[align=left] 1. Introduction Due to bidding requirements and technological advancements, many organizations explicitly request high-low-high direct-drive high-voltage frequency converters when purchasing or bidding for them. Consequently, regardless of whether they are genuine high-low-high direct-drive high-voltage frequency converter manufacturers, they all claim their products are high-low-high direct-drive high-voltage frequency converters. Since most users are not professionals in the frequency converter industry, they are easily misled. High-low-high frequency converters use transformers to reduce high voltage, convert the frequency, and then raise the voltage again through circuit combinations. A true high-low-high direct-drive high-voltage frequency converter, however, refers to a device that directly inputs, rectifies, and inverts high voltage without any high-voltage transformer in the main circuit. High-high frequency converters and high-low-high frequency converters represent two completely different technical solutions, signifying a significant gap in technological levels. Because the series connection of high-speed power switching devices such as IGBTs is a recognized global challenge in the frequency converter and power electronics industry, it is difficult to solve. Therefore, people have had to use a high-low-high approach with low-voltage frequency converters to solve high-voltage problems, but this has also brought many problems, such as transformer issues. The true direct high-voltage frequency converter was born precisely because it solved the problem of series connection of high-speed power devices such as IGBTs. The disadvantages of using high-voltage transformers in high-low-high and high-medium-high voltage frequency converters are obvious. In high-voltage frequency converter equipment with isolation transformers, there are the following four problems: (1) high energy consumption, low efficiency, large size, and heavy weight; (2) low power factor and large harmonic pollution; (3) large starting impact; (4) poor isolation effect. This article will focus on discussing the defects of high-voltage frequency converter equipment with high-voltage transformers, in order to gain everyone's attention. 2. Analysis of the disadvantages of high-voltage frequency converters using high-voltage transformers 2.1 High energy consumption, low efficiency, large size, and heavy weight. High-low-high type high-voltage frequency converters use low-voltage single-phase frequency converters in series, and each power unit is powered by a multi-winding phase-shifting isolation transformer. In high-voltage frequency converter equipment with isolation transformers, the transformer is an important component in the circuit structure. In phase-shifting transformers, 5-7 windings × 3 are required at 6kV (8 windings × 3 are required at 10kV). To improve input current harmonics, an extended delta connection with an external star configuration is used at the transformer output, as shown in Figure 1. Due to phase-shifting requirements, the connection ratio of delta and star for each winding is different. However, due to manufacturing limitations, the vector sum of the inner delta windings cannot be zero, resulting in circulating current in the inner delta. This circulating current only generates losses and does not perform work. When superimposed with the operating current, the winding loss is p = i²r. Therefore, the loss is considerable. Conventional transformers can achieve an efficiency of 98-99%, while this type of transformer can only achieve 94-96% (rated load). When the load decreases, due to the inherent losses of the transformer, the efficiency of the high-voltage frequency converter system with the transformer drops to 60-70%, as shown in Figure 2. Figure 1. Extended delta connection. Figure 2. Efficiency comparison of high-voltage frequency converters with and without transformers. Frequency converters are mainly used for energy saving, with load rates often between 50% and 90%, which is precisely the inefficient range for frequency converters with transformers. Furthermore, these transformers have many connections; one extended delta winding has 12 terminals connected, and a 20-winding transformer has 243 terminals connected. Since the extended delta winding is composed of two voltage superpositions, and the arithmetic sum of the two voltages is greater than the vector sum, this leads to increased losses. Additionally, the need for cross-phase cable connections due to so many terminals inevitably increases internal resistance and losses, resulting in a higher failure rate. Their large size and weight are unavoidable inherent defects. Our company developed this type of high-voltage frequency converter with transformers in 1998, but perhaps due to our perfectionism, we were unwilling to recommend products with many flaws to users, and thus resolved to overcome this global challenge. 