WeEngineers

Earthing switch is a mechanical switching device for earthing parts of a circuit, capable  of withstanding for a specified time short-circuits currents, but not required to carry normal  rated currents of the circuit.  Earth switch is also connected on the isolator to ground residual  current. The tubular arm of earthing switch is permanently connected with the earthed frame  by a flexible connection. In the open position, the contact arm is located along the base frame. Fig. Earthing switch at open position During closing operation, the arm rotates in the vertical plane and reach the fix contact. In the   end position, the earthing blade moves up along its axis and is inserted into earthing contact. Arcing horns are connected to contacts of earth switch. An arc can establish between the switch  contacts when contact arm and fix contact connects during a short circuit fault.The  horns are  designed to endure the arc rather than the contact surfaces of the earth swi

Isolator is an offload mechanical switch which use to disconnect an electrical circuit to ensure electrical safety during maintenance. Isolators are used in substations to isolate apparatus such as circuit breaker, transformer and transmission line at a time of maintenance. It is usually not intended for normal control of the circuit but only for safe isolation. There is mainly four types of isolators. Centre break isolator Double break isolator Pantograph isolator Knee type isolator Centre Break (CB) isolator designed with two rotating insulators mounted on an integrated base frame. Both insulators hold current carrying aluminium rods. In this, current carrying path is between the vertical cylinder rod and horizontal contact fingers. Rotation of insulators are opposite to each other. The current path opening to the side creates a bigger phase distance in parallel arrangements than with other types. Fig. Centre break isolator Double Break (DB) isolator has t

Re-ignition is re-establish of arc after arc extinction within one fourth of cycle from final current zero. It may occur because of small distance between two contacts. Due to that, contact gap breakdown and arc re-ignite. The re-ignition is not harmful as it does not give any overvoltage. At next current zero instance only, arc gets quench as the distance between two contact becomes sufficient to withstand Transient Recovery Voltage(TRV). Fig. Current waveform with re-ignition and re-strike Re-strike is re-establish of arc after one fourth of cycle from final current zero. It occurs due to capacitive current switching. In capacitive current switching, a single restike gives 4 times voltage rise. It leads to overvoltage which can damage dielectric medium. Re-striking is more severe than re-ignition.

Previously, Importance of protection against inrush current in stationary device is determined. In this post, different mitigation methods are addressed.  Pre-Magnetizing Transformer: Function:   To magnetize the transformer core at starting with regulated voltage source before connecting with main supply and prevent from saturation of core. Fig. Medium voltage side pre-magnetizing transformer Fig. Low voltage side pre-magnetizing transformer                                                                                           Application:   Marine transformer Limitation : No. of components are more, require external small size auxiliary transformer, cost is more.    Pre-Insertion Resistor/Rheostat with controlled switching: Function:   To mitigate the inrush current by providing external resistance at starting and switching control is used to bypass that rheostat after initial few cycles . Application :  MV transformer energization    

What is switchgear? In electrical system, switchgear is the combination of electrical switches, fuses and circuit breakers used to control, protect and isolate electrical equipment. Switching devices are used to  make   or   break  electric circuits      1.  Classification of switching devices according to operation: No-load switching Breaking at operating currents Breaking at short circuit currents      2. Classification of switching devices according to function: Circuit Breaker: A circuit breaker is an electrical switch which designed to protect an electric circui t  from damage caused by excess current from an overload or short circuit. Its basic function is to make, break and carry current under normal and abnormal conditions. Switches: Switches are used to switch current between two feeder or break current in circuit. Disconnectors or Isolators: Isolator is an offload mechanical device used to physically disconnect an electrical circuit

We all know that when transformer is energized at no-load condition, core will get saturated. Due to that, huge inrush current is demanded from the source voltage. This phenomenon is quite frequent in all the types of machines. In rotating machine, there are several methods like direct on line(DOL), soft starter etc used to reduce the inrush current as it effects the dynamic performance of the rotor during starting period. Transformer also has the inrush effect but it is not given any consideration yet. There are certain parameters which put light on the importance of studying this phenomenon in stationary machine.  Inrush current condition in transformer is transient phenomenon. So, it mainly understood by two parameters namely magnitude and duration. Magnitude of inrush current depends on : - Switching angle of source voltage - Rating of transformer  - Impedance of energizing system - location of windings (inner or outer) - Magnetic properties and residual flux