Practical applications of power protection systems (III) [Part 1/2: Single conversion static UPS systems for healthcare facility loads]

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 healthcare facility loads

Healthcare facilities, like hospitals, have very high power availability requirements to ensure minimal to no interruption of service in critical areas. The high sensitivity of the equipment in use in hospitals demands a high degree of power supply quality and reliability to prevent disruption of mission critical operations and procedures.

If the power supply is compromised at a hospital, the possible outcomes include:

• Loss of critical equipment like life support systems and operating rooms equipment.
• Inability to view images of X-ray machines, MRIs and other digital systems.
• Loss of critical systems needed to register patients, sterilize instruments and complete billing and coding.
• Loss of vital facility systems such as heating, air conditioning, ventilation, emergency lighting, security cameras and lifts.
• Loss of patient signalling systems.
• Loss of access to areas requiring keyless entry systems such as medication, vaccines and other medical supplies.

Requirements

Background

The hospital uses a wide range of critical equipment that require clean and consistent power at all times. Unfortunately, the electrical environment in the hospital often fail to provide the optimal conditions, leaving these critical devices at risk for damage and downtime.

The requirements from the management of the hospital for this project can be summarised as:

• Protection of critical equipment as their proper operation affects the ability of the hospital to run effectively, so they must be kept in operation in case the mains power supply fails.
• Sensitive medical equipment, require continuously tight voltage regulation and this is only possible by using a power protection system.
• Improvement of the overall power quality and energy efficiency of the hospital.

System description

The hospital has critical and noncritical equipment connected to its electric power system. The critical equipment include life support systems, ventilators, operating room equipment, medical imaging equipment and laboratory equipment. The noncritical equipment include lighting, administrative systems and other non-medical equipment.

The power for all this equipment is supplied from the electric utility and from a rooftop solar PV power plant. There is an existing standby diesel generator connected after the power transformer supplying the critical equipment.

Solution

Analysis

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

The measurement results show that the supply to the critical equipment suffers from voltage sags lasting several milliseconds. The measurements also show that the critical equipment is suffering from other power quality and energy efficiency problems. The harmonic level in the installation is high (THDi = 18%) and the power factor is low (PF = 0.91).

Proposed solution

Based on the analysis of the measurements, the management of the hospital decided to install a dedicated power protection system for the critical equipment. A single conversion static UPS system rated 500 kW 550 kVA was chosen. It uses supercapacitors as the energy storage media coupled through an energy storage inverter to enable the protected equipment to ride through voltage sags and short interruptions of up to 12 seconds, which allows enough time for the existing standby diesel generator to start.

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

Conclusions

In today’s healthcare landscape technology innovations are driving the practice of medicine, completely modernising the way in which medical care is delivered and information is shared. However, this growing dependence on technology has increased the need of these facilities for a continuous and reliable supply of power.

The benefits of using single conversion static UPS systems for healthcare facilities can be summarised as:

• Modular and scalable architecture simplifies future expansion of the facilities.
• High energy efficiency, over 99%.
• Small footprint.
• Can protect the full healthcare facility or just selected sensitive equipment.
• Ultra-fast transfer time of less than 2 milliseconds.

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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.