Discussion on Stator Insulation of Large Motors in Drainage Pumping Station

With the improvement of the flood control and drainage level of the city, various large-scale axial flow pumps, mixed flow pumps and centrifugal pumps have been put into the operation of the drainage pump station. Since the pumping station is usually built in low-lying and easy-to-open areas, environmental factors such as humidity and poor ventilation, As well as the influence of hydraulic conditions, machinery and operation modes during operation, it often causes the insulation performance of large motor stators of the unit to be reduced. According to incomplete statistics, the pump station shutdown caused by the stator winding insulation problem accounts for about one-third of all shutdown accidents, which has seriously affected the city's flood control and drainage. Therefore, analyzing the reasons for the reduction of the stator insulation resistance of the exhaust unit and finding a reasonable solution should be an urgent problem for the current pump station management personnel.

1, the damage of the stator winding insulation

In the hot state, the insulation resistance value of a small and medium-sized low-voltage motor is generally not less than 0.5 megohm, and the insulation resistance of a stator of a high-voltage motor per kilovolt is not less than 1 megohm. However, after many years of operation, we found that the insulation performance of the stator of the unit has decreased. For example, after the operation of several 250KW exhaust units installed at the Hongqipu drainage station, after several years of operation, the motor was operated by the personnel during the non-flooding equipment inspection. When the stator insulation was measured, it was found that the insulation resistance of the three motors was about 0.5 MΩ, indicating that the insulation performance of the stator winding has been degraded. The cause must be identified and repaired to avoid unnecessary losses.

If the long-term stator insulation performance of the drainage unit is low, during the flood season or during the typhoon, when the over-standard water level occurs in the discharge area, the shaft power will increase due to the increase of the lift, especially the large high-voltage motor with the power supply working voltage above 1000V. During the operation process, the stator coil turns or the phase-to-phase short circuit is easily generated to burn the motor, which directly affects the life of the unit.

2. Reasons for the reduction of stator insulation resistance

Based on years of experience in drainage operation, combined with the specific environment and working conditions of the drainage unit, we set the large motor for the pumping station.

The reason for the reduction of the sub-winding insulation is analyzed.

Taking a three-phase asynchronous motor as an example, the main indicators affecting its insulation performance include:

a, slot insulation: to prevent the winding from short-circuiting the iron core (ground);

b, turn-to-turn insulation: to prevent short circuit between winding turns;

c, interlayer insulation: for the double-layer winding, to prevent short-circuit between the layers;

d. Phase-to-phase insulation: Prevent phase-to-phase short-circuit between three-phase windings.

The main reasons for the reduction of the insulation performance of the stator winding of the motor are: environmental pollution, working conditions, pump room temperature and so on.

2.1 Environmental pollution - moisture, vacuum and other pollutants

The drainage unit is installed on the lower side of the river. The environmental factors cause the stator coil of the unit to be easily contaminated by moisture. The pollutants contain salt, acidic or alkaline substances, metal ions, water, etc. These substances stick to the surface of the stator coil. Corrosion of insulating materials leads to an increase in electrical conductivity, a decrease in resistivity, and a significant increase in polarization of the dielectric medium. Different dielectric layers are present in the medium, and a layer of polarization redistribution of charge is formed on the layer. Dielectric loss. When the coil is contaminated and the moisture is more serious, the dielectric loss is greater. The dielectric loss causes the internal heat of the insulating medium to rise, and the temperature rises, so that the dielectric loss is more serious until the insulation is aging, and finally the insulation resistance of the motor is continuously reduced.

2.2 Working conditions factors - the impact of mechanical, hydraulic, and operating modes

The drainage unit undertakes the task of flood control and drainage in the city. Therefore, during the flood season, especially during typhoon or flood, when the water level in the discharge area exceeds the standard water level and even reaches the warning water level, the pumping unit will increase the shaft power due to the increase of the lift, and the pump will often be overloaded. It is easy to cause the motor to be overloaded and cause insulation aging. In addition, during the drainage period, the pumping station often has the phenomenon of debris in the inlet channel of the water inlet channel, and the design of the inlet channel is unreasonable, which causes the upstream water to be poor, the pump submerged depth is not enough, the unit load changes greatly, the bearing is uneven, and even The phenomenon of frequent start and stop of the unit occurred. Since the stator winding is different from the Stator Core and the wedge material, the stator coil temperature rises faster than the iron core (20 to 30 °C higher than the core temperature) during startup, and the expansion degree is greater than that of the stator core and the slot wedge; When frequent start-stop or load fluctuation is large, according to the structural characteristics of the stator, the winding coil is nested in the stator channel. Due to the thermal expansion and contraction phenomenon and the change of the characteristics of the insulating material after long-term operation, the wedge will appear loose. In addition, the coil in operation vibrates due to the alternating radial electromagnetic force, and the anti-corona layer of the stator coil gradually wears, which will cause the stator coil surface of the motor slot to be lost between the slot wedge and the slot wall. Good contact produces spark discharge, causing corona and electrical corrosion, which ultimately results in reduced or damaged insulation of the motor stator. For example, the large-scale inclined horizontal water pump of the Kuiyu drainage station, the supporting 2000KW high-voltage two-speed motor found the phenomenon of aging shrinkage of the stator groove wedge material, aging and loosening of the underlying strip during the overhaul period, and then adopting the replacement aging tank during the overhaul process. Wedges, replacement of thicker wedge underlays, and then into new wedges, coated with epoxy glue and other treatments, the performance of the motor can be repaired and improved.

In addition, the motor and water pump of the pumping unit are usually connected by a coupling. During the operation, the pump often causes vibration of the unit due to various reasons such as uneven inlet flow rate and pressure distribution, load change, etc.; pump start and stop, valve opening and closing, etc. The sudden change of pressure in the water pipeline and the water hammer caused by the dynamic transition process such as working conditions and emergency shutdowns often cause the unit to vibrate, causing mechanical failures such as ablation of the bearing bush of the unit. There is an air gap between the stator and the rotor on the structure of the motor. If the vibration exceeds the maximum allowable value, the rotor of the motor will be eccentric, the Rotor Core will be “broomed”, the temperature of the core will rise rapidly, and the motor will smoke or even affect the motor. The insulation of the stator windings eventually causes the coil to burn out. For example, in January 2009, the 1# unit of Binde Pai Station was violently vibrated when it was turned on. It was visually seen that the main shaft swayed and smelled of rubber burnt smell. After inspection, it was found that the stator and rotor gaps were uneven, and the rotor was biased toward positive Y and positive. The X direction is 0.45mm, and the unit's water guiding bush is seriously worn, which may cause a big accident of burning the motor coil.

2.3 The influence of pump room temperature

The installation of the drainage unit is affected by the geographical location. The ventilation and heat dissipation conditions in the pump room are not ideal. During the flood season and typhoon, many of them are in the high temperature summer. Although certain heat dissipation measures have been taken, more large units are installed in the pump room. Air blower, but due to the lack of air guiding channels, the air convection in the pump room is not smooth, the effect is not ideal, and the operating environment temperature of the unit is still high. In the case of high temperature, the inside of the insulating material of the stator winding generates heat due to dielectric loss. When the heat of the insulating medium cannot be dissipated, the temperature of the insulating medium will continue to rise, causing a thermal change, and finally the insulation resistance performance is lowered.

3. Measures to improve stator insulation resistance

We start from the reason of the insulation performance of the stator winding. Combined with the actual operation of the pump station, we can improve the insulation performance of the large motor stator by means of motor drying, changing the environment and working conditions, and regularly measuring the stator winding and stator cleaning.

