In order to achieve the purpose of electrical energy saving and process optimization, high-voltage inverter should pay attention to in engineering design:
First, the motor characteristics of the test and re-revision of technical specifications When an ordinary motor by the frequency conversion to provide power, the inverter output voltage and current harmonic components will increase the loss of the motor, reduce efficiency, temperature rise. The increase in losses caused by higher harmonics is mainly manifested in the increase of copper loss, iron loss and additional loss of the stator and the rotor. Among them, the rotor copper consumption is the most significant, because the induction motor is always in the state of slip near 1 rotation, so the rotor copper loss is very large. In ordinary induction motors, the skin effect of the rotor increases the actual impedance to improve the starting performance of the motor, thereby increasing the copper consumption. On the other hand, due to the distributed capacitance between the motor coils, the voltage between the coils is not uniform when the harmonic voltage is input. This long-term repetitive effect causes damage to the insulation of a portion of the stator coil, resulting in The coil is aging, which is unacceptable in the insulation structure of a conventional induction motor.
In addition, it is impossible for the motor's electromagnetic circuit to be absolutely symmetrical. Therefore, the harmonic components contained in the inverter output power will interact with the space harmonic components inherent in the electromagnetic circuit to form various electromagnetic pulsations. At the same time, the motor is in the working condition that the frequency is constantly adjusted, it is easy to produce mechanical resonance with the mechanical part of the motor, resulting in damage to the mechanical parts of the motor.
Therefore, in the frequency control project, in order to avoid the above problems in the operation of the variable frequency speed regulation system, the technical design must take into consideration the technical cooperation with the motor manufacturer, carry out the speed experiment on the relevant characteristics of the motor and re-amend the original motor Technical specifications.
Second, the main points of power cable selection and laying requirements Because the connection between the output of the inverter and the motor by cable attachment, and the circuit has the capacitance of each phase, so the capacitance of the circuit current is not equal. If the cable is attached with a long distance and there is a higher harmonic current in the line, the arc extinguished by the fault capacitor current will be extinguished for a long time if single-phase grounding occurs, which will cause the cable to generate heat and cause non-fault insulation.
Therefore, in the frequency control project, for the output power cable, consider the three-phase symmetry and shielding of the cable structure, the appropriate increase in cable cross-section, laying length does not exceed the limit (100m), if the original output power cable is unshielded Or cross-section flow of less than 2, should be replaced to meet the requirements of the power cable.
The laying of the site construction of the power cables and control cables and signal cables laid separately to avoid the power cables from the high harmonics of the magnetic field to interfere with other signals.
Third, the basic requirements of the working environment of the inverter As the inverter part of the high-voltage inverter using high-voltage IGBT and other power devices, the opening and closing frequency greater than 100HZ, easy to form high-order harmonic current, making the frequency conversion device will produce a certain amount of work Heat. The top of the inverter cabinet is generally equipped with exhaust fan, which will discharge the heat inside the cabinet, which makes the indoor temperature continues to rise, and ultimately will affect the reliable operation of the various components inside the cabinet. Therefore, in the water works engineering general frequency control device is set separately in the frequency control room, the room must be equipped with spare air-conditioning facilities, control the indoor temperature in the inverter required range, at the same time with ventilation doors and windows, if necessary Dedicated duct for forced ventilation and cooling.
Fourth, the high-voltage power supply system, the outlet breaker control technology improvements High-voltage side of the transformer used in high-voltage systems with the switchgear directly connected, but the protection of the switchgear is only the supply line and the transformer low-voltage side of the short circuit, and frequency The fault of the inverter should be completed by the inverter's own detection and protection system.
When the inverter fails to issue a trip signal, the circuit breaker should trip reliably. However, there is a situation where the tripping circuit disconnection or DC control power disappears inside the ordinary circuit breaker high-voltage switchgear. When the inverter happens to be faulty (the circuit breaker is required to be tripped), the tripping coil has lost power and the circuit breaker refuses to act, thus causing the inverter Internal power device is damaged. Therefore, in the design, the circuit breaker with undervoltage tripping coil is selected. Once the tripping circuit disconnects or the control power disappears, the circuit breaker will firstly automatically trip to protect the safety of the inverter.
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