Energy Management in the Train Operation 13 Power management control in DC-electrified railways for the regenerative braking systems of electric trains Y. Okada 1, T. Koseki 1 & K. Hisatomi 2 1 The University of Tokyo, Japan 2 Shin-Keisei Electric Railway Co. Ltd., Japan Abstract Most electric trains in DC-electrified railways are presently equipped with a regenerative braking system. On braking, the traction controller of a train can convert kinetic energy into electrical energy during deceleration of the train only when other powering trains consume the electrical energy as electrical loads for the regenerating train in the electrical circuit. Therefore, the traction controller of the braking train must reduce the electrical power following squeezing control of regenerative power when the electrical loads are too small in the electrical circuit, because there are, typically, no other devices to absorb the regenerated energy in the electrical circuit. However, actual traction controllers have often reduced regenerative power excessively because they do not recognize the states of the electrical circuit, which include positions of other trains and substations and power consumption/regeneration of other trains in the electrical circuit. In this paper, the authors discuss an improvement of the squeezing control of regenerative power based on information of the electric circuit. The information includes voltage at the pantograph, estimated positions and power consumption/regeneration of other trains etc. 1 Regenerative braking in DC-electrified railway Fig.1 shows the typical power flow on braking in a DC-electrified circuit. The black solid arrows show the typical power flow in the present system, in which only the powering train consumes the power regenerated from a braking train. Therefore, the braking train must reduce the electrical power following squeezing control of regenerative power when the power consumption of powering trains is too small since there is, typically, no other device to absorb doi:10.2495/978-1-84564-498- 7/ 02
14 Power Supply, Energy Management and Catenary Problems the regenerated energy in the electrical circuit. However, there are many possible solutions for effective usage of regenerative braking. For example, brake choppers with resistances on board or in the electrical circuit contribute to maintenance reduction of trains. Another method is to install energy storage devices which include flywheels, batteries and double layer capacitors on board or in the electrical circuit, and commutated rectifiers at substations contribute to efficient energy usage. In addition, reduction of voltage regulation at substations and of feeding resistance can contribute to effective regenerative braking. However these methods require additional hardware, which mean additional cost. The other solution, which does not cause excessive cost, is to improve the squeezing control of regenerative power, which can enhances the performance of regenerative braking. Loads Reduction of voltage regulation Introduction of com m utated rectifier Pow er system S ubstation Typicalpow er flow in present system System s for only m aintenance reduction System s for efficient energy usage and maintenam ce reduction Energy storage by fly w heels,batteries and double layer capacitors Pow er consum ption w ith D C chopper and resistance R eduction of line resistance Im provem ent of squeezing controlof regenerative pow er E nergy storage by fly w heels,batteries and double layer capacitors Pow er consum ption w ith brake chopper and resistance Power Motor converter R egenerating train Power converter Motor Powering train Figure 1: Typical power flow on braking. In this paper, the authors discuss improvement of the squeezing control of regenerative power with information of the electrical circuit and brake choppers with resistances. 2 Problems of squeezing control of regenerative power On braking, the braking train converts kinetic energy to electrical energy. And other powering trains consume the electrical energy as electrical loads in the electrical circuit. Therefore, when electrical loads are too small in the circuit, the braking trains must reduce regenerative power following the characteristic shown by the solid line in Fig.2 to avoid excessive voltage at the pantograph. This control is called squeezing control of regenerative power. Computers www.witpress.com, in Railways ISSN IX, 1755-8336 J. Allan, C. (on-line) A. Brebbia, R. J. Hill, G. Sciutto & S. Sone (Editors) 2004, www.witpress.com, ISBN 1-85312-715-9
