Main switchboard (MSB)

Figure 44. Main Switchboard

Ship's main switchboard.

Main Switchboard usually situated in Engine Control Room (ECR), or in some cases, when ship is designed for High Voltage, or there are 2 or more Main Switchboards, it is located in Switchboard room, with restricted access of personnel.

When designing the Main Switchboards of sea vessels, it is generally accepted that during normal operation of the Main Power Plant, the following consumers should be fed from the main power distribution busbars through separate feeders:

1. electric drives of steering devices;

2. electric drives of anchor/mooring devices;

3. electric drives of fire pumps;

4. electric drives of ballast pumps;

5. electric drives of compressors and pumps of the sprinkler (fire) system;

6. gyro-compass;

7. switchboard of the refrigeration unit of cargo holds (if applicable);

8. electric drives of the excitation units of the electric propulsion plant (on ships with electric propulsion);

9. sectional panels of the main lighting;

10. radio station switchboard;

11. navigation instrument panel;

12. switchboard of signal and distinctive lights;

13. sectional switchboards and power distribution devices for other critical consumers, united according to the principle of homogeneity of the functions performed (for example, cargo cranes, winches, etc.);

14. distribution devices of the joint control panel;

15. automatic fire detection alarm system;

16. electric drives of mechanisms that ensure the operation of the main propulsion;

17. switchboard for electric drives of cargo, mooring, boat and other winches, ventilation and heating devices;

18. controllable pitch propeller control devices;

19. chargers for starter batteries and batteries supplying critical equipment;

20. power boards for electric drives for closing watertight doors and devices that hold fire doors open, as well as signaling boards about the position and closing of watertight and fire doors;

21. other consumers not listed above - at the request of the Register (classification society).

It is allowed to supply consumers listed in 4,.6, 10, 11, 12, 15, 16, 18, 19, 20 from switchgears specified in 13 or 14, on separate feeders having switching and protective devices.

All other power consumers must be powered through secondary Distribution Boards (DB).

In case of sectioning of the Main Switchboard Busbars, it is necessary to ensure the sectioning of the 220 V busbars in case of disconnection of the sectional ACB on the Main Switchboard Busbars. The division of the Main Switchboard into sections allows one-by-one maintenance of the equipment of the sections.

For each Generator, it is customary to allocate its own section.

At each feeder coming out from the Main Switchboard, automatic switches (ACB) are installed.

If the critical machinery with Electric Drives are installed in double or more, then at least one of these electric drives must be powered by a separate feeder from the Main Switchboard.

It is allowed to supply power to Electric Drives of other such machinery from Sectional Boards (SB) or special switchgears designed to power critical consumers.

If the busbars on the Main Switchboard are divided into sections using intersection disconnecting devices, then Electric Drives, Sectional Boards, special switchgears or panels installed in double or more numbers or fed through two feeders must be connected to different sections of the Main Switchboard. One of the power Steering Gear (SG) feeders must be supplied from the Emergency Switchboard (ESB).

For reasons of reliability, it is not advisable to connect a larger number of feeders to the MSB.

The power supply of consumers less than 45 kW is carried out from the distribution boards.

When developing the Main Switchboard scheme, the number of main switchboard panels (generator, control, consumers) is preliminarily determined. In this case, as a rule, a separate panel is required for each Generator Automatic Circuit Breaker (GACB). Consumers ACB, taking into account their dimensions, are grouped into 12-24 pieces on the panel. Some consumer switches can also be installed on generator panels (6-8 ACB each).

To solve the issue of electrification of the vessel in future, spare circuit breakers should be provided. Their number usually does not exceed 10% of the total number of switches on the Main Switchboard. The rated current of the release of the spare ACB is taken equal to the rated current of the release, which is most often found on this MSB panel.

The final number of panels is specified after the selection and verification of all devices. With small sizes of the Main Switchboard, 220 V panels are structurally combined with 440 (380, 660) V panels.

Schematic diagrams of the MSB are developed and manufactured separately for each class of ships.

Schematic of the Main Switchboard

The Main Electrical Switchboard is made as a one-piece structure, but is divided into separate sections. Having prefabricated separate standard sections, it is possible to assemble a Main Switchboard for any number of Generators and supply lines. This significantly speeds up and reduces the cost of their production, transportation and loading.

Figure 45. Single-line diagram of AC Main Switchboard (MSB) and Emergency Switchboard (ESB)

The purpose of the distribution sections is mainly to control and monitor consumers supplied by the Main Switchboard.

In this regard, automatic devices (ACB) are installed on the distribution sections, as well as ammeters (A) for measuring the load of consumers. Ammeters are installed only on the supply lines of the most powerful consumers or can be switched-over, using switches, to control the load of several consumers.

In the control section, devices are installed for monitoring and switching Generators to parallel operation: a synchronoscope, frequency meter, voltage meter, device for automatic synchronization; as well as:

1. electrical measuring instrument for measuring insulation resistance;

2. ACB for connecting the Emergency Switchboard to the Main Switchboard;

3. ACB that supplies power from shore line (in some cases it is installed on the ESB);

4. automatic bus disconnector (TIE breaker) for separate operation of Generators;

Figure 46. Main switchboard, generators and synchronization sections

 

Other devices and instruments which are not associated with the constant operation of Generators or consumers can be installed on the same section (for example, a device for automatic switching ON a backup Generator, a device for protecting against operation in the event of a wire break in one of the supply phases from the shore and a decrease in voltage from the shore.

A separate Generator section is provided for each Generator (sometimes one section for two Generators).

The control and management of the Generators is provided by devices and instruments installed on the Generator section:

1. ACB for connecting the Generator with the busbars of the MSB and protecting it;

2. AVR potentiometer for adjusting the generator voltage (usually behind the GACB panel door);

3. blocks of Generator’s automatic voltage regulation system (AVR) if not installed directly in the generator, or in Generator’s local control panel, for High Voltage (HV) Generators;

4. servomotor switch, which allows to change the supply of fuel or steam to the Generator Prime Mover (GPM) to change the active load shearing factor of the Generator’s;

5. a device for damping the Generator field, which, in emergency cases, allows operator to quickly turn OFF the excitation winding of the Generator and extinguish it Electro-Motive Force (EMF) to zero;

6. overload relay, which signals an overload of the Generator and also gives an impulse to automatic turn OFF unessential consumers in order to unload the Generator (Meyer protection);

7. the reverse power relay acts on the GACB and disconnects the Generator from the busbar if it has switched to the motor mode;

8. instrumentation: ammeter, voltmeter, kilowatt meter, phase meter and frequency meter. For connection of listed equipment, current and voltage transformers are installed.

 

High voltage main switchboards

Ships with very large electrical loads have Generators operating at High Voltages of 3.3 kV, 6.6 kV and even 11 kV. Such High Voltages are economically necessary in high power systems to reduce the size of current, and hence reduce the size of conductors and equipment required. Operating at such High Voltages is becoming more common as ship size and complexity increase, e.g. for large cruise liners. Offshore oil and gas production platforms operate at up to 13.8 kV, where equipment weight saving is important. Distribution systems at these High Voltages usually have their neutral points earthed through a resistor or earthing transformer to the ship's hull. Main Switchboards of such a ships are splitted to High Voltage and Low Voltage sides. I.e. Main Electrical Propulsion motors, Thrusters etc. are powered from High Voltage side and other consumers are powered from Low Voltage side, via step-down transformers [4].

Figure 47. Single-line diagram of High Voltage Main Switchboard