1 Fault phenomenon
The 3rd phase calcination 4 # rotary kiln of China Aluminum Qinghai Branch was newly built in May 2005, and its petroleum coke calcination capacity is 12t / h; the kiln drag system adopts frequency converter (manufactured by Siemens) speed control, and its normal production The feeding amount is generally controlled in the range of 10 ~ 11.6t / h, the working frequency of the inverter (110va) is set in the range of 38 ~ 46hz, and the drive motor of the big kiln is an ordinary three-phase asynchronous motor (type y315s-6, 75kw), The load current is between 68 ~ 92a; during the cooling of the material, the kiln is running at an ultra-low speed (the kiln body speed is working in the range of 2.5 ~ 2.9r / min), and the frequency of the inverter is about 5hz. On June 18, 2006, the material was stopped and cooled at 8 a.m., because the small kiln carried out maintenance tasks, the large kiln was operated at an ultra-low speed for more than 1 hour, and then the kiln was stopped for heat preservation. During the start of the kiln drag motor at 3 pm, smoke suddenly appeared in some low-voltage devices (such as emergency lighting chargers, walkie-talkie chargers and displays, etc.) in the operating room, and the operator immediately terminated the kiln frequency conversion device After starting the operation, the abnormal phenomenon disappeared immediately. According to the on-site operator's description of the abnormal phenomenon, the preliminary analysis may be caused by the sudden increase in the voltage of the power supply system. However, the voltage of each phase of the power supply system is normal (the phase voltage is 220v), check the operation console and the control cabinet, there is obvious burnt smell in the box, and find that multiple indicator lights burn out, ups trip, PLC and belt metering Power modules such as scales are burned. Because the abnormal phenomenon occurs during the startup of the frequency conversion device of the rotary kiln dragging kiln, check that the control converter of the kiln has no abnormalities (the panel shows normal, no damage to the electronic devices, etc.); check the neutral line (zero line) of the power supply system , No abnormal problems such as poor contact (sometimes the poor connection or overheating of the connection point of the main neutral line of the power supply transformer will cause the zero potential drift phenomenon on the neutral line). When the inverter of the kiln was restarted afterwards, the above-mentioned abnormal phenomena appeared immediately. Various monitoring measures were taken before the operation. After the inverter was started, the single-phase voltage of the power supply system rose to about 286v; the inverter braked (released Energy) The resistor is automatically put into operation (the sound of the braking resistor can be heard), and the measured temperature of the braking resistor is 72 ℃ (circle temperature 19 ℃); after the start of the kiln inverter is stopped, the grid voltage returns to normal (220v). During the start-up process of frequency conversion (the start-up process lasted 4 ~ 6s), the kiln body showed no signs of rotation, so check the motor, open the junction box and find that the winding turns black, and there is a strong smell in the cabinet. The three-phase winding of the motor is measured to ground The insulation resistance is 0, 12mω and 16mω respectively.
2 Failure analysis
In practical work, the voltage rise of the power supply system caused by the insulation damage of the inverter-driven motor is extremely rare, and it is difficult to explain with the general knowledge of electrical engineering. In response to the abnormal voltage rise of the power supply grid, we conducted a comprehensive on-site inspection and measurement, and measured that the motor grounding protection line was connected to the neutral line (zero line) of the power supply grid (resistance is 0ω), and further investigation found that the kiln motor grounding The protection line is connected to the inverter cabinet, and the cabinet is connected to the zero line of the power supply system (as a protection zero connection measure). The common connection of this grounding line and the neutral line constitutes repeated grounding of electrical equipment (also called loop-type repeated grounding). Its function is to reduce the risk of electric shock when the ground wire is disconnected or the resistance of the ground body is large; at the same time, when the live part of the device touches the shell, the short-circuit current forms a loop through the neutral line, which can accelerate the action of the line protection device. The mechanism by which the abnormal phenomenon occurs is as follows: After the insulation of the one-phase winding of the motor controlled by the inverter device is damaged (breakdown), if the inverter starts, the three-phase AC (low frequency) voltage output by the inverter will pass through the motor control power line , Motor core, grounding wire, power supply system neutral (zero line) are superimposed on the power supply transformer low-voltage power frequency (50hz) three-phase power supply. Because the frequency converter and power supply transformer form a large current path through the grounding line, neutral line and other power lines, the frequency and initial phase of the three-phase AC voltage output by the frequency converter are different from the power frequency voltage of the power grid. It will cause the superposition of different frequency power supply voltages. The degree of superposition is closely related to the capacity of the inverter, the operating frequency, the initial phase and the capacity of the power supply transformer. The larger the inverter capacity and the larger the load current at startup, the higher the superimposed voltage and the more serious the damage. The current loop formed by the inverter, power grid, neutral line and grounding line is shown in Figure 1.
