Power Convertor Building Blocks for Multi-megawatt PWM VSI Drives

POWER CONVERTOR BUILDING BLOCKS FOR MULTI-MEGAWATT PWM VSI DRIVES E A LEWIS CENG MIEE IEEE AISE ALSTOM DRIVES AW CONTROLS LTD. RUGBY, U.K., CV21 1BU

ABSTRACT

This paper details how the latest generation of high power insulated Gate Bipolar Transistor (IGBT) modules have been used to create a range of medium voltage drives with output voltages from 2200 to 6600 volts, based on a common Power Convertor Building Block (PCBB) concept.

INTRODUCTION

The development of medium voltage multi megawatt Variable Speed Drives (VSDs) using Pulse Width Modulation (PWM) in a Voltage Source Inverter (VSI) circuit, is a development based on combining three very well proven technolo,' oies:-

Q Series thyristor technology, used for many years to implement many different circuits, with motor voltages up to and above 11000 volts ac rms.

0 Two level low voltage PWM VSI technology, the most common VSD in use today.

0 High voltage IGBTs which are now in mass production for the traction industry.

SERIES THYRISTOR TECHNOLOGY

This technology uses a number of thyristor devices in series to achieve high output voltages. A typical construction using deionised water liquid cooling is shown in Figure 1.

1 P-- I

Figure I : Series thyristors with liquid cooling

41 1

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The use of thyristors in series is based on the following 4 core principles:-

* Resistors across each series thyristor for dc voltage sharing.

Resistor plus capacitor snubber circuits across each series thyristor for dynamic voltage sharing.

Extra thyristors added (per series circuit), so that the VSD can continue to operate with a thyristor failed to a short circuit condition. This is normally called “N+l” redundancy.

If a thyristor fails so that it is unable to turn on, it will fail to a short circuit condition the next time it should have fired, due to the level of applied voltage. The VSD can then continue to operate.

The development of the medium voltage VSDs using series IGBTs was based on these well proven 4 core principles.

TWO LEVEL LOW VOLTAGE PWM VSI TECHNOLOGY

This is the most common circuit used for VSD’s in the world, see Figure. 2.

L

AC

Input Diodes DC Link IGBT plus APD’s

Figure 2: 2 Level VSI PWM Circuit

The Input Diodes rectify the ac supply via the inductor L and capacitor C, to give an essentially fixed voltage on the dc Link. The IGBTs together with the Anti Parallel Diodes (APDs) switch odoff rapidly with varying pulse widths, to give a PWM ac output as shown in Figure 3.

The VSD construction is simplified as the IGBTs and APDs are mounted together in a composite base mounted package, see Figure 4.


Figure 3: PWM Output - Line Voltage Figure 4: IGBT Packaging

The IGBT shown in Figure 4 is rated at 3300 volts, 1200 amps. The development of the Medium Voltage VSDs was also based on the klly proven 2 level circuit.

ALTERHATWE POWER DEVICES

Before IGBTs were used, many PWM VSDs were built using Gate Tum Off Thyristors (GTOs). These required a high current gate drive, large snubber components and only had a low switching frequency.

The IGBT with its low current gate drive, no need for snubbers in Figure 2, and its high switching frequency has virtually replaced GTOs. For use in series circuits there are two proven power devices available, the IGBT and the IGCT. (The integrated gate commutated thyristor.) The IGCT is a development of the GTO, using increased gate drive power (typically 3000 amps for 3 micro seconds), to achieve faster switching, see Figure 5 .

Figure 5 : The first generation IGCT device from ABB. This illustrates the very large gate driving circuit required, this becomes even larger if switching frequencies similar to IGBT frequencies are required.


A comparison of IGBT and IGCT characteristics is given in Figure 6 .

ITEM

Typical Voltage Rating

Typical RMS Current Rating

Switching Speed

Gate Drive Power

Limiting of Fault Current

Control of Fault Current to Off State

Series Operation

Suppliers

Device Costs

IGBT

3300V

1200A

Very Fast

Low

Yes

Yes

Yes

Multi Source

Competitive

IGCT

4500V

1500A

Slower

High

No

No

Yes

Limited

Increased

Figure 6: IGBT - IGCT Comparison

Note - The manufacturer’s data sheet current ratings for IGBT are rms, while IGCT current ratings are the maximum allowed under fault conditions.

