Power System for New MBARI ROV

Ed Mellinger
Monterey Bay Aquarium Research Institute
Moss Landing, California 95039

Abstract - MBARI is currently developing a Remotely Operated Vehicle (ROV) capable of 4000 m depth and designed exclusively for support of oceanographic research. Performance and operational requirements led to an all electric design for the basic core vehicle. The vehicle's electric power system, described in this paper, delivers 15 kW to the various onboard loads, and allocates power using preset priorities when demand exceeds supply. Features of the power system include ground fault and overcurrent detection, switch isolation of all loads, motor regeneration control, and limited dual redundancy. A novel transformer cooling scheme minimizes acoustic emissions from the main power transformer.

I. INTRODUCTION

The Monterey Bay Aquarium Research Institute (MBARI) is currently developing a Remotely Operated Vehicle (ROV) designed exclusively for support of oceanographic research [1]. Performance features for the vehicle's scientific mission include 4000 m depth rating, 100 kg payload with +/-35 kg variable buoyancy adjustment, precision 4 degree-of-freedom vehicle control, minimized acoustic emissions, fiber optic telemetry system, and full integration of vehicle, navigation, and science sensor data streams. Operational features include a quick-change payload toolsled, and extensive onboard fault detection and isolation capability.

The vehicle electrical power system is required to deliver and manage 15 kW of DC electrical power, primarily to meet the vehicle propulsion goals of 1.5 knot free speed and 0.75 knot full depth transit (i.e., with cable drag). The principal electrical load consists of 3.7 kW (mechanical output) brushless DC permanent magnet motors (Moog, Inc., East Aurora, NY), chosen for fast, linear control characteristics and low acoustic output. Six motors (two per axis) provide propulsion, with an additional motor powering the variable buoyancy and auxiliary hydraulic systems. Up to 5 kW of lights and 3 kW of electronics form the balance of the electrical load, with a mission-specific power block of up to 5 kW reserved for the payload toolsled package. Since overall power demand can readily exceed supply, a power management task in the main vehicle computer allows power to be allocated according to pilot selected priorities.

Power is transmitted to the vehicle at 1600 V using 400 Hz three phase AC. This is stepped down and rectified aboard the vehicle to produce 240 V and 48 V unregulated DC distribution voltages. These voltages vary over a 0 to ­25% range from no load to full load, which for convenience is expressed as a nominal voltage +/- 15%. Thus the 240 V supply actually ranges from 276 V to 204 V over no load to full load, and the 48 V supply ranges from 56 V to 40 V.

The vehicle power system can be conveniently divided into transmission and distribution systems, which are described in sequence below. As of this writing, the transmission system has been designed, and the distribution system has been designed, built, and tested, as part of prototype vehicle tests.

II. POWER TRANSMISSION SYSTEM

The power transmission system is defined to include the shipboard power source, step-up and step-down transformers, vehicle cable and tether, and power conversion equipment required to produce DC distribution power aboard the vehicle.

A. System Design

The power system design is closely tied to the design of the overall vehicle. The two are designed in an iterative process that starts with goals for vehicle payload, operating depth, speed, support ship size, and vehicle and cable technologies. In MBARI's case, the payload, depth, and speed were derived from science requirements, the ship size was fixed, and most technology choices were limited to progressive but not radical approaches by schedule and resource constraints.

Payload and depth requirements yielded an approximate vehicle size and frontal area, for propulsion system design. Once vehicle size, depth, and speed were tentatively set, the main cable, power transmission system, and propulsion system were co-designed in a separate iterative loop. Additional constraints during this process included an allowable cable peak insulation stress of 2.5 MV/m (65 V/mil), a desire to use the common cable diameter of 17.3 mm (0.68"), and the existence of the 3.7 kW propulsion motors mentioned above. The main parameters of the cable, i.e. ratios of steel, copper, and insulation (and hence operating voltage and current), were determined using the methods of Wilkins [2], as well as input from the cable vendor. Both AC and DC power transmission approaches were investigated, but AC was chosen due to lower developmental risk.

A simplified one-line diagram of the power transmission system is presented in Fig. 1. This diagram shows design values for voltage, current, kVA, and power loss throughout the system, at no load and full load operating points. The system end-to-end regulation factor, or ratio of power delivered to power lost in transmission, is 3.0; accordingly, full load voltage is 75% of no load voltage. This value was adopted after a survey of load voltage requirements, as a tradeoff between voltage regulation and power delivery capability.

