Today, photovoltaic (PV) plants are receiving a significant attention due to their intrinsic ability to directly transform solar energy in electrical energy. However, electricity generated from PV plants can rarely provide immediate response to load demand, as these sources do not deliver a regular supply immediately compatible with consumers’ needs. Recently, an important attention has been devoted to the use of energy storage in grid-connected PV plants, with the objective of adding flexibility in load management and overcoming some important power quality problems of real distribution grids. This makes PV plants more useful and attractive. Several battery management techniques have been underlying as a way to create more price-responsive demand and as a way to integrate PV plants more effectively into power grid. However, the development of energy policies constraint the wider deployment of PV systems. In this paper, various sizing, modelling, maximum power point tracking (MPPT) methods have been reviewed for the efficient operation of grid-connected PV systems. Dispatch strategies for stored energy that maximize the financial value of battery-PV systems along with several optimization techniques are discussed. Power quality and control technology issues of grid-connected PV systems are also covered. The economic and environmental benefits of grid-connected PV systems are underlined and operational and maintenance issues of PV-battery power systems have been included. The present paper aims at reviewing some technical challenges on the current state of PV systems based on energy policies, various cell technologies, MPPT and converter/inverter technology, energy management and scheduling techniques, reliability, power quality and control systems issues.

A review of technical issues on the development of solar photovoltaic systems

DC–DC boost converters have been widely employed at the dc input of grid-tied photovoltaic (PV) inverters. In order to comply with grid standards, their control systems must usually work in two operation modes: Maximum power point tracking (MPPT) mode and limited power tracking (LPT) mode. MPPT algorithms reach high dynamic and static efficiencies when they operate with high-speed PV voltage control, because PV voltage is not significantly dependent on solar irradiance variations. On another hand, high-speed LPT mode can be obtained by controlling inductor current, which is proportional to the power injected into the dc bus. Both modes can be integrated in a cascade control scheme with an inner and fast current loop and an outer voltage loop. The main challenge is that both voltage and current small-signal models are highly dependent on the operation point of the PV array, temperature, and solar irradiance. This may cause control interactions between both loops and instability, especially when small film capacitors are used in parallel with the PV array. A common approach is to increase the input capacitance and design a low-speed PV voltage controller, which is not the best solution when high MPPT dynamic efficiency is necessary. To overcome these challenges, this paper proposes a cascade control structure based on an inner non-linear current controller and an outer adaptive voltage controller with fast detection of the PV array's model without requiring extra current sensor. A control design methodology and a MATLAB stability analysis tool are presented to support application engineers. The proposed control system has been experimentally validated using a PV array and a boost converter switched at 40 kHz. PV voltage settling time lower than 8 ms has been achieved for low and high solar irradiance, demonstrating the efficacy of the proposed technique.

Cascade Control With Adaptive Voltage Controller Applied to Photovoltaic Boost Converters

In this paper, the stability, stabilization, and $L_{2}$ -gain problems are investigated for periodic piecewise systems with time-varying subsystems. Continuous Lyapunov function with time-varying Lyapunov matrix is adopted. A condition guaranteeing the negative definiteness of a matrix polynomial, deriving from the Lyapunov derivative, is first obtained. Based on such a condition, an exponential stability condition is provided. Moreover, a state-feedback controller with time-varying gain is developed to stabilize the unstable periodic piecewise time-varying system. The $L_{2}$ -gain criterion for periodic piecewise time-varying system is also studied. Numerical examples are given to show the validity of the proposed techniques.

Stability and $L_2$ Synthesis of a Class of Periodic Piecewise Time-Varying Systems

Stability analysis of power converters in ac networks is complex due to the nonlinear nature of the conversion systems. Whereas interactions of converters in dc networks can be studied by linearizing about the operating point, the extension of the same approach to ac systems poses serious challenges, especially for single-phase or unbalanced three-phase systems. A general method for stability analysis of power converters suitable for single-phase or unbalanced ac networks is presented in this paper, based on linear time periodic theory. A single-phase grid-connected inverter with phase-locked loop (PLL) is considered as case study. It is demonstrated that the stability boundaries can be precisely evaluated by the proposed method, despite the nonlinearity introduced by the PLL. Simulation and experimental results from a 10-kW laboratory prototype are provided to confirm the effectiveness of the proposed analysis.