2.2 Low power factor and high harmonic pollution (1) The power factor is seriously low. Due to the large number of windings in the phase-shifting transformer and the high level of insulation between each winding, the leakage inductance is much higher than that of the ordinary transformer. This directly results in a seriously low power factor. When people discuss multiplexing, they talk about how good the current waveform is under rated conditions. However, most users do not operate under rated conditions. At this time, the input current harmonics are also not negligible. The current waveform is shown in Figure 3. Figure 3 Current waveform diagram under rated load and non-rated load (2) Harmonics of transformer The core of the power transformer has nonlinear magnetization characteristics. Its hysteresis loop (i.e., bh curve) is shown in Figure 4(a). When a sinusoidal voltage u is applied to the unloaded transformer, if the excitation current is ignored, the curve of the magnetic flux density b of the transformer core changing with time is also sinusoidal. This is because of this. At this time, the excitation current (i.e., the unloaded current io) is a non-sinusoidal waveform. This is because h=b/μ, and the permeability μ decreases as b increases. In Figure 4(b), the solid line represents the theoretical waveform of io-t, which is the no-load current of a single-phase transformer. This current has a high harmonic content, with i3/i1 being the largest at approximately 50%, followed by i5/i1 at approximately 30%. This is because the rated b value of the transformer is generally designed close to the inflection point of the bh curve. Figure 4 shows the bh curve of the transformer and the waveforms of u, b, and i0. According to literature, under normal circumstances, the high-order harmonic current content in the transformer excitation current is within the following ranges: when the core is made of cold-rolled silicon steel sheets, i3/i1 is 40%–50%; i5/i1 is 10%–25%; i7/i1 is 5%–10%; i9/i1 is 3%–6%; and i11/i1 is 1%–3%. For three-phase transformers without a zero-sequence magnetic circuit or (and) a zero-sequence current path, and for transformers with a delta winding as a zero-sequence current path but only considering the current supplied by the power source and excluding the current in the delta winding, the value of i3/i1 is significantly smaller than the above values. The lower limit of the range values ​​for i7/i1 and higher harmonic content is given as larger. The core of a three-phase core transformer lacks a zero-sequence magnetic circuit, but the magnetic circuit lengths of the three core columns are unequal. The magnetic circuits of the two outer phases also include the lengths of the upper and lower yokes; therefore, the magnetic circuit length of the outer phases is approximately twice that of the middle phase. This asymmetrical three-phase magnetic circuit results in the generation of positive and negative sequence third-harmonic magnetic fluxes and corresponding positive and negative sequence third-harmonic induced electromotive forces, causing the transformer's excitation current to contain positive and negative sequence third-harmonic currents. Therefore, when a voltage excitation is applied to an unloaded three-phase transformer, the excitation current still contains third-harmonic current; when there is an extended delta connection, the third-harmonic current is only slightly reduced. The harmonic current generated by a single transformer generally does not exceed the specified allowable value. However, the total capacity of transformers in the power grid may be more than 4 times the total capacity of generators. Their total harmonic current is very large, such as 1% to 2% of the total rated current of all generators. When the transformer winding connection method and the connection between each winding and each phase of the power grid are uniformly specified, otherwise, if the same harmonic current in the excitation current of each transformer is roughly superimposed, it will become an important source of background harmonics in the power grid. The excitation current of the transformer and its contained harmonic current increase with the increase of voltage and magnetic saturation. Since modern transformers are designed with the magnetic flux density at rated voltage close to the inflection point of the magnetization curve, when the voltage exceeds the rated value, the transformer harmonic current increases rapidly with the increase of voltage, especially the 5th harmonic current, which makes voltage adjustment difficult. (3) Abnormal harmonics and surge magnetism of transformers When DC current or low-frequency current flows through the transformer winding, it causes the transformer core to saturate in one direction, thereby generating large even and odd harmonic currents. When a rectifier is unbalanced, it can cause a small DC current to flow through the transformer, which in turn can generate a large harmonic current. When a transformer is switched on without power, or when the grid voltage suddenly rises significantly (such as when a short-circuit fault in the grid is cleared), it will cause varying degrees of abnormal magnetic saturation in the three-phase core of the transformer, resulting in extremely large excitation