Keywords: Transformer coupling, medium-voltage high power, frequency converter Abstract: This paper introduces the basic working principle and control method of a novel series medium-voltage high-power frequency converter with transformer coupling. 1 Introduction In the past 20 years, industrialized countries, especially Japan, France, and Germany, have attached great importance to the development and research of energy-saving technologies. On the one hand, they have far surpassed others in the research and application of high-performance, high-capacity AC motor drive technology; on the other hand, they have vigorously expanded the market for speed control technology, continuously broadening its application scope, making motor speed control technology a considerable industrial sector. Medium-power frequency converters have been widely launched on the market and applied in high-performance systems such as electric locomotives, ship electric propulsion, steel rolling, and papermaking. In addition, the other 70% of electric motors also employ various speed control and energy-saving technologies, applied in various fields, such as the transmission of equipment like fans, pumps, compressors, hoists, conveyors, crushers, mills, rolling mills, textile machines, cigarette machines, and air conditioners. These technologies are prevalent in metallurgy, chemical industry, power generation, machinery, building materials, petroleum, transportation, textiles, papermaking, coal, agriculture, and defense sectors, significantly reducing unit energy consumption and achieving substantial improvements in product quality, output, and environmental benefits. Since the 1980s, my country has made significant progress in the development and application of electric motor speed control technology, but it still lags considerably behind developed countries. Domestic speed control devices mainly use thyristor-based DC motor speed control, primarily used in metallurgy, chemical industry, mine hoisting, and oil drilling rigs. While these systems have mature manufacturing technology, they have many drawbacks: DC motors are expensive, require extensive maintenance, and cause severe pollution to the power grid, leading to their eventual obsolescence. In the field of high-power AC speed regulation, some research has been conducted domestically, and some domestically produced AC speed regulation devices have been manufactured. However, most of them are thyristor AC-AC frequency converters, which are costly, have poor performance, and cause serious pollution to the power grid. They also have low power factors and large reactive power losses. This article introduces a new type of transformer-coupled series medium-voltage high-power frequency converter. Its main idea is to use a transformer to superimpose the output voltages of three three-phase half-bridge conventional inverter units to achieve medium-voltage output. Furthermore, the three conventional inverters can use the same control method, which greatly simplifies the circuit structure and control method, making it very suitable for research and production in China. This frequency converter has an ingenious design and superior performance. 1. Circuit Composition and Working Principle In 2000, E. Cengelci proposed a new type of transformer-coupled series superimposed medium-voltage high-power frequency converter circuit for three-phase half-bridge inverter units in the literature. Its main principle is: using a three-phase transformer with a turns ratio of 1:1, the line voltages of the same phase of three identical three-phase half-bridge inverters are connected in series with the same polarity and superimposed to achieve medium-voltage output. Three conventional three-phase half-bridge inverters can use the same SPWM control method, which greatly simplifies the frequency converter circuit. Figure 1 shows the main circuit structure and voltage-current relationship of this new type of frequency converter, where Figure 1(a) is the main circuit structure diagram, Figure 1(b) is the transformer winding diagram, and Figure 1(c) is the voltage-current relationship diagram. This new type of frequency converter consists of the following three parts. (1) The input circuit is an 18-pulse rectifier, which provides DC power to the three three-phase inverters respectively, improves the mains input power factor of the frequency converter, reduces pollution to the mains power supply, and achieves harmonic-free input current. (2) The inverter circuit consists of three identical three-phase half-bridge inverters, which convert three independent DC power supplies into three identical three-phase AC power supplies. (3) The three-phase transformer is composed of three iron cores. Its function is to connect the line voltages of the same phase of the three half-bridge inverters in series with the same polarity to achieve medium voltage output. The transformer ratio is 1:1. Its structure is shown in Figure 1(b). The voltage and current relationship of the series-connected superimposed frequency converter of the three three-phase half-bridge inverters is shown in Figure 1(c). As shown in Figure 1(c), considering the line voltage of the AC motor, the voltage relationship of the inverter is as follows: Since the turns ratio of the three-phase transformer is 1:1, it is assumed that the three-phase current of the AC motor is balanced and the effective value of the current is I. Without considering the harmonic current, considering that the turns ratio of the transformer is 1:1 and the primary and secondary currents are equal, the three-phase output current of inverter No.1 can be calculated as follows: In addition, the three-phase output currents of inverters No.2 and No.3 also satisfy the relationship shown in equation (5), that is, equation (6). This shows that the output currents of the three three-phase half-bridge inverters are completely balanced. From the voltage and current relationship obtained above, it is easy to obtain the power relationship between the components of this inverter. Obviously, the apparent power of the three inverters is equal, and the apparent power of the entire inverter is the sum of the apparent power of the three inverters. 