Advantages of Matrix Over Cycloconverters

8/4/2019 Advantages of Matrix Over Cycloconverters

1/46

Topic 10: Cycloconverters

Spring 2004

ECE 8830 - Electric Drives


2/46

Introduction

Cycloconverters directly convert ac signals of one

frequency (usually line frequency) to ac signals of

variable frequency. These variable frequency ac signals

can then be used to directly control the speed of ac

motors.

Thyristor-based cycloconverters are typically used in

low speed, high power (multi-MW) applications for

driving induction and wound field synchronous motors.


3/46

Phase-Controlled Cycloconverters

The basic principle of cycloconversion isillustrated by the single phase-to-singlephase converter shown below.


4/46

Phase-Controlled Cycloconverters(contd)

A positive center-tap thyristor converteris connected in anti-parallel with anegative converter of the same type. This

allows current/voltage of either polarity tobe controlled in the load.

The waveforms are shown on the nextslide.


5/46



6/46


An integral half-cycle output wave is created which has afundamental frequency f0=(1/n) fi where n is the

number of input half-cycles per half-cycle of the output.The thyristor firing angle can be set to control the

fundamental component of the output signal. Step-upfrequency conversion can be achieved by alternately

switching high frequency switching devices (e.g. IGBTs,

instead of thyristors) between positive and negative limitsat high frequency to generate carrier-frequency modulated

output.


7/46


3 to single phase conversion can beachieved using either of the dual convertercircuit topologies shown below:


8/46


A Thevenin equivalent circuit for the dualconverter is shown below:


9/46


The input and output voltages are adjusted to be equaland the load current can flow in either direction. Thus,

where Vd0 is the dc output voltage of each converter atzero firing angle and p and N are the input and outputfiring angles. For a 3 half-wave converter Vd0 =0.675VLand Vd0 = 1.35VL for the bridge converter (VL is the rmsline voltage).

0 0 0cos cosd d p d nV V V V = = =


10/46


Voltage-tracking between the input andoutput voltages is achieved by setting thesum of the firing angles to . Positive or

negative voltage polarity can be achieved asshown below:


11/46


A 3 to 3 cycloconverter can be implemented using 18 thyristors as shownbelow:


12/46


Each phase group functions as a dualconverter but the firing angle of each group

is modulated sinusoidally with 2 /3 phase

angle shift -> 3 balanced voltage at themotor terminal. An inter-group reactor(IGR) is connected to each phase to restrict

circulating current.


13/46


An output phase wave is achieved bysinusoidal modulation of the thyristorfiring angles.


14/46


A variable voltage, variable frequencymotor drive signal can be achieved byadjusting the modulation depth and output

frequency of the converter.

The synthesized output voltage wave

contains complex harmonics which can beadequately filtered out by the machinesleakage inductance.


15/46


A 3 to 3 bridge cycloconverter (widely used in multi-MW applications)can be implemented using 36 thyristors

as shown below:


16/46


The output phase voltage v0 can be written as:

where V0 is the rms output voltage and 0 isthe output angular frequency. We can also write:

where the modulation factor, mf is given by:

0 0 02 sinv V t=

0 0 0 0 0cos cos sind p d n f d v V V m V t = = =

0 02 /f dm V V=


17/46


From these equations, we can write:

and

Thus for zero output voltage, mf=0 and

P= N= /2. For max. phase voltage,mf=1 => P=0, N= . See below figure

for P and N values for mf=0.5 and 1.

1

0cos [ sin ]p fm t =

N P =


18/46


The phase group of a cycloconverter can be operated in two

modes:

1) Circulating current mode

2) Non-circulating current (blocking) mode

In the circulating current mode, the current continuously

circulates between the +ve and -ve converters. Although the

fundamental output voltage waves of the individual converters

are equal, the harmonics will cause potential difference whichwill result in short-circuits without an IGR.


19/46


The equivalent circuit of a phase group with an IGR is shown

below.

The inclusion of an IGR leads to self-induced circulating

current as illustrated in the next slide.


20/46



21/46


At t=0, +ve load current is taken by the +ve converteronly (iP=i0).

From 0-> /2, rising +ve load current will create a +ve

voltage drop (vL=Ldi0/dt) in the primary winding of the

IGR. This creates a -ve voltage drop in the secondarywinding of the IGR -> DN reverse biased. no

current flow in the -ve converter.

At /2, i0 peaks at Im-> vL=0. After this vL tends to

reverse polarity inducing current in the -ve converter.Voltage across IGR becomes clamped to 0 -> self-induced

circulation current between +ve and -ve converters.


22/46


The +ve and -ve converter currents can be expressed

as:

The self-induced circulating current is simply iP-iN.

In practice, the waves will not be pure sine waves butinclude a ripple current. Practical waveforms are shown on

the next slide.

00.5 0.5 sin P m mi I I t = +

00.5 0.5 sin N m mi I I t =


23/46



24/46


Advantages of circulating current mode of operation,

over blocking mode include:

Output phase voltage wave has lower harmonic contentthan in blocking mode.

Output frequency range is higher.

Control is simple.

