www.sut.org
Design and implementation of compact and
robust medium voltage switchgear for deepwater
work-class ROV ROSUB 6000
Ramesh Raju, Vedachalam Narayananaswamy*, Muthukumaran Durairaj, Doss Prakash Vittal,
Ramesh Sethuraman, Ramadass Gidugu Ananda and Atmanand Malayath Aravindakshan
National Institute of Ocean Technology, Velacherry-Tambaram Main Road, Narayanapuram, Pallikaranai,
Chennai, Tamil Nadu, 600 100, India
Abstract
The present paper presents the design and implementation
of compact and robust medium-voltage switchgear for the
deepwater remotely operated vehicle (ROV) ROSUB 6000.
Electrical and control systems in the tether management
system (TMS) and the ROV are powered by a voltage of
6.6kV at 460Hz transmitted through 6,000m umbilical and
400m tether cables. Any fault in an ROV system could lead
to failure of the complete system. Thus the ROV needs to be
isolated by medium voltage switchgear located in the TMS.
Using conventional circuit breakers inside pressure-rated
enclosures with feed-through could be a possible solution,
but they are not attractive because they affect the volume
occupied and thus weight and costs. The proposed solution
involves adoption of a low-voltage air break contactor in an
insulating oil-filled pressure-compensated enclosure and uses
the subsea environmental conditions to operational advantage. To further ruggedise the oil-filled pressure-compensated
switchgear and make its life-time independent of load break
conditions, the switchgear opening sequences are controlled
using programmable automation controllers distributed in
the ship, the ROV and the TMS. The hardware developed is
proven to work according to the requirements. The present
paper further explains the extendibility of this idea to futuristic subsea systems that will involve high-power switching
operations.
Keywords: remotely operated vehicle, supercapacitors, oil
circuit breaker, tether management system
1. Introduction
The National Institute of Ocean Technology has
developed a remotely operated submersible ROSUB
6000 for deep-sea operations (Manecius et al., 2010;
Ramesh et al., 2010; Vedachalam et al., 2013a; 2013b),
such as emergency response situations, bathymetric
* Contact author. E-mail address: veda1973@gmail.com
Technical Briefing
doi:10.3723/ut.31.203 Underwater Technology, Vol. 31, No. 4, pp. 203–213, 2013
surveys, gas hydrate surveys, poly-metallic nodule
exploration and salvage operations. The ROSUB
6000 system comprises a remotely operated vehicle (ROV); a tether management system (TMS); a
launching and recovery system; a ship-based power
supply system; and a control console. The electric
work-class ROV is equipped with two manipulators
with a pay-load capability of 150kg. The overview of
the ROSUB 6000 system is shown in Fig 1, and the
electrical and control architecture is shown in Fig 2.
The subsea systems are operated at a voltage of
6,600V at a frequency of 460Hz. This voltage was
selected as a trade-off between the size of the Kevlar
armoured electro-optic umbilical and tether cables,
the voltage drop to be managed in the 6,000m transmission cable and the power requirements for the
system.
The umbilical cable has 3 copper conductors of
6mm2, 2 copper conductors of 2mm2 and 6 optical
fibres. The tether cable has 3 copper conductors of
1.5mm2, 4 copper conductors of 0.35mm2, 5 copper
conductors of 0.22mm2, and 3 optical fibres. A
frequency of 460Hz was selected to reduce the size
and weight of the magnetic components in the ROV.
Programmable automation controllers located in
the ROV, the TMS and the ship communicate
across optical ethernet. In addition to the optical
network between the TMS and the ROV, an RS485
network utilising twisted pair cable is in place for
critical data exchanges.
2. The need to electrically isolate the ROV
Ship power at 415V and 50Hz is converted into
6,600V and 460Hz using a combination of a standard frequency converter and a step-up transformer
(Vedachalam et al., 2013a; 2013b). Electro-optical
203
Raju et al. Design and implementation of compact and robust medium voltage switchgear for deepwater work-class ROV ROSUB 6000
Mother vessel
Umbilical cable 7,000m
Tether management system
Tether cable 400m
Remotely operable vehicle
Fig 1: View of the ROSUB 6000 system
connectivity between the ship system and the TMS
is achieved by the 6,000m electro-optic umbilical
cable, while the connectivity between the TMS and
the ROV is achieved using the 400m electro-optic
tether cable. Electrical overloads and short circuits
in the ROV and TMS are protected by the ship-side
system by programming the required protection
levels in the ship-based frequency converter.
