Wave farm flicker severity: Comparative analysis and solutions

Abstract

This paper proposes a flicker severity study for Wave Energy Converter farms. The flicker severity is introduced and the reason why it is an important constraint for a wave farm is explained. A new representation called intrinsic flicker severity is introduced which describes the flicker severity independently of the grid. The influence of device type, its control and the sea-state on average production, flicker severity and on the ratio between flicker and production are studied with three types of devices: an Oscillating Water Column and two Direct Wave Energy Converters (two point absorbers: a Heaving Buoy and the SEAREV). The influence of the size and the placement of each unit in the wave farm is presented with a farm-unit flicker ratio, compared with the square-root of unit hypothesis (noise behavior), as a function of wave direction by taking into account wave direction dispersion. Finally, solutions are presented to reduce the flicker produced to comply with grid code requirements in order to allow grid integration of wave farms.Keywords: Wave Energy, Wave Farm, Power Quality, Flicker, Oscilating Water Colum, Electrical Energy Storage IntroductionIntegration to the grid is one of the keys to commercializing Wave Energy Converters (WECs). However, some WEC technologies produce power that fluctuates at the rate of ocean waves. Some of them have intrinsic energy storage from the pneumatic or hydraulic chain, but can still have other issues, like efficiency and reliability.When the produced power fluctuates at frequencies between 5 mHz and 33 Hz, the induced rms-voltage fluctuations can cause power quality problems at the grid connection point. WECs have been identified as particularly susceptible to induce flicker [1][2][3][4]. The combination of the weak grid (which could be the case with an island or a near-shore distribution grid) and production fluctuations can cause flicker non-compliance, relative to the grid code requirements. In order to analyze this issue, a new representation called intrinsic flicker is proposed in order to represent the results under several conditions; such as grid characteristics and reactive power injection.Wave farm flicker severity depends on several influences; device type, different controls, sea-states, sizes of farms and architecture of farm. A comparison of different device types is proposed here: oscillating water column (OWC), heaving buoy, and the SEAREV (see Fig. 1). Some studies has already been * thibaut.kovaltchouk@ens-rennes.fr

Advances and Challenges in Wave Energy Park Optimization—A Review

“…By arranging the park layout such that the WECs are not excited simultaneously by the incident waves, the power output can be further smoothened (Engström et al, 2013;Sjolte et al, 2013;Göteman et al, 2014). Armstrong et al (2015), Kovaltchouk et al (2016), and Blavette et al (2016) used approximate methods for hydrodynamics and power production in the array to study flicker severity with a focus on transmission and grid aspects. The impact of a park consisting of ten OWCs and ten heaving buoys on the transmission system was studied by Armstrong et al (2015), and it was concluded that the transformers and transmission lines would remain below 45% capacity.…”

“…The impact of a park consisting of ten OWCs and ten heaving buoys on the transmission system was studied by Armstrong et al (2015), and it was concluded that the transformers and transmission lines would remain below 45% capacity. Kovaltchouk et al (2016) studied limits to voltage fluctuations for arrays of different WEC types, and compensatory actions were proposed to allow grid integration. Blavette et al (2016) estimated the flicker level as a function of grid impedance angle.…”

The Wave Energy Converter Design Process: Methods Applied in Industry and Shortcomings of Current Practices

“…The MaRINET report on demand side grid compatibility offers methods for designing control systems and array layouts to meet grid codes [169]. Flicker is a measure of voltage fluctuations which is impacted by the WEC farm size and architecture, device type, control, and sea state [170]. Although the impacts of flicker depend on the grid strength [171], Kovaltchouk et al present a method of evaluating flicker independent of the grid [170].…”

“…Flicker is a measure of voltage fluctuations which is impacted by the WEC farm size and architecture, device type, control, and sea state [170]. Although the impacts of flicker depend on the grid strength [171], Kovaltchouk et al present a method of evaluating flicker independent of the grid [170]. Blavette et al point out that because site-specific grid compatibility studies are time consuming and require a lot of detail, developers often put them off.…”

Simplified Estimation of the Flicker Level Induced by Wave Energy Farms

“…However, they help to determine whether more detailed flicker level assessment studies need to be performed. Another work has focused on developing an intrinsic flicker severity index independent of both the short-circuit ratio and of the grid impedance angle [12]. However, this paper assumed that electrical lines are lossless, which can lead to a significant error level when sufficiently resistive networks, as it is the case in distribution networks, are considered.…”