Abstract : This paper describes the hazards of reversing the high-voltage input and output cables of a voltage source type high-voltage frequency converter. Keywords: High-voltage frequency converter, high-voltage input, high-voltage output 1. Overview Currently , most high-voltage high-power frequency converters used internationally are multi-stage voltage source frequency converters with power units connected in series. Internationally, Robicon and Toshiba are representative manufacturers, while domestically, Hitachi and Leadway are representative. This type of high-voltage frequency converter is a unidirectional energy transfer device, allowing energy to be input only from the input terminal of the frequency converter (i.e., from the phase-shifting transformer terminal). After phase shifting, the energy is input to the power unit for "AC-DC-AC" conversion, and then output from the output terminal after level superposition. If energy is input in reverse from the output terminal (i.e., the high-voltage input and output cables are reversed), catastrophic consequences will occur. 2. Principle of Unit-Series Multiplexed Voltage Source Inverter 2.1 System Structure The unit-series multiplexed voltage source inverter adopts a direct "high-high" conversion method, consisting of multiple power units forming a multiplexed series topology. Each unit outputs a fixed low voltage level, and the multiple units are sequentially connected in a sinusoidal pattern to superimpose the required high voltage. Taking a 6kV six-unit series connection per phase as an example, the voltage superposition is shown in Figure 1, and the inverter circuit schematic diagram is shown in Figure 2. Each phase consists of six identical power units connected in series, with a phase voltage of 3464V. Each power unit outputs an effective value Ve = 577V, the peak output voltage. [align=center][IMG=6kV Inverter Voltage Superposition Diagram]/uploadpic/THESIS/2008/1/2008011417351024174R.jpg[/IMG] Figure 1 Schematic diagram of 6kV inverter voltage superposition[/align] The multi-stage series structure uses low-voltage devices to achieve high-voltage output, reducing the withstand voltage requirements of power devices. It has very little harmonic pollution to the power grid, with an input current harmonic distortion rate of less than 4%, meeting the harmonic suppression standard of IEEE519-1992; it has a high input power factor, eliminating the need for input harmonic filters and power factor compensation devices; the output waveform is close to a sine wave, and there are no problems such as motor heating and torque pulsation, noise, output dv/dt, and common-mode voltage caused by output harmonics. It can be used directly for ordinary asynchronous motors without the need for an output filter. [align=center][IMG=Schematic diagram of 6kV frequency converter circuit principle]/uploadpic/THESIS/2008/1/2008011417365256853C.jpg[/IMG] Figure 2 Schematic diagram of 6kV frequency converter circuit principle[/align] 2.2 Power Unit The power unit mainly consists of an input fuse, a three-phase full-bridge rectifier, a pre-charge circuit, a capacitor bank, an IGBT inverter bridge, a DC bus, and a bypass circuit. It also includes a control circuit composed of power supply, drive, protection monitoring, and communication components. The unit structure is shown in Figure 3. Each power unit has the same structure and is interchangeable. The power unit is powered by a set of secondary sides of a phase-shifting transformer. The AC input is rectified into DC by a three-phase full-bridge rectifier, and the energy is stored in the capacitor bank. The capacitor banks are connected in parallel or series depending on the unit voltage. For example, if the bus voltage is 815V, the three capacitor banks are connected in series to meet the voltage withstand requirements. The number of capacitors in each bank connected in parallel is selected according to the unit capacity. The control section draws power from the DC bus through the power supply board, receives the PWM signal sent by the main control system, and outputs the PWM voltage waveform by controlling the working state of the IGBTs. The monitoring circuit monitors the status of the IGBTs and the DC bus in real time and feeds the status back to the main control system. When a serious fault occurs in a unit, the main control will open the bypass circuit of the power unit, putting the unit into bypass mode to prevent the entire inverter from shutting down. Each unit outputs a PWM wave, and the output voltages of each phase N power units are superimposed to generate a multiplexed phase voltage waveform, resulting in 2N+1 voltage steps in the phase voltage. The PWM waveforms output by the six power units and the superimposed phase voltage waveforms are shown in Figure 4. [align=center] Figure 3 Power Unit of Frequency Converter Figure 4 Unit Output Waveform and Phase Voltage Superimposed Waveform of Frequency Converter[/align] 2.3 Phase-Shifting Transformer The electrical principle of the phase-shifting transformer is shown in Figure 5: The transformer (taking a 6kV input transformer as an example) has a 6kV primary winding and eighteen secondary windings divided into three phases. Each winding is connected in a delta configuration with phase shift angles of ±5°, ±15°, and ±25°, and each winding is connected to a power unit. This phase-shifting connection can effectively eliminate harmonics below the 35th order. Therefore, using a phase-shifting transformer for isolation and voltage reduction will not cause harmonic interference to the power grid exceeding national standards. [align=center] Figure 5 Structural Principle Diagram of Phase-Shifting Transformer[/align] 3. Hazards of Reversed High-Voltage Input and Output Cables 3.1 Correct System Wiring According to the primary circuit form of the frequency converter speed control system commonly used in China, the normal operation of the frequency converter is divided into two situations: power frequency bypass operation and frequency conversion operation. The main wiring circuit diagrams are shown in Figures 6 and 7. [align=center]Figure 6 Primary System Diagram for Power Frequency Bypass Operation Figure 7 Primary System Diagram for Variable Frequency Operation[/align] 3.1.1 Power Frequency Bypass Operation If the variable frequency drive speed control system operates according to the primary system diagram shown in Figure 6, the motor will be able to start and run at power frequency, but speed adjustment will not be possible. Motor protection is mainly achieved through the motor integrated protection device configured on the user's trolley switch. 