be made but the generator itself must be designed to run at low speeds, necessitating it being larger in size.
Although direct connection systems avoid
gearbox losses, low-speed electrically-excited synchronous shaft generators are likely to operate at lower efficiencies than their high speed equivalents. Considering further losses in power electronics, the overall efficiency, comparing shaft power with electrical power, can drop to below 90 per cent. Having undergone considerable development in recent years, permanent magnet (PM) generators now present a practical
alternative, with
higher power ratings now being possible. The technology is also capable of efficient low speed operation, allowing application to low speed drivetrains without the necessity of an up-speeding gearbox.
The absence of field windings and associated losses provides PM generators with advantages of efficiency, low weight and simplified construction. Finnish company The Switch points out that a typical electrically excited shaft generator has rotor field winding losses of up to 3 per cent of its input power and these do not exist in a PM generator. Stator losses are also lower, as the generator operates at a higher power factor, with resultant lower stator current and hence lower resistive losses. The Switch, which specialises in PM machinery, estimates that a typical electrically-excited shaft generator connected to a low speed two-stroke diesel engine will deliver a conversion efficiency of 93 to 94 per cent whereas a PM generator running in equivalent conditions will deliver efficiencies
The vacuum circuit breaker compartment
is fully segregated from other areas and has its own pressure relief channel leading into the arc chamber. The breaker can be
operated manually, if required, by
push buttons mounted on the front of the cabinet with the doors remaining fully closed. Mechanical interlocks prevent the compartment door being opened until the circuit breaker is switched off and placed into the test position. As a further safety feature, individually operated automatic earthed metal shutters for line busbar and outgoing cable connections can be padlocked in their closed positions. If the breaker is moved to either
the test or disconnect
position, these shutters close automatically to prevent accidental operator contact with live sections. Eaton’s vacuum circuit breakers are constructed with fixed and moveable contacts housed in a ceramic cylinder with
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also be prone to failure and will require periodic maintenance. Rotor construction of PM generators is also more simple than their traditional counterparts. In the case of The Switch products, the rotor is a simple hollow steel cylinder with magnets fixed to its surface. The yoke thickness is typically 30 to 50mm, with magnet thickness in the range of 15 to 20mm. The resultant rotor inertia is therefore low, due to the compact nature and low weight of the rotating element.
Permanent magnet generators provide a flexible and more efficient alternative to conventional generators (credit: The Switch)
up to 96 or even 97 per cent. A consequential advantage of this higher efficiency is the reduction in cooling capacity required, reducing loads on cooling water circuits or air flow requirements.
concept
A further benefit of the PM generator is
its simpler mechanical
construction. As it requires no separate excitation, there is no exciter assembly – a small generator in itself – in the machine construction. By comparison, synchronous generators often require an external power source although installations required to start without external power can be fitted with a small, permanent magnet generator, which adds to the complexity of the machine. Without the need for these components, and associated control electronics and diode packs, the PM generator is far simpler and less reliant on auxiliary systems, which can
actuation by bellows. Contacts are shielded against contamination from metal deposition caused by vapours produced when the breaker operates. The design also results in a large number of parallel arcs being created when contacts operate, resulting in low arc voltages, short arc ties and resultant low energy dissipation. This limits contact wear and reduces maintenance, with Eaton certifying the equipment for up to 30,000 operating cycles.
The cabinet includes an earth switch that is operated from the front of the switchgear and mechanical indicators are provided to show the switch position, along with a window being incorporated to allow a direct view of the mechanism. Mechanical interlocks with the circuit breaker are incorporated such that the switch can only be closed when the breaker is in the test or disconnect position and the circuit earth switch can be mechanically interlocked
In offering a comparison, The Switch indicates that, for a 1.5MW rated generator, the shaft weight of a PM design could be as low at 2 tonnes, compared with 6 tonnes for an electrically-excited generator. In addition, rotor inertia, of the order of 600 kgm2, is also almost an order of magnitude less than that of a conventional rotor. This provides further benefits as, for rotating equipment, the dynamics and vibration characteristics of shafts are often critical to operation. Hence, low mass and inertia are desirable characteristics. With
PM generators already well
established in high-power industrial applications, The Switch sees the technology becoming more significant to marine propulsion applications but notes that the machines have yet to make a breakthrough in ship power generation, where traditional electrically-excited synchronous generators are still the most popular option. The company anticipates this will change, however, and already offers a range of low, medium and high speed PM generators with power outputs up to 6.3MW. MP
with the cable compartment door, providing further safety protection.
The lower sections of the cabinet house
current and voltage transformers, cable terminations and the earth bar whilst the low voltage panel is mounted at a convenient height for operators, on the front of the cabinet, above the main breaker panel.
Eaton Industries Power Xpert UX MV switchgear has passed its General Design Appraisal by Lloyd’s Register (credit: Eaton Industries)
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