Nonstationary models for PQ monitoring in naval electrical systems

In many fields of electrical applications, the energy management process, is always more becoming one of the main process should be controlled and monitored. Itpsilas necessary to reach the optimum and, in the same time, the cheaper system working conditions. Particularly in the field of naval electrical system, where there are many environmental factors that stress, grater than other ones, the subcomponents of system itself, it\´s necessary to perform the predictive maintenance plan and avoid many failure modes owned to that equipments sensitive to harmonics. Guarantee for special performance of power quality demanded by the customers before the ships are build make calculations and simulations necessary, also if to reach the operative optimum conditions it is necessary compare results of simulations with measurements on board of the ships. Particularly, the problem of harmonic power quality, which has increased lately on the continental grids, is expected to deteriorate on shipboard installations, too, after the advent of electric propulsion and other All Electric Ship (AES) schemes where the harmonic pollutant power electronic devices dominate. During the last years there has been deterioration to the problem of the harmonic distortion, observed in voltage and current, in the electrical plants of modern ships. The problem is expected to deteriorate in the future constructions due to the extensive use of devices with power electronics and the perspective of implementing electric propulsion utilizing power pollutant converters feeding innovative A.C. motors as implied by All Electric Ship (AES) concept. The electric system of any ship can be regarded as a non-interconnected one, comprising a reduced number of generators and an increased number of nonlinear loads of low or high voltage, while in the case of AES, the electric power used for propulsion is significantly larger than the one used for the service loads. These considerable amounts of electric power require- - d have lead to the application of system voltage in the range 1 kV to 11 kV, therefore the electrified ship installations according to naval terminology drop in the category of "High Voltage" systems, although strictly speaking they comprise rather "Medium Voltage" ones. In this contest it becomes necessary for the optimal management of the naval electrical plants make use of "on- field" measurement systemsfor the real time monitoring of the state of the plants themselves. One of the power quality (PQ) indexes should be investigated is the total harmonic distortion THD. On big ships (for example tourist ships) to realize an efficaciousness PQ control system it\´s necessary to make use of a good data acquisition system and contemporary to have good different actuators (for example the equipments for electrical charges insertion and posting) able to start immediately if measured data reveal a emergency or however non optimal condition. So measure data drive the restore actions on the electrical systems conduction and under this fact measured data should be affected by minimum error. But for its nature the naval electrical system is characterized by a nonstationary process governing its PQ indexes, particularly THD, in fact the different phases of the ship workcicle, navigation, manouvre and port stop, determine according to their different electrical configurations, different effects on the PQ indexes. The "functional block" between the "data acquisition- block" and the "restore-block" should be constituted by a "data analysis block". The first step of this block is a control chart to verify the PQ index trend (for example the trend of the THD). For the dealt case some difficulties can occur in the operation of traditional control charts. A principal reason for this is that the data coming from the PQ process monitoring do not vary about a fixed mean, so in this paper it\´ll be shown how by using a nonstationary model a continuously updated local mean level is pro