3.1.2 Variable Frequency Operation If the variable frequency drive speed control system operates according to the primary system diagram shown in Figure 7, the motor will be able to start and run in variable frequency speed control mode. Under variable frequency conditions, the variable frequency drive control system will operate according to the set values, dynamically monitor the equipment's operation, and record data such as the variable frequency drive's operating status and fault status. Faults occurring during variable frequency drive operation can be handled promptly. If an overcurrent fault occurs at the output terminal, the variable frequency drive will automatically perform inverse time overcurrent protection or immediate protection according to the overcurrent level. The highest level is immediate protection, with the overcurrent point set within 3 times the rated output current and an action time of 10μS. Simultaneously, all IGBTs in all units are blocked, the output is cut off, and no current flows in the inverter's main circuit after 10μS. This protection function is a standard configuration for inverters, and each inverter produced by our company undergoes testing and verification before leaving the factory. Furthermore, our inverter's short-circuit protection function has been tested by the third-party certification body, Tianchuan Institute, and has a type test report. 3.2 Incorrect System Wiring (i.e., High-Voltage Input and Output Cables Reversed) If the inverter's input and output cables are reversed, as shown in Figure 8, the user can still perform inverter bypass operation at the power frequency under this condition. [align=center] Figure 7 Reverse Connection Circuit of Inverter Input Cable Figure 8 Direct Input of 10kV/50Hz Power Frequency to the Inverter Output Terminal[/align] In the above incorrect connection situation, if the inverter operation state is changed, i.e., the QS2 tool position is thrown towards the inverter output position, as shown in Figure 8, catastrophic consequences will occur. 3.3 Hazards of Incorrect System Wiring When the 6kV high-voltage input line is incorrectly connected to the inverter's isolating switch QS2, as soon as the user's trolley switch is closed, the 6kV/50Hz power supply from the high-voltage bus will be directly connected in series from the inverter's output terminal into the power unit. Looking from the output terminals of the inverter into its interior, since the capacitors inside each unit have zero charge, the instantaneous AC current is considered a short circuit. At this point, the equivalent circuit of the unit is simply a series-parallel connection of forward-connected rectifier diodes (the freewheeling diodes connected in parallel within the IGBT module), as shown in Figures 9 and 10: [align=center][IMG=Equivalent schematic diagram of the unit section with 6kV input voltage reversed]/uploadpic/THESIS/2008/1/2008011417543779743B.jpg[/IMG] Figure 9 Equivalent schematic diagram of the unit section with 6kV input voltage reversed [IMG=Equivalent circuit diagram of instantaneous reverse connection of 6kV input power]/uploadpic/THESIS/2008/1/2008011417555344500Z.jpg[/IMG] Figure 10 Equivalent circuit diagram of instantaneous reverse connection of 6kV input power[/align] The maximum permissible voltage drop for each phase is less than ±18V/DC, thus offering no resistance to high voltage. However, in the case of the incorrect connection shown in Figure 8, each phase instantaneously bears a peak voltage of ±8164V. This voltage far exceeds the permissible values ​​of the IGBTs, thyristors, and capacitors connected in parallel with the diodes within the unit. In an instant, the series connection of each phase unit becomes an equivalent pure conductor. Since all units are connected in series to the neutral point, a current path is formed, and this path has no current-limiting capabilities. The only current limitation comes from the external user's relay protection system. Excessive current will cause the freewheeling diode in the IGBT module to explode, leading to the explosion of the IGBT and diode within the unit, and simultaneously burning out other components connected in parallel. During the component explosion and burnout, the heat generated by the huge current will cause the cables and connectors in the aforementioned circuit to overheat, deform, or even burn out. Simultaneously, the huge short-circuit current will also cause the upstream user's trolley switch to trip, stopping the short-circuit damage. However, by this point, all units have already been damaged. [align=center]Figure 10 Schematic diagram of the damage range[/align] Because the frequency converter was connected to high voltage with the high voltage input and output reversed, the phase-shifting transformer at the actual input terminal of the frequency converter was connected to the motor, and could not supply power to the unit through normal means. The unit's control system did not enter the working state due to the lack of working power, and could not alarm the main control system. Therefore, the computer of the main control system will not record the time and state of this accident. 4. Case Analysis There are many such cases in China and abroad. 4.1 An accident that occurred in a certain enterprise This case occurred on the high-voltage variable frequency speed control device configured by our company for the converter blower of a certain enterprise. Due to the need to replace the cable, the maintenance personnel mistakenly connected the input cable to the output position of the frequency converter during the replacement, which caused a serious accident. At the moment of connecting the high voltage, the power unit of the frequency converter was completely damaged, resulting in serious economic losses. 4.2 Lessons Learned from the Case: A single incorrect wiring error can cause such a serious accident. Therefore, we draw the following conclusions: 4.2.1 After the frequency converter is installed and commissioned, the input and output cables should be strictly distinguished and clearly labeled. 4.2.2 If cable replacement is necessary, the entry and exit points of the two cables must be clearly distinguished, labeled, and tested. 4.2.3 Cable replacement and testing must be performed by at least two personnel, with mutual inspection. If necessary, an electrical engineer should also conduct acceptance testing. 4.2.4 All high-voltage operations must be accompanied by a work permit. 5. Conclusion We have compiled the state and causes of a serious accident caused by reversed high-voltage input and output cables in a unit-series voltage source type frequency converter. We present this to our colleagues in the industry, hoping that users and potential users in China will pay close attention to this issue to avoid significant economic losses due to basic errors. After adopting frequency converters, a strict inspection system needs to be established to prevent errors such as reversed input and output cables. Such errors can cause serious economic losses to enterprises, damaging equipment and disrupting normal production. This problem is not unique to unit-series voltage source frequency converters; it exists in all types of frequency converters.