A comprehensive survey on control strategies of distributed generation power systems under normal and abnormal conditions

Phase-locked loops (PLLs) are the most widely adopted techniques for synchronization of distributed generation units to the utility grid. However, when the grid voltage is polluted with severe unbalances and harmonics, the accuracy and performance of the conventional PLL will be deteriorated. This paper proposes a novel filtering technique, called the multiple delayed signal cancellation (MDSC), for improving the dynamic performance of the conventional synchronous reference frame (SRF)-PLL under adverse three-phase grid conditions. Theoretical developments of $dq$ -frame and $\alpha \beta$ -frame MDSC operators are investigated. The analysis shows that these MDSC operators can provide more flexibility to configure the lowest undesired harmonics. Since the MDSC operators possess multiple delayed signals in parallel, the delay time introduced by the operators can be reduced by selecting a suitable number of transfer delays. Therefore, the MDSC operators can be configured to have less delay time in comparison with the cascaded delayed signal cancellation operators with the same harmonic elimination capability. These two MDSC operators are utilized as a prefiltering stage to the conventional SRF-PLL to improve the dynamic responses. Since MDSC operators are dependent on grid frequency excursions, a new frequency adaptation scheme for the MDSC operators is also proposed for providing fast grid frequency estimations in designing the MDSC prefilter based PLLs. Experimental verifications demonstrate the effectiveness of the proposed PLLs and provide good dynamic responses when compared with the recent advanced PLLs.

Three-Phase PLLs by Using Frequency Adaptive Multiple Delayed Signal Cancellation Prefilters Under Adverse Grid Conditions

The moving-average-filter-based quasi-type-1 phase-locked loop (MAF-QT1 PLL) can provide zero steady-state phase error in the presence of frequency drifts, achieving a high filtering capability. However, in the presence of an MAF and frequency drift, MAF-QT1 PLL cannot achieve a fast transient response and frequency-dependent characteristic under adverse grid conditions. To overcome this drawback of the MAF-QT1 PLL, this paper proposes a frequency-adaptive improved moving-average-filter-based quasi-type-1 PLL (FAIMAF-QT1 PLL). Aiming at the shortcoming of the slow dynamic response of the MAF-QT-1 PLL, a correction link is introduced, and an improved moving-average-filter-based quasi-type-1 PLL (IMAF-QT1 PLL) is obtained. To improve the anti-interference ability of the IMAF-QT1 PLL in response to grid frequency changes, a FAIMAF-QT1 PLL and a corresponding digital implementation scheme are proposed. The regulator parameter setting method is proposed based on the small-signal model of the FAIMAF-QT1 PLL. Simulation and experimental results show that the FAIMAF-QT1 PLL can accurately track the grid voltage phase in the presence of power grid frequency fluctuations, phase angle jumps, distorted harmonic injections and unbalanced voltage drops and has good steady-state and dynamic performance.

/pdf/a-frequency-adaptive-improved-moving-average-filter-based-5bc9p1gyyc.pdf

A Frequency-Adaptive Improved Moving-Average-Filter-Based Quasi-Type-1 PLL for Adverse Grid Conditions

A flexible control strategy with overcurrent limitation in distributed generation systems

Synchronization is a crucial problem in the grid-connected inverter’s control and operation. A phase-locked loop (PLL) is a typical grid synchronization strategy, which ought to have a high resistance to power system uncertainties since its sensitivity influences the generated reference signal. The traditional PLL catches the phase and frequency of the input signal via the feedback loop filter (LF). In general, to enhance the steady-state capability during distorted grid conditions generally, a filter tuned for nominal frequency is used. This PLL corrects large frequency deviations around the nominal frequency, which increases the PLL’s locking time. Therefore, this paper presents an adaptive feed-forward PLL, where the input signal frequency and phase under large frequency deviations are tracked precisely, which overcomes the above-mentioned limitations. The proposed adaptive PLL consists of a feedback loop that reduces the phase error. The feed-forward loop predicts the frequency and phase error, and the frequency adaptive FIR filter reduces the ripples in output, which is due to input distortions. The adaptive mechanism adjusts the gain of the filter in accordance with the supply frequency. This reduces the phase and frequency error and also decreases the locking time under wide frequency deviations. To verify the effectiveness of the proposed adaptive feed-forward PLL, the system was tested under different grid abnormal conditions. Further, the stability analysis has been carried out via a developed prototype test platform in the laboratory. To bring the proposed simulations into real-time implementations and for control strategies, an Altera Cyclone II field-programmable gate array (FPGA) board has been used. The obtained results of the proposed PLL via simulations and hardware are compared with conventional techniques, and it indicates the superiority of the proposed method. The proposed PLL effectively able to tackle the different grid uncertainties, which can be observed from the results presented in the result section.

