Ships Cruise on Electricity

First there were steam ships, powered by burning coal in great big furnaces. Then there were motor ships, powered by large diesel engines. And now, there are electric ships, powered by on-board diesel generators. Naval vessels have, of course, used electric power as a mean of propulsion since the invention of submarines, but it has stayed mostly under water until the last 10 years. The oil crisis and environmental concerns, together with increased competition in the cruise business, made the smooth ride and economy of electric propulsion popular with naval engineers. I recently observed the electrical and propulsion system on the Sun Princess, one of the vessels in the Princess Cruises fleet (www.princess.com). The Sun Princess was built in 1995, and its propulsion is diesel-electric. (see figure below)

One of the first things you observe when looking at the system is the abundance of duplicate equipment and circuitry. A ship spends a considerable amount of time in open waters, far away from any repair shops. All major systems must be failsafe. Therefore, most critical machinery comes in pairs, and the ship can function with only one element of the pair. For example, there are six generators on board. Four are for the main power-distribution system, one is fully dedicated to the emergency-equipment system, and one is a backup that is generally not on line. The figure does not show the last two generators, which each have a power output of 550 kW, because they are not directly connected with the propulsion system. Both require less than one minute to become fully operational.

The entire ship-not just its propulsion system-can function with only two of the four main generators on line. Each unit generates 11.5 MW at 6.6-kV alternating current. Two electric motors provide the ship's propulsion. The motors are synchronous and run to match the frequency of the supply current. Each motor consumes 14 MW with a voltage of 2.3 kV, so the system requires transformers, as the figure shows. Once the electricity has been transformed to the correct voltage, it becomes the input to the propulsion converters, two for each motor. Each 12-pulse-type converter again divides into two six-pulse-type converters for redundancy purposes. By controlling the inductance in the converters, the ship engineers can control the speed of each motor. With no inductance, the motor stops; it reaches the maximum 145 rotations per minute when the inductance is 21.75 Hz. Each converter comprises an NB (network bridge) and an MB (machine bridge) connected together on the dc side through a dc link reactor. The windings of the two converters for each motor are displaced by 30 degrees to provide the shaft with a smooth angular momentum.

The NB converts the fixed-frequency ac power into dc power with variable voltage and current, and carries out the firing of the thyristor electromagnetically. The firing angle of the thyristors varies the bridge dc output current. The NB output passes into the MB through an external link reactor to provide adaptation between the two bridges and smoothing of the current in the dc link. The circuitry in the MB connects the thyristors in the reverse direction of those in the NB to provide inversion. The firing of the thyristors generates an ac output whose frequency relates to the required motor speed. A fully digitally controlled diode bridge provides the motor excitation. The motors have a 90 degrees phase difference to smooth the control curve.

There must be synchronism between the firing of the thyristors in the MB and the instantaneous position of the motor rotor to ensure correct locking of the rotor into the instantaneous direction of the rotating magnetic fields. The system automatically achieves this synchronism by operating in a "pulse-linked" mode from standstill to 10% speed and in an "autosynchronous" mode at higher speeds, in either direction. The pulse-linked mode operates when the electro-magnetic-frequency signal is insufficient for the autosynchronous mode. It ensures that the MB fires the correct thyristors, at both standstill and low speed, to pick up the actual position of the rotor and to set a corresponding direction of its field. An automatic changeover between modes occurs at 10% speed, depending on whether speed is increasing or decreasing. A transmitter on the propeller shaft provides the positional signals for the pulse-linked firing. In the autosynchronous mode, the instant of firing for each MB thyristor synchronizes with a signal representing the motor's electromagnetic frequency, which the system computes from the motor's instantaneous current and voltage. If transformer, converter, or control circuits for one half motor become faulty, propulsion may continue by using the remaining half over the same speed range. However, power is reduced by half, and shaft torque is at about 70%. The negative effect is that harmonic distortion increases significantly. If, however, a fault occurs in the actual windings of one half motor, the whole motor will be out of service, because the excitation circuitry is common to both halves.

The presence of large, powerful motors in the ship's electrical circuit generates harmonic distortions in the distribution network for the rest of the systems. If left untreated, the harmonic disturbances would result, at a minimum, in fluctuations in the ship's fluorescent lights that would be bothersome to the passengers and the crew. In worse conditions, the fluctuations could damage electrical equipment. Therefore, the two engines have a 90 degrees difference in phase. In addition, engineers have placed harmonic filters in parallel with the two main distribution transformers that provide power to the rest of the ship.

Until now, the diesel generators were located in the hold of the ship, because they are heavy and noisy. But turboelectric ships will soon be appearing in an ocean near you. Turbines mounted inside the funnel structure on the upper deck will generate the required electricity and will simplify maintenance. Better accessibility means they will be easier and less costly to replace or repair than today's generators. Modern cruise ships have the necessary space in the funnel for turbines. The structure, since the adoption of diesel power, contains a number of exhaust pipes, but it is mostly empty. Getting rid of the generators will decrease the number of pipes in the funnel and will also provide more space inside the ship for its passengers and crew to enjoy.

This article originally appeared in the 03/21/02 issue of EDN. Thanks to staff electrotechnical officer Corrado Allegretta of the Sun Princess for his help in gathering the technical information for this article.