report_seminar_spoorthi

TABLE OF FIGURES

Figure 1 Block Diagram of conventional traction converter ..................................................... 4

Figure 2Four step commutation ................................................................................................. 8

Figure 3 New configuration ....................................................................................................... 8

Figure 4 Block diagram of new configuration ......................................................................... 10

Figure 5 Block of DSP ............................................................................................................. 11

Figure 6 Overview of a flexible DSP controller ...................................................................... 11

Figure 7 Interfacing DSP for the control operation ................................................................. 12

Figure 8 PI Controller .............................................................................................................. 14

Figure 9 Basic hysteresis control ............................................................................................. 15

Figure 10 Hysteresis control .................................................................................................... 16

Figure 11 Current mode hysteresis controller .......................................................................... 17

Figure 12 Voltage mode hysteresis control ............................................................................. 18

Figure 13 Control of the traction converter as in the paper ..................................................... 19

Figure 14 Block diagram of control operation ......................................................................... 19

Figure 15 Comparison of the old configuration with the new configuration .......................... 21

New Configuration of Traction Converter APRIL 2015

Department of Electrical and Electronics, ASE, Bangalore Page 1

CHAPTER 1: INTRODUCTION

This seminar has been motivated by the industrial demand for a design of a traction converter

with a medium-frequency transformer (MFT) using matrix converters to replace the bulky main

line transformer found on board railway vehicles. The aim of this seminar is to examine the

benefits and drawbacks of the traction converter with single-phase matrix converters at the

primary side of the MFT. This seminar report is organized as follows. First, the proposed

configuration of the new traction converter with the MFT employing matrix converters at the

primary (medium-voltage) side of the transformer is presented. Second, the developed control

of both primary and secondary converters is described. Third, the behaviour of the designed

small-scale prototype of the traction converter is analysed by simulations and experiments

under steady-state and selected transient conditions. Finally, the benefits, drawbacks, and

constraints of the proposed new configuration of the traction converter are presented in the

final section of the seminar.



CHAPTER 2: LITERATURE SURVEY

2.1 Traction Drive with MFT - Novel Control Strategy Based on Zero

Vectors Insertion

Control strategy of matrix converter using the zero vectors insertion enables setting the phase

shift of input current against input trolley voltage (the phase). Default rectangular control of

input matrix converters brings spikes of taken current from the trolley line. During matrix

converter switching the current has to change polarity due to the current commutation. In the

matrix converter input current spikes appear as current commutation results. The spikes value

depends on current amplitude and MFT leakage inductance which leads to the commutation

time. It is difficult to eliminate both values therefore we have to think to modify the control

strategy to suppress current spikes. One possible reason is zero vectors insertion to matrix

converter strategy. Zero vectors are inserted at each transition matrix converter between "1"

and "-1". Using this method is reduced charge that must accommodate input filter capacity

when switching matrix converter. During the zero vector, the matrix converter input is

disconnected and the spikes are not presented in the taken current. The zero vector insertion

time determines the suppression of negative current spikes. However on the other hand zero

vector insertion brings higher demand on the input filter designing. It is necessary to balance

the request on current spikes suppression towards input filter capacitor value. Proposed control

strategy of input matrix converter based on zero vector insertion brings suppression of current

spikes of taken trolley current. Experimental verification of this control is done under steady

states in rectifier and inverter mode. Using zero vectors significantly reduce current spikes of

taken trolley current which will appear also in low stress of switching devices, input filter.

Control method of traction drive using hysteresis control of secondary active rectifier and

rectangular control of matrix converter has been proposed. These control methods ensure

sinusoidal waveform and zero phase shift of taken current from trolley line. A big advantage

is an ability of keeping accurate value of demanded phase shift even under the distorted grid

conditions and independently on all others quantities. [1]

2.2 DSP-Based Controller for Direct Torque Control of Induction Motor

Drives in Railway Traction System

A highly flexible DSP-controller has been set up and has been utilized for the implementation

of control technique for induction motor drives in railway traction systems application. The

DSP controller has been integrated in a complete system that includes a scale-model of ETR500

train. By mean of this simulator the dynamic behaviour of the railway traction system may be

simulated and all the significant quantities may be detected. The classic DTC algorithm can be

applied to a Simulator of traction bogie. This control technique is particularly appropriate for

solving the problems of dynamic performance of a three-phase AC drive in railway traction

systems because of their short response times.

