A Thermal Management Approach to Fault-Resilient Design of Three-Level

IGCT-Based NPC Converters

Presented by

Shambhu R11739

Seminar date 16/09/2014

Guided by

Ms.Mareena FrancisAsst. ProfessorElectrical and Electronics Engg. Dept.

Variable Frequency Drive

Block diagram of a typical IGCT based NPC VFD

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OVERVIEW1. IGCT

1.INTRODUCTION

2.KEY FEATURES

3. APPLICATIONS

2. NPC Converter1.THEORY

2.SWITCHING STATES

3. Case Study1.INTRODUCTION

2.OVERCURRENT PROTECTION

3.POWER DEVICE THERMAL STRESSES

4.JUNCTION TEMPERATURE

5. MANAGING THE POWER DEVICES THERMAL STRESS

6. IMPROVEMENTS IN THE THERMAL STRESS OF IGCTS

7. CONCLUSION

4. Reference

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Integrated Gate Commutated Thyristor

(IGCT)

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Introduction

• Introduced by ABB and Mitsubishi in the year 1997.

• Basically a high voltage, high power, hard driven device.

• Similar to GTO, it is a fully controllable power switch.

IGCT

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IGCT Symbol

IGCT

10 KV IGCT

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Features• Improved GTO switching characteristics

for operation without dv/dt snubbering

at high current density

• Reduced on-state and turn-off losses

through minimization of the silicon

thickness

• Reduced gate-drive requirements, especially

during conduction

• High-frequency operation for continuous

and dynamic conditions

• Reliability Improvement per MVA by reduction of complexity and components

IGCT

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IGCT

(µs)

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IGCT Turnoff Characteristics

Applications

• Variable Frequency Drives (VFD)• Static VAR Compensators (SVC)• Traction• Industrial Drives

IGCT

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NPC CONVERTER

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Theory

• Better waveform quality.

• One of the multilevel converter topologies.

• Contains 4 Power Switches in each phase.

• 2 Clamping Diodes.• Capacitors are used for realizing

voltage divider circuits.

NPC CONVERTER

Names

Neutral point clamped converter3 Level converterDiode clamped converter

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3 Level Inverter NPC CONVERTER

• Symbolic representation of 3 level inverter.

• A TPST switch is used.• At each position, different

voltage levels.

VOLTAGE LEVELS

Sa0 -Vdc/2 VSa1 0VSa2 Vdc/2 V

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Switching States NPC CONVERTER

Switching State

Device Switching Status (Phase A)

Inverter Terminal Voltage

Q1 Q2 Q3 Q4

P On On Off Off Vdc/2

O Off On On Off 0

N Off Off On On -Vdc/2

Vdc/2

Ot

V

Q1 and Q4 are called as Main devicesQ2 and Q3 are called as Auxiliary devicesDiodes Cd1 and Cd2 are the Clamping diodes

P

N

Vdc/2

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Circuit Diagram NPC CONVERTER

E

E

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CASE STUDY

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Introduction

• Availability of the converters are mandatory, as maintenance or replacement needs large downtimes incompatible with the requirements of production process.

• Main goal of the paper –if there is a malfunction of the breaker element in the case of an over current, then reduce the damage extension and to restrict the damage to the free wheeling power diodes, thus protecting the IGCTs.

CASE STUDY

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Overcurrent Protection

• Has vital importance in VFDs

• When fault condition occur in commercial drives with AFE(Active Front End) Rectifier

Protection is given by firing all the IGCTs To limit the thermal stress on each devices This mode of protection is called firing mode protection scheme In such a case the faulty converter should be quickly disconnected from

the mains Malfunction of the breaker device result in the damage of AFE

CASE STUDY

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Short Circuit Current DistributionCASE STUDY

• If firing mode protection scheme is activated, then there is chance of short circuit paths as shown in fig .

• Shaded (through clamp diodes), dotted and dashed paths (through freewheeling diodes)

• The internal IGCTs share most of the current(Q2C,Q3B)

• Similarly in other phases also As an inference we get the idea that

internal IGCTs of all phases will get affect by the thermal stress

NPC regenerative rectifier short-circuit paths during shoot-through firing protection (phases B and C)

VBN > VCN

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Power Device Thermal Stresses • Subjected to sequence of two

different types of thermal stresses Transient Thermal Stress due to dc

link capacitor Short circuit Thermal Stress due to

short circuit current from power grid

• DC Link capacitor discharge Shown in figure. With a frequency

Where

Leq –Equivalent inductance in the bridge.

Ceq- Equivalent series capacitance in the bridge

CASE STUDY

Equivalent dc bus discharging network on the shoot-through firing mode protection

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Fo=1/(2*¶*(Leq *Ceq )^1/2)

Junction Temperature• Consequence of keeping the

converter in the shoot through mode for more time is shown in fig.

• Shows the thermal stress in the freewheeling diodes is less than the other power devices.

• Result s in the permanent damage of the internal IGCTs.

CASE STUDY

Junction temperature rises on shoot-through modePhase A devices.

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Managing the Power Devices Thermal Stress• To protect the internal IGCTs,

the current through the clamping diode must be reduced.

IQ-short circuit current through internal IGCT

IFW-short circuit current through freewheeling diodes

Reduce IQ /IFW , for this the resistance clamp diode circuit should be increased Rc- resistance of the clamp

Rf - resistance of the freewheeling diode branch

Rc /Rf should be increased.

• So , add an additional resistance Zr (“resilience impedance”) is added to the clamp diode branch.

CASE STUDY

Insertion of the resilience impedances (Zr) in the Clamping Diode branches of the 3L NPC IGCT-based

converter.

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Improvements in the Thermal stress of IGCTS

• Redesigned the path of current flow in shoot through mode by addition of “resilience impedance” .

• To restrict the damage to the power diodes(free wheeling) as much as possible

.• So that IGCTs can be protected

in the converters shoot through mode even if the breaker fails to work properly.

Junction temperature rises on shoot-through mode Phase A devices of the resilient converter

CASE STUDY

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Conclusion • In this paper, the short-circuit behavior of the IGCT-based

high-power 3L NPC topology has been addressed by the damage pattern resulting from a shoot-through mode followed by a protection scheme malfunction.

• To make minimum repair cost and minimum repair time, damage should be restricted to the diodes, keeping IGCTs safe.

• By proper design of the converter bus bars and management of the thermal stresses, it is possible for the IGCTs to survive the converter shoot-through mode even in the event that the circuit breaker fails to operate.

CASE STUDY

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Anderson Vagner Rocha, Hélder de Paula, Manoel Eustáquio dos Santos, Braz J. Cardoso Filho,” A Thermal Management Approach to Fault-Resilient

Design of Three-Level IGCT-Based NPC Converters” IEEE Trans. Ind. Appl., VOL. 49, NO. 6, november/december 2013

P. K. Steimer, H. E. Gruening, J. Werninger, E. Carroll, S. Klaka, and S. Linder, “IGCT—A new emerging technology for high power, low cost inverters,” IEEE Ind. Appl. Mag., vol. 5, no. 4, pp. 12–18, Jul./Aug. 1999.

A. Nabae, I. Takahasai, and H. Akagi, “A new neutral-point-clamped PWM inverter,” IEEE Trans. Ind. Appl., vol. IA-17, no. 5, pp. 518–523,Sep. 1981.

D. Floricau, E. Floricau, and G. Gateau, “Three-level active NPC converter: PWM strategies and loss distribution,” in Proc. IEEE IECON,Nov. 2008, pp. 3333–3338.

Muhammad H.Rashid ” Power Electronics” ,2012

Reference

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