Application Case in Plastics Municipal Industry
Case I
Senlan Inverter Solutions for Constant Pressure Water Supply System of Water Works
IControl Scheme for Water Supply under Frequency Conversion and Constant Pressure
Control scheme for water supply under frequency conversion and constant pressure is a cyclical switchover scheme, and the system map is as shown in Fig. 1:
Fig. 1System Map of Cyclical Switchover Scheme of Water Supply under Frequency Conversion and Constant Pressure
In the figure: BP1—Senlan SB61P+160KW inverter, BU1—soft starter, PT－pressure transmitter, ZJ1 and ZJ2－used for startup/stop and automatic/manual transfer of control system. It can be seen from Fig. 1 that the inverter is connected to the first water pump motor. When a pump is required to be added, the inverter will stop running, and the output ports of RO1~ RO3 of the inverter will output the signal to PLC, and PLC will control the switchover process. When the switchover is started, the inverter stops output (the inverter is set to shut down freely). The first water pump will be switched to operate under power frequency by utilizing the inertia of water pump, and the inverter will be connected to the second water pump and then started and operated. By analogy, the second water pump shall be switched to operate under power frequency, and the inverter will be connected to the third water pump and then started and operated; when the pump is required to be reduced, the system will stop the first water pump, and then the second pump will be stopped, at this time, the inverter will be connected to the third water pump. When additional pump is required to be added, it will be switched to the third water pump for circulation. Such manner ensures that a water pump is always operating under frequency conversion, and any of those four water pumps can operate under frequency conversion. The pressure of pipe network can maintain basically constant no matter how the water consumption is changed, and the operation time of each water pump is basically equal, which brings convenience to the maintenance and repair, so most water supply plants are in deep love with cyclical switchover scheme. However, this scheme has its shortcoming, i.e. when only an inverter is operating and switched to operate under power frequency, the pipe network will be subject to short term loss of pressure, thus much attention shall be given to such aspect in the design. Besides, a standby system must be provided, and the soft starter in the figure is used for standby. In case of failure of inverter or PLC, the soft starter can be manually used to start each pump in turn to supply water.
IIApplication Cases
A water works of a certain city has a water supply capacity of 60,000 tons/day, the urban pipe network pressure is 0.4MPa, and the pump package consists of three 160kW water pumps and a 90kW water pump. Water supply under constant pressure and monitoring by computer are required, and the soft starter is required to be able to manually start each pump in case of failure of inverter or control system.
Figure 2Electrical Principle Chart of Water Supply under Frequency Conversion and Constant Pressure
As shown in Fig. 2, it is basically same as the aforesaid cyclical switchover scheme, and BP1 is Senlan SB61G+160KW inverter, DZ1-DZ6 are LG ABE403a400A air switches, FU1 500A and FU2 600A are quick acting fuses, KM1-KM10 is LG GMC-400 AC contactor, PT is Senex pressure transmitter, with the measurement range of 1MPa. When the system is commissioned, the water pump motor is changed from frequency conversion to power frequency. The time delay is started from 300ms, and when it reaches 500ms, no obvious impact is shown in the ammeter. The time delay is finally determined to be 600ms. The soft starter is of current limiting starting type, set as per 2.5 times. When the soft starter is started, the starting current is close to 800A, but it is reduced below the rated current within 30s. The fuse of soft starter is fused at 60th second when 1000A current is passed by referring to 600A fuse curve, so the fuse of soft starter is determined to be 600A. This system has been put into production for two years, with daily water supply of 40,000~ 50,000 tons and it operates well. According to the statistics of the factory, the power consumption/ton is reduced by 20%.
Case II
Application of Senlan Inverter in Water Works
A water works of SichuanProvince has a water supply capacity of 30,000 tons/day, the urban pipe network pressure is 0.4MPa, and the pump package consists of two 90kW water pumps and two 55kW water pumps. Water supply under constant pressure and monitoring by computer are required, and the soft starter is required to be able to start each pump in case of failure of inverter or control system.
(1)Monitoring contents of the computer
Pipe network pressure, flow, running state of the pump, open/close state of the valve, motor temperature, running current, voltage, power, power factor of each pump as well as water level, residual chorine and turbidity etc.
(2)Functional block diagram
Cyclical switchover manner is employed, and the standby system consists of a soft starter and relevant devices. The system map is as shown in Fig. 3.
