Application du variateur de fréquence moyenne tension dans la station de Hainan Danzhou
2009-12-15 9:57:09

I. Introduction
La ville de Danzhou est située dans le nord-ouest de la province de Hainan, face à la baie Nord. C’est la plus grande population et zone de la province de Hainan. Elle est dotée d’un climat de mousson tropical, avec une précipitation annuelle moyenne de 1,915mm, une température moyenne annuelle de 23°C, la plus haute température de l’année est de 38°C et la plus basse de 6°C, l’humidité relative est de plus de 98%.


Nada Water Plant supplies water for the whole city proper. The water supply pump room was equipped with three water pumps, which were driven with two motors of 10kV/60kW and one motor of 10kV/400kW. The designed water supplying capability was 100,000 tons/day, but the water consumption for this area was only 28,000 tons/day. There was a big difference between the water supplying capability and the water consumption. The original motor operating mode was industry frequency mode which water flow was regulated with the outlet valves of the pumps. A large quantity of electric energy was wasted at the valves. To solve this problem and reduce the energy losses, the plant was decided to reform the pumps with a speed regulation technique.

II. Type Selection of Speed Regulating Equipment
At present, there are the following speed-regulating methods for high-voltage, high-power asynchronous motors: Cascade Speed Control, Speed Regulation with Hydraulic Coupling and VF Speed Regulation.
Serial Speed Regulation: It is not adaptable to squirrel asynchronous motors. So the motor must be replaced and it is not adaptable to the reform project. Soft starting and soft stopping of the motor cannot be performed and the starting process is very complicated. The starting current is high and the speed regulating range is limited. The response is slow and closed control may not be implemented easily. The power factors and efficiency are low and reduced rapidly as speed decreasing. It is difficult to adapt to PLC and DCS etc. and is disadvantage to improve the total automation level of the facility and implementation of optimized control. The higher harmonic contamination to the electric network restricts its application further. It is a lagging technique.
Speed Regulation with Hydraulic Coupling: It is low efficient with a limited speed regulating range. The speed loss is 5%-10% at high speed and speed difference loss is high at low speed, the highest loss may be 15% of the normal power because the efficient is proportional to the speed. It is of low efficiency, accuracy and linearity and slow response, but high starting current and large volume. It must be assembled between the equipment and the motor, not suitable for the reform. Soft starting cannot be performed. When the coupling has faults, the operation may not be switched. Furthermore, the maintenance is complicated and costs high. It cannot meet the requirements of improving of the total automation level of the facility.
After investigation in many aspects, the above problems for serial speed regulation and speed regulation with hydraulic coupling that they cannot satisfy the technology requirements are recognized so that we discard the schema of serial speed regulation and speed regulation with hydraulic coupling.
The technique of speed regulation with medium voltage VFD is a new speed regulation technique for electrical drive, which had been developed rapidly in the 90’s. It mainly applies to the speed regulation with varying frequency of AC motors. Its performance and technique are superior to that of any other speed regulation technique. Along with the continuous technical progress of domestic manufacturers, domestic medium voltage VFD has rather high performance/price ratio already and are affordable for us. Since its obvious energy saving effect, higher speed regulating accuracy, wider speed regulating range, perfect protection in electronic devices and electrical power facilities and easy for implementation of automatic communication, it is decided to adopt the project of the technique of speed regulation with medium voltage VFD.

