It is difficult to estimate how much distributed generation (DG) capacity will be connected to distribution systems in the coming years; however, it is certain that increasing penetration levels require robust tools that help assess the capabilities and requirements of the networks in order to produce the best planning and control strategies. The work of this Task Force is focused on the numerous strategies and methods that have been developed in recent years to address DG integration and planning. This paper contains a critical review of the work in this field. Although there have been numerous publications in this area, widespread implementation of the methods has not taken place. The barriers to implementation of the advanced techniques are outlined, highlighting why network operators have been slow to pick up on the research to date. Furthermore, key challenges ahead which remain to be tackled are also described, many of which have come into clear focus with the current drive towards smarter distribution networks.

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State-of-the-Art Techniques and Challenges Ahead for Distributed Generation Planning and Optimization

Coordination of different distributed generation (DG) units is essential to meet the increasing demand for electricity. Many control strategies, such as droop control, master-slave control, and average current-sharing control, have been extensively implemented worldwide to operate parallel-connected inverters for load sharing in DG network. Among these methods, the droop control technique has been widely accepted in the scientific community because of the absence of critical communication links among parallel-connected inverters to coordinate the DG units within a microgrid. Thus, this study highlights the state-of-the-art review of droop control techniques applied currently to coordinate the DG units within a microgrid.

A review of droop control techniques for microgrid

Microgrid is becoming an attractive concept to meet the increasing demands for energy and deal with air pollutions. Distributed energy sources (DERs) in Microgrid are usually interfaced with the utility grid by inverters, so the characteristics of Microgrid stability are much different from that of a traditional grid. However, the classifications, guidelines, and analysis method of Microgrid stability are well behind of the Microgrid development. In this paper, a Microgrid stability classification methodology is proposed on the basis of the of Microgrid characteristics investigation, which considers the Microgrid operation mode, types of disturbance and time frame. Then a comprehensive review of the body of research on Microgrid stability is presented in order to identify and advance the field. Finally, some challenges and suggestions of Microgrid stability for further researches are discussed.

Microgrid stability: Classification and a review

In this paper voltage and frequency control of islanded microgrid after intentional and unintentional switching events are investigated. The weak low voltage (LV) network based microgrid consists of two inverter based distributed generation (DG) units. One unit is a storage (battery) unit and the other is a photovoltaic (PV) cell. In this case the battery inverter with rapid response is considered to act as a master and it has the main responsibility to control the voltage and frequency in microgrid when islanded from the main distribution network. The studies are performed on a PSCAD simulation software package. Simulation studies show the voltage - active power and frequency - reactive power dependency in weak LV network. The studies also show that in order to maintain frequency balance in islanded microgrid, there is need for a reference sine wave generator inside master unit which imitates the main network phase voltages and gives the input for master units' (battery storage) PLL (phase locked loop) during islanding

Voltage and frequency control of inverter based weak LV network microgrid

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.

Control of Microgrids

In this paper, a new control method for the parallel operation of one or several inverters in an island grid or the mains is described. Frequency and voltage control, including mitigation of voltage harmonics, are achieved without the need for any common control circuitry or communication between the inverters. Each inverter supplies a current that is the result of the voltage difference between a reference AC voltage source and the grid voltage across a virtual impedance with real and/or imaginary parts. The reference AC voltage source is synchronised with the grid, with a phase shift, depending on the difference between nominal and real grid frequency. A detailed analysis show that this approach has superior behaviour in comparison with the existing droop control methods, considering the mitigation of voltage harmonics, short-circuit behaviour and, in the case of a non-negligible line resistance, the 'efficient' control of frequency and voltage. Experiments show the behaviour of the method for an inverter feeding a highly distorted load and during the connection of two parallel inverters in operation.

A voltage and frequency droop control method for parallel inverters

The number of Distributed Generation (DG) units connected at low and medium voltage is evermore increasing. Due to the mostly non-dispatchable generation profile, grid performance can be ameliorated as well as deteriorated. The integration of small-scale energy units can play an important role in DG planning and distribution system benchmarking. An overview is given of storage systems as well as their major benefits. A robust, multiobjective search algorithm is presented, based on accuracy improving Monte Carlo (MC) simulations nested in an Evolutionary Algorithm (EA). Robustness is essential since load profiles and DG generation patterns have a stochastic nature and storage operation can be non-ideal. Multiple objectives are pursued to assess proper trade-offs regarding imbalance market revenues, ancillary services and energy self-sustainment amongst others.

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Long-Term Planning for Small-Scale Energy Storage Units

As part of the European energy policy goals for sustainability, security of supply and improved competitiveness, the share of Renewable Energy Sources for Electricity (RES-E) is rapidly increasing. The two main technologies driving this development in Europe are wind and solar PV, which are both characterised by variable energy output with limited controllability and predictability. The unexpected power output variations challenge the power system, as additional reserve capacity is expected to be necessary in order to balance real-time demand and generation. The aim of this contribution is to investigate the impact on system operation and costs by modelling these additional reserve requirements in a unit commitment model. Simulations reveal that this reserve capacity is particularly costly for upward fast-response reserves. Results for a case study, including a realistic reserve capacity following wind power integration, show that balancing wind power imbalances with fast-response reserves is not a feasible solution. Large shares of wind power curtailment occur together with a substantial increase of the system generation costs. It is found that a strategy which allocates part of the reserve requirements to the slow-response reserves, can reduce the cost increase to 1.3 - 3.1%.

https://lirias.kuleuven.be/bitstream/123456789/353699/1/DeVospaper.pdf

KU Leuven

Papers

A voltage and frequency droop control method for parallel inverters

Long-Term Planning for Small-Scale Energy Storage Units

Impact of wind power reserve requirements on system operation and costs