The development that DSP have had in the last years makes it feasible the design and realisation

of particular flexible electric drives. Thus, an accurate control of the drives is possible, in order



to make the output quantities of electric motors always fully satisfying the load operating

conditions and the required performance values.[2]

2.3 Matrix Converters: A Technology Review

The Matrix Converter is a forced commutated converter which uses an array of controlled

bidirectional switches as the main power elements to create a variable output voltage system

with unrestricted frequency. It does not have any dc-link circuit and does not need any large

energy storage elements. The key element in a Matrix Converter is the fully controlled four

quadrant bidirectional switch, which allows high frequency operation. The Matrix Converter

requires a bi-directional switch capable of blocking voltage and conducting current in both

directions.

The most important practical implementation problem in the Matrix Converter circuit, the

commutation problem between two controlled bi-directional switches, has been solved with

the development of highly intelligent multistep commutation strategies. Another important

drawback that has been present in all evaluations of Matrix Converters was the lack of a

suitably packaged bi-directional switch and the large number of power semiconductors. This

limitation has recently been overcome with the introduction of power modules which include

the complete power circuit of the Matrix Converter. [3]



CHAPTER 3: BASICS OF THE CONVERTER COMPONENTS

3.1. Traction transformer and converter

The three phase voltage required for operating the traction motors is generated on the vehicle

by means of two traction converters connected between the vehicles’ main Transformer (single

phase) and the traction motors. To control the tractive or braking effort, and hence the speed

of the vehicle, both the frequency and the amplitude of the three-phase converter output voltage

are continuously changed according to the demands from the drive’s cab. This allows

continuous adjustment of the driving or braking torque of the traction motors, which means

that the driving speed changes smoothly.[3] When braking electrically the traction motors act

as generators. In the converter the resulting three-phase electrical energy is converted into

single-phase energy, which is fed back into the line (regenerative brake).

Figure 1 Block Diagram of conventional traction converter

3.2. Control of motor or traction control

The torque and the speed of the motors are controlled by continuously changing both the

frequency and the amplitude of the fundamentals of the pulse-shaped motor-phase voltages.

In motoring mode (driving mode) the fundamental frequency of the motor terminal voltage is

higher than the frequency corresponding to the motor speed (positive slip), resulting in a

positive motor torque. [3]

During braking, the fundamental frequency of the motor terminal voltage will be lowered

below the frequency corresponding to the motor speed, resulting in a negative slip and therefore

producing a braking torque. [2] The whole control range of the motor voltage is subdivided

into three smaller ranges i.e., (indirect self-control), TB_DSR (direct self-control), or



“tolerance band control” as well as square-wave operation or “field-weakening”-DSR,

characterized as the follows

3.3. ISR (indirect self-control)

The ISR-range covers the range from standstill (motor voltage = 0) up to approx. 30% of the

nominal voltage and type frequency (resp. type speed) of the motors. In this range, the

relationship between the motor voltage amplitude and frequency remains roughly constant.

This is achieved using the pulse-width-modulation method. The motors are constantly

magnetized at nominal induction and can be loaded with the nominal torque over the whole

ISR-range. [4]

3.4. TB-DSR (tolerance band control)

The TB-DSR covers the range from approx. 30% up to 98% of the nominal voltage and nominal

frequency (resp. nominal speed) of the motors. Over this range the motors are fully magnetized

and therefore they can be loaded with the full torque. The torque, however, is no longer

controlled to an average value but to a set value.

3.5. “Field weakening”-DSR (square-wave operation)

The “field weakening” range is the region between the nominal speed (nominal voltage) and

the maximum speed of the traction motors. Over the whole field weakening range the amplitude

of the motor voltage is kept at its maximum value. Only the frequency is changed. This has the

effect that the motor flux and pull out torque are inversely proportional to the frequency.

