a new approach to enhance power quality

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2 2 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 33, NO. 1, SANUARYffEBRUARY 1997

Pe t e r W. Hammond, Member IEEE

Abstract- A new approach to medium-voltage variable- 11. TRENDSN LOW-VOLTAGERIVES

power factor of this new type of drive exceeds 94 at full loadand is above 90 at 10 load. Motor voltage and currentwaveforms are improved so that torque pulsations are reduced.Peak voltage stress on motor insulation does not exceed peakinput line voltage, and no zero sequence voltage is imposed.Drive efficiency exceeds 96 This paper describes the newapproach and some of the results achieved.

Index Terms- Harmonic cancellation, medium-voltage drive,motor-friendly drive, multilevel PWM, power-quality drive, se-

ries converters.

I. EXISTING EDIUM-VOLTAGERIVES

HYRISTOR (SCR and GTO) current-source circuits

have becom e the standard technology for medium-voltage

variable-frequency drives for induction motors [ ]-[2]. They

have found widespread application with centrifugal (pump or

fan) loads, where they offer the advantage of higher efficiency

than can be obtained from damp er controls or throttling va lves.

The drives are simple, relatively economical, and highly

reliable.

In spite of their many virtues, there are still some draw-

backs to these current-source drives. They inject significantharmonic currents in to the supply line and operate at a reduced

power factor as speed is decreased. Low-order harmonics at

the drive output may excite torsional resonances. And, unless a

dedicated isolation transformer is provided, the large commo n-

mode (zero-sequence) output voltage may require extra motor

insulation.

Another drawback of current-source med ium-vo ltage drives

is their cost. Fig. 1 shows comparative cost per hp trends

for 480, 2400, and 4160 VAC drives. It is clear that the

cost per hp is much greater for medium-voltage than for

low-voltage drives. One reason is that the components for

medium-voltage drives are manufactured in lower volume

than those for low-vo ltage drives. N evertheless, Fig. 1 impliesthat several low-voltage drives are less expensive than one

medium-voltage drive of equal total rating.

Paper PID 96-26, approved by the Petroleum an d Chemical IndustryCommittee of the IEEE Industry Applications Society for presentation at the1995 IEEE Petroleum and Chemical Industry Technical Conference, Denver,CO, September 11-13. Manuscript released for publication August 1, 1996.

The author is with Robicon Corp., Pittsburgh, PA 15068 USA.Publisher Item Identifier S 0093-9994(97)00188-6.

recent trend in low-voltage variable-frequency drives toward

pulse width modulation (PWM) voltage-source designs. PWM

voltage-source has inherent advantages regarding harmonics,

power factor, torque pulsations, and common-mode voltage.

This trend has seldom been extended to medium-voltage

drives, partly because the new switching devices do not have

the required voltage ratings to build a single-bridge converter

at medium-voltage. Fig. 1 shows that if a way could be found

to produce medium voltage by combining several low-voltagePWM converters, it should be cost effective. Suc h an approach

would take advantage of the high manufacturing volume of

low-voltage devices and would yield several other benefits to

be described.

111. NEW APPROACH

Fig. 2 shows such a new power circuit topology for a2400 V drive. Each motor phase is driven by three power

cells connected in series. The groups of power cells are W Y E

connected, with a floating neutral. Each cell is powered by an

isolated secondary winding of an integral isolation transformer.

The nine secondaries are each rated for 480 VAC at one-ninth

of the total power.Each cell is a static PWM power converter capable of

receiving input power at 480 VAC, three-phase, 50160 Hz,

and delivering that power to a single-phase load at any voltage

up to 480 VAC and any frequency upto 120 Hz. The cells are

constructed to 600 V standards using 600 V class components.

The power cells and their secondaries are insulated from each

other and from ground for 5 kV class service. The power

cells all receive commands from one central controller. These

commands are passed to the cells over fiber-optic cables in

order to maintain the 5 kV class isolation.

For a 3300 V drive, Fig. 2 would be extended to have fourpower cells in series in each phase, with 12 secondaries on

the integral isolation transformer.

For a 4160 V drive, there would be five power cells in series

per phase, with 15 secondaries on the integral transformer.

With three power cells in series per phase, the new drive

can produce as much as 1440 VAC line-to-neutral, or 2494

VAC line-to-line. With four power cells per phase, the drive


VAC line-to-line. With five power cells per phase, the drive


VAC line-to-line.

