A synchronverter is an inverter that mimics synchronous generators, which offers a mechanism for power systems to control grid-connected renewable energy and facilitates smart grid integration. Similar to other grid-connected inverters, it needs a dedicated synchronization unit, e.g., a phase-locked loop (PLL), to provide the phase, frequency, and amplitude of the grid voltage as references. In this paper, a radical step is taken to improve the synchronverter as a self-synchronized synchronverter by removing the dedicated synchronization unit. It can automatically synchronize itself with the grid before connection and track the grid frequency after connection. This considerably improves the performance, reduces the complexity, and computational burden of the controller. All the functions of the original synchronverter, such as frequency and voltage regulation, real power, and reactive power control, are maintained. Both simulation and experimental results are presented to validate the control strategy. Experimental results have shown that the proposed control strategy can improve the performance of frequency tracking by more than 65%, the performance of real power control by 83%, and the performance of reactive power control by about 70%.

https://www.sheffield.ac.uk/polopoly_fs/1.323811!/file/IEEEPE_SelfSynch.pdf

Self-Synchronized Synchronverters: Inverters Without a Dedicated Synchronization Unit

Recently, several advanced phase-locked loop (PLL) techniques have been proposed for single-phase applications Among these, the Park-PLL and the second-order-generalized-integrator-based PLL are very attractive, owing to their simple digital implementation, low computational burden, and desired performance under frequency-varying and harmonically distorted grid conditions Despite the wide acceptance and use of these two advanced PLLs, no comprehensive design guidelines to fine-tune their parameters have been reported yet Through a detailed mathematical analysis, it is shown that these two PLL structures are equivalent to each other, from the control point of view Then, a linearized model is developed which is valid for both PLLs The derived model significantly simplifies the stability analysis and the parameter design To fine-tune the PLL parameters, a systematic design approach is suggested afterward, which guarantees a fast dynamic response, a high disturbance rejection ability, and a robust performance Finally, the simulation and experimental results are presented to support the theoretical analysis

/pdf/dynamics-assessment-of-advanced-single-phase-pll-structures-3so6k0xa88.pdf

Dynamics Assessment of Advanced Single-Phase PLL Structures

This paper presents a new synchronous-reference-frame (SRF)-based control method to compensate power-quality (PQ) problems through a three-phase four-wire unified PQ conditioner (UPQC) under unbalanced and distorted load conditions. The proposed UPQC system can improve the power quality at the point of common coupling on power distribution systems under unbalanced and distorted load conditions. The simulation results based on Matlab/Simulink are discussed in detail to support the SRF-based control method presented in this paper. The proposed approach is also validated through experimental study with the UPQC hardware prototype.

Synchronous-Reference-Frame-Based Control Method for UPQC Under Unbalanced and Distorted Load Conditions

The amount of distributed energy resources (DERs) has increased constantly worldwide. The power ratings of DERs have become considerably high, as required by the new grid code requirement. To follow the grid code and optimize the function of grid-connected inverters based on DERs, a phase-locked loop (PLL) is essential for detecting the grid phase angle accurately when the grid voltage is polluted by harmonics and imbalance. This paper proposes a novel low-pass notch filter PLL (LPN-PLL) control strategy to synchronize with the true phase angle of the grid instead of using a conventional synchronous reference frame PLL (SRF-PLL), which requires a d-q-axis transformation of three-phase voltage and a proportional-integral controller. The proposed LPN-PLL is an upgraded version of the PLL method using the fast Fourier transform concept (FFT-PLL) which is robust to the harmonics and imbalance of the grid voltage. The proposed PLL algorithm was compared with conventional SRF-PLL and FFT-PLL and was implemented digitally using a digital signal processor TMS320F28335. A 10-kW three-phase grid-connected inverter was set, and a verification experiment was performed, showing the high performance and robustness of the proposal under low-voltage ride-through operation.

A Novel Grid Synchronization PLL Method Based on Adaptive Low-Pass Notch Filter for Grid-Connected PCS

This paper presents an alternative way for the current regulation of single-phase voltage-source dc-ac converters in direct-quadrature (dq) synchronous reference frames. In a dq reference frame, ac (time varying) quantities appear as dc (time invariant) ones, allowing the controller to be designed the same as dc-dc converters, presenting infinite control gain at the steady-state operating point to achieve zero steady-state error. The common approach is to create a set of imaginary quantities orthogonal to those of the real single-phase system so as to obtain dc quantities by means of a stationary-frame to rotating-frame transformation. The orthogonal imaginary quantities in common approaches are obtained by phase shifting the real components by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. In the proposed approach of this paper, the orthogonal quantities are generated by an imaginary system called fictive axis, which runs concurrently with the real one. The proposed approach, which is referred to as fictive-axis emulation, effectively improves the poor dynamics of the conventional approaches while not adding excessive complexity to the controller structure.

