Application of DANFOSS soft starter in low-pressure and high-power reservoir pumping station

Abstract: This article introduces the application of soft starters in low-voltage and high-power reservoir pumping stations. It provides the electrical schematic diagram of the system, the main parameter settings of the soft starter, and the soft start and soft stop of multiple motors driven by one soft starter. Solutions and main issues to pay attention to during debugging.
Keywords: City water supply pump soft starter soft start soft stop
I. Introduction:
Reservoir pumping stations in urban water supply systems are generally equipped with several high-power reservoir pumps and booster pumps. Their good and stable operation provides reliable guarantee for urban residents’ domestic water supply and socialist industrial and agricultural production. At present, many existing equipment list are aging, the daily maintenance workload is heavy, and the control principles are also relatively backward. How to rationally transform existing equipment to achieve the highest working efficiency without increasing a large amount of investment is currently a key issue that needs to be solved.
There are three 280kW booster pumps and three 280kW reservoir pumps at the Zhenbei Pumping Station under the Shibei Pumping Station Management Office of Shanghai Water Supply Company. They all use auto-coupling starting cabinets for starting, and the starting current is large. With the long-term use of the equipment, the loss of switching components and water outlet valves is relatively serious. In order to reduce operating costs, reduce equipment losses, and improve management levels, it was decided to use PLC automatic control systems and motor soft starters to transform the original equipment.
2. Working principle and transformation plan:
1.Control principle
The motor starter decided to use a soft starter. The basic principle of the soft starter is shown in Figure 1. The output voltage is controlled by controlling the conduction angle of the thyristor. Therefore, the soft starter is essentially a step-down starter that can be automatically controlled. Since it can adjust the output voltage arbitrarily and perform closed-loop current control, it is much faster than traditional step-down starting methods (such as series resistor start, autotransformer start). etc.) has more advantages. For example, it can start variable torque loads such as custom blac peek pet hair dryer fan and water pumps at full load, realize soft stop of motors, and can completely eliminate the water hammer effect when applied to water pumps.
At present, motor soft starter control systems at home and abroad generally use a control scheme with one soft starter and one motor. In order to save investment, we have conducted multiple demonstrations and worked with technical experts from Shanghai Jinxin Inverter Co., Ltd. After simulation tests, it was finally decided to adopt the solution of one soft starter with multiple motors. At the same time, in order to ensure the fault-free continuous operation of the equipment to the greatest extent, the original auto-coupling start cabinet was modified to retain all the functions of the original auto-coupling start, and the operating contactor in the original auto-coupling cabinet was used as the soft starter. Bypass contactor, bypass the soft starter after it is started, so that the soft starter can continue to control other motors. The soft stop control strategy is just the opposite. First, disconnect the running contactor in the original autocoupling cabinet, switch the motor to be controlled by the soft starter, then start the soft stop function of the soft starter, gradually reduce the output voltage, and switch the motor to It slowly comes to a stop and finally the soft starter exits, waiting to accept the next start or stop operation.
2. System block diagram:

Figure 2 is the main circuit wiring diagram of a single incoming line.
3. Main equipment selection:
The water supply system is related to thousands of households. The design of the system and the selection of equipment must consider its reliability and the credibility of the product manufacturer, so that the system can be as foolproof as possible under normal working conditions. But at the same time, the economy of the system must be taken into consideration. Therefore, in terms of equipment configuration, domestic and foreign high-quality products should be used as much as possible for the main key equipment. For some auxiliary equipment that does not affect the performance and reliability of the system, cheap and high-quality products can be used. To improve the cost performance of the entire system.
3.1 Programmable controller (PLC) selection
PLC is one of the key components of the control system. When selecting the model, in addition to meeting the above requirements, future maintenance and expansion must also be considered, as well as maintaining a unified communication interface with other pump stations, so as to prepare for future connection to the host computer for monitoring. Be prepared to wait. After comprehensive consideration, we decided to use ROCKWELL’s SLC500 series PLC.
