Abstract : This paper elucidates the mechanism of grid harmonic generation in SCR circuits, introduces the 'potential substitution method' for calculating harmonic voltage, derives the formula for calculating grid harmonic voltage using this method, and questions misconceptions about the grid harmonic mechanism. 1. Overview A non-sinusoidal current flows through the phase-controlled SCR AC circuit. This current flows through the grid impedance, forming a harmonic voltage on the grid, which is referred to as grid harmonic voltage in this paper. This is the mechanism of grid harmonic voltage generation. Based on this, a method for calculating grid harmonic voltage can be derived. The on/off state of the phase-controlled SCR can be reduced to a resistor that jumps between 0 and 0, thus obtaining the current waveform of the thyristor AC circuit. Multiplying this by the grid (short-circuit) impedance yields the grid harmonic voltage. This method is tentatively called the 'current waveform method'. Another method for calculating grid harmonic voltage is the 'potential substitution method': the on/off state of the phase-controlled SCR is equivalent to a potential source e[sub]SCR[/sub](θ), θ=ωt. In the on-state, u[sub]SCR[/sub](θ) = 0, i[sub]SCR[/sub](θ) ≠ 0, and the SCR is equivalent to a wire; in the off-state, u[sub]SCR[/sub](θ) ≠ 0, i[sub]SCR[/sub](θ) = 0, and the SCR is equivalent to a potential e[sub]SCR[/sub](θ) equal to u[sub]SCR[/sub](θ), which is the voltage appearing across its A and K terminals after the SCR is turned off. In summary, the 'potential substitution method' treats all SCRs in the circuit as harmonic potential sources e[sub]SCR[/sub](θ), and the grid harmonic voltage u[sub]net[/sub](θ) is the voltage division across the grid impedance. The two methods for calculating grid harmonic voltages are interconnected and, in principle, suitable for all AC circuits containing SCRs. While the 'potential substitution method' is less intuitive than the 'current waveform method', it has the advantage of not requiring current calculation. e[sub]SCR[/sub](θ) is clearly related to the operating state of the SCR (related to the firing angle). For a three-phase symmetrical AC circuit, whether it is e[sub]SCR[/sub](θ) or u[sub]net[/sub](θ), the current or voltage of phase BC lags phase AB by 120[sup]o[/sup]. 2. Power Grid Harmonies in Switching Transformer Soft Starter Circuit 2.1. Properties of Switching Transformer Soft Starter Circuit Switching transformer soft starter is a type of SCR soft starter. Its SCR is connected to the secondary side of the transformer. The turns ratio k >> 1 ensures that the SCR connected to the secondary side does not need to bear high voltage. In the conducting state of the SCR, what is seen from the primary side is the transformer's small leakage reactance x_tr = x₁ + x₂′, while in its off state, what is seen is the transformer's large magnetizing reactance x_m. Therefore, switching transformer soft starter can be considered similar to SCR soft starter. It obviously has the following properties: 1) It contains a phase-controlled SCR; 2) At any given moment, all devices in the circuit (including the switching transformer and the motor) can be considered linear devices; 3) Three-phase symmetry. From these three properties, two conclusions can be drawn: 1) The phase-controlled SCR is the cause of harmonic voltage generated by the soft starter of the switching transformer. No other device besides the phase-controlled SCR, whether it's the switching transformer or the motor, is the cause of harmonic voltage. 2) When calculating grid voltage harmonics, only one phase needs to be calculated, and the superposition principle can be used. 2.2 Calculation formula for grid voltage harmonics u[sub]net[/sub] (θ) caused by soft starter of switching transformer. Assume the switching transformer is connected in a Y/Δ configuration. Let the voltage harmonics of the grid phase caused by the soft start of the switching transformer be denoted as θ, and let the equivalent potential of the SCR be denoted as e[sub]SCR[/sub](θ). After ignoring x[sub]m[/sub], it is not difficult to derive the following from the voltage division relationship: (1) According to the same voltage division relationship, it is not difficult to obtain the harmonics caused by the soft start of the SCR (see [1]) (2) 2.3 Ambiguity about the mechanism of grid harmonics caused by the soft start of the switching transformer ([2]) The aim is to explain that the grid harmonics caused by the soft start of the switching transformer are much smaller than those caused by the soft start of the SCR. [2] made the following explanation about the mechanism of grid harmonics caused by the soft start of the switching transformer: "The generation of higher harmonics is due to the switching of the SCR. Therefore, we can regard the switching transformer TK as the harmonic source E, and the generated higher harmonic current flows to both sides." For the motor side, harmonic attenuation is not significant; however, for the power supply side, because x[sub]tr[/sub]/2 is greater than x[sub]short[/sub], "the harmonic voltage at the bus junction is much smaller," and "since solid-state SCR soft starters do not have x[sub]tr[/sub], ... there is no harmonic attenuation." It should be pointed out that [2] is inaccurate in the following two aspects: 1) It treats the harmonic source E as a grounded power source, hence the so-called "two sides." In fact, "harmonic source E" is not grounded. 2) It does not acknowledge the actual voltage division relationship between the harmonic source and the power grid in SCR soft starters. Even more so, the recently published [3] extends the view that "the harmonics of the switching transformer power grid are much smaller than those of SCR soft starters," arguing that one of the advantages of switching transformer soft starters over SCR soft starters is that "there is no output harmonic pollution." Based on inaccurate facts, it is impossible to correctly understand the mechanism, and based on an erroneous mechanism, it is impossible to make a fair evaluation. References: 1. Gao Yuenong, Li Yueying. Several conceptual issues in the application of soft starters for electric motors. Electrical Technology, 2008.4 (forthcoming). 2. Zhu Xiongxin, Yan Changfa, Wang Yu, Sun Xiangrui. Application of switch transformer type soft starter in soft starter of ultra-large capacity motors. Proceedings of the 8th National Conference on Electrical Technology of Petroleum and Chemical Industry (2007.11), pp.189-192. 3. Zhang Yan, Wang Huan. From the perspective of power electronics technology, soft starter control and transmission of medium voltage motors. 2008.1, pp.79-83.