High Voltage Generation and Distribution
4.8 Basic features of a Marine HV Power Supply and Distribution System
High Voltage Generation and Distribution
Figure 4.25 Isolation of the NER
Chapter 4
Figure 4.26 Typical Marine HV Power Supply and Distribution System
Large power consumers such as thrusters, propulsion motors, air-conditioning compressors and HV transformers are fed directly from the HV switchboard. An economical HV system must be simple to operate, reasonably priced and one that requires a minimum of maintenance over the life of the ship. On a HV powered ship with electric propulsion, each generator may be rated at 10 MVA or more and produces 6.6 kV, 60 Hz three-phase AC voltages.
The principal consumers are the two synchronous AC propulsion electric motors (PEMs) that individually demand 12 MW e e a a c d . Eac P
Electrical Motor (PEM) has two stator windings supplied separately from the main HV switchboard via transformers and frequency converters.
In an emergency, a PEM may therefore be operated as a half-motor with a reduced power output. A few large induction motors are supplied at 6.6 kV from the main switchboard or cargo switch board auto transformer starter, with the vacuum contactor acting as a direct-on- line (DOL) starting switch or soft starters and a variable speed drive.
High Voltage Generation and Distribution
4.8.1 High Voltage System for a Liquefied Natural Gas Carrier
One or two diesel generators and two turbo generators are generally installed; they can supply both main switchboards independently, but under normal conditions the two switchboards will be linked. However, modern vessels can operate with more than this number (of generators).
4.8.1.1 Salient Features
1. The generator is usually rated more than 3 MVA at 6.6 kV, 3-phase, 60 Hz and could be driven either by a diesel engine or a steam turbine. It is a totally enclosed, self-excited or separately excited, brushless machine.
2. The breakers are normally operated by the power management system, but also can be operated manually (at the switch board). The electric power system is thus designed with discrimination on the distribution system, so that the generator breaker is the last to open if any abnormalities occur.
3. The voltage is kept constant by controlling the excitation current to the exciter. Output power from the stator is fed into a current / voltage compound transformer and the output ec ed a d ed e e c e a d . T e agnetic field in the e c e a d ce AC e e c ed , c ec ed b e a d de a d passed to the DC main rotor windings. The initial voltage build-up is achieved by residual magnetism in the rotor. Constant voltage control is achieved by the automatic voltage
e a , c a a ab e c e e e c e d a a
de ee e AC a a e c a .
4. The generator is cooled by passing air over an integral fresh water cooler, using a closed- circuit air supply. The cooling spaces are fitted with internal baffles to prevent water from reaching the stator windings in the event of any cooler leakage. Water leakage and temperature sensors are fitted in each air cooler.
5. Space heaters which are energised when the generator circuit breakers are open, protect the generator against internal condensation during shut down periods. An embedded
Chapter 4
8. The starting of large motors is prevented until there is sufficient power available; the diesel generator will be started to meet the shortfall in case the turbo generator cannot sustain the load. Two generators will be required to operate in parallel when:
a. Discharging cargo.
b. Loading cargo.
c. Manoeuvring with the bow thruster in use.
The diesel generator will thus start and connect to the bus bar under the following conditions:
R Ge e a c c b ea e ab a ( .e., a dead bus due to a blackout situation).
The bus parameters are abnormal (i.e., high or low voltage and low frequency).
The generator will start, be synchronized and get connected to the bus bar to run in parallel with proportional load sharing in case the circuit breaker of a running turbo generator that was paralleled, trips abnormally (and the bus is still alive). In case of an over load condition, the preferential trip system will shed non-essential loads; however, rather than preferential tripping, it is preferred to have a standby diesel generator that will start, synchronize, connect to the bus bar and run in parallel with proportional load sharing.
4.8.2 High Voltage Supply Generation and Distribution in Gas Tankers
A HV Power supply system in gas tankers generally has 3,300 volt generators that are run by prime mover run by a steam turbine, with the steam generated by utilising boil-off gases in the boiler.
TG 1 supplies HV switchboard 1 and TG 2 supplies HV switchboard 2. Both the HV switchboards are connected by the bus tie breakers TB1 and TB2 which are normally connected so that it is possible to supply all the load from both the switchboards either by T/G 1 or T/G 2 or both the T/Gs running in parallel. Bus tie breakers TB 1 and TB 2 trips automatically in case of a fault such as a short circuit in any of the HV switchboards or can be manually tripped in case of an isolation requirement of the HV bus bar for maintenance.
