Introducing a classification method of Voltage dips in ship electric energy systems

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1 Journal of Marine Engineering & Technology ISSN: (Print) (Online) Journal homepage: Introducing a classification method of Voltage dips in ship electric energy systems J Prousalidis & E Styvaktakis To cite this article: J Prousalidis & E Styvaktakis (2008) Introducing a classification method of Voltage dips in ship electric energy systems, Journal of Marine Engineering & Technology, 7:1, 1-10 To link to this article: Published online: 01 Dec Submit your article to this journal Article views: 264 View related articles Full Terms & Conditions of access and use can be found at

2 Introducing a classification method of Voltage dips in ship electric energy systems J Prousalidis and E Styvaktakis, National Technical University of Athens, School of Naval Architecture and Marine Engineering, Division of Marine Engineering This paper presents an introduction to the analysis and classification of voltage dips, in a ship electric power system. The method followed covers the variety of dips occurring on shipboard installations in its entity. Thus although it has been primarily developed only for dips due to short circuit faults, it is shown that it can include the cases of sags due to motor starting, transformer energizing inrush or even pulsed loads. This method focuses on emerging and characterising the severeness of the event depending on the consequences on the operating equipment. Therefore, besides an implement of assessing the significance of each situation it can be easily assimilated in system power quality monitoring units, in protection scheme design and evaluation processes or even in event identification tools. Where simulations are required, the software package PSCAD is used to simulate a number of case studies in the power system of actual vessels with electric propulsion. AUTHORS BIOGRAPHIES Dr J Prousalidis (Electrical Engineer from NTUA/1991, PhD from NTUA/1997) is Assistant Professor at the School of Naval and Marine Engineering of National Technical University of Athens, dealing with electric energy systems and electric propulsion schemes on shipboard installations. Dr E Styvaktakis (Electrical Engineer from NTUA/1995, MSc in Electrical Engineering from UMIST/1996, PhD from Chalmers/2002) is currently with the Hellenic Transmission System Operator dealing with power system studies. NOMENCLATURE E : pre-fault voltage f max : maximum value of frequency waveform at modulation f max : minimum value of frequency waveform at modulation f n : nominal system frequency V dip : voltage dip magnitude V max : maximum value of voltage waveform at modulation V min : minimum value of voltage waveform at modulation V n : nominal system voltage Z F : impedance between the power grid and the fault (including any fault impedance) Z S : source impedance f: fluctuation of frequency at modulation V: fluctuation of voltage at modulation INTRODUCTION Electric power plant onboard has always been a rather complicated power system, comprising subsystems of several operation frequency and voltage levels with increased number of motors nearby limited number of generators, especially in sophisticated structures with electric propulsion. The aforementioned complicacy is expected to worsen further still in the All Electric Ship (AES) buildings 1-5 with full electric propulsion and extended electrification of all shipboard installations. On the other hand, similar to continental grids several steady- and transient-state phenomena emerge, particularly No. A Journal of Marine Engineering and Technology 1 Article number = K010

3 those concerning power quality problems, and their consequences have to be thoroughly studied, analyzed and investigated. The scope of the paper is to study and classify voltage dips (or sags), due to several causes in a ship electric power system. In continental grids, classification methods for the abovementioned problems have already been created and further developed 6. The most commonly used is the ABC classification method while the most theoretical approach is done by the Symmetrical Component Classification method. However, they can not be easily nor directly applied to ship electric systems due to their special network configuration, as described in 6, where a classification method for fault induced voltage dips, particularly developed for ship systems, has been presented. Thus, in order to identify the nature of voltage dips onboard, a large number of short circuit simulations have been performed for all fault types at all the three main operation profiles of a vessel 7-8. This method takes into account the particularities of ship electric energy