R. Bründlinger

Unintentional islanding in distribution grids with a high penetration of inverter-based DG: Probability for islanding and protection methods

Unintentional islanding in distribution networks due to the presence of DG is one of the major safety concerns for the grid interconnection of generators. The present paper looks at the current protection and safety requirements and new activities in selected European countries with a focus on inverter based generation. By means of a field study carried out in a real LV network with a high DG penetration, occurrence and persistence of balanced load/generation conditions - which are the basic prerequisite for islanding - are analyzed and the according probability is calculated based on different penetration scenarios protection settings and load characteristics. The results show that under realistic conditions the probability to encounter an island is not negligible. Thus additional protection methods to the standard voltage and frequency monitoring are required in order to detect a loss of mains at the generator and ensure the safety of customers and maintenance personnel.

IEC 61850/61499 Control of Distributed Energy Resources: Concept, Guidelines, and Implementation

The increasing installation of distributed energy resources during the last years has lead to a fundamental change of the power system structure. As a consequence, utility operators are faced with new challenges in grid planning and operation. New and intelligent approaches - like smart grids - show promising results in increasing the hosting capacity for distributed and renewable resources. Standardized automation, control, and communication systems are important keys to realize such intelligent methods. In this paper, a standard-based control approach for distributed energy resources is introduced and implemented. It uses the IEC 61850 interoperability approach as well as the IEC 61499 reference model for distributed automation. Elementary implementation guidelines are provided to handle the hierarchical architecture of distributed control applications. In order to show the advantages of the proposed approach, a simulation example and a laboratory test are demonstrated using a prototypical open-source-based implementation.

Photovoltaic-inverters as active filters to improve power quality in the grid. What can state-of-the-art equipment achieve?

Modern grid connected inverters offer a wide range of possibilities beyond the sole injection of active power into the grid. The behavior of inverters at various voltage quality conditions is analyzed by laboratory tests and a characterization according to their emission of current harmonics is provided. In addition, the contribution of PV inverters to an improvement of power quality is discussed. The characteristics of the injected current of inverters depends in many cases on the actual phase angle of the injected harmonic current related to the voltage harmonic present on the grid. The test results show that state-of-the-art inverters could already be capable to operate as active filters and thus improve the voltage quality in the network.

Collaborative Development of Automated Advanced Interoperability Certification Test Protocols for PV Smart Grid Integration

Photovoltaic (PV) systems and other Distributed Energy Resources (DERs) can provide voltage and frequency support to the electric grid through commanded and autonomous operating modes. These functions have been standardized in the International Electrotechnical Commission (IEC) Technical Report 61850-90-7 in 2013; and recently, in response to national and international requirements, are being added to PV inverters by the manufacturers. However there are currently large gaps in the certification standards for verifying the interoperability and functionality of the advanced DER functions. Sandia National Laboratories (SNL), the Austrian Institute of Technology (AIT), and TECNALIA—as part of a collaboration through the Smart Grid International Research Facility Network (SIRFN)—are designing and exercising test protocols to characterize the interoperability and advanced functionality of these devices. This paper discusses the ongoing development of a pre-certification standard protocol for testing advanced DER inverter interoperability functions and automation of the testing procedures in the U.S., Austria, and Spain. A non-dimensionalized comparison of the test results for a 3 kW single phase PV inverter at Sandia, a 20 kW three phase PV inverter at AIT, and a 5 kW single phase PV inverter at TECNALIA are provided.

Evaluation of system-integrated smart grid devices using software- and hardware-in-the-loop

This paper presents a concise description of state-of-the-art real-time simulation-based testing methods and demonstrates how they can be used independently and/or in combination as an integrated development and validation approach for smart grid DERs and systems. A three-part case study demonstrating the application of this integrated approach at the different stages of development and validation of a system-integrated smart photovoltaic (PV) inverter is also presented. Laboratory testing results and perspectives from two international research laboratories are included in the case study.

