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The series six-fold “phase hopping” AC-AC frequency converter is an extension of the traditional AC converter. This converter can increase the output frequency to close to the power frequency rate, and has the advantages of no circulation and no dead zone. But like most of the frequency converters, there is the problem of large harmonic. In order to effectively eliminate the adverse effects of harmonics, this brief designs a harmonic suppression method according to the special properties of this low frequency converter. Firstly, Fourier analysis and calculation are carried out on the output voltage waveform of the load end of the frequency converter, and the relationship between the harmonic content and the trigger Angle of the thyristor is obtained. After calculation, the optimal combination of the trigger Angle of the series six-fold is deduced, and the simulation is carried out with MATLAB. Finally, an experimental platform is built and the experimental analysis is carried out. The experimental results show that the theory and simulation analysis are correct. [ABSTRACT FROM AUTHOR]

• The nonlinear variation process of trigger angle is approximately fitted by the least square method, and the LCC side fault current expression with high accuracy is obtained. Similarly, the general PI controller model is used to deduce the feeding component of the AC current, and the VSC side fault current expression with high accuracy is also obtained. • Considering the factors such as the fast rising speed and large amplitude of the DC fault current, this paper proposes a new ZCLC topology and combines it with DCCB to apply to HVDC system. The DC-side fault analysis of Line commutated converter based high voltage direct current (LCC-HVDC) and Voltage source converter based high voltage direct current (VSC-HVDC) networks has been studied by most researchers and achieved fruitful results. However, the traditional fault analysis method does not consider in detail the change process of the trigger angle of the LCC converter station and the AC current feeding of the VSC converter station caused by the control strategy. Therefore, this paper proposes a new DC side fault transient analysis method suitable for hybrid multi-terminal HVDC transmission networks. In addition, considering the deficiency that the active fault current suppression method only relying on the control strategy cannot fundamentally clear the DC fault, this paper proposes a new Z-source current-limiting circuit (ZCLC) with reference to the topology of the Z-source dc circuit breaker (DCCB). Then, ZCLC will cooperate with DCCB to improve the fault clearing capacity of the whole system. Firstly, this paper establishes the equivalent Laplace transform model of the three-terminal LCC-VSC-VSC, and analyzes the short-circuit current expression on both sides. Then, the change process of trigger angle is approximately linearized by the least square method, which obtains a new expression of fault transient current on the LCC side. Similarly, the composition of AC current fed to DC side is deduced by using the commonly used inner-loop PI controller model, which also obtains a new expression of fault transient current on the VSC side. Secondly, referring to the traditional crossed-type Z-source DCCB, series-type Z-source DCCB, and parallel-type Z-source DCCB, this paper also proposes a new ZCLC and analyzes its working principle and parameter design. Then, the newly proposed ZCLC is combined with DCCB to be applied to the hybrid multi-terminal HVDC network. Finally, the simulation results based on Matlab/Simulink verify the effectiveness of the fault analysis and passive fault clearing methods, and the semi-physical experimental platform based on PLECS/RTbox verifies the superiority of the new ZCLC. [ABSTRACT FROM AUTHOR]

• The relationship between the harmonic wave and the trigger angle of the inverter side is derived in detail. • The influence of the phase-locked trigger on the commutation of the LCC-HVDC transmission system is qualitatively analyzed. • The fault timing sequence generation method is proposed. • An improvement scheme for an anti-harmonic control system is proposed. • The multiple commutation failures can be well suppressed by the improved method. Aiming at the problem of commutation failure and the poor transient response of line commutated converter-based high-voltage direct-current (LCC-HVDC) transmission systems, this paper starts from the harmonic components of an AC system fault on the inverter side. The quantitative relationship between the harmonic wave and the trigger angle of the inverter side is derived in detail, and the effect of the trigger on the commutation of the LCC-HVDC transmission system is qualitatively analyzed with a phase-locked trigger. On this basis, an improvement scheme for an anti-harmonic control system is proposed. First, a virtual separated phase-locked control method is used to determine the fault occurrence time. Additionally, the characteristics of the trigger angle of the rectifier side and extinction angle of the inverter side after the fault are used to determine the fault position and generate the corresponding fault timing sequence. Then, the generated fault timing sequence is used to control the notch filter and modify the input voltage of the voltage-dependent current order limiter to improve the robustness of the control system in terms of the harmonics. Finally, based on CIGRE benchmark LCC-HVDC transmission system model, the advantages of the improved method are verified. The results can provide a reference for actual projects. [ABSTRACT FROM AUTHOR]

Harmonic magnetic fluxes in transformer's core and harmonic currents in grid‐side winding can be restrained via the inductive filtering technology, and running noise of the transformer is reduced at the source. The A‐weighted sound levels of the measuring points are measured under DC bias. The noise test results verify this theory. When the fully‐tuned filters matched with the new converter transformer are inputted and not inputted, the noise response curves of the measured points are compared. The fitting coefficients of the noise values at 1250 and 1600 Hz with the unbalance degrees of the trigger angle are obtained. [ABSTRACT FROM AUTHOR]