8) that when the distance betweenthe wayside SC and the brake train is too long and the no-loadvoltage is too high, the charging threshold will still be lower thanthe no-load voltage.

In this paper, by predicting the relationship between waysideSC charging threshold and no-load voltage in advance, and byadjusting the state of the on-board SC, the traction current canbe changed, which can prevent the substation from charging toSC. As shown in the following expressions, uoc is the no-loadvoltage of traction net.{

case1 : ubr − irRline > uoc, Switchable Modecase2 : ubr − irRline ≤ uoc, Boost Mode

(9)

2) Coordinated Control Based on Time Prediction: Underthe topology of this paper, the charging and discharging currentcannot be adjusted. This makes it impossible to absorb the energythat the wayside SC cannot absorb in a short time, as shownbelow.

In Fig. 6, PMAX is the maximum power that can be achievedby the on-board SC. Area 1 is the maximum energy that can beabsorbed by the wayside SC. Area 2 is the energy that cannot beabsorbed by the wayside SC, which is defined as E2. Area 3 is themaximum energy that can be absorbed by the on-board SC at thistime, which is defined as E1. In order to enable the on-board SCto absorb this energy and prevent regenerative braking failure,it is necessary to make area 3 larger than or equal to area 2.

Fig. 7. Relationship between losses from each part of the system and DCvoltage. (a) Line resistor = 4 Ω, Te = 35 Nm; (b) line resistor = 2 Ω,Te = 20 Nm.

The relationship between voltage and current on both sides ofthe capacitor is shown in (7).

i = Cdu

dt(10)

At the same time, the voltage and current of SC show thefollowing relations:

i =P

UFC + u(11)

By combining (10) and (11), (12) can be obtained:

Cdu

dt=

P

UFC + u(12)

The integral points of voltage and time are placed on theleft and right sides, respectively, and the relationship betweenvoltage and time can be obtained; u0 is the current voltage of theon-board SC, u1 is the maximum voltage that the SC can chargeduring the remaining braking time, and U0 is the maximumterminal voltage of the SC.

u1 =

√(UFC + u0)

2 +2PC

Δt− UFC (13)

E1 =12Cu1

2 − 12Cu0

2 (14)

We can get the following two cases:{case3 : (u1 < U0)&(E1 > E2), Switchable Mode

case4 : (u1 ≥ U0)||(E1 ≤ E2), Boost Mode(15)

C. Optimization of System Losses

The main purpose of this part is to improve the energy storageefficiency of SC by optimizing the system losses.

As mentioned above, the on-board SC is worked in boostmode when case 2 or 4 happens. But in other cases, the waysideSC can absorb all the regenerative braking energy. At this time,as an auxiliary energy storage element, the on-board SC cantransfer the braking energy by adjusting its charging and dis-charging conditions, to minimize the system loss. Fig. 7 showsthe relationship between the losses from each part of the systemand DC voltage on the inverter side.



It can be seen that under the fixed-line impedance and motortorque, with a rise in DC voltage, line loss is becoming smallerand smaller and inverter loss is increasing. But the ratio of lineloss to total system loss is greatly affected by line impedanceand motor torque. Under different line conditions, the influenceof DC voltage change on total system loss is not a monotoniclinear relationship.

1) Loss Calculation: The loss of the system is mainly com-posed of three parts: line loss, inverter loss and motor loss.

The calculation formula for line loss and SC internal resis-tance loss is shown below.

Eline = idc2RlineTs (16)

The on-state losses of inverters and their reverse paralleldiodes are shown in (17) and (18). [33]

Econ_igbt =

(1

2π+

m cosϕ

8

)VCEImTs

+

(18+

m cosϕ

3π

)rCEI

2mTs (17)

Econ_dio =

(1

2π− m cosϕ

8

)VFOImTs

+

(18− m cosϕ

3π

)rFOI

2mTs (18)

The switching loss of the inverter is shown in (19). [33]

Esw_igbt =fs2π

∫ π0

[Eon × Vout

Vn× Im sin(ωt)

In+ Eoff

×VoutVn

× Im sin(ωt)In

]d(ωt)

=fsπ

× VoutImVnIn

(Eon + Eoff ) (19)

The loss of the motor is shown below.

