Inverter Helps Thermoelectric Enterprises Reduce Energy and Reduce Emissions

With the continuous development and improvement of frequency conversion technology, its excellent energy-saving performance, good start-up characteristics, and diverse 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 When the command is given to the inverter, the inverter will automatically adjust the speed of the pump motor to adjust the amount of water supply. Due to the incompressible nature of the liquid, the pressure control response to 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 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 motor pole pair number; f is the motor operating frequency; s is slip). It can be seen from the equation 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 of the motor n is approximately equal to the motor's synchronization. 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 as the load increases.

The inverter itself consists of three parts: 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 transformer secondary windings, the main circuit of each power unit is relatively independent, similar to a conventional low-voltage converter.

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. The PWM waveform of each unit can be reorganized to obtain the staircase PWM waveform. 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 operation commands and setpoints via terminals, I/O interface; third is DCS control < 1 2>