What types of electromechanical switching devices are used in MV VFD systems?
In this blog post we will talk about switches in variable frequency drive (VFD) systems. However, our subject are not the semiconductor switches but electromechanical switching devices (e.g. circuit breakers).
Classification of electromechanical switches
We will start with some classification to see what products exist and how their characteristics differ. Understanding the capabilities of each type of switch is crucial before deploying it in a drive system – especially when the switch is part of a safety concept.
According to voltage level
A) medium voltage
B) high voltage
C) very high voltage
D) ultra high voltage
You will rarely find C) and D) in drive applications but we list them here for completeness.
According to switching capability
A) Switches with full switching capability (circuit breakers)
B) Switches with reduced switching capability (load switches, switch disconnectors)
C) Switches with no switching capability (disconnectors, earthing switches)
Principle of actuator
A) Pneumatic
B) Hydraulic
C) Mechanical
Circuit breakers
Circuit breaker (CB) is a key element of a medium voltage drive system. It is common to have a CB located upstream on the primary side of the input isolation transformer.
Short characteristic:
- make, carry and break all types of loads
- break short-circuit currents
- can provide isolation function (safety)
- extinguishing medium
- oil filled CB
- gas filled CB
- vacuum type (VCB)
Load switch
Load switch or load break switch has following characteristic:
- make and break currents under normal condition
- short-circuit making capability
- lack of short-circuit breaking capability
Switch disconnectors
Functionality of switch disconnectors can be summarized as:
- combining properties of disconnector and load switch
- isolation function (safety feature)
- make and break normal currents
The execution of such a switch can be combined with a fuse:
- normal switch disconnector
- fuse switch disconnector
Disconnectors
Disconnector is a type of isolating switch. The characteristics are summarized below:
- isolating electrical equipment (repair, inspection, maintenance)
- safety purpose
- used for sequencing
- slow motion of contacts (up to several seconds)
- high number of operations between maintenance intervals
Sub-category of disconnectors is an earthing disconnector. It is a combination of a disconnector and earthing switch:
- connecting, disconnecting and earthing function
- no-load operation
Short-circuiters
These devices are special type of earthing switches:
- earthing switches capable to switch on a short circuit
- short-circuit making capability
What you get in premium article
We just scratched the surface here. To discover more practical information about electromechanical switches in VFD based systems, purchase our premium subscription or just the individual premium article.
Information you get:
♦ Where are the electromechanical switches located in the drive system?
A drive system may include circuit breakers, disconnectors, earthing switches and more. Where is each of them located? How does the device interface with other system components?
♦ What type of switch is required where?
What is the role of each switch?
We will cover standard drive systems as well as special setups (e.g. test stands, redundant drives in critical applications, drive with a bypass switch etc).
♦ How do the switches contribute to safety of the system?
Some switches are required for functionality, others provide protection and may be even safety relevant. What is important here?
♦ Does the drive system pose any specific requirements on the switches?
Apart from common characteristics, does power electronic based drive system pose any special requirements? If so, how are they addressed and what solutions are suitable?
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