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Topic 9: Current-Fed Inverters

Spring 2004

ECE 8830 - Electric Dr

ives

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Introduction

Current-fed inverters requires a stiffconstant current source input - thus aresometimes referred to as CSI (current

source inverters or current stiff inverters).

A large inductance can be used to change avariable voltage input to a variable current

input.

VSI-inverters and CSI-inverters are dual toeach other.

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Introduction (contd)

Power semiconductor devices used in CSIinverters must be able to withstand largereverse voltages. Therefore, power

MOSFETs, BJTs, IGBTs, MCTs, IGCTs andGTOs.

Symmetric blocking GTOs and thyristors

can be used in CSI inverters.

Generally CSI inverters are now used invery high power applications.

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General Operation of a 6-Step

Thyristor Inverter

General Schematic of Thyristor Inverter

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Thyristor Inverter (contd)

Initially, ignore commutation considerations.

Induction motor load is modeled by back emfgenerator and leakage inductance in each

phase of the winding.

The constant dc current Id is switched

through the thyristors to create a 3 6-stepsymmetrical line current waves as shown onthe next slide.

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The load or line current may be expressedby a Fourier series as:

where the peak value of the fundamental

component is given . Each thyristorconducts for radians. At any instantone upper thyristor and one lower thyristorconduct.

2 3 1 1cos cos 5 cos 7 ...5 7

a di I t t t

2 3 /dI 2 / 3

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The dc link is considered harmonic-freeand the commutation effect betweenthyristors is ignored.

At steady state the voltage output fromthe rectifier block = input voltage ofinverter.

For a variable speed drive the invertercan be operated at variable frequencyand variable dc current Id.

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If thyristor firing angle > 0, inverterbehavior.

If thyristor firing angle =0, rectifierbehavior.

Max. power transfer occurs when =.

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Inverter Operation Modes

Two inverter operation modes areestablished depending on the thyristorfiring angle:

1) Load-commutated inverter

Applies when /2

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Load-Commutated Inverter Mode

Consider =3/4. In this case vca < 0 =>thyristor Q5 is turned off by the load. Thisrequires load to operate at leading powerfactor => motoring mode of a synchronousmachine operating in over-excitation.

Vd=-Vd0cos

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Force-Commutated Inverter Mode

Consider =5/4. In this case vca> 0 andso thyristor Q5 is not turned off by the load.Thus some type of forced commutation isrequired in this case. Lagging VAR isconsumed by the load => motoring modeof an induction motor. Vd=-Vd0cos

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Force-Commutated Inverters

For driving an induction machine, aforce-commutated inverter is requiredbecause of the phase lag characteristic of

the induction motor.

The topology of a 3 bridge inverter with

an auto-sequential method of forcedcommutation is shown on the next slide.

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Force-Commutated Inverters (contd)

Ref: D.W. Novotny and T.A. Lipo, Vector Control and Dynamics of AC Drives

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The current is switched sequentially intoone of the motor phases by the top half ofthe inverter and returns to the dc link fromanother of the phases via the bottom halfof the inverter. By switching every 2/3radians, a 6-step current waveform can beapplied to the motor.

The series diodes and delta-connectedcapacitors force the commutation of thethyristors. The capacitors store a chargewith the correct polarity for commutation

and the diodes isolate them from the load.

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Since current is constant, voltage dropacross stator windings = 0 and voltage

drop across winding resistances = constant.Thus the motor terminal voltage is set bythe motor not by the inverter.

Since the motor is wound with sinusoidallydistributed windings, the voltages at themotor terminals are nearly sinusoidal.

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The current ideally follows a six-stepwaveform. However, current cannot change

instantaneously through the windinginductances and so the current transitionshave a finite slope.

During these transitions the currenttransfers from one thyristor to the next viaone of the six commutating capacitors.

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Example: Commutation from Q2 to Q4

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When Q4 is fired, Q2 is impressed with areverse voltage across the capacitor bank.=> Q2 turns off almost instantaneously. Id

flows through Q3 and D3, phases b and c,D2, the capacitor bank and Q4. Thecapacitor bank charges linearly with Id.During this time D4 is reverse-biased.

When the capacitor bank voltage equalsthe line voltage, diode D4 turns on and thecurrent Id flows through D4 and terminatesthe commutation process.

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Ref: D.W. Novotny and

T.A. Lipo, VectorControl and Dynamics of

AC Drives

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Note the large voltage spikes (Ldi/dt).

These can be suppressed either bydesigning the motor with small leakageinductance or by using a diode bridge atthe motor terminal with a zener diode load.

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Two positive features of CSI inverterscompared to VSI inverters:

1) CSI inverters are able to ride through acommutation failure and return naturally tonormal operation; costly preventivemeasures used for VSI inverters.

