  Providing frequency control services with flexible assets connected to the
low-voltage distribution grid, amongst which residential battery storage or
electrical hot water boilers, can lead to congestion problems and voltage
issues in the distribution grid. In order to mitigate these problems, a new
regulation has been put in place in Belgium, imposing a specific constraint: in
any circle with a radius of 100m, there can be at maximum 10 connection points
providing frequency control at any time.
  This paper presents an impact analysis and a coordination strategy of a
Flexibility Service Provider (FSP) that operates a pool of assets and is
exposed to this new regulatory constraint. Results show that at 5%
participation, only 90% of total control capacity can be used, with a large
difference between neighbourhoods with different population densities.
  A distributed optimization framework to coordinate the assets arises
naturally, in which the assets are able to keep their local cost functions
private and only have to communicate with neighbouring assets that are
geographically close, and with the FSP. Analysis of the proposed distributed
optimization algorithm shows a clear trade-off between optimality gap, owing to
the mixed-integer nature of the problem, and iterations to convergence.