Figure 3System Map of Water Supply under Frequency Conversion and Constant Pressure
In the figure, M1 and M3 are 90kW, M2 and M4 are 55kW, BP1 is Senlan SB12S90KW inverter. The working process of the system: the inverter is connected to the first water pump motor. When a pump is required to be added, the inverter will stop running, and the output ports of RO1~ RO3 of the inverter will output the signal to PLC, and PLC will control the switchover process. When the switchover is started, the inverter stops output (the inverter is set to shut down freely). The first water pump will be switched to operate under power frequency by utilizing the inertia of water pump, and the inverter will be connected to the second water pump and started and operated. By analogy, the second water pump shall be switched to operate under power frequency, and the inverter will be connected to the third water pump and then started and operated; when the pump is required to be reduced, the system will stop the first water pump, and then the second pump will be stopped, at this time, the inverter will be connected to the third water pump. When additional pump is required to be added, it will be switched to the third water pump for circulation. Such manner ensures that a water pump is always operating under frequency conversion, and any of those four water pumps can operate under frequency conversion. The pressure of pipe network can maintain basically constant no matter how the water consumption is changed, and the operation time of each water pump is basically equal, which brings convenience to the maintenance and repair.
Case III
Application of Senlan Inverter in Water Supply under Constant Liquid Level
ISite Conditions
A certain industrial and mining district is located in the mountainous area which is far away from the urban area, so it cannot use the urban water pipe network. The intake station for the user is built near the river at the foot of the mountain, which is above 800m to the water treatment station at the waist of the mountain in straight distance. There are three 30kW water pumps with the lift of 110m in the intake station, two for use and one standby. The start and stop of the water pumps of the intake station is also controlled by the person on duty of the water treatment station. The person on duty observes the water level of the water treatment settling basin. If the settling basin is nearly full or its pump discharge is not sufficient, such person on duty will walk about 1 kilometer to the intake station to conduct corresponding operations. When the settling basin is nearly full, if not treated timely, much water will be lost when the settling basin is full, which results in waste of electric energy and loss of water resources. The water treatment station sends the treated water to high position cistern, where there are two 45kW water pumps with the lift of 80m, one for use one standby. When the high position cistern is full, it also results in waste of electric energy and loss of water resources.
IIControl Scheme
Transformation is required due to the above problems, and the user’ requirements are as below:
1.Automatic intake station;
2.Automatic control of water level of settling basin and high position cistern;
3.Energy saving to a certain extent.
In view of automatic intake station, soft starter is selected because of its reliability, and a soft starter is provided for each water pump, totally three soft starters. There are “high”, “medium” and “low” water levels in the settling basin. PLC is used to determine whether one or two water pumps is/are running according to the water level of the settling basin, and PLC can be also used to ensure same use time of three water pumps. The operation signal and fault signal of water pump will be displayed in water treatment station sent by the cable. The pumping level of intake station is 2m lower than the installation position of the water pump. Initial startup or restart after stop by the water pump may form “vacuum”, which will cause no water can be sucked, and the previous solution is manual water injection. The water injection and air discharge must be automatically completed after automatic operation, which can be realized by means of indirect detection manner, i.e. the running current of the water pump motor shall be detected. If there is “vacuum” in the pipeline, the motor will nearly be idle running, with low current; if there is no “vacuum" in the pipeline, the motor will nearly full load running, and the current is basically equal to rated current. The difference of those two conditions is great. Whether there is vacuum in the motor or pumping pipeline can be determined by means of current detection. In case of any problem, PLC will give alarm signal to inform the person on duty for treatment.
Figure 4Control System Map of Intake Station
In view of energy saving, the water pump of water treatment station used to send water to high position cistern shall be driven by Senlan SB61P+ series inverter. The water level signal of high position cistern will be sent to water treatment station by means of cable, which will form a closed loop water level control system with inverter. Such closed loop water level control system can be properly adjusted to realize excellent energy saving effect.
IIIEnergy Saving Effect
The volume of high position cistern is 400m3, and if the discharge valve of high position cistern is closed, a 45kW water pump can fill the high position cistern if working for 4 hours, and the consumed electric energy is:
W=45×4COS=162kWh(Presumed COS=0.9)
After Senlan inverter is used, the frequency of inverter is adjusted to be 43Hz. The lift H1 after the water pump is adjusted is 74% of design lift (H) of the water pump, and there is no surplus lift. The flow Q1 after the water pump is adjusted is 0.86 of design flow (Q) of the water pump. A 45kW water pump can fill the high position cistern if working for 4.65 hours, and the consumed electric energy is:
W1=(0.86)3×45×4.65×COS=119.2kWh(Presumed COS=0.9)
Power consumption is reduced by 42.8kWh for filling a high position cistern. Under actual conditions, the discharge valve of high position cistern cannot be closed. Generally speaking, the water pump can also supply water to the user when supplying water to high position cistern. The daily energy saving can be estimated according to water consumption of the user. It is indicated on the flow meter that the daily water consumption of such user is about 800m3. Calculated as per 800m3, the daily power saving is 85.6kWh and the annual power saving is 85.6×365=31244kWh.
