Controllable grid feed-in – nothing works without flexibilityProject example of PV systems with adaptive power plant controllers in Southern Germany
Given the large number of grid operators who are responsible for the reliable supply of electric energy in the German national power grid, achieving a controlled feed into a medium- or high-voltage grid for power generating systems is a considerable challenge. Power plants have to do their part for the security and stability of the electricity grid, but the question of HOW they do it is far from uniform amongst the different grid operators. For photovoltaic systems in particular, a twofold task therefore arises with regard to controlling power plants. They not only have to respond to our rather unpredictable and completely uncontrollable primary source of energy - solar radiation, but because of their decentralised placement in the grid they must also be able to flexibly adjust to the specific power supply requirements and data exchange interfaces required by the local grid operator in charge.
Examples: PV Power Plants in 90579 Kirchfembach and 92263 Ebermannsdorf, Southern Germany, Nuremberg-based Solarpower project-invest GmbH & Co. KG planned, and in 2012 constructed, two PV power stations which are only 110 kilometres apart. With a nominal DC output of 3.2 MWp (Figure 2), the Kirchfembach-based power plant is located approximately 30 kilometres north-west of Nuremberg and lies within the N-ERGIE Netz GmbH distribution grid area. The Ebermannsdorf-based power plant has a capacity of 1.86 MWp and is located about 80 km east of Nuremberg, within an area controlled by E.ON Bayern AG. Both power plants feed into the 20 kV grid.
In these two plants, even for the fairly simple function of active power curtailment - today a standard grid stabilization requirement - the installed skycontrol Pro power plant controller (Figure 1)from skytron® energy GmbH must be able to respond differently. In one system, the grid operator uses a digital signal to dictate the limit of the active power in four fixed steps of 100% / 60% / 30% / 0%, while in the other the active power must be variable between 0% and 100% and the request comes by way of an IEC protocol from the grid operator’s remote plant control equipment. In this case, the controller also reports the actual P-value back to the power company for confirmation. The feed-in capacity at any moment is continuously stored and reported to the grid operator for the purpose of planning the grid management. Reactive power control in both plants is achieved via signals between the grid operator’s remote plant control equipment and the skycontrol Pro power plant controller. Depending on the signal, either static cosϕ values are set, or a characteristic cosϕ(U) curve is given which changes as a function of grid voltage.
These two PV systems nicely demonstrate how a superior power plant controller adapts to specific grid connection requirements and can be used universally – regardless of the type and model of inverters that are installed – to provide the flexible and efficient answer to the diverse German power grid landscape.