High-voltage VFD (Variable-frequency Drive) is used in boiler secondary air fans

Abstract: Study the application of high-voltage VFD (Variable-frequency Drive) in the secondary air fan of power plant boiler. High-voltage VFD (Variable-frequency Drive) has good application prospects for reducing the power consumption rate of boiler fans, reducing starting current, improving power factor, improving process level, and improving automation levels, and the application of High-voltage VFD (Variable-frequency Drive) has been carried out ), energy saving analysis after.

Keywords: high-voltage VFD (Variable-frequency Drive); secondary fan; energy saving analysis

1 Overview

In thermal power plants, fans and water pumps are the most important power-consuming equipment, and they have large capacities and consume a lot of power. The flow control of the wind turbine in the power plant is achieved by adjusting the opening of the baffle. This is a backward method with poor economic benefits and high energy consumption, which causes rapid equipment damage, difficult maintenance, and high operating costs. A considerable part of the power is consumed in the blocking process of the baffle, resulting in a huge waste of energy. In addition, these equipments operate continuously for a long time and are often in low-load and variable-load operation, and their energy-saving potential is even greater. The economic operation of the auxiliary motor of the power plant is directly related to the power consumption rate of the plant. With the continuous deepening of the reform of the power industry, the gradual implementation of policies such as the separation of factories and power grids and competitive bidding for the Internet, reducing the power consumption rate of factories, reducing power generation costs and improving the competitiveness of electricity prices have become the economic goals pursued by power plants.

Energy conservation and environmental protection of power plants are mainly reflected in the efficient and safe operation of the system and the effective flow control. As the economic requirements of the production process of thermal power plants increase, power plants must save energy, reduce consumption, improve economic benefits, and promote high voltage Frequency conversion has very important social and economic benefits in the optimized operation of power plant fan systems.

2. Working principle of high-voltage VFD (Variable-frequency Drive)

High-voltage VFD (Variable-frequency Drive) is a series superimposed high-voltage VFD (Variable-frequency Drive), which uses multiple single-phase three-level inverters connected in series to output high-voltage alternating current with variable frequency and voltage. According to the basic principles of electromechanics, the speed of the motor satisfies the following relationship: n= (1-s) 60f/p=n. × (1-s) (P: Number of pole pairs of the motor; f: Motor operating frequency; s: Slip) From the formula, it can be seen that the synchronous speed of the motor is n. Proportional to the operating frequency of the motor (n. = 60fp), since the slip s is generally relatively small (0-0.05), the actual speed n of the motor is approximately equal to the synchronous speed n of the motor. , so 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 greater the load, the slip increases, so the actual speed of the motor will decrease slightly as the load increases.

VFD (Variable-frequency Drive) itself consists of three parts: transformer cabinet, power cabinet and control cabinet. The three-phase high-voltage electricity enters through the high-voltage switch cabinet, and supplies power to the power units in the power unit cabinet through input voltage reduction and phase shifting. The power units are divided into three groups, one group is one phase, and the output of each phase of the power unit has the first and last phases. string. The control unit in the main control cabinet performs rectification, inverter control and detection on each power unit in the power cabinet through optical fiber. In this way, the frequency is given through the operation interface according to actual needs, and the control unit sends the control information to the power unit. Carry out corresponding rectification and inverter adjustments to output a voltage level that meets the load demand.

2.1 Phase-shifting transformer

The secondary winding of the phase-shifting transformer is divided into three groups to form the X-pulse rectification method; this multi-pole phase-shifting superposition rectification method can greatly improve the current waveform on the grid side, making the grid-side power factor close to 1 under load. In addition, due to the independence of the secondary winding, the main circuit of each power unit is relatively independent, which greatly improves reliability.

2.2 Intelligent power unit

All power modules are intelligently designed and have strong self-diagnosis guidance capabilities. Once a fault occurs, the power module will quickly return the fault information to the main control unit. The main control unit will promptly turn off the main power component IGBT to protect the main unit. circuit; at the same time, the fault location and category are accurately located and displayed on the Chinese human-machine interface. During the design, the unit modules within a certain power range have been standardized and considered to ensure the structural and functional consistency of the unit modules. When a module fails, after being notified by the alarm, a spare module with the same function can be replaced within a few minutes to reduce downtime.

The 6kV grid voltage is stepped down by multiple isolation transformers on the secondary side and then supplies power to the power unit. The power unit has a three-phase input and single-phase output AC and DC PWM voltage source inverter structure. The output ends of adjacent power units are connected in series. , forming a Y-connection structure to realize direct high-voltage output of variable voltage and frequency, and supply to the high-voltage motor. 6kV voltage level high-voltage VFD (Variable-frequency Drive), each phase consists of six rated voltages