Modern Concepts of Energy Control Technology through VVVF Propulsion Drive

Transcription

1 Modern Concepts of Energy Control Technology through VVVF Propulsion Drive Satoru OZAKI, Fuji Electric Systems Co., Ltd. Ken-ichi URUGA, Toyo Denki Seizo K.K. Dr. D.P. Bhatt, Autometers Alliance Ltd ABSTRACT This paper introduces the high power traction converter with high power IGBTs and its systematizing and control technology as the state-of-the-art technology for the propulsion system of AC electric railcar, especially by focusing on energy conservation and the electric power quality. 1. TRANSITION OF TECHNOLOGY FOR THE PROPULSION SYSTEM OF AC ELECTRIC RAILCAR The table-1 shows the transition of the propulsion system of Shinkansen that is the typical AC electric railcars. In the early stages, the propulsion system for the AC electric railcars began with the DC motor drive system fed by the diode rectifier with the tapped transformer. Now it employs the AC motor drive system fed by the VVVF inverter through the stage of DC motor drive system fed by the phasecontrolled rectifier with thyristor. Especially, in these years, the shift of power semi-conductor for VVVF inverter from early GTO to high power IGBT and the control technology for induction motor with the vector control lead to improvement of the energy conservation with the aid of regenerating brake and the quality of electric power on power factor and harmonic current. Tbale.1 Technological transition of the propulsion system for Shinkansen Series N7 Year 1964 ` 1986 ` 199 ` 1999 ` 27 ` Top 22km/hr 22km/hr 27km/hr 285km/hr 3km/hr Speed Total kW =11,84kW 48 23kW =11,4kW 4 3kW Power =12,kW kW =13,2kW 35kW 56 =17,8kW Power Diode Thyristor GTO IGBT IGBT Device Motor DC Motor DC Motor Induction Motor Induction Motor Induction Motor Control Tapped control Phase-shift control PWM Converter method + VVVF Inverter PWM Converter + VVVF Inverter PWM Converter + VVVF Inverter Braking Dynamic Dynamic Regenerating Regenerating Regenerating 53

2 They shall be nothing less than advancement of the power electronics technology. As the results of replacement of the traction motor from heavy DC series-wound motor to light squirrel-cage induction motor, considerable reduction of the weight in the traction motor itself and the unsprung weight of the bogie and improvement of energy efficiency of the motor have been achieved. For example, the series N7 Shinkansen is achieving the energy saving of 49% at 22 km/hr and 32% even at 27 km/hr compared with the series Shinkansen, the first bullet train. Fig.1 shows the comparison of power consumption among successive Shinkansen trains. 2. POWER CONVERTER Fig.1 Comparison of Power Consumption ( It is common to organize the propulsion system in units of one power car for the electric railcar. Then, it is widely used that the traction converter drives four induction motors connected in parallel to it as a unit. As the result of this, the capacity of the traction converter is going up to more than 1 MVA. Especially, the high-speed train such as Shinkansen that needs large power for acceleration to high speed and regenerating brake from high speed, has a pronounced tendency to become high power. Fig. 2 System organization of the tractor converter Besides, duty of the arm in the power supply side converter for the AC electric railcar enlarges by 5% compared with that of VVVF inverter, because the power supply side converter is a single phase while the load side VVVF inverter is three phases. In order to overcome these issues, the circuit voltage in the traction converter is raised to mitigate duty of the power semiconductor device to current and to reduce weight by downscaling the power cable and the conductor bars in the traction converter. As an example of the traction converter for AC electric train car, Table-2, Fig.2 and Fig.3 show the dominant specification of the traction converter, configuration of the propulsion system and the appearance of the traction converter for the series N7 Shinkansen respectively. 54

3 The propulsion converter consists of the single phase PWM converter (power supply side) and the three-phase PWM VVVF inverter (load side). Each VVVF inverter drives four induction motors connected in parallel to it. Both of PWM converter and VVVF inverter employ the NPC (Neutral Point Clamped type) converter with three voltage levels for its output equipped with module type IGBT of 3.3kV 1.2kA. Employing three-level converter as the power unit for Table-2 Specification of propulsion converter for series N7 Item Feeding voltage Input DC-link voltage Output Specification AC 25V 6Hz AC 152V DC 3V AC23V 158kVA Fig.3 Configuration of the power converter both power supply side converter and load side VVVF inverter puts the high power converter necessary for the high speed AC electric railcar into practice. Besides, the three-level converter for the power supply side converter contributes to reduction of electromagnetic acoustic noise caused by the main transformer and suppression of the harmonic current in power supply side, because that the equivalent carrier frequency of the three-level power converter is twice as that of individual IGBT device. 55

