Zhixiang Zou

Benchmarking of Grid Fault Modes in Single-Phase Grid-Connected Photovoltaic Systems

Pushed by the booming installations of single-phase photovoltaic (PV) systems, the grid demands regarding the integration of PV systems are expected to be modified. Hence, the future PV systems should become more active with functionalities of low-voltage ride through and grid support capability. The control methods, together with grid synchronization techniques, are responsible for the generation of appropriate reference signals in order to handle ride-through grid faults. Thus, it is necessary to evaluate the behaviors of grid synchronization methods and control possibilities in single-phase systems under grid faults. The intent of this paper is to present a benchmarking of grid fault modes that might come in future single-phase PV systems. In order to map future challenges, the relevant synchronization and control strategies are discussed. Some faulty modes are studied experimentally and provided at the end of this paper. It is concluded that there are extensive control possibilities in single-phase PV systems under grid faults. The second-order-general-integral-based phase-locked-loop technique might be the most promising candidate for future single-phase PV systems because of its fast adaptive-filtering characteristics and it is able to fulfill future standards.

Frequency-Adaptive Fractional-Order Repetitive Control of Shunt Active Power Filters

Repetitive control (RC), which can achieve zero steady-state error tracking of any periodic signal with known integer period, offers active power filters (APFs) a promising accurate current control scheme to compensate the harmonic distortion caused by nonlinear loads. However, classical RC cannot exactly compensate periodic signals of variable frequency and would lead to significant performance degradation of APFs. In this paper, a fractional-order RC (FORC) strategy at a fixed sampling rate is proposed to deal with any periodic signal of variable frequency, where a Lagrange-interpolation-based fractional delay (FD) filter is used to approximate the factional delay items. The synthesis and analysis of FORC systems are also presented. The proposed FORC offers fast online tuning of the FD and the fast update of the coefficients, and then provides APFs with a simple but very accurate real-time frequency-adaptive control solution to the elimination of harmonic distortions under grid frequency variations. A case study on a single-phase shunt APF is conducted. Experimental results are provided to demonstrate the validity of the proposed FORC.

Modeling, Analysis, and Design of Multifunction Grid-Interfaced Inverters With Output LCL Filter

The purpose of this paper is to investigate the model, control, and implementation of a multifunction grid-interfaced inverter with output LCL filter, which can provide high performance active power current and compensate the existent harmonics simultaneously in a distributed network. Equipped with LCL filter, the proposed multifunction inverter offers reduced switching harmonics and superior output current shapes. However, the multifunction inverter with output LCL filter brings some challenges, including LCL resonance, phase lag, and complexity in system design. To address these issues, this paper analyzes and develops general Thevenin/Norton models for the multifunction grid inverters with LCL filter. Based on the general models of system, this paper presents guidelines of the control and the procedure of filter design for this application. In particular, a proportional-resonant (PR) plus odd-harmonic repetitive control (OHRC) scheme is designed for the outer current loop. The phase compensation method and detailed design criteria for the proposed control scheme are presented. Furthermore, the proposed OHRC scheme is compared with the previous multiple resonant control (MRC) based on their internal relationship. Simulation and experimental results are provided to validate the effectiveness and advantages of the proposed control strategy.

The Smart Transformer: Impact on the Electric Grid and Technology Challenges

The increasing proliferation of renewable energy resources and new sizeable loads like electric vehicle (EV) charging stations has posed many technical and operational challenges to distribution grids [1]. Encouraged by attractive tax incentives and promotion policies, local grid end consumers are becoming not only consumers of electricity but, in many cases, also producers. The actual electric distribution system limits the use of renewable energy resources, offers poor EV infrastructure, and is based on a unidirectional information flow from sources to control centers.

Benchmarking of Voltage Sag Generators

The increased penetration of renewable energy systems, like photovoltaic and wind power systems, rises the concern about the power quality and stability of the utility grid. Some regulations for Low Voltage Ride-Through (LVRT) for medium voltage or high voltage applications, are coming into force to guide these grid-connected distributed power generation systems. In order to verify the response of such systems for voltage disturbance, mainly for evaluation of voltage sags/dips, a Voltage Sag Generator (VSG) is needed. This paper evaluates such sag test devices according to IEC 61000 in order to provide cheaper solutions to test against voltage sags. Simulation and experimental results demonstrate that the shunt impedance based VSG solution is the easiest and cheapest one for laboratory test applications. The back-to-back fully controlled converter based VSG is the most flexible solution for the system test under grid faults but also the most expensive one.

