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For three-phase power system, the synchronous reference frame (SRF) phase-locked loop (PLL) is probably the most widely used synchronisation technique under ideal grid condition. However, the presence of dc-offset causes fundamental frequency oscillations errors in estimated phase. To deal with this problem, delay signal cancellation (DSC) operator is utilised in SRF-PLL in recent published literature at the cost of slowing down the dynamic behaviour. The aim is to propose a rapid dc-offset rejection method in three-phase PLL for grid synchronisation. With the employment of modified DSC operator, the dynamic performance of the conventional DSC-based PLL is improved. The effectiveness of the proposed method is confirmed through simulation results.

In the new energy grid-connected power generation system, accurately extracting the grid synchronization signals such as the frequency, phase and amplitude of the grid voltage is the basis for effective control. Aiming at the requirements for detecting grid synchronization signals under unbalanced, harmonics and DC offset voltage mixed conditions, a dual second-order complex coefficient filter with DC offset rejection capability (DSOCCF dc) is proposed, combining the approach of moving average filter (MAF), a novel hybrid filter in dq-frame is designed and on the basis of this design a new synchronous reference frame phase locked loop (SRF-PLL) design approach based on the hybrid filter is proposed. The proposed approach employs moving average filter (MAF) to block the high-frequency harmonics in the grid voltage, and uses DSOCCF dc to separate the fundamental frequency positive and negative sequence and reject DC offset. It can accurately extract the synchronization information of the grid fundamental frequency positive sequence. After simulation and experiment verification, it can be confirmed that the proposed PLL can quickly and accurately lock the properties of the grid voltage under adverse grid condition, and also have high detection accuracy and strong robustness to frequency fluctuations. INDEX TERMS Dual second-order complex coefficient filter (DSOCCF), harmonic, phase-locked loop (PLL), dc offset, moving average filter (MAF).

Synchronous-reference-frame phase-locked loop (SRF-PLL) is widely used in grid synchronization applications. However, under unbalanced, distorted and DC offset mixed grid conditions, its performance tends to worsen. In order to improve the filtering capability of SRF-PLL, a modified three-order generalized integrator (MTOGI) with DC offset rejection capability based on conventional three order generalized integrator (TOGI) and an enhanced delayed signal cancellation (EDSC) are proposed, then dual modified TOGI (DMTOGI) filtering stage is designed and incorporated into the SRF-PLL control loop with EDSC to form a new hybrid filter-based PLL. The proposed PLL can reject the fundamental frequency negative sequence (FFNS) component, DC offset component, and the rest of harmonic components in SRF-PLL input three-phase voltages at the same time with a simple complexity. The proposed PLL in this paper has a faster transient response due to the EDSC reducing the number of DSC operators. A small-signal model of the proposed PLL is derived. The stability is analyzed and parameter design guidelines are given. Experimental results are included to validate the effectiveness and robustness of the proposed PLL.

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Autonomous underwater vehicle is an effective tool for humans to explore the ocean. It can be used for the monitoring of underwater structures and facilities, which puts forward more accurate and stable requirements for the system operation of the autonomous underwater vehicle. This paper studies the system and structural design, including the parameter identification design and control system design, of a novel autonomous underwater vehicle called “Arctic AUV”. The dynamic mathematical model of the “Arctic AUV” was established, and the system parameter identification method based on the multi-sensor least squares centralized fusion algorithm was proposed. The simplification of the mathematical model of the robot was theoretically derived, and the online parameter identification and motion control were combined, so that the robot could cope with the influence of the arctic water velocity and external turbulence. Based on the hybrid control scheme of adaptive PID and predictive control, the accurate motion control of the “Arctic AUV” was realized. A prototype of “Arctic AUV” was developed, and system parameter identification experiments were carried out in indoor pool water. Hybrid adaptive and predictive control experiments were also carried out. The validity of the parametric design method in this paper was verified, and by comparative experiment, the effect of the control method proposed in this paper was better than the traditional method.

The phase-locked accuracy of conventional phase-locked method is reduced when the grid voltage contains fundamental frequency negative sequence(FFNS) component, harmonic component, and DC offset component. Aiming at this problem, a novel adaptive notch filter (NANF) is proposed, and a dual NANF (DNANF) structure is designed to eliminate the FFNS component and extract the fundamental frequency positive sequence(FFPS) component based on NANF. Furthermore, dcDNANF with DC offset rejection capability is proposed by adopting DNANF. Then a novel hybrid filter in dq-frame is designed by combining dcDNANF and the cascaded delay signal cancellation operator filter whose delay parameters are 4 and 24 in dq-frame (dqCDSC 4,24). Meanwhile, a new SRF-PLL design method is proposed based on the novel hybrid filter. This proposed method employs dqCDSC 4,24 to separate the positive and negative sequences of the voltage and eliminate the high frequency harmonics in the grid voltage, and uses dcDNANF to reject the DC offset, so as to achieve the accurate acquisition of the fundamental voltage information under distortion and unbalanced grid. Simulation and experimental results show that compared with the conventional SRF-PLL methods, the proposed method can obtain faster phase tracking speed, better phase-locked effect, faster dynamic response, and better stability. INDEX TERMS Novel adaptive notch filter (NANF), harmonic, phase-locked loop (PLL), dc offset, cascaded delay signal cancellation (CDSC).

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