The solution for the generating energy derived from non-polluting sources configures a worldwide problem, which is undetermined, complex, and gradual; and certainly, passes through the diversification of the energetic matrix. Diversification means not only having different sources converted into useful energy, like the electricity, but also decentralizing the energy generation in order to fit with higher adequacy the demand, which is decentralized too. Distributed Generation proposes this sort of development but in order to increase its penetration several technical barriers must be overpassed. One of them is related to the conversion systems, which must be more flexible, modular, efficient and compatible with the different energy sources, since they are very specific for a certain area. The present study drives its efforts towards this direction, i.e. having a system with several inputs for combining different renewable energy sources into a single and efficient power converter for the grid connection. It focuses on the design and control of an 11.7 kW hybrid renewable generation system, which contains two parallel circuits of photovoltaic panels and a wind turbine. A multicell converter divided in two stages accomplishes the convertion: Generation Side Converter (GSC) and Mains Side Converter (MSC). Two boost converters responsible for the photovoltaic generation and a rectifier and a third boost, for the wind constitue the GSC. It allows the conversion to the fixed output DC voltage, controlling individually and performing the maximum power point tracking in each input. On the other side, the single-phase 4- cell MSC accomplishes the connection to the grid through an LCL filter. This filter uses an Intercell Transformer (ICT) in the first inductor for reducing the individual ripple generated by the swicthing. The MSC controls the DC-link voltage and, by doing that, it allows the power flow from the generation elements to the network.

During the 21st century there has been a rapid increase in grid-connected photovoltaic (PV) capacity globally, due to falling system component prices and introduction of various economic incentives. To a large extent, PV systems are installed on buildings, which means they are widely distributed and located close to the power consumer, in contrast to conventional power plants. The intermittency of solar irradiance poses challenges to the integration of PV, which may be mitigated if properly assessing the solar resource. In this thesis, methods have been developed for solar variability and resource assessment in the built environment on both national and local level, and have been applied to grid integration studies. On national level, a method based on building statistics was developed that reproduces the hourly PV power generation in Sweden with high accuracy; correlation between simulated and real power generation for 2012 and 2013 were 0.97 and 0.99, respectively. The model was applied in scenarios of high penetration of intermittent renewable energy (IRE) in the Nordic synchronous power system, in combination with similar models for wind, wave and tidal power. A mix of the IRE resources was sought to minimise the variability in net load (i.e., load minus IRE, nuclear and thermal power). The study showed that a fully renewable Nordic power system is possible if hydropower operation is properly planned for. However, the contribution from PV power would only be 2-3% of the total power demand, due to strong diurnal and seasonal variability. On local level, a model-driven solar resource assessment method was developed based on low-resolution LiDAR (Light Detection and Ranging) data. It was shown to improve the representation of buildings, i.e., roof shape, tilt and azimuth, over raster-based methods, i.e., digital surface models (DSM), which use the same LiDAR data. Furthermore, the new method can provide time-resolved data in contrast to traditional solar maps, and can thus be used as a powerful tool when studying the integration of high penetrations of PV in the distribution grid. In conclusion, the developed methods fill important gaps in our ability to plan for a fully renewable power system.

Transition towards low-carbon energy systems requires an increase in the volume of renewable Distributed Generation (DG), particularly wind and photovoltaic, connected to distribution networks. To facilitate the connection of renewable DG without the need for expensive and time-consuming network reinforcements, distribution networks should move from passive to active methods of operation, whereby technical network constraints are actively managed in real time. This requires the deployment of control solutions that manage network constraints and, crucially, ensure adequate levels of energy curtailment from DG plants by using other controllable elements to solve network issues rather than resorting to generation curtailment only. This thesis proposes a deterministic distribution Network Management System (NMS) to facilitate the connections of renewable DG plants (specifically wind) by actively managing network voltages and congestion in real time through the optimal control of on-load tap changers (OLTCs), DG power factor and, then, generation curtailment as a last resort. The set points for the controllable elements are found using an AC Optimal Power Flow (OPF). The proposed NMS considers the realistic modelling of control by adopting one-minute resolution time-series data. To decrease the volumes of control actions from DG plants and OLTCs, the proposed approach departs from multi-second control cycles to multi-minute control cycles. To achieve this, the decision-making algorithm is further improved into a risk-based one to handle the uncertainties in wind power throughout the multi-minute control cycles. The performance of the deterministic and the risk-based NMS are compared using a 33 kV UK distribution network for different control cycles. The results show that the risk-based approach can effectively manage network constraints better than the deterministic approach, particularly for multi-minute control cycles, reducing also the number of control actions but at the expense of higher levels of curtailment. This thesis also proposes energy storage sizing framework to find the minimum power rating and energy capacity of multiple storage facilities to reduce curtailment from DG plants. A two-stage iterative process is adopted in this framework. The first stage uses a multi-period AC OPF across the studied horizon to obtain initial storage sizes considering hourly wind and load profiles. The second stage adopts a high granularity minute-by-minute control driven by a mono-period bi-level AC OPF to tune the first-stage storage sizes according to the actual curtailment. The application of the proposed planning framework to a 33 kV UK distribution network demonstrates the importance of embedding real-time control aspects into the planning framework so as to accurately size storage facilities. By using reactive power capabilities of storage facilities it is possible to reduce storage sizes. The combined active management of OLTCs and power factor of DG plants resulted in the most significant benefits in terms of the required storage sizes.

