Multi-Stage Converters: A New Technology for Traction Drive Systems

Juan W. Dixon Department of Electrical Engineering

Pontificia Universidad Católica de Chile

Felipe Ríos, and Alberto Bretón Department of Electrical Engineering

Pontificia Universidad Católica de Chile

Abstract A multi-stage converter for electric traction applications has been implemented and tested. This technology allows modulating the output voltage by amplitude instead of pulse-width. The amplitude modulation is based on a digital cascade of “H” converters scaled in power of three, and with each converter able to generate three levels of voltage. The multi-stage converter developed in this work, has a chain of four converters per phase (4-Stage Converter), with a “Main converter” that manages more than 80% of the total power, and three “Slave converters” that take the rest of the power (less than 20%). With this technology, the pulsating torque generated by harmonics is eliminated, and the power losses into the machine due to harmonic currents are also eliminated. Another advantage of this drive is that the switching frequency of the Main converter is at the fundamental frequency, reducing their power losses at minimum values. Besides, the power ratings of the semiconductors of the Slaves become very small. The paper also shows how the problem of isolated power supplies required for each converter can be overcome, by using isolated motor windings for each phase of the traction motor, and by using bidirectional regulated power supplies for the Slaves. Simulations using PSIM (Power Electronics Simulator) have demonstrated the feasibility to build multi-stage converters for real electric vehicles or electric buses. These simulations have been compared with voltage and current waveforms obtained with conventional PWM inverters. The experimental multi-stage prototype has a rating power of 3 kW, and some experimental results are displayed and compared with the simulations. Copyright 2003 EVS20. Keywords: “Traction Control”, “Converter”, “Control System”. 1. 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 [1-4]. The problems associated with conventional adjustable PWM speed drive inverters are as follows: i) High dV/dt and voltage surge because of switching causes motor bearing failure and stator winding insulation breakdown. ii) High-frequency switching requires significant derating of switching devices and generates large switching losses. iii) High-frequency switching generates broadband (10 kHz to 30 MHz) Electro-Magnetic Interference (EMI) to nearby communication or other electronic equipment [5]. Multi-stage converters [6-8] can use control strategies to 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 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 Main Converter, takes more than 80% of the total power delivered to the load. A four-stage converter using three-state power modules (“H” modules), which gives 81 different levels of voltage amplitude, is designed. The results are compared with conventional PWM modulators working at a switching frequency of 10 kHz. All the load parameters of both types of converters are set at the same values. The multilevel converters can be applied to almost every practical situation. Some of them are active power filters, sinusoidal current rectifiers, machine drives, power factor compensators, back-to-back frequency link systems, and traction drive systems. This paper is focused in the last application, and starts with an explanation about operation principles of a multi-stage converters, simulations, and experimental result of multi-stage converters in traction applications. The advantages of this kind of topology are clear when compared with conventional PWM traction drive systems. 2. Main Characteristics of Multi-Stage Converters The circuit of figure 1, shows the basic topology of one “H” converter used for the implementation of multi-stage converters. It is based on the simple, four switches converter, used for single-phase inverters or for dual converters.

1S

'1S

dc V LOAD

'2S

2S + _

Figure 1: Three-level module for building multi-stage converters These converters are able to produce three levels of voltage in the load: +Vdc, -Vdc, and Zero. The four switches S1, S1

’, S2, and S2’ are controlled to generate the three discrete voltage outputs at the LOAD.

When S2 and S1’ are switched-on, the output is –Vdc. When either pair S1 and S2, or S1

’ and S2’ are “on”, the

output is zero; when S1 and S2’ are turned “on” the output is +Vdc.

As with the standard cascaded H-bridge inverter [9]–[10], the power quality of the cascaded multilevel H-Bridge inverter may be greatly improved through utilization of different dc voltages on each cell. In particular, according to the maximal expansion principle [11], it can be shown that the number of voltage levels is maximized if the dc voltages are set according to [12]:

)1...( ,3 ,1 )1(

1)(

1)1( −=⋅

−⋅−

=−

− pivnn

nv ixdc

ii

iixdc (1)

where ni is the number of voltage levels that the i-th H-bridge cell is capable of producing, and p is the number of multilevel H-bridge cells, each with an isolated dc source. However, this equation does not guarantee the existence of redundant levels. In the prototype, all the H-Bridge are equal (i = i-1), and have 3 different states (ni = 3), so (1) give us the follow relationship:

)1( )( 3 −⋅= ixdcixdc vv (2) With the relationship between the dc source of the different H-bridge cell (2) and the connection of four three-level modules, we got the main components to built a four-stage converter.

