Introduction to Power Generation and Distribution Systems on Commercial Vessels

Elstan A Fernandez, Senior Lecturer,

Department of Electrical Engineering

Tolani Maritime Institute

Induri, Pune

This paper is based on the following:

a) References from about 30 books – mostly international editions and numerous papers;

b) Information from Original Equipment Manufacturers (OEMs), research scholars / technologists and Classification Societies across the world.

c) My own hands-on experience in the field and also in teaching the subject for several years.

1 Importance of SOLAS Requirements

1.1 The basic difference between warships and commercial vessels

At the outset, with reference to the points in discussion, I wish to mention that the operation of foreign-going vessels that are greater that 500 grt are governed by SOLAS Regulations, requirements by Classification Societies and of course the International Maritime Organisation; warships however are exempted as they supersede these requirements. In this regard, the following extracts from the SOLAS Consolidated Edition 2004 are relevant:

Chapter 1 – Part A – Regulation 3 – Exceptions (Paragraph (a) serial (i);

Regulation 4 – Exemptions; and

Regulation 5 – Equivalents

In the commercial shipping industry, the flag states i.e. the governments of which are the contracting governments or otherwise termed as the Administration, govern the compliance (of such vessels) to the International Convention for the Safety of Life at Sea and the directives of the Secretary General of the International Maritime Organisation.

1.2 Specific requirements for passenger ships and cargo ships

As an example, a few extracts from SOLAS and ABS Rules are mentioned:

Chapter II – 1 – Part C – Machinery Installations – Regulation 26 – General Paragraph 6; and

Chapter II – 1 – Part D – Electrical Installations – Regulation 42 – Emergency source of electrical power in passenger ships Paragraph 6 are similar to an extract from American Bureau of Shipping Rules for Building and Classing Steel Vessels – 2005 (Part 4 - Vessel Systems and Machinery; Chapter 1 – General; Section 1 - Classification of Machinery)

Quote

TABLE 7

Design Angles of Inclination

	Angle of inclination, degrees⁽¹⁾
	Athwartship		Fore-and-aft
Installations, components	Static	Dynamic	Static	Dynamic
Propulsion and auxiliary machinery	15	22.5	5⁽⁴⁾	7.5
Safety equipment
Emergency power installation⁽³⁾	22.5	22.5	10	10
Emergency fire pumps and their drives	22.5	22.5	10	10
Switchgear
Electrical and electronic appliances and control systems	22.5⁽²⁾	22.5⁽²⁾	10	10

Notes

1 Athwartship and fore-and-aft inclinations occur simultaneously.

2 Up to an angle of inclination of 45 degrees, switches and controls are to remain in their last set position.

3 In vessels designed for carriage of liquefied gases and of chemicals, the emergency power installation is to remain operable with the vessel flooded to its permissible athwartship inclination up to a maximum of 30 degrees.

4 (2004) Where the length of the vessel exceeds 100 m (328 ft), the fore-and-aft static angle of inclination may be taken as 500/L degrees, where L is the length of the vessel in meters (1640/L degrees, where L is the length of the vessel in feet), as defined in 3-1-1/3.1.

Unquote

Similarly, SOLAS Chapter II-1-Part D – Electrical Installations – Regulations 40 to 45 are relevant to installation and operational aspects.

2 Power generation and distribution considerations on commercial ships

In this paper, my aim is to highlight the generic options available in the commercial shipping industry. No comparison is made to any defence related installations both afloat or ashore. Any similar viable off-the-shelf options explained here, if non-existent in the Indian Navy may be adopted if so desired.

2.1 Power Supplies Commonly Available

Modern merchant vessels usually use alternating current systems classified as three-phase three-wire insulated neutral systems, three-phase-three-wire earthed neutral systems (for high-voltage arrangements) or three-phase four-wire systems with an earthed neutral. However, direct current systems are also found to be in use. It is understood that direct current systems will once again be popular by the year 2020.

Where distribution is concerned, no current-carrying part of an insulated distribution system should be earthed other than through an earth-indicating device or through components used for radio interference suppression; an exception to the rule is acceptable for the following systems:

a) Impressed current (or sacrificial anode) protective system for outer hull protection or for installations within the main machinery space.

b) Limited and locally earthed systems, such as starting and ignition systems of internal combustion engines.

Large passenger ships have three or four large generators rated at 2MW or more to supply the extensive hotel services on board.

A cargo shipmay have two main generators typically rated from 350 to 1000kW, which are sufficient to supply the engine room auxiliaries while at sea, and the winches or cranes for handling cargo while in port.