0093-9994/97$10.00 @ 1997 IEEE



HAMMOND: A NEW APPROACH TO ENHANCE POWER QUALITY FOR MEDIUM VOLTAGE AC DRIVES

~

203

Cost per HP (A rb i t r a r y Units)

12400 VAC,

L

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Horsepower (Constant Torque)

Fig. I. Cost per hp trends for low-voltage versus medium-voltage drives.

IV. INPUTPOWER UALITY

The transformer secondaries that supply the power cells in

each output phase are wound to obtain a small differencein phase angle between them. The phase angle differs by

multiples of 20 for 2400 VAC drives, by multiples of 15

for 3300 VAC drives, and by multiples of 1 2 for 4160 VAC

drives. This cancels most of the harmonic currents drawn by

the individual power cells so that the primary currents are

nearly sinusoidal.

The schematic of a typical power cell is shown in Fig. 3.

A three-phase diode rectifier, fed by one of the 480 VAC

secondaries, charges a dc capacitor to about 600 VDC. The

dc voltage feeds a single-phase bridge of four insulated gate

bipolar transistors (IGBT 's), which generate the PWM output

of the cell.

As shown in Fig. 3, the input of one of the power cells is

a simple six-pulse diode rectifier. The dc side of the rectifier

is connected directly to the capacitor bank, while the ac side

is fed by a dedicated secondary winding with approximately

8 source reactance. This com bination results in a secondary

current spectrum much better than nominal six-step, as shown

in Fig. 4. Although the power cell creates a fifth harmonic

current slightly greater than the nominal 20 %, all higher order

harmonics are below the nominal levels. The K-factor of the

secondary currents is approximately six.

The concept is that if the low-order harmonics can be

canceled, the remaining high-order harmonics will have very

low amplitudes. The degree of cancellation will be excellentbecause the cells are identical and equally loaded.

With three cells per phase for 2400 V, the phase-shifted

secondaries cause harmonic cancellation between the reflected

secondary currents to produce an 18-pulse primary current.

The lowest harmonic that is not canceled is the seventeenth,which is less than one-third of its nominal level at 1.4%.

Table 10.3 of IEEE Standard 519-1992 [3] (reproduced as

Table I) allows 1.5%of seventeenth or nineteenth harmonic for

the most severe case. The next set of harmonics not canceled

will be the thirty-fifth and thirty-seventh, where 0.3% remains

and is allowed. The input current total harmonic distortion

INF4JT WWE C

PMSE AC

ANY VOLTAGi

ICELL

~

Fig. 2. New drive topology for 2400 VAC service.

(THD) for the 18-pulse drive is below 3%, well within the

5% allowed. Thus, an 18-pulse or better configuration assures

compliance with IEEE Standard 519-1992.

Fig. 5 shows the phase A line-to-neutral input voltage andphase A current waveforms fo r the 2400 VAC drive in Fig. 2,

under full load conditions.

The waveforms shown in Fig. 5 represent the w rst case for

the new drive, when there are only three cells per phase. When

the number of cells increases, as for 3300 or 4160 V drives,the waveforms improve. Fig. 6 shows the input voltage and

current for a 4160 V drive at full power. With five cells per

phase, the harmonic cancellation results in a 30-pulse input

current. The lowest harmonic that is not canceled is the 29th,

which is slightly under 0.5 while 0.6% is allowed. The input

current THD for the 30-pulse drive is below 1%.



204 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 33 NO. 1, JANUARYFEBRUARY 1997

--L Q C r

COr4TRcL 0C;L C O N T P O L C I R C U T S

POW ER

-b F I B E R O P T I C

I G b l i i L S TC3 + N D

R OM I , I - S T E R C O N T P O L

PERCENT OF FUNDAMENTAL AMPERES5

2 5

0 35.5 7 11 13 17 19 23 5 29 31 35 37

HAR MONIC NUMBER

Fig 6 Input waveforms for the new 4160 VAC drive at full load (2000 V,@ POWER CELL AC INPUT CLASSICAL SIX-STEP 50 Mdivision).

Fig . Power cell input current spectrum versus six-step.

Fig. 5.100 Ndivision). 10 Mdivision).

