Here is how UPS (Uninterruptible Power Supply) and Diesel Generator can work togheter.

When genset and UPS do not get along

In modern backup systems, it is not difficult to find systems that show the generating set and the Uninterruptible Power Supply in service on the same utility.

The configurations may be different. In many cases the generating set powers the Uninterruptible Power Supply which in turn supplies the user, while in other cases the UPS is independent of the Generator since it supplies a the non-connected part.

In the case in which a genset must supply a user through a UPS the installer finds himself dealing with the possibility that the UPS has tolerances relative to the electrical values that are incompatible with the generator.

The generator in operating mode has more or less stable values, specifically the voltage and frequency. The level of stability is not the same for all the generators, in fact, it can never be stressed enough, the generators are not all the same and are not distinguished only by electric power.

Usually the Uninterruptible Power Supply require a particularly rigid frequency and voltage stability, the reason is that the online double conversion UPS rebuild the voltage with its inverter. Therefore, the greater the instability of incoming values, the greater the work that the rectifier and the inverter will have to perform.

What happens when the power generator and the Uninterruptible Power Supply do not get along?

"When the generator is running the UPS issues an alarm ..."

Everyone is in crisis while trying to reason with the continuous "Beep" that haunts and prevents to focus on the solution.

When a UPS measures input voltage and / or frequency values outside the allowed limits (or rather, set by the manufacturer) it goes into alarm status and often an automatic bypass occurs.

For reasons still shrouded in mystery, the Customer and the Installer often prefer to make changes to the generator by intervening on the voltage regulator, rather than seeking a balance between the two products. This problem is nothing more than the result of incorrect sizing in the design phase, or an overly precise and rigid setup on the UPS.

It is also possible that the engineer did not consider the Harmonic component present in the system.

How to make peace between UPS and Generator.

Now that the problem is understood  we can move on to the solutions.

Mainly the problem is connected to the frequency oscillations of the diesel generator, in particular the rectifier present in the UPS continually senses the variation in the value and prevents the insertion of the inverter.

a)      The oscillations of a diesel generator are mainly due to load variations. The major problems occur when one or more loads are inserted. Example: The generator supplies a preferential line protected by UPS but also a part of the system consisting of motors and reactors. In this case, when the motor is switched on, a significant variation in frequency and voltage that could not be tolerated by the UPS will be inevitable.

b)     Many UPS allow to modify the intervention thresholds, therefore they allow to open the fan and frequency tolerated without affecting the quality of the output voltage.

c)      The generator intended for applications such as those protected by the UPS should be sized taking into account the class of the engine speed regulator. To learn more about this topic you can read an article about this topic and download the free material that we provide at the end of the article.

d)     Some manufacturers suggest the application of harmonic filters to the Uninterruptible Power Supply in order to reduce the harmonics (THD) to 10%.

e)     Do not be rushed to blame the generator or UPS, it is true that the harmonic distortion depends on the conversion system of the UPS but it is also true that the generator set has an impedance that is not comparable to that of the network.


The right generator for your UPS

The correct selection of a Diesel Generator destined to supply a Uninterruptible Power Supply begins with the definition of the operating logic criteria of the plant.

It is essential to know which loads in addition to the UPS will be powered by the emergency generator.

The main features involved in sizing are:

·        The overall power of the Generating Set.

·        The RPM regulator of the primary motor installed in the generator.

·        The power of the alternator.

The nominal power of the Diesel Generator

The nominal power of the genset is to be considered PRP, that is prime power of the ISO 8528 standard.

By definition of the standard, the PRP is the maximum power that the generator can provide during a variable power cycle.

When defining the power of the Generating Set it is important to consider the absorption values of the UPS during the recharging phases of the accumulator battery.

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Written from Andrea Orefice