currents, sometimes reaching several times (for large and medium-sized transformers), or even more than ten times (for small transformers). Moreover, the harmonic content is extremely high, especially the i2 and i3 harmonics, which can exceed the fundamental current. This excitation current decays exponentially, and the decay time constant depends on the l/r ratio of the current path. This time constant is generally on the order of 0.1s in medium and low voltage power grids. Inrush current caused by transformer inrush magnetization is the most common and frequently occurring short-term high-value harmonic current in the power grid. When the capacitive harmonic impedance of the grid is slightly greater than the transformer's harmonic excitation impedance, transformer harmonic resonance will occur, resulting in relatively stable and persistent high-value harmonic currents and voltages in the grid, which may have serious consequences. The harmonic components in transformer inrush current are very large. Therefore, when a transformer is energized without a load, a very dangerous harmonic inrush current may occur. This is also why it is necessary to avoid energizing a transformer without a load when there are operating capacitors or filters connected to the busbar. (4) Mathematical Model of Transformer Harmony The mathematical models of transformer harmonic sources reported to date are all based on simulating the u-io or bh characteristics of the transformer using hyperbolic functions, and all assume that the three-phase magnetic circuit is symmetrical and that the core has a zero-sequence magnetic circuit and a zero-sequence current path. It can be seen that such a model is not applicable to most transformers, that is, three-phase transformers with large differences in the length of the three-phase magnetic circuits, especially three-phase core transformers without a zero-sequence magnetic circuit in the core. 2.3 Problem of Starting Impulse of Isolation Transformer As we all know, if there is a transformer in a power supply system (such as an isolation transformer, autotransformer, or step-down transformer at the load input end), an excessive current surge often occurs when the system starts. Figure 5 shows the cause and process of the surge current formation when the transformer starts. Figure 5(a) shows the magnetization curve of the transformer core and the input voltage and current waveforms during normal operation; Figure 5(b) shows the transformer operating characteristics when the input voltage is lost; Figure 5(c) shows a schematic diagram of the transformer under no-load conditions when an inrush current occurs. The occurrence, amplitude, and duration of this inrush current are random, and in the most severe cases, it approaches the short-circuit current, even causing the system protection switch to trip. The inrush current decreases from large to small. The transition time also varies with the magnitude of the inrush current, with the transition process of the inrush current approaching a short circuit lasting several hundred ms. Another characteristic is that the inrush current is asymmetrical in the positive and negative half-cycles of the input voltage. If the first inrush current waveform occurs in the positive half-cycle, then there is no impact in the negative half-cycle current, and the inrush current of the entire transition process occurs in the positive half-cycle. If the first starting inrush current waveform occurs in the negative half-cycle, then there is no impact in the positive half-cycle current, and the entire transition process occurs in the negative half-cycle. Figure 5: Causes and processes of transformer starting inrush current formation. After the isolation transformer is put into operation, although the occurrence of the starting inrush current is random, it is unavoidable. The installation of isolation transformers in high-voltage frequency converter power supply systems may be for various reasons, such as reducing the system's zero-to-ground potential difference or supposedly increasing anti-interference capabilities. However, due to the inrush current of the isolation transformer during startup, the original intention of installing the transformer may be counterproductive, causing more severe interference and even damaging other equipment in the system. 2.4 Poor Isolation Effect It is often mistakenly believed that the presence of an isolation transformer in a system guarantees strong anti-interference capabilities. This understanding is not necessarily correct. In power supply systems, the causes and phenomena of interference are diverse, including high-voltage pulses, spikes, surges, transient overvoltages, radio frequency interference (EFI), and electromagnetic interference (EMI). However, in terms of their interference forms and transmission paths, they can be broadly classified into two categories: common-mode interference and differential-mode interference, as shown in Figure 6. Figure 6 shows the type of interference. Isolation transformers alone cannot guarantee interference immunity; the anti-interference capability of ordinary