/pdf/stability-boundary-analysis-in-single-phase-grid-connected-3u2g67gihl.pdf

Stability Boundary Analysis in Single-Phase Grid-Connected Inverters With PLL by LTP Theory

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Three of a kind

The dual-active bridge (DAB) topology is commonly preferred in bidirectional applications due to several attractive features, including auto-adjust of power flow, galvanic insulation, wide voltage gain, and zero voltage switching (ZVS) capability over some power ranges. However, the efficiency of the converter drops at light loads because the ZVS range is directly dependent on the circulating current. Assuming that the processed power is variable, the DAB converter's design must find a compromise between extending ZVS ranges and reducing the reactive power processing to ensure higher efficiency. Aiming this compromise, many papers propose hybrid approaches in which the modulation strategy is selected according to the processed power. However, this is not a simple solution because it demands multivariable control and offline optimizations. This paper proposes a modulation strategy to ensure ZVS and reduce the circulating current for the DAB converters. While the usual phase-shift modulation provides ZVS operation in a range of 40% to 100% of rated power, the proposed asymmetrical pulse-width modulation can obtain ZVS operation in a range of 2% to 100% of rated power. Experimental results demonstrated that the proposed strategy improves the converter efficiency for all power ranges, especially at light loads.

Asymmetrical-PWM DAB Converter With Extended ZVS/ZCS Range and Reduced Circulating Current for ESS Applications

Voltage ripple in single-phase ac–dc converters is usually disregarded to design the dc-link voltage control system, since large dc-link electrolytic capacitors are typically employed. However, replacement of electrolytic capacitors by film capacitors has been widely considered for increasing reliability. Consequently, due to cost constraints, such replacement usually employ low capacitance and may be combined with fast dc-bus voltage controllers. This paper shows that the conventional linear time-invariant (LTI) model may not represent the real converter behavior for reduced capacitance and/or faster controllers, leading to poor system performance or even instability. In this way, for any of these cases, the linear time-periodic (LTP) model is highly encouraged for stability and transient analysis as a tool for the controller design. Experimental results confirm that stability margins are precisely obtained from the LTP model but may not from the LTI model. Finally, this paper also compares both LTI and LTP models while considering the control gain and the dc-bus capacitance size. It is graphically revealed that the phase margin of the LTI model diverges from the LTP model for low dc-bus capacitances and high control gains.

Stability Analysis of AC–DC Full-Bridge Converters With Reduced DC-Link Capacitance

Power management of stand-alone photovoltaic (PV) systems with centralised dc bus is usually performed using centralised algorithms based on complex state machines. This study proposes an intuitive and modular solution using a price-based approach. The dc bus is considered a market environment where a fictitious energy price is defined according to energy scarcity. The converters that connect the PV generator, loads, and energy storages to the dc bus choose their operation modes in response to the price. The proposed approach was applied to a stand-alone PV system with energy storage. Advantages of the proposed technique are scalability and intuitive programming effort due to the market analogy, so it may be helpful to developers of off-grid PV systems. Experimental results have been carried out to validate the proposed approach, proved effective to manage this system.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-rpg.2015.0472

Price-based power management of off-grid photovoltaic systems with centralised dc bus

Most design techniques for power factor correction converters considers the cascade control technique assumes that output voltage ripple is removed from the feedback path using a compensator with low-pass characteristic. This strategy allows power factor correction and output voltage regulation but result in voltage loop with a poor dynamic performance. In order to mitigate this problem many designers have used notch filters in the control loop to increase the bandwidth of the controller without compromise the power factor characteristic. However, conventional linear time invariant approach is no longer valid to analyze the stability of these systems. This paper presents the linear time periodic systems approach to analyze the stability of these systems using harmonic transfer functions formulation. The paper details the analysis of a full-bridge power factor correction boost rectifier, including harmonic transfer function converter modeling, frequency response description, time-domain simulation and a stability analysis. Simulation results for two different controllers are carried out to compare linear time invariant and linear time periodic approaches.

Lucas Vizzotto Bellinaso

Papers

Cascade Control With Adaptive Voltage Controller Applied to Photovoltaic Boost Converters

Asymmetrical-PWM DAB Converter With Extended ZVS/ZCS Range and Reduced Circulating Current for ESS Applications

Stability Analysis of AC–DC Full-Bridge Converters With Reduced DC-Link Capacitance

Price-based power management of off-grid photovoltaic systems with centralised dc bus

Modeling and analysis of single phase full-bridge PFC boost rectifier using the LTP approach