3.6. Traction control monitoring system helps in protection of the traction converter

Protection stage 1: Monitoring of the minimum switching times of the GTO thyristors and

mutual interlocking of the gate units of a GTO-thyristor pair of arms. To ensure the safe

function of the converter, when it is alternately switching

Protection stage 2: Power and current set value limitation

Protection stage 4: Full reduction of load

Protection stage 5: Immediate converter shut-down

3.7. Auxiliary converter, battery and energy storage

Auxiliary converters are designed for connection to the auxiliary services winding of the main

transformer. Each converter is rated for 100 KVA output and has short circuit proof three-phase

output at 415 V. The output frequency of converter BUR1 & BUR2 is variable from 0 to 50

Hz while BUR3 gives fixed frequency output at 50 Hz. [1]

The battery charger is supplied from the converter BUR3. In case of the fault in the converter,

BUR2 will feed the battery charger. The battery charger with a rated output of approx. 111 V

charges the locomotive batteries and supplies the low voltage loads. The low voltage output is

electrically insulated from the input and from the three-phase output. The converters are

provided with external forced convection cooling (5 m/sec approximately).



Thermostats are provided inside the box. When the temperature goes above 50ͦ C, fans run until

temperature decrease below 35ͦ C. Both boxes and three phase output chokes are mounted in

the machine room of the locomotive. The intermediate chokes are incorporated in the main

transformer tank for convenience and in order to save weight.



CHAPTER 4: NEW CONFIGURATION

4.1. Medium Frequency Transformer

The auxiliary power supply of a railway vehicle needs galvanic insulation in a power range of

20kW up to several 100kW in order to feed compressors and fans, the air conditioning, heating,

lighting, the control for doors and breaking systems and to charge the battery of the vehicle.

Classically heavy and bulky low frequency transformers are used for galvanic insulation of the

auxiliary output power.

Realizing galvanic insulation by using power converters and transformers with an operation

frequency of several kHz is so called Medium Frequency Technology (MFT). The introduction

of MFT in combination with resonant switching converters leads to smaller auxiliary power

supplies with reduced weight and increased efficiency. Additionally the current and voltage

quality can be improved. The diversity of input voltages in different rail applications asks for

a modular and flexible design of the product family of auxiliary power converter. [3]

4.2. Single phase matrix converter

The Matrix Converter is a forced commutated converter which uses an array of controlled bi-

directional switches as the main power elements to create a variable output voltage system with

unrestricted frequency. It does not have any dc-link circuit and does not need any large energy

storage elements. The key element in a Matrix Converter is the fully controlled four-quadrant

bidirectional switch, which allows high frequency operation. The Matrix Converter requires a

bi-directional switch capable of blocking voltage and conducting current in both directions.

Unfortunately there are no such devices currently available, so discrete devices need to be used

to construct suitable switch cells. The common emitter bi-directional switch cell arrangement

consists of two diodes and two IGBTs connected in anti-parallel.

4.2.1. Current commutation

Reliable current commutation between switches in Matrix Converters is more difficult to

achieve than in conventional voltage source inverters since there are no natural freewheeling

paths. The commutation has to be actively controlled at all times with respect to two basic

rules. These rules can be visualized by considering just two switch cells on one output phase

of a Matrix Converter. It is important that no two bi-directional switches are switched on at any

instant. This would result in line-to-line short circuits and the destruction of the converter due

to over currents. Also, the bi-directional switches for each output phase should not all be turned

off at any instant. This would result in the absence of a path for the inductive load current,

causing large over-voltages. These two considerations cause a conflict since semiconductor

devices cannot be switched instantaneously due to propagation delays and finite switching

times.



Figure 2Four step commutation

The single phase matrix converter is the one possibility of input high voltage converter. Matrix

converter can be understood as an alternative today standard indirect frequency converter. The

circuit consists of input high voltage filter (reactor LF and capacitor CF) connected to the input

of the matrix converter which supplies middle frequency transformer. The outputs of the

transformer fed single phase active voltage rectifier and three phase voltage source inverter

supplying AC motor. Filter with capacitor is situated in the input of the matrix converter and

the inductive load (winding of the transformer) is connected at its output. These facts have to

be taken into account to control the matrix converter –cannot short circuit input terminals and

disconnect output terminals at the same time. Input 50 Hz(16,7 Hz) sine-wave trolley voltage

is cut to the middle frequency voltage waveform (up to kHz) which is set on the input of the

MFT. The secondary single phase voltage-source active rectifier processes the middle -

frequency voltage waveform and control the DC bus line voltage as the source for the three

phase voltage-source inverter.