Input waveforms for the new 2400 VAC drive at full load (500 V, Fig 7 Input waveforms for the new 2400 VAC drive at 10% load (500 V



HAMMOND: A NEW APPROACH TO ENHANCE POWER QUALITY FOR MEDIUM VOLTAGE AC DRIVES

~

205

TABLE I

CURRE NT ISTORTIONIMITS OR GENERAL ISTRIBUTIONYSTEMS (120 V-69 000 V )

Maximum Harmonic Curre nt Distortionin Percent of 1

Individual Harmonic Order (Odd Harmonics)

I S J L c11 ll Sh cl 7 17Sh;h<23 231;h<35 35Sh TDD

<20 4.0 2.0 1.5 0 6 0 3 5 0

20<50 7.0 3.5 2.5 1.0 0 5 8 0

50<100 10 0 4 5 4 0 1.5 0 7 12.0

100c1000 12.0 5.5 5 0 2 0 1 0 15.0

>loo0 15.0 7.0 6 0 2.5 1.4 20 0

Even harmonics ar e limited to 25 of th e odd harmonic limits above.

Curre nt distortions that result in a dc offset, e.g., half-wave con verters, are notallowed.

*All power generation equipment s limited to these values of curre nt distortion ,regardless of actua l ZJI ..

where

I = maximum short-circuit current at PCC

I = maximum demand load curre nt (fundamental frequency component)atPCC.

Fig. 8.

60 A per division).

Output waveforms for the new 2400 VAC driv e at full load (500 V,

Fig. 9.V, 50 A per division).

Output waveforms for the new 4160 VAC drive at full load (2000

Ranoe d a m 1 5 - A u ~ - - 1 9 9 4 2 32

Res B w 18 HZ VBW Off swp T i m e 6 6 sec

B SWEPT SPECTRUM

d a m

LOgMag

10 d B

/ a1v

100stop : 10 nz

S t a r t : HZ

Fig 10 Output voltage spectrum of 4160 VAC dn ve at full speed

Range d Bm 1 6 - A U g - 1 9 9 4 10 0 1

Res BW 9 1 HZ S w p T i m e 6 6 SecBW O f f

s t a r t : HZ S t o p : 10 HZ

Fig. 11. Output voltage spectrum of 4160 VAC drive at 67% speed.



206 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 33, NO. 1, JANUARYIFEBRUARY 1997

Surge Wi ths tand Capab i l i t y of the newDrive at 4160 volts for a RectangularPulse at Fu l l -Load, Nominal Voltage.

Peak Ki lovo l ts L ine- to-L ine30

25

2

15

10

5

2 2000 2

Pu lse Dura t i on in M i c r o s e c o n d s

c? o n t i n u o u s (110 V ) M a x i m u m no Trip M a x i m u m n o D a m a g e

Fig. 12. Surge withstand capability of new drive

The input power quality of the new drive type is maintained

even at light load. Fig. 7 show s the sam e drive as Fig. 5,

but at 1 0% of rated power. Th e power factor (watts over

voltamperes) is still better than 90%.

V. OUTPUT OWERQUALITY

Refer again to Fig. 3. At any instant of time, each cell has

only three possible output voltages. If Q1 and 4 4 are ON, the

output will be 600 V . If 4 2 a nd 4 3 are ON, the output will

be -600 V. Finally, if either Q1 and Q3, or Q2 and Q4, are

ON, the output will be 0 V.

With three power cells per phase, the circuit of Fig. 2 can

produce seven distinct line-to-neutral voltage levels (rt 1800,

k.1200,k600 r 0 V). With five cells per phase, 11 distinct

voltage levels are available. The ability to generate many

different voltage levels allows the new type of drive to

produce a very accurate approximation to a sinusoidal output

waveform.

Fig. 8 shows motor voltage and current waveforms for a2400 VAC drive. The voltage shown is between phase A and

the motor neutral (not the same as the drive neutral). The

motor current is shown in phase A during full speed and full

load operation.

Once again, the 2400 VAC drive represents the worst case.

Fig. 9 shows the motor voltage and current for a 4160 V drive

at full speed and full load.

The output waveforms have very little content of low-order

harmonics, so that they are unlikely to excite any torsional

resonance in the mechanical load. Fig. 10 shows the spectrum

of the output voltage of a 4160 VAC drive at full speed (4160

VAC, 60 Hz output). Fig. 11 shows the same spectrum at 67%

speed (2770 VAC, 40 Hz output). In both cases, there a re no

Fig. 13. Removing a power cell from the new drive.

components present less than 45 dB below the fundamental

frequency, between the fundamental and 4500 Hz.

