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Integrating Variable Renewables as Germany Expands Its Grid

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.

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Power Generating Plant (PGP) Certification

The unit certificates of the individual power generating units are the prerequisite for certification of the power generating plant (PGP) as a whole. Based on the submitted documentation and computation models, the specialists produce a computer simulation of the power generating plant, 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 analyses. Analyses are used to furnish evidence that the power generating plant performs in compliance with the directives and guidelines and form the basis of a detailed report. If all regulations have been considered and complied with, the PGP certificate can be issued.

When installation of the wind and solar farms is complete, conformity assessment is performed. Conformity assessment verifies that the plant as built complies with the design documentation and the technical directives and guidelines. For owners of wind or solar farms who want to apply for the feed-in tariff under the Renewable Energy Act (EEG), conformity assessment is a must.

The owners and operators of wind and solar farms must take the aspect of grid compatibility into account right from the outset of their planning and obtain expert advice if necessary. By doing so, they ensure a rapid certification process and guarantee that their plants can be taken into service on time. Certification ensures that the power-generating plants fulfil the basic requirements of grid compatibility, including correct fault ride-through performance. In addition, further aspects are becoming important. After all, the future energy solution depends on the flexible regulation of power generation, grid capacities and electricity demand.

Feed-in Management Is the Key

Rough outlines for the foundation of feed-in management have been established. Feed-in management offers grid operators the opportunity of reducing the active power of individual power generating units and plants in a targeted and gradual manner by means of remote control. In case of grid overload the grid operators send a control signal to the plant, which then reduces the active power output by a defined percentage or switches off the power generating unit completely. However, for feed-in management to function, the power generating unit must be able to receive and process the signal.

The above requirements are fundamental for ensuring grid stability. The faster the expansion of energy from renewable sources proceeds, the more important energy storage solutions, which provide a load levelling buffer, and self-regulating power grids become. In other words, in the medium to long term power consumers and generators must be connected in an intelligent grid – a 'smart grid'. This is the only possible way to manage the growing share of intermittent energy in the grid reliably.

Intelligent Solution:Smart Grids

Smart grids comprise intelligent electronic devices (IEDs) that can communicate with each other in order to control power generating plants and power consumers. Smart grids offer the opportunity of a balanced network in which electricity distribution is increasingly automated. The smart grid is composed of intelligent electronic devices (IEDs) with the required capabilities to do so. The automated control of electricity generators and consumers is increasingly supply-focused. However, for smart grids to become reality, new components must be installed at all levels of the grid and existing processes re-assessed.

Again, communication is key. Given this, we need to establish an extensive communication network parallel to the existing and planned energy grids. The technical components of this communication network must be capable of exchanging information and triggering the required control and regulation mechanisms. These features require additional sensors, instrumentation and control systems and communication and data infrastructure. While the extra-high and high-voltage transmission grids already have intelligent substations, the medium and low-voltage networks have some catching up to do. This is all the more important as the vast majority of renewable power generating plants, around 97%, feed their renewable power directly into the low- and medium-voltage distribution networks, resulting in a large amount of intermittent power supply at these levels that causes the massive fluctuations in the grid.

In view of this, the smart grid of the future must provide for every power consumer to adopt the additional role of power generator or intermediate energy storage – no small feat. This is imperative if we are to ensure effective distribution of intermittent power and high quality electricity supply. The smart grid integrates various infrastructure areas – from energy supply through to transport, industry and automated 'smart homes'.

Smart homes are a particularly clear example of the double function of consumer and generator. Gateways of smart-meter solutions, for example, permit two-way communication and control between the utility grid and the building. Equipped with photovoltaic systems and co-generation units, for example, these households not only consume power but also feed it into the grid. However, the challenge faced by experts today lies in aggregating the large number of small-scale grid participants into a virtual power plant (VPP) that can be controlled, and can thus contribute to ensuring grid stability. If uncontrolled, the large number of small-scale power generating plants and units could result in grid overload.

Networking the large number of participants into a large-scale virtual power station makes a contribution to grid stability that should not be underestimated. The balancing of peak loads and peak production in a defined integrated system offers advantages to grid operators that can improve their feed-in management on the basis of systematic networks and thus reduce the risks involved. Power failures that are critical to the system and caused by fluctuations in frequency and voltage are avoided and the overall quality of power supply improved. Furthermore, consumers obtain additional means of control which allow them to monitor their energy needs online and shift their electricity consumption to low price periods.

In spite of all these advantages, manufacturers are still hesitant when it comes to developing and launching devices that are enabled for use in smart grids. To improve market penetration, standardised communication between the IEDs in accordance with the IEC 61850 standard must be ensured. The standard plays a central role, providing a universal communication framework for smart grids. The standardised interchange of data between power generating units, grid components and power consumers helps to cross digital 'language barriers' and to avoid the energy revolution and the expansion of the grid turning into mere tilting at windmills.

Dieter Rosenwirth is head of the certification body for grid compatibility at TÜV SÜD Industrie Service GmbH. Dr. Kai Strübbe is head of embedded systems at TÜV SÜD AG.

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