"Before the owners of wind and solar farms can feed renewable electricity into the grid, they must prove to the grid operators that their systems are compatible with the electricity grid", says Dieter Rosenwirth, Head of the Certification Body for Grid Compatibility at TÜV SÜD Industrie Service. On behalf of wind and solar farm operators, Dieter Rosenwirth and his team of experts check the grid compatibility of the planned power generating units and systems and their electrical characteristics.
Today's grids were never designed for a large number of decentralised power generators. Originally, they were set up for large amounts of electricity produced at large, centralised power stations, which then had to be transported to consumers via transmission and distribution grids. Now, however, increasing numbers of decentralised power generators are connected to these grids, feeding in fluctuating amounts of electricity – as unpredictable as weather-driven wind and solar power themselves.
Every day, this poses a challenge to the owners and operators of power stations and electricity grids, who must flexibly align volatile power production and grid-capacity utilisation to the fluctuations in energy demand. This is a Sisyphean task which reaches its limits in what is technologically possible in terms of grid control. Continuous intervention to maintain control further increases the risk of a fault. Nevertheless, grid operators must ensure at all times that a possible grid failure remains localised and will not result in a general blackout.
Decentralised power generators replace large centralised power plants
Against the backdrop of the growing integration of renewable energy, large power stations are increasingly being replaced by decentralised power generators. Given this, a glance at the electrical characteristics of these new power generators is all the more important. On principle, they must perform in the same manner as large power stations in the grid and also include adequate safety and control systems. TÜV SÜD expert Dieter Rosenwirth says, "At the planning stage, the parties responsible are often insufficiently aware of the complex technical requirements regarding grid compatibility. This can result in unscheduled delays and cost-intensive retrofitting measures if generators fail to comply with all of the relevant guidelines and directives."
Basically, the relevant guidelines and directives demand that all technical components of the generation system contribute actively to maintaining stability of the grid voltage and frequency. However, power generation units and systems are subject to different directives and guidelines depending on the grid into which the electricity is fed. "The connection of wind farms to the high-voltage grid is subject to TransmissionCode 2007 in Germany", says the TÜV SÜD engineer. By contrast, units and systems connected to the medium-voltage distribution network must comply with the Medium Voltage Directive of the German Association of Energy and Water Industries (Bundesverband der Energie- und Wasserwirtschaft e.V., BDEW) which defines the electrical characteristics of solar farms and wind turbines. Wind farms must also comply with the German Ordinance on System Services by Wind Energy Plants (System Services Ordinance, SDLWindV). Small, private solar systems that feed electricity into the low-voltage network are subject to various regulations and to VDE-AR-N 4105, which describes the minimum technical requirements for the connection to, and parallel operation with, low-voltage distribution networks.
When is certification required in Germany?
Certification of grid compatibility is only required for systems connected to the medium-voltage distribution grid and high-voltage grid. Grid-compatibility certificates are issued by accredited third-party certification bodies. "An important factor in this context is that owners must furnish proof of the grid compatibility of every single generation unit and, in certain cases, also of the system as a whole. Given this, wind turbines or PV inverters need type-specific unit certificates. The wind or solar farm as a whole is covered by a plant or system certificate", says Dieter Rosenwirth.
Certification is governed by the following regulations: both unit and system certificates are required for wind farms if the point of common coupling is located in a high-voltage grid. Unit certificates are required if the point of common coupling is located in a medium-voltage distribution grid. A plant certificate is required in all cases in which the system generates apparent power of more than one megavolt ampere or where the length of the line to the point of common coupling exceeds 2 kilometres.
Certification is based on a long list of criteria, which are checked in detail within the scope of the certification process (see info boxes 1 and 2). The various guidelines and the large number of different technical systems make the certification process complex and time-consuming. We check basic requirements including continuous load, active power supply and short-circuit rating, but also the fault ride-through performance of the power generator.
It is important to note that a certification of network compatibility according to IEC 61400-21 and the procedures of IEC 61400-22 is not suitable to meet the requirements in Germany. In fact, the certification of a power generating unit in Germany requires the availability of measurements according to IEC 61400-21 supplemented by the requirements of TR 3 and a computer-executable model of the production unit. The measurement must be carried out by an EN 17025 and IEC 61400-21 accredited institution.
Certification of power generating units (PGU certification)
The certification body checks the completeness and plausibility of the submitted documentation that is required for certification. Once document review has been completed, the electrical characteristics of the generation unit are assessed and evaluated. This is done in accordance with the Technical Guidelines of the FGW, the German public association of the renewable energy sector. Analysis of the electrical characteristics including active power, control system and decoupling control and grid impact falls under the scope of FGW-TR8. The central element of the certification process is a digital simulation model of the generation unit, which is validated in accordance with the requirements of FGW-TR4. "The model permits simulation of various faults, and thus enables a detailed assessment of the electrical characteristics to be made", summarises Dieter Rosenwirth.
Certification of power generating plant (PGP certification)
The unit certificates of the individual power generating units are the prerequisite for the certification of the power generating system as a whole. Based on the submitted documentation, a computer model is applied to produce computer simulation of the entire power generating system, including its components, and a simplified model of the supply network at the point of common coupling. In accordance with FGW-TR8, the engineers use grid software for their analysis. Analysis is used to furnish evidence that the power generating system performs in compliance with the directives and guidelines and forms the basis of a detailed report. If all regulations have been considered and complied with, the plant certificate can be issued.
Following complete installation of the wind and solar farms, conformity assessment is performed. Conformity assessment verifies that the plant as built complies with the plant certificate and the technical directives and guidelines. Dieter Rosenwirth, TÜV SÜD, recommends, "The owners of wind and solar farms must take the aspect of grid compatibility into account right from the outset of planning and obtain expert advice if necessary. By doing so, they ensure rapid certification and on-time commissioning of their systems"
[Editor's note: Many thanks to Dieter Rosenwirth for this piece. Dieter has been with TÜV SÜD Industrie Service since 1998, and becameHead of the Certification Body for Grid Compatibility in 2011. He also represents TÜV SÜD and VdTÜV on the committees of DKE, FNN and FGW e.V.]
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