Ronnie Belmans

Partial shadowing of photovoltaic arrays with different system configurations: literature review and field test results

Partial shadowing has been identified as a main cause for reducing energy yield of grid-connected photovoltaic systems. The impact of the applied system configuration on the energy yield of partially shadowed arrays has been widely discussed. Nevertheless, there is still much confusion especially regarding the optimal grade of modularity for such systems. A 5-kWp photovoltaic system was installed at K.U. Leuven. The system consists of three independent subsystems: central inverter, string inverter, and a number of AC modules. Throughout the year, parts of the photovoltaic array are shadowed by vegetation and other surrounding obstacles. The dimensions of shadowing obstacles were recorded and the expectable shadowing losses were estimated by applying different approaches. Based on the results of almost 2 years of analytical monitoring, the photovoltaic system is assessed with regard to shadowing losses and their dependence on the chosen system configuration. The results indicate that with obstacles of irregular shape being close to the photovoltaic array, simulation estimates the shadowing losses rather imprecise. At array positions mainly suffering from a reduction of the visible horizon by obstacles far away from the photovoltaic array, a simulation returns good results. Significant differences regarding shadow tolerance of different inverter types or overproportional losses with long module strings could not be confirmed for the system under examination. The negative impact of partial shadowing on the array performance should not be underestimated, but it affects modular systems as well as central inverter systems.

Usefulness of DC power flow for active power flow analysis

In recent days almost every study concerning the analyses of power systems for market related purposes uses DC power flow. DC power flow is a simplification of a full power flow looking only at active power flows. Aspects as voltage support and reactive power management are not considered. However, such simplifications cannot always be justified and might sometimes be unrealistic. In this paper authors analyze the assumptions of DC power flow, and make an attempt at quantifying these using indexes. Among other, the paper answers the question of how low the X/R ratio of line parameters can be, and what is the maximal deviation from the perfect flat voltage which still allows DC power flow to be acceptably accurate.

Short-term load forecasting, profile identification, and customer segmentation: a methodology based on periodic time series

Results from a project in cooperation with the Belgian National Grid Operator ELIA are presented in this paper. Starting from a set of 245 time series, each one corresponding to four years of measurements from a HV-LV substation, individual modeling using Periodic Time Series yields satisfactory results for short-term forecasting or simulation purposes. In addition, we use the stationarity properties of the estimated models to identify typical daily customer profiles. As each one of the 245 substations can be represented by its unique daily profile, it is possible to cluster the 245 profiles in order to obtain a segmentation of the original sample in different classes of customer profiles. This methodology provides a unified framework for the forecasting and clustering problems.

Generalized Steady-State VSC MTDC Model for Sequential AC/DC Power Flow Algorithms

In this paper, a steady-state multi-terminal voltage source converter high voltage direct current (VSC MTDC) model is introduced. The proposed approach is extended to include multiple AC and DC grids with arbitrary topologies. The DC grids can thereby interconnect arbitrary buses in one or more non-synchronized AC systems. The converter equations are derived in their most general format and correctly define all set-points with respect to the system bus instead of the converter or filter bus, which is often done to simplify calculations. The paper introduces a mathematical model to include the converter limits and discusses how the equations change when a transformerless operation is considered or when the converter filter is omitted. An AC/VSC MTDC power flow is implemented using MATPOWER to show the validity of the generalized power flow model.

Generalized Dynamic VSC MTDC Model for Power System Stability Studies

In this paper, a new general voltage source converter high voltage direct current (VSC MTDC) model is derived mathematically. The full system model consists of the converter and its controllers, DC circuit equations, and coupling equations. The main contribution of the new model is its valid for every possible topology of the DC circuit. Practical implementation of the model in power system stability software is discussed in detail. The generalized DC equations can all be expressed in terms of matrices that are byproducts of the construction of the DC bus admittance matrix. Initialization, switching actions resulting in different topologies and simulation of the loss of DC lines amount to a simple calculation or recalculation of the DC bus admittance matrix. The model is implemented in Matlab. Examples on a two- and six-terminal system show that the new model is indeed capable of accurately simulating VSC MTDC systems with arbitrary topology.

