RE-ADHESION CONTROL FOR HIGH-SPEED ELECTRIC RAILWAY WITH PARALLEL MOTOR CONTROL SYSTEM

Don-Ha Hwang*, Moon-Sup Kim*, Doh-Young Park*, Yong-Joo Kim*, Dong-Hee Kim**

* Mechatronics Research Group, Korea Electrotechnology Research Institute (KERI) P.O. Box 20, Changwon, Kyungnam, 641-600, KOREA,

** School of Electrical and Electronic Engineering, Yeungnam University 2 14- 1, Daedong, Kyongsan, Kyongbuk, 72 1-749, KOREA

ABSTRACT

Parallel Induction motor drive with PWM inverter control has been developed for the High-speed Railway. This paper describes a re-adhesion control system to improve a tractive efficiency and to reduce the wheel abrasion. The adhesion characteristics of the high-speed train are analyzed to get the maximum adhesion effort and the re- adhesion control system is designed as a subsystem of induction motor control. In order to verify the performance of the proposed system, a downscaled re- adhesion control simulator is set up and running tests are executed.

1. INTRODUCTION

Recently in the field of railway traction, the interest to control slip phenomenon between the driving wheel and rail is increasing, especially for advanced electric vehicles such as a high speed train. The wheel slip occurs frequently under low adhesion conditions like a wet and oily rail. Eventually excessive slip causes heavy damages of wheel, shortens lives of relevant elements and cuts down the tractive performance. Therefore the realisation of re-adhesion control system is inevitable in developing an advanced railway vehicle.

The phenomenon of slip is closely related to the adhesion effort. Because in the end the objective of a re- adhesion control is to keep an optimal wheel slip. To design this control system, the adhesion characteristics must be considered as exactly as possible. But in practice it is impossible to know the exact characteristics due to the influence by various unknown aspects including wheel-rail surface conditions and operating conditions. Only a trend of the fundamental relation between adhesion and wheel slip under different rail and vehicle speed conditions has been found as shown in Fig. 1.

The adhesion-slip characteristics can be divided into a stable and an unstable region. In the unstable region the more wheel slip occurs, the less adhesion effort is obtained.

. .. ,

Adhesion . Oil&Smr.j

o,,

0 0 5 - ;

Water Oil Water&Sand

o o j ; ; I ; ; a ; ; 0 I O

PU Slip x 10 , Stable . Unstable region i region

0 2

2 015 - c % 8 0 1 0

F 0 1

Fig. 1. Adhesion-slip characteristic and adhesion-speed characteristic curve

So to maximize this adhesion effort a re-adhesion controller must constantly control a slip so that the adhesion coefficient between wheel and rail is near the top of the initial positive slope of the adhesion-slip curve. In addition, the re-adhesion controller must quickly and accurately be able to compensate poor rail adhesive effort without the exact information of rail conditions.

0-7803-7090-2/01/$10.00 0 2001 IEEE. 1124 ISIE 2001, Pusan, KOREA

Table. 1. High-speed Railway Specifications

Number of power axle Operating speed

Parameters I Specifications

16 ea 0-35Okdh

20 cars: 2P4M14T* (P+M+7T+2M+7T+M+P) Train formation

Control network Wheel diameter

Traction power - 1 1.100 kW x 16 ea

Train Communication Network 0.885 m

Train mass 1 About 780 t Maximum axle load I Power car: 17 t, Trailer car: 17 t

Average: 0.99 kmlhls Starting: 2.322 kmlhls Acceleration

Train resistance I 13 1.4 kN (at 350 kdh) Adhesion coefficient I 0.07 - 0.24

Regenerative, Eddy current, Rheostat and disk brakes Braking system

2. HIGH-SPEED ELECTRIC RAILWAY

The High-speed Railway specifications are summarized in Table 1. The electrical systems of a power car consist of a main transformer, two traction units, four traction motors and an electrical braking system. A traction unit consists of two four-quadrant choppers as the line-side converters, a DC-link circuit, a rheostat-braking chopper, and a PWM inverter. Traction motors are asynchronous 3-phase squirrel cage induction motors. This High-speed Railway has the 1 C2M (one PWM inverter feeds 2 traction motors) structure. This drive system has a tendency that the control system transfers slip phenomena to normal axle when a slip occurs.

