Journal of Power and Energy Engineering, 2014, 2, 612-615
Published Online April 2014 in SciRes. http://www.scirp.org/journal/jpee
http://dx.doi.org/10.4236/jpee.2014.24082
How to cite this paper: Luo, N.Z., et al. (2014) Research on Grounding of Shore-to-Ship Power Supply System. Journal of
Power and Energy Engineering, 2, 612-615. http://dx.doi.org/10.4236/jpee.2014.24082
Research on Grounding of Shore-to-Ship
Power Supply System
Ningzhao Luo, Hongjiang Li, Haolan Zhang
College of Electric Engineering, Naval University of Engineering, Wuhan, China
Received Dec emb er 2013
Abstract
To avoid pollution of generator exhaust po rt after the sh ip docked, the u se of s h ore p owe r supply
is developing direct ion of p ort . And the shore power grounding method and related sta nd ards
needed to be carried out. According to de velopin g status of foreign medium voltage ship and cha-
racteristics of navy shipboard power supply, this paper presents a shore power supply network
structu re and its gr oundi ng meth od. The simulation results show the power supply network
structu re can effectively suppress the single-phase inte rmi tte nt ground fault on the system to
produce the harm.
Keywords
Medium Voltage; Grounding; Shipboard; Cold Ironing
1. Introduction
At present, in order to keep the high power mechanical pump, other faculties and their supply s yst e m working
normally, the powerful diesel generator on shipboard must work continuously even after large-scale civil ship
has docked [1]-[3], while the diesel generator produces a great mount of sulfi d e and nitrogen oxide because of
the burning of diesel, resulting in p ollution to the surroundings. International Maritime Organizatio n (IMO)
states that NOx and SOx are the main po llutions. Ships motivated by diesel put 10 million tons of NOx and 8.5
million tons of SOx into the atmosphere per year in the whole world. And the pollutio n substances may spread
over 1000 km away. As the rapid development of civil mar ine , waste materials emitted by ships bring huge
pressure to p ollution controlling [4]-[6]. The ports eagerly call up civil ships to sh ut down donkey generator and
use shore power supply. At the same time, from the angle of economy and maintenance of equipment, the
supply of shore power is also needed when the navy ships are docking in t he base. Meanwhile, the service of the
ship with a medium voltage shipboard system brings a whole new challenge to shore power system. No matter
in civil field or militar y field, the research of medium voltage shore power system has just star ted without stan-
dard reference. The exploration of shore power supply network str uct ur e and its groundi ng met hod , as well as
related standards are becoming a hot issue among the world ship power field [7] [8].
This paper deeply discusses the shore power supply network st ruc t ure and its grou nd ing method, provides a
theoretical basis of shore power supply standards.

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2. Civil Ship Medium Voltage Shore Power System Grounding Method
Civil shore power supply system grounding protection expert Dev Paul P.E drew this conclusion after aged re-
search [9]:
Shore power supply system has excessive distributed capacitances, so the gro und-free method should not be
used. According to industry standards, if the ground capacitance current is over 10 A, the grounding system
should be low resistance grounding. However, because the distributed capacitance varies with the number of
docked ships, the low resistance grounding method should be used even though the ground capacitance current
is below 10 A. For the sake of reducing the stray current, it is necessary to measure the resista nce between the
grounding device and ship shell. And an independent connection between the ground and ship shell is needed, in
case of the excessive resista nce [10].
To prevent an open circuit or short circuit, the grounding device should be watched at all time, otherwise a
disaste r may happen.
3. Military Ship Shore Power System Grounding Method
It is very common that military ships are supplied by shore power system when docking. Generally, Shore pow-
er is neutral-point grounding. While the shore power is supplying, the shore neutral-point of power transformer
directly connects to ground, facilities on the ship are c onne c t ed to earth through the ship hull and sea eater, using
as grounding protection. In the case of multi-ship docking, every ship shore power cable is connected to dif fe r-
ent connection spot of a same transformer in parallel. Figure 1 is a typical shore power syste m for low voltage
ship for now.
When the system voltage level reaches 6kv, the pr evio us shore power system structure is no longer suitable.
Direct grounding of the transformer neutr al -point may cause fault current and step-volta ge, threatening the sa fe-
ty of people on the port.
4. Design of Shore Power Supply Network Structure and Grounding
Method Applied to Military Naval Port
According to the conclusion from Dev Paul P.E, although the high resistance grounding met ho d has advantages,
the shore power transformer is not suitable for high resistance grounding as the ship is. It is because that while
ships are docked in port and supplied in parallel, the overall capacitance is too high due to the superposition ef-
fect of each ship’s grounding capacitance. Fur t her mor e, the number of docked ships is changing all the time,
high resistance grounding met hod needs to be frequently adjusted.
After the docking, sailo rs still work and live in ship, the shore power cable could be easily damaged when
dragging, and meanwhile, the risk of single-phase grounding exists when doing some maintaining work. When
using the low resistance grounding method, if there is a malfunc t io n, the ship wo uld lose its power and influence
normal work. Besides, low resistance method causes very high current when a failure happen, it would destroy
facilities and produce powerful electric arc which brings harm to the workers around. So a shore power network
and grounding method must be designed to fulfill the requirement of docked ships with medium voltage syste m.
There is a regulation about milita ry ship: “The design of power system should ensure the continuit y of power
supply under all kinds of working conditions”. The grounding method that can attain such requirement is high
resistance grounding and resonance grounding.
Shore power line is relatively shor t, the cable part of it can be regarded as aerial cable, and its grounding ca-
pacitance is low. Grounding capacitance of aerial cable is between 0.0055 μF/km - 0.003 μF/km. So shore pow-
er system is available for high resistance grounding method which is economic and simply. This paper aims at
the characteristics of large ship and designs a flexible shore power supply ne two r k s truc ture. Figure 2 is an
Figure 1. Low voltage shore power system.
Transformer Shore power box
Vessels

