This is your go-to guide to understanding online double-conversion UPS units — explained in a way that’s easy to follow and practical to apply.
In South Africa’s unpredictable power landscape, a reliable UPS (Uninterruptible Power Supply) isn’t just a nice-to-have — it’s a critical safeguard for businesses, hospitals, and IT systems. Whether you’re dealing with load shedding, surges, or sudden outages across South Africa and neighbouring countries, protecting sensitive equipment requires more than just a basic backup. That’s why businesses across Southern Africa trust experts like Standby Systems to deliver proven UPS solutions.
In this blog, we’ll explain why online double-conversion systems are considered the gold standard for reliability and power quality.
Different Types of Static UPS Units
There are various types of static UPS units.
But wait! What is a static UPS? A static UPS unit is a type of backup power system that doesn’t have any moving parts, like an engine or motor. Instead, it uses electronics to switch between the main power and backup power instantly when there’s a power cut or voltage problem. Because it’s all electronic, a static UPS reacts very quickly and smoothly — often so fast that computers and sensitive equipment don’t even notice when the power goes out. This is different from a rotary UPS, which uses mechanical parts to keep the power running.
When it comes to static UPS systems, the highest reliability and power quality are achieved through online double-conversion technology. From there, static UPS units can be further classified in three main ways:
- Design topology
Transformer-based or transformerless (also called transformer-free) - System type within these designs
Monolithic design or Modular design - Operational modes
Online double conversion, Line Interactive, or Smart Active
What is Online Double Conversion?
An online double-conversion UPS, such as a Riello UPS system, sits between the main power supply and sensitive equipment like computers, servers, or medical devices. Its job is to ensure a clean, reliable flow of power, even if there are dips, surges, or complete failures in the main or backup power sources. This type of UPS runs continuously, with no moving parts, and includes a battery that kicks in to provide backup power for a certain amount of time if the main supply fails.
The term “online double conversion” describes exactly what the system does: first, it converts the incoming AC power into DC power using a rectifier (the first conversion) and then converts it back from DC to AC through an inverter (the second conversion). This double process keeps the power supply stable and protects connected equipment from any electrical disturbances.
This continuous operation isolates the load from voltage and frequency fluctuations in the mains supply.
What are the Different Components in a UPS System?
1. Rectifier
The rectifier converts AC mains into DC and serves two primary functions: It charges the batteries (if using a controlled rectifier bridge) and supplies power to the inverter while compensating for system power losses.
Types of Rectifiers
- Controlled Bridges use thyristors (older but reliable technology) or IGBTs (newer, costlier, and more efficient with reduced harmonic distortion).
- Uncontrolled Bridges that use diodes, with a separate battery charger of lower amperage to cut production costs.
- Separate battery chargers allow for greater control over charging methods (e.g., charge on demand, float charge). This enhances overall efficiency, as measured with fully charged batteries.
Harmonic Issues
During rectification, harmonics are introduced into the mains supply, which can lead to cables and circuit breakers overheating, malfunctioning power factor correction equipment, and increased electricity bills.
Mitigating Harmonic Distortion
Most UPS systems use 6-pulse rectifiers, which are common but unfortunately produce fairly high levels of harmonic distortion — around 30% Total Harmonic Distortion on Current (THDI). There are a few ways to reduce harmonic distortion caused by the rectification process. 12-pulse rectifiers can be used, which bring the THDI down to a much lower range of 8–12%. Another method is to use filters to clean up the harmonics. However, filters aren’t always fully effective because the electrical grid itself may already have existing harmonics. A more advanced solution is to use IGBT rectifiers. These modern rectifiers can achieve a very low THDI of around 3%, greatly improving power quality. The downside is that IGBT rectifiers are more expensive to manufacture, which can drive up the overall cost of the UPS system.
AC Ripple and UPS Capacitors
AC ripples are unwanted superimposed AC disturbances on DC voltage. They can severely damage battery systems, particularly in lead-acid technology. UPS capacitors filter these AC ripples, ensuring smooth DC power delivery. However, capacitors degrade over time, increasing ripple and threatening battery lifespan. They should be replaced every five to eight years.
Capacitor lifespan depends on operational temperature, with ideal conditions being 20 to 22°C. Extreme temperatures accelerate ageing, potentially leading to overheating or fire hazards, which could lead to more problems.
Battery Charging and Sizing Considerations
Proper battery sizing is critical for ensuring the best performance and longest life from a UPS system. The rectifier must be able to recharge the batteries efficiently without using an excessive charge rate, which could shorten battery lifespan. It’s important to optimise the recharge rate to strike a good balance between battery longevity and system performance. Additionally, the ampere-hour (Ah) capacity of the batteries plays a key role, as it directly affects the available backup time and, in turn, influences how large the rectifier needs to be. For extended backup applications, where backup times longer than 10 to 15 minutes are needed, it becomes even more important to ensure that the UPS rectifier has enough charging capacity to properly recharge the batteries after a power interruption.
For more information about batteries for your UPS units, read our blog: Everything you need to know about UPS batteries in South Africa: The Ultimate Guide to UPS Batteries.
2. Inverter
The inverter converts DC power back to AC via Pulse Width Modulation (PWM) technology, ensuring a clean sine wave output.