Figure 3 New configuration

The basic idea of this control algorithm is following: secondary voltage-source active rectifier

takes sine-wave phase current from middle-frequency transformer (two-value control) and

square-wave control of the matrix converter which put together the phase current of output



active rectifier to the 50 Hz sine wave taken from the trolley line. Amplitude and phase shift

of the trolley line current is controlled only by output active rectifier.

Block scheme of the proposed master control of the whole converter set by the secondary

voltage-source active rectifier: Description of the basic control concept.

The input sine-wave trolley line voltage of either 50 or 16.7 Hz is chopped to the medium

frequency voltage waveform in our case with a frequency of 400 Hz (in general, it can have a

frequency of up to a few kilohertz). This medium-frequency voltage is fed into the primary

winding of the MFT. [6] The secondary single-phase voltage source active rectifier connected

at the secondary winding of the MFT processes this medium-frequency voltage waveform and

secures a constant dc-link voltage at its output which is used by the TDC’s voltage source

inverter supplying the traction motors. The developed configuration of the traction converter

requires specific control of both the primary and secondary converters. Various control

strategies for this new traction converter are proposed:

1) Control of the matrix converter by means of inserting zero-voltage vectors

2) Master control of the whole converter set by the secondary voltage-source active rectifier

3) Matrix converter control based on the hysteresis control of the trolley line current.

4.2.2. Advantage of using IGBT in matrix converter:

Power electronic devices such as IGBTs require no external circuitry for turn on or turn off

purposes The IGBT is currently the most popular switching device in low voltage 480V and

575V VFD technology and for rating up to 1000 HP. This is because of its higher gain, lower

losses and fast switching up to 20 kHz. In the last two years, IGBT devices have been employed

in 2300V and 4160V medium voltage VFDs up to 5000 HP ratings and their acceptance is

growing. IGBT produces very steep voltage waveform due to its fast switching and extra

precaution should be taken for motor insulation protection particularly in retrofit applications

and long run cables.

4.3. Concept of new traction converter with MFT

In the conventional topology, the traction converter uses a heavy low-frequency transformer at

its input. There is a huge problem with the input transformer, particularly with the ac 15-

kV/16.7-Hz electrification system, where ultralow catenary frequency results in a bulky

transformer. As described in the previous section, one of the prospective methods of

overcoming the aforementioned constraints is a new traction converter using an MFT.

Configuration of a conventional traction vehicle fed from the ac electrification system with a

heavy low-frequency transformer at input. The traction converter consists of an input filter

[medium-voltage input filter (MVF)], a primary medium voltage converter (MVC) supplying

an MFT, and the main traction drive converter (TDC). The input converter regulates the input

line voltage to the appropriate waveform for the MFT (in general, it increases the frequency at

the MFT). The MFT secures galvanic insulation between the input and output and adjusts the

output voltage level. The output traction converter (TDC) connected at the secondary side of

the MFT supplies the traction motors.



First, the proposed configuration of the new traction converter with the MFT employing matrix

converters at the primary (medium-voltage) side of the transformer is presented. Second, the

developed control of both primary and secondary converters is described. Third, the behaviour

of the designed small-scale prototype of the traction converter is analyzed by simulations and

experiments under steady-state and selected transient conditions.

Figure 4 Block diagram of new configuration



CHAPTER 5: DIGITAL SIGNAL PROCESSING

A highly flexible DSP-controller has been set up and has been utilized for the implementation

of control technique for induction motor drives in railway traction systems application.

Figure 5 Block of DSP

5.1. DSP Interfacing

The DSP controller has been integrated in a complete system that includes a scale-model of

ETR500 train. By mean of this simulator the dynamic behaviour of the railway traction system

may be simulated and all the significant quantities may be detected. [2]

Figure 6 Overview of a flexible DSP controller



The classic DTC algorithm can be applied to a Simulator of traction bogie. This control

technique is particularly appropriate for solving the problems of dynamic performance of a

three-phase AC drive in railway traction systems because of their short response times.

The system presented in this paper is controlled by a Digital Signal Processor (DSP)

TMS320C30.

