a low cost approach1 to improve the performance

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A Low Cost Approach1 to Improve the Performance

of an Adjustable Speed Drive (ASD) under Voltage Sags

arid Short-Term Power Interruptions.

*I*Jest L. DurAn G6mez, Student Member IEEE, h a s a d N. njeti, Senior Member IEEE, Byeong OkW oo

*Power Quality Laboratory

Department of Electrical Engineering

Texas A&M University

College Station, TX. 77843-3128

Tel: (409) 845-7466.

Fax: (409) 845-6259

http://powerquality.tamu.edu

Email: [email protected]

Abstract - Voltage sags are a common

occurrence in industrial power distribution

systems. Although a typical sag may last only 5-

20 cycles and voltage magnitude Power than

20% of its rated value can trip an adjustable

speed ac drive (ASD). Such a riuisance tripping

of a continuous industrial process can be very

costly. In this paper, a low cost approach to

improve the performance of an ASD under

voltage sag and short term interruptions is

presented. The approach consists of a low cost

modification (addition of three diodes, D,, Ds, Ds

and an inductor L, Fig. 4) to the front end diode

rectifier topology. This modification do ng with

the dynamic braking IGB'I' (Qdb) control

(standard component in a ASD) is shown to

provide ride-through capability for voltage sags.

Further, it is shown that with the addition of the

diode Dlo and battery E (Fig. 4) the ride-

through capability can be extended to short-

term power interruptions also. The IGBT Qat, issuitably controlled in the event of a sag to

maintain rated dc-link voltage in closed loop,

thus avoiding any nuisance tripping or

momentary speed fluctuations. A 460 V, 10 hp

commercially available ASD is modified with the

proposed approach. Analysis, design and

simulation results are discussed. Experimental

results illustrating the performance of the ASDwith the proposed ride-through topology for awide range of voltage sag conditions are

presented.

0-7803-5006-5 98/$10.00@1!>98EEE.

**Power Electronics Laboratory

LG Industrial Systems

533 Hogae-dong Anyang-shi

Kyongki-do Korea

email: [email protected]

1. INTRODUCTION.A voltage sag, or voltage dip is a reduction of

the voltage (e& Fig. 1) at a customer position with

a duration of between one cycle and a few seconds.

Voltage sags are caused by motor starting, short

circuits and fast reclosing of circuit breakers.

Voltage sags normally do not cause equipment

damage but can easily disrupt the operation ofsensitive loads such as electronic adjustable speed

drives (ASDs) [l]. A sever voltage sag can be

defined as one that falls below 85% of rated

voltage. Power quality surveys are a common

practice and frequently appear in the literature

[1,2,5]. According to these surveys, voltage sags

are the main cause of disturbances.

Time

Fig. 1 (a) Typical voltage sag on one phase, (b ) dc-link voltagevo.

For example, in the survey reported in [3], 68% of

the disturbances registered were voltage sags, and

were the only cause of production loss. This loss

was caused by voltage drops of more than 13% of

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rated voltage and a durai.ion o f more than 8.3ms - 1 D(112 cycle). Reference [2 i states that a little more

than 62 % of the disturbances recorded were voltage

sags with a duration of less than half a second (30cycles). A recent study (17-month period) [ l ]

conducted at two industrial site!; with ASDs, it was

concluded that voltage sags with a duration of 12

cycles or more and lower than 20% voltage drop

will trip out the ASD irivo1vt:d in a continuous

process. comparinghis data with the

curve" published in [1,4] estatilishes that modern

Fig. !Conventional boost topology design to provide ride-

thro'lgh for "Itage sags 1

ASDs appear to be more sensitive than data

processing equipment.

brief voltage sag

may potentially cause an ASD to introduce speed

fluctuations which can damage the end product.

