Uninterruptible Power Supplies use batteries which are generally able to provide power for durations ranging from a few minutes to a few hours. Generators use an engine to generate power for prolonged periods. Fuel tanks able to provide power for weeks of continuous operation are practical, and refuelling during operation is also possible, meaning that power can be provided indefinitely.
If a truly uninterrupted power supply is required, this necessitates an instant response to a power outage, which can only be provided by a UPS. Generators have a start-up time of at least several seconds. If both continuous power and prolonged off-grid operation are required, a generator must be combined with a UPS to supply power while the generator starts up.
Because of their occasional use and critical importance, correct maintenance of backup generators is vital. However, because the engines are only used very occasionally, resulting in very little wear, maintenance can be carried out at very long intervals. A major overhaul may not be required for decades. Because of the need for power continuity, when an engine does need to be taken off-line for maintenance, rental generators must be brought in to serve the backup power duty.
Understanding Power Quality
Ensuring a consistent supply of quality electrical power means avoiding a range of issues:
● Blackouts, otherwise known as power outages, are the complete loss of electrical power. They may be caused by faults anywhere in the electrical supply system, including generation, transmission, distribution and substations.
● Frequency stability: Most electricity is still generated using rotating electromagnetic generators. As the rotating coil (the rotor) passes the heavy copper bars (the stator), an electrical current is induced. The positive and negative poles of the rotor cause current to flow in opposite directions, producing an alternating current with a frequency determined by the rotational speed of the generator. For example, a generator rotating at 3,000 rpm produces an alternating current (AC) with a frequency of 50 Hz. All the generators in a national grid must be synchronized to rotate at the same speed so this frequency remains stable. Changes in electrical demand in the grid cause changes in the electromagnetic load on the generators, changing their speed and therefore the grid frequency fluctuates. The inertia of large heavy generators acts as a damper to slow the rate of frequency changes.
● Voltage stability: Voltage is typically controlled to within 5% by producing and absorbing reactive power. The term ‘voltage surge’ is a somewhat vague term, used for several different overvoltage conditions. These are more precisely defined as transients and swells. Transients last from microseconds to a few milliseconds. An impulsive transient is a sharp rise in over-voltage which can be caused by a lightning strike or a motor being turned off. An oscillatory transient involves the voltage alternately swelling and then shrinking very rapidly. Transients may also refer to undervoltage conditions over the same time periods. They can be caused by inductive or capacitive loads being turned on or off. A swell is an overvoltage that lasts longer than a transient, typically for a few cycles, with a voltage 5 to 10% above normal. Voltage sag, or dip, is an undervoltage condition which lasts for the same time period, sag is therefore the opposite of swell.
● Brownouts are undervoltage conditions lasting for longer periods, typically minutes or hours. They are sometimes implemented intentionally, in order to reduce power consumption without more serious supply interruptions. Resistance devices, such as heaters and incandescent lights, continue to function with their power output reduced proportionately to the reduced voltage. Some motors will also reduce their power but some may draw additional current, leading to overheating and potentially burnout. Power supplies and digital devices can be affected in different ways and may malfunction.
● Noise is a high-frequency voltage disturbance, at frequencies considerably higher than the AC frequency. A time-domain plot of an AC current should appear as a smooth sinusoidal wave, noise is visible as a rough ragged waveform. Noise can often be difficult to detect but may cause overheating, wear and even failure in equipment. The ever-present thermal noise caused by resistance in distribution wires produces a very slight disturbance. Local loads such as welders and motors can cause much more significant noise.
● Harmonics are voltage or current disturbances at frequencies that are integer multiples of the AC frequency. They are caused by non-linear loads such as rectifiers, computer power supplies, fluorescent lighting and variable-speed motors. Current harmonics tend to be larger and to drive voltage harmonic effects. Harmonics can cause motors to operate at lower efficiency, generate excess heat and have reduced life. They may also cause vibrations and torque pulsations in motors, leading to reduced life of associated bearings and other machinery.
While isolation, grounding and power converters provide the foundation for quality power supply, there is a limit to what they can achieve. Backup power and in particular a UPS have a significant role to play. Power supply grounding provides the reference voltage from which all other voltages are measured. Isolation may be required to protect both operators and equipment from dangerous voltages. Surge suppression can also be used to remove transients and swells, while filters smooth the voltage waveform, removing noise and harmonics.
It is possible for a UPS to provide many of these power quality functions, although this is not necessarily the case. This depends largely on whether the UPS is online, offline or line-interactive. An online UPS continuously feeds the mains supply into the battery, with power then flowing out of the battery into an inverter which supplies power. This type of UPS gives excellent protection from transients, swells, noise and harmonics, providing very high-quality power which also has a high level of voltage and frequency stability.
The problem with an online UPS is lower energy efficiency and higher capital costs, since the components such as the battery and inverter are being used continuously. An offline UPS allows mains power to flow directly from the supply to the load and only when an undervoltage condition is detected is the battery connected to the load. Offline UPS’s therefore do not improve power quality. Line-interactive systems are something of a compromise between online and offline.
A generator cannot provide any of this power quality functionality. Even when it is supplying the load, additional UPS or filters may be required to achieve the required power quality.
UPS vs generator
For applications where power must be supplied for days or weeks, the question is whether enough energy can be stored by batteries. With an engine, you are paying for the amount of power it can generate. With a battery, it is generally easy to supply a large amount of power, but storing enough energy to provide this power for a long time is challenging (see below). Although battery systems provide many useful functions, generators remain the only viable option to supply sustained power over long durations.