Paul Cuffe

Capability Chart for Distributed Reactive Power Resources

The ubiquity of synchronous generation should not be assumed in highly renewable power systems. Various problems may consequently arise, not least of which are the difficulties entailed in maintaining regional reactive power balance. Offering a potential solution to such problems, modern renewable generator technologies offer controllable reactive power resources. As many of these generators will be embedded in distribution networks, their incorporation into transmission system operational and planning activities appears challenging. An extension of the capability chart concept offers insight here: for a given active power exchange between the transmission system and a distribution network section, the range of controllable reactive power typically available is of interest. This aggregate capability depends on the innate machine capabilities of the distributed generators and on the prevailing conditions within the distribution network. Novel optimisation techniques are useful in addressing the latter point, offering a means to identify the combination of power flow profiles within the distribution system most restrictive to reactive power provision. The capability chart thus derived gives the dependable range of reactive power available, under the assumption that each generator is operated to locally maximize its own reactive power contribution. Such a description can be applied in transmission system planning or to quantify the effects of modifications to the distribution system.

Visualizing the Electrical Structure of Power Systems

Recent work, using electrical distance metrics and concepts from graph theory, has revealed important results about the electrical connectivity of empiric power systems. Such structural features are not widely understood or portrayed. Power systems are often depicted using unenlightening single-line diagrams, and the results of loadflow calculations are often presented without insightful elucidation, lacking the necessary context for usable intuitions to be formed. For system operators, educators, and researchers alike, a more intuitive and accessible understanding of a power systems inner electrical structure is called for. Data visualization techniques offer several paths toward realizing such an ideal. This paper proposes various ways, in which electrical distance might be defined for empiric power systems, and records how well each candidate distance measure may be embedded in two dimensions. The resulting 2-D projections form the basis for new visualizations of empiric power systems and offer novel and useful insights into their electrical connectivity and structure.

Evaluation of Advanced Operation and Control of Distributed Wind Farms to Support Efficiency and Reliability

The integration of increasing penetrations of renewable energy sources is one of the key drivers for the increased deployment of control and optimization on power systems. Much of this wind generation is connected to the distribution system, which presents a range of challenges to the operation of these networks, traditionally utilized solely for power delivery. This paper describes a demonstration program in Ireland which addresses two of the key challenges in delivering high penetrations of wind energy in a cost-effective and efficient manner. First, the paper addresses a trial of the advanced reactive power control capabilities of modern wind turbines, investigating how this resource can be better utilized from a distribution and transmission perspective. The second aspect is measures to improve the efficiency of the distribution system in light of these increasing penetrations of wind energy.

Transmission System Impact of Wind Energy Harvesting Networks

The increasing emphasis placed upon renewable sources of energy requires that power systems accommodate a roll out of variable, asynchronous generators throughout transmission and distribution networks. High penetration levels of such generation will displace synchronous plant and may cause a challenging scarcity of ancillary service providers, notably in the area of reactive power provision. Consequently, the onus must increasingly be laid upon renewable generators to provide the ancillary services necessary to operate the power system. An emerging practice is to connect adjacent distributed generators in a clustered fashion to a dedicated transmission node, an arrangement that offers rich possibilities for participation in transmission-level control. Performance characterizations of such networks will be helpful in planning transmission system development for reduced synchronous plant availability. This work will examine the effect of increasing penetration of wind generators on transmission system voltage levels and voltage security.

Ireland's approach for the connection of large amounts of renewable generation

This paper discusses the connection policies adopted to facilitate the large number of wind farms seeking access to the Irish power system. A key feature is a grouped connection offer process that provides certainty for wind project developers and optimises the development of the network. A further interesting feature is the development of a “wind cluster” concept which permits semi-dedicated MV networks for the connection of a group of adjacent wind farms, together with equitable arrangements for sharing the financial burden of construction between developers.