Further a brief voltage sag also causes a momentary

decrease in dc-link voltage biggering an undervoltage trip or result in an over current trip. Such

nuisance tripping of AS D equbment employed in

continuous-process industries contributes to loss in

revenue and can incur othcr costs.Fig, I(a) shows a typical voltage sag (one

phase) and Fig. l(b) shows the corresponding dc-

link voltage. The drop in dc-link voltage exceeds

the trip-level in most ASDs and is the frequent

cause for nuisance tripping. Fig. 2 shows a boost

topology to maintain tht: dc-l ink voltage under

voltage sags [8]. Upon thc occurrence of a voltage

sag the IGBT (Fig. 2) is iurned ordoff to maintain

the dc-link voltage Vo at near rated condition. The

disadvantages of this approach (Fig. 2) are,

(a) Diode 'D' is in the series path of power

flow.

(b) Inductor 'L' s essential. is bulky and is in

the series path of power flow. Also L

carries high frequency current during the

boost mode when voltage sags occur.

Other methods to pro\ ide ride-through consists

of

(a) Motor generator sets, [ l 7,lI].(b) Flywheel energy storage I7.101

(c) Super conductor magnetic energy storage

A11 of the above options are prohibitively

expensive.

In textile and paper mills

(SMES) [7,10].

In response to thest: concerns, this paper

examines a low cost approach to improve the

performance of an ASD under voltage sags and

short-term power interruptions.

''he integrated boost converter approach (Fig.

4) c-mploys three additional diodes, inductor Lalong with the existing dynamic braking IGBT

harcware, (a standard component in a ASD) to

prolide for ride-through. Upon detection of a

voltige sag, the IGBT Qdb (Fig. 4) is suitably

controlled. Diodes D4 ,Dg, D2 and D7, Ds, Dg and

IGBT Qdb now operate in boost mode and the dc-

link voltage is maintained at its rated value. This

metitod of control provides ride-through for most

common voltage sag conditions. Fig. 4 also shows

an option to the proposed topology with the

addition of Dlo and E. This enables ride-through

undc:r short-term power interruptions also.

'('he proposed approach has the following

advi mtages:

(i) I ow cost, due to minimal additional hardware

(ii) Yo power semiconductor components in the

(iii) The proposed modification can be easily

i nd control.

inain power flow path of the ASD.

integrated into a standard ASD.

The proposed integrated boost converter

appioach [12] is connected in shunt and its VA

ratirg is a Fraction of the ASD rating. Analysis,

design and simulation results are discussed in the

pap1.r. Experimental results on a 460V, 10 hp ASD

equipment subjected to a variety of voltage sags in

a 1.1boratory is presented to demonstrate the

effe, tiveness of the proposed system.

2. INTEGRATED BOOST CONVERTERAPPROACH TO IMPROVE RIDE-

THROlJGH PERFORMANCE.The integrated boost converter approach

consists of a low cost modification (addition of

threib diodes. D7, Dg, Ds, and an inductor L, Fig. 4)

to ti e front-end rectifier topology in a commercial

ASK). These diodes supply a rectified output

voltirge to the boost converter consisting of the

induztor (L). IGBT (Qdb) and diode Ddb.As it was

17



pointed out earlier, IGBT (Qdb) ;tnd diode (Ddb) are

already available in a commerci;il ASD as a part of

the dynamic braking feature. ?'he IGBT (Qdb) is

turned o d o f fat a constant switcl ling frequency (fJ.

The switch duty cycle is varied in the event of a

voltage sag via feedback. During the off time

energy stored in the inductor (I.) is transferred to

the dc-link.

and an energy

source E such as batteries, call be added as an

option to provide ride-through of the connected

load under short-term power interruptions. The

main advantage of the integratcd boost converter

Further, in Fig. 4 diode

Three-phase Three-phase Proposed ride-through

electric utility diode rectifie r topology fo r voltage

srgslshort term interruptions.

additional components are only a fraction of ASD

rating.

DC-Link PWM

Inverter

- Iapproach is the absence of additional power

semiconductor in the main powe c flow path and its

low cost features. Further, the ratings of theFig, adjustable speed drive (ASD)

Induction

Motor

Load

Input voltageI

ense fo r sags

and short term

interruptionsfeedback

Fig. 4 Proposed Integrated Boost Co nv ere r approach with an optional diode Dlo and energy storage E for short-term power interruptions

W I .