The flexibility of the DSP-controller can be found in the modular structure of the total system

that is constituted mainly by the DSP-based motherboard, directly connected to a data-bus

accessible by a back plane.

This back plane is the flexible connection to the different modules, in particular, there are:

Two analogue to digital converters cards

Two digital to analogue converters card

A digital interface card, an encoder card

A PWM card and an hysteresis current regulated PWM

For the conversion of the analogue quantities, 12 bit A/D converters are used, they are

characterised by a fast and parallel conversion (every analogue input signal is converted in 2.7

μS).

The PWM/Digital (on/off) card contains 6 independent PWM outputs, or 6 independent digital

outputs. This card is especially designed for power electronics applications. During the

initialisation section of the DSP, into a special register is written which output is desired: Pulse

Width Modulation, Digital (on/off), or hysteresis current regulated PWM. [2]

Figure 7 Interfacing DSP for the control operation



CHAPTER 6: CONTROL SYSTEM OF THE TRACTION

CONVERTER

Controlling is the most important part to be taken care in the traction converter. The micro

grids or the traction electrifying system connected to the grid are expected to be operated at the

grid synchronism and the power quality should be taken care.

Control Operation can be classified into three major steps:

Control on the rectifier part

Control at the trolley line

Control at the traction drive system

Further the rectifier control can be classified as Hysteresis control and Rectangular control. For

this application only hysteresis control is taken.

6.1. PI Controller

PI controller will eliminate forced oscillations and steady state error resulting in operation of

on off controller and P controller respectively. However, introducing integral mode has a

negative effect on speed of the response and overall stability of the system. Thus, PI controller

will not increase the speed of response. It can be expected since PI controller does not have

means to predict what will happen with the error in near future. This problem can be solved by

introducing derivative mode which has ability to predict what will happen with the error in near

future and thus to decrease a reaction time of the controller. [3]

PI controllers are very often used in industry, especially when speed of the response is not an

issue. A control without D mode is used when:

a) fast response of the system is not required

b) large disturbances and noise are present during operation of the process

c) there is only one energy storage in process (capacitive or inductive)

d) there are large transport delays in the system



Figure 8 PI Controller

6.2. Proposed traction converter control

The proposed control of the investigated traction converter is described .The secondary

voltage-source active rectifier takes the pulse-amplitude modulated current (i.e., the bipolar

square wave current of 400 Hz modulated by a sinusoidal waveform of the fundamental trolley

line frequency) from the MFT. The current control at the secondary active rectifier is secured

by the fast and robust hysteresis control of the current on the rectifier ac side. The primary

matrix converters are operated in a synchronized square-wave control which assembles the

phase current of the secondary active rectifier to the sine-wave of the frequency which is equal

to the fundamental frequency of the trolley line (either 50 or 16.7 Hz). This “reconstructed”

current is taken from the input medium-voltage filter. The amplitude and phase shift of the

trolley line current is controlled only by the secondary active rectifier. [3]

The measured ϕ and required ϕw phase shift between the trolley line voltage and current are

led to the phase shift controller proportional-integral controller (PI). The controller output ϕsc

is used for shifting the phase current for the secondary active rectifier. The amplitude of the

phase current of the secondary active rectifier is commanded by the PI controller of the dc-link

voltage (Udsc) of the secondary active rectifier. The matrix converters on the primary side of

the MFT are simply controlled by a synchronized square-wave control. The regular switching

sequence synchronized with the trolley line voltage assembles the phase current of the

secondary active rectifier, which is composed of 400-Hz square waves modulated by a sine

wave with a frequency equal to the fundamental frequency of the trolley line, to the sine wave

having a frequency equal to that of the fundamental trolley line.

6.2.1. Hysteretic Control

If there seems to be something missing, there’s not. Hysteretic control, in its pure and

fundamental form, is extremely simple—the simplest of all control topologies. A comparator

with some small hysteresis between its terminals compares the output voltage, via its FB input,

directly to the high-accuracy, internal reference voltage, VREF.



Figure 9 Basic hysteresis control

The advantage of such a direct control over the output voltage is the speed of the control loop.