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614
Figure 2. Shore power system structural drawing.
electric bluep ri nt. This kind of system has these characteristics below:
1) Large ship is supplied by shore power sectionally becacue its large power capacity, therefore, the number
of landing distribution box should be 2 to 4, in order to fulfill different demand of different ship type.
Futhermore, outputs of transformer should not be connected in parallel, preventing from the cir culation c urr ent.
2) Transformer in land electric box is Δ/Y connection, the secondary side of the transformer is high resistance
grounding, making itself isolated from the primary side. This connection creates good situation where many
ships are docking, because if the elec tric box has not configured independent isolation transformer inside but
outputs through the same transformer, the grounding capacitance of all ships would be connected in parallel, the
maximum of failure current would be above the allowance current with high resistance ground in g meth od.
3) In terms of medium voltage ship with e xc es si ve grounding capacitance, the resonance grounding device
should be operated when the shore power system is working, making up for excessive capacitance current and
providing effective grounding protection for shipboard power system.
4) Secondary side of isolation transformer values 400 Ω - 500 Ω, making the grounding parameters consistent
in both shore power system and shore power system.
5) The output s witch of generator is breaking while the shore power system is working, making the
neutra l -point grounding device of generator isolated from the electric network.
5. Grounding Fault Simulation of Shore Power System
Because of the application of isolation transformer, the fault condition from shore power and the shipboard
power is nearly the same. The grounding fault coming from the broken shore power cable is simulated. Fault
electric arc is contin uo usly burning between the A phase and ground, burning time is the bottom of every nega-
tive period and the arc extinguishes after half period. Nomin al volta ge is 6.3 kv and resistance of electric arc is
0.1 Ω. The results are shown below in Figure 3.
10kVbus
Land power system
Hi-R
Dock
transformer
box
Hi-R Hi-R Hi-R
10kV/6.6kV 10kV/6.6kV 10kV/6.6kV10kV/6.6kV
6.6kVbusAcross switch
GGGG
Hi-R Hi-R Hi-R Hi-R
Grounding
transformer
Resonance Grounding
device
Vessel power system
Shore
power
system
Grounding
transformer
Resonance Grounding
device

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615
Figure 3. Secondary of isolation transformer simulated waveform.
Figure 3 gives us the secondary side of isolation transformer simulated waveform. We can figure out that if
the grounding device is properly confi g ure d, the grounding fault coming from shore power cable has little in-
fluenc e on power quality.
6. Conclusion
In order to keep the military shore power system safe and stable, multiple isolation transformers should be set up
in port. And the secondary neutral-points of isolation transformer should be connected to ground through high
resistance device, prevent ing the overvoltage produced by grounding arc re-burning fault. This kind of shore
power supply network can keep the setting of protection device consistent in both shore power and shipboard
power syst e m, besides, it has up sta nding contin uit y in power supply, and furthermore, it is less affected by the
change of distributed capacitance, so this shore power grounding method is suitable for military port.
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