4 On the other hand, multi-bank type transformer is useful to optimize configuration of the propulsion system for the long consist of train in which several traction converters exist such as the Shinkansen. The series N7 Shinkansen shown at Fig.4 for example connects three or four traction converters to one main transformer in order to optimize the system configuration in the consist as well as brings effect to suppress the low order harmonic currents as described later. 3. CONTROL TECHNOLOGY FOR NPC CONVERTER Fig.4 shows the PWM control of three-level converter and the operating waveform. The potential of the neutral point of three-level converter must be controlled, and be kept at the voltage equivalent to a half of the DC link voltage so that applied voltages to each IGBT are even. However, the potential of the neutral point fluctuates easily according to various error factors in the circuit and the control. The possible counter measure that explicitly compensates this voltage fluctuation by VVVF inverter s switching needs to change the switching pattern of the IGBTS according to the direction of the load current and might result in increase of the switching frequency of IGBTs. In order to overcome these issue, the potential of the neutral point is controlled with the current flew into/out of the neutral point by adjusting the duration of Mode B and Mode D in the PWM control of the power supply side converter. The neutral point control has been achieved without increase of the switching frequency of IGBTs, because it utilize Mode B and Mode D of the PWM control that corresponds to the dead time to prevent the arm short-circuit for the two-level converter. Switching Pattern PWM waveform Mode A Mode B Mode C Mode D Mode D Phase Voltage Line Voltage Fig.4 Operating waveform of three-level converter 4. QUALITY IMPROVEMENT OF POWER CONSUMPTION The quality of power consumption is a significant issue for AC electric train whose electric power is supplied through the substation from the commercial power system from the viewpoint of influence to the commercial power system. The following measures are implemented in respect of the power factor, harmonic current and the power limitation. 4.1 POWER FACTOR CONTROL In order to keep 1. the power factor of the input electric power of power supply side PWM converter, the input current to the power supply side converter is controlled based on the sinusoidal modulated reference value for the input current that is synchronized with the power-supply voltage waveform. 56

5 The high accuracy digital PLL (Phase Locked Loop) is employed to precisely synchronize it with the power supply voltage. 4.2 Suppression of Harmonic Current In order to suppress harmonic current in power supply side load current, the carrier frequency of switching for the IGBTs in power supply side PWM converter is raised to more than 1 khz. Because the equivalent carrier frequency of the three-level converter becomes twice as that of each IGBTs, it is possible to raise the order of the component of harmonics so that low order harmonic current is suppressed. In addition, the phase of the career is shifted between traction converters so that the harmonic components originated at the career frequency are canceled each other in case where a number of the traction converters exist in a consist like the Shinkansen. As a result, in the event that N sets of traction converters are equipped in the consist, the generation of harmonic current up to N times of the career frequency can be suppressed. Moreover, in order to make these compensating measures effectively, the high speed and high accuracy PLL by high resolution digital processing and the high-speed control operation provide high quality PWM control with low waveform distortion. 4.3 LIMITATION OF INPUT POWER TO TRACTION CONVERTER The load current during acceleration of the train has a significant impact on the feeding system because the maximum acceleration current is large for the high-speed railway such as Shinkansen. The power limitation function is effective to control the over current due to the voltage drop of the overhead wiring in a weak feeding system. The power supply side converter detects an increase of the input current due to the voltage drop of the overhead wiring according to the output of the voltage regulator in it, and applies feed-forward to the current reference value for the load side VVVF inverter. Then, the power limitation works in fast response without unstable interaction between the power supply side converter and the load side VVF inverter. 5. PROPULSION CONTROLLER 5.1 HARDWARE AND ITS FUNCTIONS The high-speed and high performance control platform is essential for the traction converter of the AC electric railcar for which the control of the traction motor, control of the power supply side converter and the specific control functions for AC electric railcar are implemented. 57 Fig.5 Controller To satisfy these needs, the high performance digital controller is applied to the control system for the traction converter. It employs the multiprocessor structure whose core consists of the 64 bits RISC processor and the32 bits floating point DSP. Fig.5 shows appearance of the controller. The technology for serial communication between processors and the custom integrated circuit devices are applied to the controller by taking advantage of state-ofthe-art communication technology and microelectronics technology. It provides reduction in number of parts, multifunction and high reliability.

