RECENT TRENDS IN ELECTRICAL TRACTION

Abstract:

The application of multilevel converters for traction drive systems is being investigated. The main advantage of this kind of topology is that it can generate almost perfect current or voltage waveforms, because it is modulated by amplitude instead of pulse-width. That means that the pulsating torque generated by harmonics can be eliminated, and power losses into the machine due to harmonic currents can also be eliminated. Another advantage of this kind of drive is that the switching frequency and power rating of the semiconductors is reduced considerably. The amplitude modulation is based on a cascade of N converters scaled in a “trinary” form (three-state “H” converters). In the chain of N converters of each phase (N-Stage Converter), there is a “Master converter” that manages more than 80% of the total power, and N-1 “Slave converters” that take the rest of the power (less than 20%). One important drawback of this kind of arrangement is that it needs isolated power sources for each one of the N converters, and also for each phase. This paper shows that this problem can be overcome by using isolated motor windings for each phase of the traction motor (which is easy to get in normal machines), and by using low-power high-frequency, bidirectional switching power supplies for the “slave converters”. Simulations using PSIM (Power Electronics Simulator) have demonstrated the feasibility to build drive converters for electric vehicles using multilevel inverters. They are being
compared with inverters using the conventional PWM technique. The multilevel converter used in the simulations, works with only four inverters (N=4): one Master and three Slaves. In both the cases (PWM and multilevel), the traction motors have a rating of 80 kW, and the battery pack supply is 240Vdc. The battery pack is connected to the master converters of each phase in parallel, and to the slaves through isolated bi-directional switching power supplies.























Introduction

Power Electronics technologies contribute with important part in the development of electric vehicles. On the other hand, the PWM techniques used today to control modern static converters for electric traction, do not give perfect waveforms, which strongly depend on switching frequency of the power semiconductors. Normally, voltage (or current in dual devices) moves to discrete values, forcing the design of machines with good isolation, and sometimes loads with inductances in excess of the required value. In other words, neither voltage nor current are as expected. This also means harmonic contamination, additional power losses, torque ripple, and high frequency noise that can affect the controllers. All these reasons have generated many research works on the topic of PWM modulation .

Multi-stage converters work more like amplitude modulation rather than pulse modulation, and this fact makes the outputs of the converter very much cleaner. This way of operation allows having almost perfect currents, and very good voltage waveforms, eliminating most of the undesirable harmonics. And even better, the bridges of each converter work at a very low switching frequency, which gives the possibility to work with low speed semiconductors, and to generate low switching frequency losses. The objective of this paper is to show the advantages of multi-stage converters for all kind of applications. The drawbacks of requiring isolated power supplies is solved using different techniques, which depend on the type of application, and based on the fact that the first converter, called Master, takes more than 80% of the total power delivered to the load. A four-stage converter using three-state power modules, which gives 81 different levels of voltage amplitude, is studied. The results are compared with conventional PWM modulators working at a switching frequency of 10kHz. All the load parameters of both types of converters are set at the same values.

BASICS OF MULTISTAGE CONVERTERS:


Basic Principle:

The circuit of fig.1 shows the basic topology of one converter used for the implementation of multistage converters. It is based on the simple, four switches converter, used for single-phase inverters or dual converters. These converters are able to produce three levels of voltage in the load: +Vdc, -Vdc, and Zero.

Multi-Stage Connection:

The multi-stage connection can be implemented with two, three, or any number of three-level modules. The figure 2 displays the main components of a four-stage converter, which is being analysed in this work. The figure only shows one of the three phases of the complete system. As can be seen, the dc power supplies of the four modules are isolated, and the dc supplies are scaled with levels of voltage in power of three. The scaling of voltages in power of three allows having, with only four converters, 81 (3^4) different levels of voltage: 40 levels of positive values, 40 levels of negative values, and zero. The converter located at the bottom of the figure has the bigger voltage, and will be called Master. The rest of the modules will be the Slaves. The Master works at the lower switching frequency, which is an additional advantage of this topology.

With 81 levels of voltage, a four-stage converter can follow a sinusoidal waveform in a very precise way, as shown in figure 3. It can control the load voltage as an AM device (Amplitude Modulation). The figure 3 shows different levels of amplitude, which are obtained through the control of the gates of the power transistors in each one of the four converters.



