With the continuous development and improvement of frequency conversion technology, its excellent power saving performance, good starting characteristics and various protection functions have been well reflected in the enterprise, which has saved energy, reduced costs and increased the production of the enterprise. ASD.

1. Features of the inverter

Frequency conversion speed regulation is the best performance and the most promising speed regulation technology. The frequency converter is the main product used for speed regulation of three-phase asynchronous motors in industrially developed countries. Its main advantages are: good energy-saving effect, up to more than 55%; wide speed range, speed ratio up to 20:1; good starting and braking performance, soft start, automatic smooth acceleration and deceleration and rapid braking; The protection function is perfect, which can realize multiple protection methods such as overvoltage, undervoltage, overload, overcurrent, instantaneous power failure, short circuit, stall, etc., and can realize fault judgment display; it is easy to use in electronic computer systems and can realize remote control.

2. Application of frequency conversion speed regulation technology in the control of cogeneration system

2.1 Central heat exchange station and external network control

The boiler system mainly includes system constant pressure water replenishment, system hot water (carrier) circulation, system combustion control and furnace pressure control. In cogeneration, the superheated steam generated by it can be directly output, or hot water can be output after steam-water exchange The heat energy of steam or hot water is provided to each heat user through each heat exchange station in the pipe network.

2.2 Control of external circulation pump

Ensuring that the pressure in the heating system is constant is the basic prerequisite for the normal operation of the heating system. The pipe network of the heating system is usually a closed pipe system. In theory, the water consumption is very small. However, in practice, "run, run, drip, leak" and man-made water loss will inevitably affect the stability of the system pressure. In addition, the temperature of the water flow in the system also affects the pressure changes. For the system to have stable heating, it must ensure that the highest point of the system is in full water, and the pressure cannot exceed the system's capacity. To achieve this, a constant pressure point needs to be determined on the return water pipeline of the system. During the circulation of the system, a stable pressure value should be maintained to limit the flow and head of the circulation pump to a certain range to achieve flow. 3. The automatic adjustment of the head can avoid over-temperature and over-pressure to a certain extent for the boiler system. The frequency-adjusting water pump is used to set the pressure of the system, which can smoothly adjust the speed of the water pump and timely adjust the water volume to ensure the stability of the water volume in the system. The method is to use a pressure sensor to convert the pressure signal taken from the constant pressure point of the system into a (4~20) mA current signal and send it to the pressure regulator. The regulator compares it with the preset pressure value and sends out the frequency The instruction is given to the frequency converter, and the frequency converter automatically adjusts the speed of the pump motor and then adjusts the water replenishment. Due to the incompressible nature of the liquid, the pressure control response to the liquid is fast, and it is easy to maintain the pressure at the constant pressure point at the set value.

In the heating system, water is the carrier of thermal energy, and the transmission of thermal energy is completed by the flow of water. The higher the temperature of the water, the higher the heat content per unit of water, the greater the flow of water, the greater the heat energy delivered. In order to balance the heat of the overall heating network with frequency conversion speed regulation technology to adjust the flow of the circulating pump, the flow can be adjusted under the condition of stable system pressure to ensure the safe operation of the heating system. First of all, according to the current recommendation to use the "low outlet temperature, small outlet temperature difference" principle and operation adjustment formula, adjust the system outlet temperature according to the changes in ambient air temperature. At the same time, the speed of the circulating pump is adjusted according to the temperature difference of the return water, so as to indirectly adjust the temperature by adjusting the flow of the system. The specific method is to use the temperature sensor to convert the change in temperature at the sampling point into the corresponding voltage or current value and send it to the regulator. The regulator compares it with the preset temperature value and issues a frequency command to the inverter. , The inverter automatically adjusts the speed of the pump motor.

2.3 Controlling the high-voltage frequency conversion of the induced draft fan

A high-voltage inverter is a series of superimposed high-voltage inverters, that is, multiple single-phase three-level inverters are connected in series to output high-voltage alternating current with variable frequency and variable voltage. According to the basic principles of electromechanics, the speed of the motor meets the following relationship:

Where: P is the number of motor pole pairs; f is the motor operating frequency; s is the slip). It can be seen from the formula that the synchronous speed n0 of the motor is proportional to the operating frequency of the motor (n0=60f/p). Since the slip s is generally small (0~0.05), the actual speed n of the motor is approximately equal to the synchronization of the motor The speed n0, so adjust the motor's power supply frequency f, you can change the actual speed of the motor. The slip s of the motor is related to the load. The larger the load, the slip increases, so the actual speed of the motor will slightly decrease with the increase of the load.

The inverter itself is composed of three parts: transformer cabinet, power cabinet and control cabinet. Three-phase high-voltage electricity enters through the high-voltage switch cabinet, and supplies power to the power unit in the power unit cabinet through input voltage reduction and phase shift. The power unit is divided into three groups, one group is one phase, and the output of the power unit of each phase is first and last phase string. The control unit in the main control cabinet performs rectification, inverter control and detection on each power unit in the power cabinet through the optical fiber, so that the frequency is given through the operation interface according to actual needs, and the control unit sends control information to the power unit Carry out corresponding rectification and inverter adjustment, and output the voltage level that meets the load demand.