When the ROSUB system is in operation, a complete power failure can be catastrophic because
the ROV cannot be wound back into the TMS for
docking and recovery to the topside. The presence
of electrical power in the TMS keeps the power
and control systems active so that the dead ROV
can be docked into the TMS and recovered if
power fails in the ROV alone. This process necessitates 6,600V-rated electric switchgear in the TMS
for opening and closing the power circuit leading
to the ROV.
3. Challenges and proposed solution
Present-day industrial standard circuit breakers for
6,600V voltage ratings are generally oil-filled or vacuum types. The ROV operates with a power of nearly
60kW at near unity power factor for the propulsion
and control systems. Thus the remotely operated
switchgear in the TMS has to handle a maximum
current of 10A at 6,600V.
The following are the standard industrial switchgears that can meet the functional requirements:
• Minimum oil circuit breakers are available in stan­
dard power ratings, and the available minimum
rating is for 630A at 7.2kV. A typical minimum
oil circuit breaker for this specification weighs
approximately 200kg and has dimensions of
approximately 1.0m (L) × 1.5m (W) × 1.5m (H).
204
• Vacuum circuit breakers are available in standard power ratings, and the minimum power
­rating is for 400A at 7.2kV. A typical vacuum circuit breaker weighs approximately 25kg and
has approximate dimensions of 0.40m (L) ×
0.21m (W) × 0.32m (H).
• Static switches with power electronics can be used,
which require a housing with dimensions of
approximately 0.6m (L) × 0.4m (W) × 0.3m (H).
In addition to their huge footprints, these circuit
breaker systems should be operated inside a pressure-rated enclosure with a pressure-rated feedthrough that is suitable for 6,000m water depths.
These solutions are not attractive owing to the volume occupied and the associated weight and cost.
The basic solution to this challenge is addressed
by adopting an industrial standard 690V, 100A-rated
air-break motor contactor and using it in an oil-filled
chamber. Air has a dielectric strength of 3kV/mm,
whereas Shell Diala DX transformer dielectric insulating oil has a dielectric strength of approximately
60kV/mm. The dielectric capacity of oil is 20 times
that of air, enabling the use of a 690V air contactor
at 6,600V. The dimension of the selected switchgear is approximately 0.10m (L) × 0.08m (W) ×
0.12m (H).
Fig 3 shows the failure trees developed using
TOTAL SATODEV GRIF tool calculating the probability of failure when the contactor is operated in
an oil-filled chamber. The failure rates used to calculate the switchgear hardware failure are based on
the MIL-HDBK-217F standard (US Department of
Defense (DOD), 1991), where the environmental factor corresponds to the application on board a submarine vehicle. The failure rates of other components
of the electronics system are calculated based on the
FIDES guide (FIDES, 2009) recommendations.
Fig 3 also shows the probability of failure of the
oil-filled (OF) switchgear hardware that is calculated
to be 96.68% in a 5-year period. With this failure
rate, the probability of power input failure to the
ROV from the TMS is 96.99%, with a corresponding
mean time between failure (MTBF) of 1.44 years.
The contribution of the oil-filled switchgear hardware to the overall system failure probability is very
high, and thus attempts are made to reduce the
failure rates to as low as reasonably possible.
4. Robustness improvements of the
switchgear utilising subsea environment
conditions
4.1. Challenges in harsh subsea environment
The ROV is designed for operation in 6,000m water
depths, where an ambient pressure and temperature
Underwater Technology Vol. 31, No. 4, 2013
ROSUB Power and Control Architecture
Ship system
User
interface
computer
& display
Fibre-optic
converters
4 SM
fibres
3 Ph
6,600V,
460Hz
High
frequency
3 Ph,
415V
AC 50Hz
Step up
transformer
7,000m
umbilical
cable:
3 Ph 6,600V
AC
6 single mode
fibres
Subsea systems
TMS
TMS
controller
Fibreoptic
converter
ROV power
control
Optical
link
3 Ph
6,600V,
460Hz
MV switch
24V DC
&
300V
DC
Step down
transformer
3 Ph
rectifier
400m tether
cable:
3 Ph 6,600V
AC
6 Single mode
fibres
ROV
ROV
controller
Fibreoptic
converters
Optical
link
3 Ph
6,000V,
460Hz
Step down
transformer
3 Ph
rectifier
24V
DC
&
300V
DC
Fig 2: Electrical and control architecture of ROSUB 6000
of 600bar and 2°C exist. When a system is to be
operated in a high-pressure hydrostatic environment, it must be protected inside enclosures that
can withstand the external pressure. The thickness
of the enclosure depends on the external pressure
and the material used in its construction. When the
enclosure diameter increases, the wall thickness
must increase to avoid failures caused by buckling.
