Konstantinos Papastergiou

Energy storage sizing for large scale PV power plants base-load operation - comparative study & results

With greenhouse gas emissions increasing and global environmental policies changing, renewable energy sources have gained significant attention over the last 15 years. However, the intermittent nature of renewable generation makes its integration to the grid a challenging task. Thus, energy storage systems are usually proposed as a way to overcome this problem. This paper deals with energy storage for very large PV power plants (larger than 100MW). Different operating strategies have been defined for the PV power plant and the required energy storage capacity was estimated for each of them via a sizing methodology that will be briefly described. In addition, six areas of favorable solar radiation characteristics have been chosen as case studies to evaluate the presented methodology. Finally, a time series analysis was conducted in order to estimate the confidence interval for which both PV generated energy and energy storage are sufficient to fulfill the specific operating strategy. This analysis was performed for each operating strategy and location and thus, a comparative study will be presented.

Solar PV array-inverter matching considering impact of environmental conditions

Sizing of photovoltaic (PV) generators is an important issue which might affect the PV system overall yield and performance ratio. The procedure is realized according to inverter specifications with the most important one being the operating voltage range. The parameter that is affected by this specification is the number of PV modules per string and a software tool was developed in order to study the effects of this parameter on the system performance. Moreover, the PV generator voltage is highly affected by the ambient temperature of a specific location. Thus, the optimum number of PV modules per string will differ in locations with different environmental conditions. The importance of PV string sizing is high, since improper string length can lead to significant deficiencies on the overall inverter performance. A study of the effects of PV string size on annual system energy production and yield has been conducted for several locations in Europe with significantly different weather as well as for locations with extreme climatic conditions such as high altitude and desert areas.

Photovoltaic string configuration for optimal inverter performance

In the recent years photovoltaic installations have been growing rapidly both in numbers and in size. The main reason for this growth has been the generous subsidies offered by policy makers. As financial incentives are now being gradually removed, PV plant manufacturers are in quest for more efficient plant configurations that eventually reduce the investment cost and increase the annual energy yield. This paper presents a methodology for optimally sizing the PV panel strings (series-parallel connection of PV generators) for any specific inverter equipment and for a given location weather dataset. The methodology is based on some new PV plant performance metrics presented here. Finally, the concept of maximum power point of the inverter (MPPTi) is introduced.

DC-breaker for a multi-megawatt Battery Energy Storage System

Energy storage will gain importance in the future power system due to increased amount of renewables. In this paper, a utility scale Battery Energy Storage System (BESS) for megawatt utilizing a high voltage and a high power Voltage Source Converter (VSC) is proposed to be able to inject/consume the required power and energy to the grid. The batteries are both series and parallel connected on the DC-side of the VSC. In order to obtain high reliability, high availability and safety of the BESS, this paper proposes and analyzes a distributed Solid-State DC Breaker protecting the BESS and its batteries.

A hybrid photovoltaic and battery energy storage system for high power grid-connected applications

This paper presents a hybrid PV-battery grid connected system suitable for high power PV power applications. High power PV systems have impressive voltage fluctuation with regards to their maximum power point voltage. Using DC/DC converters to regulate this voltage is inefficient in high power systems as it adds an extra power conversion stage. On the other hand switching frequency, voltage stress as inverter limitations and power quality issues should be considered in high power applications. Central PV structures because of low efficiency and high cost inverters are not suitable for high power systems. Maximum power extraction from PV together with active and reactive injected power control is another challenge in high power system. Z-source multilevel inverter for PV arrays together with battery energy storage systems is a suitable solution for high power application. This paper proposes a control strategy for this hybrid system. A simulation with PSCAD has been done to validate the control strategy. Simulation results show how proposed control and topology satisfies high power requirements for PV grid connected systems.

Power System design compromises for large-scale linear particle accelerators

This paper discusses various design aspects of a 280MW Power System for the Compact Linear Collider (CLIC), a 50km long electrons-positrons accelerator, under feasibility evaluation. The key requirements are a very high accelerator availability and constant power flow from the utility grid, considering the pulsed power nature of CLIC. Firstly, the possible power network and cabling layouts are discussed along with potential difficulties on electrical fault clearance. Following, the use of active front-end converters is examined as a means to control the power flow and power quality seen by the 400kV grid. In particular a modular multilevel converter preliminary configuration is described and the compromises related to energy storage and voltage level are discussed.

Power converter topologies with energy recovery and grid power limitation for inductive load applications

This work investigates a grid interface for power supplies used in particle accelerators for cycling loads such as large electromagnets. Two topologies are discussed integrating magnetic energy recovery. For each topology, the associated energy management strategies are examined with the objective to control the grid current profile. A model is established for each of the proposed solutions and the simulation results are presented. A critical review of the investigated energy management solutions is attempted.

Control strategies for 2-quadrant converter used in grid power flow control

This work presents the analysis of a two-quadrant regulator connected to the DC-link of a 4-quadrant magnet supply. The key objective is to present some regulation strategies for controlling the peak power required from the power network as well as to recover the magnet energy into capacitor banks. A comparative study that highlights the trade off between the size of reactive elements, and the peak current drawn from the electrical network is presented.

Design of a Modular Multilevel Converter as an Active Front-End for a magnet supply application

The aim of this work is to describe the general design procedure of a Modular Multilevel Converter (MMC) applied as an Active Front-End (AFE) for a magnet supply for beam accelerators. The dimensioning criteria for the converter and the dc-link capacitance are presented and the grid transformer requirements are set. Considering the converter design, the arm inductance calculation is based on the specifications for the arm-current ripple and the DC-link fault tolerance, but, also, on the limitation of the second harmonic and the second-order LC resonance of the arm current. The module capacitance value is evaluated by focusing on the required switching dynamics and the capacitor-voltage ripple according to a newly proposed graphical method. The loading of each semiconductor in the half bridge is calculated via simulation, indicating the unsymmetrical current distribution. It is concluded that the current distribution for each semiconductor depends on the mode of operation of the converter. The different criteria for the choice of the number of modules per arm are discussed. Finally, the complete system, including a model for the load and the converter control strategy applied to the converter is simulated and obtained results are presented.

Current Sharing inside a High Power IGBT Module at the Negative Temperature Coefficient Operating Region

This work investigates the current sharing effect of a high power Soft Punch Through IGBT module in the Negative Temperature Coefficient region. The unbalanced current sharing between two of the substrates is demonstrated for different current and temperature levels and its impact on the thermal stressing of the device is evaluated. The results indicate that the current asymmetry does not lead to a significant thermal stressing unbalance between the substrates.