How do a generator and a UPS work together? Power and transfer settings

Why both?

A UPS and a generator do different jobs. During an outage a UPS supplies power with zero or a very short transfer time, but its battery is limited. It carries the server, recorder and network devices for a few minutes or at most a few hours. A generator supplies power for hours as long as it has fuel, but it takes time to start and take over, and the power is cut at the moment of transfer.

When the two are used together, these weaknesses cover each other. When the power goes out, the UPS takes over the load, the generator starts and warms up, and the transfer panel moves the load to the generator. From this point the UPS is fed from the generator and recharges its batteries. When the mains returns, the transfer panel gives the load back to the mains, and the generator runs for a while to cool down and then stops.

In Şanlıurfa's rural districts and industrial zones, outages can last long in the summer months. This dual setup is often preferred for hospitals, cold storage, production facilities and institutions where camera recording must continue.

What does an automatic transfer panel do?

The automatic transfer panel monitors the mains. When the voltage is cut or goes outside certain limits, it gives the generator a start command. When the generator has started and its voltage and frequency have settled, it disconnects the load from the mains and connects it to the generator. When the mains returns, after seeing that the mains stays stable for a certain time, it transfers the load back.

These waiting times can be adjusted. When the mains goes off and on for a few seconds, the generator taking over immediately means unnecessary running; the UPS already covers these short outages. Switching back right after the mains returns also leads to repeated transfers if the mains is cut again.

The mechanical and electrical interlock of the transfer panel prevents the mains and the generator from being connected at the same time. If this interlock is missing, power is fed back into the mains and a technician working on the line is put in danger. The transfer panel work must be designed by an authorized electrical engineer.

How is generator power chosen according to the UPS?

Choosing the generator power equal only to the UPS's rated power is a common mistake. When the UPS starts being fed from the generator, it draws not only the load but also the charging current of the drained batteries. Some UPS types draw a fluctuating current from the generator, which strains the generator's voltage and frequency control.

That is why the generator is chosen above the UPS power. What the ratio will be depends on the type of the UPS, its input characteristics and the manufacturer's recommendation for operation with a generator. The UPS's technical document contains a recommendation for use with a generator, and the calculation is made with this document.

If the generator will also feed loads that are not connected to the UPS, all of them are calculated together:

• The UPS's input power at full load and its battery charging power
• Air conditioning and cooling devices, including their starting currents
• Lighting and outlet circuits
• Pumps, elevators or motor loads
• Loads planned to be added in the future

Expert note: Do not forget to connect the server room's air conditioning to the generator. When an outage lasts for hours, the UPS and servers keep running but the room heats up; a server room without air conditioning on a summer afternoon causes the devices to shut down from heat.

Frequency fluctuation and the UPS not accepting the generator

A problem often seen in the field is this: the generator starts, the transfer panel moves the load, but the UPS does not accept the generator and keeps running on its battery. When the battery runs out, the load shuts down. The cause is most often that the generator's frequency or voltage fluctuates outside the UPS's acceptable range.

On online UPS units, the input frequency tolerance can be adjusted. This tolerance can be widened to suit operation with a generator. Maintenance and correct adjustment of the generator's speed governor also keep the frequency stable. When a large load is put on a small generator, the frequency drops; in that case the problem is the generator's power.

If the UPS's bypass line cannot run in sync with the generator, switching to bypass during maintenance or a fault can be problematic. These settings are checked during commissioning by actually running the generator.

Placement, exhaust and fuel

If the generator will be placed in an enclosed room, adequate ventilation and safe discharge of the exhaust gas to the outside are needed. Exhaust gas is deadly; it must not be discharged near building entrances, windows or ventilation intakes. Generators in open areas are chosen with a soundproof enclosure and protected from rain and dust.

The capacity of the fuel tank is determined by the longest expected outage. In a generator that is not run for a long time, the fuel can degrade and the filters can clog. Fuel level and quality should be on the maintenance list. Also take into account the fire safety rules for fuel storage; check the relevant regulation and the fire department for details.

Testing and maintenance

A generator starting and stopping by itself is not a sufficient test. A generator running with no load does not reveal the problems that will appear when load is applied. The test should be done at set intervals by cutting the mains or by actually transferring the load to the generator through the transfer panel. This test shows whether the UPS accepts the generator and whether the transfers go smoothly.

The test is done outside working hours and with prior notice. During the test, the UPS's battery status, the generator's voltage and frequency values and the transfer times are noted. Battery, filter, oil and coolant checks are done according to the generator manufacturer's maintenance schedule.

Adding a generator to an existing system

If a generator will be added later to a building that already has a UPS, the UPS's compatibility with the generator is checked first. An old UPS may work poorly with a generator. In the electrical panel, the circuits that will and will not be fed by the generator must be separated; panel rearrangement is often done for this.

The UPS calculator on our site shows the UPS runtime for a camera and recording system approximately. In a system with a generator, the runtime expected from the UPS gets shorter; it may be enough for the UPS to last only until the generator takes over and long enough to protect the servers.

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