Electrical and electronic systems that are operated inside these enclosures must communicate
with the external systems via pressure-rated feedthroughs, such as penetrators and connectors. The
cost, weight and complexity of the feed-through
components increase with the differential pressure.
The principle of pressure compensation offers a
solution to this challenge.
4.2. Principle of pressure compensation
Pressure compensation (Mehnert, 1972) is a technique applied to subsea systems so as to eliminate
the need for thick-walled enclosures, feed-through
and associated complexities. The technique involves
the principle of maintaining the internal pressure
slightly greater than the external ambient hydrostatic
205
Raju et al. Design and implementation of compact and robust medium voltage switchgear for deepwater work-class ROV ROSUB 6000
Fig 3: Tree showing the probability of failure of a switchgear operated in an oil-filled environment
TMS subsea power
converter transformer
TMS subsea power
converter rectifier
Chamber housing
the switchgear
Pressure compensator
Output power to
ROV
Input power
from ship side
Fig 4: Pressure-compensated power system in the TMS
pressure, which is achieved by using pressure compensators, connected hydraulically to the system.
In case of a leak, seawater entry inside the enclosure is prevented. Instead, oil leaks out of the system, which can be identified by the status feedback
from the pressure compensator. Fig 4 shows the
pressure compensation applied to the TMS power
system. The chamber housing the switchgear is
connected to the existing pressure-compensated
gallery of subsea power systems.
206
The selection of pressure compensators is done
for the complete gallery with internal volume of
43L filled with oil having a compressibility factor
of 0.9615 and a coefficient of thermal expansion of
0.00075/°C (Shell, 2001; Skofteland et al., 2009).
Taking into consideration the change in volume of
oil resulting from pressure and temperatures for
surface and subsurface conditions, and safety factor of 2 for the compensator reserve capacity, two
1.5L pressure compensators are used for pressure
Underwater Technology Vol. 31, No. 4, 2013
4.4. High ambient pressure in reducing
oil degradation
When the oil-filled switchgear opens, the electrical
arc produces a localised high temperature, which
results in the degradation of the insulating properties of the oil (Mehnert, 1972; Prevost, 2008). Based
on the Clausius-Clapeyron equation, the boiling
point of Shell Diala DX increases with pressure
(Shell, 2001). The boiling point at different pressures is shown in Fig 6. An increase in the boiling
point reduces oil degradation.
4.5. Low ambient temperatures in reducing
oil degradation
The ROV is designed to be operated at a depth of
6,000m in water, where an ambient temperature of
less than 5°C exists. A lower ambient temperature
reduces oil degradation because the heat generated
caused by arching is cooled immediately during the
opening of the contacts.
Thus operating the oil-filled pressure-compensated switchgear in the designed subsea conditions provides a benign environmental condition.
The failure rates used to calculate the oil-filled
140
Breakdown voltage / kV
4.3. Switchgear operation in high ambient
pressure with improved breaking performance
When the electrically loaded contacts of a switchgear are opened in oil under pressure, the pressure
quenches the electric arc produced in the gap
between the fixed and moving contact. The oil
regains its insulation capability within a period of a
few milliseconds.
The results of the experiments conducted on
Nytro dielectric oil’s breakdown behaviour with
pressure carried out by Koch et al. (2007) are shown
in Fig 5. The results explain that the breakdown
voltage of Nytro insulating oil at 100bar pressure is
found to be double that at atmospheric pressure,
and asymptotic thereafter. The breakdown voltages
of Nytro and Shell Diala DX (which is used in the
system) have similar dielectric properties.
One possible reason for this observation is the
fast flooding of oil to the region of the contacts
during separation, which provides the advantage
that the switchgear can handle increased currentbreaking capacity. Operating the system in the subsea environment with an ambient pressure of 600bar
increases the oil dielectric insulation capacity, which
in turn helps to reduce arc formation and sustain
the lifetime of the contacts of the switchgear.
160
120
100
80
Average
60
5% confidence
Standard deviation
40
20
0
0
20
40
60
80
100
Pressure / bar
Fig 5: Nytro insulating oil dielectric strength variation with
pressure (Koch et al., 2007)
400
350
300
Boiling point (ºC)
c­ ompensating the gallery. Thus, the technique of
pressure compensation can significantly reduce
the weight, volume and cost of the pressure-rated
enclosure for the switchgear.