The input side is powered by a phase-shifting transformer for each unit. The secondary windings of the phase-shifting transformer are divided into three groups; this multi-stage phase-shifting superimposed rectification method can greatly improve the current waveform on the grid side and make the grid side under load The power factor is close to 1. In addition, due to the independence of the secondary winding of the transformer, the main circuit of each power unit is relatively independent, similar to a conventional low-voltage inverter.

On the output side, the U and V output terminals of each unit are connected in series to form a star connection to supply power to the motor. By reassembling the PWM waveform of each unit, a stepped PWM waveform can be obtained. This waveform has a good sine degree and a small dv/dt, which can reduce the insulation damage to the cable and the motor. The output cable can be made very long without the need for an output filter. The motor does not need to be derated and can be directly used for the transformation of old equipment At the same time, the harmonic loss of the motor is greatly reduced, eliminating the mechanical vibration caused by it and reducing the mechanical stress of the bearings and blades.

2.3.1 Operation and start of the inverter

There are three operation methods of the inverter: one is local control, and the operation process of frequency conversion speed regulation is realized by directly using the buttons of the man-machine interface of the touch screen of the control cabinet to start, accelerate, decelerate, reset, and stop; the second is remote control. Controller, given by switch and analog, input operation command and given value through terminal and I/0 interface; third is DCS control, directly connected with DCS, to achieve perfect combination with on-site process control system, and through on-site process control The system realizes the control. The frequency conversion control of the induced draft fan mainly adopts DCS control. There are three kinds of inverter start-up: one is normal start, after starting in normal mode, the frequency will automatically increase and run stably at the frequency set by the user; the second is soft start, after starting in soft start mode, the speed will be directly increased into the system parameters The power grid switching frequency is provided, and then the system gives the "frequency conversion / power frequency switching" command, and performs the corresponding electrical interlock control to achieve the effect of soft start; the third is the bypass function, when the inverter fails to be put into operation , Manual bypass operation is possible. Open QS2 and QS3, close QS1, the motor can be started and stopped directly by QF, and the motor is driven to work. This is the frequency conversion direct bypass function, which is convenient for maintenance and repair.

2.3.2 Analysis of energy saving effect after frequency conversion

The current environmental protection requirement in Tianjin is that all coal-fired boilers must be equipped with desulfurization and dust removal facilities. The addition of the two systems increases the resistance of the flue gas system of the boiler by about 2500Pa, increases the power of the induced draft motor by about one level, and acts as an industrial steam The heating boiler has the operating characteristics of large load fluctuations. At the same time, the operation of the wet desulfurization system will change with the change of the operating conditions of the boiler, so that the resistance of the flue gas system becomes multivariable. For example, the use of the air door adjustment system has simple and reliable air pressure. It has the advantages of constant and low investment, but the adjustment accuracy is low, the power consumption is high, and the desulfurization liquid of the desulfurization tower is adjusted with the change of the boiler flue gas, the resistance of the tower is a variable, so the damper adjustment system is not suitable for the operation of the wet desulfurization system Industrial and mining, do not meet the energy saving requirements. The inverter is used to adjust the induced draft fan according to the load change of the boiler, so that the wind pressure is within an optimal range, which not only meets the operation requirements of the boiler but also the operation requirements of the desulfurization tower, which not only saves energy but also meets the standard emissions, so that the flue gas system The operation of mines has been optimized. The frequency converter also has the characteristics of improving the power factor of the electrical equipment, which will reduce the reactive power absorbed by the motor from the grid and reduce the active loss caused by the reactive power transmitted in the grid.

By analyzing the change of boiler load by 70%, the power of the damper and inverter adjustment motor will change by 50%. The boiler load needs to be adjusted according to 40% of the time. If the annual operation is 8640h, the induced fan motor is 315kW and the electricity price is 1.00 yuan. Adjust the annual power saving by 544320 degrees, saving electricity cost of 544,000 yuan.

2.4 Inverter in water level adjustment

When the water level of the drum changes, the output signal of the transmitter will also change. After comparing this measurement signal with the given value, a deviation signal is generated and sent to the regulator. The regulator performs the corresponding operation on the deviation signal and outputs the result of the operation to the frequency converter. When the variable frequency speed regulator receives the output signal of the regulator, it will change the frequency of the pump motor power accordingly according to the signal, thereby changing the motor speed and increasing or decreasing the flow of water supply. Keep the drum level stable at the given value.

3. Conclusion

The technology of frequency conversion and speed regulation is becoming more and more extensive in all walks of life. Its energy-saving, power-saving, and easy to form significant advantages of automatic control system will surely become the central equipment for electric power drag. At the same time, the application of frequency conversion speed regulation technology is also an effective way for enterprises to transform and increase benefits.