Benchmarking of grid fault modes in single-phase grid-connected photovoltaic systems

Pushed by the booming installations of single-phase photovoltaic (PV) systems, the grid demands regarding the integration of PV systems are expected to be modified. Hence, the future PV systems should become more active with functionalities of low voltage ride-through (LVRT) and the grid support capability. The control methods, together with grid synchronization techniques, are responsible for the generation of appropriate reference signals in order to handle ride-through grid faults. Thus, it is necessary to evaluate the behaviors of grid synchronization methods and control possibilities in single phase systems under grid faults. The intent of this paper is to present a benchmarking of grid fault modes that might come in future single-phase PV systems. In order to map future challenges, the relevant detection and control strategies are discussed. Some faulty modes are studied experimentally and provided at the end of this paper. It is concluded that there are extensive control possibilities in single-phase PV systems under grid faults. The Second Order General Integral based PLL technique might be the most promising candidate for future single-phase PV systems because of its fast adaptive-filtering characteristics and is able to full fill future standards.

Design and Control of a Photovoltaic Energy and SMES Hybrid System With Current-Source Grid Inverter

This paper proposes a novel photovoltaic (PV) energy and superconducting magnetic energy system (SMES) hybrid system based on the current-source grid inverter (CSGI). The key is to integrate the SMES coil into the dc link of CSGI for PV energy and battery systems. Thus, the SMES and PV energy system can share the CSGI, and the hybrid system offers more straight forward control on the grid side. The battery is added to the system for increase of storage capacity and effective operation under quenching condition. The dc choppers are applied to exchange the power between the PV, battery, and SMES. The battery-side dc converter and the PV-side boost converters are utilized to deliver their power to the voltage bus of dc choppers. The dc choke is proposed to take place of the SMES coil while quenching condition occurs. The control schemes for the proposed hybrid system for both normal SMES and quenching conditions are presented. The operation of the hybrid system under faulty grid conditions is also given. The simulation is developed to verify the validity of the proposed system and control schemes.

Position Sensorless Control of Interleaved CSI Fed PMSM Drive With Extended Kalman Filter

The paper proposes the design of a sensorless control for driving a permanent magnet synchronous motor (PMSM). The drive is fed by a current source inverter (CSI) which adopts the paralleled configuration to increase the power level. The key is to design the interleaved operation and active damping to suppress the LC resonance and reduce the low order harmonics for motor voltages and currents. Thus, the better voltage and current waveforms could provide good performance for the extended Kalman filter (EKF) to estimate the PMSM rotor position. The proposed control scheme is designed, analyzed and compared with other methods available. It has been verified that the proposed drive system can offer good operating performance by computer simulation.

Frequency adaptive control of a smart transformer-fed distribution grid

An advanced service provided by the Smart Transformer (ST) is the decoupling between the Medium Voltage (MV) and Low Voltage (LV) grids. In the LV side, the ST can modify the waveforms frequency in order to interact with the droop controllers of the local generators to control the power demand among the sources without affecting the MV grid. However, most of the existing controllers for power converters cannot guarantee good performances under variable frequency conditions. To address this issue, a frequency adaptive control scheme based on the Fractional-Order Repetitive Control (FORC) as well as Frequency Locked Loop (FLL) is proposed in this paper. This proposed scheme provides fast online parameter tuning capability in order to be highly adaptive to variable frequencies, and it can be implemented either in a ST converter or in distributed generators. In this work simulation and experimental results are provided to demonstrate the effectiveness and advantages of the proposed scheme.

Resonance damping in a smart transformer-based microgrid

Compared with the traditional microgrid, a smart transformer (ST)-based microgrid shows advantages in terms of higher efficiency, increased hosting capacity, and enhanced reliability. However, this novel microgrid presents challenges because of the complex resonances within the system and the interactions between ST and power converter-based distributed energy resources (DERs). Both the ST and DERs might be severely affected by each other and leads to performance degradation and system instability. To solve these problems, this paper first develops equivalent circuits of a ST-based microgrid. Then, the resonance and interaction problems are investigated based on the equivalent circuits. The active damping method is used to fully address the resonance issue and mitigate the coupling effects. To guarantee superior performance, this paper also designs high-performance control strategies for both the ST and DERs. Simulation and experimental results are provided to verify the validity of the proposed control strategy.