High Voltage Generation and Distribution
TG 1 2500 kVA 3300 V HV Sw Bd 1 TG 2
2500 kVA 3300 V HV Sw Bd 2
3.3 kV Cargo Sw. Bd 2
3.3 kV Cargo Sw. Bd 1
440 V Cargo Sw. Bd 2 440 V Cargo Sw. Bd 1
440 V Sw. Bd 2 440 V Sw. Bd 1
220 V Section
220 V Section 220 V
Section Emergency Sw. Bd
No. 1
3.3 kV / 440 V Tr
No. 2
3.3 kV / 440 V Transformer
TG1 TG2
TB2 TB1
TC2 TC1
CC2 CC1
CB2 CB1 CL1
CL2
No. 2
3.3 kV / 440 V CSBD Transformer
No. 1
3.3 kV / 440 V CSBD Transformer
LL2 LB2 LL1 LB1
No.1 DG
440 V 1380 kVA No.2 DG
440 V 1380 kVA
No. 2 440 V to 220 V LV Transformer
No. 2 440 V to 220 V LV Transformer Shore Power
Connection Box
440 V to 220 V Em. Transformer Em. DG
440 V 1360 kVA Interlocks
EG1 EG2
GL2 GB2 GE2 GE1 GB1 GL1
Figure 4.27 Power Supply Distribution on a Gas tanker
HV motors such as the bow thruster and ballast pumps are directly fed from HV switchboard.
The cargo switchboard 1 is supplied from the HV switchboard 1 by the TC1 and CC1 feeder breakers. Similarly, cargo switchboard 2 is supplied from the HV switchboard 2 by the TC2 and CC2 feeder breakers. All HV motors for cargo pumps and other cargo equipme
like compressor are fed from both HV cargo switch boards.
The HV power distribution systems are so arranged that in case of fault in any of the main HV switchboard, Cargo switch board 1 and 2 can be connected by the bus tie breaker CB1 and CB 2.
Two transformers rated at 3300 V / 440 V from both cargo switch board feed 440 Volt the
Chapter 4
A similar arrangement is made for 440 V 1 and 2 auxiliary switchboards which are normally fed from two 3300V / 440 V transformers feeding independently the 440 V auxiliary switchboards 1 and 2. Redundancy for 440 V supply availability is done by GB1 and GB2 bus tie breakers in case of faults in any of the main HV switch boards.
A Further development of the HV supply distribution system is a 6600 V system which is most common in commercial vessels. Standby D/G 1 and 2 also feed the HV switchboard.
High Voltage Generation and Distribution
Chapter 4
However, there were some concerns about the availability of power supply in case of faults under certain conditions; for example, assuming that DG is under maintenance, DG 1 is on standby mode and TG 1 is running and feeding supply to all loads; if there is a short circuit fault condition in HVSB 1, TG 1 circuit breaker T1 will trip.
Even though standby DG 1 starts, the circuit breaker cannot be closed because of a short circuit condition in HVSB 1. So, running the TG and Standby DG cannot be from same the switch board. If DG 2 cannot be started, then TG 2 should be the running generator; DG 1 can be on standby so that there will not be any problem for the availability of power in case of a fault. To solve this problem and to have 100% availability of power supply, the distribution system is further modified as shown in Figure 4.29.
One generator from both the HV switchboards has the option of connecting to either switchboard; for example, DG 2 can be connected to 1 HV MSB 1 by the DGB 2A circuit breaker or can be connected to HV MSB 2 by the DGB 2B circuit breaker and similarly is the case for DG 3.
So now DG 1 can be the running generator and DG 2 can be the standby generator, which will supply HV MSB 2 in case of a short circuit fault in HV MSB 1.
High Voltage Generation and Distribution
Figure 4.29 An Alternate 6600 V Power Supply Distribution System
DG 4 DG 3 DG 2 DG 1
No.2 HV MSB No.1 HV MSB
EG Shore (SC)
EGB SHB
MESB1 MESB2
DS
CESB ESB
No. 1 LVMSB
No. 2 LVCSB No. 1 LVCSB
No. 2 LVMSB
No. 2 HVCSB No. 1 HVCSB
DGB4 DGB3 DGB2B DGB3A DGB2A DGB1
HMBTB2 HMBTB1
HMCSB2 HMCSB1
HMTRB2 HMTRB2
HCBTB2 HCBTB1
HCCSB2
HCTRB2
HCCSB1
HCTRB1
LCTRB2 LCTRB1
LCTB1 LCTB2
LCESB
LMTRB2 LMTRB1
LMESB1 LMESB2
LMBTB2 LMBTB1
Chapter 4
HV MSB
DGB2A and DGB2B are mutually interlocked only ONE of them can be closed
DGB3A and DGB3B are mutually interlocked only ONE of them can be closed
HMBTB1 and HMBTB2 are normally closed.