2. Expression of Inverter Output Voltage and Harmonic Analysis Since the voltage, current and power of the three inverters are completely symmetrical, the three inverters can adopt the same control method. At this time, the line voltage applied to the motor is equal to three times the output line voltage of one inverter. Assuming that the three inverters all adopt two-half SPWM control, taking inverter No.1 as an example, its SPWM working waveform is shown in Figure 2, where Figure 2(a) is the circuit diagram of inverter No.1, and Figure 2(b) is its SPWM working waveform. Since the inverter works in frequency conversion mode and SPWM works in asynchronous mode, the double Fourier series analysis method based on the carrier triangular wave angular frequency is adopted. For convenience, the carrier triangular wave is represented by two "piecewise linear functions" with initial values ​​of +Uc and -Uc. Thus, the mathematical equation of the triangular wave can be written in the form of equation (7). In Figure 2(b), when the second-order SPWM waveform below is obtained from the modulation waveform above, it is divided into N intervals with the amplitude point of the carrier triangular wave as the boundary. For example, the expression of voltage ub can be obtained by the same method at x=ωct. The expressions of ubc and ucn can also be obtained by the same method. In these expressions, the carrier and carrier harmonics are eliminated. In the formula, the terms where m and n are both even or odd are eliminated. There are two control methods for the three inverters: one is that the three inverters adopt the same SPWM control method; the other is that the carrier triangular waves of the three inverters are successively delayed by 120° in the SPWM control method. For the first control method, it can be seen from equation (13) that it is equivalent to a high-voltage SPWM inverter, and du/dt is too large, so it is not suitable. For the second SPWM control method, the waveform of the output voltage uAB superimposed in series can be obtained from equation (13) as shown in Figure 3. This is a seven-level voltage waveform. To effectively increase the inverter's switching frequency, reduce switching losses, and improve DC voltage utilization, a 15% third harmonic can be added to the sinusoidal modulation wave, increasing the modulation index M to 1.15. A 18-pulse triple-overlapping rectifier circuit with amplitude conversion is used in this inverter, coupled with a Δ/YYY input transformer, as shown in Figure 4. Figure 4(a) is the circuit diagram, and Figure 4(b) is the waveform diagram. For the waveform of the input current ia, the fundamental and harmonic amplitudes of the input current ia can be calculated using the Biringer formula: Solving the above three equations simultaneously yields the parameters a = 0.575, b = 0.881, and c = 0.2. The number of turns from w1 to w5 can be determined from a, b, and c. Thus, the 5th, 7th, 11th, and 13th harmonics can be eliminated from the primary input current ia of the transformer. The harmonic order in equation (16) is n=18k±l, k=0, 1, 2, 3, ... 4 Conclusion From the above analysis, it can be seen that this type of frequency converter has the following advantages. (1) The capacity of the coupling transformer is 1/3 of the total capacity of the frequency converter, which is 2/3 smaller than the capacity of the output transformer in the commonly used high-low-high circuit. (2) The high-voltage frequency converter, which is composed of three conventional three-phase half-bridge inverters as the core, can adopt the conventional SPWM control method, which simplifies the circuit; the output voltage is increased by using the coupling transformer to perform line voltage series superposition, eliminating harmonics below 3mN±1, which is ingenious, has excellent performance, and reduces the number of DC power supplies. (3) The three conventional three-phase half-bridge inverters operate in a balanced manner, each sharing 1/3 of the total power of the frequency converter. (4) The output voltage of the frequency converter is an equivalent seven-level PWM wave with low harmonic content and low du/dt. (5) The 18-pulse input rectifier circuit has a high input power factor and low pollution to the mains power supply. (6) By adding 15% third harmonic to the sinusoidal modulation wave, the modulation index can be increased to 1.15, which improves the DC voltage utilization rate. If this type of frequency converter is applied to a medium-voltage motor with a line voltage of 2300V, Ud=1090V, an inverter can be constructed using switching devices with a rated voltage of 1700V; for a voltage motor with a line voltage of 4160V, Ud=1970V, an inverter can be constructed using switching devices with a rated voltage of 3300V; for a voltage motor with a line voltage of 6600V, Ud=3130V, an inverter can be constructed using switching devices with a rated voltage of 4500V. Therefore, this type of frequency converter has strong adaptability. In addition, from the perspective of low technical difficulty, this frequency converter scheme is also very suitable for the current production level in my country.