CASE STUDY

Conversion of a Fixed speed drive

to an Adjustable speed drive

using a Slip Energy Recovery System

Author : Reinhold A. Errath Engineering Manager

ABB lndustrie AG CH 5404 Baden / Diittwil

Switzerland

Coauthor: Juan Gonzales Santillana Director de Fabrica

Hisalba Spain

ABSTRACT

A recently delivered drive system for an existing fixed speed Fan provides an example how with a Slip Energy Recovery [SER] drive system the change from a fixed speed to a variable speed drive was realized. This paper describes the reason for the change, the function and also the economical advantages of an adjustable speed drive in this application with the SER drive configuration. This paper also states and explains why this type of drive system is attractive. SER Drive Systems are accompanied by an availability almost unreachable using other configurations.

0-7803-3941 -W98 $1 0.00 0 1998 IEEE-IAS 107

1. INTRODUCTION

Nowadays, newly installed Fans in the Cement Industry are mostly equipped with adjustable speed drives. The specification and use of speed controllable Fans is required by process conditions and the pressing need to use more efficient drive technology. Better process control with lower maintenance costs is the result of using adjustable speed drives. Nevertheless, not only pure economical reasons are the source of these considerations, but also the increasing awareness for the fulfillment of environmental basic conditions influences all the decisions. For older installations like the present case in the Plant HISALBA in Spain, where an inefficient, energy consuming damper technology was still used. The energy consumption of the Fan drive and its auxiliary drives was 10.65 kwh/t raw meal.

Due to new operating approaches with a reduction of the operating air flow, the energy consumption could be reduced. However, there was still plenty of energy saving potential A Eliminating the damper control and substituting an adjustable speed

drive system. B Changing the existing impeller with an efficiency of 65% to a high

efficiency impeller with an efficiency of 72 %, but keeping the impeller housing and the fixed speed drive Changing the Complete impeller including the housing to a high efficiency system with an efficiency of 82%, and also modify the fix speed drive to a variable speed drive.

C

It was decided to make the change in several steps, beginning first with step A.

But how to change the damper technology to the efficient adjustable speed drive technology? Several approaches are possible, Hisalba decided for two of those, which were then currently studied.

1. Installing a complete new drive system including a converter transformer, the drive and a new Squirrel cage induction motor.

2. Remaining with the existing wound rotor motor, the existing secondary resistance starter as well as the control for this starter and only purchase the drive.

Hisalba took many aspects and questions into consideration and many wishes have been formulated and had to be respected, but the following three goals had to be fulfilled

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2.

0 Reaching a high degree of availability and reliability by changing the drive system.

0 Archive the amount of energy saving expected 0 Choosing the most cost-effective solution

These have been the questions when Hisalba took the decision to evaluate the proper drive system.

- ORIGINAL CONFIGURATION

Twenty years ago in 1976 the Hisalba Cement, a 3500 tlday plant was built. The plant was constructed based on the state of the art technology used at that time. The Fan with 4400 HP and 750 rpm was too big in order to be powered with reasonable costs by a DC drive. The same reason was applicable to other adjustable drive systems in discussion like the LCI. The cost versus benefit relationship based on the low energy prices was not in favor to the adjustable speed drive solution at that time.

A *4400 HP wound rotor motor with a liquid starter purely for starting purposes was installed. For the operation itself the drive was running at the maximum speed. The air flow based on the process requirements was regulated by input dampers.

2.1 - Dctsinn data Fan and Drive systems

Mechanical details

figure 1. Mechanical configuration

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2.1.1 Process parameters

Design gas flow capacity [m3/hJ Minimum gas flow capacity [m /h] Maximum production gas flow capacity [m3/h] Minimum content of dust [g/m Maximum content of dust [g/m ] Minimum production [t/h] Maximum production [tlh] Operating range [%]

5

2.1.2 Electrical parameters

Drive shaft power [HP] Motor basic speed [rev/min] Operational range [revlmin] Motor type Secondary Starter

525 000 387 000 495 000 600 650 240 300 70 to 96

4400 750 actual 520 to 700 Wound rotor Liquid Starter type

2.2 Electrical configuration

The electrical design configuration for the fixed speed drive is shown in Figure 2. The fixed speed drive consists of a wound rotor motor, a liquid resistance starter and a control system.

