Variateur de fréquence moyenne tension dans la station de production d'eau potable de Jilin
2010-1-6 11:18:18
Résumé : l’expansion de la 3^ème station de traitement d’eau de Jilin envisage 10 ans de travaux à venir. La capacité d’alimentation en eau se doit d’avoir une grande redondance. La technologie de variateur de fréquence à moyenne tension VFD peut satisfaire aux fréquents changement et ajustements d’eau dans les 10 prochaines années, maintenant l’unité économique et la praticabilité. Il réalise l’homéostasie entre la production stable et l’économie d’énergie et la protection environnementale.
Mots clés : variateur de fréquence moyenne tension VFD, économie d’énergie et protection environnementale.

Preface

With population increasing and economy developing, daily water supply of Jilin Waterworks expanded to 200000t from original 100000t to adapt development of economy society in next 10 years or longer time. 4 pumps of 355kW/10kV in adopting pump room and 3 pumps of in supplying pump room all run in fixed speed. Because of big difference of water supply in day and night, it needs to adjust valve opening. In this way, frequent start/stop brings impact which causes frequent fault of motor, pump and valve. That increases maintenance cost and brings impact on pipe. Therefore, conventional valve adjustment must result in much energy wasted. By comprehensive research, the plant decided to use Leader&Harvest VFD to control flow and constant pressure, thereby implementing energy saving emmission reduction, heightening water quality.

I.Production technics of waterworks

Please look at the process drawing:

First, choose appropriate water source and water intake, transfer water to first level pump room by pipe and add chlorin to kill alga, plant and seashell. Then transfer water to treatment system. After mix, reaction, deposition, filter and disinfection, transfer eligible water to second level pump room(water supply room) and output to city pipe networks to supply water for city life and production.

II. VFD application scheme

In order to ensure stability and security of water supply system, we choose HARSVERT-A10/045 VFD and “one drive two” scheme for water supply pump 560kW/10kV. The VFD adopts mode of “high-high”, cell-cascade multi-level structure. There are 8 power cells connecting in series in each phase. Rectifier mode of multi-level phase-shift overlap eliminates harmonic current caused by independent cell. Current waveform is improved a lot and near to sine waveform. No need to change original motor and cable, applicable to reform for existing equipment. Adopting pump 355kW/10kV chooses HARSVERT-A10/030 VFD and “one drive one” scheme. With operation experience of half year, one variable frequency system can satisfy requirement of water intake.

Following is principle of “one drive two”. It is composed of medium voltage isolators QS1-QS6. QS1and QS4, QS2 and QS5 are electrical interlock; QS2 and QS3, QS5 and QS6 are mechanical interlock.

          One drive two bypass drawing

If two power sources work at the same time, M1 works in variable frequency state, M2 works in direct on line state, QS3,QS4 and QS5 open; QS1, QS2 and QS6 close; when M2 works in variable frequency state, M1 works in direct on line state, QS1, QS2 and QS6 open，QS3, QS4 and QS5 close. When user makes maintenance for VFD, QS3 and QS6 can be in any state, other isolators all open, 2 loads can work in direct on line state at the same time; when one power source needs maintenance, opening/closing isolator can make any motor work in variable frequency state.

III. Principle of constant pressure water supply

Because this is a new expansion project, water demand requires to be adjusted and measured exactly during actual operation. We decide to adopt constant pressure system. Its principle is in drawing. Pressure transmitter produced by Germany E H is in outlet of pump, Vi is set value of constant pressure, water supply pressure V is output. These compose close loop control system. PLC inside VFD collects water supply pressure V and makes comparison & operation with given value Vi. Through PID regulation, PLC converts the result into signal and sends the signal to VFD, till reaching given value Vi. No matter how flow changes, water supply pressure V keeps close to given value Vi all the time.

Principle of constant pressure

In early term of VFD operation, water supply system adopts control mode of constant pressure close loop. We find frequency curve shows teeth waveform and vibration scope is big. We make regulation for PID coefficient. Change P from 1 to 0.01, change I from 4 to 20. After the regulation, the curve is smooth with small vibration scope. VFD adjusts PID parameter according to deviation. When operation parameter is far from object parameter, VFD accelerates adjustment. With deviation closing gradually, trace amplitude reduces gradually. When they are equal approximately, system reaches a homeostasis and keeps constant pressure.

IV. VFD operation

Adopting pump works in open loop. User adjusts frequency manually according to clean water level. Generally, it is in 38Hz-45Hz.

Water supply pump works in close loop. Parameters are following:

Through measuring, we find pump does not output water when VFD runs in 37Hz downward. So we limit the minimum frequency at 38Hz. During daily operation, we start an extra line frequency machine at 4:00 AM. When VFD frequency reaches 39Hz, we close the line frequency machine and make only one VFD run. Start the line frequency machine again at 4:00 PM which is peak time of water demand. This method can satisfy output pressure and save much energy.

V. Application effect

After reform with medium voltage VFD, motor runs stably in effective adjustment scope. In contrast with original operation in direct on line state, soft start makes start current smaller. There is no impact on machine. That prolongs equipment lifetime and reduces maintenance cost.

Effective close loop control makes pressure adjustment smooth, no vibration of speed. Motor links with multifunction valve. As long as given frequency inputs, multifunction valve starts automatically, which makes start/stop of machine easy.

After using VFD, power factor rises to 0.96. With kinds of loads, current on power grid side reduces a lot. Reactive current of motor is compensated directly by VFD.

In term of energy saving, we make a measure. In the same working condition, energy saving rate of single machine is 32%. The high energy saving rate and reasonable start/stop operation saves 1000000yuan(USD147000) in a year.

Medium voltage VFD has been running consecutively for two years since it put into service. VFD guarantees plant to supply water normally. HARSVERT-A VFD has advantages of free maintenance. It just needs to change filter of door. VFD lowers damage on pipe seal, prolongs equipment lifetime and reduces maintenance. It also heightens system automatization level and saves much energy.