Genset driven UPS (Uninterruptible Power Supplies)



UPS and Generators (marriage made in hell)

The harsh reality of many gensets used in the home is that they will enable switching between mains and battery of a line interactive uninterruptible power supply.  This will inevitably cause eventual shut down of the UPS when the battery becomes exhausted or low voltage is sensed. If your backup supply is beeping at regular intervals ensure that it’s not going onto battery mode to drive the inverter. (if in doubt read the manual).

An online supply is one in which the AC is converted into a high power DC supply, which in turn drives an inverter for alternating current out. Although much more expensive than a line interactive supply they do not succumb to switching between mains and battery when it detects low or high voltage and frequency fluctuations. Diesel generators, even smaller units, are better at driving a line-interactive UPS than petrol generators because of their higher torque at lower rpm.

UPS - rockmountables[ ATTRIBUTE: Jimmy Thomas “Dark Server Room” ]

The Kipor 6700TA or in that particular series is a popular genset used in the home and can drive most line interactive supplies without picking up problems even when the frequency runs as low as 40Hz (tested with a Vanguard 2000 rack mount). The VGD series is online and worked 100%. All the line interactive supplies tested on this generator worked 100% also, even with a 2kW load on the genset. (APC, BNT, etc). Running a 4.5kVA petrol genset caused the backup supplies all to beep when the load was set to 2kW but ran comfortably once the generator speed had stabilised (45-55Hz).

Some inverters allow for generator input, the inverter can be used then as a stabiliser, depending on what type and how it’s wired. These inverters are fed from batteries as well. With a solar array acting as a charger, batteries and a generator most home owners can run entirely off the grid even with a 5kVA system as long as there is no overload – use gas geysers and cookers. Kettles and toasters draw a fair whack so they should in theory be eliminated if possible. Who needs a toaster when there is a power outage in any event?

Electrical supplies, especially those used for backup power can be very sophisticated. 99% of this is due to the use of 50 or 60Hz a.c. and the 110 or 220V line voltages. Your supply utility frequency is usually rock steady, the voltages not so.  With small generators we have a problem with both voltage and frequency stability. The line interactive supply does not like this. The online variety can handle it.

UPS have another inherent drawback – their output wave shape. This applies to ‘modified’ sine-wave inverters as well.  Besides for the very expensive pure sine wave device most are modified square-wave (to get sine of course).  If an inverter drives a line interactive supply we may end up with the same problem as the genset line interactive supply issue. Dirty waveforms are not welcomed by the line interactive device. Again the backup supply may go into battery inverter mode.

The truth of the matter is that the best uninterruptible supply for this project is an online supply to feed switched mode power supplies and non-inductive loads (not transformers and a.c. motors). Modifying a line interactive supply by ensuring there is adequate ventilation and fan-cooling one can build a charger/DC regulator to drive the inverter stage very much like the double conversion process used in the online UPS.


 

 

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