B. Power Conversion

Power for the system originates in a 400 Hz, 60 kVA solid state converter (Pacific Power Source, Huntington Beach, CA; model 3060-MS) fed 60 Hz power from the main ship bus. This converter uses diode, as opposed to SCR, rectification in its input stage, which is expected to be less sensitive to input noise originating in the ship's 1700 kW diesel electric propulsion plant.

C. Transformers

The step-up and step-down transformers are worst-case modelled as ideal 96% efficient linear devices with purely resistive losses, though in practice there are separate no load (iron) and full load (copper) losses. The symmetrical delta-wye-delta arrangement of input, cable, and output circuits was chosen to minimize the current waveform crest factor presented to the converter [3], and also so that each transformer has a delta winding for harmonic current control and a wye high voltage winding for minimum insulation stress [4]. The shipboard step-up transformer is a conventional air cooled unit and is packaged, along with a ground fault detector for the high voltage cable segment, in a cabinet aboard ship.

The vehicle step-down transformer contributes significantly to vehicle mass and volume budgets, and of scientific importance, to the vehicle acoustic signature as well.

Transformer noise is largely due to core magnetostriction, and thus is present, and in fact maximum, when motors are off, loads are small, and input voltage is high, as during "quiet sub" operation. Reductions of transformer mass and volume are desirable, but these increase core flux level and winding current density, and thus increase both noise and thermal output. The keys to a small, light, quiet transformer thus became getting the waste heat out while keeping the noise in. This in turn meant breaking the acoustic path to seawater with an absorptive layer or a sharp discontinuity in acoustic impedance, while preserving high thermal conductivity. No suitable solid or liquid with this combination of properties could be found.

The solution adopted is to acoustically isolate the transformer using a gaseous vapor barrier, while using the vapor's latent heat of evaporation to carry the transformer's heat away. This requires a pressure housing with an inner tank, where the transformer is submersed in a suitable liquid. The liquid is boiled by heat from the transformer, and the vapor flows to the housing wall, condenses, and runs into a sump, where it is recirculated to the tank by a small pump. Very high heat transfer rates are possible since boiling carries away latent heat and also induces strong turbulence at the heat transfer surface. Tests to date on hardware models predict an overall temperature drop of less than ten degrees C from winding surface to seawater at full load (1000 W thermal).

The choice of liquid is obviously critical since it must have high dielectric strength in both phases, high latent heat, and material compatibility, not to mention low toxicity, environmental correctness, and low cost. Fortunately the Fluorinert family of perfluorocarbons (3M Industrial Chemicals Division, St. Paul, MN) meets all of these requirements except low cost ($120/liter), and is available in arange of boiling temperatures. FC-72, the lowest boiling fraction, was selected for this application.

Fig. 1 Simplified one-line diagram of power transmission system

The resulting vehicle step-down transformer assembly consists of three 8250 VA toroidal sections stacked and mounted coaxially in a cylindrical tank, which in turn is mounted coaxially inside a 27 cm (10.5 in) I.D. titanium pressure housing. (Diameter chosen for commonality with vehicle main computer housing.) The tank is supported within the housing by vibration isolators which minimize acoustic conduction. The housing outer wall has an area of roughly 0.4 m2 (4 ft2) in direct seawater contact, providing a heat sink with very low thermal impedance.

Since even a small amount of air interferes with vapor flow at the inner housing wall condenser surface, the housing is drawn down to a modest vacuum of 0.1 kPA, and then the transformer tank filled with approximately three liters of degassed FC-72. Internal pressure equilibrates at the vapor pressure of FC-72 at the temperature of the condenser surface, varying from about 10 kPa (1.5 psia) at 0 degrees C, to 110 kPa (16.3 psia) at 60 degrees C. The FC-72 flow rate at full load is approximately 0.7 kg/min, or 0.4 l/min. Two redundant small 48 VDC centrifugal pumps scavenge the sump and return fluid to the tank.

D. Cable and Tether

The main cable electrical optimization led to three 5.3 mm2 (#10 AWG) copper conductors with 0.93 mm (.036 in) wall thickness HDPE insulation. The neutral tether is more tightly constrained for copper content, and when used, will have three 1.3 mm2 (#16 AWG) conductors with 1.7 mm (.066 in) wall insulation.