250
200
150
100
50
0
0
100
200
300
400
500
600
700
Pressure (Bar)
Fig 6: Insulating oil boiling point variation with pressure
pressure-compensated switchgear hardware failure
are based on MIL standard (DOD, 1991), where
the environmental factor corresponds to benign
applications. The probability of power input failure to the ROV from the TMS is 78.9% over 5 years,
with a corresponding MTBF of 3.22 years.
5. Robustness improvements of the
switchgear by operation as a switch
The reliability of the oil-filled pressure-compensated
(OFPC) switchgear is of utmost importance in the
ROSUB system because its failure leads to failure of
the complete system. When the switchgear opens,
the electrical arc produced is extinguished with
the arc chute mechanism and the insulating oil.
Because the arc generates a high temperature, it
decomposes a portion of the oil into gases composed
of 70% hydrogen, 20% acetylene and carbon particles (Mehnert, 1972; Prevost, 2008). The produced
carbon particles are conductive, which could create
insulation breakdowns and short circuits.
207
Raju et al. Design and implementation of compact and robust medium voltage switchgear for deepwater work-class ROV ROSUB 6000
Fig 7: Tree showing the probability of failure of the switchgear operated as a switch in an oil-filled pressure-compensated
environment
The quantity of decomposed carbon is a function
of the electric arc intensity, the duration of the arc
and the temperature of the arc plasma. As the OFPC
switchgear is in a closed oil system, prolonged usage
and excessive generation of carbon particles leads
to oil contamination and subsequent failures. Therefore, the longevity and thus the reliability of the
switchgear can be improved by ensuring that the
OFPC switchgear is always opened under a no-load
condition. Then the OFPC switchgear is ensured to
operate as a medium voltage (MV) switch.
Fig 7 shows the probability of failure trees when
the OFPC switchgear is operated in an oil-filled
pressure-compensated deepwater environment and
using the control system that ensures the switchgear operation as an MV switch. The failures taken
for calculating the MV switch hardware failure were
based on MIL standard (DOD, 1991), where the
environmental factor corresponds to a benign
application and the switchgear operation is under
no-load conditions. The probability of power input
failure to the ROV from the TMS is 74.31% over
5 years, with a corresponding MTBF of 3.68 years.
6. Load break scenarios
Failure mode effect critical analysis on this system
revealed that some situations could lead to the opening of the OFPC switchgear under load, such as:
• input power failure in the TMS alone;
• control system failure in the TMS alone; and
• accidental issue of open command by the operator.
To address these situations, a possible solution
would be to delay the opening of the MV switch by
208
a few seconds, during which the electrical load on
the MV switch would be reduced. System studies
show that when the thruster control signal for
deceleration is given at 0.8V/s it requires 5 seconds
to reduce the electrical load on the switchgear if all
the ROV thrusters are operating at load. This is followed by the time required for the exchange of the
decision information between the TMS and the ROV
controllers (which is typically 5–10 seconds) through
the ethernet network. Thus, a minimum delay of
15 seconds is required to ensure that the OFPC
switchgear is opened under a no-load condition.
However, with an adequate margin, time delay hardware is designed to produce a delay of 30 seconds.
6.1. Energy storage requirements and selection
of energy storage device
This operation requires an independent energy
source that can cater for the energy demand for
holding the switchgear coil in the closed position
for 30 seconds.
Possible devices that support energy storage in the
required environment are batteries and capacitors.
Although lithium-based batteries may be attractive
in terms of size and reliability (FIDES, 2009), they
are not preferred because of safety aspects as they
are operated inside a pressure-rated enclosure with
other critical telemetry systems.
As a better alternative, supercapacitors (electrochemical double-layer capacitors) have been found
to have comparatively lower failure rates (Cooper
Bussman, 2006) and offer the best trade-off between
power density, size and reliability (Shukla et al., 2000;
Riberio et al., 2001; Ruffer et al., 2003; Lemofouet
and Rufer, 2005; Weddell et al., 2011; Cooper
Underwater Technology Vol. 31, No. 4, 2013
Fig 8: Schematic indicating the components involved
6.2. Circuit design and implementation with
supercapacitor
The MV switch operates on 24VDC supply and
requires a coil operating current (nominal) of
0.15A in the closed condition. The design criteria
involve ensuring a minimum voltage of 18V for
the continuous holding of the coil for duration of
30 seconds. The energy and hence the minimum
capacitance requirements are calculated (Weddell
et al., 2011) and are 94.5 Joules and 1 Farad, respectively. A supercapacitor of 10F was selected with a
voltage rating of 2.5V. Twelve capacitors were connected in series to obtain an effective capacitance
of 0.833F, which can hold the MV switch in closed
position up to 32 seconds.