Distributed generation: challenges and possible solutions

This contribution starts from the observation that there is a renewed interest in small-scale electricity generation. The authors start with a discussion of the drivers behind this evolution indicating the major benefits and issues of small-scale electricity generation. Attention is paid to the impact of a massive penetration of distributed generation in the grid on the system safety and protection. An overview of the impact on voltage quality and stability is given, both static and dynamic. A practical example is discussed in order to show the problems and indicate solutions. Different types of generators and grid interfaces are treated. In a final chapter, an attempt is made to correctly define small-scale generation also commonly called distributed generation, embedded generation or decentralized generation

Optimal Generation Mix With Short-Term Demand Response and Wind Penetration

Demand response, defined as the ability of load to respond to short-term variations in electricity prices, plays an increasingly important role in balancing short-term supply and demand, especially during peak periods and in dealing with fluctuations in renewable energy supplies. However, demand response has not been included in standard models for defining the optimal generation technology mix. Three different methodologies are proposed to integrate short-term responsiveness into a generation technology mix optimization model considering operational constraints. Elasticities are included to adjust the demand profile in response to price changes, including cross-price elasticities that account for load shifts among hours. As energy efficiency programs also influence the load profile, interactions of efficiency investments and demand response are also modeled. Comparison of model results for a single year optimization with and without demand response shows peak reduction and valley filling effects, impacting the optimal amounts and mix of generation capacity. Increasing demand elasticity also increases the installed amount of wind capacity, suggesting that demand response yields environmental benefits by facilitating integration of renewable energy.

Control of Microgrids

The motivation to develop microgrids, as a particular form of active networks is explained and presented as an effective solution for the control of grids with high levels of distibuted energy resources. The operation, more in particular the voltage and frequency control, is discussed. Control concepts useful with microgrids are detailed and implemented. Besides technical control aspects, also economical ones are developed. Primary, secondary and tertiary control algorithms are designed operating in a completely distributed way. The theoretical concepts are tested in a extensive laboratory experiment implementing a realistic scenario by using a setup of four inverters able to communicate through an Internet connection.

Model-based generation of low distortion currents in grid-coupled PWM-inverters using an LCL output filter

In this work, a single phase LCL output stage for grid coupled applications is built and evaluated. An accurate model and observer of the output filter and the distorted grid voltage is implemented. This paper deals with the construction of a 14-state model, and the feedback control loop to obtain good closed loop response. Simulations indicate a good performance of the controller, with a total harmonic current distortion (THD) below 1%. Experimental results confirm the simulations, and illustrate the good operation of the Kalman observer to estimate the distorted grid voltage (THD 3%). The output filter effectively reduces the PWM harmonics in the grid current.

The Feasibility of Small-Scale Residential DC Distribution Systems

The application of DC distribution of electrical power has been suggested as an efficient method of power delivery. This concept is inspired by the absence of reactive power, the possibility of efficient integration of small distributed generation units and the fact that, internally, many appliances operate using a DC voltage. A suitable choice of rectifier facilitates the improvement of the power quality as well as the power factor at the utility grid interface. Stand-by losses can be largely reduced. However, because of the inherent danger associated with DC voltages and currents, it is imperative that a considerable amount of design effort is allocated for risk analysis and the conception of protective devices and schemes, in order to guarantee personal and material (especially fire) safety. This paper consists of the following topics: topological design, buffering of the DC bus, interfacing distributed generators, efficiency analysis and safety measures. The conclusion of this work is that (at the moment) it is generally not efficient to implement a DC distribution system exclusively at the level of the end-user. Rather, further research should focus on the extension of DC power delivery to higher levels of the electricity grid