Voltage dips in ship systems J. Prousalidis, E. Styvaktakis, E. Sofras, I.K. Hatzilau, D. Muthumuni Abstract: In this paper, voltage dips (an important aspect of power quality) are presented in the context of ship power systems. Voltage dips are analyzed with respect to their origin (fault, motor starting, transformer saturation) considering the characteristics of the on-board installations. Emphasis is given to voltage dips and power quality issues of the All Electric Ship. Related issues are presented and discussed.
Simulations in PSCAD environment (the computer program of Manitoba HVDC Research Center) are used for analysis purposes.
Keywords: power quality, voltage dips(sags), ship power systems.
Voltage dips (a temporary reduction in rms voltage) are responsible for the tripping of computers, electronic equipment and process control equipment. The behavior of these types of equipment can be a reset, incorrect operation or shutdown depending on the design and the inherent protection system of the load. For adjustable speed drives, depending on the load of the drive, the reduction in speed or torque might not be tolerated, by the process driven by the drive. Problems are also caused to the drive controller or the PWM inverter. Induction motors might not also manage to reaccelerate after the reduction in speed that is caused by the voltage dip [3]. Contactors are also sensitive to dips.
I. INTRODUCTION
Ship Electric power plants similarly to continental grids suffer from several power quality problems. The particularly complicated structure of shipboard installations comprising DC and AC subsystems of several operation frequency and voltage levels is expected to worsen even further with the advent of All Electric Ship (AES) buildings (referring to full electric propulsion and extended electrification of all shipboard installations). Power quality phenomena in this context have already been studied by the authors in previous papers [1-2]. The aim of this paper is to identify and study the different types of voltage dips (sags) that occur in ship systems considering the particularities of these systems. In this way, identification of the characteristics of different types of voltage dips in ship system for planning and operating purposes is done while situations that could cause problems in ship operation are further investigated. Moreover, the extraction of voltage dip features is highlighted for electric system monitoring purposes. Protection issues relative to voltage dips and shipboard standards for voltage quality are discussed. _________________________________________ The work of this paper was supported in part by the research project "Pythagoras-II" within the "Operational Programme for Education and Initial Vocational Training - EPEAEK-II"-frame. J. Prousalidis is an Assistant Professor at the School of Naval and Marine Engineering of National Technical University of Athens, 9 Heroon Politechniou St, 15773 Athens, Greece (
[email protected]). E. Styvaktakis, PhD, is with the Hellenic Transmission System Operator HTSO SA, Amfitheas 11, N. Smirni, 17122, Athens, Greece (
[email protected]). E. Sofras is a PhD cantidate at the School of Naval and Marine Engineering of National Technical University of Athens. I. K. Hatzilau is professor with the Department of Electrical Engineering and Computer Science in the Hellenic Navy Academy, Peiraeus, Greece. D. Muthumuni, PhD is with the Manitoba HVDC Center, Manitoba, Canada.
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II. BACKGROUND
A. Voltage dips
Voltage dips are characterized by their rms magnitude and duration. Duration is the time that the rms voltage remains below a certain threshold and dip magnitude is the remaining rms voltage considering the phase with the maximum voltage. A typical threshold is 10% of the pre-event voltage. B. Ship Power Systems - The All Electric Ship Ship power systems require high levels of reliability as their operation is closely linked to human safety [4]. The increased use of electric propulsion leads to an increased installed generating capacity, the use of different voltage levels and, in general, to more complicated systems. Considering that the load demand is typically comparable to the generation capacity and that ship systems are isolated it becomes obvious that, compared to the interconnected terrestrial power systems, these systems present new challenges. The eventual complete electrification of all systems onboard according to All Electric Ship (AES) concept is expected to create a need for a closer look on the power quality aspects of these systems. Thus, considering the great variety of electric loads to be installed onboard in conjunction with their huge energy demands leads up to the conclusion that almost all AES- evolution trends would be inherently correlated to several adverse power quality phenomena. Considering voltage dips with respect to ship power systems of interest is on:
1. Large motors and the voltage dips related to them that may effect other loads in the system 2. System design and protection issues. 3. Motor drive converters applied to main and auxiliary propulsion systems and their voltage dip ride through capability. III. 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 (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): ZF E Z F +Z S
Fig. 2 shows the voltage dip in the generators terminals caused by a 3-phase fault in the 4 kV voltage level. It can be seen that voltage drops to approximately 0.4 pu (identical for all three phases) although voltage in the 4 kV 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 0.80 pu and then the voltage increases gradually towards the normal voltage due to the motor load influence. 1.2 1.1 1
(1)
where ZS is the source impedance at the PCC, ZF 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 post-fault dip as described in [3] that could cause motor stalling. A. Voltage dips and propagation The electrical power system of Fig. 1 was 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 (6 kV, 4 kV and 0.38 kV). 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). 310
0.9 Voltage rms (pu)
Vdip=
Fig. 1. Simulated Ship Power System
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Fig. 2. Voltage dip due to a 3-phase fault
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 4 kV 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 one of authors in [6]. Considering Eq.1, it should be mentioned that the magnitude of the drop depends on the available source impedance.
