Application du convertisseur moyenne tension domestique dans station de traitement d'eau
2009-11-29 23:36:09

1. Introduction
Avec le développement de l’électronique et de la technologie des composants d’inverseur, la technologie du variateur de fréquence moyenne tension haute puissance VFD s’est développée rapidement dans le pays et à l’étranger. Plus spécialement avec les avancées en matière de capacité et vitesse des puces intelligentes, la technologie de contrôle automatique intelligent est entrée dans une nouvelle étape. Combinant la technologie de contrôle automatique intelligent avec la technologie de variateur de fréquence moyenne tension haute puissance VFD, des nouveaux équipements et produits apparaissent constamment.

We have contacted with various VF speed regulation technology, such as Toshiba, Fuji, Siemens, Alston, Swiss ABB, Danvus, US ABB, GE, GTR and SGCT as well as IGBT and IGCT technology. At the same time with the VFD technology development in abroad, the domestic VFD technology has been developed, such as the medium voltage high power VFD of Beijing Leader & Harvest Electric Technologies Co., Ltd and the low voltage VFD technology by Huawei Company etc. This has brought with new possibility for the application of domestic VFD technology.

The application of high power 6KV/1000kW VFD made by Beijing Leader & Harvest Electric Technologies Co. in Tianjin Xinkaihe Water Plant is described in this paper.
Tianjin Xinkaihe Water Plant was established in 1986 and extended in 1995 as the second stage of the project. The water supply capability is one million ton per day. There are several speed regulation methods for the water pump motors of the plant. In 1986, two sets of cascade speed regulators manufactured by Xian Rectifier Factory for the 6KV/1000kW motors with winding rotor were applied for the first stage of the project. In the second stage of the project, a low voltage low power VFD made by Siemens was applied to the metering pump and a low voltage 75kW VFD made by Fuji Company was applied in the water discharging pump room. The 2200V/1000kW GTO VFD made by ABB Company was applied to the water supply pump room of the second stage of the plant. There has been a history for a certain time for application of VFD to water pumps for the water plant.

In 2001, in the reform of old equipments for the pump room of the first stage, the technique from Tsinghua University and the 6KV/1000kW medium voltage VFD of Model HARSVERT-A06/130 made by Beijing Leader & Harvest Electric Technologies Co., Ltd was applied. The driven pump is Model 32SA-10, manufactured by Changsha Pump Works in September 1983 and the motor is a conventional squirrel motor of Model Y1000-10/1430, manufactured by Xiangtan Motor Works in September 1983. The pump set was put into operation in 1983 and was a set of equipment used for 13 years when installing the VFD. The installation was simple. The VFD was connected in series between the vacuum breaker of 6KV and the motor of 6KV. No other equipment, such as transformers for raising or decreasing voltage, is needed. Installation and tests began on June 27 at site and it drove the pump and operated on June 30 to supply water.

2. Basic Electrical Principles & Features
2.1 Basic Electrical Principles
The electrical principle of the medium voltage VF speed regulation system of Model HARSVERT-A06/130 is shown in Figure 1. It is composed of three parts: Phase-shift Rectifying Transformer, Power Cell and Controller. There are 21 power cells for the 6KV series and every 7 cells are connected in series to make a phase. The basic principle is: the 6KV high voltage is applied to the voltage decreasing, phase-shift and isolating transformer, which has 7 sets of three-phase output at the second-level side, and transmitted to various power cells. The power cells (see Figure 2) have three-phase input and single-phase output. The outputs of the adjacent power cells are connected in series and through sinusoidal PWM control to the IGBT inverting bridge, the medium voltage sinusoidal wave output with varying frequency is obtained and then the motor is driven directly.

