With the continuous development and improvement of frequency conversion technology, its excellent energy-saving performance, good starting characteristics and diverse protection functions have been well reflected in enterprises, enabling enterprises to save energy, reduce costs and increase safety in production.
1. Characteristics of frequency converters
Variable frequency speed control is the best-performing and most promising speed control technology. Variable frequency drives (VFDs) are the main products used for speed control of three-phase asynchronous motors in industrialized countries. Their main advantages are: excellent energy saving (up to 55%); wide speed range (speed ratio up to 20:1); good starting and braking performance, enabling soft start, automatic smooth acceleration and deceleration, and rapid braking; comprehensive protection functions, including overvoltage, undervoltage, overload, overcurrent, instantaneous power outage, short circuit, and stall protection, with fault diagnosis and display capabilities; and ease of use in computer systems, enabling remote control.
2. Application of variable frequency speed control technology in the control of combined heat and power systems
2.1 Central heat exchange station and external network control
The boiler system mainly includes constant pressure water supply, hot water (carrier) circulation, combustion control and furnace pressure control. In combined heat and power, the superheated steam it produces can be output directly or output as hot water after steam-water exchange. The heat energy of the steam or hot water is provided to various heat users through various heat exchange stations in the pipeline network.
2.2 Control of the external circulation pump
Maintaining constant pressure within a heating system is a fundamental prerequisite for its normal operation. Heating systems are typically closed pipe networks, theoretically consuming very little water. However, unavoidable leaks and human-caused water loss inevitably affect system pressure stability. Furthermore, the temperature of the water flow within the system also influences pressure changes. For stable heating, the system's highest point must be fully filled with water, and the pressure must not exceed the system's capacity. To achieve this, a constant pressure point needs to be established on the system's return pipe. Maintaining a stable pressure value during system circulation limits the flow rate and head of the circulating pump within a certain range, enabling automatic adjustment of flow rate and head. For boiler systems, this also helps prevent overheating and overpressure to some extent. Using a variable frequency speed-controlled (VFD) makeup water pump to maintain system pressure allows for smooth adjustment of the pump's speed and timely adjustment of the makeup water volume, ensuring stability of the water volume within the system. The method involves using a pressure sensor to convert the pressure signal taken from the constant pressure point of the system into a (4~20)mA current signal, which is then sent to the pressure regulator. The regulator compares the signal with the preset pressure value and sends a frequency command to the frequency converter. The frequency converter then automatically adjusts the speed of the water pump motor, thereby adjusting the water supply. Due to the incompressible nature of liquids, the pressure control response to liquids is fast, making it easy to maintain the pressure at the constant pressure point at the set value.
In heating systems, water is the carrier of heat energy, and heat transfer relies on water flow. The higher the water temperature, the higher the calorific value per unit volume; the greater the water flow rate, the greater the heat energy transferred. To maintain the overall heat balance of the heating network, variable frequency drive (VFD) technology is used to regulate the flow rate of the circulating pump. This allows for flow rate adjustment under stable system pressure, ensuring the safe operation of the heating system. Firstly, following the currently recommended principle of "low outlet water temperature, small outlet-return temperature difference" and operating adjustment formula, the system's outlet water temperature is adjusted based on changes in ambient temperature. Simultaneously, the circulating pump speed is adjusted based on the outlet-return water temperature difference, thereby indirectly regulating the temperature by adjusting the system flow rate. Specifically, a temperature sensor converts the temperature change at the sampling point into a corresponding voltage or current value, which is then sent to the regulator. The regulator compares this value with a pre-set temperature value and sends a frequency command to the VFD, which then automatically adjusts the pump motor speed.
2.3 High-voltage frequency converter for controlling the induced draft fan
A high-voltage frequency converter is a series-connected, superimposed high-voltage frequency converter, meaning it uses multiple single-phase three-level inverters connected in series to output high-voltage AC power with variable frequency and voltage. According to the basic principles of electrical machinery, the speed of a motor satisfies the following relationship:
Where: P is the number of pole pairs of the motor; f is the operating frequency of the motor; s is the slip. From the formula, it can be seen 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 synchronous speed n0. Therefore, adjusting the power supply frequency f of the motor can change the actual speed of the motor. The slip s of the motor is related to the load; the larger the load, the greater the slip. Therefore, the actual speed of the motor will decrease slightly as the load increases.
The frequency converter itself consists of three parts: a transformer cabinet, a power unit cabinet, and a control cabinet. Three-phase high-voltage electricity enters through the high-voltage switchgear, is stepped down and phase-shifted to supply power to the power units within the power unit cabinet. The power units are divided into three groups, each group consisting of one phase, with the outputs of each phase power unit connected in series. The control unit in the main control cabinet performs rectification, inversion control, and monitoring of each power unit in the power unit cabinet via fiber optic cable. Based on actual needs, the frequency is set through the operating interface, and the control unit sends the control information to the power units for corresponding rectification and inversion adjustments, outputting a voltage level that meets the load requirements.
