Computes 2nd derivatives of complex power flow w.r.t. voltage.

def d2Sbr_dV2(Cbr, Ybr, V, lam):
    """ Computes 2nd derivatives of complex power flow w.r.t. voltage.
    """
    nb = len(V)

    diaglam = spdiag(lam)
    diagV = spdiag(V)

    A = Ybr.H * diaglam * Cbr
    B = conj(diagV) * A * diagV
    D = spdiag(mul((A*V), conj(V)))
    E = spdiag(mul((A.T * conj(V)), V))
    F = B + B.T
    G = spdiag(div(matrix(1.0, (nb, 1)), abs(V)))

    Haa = F - D - E
    Hva = 1j * G * (B - B.T - D + E)
    Hav = Hva.T
    Hvv = G * F * G

    return Haa, Hav, Hva, Hvv