Both types of devices are in use in high power applications, but for this development the fast switching speed, fault current control and the multi source availability of IGBTs made them the optimum device to achieve the high PWM frequencies required for the target applications.

The Mosfet Turn Off Thyristor (MTO), which is still in the development phase, may replace IGCTs and may also become a replacement for IGBTs in high power applications.

THE PEBB AND PCBB CONCEPTS

The Power Electronic Building Block (PEBB) concept is a very important initiative by the Office of Naval Research (ONR) in the USA, and should revolutionise power electronics. The essential features of a PEBB are shown in Figure 7 .

9

Power - 7 k output Power Supplies Control ’ Cooling ~ Monitoring

Figure 7: The PEBB Concept


For low power P W VSI drives the PEBB can be the complete convertor, produced as a composite unit.

For medium voltage (up to 6600 volts ac) VSDs a similar Power Convertor Building Block (PCBB) approach is still viable, has benefits, and has been used in this development. The PCBB concept for medium voltage drives is shown on Figure 8.

DC POWER SUPPLY

COMMOS CONTROL

COMMON COOLING

A B C

POWER OUTPUT

Figure 8: Medium Voltage PCBB Technology

The Figure 8 shows 18 identical PCBBs, based on series device technology, to implement a medium voltage P W VSD, using the low voltage two level inverter topology. The benefits are:-

*

0

*

e

0

Based on proven series device technology, with Ni-1 series redundancy.

Based on the proven two level circuit, with fully proven controllers.

Control methods and firmware are hlly proven, and it is simple to use any new developments coming from the product market.

PCBBs can be mass produced and then assembled into a VSD.

Gives modular spare holding, each unit being fully interchangeable.

Different voltage ratings can be produced by varying the number of PCBBs in series, without changing the drive topology or control.


MEDIUM VOLTAGE PCBBS

The power circuit of a medium voltage PCBB is shown on Figure 9 and is directly based on the series thyristor technology.

RC Snubber I I

3000V Voltage Sharing Resistor

Surge Supressor

for senes circui:s

1 Saturable

Figure 9: Medium VdWage PCBB Circuit

The following parts are used:-

* Voltage sharing resistor - dc sharing

9 RC snubber - Dynamic voltage sharing

Optional surge suppressor - open circuit protection

0 Optional Anti Parallel Silicon Controlled Rectifier (ASCR) - N+l Redundancy option

The power comes from the dc supply shown on Figure 8, which for a 4160 volts ac output VSD is typically 6500 volts dc. The cooling will depend on the application, but for maximum output some form of liquid cooling is used.

The control uses the hardware and firmware fully developed and proven for low voltage PWM VSI drives. The firing signals are sent to a gate board, mounted by each IGBT, via fibre optic cables for high noise immunity.

The auxiliary power supply to the gate board is supplied via a current transformer circuit working at a high frequency with a very low coupling capacitance. The gate board provides a range of IGBT monitoring and protection hc t ions , including:-

Under voltage of gate board’s power supply

IGBT short circuit detection

IGBT open circuit (overvoltage) detection

9 Firing of ASCR when used

There is also a second fibre optic serial link to retum this data to the control system.


IGBT VOLTAGE SHARING

The voltage sharing between IGBTs in series has been proven to be exceptionally well controlled due to the components of Figure 9 and gate boards with precise time delays. A technique has been specially developed to measure the voltages across IGBTs using an array of floating battery powered oscilloscopes with fibre optic triggering and a fibre optic data link to a PC. Using this technique the voltages across a set of series IGBTs on a 9000 volt dc link is shown on Figure 10.

Figure 10: IGBT Voltage Sharing Waveforms

The superimposed voltages are very well balanced, and the sharing components avoid the need to select IGBTs.

IGBT STRESS LEVELS

The same RC snubber circuits provide a semi soft switching of the power devices at turn on and turn off. This is most significant for turn on, see Fi,we 11. The well damped current overshoot after turn on is mainly due to the RC snubber discharge current.