6 The controller carries out functions for not only control of the power supply side converter and the load side VVVF inverter but also interface for operation command via serial communication, sequence control and monitoring of the operation status. It also contributes to obtaining high reliability in the system. 5.2 TORQUE CONTROL OF MOTOR 6. WEIGHT SAVING A high-performance vector control that controls the motor torque in acceleration and regenerating to brake in high-speed and high accuracy is applied to the control of the traction motor. The traditional vector control with slip frequency of induction motor is a method that independently controls the exciting current component and the torque current component with slip frequency based on the equivalent model of the induction motor, which are orthogonal relationship each other as a spatial vector in the system that controls one motor by one inverter. However, in the system where four induction motors are connected in parallel to one inverter, the slipping frequency in each motor is different. Then an existing vector control algorithm cannot be applied to this system as it is. In order to solve this issue, the vector control has been enhanced to the control method based on the phase angle of flux that is equivalent to the primary voltage of the motor that was a common state valuable regardless of the parallel number of the motor. As the substantial factor to the performance of the train, saving weight of the equipments on the train is significant challenge. 6.1 SAVING WEIGHT OF MIN CONVERTER The traction converter without the blower motor for cooling the IGBT power units has been jointly developed with Central Japan Railway Company. The power units are naturally cooled by the wind flow under the car body during train running with the radiating fin for IGBTs and the inclination as the drafty guide on the bottom of the traction converter. Air flow caused by travel Fig. 6 Cooling mechanism of Brower-less unit Fig.6 shows the flow of wind under traction converter during train running. Cooling air Aluminum cooling fin As a result, the weight of the traction converter is reduced by 12% compared with that of the same type of traction converter with the blower motor. On the other hand, dismantlement of the snubber circuit and the optimum design of the each components have reduced 3 % of the volume and 15% of the weight compared with the previous type of the traction converter, while its output is increased by 1% from the previous one. Besides, the efficiency of the traction converter has been improved by applying the lowloss type IGBT and the implementation of the snubber-less circuit with IGBT connecting conductors that are low inductance. 58

7 6.2 MAIN TRANSFORMER Employment of the aluminum coil and the polyimide film as insulating material for element wire of the coil has reduced weight of the main transformer unit. The multi-bank type transformer is also effective to optimize configuration of the propulsion system for the long consist of train in which several traction converters exist such as the Shinkansen. To take the series N7 Shinkansen as an example, the configuration of the propulsion Fig.6 Cooling mechanism of Brower-less unit Aluminum cooling fin Cooling air Air flow caused by travel system is optimized by combined the main transformers that are the type with 3 banks of secondary windings and the type with 4 banks of secondary windings. As the result, the number of the main transformer in the consist has not increased even though the number of the traction converter is increased to 14 from 12 in the previous Shinkansen. Then the mass of a main transformer per unit output has been improved by 5% from that of the previous Shinkansen. 2, [kg], [kva] 15, 1, 5, Output Capacity [k VA] Weight [kg] Series Series Series 3 7 N7 2 [kg/kva] Fig.7 Weight & Output capacity of transformers 1 1, [ton] Weight 5 Series Weight kg/kw Series Series Series Series N7.1 [kw /ton].5 Fig.7 Weight & Output capacity of transformers????????? n?????????? 7. CONCLUSION Fig.7 and Fig.8 show the transition of the weight-output capacity ratio and the relationship between total mass and total output in the consist, respectively as the example of the shinkansen. The state-of-the-art technology of the energy control for the propulsion system of AC electric railcar has been introduced. We are sure to develop and provide the propulsion system that is earth-conscious and has high reliability by applying the state-of-the-art power electronics technology. **** 59