The input side is powered by a phase-shifting transformer, whose secondary winding is divided into three groups. This multi-stage phase-shifting rectification method can greatly improve the current waveform on the grid side, making its grid-side power factor close to 1 under load. In addition, due to the independence of the transformer's secondary winding, the main circuit of each power unit is relatively independent, similar to a conventional low-voltage frequency converter.
The output side supplies power to the motor by connecting the U and V output terminals of each unit in a star configuration. By recombining the PWM waveforms of each unit, a stepped PWM waveform can be obtained. This waveform has good sinusoidal properties and a small dv/dt, which reduces damage to the insulation of cables and motors. It eliminates the need for output filters, allowing for long output cables and eliminating the need for motor derating, making it suitable for retrofitting existing equipment. At the same time, it significantly reduces motor harmonic losses, eliminating the resulting mechanical vibrations and reducing mechanical stress on bearings and blades.
2.3.1 Operation and Start-up of the Frequency Converter
There are three methods for operating a frequency converter: First, local control, using the start, acceleration, deceleration, reset, and stop buttons on the control cabinet's touchscreen HMI to achieve variable frequency speed control. Second, remote control, using a remote controller with switches and analog inputs, and inputting operation commands and setpoints through terminals and I/O interfaces. Third, DCS control, directly connecting to a DCS to achieve perfect integration with the field process control system, and controlling the induced draft fan primarily using DCS control. There are three ways to start a frequency converter: First, normal start, after starting normally, automatically increasing the frequency and running stably at the user-set frequency. Second, soft start, after starting soft start, directly increasing the speed to the grid switching frequency provided in the system parameters, then the system issues a "frequency converter/power frequency switching" command and performs corresponding electrical interlock control to achieve the soft start effect. Third, bypass function, allowing manual bypass operation in case the frequency converter malfunctions and cannot be put into operation. With QS2 and QS3 disconnected and QS1 closed, the motor can be directly started and stopped by QF, driving the motor to work. This is the direct bypass function of the frequency converter, which also facilitates maintenance and repair.
2.3.2 Analysis of Energy-Saving Effects After Variable Frequency Drive Retrofit
Tianjin's current environmental protection requirements mandate that all coal-fired boilers must be equipped with desulfurization and dust removal facilities. The addition of these two systems increases the boiler's flue gas system resistance by approximately 2500 Pa and the induced draft fan motor power by approximately one stage. Furthermore, as industrial steam heating boilers, they exhibit significant load fluctuations, and the operation of the wet desulfurization system varies with the boiler's operating conditions, making the flue gas system resistance a multivariable factor. While damper regulation systems offer advantages such as simplicity, reliability, constant air pressure, and low investment, they suffer from low regulation precision and high power consumption. Additionally, the desulfurization liquid in the desulfurization tower is adjusted according to changes in boiler flue gas, making the tower's resistance a variable. Therefore, damper regulation systems are unsuitable for wet desulfurization systems and do not meet energy-saving requirements. Using frequency converter regulation, the induced draft fan is adjusted based on boiler load changes, keeping the air pressure within an optimal range. This satisfies both the boiler's and the desulfurization tower's operational requirements, achieving energy savings and meeting emission standards, thus optimizing the flue gas system's operation. Inverters also improve the power factor of electrical equipment, reducing the reactive power absorbed by the motor from the power grid and reducing active power loss caused by reactive power transmission in the power grid.
Analysis shows that a 70% change in boiler load will result in a 50% change in the power of the damper and the frequency converter regulating motor. Assuming that the boiler load needs to be adjusted 40% of the time, and assuming an annual operation of 8640 hours, an induced draft fan motor of 315kW, and an electricity price of 1.00 yuan, using frequency converter regulation will save 544,320 kWh of electricity annually, resulting in a saving of 544,000 yuan in electricity costs.
2.4 Frequency converters in water level regulation
When the water level in the steam drum changes, the transmitter's output signal also changes. This measured signal is compared with the setpoint to generate a deviation signal, which is sent to the controller. The controller performs corresponding calculations on the deviation signal and outputs the result to the variable frequency drive (VFD). When the VFD receives the controller's output signal, it adjusts the frequency of the water pump motor's power supply accordingly, thereby changing the motor speed and increasing or decreasing the water supply flow rate. This ensures that the water level in the steam drum is stably maintained at the given value.
3. Conclusion
Variable frequency drive (VFD) technology is being increasingly adopted across various industries. Its significant advantages, including energy saving, power efficiency, and ease of integration into automated control systems, will undoubtedly make it a central component of electric drives. Furthermore, applying VFD technology is an effective way for enterprises to upgrade their operations and increase efficiency.