4/7


At turn on the circuit used enables the voltage to fall to a low value before the IGBT current rises giving a large reduction of IGBT switching losses. Tne RC snubber does have some switching losses, and the net VSD Iosses are approximately the same as VSDs using hard switched IGBTs. The RC snubber action has major benefits to the rating of the IGBT and therefore the VSD. Typical results are:-

* Hard switched IGBTs have typically 50% more losses than IGBTs using the semi- soft-switching circuit shown on figure 9.

For a ATj = 50°C the semi-soft-switching circuit can deliver 40% more current or power than hard switched IGBTs.

0 For a Tj = 120°C the semi-soft-switching circuit can deliver 30% more current or power than hard switched IGBTs.

The use of the snubber shown in Figure 9 has thus given:-

* Lower absolute junction temperature (Tj)

0 Lower values of ATj for cyclic duties

0 Reduced IGBT stress levels

0 Improved IGBT device life

Improved reliability

Ability to switch at faster rates which is essential in limiting the size and cost of output filters .

Circuits which are less critical in achieving a low inductance.

The use of the RC snubbers reduces the transient energy pulses at turn on and turn off and this is a significant factor in improving reliability. In the longer term the reduction of the turn on and tum off energies could enable ZGBTs to be specifically tailored to the semi-soft- switching circuit with reduced conduction losses by up to 50%.

IGBT PROTECTION

With the use of GTO type of devices in the basic 2 level circuit of figure 2 there are two major protection topics:-

* If a fault current occurs above the rated GTO current, the GTO cannot be turned off to limit the fault current. If it is turned off it will be destroyed.

0 If a GTO goes to a short circuit condition, e.g. A1 in figure 2, the next time the associated GTO is fired, e.g. A2 in figure 2, a short circuit is put across the capacitor C with very high fault currents.

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The use of IGBTs has many protection advantages as the IGBT is a linear device, where for high levels of current the IGBT’s voltage rises from its normal low voltage on state. Thus if overloads occur the IGBT firstly de-saturates to limit the fault current, which is easily detected. The IGBT can then be turned off very rapidly to provide protection. The peak currents will be limited to typically 6 times the IGBT’s rating for 10 microseconds without device damage.

For the two level series device circuit shown in Figure 8 the protection is again improved:-

0 If an IGBT goes short circuit then the other IGBTs are rated to cary the voltage stress levels, but unlike Figure 2, no overcurrents will occur, and the drive can continue to operate. This is identical to the series thyristor concept.

0 If an IGBT goes open circuit then the surge suppressor of Figure 9 will operate and permit the circuit to continue in operation for a short time. If a shut down is permitted for an open circuit failure then the ASCR is not required. If continued operation is required following an open circuit device failure, then the ASCR is fitted, and fired by the gate board to provide a shorted device mode.

e If overloads occur, e.g. output short circuit, then the inherent IGBT protection systems work to limit the fault current and give protection.

For the 2 level medium voltage series IGBT circuit, the possibility of any significant overcurrents is inherently very low, as set out in the above points, especially as any single short circuited device will not lead to overcunents.

The possibility of open circuit IGBTs due to the rupturing of the internal bond wires, is normally caused by fault currents. Because the 2 level series IGBT circuit is inherently free of overcurrents produced by chip short circuits, then the circuit also has a very low probability of open circuit IGBTs. This is a very major advantage of this circuit and gives increased reliability .

The probability of an open circuit IGBT module is so low that for most industrial applications only the surge suppressor of Figure 9 is required to provide a guaranteed safe shutdown. This is called a “N+l Safe Shutdown Design”.

For more critical applications the ASCR of Figure 9 is fitted, to permit continued operation. The devices are rated to do this on a continuous basis. This is called a “ W 1 Fully Rated Design”.

Both design options are proven and available, with most drives sold as a ‘“+I Safe Shutdown Design” type.

A number of reports of IGBT plastic cases rupturing under severe faults, have been reported in the technical press. The measures taken for the series IGBT circuit make this very unlikely to occur, however, as a back up policy a set of simple containment barriers are fitted to contain any such ruptures which might occur, so that a spare can then be fitted.


