Hossein Dehghani Tafti

NPC photovoltaic grid-connected inverter using proportional-resonant controller

Multi-megawatt photovoltaic (PV) power plants are widespread in electrical power systems because of the drastic decrease in the PV panel prices. The multi-string structure is considered for high power PV systems due to the increase in the overall energy conversion efficiency and the modularity when compared to the centralized topology. High power conversion can be achieved easily using parallel connection of several grid-tied two-level voltage-source inverters (2L-VSIs). However, this configuration complicates the control designs. Thus, this issue can be overcome by replacing these 2L-VSIs with a single high power three-level neutral point clamped inverter (3L-NPC). Moreover, the 3L-NPC inherits several attractive merits such as improved power quality with reduced total harmonic distortion (THD), increased overall efficiency as well as lower costs. In addition to that, the voltage oriented control using proportional-resonant (VOC-PR) in conjunction with the adaptive space vector modulation (ASVM) technique is proposed for the large-scaled PV system with grid-tied NPC inverter. The feasibility of the proposed control for the high power system is validated through the simulation results under various environmental conditions. Besides that, the dynamic mance of the system under the proposed VOC-PR is greatly enhanced in comparison with the conventional method using PI controller.

NPC photovoltaic grid-connected inverter with ride-through capability under grid faults

Fault ride through (FRT) capability is one of the challenges faced in the medium to high voltage grid-tied large-scale photovoltaic (PV) power plants. This paper proposes a novel control strategy to enhance the FRT capability of a two-stage multi-string PV plant which consists of DC/DC converters and NPC inverter. The proposed control is implemented to the DC/DC converter to maintain the DC-link voltage under any grid fault conditions by adjusting the extracted PV power to the inverter output power. This is achieved by changing the PV string reference voltage from its maximum power point voltage to a new voltage point with less power. On top of that, a novel voltage-oriented control (VOC) control structure using proportional-resonant (PR) controller is proposed for the NPC inverter. Additionally, the DC-link capacitor voltages are remained balanced at all times due to the adaptive space vector modulation (ASVM) scheme. The evaluation results have verified the feasibility and the FRT capability of a 14kW PV plant using the proposed control scheme under different grid fault conditions.

A General Constant Power Generation Algorithm for Photovoltaic Systems

Photovoltaic power plants (PVPPs) typically operate by tracking the maximum power point (MPP) in order to maximize the conversion efficiency. However, with the continuous increase of installed grid-connected PVPPs, power system operators have been experiencing new challenges, such as overloading, overvoltages, and operation during grid-voltage disturbances. Consequently, constant power generation (CPG) is imposed by grid codes. An algorithm for the calculation of the photovoltaic panel voltage reference, which generates a constant power from the PVPP, is introduced in this paper. The key novelty of the proposed algorithm is its applicability for both single- and two-stage PVPPs and flexibility to move the operation point to the right or left side of the MPP. Furthermore, the execution frequency of the algorithm and voltage increments between consecutive operating points are modified based on a hysteresis band controller in order to obtain fast dynamic response under transients and low-power oscillation during steady-state operation. The performance of the proposed algorithm for both single- and two-stage PVPPs is examined on a 50-kVA simulation setup of these topologies. Moreover, experimental results on a 1-kVA PV system validate the effectiveness of the proposed algorithm under various operating conditions, demonstrating functionalities of the proposed CPG algorithm.

Proportional-resonant controlled NPC converter for more-electric-aircraft starter-generator

More electric aircraft (MEA) technology is achieved by exchanging various mechanical and pneumatic elements of a conventional aircraft with their equivalent electrical devices in order to increase the reliability and decrease the maintenance. This paper proposes a proportional resonant (PR) controller together with the adaptive space vector modulation (ASVM) technique for the more electric aircraft (MEA) starter-generator neutral-point clamped (NPC) converter. The proposed controller is implemented in the stationary (αβ) frame where the calculated reference voltages of the PR controller can be directly fed into the ASVM. Hence, the main objectives of the proposed controller are to reduce the computational complexity and the steady state error by decreasing the required number of framework transformations units. On top of that, the ASVM technique provides a proper DC-link capacitor voltage balancing with improved output power quality. The dynamic performance of the proposed controller is evaluated under both initial starting interval and generating mode.

Control of bidirectional DC/DC converter for back to back NPC-based wind turbine system under grid faults

Viability and reliability of the wind power has made Wind Turbine Systems (WTSs) a popular renewable energy resource. A bidirectional DC/DC converter is proposed in this study for the WTS. The proposed DC/DC converter controls the DC-link voltage by injecting the difference between the extracted power from permanent magnet synchronous generator (PMSG) and the output power of the grid-tied inverter, to the energy storage system. The proposed DC/DC converter controller compensates the shortage of extracted active power from PMSG under wind reduced speed condition in order to inject the constant power to the grid during normal operation. The back to back (BTB) neutral point clamped (NPC) converter is proposed to extract the maximum power from PMSG and inject the extracted power to the grid. The proposed WTS structure achieves fault ride through capability and it can inject reactive power to the grid in order to enhance the PCC voltages under voltage sags. The proportional resonant (PR) current controller is implemented for both rectifier and grid-tied inverter due to its fast response and low steady state error. The adaptive space vector modulation (ASVM) is used to generate the switching signal while balances the DC-link capacitor voltages. The performance of the proposed controller is investigates under grid faults as well as reduced wind speed condition and results have proven the applicability of the proposed controller.

