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With the continuous improvement of industrial automation level and the development of power electronics technology, more and more high-voltage variable frequency speed control technology is adopted in water engineering. Guangxi Branch plans to use 12-pulse voltage source high-voltage frequency converter in the pumping house. To control the pump motor with power of 355KW, upgrade the frequency conversion speed regulation technology of the pump motor. In order to achieve the purpose of electrical energy saving and process optimization, the high-voltage inverter should pay attention to the engineering design:
First, the motor's characteristic test and technical specifications are revised
When an ordinary motor is powered by frequency conversion, the voltage and current harmonic components at the output of the inverter will increase the loss of the motor, reduce the efficiency, and increase the temperature. The increase in loss caused by higher harmonics is mainly manifested in the increase of copper loss, iron loss and additional loss of the stator and rotor. Among them, the copper loss of the rotor is the most significant, because the asynchronous motor always rotates in a state where the slip is close to 1, so the copper loss of the rotor is very large. In a conventional asynchronous motor, in order to improve the motor starting performance, the skin effect of the rotor increases the actual impedance, thereby increasing the copper consumption.
On the other hand, due to the distributed capacitance between the motor coils, when the higher harmonic voltage is input, the voltage between the coils is not uniform, and this long-term repeated action causes damage to a certain portion of the stator coil, thereby generating damage. The coil is aged, which is unacceptable in terms of the insulation structure of a conventional asynchronous motor.
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 spatial harmonic components inherent in the electromagnetic circuit to form various electromagnetic pulsations. At the same time, the motor is easily mechanically resonated with the mechanical part of the motor due to the constant adjustment of the frequency, resulting in damage to the mechanical part of the motor.
Therefore, in the variable frequency speed regulation reconstruction project, in order to avoid the above problems during the operation of the variable frequency speed control system, the technical design must consider technical cooperation with the motor manufacturer, carry out the speed regulation experiment on the relevant characteristics of the motor, and re-revise the original motor. Technical specifications.
Second, power cable selection points and laying requirements
Since the connection between the output end of the inverter and the motor adopts the cable attachment mode, and the capacitance of the line exists in each phase of the line, the capacitance current on the line during operation is not equal. If the cable is attached for a long distance and there is a higher harmonic current in the line, once the single-phase grounding occurs, the arc extinguished by the fault capacitor current is too long, which will cause the cable to heat up and cause non-fault insulation.
Therefore, in the variable frequency speed regulation transformation project, for the output power cable, consider the three-phase symmetry and shielding on the cable structure, increase the cable cross section appropriately, and the laying length does not exceed the limit value (100m), if the original output power cable is unshielded Or the cross-section of the planting flow margin is less than 2, should replace the power cable that meets the requirements.
When laying the facility on site, separate the power cable from the control cable and signal cable to avoid interference with other signals caused by the magnetic field generated by the higher harmonics in the power cable.
Third, the basic requirements of the working environment of the inverter
Since the inverter part of the high-voltage frequency converter adopts a power device such as a high-voltage IGBT, the opening and closing frequency thereof is greater than 100HZ, and a high-order harmonic current is easily formed, so that the frequency conversion device generates a certain amount of heat during operation. Generally, the exhaust fan is equipped with an exhaust fan at the top of the inverter cabinet, which discharges the heat inside the cabinet into the room, which causes the indoor temperature of the room to continuously rise, and finally affects the reliable operation of each device in the cabinet. Therefore, in the water plant engineering design, the general frequency conversion speed control device is separately installed in the frequency conversion speed control room. The indoor air conditioning facility must be installed in the room to control the indoor ambient temperature within the range required by the frequency converter. At the same time, ventilation windows and doors are provided, if necessary, Special air ducts for forced ventilation and cooling.
Fourth, the technical improvement of the control of the outlet circuit breaker of the high voltage power supply system
The high-voltage side of the transformer used in the frequency conversion device should be directly connected to the switchgear in the high-voltage system, but the protection range of the switchgear 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 depend on the detection and protection system of the inverter itself. carry out.
When the inverter has a fault and a trip signal is issued, the circuit breaker should trip reliably. However, when the trip circuit is disconnected or the DC control power supply disappears inside the high-voltage switchgear of the ordinary circuit breaker, when the inverter happens to be faulty (requires the circuit breaker to trip), the trip coil has lost power, and the circuit breaker refuses to operate, thus causing the inverter The internal power device is damaged. Therefore, the circuit breaker with undervoltage trip coil is selected in the design. Once the trip circuit is disconnected or the control power supply disappears, the circuit breaker first automatically trips to protect the safety of the inverter equipment.