Today’s power systems are based on a centralised system and distribution networks that are considered as passive terminations of transmission networks. The high penetration of Distributed Generation (DG) at the distribution network level has created many challenges for this structure. New tools and simulation approaches are required to address the subject and to quantify the dynamic characteristics of the system. A distribution network or part of it with DG, Active Distribution Network Cell (ADNC), can no longer be considered as passive. An equivalent dynamic model of ADNC is therefore extremely important, as it enables power system operators to quickly estimate the impact of disturbances on the power system’s dynamic behaviour. A dynamic equivalent model works by reducing both the complexity of the distribution network and the computation time required to run a full dynamic simulation. It offers a simple and low-order representation of the system without compromising distribution network dynamic characteristics and behaviour as seen by the external grid. This research aims to develop a dynamic equivalent model for ADNC. It focuses on the development of an equivalent model by exploiting system identification theory, i.e. the grey-box approach. The first part of the thesis gives a comprehensive overview and background of the dynamic equivalent techniques for power systems. The research was inspired by previous work on system identification theory. It further demonstrates the theoretical concept of system identification, system load modelling and the modelling of major types of DG. An equivalent model is developed, guided by the assumed structure of the system. The problem of equivalent model development is then formulated under a system identification framework, and the parameter estimation methodology is proposed. The validation results of the effectiveness and accuracy of the developed model are presented. This includes the estimation of the parameter model using a clustering algorithm to improve the computational performance and the analysis of transformer impedance effects on the ADNC responses. The evaluation of probability density function, eigenvalue analysis and parameter sensitivity analysis for the model parameters are also presented. Typical model parameters for different network topologies and configurations are identified. Finally, the developed equivalent model is used for a large power system application. The accuracy and robustness of the developed equivalent model are demonstrated under small and large disturbance studies for various types of fault and different fault locations.

Recent research shows that combining distributed generation (DG) with renewable resources will reduce fossil fuel dependency and carbon dioxide (C02) emissions. This thesis presents a framework to evaluate the benefits of DG in terms of C02 emission and transmission line losses with respect to the use of centralized power production through 2050. Due to availability of complete data, Sacramento Municipal Utility District (SMUD) in California is the main focus of this thesis; however other utility companies such as PG&E, SDG&E and SCE are also discussed. The test results based on SMUD show a decrease of about 11% to 4% in line losses when a 500 MW DG is placed at the consumption site. This thesis also shows that by adding a 40 MW DG at the central location, C02 can be reduced by 71% when compared to current standard business practices. By adding 40 MW DG every year near consumers, SMUD can eliminate inhouse electricity generation thus completely eliminating C02 emissions by 2034.

The electricity generation composition in the South African national grid has changed in recent years from mostly thermal generation to a combination of thermal generation plants and a variety of plants owned and operated by Renewable Energy Independent Power Producers (REIPPs). The need arises to determine whether the existing planning and design guidelines of distribution networks in South Africa are sufficient in terms of equipment specifications and general sizing and rating principles, used during the network planning process, under increasing penetration levels of embedded generation. The correlation between increases in embedded generation penetration levels and voltage variation, unbalance and harmonic emissions are determined by simulating various operating scenarios of varying load and short circuit level for penetration levels of 10%, 25% and 40%. The existing distribution grid planning standard NRS 097 allows for a 25% penetration level where several consumers share one feeder or distribution transformer. Some of the limits contained in the South African power quality standards NRS 048 and the distribution grid planning guidelines NRS 097 are exceeded when penetration levels of grid connected Photovoltaic (PV) generation exceeds certain levels. - Switching embedded generation in or out of service does not cause voltage variations that exceed the planning limit of 3% at the shared feeder. - Voltage unbalance due to embedded generation connected to the same phase does not cause the compatibility limit of 3% to be exceeded. - Current unbalance should be monitored as it is very likely that equipment ratings may be exceeded when the integration of embedded generation is not coordinated. - Voltage harmonic limits of the odd harmonic which are multiples of 3 are exceeded. - Current harmonic planning limits of several harmonics are exceeded for penetration levels of 25%. The criteria and limits contained in the standards and guidelines relating to current unbalance and harmonic currents should be reviewed to ensure that future grids with high penetration levels of embedded generation can withstand the inherent power quality challenges without having an adverse effect on distribution equipment. Distribution transformers can age faster when they are subjected to harmonic currents and voltages exceeding their design parameters [12]. The distribution transformer isolates the Medium Voltage (MV) distribution grid from the 400 V residential grid. The voltage harmonics and voltage unbalance on the Low Voltage (LV) grid therefore do not permeate to the MV grid. Proposed future work includes translating the qualitative suggestions made in this dissertation into quantitative terms that can be included in revisions of the distribution equipment standards and grid planning guidelines. / Dissertation (MEng)--University of Pretoria, 2017. / Electrical, Electronic and Computer Engineering / MEng / Unrestricted