3.2 Soft starter selection:
Choose the MCD3000 industrial soft starter produced by Danfoss Industrial Co., Ltd. in Denmark. This product has the following advantages:
​ 1. It has a dedicated bypass terminal and current monitoring during bypass, which can provide overload protection;
2. Danfoss soft starters have many proprietary functions and are particularly suitable for this transformation;
3. Danfoss soft starter has built-in soft stop function;
4. Danfoss soft starter is a true constant current closed-loop control, which can start the fastest with the set starting current value, and the logical judgment of the start completion is also more reasonable.
The water pump motor model is JS137-6, 280kW, 493A, 983rpm. The pumping station is powered by two high-voltage incoming lines, each with three motors. Therefore, it was decided to use two 300kW soft starters (model MCD3300). Each power supply is connected to one soft starter. Each soft starter is responsible for the soft start and soft stop of three motors. The starting control principle is shown in Figure 3. It is completely controlled by current closed loop during starting. When the motor accelerates to the rated speed, the current also drops to the rated current, the thyristor is fully opened, and finally the soft starter sends out a running signal.
3. Programming
Using ROCKWELL’s RSLOGIX500 programming software, the program block diagram is as follows:
4. Precautions for installation and debugging:
On-site installation and debugging are carried out without affecting the normal water supply of the entire pump station, so a detailed construction plan must be formulated in advance, and the engineering company is required to have:
-Have strong technical strength
-Ability to coordinate large quantities of materials, equipment ordering, transportation and purchase issues
-Have experience in the design and construction of corresponding projects
-Have a good management system
This construction was carried out pump by pump after full coordination with dispatch. The same is true when debugging. First, select a time when all three pumps on a certain power supply are not in use, and then debug the soft starter of this power supply. Then select the controlled water pump, and completely close the water outlet valve of the selected water pump. The debugging work will affect the pipe network; and then the formal debugging action will begin. Note that the water pump is pumping water at this time, and the running time should not exceed 1 minute; after these debugging tasks are successfully passed, the water outlet valve can be opened and the full load soft water can be operated. Start the soft stop test. At this time, because it is full load, the soft start time will be longer than that at no load. Therefore, the parameter setting of the soft starter must have a certain margin; finally, through the test, summarize the data and obtain the best The control method and control parameters are ready for trial operation.
In addition, the power distribution of low-voltage high-power motors is relatively compact, so most debugging should be completed under simulated conditions as much as possible to reduce the chance of failure during formal debugging.
5. Operational effects and significance:
This control system has been officially put into operation at the Zhenbei Pumping Station. The water outlet valves of the six water pumps are fully opened, and they are no longer operated under normal circumstances. The starting current of the soft starter is set to 330%, and the starting time is about 5-6 seconds. The starting impulse current endured by the power supply system is similar to that when starting with an autotransformer; the maximum current during soft stop does not exceed 170%, and the soft stop time is about 7 seconds without any impact on the check valve. The system has been running normally since it was put into use.
The successful application of this system will bring far-reaching significance to the design and transformation of booster pump stations and reservoir pump stations in urban water supply systems in the future:
1. Since the use of soft starters can realize full-load start and soft stop of the water pump motor, the water outlet valve of the water pump can be completely eliminated;
2. The successful application of high-power soft starter “One Belt and More” will significantly reduce the cost of motor starting control systems for multi-pump stations in the future;
3. The soft starter itself has voltage and current protection functions, and has inherent zero-current switching characteristics, which strengthens the protection of the motor and maximizes the
Greatly reduces contactor losses;
4. Because this type of system has PLC as the main controller, its functions can be further expanded in the future.
6. Conclusion:
Due to the use of advanced PLC and motor soft starter, the original electrical control has been successfully transformed, and the electrical control and management level of the entire pumping station has reached a new level. With the continuous updating of power electronics technology and the continuous development of automation control theory at home and abroad, I believe that there will be more and better solutions in the future.