Renewables and grid stability – a contradiction?skycontrol power plant controller
Much is reported about the supposedly devastating impact of renewables on grid stability. Are renewables and grid stability actually a contradiction in terms? Today, the contribution of PV systems to grid security management is standard. Moreover, quite often, fully-fledged PV power plants with a capacity of more than 50 MWp feed directly even into high-voltage grids. To control these plants is quite a challenge, especially since PV power plants of this size often consist of generators of different topologies and are typically equipped with components, such as inverters, from different manufacturers, exhibiting dissimilar control behaviour. Such heterogeneous architecture is no stumbling block for the sophisticated skycontrol power plant controller from skytron® energy. Its underlying closed-loop control (Figure 1) incorporates the entire power plant as a process that needs to be controlled. The current grid conditions are monitored by constantly measuring the actual feed-in parameters at the grid connection pointand correcting the PV system accordingly by a comparison with setpoint values. Spurious control responses of different inverters, and losses caused by long cable runs and passive components are thus compensated by the control process (Figure 2).
Controlled feed into the high-voltage grid
The special challenges are illustrated by a 34 MWac PV power plant built by GP JOULE, a German EPC company responsible not only for construction, but the entire project. The plant was connected to the French high-voltage grid in March 2012. During the day, in normal power generating mode, the Q(U) controlled consumption and supply of reactive power actively help to stabilize the grid voltage.. At night, however, the inverters do not generate any power and the plant’s reactive power output is regulated down to Q = 0 Mvar. In order to cope with this demanding night mode, the functional range of the Refusol 630 kW inverters installed in the plant was expanded.
Reactive power compensation and frequency controlThe long distances between the four power plant blocks, and from this cluster of power plants to the common grid connection point, proved to be particularly challenging. These long cable runs required a compensation of the reactive power load. This and the response to changing grid frequencies are also handled by the skycontrol power plant controller. Frequencies exceeding 50.5 Hz require power production to be reduced in a controlled manner and at a critical frequency of 52 Hz the plant must be completely shut down. The power plant automatically starts up again as soon as the grid frequency recovers and settles back to normal operating values..
Grid stabilization potential of Renewables
This example refutes the claim that the ever growing feed of photovoltaically generated power threatens the stability of our electricity grids. On the contrary, as the trend points towards distributed power generation combined with full controllability, photovoltaic power plants offer the prospect of stabilizing today’s power grid and putting the often heated discussion about the need for costly grid expansion into perspective.