When the output voltage changes due to a transient, the time it takes for the control loop to

begin to react is limited only by the propagation delays in the comparator and gate driver. There

is no low-bandwidth error amplifier for an error signal to travel through. Thus, the hysteretic

topology is the fastest control topology. [3]

Additionally, its simplicity of operation makes it inherently stable without any loop

compensation. This simplicity makes it a low-cost topology as well. There is no oscillator or

error amplifier in the power supply to design, build, and test. Just a basic comparator is needed

to control the switching action.

The main practical disadvantage of the hysteretic topology is its switching frequency variation.

There is no clock or synchronization signal setting the switching frequency. Instead, the

switching frequency is set by the hysteresis amount, as well as the external components and

operating conditions. [5]

Large frequency variations over input voltage and load are expected with a pure hysteretic

converter. Moreover, without a high-gain error amplifier, the dc setpoint of the output voltage

may not be as precise as achieved with voltage-mode control. Lastly, hysteretic control requires

equivalent series resistance (ESR) in the output capacitor. Therefore, ceramic output capacitors

with little ESR generally cannot be used with the pure hysteretic topology. [2]

But in some low-power, very low-cost applications such as toys, a hysteretic converter might

be acceptable due to the very low price point of such end equipment, as well as their low power,

which produces low levels of electromagnetic interference (EMI) across the hysteretic power

supply’s wide switching frequency range. Also, systems with very harsh transients require a

hysteretic or hysteretic-based topology to keep acceptable regulation of the output voltage. If

the input voltage, output voltage, and other operating conditions of these systems remain well

controlled, the switching frequency is kept within an acceptable range. This makes hysteretic



control a valid choice for applications operating from a fixed input voltage bus and creating a

fixed output voltage. [6]

This sawtooth is not created from a clock. Instead, it is created through a special circuit

connected directly to the output voltage at the VOS input pin. Essentially, the hysteretic

comparator still has a direct connection to the output voltage through this VOS pin while a

high-gain error amplifier is present to provide very good output voltage setpoint accuracy.

Besides combining the hysteretic comparator and error amplifier from the hysteretic and

voltage-mode topologies, DCS-Control incorporates an on-time circuit to control the switching

frequency. Finally, loop compensation is required and included for stability. [6]

The main advantage of DCS-Control is keeping the very fast transient response of the hysteretic

converter and the output voltage accuracy of the voltage-mode converter, while addressing

other key disadvantages of both topologies: slow response time, limited control loop

bandwidth, and frequency variation. [3]

6.2.2. Types of Hysteresis Control

The benefits of hysteresis controllers are primarily the linear modulation caused by the

sawtooth-shaped carrier with ideally straight slopes, and by the infinite power supply rejection

ratio, PSRR, if the supply variation can be considered very slow compared to the switching

frequency. Power supply variations at higher frequencies are not suppressed totally, and will

result in sum and difference products of the reference signal and the power supply variation,

but these still meets high suppression.

Figure 10 Hysteresis control

For use in audio amplifier applications, the hysteresis controller is very desirable due to the

high linearity and simple design. However, hysteresis controllers suffers from a switching



frequency dependent on the modulation index, M, of the amplifier. All other basic types of

self-oscillating modulators suffer from this phenomena too.

The basic operation of the current mode hysteresis controller is: The output inductor integrates

the differential voltage between the output voltage of the power stage and the output voltage

of the amplifier. [5] If the output voltage of the amplifier can be considered constant within

one switching period, the integration results in a sawtooth shaped inductor current, which is

subtracted from the reference current programming voltage, and fed into a hysteresis window

to control the switching frequency by controlling the time-delay trough the controller loop. The

voltage mode hysteresis controller differs from the current mode controller by integrating the

difference between the output voltage of the power stage and the input reference voltage with

an active integrator, which again results in a sawtooth shaped carrier which is fed to a hysteresis

window. [3]

The major functional difference between the two is that the current mode controller is a voltage

controlled current source with an integrated output filter. The voltage mode controller is a

voltage controlled voltage source without output filter. Both controllers have a first order

closed loop function. [2]

In switch mode audio amplifier applications some additional control feedback loops are often

desired to reduce distortion caused by non-linearities in the circuit. Furthermore if a current

mode controller is used, a voltage feedback control loop is required since an audio amplifier as

often is a voltage amplifier.