The purpose of the secondary starter is to limit the starting current to an acceptable level. When the starting period is finished and the maximum speed is achieved, the secondary starter is short circuited. From this condition on, the motor behaves like a Squirrel cage induction motor.

"Breaker

Wnund rmlmmotor

Figure 2. Electrical configuration of a wound rotor motor with a secondary starter

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2.3 Flow control

The flow from the Fan was controlled by a damper control. Figure 3. shows the power consumption as a function of the flow through the fan. Dalmper control generally results in bad efficiency and high power consumption.

In ,a system where the pressure is proportional to the square of the flow, fan efkiency remains constant at all speeds, and the power consumed by the fan closely follows the fan law. This law states that the torque is a function of the square and Wed.

figim 3. Power consumption in comparison to the gas flow, when the flow is regulated with a damper

The plant was working properly without any technical problem. Nevertheless, pressing needs to improve the cost situation in the plant has resulted initially into better process control as well as improving and optimizing the thermal energy consumption and searching for substantial electrical energy saving. However, not only the economical point of view has been a source of the considerations, but also the increasing awareness for the fulfillment of environmental basic conditions has driven the decision. The equation

energy saving results in better environment

is valid and is in the present case free of charge. Beside savings in thermal process, a substantial source of energy saving was predicted in the change of the! fixed speed fan to an adjustable speed drive.

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3.

3.1 Proceedinas

0 First, it was observed at what speed the operation was realized and for how many hours per year

0 It was noted at what closing position the damper was usually operating and the resulting gas flow which was achieved The respective power drawn from the network was measured The power factor in the feeder line was measured The reactive power was measured

o The energy consumption of the Fan operation per mT of raw meal production was calculated

flow I power I damper I hours I

Table 1. Result of the investigation, in an operation with damper control

3.2 Prediction

Already at the first glance, the prediction using an adjustable speed drive system and regulating the flow with the speed instead of the damper was indicating a very positive effect. Considering the measured values the possible potential of energy saving was calculated. Figure 4. shows the relationship between the air flow and the speed with the damper completely open.

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Power demand [%] 100

90

80

70

60

60

40

30

20

10

0 0 Z R Z % ! % S E Z 8 8

Flow ate fi]

FigiUe 4 Power consumption in comparison to the gas flow, when the flow is regulated with the speed of the drive

The measurements, made with the fixed speed drive have been taken as a base for the calculation. The fan law is again the reference for the calculation. This law state that the torque is a function of the square and the power is a function of the cube of the speed. The speed is proportional to the flow. The Table 2. shows what power is required in order to provide the flow. The figures in the table are calculated numbers and serve to calculate the expected energy saving.

1 speed I flow I power I hours I [per hours]

495 000 80 442 000 2600 70 387 ooo 1800

Tahlle 2. Result of the calculation flow regulated with adjustable speed versus power with the damper in open position

3.3 mlculation of the predicted enerw saving

The predicted energy saving is shown in Figure 5. The theoretical energy saving is the difference between the energy required with operation in darnper control 7) and the operation with adjustable speed drive 2).

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Figure 5

Flow power

Predicted energy saving

power hours power production

Joining the results of both tables together, the theoretical energy saving can be quantified by taking into account the different operational points over a period of one year. Furthermore, the energy price plays a major role in the ROI calculation.

[m31h] 495000 442000 387000

Local Electrical power costs 0.05 US$ / kWh

PA1 [kwl [peryeafl WWhI [kWhh 96 2735 1500 41 02 9.1 1 92 262 1 2600 681 4 9.70 88 2507 1800 451 2 10.45

Production energy calculation

L I I I I I I

Table 3. Damper operation for one year

Sum of energy used [Mwh] Average production energie of the Fan [kwh/t]

15 428 9.74

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Table 4. Adjusfable speed drive operafion for one year

Sum of energy used [Mwh] Average production energy of the Fan [kWh/t]

12 746 8.00

Aflter all the calculations, we want to remind how the energy consumption for raw meal did develop. Without any modifications the energy consumption for the Fan and its auxiliary drives was 10.65 k W t . A reduction of the air flow demanded by the process did reduce the overall production energie to 9.89 kVJh/t. The expected production energy for the SER driven fan and its additional drives is 8.16 kwh/h.