The limited load required during an emergency demands that the emergency generators be rated from about 10kW for a small coaster to about 300kW or more for a cargo vessel. The shipbuilder must estimate the number and power rating of the required generators by assessing the power demand of the load for all situations whether at sea or in a port. Electrical power onboard a ship is commonly generated at 440V, 60Hz (sometimes 380V, 50Hz). These values have been adopted because they are standard shore supplies in the Americas and in Europe. A frequency of 60 Hz is generally a US standard and 50 Hz is a UK standard (the effects of changes in applied voltage and frequency on torque and speed must be kept in mind, especially when attempting to use one source for the other type of equipment).

Ships with a very large electrical power demand may be designed to operate at 3.3kV and even 6.6kV or higher. Lighting and other domestic supplies usually operate at 110, 115V or 220V a.c. (single-phase). Transformers are used to reduce the generated voltage to this safer voltage level. Where portable equipment is to be used in dangerous, hot and damp locations, it is advisable to operate at 55V or even 25V, supplied again by a step-down transformer. Occasionally, transformers are also used to step up voltages, e.g. supplying a large 3.3kV bow thruster motor from a 440V switchboard supply. Batteries for various services operate at 12V or 24V but sometimes, higher voltages are used.

2.2 Maximum Voltages

The Institution of Electrical Engineers’ Regulations for Electrical and Electronic Equipment of ships (1990) and the 1994 Supplement recommend for alternating current installations as follows:

+ 500 Volts for generators, power, galley, and heating equipment permanently connected to fixed wiring;

+ 254 Volts (440/Ö3 = 254, where 440 is the generated line voltage in a three-phase system and 254 is the phase voltage or single-phase supply derived from it) for lighting, heaters in cabins, public rooms and other applications;

+ 15000 Volts (but 3.3kV or 6.6kV are often used) in very large a.c. installations; the generation and limited distribution being subjected to special considerations.

2.3 Specific Systems for Ship’s Service

2.3.1 Generation

The following systems are recognised as standard types for a.c. generation:

a) 3-phase 3-wire insulated

b) 3-phase 3-wire with the neutral earthed

c) 3-phase 4-wire with the neutral earthed

2.3.2 Primary Distribution

The following systems are recognised as standard types for primary a.c. distribution:

a) 3-phase 3-wire insulated

b) 3-phase 3-wire with neutral earthed

c) 3-phase 4-wire with neutral earthed

d) Single-phase 2-wire insulated

2.3.3 Secondary Distribution

The following systems are recognised as standard types for secondary a.c. distribution:

a) 3-phase 3-wire insulated

b) 3-phase 3-wire with neutral earthed

c) 3-phase 4-wire with neutral earthed

d) Single-phase 2-wire insulated

e) Single-phase 2-wire with one pole earthed

f) Single-phase 2-wire with the mid-point of the system earthed

2.3.4 Frequencies

The following frequencies are recognised as standard types:

a) 50 Hz is a European standard frequency.

b) 60 Hz is the most common frequency adopted for use onboard ships, the higher frequency thus enabling motors and generators to run at higher speeds with a consequent reduction in size for a given power rating.

2.3.5 Voltages

The standard values for specific requirements are mentioned in Table 1 below:

Sl. No.	Applications	Recognised Standard Voltages
1	Power, heating and cooking equipment securely fixed and permanently connected	Three-phase	380
			415
			440
		Single-phase	115
			220
			240
2	Fixed Lighting	Single-phase	115
			220
			240
3	Socket outlets for use where extra precautions against electric shocks are necessary a) Where a safety-isolating transformer supplying only one socket outlet is used	Single-phase	55
			115
			220
			240
	b) Supplied without the restrictions in (a) above.	Single-phase	24
			48
			55
		Single-phase	115 with mid-point earthed
4	Supply to equipment for general purposes, connected by flexible cable, other than for applications in 3 and 5	Single-phase	115
			220
			240
5	Socket outlets rated at 15 amperes or more and intended for equipment, the connection to which incorporates an earth-continuity conductor ³ 1.5mm² in cross-sectional area	Three-phase	415 440
6	Internal communications	Single-phase	12
			24
			115
			220
			240

Note: - Voltages stated are the nominal system voltages (these are root mean square values unless otherwise stated) - All 3-phase socket-outlets should be of the same phase rotation

Table 1 – Voltages for AC Systems on Ships

2.4 Motor Controls

It is often convenient to group motor driven auxiliaries according to their function, e.g. fuel and lubrication oil services, accommodation ventilation systems, machinery ventilation systems and domestic service systems.

The auxiliary motors would be supplied from grouped motor controllers located either in the engine room, in a machinery control room or in a convenient location close to the auxiliary motors. This can often simplify the machinery control functions and required protection systems’ circuitry.