Power Distribution:


One of the good advantages of the strategy described here for multiconverters is that most of the power delivered comes from the Master. The example of figure 5 shows the power distribution in one phase of the four-stage converter, feeding a pure resistive load with sinusoidal voltage. A little more than 80% of the real power is delivered by the Master converter, and only 20% for the Slaves. Even more, the second and third slave only deliver 5% of the total power. That means, the dc power sources needed by the Slaves are small.



This characteristic makes possible to feed the Slaves with low power, isolated power sources, fed by a common power supply from the Master. These power sources need to be bi-directional, because the power factor of the load can produce negative active power in some of the Slaves. The figure 6 shows a bi-directional dc-dc power supply, which can be used for this purpose .
Another attribute of the multi-stage configuration, which is possible to see in the oscillograms of figures 4 and 5, is the very low switching frequency of each converter. But even better, the Master, which carries most of the power, operates at the lower switching frequency. Then, the larger the power of the unit, the lower the switching frequency. In large traction applications, like buses, electric locomotives or ships, the Master can be implemented with GTOs, and the Slaves with IGBTs.To avoid the three Masters having to be isolated one from each other, the three windings of the machine have to be fed independently (no electrical connection between them). The complete configuration, using Bidirectional DC-DC Converters, and isolated windings for the traction motor is shown in figure 7.
Traction system configuration using a four-stage multilevel converter

Simulation Results:

The following results show a comparison between PWM strategy and a four-stage multilevel converter. These results have been obtained using the software called PSIM , which has demonstrated its reliability for almost 10 years of simulations, which have been corroborated with real experimental results. Shunt active power filters, static var compensators, sinusoidal voltage power supplies, high power rectifiers, and machine drives have previously been simulated with PSIM.

Machine drives with sinusoidal supply:

With PWM techniques, it is not possible to implement a sinusoidal voltage power supply. The multiconverter topology, scaled in power of three, with a few quantity of inverters, can generate a very good sinusoidal voltage waveform. A four-stage converter can generate 81 steps of voltage levels, as was shown in figure 3. Forty positive levels, forty negative levels, and zero. The figure 8 shows the current of the load when is fed with a PWM power supply, and with a four-stage power supply. The switching frequency of the PWM inverter is 15 kHz, and the supply frequency is 50 Hz. It is clear the difference: the current in the four-stage converter is almost harmonic-free. This system can operate at all output frequencies.
Armature currents from a pwm voltage source and a four-stage voltage source
Despite the system looks complicated, it can be adequately integrated. It is important to remember that the DC-DC converters are small power devices. For example, for 60 kW traction system for an electric vehicle, the DC-DC converters for the first Slave are only 3 kW each. For the second Slave are 0.8kW, and for the third Slave only 200 W each. These converters can be small today with a switching frequency link of hundreds of kHz.

As the quality of sinusoidal current waveforms was already showed in figure 12, the figure 13 shows a comparison of current for a brushless dc motor, using PWM converter, and four level converter. Again, the quality of the current obtained with the last technology is superior.

Machine drives with non-sinusoidal supply.
Multilevel converters can also be used as choppers for controlling dc motors. In this case, only one phase is needed, and is used for armature control. With the four-stage converter, the dc voltage of the armature can also be controlled with 81 levels: 40 for motoring, 40 for regenerative braking, and zero.Another important application is with brushless dc motors, because they need a special voltage modulation to get the typical trapezoidal waveform of the armature current. In figure 9, a comparison between PWM and a four-stage multilevel converter is displayed.



Conclusions:

A four-stage multilevel inverter, using three-state converters for electric traction applications, has been analyzed. The advantages and drawbacks of this kind of converter have been displayed. The problem related with galvanic isolation, have been overcome by using isolated, bidirectional dc power supplies,which can be fed from a common power source from the Master. This solution becomes practical because the Master takes more than 80% of the total active power required by the system. The rest of the converters, called “Slaves”, need to convert very low power, and then those dc supplies are small. Different simulations were shown and compared with similar results obtained with conventional PWM converters. The topology looks applicable not only for electric vehicles, but also for large traction equipment such as electric buses, electric locomotives and ships.