With the continuous development and improvement of frequency conversion technology, its excellent power-saving performance, good start-up characteristics and various protection functions have been well represented in the enterprise, enabling enterprises to save energy, reduce costs, and increase ASD.
1, the characteristics of the inverter
Frequency control is the best and most promising technology for speed control. The frequency converter is the main product for the three-phase asynchronous motor speed regulation in industrialized countries. Its main advantages are: good energy-saving effect, up to 55%; wide speed range, speed ratio up to 20:1; good start and braking performance, can achieve soft start, automatic smooth acceleration and deceleration and rapid braking; The protection function is perfect, and it can realize various protection modes such as overvoltage, undervoltage, overload, overcurrent, instant power failure, short circuit, stall, etc., and can realize fault judgment display; it is easy to use in the electronic computer system and can realize long-distance control.
2. Application of Frequency Control Technology in Control of Cogeneration System 2.1 Central Heat Exchange Station and External Network Control
The boiler system mainly includes system constant pressure water supply, system hot water (carrier) circulation, system combustion control and furnace pressure control. In the cogeneration, superheated steam produced by it can be directly output, and can also be exported after steam and water exchange. The heat energy of steam or hot water is provided to each heat user through various heat exchange stations in the pipe network.
2.2 External network circulation pump control
Keeping the pressure in the heating system constant is the basic prerequisite for the normal operation of the heating system. The heating system's pipe network is usually a closed pipe system. Theoretically speaking, the water consumption is very small. However, run, blow, leak, and artificial water loss that are difficult to avoid in practice will inevitably affect the stability of system pressure. In addition, the temperature of the water flow in the system also affects the pressure changes. In order to stabilize the system's heat supply, it must be ensured that the highest point of the system is full of water and the pressure cannot exceed the capacity of the system. To achieve this, a constant pressure point needs to be determined on the return pipe of the system. In the process of system circulation, a stable pressure value is maintained, so that the flow and the head of the circulation pump are limited within a certain range, and the flow is realized. Automatic adjustment of head and head can also avoid over-temperature and over-pressure for boiler system to a certain extent. The constant-pressure water-supplying replenishing water pump is used to set the pressure of the system, which can smoothly adjust the speed of the replenishing water pump, adjust the replenishing water volume in time, and ensure the stability of the water volume in the system. The method uses 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. After the regulator compares it with the preset pressure value, the frequency is sent out. When the instruction is given to the frequency converter, the frequency converter automatically adjusts the speed of the water pump motor to adjust the amount of replenishment. Due to the incompressible nature of the liquid, the liquid pressure control responds quickly 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 transport of thermal energy is accomplished by the flow of water. The higher the temperature of the water, the higher the heat content per unit of water, and the greater the flow of water, the greater the heat energy delivered. In order to regulate the flow of the circulating pump by using the frequency conversion speed regulation technology for the heat balance of the overall heat network, the flow rate can be adjusted under the condition of stable system pressure to ensure the safe operation of the heating system. First of all, in accordance with the current recommendation to use low water temperature, the principle of small temperature difference and the operating adjustment formula, according to the changes in ambient temperature and temperature to adjust the system's water temperature. At the same time, according to the return water temperature difference to adjust the circulation pump speed, so as to adjust the system's flow to indirectly adjust the temperature. The specific method is to use a temperature sensor to convert the temperature change of the sampling point into a corresponding voltage or current value and send it to the regulator. After the regulator compares it with the preset temperature value, it sends a frequency command to the inverter. The frequency converter automatically adjusts the speed of the pump motor.
2.3 control of induced draft fan high-voltage frequency conversion
The high-voltage frequency converter is a series superimposed high-voltage frequency converter, which uses multiple single-phase three-level inverters connected in series and outputs variable-voltage high-voltage alternating current. In accordance with the basic principles of electrical machinery, the rotational speed of the motor satisfies the following relationship:
Where: P is the number of motor pole pairs; f is the motor operating frequency; s is slip). From the formula, 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 speed of the motor. Speed ​​n0, so adjust the motor power supply frequency f, can change the actual speed of the motor. The slip s of the motor is related to the load. The greater the load is, the greater the slip is, so the actual speed of the motor will decrease slightly with the increase of the load. The inverter itself consists of a transformer cabinet, a power cabinet, and a control cabinet. The three-phase high-voltage electricity enters through the high-voltage switchgear and is powered by the power unit in the power unit cabinet via the input voltage step-down and phase-shifting. The power unit is divided into three groups, one group is one phase, and the output of each phase power unit is first and last phase. string. The control unit in the main control cabinet conducts rectification, inverter control and detection of 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 the control information to the power unit. Perform 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 to each unit, and the secondary windings of the phase-shifting transformer are divided into three groups; this multi-level phase-shifting superimposed rectification method can greatly improve the current waveform on the grid side, making the net 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 loop 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 and the star connection is used to supply power to the motor. By reorganizing the PWM waveforms of each unit, the staircase PWM waveform can be obtained. This sinusoidal waveform is good, dv/dt is small, can reduce the insulation damage to the cable and the motor, without output filter can make the output cable length is very long, the motor does not require derating, can be directly used for the transformation of old equipment At the same time, the harmonic losses of the motor are greatly reduced, the resulting mechanical vibrations are eliminated, and the mechanical stress of the bearings and blades is reduced.