systems with emphasis on making the information easy to assimilate. The paper also includes case studies of voltage dips due to: motor starting transformer energizing pulse load operation. It is shown, that all cases can be categorized in terms of voltage quality and more specifically according to: symmetry among the three AC phases rms magnitude waveshape. While some extra features characterizing certain voltage dip types are also highlighted and discussed. The electric grid of a car-passenger ferry with electric pod propulsion as well as bow thrusters driven by electric motors, has been used for the case studies considered. Furthermore, for the simulation of the complete power system, the power system simulation software PSCAD has been exploited PARTICULARITIES OF SHIPBOARD INSTALLATION The electric power grid of a ship can be regarded as a small scale, autonomous, industrial type compact power system, although several differences between a conventional continental grid and a shipboard installation can be enumerated as follows: The nature of a ship electric energy system is highly hybrid comprising DC and AC subsystems operating at various voltage and frequency levels Referring to prime movers, their relative rotational inertia with respect to electric load demand is fairly small. However, the operating frequency of 50 or 60Hz is controlled via fast regulators, as the time constants of the prime movers (often 4-stroke diesel engines) are small The total power installed per volume unit is large especially in the electric propulsion applications. The installed power system can be of 40-80MVA in an area of few dozen m 2. The largest amount of energy is demanded by electric motors (acting either as main propulsion or as drivers of auxiliary engines). Furthermore, besides rotating electric motors comprising dynamic loads, there is a significant amount of non-linear loads with non-conventional behavior e.g. pulsed loads of weapon or navigation systems There is no significant power transmission-distribution cabling system as the electric power grid is composed of cables of small length ( m) There is limited number of transformers installed Adopting insulated neutral, i.e. unearthed systems is a common practice. Generators, motors and distribution transformer windings are either ungrounded wye- or delta- connected, whereas the great majority of continental grids are grounded, either directly, or via resistive elements The power system is completely autonomous, thus its reliability is of high priority. The only available back up power is the emergency generator supplying only few loads critical for the vessel survivability A considerable number or electronic devices installed onboard (automation systems, controllers, navigation systems) are sensitive to power quality and EMI problems provoked, in particular, by the extensive use of power electronics. Hence, the power quality problems are of extreme importance and have to be analyzed thoroughly Referring to short-circuit calculations, as IEC pinpoints, generators can not but be always considered nearby the faults, resulting in fairly high fault currents. Consequently, the voltage sags due to short-circuit faults are expected to be more severe. Power quality problems onboard are of different significance compared to the same problems that occur on a continental power grid. For inland, power quality problems, apart from the fact that they result in a problematic production process, it is possible to have a significant impact on the pricing relations (tariffs applied and penalties) between the utility and its clients. This is meaningless onboard, where the most important issue is the continuous operation of the system and its redundancy. A possible malfunction in a critical load may lead to a total loss of the whole vessel, resulting in possible human casualties and environmental pollution. INTRODUCING POWER QUALITY CLASSIFICATION ON SHIPBOARD GRIDS Fault-induced voltage dips Faults (due to the short circuit current) lead to voltage dips of a magnitude that depends on the impedance of the source 2 Journal of Marine Engineering and Technology No. A