Characterisation of the reactive power capability of diverse distributed generators: Toward an optimisation approach

The highly renewable power system cannot assume the ubiquity and constant availability of synchronous plant. For this reason, the provision of ancillary services must shift to renewable generators, concomitant with their rising penetration levels. This work will focus on one aspect of this challenge: the provision of reactive power from distributed generation. As such resources become increasingly important, their incorporation into transmission system operation and planning activities becomes vital. To that end, a capability characterisation appears useful, delineating the range of controllable reactive power available for a given power flow onto a section of distribution network. Work to date has used time series techniques to provide a proxy to this type of capability chart. Distribution system optimisation techniques offer the potential for a more rigorous and general-purpose characterisation methodology. This work will set out how such an optimisation problem may be formulated, and will provide some initial results and validations.

Embracing an Adapatable, Flexible Posture: Ensuring That Future European Distribution Networks Are Ready for More Active Roles

The challenge for EU member states is to ensure the timely evolution of DNOs to their future, more active roles, which are necessary to make low-carbon societies a reality. While the corresponding regulatory actions will largely depend on the context and characteristics of the distribution networks within each member state, it is clear that incentivizing innovation should be key in this endeavor. Equally critical is defining the roles of potential future players and technologies in the provision of flexibility (e.g., consumers, aggregators, and storage) to prevent new business models being hindered by regulatory barriers. This transition of DNOs to DSOs in the next few years will certainly bring exciting new regulatory environments where the envisioned smart grids are likely to finally emerge.

Voltage Responsive Distribution Networks: Comparing Autonomous and Centralized Solutions

Power system voltage control has traditionally been the responsibility of transmission-connected reactive power resources. Accordingly, high penetration levels of distributed generation present new challenges for reactive power management. Simply stipulating voltage control operation for distributed generators will not generally deliver voltage-responsive reactive power flows at the transmission system level, due mainly to the voltage-isolating effects of tap-changing bulk supply transformers. Additionally, the resistance of distribution system conductors establishes an unhelpful interaction between active power flows and voltage magnitudes. This work uses optimal power flow techniques to explore two ways to overcome these challenges. Most innovatively, a methodology is presented to optimally select static voltage control settings for distributed generators and transformers, such that they will provide an autonomous voltage-responsive behavior without supervisory control systems. In this scheme, distributed generators are exposed to transmission voltage fluctuations as far as is feasible, by blocking the tapping of the bulk supply transformer when operating within an optimally-determined range of transmission voltages. Comparatively, an active control scheme is presented, where reactive power and tap positions are dispatched period-to-period to support the transmission system voltage. Comparing these approaches suggests the level of smart-grids investment required to effectively harness the reactive power available from distributed generation.

Capability chart for distributed reactive power resources

The ubiquity of synchronous generation should not be assumed in highly renewable power systems. Various problems may consequently arise, not least of which are the difficulties entailed in maintaining regional reactive power balance. Offering a potential solution to such problems, modern renewable generator technologies offer controllable reactive power resources. As many of these generators will be embedded in distribution networks, their incorporation into transmission system operational and planning activities appears challenging. An extension of the capability chart concept offers insight here: for a given active power exchange between the transmission system and a distribution network section, the range of controllable reactive power typically available is of interest. This aggregate capability depends on the innate machine capabilities of the distributed generators and on the prevailing conditions within the distribution network. Novel optimisation techniques are useful in addressing the latter point, offering a means to identify the combination of power flow profiles within the distribution system most restrictive to reactive power provision. The capability chart thus derived gives the dependable range of reactive power available, under the assumption that each generator is operated to locally maximize its own reactive power contribution. Such a description can be applied in transmission system planning or to quantify the effects of modifications to the distribution system.

Transient stability impacts from distribution connected wind farms

Wind generation penetration levels are increasing in power systems across the world. Along with transmission connected wind farms, distribution connected wind farms are becoming more prevalent in power systems. How these distribution connected farms control reactive power is of concern to the transmission system operator. This paper examines a hybrid system, where the transmission system is modeled with a significant penetration level of radial distribution feeders connected to a collection of small wind farms. By varying control strategies at the distribution farms the impacts of the reactive power control strategy implemented by the wind farms are observed. It aims to show that the additional impedance of the distribution system will have unintended consequences on the transmission system and the application of voltage control is less critical.