1 Introduction In the application of high-voltage frequency converters, there are many situations where the "fly start" function is needed. "Fly start" is simply speed tracking. That is, restarting a rotating motor. For example: (1) Situations with relatively small moment of inertia. Typical loads such as fans have relatively small mechanical inertia. A slight natural wind may cause it to rotate naturally. For frequency converters without this function, overcurrent protection may occur when putting them into operation. It is necessary to wait for the motor to come to a complete stop before starting, which will inevitably delay the debugging or production time. (2) In the case of a duct using two fans for induced draft, when one fan stops, the other will also drive the stopped fan to rotate. To stop it, both fans must be stopped, which is quite troublesome. (3) In the case of one-to-many, although this situation may be less common, sometimes frequency converters are used to switch from power frequency to frequency converter, and vice versa. It is also necessary to detect the speed and implement tracking control. (4) The “star point drift” function is used in the operation of high voltage frequency converters. When one of the units fails, it can be temporarily bypassed and derated due to the unit bypass function. However, the voltage of the damaged phase will inevitably drop, which will be unbalanced with the voltage of the other two phases, resulting in unbalanced motor terminal current and inability to operate for a long time. Therefore, the “fly-start” and “star point drift” functions are of great significance in the installation, commissioning and industrial production of frequency converters. With the continuous progress of technology and the deepening of scientific research, these two functions are gradually being applied to frequency converter practice, forming a feature of high voltage frequency converters. 2 The principle and difficulties of “fly-start” The “fly-start” of high voltage frequency converters is when the motor stator is disconnected from the frequency converter or the power grid, the motor stator is “passive” and the motor rotor is in a rotating state, but the speed is random and uncertain. The high voltage frequency converter is connected to the motor stator, so that the motor stator goes from “passive” to “active”, the rotating magnetic field of the motor stator goes from non-existent to active, and finally the rotating magnetic field of the motor stator drives the motor rotor to enter the normal drive process. As we know from the principles of motor operation, when the speed of the rotating magnetic field of the motor stator differs significantly from the speed of the motor rotor (i.e., when the slip is large), a large current will be generated, but the electromagnetic torque will be small. For example, when a motor is directly started at full voltage under power frequency, the stator current will reach 5 to 7 times the rated value. However, the capacity of a high-voltage frequency converter is generally not selected to be 5 to 7 times the rated current of the motor. A similar situation occurs if the high-voltage frequency converter has a high output frequency (50Hz) during a "fly start" while the motor rotor speed is very slow, inevitably leading to overcurrent tripping. Conversely, if the high-voltage frequency converter has a low output frequency during a "fly start," and the speed of the rotating magnetic field of the stator is lower than the speed of the motor rotor, the motor is in a generating state. The motor rotor will send energy back to the stator side to charge the frequency converter capacitor, causing the frequency converter to trip due to overvoltage pumping from the capacitor. Therefore, the key to the success of a high-voltage frequency converter's "fly start" is that the output frequency and the rotor speed (frequency) are the same. The motor rotor frequency is random, so it is necessary to search for the motor rotor frequency. That is, the motor rotor frequency is searched first when the "fly start" starts. After the motor rotor frequency is found, the frequency converter uses the searched rotor frequency as the output frequency. In this way, there will be no overcurrent or capacitor voltage pumping overvoltage phenomenon. [b]3 Implementation Methods 3.1 There are two methods for rotor frequency search[/b] For the V/F control mode without speed sensor, the Siemens frequency converter user manual mentions two methods for rotor frequency search: (1) One can be called the "V/F curve voltage comparison method with constant rated current of stator input". During the search, the stator is always kept at a constant rated current. The output voltage of the frequency converter is compared with the voltage value on the V/F curve. When the two are equal, it means that the output frequency at this time is the rotor frequency. (2) Another can be called the "DC bus minimum current method". That is, when the speed of the stator rotating magnetic field is the same as the speed of the motor rotor, the DC bus current of the frequency converter is the minimum. The rotor frequency is indirectly detected by detecting the DC bus current. The former approach is theoretically feasible, but in practice, the physical relationship between the v/f curve and the stator rated current is unclear. Furthermore, at low frequencies, a boost voltage is added for voltage compensation, making it difficult to guarantee the accuracy of voltage comparison using the v/f curve. Additionally, the dynamic response of constant rated current control directly affects the accuracy of voltage comparison and frequency search. The latter approach has a clear physical concept but cannot be directly applied. In this high-voltage inverter power unit, there is no DC bus current detection, so the scheme of indirectly detecting the rotor frequency by detecting the DC bus current cannot be used. However, the stator current generated after applying the search voltage to the motor can be vector-decomposed to extract the torque current component, and the rotor frequency can be indirectly observed by observing this torque current component. When the stator rotating magnetic field speed is the same as the motor rotor speed, the motor rotor speed is the synchronous speed. At this point, the torque current component should theoretically be zero. However, in