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.

def _getBusVoltageLambdaSensor(self):
        """ 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. """
        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