Frequency conversion speed regulation

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Frequency conversion speed regulation [Copy link]

With the continuous improvement of industrial automation level and the development of power electronics technology, more and more high-voltage variable frequency speed regulation technology is used in water projects. Guangxi Branch plans to use 12-pulse voltage source type high-voltage variable frequency speed regulator in the water intake pump room to control the 355KW water pump motor, and upgrade the variable frequency speed regulation technology of the water pump motor. In order to achieve the purpose of electrical energy saving and process optimization, the high-voltage frequency converter should pay attention to the following in engineering design: 1. The characteristic test of the motor and the revision of the technical specifications. When an ordinary motor is powered by frequency conversion, the voltage and current harmonic components at the output end of its frequency converter will increase the loss of the motor, reduce the efficiency, and increase the temperature. The increase in loss caused by high-order harmonics is mainly manifested in the increase of copper loss, iron loss and additional loss of the stator and rotor. Among them, the rotor copper loss is the most significant, because the asynchronous motor always rotates in a state where the slip is close to 1, so the rotor copper loss is very large. In ordinary asynchronous motors, in order to improve the starting performance of the motor, the skin effect of the rotor increases the actual impedance, thereby increasing the copper loss. On the other hand, due to the distributed capacitance between the motor coils, when the high-order harmonic voltage is input, the voltage between the coils is uneven. This long-term repeated action causes damage to the insulation of a certain part of the stator coil, resulting in coil aging, which is unacceptable in terms of the insulation structure of ordinary asynchronous motors. In addition, the electromagnetic circuit of the motor cannot be absolutely symmetrical, so the harmonic components contained in the output power of the inverter will interact with the inherent spatial harmonic components in the electromagnetic circuit to form various electromagnetic pulsations. At the same time, because the motor is in a working state of continuous frequency adjustment, it is easy to produce mechanical resonance with the mechanical part of the motor, causing damage to the mechanical part of the motor. Therefore, in the frequency conversion speed regulation transformation project, in order to avoid the above problems during the operation of the frequency conversion speed regulation system, technical design must consider technical cooperation with the motor manufacturer, conduct speed regulation experiments on the relevant characteristics of the motor, and revise the technical specifications of the original motor. 2. Key points for power cable selection and laying requirements Since the connection between the output end of the inverter and the motor adopts the cable attachment method, and each phase of the line has capacitance to the ground, the capacitive current on the line is unequal during operation. If the cable is installed over a long distance and there are high-order harmonic currents in the line, once single-phase grounding occurs, the arc ignited by the fault capacitor current will extinguish for too long, causing the cable at this end to heat up and cause non-fault insulation. Therefore, in the frequency conversion speed regulation transformation project, for the output power cable, the three-phase symmetry and shielding of the cable structure should be considered, the cable section should be appropriately increased, and the laying length should not exceed the limit value (100m). If the original output power cable is unshielded or the cross-sectional flow margin is less than 2, a power cable that meets the requirements should be replaced. During on-site installation, the power cable should be laid separately from the control cable and the signal cable to avoid the magnetic field generated by the high-order harmonics in the power cable interfering with other signals. 3. Basic requirements for the working environment of the frequency converter Since the inverter part of the high-voltage frequency converter uses power devices such as high-voltage IGBT, its on-off frequency is greater than 100HZ, which is easy to form high-order harmonic currents, so that the frequency converter will generate a certain amount of heat when working. Generally, there is an exhaust fan on the top of the inverter cabinet, which discharges the heat in the cabinet to the room, which makes the indoor ambient temperature rise continuously, and eventually affects the reliable operation of each device in the cabinet. Therefore, in the design of water plant engineering, the frequency conversion speed regulation device is generally set separately in the frequency conversion speed regulation room. The room must be equipped with spare air conditioning facilities to control the indoor ambient temperature within the range required by the inverter. At the same time, ventilation doors and windows are provided. When necessary, special air ducts are used for forced ventilation and cooling. 4. Technical improvement of the control of the circuit breaker at the outlet of the high-voltage power supply system The high-voltage side of the transformer used in the frequency conversion speed regulation device should be directly connected to the switch cabinet in the high-voltage system, but the protection range of the switch cabinet is only the short circuit between the power supply line and the low-voltage side of the transformer, and the fault of the inverter should be completed by the detection and protection system of the inverter itself. When the inverter fails and sends a trip signal, the circuit breaker should trip reliably. However, if the tripping circuit is disconnected or the DC control power disappears inside the ordinary circuit breaker high-voltage switch cabinet, and the inverter happens to fail (requiring the circuit breaker to trip), the tripping coil has lost power and the circuit breaker refuses to operate, thus causing damage to the power devices inside the inverter. Therefore, a circuit breaker with an undervoltage tripping coil is selected in the design. Once the tripping circuit is disconnected or the control power disappears, the circuit breaker will automatically trip first to protect the inverter equipment safety.