4 (system s strength), system configuration, fault impedance, fault type and distance to the fault. Fault-induced events present the most severe characteristics. Their duration depends mainly on the protection system operation. That varies from half-cycle (fuse operation) to several cycles (operation of circuit breakers). Advanced classification methods have been proposed that consider all three phases, motivated by the fact that the relation between the phases is important for the equipment performance 3 or by the fact that valuable information can be extracted considering all three phases 5. The calculation of the dip magnitude for a fault somewhere within a radial distribution system requires the point of common coupling (PCC) between the fault and the load to be found. The dip magnitude (%) at the load position equals the voltage at the PCC (neglecting all load currents): V dip ¼ Z F Z F þ Z S E (1) where Z S is the source impedance at the PCC, Z F is the impedance between the PCC and the fault (including any fault impedance), while E the corresponding pre-fault voltage. Motors that experience a voltage drop will temporary operate as generators supporting voltage. This shows up in the voltage recording as a slow decay in voltage magnitude, during the fault. After fault clearing, motors re-accelerate delaying the full recovery of voltage and creating a postfault dip as described in 3 that could cause motor stalling. The electrical power system of a car-passenger ferry with electric pod propulsion and bow thrusters, (see Fig 1) has been simulated in order to investigate the characteristics of fault-induced voltage dips in naval systems. This system consists of four generators and three different voltage levels (6kV, 4kV and 0.38kV). Induction motor load is a significant part of the available generation. Faults in the lower voltage level effect mainly loads fed by the same busbar as the resulting voltage dips do not propagate upwards in the system (the impedance of the transformer is large compared to the source impedance in medium voltage). Fig 2 shows the voltage dip in the generators terminals caused by a 3-phase fault in the 4kV voltage level. It can be Fig 2: Voltage dip due to a 3-phase fault seen that voltage drops to approximately 0.4pu (identical for all three phases) although voltage in the 4kV system becomes zero at the fault point. The influence of the motor load can be seen both during the fault and after fault clearing. Fault clearing causes a fast voltage increase to 0.80pu and then the voltage increases gradually towards the normal voltage due to the motor load influence. Voltage dips propagate in the system and their characteristics change as they propagate through transformers. Fig 3 shows the voltage dip in the generators terminals caused by a 2-phase fault in the 4kV voltage level. The delta-star transformer that connects the two voltage levels transforms the relationship of the voltage drop in the three phases. A characterization method has been proposed in 3 for voltage dips due to faults that takes into account the different transformer connections. An extensive analysis of voltage dips in naval systems, based on this classification, has been presented by the first author in 6. Considering Equation 1, it should be mentioned that the magnitude of the drop depends on the available source impedance. Therefore, the number of generators in operation affects significantly the magnitude of the voltage dip experienced by the loads. Fig 1: Simulated Ship Power System Fig 3: Voltage dip due to a 2-phase fault No. A Journal of Marine Engineering and Technology 3

5 Finally, single-phase faults consist a specific case of voltage dips in most ship power systems due to their ungrounded nature. In case of a faulted phase, the load is still supplied via the healthy ones which is the main reason of installing the ungrounded system onboard- but at a significantly higher voltage level for both phases, namely ˇ3 or 1.73p.u (see Fig 4). Therefore, the system suffers from a voltage swell rather than a dip, which stresses equipment insulation. Summarizing, voltage dips due to faults are: rectangular: voltage recovers fast after fault clearing operation symmetrical or asymmetrical: depending on the type of fault that caused them. CLASSIFICATION METHOD OF FAULT INDUCED VOLTAGE DIPS The Classification Method developed in 6, for the system of Fig 1, is summarized in Table 1. There are eight different categories introduced based on symmetry among phases and voltage rms magnitude in all three phases. This classification approach aims to provide the system operator with a practical tool to quickly form a clear picture of the system state. The limit value of 80% has been rather arbitrarily set as the minimum voltage level required for the affordable operation of most electric equipment (including both static and rotary, three- phase and single-phase loads), but is subjected to modification. Thus, common practice suggests that this threshold value could vary from 75% up to even 90%. It is worth noting that I0, I3 and IV could refer to more symmetrical operating conditions, although in I3 and IV the three phases can be within the limits but of different values. On the other hand, the other cases refer to highly unbalanced ones with at least one zero value - due to short circuit fault. Moreover, while