On small ships, e.g. tugs, etc., such grouping is not economical and the major ship’s auxiliaries are normally supplied directly from the main switchboard. In this case the motors would be provided with individual starters located adjacent to the motor. For high-speed vessels where weight is important, minimum cable weight may be achieved using a ‘non-distributed’ distribution scheme. Auxiliary motor controls should be arranged in compliance with the general control philosophy applied to machinery control systems.

For ship’s that do not have automated machinery operation, the most economic method of control is to provide local starters for each auxiliary motor supplied from power panels located in the same or adjacent spaces. These motors would be manually controlled (started and stopped), locally at the motor’s controller (e.g., a starter). This arrangement minimises cable costs.

When a centralised machinery control system is required, cables for the motor control functions can be installed back to the machinery control room and the starter push buttons located on a centralised machinery control console. Alternatively, the motors may be grouped together at motor control centres located inside the control room. The motor control functions can then be left on the motor’s starter at the MCC or again wired back to a central control desk.

When hard-wired systems are used, the installation is prone to mechanical problems, which may cause loose or broken connections; and the marine environment can also cause corroded connections. These problems have been overcome to a certain extent by using microprocessors and digital control systems. When fully automatic machinery control is required, these techniques are now in common use and microprocessor devices control the ship’s machinery through video display units (or desktop computers) located in the machinery control room or on the bridge.

The ship’s auxiliaries are generally controlled with programmable logic controllers (PLCs) installed inside the motor control centres and linked through a data bus to the machinery control location. When this type of system is used, the motor control centres can be located either together in the machinery control room or alternatively, distributed throughout the ship close to the motors being controlled. There is little difference in the cabling requirements of either method, however when motor control centres are located outside a dry, atmosphere-controlled space such as the machinery control room, a higher degree of mechanical protection is required (IP 44 instead of IP 22) and consequently adds to the MCC costs

2.5 Emergency Services

To ensure that electrical supplies are available to emergency and safety systems, in the event of a main power failure, an emergency generator and associated emergency switchboard will be located above the main deck in a separate space, completely isolated from the main machinery spaces. Emergency services would be supplied from the emergency switchboard using distributed panels for navigation, safety and emergency lighting services. These distribution panels are also generally arranged to be above the bulkhead deck.

Where lighting systems are concerned, it is important to ensure that a fire or flooding in one area will not cause the loss of lighting supply in other areas or along escape routes and the circuitry must be designed in compliance with the ships general arrangements.

2.6 Emergency Stop Panel

According to the requirements of the classification societies, additional emergency stop circuits have to be installed for certain consumers at a location that is separate from their installation position, e.g., in order to switch off fans, the fuel or oil pumps in the endangered area if there is a fire

2.7 Ship’s Auxiliary Services

Auxiliary services on board a ship range from engine room pumps and fans, deck winches and windlasses to general lighting, catering and air-conditioning. Electrical power is used to drive a majority of these auxiliary services. The electrical power system on board a ship is designed to provide a secure supply to all loads with adequate built-in protection for the equipment and operating personnel as well. The general scheme of a ship’s electrical power system is common to nearly all ships.

2.8 Load Analysis

Extract from Abs Rules for Building and Classing Steel Vessels - 2005

Part 4 Vessel Systems and Machinery; Chapter 8 Electrical Systems; Section 1 General Provisions

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5.1.5 Load Analysis (2002)4-8-1/5.1.5

An electric-plant (including high voltage ship service transformers or converters, where applicable per 4-8-2/3.7) load analysis is to cover all operating conditions of the vessel, such as conditions in normal sea-going, cargo handling, harbor maneuver, and emergency operations.

Unquote

The power rating of the generators is determined by the power demand of the electrical load. Principle design documents for the ship’s auxiliary services should include a load list, load analysis and short-circuit current analysis. In consultation with the client, all electrical services on the vessel are identified. Approximate horsepower or kilowatt ratings are obtained for motors. Lighting loads are estimated from the ship’s general arrangements and electronic aids are obtained from similar vessels, and then a complete load list compiled. The electrical load analysis uses the load list in order to estimate the expected power demand of the electrical system under specific operating conditions of the ship. Typical operating conditions would be with the ship, ‘in transit’, ‘at anchor’, ‘manoeuvring’, etc. For special vessels, other operating conditions would be appropriate such as ‘towing’ for a tug, ‘drilling’ for a drill ship.

Load analysis helps to calculate the expected power demand by multiplying each service power by a ‘demand’ factor. The demand factor is a combined load factor and diversity factor and is the ratio of the estimated power consumption of a service to its normal full load power consumption. It is determined by an experienced assessment of the estimated power during a four to six hour period when loads may be at their maximum utilisation.