Energy Management in the Train Operation 15 M otor current 0 V oltage of pantograph[v ] 1650 1750 1900 Maximalline voltage Conventionalcharacteristic Figure 2: Typical characteristic of squeezing control. However, actual traction controllers often reduce regenerative power excessively [1]. The reasons for the excessive reduction are as follows; 1. traction controllers reduce regenerative power excessively in low-speed range because they reduce AC motor current directly instead of their DC current, 2. traction controllers reduce motor current at lower voltage than maximal voltage limit of feeding circuit as shown by the solid line in Fig.2 and, 3. actual traction controllers often reduce motor current at lower voltage than the conservative voltage limit shown by the solid line in Fig.2. In these problems, squeezing DC current of traction controller instead of AC motor current can solve the problem in 1(above). However, a traction controller needs to recognize the state of the electrical circuit in which braking trains exist to solve the problems in 2 and 3. When the traction controller cannot recognize the states of the electrical circuit, it must control regenerative power with statically conservative characteristic shown by the solid line in Fig.2 to avoid excessive voltage at the pantograph, because the voltage at the pantograph rises when a powering train, which exists in the electrical circuit, reduces its power consumption. The faster the reduction of power consumption is, the higher the voltage of the pantograph rises. Therefore, the traction controller must squeeze regenerative power regarding reduction of power consumption of registercontrolled trains, which reduce their power consumption faster than any other train, in the electrical circuit. However, the reduction of power consumption of trains controlled by VVVF-inverters, armature choppers or a field chopper is slower than that of resistor-controlled trains. Traction controller squeezes, consequently, regenerative power excessively when powering trains controlled by these methods to reduce their power consumption. 3 Improvement of squeezing control The improvement of electrical circuits, power management with data communication in an electrical circuit etc. are proposed to improve squeezing control of regenerative power [1], [2], [3]. In this paper, the authors propose squeezing control of regenerative power whose characteristics vary according to states of the electrical circuit. It is necessary to know the behaviour of the
16 Power Supply, Energy Management and Catenary Problems pantograph voltage rising quickly at the stop of the power consumption of powering trains in the same electric circuit for improving the squeezing control of the braking train. For that purpose, the traction controller must have the following information; 1. the position and velocity of the trains, voltage at pantograph, DC current of traction controller and power regeneration of the regenerating train, 2. running profile of the line on which the regenerating train exists, 3. control method of every train on the line, 4. the time when powering trains in the electrical circuit reduce their power consumption and 5. distance between the braking train and the powering trains. In the above, the information in 1 can easily be measured, and the information in 2 and 3 can be stored on board as data of traction controller. However, the information given in 4 and 5 needs to be estimated from the information in 1, 2 and 3. And, the characteristics of squeezing control of regenerative power must be determined, based on the information. One must propose how to estimate the information in 4 and 5 and how to determine the characteristics of squeezing control of regenerative power. The voltage regulation at the pantograph in the case of powering trains with various control methods, reduce their power consumption for determining characteristics of squeezing control of regenerative power in the following part of this paper. Internal resistance S ubstation Feeder line (1km ) Il Powering train Feeder line (variable) E s Filter reactor Filter capacitor Braking train I s E fc I ch I r Braking resistor Traction controller E fc I r I 0 Squeezing control of regenerative pow er E fc I ch Brake chopper operation I 0 : C urrent operation from braking operation Figure 3: Electrical circuit for examination. 4 Voltage regulation at the pantograph 4.1 Electrical circuit for examination of voltage regulation Fig.3 shows the electrical circuit to calculate voltage regulation at the pantograph. The electrical circuit consists of a substation, a powering train and a braking train controlled by VVVF-inverter. The powering train is controlled by
