Anne Blavette

Impact of a Medium-Size Wave Farm on Grids of Different Strength Levels

Power fluctuations generated by most oscillating wave energy converters may have a negative impact on the power quality of the local grid to which the wave farms will be connected. Hence, assessing their impact is an important step in the selection process of a suitable deployment location. However, site-specific grid impact assessment studies are relatively time-consuming and require a high level of detail on the local network. Both of these constraints mean that grid impact studies are usually not performed in the preliminary stages of the site selection process, despite the extremely negative consequences resulting from poor power quality. This paper details a comprehensive study based on a relatively typical wave farm design connected to networks of different strength levels. The study was performed using experimental electrical power time series of an oscillating water column (OWC) device generated under the framework of the European FP7 project “CORES”. Simulations were performed using DIgSILENT power system simulator “PowerFactory”.

Dimensioning the equipment of a wave farm: Energy storage and cables

Still largely untapped, wave energy is particularly abundant and may represent an important share in the energy mix of many countries in the future. However, the power fluctuations generated by most wave energy converters with little to no storage means or without suitable control strategies may deteriorate the power quality of the local network to which wave farms will be connected. They may in particular generate an excessive level of flicker. The minimum amount of storage required for a wave farm to be grid compliant with respect to typical flicker requirements was investigated and is presented in the first part of this study. Besides giving rise to power quality issues, the rapid and high amplitude power peaks generated by wave devices may also render more complex the optimal dimensioning of the wave farm electrical components, whose cost is highly dependent on their power rating. This statement applies also to submarine cables, as the maximum current flowing potentially through them seems to be no longer a relevant criterion for determining their optimal current rating. Hence, the second part of the presented study focuses on the minimum current rating required from a submarine cable to avoid its thermal overloading.

A novel method for estimating the flicker level generated by a wave energy farm composed of devices operated in variable speed mode

The output power of wave energy farms may be very fluctuating, which may give rise to power quality issues such as flicker. However, although there existed a method for estimating the flicker level generated by a wave energy farm in relation to its short-circuit ratio (as described in IEC standard 61400-21), until recently, no method had been defined yet regarding the two other major parameters on which flicker level is highly dependent: the impedance angle at the point of connection and the rated power of the farm. In a previous work, the authors had presented a method for estimating the level of wave farm-induced flicker as a function of these latter parameters. They had identified two relationships regarding the impedance angle and the rated power in the case where the wave energy devices composing the farm are operated in fixed speed mode. This article presents the results of a follow-on work regarding the generalization of this method in the case of wave energy devices operated in variable speed mode.

Dimensioning the Equipment of a Wave Farm: Energy Storage and Cables

Still largely untapped, wave energy may represent an important share in the energy mix of many countries in the future. However, the power fluctuations generated by most wave energy devices with little to no storage means, or without suitable control strategies, may cause power quality issues that must be solved before large wave energy farms are allowed to connect to a network. For instance, large power fluctuations may induce an excessive level of flicker in the distribution networks to which they are currently envisaged to be connected. Although storage appears to be a technically feasible solution, the minimum amount of storage required for a wave farm to become grid compliant with respect to typical flicker requirements is still unknown and is therefore investigated. This study constitutes the first part of this paper. Another issue, on which the second part of this paper focuses, concerns the optimal dimensioning of wave farm electrical components, which is traditionally performed assuming steady-state conditions (i.e., a constant current level), and is thus irrelevant in the case of wave farms outputting power fluctuations of significant amplitude. Hence, a second study, the results of which are presented in this paper, focuses on the minimum current rating for which a submarine cable may be safely operated without thermal overloading. Addressing both these issues is essential to the economic viability of a wave farm as the cost of both storage means and electrical components is highly dependent on their rating and may represent a significant percentage of the capital expenditure.

Grid Integration of Wave and Tidal Energy

Wave and tidal energy provide a renewable source of electricity. However, their inherent fluctuations may have a negative impact on the power quality of a local electrical network. Grid operators assess this impact through the use of dynamic models of the generation units, which are inserted into the overall power system model. Providing these models is a compulsory step for any power generator to procure a grid connection above a specified power capacity. Significant issues were encountered in the wind energy industry regarding the dynamic modelling of devices, among which were model numerical instability, poor dynamic model quality and model incompatibility. Considering the large diversity of device types in the emerging ocean energy industry, these problems are considered as a major barrier to the larger scale grid-integration of marine energy converters. Dynamic models must clearly demonstrate the compliance of the actual power generation device and array of devices to the grid code requirements for grid-connection to be allowed. A further barrier to grid connection of ocean energy devices is that existing grid codes — mainly written in the context of wind energy — may be irrelevant or inadequate for ocean energy devices. This paper presents an overview of these issues, and details a radically different approach to the dynamic modelling of ocean energy devices that will assist in overcoming the issues previously encountered in the development of wind turbine models. It also highlights the gaps and inadequacy regarding grid code requirements for ocean energy devices, and provides some recommendations for a new ocean energy grid code.Copyright

Wave-induced flicker level emitted by a tidal farm

The inherently fluctuating nature of sea waves can be reflected to a significant extent in the power output of tidal turbines. However, these fluctuations can give rise to power quality issues such as flicker. Hence, it is important to assess the impact which tidal farms may have on their local network before such power plants are allowed to connect to the grid. This paper describes the influence of the wave climate on the short-term flicker level induced by a tidal farm on the point of common coupling. It analyses also under which conditions the tidal farm breaches the grid code requirements in terms of short-term flicker level.

An initialisation methodology for ocean energy converter dynamic models in power system simulation tools

Environmental concerns, coupled with dwindling fossil fuel supplies, and growing government support, have resulted in increasing amounts of electricity generated from renewable energy. As the number of these installations rise, their influence on the behaviour of the electrical power system becomes more apparent. Transmission System Operators (TSO) carry out power system dynamic simulations in order to investigate the effect of renewable energy systems on the stability and reliability of the grid under varying operating conditions. Correct initialisation is an essential step in the dynamic model. This paper describes a three step methodology for correct initialisation of power systems dynamic model for an ocean energy system. The methodology is validated using DIgSILENT Power Factory simulations.

Grid Impact of a Wave Farm on its Local Network: Analysis of Voltage and Flicker Levels

Most oscillating wave energy converters without significant amounts of energy storage capacity generate significant electrical power fluctuations in the range of seconds. Because of these fluctuations, a wave farm may have a negative impact on the power quality of the local grid to which it is connected. Hence, the impact of these devices on both distribution and transmission networks needs to be well understood, before large scale wave farms can be allowed to connect to the grid.This paper details a case study on the impact of a wave farm on the distribution grid around the national wave test site of Ireland. The electrical power output of the oscillating water column (OWC) wave energy converters was derived from experimental time series produced in the context of the FP7 project “CORES”. The results presented in this paper consider voltage fluctuation levels and flicker levels for a typical time series. Simulations were performed using DIgSILENT simulation tool “PowerFactory”.Copyright