The presence of renewable power generation technologies increases the need for system flexibility due to their variable nature. The increasing share of variable renewables in European power systems create a downward adequacy problem, which deals with the ability of power systems to cope with periods of excess generation. The occurrence of negative prices on Central Western European electricity markets confirms the relevance of this issue, which is referred to as "incompressibility of power systems" and is assessed as a barrier for further renewable power integration. The objective of this article is to identify the main drivers of negative price periods in European balancing markets, by means of both an empirical and regression analysis. Results confirm a positive relation with the scheduled generation of renewables and inflexible base load, as well as a negative relation with the scheduled system load. Furthermore, the occurrence of negative prices is related to the positive and negative forecast error of renewable generation and demand, respectively. It is concluded that negative balancing market prices provide a market signal for investments in flexibility sources such as flexible generation, demand response, electricity storage, and interconnector capacity. (C) 2015 Elsevier Ltd. All rights reserved.