Dieter Rosenwirth and Kai Strubbe, Contributors
March 21, 2013 | 13 Comments
Until some years ago, the electricity in German grids flowed in only one direction – from a large-scale power station to transformer stations and from there to nearby consumers.
Matching power supply to power demand was easy. However, for some time now smaller-scale power generating plants (PGPs) have been feeding electricity into the medium- and low-voltage grids throughout Germany, making the situation increasingly complex. And new, decentralised power generating plants and units that are located in different regions and, depending on the weather, produce larger or smaller amounts of power are integrated into the grid on an almost daily basis. In addition, Germany's Renewable Energy Act obliges grid operators to give priority to the purchase of electricity from renewable energies.
Every day, this poses a challenge to the owners and operators of power stations and electricity grids, who must flexibly align intermittent power production and grid-capacity utilisation to fluctuations in energy demand. Meanwhile, the power infrastructure has reached its limits in terms of capacity and load. Matching demand and supply is a Sisyphean task that has also reached its limits in what is technologically feasible 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 possible grid failures remain localised and will not result in a general blackout.
Decentralised Generation Replacing Large-scale Power
The change primarily makes higher demands on individual power-generating plants such as wind and photovoltaic (PV) farms. In light of this, before a new wind or PV farm can feed renewable energy into the grid, its owner must furnish proof of its grid compatibility to the grid operators. For this purpose, accredited third-party certification organisations review the design documentation to ensure the PGPs will not cause any problems once they are hooked up to the power grid – which in its present form was never designed for decentralised feed-in at various grid levels.
In principle, the new power generating plants must perform in the same manner as large power stations in the grid, and also include adequate safety and control systems. They must also meet the high technical demands imposed on grid compatibility in Germany. Generating plants that fail to comply with all relevant guidelines and directives can cause unscheduled delays and cost-intensive retrofitting measures.
Electrical Characteristics Are Critical
Basically, the relevant guidelines and directives demand that all technical components of the generating plant contribute actively to maintaining the stability of grid voltage and frequency. However, the applicable directives and guidelines differ depending on the level of the grid into which the electricity is fed. While, for example, the Transmission Code 2007 governs the connection of wind farms to the high-voltage grid, the Medium Voltage Directive of the German Association of Energy and Water Industries (BDEW) defines the electrical characteristics that must be verified if solar farms and wind turbines are hooked up to the medium-voltage system. Wind farms must furthermore comply with the Ordinance on System Services by Wind Energy Plants (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. If the power generating plants are in compliance with the relevant guideline or directive, the certification organisation will issue a certificate.
What Type of Certificate Applies When?
To become certified, components must fulfil a host of requirements which the experts check in detail during the certification process (see boxes 1 and 2). The different directives and the large variety of technological solutions render the certification process complex and time-consuming. To complicate matters even further, the certification process also differs from those of other European countries. Certification professionals check the basic requirements including continuous load, active power supply and short-circuit rating, but also the fault ride-through performance of the power-generating plant or unit. The power-generating plant, for example, must have low-voltage ride-through (LVRT) capability, which keeps the plant operating even in case of a drop in voltage, providing sufficient reactive power to do so and to avoid disconnecting the generator from the grid.
However, certification of grid compatibility is only required for systems that are hooked up to the medium-voltage distribution network or the high-voltage grid. An important aspect in this context is that owners must furnish proof of the grid compatibility of every single generating unit and, in certain cases, also of the plant 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 certificate.
In detail, certification is governed by the following regulations: Wind farms that are connected to the high-voltage grid require both unit and plant certificates. Wind or PV power plants hooked up to a medium-voltage distribution network need unit certificates. All plants require a plant certificate if they generate apparent power of more than one megavolt ampere or if the length of the line to the point of common coupling exceeds 2 km.
Power Generating Unit (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 experts assess and evaluate the electrical characteristics of the generating unit. 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 generating unit, which is validated in accordance with the requirements of FGW-TR4. The model enables simulations of various faults to be run and thus a detailed assessment of the electrical characteristics to be made.