Emotor = Rs(isd

2 + isq2)Ts (20)

2) Prediction of Current: Current prediction can be carriedout in two ways. On the one hand, when the output currentof the motor is unchanged, the magnitude of the current willbe predicted when the DC-side voltage changes. On the otherhand, when the output torque of the motor is unchanged, the d-qaxis current of the motor will be predicted before and after fluxweakening.

For the first aspect, assuming that the input power of theinverter is constant, and the current and voltage at the currentmoment are known, the voltage and current of the on-boardSC can be predicted in the next three states, respectively. Thepredicted current of idc is shown in Table II.

Concerning the second aspect, under MTPA (maximumtorque per ample) control, by combining (2) and (3), and cal-culating the tangent point of two curves, the d-q axis currentsunder MTPA control can be obtained as (21).⎧⎨

⎩isd = isq =

√TeLrnpL2m

(Te > 0)

isd = −isq =√

TeLrnpL2m

(Te ≤ 0)(21)

TABLE IIPREDICTED CURRENT

Fig. 8. Online optimization control strategy considering loss.

Under the control of single current loop, by combining (1)and (3), and calculating the intersection point of two curves, thed-q axis currents under control of a single current loop can beobtained as (22).

⎧⎪⎨⎪⎩isd =

√(Usmaxωe )

2−(σLsIsmax)2L2s−(σLs)2

isq =LrTe

npLm2isd

(22)

3) Loss Optimization Strategy and Parameter Optimization:In this part, based on the calculation of loss and current predic-tion, an online loss optimization strategy is proposed, and theparameters K1, K2 and K3 in the strategy are optimized.

The online optimal control block diagram with minimumsystem loss as objective function is shown below.

In the figure, from left to right are the current predictionmodel, loss calculation model, scaling model and comparisonmodel. Inputting the current and voltage at the current moment,using the previous section for the current prediction method, thecurrent under different states is predicted. Subsequently, the lossof the three states is calculated by the loss calculation model.Then, the three losses are scaled up or reduced, and the final lossis compared, so that the state of the minimum loss is the state ofthe next moment. Supposing mode 1 is the current mode, mode 2is the selectable mode, which can be matched in Fig. 11, andmode 3 is a not recommended mode, the meaning and selectioncriteria of K1, K2, K3 are as follows.

K3 is the elimination coefficient, the purpose of which is toeliminate the state that is not recommended or not allowed, andgenerally takes infinity. K2 usually takes 1. K1 is a stability factor.In some special cases, if the value of K1 is unreasonable, itwill cause repeated state switching, resulting in instability ofthe system. In order to maintain the stability of the system, theselection of K1 is important. The following is an analysis of thevalue of K1.



Fig. 9. The choice of K1.

From (13) to (17), the total loss for the system is shown:

Esystem=Eline+Econ_igbt + Econ_dio + Esw_igbt + Emotor(23)

Among them, the sum of on-state losses of the IGBT anddiode, as well as the internal resistance loss of the motor,have little relationship with DC-side voltage and current. Thedifferent values of K1 mainly affect line loss and IGBT switchingloss, namely Eline and Esw_igbt. The simplified system loss canbe expressed as follows:

Esys∗ = A ∗ i2 +B ∗ Pin

i(24){

A = (Rline +Rsc)Ts

B = fsπ · ImVnIn (Eon + Eoff )(25)

Subscript band 0 is the voltage and current in the standby stateof the SC, subscript band 1 is the voltage and current in the boostmode of the SC, and subscript band 2 is the voltage and currentin the step-down state of the SC.{

idc1 =UDC0

∗idc0UDC0+Uc

idc2 =UDC0

∗idc0UDC0−Uc

(26)

According to the relation of current, the relation between lossand current I0 in the three modes can be listed at the same time.If the difference between the three loss equations is zero, thethree intersection points i0, i1 and i2 of the three curves can beobtained as follows:⎧⎪⎪⎪⎪⎨

