Which One Can Feed Capacitive Power Factor Static or Rotary Ups

After the introduction of power protection systems and their design, components and functions, this eighth article will provide an overview of static UPS systems.

Static uninterruptible power supply systems (SUPS for short) have been around since the 1960s. Description of their topology and operating principle can be found as far back as 1965. They store power typically in the form of electrochemical energy (rechargeable batteries, flow batteries or fuel cells), electrical energy (supercapacitors) or mechanical energy (flywheels).

Static UPS systems are the most commonly used UPS systems. They have a wide variety of applications ranging from low power personal computers to medium power industrial machinery to high power critical facilities. Depending on their design, static UPS systems used in applications over 10 kVA can be divided into:

• Single conversion static UPS systems (SC SUPS).
• Double conversion static UPS systems (DC SUPS).
• Delta conversion static UPS systems.

These UPS systems are called "static" because throughout their power path they have no moving parts involved in transferring power to the protected equipment or facility (although they may have auxiliary moving parts, such as cooling fans).

Static UPS systems protect against full range of power disturbances affecting equipment and facilities thanks to the seamless transition between the electrical supply and the energy storage. There is almost no transfer or switching time delay (typically around 2 milliseconds) involved during a power supply event like an interruption.

One of the major selection criteria when considering static UPS systems for any application is their efficiency. Double conversion and delta conversion SUPS have lower efficiency than single conversion SUPS. This is due to the two power conversion stages used in these devices (rectifier/inverter or delta converter/inverter) plus the needed transformers, compared to the single conversion SUPS design that uses just one power conversion stage.

Single conversion SUPS (SC SUPS)

A single conversion static UPS system (SC SUPS for short) typically works both as a controlled current source providing any kind of compensation current waveform in real time and as an energy storage device able to supply backup power for a certain period of time when needed. Their design usually incorporates additional power quality and energy efficiency improvement features and functionalities not found commonly in conventional double conversion or delta conversion SUPS systems.

Design

The main components of single conversion static UPS systems are

• Connection module: It contains a static switch that separates the device and the protected equipment or facility from the supply in case of an event in the supply occurs, and a bypass switch to automatically bypass the device when an internal fault occurs. It also contains current transformers to measure the supply side and the equipment side currents.
• Step-up transformer: It is used if the device needs to be connected to a system over 690 V.
• Energy storage inverter: It produces the needed currents to take care of installation's power quality problems and energy efficiency requirements. It also converts the energy storage's DC energy to three-phase AC power and recharges the energy storage as needed.
• Energy storage: It provides power to the protected equipment or facility when needed. To suit required autonomy several energy storage media can be used. The most common media used are supercapacitors or rechargeable batteries.
• Control & protection system and HMI: It detects disturbances in the power supply side or in the equipment or facility side and handles the operation of the energy storage inverter and the energy storage media.

The modular design of the energy storage inverters and the energy storage media of these devices together with the electromechanical bypass switch provides high system availability. The modular design also reduces the footprint of the device and enables fast and simple maintenance.

Single conversion SUPS offer several benefits compared with double conversion or delta conversion SUPS systems. They have a very high efficiency (typically over 99%) resulting in very low operational costs and reduced total cost of ownership. They also do not limit system's short circuit current, so it is the same at the protected equipment or facility as fed by the electric utility or local generator/s.

Basic schema of a single conversion SUPS

Operation

When the supply voltage is within a user defined range (normal situation), the power to the protected equipment or facility is supplied directly by the electric utility or local generator/s. In this situation, the energy storage inverter is taking care of the power quality problems or energy efficiency requirements of the equipment or the facility. At the same time, the energy storage inverter maintains synchronization with the supply voltage to allow instant operation of the device in case a disturbance in the supply occurs. It also maintains the state of charge of the energy storage media.

When the supply voltage deviates from the user defined range, the device immediately begins supplying power to the protected equipment or facility from its energy storage media through the energy storage inverter guaranteeing maximum system availability and reliability. At the same time, the supply from the electric utility or local generator/s is disconnected by opening the static switch, isolating the device output and the protected equipment or facility from the supply. Typically, maximum time required by the static switch to open is 1.8 milliseconds.

When the supply voltage returns within the user defined range, the device synchronizes its voltage with the supply voltage and closes the static switch. This provides a seamless transfer of the protected equipment or facility from the energy storage supply back to the electric utility or local generator/s supply. After that, the energy storage media is rapidly recharged by the energy storage inverter.