I0 corresponds to balanced three-phase faults, category III mainly refers to single-phase fault conditions in the unearthed ship grounding system, which as mentioned leads to an overvoltage to the remaining healthy phases. Furthermore, as it can be observed, there is also an inherent hierarchy introduced in terms of how significant - in descending order - each situation is. Therefore the classified categories, can be written in the following order: I0. I1. I2. I3. II1. II2. III. IV (2) This method can be extended and include other types of voltage dips as explained in the following. Induction Motor Dips The start-up of an induction motor takes current five to six times larger than normal. This current remains high until the motor reaches its nominal speed; this lasts from several seconds to one minute. The characteristics of the corresponding voltage dip depend on the induction motor data (size, starting method, load, etc) and the strength of the system at the point the motor is connected. The magnitude of the dip depends strongly on the system parameters. The duration of the voltage dip due to motor starting depends on a number of motor parameters with the most important being the motor inertia 3. The duration of the dip is prolonged if other motor loads are connected to the same busbar, as they keep the voltage further down. Fig 5 shows a voltage dip caused by the starting of a medium voltage auxiliary propulsion motor of 1.4 MW by an 8MVA generator. It can be seen that voltage drops to 0.85pu and gradually recovers approximately 1 sec after. The voltage dip is the same for all three phases. Large power motors starting consecutively during ship maneuvering (e.g. driving thruster systems) can cause such severe voltage dips. Fig 4: Voltage dip (swell) due to a 1-phase fault Category Phase A (% Vn) Phase B (% Vn) Phase C (% Vn) I I , 80% I2 0.00, 80%, 80% I3, 80%, 80%, 80% II1, 80%, 80% 100% II2, 80% 100% 100% III %. 100% IV.80% and,100%.80% and,100% Table 1: Voltage dips of Classification Method.80% and,100% Fig 5: Voltage dip due to motor starting 4 Journal of Marine Engineering and Technology No. A

6 Summarizing, voltage dips due to induction motor starting are: non-rectangular: voltage recovers gradually symmetrical: all phases present the same behaviour. Consequently, considering the classification method presented above, the voltage dip due to motor starting can be a symmetrical category, i.e. either I3 or IV (symmetrical case I0, as already mentioned, with zero voltage in all threephases corresponds to faults rather than other cases). Transformer Saturation Dips When a transformer is energised under a no-load condition, the magnetizing current necessary to maintain the magnetic flux in the core is in general only a few percent of the nominal rated load current. During transformer energising, a transient occurs to change the flux in the core to the new steady state condition. In general this will cause the flux to go above the saturation value once each cycle until the average value of the flux over a cycle has decayed to nearly zero. This temporary over-fluxing of the transformer core causes high values of the magnetizing current, which is highly asymmetrical and decays exponentially. This phenomenon is known as magnetising inrush current and its magnitude depends on the point on the wave where the energization switching takes place and the core residual flux. As the core is forced into saturation the transformer draws a large current from the supplying network. When the voltage reverses its polarity in the next half cycle, the maximum flux in the core is less than the maximum flux density in the no-load situation. The transformer inrush current is therefore asymmetrical and contains a DC component, which might take seconds to disappear depending on the damping of the system 5. The voltage dip caused by the magnetising inrush current can be long in duration and drive other near-by transformers into saturation (sympathetic saturation 7,8 ). In general, any voltage change in the transformer terminals (like a voltage dip) could lead it into saturation due to the resulting transient in the core. Fig 6 presents a transformer saturation voltage dip caused by the energizing of a transformer in the network of Fig 1. A sharp voltage drop (approximately 0.2pu for the worst phase) is followed by a gradual recovery. As can be seen in Fig 7, the voltage presents temporary harmonic distortion. The Short Time Fourier Transform has been used for the estimation of the harmonics (from 2 nd to 5 th ) of the voltage of one of the phases of Fig 6. The 2 nd harmonic is contributing the most. This increased harmonic distortion can cause undesired protection operation. Summarising, voltage dips due to