⎪⎪⎪⎪⎩

i0 =3

√BA

(UDC0+Uc)2

(2UDC0+Uc)UDC0Pin

i1 =3

√BA

(UDC0−Uc)2(2UDC0−Uc)UDC0 Pin

i2 =3

√BA

(Udc0+Uec)2(Udc0−Uec)2

2Udc04 Pin

(27)

It can be seen that the three intersections are affected byvoltage, power and line resistance at the same time. If idc0 isnear the three intersections, because of current jitter, there willbe repeated switching. A control strategy is needed to adjustK1, as shown in the Fig. 9. And in the figure, ierror_ref is thereference value of current error. When the minimum value of �iis less than ierror_ref , it decreases the value of K1 by adjustingPI and tend to not switch. If the minimum value of �i is largerthan ierror_ref , it increases K1 and tends to switch. KMAXis generally set to 1, and value of KMIN should be balancedbetween loss and vibration. And in the paper, KMIN is 0.6.

D. Control Process of Hierarchical Optimization Strategy

The three optimization objects mentioned above have a certainpriority. The highest priority is coordinated control of wayside

Fig. 10. Control flow in the train braking process.

Fig. 11. Selectable mode corresponding to the optimum conditions.

SC and on-board SC, which ensures that all regenerative brakingenergy can be absorbed. The second highest priority is theimprovement of braking characteristics, which ensure that morebraking energy will be generated. On the premise that both ofthose can be satisfied, the third optimization object, namelythe optimization of system loss, is considered. The conditionin which the SC can be switched is called the optimum condi-tion. Considering the train speed and the maximum torque, sixoptimum conditions can be selected in the braking process. Thespecific step control flow is shown in Fig. 10. Tmax1, Tmax3 andTmax5 are the maximum torque in standby mode under differentspeed regions. Tmax2, Tmax4 and Tmax6 are the maximumtorque in discharge mode under different speed regions, case1to case4 correspond to the cases in (6) and (12), and conditions1© to 6© are shown in (28).⎧⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨

⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩

©1 : (ω > ω1)&(Tmax 1 > Te_ref > Tmax 2)©2 : (ω > ω1)&(Te_ref < Tmax 2)©3 : (ω < ωbase)&(Tmax 3 > Te_ref > Tmax 4)©4 : (ω < ωbase)&(Te_ref < Tmax 4)©5 : (ω > ωbase)&(Tmax 5 > Te_ref > Tmax 6)©6 : (ω > ωbase)&(Te_ref < Tmax 6)

(28)

In some conditions, some modes will not be allowed or recom-mended to meet the braking torque requirements. Specifically,the selectable mode corresponding to the optimized workingconditions is shown in the following figure.

IV. ALGORITHM VERIFICATION

The experimental part of this paper is verified by the RTLABsemi-physical platform, as shown in Fig. 12 below. RTLAB is amodular real-time simulation platform. The experimental setupis composed of the DSP controller, OP5600 HIL Box, related



Fig. 12. RT-LAB experimental setup.

TABLE IIISELECTION OF EXPERIMENTAL PARAMETERS

TABLE IVPARAMETERS IN THE EXPERIMENT

cables, a software component and an upper computer monitoringinterface. TMS320F28346 is selected as the DSP controller,which is used to operate the energy management strategy; andRT-LAB OP5600 is used to simulate the traction drive systemwith wayside SC-ESS and on-board SC-ESS.

In actual subway line, the DC traction network is generally750 V grade or 1500 V grade, but the experimental cost ofthe same voltage in the laboratory is relatively large, whichis difficult to achieve, and the simulation of equal power isdifficult to verify through the experimental platform. Therefore,this paper selects the parameters of the laboratory low-powerplatform, and scales the DC side voltage and the braking resistorstarting voltage in equal proportions, as shown in the followingtable.

Among them, the rated voltage of the low-power motor is380 V, which corresponds to 540 V on the DC side. Comparedwith 750 V, the scaling factor is 0.72, and the starting voltageand no-load voltage of braking resistor are scaled according tothis ratio. The complete parameters in the experiment are shownin Table IV.