Driver

Driver

Driver

Driver

LOAD

Vdc + _

3·Vdc

+ _

9·Vdc + _

27·Vdc + _

3rd Slave

2nd Slave

1st Slave

Main Converter

Figure 2: Main components of the four-stage multiconverter. The figure 2 displays the configuration of one phase leg of the complete prototype. 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 (34) 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 3 has the bigger voltage, and will be called Main Converter. The rest of the modules will be the Slaves. The Main Converter works at the lower switching frequency (fundamental frequency), which is an additional advantage of this topology.

In general, a multilevel inverter with ‘n’ equal dc voltage levels can offer 2n + 1 distinct voltage levels at the phase output. The performance attributes of the output waveform in terms of number of levels can be further enhanced by using unequal dc voltage levels. For instance, a set of cascaded inverters with dc voltages varying in binary and trinary fashion gives an exponential increase in the number of levels. For ‘n’ such cascaded inverters, with dc voltage levels varying in binary fashion, one can achieve 2n+1 - 1 distinct voltage levels. With dc voltage levels varying in trinary fashion, one can produce 3n distinct voltage levels [13]. The advantage of this topology is that it provides flexibility for expansion of the number of levels easily without introducing undue complexity in the power circuit, for example, the number of steps can be increased to 243 with one more converter in the chain (five-stage H-converter), but the quality of waveforms with only four converters is good enough. However this kind of configuration requires multiple dedicated dc buses which makes it an expensive solution. With 81 levels of voltage, a four-stage converter can follow a sinusoidal waveform in a very precise way. 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. As seen on figure 3, at 100% of voltage amplitude modulation, the output voltage have 40 positive levels, which added with the 40 negative levels, and zero, to obtain the maximum of 81 levels for this converter. For de 75% of amplitude modulation there are 30 positive levels, at 50% of amplitude modulation we have 20 positive levels and so on. At very low voltages (less than 10%) PWM strategy can be applied to avoid current distortion at the load.

Figure 3: Voltage AM using four-stage converter.

Figure 4: a) Voltage Modulation in each converter b) Active power distribution in each converter. One of the good advantages of the multi-stage configuration, which is possible to see in the oscillogram of figure 4(a), is the very low switching frequency of each converter. But even better, the Main Converter, 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 Main Converter can be implemented with GTOs, and the Slaves with IGBTs. Another attribute of the strategy described here for multiconverters is that most of the power delivered comes from the Main Converter. The example of figure 4(b) shows the power distribution in one phase of the four-stage converter, feeding an inductive load with sinusoidal voltage. A little more than 80% of the real power is delivered by the Main 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 Main Converter. 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 as shown in figure 4 b). The figure 5 shows a bi-directional dc-dc power supply, which can be used for this purpose [14].

DC Input from Main Converter

DC Output

to Slave

High Frequency Switching Square Wave

N1:N2

Figure 5: Bidirectional DC-DC Power Supply

One more advantage of the multi-stage configuration, which is possible to see in the oscillograms of figures 4 a) and b), is the very low switching frequency of each converter. But even better, the Main Converter, 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 Main Converter can be implemented with GTOs, and the Slaves with IGBTs. To avoid the three Main Converters 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 6.

Figure 6: Traction system configuration using a four-stage multilevel converter. Despite the system looks complicated, it can be adequately integrated. It is important to remember that the DC-DC converters required to feed the Slaves, 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 0.8 kW, and for the third Slave only 200 W each. These converters can be small today with a switching frequency link of hundreds of kHz.

3. 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 [15], which has demonstrated its reliability for almost 10 years of simulations, and has 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.

3.1. 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 7 shows a current comparison of the load when it 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.