3.4 Comparison between the 2 operation modes of the Fan

Average production energy with damper operation Average production energy with SER operation

Difference in production energy

9.89 [kVVh/t]

for one year 8.16 [ k W t ] 1.73 [kWh/t]

A f'inal cost calculation will clarify the results given above. With a production of 1 570 000 t per year, the saved money will be reasonable.

The predicted amount of cost savings did justify a change of the energy consuming, damper operated fixed speed drive to an adjustable speed drive. This without any doubts for saving in equivalent magnitude. But how should the change be realized?

Taking out the existing installation and applying a complete new drive system with a converter transformer, Drive and motor,

0 Keep the existing drive, a wound rotor motor and a secondary starter and (adapt a SER Slip Recovery Drive system, a feedback transformer and a transfer switch to it? Many aspects had to be regarded in this evaluation ;and comparing process.

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Investment costs

figure 6.

0 50 100 150 200 250 300 350 400 450 500

ur(ts

Investment cost for the different concepts

Installation costs

Motor

Transformer

0 2 4 6 8 10 12 14 16 18 20

units

Figure 7. Installation costs of the different concepts

N E W DRIVE SER

Installation time Pays1 20 8

Commissioning time [Days1 3 8

5YPSS no Yes €xpected live time r/ea=l 25 25

Efficiency 94 93.5

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Analyzing the data that is given above. Hisalba came to the conclusion to remain with the existing motor, the existing secondary starter and the existing control for the starter and install a new SER system, a change over switch arid a feed back transformer.

The justification for the decision to power the Fan with a SER system, taken from the client after this detailed evaluation process, is to be found in the following facts:

0 the lower investment costs compared to other drive systems the possibility to operate the mill in fixed speed mode, if the SER system should be out of order

0 thus having a utmost system availability

I C a l c u l a t e d ROI t i m e : about 2 . 4 y e a r s

The installation was realized in the summer of 1996. Since that time the drive has been working satisfactorily.

Aflter one year operation Hisalba wanted to know whether the change from a fixed speed drive with damper operation to an adjustable speed drive system did really fulfill the expectations.

Ac:cording to this, measurement on the relevant parameters have been realized once again.

Thie results were similar to the predictions in terms of energy saving, but the results in the installation costs did show a different picture. The investment for installation costs are higher than the predicted costs.

r Archived ROI t i m e : about 3 y e a r s

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4. CHANGE THE FIXED SPEED DRIVE TO ADJUSTABLE SPEED DRIVE

4.1 How a SER system functions

The operating speed range of Fans is generally small. The effective utilization of the adjustable speed range usually is limited to values among 50 to 96% of the drive’s nominal speed, in the present case 70 to 96%. All drive systems, apart from the Slip Energy Recovery system, are characterized by the fact that the converter transformer and the converter must be designed for 100% of the motor power. Only this configuration can offer a fixed and variable speed control without additional investment. With all other configurations this is either not possible, or can only be realized with a significant investment.

The drive configuration is unique since both the transformer power and the converter power need only be designed to deliver slip power. It is the only configuration in which the physics provides full bypass operation in the fixed speed mode. Large fans have used this configuration successfully over decades, in new installations as well as in revamping jobs, as this paper shows in Hisalba, Carboneras Spain.

4.2 SER Drives in General

The Slip Energy Recovery drive system is an Adjustable Speed Drive system that will operate usually in a limited operating range. The rated power of the drive is ideally from 2 MW up to 20 MW.

The drive does support both operating modes. Therefore, it either can be used in fixed speed or in variable speed operation, the mode can be pre- selected by a selector switch. The drive system with a wound rotor motor, will always be started with a secondary resistance starter. With reaching the minimum speed of the operating range, it will be switched over to the SER System.

Instead of burning the slip energy in the secondary starter, it will be recuperated back into the network - the slip energy is recovered.

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4.3

4.3.1

4.3.2

4.3.3

- SIER Drives As Fixed Speed Drive

The SER Drive system is equipped with a built in bypass. Operation with the built in bypass is only possible in the fixed speed mode of operation. In the fixed speed operation the motor will run at asynchronous speed.

- Mein components for fixed speed operation

MV Beaker Wound rotor motor Secondary starter

MV Breaker

Starting b rartrtor

Figlure 8. Fixed speed operation

Motor protection philosophy

The Motor is protected with the Motor protection relays mounted inside the medium voltage panel.