In all experiments, the charge threshold of the wayside SCis set as in (5), which needs to use the starting voltage of thebraking resistor. And in the calculation process, the startingvoltage of the braking resistor in formula 8 is fixed to 650 V.When the braking train is close to the wayside supercapacitor,the voltage at the traction network of the braking train willbe stable around 650. If the actual starting voltage of brakingresistor is 650 V, the phenomenon is not obvious. In order tomake the phenomenon more intuitive when the brake resistor isstarted, the actual starting voltage of the brake resistor is set at700 V. When the braking resistance is activated, the voltage atthe traction network of the braking vehicle (DC bus voltage inthe figure) will be quickly limited to 700 V, as the Fig. 14(a)shown.

It should also be noted that the traction network current inthe experiment is smaller than the real train current. In order tomake the loss in the experiment close to the real situation, it isnecessary to maintain the same proportion of line loss and trainpower. The actual line loss is generally 0.016 Ω/km, which isconverted to 4.4 Ω/km in this paper.

And in order to reflect the restriction of high-priority controltarget on low-priority control target, the coordination control partwith the highest priority was put to the last part in the experimentpart. In the experiment in Section A, only the energy manage-ment strategy to improve the braking characteristic curve wasapplied, which was compared with the situation that on-boardsupercapacitor was not activated. In the experiment in SectionB, the energy management strategy considers both the improve-ment of braking characteristics and optimization of loss, but notconsiders the coordinated control strategy to prevent the failurepart of regenerative braking. In the experiment in Section C,the energy management strategy considering three optimizationobjectives is applied, and the effectiveness of the optimizationobjectives is verified under different line resistances.

A. Brake Characteristic Curve Improvement Experiment

In this section, keeping the line resistor unchanged, the exper-iment under the braking characteristic curve of this paper withor without the control strategy for improving the braking char-acteristic curve is shown in Fig. 13(a) and (b). In the diagram,the purple curve is the inverter-side DC voltage, the dark bluecurve is the DC voltage of the traction grid, the light blue curveis the on-board SC terminal voltage, and the green curve is themotor output torque.

It can be seen that the motor cannot output the brakingcharacteristic curve in the braking high-speed region, and theoutput torque of the motor cannot maintain stability withoutthe control strategy. And with the control strategy, the electricbraking area of the train is improved, and more regenerativebraking energy can be output.

B. Loss Optimization Experiment

In this section, only the brake curve improvement algorithmand loss optimization algorithm are applied, and the coordinatedcontrol algorithm is not applied. In this section, the no-load



Fig. 13. Brake characteristic curve improvement experiment: (a) withoutcontrol strategy for on-board SC and with the improved braking curve, (b) withcontrol strategy for on-board SC and with the improved braking curve.

Fig. 14. DC voltage variation under line resistance change under control ofloss optimization algorithm: (a) line resistance is 1 Ω, (b) line resistance is 6 Ω.

voltage is set to 540 V. Fig. 14(a) and (b) show the experimentalwaveforms for line resistance of 1 and 6 Ω, respectively. InFig. 14(a), it can be seen that in region 1, in the condition oftraction, when the train power is small, the line loss proportionof the total system loss is small, and using the traction-chargingmode in Table I to reduce the inverter-side voltage can effectivelyreduce the system loss; In region 2, the train power gradually

Fig. 15. Change of SC terminal voltage with (a) or without (b) loss optimiza-tion algorithm.

increases, the line loss is the main part, and using the traction-discharge mode in Table I to boost the inverter-side voltage caneffectively reduce the system loss; Region 3 is the train idle zone,when the SC is in standby state; In region 4, brake power is large,line loss is the main part, and the Table I brake-charge mode isused on the inverter side. Voltage boosting can effectively reducethe system loss; In region 5, braking power is small, line-lossratio is small, and using the brake-discharge mode in Table I toreduce the inverter-side voltage can effectively reduce the systemloss. But at the same time, without considering the coordinatedcontrol, too small a line resistance leads to too large a region 5,so that the on-board SC releases energy when it should absorbenergy, resulting in opening of the brake resistance. In Fig. 14(b),it can be seen that, with an increase of line resistance, the junctionarea of regions 1 and 2 moves forward gradually, and the junctionarea of regions 4 and 5 moves backward gradually.