3. RE-ADHESION CONTROL SYSTEM

Fig. 2 is a simple block diagram of the re-adhesion control scheme. Four axle speeds are obtained from wheel speed sensors. All speed values are used to estimate a reference speed, accelerations and wheel slips. If an inordinate wheel slip occurs, immediately a torque reduction is executed by the control of current to the traction motor. The proposed re- adhesion control scheme improves particularly the reference speed estimation and torque reduction method, which plays a very important role in the performance of controller. The re-adhesion control system is programmed as a part of TCU(Traction Control Unit) software.

I

v5

I I -- Accelerations

Reference W e d Estimation

RAW Speed = Minimum Wheel Speed

RAW Speed > V5

c l , c2 Defined rate for correction

VREF = RAW + C l I VREF = RAW - c2

I I Reference Speed

Fig. 2. Re-adhesion control flowchart

3.1. Reference Speed (Pseudo Train Speed)

An exact reference speed is very important to detect a correct wheel slip. The more wheels are used to get the minimum velocity as a reference speed, the higher becomes the accuracy of the reference speed. So collecting rotational speeds of as many wheels as possible is recommended. The problem is how to transmit so much information accurately and quickly. This is solved by transmitting the data through a control network system of the High-speed Railway.

In the proposed scheme, the raw train speed is calculated from four axle speeds of a bogie to be controlled and an adjacent bogie. When the train is in acceleration, the minimum of all wheel speeds is selected as the raw train speed. The actual train speed is obtained by comparing the raw train speed with the output of an extra train speed indicator including speed information of a trailer car. When the raw train speed is less than the wheel speeds of a trailer car, it is allowed to increase at a rate defined by train acceleration. On the contrary, when the raw train speed is greater than the wheel speeds of trailer car, it is allowed to decrease at a rate defined by train acceleration.

1125 ISIE 2001, Pusan, KOREA

v4 -

From Bogie 1 V l Acceleration criterion 1 - Aliz Vbooie- - 1 * a1 n AI 1 Acceleration calculation a2 From Bogie 2 V3 * each bogie -

v2 for V F ~ 2,

t a c normal acceleration limit

calculation

Lc

---c ---+I Reference

speed estimation Wheel slip

Speed indicator v5 From Extra

Fig. 3. Block diagram ofre-adhesion control (Per 1 powered bogie)

This reference .estimation method compensates defects of conventional methods such as the oscillation and the rapid increment of reference speed caused by the whole wheel slip.

3.2. Improved Re-adhesion Control

The proposed re-adhesion control scheme is based on the conventional methods employing slip criterion such as a pattem control and a speed difference control. The pattem control is a method that the motor current of slipping axle is reduced according to a specific control pattem when the wheel slip becomes greater than the threshold value as shown in Fig. 4 (a). And the speed difference method is a feedback control by slip speed. The motor current is reduced through a current reduction function and a time delay of first order. Fig 4 shows characteristics of each control method under the same slip phenomenon. Though the pattern control can give a positive re-adhesion of wheels, relatively this method causes the frequent oscillations and drop of traction effort. On the other hand the speed difference control is ideal for early inhibition of a wheel slip development due to an attribute of feedback control, but this method can not guarantee re-adhesion if a bad road condition continues. Therefore the proposed control adopts a hybrid control scheme by blending two conventional control methods for re-adhesion control [ 11.

Fig. 3 shows the block diagram of proposed re- adhesion control [2]. At the first step, to prevent the development of a wheel slip, the speed difference control method is separately activated after the dead zone.

