2 Learning ObjectivesExplain the rating of a Ships Service Diesel GeneratorDraw and explain the simplified diagram of a warship’s 450-VAC/120-VAC distribution system.Discuss the concept of vital and non-vital busses and the loads typically powered by each.Explain the purpose of shore power and how it is paralleled with ship’s power.Define a bus, bus-tie breaker, split-plant operation, and parallel plant operation.
3 Bus (Switchboard)A Bus is a conductor that connects multiple circuits or loads to a common voltage supply.Large Loads and Power Distribution panels are connected to a bus via a Breaker.A bus is physically heavy gauge wire or a metal bar.You can think of it as a VERY LARGE circuit breaker box.
4 Bus Supply breaker Load breakers Reactor Coolant Pump Main Seawater PumpPower Distribution panelPort SSDG2S BUS
5 Busses… Busses are categorized as VITAL or NON VITAL Typically located in a SWITCHBOARD, which is a watertight enclosureUsually numbered 1S, 2S, 1B, 2B, 1E, 2EOdd numbers are Starboard bussesS=Ship Service (450 VAC, 60 HZ)B=Battery Bus (High Voltage DC)E=Emergency Bus
6 Power Distribution panel Supplies groups of loads in close proximity or of similar typeCan also feed other distribution panelsTypically look like a circuit breaker box
7 Vital LoadsVital loads are electrical loads deemed critical to safe operation of the vesselbilge pumphydraulic steering pumps1MC systemReactor Coolant PumpsLighting (required to be able to successfully combat any casualty)For maximum reliability, vital loads should be capable of being powered by different sources.
8 Non-Vital LoadsNon-Vital loads are not critical to safe operation of the vesselgalley powerHot water heaterFans and ventilationBecause reliability isn’t critical, non-vital loads are typically powered by a single source
9 Typical Bus Arrangement Vital power panel#2 (Port) SSDGNorm.Alt.Shore Power2SNon Vital power panel#1 (Stbd) SSDG1S
10 Circuit Breakers Designed to trip on overcurrent Usually have both short-term and long-term time-current characteristicsA breaker will trip on 100 amps after 1 sec, but will allow 15 amps to pass for 10 seconds before trippingThis allows loads requiring short-duration large startup currents to operate (such as AC motors)
11 Motor Operated Breaker Adjustable Over Current and Time Delay Settings
12 Circuit BreakersSupply Breakers allow the source to be started or secured while isolated from the network. They also protect the supply from faults on the network.Bus-Tie Breakers connect buses together.Feeder Breaker connects busses to power distribution panelLoad Breakers Isolates a component from the Bus
13 Typical Bus Arrangement Feeder BkrSupply BkrVital power panel#2 (Port) SSDGNorm.Alt.Individual loadBus Tie BkrShore Power2SLoad BkrNon Vital power panel#1 (Stbd) SSDG1S
14 Split Plant (Full Power Lineup) Port & Stbd SSDGs operating, Bus Tie Bkrs OpenVital power panel#2 (Port) SSDGNorm.Alt.Shore Power2SNon Vital power panel#1 (Stbd) SSDG1S
15 Split Plant Operations Port & Stbd busses isolated from each otherElectric Transient on one bus does not affect the other busVital Power Distribution Panel can be quickly powered from stbd bus if the port bus is lostUsual mode for maximum reliability
16 Cross Connected (Parallel) Plant Ops Port & Stbd SSDGs operating, Bus Tie Bkrs ShutVital power panel#2 (Port) SSDGNorm.Alt.Shore Power2SNon Vital power panel#1 (Stbd) SSDG1S
17 Cross Connected Operations Electric Transient on one bus can affect the other bus, possibly resulting in loss of all ACIf SSDGs are not balanced correctly, uneven load sharing may occur
18 Half Power (Single SSDG) Plant Ops Stbd SSDG operating, Bus Tie Bkrs ShutXVital power panel#2 (Port) SSDGNorm.Alt.Shore Power2SNon Vital power panel#1 (Stbd) SSDG1S
19 X X Shore Power Plant Ops Both SSDGs secured, Bus Tie Bkrs Shut Vital power panel#2 (Port) SSDGNorm.Alt.Shore Power2SNon Vital power panelX#1 (Stbd) SSDG1S
20 Shore Power LineupIn port operations; used because less wear on rotating equipment and a smaller watchsection requiredNo inherent redundancy or reliability, but no combat operations in port = not required.
