Mitra Mirhosseini

Single- and Two-Stage Inverter-Based Grid-Connected Photovoltaic Power Plants With Ride-Through Capability Under Grid Faults

Grid-connected distributed generation sources interfaced with voltage source inverters (VSIs) need to be disconnected from the grid under: 1)excessive dc-link voltage; 2)excessive ac currents; and 3)loss of grid-voltage synchronization. In this paper, the control of single- and two-stage grid-connected VSIs in photovoltaic (PV) power plants is developed to address the issue of inverter disconnecting under various grid faults. Inverter control incorporates reactive power support in the case of voltage sags based on the grid codes (GCs) requirements to ride-through the faults and support the grid voltages. A case study of a 1-MW system simulated in MATLAB/Simulink software is used to illustrate the proposed control. Problems that may occur during grid faults along with associated remedies are discussed. The results presented illustrate the capability of the system to ride-through different types of grid faults.

A three-phase frequency-adaptive phase-locked loop for independent single-phase operation

This letter proposes a phase-locked loop (PLL) able to operate as an independent single-phase PLL in the context of three-phase systems. Its key advantage is that the extracted information allows the independent control of both the active and reactive powers for each phase. The proposed PLL utilizes moving average filters and, hence, remains robust under unbalanced and highly distorted voltages. A frequency estimator is also presented and incorporated in the PLL angle detector path to make it frequency adaptive. The performance of the proposed PLL is verified experimentally and results for frequency change, phase-angle jump, and unbalanced voltage sag are reported as a confirmation of its potential.

Resonant Versus Conventional Controllers in Grid-Connected Photovoltaic Power Plants Under Unbalanced Grid Voltages

This paper discusses the control of large-scale grid-connected photovoltaic power plant (GCPPP) operating under unbalanced grid voltages. The positive and negative sequences of the grid currents need to be controlled to regulate the power injected into the grid during unbalanced grid voltages. This paper shows that the use of conventional proportional-integral-based controllers compromises stability and dynamic performance of the inverter. The reason is the delays introduced by the filters needed to extract the sequences of the transformed grid currents. Because of such delays, there is a strong restriction on choosing the parameters for the current and voltage controllers, which forces the GCPPP to perform slowly. This can be improved by using resonant controllers instead, which avoid the need for filtering the transformed grid currents. Additionally, a new overcurrent protection is proposed for the GCPPP when it is providing grid voltage support during voltage sags. Simulation and experimental results are presented to evaluate and compare the performance of the GCPPP when operating with the different controllers.

Single-stage inverter-based grid-connected photovoltaic power plant with ride-through capability over different types of grid faults

In this paper the control of a single-stage grid-connected photovoltaic power plant (GCPPP) is developed to address the issue of inverter disconnection under various grid faults. There are three main reasons for inverter disconnection which are (i) excessive dc-link voltage, (ii) excessive ac currents and (iii) loss of grid-voltage synchronization. The control of the inverter incorporates reactive power support in the case of voltage sags based on grid code requirements to ride-through the faults and support the grid voltages. Accordingly, another requirement of the grid codes is to control the reactive current injection under unbalanced voltage sags so that the voltages in the non-faulty phases do not exceed the specified limitations. All these issues are discussed in this paper for a case study of 1-MVA GCPPP using MATLAB/Simulink. The results illustrate the capability of the developed GCPPP to ride-through different types of faults occurred on the grid side.

Performance of large-scale grid-connected photovoltaic system under various fault conditions

The paper discusses the performance of a 10 MVA grid-connected photovoltaic system (GCPS) under various grid faults. The faults include the typical three-phase symmetrical fault (3LG), the single-phase to-ground fault (LG), two-phases-to-ground fault (2LG) and the phase-to-phase fault (2L). The system is studied with and without reactive power support in the case of voltage sags. The faults that may lead to the inverter protection triggering from both DC and AC sides are identified based on grid codes, fault-ride-through (FRT) capability and according to the datasheet of the inverter. The evaluation is performed through electromagnetic transients (EMT) simulations using DigSILENT PowerFactory software. The results indicate that the reactive power injection in asymmetrical faults triggers inverter protection systems apart from the AC over-current protection.

Individual Phase Current Control With the Capability to Avoid Overvoltage in Grid-Connected Photovoltaic Power Plants Under Unbalanced Voltage Sags

Independent current control of each phase of a three-phase voltage source inverter under unbalanced voltage sags is proposed to effectively meet grid code requirements for grid-connected photovoltaic power plants (GCPPPs). Under current grid codes, GCPPPs should support grid voltages by injecting reactive currents during voltage sags. Such injection must not allow the grid voltages of the nonfaulty phases to exceed 110% of their nominal value. However, grid overvoltages can occur in the nonfaulty phases, especially if the currents injected into the grid by the GCPPP are balanced. Based on a new requirement of the European network of transmission system operators published in 2012, a transmission system operator is allowed to introduce a requirement for unbalanced current injection. In this letter, this grid code is addressed by controlling individual phases and injecting unbalanced currents into the grid during voltage sags. Experimental results from a 2.8-kV.A inverter are presented, confirming the effectiveness of the proposed control method.

Positive- and negative-sequence control of grid-connected photovoltaic systems under unbalanced voltage conditions

This paper discusses the control of the positive- and negative-sequence components of a large-scale grid-connected photovoltaic system (GCPS) under unbalanced voltage sag conditions in the grid. Some issues regarding stability and dynamic performance of the system occur when applying PI controllers in the current control loops. The reason is the delay that the filtering method imposes when extracting the current/voltage sequences. Because of such a delay, the dynamic response of the system becomes slower compared with the case when no filtering technique is needed. Furthermore, there is a strong restriction on choosing suitable parameters for the current/voltage loop controllers without compromising system stability. All these issues are discussed in this paper on a 1-MVA GCPV system using MATLAB/Simulink software.

Low-voltage ride through capability of three-phase grid-connected photovoltaic inverters with slim film capacitors

In grid-connected photovoltaic (PV) power stations, improving the life expectancy and long-term reliability of three-phase PV inverters is urgently needed to match the significantly higher lifetime of the PV modules. A key contribution towards such improvement is replacing the conventional electrolytic film capacitors inside the inverter with metallized polypropylene film ones. With more and more PV power generation systems being installed, the utilities require these generation systems to remain connected to the ac grid during grid faults/voltage sags to ensure the operating stability of the ac power system. This paper presents a detailed analysis of how the control of PV inverters deploying film capacitors can be improved to enable them comply with the low voltage ride through capability demanded by the stringent current grid codes in the events of voltage sags/grid faults. The simulation and experimental results obtained in this study confirm that this modified inverter configuration can meet the grid codes and retain longer lifetimes expected from the use of slim film capacitors.

Interconnection of large-scale photovoltaic systems with the electrical grid: Potential issues

The interconnection of photovoltaic (PV) systems with electricity grid continues to increase significantly worldwide. These systems benefit firstly the grid by improving the reliability of the supply and secondly the owners through revenue earning. However, PV systems without energy storage are intermittent power sources. As a result they bring about some new issues to the utility, particularly as the penetration levels of renewable energy systems all over the power system continue to increase. Technical requirements from both PV system and grid side need to be fulfilled to maintain the reliability of the power system. This paper reviews the grid interactions with grid-connected PV systems.