III. Why Chose LEADER & HARVEST’s Inverters
Since the water plant uses voltage of 10kV, if inverters of 6kV are selected, a transformer for 10kV/6kV, and switching board of 6kV etc. would be needed additionally and there is no space in the site. A large quantity of fund would be held. In addition, the selected inverters must be compatible with the original conventional squirrel asynchronous motors, have no contamination to the electricity network, have a good man-machine interface and well service is also necessary. At last, it decided to adopt LEADER & HARVEST’s medium voltage VFD of 10kV
IV. Installation, Tests and Operation
In January 2002, the water plant ordered a medium voltage VFD of 10kV/400kW from Beijing Leader & Harvest Electric Technologies Co., Ltd. The inverter was delivered in April 2002 after a week’s installation and test, and passed the site tests in May 2002. Afterwards, since the outside pipelines were reformed, it did not provided with the operating condition of the inverter on the site so that the inverter was not put into operation. The inverter was just put into operation until October 2003. Since then, it has been operated stably for more than half a year and energy saving effects is obvious with a simple operation.
When the inverter was just transported to the site, it had a heavy rain in Danzhou and the inverter was rinsed in the rain for a night. But since it was well packed, it was found no any wet inside when opened to check, thus direct economic losses were avoided.
The inverter can be operated without failure after placing for one year and there was no any corrosion stains inside, it showed that Beijing Leader & Harvest has adopted effective measurements to make the inverters to adapt to the distinctive climate of Hainan Island, such as plastic spray painting for the cabinet body, nickel coating for all screws, bolts, nuts and other connecting components, dried transformers vacuum soaked with paint.

V. Operation Effects
After the pump was controlled with the inverter, the shock to the water supply pipeline network made by the starting/stopping of water pumps and the water flow regulating has been reduced. The outlet valve opening is kept to maximum and the water flow is regulated with the varying frequency output of the inverter (motor speed). This saves the energy loss on the valve when regulating the water flow with the varying opening of the valve originally. Through closed control for the water pressure with the inverter, the water pressure in the pipeline network is kept constant. Soft starting and soft stopping for the motor have been implemented and there is no any shock to the electricity occurred.

VI. Energy Saving Data
The motor current is 24A at operating with industry frequency.
After applying the inverter:
The input current is 16.2A and the flow is 2,200 tons/hour when operating frequency is 47.6Hz.
The input current is 5.9A and the flow is 580 tons/hour when operating frequency is 47.6Hz.
The peak water consumption time period is 7:00-9:00, 10:00-12:00 and 18:00-22:00. The input current of the inverter is 12-16A during peak time period and is 5.9-10A during other time period.
According to operation for 300 days with an electricity price at RMB 0.65Yuan, the theoretical calculated energy saving comparison for operating with inverter and operating with industry frequency is:
The electricity cost for operating with industry frequency is
1.732×10×0.8×24×300×0.65=1,556,305.92 (Yuan)
The electricity cost for operating with inverter is calculated according to the average value: the current for peak time is 14A and the current for valley time is 8A:
1.732×14×0.8×24×100×0.65 1.732×10×0.8×24×200×0.65
= 302,615.04 345,845.76 = 648,460.8 (Yuan)
The saved electricity cost for one year is
1,556,305.92 - 648,460.8 = 907,845.12 (Yuan) (USD 133506)

At the beginning of 2002, when we signed the contract with Beijing Leader & Harvest Electric Technologies Co., Ltd., the number of contracts signed with Beijing Leader & Harvest Electric Technologies Co., Ltd. was only 20. At present, the number of contracts signed with Beijing Leader & Harvest Electric Technologies Co., Ltd. has reached more than 170. From this number variation, it is showed that we have chosen an excellent company and good products. Since the inverter put into operation, it has been operated stably and reliably and it is easy for operation, alleviating the working load of the operating personnel at site. It provides reliable guarantee for the energy saving and loss reducing. Although the initial investment for the medium voltage VFD is rather high, but because it has obvious energy saving effects, high return for investment, reliable operation and high automation level, it has replaced other speed regulation technique gradually. It is believed that along with the growth of domestic inverter manufacturers, which are similar to Beijing Leader & Harvest Electric Technologies Co., an era of medium voltage VFD will be coming.

References
1. Documentation of Hainan Danzhou Nada Water Plant for technical reform with medium voltage VFD
2. The technical principles of medium voltage VFD of type HARSVERT-A

Author’s Brief Introduction
Wang Tongmin, male, born in 1956
Director of Hainan Danzhou Nada Water Plant