Hariharan Krishnaswami

With the increasing penetration of Photovoltaic (PV) systems there is an increase in the variability of power generation. Grid operators tackle this problem by implementing advanced control functions such as frequency regulation. The focus of this paper is to develop a new control strategy that provides frequency regulation in a modular single-phase string configured two-stage grid-connected PV inverter. The control strategy is implemented for the PV systems without the use of energy storage. This frequency control is performed by regulating grid current based on droop curve control for PV

A modular multilevel power converter configuration for grid connected photovoltaic (PV) systems is proposed. The converter configuration replaces the conventional bulky line frequency transformer with several high frequency transformers, potentially reducing the balance of systems cost of PV systems. The front-end converter for each port is a neutral-point diode clamped (NPC) multi-level dc-dc dual-active bridge (ML-DAB) which allows maximum power point tracking (MPPT). The integrated high frequency transformer provides the galvanic isolation between the PV and grid side and also steps up the low dc voltage from PV source. Following the ML-DAB stage, in each port, is a NPC inverter. N number of NPC inverters’ outputs are cascaded to attain the per-phase line-to-neutral voltage to connect directly to the distribution grid (i.e., 13.8 kV). The cascaded NPC (CNPC) inverters have the inherent advantage of using lower rated devices, smaller filters and low total harmonic distortion required for PV grid interconnection. The proposed converter system is modular, scalable, and serviceable with zero downtime with lower foot print and lower overall cost. A novel voltage balance control at each module based on power mismatch among N-ports, have been presented and verified in simulation. Analysis and simulation results are presented for the N-port converter. The converter performance has also been verified on a hardware prototype.

Neutral point clamped (NPC) multilevel inverter is being used for medium and high power applications since it can synthesize more voltage levels with smaller output voltage steps compared to two level full-bridge inverters. An NPC based multi-level dual-active bridge (ML-DAB) dc-dc converter has been proposed where the low voltage side consists a full-bridge producing a two-level voltage and the high voltage side has two three-level neutral point diode clamped (NPC) switching poles which produce a five-level voltage waveform across the high frequency transformer. Capacitor voltage unbalance has been an important concern for NPC topology. A neutral-point current control scheme has been proposed to control the imbalance between the capacitor voltages in the NPC side. Experimental results for steady-state operation and the capacitor voltage balancing control have been shown in simulation.

With increasing use of photovoltaic (PV) power generation by utilities and their residential customers, the need for accurate intra-hour and day-ahead solar irradiance forecasting has become critical. This paper details the development of a low cost all-sky imaging system and an intra-hour cloud motion prediction methodology that produces minutes-ahead irradiance forecasts. The SkyImager is designed around a Raspberry Pi single board computer (SBC) with a fully programmable, high resolution Pi Camera, housed in a durable all-weather enclosure. Our software is written in Python 2.7 and utilizes the open source computer vision package OpenCV. The SkyImager can be configured for different operational environments and network designs, from a standalone edge computing model to a fully integrated node in a distributed, cloud-computing based micro-grid. Preliminary results are presented using the imager on site at the National Renewable Energy Laboratory (NREL) in Golden, CO, USA during the fall of 2015 under a variety of cloud conditions.

With the increasing demand for PV technology, low cost, highly efficient, modular PV inverter system has become a pertinent issue. As a solution, modular two-stage power conversion with isolation and string-level power conversion has become an attractive choice recently. However, the complexity of the control algorithm in addressing power imbalance between modular power conversion units becomes a disadvantage to these type of configurations. This paper proposes a new control scheme for the two stage, N-port, string-connected modular multilevel cascaded inverters. Control algorithm ensures the maximum extraction of solar power along with the per port power balancing of the PV systems. Simulation of N-port multilevel dual active bridge stage along with the cascaded H-bridge stage for one phase is presented to verify the feasibility of the proposed algorithm.

A modular, multi-port converter configuration realizes DC-AC conversion for a high-power grid-connected photovoltaic system; but one or more faults can occur in any of the highly numerous components. In this paper, a reconfigurable multi-port control is proposed to achieve fault detection and resolution among cascaded ports. A series of switching algorithms are applied, thus enabling a continued operation under faulty conditions. A detailed analysis and simulation results are presented for the control to detect the fault, localize it, and reconfigure the system for robust operation. The projected strategy is demonstrated using MATLAB/SIMULINK.

A multi-port converter configuration which achieves a high-power DC-AC conversion for a large-scale grid-connected photovoltaic system is utilized. A novel multi-port nonlinear control is proposed to realize a voltage and current control strategy among cascaded ports. Each port consists of solar panels, a multilevel dual-active bridge DC-DC converter utilizing a secondary step-up high-frequency transformer, and a high-voltage DC-AC sinusoidal pulse-width modulated inverter. This control is modelled and verified in Matlab/Simulink; detailed analysis and simulation results are provided.

This paper presents an intra-hour cloud forecasting methodology for distributed solar generation by predicting irradiance changes in five minute intervals throughout the day. This approach is designed to assist in the creation of a low-cost, sky imaging system for experimental field-testing. The image processing techniques are presented in the following sections: Forecasting Inputs Required, Masking, Segmentation, Filtering & Binary Conversion, True North Orientation, Optical Flow & Cloud Motion, and Ray Tracing & Irradiance Predictions. Sky/Cloud images were acquired on site at the National Renewable Energy Laboratory (NREL) and were processed by the above procedure for five individual days under a variety of cloud conditions. Lastly, the irradiance change predictions were validated using NREL's direct solar irradiance measurements* for comparison.

With the increase in demand for clean and green power, several energy utilities are increasing renewable energy in their generation portfolio. The high penetration of PV can create problems of voltage fluctuation and frequency variation. The two major smart functionalities that address high PV penetration include dynamic Volt/VAR control for voltage fluctuations and active power control and curtailment to achieve frequency regulation. Quasi-static time series simulation of the power grid that includes the two smart functionalities of the Photovoltaic (PV) generator is needed. In this paper, the modeling and analysis of the power grid with PV generators is presented using the open source simulation tool OpenDSS. Specifically, the COM interface capability of OpenDSS is used to interface with Matlab to simulate the advanced functionalities of Volt/VAR and active power control and at the same time observe the effect of these controls in the power grid. The IEEE 13 bus feeder is used to demonstrate the usefulness of the proposed simulation methodology interfacing both Matlab and OpenDSS.

Dual active bridge (DAB) converters have been popular in high voltage, low and medium power DC-DC applications, as well as an intermediate high frequency link in solid state transformers. In this paper, a multilevel DAB (ML-DAB) has been proposed in which two active bridges produce two-level (2L)-5L, 5L-2L and 3L-5L voltage waveforms across the high frequency transformer. The proposed ML-DAB has the advantage of being used in high step-up/down converters, which deal with higher voltages, as compared to conventional two-level DABs. A three-level neutral point diode clamped (NPC) topology has been used in the high voltage bridge, which enables the semiconductor switches to be operated within a higher voltage range without the need for cascaded bridges or multiple two-level DAB converters. A symmetric modulation scheme, based on the least number of angular parameters rather than the duty-ratio, has been proposed for a different combination of bridge voltages. This ML-DAB is also suitable for maximum power point tracking (MPPT) control in photovoltaic applications. Steady-state analysis of the converter with symmetric phase-shift modulation is presented and verified using simulation and hardware experiments.