Beijing Leader & Harvest Electric Technologies Co., Ltd.

Variable Frequency Drive

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A Variable Frequency Drive

System Principle

System configuration
Multi-level voltage source VFD with the technology of cascade-connected power cells can directly output medium voltage, i.e. 3kV, 3.3kV, 4.16kV, 6kV, 6.6kV, 10kV and 11kV. VFD is composed of phase-shifting transformer, power cells and controller.

3kV series VFD
There are 12 power cells for 3kV series and every 4 power cells constitute a phase. Three phases are ' star' connected and supply the power to the motor directly.

3.3kV series VFD
There are 12 power cells for 3.3kV series and every 4 power cells constitute a phase.

4.16kV series VFD
There are 15 power cells for 4.16kV series and every 5 power cells constitute a phase.

6kV series VFD
There are 15 power cells for 6kV series and every 5 power cells constitute a phase.

6.6kV series VFD
There are 18 power cells for 6.6kV series and every 6 power cells constitute a phase.

10kV series VFD
There are 24 power cells for 10kV series and every 8 power cells constitute a phase.

11kV series VFD
There are 27 power cells for 11kV series and every 9 power cells constitute a phase.

Configuration of power cell

It is a basic single phase AC-DC-AC inverter circuit. The rectifier is a three-phase diode bridge. By controlling the IGBT inverter in sine wave PWM, get single phase AC output.
Each power cell is completely identical in structure, and can be replaced with another.

Configuration of input side
Phase-shifting transformer supplies the power to power cells. The secondary windings of the transformer are divided into three groups. Based on the number of the cells per phase, the rectifier mode of multi-level phase-shifting overlap includes 24-pulse, 30-pulse, 36-pulse, 48-pulse or 54-pulse, which greatly improves the current waveform of the power grid (The THD of voltage and current in the power grid side can directly satisfy IEEE519 harmonics standard), and the input power factor is near 1, no harmonic filter and power factor correction capacitor required.
In addition, the independent secondary windings of the phase-shifting transformer isolate electrically the power cell from another. The power cell is similar to low voltage PWM inverter. Many inverters are connected in series to form step PWM sine wave. This PWM technology is mature and well proven.

Configuration of output side
U and V output of each power cell are connected in series to form a star connection to supply the power to the motor. By restructuring PWM waveform of each cell, we can get the step sine PWM waveform. It has a good sine-shape, small dv/dt, the long motor cable is available without output filter . Meanwhile the output harmonic is very less, which greatly reduces mechanical vibration and mechanical stress of the bearings and the vanes.
[OPTIONAL FUNCTION] When one power cell is in failure, the system will automatically bypass the faulty cell. All other cells will continue to work and keep the same output line voltage to the motor. The VFD will continue to run but in de-rating mode without the trip-off and the motor will continue to work without any stop. The production loss caused by accidental shut-down of the motor can be avoided in many cases.

Controller is composed of high speed SCM, HMI and PLC. SCM realizes PWM control. HMI based on Windows CE is friendly touch screen interface. Embedded PLC treats with the logic of digital signals and communicates with the controller system of the user with great flexibility.
Controller communicates with power cells through optical fibers. So medium voltage part and low voltage part are isolated, which makes system extremely safe and away from electromagnetic disturbance. In addition, when the auxiliary power supply is in failure, UPS will supply the auxiliary power to the controller for about 30 minutes, VFD can continue running.

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Variable Frequency Drive
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