The designed circuit is integrated with the TMS
data telemetry system (DTS). The schematic diagram
is shown in Fig 8. When the 24V control command
from TMS DTS is given, the MV switch coil is energised. In the event of 24V control command failure,
supercapacitors will continue to hold the MV switch
coil by supplying the stored energy through the
diode. They will also continue to energise the switchgear coil and hold the power contacts of the switch
in the closed condition for a period of 32 seconds.
The control system utilises this period to reduce
the electrical load on the MV switch so that it opens
at no-load.
An uncharged supercapacitor draws high charging current when energised and continues until it
builds up the rated voltage; this produces a voltage
drop in the upstream 24V circuit. A voltage drop of
30
25
Charging voltage (V)
Bussman, 2006). A high-power density is achieved
as no chemical reactions are involved during the
charging and discharging processes, thus ensuring
a fast response during the period of discharge. The
amount of stored energy is a function of the available electrode surface, size of the ions and level of
the electrolyte decomposition voltage.
20
15
10
5
0
0
50
100
150
200
250
300
350
400
450
500
Charging time (seconds)
Fig 9: Supercapacitor charging characteristics
up to 11V was observed, which resulted in the interruption of other circuits in the network. Thus, the
supercapacitor charging current is limited by a
50-Ohm, 10-Watt resistor. The diode placed in parallel to the 50-Ohm resistor is used to provide a lowresistance path during the supercapacitor discharge.
The recorded charging and the discharging characteristics of the supercapacitor bank with an MV
switch are plotted in Figs 9 and 10. The diode in
the 24V input side is used to block the power flow
from the supercapacitor to the 24V power source,
which is the subsea power converter.
When the MV switch power contacts open in oil,
the dielectric strength of oil in the arcing location
decreases, and it takes some time for the insulation
to recover. If the next operation is initiated before
the insulation recovery period, there are increased
possibilities that the electric arc will be maintained,
degrading the insulating oil and damaging the MV
switch and the associated circuitry.
Therefore, a sufficient interval has to be given
between subsequent opening operations. IEEE
Standard C37.010 (Institute of Electrical and Electronics Engineers, 1999) and industrial practices
209
Raju et al. Design and implementation of compact and robust medium voltage switchgear for deepwater work-class ROV ROSUB 6000
30
Amplitude in 5V/div
Discharge voltage (V)
25
20
Switch closed
Switch
Switch
Switch
re-close 1 re-close 2 re-close 3
15
10
Switch
open
Switch
open
5
1->
0
0
5
10
15
20
25
30
35
Time (seconds)
Time in 50 mS/div
Fig 10: Supercapacitor discharging characteristics
Fig 11: Behaviour of the MV switch during the end of the
discharge period
recommend a period of 5 minutes for oil-based circuit breakers. This duration is programmed in the
control system so that the time between two subsequent close-open operations is 5 minutes. When
there is a failure in the 24V input command from
TMS DTS, the supercapacitor starts supplying the
energy momentarily to the MV switch coil. The supercapacitor voltage decreases as a function of time,
and when the voltage falls below 18V after 32 seconds,
the MV switch coil opens as a result of under voltage. Fig 10 shows the laboratory setup to record the
performance of the supercapacitor and the associated circuitry. Fig 11 shows the location of the TMS
DTS where in the supercapacitor circuitry is placed.
The failure of the MV switch may paralyse the
complete system. Failure mode effect critical analysis
indicates that MV switch power contact chatter
could result in re-closing situations before the insulation recovery period. The following could be the
possible reasons for the MV switch chatter:
The following are the observations concerning
the waveform in Fig 11:
• power system studies indicate that the system could
experience voltage variations during transient
loading conditions and electrical fault clearing
situations, which in turn may affect the 24V supply
in the TMS; and
• the residual energy in the supercapacitor could
result in chattering of the contacts during the
end of the discharge period.
Experiments were performed in the laboratory to
identify the behaviour of the power contacts during
the end of the supercapacitor discharge period,
which can be understood with the captured waveform shown in Fig 11. The waveform was captured
using a digital oscilloscope (Tektronix TDS 3014B),
which has an amplitude of 5V/div and a time base
of 50ms/div.
210
1) [YTSheet(1)].Ch2 5 V 50 mS
• The MV switch closing and opening voltages are
17V and 13V, respectively.
• As the voltage goes below 13V, the switch coil
opens for a period of 20ms; during this interval,
the supercapacitors voltage builds up to 17V,
which is sufficient for the switch coil to re-close.
