Do you know how an uninterruptible power supply (UPS) ensures that the power for your critical load is stable? If you have read my earlier posts on flywheels and kinetic energy storage modules, you understand how rotary UPS systems store energy and bridge power interruptions. Let’s take it one step further and look at an essential feature of any UPS: keeping the supply voltage up.
Double conversion UPS
The majority of the UPS systems nowadays comprise power electronics devices and batteries. Most of these systems are so-called “double conversion” types. The utility supply is first rectified to a DC voltage, and it is then converted back to the desired AC voltage. The battery is connected in between. The UPS generates the output of the UPS from the DC circuit, regardless of whether the rectifier or the batteries are the source of power. Thus the height of the output voltage is independent of the mains voltage.
In double-conversion UPS systems, power is continuously being converted, and most of the time, it’s even converted twice. But since each conversion has losses, efficiency is a concern.
Double conversion rotary UPS
A motor generator set with an attached flywheel can be seen as a rotary double-conversion system. In such sets, the mains powers an electrical motor. It drives an alternator which generates AC voltage for the load.
Such configurations can be useful if the mains frequency differs from the load frequency, for example, to power aircraft (400 Hz) or to service boats (50/60 Hz). But in most applications, the frequency of the load is the same as that of the mains supply. That is why most rotary UPS systems nowadays use a more sophisticated design that is efficient and suitable for power ratings in the megawatt range.
Line interactive rotary UPS
This design is often referred to as “parallel” or “line interactive.” But these terms may lead to confusion, since they are also used for different configurations. What is meant here is a design in which an alternator is connected in parallel to the load, not in series, like in a double-conversion system. The alternator does not feed the load but merely interacts with the mains power supply to stabilise it.
A reactor between the mains and load provides the necessary isolation. Otherwise, it would be impossible to regulate the output voltage independently of the mains. The only time that the alternator generates power is when the utility power goes off. Then it uses the kinetic energy module (KEM) as a power source, followed by the diesel engine, if one is present. Most of the time, however, when utility voltage is present, the alternator runs as an electrical motor, keeping the KEM up to speed.
This part may be difficult to understand. How can a motor stabilise the output voltage? Intuitively, most people understand that if you drive an alternator with a diesel engine, like in a diesel generator set, the output voltage is regulated. An automatic voltage regulator (AVR) measures the generated voltage and controls the alternator so that it generates the desired voltage level. But how does this work if the alternator is working as an electrical motor on the utility voltage?
The point is that for an alternator to generate voltage, it does not matter what drives it. If you run it electrically, as a motor, it still generates voltage! This phenomenon is known as counter-electromotive force. If that didn’t exist, an electrical motor would continuously draw a very high current. And similar to when you drive the alternator with a diesel engine, you can control the height of the generated voltage with the AVR.
This UPS design is popular, because it is simple, reliable and rock-solid, all of these being important features for systems which power critical loads. However, the biggest advantage of this configuration is that it is efficient. When utility voltage is present, and that is most of the time, no power conversion takes place. The grid supplies power to the load via the reactor, with minimum losses. In particular, in megawatt-size applications, conversion losses accumulate into considerable operational cost. It is not surprising, therefore, that this design has become the de facto standard for rotary UPS systems.
This is the secret of rotary UPS. It is efficient because it does not convert energy all the time. A rotary UPS only generates power when the utility fails. At that moment, the alternator is already running and generating voltage, so this transition is fast. You won’t notice it, and even better, nor will your critical load.
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