1 Introduction As is well known, in a typical general-purpose frequency converter as shown in Figure 1, the input terminals (r, s, t) and output terminals (u, v, w) cannot be reversed. Otherwise: when the frequency converter is not running, the three-phase power supply will pass through a three-phase rectifier bridge composed of 6 reverse freewheeling diodes and then be connected to the uncharged intermediate circuit capacitor. Since there is no current-limiting resistor on the output side, the excessive charging current (note that the uncharged capacitor is equivalent to a momentary short circuit to the external circuit) will burn out the fast fuse on the output side of the intermediate circuit. Figure 1 shows the schematic diagram of the main circuit of a general-purpose frequency converter. When the frequency converter is running, if another power source (three-phase mains power or the output of another frequency converter) is connected to the output side of the frequency converter before the frequency converter is completely stopped, the output side circuit of the frequency converter is equivalent to a three-phase reversible rectifier bridge. However, the positive group of the bridge is composed of uncontrollable diodes. Therefore, as long as one IGBT in the reverse group of the IGBT is turned on, it will cause a short circuit between one tube of the common anode group of the IGBT and one tube of the common cathode group of the diodes (equivalent to the failure of the forward and reverse switching in the DC reversible circuit). Since the output side of the frequency converter generally does not have fast fuses, it can only burn out the IGBT. In PWM control mode, the commutation pulse switching of the IGBT is very fast. Therefore, once this situation occurs, the IGBT on the output side of the frequency converter will be damaged instantly. The current flow path when the output side power supply is short-circuited is shown in Figure 2. Figure 2 shows the path of short-circuit current when the output side is connected to a power source. A special case where the output side of the inverter is mistakenly connected to another power source is when the other power source connected to the output side comes from the same transformer as the input power source of the inverter (as shown in a, b, and c). For example, when using the inverter as a soft-start device, if the bypass contactor is closed before the inverter has been taken out of operation, the power source on the input side of the inverter will be short-circuited through the input rectifier bridge, the DC intermediate circuit, and the IGBT on the output side. Based on the above analysis of the parallel connection of the output of the voltage-type inverter with other power sources, the following two questions can be derived: (1) If a voltage-type inverter is used as a soft starter, is it permissible to connect the motor to the public power grid while the inverter is still running? (2) Are two independent AC-DC-AC voltage-type inverters allowed to operate in parallel? 2. Can a voltage-type frequency converter, used as a soft starter, achieve a completely seamless switching between startup and operation? In centralized water supply systems, to simplify design, a single frequency converter is often used both as a soft starter to start water pump motors at different times and as a flow regulator for a specific pump through speed control, ensuring a constant mains pressure in the water supply system. Ideally, the soft start process should involve detecting complete synchronization between the soft starter's output three-phase voltage and the public grid's three-phase voltage (voltage amplitude, frequency, and phase), then first closing the bypass contactor to connect the soft starter output and the public grid power supply in parallel, and then disconnecting the soft starter from operation. If a soft starter is used, since it is essentially a three-phase AC voltage regulator and does not change the frequency and phase of the input voltage, when the thyristor firing angle is 0°, the output voltage and the input voltage (i.e., the public grid voltage) are the same. Therefore, a startup process of connecting in parallel and then disconnecting is permissible. However, when using a voltage-type frequency converter as a soft starter, the frequency converter must be shut down before the bypass contactor is closed. Otherwise, a short circuit will occur. The schematic diagram and program control requirements for the main circuit of the voltage-type frequency converter as a soft starter are shown in Figure 3. Figure 3 Schematic diagram of the main circuit when the frequency converter is used as a soft starter. The program design requirements are as follows: (1) The closing logic of contactors km1 and km2 must be interlocked and they cannot be closed at the same time. (2) km1 cannot be disconnected with current. Otherwise, the frequency converter will alarm due to large current changes or overvoltage, and may even break down the IGBT due to overvoltage. When switching, the free stop command of the frequency converter must be issued first, and km1 should be disconnected and km2 closed after confirming that the frequency converter has stopped running. (3) If there is a requirement to switch back from power frequency to inverter operation (such as in a water supply system, where the water consumption decreases and the pipeline pressure increases, and the motor needs to switch back from power frequency operation to inverter operation), the contactor km1 on the output side must be closed first. Otherwise, if the inverter runs first and then the contactor km1 is closed, the inverter will trip due to overcurrent caused by the current surge when the motor starts. To achieve seamless switching as much as possible, considering that although the magnetic field energy on the stator side of the motor is released the moment the contactor opens when switching from the frequency converter to the mains frequency, the magnetic field energy on the rotor side of the motor requires a certain amount of time to dissipate (generally 1-3 seconds, depending on the motor capacity). The rotor's magnetic field will induce an