Yoash Levron

Optimal Power Flow in Microgrids With Energy Storage

Energy storage may improve power management in microgrids that include renewable energy sources. The storage devices match energy generation to consumption, facilitating a smooth and robust energy balance within the microgrid. This paper addresses the optimal control of the microgrids energy storage devices. Stored energy is controlled to balance power generation of renewable sources to optimize overall power consumption at the microgrid point of common coupling. Recent works emphasize constraints imposed by the storage device itself, such as limited capacity and internal losses. However, these works assume flat, highly simplified network models, which overlook the physical connectivity. This work proposes an optimal power flow solution that considers the entire system: the storage device limits, voltages limits, currents limits, and power limits. The power network may be arbitrarily complex, and the proposed solver obtains a globally optimal solution.

Performance of Power-Limited Differential Power Processing Architectures in Mismatched PV Systems

Differential power processing (DPP) architectures employ distributed, low power processing, submodule-integrated converters to mitigate mismatches in photovoltaic (PV) power systems, while introducing no insertion losses. This paper evaluates the effects of the simple voltage-balancing DPP control approach on the submodule-level maximum power point (MPP) efficiency. It is shown that the submodule MPP efficiency of voltage-balancing DPP converters exceeds 98% in the presence of worst-case MPP voltage variations due to irradiance or temperature mismatches. Furthermore, the effects of reduced converter power rating in the isolated-port DPP architecture are investigated by long-term, high-granularity simulations of five representative PV system scenarios. For partially shaded systems, it is shown that the isolated-port DPP architecture offers about two times larger energy yield improvements compared to full power processing (FPP) module-level converters, and that it outperforms module-level FPP approaches even when the power rating of DPP converters is only 20-30% of the PV system peak power. In the cases of aging-related mismatches, more than 90% of the energy yield improvements are obtained with DPP converters rated at only 10% of the PV peak power.

Active balancing system for electric vehicles with incorporated low voltage bus

Electric-drive vehicles, including hybrid, plug-in hybrid, and electric vehicles, require a high-voltage (HV) battery pack for propulsion and a low-voltage (LV) dc bus for auxiliary loads. This paper presents an architecture that uses modular dc-dc bypass converters to perform active battery cell balancing and to supply current to auxiliary loads, eliminating the need for a separate HV-to-LV high step-down dc-dc converter. The modular architecture, which achieves continuous balancing of all cells, can be used with an arbitrary number of cells in series, requires no control communication between converters, and naturally shares the auxiliary load current according to the relative state-of-charge (SOC) and capacities of the battery cells. Design and control details are provided for LV low-power dual active bridge (DAB) power converters serving as the bypass converter modules. Furthermore, current sharing is examined and worst-case SOC and current deviations are derived for mismatches in cell capacities, SOCs, and parasitic resistances. Experimental results are presented for a system consisting of 21 series 25 Ah Panasonic lithium-ion NMC battery cells and 21 DAB bypass converters, with combined outputs rated to supply a 650-W auxiliary load.

Design of EMI Filters Having Low Harmonic Distortion in High-Power-Factor Converters

This paper studies how electromagnetic interference (EMI) filters affect the harmonic distortion in high-power-factor converters. The EMI filter presents a tradeoff: higher noise attenuation results in higher harmonic distortion. In many modern converters, the EMI filter is designed to meet certain noise requirements. However, when tested in practice, the filter, while attenuating the EMI noise well, is found to generate high harmonic distortion, and should be redesigned. The cause of this distortion is a nonlinear interaction of the filter and the bridge rectifier. The current understanding of this process is incomplete, especially in modern converters that use high-order LC filters, and modern low-cost inverters. As a result, harmonic distortion is typically tuned by repeated trial-and-errors. In this paper, we provide a simple model for evaluating the harmonic distortion. The harmonic distortion is shown to be dominated by a single parameter: the filter total capacitance. The total harmonic distortion and capacitance are shown to be to be related by a simple power-law function. This relation is proved to be accurate if the total filter inductance is smaller than a given threshold. Experimental results show that total harmonic distortion can be estimated by the capacitance, with a normalized error of 0.09%.

Control of Submodule Integrated Converters in the Isolated-Port Differential Power-Processing Photovoltaic Architecture

Recently, a variety of differential power-processing (DPP) architectures have been shown to improve the efficiency of photovoltaic (PV) systems. This paper proposes a simple control strategy for the isolated-port DPP architecture, and provides a comprehensive stability analysis for this system. The proposed controller drives the duty-cycle of the differential submodule integrated converters (subMICs) in proportion to a voltage difference between the submodule and the isolated-port. This method requires no additional sensing, complex processing, or communication between subMICs, and is therefore well suited for low-cost integrated hardware solutions. Stability of the resulting high-order nonlinear system is analyzed both in the time and frequency domains. A decoupled model is developed that reduces the high-order system dynamics to a 1-D control loop, which allows stable, well-behaved responses using a proportional or a lag compensator. Experimental results for a 72-cell PV module with three subMICs verify static and dynamic operation, and show that overall PV module efficiency exceeds 99% with no shading, and is higher than 96% under significant (50%) shading.