- Startinn procedure

1. Command “start drive” 2. The liquid starter has to be available and positioned in the starting

position, with the full resistor connected. 3. If MV Breaker available, close Breaker 4. The Motor starts 5. The liquid starter connected in accordance with the starting program 6. When maximum speed is reached, the short circuit contactor is closed 7. Drive svstem is cleared for oroduction

4.4 SER Drives as Variable Speed Drive

The SER Drive system is an adaptation to the fixed speed drive system. In addition to the fixed speed drive system a change over switch, a rectifier, a inverter and a feedback transformer have to be added.

4.3.1 Main components for variable speed operation

MV Breaker Wound rotor motor Secondary starter Change over switch Rectifier Intermediate DC link Inverter Feedback transformer

Figure 9. Single line diagram with Variable Speed Operation

4.4.2 Drive thermal and over current protection philosophy

The motor is protected with the motor protection relays which are mounted inside the MV panel. It will protect both, the thermal and the instantaneous value. The current is measured by a current transformer [CT] mounted inside the MV panel as well. The feedback transformer instantaneous value is protected with a CT mounted inside the MV panel. The thermal protection will be implemented in the SER panel. The CTs are likewise mounted inside the SER panel.

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Tihe converter and the inventer have not to be protected at all. Both are of a fuseless design. A short circuit on a thyrktor or in the DC link will not result in damage of the fhpstor or associated equipment since the thyristors are dimensioned to withstand a current 8 times the slip rated current for 700 msec. Other protective elements will trip before the fhyristors are damaged. For instance, in this time the transformer instantaneous protection will activate arid disconnect the drive from the line.

Figure 70. Drive protection single line diagram

4.4.3 S&rtina procedure

1. 2.

3. 4. 5.

6. 7. 8.

9.

Command “start drive” The liquid starter has to be available and positioned in the starting position, with the full resistor connected. SER drive system is ready Change-over switch positioned in secondary resistor position Operating mode switch fdadjustable speed, is positioned for adjustable speed. Close MV Breaker if available Motor starts Resistor steps are connected in accordance with the designed starting program As the preset transfer speed is reached, the change-over switch connects to the SER system. For a short time both systems, the SER system and the secondary resistor will be connected together to guarantee a current- free transfer phase. When the current from the SER system is built up, the secondary resistor is disconnected.

I O . The speed of the drive follows the preset value.

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Figure 11. Swifch-over process - Current behavior

4.4.4 why it works

The voltage across the rotor ( U20) v, f and the rotor hquency is inversely proportional to the slip. In other words:

when the rotor is at standstill, the highest voltage and frequency is created on the rotor output when the motor speed reaches its asynchronous level, the lowest voltage and frequency is created M Pkl

--+L m 97.0 "N

Figure 12. Relationship between the speed, the rotor frequency and the rotor voltage

Rectifier

The induced rotor voltage will be rectified by a six pulse thyristor rectifier. The Thyristor pulse firing control is synchronized to the rotor voltage. Basically the circuit is functioning like a DC drive circuit in the first quadrant. The rectified voltage is connected to a DC intermediate link.

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Converter

OC Intermediate link

Figure 13. Rectifier with Energy Flow to the intermediate link

Inverter

The inverter consists basically of a 6 pulse thyristor rectifier. The pulse firing coritrol is synchronized to the line frequency, the inverter is thus line- cornmutated. The systems takes the energy out of the DC intermediate link anti recuperate the energy into the network via the feedback transformer. The system is basically functioning like a DC drive in the third quadrant.

Figure 14. Inverter with feedback transformer SER Energy flow from DC link via Inverter via Converter transformer to network

The amount of energy that can be recuperated into the network is basically the amount that is shown in Figure 15, where an example with 89 % of rated speed is illustrated.

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4.5 Energy flows

Where does the power flow?

Figure 15,

The Figure above clearly indicates why the system is called a SER Drive system. The given data are approximate figures with a 89 % of the motor nominal speed, thus with 10 % slip.