3. ANALYSIS OF THE PROPOSED

APPROACH.Fig. 4 illustrates the proposal approach. In Fig.

4, diodes D7, D8, D9 and D4, D6, D2 form an

additional three-phase rectifier bi idge. The inductor

L along with Qdb and Ddb form a boost converter.

Upon the detection of an input voltage sag, control

of IGBT Qdb is initiated. Hence. during a sag the

dc-link power is supplied by the I ectifier diodes D7,

D8, D9, D4, Dg, D2 and the boosi converter (L , Qdb

and Ddb). In this section, analy:.is is presented to

limit the peak inductor currcnt and calculate

component ratings.

3.1 Peak Inductor Current.

Under voltage sag the IGBT Qdb is turned odoff

with a duty ratio 'D' adjusted in closed loop.

Assuming high switching frequency, discontinuous

operation of the boost stage

to nL L =L Jz vu*, (1)

Where,

VIL,sss RMS line to line voltage under voltage sag

condition.

IL,peakPeak inductor current of L.

t - On time of IGBTQdb.

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Equation (1 ) can be modified a:;

and

L = i * v u , s a g * D m m (3)

f T * L p r t r k

Where,

switch.

stage.

D - Maximum duty cj.cle of the boost

f, - Switching frequency of the boost

For a given , D,,,, f, and ILpeak,theinductor ‘L‘ alue can be computed. Also IL,peaks

the peak current rating of the IGB?’Qdb andDdb.

3.2 Design Example.

a 460V , 10kW, ASD system.

In this section a design example is illustrated for

Assuming:

vu,,,,,,line to line rms voltage) =46QV

4, (output power) = l o kw .

v, (dc-link voltage) = 1.35*v, = 620v

I , (dc-linkcurrent) - 0 -17 39Amp

I, (input line current rms)= - * I = 1 5 A

P

v,,

t oAssuming I L ,pea l = 2 * I = ~ O A

and a switching frequency of

f , = 1O.OkHz,Dmax 0.5 , W i have from

equation (3 )

L=0.542mH (4)

4. SIMULATION RESULTS

Simulation of the ASD system with the

integrated boost converter approa:h is performed

on PSPICE. An ideal switch was used in place of

the IGBT, Qdb. Fig. 5 shows the results. Fig. 5 fa)

illustrates the occurrence of the sag on phase A line

to neutral voltage at 0.5 sec. A reduction in voltage

magnitude of 50 To lasting 30 cycles is shown. Fig.

5 (b) shows the dc-link voltage before and during

the occurrence of the sag. Notice that the boost

module is successful in maintaining the dc-link

voltage: at the required dc-level. Fig. ,5 (c) to (e)

show the input line currents. Line current

magnitude in phase ‘b’ and ‘c’ increase as phase A

is experiencing a sag.

5 0 ~ ) ;

Voltage sag ma ni tude = 50 Rof rated l i n e 10 neutral voltaae

I

I

I& 1.55 1.65 0.75 1.15 1.95 1.85

U I ( L I ) (11

line

Fig. 5 Simulation results of the proposed approach (Fig. 4), (a)

voltage sag on phase ‘a’ line to neutral voltag e, (b) dc-link

voltage, 1.c) line current i,, (d) line current ib, (e) line curre nt i,.

5. EXPERIMENTAL RESULTS.

A 10 hp, 460 V commercially ASD was

modified with the integrated boost converter

approach (Fig, 4). Fortunately, sufficient room was

available on the heat sink for the placement of

diQdesD7, D8,Dg. he dynamic braking IGBT Qdb

gating signals were re-routed to a sag correction

control hardware shown in Fig. 4. A three phase

programmable ac power source (480V, 54 kVA)

was employed to create a sag on phase A when the

ASD was powered.

