Solid-state switches are entering high voltage and high current applications that were previously the exclusive domain of electromechanical switches. The additional design freedom offered by having two types of switches available with different performance characteristics is valuable to design engineers; this article will discuss some of the advantages and disadvantages of mechanical vs solid-state switches for applications at or above line voltages (>100 V).
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Electromechanical switches are composed of an electromagnet that actuates a mechanical switch opening and closing electrical contacts. These switches contain moving mechanical parts that are subject to wear and contact tips that are subject to damage from the opening and closing arcs. Solid-state switches operate by changing a semiconductor between from an &#;off&#; state (insulating) and an &#;on&#; state (conducting) depending on the applied control voltage. This process is completely reversible and, under the right conditions, these switches can operate nearly forever.
Both electromechanical and solid-state switches have some operational consequences arising from their design. Electromechanical switches take time to open and close, frequently on the order of tens of milliseconds, and tend to work more easily with AC power, as it is easier to extinguish an AC arc compared to a DC arc[1]. Contrastingly, solid-state switches change state faster and work more easily with DC power due to inherent directionality of semiconductor junctions. When on, the solid-state switch will always have a voltage drop across the switch that is independent of the voltage applied and when off solid-state switches will leak a small amount of current, e.g. the circuit will never completely de-energize. Additionally, solid-state switches tend to have a lower breakdown voltage and higher internal resistance versus the contact resistance within electromechanical switch. The thermal rise associated with a solid-state switch is higher, necessitating a heatsink and results in faster derating with temperature when compared to an electromechanical switch. An example of the derating curve of a solid-state switch can be found in Figure 1. A general comparison between similar commercially available solid-state and electromechanical switches can be found below in Table 1. These switches were selected from in stock items sold by major online electronics retailers, at the time of publication the solid-state switch was more expensive vs the electromechanical switch.
Figure 1: Thermal derate curves for load capacity of the solid-state switch described in Table 1 at three heatsink sizes².
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Table 1: Comparison of performance values for example solid-state and mechanical switches.
One of the largest differences between electromechanical and solid-state switches is the causes and consequences of switch failure. Electromechanical switches usually fail due to degradation of the contact points which results in an unacceptably high resistance up to the point of the circuit being permanently open. This is usually a gradual and predictable failure primarily driven by the number of switching cycles completed[1]. Solid-state switches on the other hand have no moving parts and have failure modes that are independent of switching cycles. Their main failure modes have to do with exceeding their rated voltage, current, or temperature resulting in a material breakdown causing the switch to fail closed and energizing the circuit[4]. This tendency for solid-state switches to fail closed is often undesirable, as the unexpected energizing of a circuit can present safety and performance risks. Electromechanical and solid-state switches have distinct performance differences making them suitable for different applications. An understanding of the fundamental differences of the two switches and engineering judgement is needed to understand what type or combination of switches is needed to provide a robust, safe and cost effective solution.
References:
[1] Slade, Paul G., ed. Electrical contacts: principles and applications (2nd ed.). CRC press, .
[2] crydom, &#;Power Plus DC Series&#;. http://www.crydom.com/en/products/catalog/power-plus-dc-series-200-dc-panel-mount.pdf
[3] OMRON, &#;G6RL PCB Power Relay&#;, https://omronfs.omron.com/en_US/ecb/products/pdf/en-g6rl.pdf
[4] &#;Safety Precautions for Solid-State Relays&#; OMRON, 12 Dec , www.ia.omron.com/product/cautions/18/safety_precautions.html
There are many disconnect switch designs (>4) and mounting options for HV for transmission systems application. Disconnect switches are manufactured to meet the specifications of different markets. The following are quick descriptions: 1) The vertical break disconnect switch is one of the most popular switch in both ANSI & IEC market places. There are more data available for this type of switch than any other type including performance in shake table test for heavy seismic zones and sub-zero temperatures with Icebreaking capability used for rough extreme weather conditions. This switch can be equipped with an arcing horn, quick break whip . Although none of the HV disconnect switches are designed for load break, there are optional load break devices available for T. Line and transformer switching operations without circuit breakers. 2) Centre side break type switch requires at least the same clearance and is less strong than the vertical break type. 3) Double break disconnect switch requires less phase-to-phase clearances than any other switch type. It can be specified with arcing horns. 4) Pantograph disconnect switch is popular in the IEC marketplace. However, seldom or not use in the ANSI/IEEE marketplace. 5) Vertical break with folding arm switch are popular in Canada. There are seldom used in the USA and ANSI marketplaces.[sub]Disconnect switches manufactured for the IEC or other foreign standards may not conform with more stringent requirements of the ANSI/IEEE Standards.[/sub]
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