From the electrical principle schema, the input side of 6KV is of full wave diode rectifier. The rectifying mode of multi-pole phase-shift superposition of 42 pulses improves the current waveform at the network side considerably and makes the power factor under load at network approaching 1.0. The output side supplies the motor in star connection formed by U, V output terminals of each power cell. Through reforming the PWM waveform of each cell, step-style PWM waveform is obtained. It has a good sinusoidal shape with low dv/dt, and may decrease output harmonic, which causes motor vibration, motor heating and noise etc., so that reduces damage to the insulation of output cables and motors. Filters are unnecessary and the motor needs not to be operated with deratings. Conventional squirrel motors may be driven directly. It may be used for reform of old equipments.

A touch screen of industrial site type and a color LCD display screen are set on the panel of the control console and a PLC is built in the equipment to perform the logic processing of digital signals and coordination of various operating signals and status, enforcing the controllability and flexibility of the system. Since optic fiber communication is used between the controller and power cells, the low voltage is isolated from the high voltage completely, making very high safety and excellent anti-electromagnetic interference property for the system. The control device may be linked with the industry control computer through standard computer interfaces, provide all-Chinese Windows 98 monitoring and operating interfaces and perform coordinated calculation with the built-in controller, achieving the optimal operating performance. At the same time, remote monitoring and network control may be performed. The performed functions include: setting of system functions, parameter setting, failure information inquiring, operating waveform displaying, operating data recording, operating mode setting, alarming and alarming releasing, and system resetting etc.

2.2 Features of Equipments
2.2.1 Design adaptable to situation of China: Since each power cell has filtering capacitors and the structure of multi-sets in series, it is suitable for the present Chinese electric network situation of large voltage fluctuation. The normal input voltage for the equipment is 6KV 5%~10% and the actual operating range is wider.
2.2.2 Both the controller of the equipment and the linked industry PC have all-Chinese man-machine interface, particularly suitable for workers operating at site.
2.2.3 Since the system is not the traditional structure of high-low-high or high-low, i.e. using decreasing voltage transformer or raising voltage transformer, or using motor with lower voltage, the system has considerably higher efficiency. The transformer efficiency under normal load is >0.96.
2.2.4 Since power cells use diode full wave rectifying and have filtering capacitors, so the system has high power factor, cosф>0.95, even approaching 1.0.
2.2.5 Since power cells are divided into 7 different phase sets with difference of 8.6 degrees in electricity, so the diode full wave rectifying forms 42 pulse wave and harmonic under 41 th is not present theoretically. Thus, the harmonic interference is very low.
2.2.6 The system has a high speed PLC controller. It is suitable for different control demands and control modes.
2.2.7 The system has a good sinusoidal wave with varying frequency with a low ripple factor so that the driven motor has low noise, low vibration and low heating. It is especially suitable for the reform of the old equipment.
2.2.8 Protection for overload and over current may be performed for the driven motor.
2.2.9 The system has a high reliability and high an anti-interference ability.