Flexible Control of Photovoltaic Grid-Connected Cascaded H-Bridge Converters During Unbalanced Voltage Sags

Multilevel cascade H-bridge (CHB) converters are one of the promising solutions for medium- and large-scale grid-connected photovoltaic power plants. However, there is a lack of a complete study about their operation during voltage sags. This paper proposes a flexible control strategy for the operation of photovoltaic grid-connected CHB inverters during unbalanced voltage sags. The key novelty is that the proposed strategy is able to inject both active and reactive powers to the grid with either balanced or unbalanced currents, while ensuring that all dc-link capacitor voltages remain balanced. The simulation and experimental evaluations of a 9-kVA grid-connected seven-level CHB illustrate and validate the performance of the proposed strategy for the operation of the grid-connected CHB converter during different unbalanced voltage sags.

Study on the unbalanced current injection capability of grid-connected photovoltaic neutral-point-clamped inverter

Due to high penetration of distributed generation units in power system, fault ride through (FRT) capability is one of the new requirements of the medium-scale grid-tied photovoltaic power plants (PVPPs). This paper proposes and investigates a control strategy for the neutral-point-clamped (NPC) inverter in order to inject proper unbalanced reactive currents to the grid during unbalanced grid faults. The proper unbalanced current injection reduces negative sequence of grid voltages and currents. The current references are scaled up/down individually, based on the grid phase rms voltages and inverter nominal current. The performance of the implemented control algorithm is investigated on a 150-kVA PVPP connected to 12.47-kV medium-voltage grid simulation model under various voltage sag conditions. Results from an experimental setup of grid-tied NPC inverter are presented in order to demonstrate the effectiveness of the proposed unbalanced current injection algorithm.

Multiple-pole multilevel diode clamped inverter for permanent magnet synchronous motor drive

A five-level reduced device multilevel inverter is proposed for driving permanent magnet synchronous motor. The proposed multilevel inverter drive consists of lesser number of clamping diodes compared to the conventional five-level diode clamped inverter. An active balancing circuit is used to provide balanced voltages across the four dc-link capacitors to ensure five-level voltage waveform in all operating conditions. The field oriented control of a permanent magnet synchronous motor using space vector modulation technique is implemented to investigate the performance of the proposed inverter drive and results are presented based on simulation done in Matlab/Simulink® and PSIM environment.

A general algorithm for flexible active power control of photovoltaic systems

The maximum power point tracking (MPPT) is generally implemented in grid-connected photovoltaic (PV) power plants to maximize the energy yield. However, as the penetration level increases, challenging issues such as overloading and overvoltage arise in PV applications. Accordingly, a constant power generation (CPG) operation, in which the PV output power is limited to a specific value, has been imposed by some grid regulators to alleviate the integration challenges. In that case, the combined operation of MPPT and CPG is required, which increases the complexity of the controller design. To generalize the two control objectives, a flexible active power control algorithm that combines both MPPT and CPG operating modes is proposed. The proposed algorithm can optimize the performance in both modes (i.e., MPPT and CPG). By adjusting the voltage-step between two consecutive operating points, fast dynamics and low-power oscillations can be obtained. The performance of the proposed strategy is evaluated through simulations and experiments under different irradiance and power reference profiles.

An Adaptive Control Scheme for Flexible Power Point Tracking in Photovoltaic Systems

One of the major concerns associated with the increasing penetration of grid-connected photovoltaic (PV) power plants is the operational challenges (e.g., overloading and overvoltage), imposed due to the variability of PV power generation. A flexible power point tracking (FPPT), which can limit the PV output power to a specific value, has thus been defined in grid-connection regulations to tackle some of the integration challenging issues. However, the conventional FPPT algorithm based on the perturb and observe method suffers from slow dynamics. In this paper, an adaptive FPPT algorithm is thus proposed, which features fast dynamics under rapidly changing environmental conditions (e.g., due to passing clouds), while maintaining low power oscillations in steady state. The proposed algorithm employs an extra measured sampling at each perturbation to observe the change in the operating condition (e.g., solar irradiance). Afterwards, the voltage-step is adaptively calculated following the observed condition (e.g., transient or steady state) in a way to improve the tracking performance. Experimental results on a 3-kVA grid-connected single-phase PV system validate the effectiveness of the proposed algorithm in terms of fast dynamics and high accuracy under various operational conditions.