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Certified grid integration

10. September 2012 By:  Dieter Rosenwirth, TÜV SÜD Industrie Service

The major challenge of Germany’s energy transition is integrating the growing number of photovoltaic and wind energy plants into the existing grid. Grid integration requires a compulsory certificate of grid compatibility. The associated processes for unit and plant certification are complex and require a high level of expertise, writes Dieter Rosenwirth, TÜV SÜD Industrie Service.

Producers of renewable electricity are already making a significant contribution to our energy supply. In the future, with the increasing integration of a wide variety of plants, distributed electricity input will become the standard alongside an ever-decreasing number of centralised large power plants. However, the existing grids are basically not designed for distributed input and in the meantime, are reaching their maximum loads. Added to this, the input of renewable energies is dependent on wind and weather and, therefore, are more difficult to predict.

The ability to adapt fluctuating electricity production and network capacity utilisation to changing energy requirements represents an enormous challenge for plant and grid operators, given the current technical facilities for grid control. The reliable operation of distributed plants is of maximum importance. Thus, it is imperative to ensure security of supply and to keep any grid breakdowns (line faults) within local limits.

The cornerstone of supply security is grid compatibility, even for the distributed electricity generators. The plants must actively contribute towards keeping the voltage and frequency in the grid steady. As a result, certification of the grid compatibility of photovoltaic and wind energy plants, which ensures conformity to the industry’s complex technical codes and standards, is very important for all responsible parties.

Verifiable grid compatibility

Producers or operators must provide the grid operator with evidence of grid compatibility, which has been supplied by independent testing institutes, before renewable electricity from new wind energy and PV plants can be fed into the grid. TÜV SÜD Industrie Service supports this process from planning to declaration of conformity.

The German Accreditation Body [Deutsche Akkreditierungsstelle (DAkkS)] has approved the TÜV SÜD Certification Body for Grid Compatibility for the testing of power generation plants and units. The global service provider has also been recommended as a certification body by the Association for the Promotion of Wind and Other Renewable Energies [Fördergemeinschaft Wind und andere Erneuerbare Energien e.V. (FGW)]. The proof required is supplied in the form of the project-specific plant and type-specific unit certificate.

This certificate provides evidence of the generating units and the generating plants' electrical properties. Currently, this relates primarily to wind and solar plants, but in future will also include biogas plants and combined heat and power plants. Demonstration of grid compatibility also forms the requirement for obtaining the feed-in tariff guaranteed by the Renewable Energy Sources Act.

The outcome of the high level of dynamic innovation and the huge number of guidelines for different technical solutions offered by various producers is an extensive and demanding catalogue of requirements, which must be examined in detail during the certification process. Basic requirements such as continuous current load, provision of active power and short-circuit withstand capability are put on the test bench as is appropriate conduct of the electricity producer in the event of a grid failure.

Applicable guidelines for grid integration

Unit and plant certificates demonstrate the grid compatibility of a power generation plant. Individual power generation units (e.g. the wind turbine) receive a type-specific unit certificate, which provides the grid operator with evidence of grid compatibility. The plant certificate verifies guideline-compliant behaviour of the plant as a whole (e.g. the wind farm). In this case, different guidelines are applicable depending on the grid level where feed-in will take place.

If the grid coupling point of a wind farm is in the high-voltage grid, then Transmission Code 2007 is relevant and in that case, the power plant which is feeding in requires both a unit certificate and a plant certificate. The Ordinance on System Services by Wind Energy Plants (SDLWindV) additionally applies to recently commissioned wind farms.

For a grid coupling point in the medium-voltage grid, wind and PV plants require unit certificates in accordance with the Guideline of the Federal Association of the Energy and Water Industry (BDEW Guideline). Likewise, a plant certificate is required if a plant generates an apparent power greater than one megavolt ampere (MVA), or if the length of the line up to the grid connection point is greater than two kilometres.

Only when connecting to the low-voltage grid is there no requirement to provide any certificates. However, it is necessary to comply with normative specifications, in particular with VDE-AR-N 4105, the follow-up regulation to the VDEW guideline "Power generation systems connected to the low-voltage distribution network".

Unit Certificates (UCE) and Plant Certificates (PCE)

For certification of the power generation unit (UCE), the required documents compiled prior to the actual test process are submitted to the accredited certification body. Since we know from experience that missing documents lead time and again to delays in grid integration, TÜV SÜD Industrie Service checks the documents for completeness and feasibility early in the process. Then an assessment is made of compliance with the legal specifications.

In this case, the Technical Guidelines of the Association for the Promotion of Wind and Other Renewable Energies [Fördergemeinschaft Wind und andere Erneuerbare Energien e.V. (FGW)] are applicable in addition to the analysis of output, protective equipment and line harmonic distortions (FGW-TR8).

The next step is to validate a digital simulation model of the UCE with steady-state load flow calculations, as well as symmetrical and asymmetrical fault simulations in accordance with FGW-TR4. This must be submitted together with the documents before certification begins. The unit certificate is issued once all the guidelines have been complied with and the specifications have been met.

The unit certificate provides the basic data for the plant certificate, which then provides the distribution grid operators with evidence of grid compatibility in accordance with applicable guidelines. The documents submitted are first checked in the same way as for the unit certificate. Among other things, checks are made on the completeness and feasibility of the following documents:

• Project-specific unit certificate and analytical model
• Overview drawing of the solar farm (single line diagram)
• Overview drawing of the farm’s control system
• Protection concept ("grid decoupling protection")
• Control concept for provision of reactive power and active power
• Certificates and declarations of conformity for operating equipment
• Circuit diagrams of medium-voltage plants
• Technical data relating to cable installation, transformers and operating equipment (switchgear, controllers or dynamic power factor correction)
• Data query forms (PV farm operator/grid operator)

A plant or data model, which includes all the UCEs and components of the whole plant up to the grid connection point is also created. This model enables steady-state and dynamic simulation of the plant’s behaviour. The technical properties are included in the conformity report by means of advanced, in-depth calculations and analyses.

When the power generation plant has been completed, the plant and all the certificates and reports are examined once more by TÜV SÜD experts. The basic functions are also tested on a spot check basis in cooperation with the grid operator. The concluding declaration of conformity is issued after final acceptance if compliance between planning and execution of construction work has been confirmed.

Safely connected to the grid

A great deal of attention must be paid to the challenging technical requirements of grid compatibility as early as in the planning phase. This is the only sure way to avoid project delays, unplanned investments and expensive retrofits. The key argument for producers, operators and management with technical and commercial responsibility is to clarify risks and detailed questions regarding certification as early as possible.

About the author

Dieter Rosenwirth has worked with TÜV SÜD Industrie Service since 1998. Since 2006, he has served as Head of Electrical Engineering in the Electrical and Building Services Engineering business are, and since 2010 as Head of the Certification Body for Grid Compatibility. Rosenwirth also represents TÜV SÜD and VdTÜV on the committees of DKE, FNN and FGW e.V.

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Disclaimer: The views and opinions expressed in this article are the authors own, and do not necessarily reflect those held by pv magazine.