transformers is limited. Besides its transformer function, isolation transformers also provide electrical isolation between circuits, solving the common ground problem between devices. They also offer some suppression of mutual interference caused by ground loops. However, due to the distributed capacitance between the windings, their suppression effect on common-mode interference decreases as the interference frequency increases. Transformers rely on magnetic coupling to achieve voltage transformation between the primary and secondary sides, thus lacking differential-mode interference immunity. In the interference frequency range of 1kHz to 100MHz, the attenuation capability of ordinary isolation transformers for both common-mode and differential-mode interference is negligible. Analysis of the common-mode suppression capability of ordinary isolation transformers shows that improving this capability hinges on reducing the inter-turn coupling capacitance of the transformer windings; this requires adding a shielding layer between the primary and secondary windings. To enable isolation transformers to simultaneously possess good immunity to both differential-mode and common-mode interference, they must be manufactured as super-isolated shielded transformers, as shown in Figure 7. The anti-interference capabilities of various isolation transformers are shown in Figure 8. Figure 7 shows a super isolation transformer. Figure 8 shows the anti-interference function of various isolation transformers. Figure 9 shows the circuit topology of an IGBT direct series high-voltage frequency converter. As can be seen from Figure 8(a), ordinary isolation transformers have very little suppression effect on differential-mode and common-mode interference; while Figure 8(b) shows that shielded isolation transformers have a significant suppression effect on common-mode interference. The curve shown in Figure 8(c) represents the anti-interference performance of the super isolation transformer. As can be seen from Figure 8, its suppression effect on common-mode interference is above 80dB over a wide frequency range. When the interference frequency exceeds 100kHz, its suppression effect on differential-mode interference can also be above 60dB. Direct high-voltage frequency converters use different electromagnetic compatibility (EMC) schemes and circuit topologies depending on the user's power supply capacity, as shown in Figure 9. Their anti-interference capability, power supply interference and power factor, and system efficiency are incomparable to those of high-voltage frequency converters with transformers; they are products of completely different levels. In conclusion, the view that "all isolation transformers have anti-interference function" is a misunderstanding of the function of isolation transformers. 3. A True Direct High-Voltage Frequency Converter with IGBTs Directly Connected in Series : This type of converter receives high-voltage power directly from the grid via a high-voltage circuit breaker. After passing through a high-voltage diode full-bridge rectifier, a DC smoothing reactor, and capacitor filtering, it undergoes inversion by an inverter. With the addition of a sine wave filter, it easily achieves high-voltage frequency conversion output, directly supplying power to the high-voltage motor. This two-level voltage-type high-voltage frequency converter with IGBTs directly connected in series utilizes existing mature frequency converter technology and unique, simple control techniques to successfully design a transformerless, IGBT-direct-connected inverter system with an output efficiency of 98%. 4. Conclusion Unit series multiplexing was a transitional measure adopted when the IGBT series problem could not be solved. This solution introduces a series of serious problems due to the use of transformers, as mentioned above. While some vendors claim perfect harmonic-free operation, this is unrealistic. Even under static and rated load conditions, the serious problems mentioned above cannot be masked under actual operating loads, transients, and transient conditions. Therefore, eliminating the transformer and using IGBTs in direct series connection with appropriate input measures results in a far smaller impact on the power grid compared to using a transformer. Extensive practical experience has proven this to be an indisputable fact. Thus, IGBT direct series high-voltage frequency converters, due to their new circuit structure and control technology, eliminate the high-voltage transformer at the power input, reducing the size and weight of the high-voltage frequency converter and significantly lowering costs; reducing component losses and improving operating efficiency; facilitating the improvement and addition of new control functions, improving dynamic characteristics, and significantly reducing operating noise; and facilitating integration and modularization in structure, effectively improving equipment availability. Therefore, eliminating the input transformer in high-voltage frequency converters represents a significant advancement in frequency conversion technology.