Effect of module degradation on inverter sizing

The effect of amorphous Silicon (a-Si) module degradation on inverter sizing is investigated in this paper to identify appropriate sizing ratios even if only undegraded data-sheet values are available. The seasonal degradation and annealing pattern of a-Si modules requires special attention to the sizing of inverters for these devices, as is demonstrated in this paper for three types of modules with different degradation rates. The efficiency of the inverters depends on the sizing ratio as well as the DC input voltage. Here data of an inverter with relatively dependence on operating voltage is used. As modules degrade, the optimum ratio of system rated power with respect to inverter nominal power increases by 10 to 15% for the specific inverter. Considering the module life-time, the inverter size chosen to be matched to the degraded power and voltage rating achieves high efficiency over the life-time of the modules, while the inverter chosen to match initial values, as given by some manufacturers on their datasheets, can add about ten percent losses to the operation.

Examination of LV grid phenomena by means of PHIL testing

This work is giving an approach of comparison between different commonly used methods to evaluate investigations of generation units in electrical grids. State-of-the-art simulation tools are utilized for pure numerical simulation, while physical laboratory tests are conducted as a data reference and validation. Introducing the established Power Hardware in the Loop (PHIL) method, all results are compared one to each other. This composite simulation technique (PHIL) features advantages in terms of setup and simulation flexibility, while its overall validation is up for discussions. This validation is heavily dependent on the quality of the used equipment conjoined with the chosen experiment of interest. Profound know-how in the field of control technique, system theory and measuring method is necessary to obtain clean, useful results out of a valid PHIL simulation. While every method has its advantages in use, time, costs and applicability, it is of importance to know when to use which domain (software or hardware) in order to get the intended answers to arising questions. As a validating case study, a low voltage grid with different grid impedances and two small scale generators connected to two different nodes each is simulated. Thereby, the reactive power control is under examination and the results of the different methods are compared one to each other.

A low cost open source-based IEC 61850/61499 automation platform for distributed energy resources

The integration of distributed renewable energy resources into active power distribution grids (Smart Grids) is an important activity nowadays. Advanced but also standardized remote control functions for such renewable sources are necessary to allow a higher penetration degree of them in power distribution grids. These functions are defined by the power utility automation approach IEC 61850 but their implementation is not really covered by this important power system standard. IEC 61499 know from distributed automation is a feasible way to implement IEC 61850 functions but a low cost automation platform for research projects - especially master and PhD projects - which integrates both approaches is not really existing. This paper addresses this point and introduces an IEC 61850/ 61499 open source-based environment together with a low cost controller platform for upgrading off-the-shelf distributed energy resources in Smart Grid applications.

An Integrated Research Infrastructure for Validating Cyber-Physical Energy Systems

Renewables are key enablers in the plight to reduce greenhouse gas emissions and cope with anthropogenic global warming. The intermittent nature and limited storage capabilities of renewables culminate in new challenges that power system operators have to deal with in order to regulate power quality and ensure security of supply. At the same time, the increased availability of advanced automation and communication technologies provides new opportunities for the derivation of intelligent solutions to tackle the challenges. Previous work has shown various new methods of operating highly interconnected power grids, and their corresponding components, in a more effective way. As a consequence of these developments, the traditional power system is being transformed into a cyber-physical energy system, a smart grid. Previous and ongoing research have tended to mainly focus on how specific aspects of smart grids can be validated, but until there exists no integrated approach for the analysis and evaluation of complex cyber-physical systems configurations. This paper introduces integrated research infrastructure that provides methods and tools for validating smart grid systems in a holistic, cyber-physical manner. The corresponding concepts are currently being developed further in the European project ERIGrid.

An Open Source-Based and Standard-Compliant Smart Grid Laboratory Automation System: The AIT SmartEST Approach

The large-scale roll out of distributed renewable energy resources has led to a change in the planning and operation of the electric power grids. Proper approaches and concepts are necessary in order to cope with the higher complexity in such systems. The Smart Grid vision has the potential to significantly contribute to a more stable and secure electricity supply but technologies, methods as well as corresponding validation approaches need to be further developed. This includes also the necessary laboratory equipment. The SmartEST laboratory established by AIT provides such a necessary research infrastructure and corresponding testing methods. Its automation and control system is based on open source tools as well as on international Smart Grid standards which is discussed in this paper. Therefore, this environment provides the necessary basis for the realization of a flexible and highly configurable laboratory automation supporting the design and validation of new methods and concepts in the domain of Smart Grid systems.