Single conversion static UPS systems for mission critical applications [Part 3/9: Design]

After the introduction of single conversion static UPS systems, this article will discuss the design of these devices.

A single conversion static UPS system is a power electronics-based device connected in series with the equipment or facility that has to be protected, that is generating power quality problems or that has issues to comply with certain grid code and energy efficiency requirements.?

The basic design of the device can be described as an energy storage media connected to the electric power system through a power electronics inverter. Depending on the needed backup time, the device is able to use different energy storage technologies. A control and protection system enables the device to safely inject the required current/power/energy into the electric power system as demanded by the application.

The modular design of the energy storage inverter and the energy storage media of these devices together with the static switch provides high system availability. This design also reduces the footprint of the device and enables fast and simple maintenance. The compact and small footprint allows the use of these devices for new installations or as a kit to retrofit existing backup power systems.

By their connection capability, they can be classified into 3-wire and 4-wire devices.

• 3-wire devices are typically used in industrial applications.
• 4-wire devices are typically used for applications in buildings.

Voltage range

When connected to an AC electric power system, single conversion static UPS systems are offered in a range of voltages. Most common range is 200 V up to 690 V as the energy storage inverters are usually built using low voltage IGBT switches. Many manufacturers offer devices that can be connected directly to the electric power system within this range.?

Typical single conversion static UPS system design for low voltage connection

It is possible to connect the devices to higher voltages using a suitable step-up transformer for the energy storage inverter. Transformers should be studied carefully when designing single conversion static UPS systems. Step-up or step-down transformers could reduce operational performance due to increased impedance in between the device and the electric power system.

Typical single conversion static UPS system design for high voltage connection

Power and energy sizing

The power rating of a single conversion static UPS system is basically the amount of load that it is designed to handle. These devices have both watt ratings and volt-ampere (VA) ratings. Typically, the watt rating is approximately 60% to 80% of the VA rating, this coming from the typical power factor of common loads. Single conversion static UPS systems are typically available with ratings as low as 20 kVA and as high as 3 MVA.

The energy rating of a single conversion static UPS system guarantees that loads being protected are adequately served for its designed autonomy time. An accurate energy rating is important for selecting and sizing the energy storage media.

Unless having very precise load data for the equipment to protect, the basic method to determine the approximate power and energy rating of a single conversion static UPS system can be summarised as:

• List all equipment that needs to be protected by the single conversion static UPS system.
• List the amps and volts that each piece of equipment draws. These ratings can typically be found on the rating plate of the equipment.
• For each piece of equipment multiply amps by volts to determine its VA rating (apparent power). Some devices may list their power requirements in watts. To convert watts to VA, divide the watts by the power factor.
• Decide on the autonomy time (in hours) for each equipment. Some devices may only be required to ride through short interruptions or to have enough time for proper shut down. Other critical devices may need to operate for as long as possible.
• Multiply the VA and autonomy time for each device to get the energy requirements (VAh).
• Add all of the VA and VAh ratings together.
• Multiply the total by 1.2 to add the design margin factor. This step accounts for any inaccuracies in estimating the protected load that may lead to overloading the single conversion static UPS system.
• If needed, multiply the result by 1.2 to add the load growth factor. This step accounts for future expansion of the facility.
• Use the grand total to select a single conversion static UPS system. When choosing a single conversion static UPS system, it is important that the total VA and VAh requirement of supported equipment does not exceed the VA and VAh rating of the single conversion static UPS system.

Some equipment like motors and compressors produce an inrush of current during start up or draw higher currents in normal operation, which can cause the single conversion static UPS system to overload. For this equipment, good practice suggests two options: Removing them from the single conversion static UPS system (if they can safely power down on mains failure) or oversizing the single conversion static UPS system by a certain safety factor to take their operation into account.

Typical worksheet for sizing a single conversion static UPS system

Scalability and modularity

Implementing a robust power protection solution takes time and money. To get as much value as possible from that investment, facilities should estimate their needs for the future at design stage. If the facility is likely to have significantly larger power requirements in the coming years, they must design a larger power protection system.

Full scalability of single conversion static UPS systems is achieved through modularity in the energy storage inverters and switches used as well as in the energy storage media selected. Facilities with fluctuating needs or future requirements that are difficult to forecast can employ two strategies to increase the scalability of their single conversion static UPS systems:

• Install single conversion static UPS systems in parallel: Parallel arrangements boost scalability and redundancy. As their power needs increase, facilities can simply add more devices in parallel to existing systems rather than replace old devices with new ones.
• Use modular single conversion static UPS systems: Some devices feature modular designs that allow to add capacity incrementally as requirements increase. This is an scalable and efficient approach to keeping up with escalating power needs that also lowers upfront capital spending and conserves floor space.

Availability

Despite all the precautions taken during the design and operation of critical facilities or industries, situations can arise in which power supply is compromised in terms of availability. Such events could result for example in production and data losses, interruption of essential services, risk to personnel and equipment and high financial losses. This makes highly dependable power protection systems essential components to maintain the availability of these critical facilities or industries.

The level of protection given by a single conversion static UPS system is best quantified by the metric power availability. It represents the fraction of time a system is operational during its expected lifetime. Availability is formally defined as:

Availability (%) = MTBF / (MTBF + MTTR) x 100%.

• MTBF is short for mean time between failures and it is a measure of reliability. It measures the ability for a system to run without failures ensuring correct operation of equipment and processes.
• MTTR is short for mean time to repair and it is a measure of maintainability. It measures the easiness and speed at which maintenance and service can be performed including the time to detect the cause of the failure, repair it and start the system up again.

The availability of a single conversion static UPS system can be increased in two ways, increasing the reliability of the system or increasing its maintainability.

The reliability of the system can be increased using several methods including:

• Adding redundancy by using a modular design or implementing a decentralized parallel architecture to eliminate single points of failure.
• Minimising the chance for human error during operation and service of the system by reducing its complexity and using standardised components.
• Using proven and reliable components.

The maintainability of the system can be also increased using several methods including:

• Minimising or eliminating the need for downtime during maintenance activities that can be achieved by using modular designs.
• Simplifying and standardising the service concept across the facility.

Continuous power supply systems

An special application where single conversion SUPS can be used is in the design of continuous power supply systems (CPS for short) as the main component of diesel/gas static UPS systems (D/G SUPS for short).

These systems combine the advantages of static UPS systems together with the almost unlimited power backup time offered by standby generators. They are formed by a single conversion SUPS integrated together with an standby generator (typically a fossil fuel generator) through an automatic transfer switch combining both short term protection against momentary power supply disturbances with longer term emergency power generation.

The next article of this series will discuss the main components of single conversion static UPS systems.

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.