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«Project management: Deutsche Energie-Agentur GmbH (dena) – German Energy Agency Project partners: 50Hertz Transmission GmbH, ABB AG, Amprion GmbH, ...»

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With regard to the security and reliability of the electricity system, the stability and the level of the grid voltage must be guaranteed both in normal operation and in the event of failures. At the same time, to cope with voltage drops in the event of major failures, sufficient short circuit power must also be provided, among other things. In addition to this, stable system properties in normal operation and during failures also depend on suitable coordination of the voltage controllers in the electricity grid.

4.1 Provision of reactive power for static voltage control.

For stable grid operation and to protect people, operating equipment and end consumer devices, the voltage is kept in the permitted voltage range of +/- 10 percent of the nominal voltage at the end consumer via a variety of means. That is currently largely implemented via grid planning and operationally via provision of reactive power by conventional power plants and intentional stepping of transformers. In addition to this, reactive power compensators and voltage controllers are used in the electricity grid. Some of the redispatch measures in the transmission grid are taken for voltage control reasons.

Development of the demand until 2030.

Due to the increasing transport distances and international power transit, the demand for reactive power in the transmission grid will increase significantly by 2030. The reactive power range to be provided at the extra high voltage level, i.e. the range of reactive power demand at the respective grid nodes at different times, will increase overall.

With the increasing fluctuating feed-in of renewable energy and the increasing use of underground cables, the demand for regulating reactive power and thus voltage is growing to prevent violations of the permitted voltage range and restrict the grid expansion requirements.

Page 12 of 21 dena Ancillary Services Study 2030: Summary of the results of the project steering group.

Alternative provision of reactive power.

By 2030, an increasing number of alternative solutions to the current provision of reactive power in the

power supply system via conventional power plants will be required. From a current perspective, the following alternatives are technical options to meet the demand for reactive power in the electricity grid:

Installation of additional reactive power compensators (inductors, capacitors, SVCs and STATCOM4)  The inverter stations of the planned high voltage direct current (HVDC) transmission lines  Reactive power provision from decentralised generation systems in the electricity distribution grids  Modification of disused power plants for phase shift operation, equipping new power plants for decou pled phase shift operation and building standalone phase shifters In addition to this, voltage problems can also be solved via redispatch, i.e. starting individual power plants which then regulate voltage, if the demand only arises in individual hours and corresponding generation systems are available.

The analyses show that in the studied sample electricity distribution grids, targeted controllable provision of reactive power from wind turbine and photovoltaic system inverters, which is possible irrespective of the active power feed-in, technically permits reactive power-neutral operation at all distribution grid levels. This reduces the strain on the transmission grid, which was previously used to exchange reactive power in the distribution grid. The available potential of the provision of reactive power by renewable energy sources in the high voltage grid (110 kV level) in 2030 can also be used to meet the reactive power demand of subordinate grid levels, in addition to its own demand. In addition to this, the grid regions examined in the study have the potential to provide reactive power from the high voltage grid for the superordinate extra high voltage grid.

However, results from studying individual grids cannot be extrapolated to indicate the availability and total calculated potential to provide reactive power. This must be calculated in individual cases for specific grid nodes taking the grid topology and the connected generation systems into account. A major influencing factor on the ability to provide reactive power from the electricity distribution grids for the transmission grid is the position of the grid connection points of the renewable electricity generation systems. The nearer these systems are to the transformer, the better they can be used to provide reactive power for the superordinate grid level, and the loads on the grids are reduced. For decentralised provision of reactive power, the incorporation of the tap changers from transformers to the upstream grid must be taken into account in the control concept.

Economic viability of alternative provision of voltage control From an economic perspective, the converter stations of the planned HVDC lines are initially to be used to provide reactive power in the transmission grid. A redispatch can be implemented at grid nodes with remaining reactive power demand in individual hours only, if conventional power plants are available. For more frequent reactive power demand at grid nodes or if there is no local availability of suitable power SVC = Static Var Compensator, STATCOM = Static Synchronous Compensator.

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plants for redispatch, the most economical and currently established technology available is to build reactive power compensators.

Further provision of reactive power for the extra high voltage grid via decentralised energy units from the distribution grid can be an alternative to building reactive power compensators. For this, the corresponding requirements must be available locally (presence of sufficient capacity from decentralised energy units, grid topology and capacities). In addition, an evaluation is required in individual cases as to whether the provision of reactive power via decentralised energy units is more economical than building and operating a compensation system. The study sees considerable potential for decentral provision of reactive power which also appears economically viable. This applies in particular for large ground-mounted solar power plants and wind farms. Taking the above mentioned requirements into consideration, continuous provision of minimum generation from conventional power plants is not necessary to meet the demand for reactive power.

Recommended actions.

With the increasing shift of electricity generation to the distribution grids 5, optimisation of voltage control by providing reactive power from decentralised generation systems at all distribution grid levels must be assessed under technological and economic aspects. Where it makes sense, a reduced transfer of reactive power between the grid levels should be the goal, to relieve the strain on the overlying grid levels and in particular the electricity transmission grid. The resulting costs must be economically acceptable both for the system operators and the operators of decentralised energy units.

