Disconnector switch
High-voltage disconnectors are used in electrical substations to allow isolation of apparatus such as circuit breakers, transformers, and transmission lines, for maintenance. The disconnector is usually not intended for normal control of the circuit, but only for safety isolation. Unlike load switches and circuit breakers, disconnectors lack a mechanism for suppression of electric arcs which occur when conductors carrying high currents are mechanically interrupted. Thus, they are off-load devices, with very low breaking capacity, intended to be opened only after the current has been interrupted by some other control device. Safety regulations of the utility must prevent any attempt to open the disconnector while it supplies a circuit. Standards in some countries for safety may require either local motor isolators or lockable handles (which can be padlocked).
IEC standard 62271-102 defines the functionality and features of a disconnector.
Disconnectors have provisions for a lockout-tagout so that inadvertent operation is not possible. In high-voltage or complex systems, these locks may be part of a trapped-key interlock system to ensure proper sequence of operation. In some designs, the disconnector has the additional ability to earth the isolated circuit thereby providing additional safety. Such an arrangement would apply to circuits that inter-connect power distribution systems where both ends of the circuit need to be isolated.
Types of disconnectors
Disconnectors can be classified into various types based upon their constructional features and mounting arrangement. The major types of disconnectors are:
Centre-break disconnectors
Double-break disconnectors
Pantograph disconnectors
Horizontal break knee disconnectors
Vertical break disconnectors
Coaxial disconnectors
These are selected based upon the sub-station layout, clearances available and space constraints.
Switch disconnector
A switch disconnector combines the properties of a disconnector and a load switch,[1] so it provides the safety isolation function while being able to make and break nominal currents.
Integrated disconnecting switch
edit
In a disconnecting circuit breaker the disconnectors are integrated in the breaking chamber, which eliminates the need for separate disconnectors. The intention of this combined device is to decrease maintenance and increase availability and reliability. The usage of this device instead of a disconnector is limited due to the fact that the open gap is not clearly visible and many doubts in term of safety arise during maintenance activities. Where it is adopted the earthing switch must be used and the performance must be increased by the typical value.
The open-air disconnecting switches typically need maintenance every five years (every two years under very polluted conditions), while circuit breakers have maintenance intervals of 15 years
Introduction
A disconnector (disconnecting switch) is a switching device used to provide safe isolation by de-energising parts of an electrical network e.g. an overhead line, transformer, or bus bar etc. By virtue of their isolating function, disconnectors are sometimes also called isolators. The primary function of a disconnector is to serve as a visual indicator of whether an electrical connection is open or closed; this lets network operators know if the circuit/equipment is either energised or de-energised. It is important operators can confirm the energised state of the circuit/equipment in order that they can safely perform inspections, maintenance, and repairs, without unnecessary risk
Disconnector Switch (Courtesy of SDCEM)
Moreover, earthing switches are employed in conjunction with disconnectors to earth the de-energized parts of the system, by providing an effective bonding/connection to ground.
Application of disconnecting switches to provide safe working isolation in power systems dates back to the early 1900s. At that time, safety codes required relevant disconnectors to be opened so that a ‘visible break’ was provided; the break was then blocked to prevent unauthorised closing. In addition to safe electrical isolation, protective grounding was also made mandatory; this requirement led to the development of earthing switches.
Substation Disconnectors (Elevation View)
Enjoying this article? Then be sure to check out our Engineering Video Courses! Each course has a quiz, handbook, and you will receive a certificate when you finish the course. Enjoy!
Disconnectors
Different types of air-break disconnectors are used in substations, these are the centre break, double break, vertical break, knee break and pantograph types. The type of disconnector switch used depends on the substation layout, design, and spatial constraints.
Parts and Construction
All disconnector types consist of the same basic components:
Current/live part – the part of the disconnector that carries electrical load.
Contact system – the point where the disconnector makes or breaks the electrical circuit.
Supporting and rotating insulators – insulators reduce leakage/creepage current and reduce the likelihood of flashover.
Operating drive and connecting rods – used to actuate (make/break) the disconnector (usually an electric motor is used).
Base frame – allows for the disconnector to be mounted or installed easily.
What is the difference between disconnectors and circuit breakers?
The feature that differentiates disconnectors from circuit breakers is that disconnectors do not possess the capability of current interruption. This means that a disconnecting switch cannot be opened when it is conducting current and cannot be closed when there is voltage across the terminals. The key working concept for a disconnector is the electrical distance it creates when opened, thereby providing a visible isolation gap.
Note: A disconnector can interrupt a small current when, after opening, a negligible voltage appears across the contacts.
