ELECTRICAL EASY

Rate of Change of Frequency (ROCOF or df/dt) relay is used for fast load shedding, to speed up operation time in over- and under-frequency situations and to detect loss of grid. For better understanding of role and operation of df/dt relay, let us first study the variation of frequency with load for a Grid. Graphical relationship between power and frequency of a Grid is shown in figure below. We can have two things to be noted from the above graph: a)     If the power available in the Grid increases i.e. in other words generation is more than the load, frequency will go up. b)     If the generation is less than load i.e. power deficient in the Grid then frequency will decreases. Large power grids (here large power grid means having large installed capacity in MW) are characterized by a very high stiffness constant which means that a large perturbation (load generation mismatch) is required to cause the grid frequency to change by 1...

Frequency all over a synchronous Power Grid is the same in steady state. Mind the word Synchronous Power Grid, as there may be two different grids operating at slight different frequencies. It is very important to maintaining a constant frequency or to frequency to vary over a very narrow band in a power system operation. Frequency in a power system is intimately related to the electrical speed of synchronous generators. As we know that the acceleration of a Generator is solely governed by the difference between mechanical and electrical torques, therefore to maintain a constant speed, mechanical input and electrical output power need to be continually matched. Electrical load can vary randomly, but fortunately the total load versus time roughly follows a trend. Frequency of Grid is depends upon many factor like load variation, prime move control of Generator etc. Frequency needs to be maintained near 50 Hz (For India) for the following reasons: 1) Steam turbine blades ...

This test is necessary for all indoor and outdoor breakers. The test is carried out as follows: Standard impulse wave of specified amplitude is applied five times in succession. If flash-over or puncture of insulators does not occur, the circuit-breaker is considered to have passed the test. If puncture occurs or if on two or more applied test wave flash-over occurs, the circuit breaker is considered to have failed the test. During the test some waves should be applied with reversal of polarity. The impulse voltage wave is generated in an Impulse Voltage Generator. During the test one terminal of the impulse generator is connected to the terminal of the circuit breaker pole. The other terminal is connected to the earth and the frame of the circuit breaker. Standard lightning Impulse is a full impulse having a front time 1.2 μsec and time to half value of 50 μsec. It is described as 1.2/50 impulse as shown in figure below. Standard switching impulse wave is ...

Endurance Test of Circuit Breaker is conducted to check the healthiness of its mechanical parts i.e. operating mechanism. In this test, Circuit Breaker is operated several times and checked for any damage of its mechanical parts / contacts. The breaker should be in a position to open and close satisfactorily.  This test is also called Mechanical Test. In mechanical tests, the circuit breaker is opened and closed several times (1000). Some operations (about 50) are conducted by energizing the relays, remaining are by closing the trip circuit by other means. Mechanical tests on high voltage AC circuit breakers are conducted without current and voltage in the main circuit. Out of the 1000 operations, about 100 operations are made by connecting the main circuit (contacts) in series with trip circuit.  No adjustment or replacement of parts is permitted during the mechanical tests. However, lubrication is permitted as per manufacturer’s instructions. After the test...

Rated Normal Current: The rated normal current of a circuit-breaker is the r.m.s. value of the current which the circuit breaker can carry continuously and with temperature rise of the various parts within specified limits. The design of contacts and other current carrying parts in the interrupter of the circuit breaker are generally based on the limits of temperature rise. For a given cross section of the conductor and a certain value of current, the temperature rise depends upon the conductivity of the material. Hence, high conductivity material is preferred for current carrying parts. The cross-section of the conductors should be increased for materials with lower conductivity. The use of magnetic materials in close circuits should be avoided to prevent heating due to hysteresis loss and eddy currents. The rated current of a circuit-breaker is verified by conducting temperature-rise tests. Rated Short Circuit Breaking Current: The rated short-circuit-breakin...