transformer saturation are: non-rectangular: voltage recovers gradually as the inrush current decays non-symmetrical: each phase presents a different degree of saturation rich in harmonics: due to the asymmetry of the inrush current. Fig 6: Voltage waveforms during transformer energizing (top) and corresponding voltage rms magnitude Fig 7: Voltage harmonics in time due to transformer saturation Consequently considering the classification method presented above, the voltage dip due to transformer energizing inrush can be any non-zero but also non-symmetrical category i.e. I3, II1, II2 or IV (in case I3 and IV). The harmonic distortion caused by transformer saturation can lead to more severe dips if any system resonance is excited. Fig 8 shows the voltage dip occurring when the same transformer as above is energized through a busbar where a 3 rd harmonic filter (single-tuned filter) is con- Fig 8: Voltage waveforms during transformer energizing (top) and corresponding voltage rms magnitude No. A Journal of Marine Engineering and Technology 5

7 nected. The 3 rd harmonic filter causes a parallel resonance in a frequency slightly lower than the 3 rd harmonic and the harmonics of the transformer inrush current are amplified leading to a more severe dip (0.1pu lower than before). Considering that the use of harmonic filters is extended in ship systems due to the multiple use of power converters this example shows that these filters must be disconnected during transformer energizing, a practice followed in many industrial power systems. Voltage dips due to Pulsed Loads The recently introduced concept of pulsed loads can be regarded as a particular voltage dip case. More specifically 10-11,20, a limited but increasing number of loads installed aboard tend to have an extremely high power (or current) demand for a very short time interval, followed by almost zero demands for longer intervals. This behaviour is repeated on a periodic or quasi-periodic basis. As representative examples the railguns, radars and the electromagnetic launchers (EMALS) can be referred. In Fig 9 a typical phenomenon caused by a pulsed load is shown. The aforementioned high valued current flowing through the system impedances provokes a significant voltage drop, in a manner similar to the one discussed in the previous sections. Furthermore, in this particular case, due to the finite ship electric energy generation capacity, the high power demand affects also system frequency, resulting thus in a frequency drop, too, (see Fig 9). The periodic fluctuation of voltage and/or frequency is often met in many shipboard standards (STANAG 1008, IEEE 45 and USA MIL-STD-1399) as modulation. Concerning quantifying voltage or frequency modulation the difference between maximum and minimum value is used as a percentage of the double of the nominal value as shown in Equation (3): V ¼ V max V min, f ¼ f max f min (3) 2V n 2f n Frequency and voltage modulation may affect the operation of several subsystems of a ship such as radarscopes, communication equipment, missile guidance systems, weapon systems, gear systems etc 3. Anyhow, by investigating several case studies, it can be elicited 20 that, voltage dips due to pulsed loads starting are: non-rectangular: voltage and frequency recover gradually symmetrical: all phases present the same behaviour periodical or quasi-periodical : the phenomenon is repeated on a rather constant time basis, hence, modulation in frequency and voltage waveforms is noticeable. Consequently considering the classification method presented above, the voltage dip due to pulsed loads can be any symmetrical category, i.e. either I3 or IV (symmetrical case I0, as already mentioned, with zero voltage in all three-phases corresponds to faults rather than any other case). Finally, it is highlighted that this kind of voltage dip is provoked by specific equipment type well located in the electric grid, while in all cases the pulsed load is installed along with a dedicated intermediate power supply unit (e.g. a flywheel) aiming at alleviating the adverse consequences of the entire phenomenon. EXPLOITATION OF DIP CLASSIFICATION METHOD The classification method discussed so far can be exploited among others as a handy implement for assessing the operation status of the ship system based on representative measurements of limited number. More specifically, it can be integrated into a Power Quality Monitoring (PQM) module which can be part of the Electric Power Control and Management System (EP- CAMS). In this way, all major electric quantities can be monitored and recorded leading to protection scheme evaluation, damage or even maintenance assessment. Thus, any event occurred as soon as identified, analysed and located can trigger possible scenarios of courses of actions. For instance, the fault induced voltage dips are the most severe ones as they lead to the most adverse consequences comprising, insulation failures, fuse blowing, equipment or network damages, etc, necessitating urgent maintenance actions with several economical implications. Fig 9: Voltage and frequency modulation 6 Journal of Marine Engineering and Technology No. A