Types of Inverters
Transformer-based UPS units, such as the AROS Riello MHP UPS, are an excellent example of a high-quality Riello UPS unit that uses a transformer for better isolation and reliability, especially in high-peaking or dirty loads (e.g., medical scanners and industrial printers).
Transformerless uses higher DC voltage, improving efficiency by up to 97%, but increasing stress on components and susceptibility to faults in harsh environments. A recommended transformerless UPS unit is the AROS Riello NextEnergy UPS.
Transformer-based inverters are more reliable for industrial applications, while transformerless inverters are preferred for high-efficiency applications like data centres and IT rooms.
3. Static or Manual Bypass Switch
A Static Bypass Switch (SBS) allows seamless power transfer between the UPS and mains supply in case of faults or overload conditions. It is essential for uninterrupted operations.
A Manual Bypass Switch mechanically connects the mains to the load, enabling maintenance without shutting down any critical loads or disrupting industrial operations.
4. Power Factor and UPS Ratings
Understanding the power factor is essential when selecting a UPS system. In simple terms, real power (measured in kilowatts, or kW) is calculated by multiplying apparent power (measured in kilovolt-amperes, or kVA) by the power factor. A common issue is that some manufacturers advertise only the kVA rating without clearly disclosing the power factor, which can mislead buyers into choosing a UPS that seems sufficient but is actually undersized. This can cause serious problems if the mains fail, as the UPS may not be able to handle the full load. In most industries, the standard power factor ranges from 0.9 to 1.0, with data centres increasingly demanding a unity power factor of 1.0 to support highly sensitive and high-demand equipment.
5. Software Monitoring and Diagnostics
Most modern UPS systems include remote monitoring options via Ethernet, Modbus, or Jbus.
But wait! What do Ethernet, Modbus, and Jbus mean?
In a UPS system, Ethernet means the UPS can connect to a computer network, just like your laptop or printer would. Through this network connection, you can monitor the UPS remotely (checking things like battery status, load levels, and alerts) using a web browser or specialized software. It’s very common for modern UPS systems to have a network card that makes this possible.
Modbus is a communication protocol (basically a “language” for devices to talk to each other) that’s often used in industrial settings. If a UPS supports Modbus, it can easily communicate with other systems like building management systems (BMS) or industrial automation networks. This allows technicians to monitor and control the UPS as part of a larger system.
Jbus is very similar to Modbus—it’s actually a variant of the Modbus protocol, originally developed mainly for use in industrial equipment in France. In UPS systems, supporting Jbus simply means the UPS can talk to certain types of industrial monitoring and control systems using a slightly different “dialect” of Modbus.
In short, all three (Ethernet, Modbus, Jbus) allow the UPS to be monitored remotely, but Ethernet is for standard computer networks, while Modbus and Jbus are more common in industrial or building management environments.
Although they are helpful, software features should take a back seat to reliability when you choose your UPS system.
Standby Systems offers a range of Riello UPS units equipped with advanced remote monitoring features, giving you complete visibility and peace of mind.
6. Monolithic vs. Modular UPS Technology
When selecting a UPS system, it’s important to consider the type of technology used. Monolithic UPS systems are built as a single, solid unit. They are generally more affordable in terms of total cost of ownership, offer simpler repairs, and are often praised for their reliability. On the other hand, modular UPS systems are made up of smaller, interchangeable units, which makes them highly scalable and provides built-in redundancy — a major advantage for critical applications. However, modular designs can come with a higher upfront cost, and repairs can sometimes be complicated or expensive, especially if a fault requires replacing an entire frame rather than just a single component.
7. Load Backfeed and Regenerative Loads
Some electrical loads, known as regenerative loads, can push excess power back into the UPS system instead of simply consuming it. If this backflow of energy isn’t managed properly, it can cause serious failures or even damage the UPS. To prevent this, UPS systems designed for environments with regenerative loads must include load banks or power absorbers. These components safely dissipate the excess energy and protect the UPS and connected equipment from harm.
8. Cooling Considerations
UPS systems, especially modern high-efficiency models, generate significant amounts of concentrated heat during normal operation. Effective cooling is critical to maintaining system reliability and extending component lifespan. Most UPS units use high-speed fans to prevent heat buildup, but like all mechanical parts, fans wear out over time and need to be replaced periodically. Maintaining an optimal ambient temperature — ideally between 20°C and 22°C — not only prolongs the life of cooling fans but also helps ensure the overall health and performance of the UPS system.
Conclusion
The online double-conversion UPS design provides maximum load protection. While it may introduce harmonic distortion and be slightly less efficient than other topologies, reliability and power quality must remain the top priority when selecting a UPS in South Africa.
For critical loads, invest in a high-quality, double-conversion, three-phase Riello UPS system, such as the MPT, MHT, MHE, or Sentryum series. These trusted Riello UPS units, available through Standby Systems, are specifically suited to handle the demanding power conditions often experienced across South Africa and neighbouring countries.
Should you require professional help, contact Standby Systems.
Our experienced technical team is ready to advise you on the best UPS unit and battery solutions for your business or industry’s needs and assist in servicing your UPS. Visit Standby Systems’ website or contact us at 011 794 3406 for services, 011 794 2541 for UPS systems for sale, or our UPS contact number at 082 450 2361.
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