2) CSI inverters can be switched toregenerative mode simply by reversing thepolarity of the dc rectifier output voltage.This is automatically accomplished when aninduction motor operates in a negative slipmode. In the VSI inverter, the current flowmust be reversed - much harder.

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On the other hand, CSI drives cannot beoperated in open loop operation as can VSIdrives. The torque-speed characteristics ofan induction motor driven by a voltagesource and a current source are shown below:

Ref: D.W. Novotny and T.A. Lipo,

Vector Control and Dynamics of

AC Drives

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A distinct peaking occurs in the currentsource case.

Two possible operating points:1) One on the stable, negatively slopedregion, and

2) one above breakdown torque on the

positively sloped region where operationis generally unstable (depending on loadtorque vs. speed characteristics).

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On the stable side, the working flux in themachine is is high => saturated operationand excessive magnetizing current and

iron losses. Thus, continuous operation isnot feasible on this side.

On the unstable side, the flux in the

machine is near its rated value and lossesare reasonable. However, being on theunstable side, feedback control must beused to maintain the operating point.

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One system uses a motor voltage controlloop (see next slide) which regulates themotor voltage by controlling the input

phase controlled rectifier. Also, an internalcurrent control loop is used with thevoltage error serving as a reference signalfor the current regulator. Some IR drop

compensation is often added as areadditional compensating circuits to improvesystem dynamics.

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ASCI inverter-fed induction motor drivesfor medium to high power applicationswere popular.

However, the size and cost of thecommutating capacitors and the dc linkinductor are the major disadvantages of

this type of inverter.

ASCI inverters are being replaced withinverters using self-controlled devices(e.g. GTOs).

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Six-Step CSI with Self-Commutated

Devices

Self-controlled symmetric blocking devices,e.g. GTOs can be turned on and off by gate

current pulses. This allows the 6-step

waveform to be directly controlled.

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Devices (contd)

In this circuit, the capacitors are freedfrom their commutating requirement andare simply placed across the terminals of

the induction motor. These capacitors aremuch smaller and serve two roles:

1) primarily, to allow commutation fromthe outgoing GTO to the incoming GTO,

2) secondarily, to load filter higherharmonics

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Devices (contd)

Example: Commutation from Q1 to Q3.

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Devices (contd)

Initially current flows through Q1, phase a,phase c, and Q2. The equivalent capacitanceCeq and polarity of vba are as shown.

Next, Q3 is turned on at time A. But becauseof voltage across Ceq, Q1 does notautomatically turn off.

Next, Q1 is turned off.The current Id transfers

to Q3 and through Ceq.Ceq charges up overcoming the motor backemf b/w phases a and b. Gradually thecurrent transfers to phase b. Commutation is

completed when ib=Id.

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Devices (contd)

Total commutation time is tc.

Once commutation is complete, current

can be commutated back to Q1. Thisback and forth current commutation canbe used to create a PWM current waveand with suitable selection of notch

angles, can be used to suppress higherharmonics (just as in the VSI inverter).

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Six-Step CSI with Self-

Commutated Devices (contd)

A major disadvantage of this scheme is thepotential for resonance between thecapacitors and the motor inductance. Care

must be taken to avoid impressing currentharmonics into the motor/capacitornetwork which will excite one of thesystem resonance frequencies. This can be

avoided by careful use of PWM. However,since the motor parameters must beknown to implement such an approach,this drive is not popular for general-

purpose applications.

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PWM Inverters

The six-step CSI inverter has severaldisadvantages primarily associated withharmonics in the current waves. Pulse

width modulation can be used to reduce theharmonic content of the current waves. ThePWM methods are somewhat different fromthose for the voltage-fed inverters.

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Trapezoidal PWM

Similar to the sinusoidal PWM method forvoltage-fed converter. This method isshown below:

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Trapezoidal PWM

Trapezoidal wave has max. amplitude of Band is compared to a triangle wave ofamplitude A.

For the first /3 radians both waves arecompared. For the next /3 radians notriangular wave is applied. For the final /3radians both waves are compared again.

Two variables: 1) modulation index m=B/A

2) pulse number M in half-

cycle of inverter operation.

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Trapezoidal PWM (contd)

For M=21, harmonics vs. m is as shown below:

At m=0.82,

5th harmonic =0, 7th harmonic=4%,

11th harmonic=1% and 13th harmonic=2%.

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The output current waves for theseconditions is shown below:

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To limit switching losses it is necessary tocontrol the device switching frequency,irrespective of the fundamental frequency

of the current waveform. This can beachieved by making the parameter Mconstant in many segments of thefundamental frequency (see next slide for

switching frequency 1kHz).

Note: In multi-MW GTO inverters theswitching frequency generally does not

exceed a few hundred Hz.