1126

I Speed difference method I I

Pattern method

threshold 1 threshold 2

3

. . i i ; i exceed slip ~ i : I

detection

. . ! i i 1 i i , . (b) j

current command by palern control 2

Fig. 4. Re-adhesion control methods

This zone is determined by the wheel wear error and speed sensor error. If the re-adhesion control fails in spite of the first step, that is, if the wheel slip reaches the threshold 1 of Fig. 4 (a), simultaneously the second step by the pattem control begins. This control method reduces quickly and forcibly a large wheel slip under a serious road condition. The used pattern in the proposed control is an improved type that has the same gradient of current command but has variable periods as shown in Fig. 4 (b). If the above two steps fail in re-adhesion control, the last step by acceleration criterion is activated. Usually this happens when all wheels are simultaneously slipping. Then,

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because the controller can not estimate an accurate reference speed any more and the calculated wheel slip becomes much smaller than the real one, two control methods based on slip criterion have poor ability to control a wheel slip. However, in the third step, the separately calculated acceleration is compared to the normal acceleration limit. And when the wheel acceleration exceeds the limit, the motor current command is exponentially reduced. This proposed control method continuously secures good adhesion characteristics regardless of the severity of a wheel slip.

4. RUNNING TEST

4.1. Re-adhesion Control Simulator

Fig. 5 and Fig. 6 show the downscaled simulator of the High-speed Railway. It consists of seven flywheels, a DC generator and two disc braking system. A main flywheel is for the train inertia, two flywheels for rails and four flywheels for the train wheel. A DC generator is used to simulate the running resistance. The adhesion effort can be controlled to an arbitrary value by hydraulic servo valves. The water sprinklers can simulate a change of adhesion coefficient by injecting water to the contact points.

Fig. 7 shows electrical systems of the simulator. Electrical parts of the system consist of a main transformer, two traction units and four traction motors and electrical braking systems. The transformer has four secondary traction windings and two auxiliary windings.

Fig. 5. Mechanical system layout of re-adhesion control simulator

Fig. 6. Re-adhesion control simulator

AC-DC Converter

, - - - - - - -I m 1 -

_ _ _ _ _ _ 1

["h Power

..... - . lo be added

Fig. 7. Electrical systems of the simulator

Fig. 8. Control block diagram of the inverter

The traction unit consists of two four-quadrant chippers as the line-side converters, DC-link circuit, a rheostat-braking chopper, and a PWM inverter.

1127 ISIE 2001, Pusan, KOREA

The PWM inverter generates a three-phase VVVF source for two induction traction motors. Instead of IGCTs that will be used in real system, IGBTs are used as the switching devices. Traction motors of the simulator are asynchronous 3-phase squirrel case induction motors. The specifications of the simulator are shown in Table 2.

Table. 2. Comparison of traction system specifications

Traction Motor Rated Voltage, cwent Maximum Speed

Inverter Input

output

Converter Input

OutDut Switching Device

DC Link

Transformer

7.5kW x 4 1 lOOkW x 16

220V, 26A I 2183V, 379A

4500rpm I 4222rpm

22kVA x 2 400V 0-220v

lOkW x 4 220v 400V

2824kVAx 8 2800V 0-22oov

1250kW x 16 1400V 2800V

400V. 5500uF 2800V. 8000uF

1 : 220V, 70kVA I 1 : 25kV, 8900kVA 2: 1400V, 2: 220V, ISkVA

152V, 1 lOV, SkVA I 1250 kVA x 6

35OV, 350kVA x 4

The vector control block diagram of the traction inverter is shown in Fig. 8. The traction motor is operated by either torque control loop or speed control loop with vector control method [3].

4.2. Test and Simulation Results

To verify the proposed control method, a simple simulation was performed using simulation tool software and the re- adhesion control simulator was constructed and running tests were performed [4]. An average speed of two wheel speeds was used as an input of motor control per one bogie. To generate a wheel slip the soapy water was sprayed between only one wheel of front bogie and rail since 50 seconds after start. But In simulation, the condition of slipping rail was set up since 25 seconds after start due to the time required of simulation. The various control methods are applied under the same condition to show the improved

performance. In case of the pattern control, the threshold for excessive slip detection was set up at 10 rpm (1.67 M). The running test results show remarkable re- adhesion properties as shown in Fig. 9. This figure includes speed, speed difference, Q-axis current reference and measured current of slipping wheel. The simulation results as shown in Fig. 9 (e)-(g) and the following running test results are alike in the adhesion characteristic.

