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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Electrical aspects and grid integration' taking place on Thursday, 13 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Stavros Papathanassiou National Technical University of Athens, Greece
Co-authors:
Sotirios Nanou (1) F Stavros Papathanassiou (1) Panagiotis Anagnostopoulos (1) Stavros Papathanassiou (1) Eleni Kapolou (2) Stavros Stroumpoulis (2) Dimitra Telaki (2) Nikolaos Drosos (2)
(1) National Technical University of Athens, Kifissia, Greece (2) HEDNO S.A., Athens, Greece

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Abstract

Interconnection of small wind turbines to the LV network: technical issues and evaluation methodology

Introduction

The favorable Feed-in-Tariff scheme adopted for Small Wind Turbines (SWTs) in Greece, along with the growing interest for installations of this type, pose the need for the determination of technical requirements and suitable evaluation methodologies for their connection to the Distribution Network. This paper presents the main results from a study performed on behalf of the Greek DNO (HEDNO S.A.), with the objective of formulating a technical evaluation framework for the connection of small wind turbines to the Low Voltage (LV) network.

Approach

First, an extensive research was performed on a large number of commercially available wind turbines (WTs) (approximately 250 samples), in the range of 1 – 100 kW, in order to identify the current trends of the small wind turbine market. Specifically, the research topics cover technology related issues, such as available electrical schemes (generator type, inverter-interfaced or directly connected equipment), as well as certification related issues, such as the number of SWT manufacturers who possess a valid type certificate or power quality test certificate. This investigation provides thus a comprehensive overview of the technological maturity of the SWT market.
Then, suitable interconnection criteria and guidelines were developed, taking into account the extensive set of International Electrotechnical Commission (IEC) power quality standards (part of the IEC 61000 series of Electromagnetic Compatibility (EMC) publications [1]), the European standard EN 50438 [2], the existing technical guidelines for the interconnection of photovoltaic (PV) stations to the LV grid, as well as established Distribution Network Operator (DNO) practices. Other important considerations, such as type certification requirements, have been also considered, based on the IEC 61400 series and other established practices [3].
SWTs were divided into two categories, based on their rated power (up to 10 kW and 10-50 kW), a classification based on limits adopted in the relevant IEC standards (16 A/ phase output current).
Α set of technical criteria and requirements, which permits the efficient evaluation of small wind turbine interconnection projects, is derived, with particular emphasis placed on power quality issues, namely slow and fast voltage variations, flicker and harmonic emissions, as well as on interconnection protection requirements [4].

Main body of abstract

Based on the study of approximately 250 SWTs, 80% of the commercialized WTs have horizontal axis orientation, while only 20% adopt the vertical design. As for the electrical scheme, the vast majority of SWTs is equipped with permanent magnet synchronous generators using full power converters. The rest mainly utilize asynchronous generators directly connected to the grid. It is noteworthy that most WTs of this type have a rated power above 20 kW. Concerning type certification, 18% have a valid type certificate, whereas 18% of the SWT manufacturers declare that they have initiated a certification process for their products, clearly showing that the number of certified SWTs is steadily increasing.
In the following, a brief overview of the main technical requirements imposed to SWTs is discussed, namely slow voltage variations, fast voltage changes, flicker and harmonic emissions, and interconnection protection requirements.
The main criterion for slow voltage variations is that the maximum steady-state voltage change at the Point of Common Coupling (PCC) should remain below 3%. The evaluation procedure can be simplified using power – distance graphs, as the one depicted in Fig. 1, from which the maximum permissible rated power of a SWT can be estimated as a function of the distance from the substation, assuming a 4x120 mm2 Al bundled overhead cable LV line.

Fig. 1: Power–distance evaluation graph concerning slow and fast voltage variations, for the interconnection of a SWT to the LV network.

