A frequency converter is a control device that uses the switching action of power semiconductor devices to convert power frequency into electrical energy of another frequency. With the rapid development of modern power electronics and microelectronics technologies, high-voltage, high-power variable frequency speed control devices have become increasingly sophisticated. The high-voltage problem, which was previously difficult to solve, has been well resolved in recent years through device series connection or unit series connection.
I. Basic Components of a High-Voltage Frequency Converter:
1. Composition of the high-voltage frequency converter: Internally, it consists of eighteen identical unit modules, with six modules forming a group, each corresponding to one of the three phases of the high-voltage circuit. The unit power supply is provided by a phase-shifting transformer. (Schematic diagram)
2. Power Unit Composition: The power unit is a single-phase bridge converter, powered by the secondary winding of the input split transformer. After rectification and filtering, it is controlled by four IGBTs using PWM to generate a set frequency waveform. All power units in the inverter have the same circuit topology, implementing a modular design. Control is transmitted via optical fiber. The control optical signal from the main controller is converted from optical to electrical signals and sent to the control signal processor. After receiving the corresponding instructions, the control circuit processor issues the corresponding drive signal to the IGBT. Upon receiving the corresponding drive signal, the drive circuit issues the corresponding drive voltage to the IGBT control electrode, operating the IGBT to turn off and on, outputting the corresponding waveform. The status information in the power unit is collected and processed in the response signal circuit, then converted by the electrical-to-optical converter and sent as an optical signal to the main controller.
II. Operating principle of high-voltage frequency converter:
Each power unit of the high-voltage frequency converter is equivalent to a three-level two-phase output low-voltage frequency converter. These are superimposed to form a high-voltage three-phase AC power supply. The neutral point of the frequency converter is not connected to the neutral point of the motor.
The inverter output is actually a line voltage. The UAB output line voltage generated by the output voltages of phases A and B can reach 6000V, which is a 25-step waveform. As shown in the figure below, the stepped waveforms of the output line voltage and phase voltage are as follows. The UAB not only has a sinusoidal waveform, but also the number of steps is multiplied, so the harmonic components and dV/dt are relatively small.
III. Operation of a multi-level unit series superimposed high-voltage frequency converter:
The inverter converts the input three-phase high-voltage AC power at the industrial frequency into frequency-adjustable three-phase AC power. The voltage and frequency are adjusted according to the V/F setting, allowing the motor to operate at different frequencies while maintaining the main magnetic flux in the stator core at the rated level, thus improving the motor's conversion efficiency. On the inverter input side, due to the uniform displacement of multiple secondary windings (e.g., +250°, +150°, +50°, -50°, -150°, -250° for 6kV output), the corresponding current components in the inverter's primary current also shift uniformly, forming an equivalent 36-pulse rectifier circuit. The harmonics generated during the conversion cancel each other out. The power factor during operation is above 0.95, eliminating the need for additional power filters or power factor compensation devices. It also avoids resonance with existing compensation capacitors and causes no interference to electrical equipment operating on the same power grid.
IV. Performance characteristics of high-voltage frequency converters:
1. Application Scope: Wide speed regulation range, allowing smooth adjustment from zero speed to mains frequency speed. It enables soft starting with low current on large motors, with starting time and method adjustable according to site conditions. Frequency adjustment is based on the motor's voltage-to-frequency ratio at low frequencies, resulting in lower heat generation and lower input voltage, which slows down the aging of motor insulation.
2. The innovative series multiplexing superposition technology enables true high-to-high voltage power conversion, eliminating the need for step-down and step-up conversion, reducing device losses, improving reliability, and solving the difficulties of high-voltage power conversion. The application of series multiplexing superposition technology also opens up new avenues for achieving pure sine waves and eliminating grid harmonic pollution.
Phase-shifting transformer
Phase-shifting transformers are one of the key components in unit-series multilevel high-voltage high-power frequency converters.
There are generally two methods for using low-voltage power electronic components to make high-voltage frequency converters: one is to directly connect low-voltage components in series, and the other is to connect independent power units in series, which is called a unit-series multilevel high-voltage high-power frequency converter. The latter has become the mainstream of high-voltage high-power frequency converters because it has more advantages than the former.
Taking a 6kV frequency converter as an example: each phase consists of 6 independent low-voltage power units connected in series, with a rated voltage of Ve=577V (peak voltage of 816V). The output phase voltage is 3464V, and the line voltage can reach approximately 6000V. Each power unit bears the entire output current but only provides 1/6 of the phase voltage and 1/18 of the output power. Each power unit is powered by a set of secondary windings of a transformer, and the power units and the secondary windings of the transformer are mutually insulated.
Clearly, the phase-shifting transformer plays two crucial roles in this frequency converter: first, its electrical isolation ensures the independence of each power unit, enabling voltage superposition and series connection; second, the phase-shifting connection effectively eliminates harmonics below the 35th order. (Theoretically, it can eliminate harmonics below the 6n-1th order, where n is the unit stage number.)
