Today an increasing amount of intermittent PV electricity is being pumped into the grid. As grid capacity nears its limits, the issue of grid compatibility is gaining in significance. Solar-farm owners must prove that - once connected to the electricity grid - their plants will behave in the same manner as conventional power stations have done for years. Failure to ensure solid preparation of the comprehensive certifications required in this context may delay the launch of service for their solar power system.
Too much of a good thing can lead to grid stability issues
Dieter Rosenwirth | TÜV SÜD Industrie Service
In late 2010, Germany had 17,000 megawatts of installed solar capacity, i.e. just under one-quarter of the 80000-megawatt peak load. A further 3,000 MW of solar capacity were added to the grid in December 2010 alone. Distribution networks are experiencing increasing problems in integrating the growing amount of intermittent electricity fed into the grid. While wind power, for example, is fed mainly into the high- and medium-voltage grids, most of the power produced by solar power systems is pumped into the low- and medium-voltage grids. Unfortunately, awareness of capacity issues on distribution networks is still at a low ebb. Given this, it is all the more important to ensure that all technical components of the generation system contribute actively to maintaining stable voltage and frequency in the network.
Proof of electrical characteristics must be furnished for both the power generation unit (PGU) – in the case of a solar farm, the inverter – and the entire power generation plant (PGP), i.e. all inverters of a solar farm including the other installed electrical components i.e. cables, switchboards, transformers. For this purpose, the experts assess the basic requirements including continuous load, active power supply and short-circuit rating, as well as performance of the power generation unit in the case of grid disturbances. All these characteristics are certified by an independent third party. The central documents for grid compatibility and thus grid integration are the 'unit certificate' and the 'plant certificate' or 'plant-related expert report'. However, another important document of all is the declaration of conformity. Within the scope of conformity assessment, the experts verify that the plant conforms to the certificate and technical guidelines and directives.
Familiarity with guidelines and directives
Connection to the medium-voltage grid is subject to the Medium Voltage Directive of the German Association of Energy and Water Industries (BDEW-RL), while energy generators connected to the low-voltage network need neither a PGU nor a PGP certificate but must comply with standard provisions and the VDE-AR-N 4105 application guide – the successor document to the "Generators connected to the low voltage distribution network" guideline of the German Association of the Electricity Industry.
Solar farms with an apparent power of more than one megavoltampere (MVA) require a plant certificate. The apparent power comprises the active power fed into the grid and the 'reactive power'. The latter is needed for grid operation but is not subject to remuneration. A second critical criterion is the length of the cable network leading from the generation units to the point of grid connection. Long cables can significantly impact on system performance at the point of common coupling (PCC). Given this, units or plants with an apparent power of under 1 MVA but a cable system over 2 km long still require a certificate – even if the generation unit and the cable leading to the grid connection point are located on the premises of the same company.
Unit certification in five steps
In solar farms, the power generation unit (PGU) subject to certification is the inverter. Within the grid, it is perceived as the unit that actually generates electricity. The inverter is connected to the PV modules which supply direct current and must comply with all relevant guidelines and directives. In addition to the BDEW Directive, the Technical Guidelines of the FGW, the German public association of the renewable energy sector – form the test basis. Documents that must be submitted include but are not limited to:
- A manufacturer's certificate of the specific data of the PGU in accordance with FGW-TR3
- A detailed technical description of the PGU
- The complete documentation, including reports of measurements and type approval in accordance with EN 17025 by an accredited testing laboratory
- A simulation model of the PGU including a description of the model in accordance with FGW-TR4
- The measured data underlying the test report in accordance with FGW-TR3
- Component certificates in as far as relevant for PGU certification
- Declarations of conformity and technical descriptions by the manufacturers of other relevant operating equipment (e.g. control, protection devices)
- At minimum a single-line diagram of the PGU
- Evidence of a certified quality management system in accordance with EN ISO 9001 for the manufacture of the PGU
The TÜV SÜD experts apply a step-by-step approach to the certification of grid compatibility:
- Review of all documents submitted by the owner for completeness and plausibility.
- Comparison of the electrical characteristics against the technical requirements such as active power output, supply of reactive power and/or grid impact. To prepare for the further certification process, the results achieved up to this stage are summarised in an intermediate report.
- Validation of the existing PGU model, using special grid calculation software.
- Evaluation of all results of model validation, including a validation plan, load-flow analyses and fault simulations for stationary applications.
- Recommendation for or against certification marks the last step in the process. The recommendation depends on the results of conformity assessment and model simulation.
Criteria for PGP certification
Plant certificates are based on the results of the unit certificates and the installed electrical components and submitted to the owners of distribution grid. PGP certification also uses validated models and starts with a document review and plausibility check of the project-specific unit certificate and calculation model, a single-line diagram of the solar farm and a diagram of its control system and decoupling control.
In addition, the experts assess the solution for regulating the supply of reactive and active power and the certificates and declarations of conformity of operating equipment. Also reviewed within the scope of this process are the wiring diagrams of medium-voltage systems, the data sheets (owners of solar farms / owners of distribution grid) and the technical data of the associated cables, transformers and operating equipment such as switchgear, controllers or dynamic reactive power compensation.
Subsequently, the certifiers work closely with the owners and the constructor of the power generation plants (PGP) and with the network owners to discuss, and agree on, further steps. The certification professionals check whether the components conform with the technical guidelines and standards and the specific requirements of the grid owners. Once conformity has been established, they carry out the analyses in accordance with the Technical Guideline FGW TR 8. Similar to unit certification, they also develop a data model which includes the operating equipment and a simulation model of the utility network at the grid connection point. This data model can be used to simulate the stationary and dynamic behaviour in accordance with the FGW Technical Guideline and BDEW Directive. The results of simulation and further analyses are included in the conformity report, which again documents the technical characteristics of the power generation unit. As in PGU certification, a certificate is only issued if the system complies with the technical guidelines and directives. After the solar farm has been installed, the experts carry out an on-site assessment, verifying whether the installed plant conforms to the documentation. If this is the case, the experts confirm this fact in the declaration of conformity which marks the end of the process.
Since certification of grid compatibility is extensive and complex, experienced partners are essential. TÜV SÜD's expert teams offer years of practical experience, multidisciplinary know-how and familiarity with all relevant guidelines, directives and standards. Foresight and high data quality are guarantors of smooth and quick certification. TÜV SÜD's experts advise solar farm owners to clarify detailed issues or interpretations concerning grid compatibility at an early stage of their project. To do so, the manufacturers of inverters must work closely with their external contractors and the accredited testing laboratory. Particularly important aspects are consistent, plausible information in the pertinent documentation and well-documented easy to use simulation models.
Dipl.-Ing. Dieter Rosenwirth, Head of the Certification Body for Grid Compatibility at TÜV SÜD Industrie Service GmbH
Dieter Rosenwirth also represents TÜV SÜD and VdTÜV on the committees of DKE, FNN and FGW e.V.
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