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Trapezoidal PWM can reduce harmoniccomponents up to order n=1.5(M+1)for M > 9 but does produce a pair of

harmonics of order 3(M-1)1.

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Selected Harmonic Elimination PWM

SHE-PWM can both lower the harmoniccontent of the output current and, moreimportantly, remove the resonant

harmonic. Unlike SHE-PWM for voltage-fedinverters, several restrictions apply forapplication of SHE-PWM to current-fedinverters.

Consider the 3 current waveforms forM=5 shown in the next slide.

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Selected Harmonic Elimination

PWM (contd)

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PWM (contd)

Angles 1 and 2 are the variables and all theother switching angles are in terms of thesetwo variables. With two variables, two

switching harmonics (e.g. 5th

and 7th

) can beeliminated. The fundamental is controlled bythe dc link current frequency. The generalrelation between # of harmonics removed (K)

and # of pulses per half cycle (M) is given by:

K=(M-1)/2

Both K and M are odd numbers.

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PWM (contd)

For M=3, only one harmonic (e.g. 5th)can be eliminated and for M=7, threeharmonics (e.g. 5th,7th and 11th) can beeliminated.

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PWM (contd)

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Double-Sided CSI Converter

As mentioned earlier, the CSI convertercan easily be used to send power back intothe rectifier when the machine acts as agenerator. In this case the load-side

converter acts as a rectifier and the line-side converter acts as an inverter.

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Duality of Current-Fed and

Voltage-Fed Inverters

Ref: D.W. Novotny

and T.A. Lipo,

Vector Control andDynamics of AC

Drives

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Current-Fed vs. Voltage-Fed

Inverters

Current-Fed Inverters Voltage-Fed Inverters

1.More interactive with Not so interactive with

the load and hence machine and can thus

require a close match be designed to be moreto the machine. general purpose.

2. Inherent 4-quadrant Requires additional

operation. circuitry to operate in

all 4 quadrants.

3. Robust through load Shoot-through faults

short circuits/inverter need to be avoided (use

misfirings. freewheel diodes).

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Inverters


4.Devices must be Devices must be

symmetric blocking. assymmetric blocking.

5. Multi-machine or Normally used for

multi-inverter system multi-machine or multi-

inverter system very inverter system

difficult to implement. applications.6. Relatively sluggish PWM inverters can

response. demonstrate relatively

fast dynamic response.

C

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Inverters


7. Cannot be operated Can be operated open-

open-loop. loop.

8.Minimum load required. Can operate at no-load.

Based on these differences, PWM voltage-fedinverters are most widely used for motor

drives. However, current-fed inverters areused for high-power applications, particularlyload-commutated synchronous motor drives.

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d,q Model for CSI Inverter

The duality of VSI and CSI systems impliesthat the switching function models for CSIsystems should be the duals of those for

the VSI systems.The exact dual of a VSI feeding a Yconnected load is a CSI feeding a connected load. However, since wegenerally want to consider Y connectedloads, the model for the CSI inverter willnot be the exact dual of the VSI inverter(but it will be close).

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d,q Model for CSI Inverter (contd)

The d,q equations for each switching modefor the CSI inverter are obtained in thesame way as for the VSI inverter. The sixswitching modes for the CSI inverter areshown on the next slide.

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Ref: D.W. Novotny and T.A.

Lipo, Vector Control and

Dynamics of AC Drives

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The d,q equations, in the stationary statorreference frame, can be written in termsof CSI inverter switching functions, h as:

; ;

where the switching functions (shown on

the next slide) can be expressed asFourier series by:

2 3s sqs qs ii h i

2 3s sds ds ii h i

3 3

( )s s s si qs qs ds dsv v h v h

1 1cos cos 5 cos 7 ...

5 7

s

qs e e eh t t t

1 1

sin sin 5 sin 7 ...5 7

s

ds e e eh t t t

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Ref: D.W. Novotny and

T.A. Lipo, Vector

Control and Dynamics

of AC Drives

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These equations can be written in complexform as:

where

Note that these are very similar to the VSIequations. In particular, only the signs in

hqdss

are altered.

2 3 2 3( )

s ss s

qds qdsi qs ds ii i h jh i h

3 3Re[ ]

s s

qds qdsiv v h

5 71 1...5 7

e e e

s j t j t j t

qdsh e e e

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As before we observe that the complexvector current is constant in each modeand simply shifts by 60 at each mode

transition. We can write:

for k=1, 2, 3, 4, 5, and 6. The six vectorscorresponding to the switching of a CSIinverter are shown in the next slide.

[ / 6 ( 1)( / 3)]2

3

s j k

qds ii i e

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See handout for d,q model for CSIinverter in stationary reference frame.

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