Vector Control of Single-Phase Voltage-Source Converters Based on Fictive-Axis Emulation

This paper proposes a novel phase-locked loop (PLL) control strategy to synthesize unit vector using the modified synchronous reference frame (MSRF) instead of the traditional synchronous reference frame. The unit vector is used for vector rotation or inverse rotation in vector-controlled three-phase grid-connected converting equipment. The developed MSRF-PLL is fast in transient response compared to standard PLL technique. The performance is robust against disturbances on the grid, voltage wave with harmonic distortion, and noise. The proposed algorithm has been analyzed in detail and was fully implemented digitally using digital signal processor TMS320F2812. The experimental evaluation of the MSRF-PLL in a shunt active power filter confirms its fast dynamic response, noise immunity, and applicability.

A Digital PLL Scheme for Three-Phase System Using Modified Synchronous Reference Frame

In PWM inverters, a time delay between consecutive semiconductor switching is introduced to prevent a short circuit in the DC link. This action causes the dead-time effect, which is detrimental to the performance of inverters. This paper deals with a new technique that speedup the feedback loop in Shunt Active Power Filter, in order to compensate the dead-time effect. A simple method based on an average value theory can be used to compensate this effect. For Shunt Active Power Filters (SAPF), the compensation can be done in two different ways, one for the feedforward configuration and the other for the feedback one. This paper presents both techniques and discusses the details, advantage of the feedback implementation with a new fast feedback loop that guaranties the dead-time compensation and the overall stability. Experimental results are presented showing the effectiveness of the proposed technique.

Dead-time compensation in Shunt Active Power Filters using fast feedback loop

This paper describes an algorithm to implement a full digital adaptive current hysteresis control. The algorithm can be applied in all types of inverters. The proposed strategy takes care of DC link voltage fluctuations, load parameters variations, and it guaranties constant switching frequency operation keeping the ripple at the smaller possible value. The Fuzzy based algorithm has been developed in such way that no hysteresis-band calculations are needed. The constant switching frequency is important to minimize the power semiconductors losses and to maintain the optimal operation of the output ripple filter. The algorithm is full digitally implemented in DSP and it doesn't need any additional circuits to do the job. It is easy to be programmed, fast and extendable to three-phase applications. Simulation and experimental results have shown the good performance of the algorithm in all practical situations: normal operation, fast load transient, load parameters variation and harmonics follow-up.

A Simple Full Digital Adaptive Current Hysteresis Control with Constant Modulation Frequency for Active Power Filters

Partial discharge detection in power transformers is discussed using a new approach that exploit the broad band of the Rogowski coils and the potential of two signal processing tools: discrete wavelet transform and empirical mode decomposition. Detecting and analyzing incipient activities of partial discharge can provide useful information to diagnostics and prognostics about transformer insulation. So, partial discharge signals embedded in the electric current at ground conductor are measured using the Rogowski coil. These signals are submitted to noise suppression and the partial discharges waveforms are extracted through different ways: using discrete wavelet transform and using empirical mode decomposition. These two methods are compared and their advantages discussed.

Detection of partial discharge in power transformers using Rogowski coil and multiresolution analysis

This paper introduces the development and implementation of an algorithm for real-time measurement of powers under sinusoidal, nonsinusoidal, balanced and unbalanced conditions. The power calculations are based on the definitions given by the IEEE Std. 1459–2000 and the adaptive filtering. The results are obtained in two ways: through simulation methods and practical implementation of the algorithm on DSP TMS320F2812. These results are compared and the efficiency of the algorithm is proven.

Se Un Ahn

Papers

A Digital PLL Scheme for Three-Phase System Using Modified Synchronous Reference Frame

Dead-time compensation in Shunt Active Power Filters using fast feedback loop

A Simple Full Digital Adaptive Current Hysteresis Control with Constant Modulation Frequency for Active Power Filters

Detection of partial discharge in power transformers using Rogowski coil and multiresolution analysis

Adaptive algorithm for real-time power quality measurement according to IEEE Std. 1459–2000