8 The other types of voltage dips which are more equipment dependent, do not involve the maintenance and cost components to that extent referring more to abnormal operating conditions rather than possible destructions. Anyhow, the electric system operator can be assisted to assess the importance of the situation locating and identifying the problems due to the events monitored in conjunction with other monitoring systems, e.g. insulation monitoring systems which mostly help identifying single-phase faults. On identifying voltage dips In order to show how the voltage dip classification method presented can be further exploited e.g. by being integrated in a PSQ Monitoring sytems, Table 2 summarises the dip categories versus possible causes as presented in the previous sections. It can be seen that in most cases there more than causes resulting in the same dip type. However, as shown in Table 3, there are more than one ways of identifying the actual events via some extra features of theirs, e.g. the waveshape and the symmetry among phases. Furthermore, the features in italics are the ones that can be exploited in the easiest way as a means to identify the phenomenon causing the voltage dip. Hence, for instance, while transformer energization is mainly characterized by the transient 2 nd harmonic developed, still only the voltage waveshape in conjunction with the lack of balance among phases suffice for its identification. On the other hand, dips due to pulsed loads can be uniquely traced via the periodic nature of their associated modulation. Anyway, following this approach an expert system could identify the event type based only on measurements of phase voltages, as figuratively shown in Table 4. As already mentioned, 2 nd harmonic criterion is not really necessary as the combination of non-symmetrical non-rectangular voltage dip suffices for identifying transformer voltage dip. Furthermore, frequency modulation identification can be done fairly easy by a frequency monitoring Dip Category Phase A Phase B Phase C possible cause (% Vn) (% Vn) (% Vn) fault motor starting transformer pulsed load I Yes No No No I ,80% Yes No No No I2 0.00,80%,80% Yes No No No I3,80%,80%,80% Yes Yes Yes Yes II1,80%,80% 80%-100% Yes No Yes No II2,80% 80%-100% 80%-100% Yes No Yes No III %.100% Yes No No No IV 80%-100% 80%-100% 80%-100% Yes Yes Yes Yes Table 2: Dip categories versus possible causes event waveshape symmetry among 2nd harmonic Voltage-Frequency Modulation phases fault rectangular yes/no no motor starting non-rectangular yes no transformer energization non-rectangular no yes pulsed load non-rectangular yes yes Table 3: Characteristic features of dip provoking events Dip Category waveshape symmetry among Voltage-Frequency Event type recognised phases Modulation I0 Fault I1 Fault I2 Fault I3 Rectangular Fault I3 Non-Rectangular Yes Motor Start I3 Non-Rectangular No Transformer Energization I3 Yes Yes Pulsed Load II1 Rectangular Fault II1 Non-Rectangular Transformer Energization II2 Rectangular Fault II2 Non-Rectangular Transformer Energization III Fault IV Rectangular Fault IV Non-Rectangular Yes Motor Start IV Non-Rectangular No Transformer Energization IV Yes Yes Pulsed Load Table 4: Event type identification based on voltage dip classification No. A Journal of Marine Engineering and Technology 7

9 system always installed onboard- in the case of pulsed loads. On the other hand, the fault type can not be traced unless the system topology is well known and several shortcircuit conditions have been thoroughly studied. As shown in the following this thorough approach also assists in locating the fault incident. Thus, in the case-study of the carpassenger ferry of Fig 1, the results of the fault induced voltage dips analysis are presented in Table 5. More specifically, the results of voltage dip classification for all buses and all fault combinations are summarized. It is noted, that letters A, B, C stand for the corresponding phases and G stands for the ground, i.e. any ship hull metal part. Moreover, in operation at sea no thruster buses are taken into consideration, while in Harbor operation, no propulsion buses are considered at all. As can be deduced from Table 5, the voltage dip types occurred are more dependent on the network topology rather than the loading conditions according to the operation