Double conversion SUPS (DC SUPS)

The double conversion static UPS system (DC SUPS for short) is the most commonly used type of static UPS system above 10 kVA. They convert power twice during normal operation providing both, conditioned power and protection from power supply interruptions to critical equipment and facilities.

Design

The main components of double conversion static UPS systems are a rectifier/charger, an inverter, an energy storage media (typically rechargeable batteries), a static bypass switch and a electromechanical bypass switch.

In their design, first an input rectifier/charger converts AC power into DC power in order to charge the energy storage media and feed power to the output inverter. The output inverter then processes the DC power back to AC power before sending it on to the protected equipment or facility.

Because double conversion static UPS systems rectify the input power supply, they can accommodate a certain amount of variations in the supply voltage and frequency and continue to operate normally without going into backup power operation mode.

The main advantage of double conversion UPS systems is their ability to provide an electrical firewall between the incoming power supply and sensitive equipment or facilities. They allow control of output voltage and frequency regardless of input voltage and frequency.

The major disadvantages of double conversion UPS systems are the increased costs and increased power consumption due to losses in the two power conversion stages (the rectifier/charger and the inverter), with the corresponding increase in heat generation. Depending on design, they can also produce harmonic currents that affect the electric power system. The process of drawing power from the upstream power source through the rectifier/charger creates harmonic distortion. This distortion causes both current and voltage harmonics and can reduce the power factor of the installation.

Basic schema of a double conversion SUPS

Operation

When the supply voltage is within a user defined range (normal situation), the power to the protected equipment or facility is supplied directly by the electric utility or local generator/s through the rectifier/charger - inverter system.

When the AC input power supply falls out of predefined limits, the input rectifier/charger shuts off and the output inverter begins drawing power from the energy storage. The device continues to utilise energy storage power until the AC input returns within normal tolerances or the energy storage runs out of power, whichever occurs sooner. When the power supply is restored, the rectifier/charge resumes supplying the load and begins charging the energy storage media.

In case of a severe overload of the inverter, or a failure of the rectifier/charger - inverter system, the static bypass switch path is turned on immediately to support the protected equipment or facility.

Delta conversion SUPS

Delta conversion static UPS systems were developed to improve one of the major weaknesses of double conversion static UPS systems, their low efficiency. Their design does not require two power conversion stages; thus, their operating efficiency is improved.

Design

The main components of delta conversion static UPS systems are a delta transformer, a delta converter, an inverter, an energy storage media (typically rechargeable batteries), a static bypass switch and a electromechanical bypass switch.

The delta conversion static UPS system differs physically from other static UPS system designs because of two main components, the delta transformer and the delta converter. The delta transformer is typically formed by three single-phase isolation transformers, one for each phase. The delta converter performs two major functions:

• Regulates the input power and current acting like a variable current source in the secondary circuit of the delta transformer. This allows the control of current magnitude, wave shape and power factor.
• Charges the energy storage media by converting AC voltage to the suitable DC charging voltage.

Delta conversion static UPS systems have certain advantages over the conventional double conversion static UPS systems like higher power factor, lower harmonic generation, lower operating losses and better overload capability during normal operation.

Basic schema of a delta conversion SUPS

Operation

Under normal operating conditions, when the input voltage is within acceptable limits, power is supplied to the protected equipment or facility through the delta transformer from the electric utility or local generator/s. The delta converter and the inverter are both on, so that the delta converter can control the delta transformer secondary current, including its magnitude, phase and wave shape. The delta converter takes also care of the charge level of the energy storage media.

When the supply voltage deviates from the acceptable limits, the delta converter shuts off, forcing the primary current in the delta transformer to zero, and the energy storage media discharges through the inverter to support the protected equipment or facility.

The next article of this series will discuss rotary UPS systems.

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 pedro.esteban@meruspower.com.

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

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About the author:

Pedro Esteban holds since 2002 a broad global experience in sustainable energy innovation and transition including renewable energy, power electronics solutions, energy storage, microgrids and their Smart Grid integration. He has been a leading expert in several marketing, strategic planning, business development and communications positions at Areva T&D, Alstom Grid and General Electric. He is based since 2012 in Singapore and currently working at Merus Power Plc.

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Source: https://www.linkedin.com/pulse/how-choose-right-power-protection-system-your-part-811-pedro-esteban

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