PWM

Four-Stage

Figure 7: Armature currents from a PWM voltage source, and a four-stage voltage source. The figure 8 shows the direction of the power in the Main, and in each Slave converter, for a 60 kW induction traction motor. The simulation shows the total power per phase delivered by the machine, as a function of stator frequency. It can be noticed that the Slaves absorb power at some particular frequencies, and for this reason the bi-directional, isolated power supplies are required.

-5000

0

5000

10000

15000

20000

25000 Total Power per phase Main Converter Slave 1 Slave 2 Slave 3

f=Hz

10 20 30 40 50 60 70 80 90

Figure 8: Power delivered by each converter as a function of frequency in a 60 kW traction motor.

3.2. 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. Again, the quality of the current obtained with the multi-stage technology is superior.

Figure 9: Current comparison for brushless-dc motor 4. Experimental Results An experimental multi-stage prototype, with a rating power of 3 kW, has been implemented. This prototype was built using Power Mosfets, and is controlled using DSP technology. The figure 10 shows some photographs of this four-stage 81-level converter.

Figure 10: Prototype of the four-stage, 81-level multiconverter. In order to validate the proposed concept, the inverter of figure 10 was constructed and tested. The prototype has a nominal power of 3 kW ( 1000 Watts per phase). A 3-kW induction machine was connected to the output voltage for the tests. The dc voltage was supplied by a transformer/rectifier power source and adjusted to obtain an ac voltage output of 100 V.

Figure 11: Simulated and Experimental load currents and voltages from a PWM converter, and a four-stage multilevel converter. In the figure 11, it can be seen the great difference between the quality of the output currents and voltages of the two technologies. The multi-stage converter have the advantage in current and voltage quality, so the multilevel topology is an attractive solution for machine drives. Finally, simultaneous experimental measures (voltage and current waveform) of the multilevel converter with a RL load are presented in the figure 12. In that figure, the phase angle between current and voltage can be appreciated.

CurrentVoltage

2 ms/DIV Figure 12: Voltage and Current waveforms of Multilevel converter with a RL Load. This waveforms are obtain with a load current of 3 Amperes, giving a total power of 840 Watts (280 Watts per phase). Experimental result about the phase angle, give a result of 0,63 radian (36°) without harmonics distortion, which shows the advantage of this topology. 5. Conclusion This paper has presented the concept of using multilevel H-bridge cells in cascaded inverters. 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 bi-directional dc supplies for the Slaves and isolated machine windings to keep the three Main Converters fed by a common supply. Different simulations and experimental result 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. 6. References

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[15] Powersim Technologies. PSIM Version 4.1, for Power Electronics Simulations. User Manual. Powersim Technologies, Vancouver, Canada, Web page: http://www.powersimtech.com.

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7. Ackowledgements The authors want to thank Conicyt through Project Fondecyt 1020982, for the support given to this work.

8. Authors

Juan W. Dixon, Ph.D. Department of Electrical Engineering, Pontificia Universidad Católica de Chile, Casilla306, Correo 22, Santiago, Chile; Phone 56-2-686-4281, Fax 56-2-552-2563, E-mail:jdixon@ing.puc.cl J. Dixon got the Ph.D. degree from McGill University, Montreal, Canada; From 1977 to1979 he was with the National Railways Company (Ferrocarriles del Estado). Since 1979he is Associate Professor at Pontificia Universidad Católica de Chile.

Felipe E. Ríos Department of Electrical Engineering, Pontificia Universidad Católica de Chile, Casilla 306, Correo 22, Santiago, Chile; Phone 56-2-686-4223, Fax 56-2-552-2563, E-mail: ferios@puc.cl F. Ríos is working towards the Engineering degree from Pontificia Universidad Católica de Chile, Santiago, Chile

Alberto A. Bretón Department of Electrical Engineering, Pontificia Universidad Católica de Chile, Casilla 306, Correo 22, Santiago, Chile; Phone 56-2-686-4223, Fax 56-2-552-2563, E-mail: abreton@puc.cl A. Bretón is working towards the Engineering degree from Pontificia Universidad Católica de Chile, Santiago, Chile