Fig. 15(a) and (b) shows the experimental curves before andafter applying the loss optimization strategy, respectively, tomaintain line resistance of 2 Ω. In the diagram, the purple curveis the on-board SC voltage, the dark blue curve is the wayside SCvoltage, and the green curve is the motor output torque. Onceunder braking condition, the terminal voltage of the waysideSC varies from 180 to 336 V, and the on-board SC varies from65 to 110 V without the energy management strategy proposedin this paper. The total energy absorbed by the on-board SCis 118514 J. With the proposed energy management strategy,the terminal voltage of the wayside SC varies from 180 to357 V, and the on-board SC varies from 82 to 100 V, absorbing132411 J of energy. As can be seen from the foregoing, afterimproving the braking characteristic curve, the single brakingenergy is 0.039 kWh, i.e., 140400 J. It can be seen that underline resistance of 2 Ω, the system loss can be reduced by 9.8%.



Fig. 16. Relationship between system loss reduction and line resistance.

Fig. 17. Changes of DC voltage before (a) and after (b) application of systemK1 online adjustment strategy.

Fig. 16 shows the relationship between line resistance and theloss which can be reduced by the control strategy. It can be seenthat the loss is similar to the inverse proportional function, but itis about 10%. It is noteworthy that the brake resistance will startif the on-board SC is not started under the experimental settingconditions in this paper. Therefore, in the cases of two kinds ofloss calculation, on-board SCs are used. The difference is thatwithout this strategy, the whole braking process of the on-boardSC is in the absorptive capacity, in the boost mode.

Fig. 17 shows the voltage curve of the inverter side and thesupporting capacitor side with or without the K1 optimizationstrategy. The purple curve is the voltage of the inverter side, andthe dark blue curve is the voltage of the supporting capacitor. Itcan be seen that before applying the K1 optimization strategy, theswitching point of the system happens to be near the oscillationpoint, at the switching points of regions 1 and 2, and at theswitching points of regions 4 and 5, the operating conditionsare repeatedly switched, resulting in voltage oscillation. Afterapplying the K1 optimization strategy, although the boundarybetween regions 1 and 2 moves forward and the boundarybetween regions 4 and 5 moves backward, the oscillation caused

Fig. 18. Experimental results after applying the hierarchical optimizationstrategy with (a) line resistance is 1 Ω, (b) line resistance is 6 Ω.

by repeated switching of working conditions can be effectivelysuppressed.

C. Coordinated Control Experiment

In this section, the no-load voltage is set at 600 V. Fig. 18(a)and (b) shows the experimental waveforms for line resistanceof 1 and 6 Ω, respectively. Firstly, compared with Fig. 13(a),as can be seen from region 5 in Fig. 18(a), after applying thecoordinated control algorithm, when the on-board SC predictsthat the wayside SC cannot absorb all the regenerative brakingenergy, the braking boost mode is turned on ahead of time toabsorb the braking energy and prevent the braking resistancefrom starting. As can be seen from Fig. 18(b), with the coor-dinated control algorithm, region 4 moves backward comparedto Fig. 14(b). This is because in this case, the line resistance islarge, and when the no-load voltage is high, in order to preventa wayside SC charging threshold below the no-load voltage, inFig. 14(b) the switching point is initially selected not to switch,forced to boost mode and reducing the current, thereby reducingthe charging threshold of the wayside SC.

