Side Pane 1
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Main Pane
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System Principle
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1. System Structure
The structure of HARSVERT-A series medium voltage VFD is shown in fig 1. The system is composed of phase shifting transformer, power cells and controller. The 3000V series have 12 power cells, and every 4 cells make up a phase in series. The 6000V series have 15 (or 21) power cells, every 5 (or 7) cells make up a phase in series. The 10000V series have 24 power cells, and every 8 cells make up a phase in series. The structures of 3000V, 6000V and 10000V series VFDs are shown in fig 1.(a),1.(b) and 1.(c).
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Fig 1 the Structure of HARSVERT-A series Medium Voltage VFD
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2. Power Cell Structure
The power cells are completely identical in structure, and can be replaced with each other. The circuit structure is shown in fig 2.(a), it is a basic three phase input, single phase output AC-DC-AC VFD circuit. The rectifier is a three-phase diode bridge. Controlling the IGBT inverter By sinwave PWM, the output voltage waveform is shown in fig 2.(b).
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3.Input Side Structure
The phase shifting transformer supplies power to each power cell on the input side. The transformer's secondaries are divided into three groups; for 3000V series, 24-pulse rectifier mode is formed; for 6000V series, 30-pulse or 42-pulse rectifier mode is formed; for 10000V series, 48-pulse rectifier mode is formed. This multi-pulse overlap rectifier mode greatly improves the line current waveform quality, and the load power factor can be improved up to near 1.
In addition, the independence of the transformer's secondaries makes the power cells' main circuit relatively separate. It's similar to the normal low-voltage VFD, therefore, easy to utilize the mature technologies. | |
،،،،[fig 2.(a)circuit of power cell] |
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4.Output Side Structure
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[fig 2.(b) the multi-level sinusoidal PWM Waveform of the Output phase voltage of medium voltage VFD] | |
See fig 2.(a). At the output side, each cell's U and V output terminals are connected in series to form a star (wye) connection to supply power to the motor. By overlapping each cell's PWM wave, we can get the multi-level SPWM waveform shown in fig 2.(b). It has a excellent sine-shape, small dv/dt, no cable and motor insulation destruction, no output filter for long output cable, no motor derating, and can be used in driving the old motor directly. Meanwhile the motor harmonic is reduced greatly, the mechanical vibration is eliminated, and the mechanical stress of shaft and vanes is reduced.
When a certain power cell has a fault, by automatically closing the relay K shown in fig 2.(a), the fault cell can be bypassed out without effect on other cells and the VFD can run at a derated mode; so the loss due to the motor's suddenly shutting down can be reduced in many situations.
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5. Controller
The core of the VFD's controller is implemented by a high speed Single Chip Microcomputer. The well-designed algorithm ensures that the motor is operating with the optimum performance. The controller also contains an embedded PLC, which deals with the logic of switching signals in the control cabinet and cooperates with the various operation and status signals. It ensures the flexibility of the system.
The controller adopts a standard VME box structure. Surface welding technology and large-scale integrated circuit such as FPGA and CPLD are used in each control board, which make the system extremely reliable.
Furthermore, the controller communicates with the power cells through fibers. So the medium voltage part and low voltage part are reliably isolated electrically, which makes the system extremely safe and away from electromagnetic disturbance.
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Side Pane 2
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