One of EU’s actions against climate change is to meet 20% of our energy needs from renewable resources. Given that the renewable resources are becoming more economical to extract electricity from, this will result in that more and more distributed generation (DG) will be connected to power distribution. The increasing share of DG in the electricity networks implies both increased costs and benefits for distribution system operators (DSOs), customers and DG producers. How the costs and benefits will be allocated among the actors will depend on the established regulation. Distribution networks are traditionally not designed to accommodate generation. Hence, increasing DG penetration is causing profound changes for DSOs in planning, operation and maintenance of distribution networks. Due to the unbundling between DSOs and electricity production, DSOs can not determine either the location or the size of DG. This new power distribution environment brings new challenges for the DSOs and the electric power system regulator. The DSOs are obliged to enable connection of DG meanwhile fulfilling requirements on power quality and adequate reliability. Moreover, regulatory implications can make potential DG less attractive. Therefore regulation should be able to send out incentives for the DSOs to efficiently plan the network to accommodate the increasing levels of DG. To analyze the effects of regulatory polices on network investments, risk analysis methods for integrating the DG considering uncertainties are therefore needed. In this work, regulation impact on network planning methods and network tariff designs in unbundled electricity network is firstly analyzed in order to formulate a realistic long-term network planning model considering DG. Photovoltaic (PV) power and wind power plants are used to demonstrate DG. Secondly, this work develops a deterministic model for low-voltage (LV) networks mainly considering PV connections which is based on the worst-case scenario. Dimension the network using worst-case scenario is the convention in the long-term electricity distribution network planning for the reliability and security reason. This model is then further developed into a probabilistic model in order to consider the uncertainties from DG production and load. Therefore more realistic operation conditions are considered and probabilistic constrains on voltage variation can be applied. Thirdly, this work develops a distribution medium-voltage (MV) network planning model considering wind power plant connections. The model obtains the optimal network expansion and reinforcement plan of the target network considering the uncertainties from DG production and load. The model is flexible to modify the constraints. The technical constraints are respected in any scenario and violated in few scenarios are implemented into the model separately. In LV networks only PV connections are demonstrated and in MV networks only wind power connections are demonstrated. The planning model for LV networks is proposed as a practical guideline for PV connections. It has been shown that it is simple to be implemented and flexible to adjust the planning constraints. The proposed planning model for MV networks takes reinforcement on existing lines, new connection lines to DG, alternatives for conductor sizes and substation upgrade into account, and considers non-linear power flow constraints as an iterative linear optimization process. The planning model applies conservative limits and probabilistic limits for increasing utilization of the network, and the different results are compared in case studies. The model’s efficiency, flexibility and accuracy in long-term distribution network planning problems are shown in the case studies. / <p>QC 20140217</p> / Elforsk Risknanlys II

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Wind power often is a source of voltage fluctuations and possible voltage issues are raised when considering interconnecting wind turbines to an electric grid. Also, the power electronics introduced in the wind turbines might insert more fluctuations and different PQ problems. Distributed generation seems to be a good option in order to try to mitigate these problems. The first goal of the work is to create a model of a small electric grid, using MATLAB/Simulink. The models aims to simulate various DFIG wind turbines coupled to the grid in different conditions of location and wind. Then, the main objective is to analyze the PQ in the grid with this type of turbine. For this, once the simulations have been done, the results obtained have allowed calculating different indices to study PQ in the model. Afterwards, a comparison of those indices in the different conditions is made. / StandUp

Solar PV-based distributed generation has increased significantly over the last few years, and the rapid growth is expected to continue in the foreseeable future. As the penetration levels of distributed generation increase, power systems will become increasingly decentralized with bi-directional flow of electricity between the transmission and distribution networks. To manage such decentralized power systems, planners and operators need models that accurately reflect the structure of, and interactions between the transmission and distribution networks. Moreover, algorithms that can simulate the steady state and dynamics of power systems using these models are also needed. In this context, integrated transmission and distribution system modeling and simulation has become an important research area in recent years, and the primary focus so far has been on studying the steady state response of power systems using integrated transmission and distribution system models. The primary objective of this dissertation is to develop an analysis approach and a program that can simulate the dynamics of three phase, integrated transmission and distribution system models, and use the program to demonstrate the advantages of evaluating the impact of solar PV-based distributed generation on power systems dynamics using such models. To realize this objective, a new dynamic simulation analysis approach is presented, the implementation of the approach in a program is discussed, and verification studies are presented to demonstrate the accuracy of the program. A new dynamic model for small solar PV-based distributed generation is also investigated. This model can interface with unbalanced networks and change its real power output according to the incident solar irradiation. Finally, application of the dynamic simulation program for evaluating the impact of solar PV units using an integrated transmission and distribution system model is discussed. The dissertation presents a new approach for studying the impact of solar PV-based distributed generation on power systems dynamics, and demonstrates that the solar PV impact studies performed using the program and integrated transmission and distribution system models provide insights about the dynamic response of power systems that cannot be obtained using traditional dynamic simulation approaches that rely on transmission only models. / Ph. D.