electromotive force on the stator side of the asynchronous motor. Therefore, a more reasonable switching circuit is to use a synchronous detection unit and an ATS dual-power transfer switch (ATS is used because its built-in mechanical interlock is more reliable for circuit switching). When the synchronous detection unit detects that the frequency, amplitude, phase, and phase sequence of the frequency converter output voltage are within the allowable range of the mains frequency grid, it issues an ATS switching command to switch to mains frequency operation. The main circuit schematic diagram is shown in Figure 4. Figure 4: Main circuit schematic diagram of the frequency converter as a soft starter to reduce disturbances during switching. The control requirement for this switching circuit is that after detecting voltage synchronization, the frequency converter output must be blocked before issuing the ATS switching command. Incidentally, LCI frequency converters are often used for soft starting of large synchronous motors. Since the main circuit of an LCI is an AC-DC-AC current-type frequency converter, it allows the output side to be connected in parallel with the public grid voltage before the frequency converter is taken out of operation. Therefore, in the synchronous grid-connected control of LCI soft starting, after the synchronous detection equipment and the frequency converter control system cooperate to complete the voltage synchronization control, there are two methods: one is to first block the frequency converter output and then connect the grid circuit breaker, and the other is to first connect the grid circuit breaker and then block the frequency converter output. Of course, the latter has a smaller impact on the grid when connected to the grid, because the frequency converter and the grid share the instantaneous current of the synchronous motor during grid-connected operation. 3. Can two independent AC-DC-AC voltage source inverters operate in parallel? From a circuit principle perspective, in a linear circuit, two voltage sources are allowed to be connected in series, but not in parallel (similarly, two current sources are allowed to be connected in parallel, but not in series). Most inverters are AC-DC-AC voltage source inverters, which are voltage sources. Therefore, using them in parallel will cause a short circuit. Intuitively, as mentioned in the first question, no other voltage source should be connected to the output side of an inverter that is in operation. Of course, there are exceptions in nonlinear circuits. For example, in AC-DC-AC current-type inverters or LCI circuits, two unidirectional rectifier bridges powered by a three-winding split transformer are often connected in parallel on the output side to reduce harmonics from the rectifier circuit to the power grid. This is because the unidirectional conductivity of the unidirectional rectifier bridge limits the current flow to the load of the rectifier circuit. Even if the two voltage sources are connected in parallel, it will not cause a short circuit in the power supply. The problem is only the load balancing of the two voltage sources, which can be solved by master-slave control, so that the two rectifier bridges controlled by the current loop use the same current command signal. The structure of this circuit is shown in Figure 5. Figure 5 Schematic diagram of the rectifier side of a 12-pulse AC-DC-AC current-type inverter. I also often see another application in DC drives: two sets of DC drive devices are connected in parallel to drive a DC motor, forming a pseudo-12-phase reversible rectifier circuit. This is actually equivalent to two reversible rectifier bridges connected in parallel on the output side. Of course, the purpose is also to reduce harmonics to the power grid, and at the same time, the parallel connection increases the capacity of the motor that can be driven. Why can the voltage sources be connected in parallel in this case? The reason is that the DC drives currently in use all employ non-circulating current logic circuits. The reverse rectifier circuit is only allowed to conduct after the positive rectifier circuit must be completely turned off. In fact, at any given moment, it is equivalent to only one unidirectional rectifier bridge driving the motor (the positive group provides forward current, and the reverse group provides reverse current). If it is necessary to change the current (or torque direction), the two DC drive devices must be logically interlocked. Only after the positive groups of both DC drive devices are turned off can the reverse group be turned on simultaneously (in Siemens' 6RA24, the control output baf32 and the control input bef60 implement such interlocking logic). In this way, even if two reversible rectifier bridges are connected in parallel, at any given moment, it is equivalent to two unidirectional rectifier bridges connected in parallel, which will not cause a power supply short circuit. Having analyzed this, let's return to Figure 1. On the inverter side of the general-purpose AC-DC-AC inverter, due to the presence of the reverse freewheeling diode, the current path of the reverse group is always open. Therefore, any connection in parallel with other power sources will cause a short circuit. Could the reverse freewheeling diode be replaced with a controlled IGBT, confirming that the positive group current is turned off before turning on the reverse group? Theoretically, this is possible, but the dead zone for determining the zero-current signal limits the inverter's maximum output frequency. Moreover, the target of AC-DC-AC voltage-type inverters is the output voltage, not the direction of current flow. Therefore, AC-DC-AC voltage-type general-purpose inverters cannot be used in parallel. 4. Conclusion Because AC-DC-AC voltage-type inverters are not allowed to be connected in parallel with other independent voltage sources on the output side, when a voltage-type inverter is used for soft starting, it is not allowed to connect two voltage sources in parallel first and then take the inverter out of operation, nor is it allowed to use two independently operating AC-DC-AC voltage-type inverters in parallel.