For each service, data banks are searched to determine the service full-load current and power factor dependent upon the operating voltage. The information thus derived, is used to determine the services’ kilowatt and kilovar demand from which the kilovolt-amps required is finally computed.

Also, by applying the demand factor to each load (kW and kVAr) and summing all loads for specific operating conditions, the expected kilowatts, kilovolt-amps and power factor can be computed. By comparing the expected load for the different operating conditions of the ship, the number and rating of the main generators can be assessed.

Preliminary short-circuit current calculations can be completed once the load analysis and number and rating of generators have been determined. The principle purpose of short-circuit current calculation is to ascertain the short-circuit rating of the systems’ protective devices. Several types of short-circuit current calculations are applied under different circumstances at various stages of the design process. The major contributors to short-circuit current are the generators and motors.

The most simple short-circuit current analysis is based on an assumed value of the generator’s sub-transient reactance and an approximate estimate of the worst case motor loading can be obtained from the load analysis.

The ‘second stage’ short-circuit current analysis is completed when the electrical system’s conceptual one-line diagram is finished. For this calculation actual sub-transient data is used together with cable transformers and other system parameters. This calculation generally results in lower values of short-circuit current.

When complete system information is available, a ‘third-stage’ short-circuit analysis is completed. This is the most accurate and also determines the decrements of the short-circuit current over a 3 and 5 cycle period.

2.9 Power Management Systems

A vessel may have a number of generating sets, a split bus bar and a variable load. Since UMS classification, automatic control has become common to ensure continuity of service and an efficient management of both generating and load equipment. Power management systems (PMS) have in the past been relay based, but a programmable electronic system (PES) using a programmable logic controller (PLC) is now more commonly used. This can also be integrated into a distributed control system (DCS) for supplying information to the bridge.

A typical power management system will incorporate the following functions:

2.9.1 Load Demand Monitoring

The total load is monitored and compared with available on-line generating capacity.

2.9.2 Generator Management

In response to varying load, the system will start up a standby generator and initiate the synchronising sequence to bring it on-line, or off-load a generator, disconnect it from the bus and stop it when no longer required.

2.9.3 Load Sharing

The load demand is shared between generating sets which are on-line. This can be done on a proportional basis according to the rated power of each set.

2.9.4 Frequency Control

The frequency of the bus is monitored and kept within a specified limit by means of raise / lower signals to each generator governor.

2.9.5 Load Inhibit

Some systems operate a load inhibit to prevent a sudden increase in load when generating capacity is not available.

Load shedding, to trip selected loads in a preset sequence if generating capacity is not sufficient, is often undertaken by a system separate from the PMS automatic control. It can be taken care of by a preferential tripping device.

Each of the earlier mentioned PMS functions will be defined within the system software. The necessary inputs are provided as on/off status inputs from circuit breakers and generator status signals, analog inputs of generator power and current, power of each load and voltage and frequency of each bus section. Outputs are given to raise / lower generator governor settings, close circuit breakers or initiate an auto synchronising system.

An operator interface will provide selection of duty and standby generating sets, and the ability to set levels at which generating sets are introduced or shut down. The operator will be provided with information such as power generated, power demands, running and standby capacity. An interface to the ship’s main alarm system is likely, giving system alarms and indications of any faults within the PMS itself. The main advantage of using a programmable electronic system / PLC for a PMS is the mathematical functions that it can perform. For example, if the power input from the generator fails, generator power can be calculated using the current input and an assumed or actual power factor. Overloads can also be calculated and hence the load reduced by a minimum amount, or an increase in load calculated in advance and the required generating capacity brought on.

3 Examples of systems on Modern Commercial Vessels

a) Power Plants and Power Management

A Basic Power Generation and Distribution System (on a modern merchant ship)

Note: It may be noted that higher voltages are used for the simple fact that overall weight of equipment may be reduced thereby permitting increased cargo-carrying capacity. Some motors like thrusters may operate at voltages as high as 6.6kV also.

The Power Plant Concept

(Courtesy - ABB Norway)

b) Various Types of Modern Electrical Propulsion Systems

Schematic Diagram of a Cycloconverter-based Propulsion System

(Courtesy ABB Norway)

(Courtesy Schottel GmbH and Co. KG)

Contra-rotating Props

The above methods of propulsion with a single skeg arrangement can help to boost the overall thrust and even eliminate the need for a dedicated steering system. The other systems used are azipods,

The various types of power generation, distribution and propulsion systems, including some of the latest systems are designed by renowned manufacturers like Siemens AG (Hamburg), ABB (Norway) and Rolls-Royce Commercial Marine.

c) Addendum:

If so desired, any the following topics related to merchant ships can be discussed to some extent: