Application of Frequency Conversion Technology for Low-adding Drain Pumps in Thermal Power Plants


The steam turbine of the Jilin Petroleum Group Thermal Power Plant is a CC50-8.83/1.28/0.118 steam turbine manufactured by Harbin Steam Turbine Works. The heat recovery system consists of six steam extraction units, condensers, two high-pressure heaters (hereinafter referred to as high-pressure heaters), and three low-pressure units. Heater (hereinafter referred to as low plus) and deaerator. The high-pressure portion of the hydrophobic system flows from high-pressure to low-pressure self-recirculation to a high-pressure deaerator, low-water-filled from 3" to 2" low, and then drained via a hydrophobic pump to a 3" low-increased condensate inlet tube, 1" low. Add water to the condenser. The hydrophobic pump is 3N6×2 type, lift head is 128m, power is 17.8kW, rotation speed is 195Or/min, motor is Y180M-2 type, power is 22kW, speed is 2940r/min. The outlet flow rate of the hydrophobic pump is adjusted by an electric regulating valve.

The problems existing before the frequency conversion of low-adding hydrophobic pumps in our factory are as follows: 1 Low-adding hydrophobic pumps due to constant speed, large throttle flow during operation, high outlet pressure, and frequent loss of working medium caused by leakage of the seal shaft at the pump shaft. At the same time easily lead to bearing water, not only affect the normal operation of the pump and even damage to the bearings and pumps, but also a large energy loss; 2 pump outlet control valve linearity is not high, affecting the stability of automatic adjustment put into operation, 3 due to poor regulation, Frequently, the low-water pump with low water level or low water level does not operate, causing cavitation of the hydrophobic pump, large vibration of the water-removal pipeline, large amplitude of current change, damage to the pump impeller, impeller, pump shaft, bearings, packing, etc. The service life is long, the maintenance work is heavy, and the cost is high; when the water level is too high in severe cases, a high level of low water level protection action must be used to affect the safe operation of the unit.

In recent years, with the increasing perfection and maturity of frequency conversion technology, and at the same time, in order to save energy, improve the characteristics of various water level adjustment methods and realize the adjustment of low water levels, after field technical demonstration, our factory decided to implement frequency conversion control for low-adding hydrophobic pumps. .

First, the basic principle of variable speed regulation

Proportional theorem based on similar law of pump flow:

Q1/Q2=n1/n2 (1)

Where: Q1 is the prototype pump flow; Q2 is the model pump flow; n1 is the prototype pump speed; n2 is the model pump speed.

The flow rate of the pump is proportional to the primary speed of the rotation speed, so long as the pump rotation speed is changed, the output of the pump can be changed. Asynchronous motor speed formula:

n=ne(1-s)=60f(1-5)/P (2)

Where: n is the motor speed; ne is the motor synchronous speed; s is the motor slip; f is the power frequency; P is the number of motor pole pairs.

From equation (2), it can be seen that the rotation speed of the three-phase AC motor depends on the synchronous rotation speed ne and depends on the power supply frequency f. When the power supply frequency f is changed, the synchronous rotation speed changes in proportion to the frequency, and the rotation speed of the asynchronous motor n also follows. The change, so if you can smoothly change the power frequency, you can smoothly adjust the speed of the asynchronous motor.

The control principle of low-adding hydrophobic pump frequency conversion speed control is through its water level automatic control unit (located in the internal set of the frequency converter) to output the signal quantity to the frequency converter. The frequency converter controls the frequency converter according to this signal quantity to change the rotation speed of the motor and thus change the pump. The output, when the water level is low, increases the output frequency of the inverter and achieves the purpose of controlling the low water level.

Second, low-drainage pump frequency control system design program

Hydrophobic pump motor for frequency conversion speed conversion, that is, through the differential pressure sensor to reflect the 2 "low water level changes, through the inverter automatically adjust the motor speed, change the flow of the hydrophobic pump to control the 2" low water level in the set range. In order to facilitate debugging and maintenance, the transformation system has a start/stop switch in place.

1 The original manual start and stop and interlock, interlocking retention, to control the power switch of the two devices.

2 The frequency changer control cabinet is installed in the main factory building, there are two kinds of control modes of full-automatic, manual speed-adjustment running. In the centralized control room, 2 "low pressure heater water level monitoring and hand, automatic transfer switch, remote operation device, and retain the original hydrophobic pump circuit breaker and control switch. In the automatic state, through the internal function of the inverter settings, According to the requirements of the operating personnel on the low water level, set the parameters of the PJ knife reasonably, combine the collected water level value to compare the water level, and adjust the speed automatically; in the manual state, start the switch and switchboard switch and intelligent manipulator According to the actual value of the low water level displayed on the disk table and the actual operation of the unit, the corresponding adjustment shall be made, and the wiring shall be changed from the original drain pump power disk cable to the frequency conversion control cabinet, and then the power cable from the frequency conversion control cabinet to the motor .

3 The normal operation mode is 1 "the automatic frequency conversion running of the hydrophobic pump, and 2" the pump is used as the standby mode of the power frequency. When 1" drain pump frequency conversion speed control system fails to trip, 2" hydrophobic pump starts automatically to ensure stable operation of low-add hydrophobic system.

4 After the inverter fails, the internal output fault signal is used to start the communication of another drain pump and the water level control is switched to the pump outlet control valve.

In order to improve the reliability of the system, it is ensured that the frequency converter can operate normally under various conditions, and it will not cause loss of control. The redundant control method using analog quantity and digital quantity is coexisting. When the system is normal and automatic, a 4~20mA analog analog control method is used. When the system is faulty (mainly considering AO fault, loss of power, or disconnection) or manual operation, a switching stepping operation mode is adopted.

In order to simplify the control system and improve reliability, the inverter's built-in PID is used to achieve low water level control.

Normal start and stop, including manual start, stop and interlock start and stop, are controlled by the central control room inverter. Emergency shutdown is achieved by electrical disconnection of the MCC switch.

Third, the frequency conversion speed control system composition

The frequency conversion speed control system is mainly composed of an automatic control unit (located in the inverter), a frequency converter and a hydrophobic pump. The inverter is of type ACS800-01-0030-3 manufactured by ABB. The motor power of the hydrophobic pump is 22kW, taking into account the high summer temperatures, it will lose a little power, set aside a certain margin, using 3OkW inverter. Frequency control system shown in Figure 1.

Technical specifications of the inverter: 1 model is ACS800-01-0030-3+P9O1; 2 output current over-temperature derating characteristics, the rated current limit temperature is 40 °C, each increase of 1 °C, the rated output current is reduced by 1.5%, the highest For 50 °C, according to 22kW output, the maximum temperature is 48 °C.

Environmental indicators: 1 temperature range of 0 ~ 48 °C; 2 relative humidity range of 5% ~ 95%; 3 does not allow conductive dust; 4 maximum vibration of 0.3mm.

It is mainly divided into two parts: the main loop and the control loop. The main loop consists of rectifiers, filter links, and inverters. It is used to provide voltage-regulating and frequency-modulating power supplies for the motor. The rectifier converts the power-frequency power supply to a DC power supply, and the filter links absorb the voltage fluctuations between the rectifier and the inverter. The converter converts the DC power supply back to AC power. The control loop provides signals to the main loop, from the frequency and voltage operation loops, the main loop voltage/power detection loop, the motor speed detection loop, the drive loop that amplifies the control loop control signals, and the inverter and motor Protection circuit composition.

During the implementation of the project, we mainly carried out the following two tasks: the collection of local analog signals and signal distribution; according to the situation of our factory, the function of ABB inverter was set to PID control mode, and after discussion with the operating personnel, Specifically set the P/I value, as well as the given value. After commissioning, the system has been stable since the system was put into operation. The automatic input rate is 100%.