First, the motor's characteristic test and technical specifications are revised
When an ordinary motor is powered by frequency conversion, the voltage and current harmonic components at the output of the inverter will increase the loss of the motor, reduce the efficiency, and increase the temperature. The increase in loss caused by higher harmonics is mainly manifested in the increase of copper loss, iron loss and additional loss of the stator and rotor. Among them, the copper loss of the rotor is the most significant, because the asynchronous motor always rotates in a state where the slip is close to 1, so the copper loss of the rotor is very large. In a conventional asynchronous motor, in order to improve the motor starting performance, the skin effect of the rotor increases the actual impedance, thereby increasing the copper consumption.
On the other hand, due to the distributed capacitance between the motor coils, when the higher harmonic voltage is input, the voltage between the coils is not uniform, and this long-term repeated action causes damage to a certain portion of the stator coil, thereby generating damage. The coil is aged, which is unacceptable in terms of the insulation structure of a conventional asynchronous motor.
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 spatial harmonic components inherent in the electromagnetic circuit to form various electromagnetic pulsations. At the same time, the motor is easily mechanically resonated with the mechanical part of the motor due to the constant adjustment of the frequency, resulting in damage to the mechanical part of the motor.
Therefore, in the variable frequency speed regulation reconstruction project, in order to avoid the above problems during the operation of the variable frequency speed control system, the technical design must consider technical cooperation with the motor manufacturer, carry out the speed regulation experiment on the relevant characteristics of the motor, and re-revise the original motor. Technical specifications.
Second, power cable selection points and laying requirements
Since the connection between the output end of the inverter and the motor adopts the cable attachment mode, and the capacitance of the line exists in each phase of the line, the capacitance current on the line during operation is not equal. If the cable is attached for a long distance and there is a higher harmonic current in the line, once the single-phase grounding occurs, the arc extinguished by the fault capacitor current is too long, which will cause the cable to heat up and cause non-fault insulation.
Therefore, in the variable frequency speed regulation transformation project, for the output power cable, consider the three-phase symmetry and shielding on the cable structure, increase the cable cross section appropriately, and the laying length does not exceed the limit value (100m), if the original output power cable is unshielded Or the cross-section of the planting flow margin is less than 2, should replace the power cable that meets the requirements.
When laying the facility on site, separate the power cable from the control cable and signal cable to avoid interference with other signals caused by the magnetic field generated by the higher harmonics in the power cable.
Third, the basic requirements of the working environment of the inverter
Since the inverter part of the high-voltage frequency converter adopts a power device such as a high-voltage IGBT, the opening and closing frequency thereof is greater than 100HZ, and a high-order harmonic current is easily formed, so that the frequency conversion device generates a certain amount of heat during operation. Generally, the exhaust fan is equipped with an exhaust fan at the top of the inverter cabinet, which discharges the heat inside the cabinet into the room, which causes the indoor temperature of the room to continuously rise, and finally affects the reliable operation of each device in the cabinet. Therefore, in the water plant engineering design, the general frequency conversion speed control device is separately installed in the frequency conversion speed control room. The indoor air conditioning facility must be installed in the room to control the indoor ambient temperature within the range required by the frequency converter. At the same time, ventilation windows and doors are provided, if necessary, Special air ducts for forced ventilation and cooling.
Fourth, the technical improvement of the control of the outlet circuit breaker of the high voltage power supply system
The high-voltage side of the transformer used in the frequency conversion device should be directly connected to the switchgear in the high-voltage system, but the protection range of the switchgear 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 depend on the detection and protection system of the inverter itself. carry out.
When the inverter has a fault and a trip signal is issued, the circuit breaker should trip reliably. However, when the trip circuit is disconnected or the DC control power supply disappears inside the high-voltage switchgear of the ordinary circuit breaker, when the inverter happens to be faulty (requires the circuit breaker to trip), the trip coil has lost power, and the circuit breaker refuses to operate, thus causing the inverter The internal power device is damaged. Therefore, the circuit breaker with undervoltage trip coil is selected in the design. Once the trip circuit is disconnected or the control power supply disappears, the circuit breaker first automatically trips to protect the safety of the inverter equipment.
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