Using a UPS in conjunction with an electric generator

Power outages are not uncommon. The grids are overloaded, and the substations do not cool down even at night. For some, this is romance with candles, but for others, it is a critical situation. For example, for medical institutions, where any power surge can damage equipment and become a threat to human life. To eliminate possible incidents, organizations duplicate the network using electric generators. But for this they have to use uninterruptible power supplies. What and why – we figure it out.

An ordinary person can somehow exist without electricity. We’ll have to forget about the refrigerator, the Internet and other delights of modern life. This is uncomfortable and unusual, but not a matter of life and death. If large organizations, banks and shops are left without energy, then the end of the world will begin ahead of schedule. It is enough to leave the supermarket without electricity for a couple of days – and the food is gone: most of the food will go to the landfill, and the residents of the nearby districts will be left without fresh food. It is scary to imagine what would happen if polyclinics and hospitals with their sophisticated technology, on which the lives of patients depend , are left without electricity .


Then the count goes not for days or hours, not even for minutes – just a few seconds without 220 V can be fatal. Therefore, some institutions use their own substations or supply additional feed lines from other directions. But not everyone has the opportunity to use the “dedicated line”, so you have to get out of the situation with the help of backup systems. For example, use a generator .

Without him as without hands
An electric generator is a mini-power plant that runs on liquid fuel. Unlike real power plants, which generate electricity with huge turbines, civilian laborers use generators similar to automobiles for this. Naturally, such a system does not accumulate electricity, but gives out in fact. Therefore, in order for a stable sine to appear at the output of the diesel generator set , it is necessary to wait until the unit reaches operating speed.


The start time depends on many factors. This is influenced by the power of the coils as well as the type of motor that drives the power generation system. For example, home installations with up to 10 kW of power are ready for work and defense almost immediately after launch. But industrial generators with a capacity of 100 kW and above take longer to start. Such stations can accelerate to an operating state for 20-30 seconds, given that the automation itself starts the engine in the absence of a signal from external lines.

Some systems are not able to monitor the line voltage, so their response speed in an emergency will be reduced. Add to this the time for refueling, in spite of a jammed starter – and 10-15 minutes without electricity are guaranteed. In some situations it is uncritical: “and let the whole world wait.” But in most cases, owners rely on stations and expect lightning-fast response when zero at the sockets.

To fill the gap between the loss of electricity and the start of generating its own energy, uninterruptible power supplies are used.

UPS plus diesel generator set
Modern technology is sufficiently prepared to work in unstable conditions. For example, a computer power supply can operate within 180-270 V. At the same time, the capacitors of the power supply store enough energy so that the computer does not turn off during short-term power surges. But capacitors do not save you from completely turning off the light. Therefore, for a seamless transition to power from the generator, special devices have to be used.

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Uninterruptible power supplies , APCs, or simply “uninterruptible power supplies ” are also generators of electricity. Only they do not use gasoline or diesel fuel as fuel, but batteries and the charge accumulated in them. This is their main advantage – in the event of a power outage, the UPS simply switches to operation from the internal battery . The switching time is a few milliseconds. This is enough to keep your computer running smoothly. An ordinary office uninterruptible power supply can withstand up to 10-20 minutes in battery mode. Therefore, each computer with a UPS “painlessly” waits for the generator to start. The same thing happens with the rest of the equipment that is sensitive to voltage surges: routers, modems, servers.

Margin of safety
It would seem that the current flowing through the link “computer power supply unit – uninterruptible power supply – generator” should be ideal, but in reality everything turns out to be sad. If only because an ill-conceived power system will all the time remind of itself by the eternal “rush-rush” from an uninterruptible power supply.

The generator delivers a current of a certain frequency. It can vary within acceptable limits , which is regulated by the manufacturer and is allowed by most consumer electrical appliances. Only the UPS is scrupulous about this indicator and is ready to respond to even the most imperceptible fluctuations by timely switching to batteries. As a result, the user gets a constantly clinking and beeping UPS with a working 220 V in the outlet, and later – a nervous tic.

This is most often due to insufficient generator power. In an idle load, low-power installations supply current at an overestimated frequency. The electric kettle does not notice this, but the UPS considers the mains frequency 55-56 Hz unsafe for the operation of equipment and automatically switches to battery operation. The same happens when the frequency falls below the permissible level. According to the UPS, the 49 Hz current is a good reason to switch back to the battery.

Therefore, it is recommended to select the power supply based on the maximum power of the network. The golden rule is that the generator should be three times more powerful than the consumer. It is necessary to find the total power of the entire network and multiply the resulting figure by three. Let’s say the generator must provide energy for five office computers of 200 W, two lamps of 100 W and various peripherals with a total consumption of 200 W. Total – 1.4 kW. Hence, in accordance with the golden rule, the generator must produce at least 3-4 kW.

Online or online
Choosing a backup power system , you need to take into account not only the total power of the network, but also the type of consumers. The office scenario is a primitive option. As a rule, computer power supplies cope well with fluctuations in frequency and amplitude of current, the same goes for the rest of office electronics. Another thing is a technique that is sensitive to voltage drops. For example, automatic gas boilers.

They instantly react to short-term changes in the network and turn off at the slightest voltage surge. It is better not to think about what the heating shutdown in winter will lead to when everyone is at work. Therefore, competent specialists take into account the specifics of the configured network and select backup power individually for groups: Off-line, Line-Interactive and On-line.

Off-line UPSs are characterized by a simplified operation diagram. These are common uninterruptible power supplies that are used in tandem with home or office computers , printers , laptops . The switching time of such a UPS from mains voltage to battery power is 10–20 ms. This is enough so that the equipment does not have time to turn off during a voltage drop or a power outage. Off-line UPSs do not have built-in protection against interference, as well as a voltage stabilizer.

Line-Interactive series devices are distinguished by the presence of “intelligence” in the design of the UPS. They work on the same principle as Off-line, but they are able to stabilize and filter the voltage. With an unstable incoming current, such an uninterruptible power supply will partially extinguish current fluctuations, although it does not protect against strong frequency surges.

Advanced type of power supplies – On-line . These UPSs operate on the double conversion principle. To do this, the system converts the incoming alternating current into direct current, filters and stabilizes it, and then converts it to alternating current. After that, at the exit from the on-board sockets, the technician receives a pure sine. This is important if the UPS is paired with a generator that is not able to smoothly regulate the voltage. The second key feature of these UPSs is zero transfer time. On-line devices switch between power sources instantly – what is needed for boilers, processor control systems, data centers, servers.