This instance is indicated as switch re-close 1.
• The switch remains for a period of 12ms, during
which the voltage drops back to 13V and the
coil opens.
• Auto re-closure is noticed to occur three times,
after which the energy content in the supercapacitors can no longer rebuild the voltage
levels.
• The switch open durations are 20ms, 56ms
and 60ms, which clearly explains the decreasing
energy content over time. In other words, the
time taken to rebuild the re-closing voltage
increases.
Experiments demonstrated that out of 50 operations, 90% of the operations resulted in 3 re-closures,
and 10 % had 4 re-closures with the open and close
durations varying by ±20%. Because the MV switch
is a coil-actuated spring-loaded mechanical device
and supercapacitors are electro-chemical devices,
minor inconsistencies in the behaviour are noticed.
To overcome this challenge, an anti-chatter circuit was designed and incorporated. The anti-chatter protection circuit uses a voltage regulator, which
allows the supercapacitor to power the switch coil
only until the supercapacitor voltage is above 18.1V.
Below this set voltage, the regulator turns off and
stays off until it is reset with the command from the
DTS command supply.
Underwater Technology Vol. 31, No. 4, 2013
7. Control logic implementation
The control logics related to the MV switch are programmed in the TMS, the ROV, and the shipside
control and power systems. The load break scenarios
identified by failure mode effect critical analysis and
discussed earlier are managed as indicated in the
following sections.
ROV power shutdown request
from ship control system
Send command from ROV
controller to disable loads
on highest priority
Yes
300V DC loads
active in ROV?
No
7.1. Switch trip request by the operator
The operator demands to open the switch during
the following situations:
• low electrical insulation in the ROV power system;
• the need to locate an electrical problem in the
overall system; and
• low insulation or shorts in the 400m tether cable.
In this situation, the control logic is programmed
so that the switch power contacts open only after
the ROV loads are reduced. The flow of information
between the control systems is shown in the flowchart in Figs 12 and 13.
7.2. Loss of data exchange with TMS
This situation arises during the following situations:
• a TMS controller electronics failure, including
the processor and the input-output module;
• a 24VDC operating power supply input failure to
the TMS controller electronics; and
• failure of the fibre-optic media converter or its
operating power supply.
The TMS control system keeps the MV switch energised so that the ROV system is powered up. In this
situation, if the TMS controller fails it automatically
opens the MV switch. If the ROV system is in operation, then the switch has to operate under load.
To overcome this situation, the shipside control
system continuously exchanges data with the TMS
controller. When a loss of communication/failure
is detected within the stipulated time of 20 seconds,
the shipside systems issue the command to switch
off the main ship power input to both the TMS and
ROV, which ensures that the TMS switch opens
without load. This same control logic is implemented in the controllers in the TMS, the shipside
control system and the power electronics system.
7.3. Loss of ethernet communication in the
TMS and the ROV
Communications from the ship control system to
the ROV and the TMS ethernet links are essential
to ensure that ROV loads are switched off before
the switch opens. In addition to the optical ethernet
links between the TMS and the ROV, an RS-485
serial link is provided between the TMS and the
Send open command from
TMS controller to MV switch
Fig 12: Information exchanges on the switch open command
ROV controller, which is realised using shielded
twisted cables in the tether cable.
Whenever there is an ethernet link failure
between the ROV DTS and the shipside system, the
ROV communicates the critical data to the TMS
controller by the RS-485 link at a baud rate of 9,600.
This is then re-transmitted to the ship by the existing healthy optical ethernet link between the TMS
and the shipside system. Whenever an ethernet
link failure occurs in the TMS DTS, the TMS switchopen command from the pilot is sent to the ROV
controller through the ship control system, and the
data are transmitted to the TMS controller through
the RS-485 serial link.
The control logics have been implemented in the
ROV, the TMS and the ship control system. Failures
were simulated in the laboratory and during the
deep-sea trial of the ROSUB system, and it was
found that the performance was as set out in the
requirements.
8. Use of pressure-compensated oil circuit
breakers in high-power subsea applications
With the increasing demand for oil and gas, it is
required to establish deepwater installations with
long tie-backs. In the case of enhanced oil recovery
systems (Vedachalam, 2013a), power from the shore
installation is stepped to a voltage level suitable for
long-distance transmission and transmitted through
subsea umbilical cables. At the subsea end, the
voltage level is stepped down to suitable levels and
distributed to subsea consumers.