Note that the modulation sidebands in Figs. 10 and 11

are centered on 6000 Hz. The actual switching frequency

of the IGBT’s in the power cells is only 600 Hz. The



HAMMOND: A NEW APPROACH TO ENHANCE POWER QUALITY FOR MEDIUM VOLTAGE AC DRIVES 2 7

Fig. 14. Complete lineup for a 4160 V drive.

control is arranged to interdigitate the switching events of

individual cells, so that the apparent switching frequency

is much higher. This also minimizes the amplitude of the

unwanted components, which in Fig. 10 are at least 25 dB

below the fundamental.

The spectra of Figs. 10 and 11 imply low-acoustic n oise

emissions from the motor. Some motors, in fact, sound no

different operating from the new drive than from the util-

ity.

VI. OTHERADVANTAGES

One important advantage of PWM voltage-source designs is

their surge-withstand capability. Any lightning-induced surge

arriving a t the input of the new drive will have its prospective

current limited by the transformer impedance. Surge current

that does reach the power cells can easily be absorbed by

the diode rectifiers and large capacitor banks. This contrasts

favorably to current-source designs, which are inherently high

impedance. Fig. 12 shows the surge-withstand capability of

the new drive type at 4160 V.

One area of concern with PWM voltage-source drives is

extra stress on the first-turn insulation of the motor, due to

fast-switching steps on the output voltage. This problem is

exacerbated by long cable runs, where wave reflections can

nearly double the step voltage. However, the new drive allows

only one cell at a time to switch in each phase, imposing ab out

a 600 V step. Even if reflective doubling should occur, the

added stress on 5 kV insulation is minimal.

The modular nature of the new drive allows two optional

degrees of redundancy. An electronic bypass circuit can short

the output of a defective power cell, so that current from

the remaining cells can reach the motor. A 4160 VAC drive

can still generate 80% voltage and 100% current under these

conditions, enough for 92% speed with a centrifugal load. If,

in addition, a set of redundant cells is provided, the drive can

still attain full speed.

If problems do occur, microprocessor diagnostics allow

quick identification of the location. The drive is packaged so

that any pow er cell or any printed circuit board ca n be replaced

in less than ten minutes. The photograph in Fig. 13 shows a

power cell being replaced in a 1000 hp 4160 V drive.

Fig. 14 shows a complete lineup for a 4160 V drive. The

four compartments from left to right contain the load-break

fused disconnect, the transformer, the power cells, and the

control plus blower.

VII. CONCLUSIONS

A new design approach for medium-voltage variable-

frequency drives has been described. Examples have been

given of the improvement in power quality offered by thenew approach. More than 100 drives of this new design

have been delivered as of this writing, with favorable field

experience.

REFERENCES

H. N. Hickok and M. R. Wickiser, “The gate-turn-off thyristor: Abreakthrough for the retrofit of existing induction motors from fixedto variable speed,” IEEE Trans. Ind. Applicut. vol. 25, May/June 1989.B. Wu, G. R. Slemon, and S B. Dewan, “PWM CSI inverter forinduction motor drives,” IEEE Trans. Ind. Applicat. vol. 28, Jan./Feb.

1992.

IEEE Recommended Practices and Requirements For Harmonic Controlin Electrical Power Systems IEEE Standard 519-1992, 1993.



208 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 33 NO. 1, JANUARYPEBRUARY 1997

[4] B Wu, G R Slemon, and S B Dewan, “Eigenvalue sensihvity analysisof GTO-CSI induction machine drives,” IEEE rans Ind Applzcat., vol30, May/June 1994.

[S F. A. DeWinter and L. M. Benke, “Systems engineenng for largeinduction motor adjustable frequency drives,” presented at the IEEEPetroleum and Chemical Industry Technical Conf., 1991.

[6] F A . DeWinter and L. G. Grainger, “A practical approach to solvinglarge drive harmonic problems at the design stage,” IEEE Trans Ind

Applicat., vol. 26 Sept./Oct. 1990.

Peter W. Hammond (M’71) received the B S E E

degree from the California Insbtute of Technology,Pasadena, in 1962 and the M.S.E.E. degree fromCase Institute of Technology, Cleveland, OH, in1966.

He joined Robicon Corporation near Pittsburgh,PA, in 1977 and has held the positions of Senior En-gineer and Supervising Engineer in the AC DnvesGroup He is currently Manager of Advanced Prod-

uct Development He has been involved in powerquality issues throughout his career at Robicon In

1993, he conceived of using multiple low-voltage cells in series to achievemedium-voltage output, with very high power quality. A patent is now pendingfor this idea. Since then, he has been responsible for developing Robicon’s“Perfect Harmony” line of ac drives, based on this concept.

a new approach to enhance power quality

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