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 Power Swing

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 Effect of Load Encroachment on Distance Protection

Outfeed and Infeed Effect

We will discuss here the Outfeed and Infeed Effect and its implication on Distance protection. Consider the operation of distance relay R1 for a fault F close to remote bus on line BC as shown in the figure below. Therefore fault current will be feed by Source G1, G2 and G3. Therefore, IBF= IAB + … Read more Outfeed and Infeed Effect

Why Zone-1 Setting 80% and Zone-2 150% in Distance Protection?

As discussed in my earlier post on Distance Protection Philosophy, Zone-1 setting is kept at 80% of the total impedance of the Protected Line. So question will come in your smart brain that why 80% even though Zone-1 is meant for primary protection? Zone-1 is meant for protection of the primary line. Typically, it is … Read more Why Zone-1 Setting 80% and Zone-2 150% in Distance Protection?

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 Distance Protection Philosophy

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 Reactance and MHO Relay

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 Impedance Relay

Directional Earth Fault Protection

Generally earth faults are Single Line to Ground (SLG) and Line-Line to Ground (LLG) faults. Earth faults are characterized by the presence of Zero Sequence Curren I0. Since, except for unbalance, normal system operation is not having Zero Sequence Current I0, much more sensitive pick-up is possible for earth fault by using zero sequence current … Read more Directional Earth Fault Protection

How Directional Feature Incorporated in a Relay?

Why Directional Relay? For understanding the need of Directional Relay we consider a power system as shown in figure below. For fault at F1, Relay R2 is supposed to pick-up and operate the circuit breaker. Similarly for fault at F2, Relay R1 shall operate to isolate the faulty section from the healthy section. Now suppose … Read more How Directional Feature Incorporated in a Relay?

Over Current Relay and Its Characteristics

Protection schemes can be divided into two major groups: a) Unit schemes, and b) Non-unit schemes. a) Unit Scheme: Unit type schemes protect a specific area of the system i.e. a transformer, transmission line, generator or bus bar.The unit protection schemes are based on Kirchhoff’s Current Law – the sum of the currents entering an … Read more Over Current Relay and Its Characteristics