Nonlinear control design for the photovoltaic isolated-port architecture with submodule integrated converters

Differential power processing (DPP) isolated-port architecture with submodule integrated converters (subMICs) enables improved energy capture in photovoltaic (PV) power systems. This paper describes a control scheme for flyback subMICs operating in discontinuous conduction mode (DCM). The duty-cycle is driven in proportion to a voltage error, with no further processing, thus eliminating the need to measure or to control the current explicitly. The approach is well suited for very simple, analog PWM controller implementation, but the resulting control loops become nonlinear. The system stability is demonstrated using the Lyapunov stability theorem. Experimentally measured module-level efficiency is greater than 95% for a 72-cell PV module having 3 substrings with solar irradiation of 80%, 60% and 40%, respectively.

Bus Voltage Control With Zero Distortion and High Bandwidth for Single-Phase Solar Inverters

Single-phase inverters must include an energy storage device, typically a high-voltage bus capacitor, to match the inverter constant input power to its pulsating output power. Because of its increased cost, the size of this bus capacitor must be minimized. However, when the bus capacitor is small, the bus voltage includes a high ripple at the ac line second harmonic frequency, which causes harmonic distortion. The bus voltage controller must filter this ripple, while regulating the bus voltage efficiently during transients, and must therefore balance a tradeoff between two conflicting constraints, low-harmonic distortion and high bandwidth. This paper analyzes this tradeoff, and proposes a new control method for solving it without using addition hardware. Instead of reducing the distortion by lowering the loop gain, the new controller employs a digital FIR filter that samples the bus voltage at an integer multiple of the second harmonic frequency. The filter presents a notch that removes the second harmonic ripple, enabling a design that operates with zero distortion and high bandwidth simultaneously, and is suitable for inverters with small bus capacitors. The proposed controller is tested on a microinverter prototype with a 300-W photovoltaic panel and a 20-μF bus capacitor.

High Weighted Efficiency in Single-Phase Solar Inverters by a Variable-Frequency Peak Current Controller

This paper discusses a control method that achieves high weighted efficiency in solar microinverters. A challenge in microinverters is to achieve high efficiency over a range of output powers. To address this challenge, the proposed controller presents two primary benefits that enable such an efficiency profile, a switching frequency that scales with power, and a low peak current that enables efficient magnetic design of the inductor. At high powers, the switching frequency increases to minimize the root-mean-square (rms) current, and at low powers, the switching frequency decreases to minimize the switching loss. Since the peak inductor current is low, the inductor may be designed with fewer turns of wire, or with lower flux density, and is thus highly efficient. The proposed constant peak current switching scheme is implemented by a cycle-by-cycle predictive controller that uses a fast integrator to control the switching period, achieving high bandwidth and stability. This controller senses only the peak inductor current and, therefore, does not require expensive average current sensors. We demonstrate a low-cost inverter prototype with a 300-W solar panel. The prototype uses standard silicon devices and a small inductor of 360 μH to achieve a weighted efficiency of 99.15%.

Distributed Series Static Compensator Deployment Using a Linearized Transmission System Model

Distributed static series compensators (DSSCs) are power-electronics devices that can inject positive or negative reactive impedance into a transmission line by connecting to it in series through a transformer suspended from the line. This paper presents a method for linearizing a transmission network, allowing the relationships between the network operating points and the injected reactances to be easily derived and thereby illuminating how best to deploy DSSCs. The model, which is general and applies to arbitrarily complex systems, is simplified into a form that provides additional insights without knowledge of the system topology or operating state. Using standard IEEE test systems, examples are given where DSSCs are used to raise load bus voltage, reduce line current, or reduce generator reactive power.

State-of-charge estimation based on microcontroller-implemented sigma-point Kalman filter in a modular cell balancing system for Lithium-Ion battery packs

Cell balancing in large battery packs requires accurate state of charge (SOC) estimation for individual cells. This paper presents a low complexity sigma-point Kalman filter to estimate the state-of-charge (SOC) of Lithium-Ion battery cells. The proposed sigma-point Kalman filter is of 1st order, and can be easily implemented on a simple microcontroller around a dc-dc converter in a modular cell balancing system. The approach is verified experimentally on a battery pack containing twenty-one balancing converters and twenty-one 25 Ah Lithium-Ion cells under high-current (up to 100A) cycling.