The switch to an energy supply, via renewable energies, that is climate-friendly and affordable in the long term poses huge challenges for the energy industry.

Last year alone, 22,297 wind turbines totaling 29,075MW and 1,090,000 photovoltaic systems totaling 24,800 MWp fed into the German power grid.

New electricity producers are being added all the time. In the meantime, the grid infrastructure is reaching its load limit – the existing structure has only limited suitability for the fluctuating input of the distributed electricity producers.

The ability to adapt fluctuating electricity production and network capacity utilisation flexibly to changing energy requirements is a demanding task associated with a high risk of error, given the current technical facilities for grid control.

Priority is given to reliable operation of the renewable energy systems. It is imperative to ensure security of supply at all times and to keep any grid breakdowns (line faults) within local limits.

The basis for security of supply is grid stability and therefore grid compatibility of the renewable energy plants to be integrated. A dependable demonstration of grid compatibility – which provides evidence of conformity to the existing technical codes and standards – is therefore very important both for those technically and commercially responsible within the energy suppliers and also for the manufacturers of technical components and investors.

Complexity

The certification process has become an extensive and very complex process due to the large number of different technical guidelines and requirements in an extremely innovative dynamic environment.

In this case, for example, different regulations are applicable depending on the grid level where feed-in will take place. If the grid coupling point of a windfarm is in the high-voltage grid, then Transmission Code 2007 is the determining factor and in that case the power plant requires unit certificates and plant certificates.

In addition, the Ordinance on System Services by Wind Energy Plants (SDLWindV) applies to recently commissioned wind farms. For a grid coupling point in the medium-voltage grid, wind and PV plants require unit certificates, regulated in accordance with the Guideline of the Federal Association of the Energy and Water Industry (BDEW Guideline).

Likewise, a plant certificate is required if the length of the line up to the grid connection point is greater than two kilometres, or if an apparent power greater than one megavolt ampere (MVA) is generated.

In contrast, connection to the low-voltage grid requires no certificates. However, normative specifications are still applicable, in particular VDE-AR-N 4105, the follow-up regulation to the VDEW guideline “Power generation systems connected to the low-voltage distribution network”.

It is advisable to call in experienced, independent test institutes in order to guarantee compliance with all relevant guidelines. TÜV SÜD Industrie Service GmbH supports certification processes from planning to the declaration of conformity and commissioning of the plant. These experts are accredited by the German Accreditation Body (Deutsche Akkreditierungsstelle or DAkkS) for testing of the grid compatibility of power generation plants. And they are 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., otherwise know as FGW).

Certificates requirements

The demonstration of grid compatibility of wind and solar energy plants is based on the unit and plant certificates which identify the electrical properties of the generating unit and generating plant. The type-specific unit certificate is issued for the individual power generation unit (PGU – e.g. wind turbine, photovoltaic inverter).

The project-specific plant certificate verifies guideline-compliant behaviour of the plant as a whole, such as a wind farm. The scheduled commissioning of a plant is at risk if these certificates are missing or if they are not submitted in due time.

To award unit certificates, all the required documents compiled prior to the actual test process must first be submitted and examined by experts from an organisation like TÜV SÜD for completeness, feasibility and compliance with the legal specifications.

In this step, 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 decisive, among other things, to the analysis of output, protective equipment and line harmonic distortions (FGW-TR8).

A detailed technical description of the type according to FGW-TR3 is required in addition to the manufacturer's certificate containing the generating unit's specific data. A certified quality management system in accordance with DIN ISO 9001 must also be demonstrated for production of the PGU. Added to this, all test reports of measurements and type tests, carried out in accordance with DIN EN 17025, are inspected.

Furthermore, component certificates, declarations of conformity and technical descriptions of other operating equipment (e.g. control system, protective equipment) are also relevant. The validation of a digital simulation model of the PGU with steady-state load flow calculations as well as symmetrical and asymmetrical fault simulations in accordance with FGW TR4 are crucial. The unit certificate is only issued once all the documents are complete, all the guidelines have been complied with and the specifications have been met.

The type-specific unit certificates with the associated, validated simulation models and an overview drawing of the generating plant (single line drawing) are required for the plant certificate. The documents to be submitted also include:

• completed data query forms of the grid operators (wind or PV farm operators);
• circuit diagrams of the medium-voltage systems,
• documents relating to the protection concept;
• and an overview drawing of the control mechanisms (particularly the control concepts for the reactive and active power).

Furthermore, component certificates and declarations of conformity for the operating equipment are just as important as the technical data of the cable installation, transformers, controllers or switchgear.

A plant or data model which takes account of all the generating units (PGUs) and components of the whole plant up to the grid connection point is created after all the documents have been analysed. This model enables steady-state and dynamic simulation of the whole plant.

Advanced, in-depth calculations and analyses make up the conformity report which brings together the plant's technical properties and all the results. The experts issue the plant certificate once all the technical details and mandatory specifications have been taken into consideration and satisfied.

However, the final process of the conformity evaluation, which is specified by FGW-TR8, must be satisfied before the plant can finally be commissioned. Various basic functions, commissioning documents and circuitry documents are spot-checked after completion of the plant for this purpose. This ensures that the power generation plant is connected to the grid in compliance with the certificates. Only then can a payment be claimed in accordance with the Renewable Energy Sources Act.

Early planning

The high technical requirements for grid compatibility should be taken into account at the planning stage of wind farms and solar energy parks. In this way, project delays, unplanned investments and expensive retrofits can be avoided from the outset.

Those responsible technically and commercially are well advised to deal with the detailed questions of certification in the earliest possible phases of the project. Moreover, they should consider taking advantage of the support offered by independent, recognised certification experts.

About: Dieter Rosenwirt is from TÜV SÜD Industrie Service