HV. ISOLATORS OR DISCONNECT SWITCHES.

Group members:Gulfam Hussain 10El85.M. Ummair 10El84.Amaar Ali 1El119.M. Alaryaudah 10El82.

Introduction To Disconnectors. Definition:A disconnect switch is a switch which has the

capability to interrupt power to an electric circuit or group of the electric circuits.

General functions:The main function of the disconnect switch is to

isolate one part of the system, so that it can de-energize it for operational reasons or for maintenance and repair purposes.

Cont…One major application is in power distribution. Disconnect switch can be used to manage the

distribution network, to shift loads across the network, to interrupt power while maintenance occurs, and to turn off an area of the network if a safety threat arises.

In closed position disconnectors must be able to carry their rated load current as well as their rated short circuit current.

In open position they must provide isolation against the rated power frequency voltage and overvoltage due to switching or lighting impulse voltages.

Cont…Therefore the reliability of disconnectors is very

important for the personnel safety as well as for operational reasons.

The major difference between an isolator and a circuit breaker is that an isolator is an off-load device intended to be opened only after current has been interrupted by some other control device.

Construction Elements.A disconnecting switch consists of movable and stationary

contacts mounted on insulators. A movable contact is actuated by using an insulator to connect it with a conductor, the insulators that conform to the common insulator requirements of BIL coordination to withstand common voltage surges.

Electrical Contacts:

An electrical contact is defined as an electrical connection between two conducting elements.

The purpose of contacts in an electrical circuit is not only to provide current path between conducting elements or isolate current paths from each other. But also, when closed, to carry current and to interrupt current when open.

Cont…The connection between the two contact parts must be good.

Types of Electrical Contacts:

Static contacts: Static contacts are the ones with permanent electrical connections over their entire life span.

Switching contact or make- and- break contacts: Switching

contacts consist of pair of contacts that open and close on demand.

Sliding contacts: Sliding contacts consist of at least one fixed part and one moveable part that slide over the fixed part.

Cont…Disconnector contacts are of the make and break type,

and are often made of silver-plated copper contacts designed to carry load currents without excessive temperature rise.

Figure in next slide show a typical dissconnetor contacts to relive your confusion.

Construction.

Diagnostics for disconnectors.

To ensure that disconnectors fulfill the function of isolation and thus, guarantying the safety of the workers, and eliminating any malfunctioning or failure of the component, it is essential that they are properly maintained.

There are fundamentally three different maintenance strategies in practice.

Corrective maintenance: This is done after failure of the equipment.

Time-based maintenance or preventive maintenance: maintenance done at predefined time intervals to avoid failure.

Cont… Condition based maintenance also known as

predictive maintenance: Condition assessment is performed on equipment followed by appropriate action to avoid the consequence of the failure .

Types Of Disconnectors.• Disconnecting switches are classified according to

the type of installation (indoor or outdoor), the number of poles (for example, single-pole or three-pole), and the method of control (manual or remote).

Types cont…Four most common types encountered are briefly

reviewed.

Horizontal Centre - Break Type:

Figure # (1) illustrates a centre break disconnectors. This type is the most commonly used. The active parts are the two blades which make and break at the centre.

Picture show the centre-break type isolators in 132kv Qasimabad Grid station.

Cont….(fig.1)

Cont…Salient Features:

Self wiping contacts.Moving blade arms are made out off hard drawn

electrolytic copper/aluminum alloy.Suitable for all typical substation layouts. Motor/Manual operating mechanisms.

Cont… Double-Break Disconnector:

The double side break disconnector, see figure (2) is a variant of the centre break disconnector. The active parts consist of two jaw assembly, one at each end and a rotating blade. The centre rotates to open or close the disconnector. Contrary to the centre break type, this type provides two breaks in series, requires minimal inter-phase distance and allows higher loads on high voltage terminals.

Cont…fig(2)

Cont…Salient Features:

Turn & Twist type contacts for creating high pressure contacts for higher current rating.

Manual reduction gear or motor operating mechanism. 

Straight banging type contacts with Manual lever operating mechanism for low current rating.

Vertical or horizontal terminal take-off arrangement.

Cont…

Pantograph Type :

The active parts consist of fixed arrangements attached to a busbar, a blade and a hinge. The pantograph type is shown in figure (3).

This type requires the least inter phase spacing compared to the two types mentioned previously.

Figures are shown in next slide.

Cont… fig(3)

Cont…Salient Features:

The optimizing design of the pantograph arm ensures excellent operating performance.

Contact strips on moving/static contact is made of special silver coating copper(AgSnO2), this contact strips can endurance high temperature electric arc and switch 1600A current of bus transfer directly.

Using delrin bushing(one kind of special nylon) as the mechanic rotating bearing , without any risk of rust, no any lubricating requirement.

Delivery of the disconnector in pre-assembled and pre-adjusted main components, Easy installation and commissioning in site.

Running time is more than 40 years. Spare parts are available during the whole running time.

Cont… Vertical Break Disconnector:

The vertical break disconnector is of simple design of two post insulator and one operating rod insulator. Figure (3) shows the vertical break disconnector.

The arm moves vertical position by the operating rod insulator for disconnecting the line. This design is mostly used in Railways electrification / traction. 

The disconnector is suitable for mounting Horizontally or Vertically as per sub-station layout.

Cont…fig(3)

Main Advantages..Circuit breaker always trip the circuit but open

contacts of breaker cannot be visible physically from outside of the breaker and that is why it is recommended not to touch any electrical circuit just by switching off the circuit breaker. So for better safety there must be some arrangement so that one can see open condition of the section of the circuit before touching it. Isolator is a mechanical switch which isolates a part of circuit from system as when required.

Electrical isolators separate a part of the system from rest for safe maintenance works.