# Overcurrent protection relay pdf

For simplicity in explaining the key ideas, we consider three phase bolted faults. We can call them as overcurrents. This protection scheme finds usage in radial distribution overcurrent protection relay pdf with a single source.

It is quite simple to implement. Figure 1 shows a radial distribution system with a single source. The fault current is fed from only one end of the feeder. IF1 as well as IF2 pass through CT of R1. This is because no component of IF1 passes through CT of R2. Relaying decision is based solely on the magnitude of fault current. Such a protection scheme is said to be non-directional.

Figure 2 shows such a case for a radial system with source at both ends. Consequently, fault is fed from both the ends of the feeder. To interrupt the fault current, relays at both ends of the feeder are required. In this case, from the magnitude of the current seen by the relay R2, it is not possible to distinguish whether the fault is in the section AB or BC.

To obtain selectivity, a directional overcurrent relay is required. It uses both magnitude of current and phase angle information for decision making. It is commonly used in subtransmission networks where ring mains are used. 0, and the apparent impedance seen by the relay is infinite. ZL is the load impedance and Zline is the line impedance. Zline as shown in figure 3 below.

Because, impedance is a complex number, the distance protection is inherently directional. The first quadrant is the forward direction i. However, if only magnitude information is used, non-directional impedance relay results. The impedance relay trips if the magnitude of the impedance is within the circular region. Since, the circle spans all the quadrants, it leads to non-directional protection scheme. In contrast, the mho relay which covers primarily the first quadrant is directional in nature.

As shown in the Figure 5, θ is the angle of transmission line. Thus by comparing the two currents either in magnitude or in phase or both we can determine a fault and issue a trip decision if the difference exceeds a predetermined set value. Then under no fault condition, phasor sum of currents entering the device is zero i. Thus, we can say that differential current under no fault condition is zero.

This principle of checking the differential current is known as a differential protection scheme. In case of transmission line, implementation of differential protection requires a communication channel to transmit current values to the other end. It can be used for short feeders and a specific implementation is known as pilot wire protection. Differential protection tends to be extremely accurate. Its zone is clearly demarcated by the CTs which provide the boundary. Consider an ideal transformer with the CT connections, as shown in Figure 8.

To illustrate the principle let us consider that current rating of primary winding is 100A and secondary winding is 1000A. CT currents will match in magnitudes. No current will flow through the branch having overcurrent current relay because it will result in violation of KCL. Now if an internal fault occurs within the device like interturn short etc. Under this condition, the CT secondary currents of primary and secondary side CTs will not match. The resulting differential current will flow through overcurrent relay.

In practice, the transformer is not ideal. Thus, a differential current always flows through the overcurrent relay. Therefore overcurrent relay pick up is adjusted above the no load current value. Consequently, minute faults below no load current value cannot be detected. If the fault is external to the bus, it can be seen that algebraic sum of the currents entering the bus is zero.

Thus, differential protection can be used to protect a bus. 5, which I never haired about, would you please advice me when we have CT ratio , what it means differential scheme ratio ? Thank you so much for this great articles. Your posts are always found very useful for understanding. About this post, I would like to ask you three things, exclusively for the distance protection. At the end of this principle it shown an equation, what is or what does mean Zset, Vr and Ir? Second question is, what is or what does mean Zn?