slides topic9

PWM CSI fed MV drive
Topic 9
PWM Current Source Inverters (CSI)
Courtesy of
Rockwell Automation

Lecture Topics
Single Bridge Inverter
Trapezoidal Modulation (TPWM)
Selective Harmonic Elimination (SHE)
Space Vector Modulation (SVM)
Dual Bridge Inverter
PWM Current Source Inverters

Inverter Topology
Features:
Simple topology – no antiparallel diodes
Reliable short circuit protection – constant dc current
Very low dv/dt on motor terminals
Symmetrical GCTs
Function of Cf :
- Assist GCT commutation
- Reduce harmonic distortion
Single Bridge Inverter

DC Current Source Id
Implementation of dc current source:
use a large size dc choke – making Id smooth;
use dc current feed back control – keeping Id constant
Single Bridge Inverter

Trapezoidal PWM (TPWM)
Selective Harmonic Elimination (SHE)
Space Vector Modulation (SVM)
Constraints on Switching Pattern Design
- dc current Id cannot be interrupted
- inverter output current waveform
must be defined.
At any time instant, only two switches are
turned on, one connected to the positive
dc bus, and the other to the negative dc bus.
Modulation Techniques
Single Bridge Inverter

Trapezoidal PWM (TPWM)
Number of pulses per half cycle: Np = 7
Trapezoidal Modulation

Harmonic Content (TPWM)
Np = 15
Fundamental does not
change much with ma
ma = 0.85
Trapezoidal Modulation

Waveforms
(TPWM)
Trapezoidal Modulation

Principle
Not all the switching angles are independent.
Only two harmonics can be eliminated with Np = 5.
No modulation index control – the magnitude of inverter output
current is controlled by Id
Number of harmonics to be eliminated:
Selective Harmonic Elimination (SHE)

Fourier Analysis
Selective Harmonic Elimination

Expression for an
Fourier Analysis
Selective Harmonic Elimination

Switching Angle Calculation
To Eliminate nth harmonic, set an = 0
For 5th, 7th and 11th harmonic elimination:
Selective Harmonic Elimination

Switching Angles (SHE)
Selective Harmonic Elimination

Inverter Waveforms (SHE)
Harmonic eliminated: 5th, 7th and 11th; fsw = 420Hz
Selective Harmonic Elimination

Waveforms (SHE)
Selective Harmonic Elimination

Space Vector Modulation
Six active current vectors and one zero vector Six sectors
Space Vector Modulation

Switching State
Space Vector Modulation

Switching States & Space Vectors
[1]: Upper switch on
[-1]: Lower switch on
[0]: None of the switches
in a leg turned on
[2]: Both switches in a
leg turned on
(bypass mode)
Space Vector Modulation

Dwell Time Calculation
Following the same principle
of SVM for the NPC inverter
Space Vector Modulation

Dwell Time Calculation
→
Space Vector Modulation

Switching Sequence
Requirements:
Transition from one switching state to the next involves only
two switches
2) At any time only two switches are on.
Space Vector Modulation

Switching Sequence (Over one cycle)
Space Vector Modulation

Inverter Output Waveforms (SVM)
f1 = 60Hz, fsw = 540Hz, Np = 9 and ma = 1
Space Vector Modulation

Harmonic Spectrum (SVM)
f1 = 60Hz, fsw = 540Hz, Np = 9 and ma = 1
Space Vector Modulation

Harmonic Content (SVM)
Space Vector Modulation

Converter Configuration
Dual-Bridge Inverter

Space Vector Diagram
19 vectors and 51 switching states
Dual-Bridge Inverter

Current paths with switching state [16;56]
DC Current Balance Control
Dual-Bridge Inverter

DC Current Balance Control
Current paths with switching state [16;56]
Dual-Bridge Inverter

Space Vector Diagram
Dual-Bridge Inverter

PWM Scheme Comparison
Summary