Energy Management in the Train Operation 17 VVVF-inverters, field-current choppers or resistor controllers. Fig.4 shows the equivalent circuits of the powering train and Fig.5 shows characteristics at reduction of power consumption at the powering train. The line voltage at the electrical circuit is limited up to 1900V. The authors will monitor the voltage at a filter capacitor of a braking train instead of that at pantograph. I l Filter reactor Filter capacitor Traction controller I a I l I a Traction controller (a) V V V F -inverter control (b) F ield chopper control R esistor control Figure 4: Equivalent circuits of a powering train. Ia[A ] Ia[A ] Ia[A ] 1600 1600 1600 1.0s 0.6s 50m s 800 50 50 50 0 2.5 3.5 5 0 2.5 3.1 5 0 2.5 2.55 2.60 2.65 5 Time[s] Time[s] Time[s] (a)vvvf-inverter control (b)field chopper control (c)resistor control Figure 5: Characteristics at reduction of power consumption. E fc I 0 I C haracteristic of 00 squeezing control I 00 >I 0 I=I 00 I I 00 <I 0 I=I 0 1 I 1+Ts r (T=30[m s]) I 00 [A ] 0 10V E max E fc [V ] -2000 (a) O peration logic for squeezing controlof regenerative pow er (b) C haracteristic of squeezing control Figure 6: Squeezing control for VVVF-inverter and chopper controlled train. 4.2 Voltage regulation at pantograph of the braking train 4.2.1 Case of VVVF-inverter controlled powering train Fig.6 (a) shows the operation logic of how to reduce the regenerative power when the powering train controlled by VVVF-inverter stops its power consumption. In this logic, the V-I characteristic in Fig.6 (b) is assumed as the characteristic of squeezing control in Fig.6 (a). The first order delay, the time constant of which is assumed T=30[ms], represents the response of the traction
18 Power Supply, Energy Management and Catenary Problems motor current. In addition, the distance between the powering and the braking trains is 2 km. Fig.7 shows voltage at the filter capacitor of the braking train. It also shows that the braking train can keep electric braking action by reducing its regenerative power continuously for avoiding excessive pantograph voltage, even if the other train stops its powering in various cases from E max =1600[V] up to 1850[V]. In addition, Fig.8 demonstrates the relation between the voltage at the filter capacitor and the DC current from the braking train while the powering train reduces its power consumption in the case that E max is 1850V. And Fig.8 illustrates that the traction controller of the braking train can reduce its regenerative power following the design of its squeezing control. Figure 7: Voltage at the filter capacitor (VVVF-inverter). Figure 8: Following characteristic of squeezing control (VVVF-inverter). 4.2.2 Case of powering train controlled by field-current chopper Fig.6 (a) shows operation logic for squeezing control of regenerative power when a powering train controlled by a field-current chopper stops its power consumption. In addition, the distance between the powering and the braking trains is 2 km.
Energy Management in the Train Operation 19 Fig.9 shows voltage at the filter capacitor of the braking train. It also shows that the braking train can keep electric braking action by reducing its regenerative power continuously for avoiding excessive pantograph voltage, even if the other train stops its powering in various cases from E max =1600[V] up to 1850[V]. In addition, Fig.10 illustrates the relation between voltage at the filter capacitor and the DC current of the traction controller of the braking train while the powering train reduces its power consumption in case that E max is 1850V. And Fig.10 illustrates that traction controller of the braking train can reduce its regenerative power following the design of its squeezing control. Figure 9: Voltage at the filter capacitor (Chopper). Figure 10: Following characteristic of squeezing control (Chopper). 4.2.3 Case of resister-controlled powering train Fig.11 shows operation logic for squeezing control of regenerative power in case the powering train, which is resister-controlled, reduces its power consumption. In addition, the first order delay, whose time constant is 1.0 ms, is used to suppress vibration of I 00 and the other first order delay, whose time constant is 30 ms, indicates characteristic of response of current at traction motor. Moreover,
20 Power Supply, Energy Management and Catenary Problems the limiter 1 makes its output zero when its input is negative and the Limiter 2 makes its output zero when its input is positive. d dt Proportionalgain 0.3 Limiter 1 E fc I 0 1 I C haracteristic of 00 squeezing control 1+T I 1 s 00 I 00 >I 0 I =I 00 (T 1 =1[m s]) + I 00 <I 0 I = I 0 + Limiter 2 I 1 1+T 2 s (T 2 =30[m s]) I r Figure 11: Operation logic for squeezing control (2). Fig.12 shows voltage at the filter capacitor of braking train when the E max indicated in Fig.6 (b) is 1600V and the distance between the powering and the braking trains is 2km. Fig.12 also illustrates that the filter capacitor of braking train rises drastically because the powering train spontaneously reduces its power consumption in several milliseconds. Therefore, E max must be less than 1600V so that traction controller