7/27/2019 ENERGY SAVINGS WITH VARIABLE SPEED DRIVES.pdf

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ENERGY SAVINGS WITH VARIABLE SPEEDDRIVES

K M Pauwels

Energy auditor, Laborelec, Industrial Applications,Belgium

SUMMARY

Ths paper focuses on the econo mic benefits that can beobtained by replacing mechanical flow control ofpumps and fans with variable speed drives.Firstly, different topologies of variable speed drives arediscussed: rectifier topologies (thyristor, IGBT) as wellas the control of invertors with Pulse WidthModu lation. A sh ort overview of the advantages for thedifferent topologies is given.The mo st important restrictions are also mentioned.The core of the paper consists of three case studies,which are taken from our energy consulting experiencein Belgian industry.Case study 1 discusses VSD installation on thecirculation pumps of the electrocoat bath in a carassembly factory. Before, flow was controlled by valveregulation. Installing a variable speed dnve insteadgenerated a yearly saving of 176.400 kWh for onepump. The payback time amounts to approximately 14months.Case study 2 explains the economic benefits of V SD onlubricated screw compressors.Depending on the load profile, energy savings up to 41Y an be obtained by applying frequency regulationinstead of loadu nload regulation.Case study 3 shows the case of a convertor in astainless steel factory.Two 800 kW fans blow air into the convertor.The air flow is controlled by a valve.Replaclng the valve with a VSD implies a yearly savingof4.188.277 k w h .

TECHNOLOGY

A variable speed dnve (VSD) essentially consists oftwo main parts: rectifier and invertor (fig. 1).

Fig. - oncept of a VSD

Rectifier

Nowadays, two main topologies are used for mediumpower rectifier units:

diode rectifiers1

The rectifjmg unit consists of a diode bridge. Thismeans that .the DC link voltage is fully dependingon the AC supply voltage. Some typicalcharacteristics of diode bridges are:- Good power factor: phase shift betweenvoltage and current is very low.- No control needed.- Introduction of line-side harmonics: a diodebridge is a non-linear load. This means that anon-sinusoidal current is taken from thefeeding line. Current harmonics cause voltageharmonics. These voltage harmonics candisturb nearby charges.

IGBT rectifiersTo eliminate the disadvantages of diode (andthyristor) rectifiers, new solutions are developed.

CIRED2001 18-21 June 2001 Conferen ce Publication No 482 2 1

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These solutions are all based on the p rinciple thatthe diode bridge is substituted by an inverterconsisting of IGBT components (Fig. 2). IGBTstands for Insulated Gate Bipolar Transistor.Invertors with IGBT rectifiers are called ActiveFront End M E ) invertors.Some features of Active Fron t Ends are:--

undamental sinewinr.

good power factor, even better then dioderectifier bridgerectifier is con trolled by m eans of Pulse W idthModulation (PWM )PWM technology redu ces harmonic distortionto a very low level-

Fig -Pulse Width Modulation

Fig. 2 - ctivefiont end

Invertor

The invertor uses the DC voltage from the DC bus tocreate an AC voltage with variable frequency (andamplitude).Nowadays, all invertors are equiped with IGBT (orsimilar) components. PWM control (Pulse WidthModulation) is widely used for control of the IGBTswitches. PWM control consists in rapidly switching onand off the IGBT switches in such a way that pulseswith variable width constitute a sinusoidal waveform(Fig. 3).

RESTRICTIONS

In spite of all the advantages, some problems can occurwith Variable Speed Drives. The problems are allrelated to a deteriorated power quality.

Production of harmonics

Power e lectronics always take a distorted current fromthe feeding line. This causes line-side currentharmonics. Some problems that may occur with line-side harmonics are:Harmonics cause supplementary heating intransformers, cables, . This fastens the agingprocess.Nearby cap acitor banks can be damaged severelyby harmonics.Older types of thyr istor control (based onswitching at z ero voltag e) can be disturbed....

Next to line-side harmonics, motor-side harmon ics arealso generated. One of the big problems caused bymotor-side harmonics is overvoltag e: overvoltageappears due to reflections of the voltage waveform atboth ends of th e transm ission cable. Transmission linetheory tells us that matching impedances for allcomponents are required to reduce reflection as muchas possible.

Sensitivity to voltage dips and shor t interruptionsAn invertor basically consists of a rectifjmg and aninverting unit, linked by a DC-circuit (Fig. 1). In theDC-circu it, the cap acitor is the only energy storingelement. Even with a big capacity, the sto red energy islow.

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Example: the stored energy in a 10 mF capacitor (600 Measurement resultsV D C ) i s 001,Oo = 1800J = 0 0005kWh .Furthermore, the capacitor in the DC-link is limited toavoid too big inrush currents at switching on.Most important consequence of the low capacity tostore energy.is the fact that invertors are very sensitiveto short interruptions and voltage dips. In the worstcase, the control of the V SD sim ply blocks. Nowadays,a lot of constructors provide different kinds oftechniques that improve this weak point. However,none of this solutions guarantees complete protection toshort interruptions and voltage dips.