FILTERING MOTOR VOLTAGES

All medium voltage PWM VSI inverters will produce very high levels of dv/dt and Electro magnetic Compatibility (EMC) effects if the inverter is directly connected to the motor. The design must limit these to acceptable levels to meet the relevant standards, and must also avoid excessive extra costs in the motor’s insulation system. A large number of factors must be considered, including:-

*

0

Q

*

Q

Q

e

Life of motor insulation

Permitted motor dv/dt levels

Motor derating if a higher than normal grade of insulation is required

Overall cost impact of insulation used

Currents induced into the motor’s frame due to the common mode components of the PWM voltage.

Effects on motor iron and copper losses, which may require motor derating, or an increase in a frame size.

Effects on the motor’s acoustic noise

Induced rotor currents and means to avoid (shatlhearing) currents, which can lead to premature bearing failures

EMC effects of cables and ground currents

Effects on rating on size/cost of motor cables

When all of these factors are considered some form of output filter will be required for an optimum design.

The filter can be of two types:-

* SINEWAVE - Giving very near to sinewaves of voltage at the motor terminals for both differential and common mode voltages. A typical total harmonic voltage distortion factor is 5%.

DV/DT TYPE - This limits the dv/dt to typically 5OOV/microsecond or some other agreed value, but lets the PWh4 waveshape remain.

For many applications the use of a full sinewave filter is totally justified as it gives the minimum of cost effects for the motor, and gives the lowest overall net cost with maximum security. All related application considerations are minimised, especially EMC, ground currents and bearing currents.

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The waveforms of a typical sinewave output at the motor’s terminals is shown on Figure 12. The motor voltages and currents are better than any other type of power circuit.

The Sinewave filter is included in the cubicles of the VSD, see Figure 13.

-- -MotoYchent 200Ndiv - -

Figure 12: Motor current and line voltage. DC link voltage = 5200V, Motor frequency = 50Hz

Figure 13 : 1 OOOA Filter Hardware

The development of optimised output filters is an essential part of this new technology. By using a fast switching PWM waveform the filter size can be kept to acceptable proportions, and its components only need to act to suppress frequencies above 1000Hz. The filter will thus have no significant impact on the dynamic response of the VSD. The present analysis suggested that a dv/dt filter is less cost effective for normal industrial applications, and that a full sinewave filter gives the optimum solution.

ALTEKNATIVE TOPOLOGIES

For medium voltage PWM VSI drives there are a number of circuit topologies which could be used, see below and figure 14.

3 (or more) Levels neutral point clamped circuit (Figure 14a)

Q Multi level capacitor clamped circuits, also known as Foch circuit (Figure 14b)

2 Level series device circuit (Figure 14c). This development.

0 Multi level circuit formed from a number of separately fed “H” bridges (Figure 1 4 4 .

4/11


The circuits shown on Figure 14 are one phase of a three phase system.

r

:. . . . ..__I.. . . .... ~. . . . .._.. 1::.

...............................

14a 14b 14c 14d

Figure 14: Medium Voltage P W M VSI Topologies

Several MV PWM VSI convertors recently introduced to the market are based on 3-level designs. A 3-level MV topology makes good use of the flexibility offered by a high number of switches per arm by asynchronous switching.

However, multi-level solutions were avoided in this development, for several very practical reasons:

They involve many steering diodes or flying capacitors, particularly at higher supply voltages, say 6600 volts.

0 Similarly, series H-bridges employ a large number of power devices and require many isolated power supplies.

0 Multi level designs must use different circuits at different voltages.

0 It is not as easy to add N+1 redundancy to multi level circuits, to keep the VSD operating with faulty devices.

0 Many multi level circuits will give a large overcurrent for one failed device unlike the 2 level circuit in Figure 14c.

A more complete comparison of the different topologies is given in Reference No.1.

A common claim is that the wavefonn of 3 level or other multi level circuits is significantly superior to that from a 2 level circuit. However, all such medium voltage circuits require a filter between the inverter and the motor to limit the effects detailed in the section on filter

4/12


motor voltages. When the filter circuit is added the motor sees essentially the same waveform whatever power circuit is used.

