Uninterruptible Power Supplies (UPS) can appear in many places around a superyacht. From providing back up power supplies for essential navigation and communications equipment, to cleaning up the AC waveforms for some other particularly sensitive computers and entertainment equipment on board, to providing power for lighting in guest areas where definitely the lights ‘flickering’ as generator loading changes is to be avoided.
This last problem turned out to be a very stubborn one to solve on one superyacht I have worked with for several years. Flicker of lights (down/up, or on/off) in guest areas is both ‘not very superyacht’ but also can be quite disturbing to guests on board who might equate lights flickering to a serious problem in the engine room. The solution boiled down to an incompatibility between the type of UPS used for smoothing the mains supply from the generators and the Crestron lighting system used throughout the boat.
So what are these different types of UPS?
The three major types of UPS system configurations are defined by how power moves through the unit and are, in increasing complexity and price:-
- Off-line (also called standby and battery backup)
- Online double conversion.
In an off-line (“standby”) UPS system, the load is powered directly by the input power, and the backup power circuitry will only be invoked when the utility power fails after a short delay of a few milli-seconds (most computers can ride through this without noticing):-
A line-interactive UPS (below) maintains the inverter in line and redirects the battery’s DC current path from the normal charging mode to supplying current when power is lost. Once the input power fails, the transfer switch will open and the power will flow from the battery to the UPS output. This design offers additional filtering and yields reduced switching transients since the inverter is always on and connected to the output. NB: the mains frequency normally feeds straight through to the output (this is a crucial point for later):-
An online UPS, type iii uses a “double conversion” method of accepting AC input, rectifying to DC for passing through the rechargeable battery (or battery strings), then inverting back to 120 V/230 V AC for powering the protected equipment. The output frequency is therefore normally completely unaffected by the input frequency which may be moving around slightly as generator loading varies.
So why the flickering lights? On the superyacht in question, the type of UPS used to ensure clean power to the Crestron light controllers was of a Line-interactive type (ii above), in which the input frequency fed directly though to the output frequency, whilst the batteries were normally being charged from the inverter bridge in a rectifier mode. The generators on this superyacht had a slight ‘droop’ characteristic meaning that as they loaded up from other loads on board being turned on, the frequency would drop slightly and this upset the phase controllers’ action within the Crestron dimmer channels, meaning that complete mains cycles were occasionally missed and this caused very noticeable flickering on any dimmer channel in the guest areas at certain, seemingly random times.
The big clue was that this effect did not happen when connected to shore power in port. The solution was to change the UPS for the dimmer channels to type iii), the online double conversion type, where the output mains frequency is maintained at exactly 50.0Hz (in this case), whereas the input frequency could be seen to move around a few 1/10th of a Hertz, enough to upset the Zero Crossing Detectors in the Crestron dimmers, causing the missing half- and full-cycles.
It was left as a separate task, at another time, to investigate the speed control and droop characteristics of these generators on board.
[(*) For much more in-depth information and training on inverters and generator systems, see module 6, 8 and 9 of our course: Electrical Control Systems for superyacht engineers.]