To reduce distortion as much as possible, it is desired to apply the voltage feedback after the

output filter, so errors from both power stage and output filter will be reduced. The biggest

drawback with basic hysteresis controllers used in audio and a lot of other applications is the

non-constant switching frequency.

Figure 11 Current mode hysteresis controller



Figure 12 Voltage mode hysteresis control



6.3. Block Diagrams of Control Operation

Figure 13 Control of the traction converter as in the paper

Figure 14 Block diagram of control operation



CHAPTER 7: IMPROVEMENTS PROPESED

The seminar paper was studied and the followed improvements were noted with have their own

benefits:

HV IGBT transistor modules with the latest generation of excellent thermal cycling

capability which simplify the high power circuitry and converter control systems.

Amorphous and nano-crystalline magnetic cores which allow a significant reduction in

the size and weight of inductive components as well as a reduction of dissipated power.

Composite technologies which provide increased protection against shocks and

vibrations acceptable of coaches’ converters. [5]

Fibre-optical technology, which provides high insulation level and eliminates the

impact of electromagnetic interferences

Dynamic multi-level short-circuit protection which increases the reliability of

performance of converters, battery chargers and inverters Pulse Width Modulation

technology with built-in LC filters, special control algorithms to keep THD low (below

5%) in the voltage supplying asynchronous motors and other AC devices. This

increases the reliability of devices and limits over-voltage spikes in the coils. It also

increases durability of the wire insulation and decreases the level of noise emission.

Multilevel systems for over voltages occurring in the traction network, RLC passive

filters, non-linear voltage limiters (varistors) and active protection systems (crowbar

circuits). In case the input voltage exceeds a safety level, a relevant electronic system

shorts the input circuits causing fuse blowing in the converter power supply circuit.

Automatic thermal compensation of the battery charging voltage (implemented in

accordance with the requirements of battery manufacturers) which allows the batteries

to be charged optimally depending on ambient temperature thus prolonging their

lifetime with minimum maintenance. [6]

DSP (Digital Signal Processor) microprocessor control systems, which provide

automatic diagnostics of the performance of converters, battery chargers and inverters

The converters, battery chargers and inverters fitted with RS232/485 and CANbus or

MVB interface, which provides the possibility of continuous monitoring of the

operation of devices. [1]

Aluminium housings with heat sinks of significantly lowered thermal resistance,

welded and powder painted. The elements of housings are cut out with a water stream

(Water Jet), owing to which, tensions and defects of material structure do not occur (as

is in case of laser or plasma cutting). Further processing of elements are carried out on

the automatically controlled milling machining centre (CNC) produced by an US

company - Hass.

The welding is performed by means of the newest computer controlled. Semi-automatic

Kemppi welders in any technology (TIG, MIG, MAG). Special heat sinks of low

thermal resistance or water cooling services serve the purpose of good heat transfer

from electronic power modules.

PR controllers can replace the PI controllers and these track AC references in the

stationary reference frame with zero steady-state error. However, they suffer from



several drawbacks, e.g., sensitivity to grid frequency variations, exponentially decaying

transients during step changes, and being pushed toward instability margins even by a

small phase shift introduced by the adopted current sensors Moreover since PR-

controllers have more poles than Proportional Integral (PI)-controllers, they introduce

a greater phase lag in the Nyquist plot of the open loop transfer function, which makes

their tuning more complicated compared to PI-controllers. Furthermore, the choice of

the damping ratio of non-ideal PR-controllers, which are used in practice instead of

ideal PR-controllers, is itself another design issue. Generally speaking, PI control is

unable to remove the low current harmonics due to the bandwidth limitation. In order

to increase the bandwidth it would be required such a high proportional gain that the

stability of the system becomes a critical issue. Especially when LCL filters are used to

attenuate the switching ripple the stability problem is difficult to solve as higher gains

could excite resonance and lead to instability. [2]

For three-phase systems proportional integral (PI)- dq control with voltage feed-

forward is commonly used and it requires a lot of transformations to change the

reference frame increasing so the complexity of the implementation, especially when

low-cost fixed point DSP technology is used. Thus the need for a different control

structure exhibiting better performances and lower implementation burden has become

very actual. [3]