The choice of the power circuit topology for practical medium voltage drives does not significantly affect the quality of the motor voltage waveform, but it does affect the size and the cost of the filter circuits. However, when the costs of the simpler two level power circuit are taken into account, it remains the optimum solution. The use of fast switching IGBTs is essential in minimising the size and cost of the filter shown in Figure 13, and a viable filter could not be designed if IGCTs were used in Figure 8 due to their lower switching frequencies. If IGCTs were selected then one of the more complex multi level circuits of Figure 14 would have to be used.

POWER CONVERTOR DEVELOPMENT

The development program for these medium voltage power convertors started in 1995 based on the three proven technologies previously detailed.

The development process required the proving of the series IGBT circuit, and a dedicated high voltage test facility was developed.

The major developments that had to be completed were:-

Monitoring facilities compatible with EMC levels IGBT static and dynamic voltage sharing IGBT protection for all conditions IGBT snubber circuit components Design of new IGBT gate pcb Design of an efficient cooling system Design of interconnection busbars with low inductance Design of DC Link Capacitors, a self healing non electrolytic type was selected Design of output filtering circuits

The design of the controller and its firmware did not have to be changed as they were already proven and in use on other products.

The development work has all been completed and proven and an industrial drive range called VDM5000 has been produced. At the time of writing this paper the following VDM5000 systems are at various stages of use with ratings up to 6.0MW.

e 1 as a prototype for full testing (completed)

e 14 for Ship propulsion of Thruster drives. So far 7 are in service on board ships.

e 1 for a long conveyor

0 1 for a large pump in the USA

All 17 VDM.5000 drives will be in service before the end of 1999. In addition to the VDM.5000 actually built, and in or near to being in service, there are more than 50

4/13


VDM5000s quoted for a wide range of industries including metal drives, ship propulsion and linear motors.

The typical industrial ratings range from 3300 volts at 700 amps (4MVA), up to 41 60 volts at 1000 amps (7.2 MVA). If required, ratings to above ZOMVA are available.

DESIGN OF PCBB FOR THE INVERTER

The basic PCBB hardware is shown on Figure 15. The PCBB consists of:-

* Power Input = Busbar shown on top of IGBT

Cooling = De-ionised water cooled heatsink with pipe interfaces

Power Output = Busbar shown on top of IGBT

Control = Gate board with fibre optic control input and diagnostic fibre output

Auxiliary power = Small ring ac CT on left side of Figure 15

Circuit = IGBT plus the simple RC snubber

Figure 15: VDM5000 - PCBB Hardware

This PCBB uses 3300 volts, 1200 amp IGBTs.

The component group shown in Figure I5 is the PCBB which can be manufactured in quantity for use in inverters of different ratings. The PCBBs are assembled into a phase leg chassis as shown in Figure 16.

The cooling system uses a de-ionised water technology, which is fully proven in series thyristor convertors. The heat exchanger is included in the suite and permits the use of extemal water or extemal air cooling.

The use of de-ionised water cooling is a major benefit in keeping atmospheric pollution &om entering the power cubicles, and uses redundant pumps for most applications.

4/14


Figure 16: Inverter phase leg chassis

1

Figure 17: A complete inverter

The chassis of Figure 16 holds 8 PCBBs and forms two arms, like Al . 1 to A2.3 of Figure 8, except in this case 4 IGBTs in series per arm are used rated at 6500 volts dc 1000 amps. The number of series IGBTs per arm can be changed to vary the voltage rating, while the control electronics stays unchanged. Such an approach will benefit from the future development of high voltage devices.

The inverter module series connection is made by a low-inductance, medium voltage busbar technology, developed for this project with near zero partial discharge performance. A number of the standard PCBB based chassis can then be installed in a cubicle to produce a complete inverter see Figure 17.

The VSD inverter shown in Figure 17 has the overall circuit shown in Figure 18.

AC Input Converter DC Link PWM Inverter Output Filter

Transformer 12 pulse

Motor

T i

411 5


Figure 18: The VSD PWM Circuit

The Input Convertor can be 12, 18 or 24 pulse to minimise ac supply harmonics. Normally a 12 pulse convertor is sufficient due to the circuit giving very low ac current harmonics.

