  Electricity markets are experiencing a rapid increase in energy storage unit
participation. Unlike conventional generation resources, quantifying the
competitive operation and identifying if a storage unit is exercising market
power is challenging, particularly in the context of multi-interval bidding
strategies. We present a framework to differentiate strategic capacity
withholding behaviors attributed to market power from inherent competitive
bidding in storage unit strategies. Our framework evaluates the profitability
of strategic storage unit participation, analyzing bidding behaviors as both
price takers and price makers using a self-scheduling model, and investigates
how they leverage market inefficiencies. Specifically, we propose a price
sensitivity model derived from the linear supply function equilibrium model to
examine the price-anticipating bidding strategy, effectively capturing the
influence of market power. We introduce a sufficient ex-post analysis for
market operators to identify potential exploitative behaviors by monitoring
instances of withholding within the bidding profiles, ensuring market
resilience and competitiveness. We discuss and verify applicability of the
proposed framework to realistic settings. Our analysis substantiates commonly
observed economic bidding behaviors of storage units. Furthermore, it
demonstrates that significant price volatility offers considerable profit
opportunities not only for participants possessing market power but also for
typical strategic profit seekers.
