code: def _getBusVoltageLambdaSensor(self):
          
          muVmin = array([b.mu_vmin for b in self.market.case.connected_buses])
          muVmax = array([b.mu_vmax for b in self.market.case.connected_buses])
          muVmin = -1.0 * muVmin
          diff = muVmin + muVmax
          return diff
docs: Returns an array of length nb where each value is the sum of the
          Lagrangian multipliers on the upper and the negative of the Lagrangian
          multipliers on the lower voltage limits.