21 Designed for Reliability Electric Plant is designed to be reliableA faulted piece of equipment will not bring down the entire distribution system.Fault: A piece of equipment is shortedShort circuit causes generator to see near zero resistance; generator must supply near infinite current.Too much current is an overload; with no protective action (breaker opening, fuse blowing) excessive heat and fires can result.
22 Designed for Redundancy Failure of any single component does not cripple the ship.Failure of a single supply will not cause a complete loss of power.
23 Designed for Redundancy Considerations For Plant RedundancyMultiple Power SourcesPort and Starboard GeneratorsMBT or ABT bus transfer switches ensure power is always available to vital componentsVital electric equipment redundantEx: 2 lube oil pumps, different and independent power supplies to each.Ex: 1 power plant control computer, two independent power supplies.
24 Ships Service Power 450-V, 60-Hz, 3-phase AC is Navy Standard Prime Movers onboard US Ships:Turbine Generators (SSTG)Diesel Generators (SSDG)
25 Shipboard AC Overview Majority of the loads are AC powered 60 Hz 450 VACProduced by generatorsRequired by large machines: pumps, air conditioners120 VACStepped down by transformersRequired by lighting and most electronic equipmentMany 120 VAC loads are single phase
26 120 VAC Power Distribution Three Phase transformer has 450 Vac on primary side and 120 Vac on secondary.What is the turns ratio?
27 Requirements prior to Parallel AC Power Source When two different sources or loaded busses need to be connected, we must Parallel them.This means Frequency and Phase Angle must be matched before shutting a breakerShutting a breaker with a large phase difference:Arcing and sparking = fireEquipment could be reverse powered (running equipment could be overloaded).
28 Requirements prior to Paralleling Voltage matchedPhase sequence matchedIncoming Frequency slightly greater than running (so incoming machine will take load)In phase
29 Requirements prior to Paralleling A Synchroscope is used to compare the frequency difference between the two sources.
30 Navy Ungrounded Systems Navy ship-service equipment is UNGROUNDEDThe neutral wire (center of Y) is not bonded (grounded) to the ship’s hull or electrical cabinetsThis is for EQUIPMENT RELIABILITY, not personnel safety
31 Grounded System: Personal Safety The person is touching cabinet when one of the electrical phases shorts to it.The current from the fault flows directly back through the grounding strap to the neutral line on the generator WITHOUT passing through person’s body.Insulated wiresCircuit breaker cabinetMachine casingSSDGLoadGrounding strapGrounding strapShip’s Hull, deck plating, etc.
32 Ungrounded: No Personal Safety The person is touching cabinet when one of the electrical phases shorts to it.A small current will flow through the impedance of the ship’s wiring.The current from the fault will seek the least resistant path, which may include the person’s body.IT ONLY TAKES 100 mA TO KILL YOU!SSDGLoad
33 Navy Ungrounded Systems EQUIPMENT RELIABILITY is the issue!If a piece of equipment shorts to the hull, that piece of equipment usually will continue to operateSince the hull is not grounded to the neutral of the generator, only a very small current flows.Small current = no fire!Small current = circuit breakers don’t trip, equipment continues to run!Electrical plant operator checks for grounded equipment once per hour. If a ground is discovered, equipment is sequentially switched around until the grounded equipment is found.If two different pieces of equipment on different phases are shorted, circuit breakers trip and/or a fire occurs!