Etudes et applications du variateur de fréquence dans les pompes à eau
I. Introduction
Le coût d’électricité annuel pour la compagnie de distribution d’eau de Daqing Petroleum Administration prend environ 30% du coût total de la fourniture d’eau et le coût de l’électricité pour la transmission et la distribution d’eau coûte environ 70%. Aussi, ajuster les opérations journalières de la pompe à eau pour conserver la consommation électrique à chaque tonne d’eau dans une gamme plus économique et raisonnable est là ce sur quoi se focalise l’économie d’énergie. Nous avons toujours explorés et étudiés avec sérieux, les mesures efficaces pour résoudre ces problèmes.

II. Problems
Longhupao Water Plant of the Company is situated at the north bank of Longhupao. It has a daily water supply capability of 500,000 m3/day. When it was designing, in consideration of the middle term and long term planning and the worst water quantity and level, two pump set types were selected, in which one is a motor of 1400kW/10kV and a water pump of 900HR (H62m, Q6360 m3/h), another one is a motor of 900kW/10kV and a water pump of 700HR (H62m, Q3900m3/h). The designed water supply for the first stage and second stage of the project of the second level pump station and middle water drawing plant is 500,000 m3/day. Since the oilfield yield is restricted, the water consumption is reducing year by year. At present, the actual water supplied of the water plant is only 250,000-350,000m3/day. The total flow of the pump Q is 10,40,000,000-12,50,00,000 m3/h only. The water pressure loss for the water-transmitting pipeline is 18-20m. The normal pumping level of the pump is 62m. If the pump pressure is to be decreased to adapt to the resistance of the pipeline network, then the pump will operate with overload. Therefore, the pump pressure can only be controlled with adjusting the outlet valve to make the pump operating in the high efficiency zone. Such operation not only increases the pressure loss at the valve and wastes the a large quantity of electric energy, but also the pump is easy to deviate from the high efficiency section or operates with overload, leading to the increase of failure rate of the pump and to shortening of the pump life, if the valve is adjusted not improperly. For the late two years, the pump set had been maintained for more than ten times and the cost for maintenance of more than one million was paid. The middle water drawing plant is important for the company. It has an important function to ensure the water consumption of the oilfield. Therefore, it is necessary to perform the technical reform of the pump sets of the Longhupao Water Plant.

III. Preliminary Calculation and Discussion for the Project of Reform with VFD
Usually, pumps operate under a normal operating condition. The typical operating curve is shown in Figure 1. AB is the performance curve of the pump. It is matched to the normal system pressure curve OB. The normal flow under the normal system pressure may be obtained at point B. The pump has the highest efficiency at this point. Closing the valve may control the flow. When the flow reduces, the pump operates at point P, Q, R and S respectively. At this time, the pump is to be operated under very high pressure-difference. Therefore, the energy output of the pump is much more than the actual need of the system. The surplus energy is lost at the valve in the form of heat and brought away by the flow. Divide the output energy of the pump by the efficiency of the pump. The total input energy of the pump may be obtained. Regulating speed with a VFD, the pump is driven by the motor and operates with a varying speed and the characteristic curve of the pump is matched to the need of the system under the condition of any flow. The flow is proportional to the motor speed and the pressure difference is proportional to the square of speed, as shown in Figure 2. Adopting stepless VF speed regulation, countless characteristic curves of the pump AB~CD may be obtained. Any cross point in the shadow area formed by the pipe characteristic curve and ABCD may become the working condition point, adapting to the variation of the water network flow and the flow corresponding to P, Q, R and S may be obtained with lower pumping level, the energy loss is very low and energy saving effect is obvious.
For the same vane pump the rule is as follows:
Q1/Q2=n1/n2 H1/H2=(n1/n2) 2 N1/N2=(n1/n2) 3
The pump 1#, 3# and 6# of Longhupao Water Plant all are imported pumps. The motor speed is 748rpm, 745rpm and 750rpm respectively.
If the speed of pump 3# is regulated to 500rpm, then the flow, pumping level and power are as follows:
Qorig. /Qvar. W=n.n=748/500=1.496
Q=4251.3m3/h
Horig/Hvar=(norig/nvar) 2=1.496 2
Hvar=28.59m
Norig/Nvar=(norig/nvar) 3=1.496 3
Nvar=412.2kW
If the speed is regulated to 550rpm, then Q=4663m3, H=34m, N=547kW and the parameters after the speed regulation suits the present working condition basically. Of course, the above calculation is performed under the assumption of that the system pipeline characteristic curve is not changed. After installing of the VFD, the outlet valve of the pump may be opened fully and the system pipeline characteristic curve will move to outside. Then it is more possible that the operating parameters of the pump can satisfy the requirements of the actual working conditions. Therefore, it is confirmed that the scheme is feasible.