profile. This is mainly due to the wide range of values set at the categorization. Moreover, it can be deduced that the closer to the faulted bus a monitored point is, the severest voltage dip is subjected to. The most representative case is covered during manoeuvering where bow thrusters are also active. Anyhow, each fault situation is characterized and, hence can be identified in a sole unique way by the combination of category type per monitored bus. For example, the case of: Main Bus at IV Pod1 at IV Pod2 at IV Bow thruster 1 at I0 Bow Thruster 2 at IV Service Load at IV corresponds only to three-phase fault at the bus of Bow Thruster 1. Furthermore, in this way, besides the fault type, the faulted point is located also. On integrating the voltage dip classification method in monitoring systems For a shipboard system which is relatively small compared to terrestrial power systems, a small number of monitors are capable to provide the necessary information for identifying the type and the origin of an event by combining the measurements from different locations. Although there is a num- Main Bus Pod1 Pod2 Bow Thruster1 Bow Thruster2 Service Load At Sea Fault at AG III IV IV IV main bus AB I3 I2 I2 I2 ABG I1 I2 I2 I2 ABCG I0 I0 I0 I0 Fault at AG IV III IV IV Pod1 bus AB I3 I3 I3 I3 ABG I3 I1 I3 I3 ABCG I3 I0 I3 I3 Fault at AG IV IV IV III Service AB I3 I3 I3 I3 Load bus ABG I3 I3 I3 I1 ABCG I3 I0 I0 I0 Maneuvering Fault at AG III IV IV IV IV main bus AB I3 I2 I2 I0 I2 ABG I1 I2 I2 I0 I2 ABCG I0 I0 I0 I0 I0 Fault at AG IV III IV IV IV Pod1 bus AB I3 I3 I3 I3 I3 ABG I3 I1 I3 I3 I3 ABCG I3 I0 I3 I0 I3 Fault at AG IV IV IV III IV IV Bow AB IV IV IV I3 IV IV Thruster ABG IV IV IV I1 IV IV 1 bus ABCG IV IV IV I0 IV IV Fault at AG IV IV IV IV III Service AB I3 I3 I3 I1 I3 Load bus ABG I3 I3 I3 I1 II1 ABCG I3 I0 I0 I0 I0 Harbor Fault at AG III IV main bus AB I3 I2 ABG I1 I2 ABCG I0 I0 Fault at AG IV III service AB I2 I3 load bus ABG I2 I1 ABCG I0 I0 Table 5: Analysis of fault induced voltage dips in the case study of Fig 1 8 Journal of Marine Engineering and Technology No. A

10 ber of reasons for measuring current, in terms of the events presented in this paper, voltage measurements are adequate for analysis and identification purposes. This simplifies significantly the requirements for the equipment. In recent years, the developments in microprocessing and communications led to monitoring systems of low cost. Fig 10 presents a monitoring system for voltage dip events for a shipboard power system. It consists of monitoring equipment, voltage transformers (if not already available for protection purposes), communication links for transferring the monitoring data and a central process unit that will accommodate and process the data. Data mining software (for example an expert system) can be used to combine the data in order to provide decision support information (type of event, origin, impact of the event etc) for the system operator. As the cost of storing the recordings is low (the cost of data storage technology presents a decreasing trend), the utilization of such a system will create a database of recordings-measurements that can be exploited in many ways. Firstly, the recordings can be used to evaluate and tune the available models of the power system components used for simulation purposes. By making the necessary additionschanges in these models more accurate results can be obtained when for example, predicting the impact of new equipment on board. Furthermore, useful knowledge can be extracted by this database for the evaluation of the protection system operation of the power system. Voltage dips are linked with protection in two ways: (a) the duration of fault induced dips depends on the time it takes for protection system to isolate the faulty part of the system (b) transformer and motor dips can cause protection maloperation. The recordings can reveal situations of slow protection operation or situations where protection operated under non-fault conditions. This knowledge can be used to improve the protection system performance. Voltage dip classification in terms of origin (fault, transformer, motor) can be automated using signal processing techniques to extract voltage rms magnitude, segment the voltage signature by detecting points of change and by processing the different segments in an expert system framework. This work is presented in 21. Such methods simplify considerably the analysis of the measurements obtained by monitoring systems. As power quality becomes critical in shipboard systems, a monitoring system apart from voltage events can record (by adding a few more elements) and store periodically: Voltage rms Harmonics (individual harmonics and total harmonic distortion by performing