Disadvantages

Bulky IGR increases cost and losses.

Circulating current increases losses in thyristors.

Over-design increases cost.


25/46


In the blocking mode of operation, no IGR is usedand only one converter is conducting at any time.

Zero current crossing detection can be used to select

+ve or -ve converter conduction as shown below:


26/46


Since the cycloconverter is usually connecteddirectly to a motor, the harmonics from theconverter will induce torque pulsations andmachine heating resulting in increased machine

losses. Also, since the cycloconverter is essentiallya matrix of switches without energy storage(neglecting IGR) Pin=Pout . Thus distortions in theoutput voltage waveform reflect back into the line

input. See text for a discussion of the load voltageand line harmonics.


27/46


A major disadvantage of cycloconvertersis poor DPF (displacement power factor). Tocalculate DPF, consider a phase group of an

18-thyristor cycloconverter shown below.


28/46


Assume the +ve converter is operating incontinuous conduction and is connected to a highinductance load and assume that thecycloconverter is operating at low frequency.Segments of the output current and voltage wavesare as shown below:


29/46


The Fourier series of the line current isgiven by:

where i0 is the load current and is thesupply frequency. The current wave has a dc

component and a fundamental componentwith a lagging phase angle, P.

0

0

3 1 1 1[sin( ) cos 2( ) cos 4( ) sin 5( ) ...]

3 2 4 5 P P P P

ii i t t t t

= + +


30/46


Since the supplys active and reactive power components are

contributed only by the fundamental current, the instantaneous

active Pi and reactive power Qi for the positive converter is given

by:

where Vs =rms line voltage.

' 033 cos2

i s P

iP V

=

' 0

33 sin

2i s P

iQ V

=


31/46


These equations can be rewritten as:

If the firing angle is constant, the converter acts as arectifier and Vd=Vd0cos P and i0=Id.

In a cycloconverter P and i0 vary sinusoidally and so Piand Qi are also modulated. We need to average these

parameters to determine loading on the source.

'

0 0 0 0 0(1.17 cos ) cosi s P d P P V i V i v i = = =

'0 0 0(1.17 sin ) sini s P d P Q V i V i = =


32/46



33/46


The expression for the average reactive power contributed by the line, Qi, isgiven by:

where = load power factor angle.

Performing the integration above yields:

where P0, Q0 are the real and reactive output power per phase, respectively.

1

2

0 0

0

1(1.17 sin )

cycle

i s PQ V i d t

=

/ 20 0 0 0 0

/ 2

1(1.17 cos )( sin( )) ( 1.17 cos )( sin( ))

s m s mV t I t d t V t I t d

+= + +

20 02 2 1cos sin 22

i

P QQ

= + +


34/46


P0 and Q0 are given by:

and

Since the real output power = real input power, we can write:

The input DPF can be expressed as:

DPF = cos i =

0 0 0 cos P V I = 0 0 0 sinQ V I =

2

0

2 1cos sin 2

2i i i i P jQ P j P Q

+ = + + +

i

i i

P

P jQ+


35/46


DPF =

=

=

where tan = Q0/Pi (=Q0/P0)

2 0

1

2 11 cos sin 2

2i

Qj

P

+ + +

2

1

2 11 cos tan sin 2

2j

+ + +

12

1 (1 tan )j

+ +


36/46


This equation for DPF applies when additionalphase groups are added or if a 36-thyristorimplementation is considered.

mf=1 was assumed in this derivation. For mf1:

The maximum value of line DPF is 0.843.

21 (1 tan )

fmDPF

j

=+ +


37/46


See Bose text pp. 180-184 for methodsto improve DPF.


38/46


The control of a cycloconverter is verycomplex. A typical variable speed constantfrequency (VCSF) system is shown below:


39/46


Generator bus with regulated voltage but variable

frequency (1333-2666 Hz) is fed to the cycloconverter

phase groups. (A generator speed variation of 2:1 is

assumed). The dual converter in each phase group uses a

low-pass filter to generate a sinusoidal signal.

modulator receives biased cosine waves from

generator bus voltage and sinusoidal control signal

voltages to generate thyristor firing angles.


40/46


3 sinusoidal control signals are generatedthrough the vector rotator. The feedbackvoltage Vs is generated from the output

phase voltages.


41/46

M t i C t


42/46

Matrix Converters

These types of cycloconverters use high-frequency, self-controlled ac switches (e.g.IGBTs). A 3 to 3 converter is shown below:


43/46

Matrix Converters (contd)

A matrix of nine switches where any inputphase can be connected to any output phase.The switches are controlled by PWM to

fabricate an output fundamental voltagewhose amplitude and frequency can be

varied to control an ac motor.

The output waveform synthesis is shownon the next slide.

C ( )


44/46


M i C ( d)


45/46


Matrix converters offer the advantage over

thyristor cycloconverters of being able to produceunity PF line current.

However, compared to PWM voltage-fed

converters, the parts count is significantly higher.


46/46

High-Frequency Cycloconverters

See Bose text pp. 186-189

Advantages of Matrix Over Cycloconverters

Documents

Transcript of Advantages of Matrix Over Cycloconverters