Diesel/gas static UPS systems for mission critical applications [Part 5/9: Other components - Switches]

After the introduction of diesel/gas static UPS systems, this article will discuss the switches used in these devices.

Power electronic switches

Power electronic switches can be opened and closed through a control signal. They can be broadly classified into two major groups depending on their controllability.

Line commutated switches can be turned on when the anode-cathode voltage is positive, and an adequate firing signal is sent to the gate. However, once the conduction state is established, they can only be turned off when the current is almost zero and the anode-cathode voltage is negative. Therefore, these devices are characterized by a single degree of freedom, namely, the firing angle. The thyristor switch (also known as silicon control rectifier or SCR) is the most relevant switch within this group. Static switches used in diesel/gas static UPS systems are built with thyristor switches.

Self-commutated switches can be turned on by applying a suitable control signal to the gate when the collector-emitter voltage is positive, and then turned off just by removing the control signal. This brings an additional degree of freedom compared to line commutated switches. IGBT, GTO and IGCT are the most relevant switches within this group. Diesel/gas static UPS systems use IGBT switches in their energy storage inverters.

Static switches

A static switch (SS for short) is a device that executes fast switching operations without the presence of moving parts. Their main mission is to separate the diesel/gas static UPS system and the protected equipment or facility from the electric supply in case of an event or disturbance affecting the supply.

Static switches are electronic switches that are usually built with thyristor switches. Each phase of the static switch is formed by a matched pair of inversely mounted thyristor switches. A three-phase static switch consists of three single-phase static switches. One of the thyristor switches conducts during the positive voltage half cycle and the other conducts during the negative half cycle. Thyristor switches are able to conduct when the anode-to-cathode voltage is positive and a gate pulse is applied.

In static switches, the thyristor switches are used for simply connecting or disconnecting the input and the output of the static switch. They do not have control over the power delivered through them. Hence, the power delivered through the static switch is not changed.

Static switches offer several advantages compared to conventional electromechanical switches. Some of the advantages are:

• Fast switching (within 3 to 5 milliseconds).
• No limitations in the number of switching operations.
• No mechanical movements of contacts when switching, this means no arcing or sparking takes place and no audible switching noise is produced.
• Due to the absence of moving parts, there is less wear and tear thus maintenance is reduced.
• No system perturbation by switching operations due to zero cross over switching techniques. Transients can be controlled within safe limits.

Electromechanical switches

Electromechanical switches like contactors or circuit breakers are automatically operated switches designed to make or break the connection between their input (usually the power supply) and their output (usually the connected equipment). They are typically controlled by using electromagnetism to operate a mechanical switching mechanism that consists of fixed and moving contacts.

Bypass switches

Bypass switches, also known as maintenance bypass switches, are an external wiring path to which the protected equipment or facility can be transferred to in order to upgrade or perform service on the diesel/gas static UPS system without powering down the protected equipment or facility. Circuit breakers are typically selected as bypass switches. They usually work together with electromechanical input and output switches to ensure that the diesel/gas static UPS system is electrically isolated so that personnel can perform the required work safely. Without a bypass switch, the only safe alternative is to power down the entire equipment or facility, not just the diesel/gas static UPS system.

Bypass switches can incorporate mechanical or electrical interlocks to make sure the bypass operates in the correct sequence and does not damage the diesel/gas static UPS system or the installation.

Automatic transfer switches

Automatic transfer switches (ATS for short) are self-acting power switching devices governed by a dedicated control logic that ensures the continuous delivery of electrical power from two or more power sources to a certain equipment or facility.

The control logic monitors constantly the voltage and frequency of the primary and the alternative power sources. Upon failure of the connected primary power source, the ATS will automatically transfer (switch) the equipment or facility to an alternative power source. The switching mechanism is the part of the ATS that is physically responsible for carrying the rated electrical current and shifting the equipment or facility connection from one power source to another. These switching mechanisms are typically contactors or circuit breakers.

While the switching happens automatically, the transfer can take up to 100 milliseconds or longer because of the operating characteristics of contactors and circuit breakers. This transfer period, particularly when dealing with big loads, can cause problems for sensitive and critical equipment and processes.

The typical transfer sequence of an automatic transfer switch can be described as:

• The primary power source supplying the equipment or facility (typically the electric utility) fails.
• The ATS shifts the load to the alternative power source when power from this source is stable and within prescribed voltage and frequency tolerances.
• The ATS returns the load from the alternative power source to the primary power source when desired voltage and frequency is restored.

A variety of ATS configurations are available typically using two or three power sources. These power sources can be renewable or non-renewable. The standard ATS configuration is formed by the electric utility supply as the primary power source and a standby generator as the alternative power source. This configuration is usually referred to as an emergency standby generator system.

Critical facilities with an emergency standby generator system will often include provisions for a third power source connection to serve as a redundant emergency backup in case the primary electric utility supply fails.

The next article of this series will discuss other components of diesel/gas 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.