Application du variateur de fréquence moyenne tension dans les pompes d\'alimentation en eau de système de distribution d\'eau
2009-11-29 23:35:03

I. Préface
Il y a trois ensembles de pompe de fourniture de d’eau dessinés pour Shishi Water Supply Co., Ltd. Durant la 1^ère période, deux ensembles de pompe de fourniture d’eau (#2 et #3) fonctionnent à fréquence industrielle (une réforme technique a été appliquée sur l’ensemble de pompe de fourniture d’eau #1). La puissance de l’ensemble de pompe de fourniture d’eau #2 est de 200kW/10kV et le débit normal est de 4200 m³/h. La puissance de l’ensemble de pompe de fourniture d’eau #3 est de 200kW/10kV et le débit normal est de 8200 m³/h.

Since the electricity cost is different for each time period of a day and the water consumption of the water plant varies rather greatly, the water supply needs to be controlled by starting/stopping the water pump sets usually. This may lead to a shock to the pump sets and the water supply pipelines when starting the water supply pumps with industry frequency, and a large quantity of electric energy will be consumed for the valve operating, since the water supply is regulated with the opening of the valve. Considering all of these, the water supply station decided to make technical reform for the water-supply pump set #1.

II. Reform Project of Variable Frequency
After investigation and consideration, it is decided to apply the variable frequency device made by Beijing Leader & Harvest Electric Technologies Co., Ltd. to the water-supply pump set #1. Comparing with other speed regulating devices, speed regulating with variable frequency drives has incomparable advantages in energy saving, speed regulating accuracy, and speed regulating range, and communication with automatic control systems (such as DCS system) can be implemented easily. At the same time, the technical reform project for the pump set #2 and pump set #3 are performed to add soft starting.
1. Omitted
2. Type Selection for the Equipment
For the pump set #1, a water pump with water pumping level of 45M and flow of 8,600 m3/h, a driving motor of 1600kW/10kV and a medium voltage variable frequency drive of 1600kW/10kV made by Beijing Leader & Harvest Electric Technologies Co., Ltd. are equipped. The installation and test of the variable frequency drive were completed in July 2004 and began to operate.
Since water supply from the water station began, the pipeline characteristic has been deteriorated continuously. Afterwards, pre-chlorinating was adopted and the pipeline characteristic began to get better, but the designed values have not been approached. When selecting types, the water pumping level of the water pump can only be calculated according to the practical conditions. The designed water pumping level of the water pump is reduced from 50M to 45M based the practical conditions. When operating, the actual normal water pumping level is 45 M and flow is 8,600 m3/h, ensuring the water pump operates in the range of the highest efficiency.

III. System Operation after Reform
1. Operating with Variable Frequency Drives
When trial operating, mechanical resonance of motor occurred between frequency of 24-26 Hz. After setting a varying frequency jumping point to evade from the resonance zone, the pump set operated stably. When regulating speed, the pump speed may decrease to the 40% of the normal speed, i.e. 198 rpm and operates stably. Various parameters conform to its characteristics.
2. Frequency Varying Operating and Energy Saving
The working load of the working personnel has been reduced through the technical reform and the pipeline has been protected, electric energy has been saved. Efforts have been put into the type selection and the feasibility verification. The application of variable frequency drives is use of speed regulating and it does not change the pipeline characteristic. When the pump operates, one pump operates, the water passes through one pipeline and the pump with variable frequency drive does not operate with the pump with industry frequency in parallel. It is rather simple and saves electricity mostly. But actually, because of the difference between peak and valley electricity cost, the level regulation of the regulating reservoir and the variation of the water consumption of the water plant (it can only be regulated with switching the pump), it can not be simply regulated according to the flow, but is necessary to establish a efficient mathematical model to perform frequency-varying control. The frequency-varying control is described as follows:
(1) The data sampling for the variable frequency drive is performed with the PLC module of the variable frequency drive own. The PLC module communicates with the industrial control machine through 485 cards. Modbus communication between the water station and the Simens PLC of the variable frequency drive is made through the PLC of the original control system AB. After communicating successfully, the signal is very stable.
(2) Control with variable frequency drives is classified into open-loop control and closed-loop control (water supplying with constant pressure). The operation mode for the water station is of open-loop operation, i.e. the calculated values in the PLC of AB are transmitted to the variable frequency drive to perform control. The control of the upper-level machine of the control system of the pump station is classified into manual and automatic control. For manual control, the frequency is input manually. For automatic control, the PLC of AB calculates the variable frequency based on the level of the regulating reservoir, peak and valley electricity cost and the total water consumption and performs frequency-varying control. For automatic control mode, there are some aspects influenced. To implement automatic control, the software of the main computer for central control is modified first to stabilize the data transmission and signal of water level is added in regulating reservoir.
Before the signal has been stabilized, frequency-varying control is performed manually. When the electricity cost is at valley price, pump set #1 and #3 are put into operation and set #1 and pump set #2 operate for other cases. The frequency for varying frequency is set to 30Hz for peak price and is regulated manually based on the water level for normal price. Thus, more than RMB 1 million will be saved annually for the whole water supply system (for water supplied of 300,000 ton daily). But there are still some problems, such as the flow cannot be distributed reasonably for normal electricity price, errors in time control and inaccurate estimation of daily water supply. Therefore, a model has been constructed for water supplied of 275,000- 310,000 ton daily. The only difference compared with the manual control is that the frequency is regulated based on the level of the regulating reservoir and the total water consumed (water from Shandou + 200,000 ton). The level of the regulating reservoir is set to 3.3m. After operating for a week, comparing with manual control, the electricity consumption for water of 1,000 ton was reduced from 112 kWh to 105 kWh (for water supplied of 300,000 ton daily). According to the average electricity price of RMB 0.51 /kWh, the daily saving is RMB 1,071 and the annual saving is RMB 380,000. When operating automatically, the level of the regulating reservoir is kept between 3.28-3.38m at 11:00 pm. It is the ideal designed value. If the level of the regulating reservoir is kept under 3.3m, the electricity saving would be more distinctive.
When the daily water supply is lower than 275,000 ton or greater than 310,000 ton, the operating time of the pump sets 2# and 3# need to be regulated for the best electricity saving. It is more complicated. Efforts are made to achieve a best project. The key for saving electricity is a feasible efficient model. In addition to the daily maintenance, the operating model need to be perfected based on the pipeline parameters and the operation parameters of water pumps to make the frequency-varying control play the best role.

IV. Conclusion
The medium variable frequency drive has obvious energy saving effects. After applying speed regulating with variable frequency drives, soft starting of electric motors is performed and the life of electric motors has been extended and shock to water pipeline is reduced. The flap in the water pipeline is opened fully and vibration and wearing of the pipeline are decreased. The speed regulating system with medium voltage variable frequency drives, which is made by Beijing Leader & Harvest Electric Technologies Co., has created economic and social benefits for Shishi Water Supply Co., Ltd. and other users with its reliable operating performance and good energy saving effects.