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 reflected in the enterprises, enabling enterprises to save energy, reduce costs and increase ASD.

1, the characteristics of the inverter

Frequency control is the best performance and the most promising speed control technology. Inverter is industrialized countries for three-phase asynchronous motor speed control of the main products. Its main advantages are: energy-saving effect, up to 55% or more; speed range, speed up to 20: 1; start and braking performance, enabling soft-start, automatic smooth acceleration and deceleration and rapid braking; The protection function is perfect, can realize overvoltage, undervoltage, overload, overcurrent, instantaneous power failure, short circuit, stall and other protection methods, and fault diagnosis can be displayed; easy to use in electronic computer system, can achieve long-distance control.

2, Frequency Control Technology in the cogeneration system control applications

2.1 Center heat exchanger station and the external network control

The boiler system mainly includes the system constant pressure replenishment, the system hot water (carrier) circulation, the system combustion control and the furnace pressure control. In the cogeneration, the superheated steam generated by the superheated steam can be directly output, and the hot water can also be output after the exchange of the soda water , Steam or hot water through the heat pipe network heat transfer stations available to all hot users.

2.2 External network circulation pump control

Ensuring that the pressure inside the heating system is constant is a basic precondition for the proper functioning of the heating system. The pipe network of a heating system is usually a closed pipe system, which theoretically consumes little water. However, in practice, "running, running, dripping, leakage" and artificial water loss inevitably affect the stability of system pressure. In addition, the temperature of the water flow in the system also affects the pressure changes. To stabilize the system heat, you must ensure that the highest point of the system is full of water, while the pressure can not exceed the system's ability to withstand. To do this, it is necessary to determine a constant pressure point on the return pipe of the system to maintain a stable pressure value during the circulation of the system so that the flow and lift of the circulation pump are limited within a certain range to achieve the flow rate , Head automatically adjusts, to a certain extent, the boiler system can also avoid over-mild overpressure. The use of frequency control of the fill pump on the system constant pressure, you can smoothly adjust the speed of the make-up pump, timely replenishment, to ensure the stability of the volume of water within the system. The method is to use a pressure sensor to convert the pressure signal taken from the constant voltage point of the system into a (4-20) mA current signal and send the signal to a pressure regulator. The regulator compares the pressure signal with a preset pressure value and then sends the frequency Command to the inverter, the inverter will automatically adjust the pump motor speed, and then adjust the amount of water, due to the liquid incompressible characteristics of the liquid pressure control response speed, it is easy to maintain the constant pressure point pressure set point.

In heating systems, water is the carrier of thermal energy and the transport of heat energy is done by the flow of water. The higher the temperature of 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 whole heating network, the frequency control technology is used to adjust the circulating pump's flow rate. Under the condition of stable system pressure, the flow can be adjusted to ensure the safe operation of the heating system. First of all, according to the current recommended use of "low water temperature, small back to the temperature difference" principle and operating adjustment formula, according to changes in ambient temperature temperature to adjust the system's water temperature. At the same time according to the temperature difference between the backwater to adjust the circulating pump speed, so as to regulate the flow of the system to achieve the purpose of indirect regulation of temperature. The specific approach is to use the temperature sensor sampling point temperature changes, converted into the corresponding voltage or current value to the regulator, the regulator will be pre-set with the temperature value of comparison, the frequency command issued to the inverter , The frequency converter will automatically adjust the pump motor speed.

2.3 Control Fan Fan high-voltage inverter

High-voltage inverter is a series of superimposed high-voltage inverter, that is, using more than one single-phase three-level inverter connected in series, the output variable frequency variable voltage high-voltage AC. In accordance with the basic principles of motor science, the motor speed to meet the following relationship:

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

The inverter itself consists of transformer cabinet, power cabinet, control cabinet composed of three parts. High-voltage three-phase high-voltage switchgear into the input step-down, the phase shift to the power unit cabinet power supply unit, the power unit is divided into three groups, one for each phase of the power unit output string. The control unit in the main control cabinet rectifies and inverts each power unit in the power cabinet through the optical fiber so that the frequency is given through the operation interface according to the actual needs and the control unit sends the control information to the power unit The corresponding rectifier, inverter adjustment, the output voltage level to meet the load requirements.

The input side of the phase-shifting transformer to each unit power supply, phase-shifting transformer secondary windings are divided into three groups; this multi-stage phase shift superimposed rectification can greatly improve the network side of the current waveform, the load side of the network Power factor 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 conventional low-voltage inverters.

The output side of each unit by the U, V output terminals connected in series with each other as a star connected to the motor power supply, by the PWM waveform for each unit to be reorganized, you can get the ladder PWM waveform. The good sinusoidal waveform, dv / dt is small, can reduce the insulation damage to the cable and motor, the output filter can be long without the output filter, the motor does not require derating, can be used directly for the transformation of the old equipment ; At the same time, the motor harmonic loss is greatly reduced, eliminating the resulting mechanical vibration, reducing the bearing and the blade mechanical stress.

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