IV. Evaluation of Energy Saving for the VF Speed Regulation Scheme
The formula for calculating energy consumed by the pump is: P=(K′H′Q)/ h
in which K is margin factor, h is efficiency
If the actual pressure of the pump decreases from 5.8 kgf to 2.5 kgf and the flow not changed, then
P25/P28=25/28=43%, i.e., the power consumed is less than the original for 1-43%=57%. According to the electricity price of 0.41Yuan/kWh, motor efficiency 0.95, operating for 330days/year, then the original annual electricity cost is
(1400kW′24′330′0.41)/0.95=4,780,000Yuan
After VFD speed regulation and the pressure decreasing to 25kgf, the annual electricity cost is
478′43%=2,055,400Yuan. The electricity cost saving for a year is
4,780,000-2,055,400=2,725,000Yuan

V. Determination of Scheme
In consideration of the practical conditions of the first level pump station and middle water drawing plant and through careful calculation, discussion, investigation and comparison for many manufacturers, it was decided to apply the medium voltage high power VFD made by Beijing Leader Harvest Electrical Technology Co., Ltd. The performance specifications of this VFD satisfy the technology requirements of the pumps and it has a high reliability. The reform period would be short and effects would be seen in short period.
The specific scheme is to close the connecting valve between the first stage and the second stage. The pipeline network of the second stage would operate interdependently. The VFD drives the pump 6# in one-one mode. When the pump 6# has failures, it may operate with industry frequency, the switching of industry frequency /VF may be manually performed through the bypass cabinet. The switching of industry frequency/VF may have electrical interlock and PLC logic interlock to ensure the safe operation of the pump set. Constant pressure control and closed loop operation. The pressure of pipeline network of the second stag is 0.1Mpa: automatic control with linked valve. It has the monitoring and protecting function for the cooling water flow.

VI. Trial Operation of Medium Voltage VFD for Pump 6# after Reform
On January 6, 2002, technical personnel of Beijing Leader Harvest Electrical Technology Co., Ltd. and related personnel of the water plant performed trail operation of the VFD of the pump set 6#. The operating data for the trial test is as follows:

Before a trial operation, the pump set 3# and 7# operated in parallel. The operating parameters are listed in the following table:

To ensure the stable operation of the water plant during trial operation, the transformer station and the middle water drawing plant were informed to make preparation for trial operation. The pump set 7# was first stopped, the linking valve between pump set 73# and pump set 6# was closed, the operation of the pump set 3# was adjusted, the outlet valve opening for the first stage was kept to 12% unchanged, the frequency of the VFD of the pump set 6# was set to 37Hz. Then, the pump set 6# was started. After it operated stably, the outlet valve opening for the second stage was opened to 70%, then the operating conditions of the pump sets at frequency of 35Hz, 33Hz, 30Hz and 28Hz were observed.

After a trial operation, the frequency was set to 30Hz and the pump set operated in open loop. Records were made for the operation of the pump set 6# individually. Through an operation for several days, it was shown that the VFD operated stably with a complete function, operating easily, which satisfied the requirements of the actual working conditions to get the expected effect.

