How to Rate an Uninterruptible Power Supply (UPS)
The UPS (Uninterruptible Power Supply) is rated in kVA or VA, reflecting its output voltage and current. It typically has a power factor (PF) which influences its true output kW rating. Common examples include 20kVA at 0.8PF resulting in 23A output current per phase, while 10kVA and below often use a 0.7PF yielding 30A. The main role of a UPS is to provide a stable, 'clean' power supply to protect sensitive computer equipment from voltage spikes and fluctuations that can arise from the electrical mains, which may suffer distortions due to high loads or irregularities. Systems over 6kVA feature redundant mains inputs for reliability. When using diesel generators as backups, it is advised to have a generator rated at least double the UPS capacity to ensure reliable power during outages. Overall, a UPS enhances equipment protection and operational continuity.
The UPS is rated in kVA, or VA for smaller models. This is basically the product of the UPS rate output voltage and current. However, due to the nature of the electrically complex load that uninterruptible power supplies support, manufacturers often derate the UPS. It is often stated, for example, a 20kVA UPS has a power factor of 0.8. (20kVA at 0.8PF). This will give the true output rating in kW. For uninterruptible power supplies with ratings typically 20kVA three-phase input, three-phase output UPS here’s the equation below:
20000 x 0.8 (kW) /3 (power per phase) / 230 (output voltage) = output current per phase, which in this case would be 23A maximum.
For uninterruptible power supplies with ratings of 10kVA and below, the output power factor is often stated as being 0.7. Therefore a 10kVA single phase input, single phase output UPS at 0.7PF would have a maximum output current capacity of 30A. Manufacturers have many different ways of calculating their power factor rating; some state 0.9, 0.95,0.99 etc. Only by carefully comparing their methodology can the true factor be calculated. The figures themselves shouldn’t be taken on face value without further investigation.
UPS General Operation
The role of a UPS system is to provide a stable and consistent power supply, commonly referred to as a "clean supply," to the connected load. This term can be better understood by delving into the operation of UPS systems and the potential challenges associated with the utility mains supply.
The utility mains supply, which is provided by the local electricity supplier, can become distorted due to various factors, including the load it serves. For instance, in manufacturing processes where multiple motors are frequently turned on and off, it can result in the generation of spikes of extremely high voltages. These voltage spikes pose a significant risk to delicate computer equipment, as they can lead to damage, malfunctions, or data loss. In addition to the distortion caused by the load itself, another challenge arises from the fact that a significant portion of computer equipment is manufactured overseas. Power supplies associated with such equipment may not be designed to withstand the voltage ranges experienced in a particular country. For example, in some regions, the nominal supply voltage may be 230V, but voltages can reach as high as 260V. This discrepancy becomes more apparent during the summer months when there is a higher demand for air conditioning, causing the local substation transformer taps to be raised to cope with the increased load. Similarly, during low-demand periods, such as at night, the supply voltage can increase. These fluctuations in supply voltage can negatively impact sensitive computer equipment, often leading to performance issues, component failures, or even permanent damage. In this context, the importance of a UPS system becomes evident. It serves as a protective barrier, shielding the connected equipment from irregularities in the utility mains supply.
By actively monitoring the incoming voltage, a UPS system can regulate and stabilise the supply, providing a consistent and safe power source to the load. It filters out spikes, surges, and unwanted fluctuations, ensuring that the equipment receives a "clean supply" of electricity within the acceptable voltage range, regardless of variations in the utility mains supply. Overall, the UPS system plays a crucial role in protecting delicate computer equipment from the adverse effects of voltage spikes, fluctuations, and distortions that can arise from the utility mains supply. It safeguards against potential damage caused by manufacturing processes and ensures that the connected load receives a stable and reliable power supply, even in regions where the supply voltage can vary. Conventional UPS systems operate in the following way, broken down into a number of building blocks for clarity of description.
Mains Input
When it comes to UPS systems with a capacity greater than 6kVA, it is common to have provisions for two separate utility mains inputs to enhance security and reliability. These inputs are typically protected by individual fuses or circuit breakers. One input is dedicated to powering the rectifier/charger and, consequently, the inverter of the UPS system. The other input is reserved for the backup power supply, responsible for seamlessly switching the load in case of a mains failure. By having two utility mains inputs, the UPS system ensures redundancy and minimises the risk of downtime or power disruption. In the event of a fault or failure in one input, the UPS can seamlessly switch to the alternate input, maintaining a continuous and uninterrupted power supply to the load.In situations where a diesel generator is required as a standby power protection measure, it is recommended to have a generator with a capacity that is at least twice the size of the UPS system. This is to ensure that the generator can handle the maximum power load of the connected equipment during an extended power outage or blackout. Oversizing the generator provides an additional safety margin and allows for potential future expansions or increases in power demands. With a generator capacity twice that of the UPS system, there is ample headroom to support the load without overburdening the generator or risking performance issues.
Having a properly sized and adequately rated generator ensures efficient and reliable backup power protection for critical systems. It provides peace of mind knowing that in the event of a utility mains failure, the UPS system is capable of seamlessly switching to the backup power supply without any disruptions or compromises in power quality.