/pdf/an-adaptive-feed-forward-phase-locked-loop-for-grid-4f4bt4mjcd.pdf

An Adaptive Feed-Forward Phase Locked Loop for Grid Synchronization of Renewable Energy Systems under Wide Frequency Deviations

A Hybrid Filtering Stage Based Quasi-type-1 PLL under Distorted Grid Conditions

The invention discloses an active power filter control method for self-adaptive parameters. According to the method, an APF main circuit, a self-adaptive parameter identification algorithm and an SVPWM control algorithm are included. By means of the self-adaptive parameter identification algorithm, the problem that the compensation effect of an active power filter is reduced due to reactance value changes caused by the reasons such as materials, a processing technology and electric reactor ageing can be thoroughly solved. Meanwhile, the SVPWM algorithm is adopted, and compared with a traditional method, the algorithm has the advantages of being few in detection link, low in calculation amount, high in precision and the like; by means of trigger pulse generated through the algorithm, the direct-current side voltage reaches a stable state within two power cycles, and a three-phase power grid current is well controlled.

Active power filter control method for self-adaptive parameters

The invention discloses a novel hybrid parallel-connection type active power filter. The main circuit employs a passive filter and an active power filter, which are in parallel connection, wherein the passive filter filters fifth, seventh and eleventh harmonics, the active power filter is used to compensate for the rest of the harmonics. A soft start circuit is connected in series in the active power filter to prevent high current generation during an APF start process and prevent damage of an IGBT. By using the novel hybrid parallel-connection type active power filter, the fifth, seventh and eleventh harmonics can be effectively filtered through the parallel passive filter. The capacity level of the active power filter can be reduced, and the capacity of the whole system can be reduced. The system has a simple structure, and is easier to control. In addition, the distortion rate of the grid current can be effectively reduced to meet the distortion rate requirement of being within 5% in China.

Novel hybrid parallel-connection type active power filter

The invention discloses a current prediction-based parallel APF (Active Power Filter) dead-beat control system and a method. The method comprises the following steps of: firstly, predicting a harmonic current according to a dead-beat control method and by combining an adaptive prediction method and a linear prediction method; secondly, generating a pulse control main circuit by using SVPWM (Space Vector Pulse Width Modulation) to enable an alternating-current side to send out a corresponding compensating current; and finally, controlling capacitance and voltage of a direct-current side by adopting a conventional PI (proportional-integral) control technology. The system comprises four parts including an APF main circuit, a dead-beat controller, a current predictor, an SVPWM modulator. The method can enable the prediction of the harmonic current to have relatively good steady state accuracy and dynamic convergence characteristic, and have a relatively strong noise resisting ability; and accurate prediction of an instruction current is achieved, time delay errors generated by numerical control to an APF is solved, the instantaneity of system harmonic detection is improved, a reliable reference voltage vector is obtained, and real-time compensation of the harmonic current is achieved in combination with an SVPWM mode of seven-section control.

Current prediction-based parallel APF dead-beat control system and method

The invention discloses a slotting permanent magnet speed regulator stuffed with steel blocks and a preparation method for a slotting permanent magnet speed regulator A conductor plate in the permanent magnet speed regulator is improved, a design method for the slotting conductor plate is disclosed, wherein a steel plate on the back of the slotting conductor plate is designed into a steel plate with a slotted bump, wherein the bumped steel block is just inserted in to a stepped slot of the conductor plate; due to the structural complexity of the slotting magnetic path structure stuffed with the steel blocks, ANSYS software is utilized to perform two-dimensional dynamic simulated analysis on the slotting magnetic path structure, the distribution rules of electromagnetic fields are researched, and the torque characteristic curve of the slotless magnetic path structure, the slotting magnetic path structure and the structure is compared According to the summary, the slotting permanent magnet speed regulator stuffed with steel blocks can be used for effectively improving the transmission torque by more than 28%

Slotting permanent magnet speed regulator stuffed with steel blocks and preparation method for slotting permanent magnet speed regulator

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