Uninterruptible Power Supply (UPS) Systems
Overview
An Uninterruptible Power Supply (UPS) ensures continuity of the power supply regardless of fluctuations or interruptions in the utility supply. This is an essential requirement for critical applications such as IT/data centers, stock exchanges, medical scanners, radar systems etc. Such fluctuations and interruptions can have major consequences if there is even a momentary break in the supply.
In addition, the UPS needs to provide a clean and stable power supply, free from voltage distortion, frequency variations, electrical noise, harmonics, spikes, brownouts, and surges. If any of these issues occur in the mains supply at a significant level then critical loads and computer systems can fail.
Types of Uninterruptible Power Supply (UPS) Systems
UPS systems are generally static or rotary. These are fundamentally different in their construction, method of operation, and protection of the load. Almost 98% of UPS systems are static, due to their superior topology, size and resilience, and lower costs of ownership and maintenance.
Static UPS system
Static UPS systems provide a power range from approximately 100 VA to 1,100 KVA per unit. Static systems utilize a frontend rectifier and a DC link connected to the output stage inverter module. When the input utility mains supply is within acceptable limits, the input power is converted from AC to DC by the rectifier. The DC link is utilized for recharging the battery bank. The majority of the DC power is designated for the inverter unit, which converts the DC power into tightly regulated clean AC output.
Rotary UPS system
A rotary UPS generally incorporates a motor and/or alternator unit plus a diesel engine and a kinetic energy storage unit. The way in which a rotary UPS continues to drive the alternator in the event of a mains failure depends on the individual system. Rotary units generally contain a high number of mechanical components. This results in a higher rate of equipment failure than static UPS systems. Repairs can also take longer since some of the components are rather bulky. Initial cost of rotary units can be high (40-50% higher than similar static UPS systems) and their scalability is limited. There are three basic types: diesel, hybrid, and simple mechanical flywheel backed UPS units.
Diesel rotary UPS systems
Diesel rotary units are generally noisy and are only available in higher (500 to 1,600 kVA) power ranges. They contain a device often referred to as an induction type coupling. This is an electro-mechanical eddy current based flywheel that stores kinetic energy able to last for a few seconds (3 to 6). In the event of a mains failure, the energy stored in the induction type coupling is used to maintain the required alternator shaft speed while the diesel engine is started and brought up to speed. Once the diesel engine speed has stabilized, it can then commence to support the load.
Hybrid rotary UPS systems
Hybrid rotary UPS systems do not incorporate a diesel engine or induction type coupling. Instead, they use a rectifier, batteries, and an inverter to provide the AC power needed to support the motor alternator in the event of a mains disturbance or failure. Such systems range from 300 kVA to 800 kVA. A standby diesel generator will be required for the long-term support of critical loads that require power at all times regardless of the mains blackout period.
Simple mechanical flywheel backed UPS units
These units are mainly designed to provide a ride-through lasting only 10-15 seconds. This is sufficient to enable the standby generator to start up and provide support for the critical load. Ratings are available from 60 kVA to 250 kVA. Their market share is extremely small, as flywheel energy recharge time can be a risk, recovery from possible step load condition is slow, and repair times can be long and expensive.
Main Components of a Static Uninterruptible Power Supply (UPS) System
Rectifier
The rectifier provides the necessary float charging to the battery and simultaneously the stable DC power via the DC link for the inverter. Most UPS units are fitted with temperature compensated rectifiers to avoid damaging the battery at high ambient temperature. Over the past few years, most major manufacturers have begun to offer new transformer-less UPS systems with an active front-end rectifier. Such a design utilizes a boost-converter type switched mode power supply offering benefits such as very low harmonic distortion. Such rectifiers can save space and accept a wide range of input voltages and frequencies.
Inverter Block
The inverter block converts the DC link voltage into an AC output with a tight control on tolerance to suit critical load applications. Over the years there has been a major shift in inverter power component technology, with the latest being the use of Insulated Gate Bipolar Transistors (IGBT) with very high switching frequencies, typically 2.2 kHz/sec. These changes have helped improve the efficiency of UPS systems and reduce noise levels and ecological footprint.
Static Bypass Switch
A static bypass switch is necessary to protect the load with a feed from the inverter or utility supply, either due to overload or UPS malfunction. Built-in control and intelligence constantly monitors the mains condition and the phase angle. This is critical in order to achieve a transfer without a break, i.e. from inverter to mains or from mains back to inverter.
Maintenance Bypass
A maintenance bypass is essential and is normally built into the UPS to provide isolation during maintenance or repair, as well as meet health and safety requirements.
Batteries
Batteries (accumulators) are one of the key components of static UPS systems. They provide necessary storage for backup energy when a utility fails or is outside the agreed tolerance level. Typical autonomy times vary from 10 to 20 minutes. The type of battery used and the maintenance procedures required have a significant influence on cost. Stationary batteries are used when weight is not important, and are usually of the sealed lead acid (SLA) type because of their lower cost. For larger UPS systems it is recommended that a battery has a 10-year life. There is a trend towards the use of the sodium-nickel (zebra) battery for UPS applications. This has more benefits compared to the standard SLA unit, but still has technical and commercial limitations.
Selecting the Right Uninterruptible Power Supply (UPS)
Factors to consider when choosing a UPS system include performance, efficiency across the load range, reliability, Total Cost of Ownership, weight, size, and ease of maintenance. While scalable UPS systems and sleep mode options can significantly raise system efficiency, such solutions are only appropriate if they do not sacrifice system resilience.
Apart from normal basic information – utility voltage, frequency, and load current – consideration must be given to the load power factor across the entire load range. Both the apparent (kVA) and the active (kW) power load consumption – with expected load variation over time – need to be considered. If lighting loads made up of gas discharge and fluorescent fittings are to be supported, then the UPS will have to be sized for very high inrush current demand. Allowance should also be made for redundancy, expansion, and scalability.
Choosing the Optimal Location
The location for a UPS system should be safe from flooding, since sites selected by architects for electrical power and UPS installations tend to be in basements. If possible, place the unit on steel plinths to help protect from flooding. This will also ease system cabling as most UPSs are suitable only for bottom cable entry.
Adequate all-round access is essential to ease servicing. Overhead water pipes should be avoided since leaks may damage the UPS system. Air conditioning may be necessary to keep the environmental temperature between 10 and 25 °C.
Some applications may require a special type of UPS to meet environmental demands. For example, a marine application may need a unit suitable for a salty atmosphere. A UPS system for a semiconductor/chip manufacturer will need units with special anti-vibration mountings to limit vibrations being passed on to the production platform.
Conclusion
UPS systems now form part of the value chain for most companies since power quality and availability have direct impact on the continuity of their operations. In some cases, a major discontinuity may even jeopardize survival of the business itself. In recent years, the utilization of UPS systems has undergone substantial evolution and innovation. Since the IT industry – a major UPS customer – continues to grow at an exponential rate, UPS demand will also continue to grow strongly.
Further Information
References
- IEC62040-3 UPS Topology
- EN50091-1: UPS-Safety, EN50091-2: UPS-EMC, EN50091-3: UPS—Performance
- "Three of a Kind-UPS", Shri Karve, IEE review, March 2000