Table V summarizes the changes of volume, voltage reg-ulation, utilization of braking energy, braking characteristicsand system loss before and after the application of topologyand energy management strategy. It can be seen that, withthe application of the topology and energy management strat-egy in this paper, in addition to the slightly increased volumeand weight of on-board devices, the utilization of renewableenergy, braking characteristics and system loss of the systemhave been effectively improved. Combined with the analysisand experimental results, the main reasons for performanceimprovement can be summarized into two aspects: (1) Usingon-board supercapacitor, the regenerative braking energy can



TABLE VCOMPARISON BEFORE AND AFTER TOPOLOGY APPLICATION

be captured when energy transmission capacity is limited. Thiscannot be achieved by using wayside supercapacitor. (2) Theseries topology increases the voltage on the inverter side, whichcan improve the high-speed braking performance of the train.

V. CONCLUSION

Based on a series topology, an on-board SC control strategyis proposed in this paper. Experimental results show that thestrategy has the following three advantages:

1) The strategy can predict the maximum voltage corre-sponding to the maximum torque at the current speed inreal time. When the voltage is insufficient, the inverter-sidevoltage can be increased by adjusting the working mode ofthe on-board SC, so as to enhance the high-speed brakingforce. Through experiment, we found that the regenerativebraking energy can be increased by about 10%.

2) Considering the limitation of charging power and the in-fluence of no-load voltage, a coordinated control strategyis proposed. Under this coordinated control strategy, theESS has high robustness to line resistance and no-loadvoltage and can absorb regenerative braking energy underany circumstances to prevent the brake resistor activating.

3) By calculating the system loss and predicting the current,system loss can be optimized by adjusting the workingmode of the on-board SC in real time, and the systemloss can be reduced by about 10%. Compared with theprevious loss-reduction methods, the proposed method notonly considers line loss, but also inverter loss and motorloss. In this method, the value of K1 is very important. Andthe theory and experiment show that real-time correctionstrategy for K1 parameter proposed in this paper can ensurethe stability of the system.

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Zhihong Zhong (Student Member, IEEE) receivedthe B.S. degree in electrical engineering from BeijingJiaotong University, Beijing, China, in 2015. He iscurrently working toward the Ph.D. degree with theSchool of Electrical Engineering, Beijing JiaotongUniversity, Beijing, China. His research interests in-clude modeling and optimization of energy storagesystems, traction converter and motor drives.

Zhongping Yang (Member, IEEE) received theB.Eng. degree from the Tokyo University of Mercan-tile Marine, Tokyo, Japan, in 1997, and the M.Eng.and Ph.D. degrees from the University of Tokyo,Tokyo, Japan in 1999 and 2002, respectively, all inelectrical engineering. He is currently a Professorwith the School of Electrical Engineering, BeijingJiaotong University, Beijing, China. His research in-terests include high-speed rail integration technology,traction & regenerative braking technology, and wire-less power transfer of urban rail vehicles.

Xiaochun Fang (Member, IEEE) received the B.S.and Ph.D. degrees from Beijing Jiaotong University,Beijing China, in 2010 and 2016, respectively, bothin engineering. He is currently a Lecturer with theSchool of Electrical Engineering, Beijing JiaotongUniversity, Beijing, China. His research interests in-clude traction converter and motor drives, energymanagement for railway systems, IGBT fault mech-anism and failure prediction.

Fei Lin (Member, IEEE) received the B.S. degreefrom Xi’an Jiaotong University, Xi’an, China, theM.S. degree from Shandong University, Jinan, China,and the Ph.D. degree from Tsinghua University,Beijing, China, in 1997, 2000, 2004, respectively, allin electrical engineering. He is currently a Professorwith the School of Electrical Engineering, BeijingJiaotong University, Beijing, China. His research in-terests include traction converter and motor drives,energy management for railway systems, digital con-trol of power-electronic-based devices.

Zhongbei Tian (Member, IEEE) received the B.Eng.degree from the Huazhong University of Scienceand Technology, Wuhan, China, in 2013, and theB.Eng. and Ph.D. degrees in electrical and electronicengineering from the University of Birmingham,Birmingham, U.K., in 2013 and 2017, respectively.He is currently a Research Fellow with the Universityof Birmingham. His research interests include railwaytraction power systems modeling and analysis, energysystems optimization, advanced traction power sys-tems design, energy harvesting, and “Rail to Grid”

energy systems integration and management.


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