Fig. 9 (a) shows the sudden occurrence of large inordinate wheel slip by a rapid drop of adhesive effort without the re-adhesion control. The maximum slip speed is about 30 rpm ( 5 km/h). Test results by the pattern control appear in Fig. 9 (b). The result by the pattern control shows that although the slip speed is close to threshold, this method has a serious fault to cause the frequent oscillation of wheel speed under the constant low adhesion coefficient. This phenomenon is injurious from the viewpoint of abrasion and shock. Fig. 9 (c) shows that the wheel slip is reduced only by the speed difference control. This method has no system oscillation, but the excessive wheel slip of approximately 3.2 km/h is still continued. Fig. 9 (d) shows the test results by the proposed hybrid control scheme. Compared with Fig. 9 (a), the proposed hybrid control with variable pattern reduced the maximum slip difference more than about 50%. The slip difference is almost restricted within threshold, and moreover, the oscillation of the whole system is not noticeable. The reduced oscillation helps to reduce the abrasions of wheels and the shocks to the system.

5. CONCLUSIONS

This paper presents the design and implementation of an improved re-adhesion control system for the High-speed Railway. This system provides an improvement of adhesion characteristics regardless of the severity of wheel slip under a continuous slipping rail condition by blending two conventional control algorithms and acceleration control scheme. To verify the validity of the proposed hybrid control scheme, the downscaled re-adhesion control simulator was set up.

The running tests under the conditions very close to the real state were performed. Compared with an individual operation of the conventional control scheme, the proposed re-adhesion control scheme shows a superior ability to reduce wheel slip and the vehicle vibration accompanied by the re-adhesion control. It can be said from this result that the proposed control has the advantages of reducing the wheel abrasion and improving the tractive performance. Therefore it can be expected that the proposed control scheme be applied to the high- performance vehicles.

,

1128 ISIE 2001, Pusan, KOREA

Speed 60

Wheel Speed

20 Wheel Speed

0 5 10 15 20 25 30 35 40 45 50

Tune (sec) (e) Pattem control (with variable pattem)

t

Slippmg wheel speed

(a) Without re-adhesion control

(0 Speed difference control

(b) Pattem control (with variable pattem)

Y (c) Speed difference control

(d) Improved re-adhesion control

(8) Improved re-adhesion control

(Oscillogram Unit - Time: lOseconddiv, Current: 15 Ndiv, Slip speed: 15 rpddiv, Wheel speed 125 rpddiv)

Fig. 9. Running test and simulation results

6. REFERENCES

[ I ] T. Hariyama, K. Hoshi, S. Nakamura, S. Toda, Y. Nakazawa and E. Takahara, “Wheel Slip Recovering Applying Vector Control for Shinkansen Train with Individual Motor Control System”, Transactions of the Institute of Electrical Engineers of Japan, Vol. 118-D,No. 9, pp. 1081-1088,1998. [2] M. S. Kim, D. H. Hwang, J. W. Jeon, D. Y. Park, Y. J. Kim, H. J. Ryu, J. S. Kim, “Hybrid Anti-Slip Control System of High Speed Trains”, Proceedings of IPEC 2000, Tokyo, Japan, Vol. 3, pp. 1379-1384, April 2000. [3] D. C. Lee and G. M. Lee, “A Novel Over-modulation Technique for Space-Vector PWM Inverters“, IEEE Transactions on Power Electronics, Vol. 13, No. 6, pp. 1144-1 151, November 1998. [4] M. Grassi, E. Pagan0 and 0. Veneri, “A Scale-model of a Train for the Validation of Mathematical Models”, Proceedings of WCRR’97, Firenze, Italia, pp. 16-19, November 1997.

1129 ISIE 2001, Pusan, KOREA