Case IV
Application of Senlan Inverter in Heat Exchange Station
IIntroduction
The residual heat of steam turbine of thermal power plant after power generation shall be fully utilized to supply heat to northern cities in winter in a concentrated way, so as to save energy sources and reduce urban pollution. When the hot water sent from power plant arrives at heat exchange station in the city, the primary supply temperature of hot water is higher than 90℃, and the primary return water temperature of the hot water will reduced to be 60℃ after passing the heat exchanger, and then the hot water will flow back to the power plant. The secondary return water temperature of the hot water sent to the urban residents after passing the heat exchanger of the heat exchange station will be higher than 50℃, and the secondary supply water temperature will be higher than 60℃. There are many similar heat exchange stations in Baoji, Shaanxi, of which a heat exchange station consists of four heat exchangers, a circulation pump package composed of four 37kW line pumps, a 3.7kW makeup pump. The flow of circulation pump and makeup pump is controlled by manually opening and closing the valves, which results in increase of pipeline damping, thus causing waste of electric energy.
IIControl on Frequency Conversion Speed Regulation of Heat Exchange Station
1.Control on frequency conversion speed regulation of makeup pump
In order for further energy saving, the heat exchange station shall be subject to automation transformation, and the circulation pump and makeup pump shall be adjusted under frequency conversion, and the whole urban heat supply system shall be monitored by the computer to realize automatic control of the heat exchange station. The hot water is running in the heat supply system by means of circulation pump, and the leakage of pipeline or valve may cause the pressure drop of the circulating water. If no water makeup is provided timely, the heat supply system will operate abnormally. The water makeup manner of the makeup pump is simple, which employs water supply under constant pressure, with the setting pressure of 4kg. A Senlan SB12S3.7KW inverter and a Senex DG130W-BZ-A 1MPa pressure transmitter are selected in this case. The frequency conversion speed regulation water makeup system is as shown in Fig. 7:
Figure 7Schematic Diagram of Frequency Conversion Speed Regulation Water Makeup System
2.Control on frequency conversion speed regulation of circulation pump
The ultimate objective of heat supply system is to maintain the stability of indoor temperature of the heat user, however, there is no room temperature regulator in the heat user’s room, and the room temperature of most heat users cannot form closed loop control. The most effective method is to control secondary supply water temperature of the heat exchange station according to the demands in order to ensure economic operation and quality of heat supply. The secondary supply water temperature under steady state condition can be obtained according to the principle that the heating capacity of system under steady state condition, the heat dissipating capacity of the heat radiator and the heat consumption of the user are same:
（1）
Making amendment to Formula (1) and considering that the indoor temperature , ratio of actual flow and design flow of secondary pipe network and return water temperature are approximately constants, then:
（2）
Where: a, b and c are relevant meteorological parameters of the district where the pipe network is located.
Formula (2) is the calculation method for setting value of secondary supply water temperature. determined by Formula (2) can track the change of outdoor temperature , which enable the indoor temperature of the heat user not affected by the change of , so as to realize stable heat supply.
If the outdoor temperature is changed, in order to maintain indoor temperature basically constant, a control strategy is to control the rotation speed of inverter of circulation pump by means of the temperature difference of secondary intake water and return water, and the temperature difference of secondary intake water and return water is set at 12. When the temperature difference of secondary intake water and return water is greater than 12, the inverter of circulation pump will accelerate; when the temperature difference of secondary intake water and return water is less than 12, the inverter of circulation pump will decelerate. Frequency conversion speed regulation system map of inverter of circulation pump is as shown in Fig. 8:
In the figure, BP1 is Senlan SB12S37KW inverter, BU is soft starter (auto-transformer starter). The system employs cyclical switchover manner to send the temperature difference signal into PLC, which will be taken as frequency conversion speed regulation signal in the inverter after processed by PLC. The operation information of the whole system will be sent into the computer by means of PLC.