E. Rectifiers and Filters

The vehicle power transformer has dual secondaries for the 240 VDC and 48 VDC distribution voltages. Each transformer secondary feeds two full wave bridge rectifiers that in turn supply separate A and B busses for each voltage. The four rectifier bridges actually contain Silicon Controlled Rectifiers (SCRs) which are fired by zero-crossing circuits and operate in on/off mode as electronic circuit breakers for their associated power busses. Fast fuses at each rectifier input protect against SCR or other catastrophic failure.

Each rectifier bridge is followed by an L-C filter that reduces output ripple voltage, and reduces harmonic currents drawn from the power transmission system. Filter design was dominated by a requirement that the 240 V bus output ripple be less than 0.5 V RMS, in order to avoid ripple current heating in bypass capacitors internal to the motors. This, plus cable power factor considerations explained below, led to the filter component values shown for the 240 V outputs. 48 V component values were less critical and were based partly on physical sizes that would fit into gaps between 240 V components; still, the 48 V bus ripple value is expected to be less than 24 mV RMS. Positive Temperature Coefficient (PTC) thermistors are used as constant-power capacitor bleeders, and can discharge the 220 J main bus capacitors to 0 V in about 30 seconds, while contributing only 10 W of heat to the housing thermal load at full voltage.

Rectifiers draw harmonic-rich currents from their input circuits [5]; any harmonics above the AC fundamental flow through, load, and stress the power transmission system, but do not deliver useful power. The effective power factor varies depending upon the filter impedance at the rectifier output, being a maximum of 0.96 for the six-pulse rectifiers employed here when operating into an infinitely inductive filter. Recent work [6] has quantified the relationship between power factor and filter inductance for more practical inductor values. This method was used, with a target power factor of 0.94, to size the filter inductors for each distribution voltage on the vehicle.

F. AC System Packaging

The rectifiers and filter inductors are high dissipation components, and are naturally related to the power transformer. Therefore, the inductors are mounted in the Fluorinert tank with the transformer. The rectifiers have associated fuses which cannot be submerged, so they are attached to the tank aluminum end wall, and dissipate heat through it into the liquid within. The assembly occupies a 41 cm (16 in) length in the 27 cm (10.5 in) diameter power housing, which is dubbed the "AC Side" of the housing. Input high voltage AC power, or low voltage DC for shore testing, are supplied through underwater connectors in one end of the housing. Raw DC power from the filter inductors leaves the AC section through eight high current blind mate contacts in a gas tight bulkhead mid-housing; the bulkhead divides the AC and DC sections of the housing, and the bus filter capacitors are located in the DC section along with other power distribution system components.

G. Redundancy and Fault Tolerance

Two design features increase the operational availability of the vehicle power transmission system. The first is the use of dual power busses for each distribution voltage. The system is not fully redundant, although critical loads such as the main computer draw power from both A and B busses through diode-OR circuits. Even without full redundancy, however, less than half of the vehicle functionality is lost through the loss of any one power bus. For example, thrusters are arranged so that failure of one 240 V bus leaves one vertical plus two horizontal thrusters available (lateral or fore-aft), which allows yaw control, translation, and vertical motion.

A second design feature is fault tolerance, which is achieved through coordinated overload protection plus the ability to selectively isolate loads using switches in the distribution system. Coordinated overload protection simply means that fuse and circuit breaker current-time characteristics are selected so that the overcurrent device closest to the faulted load trips first, allowing operation on the non-faulted part of the system to resume with minimal interruption. The circuit breakers also function as controlled switches, and are commanded to disconnect loads when a ground fault is sensed on the associated supply bus, again allowing operations to continue.

III. POWER DISTRIBUTION SYSTEM

The power distribution system is defined to include the four DC busses, power switches, ground fault detection system, and motor regeneration control system.

A. System Design

Requirements for the distribution system design were to distribute and control 15 kW of 240 VDC power and 2 kW of 48 VDC power on each of the A and B busses. This allows either A or B bus to operate to capacity if the other bus is faulted, and leaves room for future upgrades to the transmission system as well. General goals included support for the distributed Data Concentrator architecture adopted for the vehicle data system [7]; the ability to detect ground fault conditions on any circuit passing through seawater; the ability to switch off and fully isolate any faulted load circuit; and minimization of personnel exposure to 240 VDC circuits and wiring.

