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Cost-effective grid stabilisation

Renewable energy is here to stay but there are technical challenges. Maren Gast and Hans-Peter Pircher from Phoenix Contact explain why.

Feeding renewable, and therefore fluctuating, energy into the power grid causes variations in supply voltage and electric load. The use of an energy storage system for control reserve, as implemented at the campus Garching (Technical University of Munich), can be a solution for stabilisation.

Approximately 30,000 wind turbine generators have been operating in Germany, supplying about 19 per cent of the total power with a power output of 56GW. Also, the number of photovoltaic systems has been increasing to 1.6 million over the past few years.

This source of renewable energy currently covers seven per cent of the energy needed in Germany. In total, renewable energies had a share of almost 39 per cent in 2017’s net power generation. Feeding in these heavily fluctuating energies from renewable sources results in increasing technical challenges for the German power grid. This is why the supply
of green power to the grid is already subject to political regulation.

Under specific circumstances, network operators may limit the power output of renewable energy systems or shut them down completely. With regard to efficiency, limiting the output of wind turbines, photovoltaic and other renewable energy systems is not necessarily the best solution for stabilising the networks. Large battery storage systems are more suitable for the compensation of voltage and load variations in the power grid. At the same time, energy storage systems for control reserve help limit the power distribution infrastructure to a necessary and acceptable extent.

With all this in mind, German energy storage specialist, Smart Power, deployed the first energy storage solution for control reserve in June 2017 in the area of the Bayernwerke distribution grid operator at the campus Garching of Munich’s Technical University.

The storage system, with a capacity of 1.2 MWh, analyses the grid frequency, automatically detects rising and falling feed-in power values in the grid and immediately initiates automatic adjustment. It is therefore an important solution for the stability of the power grid.

Centralised control by means of one control unit

For effective grid stabilisation, a variety of different parts of the PCR energy storage system need to work optimally together. The components include NEC energy storage devices, Bonfiglioli inverters and a control unit supplied by Phoenix Contact.

Another task is the implementation of connections and communication interfaces to the medium-voltage system, the direct sellers, and finally, to the grid connection point. Centralised control is realised through the control unit. It is a modular small-scale controller from Phoenix Contact’s Inline product family with digital and analogue inputs and outputs.

The modularity and flexibility of the hardware is reflected in the related software. Additionally, the company provides detailed pre-programmed function block libraries in accordance with IEC 61131, the use of which facilitates the communication between the individual system components. The control unit then performs the essential and complex operations that are detailed below.

The battery energy controller reacts within milliseconds

While traditional power plants need to be operated outside of their optimum operating range, battery storage systems for control reserve are able to react within a few milliseconds to variations in load and voltage. To this effect, the grid is balanced by means of a battery energy controller, i. e., a software running on the Inline controller.

The software block was developed by Smart Power on the basis of Phoenix Contact’s PC Worx engineering environment. Deviations from the mains frequency of 50 Hz must not be larger than 20 mHz. If, for instance, the frequency value is 49.8 Hz, energy is fed into the grid. However, if the mains frequency exceeds 50.2 Hz, the battery energy controller activates, and the surplus energy is stored in the battery.

Secure data transmission to the direct seller

Regarding the grid, the PCR energy storage system also serves as a decentralised energy generator when energy is being fed into the grid.

Therefore, the network parameters need to be recorded at the grid connection point by measuring devices and transmitted to the PGS controller in the central control unit. Then, the actuating values calculated from the different types of reactive power regulators are transmitted to the inverter of the system.

Another task is realising a data connection to the control system of the company in charge of direct marketing. For the PCR energy storage system at campus Garching, Entelios AG, an energy trading and services company, manages the distribution of control reserve. The data is exchanged between the control unit and the direct seller via a VPN network connection by means of a secure remote maintenance router, combined with a GPS antenna.

Furthermore, the data can be forwarded via Modbus/TCP or via the IEC 60870-5-101 remote control protocol. With that said, Phoenix Contact not only offers numerous industrial components to the operators, but also provides support in the individual design of each application.

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