practice, the angular frequency of the rotating magnetic field changes during the search for the motor rotor frequency. The vector transformation formula for decomposing the torque current is for a specific angular frequency, and the step size during frequency search cannot be infinitesimally small. It's possible that one step is higher than the rotor frequency, and the next step is lower. Therefore, the search should be based on the torque current component being "close to zero." That is, the search should be based on the minimum torque current component, with a minimum torque current comparison value given. Because when the stator rotating magnetic field speed is lower than the motor rotor speed, the motor is in a generating state, and the motor rotor will send energy back to the stator side to charge the inverter capacitor, causing the inverter capacitor voltage to rise to overvoltage. Therefore, the search process must start from a frequency higher than the motor rotor frequency, considering all possibilities and starting from the highest 50Hz. Thus, the frequency search monotonically decreases from high to low. If the search process starts from a frequency higher than the motor rotor frequency (50Hz), directly following the v/f curve will output a "full-scale" voltage, similar to "direct full-voltage start," resulting in extremely large current and torque surges. Therefore, the voltage output is 5% to 20% of the "full-scale" voltage. After a successful search, the voltage is gradually increased back to the "full-scale" voltage at this frequency. This weakens the voltage-limited current and torque impact. Although the search process outputs 5% to 20% of the "full-scale" voltage, overcurrent may still occur due to an inappropriately set weakening voltage coefficient. Therefore, the search process also needs current limiting to form negative feedback on the voltage output to automatically suppress overcurrent. Considering that the motor rotor's free rotation direction may be opposite to the normal operating direction, the frequency converter also needs a bidirectional search function. If the minimum torque current cannot be found from 50Hz to 0Hz in the normal operating direction, the reverse search process is initiated. After finding the motor rotor frequency, the speed is first reduced to 0 and then accelerated forward to the given frequency. 3.2 Implementation of "Fly-Start" of High-Voltage Frequency Converter Based on the previous analysis and the original system, the control logic for rotor frequency search can be determined as shown in Figure 1. [align=center] Figure 1 Functional diagram of the inverter's "fly-start" function[/align] After activating the "fly-start" function, the normal frequency setpoint will be automatically disconnected, and the frequency setpoint will be searched for from 100% (50Hz) to 0% (0Hz) of the setpoint. The v* generated by the v/f curve converter will be multiplied by the step-down factor (<20%). The real-time sampled value of the motor stator current is compared with the maximum current imax to form negative feedback on the voltage u*act, so as to limit the overcurrent in the frequency search. When the real-time sampled value of the motor stator current is less than or equal to the set minimum torque current, it indicates that the frequency search is successful. Then, the process of "setting" the frequency and "smoothing v* recovery" will begin. After v* recovers, the frequency will be automatically adjusted to the manually set frequency point according to the integral process. If the real-time sampled value of the motor stator current is always greater than the set minimum torque current and the search delay has expired, the search will stop and a search failure message will be given. [b]4 Experimental process and problems encountered 4.1 Fly-start test[/b] The method of the fly-start test is shown in Figure 2. [align=center] Figure 2 Experimental Flowchart[/align] First, use a low-voltage frequency converter to drive a low-voltage motor to drag the high-voltage motor to a certain speed. Then, disconnect the low-voltage frequency converter and start the high-voltage frequency converter to drive the high-voltage motor. Let the high-voltage frequency converter search for the speed of the high-voltage motor. After finding the synchronous speed, start normally. At the same time, detect the change in the output current of the high-voltage frequency converter during this process, as shown in Figures 3 and 4. [align=center]Figure 3 Output Current Changes[/align] [align=center]Figure 4 Current Changes During High-Voltage Start-up[/align] Each current increment is 20A, and the motor's rated current is 60A. Based on the measured current, the peak current during motor synchronization is 20A, and the maximum current during motor acceleration is approximately 50A. 4.2 Another Idea and Considerations for High-Voltage "High-Voltage Start-up" In the high-voltage start-up function, when the motor is running at high speed, the inverter outputs the rated frequency (50Hz), and the output voltage gradually increases from its lowest value at a certain rate. During this voltage increase, the output current is simultaneously monitored. If the output current exceeds 1.5 times the rated current, the output voltage remains constant. After the current decreases, the voltage is increased again at a certain rate. If the voltage reaches the voltage corresponding to the rated frequency (50Hz), the "high-voltage start-up" is considered complete, and the motor can then operate normally at the set frequency. 4.3 Issues to be verified (1) When the motor rotates again, will the current change not exceed 1.5 times the rated current during the voltage increase process? (2) At what rate will the voltage increase? (3) If the output current is greater than 1.5 times the rated current, will the current decrease during the voltage stabilization process? If the current does not decrease, will the current decrease if the output frequency is reduced? 