Distribution voltage selection was based on vehicle performance and personnel safety issues. 240 VDC was selected as a logical next step upward from the 120 VDC of manned submersible practice, after it became apparent that the 5 kW demanded by the largest loads would require large and heavy switches, connectors, and wiring at 120 V. The 240 V bus voltage is in line with emerging practice for 270 VDC aircraft power distribution, and 270 VDC is also the natural product of full wave rectification of 120/208 three phase AC. 48 VDC was selected as the highest industry standard voltage that can be considered "low voltage" for safety purposes.

Safety was a major factor in the design of the distribution system. 240 V circuits are restricted to high power loads that are not frequently opened, and the circuits appear in only a limited number of wiring junction boxes. Both the 240 V and the 48 V systems are fully isolated from frame ground, and ground fault monitor circuits (described below) warn if the impedance to ground falls low enough to cause a hazardous condition. Finally, in the event of contact with a live circuit, DC voltage is safer than equivalent voltage AC. This is because the body current "let-go threshold", beyond which voluntary muscular control is lost, is much higher for DC (76 mA) than for AC (16 mA) current at power frequencies [8].

B. Power Switching, Telemetry, and Control

For reasons of fault tolerance mentioned above, power switches were required for each load, or group of loads, on the vehicle. Ground fault isolation required that both sides of the load be switched. Switch control and telemetry had to be part of the vehicle's distributed data system, which is implemented as multiple small "Data Concentrator" computers based on the 15 cm (6 in) diameter Instrument Bus Computer (IBC) backplane and form factor [9]. Based on these requirements, Low Power and High Power Switch cards were designed for use in the Data Concentrators.

The Low Power Switch card has four DPST switches of 4 A capacity, rated for 60 V. The switch elements are power MOSFETs, driven directly by photovoltaic optoisolators. Shunt resistors allow current to be sensed by an onboard A/D converter and reported over the backplane. The switches share a common input power bus, which can be supplied externally, or generated onboard by a 35 W (thermal limit) DC-DC converter.

High power DC switching is more difficult, due to two practical issues. The first is that mechanical switching elements require elaborate arc suppression measures (vacuum or arc blowout), since unlike AC current, DC has no naturally occurring zero crossings that allow the arc plasma to dissipate. The second issue is that solid state switching elements inevitably have a few volts of "on" state voltage drop, and generate dozens of watts of waste heat. Both problems make compact packaging difficult.

The High Power Switch card has one DPST switch of 25 A capacity, rated for 300 V. The solution adopted is to parallel a conventional AC relay with solid state devices, in this case Insulated Gate Bipolar Transistors (IGBTs). The IGBTs provide arcless make and break for the DC current, while the relay contacts carry the steady state load with only a few watts loss. An additional set of current limited IGBTs are switched in advance and precharge any capacitance in the load, for example, the 150 uF internal bypasses in the thruster motors. Logic on the card sequences the switching events and responds to overloads, and a shunt resistor and A/D converter allow current to be sensed and reported.

C. Ground Fault Detection

As mentioned above, all vehicle electrical systems are fully isolated from frame (seawater) ground. The insulation resistance must be continuously monitored for reasons of safety, and also to provide early warning of seawater intrusion. The "floating ammeter" ground fault detector shown in Fig. 2 is used. Conceptually, each side of each supply voltage is alternately connected to frame ground through a current limited ammeter. If a ground fault exists on the opposite supply rail, current will flow through the meter. This approach can be extended to monitor several supplies of differing voltages with a shared common rail, at the expense of a more complex troubleshooting flowchart.

The ground fault detector is implemented as a four channel unit and shares an IBC card with the 5 V backplane logic supply, since both functions are required in every Data Concentrator. A shunt resistor, isolation amplifier, and A/D converter form the floating ammeter. Four photovoltaic relays form a SP4T switch that allows four supply rails to be alternately connected to frame ground. Switch sequencing and A/D converter operation is controlled over the IBC backplane. Typically three supplies plus common, or two fully independent supplies, can be monitored.

A few wrinkles affect the ground fault detector operation. Stray or bypass capacitance, between any monitored supply rail and frame ground, requires time to charge through the ammeter current limit of 1 mA (chosen for safety reasons). The time is usually a few tenths of a second, but can be several seconds on the motor circuits due to large 4 uF frame bypass capacitors internal to the motors. This slows the detector's scan rate. A second minor problem is that the switch transistors in both the Low Power and the High Power Switches have integral body diodes that can conduct during ground fault sensing. This does not cause danger or damage, but does further complicate the ground fault isolation algorithms.

