Three Point DC Motor Starter

Three Point Starter for DC Motor have three point for electrical connection namely Line (L) to which positive / negative terminal of DC Voltage is connected, Field (F) which is connected to the field winding of DC Motor and Armature which is connected to the Armature of DC Motor. As it is having three points for … Read more

Insulation Resistance Test and Polarization Index

Megger is an electrical device which is used to measure the Insulation Resistance by performing Insulation Resistance Test (IR Test). Basically Megger injects High DC Voltage across the Insulator and ground due to which leakage current flows through the Insulator to the ground. By measuring this Leakage Current, Megger calculates the Insulation Resistance. Suppose the … Read more

Effect of Power Swing on Distance Protection

In last post Power Swing, we discussed Power Swing and behavior of machine on Power Swing. Now we will discuss the impact of Power Swing on Distance Relaying. We consider the two machine system connected by Transmission line of Impedance ZL as shown in figure below. For making the calculation easy, assume ER as reference and its angle … Read more

Power Swing

In last post Load Encroachment on Distance Protection, we observed that low value of Impedance Z arise due to voltage instability or transients associated with electromechanical oscillations of rotors of synchronous machines after a major disturbance like the faults. This can introduce nuisance tripping. Such tripping is known as tripping on Power Swings. In this post … Read more

Effect of Load Encroachment on Distance Protection

For discussing the effect of Load Encroachment on Distance Protection, we make a steady state model of Transmission Line as shown in figure below. In the figure Vi is Sending End voltage and Vj is Receiving End voltage. Power Pij+jQijis flowing in the direction shown in figure. We know that,   Pij – jQij= VixI … Read more

Distance Protection Philosophy

Distance protection is a non-unit system of protection offering considerable economic and technical advantages. Unlike phase and neutral over current protection, the key advantage of distance protection is that its fault coverage of the protected circuit is independent of source impedance variations. Let us take an example of this to understand how distance protection is … Read more

Reactance and MHO Relay

Reactance Relay: In Reactance Relay the operating torque is obtained from the current and the restraining torque by the current and voltage of the directional element. This implies that Reactance Relay is an over-current Relay with directional Restraint. The directional element is so designed to give Maximum Torque Angle (MTA/RCA) of 90 degree.  Therefore from … Read more

Impedance Relay

Distance Relays are of three types: ·         Impedance Relay ·         Reactance Relay ·         MHO Relay. Impedance Relay: As from Universal Relay Torque Equation, T = K1I2+K2V2+K3VICos(Ɵ-Ƭ)+K4 Where K1 = Constant             K2 = Constant             K3 = Constant             K4 = Spring Torque             V = Voltage fed to the Relay             I = Current … Read more