2.3.1 Frequency Converter Operation and Startup
There are three kinds of frequency converter operating methods: First, local control, the direct use of the control cabinet touch screen man-machine interface of the start, acceleration, deceleration, reset, stop and other buttons to achieve the frequency control process; Second, remote control, through the remote Controller, using switches and analog inputs, input commands and setpoints via terminals, I/O interface; third, DCS control, direct connection to DCS, perfect integration with on-site process control systems, and on-site process control The system realizes control, and the frequency conversion control of the induced draft fan is mainly controlled by DCS. There are three types of inverter start-up: First, after normal start-up, after starting in the normal mode, it will automatically increase the frequency and run stably with the frequency set by the user. Second, it will start up in soft start mode and then directly increase the speed to the system parameters. The provided grid switching frequency, then the system to frequency / frequency switching instructions, and the corresponding electrical interlock control, to achieve the effect of soft start; third is the bypass function, in the case of inverter failure can not be put into operation, can Manual bypass operation. The QS2 and QS3 are disconnected and the QS1 is closed. The motor can be started and stopped directly by the QF and the motor is driven. This is a frequency conversion direct bypass function, which is convenient for maintenance and overhaul.
2.3.2 analysis of energy-saving effects after frequency conversion
The current environmental protection requirement in Tianjin is that all coal-fired boilers must be equipped with desulphurization and dust removal facilities. The addition of the two systems will increase the resistance of the boiler's fume system by approximately 2500 Pa, increase the power of the induced draft fan motor by approximately one degree, and add it as an industrial steam. Heating boilers have the characteristics of large load fluctuations. At the same time, the operation of the wet desulfurization system will change with the operating conditions of the boiler. The resistance of the smoke and wind system will become multivariable. If the damper adjustment system is adopted, it is simple and reliable, and the wind pressure Constant, less investment advantages, but low adjustment accuracy, high power consumption, and desulfurization tower desulfurization liquid with the boiler flue gas to adjust the adjustment, the tower resistance is a variable, so the throttle control system does not apply wet desulfurization system operation Industrial and mining, does not meet the energy-saving requirements. Inverter adjustment is adopted to adjust the induced draft fan according to the boiler load change so that the air pressure is within an optimal range, which not only satisfies the boiler operation requirements, but also satisfies the operation requirements of the desulfurization tower. It is energy-saving and meets the standard emission, making the smoke and wind system The operation of mines has been optimized. The inverter also has the characteristic of improving the power factor of the power equipment, which will reduce the power absorbed by the motor from the power grid and reduce the active power loss caused by the reactive power transmission in the power grid.
By analyzing the boiler load change of 70%, the damper and inverter adjust the motor power will change by 50%, according to 40% of the time to adjust the boiler load, such as the annual operation of 8640h, the induced draft fan motor is 315kW, the electricity price is 1.00 yuan, using the inverter The annual electricity saving rate is 544,320 degrees, and the electricity bill is saved by 544,000 yuan.
2.4 Inverter in Water Level Regulation
When the water level of the drum changes, the output signal of the transmitter will also change. After this measurement signal is compared with a given value, a deviation signal is generated and sent to the regulator. The regulator performs a corresponding operation on the deviation signal and outputs the result of the calculation to the frequency converter. When the frequency converter receives the output signal of the regulator, it will correspondingly change the frequency of the pump motor power according to the signal, so as to change the motor speed and increase or decrease the flow of the water supply. Make the drum level stable at a given value.
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The technology of variable frequency speed regulation is becoming more and more extensive in all walks of life. Its energy-saving, power-saving, and easy to form a significant advantage of the automatic control system will surely become the central equipment for power drag. At the same time, the application of frequency control technology is also an effective way for companies to transform and increase profits.
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