Power flow and losses associated with slip recovery drives,

4.6 Efficiencv and Power Factor

The Power factor in SER mode operation is worse when compared to the operation in the fixed speed mode. The reason is to be found in the firing angle shifting. The larger the operating range becomes, the worse the Power Factor. In the above case the power factor is quite satisfactory because of the relatively smatl operating range.

4.7 Power Paths

Current Harmonics / Voltage distortion

The SER drive system is configured basically as two converter systems: One on the rotor side and one on the network / transformer side.

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All 6 pulse converters produce harmonics of fixed orders, the 5th, 7th, 1 1 th, 13tlh, etc.

In the case of the SER drive system, the converter on the rotor side is commutated to the rotor frequency that is inversely proportional to the speed. Thus the converter is producing current harmonics that are based on the rotor frequency. Since the rotor frequency is not a fixed value, the current harmonics are not fixed as well.

A simple example shows in more detail the relationship between current harmonics and motor speed. The rotor frequency is 5 Hz at 90 % of the motor speed.

The 5th harmonic of 5Hz will be at 25Hz, the 7th at 35 Hz, etc. In other words the rotor dependent harmonics are changing as often as the speed of the motor is changing.

When designing filter equipment, its a fact that filtering out any frequency is no problem as long as the frequency spectrum is fixed and defined. However on the rotor side the expected spectrum is neither fixed nor defined at all. The reaction of the current harmonics will be transferred from the rotor over the magnetic air gap to the stator winding and back into the network.

Tho dimensioning of a filter that should dampen the current harmonics is a significant challenge.

The Inverter side converter is line-commutated. That means that the 5th harmonic will react with 250 Hz, the 7th with 350 Hz, etc. The reaction of the current harmonics will be transferred via the transformer back to the network.

The solution is a so-called "wide-band filter" that takes into consideration the rotor effects with its varying behavior as well as the transformer effects which are "fixed.

4.8 Drclue Harmonics

As in any other drive system in the higher power range, special attention has to tie paid to the torque harmonics. Torque harmonics by themselves are not destructive. They do however become an important factor when they act as an exciting element in the range of the drives natural frequency.

Consider an ideal sinus wave form from the supply network. In addition to the torque that is produced, the drive system will retum torque harmonics. The

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origin of these torque harmonics are the current harmonics created on the rotor side, called pendulum momentum. These pendulum momentum are superimposed on the normal torque harmonics.

The following torque harmonics can be found: A) Natural harmonic of the stator current, together with the higher current

harmonic of the rotor 8) Natural harmonic of the rotor current, together with the higher current

harmonic of the stator C) Higher harmonic of the stator current, together with the higher harmonic

of the rotor currenf

The amplitude of the case A) torque is result only according to the load and does not depend on the slip. The amplitude of the case B) torque is caused by the load as well as the slip. The amplitude of the case C) torque can be neglected with higher harmonics: The higher the harmonic level, the less the developed amplitude. Therefore most attention must be paid to the lowest harmonic levels. The 5th and 7th current harmonic causes a 6th torque harmonic that reaches its maximum amplitude at 1/6 slip.

The 11 th and 13th current harmonic causes a 12th torque harmonic that reaches its maximum amplitude at 1/12 slip. With the avoidance of these torque critical speeds of the mechanical system in the range of certain slip values, inadmissible mechanical stressing is avoided.

A torsional analysis is essential for drives above 2 MW drive power. In the Hisalba installation a torsional analysis was made as well.

The result was, that the existing coupling did fulfill the requirements up to a speed of 92%, above this speed, operating with the SER system, the coupling would have to be substituted by a damping coupling with other characteristic. The goal of using a damping coupling is to transfer possible exciting frequencies close to the system natural frequency into a range where no resonance can appear.

The coupling was not changed, because the required operational conditions are not in this speed range.

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5. - SUMMARY

New specifications of the process, pressing economic needs as well as higher corisciousness of nature conservation made discovering of energy saving resources necessary.

First of all the energy consuming damper operation had to be substituted by an energy economic adjustable speed drive. In the selection of this drive two possibilities have been discussed. Either replacing the already running fNed speed drive with keeping the wound rotor motor and the secondary resistor starter untouched or apply a new drive and a new motor.

The predicted ROI has been confirmed with measurements which have been malde after the drive had been in operation for one year.

A $;ER Drive is attractive either for new applications or to replace inefficient and environmental unfriendly fixed speed drives with damper operation.

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