Overview of the distribution of energy generation, annual energy yields and installed capacity and full-load hours
© Fraunhofer-Institut für Windenergie und Energiesystemtechnik IWES, Kassel
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Local voltage maintenance ensures stable grid

Within a given grid level, the voltage may fluctuate by a maximum of ten per cent. If it exceeds that level, insulators and components can suffer damages. The risk of short circuits increases as well. If the voltage drops too far, the system can become unstable and collapse. So an important task of grid operators is to maintain the voltage level. In contrast to frequency stability, voltage maintenance measures are carried out locally. This requires reactive power. While it creates a current flow, it cannot perform work. By selectively adding positive or negative reactive power, grid operators can affect the voltage.

The voltages of the individual grid nodes also influence each other. That means: for all voltage maintenance measures, adjacent nodes need to be considered in the calculations. As part of the project, the developers designed a proportional control scheme, with which it is possible to limit the voltage within the specified voltage range at all times. Only some feed-in nodes exhibited a voltage slightly in excess of the tolerance range. Overall, and as a result of the control scheme, the researchers certified that the scenario was able to keep the voltage range within the limits even when using renewable energy sources, and regardless of the season. The planned HVDC transmission lines can also help lower high reactive power during transmission.

Supply restoration from the bottom up

Currently, grid operators restore voltage after a power failure by starting at the highest voltage level and finishing at the lowest voltage level. That means: first, large power plants ensure a stable grid at the 400-kV level before the next lower grid levels are reconnected. This method will not work with a supply coming from many generators in the low and medium voltage levels. This is why the engineers looked into supply restoration from the bottom up. This is how it works: after a power failure, a number of isolated grids form without a live high-voltage grid level. Once the isolated grids are stable, they can be connected. They then connect to the transmission system.

To test this, the developers implemented an experimental setup. It consisted of a typical distribution network with a biogas plant, a wind turbine and controllable and non-controllable loads. At the beginning of the test, the plant was disconnected from the grid and gradually, the engineers were able to activate the individual elements – until the grid was finally stable and able to synchronise with the interconnected power system. The conclusion: supply restoration from the bottom up and voltage maintenance with renewable energy sources are both possible.

Storage technologies for periods without sunshine and wind

But what if there is neither wind nor sunshine? Storage technology is the answer. In addition, permanently available systems such as biomass power plants are utilised in emergency cases. Surplus energy must be stored to ensure a stable supply even under continued bad weather. The researchers focus mainly on the so-called power-to-gas process. Excess energy splits water into hydrogen and oxygen. Hydrogen can be methanised in a chemical reaction, and fed into the natural-gas grid. Under bad weather, gas-fired power plants then reconvert it.

Effective grid expansion can help support renewable energies. The necessary backbone can be created in conjunction with new energy storage systems. The researchers assume that large storage systems will be technically mature and economically feasible by 2050. In the simulation, every third photovoltaic system on a building is equipped with a battery. The already successfully employed pumped storage power plants support the new energy storage systems.

Innovations still required by 2050

The researchers see a further need for development, amongst others, in the share of grid-forming converters, voltage maintenance measures from subordinate grid levels in combination with the transmission system, in investigations into short-circuit current and further grid stability simulations. They also consider the continued coupling of power supply, transport and heating networks as key to the future energy supply. In addition, the existing electricity grid must be expanded and optimised. Ongoing research projects on these issues are being carried out, amongst others, by the Energy Storage Research Initiative and the Future-proof Power Grids initiative.

Projektinfo 06/2015:
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