can reduce regenerative power conservatively when resister-controlled train, instead of a VVVF-inverter controlled train or a fieldchopper controlled one, cuts off its power. Figure 12: Voltage at the filter capacitor (3). In addition, Fig.13 shows maximal voltage at the filter capacitor of the braking train when the distance between the powering and the braking trains varies if E max is 1600V. This figure means that the longer the between the powering and the braking trains is, the lower the maximal voltage at the filter capacitor of the braking train is, since the line resistance proportional to the distance between the two trains restricts the power to be transferred from the braking to the powering train. Fig.13 also demonstrates that the longer the distance between the powering and the braking trains is, the higher the E max can be. Fig.14 shows maximal E max to avoid excessive voltage at the filter capacitor of the braking train. This figure means the longer distance between the powering and the braking trains allows
Energy Management in the Train Operation 21 higher E max, since the influence from the action of the powering train is substantially smaller when the distance between the two trains is longer. The logic indicated by Fig.14 (b) determines the possible E max to avoid excessive voltage at the filter capacitor of the braking train. Figure 13: Voltage rise. START E max =1600[V ] E max = E max +10 Circuit simulation Yes MaximalE fc < 1900V (during simulation) No E max = E max -10 End (a) T he possible E max to avoid excessive voltage (b) T he logic to determ ine the possible E max Figure 14: Possible E max to avoid excessive voltage. I ch [A ] 150 0 1850 1860 E fc [V ] Figure 15: V-I characteristic for a chopper-control of a braking resistor. If the braking train has supplemental braking resistor, whose characteristic for operation is assumed as Fig.15, E max =1850[V] is possible for all the investigated
22 Power Supply, Energy Management and Catenary Problems train distance, since the braking resistor can effectively absorb the power deviation from the spontaneous action of the powering train. In this case, maximal power consumption of the braking resistor at all the investigated train distance is 220kW, which is approximately 7% of maximal power consumption of typical electric train on powering. 5 Conclusion In this paper, the authors have proposed squeezing control of regenerative power whose characteristics vary according to states of electrical circuit. They have examined the voltage at the filter capacitor of the braking train when the different three kinds of powering trains stop their power consumption. They have concluded: 1. when a powering train, which is controlled by VVVF inverter or field chopper, stops its power consumption, braking train can successfully reduce its regenerative power with squeezing control whose E max is close to maximal voltage limitation, 2. the controller of the braking train must reduce its regenerative power conservatively when a resister-controlled powering train close to the braking train stops its power consumption, 3. longer distance between the powering and the braking trains allows higher E max, since the influence from the action of the powering train is substantially smaller when the distance between the two trains is longer, and 4. the braking resistor, whose power consumption approximately 7% of the maximal power consumption of typical electric train on powering enables E max to be 1850[V] for all the investigated train distance. 6 Future work The authors have studied only the squeezing control of regenerative power on board. However, they must also investigate how to estimate and use the following information to introduce a better squeezing control of regenerative power whose characteristics vary according to the states of electrical circuit; 1. the time when powering trains in electrical circuit stop their power consumption, and 2. distance between the braking train and the powering train which cuts off its power consumption. Acknowledgements The authors are grateful to Prof. Satoru Sone at Kogakuin University and Mr. Hideki Iida at Shin-Keisei Electric Railway Co., Ltd. for their assistance and cooperation in the investigation in this paper.
Energy Management in the Train Operation 23 References [1] S. Sone, Re-examination of Feeding Characteristics and Squeezing Control of Regenerative Trains, Joint Technical Meeting Transportation and Electric Railway and Linear drives, TER-02-49/LD-02-64, 2002. [2] Y. Okada, T. Koseki, Evaluation of maximal reduction of electric energy consumed by DC-fed electric trains, NATIONAL CONVENTION RECORD I.E.E. JAPAN, 5-219, pp307-308, 2003. [3] Y. Okada, T. Koseki, S. Sone, Energy Management for Regenerative Brakes on a DC Feeding System, STECH 03, pp 376-380, 2003.