Regarding the rapid voltage changes and flicker emissions two alternative evaluation procedures have been identified. The first one stems from the requirements of the relevant IEC 61000 series documents, whereas the second one is a simplified method, which relies on using power – distance graphs, as in the case of slow voltage variations.
IEC standards 61000-3-3 and 11 are product standards permitting the operation of distributed generation (DG) equipment without further assessment, as long as predefined limits for rapid voltage changes and flicker emissions are satisfied. Detailed testing procedures and parameter values are described, i.e. grid impedance values under laboratory conditions, in order to evaluate the operation of the equipment under test. However, if the limits are exceeded, the equipment shall be tested under less strict conditions in order to meet the requirements imposed by the standard. In that case, the equipment is subject to conditional connection, and an evaluation procedure should be performed by the DNO. Specifically, the connection is acceptable only if the short-circuit impedance, calculated at the PCC, is below the value declared by the manufacturer.
An alternative evaluation procedure relies on employing the assessment methods applied to large WTs, connected to the MV network. The expected rapid voltage changes and flicker emissions at the PCC can be calculated using the voltage change factor kU(ψk) and the flicker coefficient c(ψk,va) respectively [4], which are included in the test certificate of the WT, in accordance with IEC 61400-21. However, testing of SWTs in accordance with IEC 61400-21 is rather difficult and costly procedure and very few machines have such a certificate. Hence a simplified calculation method is needed. Based on power quality measurements of commercial SWTs available in the literature [5], it has been found that if a SWT is evaluated with respect to the fast voltage changes using a typical kU value, e.g. as found in the German technical guideline [6], then it can reasonably be expected that rapid voltage change and flicker emission limits will also be satisfied. Consequently, a similar evaluation procedure can be established, using power – distance graphs (Fig. 1), as in the case of slow voltage variations.
As for harmonic emissions, a common index used to evaluate the harmonic content of the output current is its Total Harmonic Distortion (THD) factor. For LV systems, specific compatibility levels are given per harmonic order in IEC 61000-3-2 and 61000-3-12 standards, while a typical THD limit adopted by several DNOs is 5%.
Typical minimum protective functions of the interconnection protection system include over-/under-voltage, over-/under-frequency and Loss of Mains (LoM) protection. The Rate of Change of Frequency (RoCoF) and Vector shift protection are common methods used to detect loss of mains, though a variety of other islanding detection techniques, both passive and active, are also acceptable.




Conclusion

The paper includes a presentation of important technical considerations for the interconnection of SWTs to distribution networks. Technical requirements and simplified assessment criteria are presented for power quality related issues, including steady-state and rapid voltage variations, flicker and harmonic emissions. Interconnection protection requirements are also discussed. The aforementioned criteria and procedures are largely based on the set of relevant IEC publications, the European standard EN 50438, the existing technical guidelines for the interconnection of PV stations to the LV grid, as well as on current DNO practice.
Based on the study performed on commercial SWTs, it can be inferred that the SWT market is still relatively immature, compared to their large relatives, since the majority of SWT manufacturers do not possess a valid type certificate or power quality test certificate. Yet, the situation is improving fast, with better designs already in the market and several manufacturers embarking on the certification process. Regarding the technology used, it should be mentioned that the vast majority of SWTs use power electronic interfaces, though there are still SWTs of relatively high ratings (above 20 kW) which adopt a direct connection scheme using asynchronous generators.
Concerning the technical requirements for interconnection to the network, the necessary evaluation framework for SWTs is inevitably more complex, compared to other DG technologies such as PV stations. For instance, flicker emissions, a standard concern for WTs, especially the small ones, as well as cut-in transients, are considerations that warrant particular attention, a task made more difficult by the absence of standardized and widely accepted testing procedures.



Learning objectives
The main objective of this study is to illustrate trends of the SWT market, to identify the important technical requirements applicable to the interconnection of SWTs to the network and to propose a suitable technical evaluation framework.


References
[1] IEC 61000 & 61400 series standards, available online at: http://webstore.iec.ch.
[2] CENELEC EN 50438, “Requirements for the connection of micro-generators in parallel with public low-voltage distribution networks”, 2007.
[3] MCS 006: Product Certification Scheme Requirements: Micro and Small Wind Turbines, Department of Energy and Climate Change, 2008.
[4] S. A. Papathanassiou, “A technical evaluation framework for the connection of DG to the distribution network”, Electric Power Systems Research, March 2006.
[5] Technical Report, “Wind Turbine Generator System Power Quality Test Report for the Gaia Wind 11-kW Wind Turbine”, National Renewable Energy Laboratory (NREL), July 2011.
[6] VDE-AR-N 4105, “Power generation systems connected to the low-voltage distribution network. Technical minimum requirements for the connection to and parallel operation with low-voltage distribution networks”, 2011.