3.1 motor drying

In view of the wet geographical environment and natural working conditions of the pumping station, the stator coil of the pumping unit is easily damp, especially in the state of non-flooding unit for a long time out of operation, and must be dried before starting. In addition to setting the heater for the motor, it can also be manually and quickly dried: the stator coil of the motor is connected to the output end of the AC welding machine, and the current of the welding machine is controlled and adjusted so that the maximum current distributed by each phase winding of the drying motor is not Exceeding 50%-60% of the original rated current (due to the specific conditions of various motors, the general required drying current should be such that the stator core temperature should reach 70-80 °C within 3-4 hours of energization) After the power is turned on, the copper loss of the motor itself is used to dry the motor. Generally, the insulation resistance of the stator coil can be improved by 10-20 hours. If the motor heater power is sufficient, the small pumping station can also use the non-flooding light-load test machine to dynamically dry. For example, the management mode of our current pumping station is required to test the machine twice per month according to the water level. 30 minutes to 60 minutes, so that you can dry the tide, but also through the test machine to judge the working conditions of the unit, killing two birds with one stone, the effect is very good.

3.2 Changing the environment and working conditions

Improve the pump room environment, improve equipment hygiene, reduce dust, oil and other pollution; in addition, the pump room adds ventilation facilities to enhance air convection and reduce operating temperature; rationally design and improve the inlet and outlet channels, adjust the operation mode of the unit, try to avoid Stator coil damage caused by hydrological hydraulics and pump mechanical reasons, reducing unit vibration, reducing the number of opening and closing, and so on.

3.3 Measuring stator windings

Regularly measure the stator insulation resistance and arrange preventive tests reasonably. The stator insulation resistance measurement is generally measured once every quarter, and the cycle is shortened during the rainy season. The submersible pump should increase the frequency of measurement. In particular, the high-pressure submersible pump should preferably measure the insulation before each startup. Each pumping station should archive and compare the data for each measurement so as to grasp the condition of the stator windings through data changes at any time.

3.4 stator cleaning

Stator cleaning is carried out regularly, at least once a year, once a year for core cleaning. The cleaning method is as follows:

Remove the dustproof casing of the motor, purge the stator coil of the motor with 7-15 bar of compressed air, blow out most of the dust, and then mix the stator with compressed air with HS-25 type motor cleaning agent, so that the bond can be compared. Clean the dust and oil, and then spray the NC-123 insulation protection agent after cleaning. If the stator is too oily, use more cleaning agents and you will not be able to clean it.

4. Monitoring and repair of stator insulation

4.1 Monitoring measures

Motor stator winding fault monitoring methods are mainly divided into two categories: one is based on fault feature information extraction and monitoring, the signal is mainly from the motor's current, voltage, temperature and flux, vibration, etc.; the other is based on accurate establishment A fault identification method for a mathematical model of motor faults, such as a parameter estimation method. At present, different types of motor stator winding insulation detecting devices have been produced at home and abroad according to the above monitoring methods. They can not only monitor the initial fault of the stator winding short circuit of the asynchronous motor on-line with high sensitivity and high reliability, but also can judge the short circuit phase. Effectively ensure the safe operation of the motor.

4.2 Repair of stator windings

In repairing the insulation resistance of the stator coil, the dumping unit is responsible for the urban flood control and drainage. If the faulty unit returns to the factory for overhaul and has a long time and high cost, it will inevitably affect the flood prevention work. Therefore, we can learn from the valuable experience of the power plant, know in advance the production process and insulation composition of the equipment manufacturer, and make some necessary fixtures under the premise of ensuring the maintenance process. After determining the fault point, the following maintenance methods can be used: First use the blade to open the fault point, remove the original insulation layer and clean it. If the fault area is large, it can be wrapped with powder mica tape, but it should be noted that the mechanical strength and size of the bandage insulation tape should be matched with the original factory condition. A small area of ​​the fault can be inserted into the epoxy fiberglass board with a thickness of 0.15mm. After immersion and drying, the motor should be subjected to the corresponding level of short-circuit test and preventive test.

Conclusion

With the improvement of the role of the drainage unit in the urban flood control business, the problem of large-sized motor stator insulation has been paid more and more attention in the pumping station. For the motor manufacturer, how to monitor and protect the stator insulation resistance and the operation of the pump station The combination of working conditions is an important issue at present. A large number of related research work has been carried out at home and abroad. For example, the use of ultrasonic pulses to scan the stator insulation material in operation, and the analysis of the aging of the main insulation according to the echo will be a new research direction.