Fourier Transform on the voltage waveforms) Unbalance (by calculating the indices described in the relevant standards). CONCLUSIONS This paper presents an introduction to the analysis and classification of voltage dips in ship electric power systems. The method followed takes into account all major voltage dip types occurring in ship electric power systems and can be a handy tool for assisting the system operator to assist the operating conditions. Furthermore, it is shown that this method can be used as an implement to identify the voltage dip type and location by an expert Power Quality monitoring system or as an evaluation tool of the protection scheme performance. The only prerequisite information is a nonexpensive multi-phase on-line voltage monitoring system of major ship network buses - recording voltage rms magnitude and waveshape as well as symmetry among phases. This monitoring system has to be accompanied by a series of off-line voltage dip incident simulations which help to locate the incidents in a precise manner. ACKNOWLEDGEMENTS The work of this paper is part of the research project Pythagoras-I within the Operational Programme for Education and Initial Vocational Training - EPEAEK-II - frame. The Project is co-funded by the European Social Fund (75%) and Greek National Resources (25%). Fig 10: Monitoring system for onboard power systems REFERENCES 1. Hatzilau IK, Prousalidis J, Styvaktakis E, Kanellos F, Perros S, Sofras E Electric power supply quality concepts for the All Electric Ship (AES), Proceedings of 2006 World Marine Transport Technology Conference, London (UK). 2. Hatzilau IK, Prousalidis J, Styvaktakis E, Sofras E Voltage and Current Spikes & Transients - Power Supply Quality aspects for the AES, Proceedings of 2005 All Electric Ship Symposium (AES2005), Paris (France). 3. Bollen MHJ Understanding power quality problems: voltage sags and interruptions, IEEE Press, N.Y. 4. Amy JV Considerations in the design of naval electric power systems, Proceedings of the Power Engineering Society Summer Meeting, vol. 1, pp , Styvaktakis E, Bollen MHJ Signatures of vol- No. A Journal of Marine Engineering and Technology 9

11 tage dips: transformer saturation and multistage dips, IEEE Trans. on Power Delivery, 18-1: Spagkouros S, Prousalidis J Electric Power Quality problems in ship systems: A classification method, IASME Transactions, 1-2: Bronzeado HS, Brogan PB, Yacamini R Harmonic analysis of transient currents during sympathetic interaction IEEE Transactions on PowerSystems, 11-4: Prousalidis JM, Georgopoulos AG, Hatziargyriou ND, Papadias BC Investigation of Transformer Sympathetic Inrush, Proceedings of 2 nd International Conference on Electromagnetic Transients, IPST 97, Seattle (USA), pp IEEE Standard , IEEE Recommended Practice for Electrical Installations on Shipboard. 10. STANAG 1008, Characteristics of Shipboard Electrical Power Systems in Warships of the North Atlantic Treaty. 11. USA MIL-STD-1399(NAVY), Interface standard for Shipboard systems Section 300A Electric Power, Alternating Current. 12. IEC-60092/ Electrical installation in Ships - Definitions and General Requirements. 13. TC3 of the Information Technology Industry Council (ITIC) ITIC (CBEMA) Curve. 14. Samineni S, Johnson B, Hess H, Law J Modeling and Analysis of a Flywheel Energy Storage System with a Power Converter Interface, Proceedings of International Conference on Power Systems Transients (IPST), New Orleans (USA). 15. Moschakis M, Prousalidis J, Hatziargyriou N Performance Assessment of STC used for Alternative Naval Power Supplying Units, IASME Transactions, 1-2: Whitehead D, Fischer N Advanced Commercial Power System Protection Practices Applied to Naval Medium Voltage Power System. Proceedings of Electric Ship Technologies Symposium, Philadelpia (USA). 17. Butler KL, Sarma NDR, Whitcomb C, Do Carmo H, Zhang H. 1998, Shipboard systems deploy automated protection, IEEE Computer Applications in Power, 11-2: Vallianatos P, Prousalidis J, Styvaktakis E On starting-up large power motors rotating high inertia loads in autonomous systems, Proceedings of International Conference on Electric Machines (ICEM-2006), Chania (Greece). 19. Prousalidis J, Styvaktakis E, Sofras E, Hatzilau IK, Muthumuni D Voltage dips in ship systems, Proceedings of 2007 Electric Ship Technology Symposium, Anaheim (USA). 20. Kanellos F, Hatzilau IK, Prousalidis J, Styvaktakis E Simulation of a Shipboard Electrical Network (AES) Comprising Pulsed Loads, Proceedings of International Symposium Engine as a Weapon II, London (UK). 21. Styvaktakis E, Bollen MHJ, Gu IYH Expert system for classification and analysis of power system events, IEEE Transactions on Power Delivery, 17-2: Journal of Marine Engineering and Technology No. A