4.4 Start working on the "fly-start" function and program structure in the high-voltage frequency converter The key to "fly-start" is current detection. The appropriate current judgment basis is the core. At present, it has not been considered in detail. Only the frequency scanning process has been considered: the frequency gradually decreases at 50Hz until the current is appropriate, then the frequency remains unchanged, the output voltage is gradually increased, and then it runs to the set frequency. This process needs to be further refined. Considering that there is a motor generating state in the experiment, it needs to be tested on the high-voltage elevator frequency converter. The appropriate current point also needs to be determined by actual current measurement. [b]5 "Star Point Drift" Function 5.1 Principle Analysis[/b] When a frequency converter unit fails, the common practice is to bypass the faulty unit and simultaneously bypass the corresponding units in the other two phases. This balances the three-phase output voltage of the frequency converter, thereby balancing the three-phase current of the motor. However, this reduces the output power of the frequency converter, making it unsuitable for long-term operation. Therefore, after analysis, technicians proposed the "star point drift" method. As shown in Figure 5, under normal conditions, the three-phase outputs a, b, and c are balanced, and the neutral point, i.e., the star point, is at point o. When one unit in phase a fails, the star point is transferred from point o to point o1. Phases b and c, after calculation, must ensure that their outputs to o1 are consistent with those of phase a. When two or three units fail, the same complex calculation is required to make them consistent. Currently, the star point transfer function can handle up to three unit failures. [align=center] Figure 5 Schematic diagram of star point transfer[/align] 5.2 Test Situation The star point drift situation is tested when a high-voltage power unit fails. The faulty phase current is shown in Figure 6. [align=center]Figure 6 Fault Phase Current[/align] When one unit fails, the output three-phase current is unbalanced. At 5kV, the currents ia=5A, ib=10A, and ic=12A. The faulty unit is a6. When the machine stops due to a single faulty unit, the phase current at the time of shutdown is shown in Figure 7. [align=center]Figure 7 Phase Current at Shutdown[/align] At this time, during the motor's stopping process, when the frequency reaches around 30Hz, overvoltage protection occurs at a5. The same phenomenon occurs when the faulty unit is switched to phase b. When switched to phase b, overvoltage protection occurs in other units of that phase. The cause needs further analysis (it is possible that the 5kV back EMF of the motor is directly applied to the five power units). With only one faulty unit (a6), the current situation at around 40Hz when the voltage is 5kV is shown in Figure 8. [align=center]Figure 8 Current Waveform at 40Hz[/align] To balance the line voltages of the three output phases, the output current is shown in Figure 9. [align=center]Figure 9 Current waveform after star point transfer[/align] In this case, the current situation during shutdown is shown in Figure 10. [align=center]Figure 10 Shutdown current under star point transfer[/align] 5.3 Reasons for poor sinusoidal current waveform during the test The reason for poor sinusoidal current waveform during the test may be that the third harmonics in the three phases cannot cancel each other out. The actual measurement also shows this. The harmonic currents are shown in Table 1. When the third harmonic is removed from the output waveform and the output sine wave is displayed, the harmonic currents are shown in Table 2. In this case, the current waveform after offset is shown in Figure 11. [align=center]Figure 11 Processed star point offset current[/align] The phenomenon after two units fail simultaneously is the same as above. 6 Method for setting the "flyaway start" function of wind and solar high voltage frequency converter To use the flyaway start function, first turn the DIP switch 7 on the main control board of the frequency converter to the ON position. Otherwise, the flyaway start function cannot be started. Then set the parameters for flyaway start. (1) In the inverter parameter settings, there is a starting frequency setting. This parameter can be used for a simple "flying start". If the motor is rotating freely before the inverter starts, and the direction of rotation is the same as the direction the inverter needs to start, if the motor speed is roughly known, the frequency that the inverter needs to output can be estimated based on the rated speed of the motor, and then a frequency slightly higher than that can be used as the starting frequency. After setting this parameter, the inverter can be started. When the voltage at this frequency is reached after the inverter starts, it means that the start is complete. After the start is complete, a frequency lower than this frequency can be set for operation. (2) Frequency search time The time for scanning the motor speed from high frequency to low frequency during flying start is 30s by default, and the setting range is 3 to 3200s. This value needs to be set according to the actual site conditions. If the free stop time of the motor is long, this value should be large; if the free stop time is short, this value should be small. (3) Current detection value Setting The current detection value during the flying start of the inverter is used in conjunction with the "flying start" function. Setting it during operation is invalid, and the default value is 0. 7 Conclusion The successful development and application of the "flying start" and "star point drift" functions of high voltage frequency converters will bring greater convenience to the installation and commissioning of frequency converters and users. This is a major feature of high voltage frequency converters. In application, as long as the parameters are set correctly and the operation is performed correctly according to the instruction manual, this function can be applied correctly. In the future, with the development of science and technology and the progress of technology, the functions of frequency converters will be further expanded, bringing greater convenience to users. About the author Yin Pengfei, senior engineer, graduated from the Mechatronics major of Northeast Polytechnic University and is currently the deputy chief engineer of Shandong Xinfengguang Electronic Technology Development Co., Ltd. References [1] Bai Defang, Li Kai. Realization of flying start function of high voltage frequency converter. Frequency converter world, 2007 (4) [2] Shandong Xinfengguang Electronic High Voltage Frequency Converter Instruction Manual