Preface

Cangnan Water Plant was set up in 1998. It provides water to 600000 population with daily output 100000t. There are 7 pumps, 10kV/250kW×2，380kV/160kW×2，380kV/90kW×2，380kV/55kW×1. Pumps work in direct on line. But actual water demand is 60000~70000t/day. Flow is adjusted by valve opening and frequent start/stop of pumps. In this way, it is difficult to control and it also wastes energy. According to forecast, water demand will not reach to 100000t/day. Thereby, plant decided to adopt medium voltage VFD to ensure pipe safety and utilize energy reasonably. Reform began in March, 2003. HARSVERT-A10/015 VFD put into operation in December, 2003. The application experience of 5 years proves that medium voltage VFD is feasible. Booster pump reduced energy consumption a lot. Work efficiency was heightened.

I. Theory basis

Due to factors of weather, city life and production, water demand is uneven. Flow changing affects throttle loss changing. Especially in smooth area of small lift, outlet pressure of delivery water pump station changes with flow changing obviously. Devices model are decided as per the most disadvantageous condition, max flow and related lift. In actual fact, max flow only presents quite short time every day. Pump station works in small flow mostly. Originally, close valve opening smaller to adapt flow changing. But pressure difference of outlet is wasted. Furthermore, it is difficult to manage for many pumps. Meanwhile, build area is enlarged and project cost rises. Not even in this way, can it adapt flow changing well. Technology of variable frequency can change pump speed consecutively to make flow and lift adapt flow changing. This method heightens efficiency, keeps pressure constant and saves energy.

II. Scheme

Choose 1#, 2# pump as variable frequency pump. One works and one is spare. Switch mode between two pumps is manual. Two pumps must not work in variable frequency state or in direct on line state simultaneously. Thereby, Leader&Harvest designed mechanical interlock for bypass switch, with open loop/close loop constant pressure water supply and valve interlinks.

VFD connecting with site

Medium voltage trip-off signal connects with trip-off circuit of KM1 breaker in parallel. Medium voltage allowable signal connects with close circuit of KM1 breaker in series. M1 and M1 open fully.

primary connecting of VFD and site

III. Principle of constant pressure water supply

Pressure transmitter is in outlet of pump. Vi is set value of constant pressure. Water supply pressure is V as 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 transfers 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.

In early term of VFD operation, water supply system adopts constant pressure close loop control. We find frequency curve shows teeth waveform and vibration scope is big. We make regulation for PID coefficient. Change P from 1 to 0.06, change I from 0.2 to 0.5. 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 two parameters are equal approximately, system reaches a homeostasis and keeps constant pressure.

IV. Energy saving

According to month report, before reform, power consumption of 1000t is 98kW.h; after reform it is 92kW.h. Yearly power saved is (98-92)×70×360=15120kW.h. With water increasing, system adopts mode of one variable frequency pump and one line frequency pump. Energy saving effect of this mode is more obvious. With economy developing, daily water supply is 95000t. Test result is following:

V. Other effects of VFD

1. VFD improves technics. In actual production, parameters of pump usually need to be adjusted. Spare equipment usually needs switch. Parameters are controlled by adjusting valve. Closing/opening valve manually is not only difficult with slowly speed but also shortens valve lifetime. With VFD, there is no need to adjust valve. It only needs to adjust motor speed. Speed rises from zero gradually. So there is no big start current. phenomenon of stopping valve first then start up is avoided.

2. VFD reduces maintenance. With VFD, vibration resulted from pump far from work range can be avoided. Generally, main purpose of VFD application is to reduce pump speed. Because of slow start-up and lower speed, lifetime of many components is prolonged, such as seal, bearing.

3.VFD lowers work intensity. Because variable speed system implements computer interlock control between running equipment and spare equipment, machine can implement automatic operation, related protection and fault alarm. Personnel work changes to monitor from manual work, which realizes unmanned operation. It lowers work intensity a lot, heightens production efficiency and ensures favourable operation.

4. VFD reduces impact on power grid. With VFD, system realizes soft start. Start current is just rated current. Start-up time is longer, no big impact on grid. It reduces mechanical damage on motor, and prolongs motor lifetime.

VI. VFD problems and solutions

VFD operation temperature is high in summer, which causes its frequent over-heat protection. Air conditioner system would affect VFD stability and its cost is too high. According to working condition and cooling analysis, we decide to install duct and air conditioner. Take advantage of fan of VFD and additional duct to discharge hot air outside. In addition, because pump station locates in plain, there is frequent thunder and typhoon. We improve earth and add lightening arrestor to avoid accident.

Concluding remarks

Controlling pumps by VFD improves water supplying, water purifying and site environment. Perfect integration of PLC control, Profibus communication and variable frequency technology realize high automatization level, stable operation, easy control and considerable enery-saving effect.

Successful application of VFD in Cangnan Water Plant has high economic value in saving energy, heightening efficiency and innovating technology.