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.

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 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. A high-voltage VFD (Variable-frequency Drive) with a 6kV voltage level. Each phase is composed of six power units with a rated voltage of 600V connected in series. The output phase voltage can reach up to 3464V, and the line voltage can reach about 6000V. By changing the number of series connection of each phase power unit or the output voltage level of the power unit, high voltage output of different voltage levels can be achieved. Each power unit is powered by a set of secondary sides of the input transformer, and the power units and the secondary windings of the transformer are insulated from each other. The secondary winding adopts the Yanbian delta connection method to achieve multiplexing to achieve the purpose of reducing the input harmonic current. VFD (Variable-frequency Drive) with 6kV voltage level, the 18 secondary windings that supply power to 18 power units are divided into groups of three, divided into 6 different phase groups, with an electrical angle difference of 10 degrees from each other, forming a 36-pulse Due to the rectifier circuit structure, the input current waveform is close to a sine wave. This equivalent split-phase power supply method greatly reduces the total harmonic current distortion, and the power factor of the VFD (Variable-frequency Drive) input can reach more than 0.95.

3. Dual DSP control system

The core of the main controller is a dual-DSP CPU unit, which enables instructions to be completed in nanoseconds. In this way, the CPU unit can quickly calculate the control information and status information based on the operating commands, given signals and other input signals, and quickly complete the monitoring of the power unit.

4 GPRS remote monitoring

Through the FTU distribution network device, the collected data such as "actual frequency", "stator voltage", "stator current", "pressure", as well as system operation status and alarm information are sent to the backend server using the GPRS network. The server can perform corresponding processing operations based on the analysis results of the received data information, including monitoring the working status, system operating parameters, current, and voltage exceeding the standard alarm, so that the site can be monitored in real time to determine the safety situation and operating conditions. . It greatly improves the reliability of system operation, makes the operation more flexible, and reduces maintenance costs.