3. Practical Application in Tianjin WaterWorks (Group) Company Ltd.
3.1 The VFD has an integrated design, small volume and easy to install and test
Additional transformers, reactors, filters, compensative capacitors and starting devices etc. are not required for HARSVET-A06/130 VFD. It is integrated that all devices, components are built in itself. The integrated design has a small volume and compact structure. It takes a small area on the site with few connection, simple installation and tests and an only short period. The installation and tests were completed in 3 days and the pump was put into operation.
3.2 The VFD operates stably with a good performance and low harmonic contamination to the network.
Since the VFD was put into operation, it has been operated stably. The voltage and current were stable without any fluctuation. The speed was regulated smoothly and reliably. Speed rising and decreasing are stable. The VFD performed under good conditions. The China Electrical Power Science Institute made a harmonic measurement during operating. The results showed that in the whole speed regulating range of the pump set, the power factor of the VFD at the network side was greater than 0.97, the efficiency was greater than 96%and the total harmonic components of the current at network side under full load were less than 3%. It is superior to the imported equipment of the same class in Tianjin WaterWorks Company.
3.3 The motor of water pump operates stably.
The HARSVET-A06/130 VFD drives the pump set 2# in the water-supply pump room. The motor is an old squirrel motor, which was manufactured in September 1983 and put into operation in 1987. Since it operated with VFD, the insulation has not been affected. It has no abnormal vibration and noise. The temperature is in normal range. The starting current and the mechanical shock were improved.
The VFD has Good adaptability and operates reliably
In the middle of July 2001, there was continuous high temperature weather in Tiajin. The ambient temperature was higher than 40℃. The temperature of the input windings of the transformer reached to 110 ℃ for a time, but the transformer operated normally without any abnormality.
In addition, the voltage fluctuation of the 6KV electricity network for the water plant surpassed 5% frequently at electricity consumption peak, even reached 10%. At that time, some imported VFD frequently stopped for the voltage fluctuation, but the HARSVET-A06/130 VFD operated normally, stably and reliably.
3.5 Failures in the operation
Some practical problems happened during operation of the VFD. After the VFD was installed and operated, it was in summer. The temperature rose too high, then the manufacturer modified the ventilating configuration of the transformer cabinet, making the temperature rising kept in a reasonable range.
Since the VFD is of air-cooled structure, there is stronger air circulation in the power cabinet. The VFD is installed in the open pump room directly, there is too much dirty in the cabinet and the connector of the optic fiber was clogged, causing optic fiber communication failure. Then the sealing of the power cabinet was improved and the problem was solved.
The upper level computer of the VFD is installed in the control room of the pump room and it communicates with the VFD through RS485. The communication line is not laid scientifically at first, which is close to the power lines, so that the communication interface board was damaged. After replacing the interface board, the lines were re-laid reasonably and the problem was solved.
For the water plant, the VFD is of high-tech products. During daily operation, the level of the operating and service personnel needs to be improved constantly and then the VFD will be used better and develops more benefits.
Benefits
With the VFD, the technology requirements for the water supply pressure and flow may be satisfied conveniently. It is only necessary to adjust the VFD output frequency without the need to switch on or off the high voltage breaker and adjust the outlet valve opening frequently. The working load of the personnel on duty is reduced.
Based on the measured water pump curves, the high efficiency operating zone was determined and the speed regulation of the water pump was limited to this zone. Obvious electricity saving effect was obtained. Preliminary record: in May and June, Two pumps with ABB VFD and two Pumps at industry frequency were operated in parallel, and the electricity consumption for water of 1,000ton was 138.63kWh and 134.80kWh respectively. In July and August, the HARSVET VFD was applied, and the electricity consumption for water of 1,000ton is 128.61kWh and 129.51kWh respectively. The comprehensive electricity consumption decreased for 5.6%. The potential for electricity saving is very high and the benefit will be better, if comparing with the pumps with fixed speed.
It has been 16 years since the Xingkaihe Water Plant was put into operation in June 1986. During this period, the operation mode has also varied much. From 1990 to now the water output pressure has reduced from the highest value of 41.1m to about 31m, the average value at present. Since the VFD of the pump room of first stage was tested in June 2000, the main operating water supply pump was the pump set with VFD. In addition a pump set with fixed speed was used simultaneously. For the second stage, all the operating pumps were pump sets with VFD and the pump set efficiency improved considerably. The operating data before and after operation with the VFD are used to compare the energy saving:

Table 1 Electricity Consumption for Pump Set 5# & 6# (Speed Regulation with Cascade Pole) in the Pump Room of First Stage

Table 2 Electricity Consumption for Pump Set 2# ( VF Speed Regulation) in the Pump Room of First Stage

Comparing the data in Table 1: Under the condition of basically same supplied water pressure, the energy saving rate for pump with varying speed 5# is:
(140.11-127.47)/140.11=9.02 %
The energy saving rate for pump with varying speed 5# and 6# is
(140.11-122.94)/140.11=13.68 %
Comparing the data in Table 2, the energy saving rate for pump with varying speed 5#, 6# and 2# operated in parallel is:
(140.50-123.46)/140.50=12.13 %
Through the analysis for the above data, the following conclusion may be obtained: when the supplied water pressure (pumping level) is far from the high efficiency zone, application of speed regulation technique changes the operating points, making the efficiency of pump set improved. The energy saving rate under operation in parallel may be about 10%. Because it is limited by the operating condition, the energy saving for single pump set is not measured. The data listed in the above tables are the operating data under operation in parallel.

