  Historically, with mostly radial power distribution and one-way power flow,
it was only necessary to evaluate the envelope of design conditions, e.g., peak
loads or fault currents, rather than continually observe the operating state.
But the growth of distributed energy resources introduces variability,
uncertainty, and opportunities to recruit diverse resources for grid services.
This chapter addresses how the direct measurement of voltage phase angle might
enable new strategies for managing distribution networks with diverse, active
components.