For the system of Figures 17 & 18 a Dynamic Braking (DB) chopper is fitted as a 4th chassis to give controlled regeneration. If high levels of continuous regeneration is required the input convertor can take one of two forms.

Antiparallel Thyristors PWM Inverter, like the output inverter

The complete VSD suite for Figure 18 is shown in Figure 19.

FRONT V!EW

Figure 19: VDM5000 converter suite arrangement

COMBINED TEST:

To validate the new technology, an extensive combined test programme was carried out using a set of production cubicles shown in Figure 21. The objectives were:-

* To test the convertor drive at realistic operating conditions To test the convertor protection under fault conditions

e To test the output filter over its specified speed and power range To quantify total harmonic distortion on supply and motor To quantify system earth currents To assess drive speed and torque dynamic responses.


Fi,oue 2 1 : Converter cubicle, rated at 4.2kV, lOOOA RMS

The test arrangement is shown in Figure 22 and comprises a 6 to

1200 volts, 2MW generator; a 3MVA, 2 x 3 300 voltsI2 200 volts, step-down transformer; the dnve including its output filter; and a standard 3 300 volts, 2MW, 50Hz induction motor, coupled mechanically to a dc load generator. For operation above 1000 amps rms current, an inductive load was used.

3.3kV, 2.3MW 12kV 1 2 - 6 k V VDM5000 12 pole, 50 Hz

-g z G 12 Puse transformer

2 x 3.312.2 kV 3MVA

Prl Resistor

Figure 22: Combined-test set-up

The full range of tests were carried out with excellent results.

APPLICATION OF MV VSI PWM DRIVES

This new range of medium voltage VSI PWM drives have a very wide range of features making them suitable for almost all applications. The main benefits are:-

Can use standard motors, or use for retrofit applications

Excellent EMC design

High quality near to sinewaves applied to the motor

High ac supply power factor (greater than 0.95)

The ac supply power factor does not vary with motor load or speed

High dynamic control 1000: I speed range, 0.01% accuracy

Very Low ac supply harmonics

High tolerance to ac supply dips and disturbances

Compact suites

4/17


Water Cooled

0 Fully standardised

0 Reduced stresses on IGBT by snubber for long IGBT life.

0 Reduced costs compared to many other types of VSDs, e.g. cycloconvertors

No significant torque pulsations

0 Low drive and motor acoustic noise

No risk to motor insulation or bearings

0 High reliability verified by extreme simulations, analysis and testing

4/18


The new technology can be applied to a very wide range of applications:

Sector Application

Metal Industry Rolling mill drives, coilers and winders

Marine Propulsion, thrusters

Mining Conveyor belts, crushers, mills, mine hoists

Power Industry Soft starters, DVR, energy storage

0 Process Control Fans, pumps, compressors

Petrochemical Down-hole pumps, pipeline pumps, gas compressors, water injection pumps

The new VSD system is the Universal solution to all these applications.

THE FUTURE

A wide range of developments in power devices and packaging is taking place and these will be incorporated into this development as they become viable for commercial use

COXCLUSIONS

This paper describes a PWM VSI IGBT convertor which is both technically and commercially viable for medium voltage adjustable speed drive applications in industrial and marine applications.

The use of a sinewave filter, combined with high switching frequency of power devices, makes it possible to design a medium voltage variable speed drive which may be used with standard ac motors, and which complies fully with EMC standards.

It is anticipated that this technology will be a strong contender to its rivals and is expected to be the new technology to produce a standard for the industrial drive market - a sinusoidal output medium voltage drive, with little or no impact on the supply system.

Acknowledgments

The author wishes to thank the management of ALSTOM Drives & Controls Ltd. for their permission to publish this paper.

References

1. Shakweh, Y., Lewis, E.A. Assessment of MV P WM VSI topologies, PESC 99, Charleston, South Carolina, USA, 1st July 1999

2. Crane, A., EMC design for a I9MW PWMmotor drive. 1999 IEEE Industry Applications Society Annual Meeting

0 2000 T h e Institution of Electrical Engineers. Printed and published by the IEE, Savoy Place, London WC2R OBL, UK. 4/19


Documents

Power Convertor Building Blocks for Multi-megawatt PWM VSI Drives