Fourth, after the transformation of the frequency control system

The transformed frequency conversion speed control system has the functions of frequency conversion operation, power frequency operation, and protection.

4.1 Frequency conversion operation function

1 Place the control/remote control selection switch on the control cabinet in the near-control position so that you can operate it manually. Local operation is only allowed for inspection and commissioning. During normal operation, it must be driven to a remote control position and operated by the operation buttons on the panel.

2 When the drain pump needs to be activated, the operation button on the panel triggers a start pulse. At this time, the frequency conversion AC contactor pulls in, the frequency converter starts, and the pump operation signal is output. The pump motor will automatically adjust the water level according to the set water level to keep 2" low. The water level is within the set range.

3 When the pump is stopped, a stop pulse is sent out from the operation button on the panel. At this time, the inverter stops working and the ice pump stops running. The frequency conversion AC contactor is disconnected and the pump operation signal disappears.

4 Low plus hydrophobic pump operation, the frequency conversion system can be adjusted by its own PID water level, automatic speed adjustment; also through the switch switch from the disk operating system to 2 "low water level adjustment.

4.2 Power frequency operation function

When the hydrophobic pump is running in a variable frequency mode, if the frequency converter fails or the power supply for the frequency conversion operation fails, the motor automatically switches to the power frequency operation state. Water pump frequency operation signal output, frequency conversion fault signal output at the same time. The drain pump cannot automatically adjust the speed at this time. The operating personnel should adjust the valve to control the 2" low level.

4.3 Protection Functions

1 When a low-adding hydrophobic pump is running, if there is a sudden increase in the 2" low plus water level and the set water level is reached, then the power frequency standby pump is started and the two pumps are operated simultaneously. When the water level drops to the low water level setting value , stop the backup pump.

2 If the running pump trips after a fault, the standby pump starts automatically.

3 The control method of the frequency converter adopts the redundant control method in which the analog quantity and the switch quantity coexist. When the system is normal and automatic, 4~2OmA analog control mode is used. When the system is faulty (mainly considering AO fault, loss of power, or disconnection) or manual operation, the switchable step operation mode is adopted.

In order to simplify the control system and improve the reliability, the inverter's built-in PID is used to achieve low water level control. Normal start and stop, manual start and stop and interlock start and stop are all controlled by the central control room inverter. The emergency stop button is set on site, and emergency stop is achieved by pulling off the power supply of the inverter.

V. Conclusion

After the transformation, the low-adding water pump variable frequency speed control system has been put into trial operation and put into operation, which can greatly improve the water level adjustment characteristics and successfully realize a low-level automatic adjustment system. At the start of the motor, both current and flow rise steadily. Avoid inrush current and water hammer phenomenon and prolong the service life of the system. It can prevent other equipment from operating abnormally due to instantaneous voltage drop caused by inrush current. The door can be fully opened, throttling loss can be reduced, and electric energy can be saved. At the same time, the system is easy to operate and has a high degree of automation. Has a number of protection functions (such as over-current, over-voltage, under-voltage, phase loss, overload, etc.) to protect the motor. Reduce equipment failure rate. The low-add hydrophobic pump adopts variable frequency control mode, the automatic control performance is excellent, the low water level is basically stable, the general fluctuation range is within 5Omm; the hydrophobic pump operates stably, low noise, low current, low current fluctuation value; low plus system operation stability Reliable, no low or high water level operation has occurred. The hydrophobic pump adopts variable frequency control method, which avoids the phenomena of low plus water level operation, cavitation of the hydrophobic pump blade, air pump, stern shaft, etc., reduces heat and wear of the pump bearing, and prolongs the life of the hydrophobic pump. Since the transformation, there have been pump vibrations at the outlet of the pump and damage to the pump impeller, guide impeller, and bearings. There is basically no maintenance. After the variable frequency control of the hydrophobic pump, the energy loss caused by throttling of the pump outlet regulating valve is eliminated, and the no-load loss of the pump and the motor is effectively reduced.


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