An uninterruptible power supply (UPS) is a device that allows an electrical load to operate for a period of time when the input power is cut off, for example, during ESKOM’s load shedding. A UPS can be used to power loads in any of the following applications: banking, commercial, computer rooms, data centres, emergency lighting, industrial, medical, telecoms, and any office or home.
As long as electricity flows from the utility, it also supplies and maintains energy storage. The more energy is stored, the longer power can be provided, within certain parameters, of course. Energy can be stored in different ways. Rechargeable batteries are the most common.
What are the different types of UPS?
The differences between UPS systems lie in the technology that allows them to do their jobs.
The most popularly used kind of UPS is also the most effective. It is commonly referred to as a double-conversion UPS.
For each UPS, the input supply current is alternating current (AC), which is what is required to power the load. Batteries, on the other hand, use direct current (DC), so all battery-type UPSs must rectify (convert/correct) the incoming AC current to DC to charge the batteries. The UPS must also provide AC power to the load using the rectifier (inverter).
In a double-conversion UPS, current flows through the rectifier to the load continuously. The output voltage and frequency are completely isolated and independent of the input voltage and frequency.
When the frequencies differ in input, the system is classified technically as voltage and frequency independent (VFI).
Voltage and frequency independent
Deviations in input power are dealt with in two ways. A surge protection device absorbs particularly low voltage surges. These can be caused by lightning on power lines, large motors such as those used in elevators or medical electronics, welding machines, or numerous other sources. But even the smallest fluctuations, such as voltage drops or voltage failures, never pass through a VFI UPS at the output.
Batteries are excellent electric shock absorbers, and they maintain a constant voltage to the inverter to ensure a clean supply to the load. Connecting large motors or air conditioners to a UPS can contaminate its clean output power and is thus not recommended.
Considerations for Selecting and Using UPS Systems
There are several important considerations when choosing a UPS system:
Modularity
Most modern battery-powered UPSs are modular. They consist of multiple smaller UPSs and battery units that can be combined as needed to provide capacity, redundancy, or both.
Modules can be procured and installed as needed, with one or two add-on modules being installed to ensure redundancy. For example, a 120 kW UPS can have seven 20 kW modules to provide N+1 redundancy.
Battery capacity can be added in a modular way in the same way. In addition, modules in most systems are hot-swappable, allowing a defective module to be removed and replaced without interruption.
Managing the UPS load
If large appliances are suddenly applied to the load, the current may become temporarily unstable, for example, when power is restored in homes and lights flash or when large motors start, and the lights temporarily dim. In extreme cases, the UPS will fail. This is especially concerning when there is no redundancy, so no backup is available to immediately absorb the extra load. This also happens when heavy equipment starts up with a surge.
For this reason, one should carefully evaluate load data to ensure the right UPS and battery for your particular application. Standby Systems can help you with this assessment.
Batteries and battery life
Batteries are an evolving technology. Today, three types of batteries are commonly used:
- Flooded lead acid or wet cells have the longest lifespan (usually 25 years or more), and they are the most expensive. However, they require separate, fire-resistant rooms with acid drains, hydrogen detection alarms, exhaust fans, eye washes, flood showers, and dangerous goods protective equipment. They also require regular maintenance, are the heaviest, and are usually used in larger and more advanced installations.
- Valve-controlled lead acid (VRLA), also known as sealed cells, uses an electrolyte where the liquid is in suspension inside an absorbed glass mat, also called an AGM, and comes in sealed packaging with small vents. They charge more slowly than wet cells to avoid hydrogen emissions and can therefore be used in any room without special construction or protection. Design life is usually three to five or ten years, but the actual lifespan is often only three to five years, depending on the stability of the local power supply and how often and to what extent of capacity the batteries are partially discharged and charged.
- Lithium-ion (Li-ion) batteries are the newest batteries available and can be used in most cases without special chambers or constructions. Lithium-ion batteries are smaller and lighter than VRLAs, can be partially discharged and charged without degradation, and should have a longer lifespan than VRLAs.
Battery monitoring and maintenance
Many newer UPS systems have third-party battery monitoring. Batteries tend to fail when they are most needed. There are different types of battery monitoring systems, but maintenance remains the most effective. Wet cells need regular maintenance. Batteries should be replaced if monitoring indicates a weak cell. Batteries play an incredibly important role in the success of your UPS, so if you need to know more about their function and how to maintain them, be sure to read this article: A UPS is only as good as its battery.Standby Systems has a team of experts to advise you on which UPS systems and batteries are best for your needs. Visit Standby Systems’ website or contact us at 011 794 2541 or 011 794 3406 or WhatsApp 082 450 2361.