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.
1.2 1.1
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Fig. 5 shows a voltage dip caused by the starting of a medium voltage auxiliary propulsion motor of 1.4 MW by an 8 MVA generator. It can be seen that voltage drops to 0.85 pu and gradually recovers approximately 1 sec after. The voltage dip is the same for all three phases.
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Fig. 3. Voltage dip due to a 2-phase fault
Therefore, the number of generators in operation effects significantly the magnitude of the voltage dip experienced by the loads. 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 √3 or 1.73 p.u., see Fig. 4. Therefore, the system suffers from a voltage swell rather than a dip, which stresses equipment insulation.
Large power motors starting consecutively during ship maneuvering (e.g. driving thruster systems) can cause such severe voltage dips.
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Fig. 5. Voltage dip due to motor starting
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Summarizing, voltage dips due to induction motor starting are: • non-rectangular: voltage recovers gradually. • symmetrical: all phases present the same behavior.
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V. TRANSFORMER SATURATION DIPS
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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 few percent of the nominal rated load current.
4
Fig. 4. Voltage dip (swell) due to a 1-phase fault
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. IV. INDUCTION MOTOR DIPS
During 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 311
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
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.
Voltage (pu)
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Fig. 7. Voltage harmonics in time due to transformer saturation
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 (pu)
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 0.2 pu 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 2nd to 5th) of the voltage of one of the phases of Fig. 6. The 2nd harmonic is contributing the most. This increased harmonic distortion can cause undesired protection operation.
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Voltage magnitude (pu)
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.
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Fig. 8. (up): Voltage waveforms during transformer energizing, (down): corresponding voltage rms magnitude
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VI. SHIPBOARD STANDARDS
1.1
Table 1 shows the limits proposed by several classification societies and international associations regarding the limits of voltage transients. Voltage dips fall in this category.
1 0.9 0.8 0.7
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Fig. 6. (up): Voltage waveforms during transformer energizing, (down): corresponding voltage rms magnitude
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 3rd harmonic filter (single-tuned filter) is connected. The 3rd harmonic filter causes a parallel resonance in a frequency slightly lower than the 3rd harmonic and the harmonics of the transformer inrush current are amplified leading to a more severe dip (0.1 pu lower than before). 312
TABLE I. Classification Society Rules & Standards ([8-11]) REGISTER VOLTAGE TRANSIENT ABS (2005) BV(2003) DNV(2001) GL(2004) ±20% (1,5s) PRS (2002) RINA(2005) (LRS) (2001) +20%, -15% (1,5s) IEEE Std 45-1998 ±16%(2s) STANAG 1008 (Ed.8, Ed.9) ±16%(2s) [ ±22%(2s) ]* ( like ST.1008 ) USA MIL-Std-1399 * under some circumstances
IEEE Std 45-1998 [9], additional to the values of Table 1, provides guidelines regarding motor starting and limits for the resulting voltage dips. The lower allowed voltage dip is 30% (70% remaining voltage) in the case of a group of
motors that automatically restart upon closing a feeder breaker. Special attention is given to the generator sizing when large motors are connected to the system.
Load immunity is an issue that, at the moment, is not covered extensively by standards and regulations. As the use of sensitive equipment increases in naval systems, this issue will become more important. For power converters that are used for propulsion purposes, the voltage ride through capability is, up to a certain extend, cost related. By adding capacitance (in low power electronics and AC drives) and with appropriate control algorithms (for DC drives) the sensitivity of the equipment to voltage dips is reduced [3].
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S p ike & T ra n sien t Bo un d ary area a cc ord in g to S T AN AG 1 00 8 E d .9
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VII. VOLTAGE DIP RELATED ISSUES
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automatic transfer switches, ferroresonance transformers, series connected voltage source converters, etc [3,14,15].