Fig. 2 "Floating Ammeter" ground fault detector

D. Regeneration Control

Motors act like generators during braking, i.e., when motor torque and shaft rotation are of opposite sign. An analysis by the thruster motor designer [10] predicted that a 50 msec stop from full speed with a prop in air would generate a peak power of 3.8 kW and total energy of 90 J; with the prop in water, power and energy predictions were about half these values. This regenerated ("regen") energy must go somewhere, and has the potential to drive the 240 V bus voltage to destructive levels. High bandwidth thrust control, necessary for precision vehicle control, is expected to require frequent and repetitive motor braking, in order to minimize thruster response time. Since little was known about actual regen loads in a real vehicle, a capable regeneration control system was designed.

A separate regeneration controller, conceptually a 300 V, 18 kW zener diode, protects each of the 240 V A and B busses. Each regen controller occupies a single IBC card, and has two separate but synchronized channels to provide a degree of fault tolerance. Each channel has an IGBT switch which is Pulse Width Modulated (PWM) at approximately 1 kHz. The PWM controller varies the switch duty cycle from 0 to 100% over the input voltage range of 295 to 305 V. With a 10 ohm power sink, this means that the current drawn from the bus varies from 0 to 30.5 A over this voltage range. The 10 ohm resistances are 5 kW industrial cartridge heaters mounted in thermal contact with seawater. The two channel regen board can sink 61 A (or about 18 kW) peak, but is thermally limited by IGBT heat sink capacity to a continuous (1 minute) average of about one sixth this value, for 125 degree C junction temperatures. The PWM control circuits operate directly off the 240 V bus and do not require IBC logic power to operate. An A/D converter and isolation amplifier on the card allow the IBC to monitor the bus voltage, switch duty cycle, and heatsink temperature for telemetry purposes.

E. DC System Packaging

The DC power control system occupies the second half of the power housing mentioned above, requiring 38 cm (15 in) in the 27 cm (10.5 in) diameter housing. A photo of the power control assembly is shown in Fig. 3. Raw DC power from the AC side of the power housing enters through the high current blind mate connector pins visible on the right hand side of the assembly. These pins are bolted directly to two "Filter/Distribution Boards", one each for A and B busses, one of which is visible on top of the assembly. These boards use full width copper traces to conduct 75 A at 240 V and 50 A at 48 V, to rows of terminal strips at the back and side of the board (not visible). The boards also support feeder fuses and the bus filter capacitors, visible in a row on the top board.

The DC section also contains two Data Concentrators mounted back to back, one each for A and B power busses, one of which is visible in the bottom center of the photo. Distribution wiring runs from the Filter Board terminal strips, to the Power Switch cards in the Data Concentrators, and then to underwater connectors on the power housing end bell, on the left side of the photo. A 0.6 m3/m (20 cfm) fan inside the end bell circulates air in a serpentine path throughout the assembly, at a rate dependent on internal air temperature.

F. DC System Test Results

The DC power distribution system described has been built and tested in support of prototype ROV tests at MBARI. The high and low power switches were bench tested over 10,000 full load make/break cycles, and the regen controller over 300,000 simulated full load regen events. The power control assembly pictured was bench tested at full load current, inside a plastic simulated pressure housing, to verify thermal loads. The system was integrated and tested on the vehicle using DC power supplied over a short test tether; this revealed a common mode noise coupling problem between the regen controller and the high power switch, which was fixed. As of this writing the power distribution system is operational and is being used daily in support of dock tests of the prototype vehicle.

ACKNOWLEDGMENTS

Ned Forrester of WHOI provided much helpful discussion and advice. At MBARI, Mark Chaffey shared in the design of the power distribution architecture, Mike Matthews designed the High Power Switch, and Janice Tarrant designed the power management software. Fitzgerald Smith and Carolyn Todd expertly procured and built the hardware. Funding was provided through the generosity of the David and Lucile Packard Foundation.

Fig. 3 DC power control assembly

REFERENCES

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      10. C. Albrecht, private communication.
      11. Assembled DC and AC Power Assembly Power Transformer Assembly