In Switchyard different Bus Bar arrangements are used for evacuation of power generated but the two most used schemes are One and Half Breaker Bus System and Double Bus Bar arrangement. In high voltage Switchyard like in 400 kV Switchyard One and Half Breaker Bus System is used due to many advantages of this scheme. The advantages of this scheme will be dealt latter in this post. Bus Bar arrangement is nothing but a combination of Bus and Circuit Breaker. Normally in Switchyard, Bus are made of hollow tubular aluminum called IPS Tubes (here IPS stands for Iron Pipe Size, a code for selection of tubes). Corona and electrostatic field performance is better for tube bus, therefore Aluminum Tubes are used for Buses. Now coming to One and Half Breaker Bus System, carefully observe the figure below. In the figure above, CB stands for Circuit Breaker, LA for Lightening Arrestor, ES for Earth Switch and DS for Disconnect Switch also called Isolator. You may like to Read, Difference between Is...

What is Circuit Breaker? Circuit Breaker is switch capable of making or breaking the circuit under no-load as well as on-load condition. It can make or break circuit either manually or by remote control. A Circuit Breaker in conjunction with Relay can break the circuit under fault condition. You may also like to read,  Basic Principle of Relay Operation Operating Principle of Circuit Breaker: A Circuit Breaker CB consists of two contacts which are called electrodes, one of which remain fixed, called fixed contact and another moving contact. Under normal operating condition, this contact will remain closed to supply power but as soon as fault is sensed by the Relay , trip coil of Circuit Breaker energizes and the moving contact of CB is pulled apart by some mechanism to open the CB. When contacts of CB are separated under fault condition, an arc is stuck between the fixed and moving contacts. The current is thus able to continue till the arc persists. The production of arc not only...

Restricted Earth Fault (REF) protection is basically a Differential Protection. The only difference in between the Differential Protection and REF Protection is that, latter protection is more sensitive as compared to the former protection scheme. In earlier posts we have already discussed Differential Protection of Transformer and various characteristics of Differential Protection. In this post we will focus on Restricted earth Fault protection. Also Read, Transformer Differential Protection Percentage Differential Protection - Slope in Differential Protection Harmonic Restraining in Differential Protection For the sake of understanding REF Protection, we take a Transformer of configuration DYn i.e. HV side of Transformer is Delta connected while the LV side is Start connected and neutral is grounded solidly. As shown in figure above, there are a total of four Current Transformers (CTs), three CTs connected in each phase i.e. R, Y and B and one CT connected in neutral. The secondary o...

We have already discussed on the Potential Distribution across the various discs in a Suspension type Insulator .  When Suspension type insulator is connected to hold a power conductor carrying electrical power at high voltage, a charging current will flow though the series connected Self Capacitors. As the charging current through each of the Capacitor (Porcelain disc) is same, therefore the potential distribution across each Porcelain disc will be same i.e. each Porcelain disc will have equal voltage stress.   Also Read, String Efficiency of Suspension Insulator How does a Capacitor Stores Charge Charging Current in Capacitor But actually, in addition to self capacitance of porcelain disc there also exists Capacitance in between the metallic link of suspension insulator and grounded tower body. This capacitance is known as Shunt Capacitance. Now due to this Shunt Capacitance, the charging current through each porcelain disc will no longer be same rather it will decrease as w...

A Suspension Insulator is basically a string of number of porcelain disc connected in series though metallic link. It shall be noted that the number of discs in Suspension Insulator can be increased or decreased by adding extra discs or removing a disc. Figure-a below shows a typical Suspension Insulator . As discussed in above paragraph, porcelain disc remain in between the metallic links, therefore each disc acts as a Capacitor . Therefore, if we draw the equivalent circuit for a suspension insulator, it will be as shown in figure-b. Thus when such type of insulator is connected to hold a power conductor carrying electrical power at high voltage, a charging current will flow though the series connected Capacitors . As the charging current through each of the Capacitor (Porcelain disc) is same, therefore the potential distribution across each Porcelain disc will be same i.e. each Porcelain disc will have equal voltage stress. Let us assume that the voltage at which power conductor c...