HMTRB1/ HMTRB2 will be open when LMTRB1/ LMTRB2 is open
If there is a Bus short circuit fault #1 or #2 HVCSB, then following breakers will be tripped condition.
Short circuit fault in # 1 HVMSB: DGB1, DGB2A, DGB3A, HMBTB1, HMBTB2 Short circuit fault in # 2 HVMSB:
DGB3, DGB 4, DGB2B, HMBTB1, HMBTB2
HV CSB
HCBTB1 and HCBTB2 are normally open HCCSB1, HCCSB2, HCBTB1 and HCBTB2
are mutually interlocked only three of them can be closed
HCTRB1/ HCTRB2 will be open when LCTRB1/ LCTRB2 is open
If there is a Bus short circuit fault #1 or #2 HVCSB, then following breakers will be tripped condition.
Short circuit fault in # 1 HVCSB: HMCSB1, HCCSB1, HCBTB1 and HCTRB1
Short circuit fault in # 2 HVCSB: HMCSB2, HCCSB2, HCBTB2 and HCTRB2
LV MSB
1) LMBTB1and LMBTB2 are normally open 2) LMTRB1, LMTRB2, LMBTB1 and LMBTB 2
are mutually interlocked only three of them can be closed
3) LMTRB1/LMTTRB2 can be closed only when HMTRB1 / HMTRB2 is closed
LV CSB
1) LCTB1 and LCTB 2 are normally open
2) LCTRB1, LCTRB2, LCTB1 and LCTB 2 are mutually interlocked only three of them can be closed
3) LCTRB1/LCTTRB2 can be closed only when HCTRB1 / HCTRB2 is closed
ESB
1) MESB1, MESB2, ECB andSHB are mutually interlocked only one of them can be closed.
Interlocked is released when EG or SC feedback to LVMSB.
2) MESB1, MESB2and CESB are mutually interlocked so that only one of them can be closed.
3) When feedback mode, LMTB1 and LMTRB2 cannot be closed.
Table 4.1 – Interlocks in the HV supply Distribution as Shown in Figure 4.29
High Voltage Generation and Distribution
4.8.3 Radial / Tree-type Electrical Power Distribution System
A radial electrical power distribution system has one major drawback that in case of any feeder failure, the associated consumers would not get any power as there is no alternative path to feed the transformer. In case of a bus failure in either HV switchboard, the power supply is interrupted. In other words, the consumer in a radial electrical distribution system would be in deprived of the power supply until the feeder or transformer is rectified.
With reference to Figure 4.30, considering that there is a fault in the HVSB 1 bus bar, DG 1 and DG 2 b ea e a d e b e b ea e be ed b e ec e a . Hence there will not be any power supply to the 6600 / 440 V reefer transformers (that feed the reefer containers). It will be a matter of concern to maintain the temperature of the reefers;
there are also limitations in feeding these containers by connecting portable cables from other spare supply sockets of the transformers that are fed from HVSB 2.
Engine Room 1.5 MW Bow Thruster
Engine Room 6.6 kV / 440 V
6.6 kV / 440 V 400 kW
Main LO Pump
Passageway
Passageway
Passageway Passageway
Passageway
DG2 DG3 DG4
M
um Voltage SwitchboardBus tie
Forward
M
440 V 60 Hz Main Switchboard
Bus tie
4 MVA
2 MVA
Distribution Boards for Reefer Container Sockets Engine Room
DG1 4 MVA
2 MVA
SC
Distribution Boards for Reefer Container Sockets
Chapter 4
4.8.4 Ring Net Topology
The drawback of a conventional tree / radial electrical power distribution system can be overcome by introducing a ring net electrical power distribution system. Here, one ring net network of distributors is fed by more than one feeder. In this case, if there is a fault in one feeder or it is under maintenance, the ring net distributor is still energized by other feeders that are connected to it. In this way, the supply to the consumers is not affected even when any feeder is out of service.
In addition to that, the ring net system is also provided with different sections that are isolated at suitable points. If any fault occurs in any section of the ring, that section can be easily isolated by opening the associated section isolators on both sides of the faulty zone. In this way, supply to the consumers connected to the healthy zone of the ring, can be easily maintained even when one section of the ring is shutdown.