Distribution with controlled switching and protection is accomplished using industrial standard
circuit breakers housed inside pressure-rated enclosures with pressure-rated feed-throughs. For switching high voltages on the order of 22kV, current and
proposed subsea installations (Bjerkreim et al., 2009;
Skofteland et al., 2009; Henri et al., 2010) utilise
state-of-the-art Sulphur hexafluoride (SF6) circuit
breakers mounted inside pressure-rated enclosures.
211
Raju et al. Design and implementation of compact and robust medium voltage switchgear for deepwater work-class ROV ROSUB 6000
TMS Ethernet link status
Healthy
TMS Ethernet
link Failed?
Controller command
(open/close)
from TMS controller
Yes
Yes
Controller voltage
from super capacitor
in TMS
300V DC loads
active in ROV?
24V DC power from
TMS
No
Send command from ROV
controller to disable loads
on highest priority
Send open command from
TMS controller to MV switch
MV switch
Fig 13: Information exchanges between the control systems on the switch open command
A typical enclosure for housing six 22kV, 1250Arated SF6 circuit breakers and the associated control systems is approximately 3m in diameter and
6m in length and can weigh approximately 100 tons
when rated for a water depth of 1,000m. The same
enclosure with the increased wall thickness should
weigh around 800 tons when used for 6,000m
water depth (Vedachalam, 2013). This poses challenges in design, fabrication, handling, deployment, recovery, maintenance and logistics. Thus,
demand arises for circuit breakers of near-similar
electro-mechanical characteristics that are capable of being operated in a pressure-compensated
environment.
Minimum oil circuit breakers are also rated for
similar voltage and power levels with dimensions
and performances on a par with SF6 counterparts.
However, frequent oil maintenance is the Achilles
heel of high-voltage minimum oil circuit breakers,
and, as a result, oil circuit breakers are becoming
obsolete in the market.
Using the methodology discussed in the present
paper, multiple minimum oil circuit breakers can
be operated in an oil-filled pressure-compensated
enclosure, thus eliminating the need for pressurerated enclosures and feed-through. Thus, pressurecompensated oil circuit breakers could be a potential
alternative for future deepwater switching systems
with the following potential advantages:
• operation without the need for pressure-rated
enclosures and feed-through;
• enhanced breaking capacities;
• prolonged dielectric life; and
• enhanced breaker contact life.
212
9. Conclusion
The design and implementation of compact and
robust medium-voltage switchgear for a deepwater
submersible has been presented. A low-voltage air
break contactor is adopted to operate an MV power
circuit in the deepwater submersible system by taking
advantage of the subsea environmental conditions
of pressure and temperature so as to make it compact and rugged. The ruggedness of the switchgear
is further increased by incorporating systems to
ensure operation at no-load conditions.
Table 1 shows that the probability of failure of
power input to the ROV is reduced from 96.99% to
74.31%, which plays a significant role in increasing
Table 1: Probability of failure of the switchgear and power
to the ROV
Operating condition
Probability of failure
in 5 years
Power to ROV
Power
switching
input failure
failure in TMS
to ROV
When the switchgear is
operated in an oil-filled
environment (OF)
When the switchgear is
operated in an oil-filled
pressure-compensated
environment (OFPC)
When the switchgear is
operated in an oil-filled
pressure-compensated
environment (OFPC)
while ensuring no-load
breaking condition
96.68%
96.99%
76.75%
78.9%
71.69%
74.31%
Underwater Technology Vol. 31, No. 4, 2013
the MTBF from 1.44 years to 3.68 years. The possibilities of using the concept for other applications
in deepwater high-power switching systems and their
potential advantages in future deepwater systems
are worthy of further investigation.
Acknowledgements
The authors gratefully acknowledge the support
extended by Ministry of Earth Sciences, Government
of India, for funding this research. The authors wish
to thank all team members of the Submersibles and
Gas Hydrates group of National Institute of Ocean
Technology for their contribution and support.
References
Bjerkreim B, Frydelund S, Lie JA, Staver KO, Haram KO,
Nystad B, Skofteland H, Gedde H, Tesei A and Postic M.
(2009). Ormen Lange subsea compression pilot system.
Offshore Technology Conference, 4–7 May, Houston,
Texas, Paper no. OTC 20028-MS, 1–19.
Cooper Bussman. (2007). Design Considerations in Selecting
Aerogel Supercapacitor. PS-5501, 03/07.
FIDES. (2009). Reliability Methodology for Electronic Systems.
FIDES. 465pp.
Henri B, Truls N and Terene H. (2010). Ormen Lange
subsea compression station pilot. In: Petroleum and
Chemical Industry Committee Conference, Europe,
15–17 June, Oslo, Norway, 1–9.