Figs. 6(a) and 6(b) show the experimental

performance of the ASD under voltage sag when

the proposed boost module was dis-enabled. Notice

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the reduction in the dc-link vo1t;ige during the sag.

Also from Fig. 6(b), notice the line current i,

collapses to zero during the sag an phase A due to

the reverse bias of the diodes in phase ‘a’.Fig. 7 shows experimental results for the

performance of the ASD under a 50 % voltage sag.

The top trace in Fig. 7 shows thz compensation of

the dc-link voltage on the event 13f the voltage sag,

(middle trace in Fig. 7), with a duration of 0.500

sec (30 cycles). The dc-link voltage was

compensated by the ride-through approach by

operating the integrated boost c( nverter during the

sag. That is, the IGBT (Qdb) is switched at a

switching frequency of 10 kHz t’.)operate the boost

module in Fig. 4. Also, the 1ou;er trace in Fig. 7

shows the boost inductor current IL on the turn

o d o f fconditions.

Tek pIIw l.OOkS/s 17 8 AcqsE -T-- ___._.._I. . . , . . . . , . , . . , . . . , . . . . , . . , . , . -

‘ 1

(b)Fig. 6 Experimental results on an ASI.? for a voltage sag on

phase ‘a’ without the integrated boost converter approach.(a ) 1+ oltage sag on phase ‘a’, 23 c-link voltage, (b) 1 -+

line curr ent i,, 24 ine current it,. 3+ .ne current i,.

1*

1

Fig. 7 Experimental results for a 50 % voltage sag magnitudewith the integrated boost converter approach 1+ oltage sag

on phase ‘a’, 2.1 dc-link voltage with an increased IGBT duty

cycle 4 -+boost inducto r curre nt iL,

6. CONCLUSIONS.In this paper a low cost approach to improve the

performance of an ASD under voltage sag and

short-term power interruptions has been proposed.

The approach requires the addition of only three

additional diodes and minimal control

modifications. Simulations and experimental results

demonstrate performance improvements.

AKNOWLEGMENTThe author, J.L. Du rh G6 me z would like to

recognize to CONACYT (Consejo Nacional de

Ciencia y Tecnologia ) Mexico and ITCH (Instituto

Tecnoldgico de Chihuahua) Mexico for the support

given during his doctoral studies and research at

Texas A&M University. Also ASD equipment

donation from Toshiba industrial drives, Houston,

TX is acknowledged.

REFERENCES

[ l ] H. G. Sarmiento, E. Estrada, “A voltage sag

study in an industry with adjustable speed

dr ives”, IEEE Industry Applications Magazine,

JanuaryEebruary 1996.

[2] Wandell W. Carter, “Control ofp ow er qual i ty

in modern industry”, IEEE Annual Textile

Industry Technical Conference, Cat.

[3] J. E. Flory, et al, “The electrical utility-

industrial user partnership in solving power

quality problems”, IEEE Transactions on Power

Systems, vol. 5, no. 3 (August 1990), pp 878-

886.

#89CH2697-1, 1989, pp. 11/1-4.

20



[4] V..E.Wagner, IEEE Recomme ided practice foremergency 3&d itindby power for commercial

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[ 5 ] V. E Wagner, A. A. Andreshak and J. P.

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[6] M. H. J. Bollen, “Voliagc: sags: effects,

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[7] A. Braz, P. ::lofmann, R. Miiuro, C. J . Melhorn,

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customers on underground networks”, Industrial

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[8] EPRI Powcr Electronics Application Center,

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. > I , . . .

446-1987).

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[9] EPRI Power Electronics Application Center,

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[ lo N. S . , Tunaboylu, E. R. , Collins, Jr . and S .

Y., Middlekauff, “Ride-through issues for dc

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<howmil<, “Voltage sag ride-through fo r

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>rive under voltage sags and short-term power

nterrupt oris", US. patent pending, Texas

\&M University System.

[I 1

r

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a low cost approach1 to improve the performance

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