An ultrasonic flow rate measurement (case of valveregulation) revealed a flow rate of 194 m3/h.Taking into account the nominal flow rate of 280 mh,the frequency of the drive needed to be set to 34.8 Hz(nominal frequency = 50 Hz) in order to obtain therequired flow rate.In the case of valve regulation, the absorbed poweramounts to 6 kW.In the case of frequency regulation, the absorbed poweramounts to 15 kW.

CASE STUDY 1:CIRCULATION PUMPGain - payback time

Short description of the original set-up)The pump functions continuously during 50 weeks ayear.The use of VSD thus implies a yearly saving of:

+,car assembly factory paints the body works by meansof an electrocoat bath.The paint is attached to the body work by applying aTo maintain temperature and concentration of the paintin the bath, the paint is continuously circulated bypumps, even when theres no production. For thls case, the payback time (investment +Flow regulation is effectuated by means of valves.

DC voltage to the body work. 36 - 15)kW x 24 h/d x 7 dlw x 50 W Y = 176.400kWWy.

installation) amounts to approximately 14 months.

Measurements

-To be able to know the energy sav ing with VariableSpeed Drive, the following parameters have to bemeasured:1 flow rate: In order to have a reliable comparisonbetween the two regulation methods, the flow hasto be the same in the two cases.Power: for the two regulation methods, the power

has to be measured.2)

The measurements were limited to one circulationP-P.

Characteristics of the pump

Nominal power: 45 kWNominal flow rate: 280 m 3h

CASE STUDY 2: LUBRICATED SCREWCOMPRESSORS

Introduction

In industry, lubricated screw compressors are widelyused.For these compressors, three regulation methods exist:I Loadhnload regulationAir flow is regulated by opening and closing theinlet vane. During the time that the vane is closed,the motor is running unloaded.2 Valve regulationAir flow is regulated by a valve that can be put inall positions.3) Frequency regulationAir flow is regulated by controlling the speed ofthe electric motor.To compare the energy consumption for the threemethods, measurements were carried out.

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To do so, three compressors of the same installedpower were taken from one constructor, each with adifferent flow regulation.The pow er of the comp ressors is 45 kW.

Load profiles

Each of the compressors has been tested for threedifferent load profiles:1) profile A: compressed air consumption all day,even during w eekends (fig. 4)2) profile B: comp ressed air consump tion all day, notduring w eekends (fig. 5)3) profile C: compressed air consumption duringworking hours (5 am to 20 pm), not duringweekends (fig. 6 )

- m o , t i l l i l l

weekly profile Sunday o Saturday)

Fig 4 --profileA

120 II

Weekly prof i Sundi

Fig 5 -profile B

II

to Saturday)

Weekly profile Sunday to Saturday)

Fig. 6 -profile C

These profiles have been simulated by using controlvalves of different flow rate connected to a compressedair vessel 2000 1).The control was made by PLC fig.7 .

Fig 7 -pictur e of the vessel + valvesMeasurements

For each of the three compressors, the three loadprofiles were simulated during one week.This makes a total measuring time of 9 weeks 3compressors- 3 profiles).The power (1/4 h average) was measured andregistered for each of the 9 situations.

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Results

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Measurements

Figure 8gives an overview of the o btained results:Depending on the load profile, a valve regulationgives a saving from 3 9 up to 8,8 YO ncomparison with loadunload regulation. Theaverage saving amounts to 8, l .Depending on the load profile, a variable speeddrive gives a saving from 15,7 up t o 41, l YO ncomparison with loadunload regulation. Theaverage saving amounts to 37,2 .

During one week, the position of the valve of 1 fan wasregistered.The opening of the valve appeared to be 60 onaverage.

Software simulation

Simulation on software (fig. 9) gives a yearly saving of4.188.277 kW h (for one fan).6

- *- Taking into account an investment of 105.000 euro, thepayback time amounts to 8 months.Including the estimated installation cost, the paybacktime comes to 14 months.c 5-c

P m

8 oF5: o

ISE

.sprofile A profile B profile C average

Fig. 8 verview savings

CASE STUDY

Fig 9 - software simulation 8 kWfanntroduct ion

A large stainless steel manufacturer uses an AC arcfurnace to melt the steel.The molten steel goes from the arc furnace in aconvertor to reduce the percentage of carbon .Essentially, air is blown into the convertor to oxidisethe carbon.Two 800 kW fans blow the air into the convertor.The air flow isregulated by valves.

CONCLUSIONS

Invertor technology is improving continuously: moreperformant components in combination with moreperformant control increase accuracy and speed ofcontrol, and reduces current harmonics.The three case studies taken from industry show clearlythat installing variable speed drives generates importantenergy savings. Screw compressors, centrifugal pumpsand fans with mechanical flow regulation are especiallysuited for variable speed regulation.However, one has to b e aware of the fact that electronicspeed regulation also implies some restrictions,especially in relation with deteriorated power quality.Line-side as well as motor-side harmonics are created,invertors are in the mean time very sensitive to shortinterruptions or voltage dips.

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