VII. Inspection for Energy Saving after Reform
The energy saving effect was inspected with instruments finally. The most direct and reliable method is to make a comparison for the energy consumptions before and after the reform with VFD by using a kWh-meter.
Originally the pump set 3# and the pump set 6# operated at industry frequency. Take the operating record for August 1 –20, 2001 to calculate:
Total electric consumption: 1,214,700kWh;
Total water supply: 5,877,400m3;
The electric consumption per m3 of water supply for the pump station: 0.2064kWh/m3;
The pump set 3# and the pump set 6# operated at industry frequency. Take the operating record for July 8 –19, 2001 to calculate:
Total electric consumption: 667,500kWh;
Total water supply: 3,115,200m3;
The electric consumption per m3 of water supply for the pump station: 0.2143kWh/m3;
The electric consumption per m3 of water supply for the pump station for 32days is:
(0.2064 0.2143)/2=0.21kWh/m3;
The pump set 3# operated at industry frequency, and the pump set 6# operated with VFD (30Hz), and the converged valve closed and the pump set operated individually. Take the operating record for 19:00 of January 9 to 7:00 of January14, 2002 to calculate:
Total electric consumption: 154,200kWh;
Total water supply: 1,064,700m3.
The electric consumption per m3 of water supply for the pump station: 0.143kWh/m3;
When the original pump set 6# operated at industry frequency, the electric consumption per m3 of water supply is 0.243kWh/m3. The electric consumption per m3 of water supply for pump set operated with VFD is 0.06kWh/m3;
From 19:00 of January 9 –7:00 of January14, 2002, total 4.5days, the daily average water is 1,064,700m3/4.5=236,600m3;
Annual electricity saving is 0.07*23.66*365=6,040,000 kWh, Cost RMB: 2,687,800Yuan.

VIII. Technology of Water Plant having been Improved after Reforming with VFD
1) A set of PLC of Siemens S7-2000 Series is built in the medium voltage VFD made by Beijing Leader Harvest Electrical Technology Co., Ltd.. When an outside logic control is performed at site, it is very convenient. For example, the valve inter-link function makes the personnel on duty start or stop the pump set without operating the valve, thus the work load and error may be reduced; and the protective function for cooling water flow ensures the safe and stable operation of the pump set.
2) After the reform of the pump set 6# with VFD, the speed, current, outlet pressure of the water pump all decrease obviously, and the operation condition of the pump set improves obviously. Not only the electricity consumption saves directly, but also the shock to the motor and network are reduced, extending the operating life of the equipment and decreasing the equipment maintenance costs since the soft starting function of the VFD.
3) Closed-loop pressure control is used by the VFD, which ensures the pressure of the pipeline network kept to 0.1Mpa. Since the VFD regulates the speed smoothly and has high control accuracy, the pressure fluctuation range is very small. It can satisfy the site water supply technology requirements fully.
4) The VFD made by Beijing Leader Harvest Electrical Technology Co., Ltd. has perfect, sensitive failure monitoring, diagnosing, alarming and switching function etc., ensuring the safe operation of the pump set.
5) The monitoring function and the remote dialing monitoring function of the upper level computer create the necessary condition for users to implement the centralized monitoring.

XI. Conclusions
The practice proves that the reform of the water pump set with VFD in the Longhupao Water Plant is successful and has gained obvious economic and social benefits. First, a large quantity of electric energy has been saved; Second, the adjustable range of the system has increased and the flexibility of the system operation has been improved; Third, the speed of the motor has been reduced and the starting shock, mechanical wearing and vibration have been decreased so that the operating life has been extended; Forth, the operating noise of the pump set has been reduced and the working environment has been improved; Fifth, the intelligent inter-link function and perfect failure protective function have improved the working efficiency and increased the safety and reliability of the system.
Therefore, the medium voltage high power VFD technology is a new technology, which has extensive prospect for application and benefits to the country and people. It has an important realistic meaning for the pump speed regulation and constant-pressure water supply in water supply trade. This type of medium voltage high power VFD will be the first-selected equipment for the energy saving and reform of the water supply trade.