Period for Returning of Investment of Equipments
Based on the statistics from “Dispatch Daily”, the electricity saving for the water pump 5# was 12.64 kWh/km3 and the supplied water from July 2000 to June 2001 was 128,641.87km3. The comprehensive electricity price for the water plant is 0.54Yuan/kWh, then the electricity cost saving for the speed regulation system per year is: annual electricity saving* electricity price= 8,780,580,000Yuan/year. The time needed for returning the equipment investment is 2,880,910,000Yuan/8,780,580,000=3.9month (2,880,910,000Yuan is the cost for the speed regulator in 1986). The cost for the speed regulator of water pump 5# and 6# may be returned in the same year.
The water pump 2# is connected with the water pump 5# and 6# in parallel, saves electricity of 17.04 kWh/km3 totally. From July 2001 to June 2002, the supplied water was 127,945.81km3. The comprehensive electricity price for the water plant is0.54Yuan/kWh, then the electricity cost saving for the speed regulation system per year is: annual electricity saving* electricity price=17.04*127945.81*0.54=1,177,306Yuan. If these benefits are considered as the energy saved by the pump 2#, then the time for returning the equipment investment is 1,251,504Yuan/1,177,306Yuan=12.8months. The equipment investment returning period is short and the energy saving effect is obvious.

4. Measurement & Evaluation of the Specifications of the Equipment
4.1 Specifications
Input frequency (Hz): 45Hz~55Hz
Normal input voltage: 6KV ( 5%~-10%)
Input power factor: 0.95 (>20% load)
Efficiency of VFD: >0.96 for normal load
Output frequency range (Hz): 0~120Hz
Resolution of output frequency (Hz): 0.01 Hz
Overload capability: 120% 1min.
150% protecting instantly
Time for acceleration and deceleration: 0.1~3000sec.

4.2 Measurement and evaluation of the specifications of the equipment
During the operation of the equipment in Tianjin WaterWorks Plant, performance inspecting and measuring were performed, chaired by the National Quality Supervision and Inspection Center for Electrical Control and Distribution Center and China Electrical Power Science Institute. The parameters, such as non-symmetry of output voltage, frequency stability, resolution, power factor and efficiency etc., especially harmonic component contents, were measured. The examination and measurement for the harmonic components was performed according to the National Standard GB/T14549-93 . The results showed the harmonic in the electric network caused by the VF system conformed to the national standard. The relative harmonic current contents of input current satisfied the requirements of IEEE Standard 519-1992. The relative harmonic voltage contents of output voltage satisfied the requirements of IEEE Standard 519-1992. These results are superior to the index of the imported foreign VFD.

5. Conclusions
The operation of the medium voltage high power VFD made by Beijing Leader & Harvest Electric Technologies Co., Ltd in Xinkaihe Water Plant of Tianjin WaterWorks Company shows: the equipment has complete functions, advanced performance characteristic, high reliability and is easy to maintenance. It is in the lead in the country and has reached the level of the international products of the same class. Especially, its distinctive features are more suitable for the Chinese situation and it suits to the reform and level rising of water industry. It is expected more products of Beijing Leader & Harvest will be developed with more series, more diversified and higher quality to make it become the main products of VF speed regulation for medium voltage high power motors.

The above is my personal understanding for the technical product. There may be some errors or omissions contained herein. All suggestions and comments from experts on this paper are most welcome.