− Haulage increased by 9% to 5’500 tons

− Better energy-efficiency and less heat generation

− Higher redundancy in case of motor failure

− Easy maintenance and reduction of operating cost

− Reliable long-term supply for traction transformers

Figure 15 Comparison of the old configuration with the new configuration



CHAPTER 8: CONCLUSION

Through this seminar we have introduced the new configuration of a main traction converter

with an MFT using matrix converters intended for locomotives and particularly for suburban

units supplied by a 25-kV/50-Hz and/or 15-kV/16.7-Hz ac electrification system. Single-phase

matrix converters are employed in the primary medium-voltage converter which is directly

connected to the ac trolley line. The described converter configuration with cascaded matrix

converters presents a new research direction in the field of traction converters with an MFT.

A major advantage of the proposed control is the simple and powerful square-wave control of

the input matrix converters. [5] The amplitude and phase shift of the input trolley line current

are controlled by the secondary voltage-source active rectifier. The proposed control of the

secondary active rectifier is based on the hysteresis control of the current in the secondary

winding of the MFT which is easy for implementation, is robust, and provides the converter

with excellent dynamic properties.

The introduced traction converter topology has a good performance within the entire operating

range. Considering the converter performance, reliability, and size of the matrix converters, it

can be concluded that the cascaded matrix converters are an interesting alternative to more

conventional cascaded indirect frequency converters in the primary medium-voltage converter.

However, eligible power electronics devices with appropriate parameters making it possible to

develop a full power traction prototype (e.g., 2–6 MW) are the biggest drawback of the

presented matrix converter-based technology. At the present time, there are only a few

semiconductor devices specifically designed for matrix converters (i.e., two IGBTs as a

bidirectional switch) with minimal parasitic inductance. Moreover, the existing devices usually

have smaller voltage and power ratings—e.g., SK60GM123 by SEMIKRON with 1200 V/50

A. Perspective switching components for matrix converters should be medium-voltage high-

power press-pack IGBTs that can easily eliminate parasitic inductance in the power circuit in

comparison with IGBT modules common in traction. [6]



CHAPTER 9: WORK PLAN

The seminar paper inclines our interest towards the recent innovations in power electronics.

We are looking forward for major projects in this area. Having a thorough study on the matrix

converter applications, we plan to implement the matrix converter with the improvements

proposed for a preferably three phase application. There is a vast ocean of opportunities in the

field of renewable converter application. We are looking forward to implement the converter

on a renewable application.



CHAPTER 10: REFERENCES

[1] P. Correa, J. Rodriguez, M. Rivera, J. R. Espinoza, and J. W. Kolar,“ Traction Drive with

MFT - Novel Control Strategy Based on Zero Vectors Insertion” IEEE Trans. Ind.Electron.,

vol. 56, no. 6, pp. 1847–1853, Jun. 2009.

[2] F. Iturriz and P. Ladoux, “DSP-Based Controller for Direct Torque Control of Induction

Motor Drives in Railway Traction System” IEEE Trans. Power Electron., vol. 15, no. 1,pp. 92–

102, Jan. 2000.

[3] P. W. Wheeler, J. Rodriguez, J. C. Clare, L. Empringham, and A.Weinstein, “Matrix

converters: A technology review,” IEEE Trans. Ind.Electron., vol. 49, no. 2, pp. 276–288, Apr.

2002.

[4] G. Abad,M. A. Rodriguez, and J. Poza, “Three-level NPC converter based predictive direct

power control of the doubly fed induction machine at low constant switching frequency,” IEEE

Trans. Ind. Electron., vol. 55, no. 12, pp. 4417–4429, Dec. 2008.

[5] S. Kouro, M. Malinowski, K. Gopakumar, J. Pou, L. G. Franquelo, B.Wu, J. Rodriguez, M.

A. Perez, and J. I. Leon, “Recent advances and industrial applications of multilevel converters,”

IEEE Trans. Ind. Electron., vol. 57, no. 8, pp. 2553–2580, Aug. 2010.

[6] Pavel Drábek, Zdenˇek Peroutka, Martin Pittermann, an “New Configuration of Traction

Converter WithMedium-Frequency Transformer Using Matrix Converter” IEEE Trans Ind.

Electron, VOL. 58, NO. 11, Nov 2011 5041

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