def update_gen_constrains(gen_constraints_user):
    """Generator constraint passed by user
    
    Parameters
    ----------
    gen_constraints_user : dict
        Valid keys p_max_pu, p_min_pu
    
    Returns
    -------
    dict
        Updated gen dict constratins
    """
    gen_constraints = {'p_max_pu': None, 'p_min_pu': None}
    gen_constraints.update(gen_constraints_user)
    return gen_constraints