The provision of reactive power via the inverter stations of the planned HVDC lines, should become a fixed component of coordination of voltage control in the transmission grid.

When planning the grid, the option of providing reactive power from the high voltage grid for the extra high voltage grid from RE systems is to be assessed as an alternative to building new reactive power compensators.

4.2 Provision of short circuit power for dynamic voltage control.

The provision of sufficient short circuit power is necessary to guarantee secure response to short circuit events by the corresponding protective devices. Further, short circuit power is needed to guarantee the transient stability of electric machines and to restrict the voltage drop to an area as small as possible if a failure does occur. However, the short circuit power may not be excessively high, as otherwise operating equipment could be damaged due to excessive short circuit currents and power switches may not be able to securely deactivate the high short circuit currents in the event of a failure.

In 2012, 96 percent of all electricity generation systems based on renewable energy were installed in the electricity distribution grids.

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Development of the availability between now and 2030.

The analyses of the short circuit power available in future show that by 2030, the range between the minimum and maximum short circuit power will hardly change from today’s levels, taking the assumptions made into account. However, significant changes can be observed at individual grid nodes compared with today. The procurement of short circuit power from other countries will not increase significantly overall in spite of the changes in the power supply between now and 2030. However, the countries of origin could change.

Systems connected via inverters contribute to the short circuit power to the amount of their operating current. The short circuit power available in 2030 is therefore subject to major weather and timedependent fluctuations. In individual cases, a review is required to assess whether the protection concept permits this bandwidth.

Provision of short circuit power can be homogenised regionally with renewable energy plants, by enabling the provision of short circuit power from inverters even without active power feed-in. This results in a decoupling from the weather and time-dependent availability of systems for short circuit power.

Recommended actions.

With regard to the regulatory framework of grid operation, conditions must be created to allow system operators to claim short circuit power from the renewable energy electricity generation plants even in times when there is no active power feed-in.

In addition to this, an analysis is also required of the effects the short circuit power changes have on the existing protection concepts and other operating aspects of the system operators.

5 System restoration in 2030.

According to the current regulations, in the event of a full or widespread power failure in the European integrated grid, the system restoration is implemented using a central concept by starting large-scale power plants with black start capabilities in the transmission grid. At the start of the grid re-establishment process, temporary standalone grids are established around these power plants with black start capabilities. Large-scale hydroelectric power plants (especially pumped-storage) and gas turbines are current examples of black start capable power plants, which can be started with batteries or emergency power systems even in the event of a blackout. Loads are added while connecting further generation capacity. Building on that, the standalone grids are gradually synchronised and connected as part of the reestablishment.

Development of the demand until 2030.

In order to guarantee the supply security, a sufficiently dimensioned secure generation capacity will still be required. In accordance with the scenario framework for the 2013 Network Development Plan, there

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will still be sufficient pumped-storage and gas power plants to implement the current concept of central system restoration also in 2030.6 Before connecting further grid areas during the re-establishment of power supply, the extent to which electricity consumption or electricity generation are being supplemented must be known. Therefore, the weather situation and other generation-relevant forecasts must be incorporated in the power supply reestablishment concept. In addition to this, for a controlled re-establishment of the grid, the communication technology option of intentionally throttling the electricity generation from decentralised generation systems is necessary to avoid difficult to predict load changes when reconnecting grid lines or afterwards.

Alternative concepts for re-establishing the power supply.

As an alternative, the present study examines the options of decentralised concepts for re-establishing the grid. Following this concept, in the event of a widespread failure of the European integrated grid, individual electricity distribution grids autonomously permit the supply of the loads based on local generation.

After elimination of the cause of the failure, the individual standalone grids are then connected and synchronised with each other in order to form the integrated grid again. As re-establishment of the grid is only needed extremely rarely, implementing a decentralised concept would mean to implement a highly complex and cost-intensive system is implemented in this case, which is only used in very rare cases. That is why a decentralised grid re-establishment concept is inefficient from a macroeconomic perspective.

Recommended actions.

Due to the extreme technical complexity and the associated investment costs, decentralised system restoration is not recommended for the future. Where corresponding options are planned by initiatives of industrial grids or individual municipal utility companies for standalone operation in subordinate grid levels, they should be incorporated in the superordinate supply re-establishment concept. Corresponding technical regulations must be issued for this.

In order to guarantee the supply security, it must be ensured that a sufficient amount of secured power plant capacity continues to be available in Germany. Centralised grid re-establishment should be implemented on the basis of these power plants, some of which need to be black start capable.

Technical solutions must be available to transmission and distribution system operators to control or limit decentralised energy systems after a grid collapse for a controlled re-establishment of the grid, even if public communication networks are not available at the time. Alternatively, the grid connection codes must ensure that generation units’ responses after a blackout are suitable for a controlled system restoration.

Note that the conventional power stations listed in the 2013 Network Development Plan are based on an exogenous assumption.

Against the background of the decreasing economic viability of conventional power stations in today’s electricity system, and the present uncertainty on the future energy law conditions, it is impossible to forecast the conventional power station capacities actually available in 2030.

Page 16 of 21 dena Ancillary Services Study 2030: Summary of the results of the project steering group.

6 System control in 2030.

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