Practical applications of power protection systems (I) [Part 1/2: Single conversion static UPS systems for chiller plants]

Single conversion static UPS systems

Single conversion static uninterruptible power supply systems (SC SUPS for short) have been around since the 2000s. Description of their topology and operating principle can be found as far back as 2005. They were developed to take care of the increasing power quality problems and energy efficiency requirements of equipment and facilities that conventional solutions like power and voltage conditioners, double conversion or delta conversion static UPS systems or rotary UPS systems could not handle.

Single conversion static UPS systems can be applied to small, medium or large installations in a wide range of segments. Thanks to their flexibility and their availability in a wide range of power and voltage ratings, they have many low and high voltage potential applications where their use offers many benefits including continuous production processes, mission critical facilities, sensitive machinery, and their integration together with standby generators to build continuous power supply systems.

Functions

Single conversion static UPS systems combine the functionalities and benefits of various power protection, power quality improvement and energy storage technologies into a single robust system without the drawbacks of conventional solutions. They can fully protect equipment and facilities from short and long duration voltage variations of any magnitude. In addition, they can take care of several other power quality problems and support the development of clean energy by combining different control functions in a single device.

Connection

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.?

By their connection capability to the electric power system, they can be classified into 3-wire or 4-wire devices.

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

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. It is possible to connect these devices to higher voltages using a suitable step-up transformer for the energy storage inverter.

SC SUPS for chiller plants

Data centers are nowadays one of the most dynamically growing market sectors as they constitute a crucial element for the development of digital platforms, cloud services, remote work and e-commerce. Gathering, storing and managing data requires extremely high standards. Therefore, the most important concern for a data center is continuous reliable operation.

The purpose of data center cooling technology is to maintain environmental conditions suitable for information technology equipment operation. Achieving this goal requires removing the heat produced by this equipment and transferring that heat to some heat sink by using for example computer room air conditioners, heating, ventilation and air conditioning (HVAC) systems, and exhaust systems. Chillers are typical cooling systems used in data centers to remove heat from one element and deposit it into another element. Chillers are used for example to cool the water used in the HVAC systems.

Data center design needs to take into account the cooling equipment performance in the event of utility power failure. Planning for an eventual power shut down becomes critical due to the resulting IT equipment temperature rise. There are different strategies to deal with an eventual utility power failure and the transition period to get the standby generators online including:

• Connect critical cooling equipment to power protection systems.
• Maintain enough reserve of cooling capacity.
• Design and use cooling systems with short start up times.
• Design and use thermal storage to cover for the cooling equipment re-start times.

Requirements

Background

Over time, increasing power density and growing number of installed servers had increased the heat load on the data center cooling system which operates on a 24/7 basis 365 days a year. Existing chillers installed during the building’s construction could not meet the cooling demand and provide the necessary redundancy required. As a result, the cooling system needed to be upgraded to keep pace with the data center’s growth.

After an analysis of the potential energy efficiency improvements and costs, the owner of the data center decided to replace original chillers with last generation variable-speed chillers.

At the same time, the owner is looking for a solution that would protect the operation of the chiller plant against short duration voltage variations and provide the necessary backup power until the standby diesel generator comes online and reaches operating speed, a period of 10 to 12 seconds.

System description

The chiller plant is a closed-loop system connected at 415 V including chiller units, condensers and primary and secondary pumps. There is an existing standby diesel generator connected after the power transformer supplying the chiller plant.

Solution

Analysis

To be able to dimension a solution it is necessary to collect power quality measurement data from the chiller plant over a period of time by using a power quality analyser.

The measurement results show that the chiller plant suffers from voltage sags lasting from several milliseconds to up to 1 second. The measurements also show that the chiller plant is suffering from other power quality and energy efficiency problems. The harmonic level in the installation is high (THDi = 30%) and the power factor is low (PF = 0.86).

Proposed solution

Based on the analysis of the measurements, the owner decided to install a dedicated power protection system for the chiller plant. A single conversion static UPS system rated 1,500 kW 1,500 kVA was chosen. It uses supercapacitors as the energy storage media coupled through an energy storage inverter to enable the protected chiller plant to ride through voltage sags and short interruptions of up to 12 seconds, which allows enough time for the standby diesel generator to start.

Based on the values monitored, the following functions are proposed for the SC SUPS.

When the voltage is within allowed limits, the single conversion static UPS system will also take care of other power quality and energy efficiency problems of the installation. It will reduce the harmonics produced by the chiller plant and at the same time it will improve its power factor.

The single conversion static UPS system was also the preferred solution due to its very high energy efficiency and small footprint. Efficiency and space are always important considerations in data centers, but restricted space and cooling power in this certain case meant they were essential features.

Conclusions

Modern data centers are becoming very large and are demanding more power. Server density is increasing rapidly and power distribution systems are consuming more space. When it comes to the server rooms, it is imperative that the servers and other IT equipment are kept in optimal temperature conditions at all times. Any power supply disruption affecting critical cooling equipment at the chiller plants might be particularly harmful.

Voltage variations are not the only problems present at chiller plants. Harmonics and low power factor cause increased apparent power loading on the electrical supply often resulting in higher electricity charges from the utility. These problems if not taken care of properly they can also affect the operation and damage other equipment in the data center.

The benefits of using single conversion static UPS systems for chiller plants at data centers can be summarised as:

• Protection against interruptions, voltage sags and voltage swells.
• Protection against utility reclosure events.
• Backup power supply during standby generator start up time following a utility supply failure.
• Very high efficiency (typically 99%).
• Very high fault capacity compared with conventional double conversion static UPS systems.
• Modular design providing high reliability and short repair times.
• Small footprint.

If you would like to receive any of my publications on the topic or to explore how #PowerProtectionSystems can benefit your application, feel free to reach me at [email protected].?

You are also welcome to join my running series of weekly #FreeWebinars for Asia-Pacific region on cutting edge #PowerElectronics solutions and their applications.

--------------------

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.