Passageway M
RTR 2/3
Ring 2
RTR 2/2
RTR 2/1 DS 2/2
DS 2/3 Ring 1
R2
R1 R3
HVSB2HVSB1
RTR 2/4
DG4
DG3
DG2
DG1
M
Lighting Hold Fan
Deck Machinery Fwd Lighting Passageway
Hold Fan
Deck Machinery Fwd
Passageway
Passageway
Passageway Passageway
Passageway Passageway
Main Switchboard
Shi Load Shi Load
NC Medium Voltage Switchboard NC
Circuit Breaker Load Connector
Distribution Board for Feeder Sockets M
Main L.O. Pump
Main L.O. Pump Earth Point
Earth Point
Earth Point Earth Point
Bow Thruster
NC
NC
NC
NO
NO
NC
NC
NC
NO
NO
NO
NO
NC NC NC
NC
NC
NO
NO
Figure 4.31 A Ring Net Power Supply Distribution
High Voltage Generation and Distribution
It can be seen in Figure 4.31 for the ring net power supply distribution, there are two ring net power distribution circuits; Ring 1 supplied by R1 form HVSB 1 busbar and supplied by R3 form HVSB2 Busbar. Similarly Ring 2 supplied by R3 form HVSB 1 busbar and also supplied by R4 form HVSB2 Busbar. The reefer transformer rooms are fed in series by the disconnectors (NC). DS are normally always closed other than any time there is fault in the ring. This can be used for isolation of a faulty reefer transformer room in that ring.
Considering Ring 1 is supplied by the R1 circuit breaker, now if there is short circuit fault in HVSB 1 bus bar, DG1, DG2 and the bus tie breaker will trip; R1 circuit breaker will also trip due to under voltage. Now Ring 1 can be supplied by R2 circuit breaker from the live HVSB 2 bus bar.
Another advantage of a ring main system is that if any fault occurs in any section of the ring, this section can easily be isolated by opening the associated section isolators on both sides of the faulty zone.
Let us consider a fault condition in one of the transformers in Ring 2 reefer transformer room RTR 2/2. R4 circuit breaker which was feeding that ring 2 will be tripped by the protection relay. In order to isolate the fault in Ring 2, first R3 and R4 circuit breakers isolation procedures will be carried out and disconnectors DS2/2 in the RTR 2/1 and DS2/3 in the RTR 2/3 reefer transformer room can be switched off so that the faulted transformer in transformer room RTR 2/2 is isolated.
Under this condition as ring 2 is open circuited, power supply to reefer transformer room RTR 2/3 and RTR 2/4 can be supplied by switching on R4 circuit breaker and reefer transformer room RTR 2/1 can be supplied by switching on circuit breaker R3 as a conventional / radial system. Whenever a fault in the reefer transformer is solved or the faulty transformer is isolated, Ring 2 can be once again reverted back by connecting disconnectors DS2/2 in the RTR 2/1 and DS2/3 in RTR 2/3 reefer transformer rooms.
4.8.4.1 Interlocking of a Circuit Breaker for the 6600 / 440 V Transformer With reference to Figure 4.32, the following interlocks function:
Chapter 4
3. MBT can be closed with manual synchronisation when both sides of MBT are alive.
4. LR1 (LR2) can be closed when HR1 (HR2) is closed.
5. LR1 (LR2) can be opened automatically when HR1 (HR2) is opened.
6. HR1 (HR2) can be opened automatically when LR1 (LR2) is opened.
Shore Gen
D2 D1 D3
D4
MBT
B Bus A Bus
HR2 HR1 AMP
TR2 TR1
LR2 LBT LR1
EBS EB1
Spare EB2
EG SC
AMP Port
AMP Stbd
Port Socket
Stbd Socket
AMP Container
Only one line can be connected AMP
Changeover Panel ESB
440 Sw Bd 6.6 kV Sw Bd
Shore Side AMP Container
Breaker
DG4 DG3 DG2 DG1
EG
Figure 4.32 Interlocking of a Circuit Breaker for a 6600 / 440 V Transformer The 6.6 kV switchboard is fitted with a short circuit fault detection system. In such a case, the following breakers cannot be electrically closed:
1. Short circuit fault on Bus A: D1, D2 and MBT 2. Short circuit fault on Bus B: D3, D4 and MBT
3. SHB cannot be closed when the DG feeds the 440 V switchboard.
High Voltage Generation and Distribution
Figure 4.33 A Ring Net Distribution System in Cruise Ships / Offshore Vessels
Ring main Breaker
Ring main Breaker
Ring main Breaker
Ring main Breaker
Chapter 4
Figure 4.34