Institute of Electrical and Electronics Engineers (1996).
IEEE Guide for diagnostics and failure investigation of
power circuit breakers. Piscataway, New Jersey: IEEE
Press.
Lemofouet S and Ruffer A. (2006). Hybrid Energy Storage
system based on compressed air and supercapacitors
with maximum energy point tracking. IEEE Transactions
on Industrial Electronics 53: 1105–1115.
Koch M, Fischer M, and Tenbohlen S. (2007). The breakdown voltage of insulation oil under the influence of
humidity, acidity, particles and pressure. In: Proceedings
of International Conference APTADM, 26–28 September,
Wroclaw, Poland, 1–6.
Manecius Selvakumar J, Ramesh R, Subramanian AN,
Sathianarayanan D, Harikrishnan G, Jayakumar VK,
Muthukumaran D, Murugesan M, Chandresekaran E,
Elangovan S, Doss Prakash V, Vadivelan A, Radhakrishnan
M, Ramesh S, Ramadass GA, and Atmanand MA. (2010).
Technology tool for deep ocean exploration – Remotely
Operated Vehicle. In: Proceedings of Twentieth International
Offshore and Polar Engineering Conference, 20–25 June, Beijing,
China. Cupertino, California: ISOPE, 372–385.
Mehnert TH. (1972). Handbook of fluid-filled, depth/pressure
compensated systems for deep ocean applications. Deep Ocean
Technology Program. Annapolis, Maryland: Naval Ship
Research and Development Center, 292pp.
Prevost TA. (2008). Oil Circuit Breaker Diagnostics. In:
Proceedings of 7th Annual Wiedmann Technical Conference, 15–17 September, New Orleans, USA, 1–10.
Ramesh R, Jayakumar VK, Manecius Selvakumar J, Doss
Prakash V, Ramadass GA and Atmanand MA. (2010).
Distributed Real Time control systems for deep water
ROV (ROSUB 6000). Communications in Computer and
Information Science 103: 33–40.
Riberio PF, Johnson BK, Crow ML, Arsoy A and Liu Y.
(2001). Energy Storage Systems for Advanced Power
Applications. IEEE Proceedings 89: 1744–1756.
Ruffer A, Hotellier D and Barrade P. (2003). A supercapacitor
based energy storage substation for voltage compensation
in Weak Transportation networks. In: IEEE Bologna Power
Technology Conference, Italy. 629–636.
Shell. (2001). Shell Diala Oil DX – High performance insulating oil, Datasheet. Available at www.vibarja.in/images/
products/shell/electrical_oil/Diala%20DX.pdf,
last
accessed <20 September 2013>.
Shukla A K, Sampath S and Vijayamohanan K. (2000). Electrochemical capacitors: Energy Storage and beyond batteries.
Journal of Current Science 79: 1656–1661.
Skofteland H, Hilditch M, Normann T, Eriksson KG, Nyborg
K, Postic M and Camatti M. (2009). Ormen Lange subsea
compression station pilot – subsea compression station.
Offshore Technology Conference, 4–7 May, Houston,
Texas, Paper no. OTC 20030-MS, 1–15.
US Department of Defense (DOD). (1991). MIL-HDBK217F Military Handbook: Reliability Prediction of Electronic Equipment. Washington, DC: US Department of
Defense, 150pp.
Vedachalam N, Ramesh R, Ramesh S, Sathianarayanan D,
Subramaniam AN, Harikrishnan G, Pranesh SB, Jyothi
VBN, Chowdhury T, Ramadass GA and Atmanand MA.
(2013a). Challenges in realizing robust systems for deep
water submersible ROSUB6000. In: Proceedings of IEEE
International Underwater Technology Symposium, 5–8
March, Tokyo, Japan, 1–10.
Vedachalam N, Ramesh R, Muthukumaran D, Aarthi A,
Subramanian AN, Ramadass GA and Atmanand MA.
(2013b). Reliability Centered Development of Deep
Water ROV ROSUB 6000. Marine Technological Society Journal
47: 55–71.
Vedachalam N. (2013). Review of challenges in reliable
electric power delivery to remote deep water enhanced
oil recovery systems. Journal of Applied Ocean Research 43:
53–67.
Weddell AS, Geoff MV, Kazmierski TJ and Al-Hashimi BM.
(2011). Accurate supercapacitor modeling for energyharvesting wireless sensor nodes. IEEE Transactions on
Circuits and Systems II 58: 911–915.
213