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Fig. 9. ITIC compared to STANAG 1008 (dashed line) (applicable to 120 V equipment - 60 Hz)
STANAG 1008 [10] (a standard referring to the electrical power plants in NATO naval vessels) states that:«The definitions, user information and characteristics in this agreement refer essentially to a healthy electrical power supply system and include transient conditions that result from normal system operations, such as motor starts and switching events. The STANAG does not describe system behavior under abnormal conditions, e.g. short circuit faults, loss of generator sets or malfunction of associated controls, as this behaviour depends heavily on the design characteristics of individual systems». Consequently, STANAG 1008 does not include limits and guidelines with respect to abnormal conditions for the design of naval systems although these conditions produce transients and spikes are even more severe than the ones in normal system operations. Comparing the voltage limit curve provided by STANAG
1008 with the ITIC [13] (a curve often applied for assessing the performance of power systems in terms of power quality) one can see (Fig. 9) that STANAG does not accommodate dips of short duration and large magnitude caused mainly by faults (up to 2 seconds the curve equals to 80% of nominal voltage). For shipboard standards, it is important to take into account abnormal conditions: (a) in the design stage in order to minimize their impact by making appropriate decisions from a system point of view (b) in coordinating load immunity and mitigation devices (c) in designing appropriate protection devices and equipment with higher immunity to power quality parameters. Mitigation devices for voltage dips are ups, flywheel, 313
As mentioned above, the duration of fault -induced dips is mainly depended on the response of the protection system (fault clearing) which, in the event of a fault, should isolate the part of the system where the fault is. Protection of ship power systems is typically based on overcurrent elements (relays or fuses). However, the protection coordination in these systems is not an easy task. As the length of the cables is typically short, the different parts of the system, in terms of impedance, are separated mainly by the transformers. This makes selectivity difficult and leads to unnecessary protection operations and load interruptions. The variations in source impedance is also a parameter that naval protection systems must take into account because these variations can lead to incorrect protection operations. At this point it should be also mentioned that different grounding practices are used in ship and this certainly influences protection operation. For ungrounded systems, for example, detection of single phase faults can be a difficult task. Faster protection operation and efficient protection coordination will reduce the duration of fault-induced voltage dips. Additionally, it is important to highlight that by improving the protection of the ship systems the risk of unwanted protection is minimized in the case of the other two types of events (motor and transformer related). Sophisticated protection methods are available that can accommodate the special characteristics of the naval system. For example, differential protection (unit protection) improves significantly protection performance by discriminating faults by other types of disturbances. This type of protection is used for transformers in terrestrial systems. Furthermore, adaptive protection is able to take into account the changes in the system (like the variations in source impedance) and adjust relay settings to the new conditions offering better performance [16-17]. In terms of design and operating practices, the increase in the size of generating capacity of ship systems rises the issue of short circuit current. As more generation is installed on-board the equipment has to cope with higher currents in the case of a fault. To avoid the cost of higher rating equipment it is
common practice to split the network into different parts. Although this practice reduces the short circuit current, it influences significantly the voltage quality of the network, not only in terms of dips but also in terms of harmonics and other power quality aspects. Stiffer systems (higher short circuit current) are less sensitive to power quality phenomena and in many cases the extra cost of higher rated equipment is justified by the benefits in power quality.
systems, protection is probably the most critical issue as it influences the dip duration, and up to a great extent, the ability of loads to operate without problems during faults. Faster protection schemes with efficient algorithms can reduce the dip duration (by reducing the fault clearing time) and minimize the risk of incorrect operation.
Regarding motor dips, a number of technical solutions for staring are available. They prevent severe voltage dips and allow motor starting without problems, by limiting the inrush current. These measures are [18]: 1. Starting-up via autotransformer 2. Starting-up via autotransformer and capacitor 3. Soft starting via power electronic devices
The work of this paper is part of the research project "Pythagoras-II" 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%).
For transformer saturation events, as mentioned above, special attention must be given to their interaction with harmonic filters to avoid severe dips, increased harmonic distortion, and protection maloperation. VIII. CONCLUSIONS
In this paper an important aspect of power quality, voltage dips, is presented with respect to on-board installations. Voltage dips are categorized in terms of their origin into three classes: fault-induced, motor starting and transformer saturation. Voltage dips due to faults can be severe depending on the voltage level where the fault takes place. The influence of induction motor load on the voltage dip is important as it can prolong its duration. The duration of this type of dips depends on the protection system response to the fault. Motor starting can be of considerable magnitude depending on the motor, its load, other motors near-by, etc. Transformer saturation dips are due to the inrush current upon energizing. There are associated with increased harmonic distortion that could lead to more severe voltage dips in the presence of single-tuned harmonic filters. Therefore, transformer energizing should be avoided when harmonic filters are connected. In terms of characterization, voltage dips due to faults present an almost rectangular shape in their rms and can be either symmetrical or asymmetrical depending on the type of fault and the transformer connections between the fault and the point where the dip is measured. Induction motor dips are symmetrical and non-rectangular as the three phases behave the same and voltage recovers back to normal gradually. Transformer saturation dips are non-rectangular and non-symmetrical as the voltage dip of each phase depends on the point-on-the-wave where the switching takes place. A presentation of naval standards on voltage dips is also given. Special attention is paid to STANAG 1008, while it is highlighted that standards must take into account power quality issues towards the complete electrification of ships. Finally, considering the characteristics of ship power 314
IX. ACKNOWLEDGEMENTS
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