UPS Excessive Load

[cloving]

I know in an islanded system when load exceeds generation, the frequency will drop (due to generator droop characteristics) and make up for deficient generation. What happens when load exceeds the rating of an inverter or UPS when that is the only source of power? I know frequency would not change. Would voltage drop?

 

[mconroy]

Cloving,

It depends on the UPS system. Most UPS systems are designed to switch to static bypass when the load exceeds 103%-125% (assuming bypass is available). I have also seen UPS systems current limit their rectifier (DC buss voltage drops) when overloaded and this drains the battery until the UPS inverter drops the load. The typical voltage drop I have seen with an overloaded UPS is in the DC section, if the inverter gets overloaded it simply drops the load. The exception to this is the older ferro-type UPS that will drop output voltage on an overload.

I suggest asking your UPS vendor what the overload characteristics are when input and bypass is available and when running on battery.

 

[sibeen]

It does, to some extent, depend upon the manufacturer of the UPS system and how it is setup on the site.

Some manufacturers have an algorithm within the software that adjusts the load capacity of the UPS depending upon ambient temperature. At 40 degrees C the UPS will be rated at its nominal load, whilst at 25 degrees C the inverter is able to supply 110% of In indefinately.

For higher overloads the system can remain on-line depending upon the duration of the overload, for example:

125% load - 10 minutes
150 % load - 10 seconds
200% load - 1 second.

The UPS voltage and frequency will be stable during these overload events (at least for a decent UPS) and the limits are set for thermal considerations of the output switching devices and magnetics. At the end of these periods the UPS will then switch to the bypass (reserve) supply if this is available. If not then the inverter will shut down and load will be lost.

As mconroy mentions, most UPSs also have the facility to current limit the incoming rectifier current. If the nominal current of the rectifier is exceeded then the rectifier will lower its voltage until current is being drawn from the battery bank. As the batteries discharge the rectifier voltage will keep going down until the minimum battery voltage is reached and again the inverter will shut down.

Some UPSs also have the facility to current limit the output of the inverter for situations where the Bypass isn't available and an extremely high load (>300%)or a downstream fault is applied to the output of the UPS. In this case the output voltage will fall dramatically until the fault is cleared or the overload removed.

 

[ruggedscot]

Ive seen something like this a while back - Generator used to supply backup power, when the generator was loaded up by the starting current of a large motor the UPS sensed the frequency drop grabbed the load and this resulted the frequency recovering. As soon as it stabilised the UPS dumped the load back to the diesel and this was enough to cause a frequency dip that resulted in the load being supported by the UPS once again. This continued for some time as the load transferred back and forth between the UPS and the Generator. Not a good situation to be in and was resolved by dumping enough load for a few minutes to allow the diesel to recover and then reapply load gradually.

Rugged