According to the principle of power supply superposition, in the case of forming a current path, two power supplies with different AC voltage amplitudes, frequencies and initial phases will overlap each other, and the voltage waveforms of the two power supplies will change greatly [1 ] (The peak value and effective value of the voltage waveform change accordingly). The frequency converter is a device that rectifies, filters, and inverts the power-frequency three-phase AC voltage into three-phase AC voltages of different frequencies. Its frequency converter is equivalent to a
For an independent power supply, the initial phase and frequency of the output voltage waveform are different from the power frequency power supply grid; during the startup process of the inverter, the voltage frequency of the inverter output is a process of slowly increasing from 0hz. Therefore, when the single-phase winding of the 4 # rotary kiln drags the motor to the ground and the insulation is broken, it causes the power supply transformer winding, the inverter motor (including the power line, iron core), the grounding line and the grid neutral conductor to form a current path. When the inverter starts, it will cause the problem that the 0 ~ 50hz low-frequency AC voltage output by the inverter and the 50hz AC voltage of the power supply system superimpose each other, causing the three-phase power supply voltage of the power supply grid to rise to 286v, causing single-phase (220vac) electrical appliances to work The voltage is too high, and overheating (smoke) or burning failure occurs; at the same time, because the input AC voltage of the inverter increases, the rectified DC voltage will also increase, so the inverter DC overvoltage protection circuit automatically starts working (That is, the braking resistor consumes power). The 5hz AC voltage waveform (low frequency output by the inverter) is superimposed on the 50hz AC voltage waveform (grid power frequency). Its principle is shown in Figure 2.
(a) b 'phase voltage waveform (220v5hz) output by the inverter (b) power supply a phase voltage waveform (220v50hz) (c) a and b superimposed power supply network a phase voltage waveform
Figure 2 The principle of superposition of the b 'phase voltage output by the inverter and the a-phase voltage waveform of the power supply grid
Figure 2a) is the b 'phase AC voltage waveform output by the inverter, the peak value of the waveform is e1 (the measured effective value is about 220v), and the frequency is 5hz (other waveforms with different frequencies can also be selected for superposition, and the result is the same); Figure 2b) is the power phase a-phase power frequency AC voltage waveform, the peak value of the waveform is e2, and the frequency is 50hz; Figure 2c) is the waveform of the b 'phase (5hz) waveform output by the inverter superimposed on the a phase of the power supply grid. The peak value of the waveform after superposition increases from e1 before superposition to e1 + e2, and the corresponding effective value of the voltage must also increase; the waveforms of the b and c phase voltage waveforms of the power supply system and the superimposed waveform are the same as the phase a. It is just that the initial phase of the superimposed waveform peak is different, but the effective values ​​are equal [1].
3 Troubleshooting
The 4 # rotary kiln driving motor of our factory is an ordinary three-phase asynchronous motor. Due to the inverter running in the ultra-low frequency state of 5hz for a long time, its motor cooling fan runs at a low speed, and the motor cooling air volume is greatly reduced, resulting in serious overheating of the motor. The motor stator winding insulation is burnt out. After replacing the motor, the large kiln control inverter was started many times, and the voltage of the power grid did not rise again.
In order to prevent similar faults from causing the voltage abnormality of the power supply system to occur again, we have taken the protection of the 4 # rotary kiln body (including the motor grounding wire) to connect the zero line, as shown in Figure 3. The neutral line n (neutral line) of the power supply system and the grounding line of the kiln body are independent of each other (the actual ground is still connected, but the resistance is relatively large), and the grounding devices are separately provided for grounding, so that the grounding line and No large current path will be formed between the neutral lines of the power supply system. It can avoid the phenomenon that the voltage of the power grid system rises after the output of the frequency conversion device is grounded. In practice, when one phase of the frequency conversion device is grounded, its grounding current forms a loop through the grounding wire, the earth, and the neutral line of the power supply system, and the superimposed voltage drop (increased part) will be fully or partially borne by the earth However, the voltage of power equipment such as power supply system and frequency conversion device will not rise significantly. In order to prove the reliability of the processing results, we conducted a fault simulation test. The inverter was started by one-phase grounding of the inverter, and the frequency conversion device was started. It was measured that the three-phase power supply of the power supply system did not change the ground voltage during the startup of the inverter. The braking resistor also did not appear to be automatically in operation.
4 Conclusion
With the rapid development of science and technology, inverters, intermediate frequency circuits, ups power supplies and other power supplies with inverter functions are widely used in industrial production, and their abnormal work will affect other equipment to varying degrees. Therefore, in the actual equipment installation, maintenance and use, we must pay more attention to the impact of different equipment. Only by fully understanding the role of each other and taking the necessary precautions can we ensure the safe and reliable operation of all equipment.
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