ELECTRICAL EASY

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...

Most of us are aware of Electromagnetic Relays and Static Relays but most us may not be well acquainted with Numerical Relay. If I define a Numerical Relay, honestly speaking it will seem to be quite tough but in reality they are very user friendly and easy to implement different types of protection scheme. However I am going to define a Numerical Relay. Numerical Relays are device in which measured electrical quantities are sequentially sampled and then converted into numerical data which are mathematically or logically processed to take decision for issuing trip command. Numerical Relays are basically Digital Relays for which manufacturers have developed specified hardware which can be used in conjunction with suitable Software o meet different protection needs.  A Digital Relay comprises both Hardware and Software. The Hardware part is briefly described below. CPU: CPU stand for Central Processing Unit which is responsible for the processing of protection algorithms and digita...

Relay is a switch which senses fault in a system and once fault is sensed by the Relay, it issues trip command to the Circuit Breaker, CB to isolate the faulty section of the network from the healthy section. The Relay detects the abnormal condition by continuously monitoring electrical quantities which are different for healthy and faulty condition. The electrical quantities which may change during fault condition are voltage, current, frequency and phase angle. If one or more of the above electrical quantities change, that signals the presence, type and location of the fault to the Relay. After detecting the fault condition, Relay pick-up, its contact will change from NO to NC or vice versa. So we can wire up a particular kind of Relay contact to Breaker tripping circuit. So whenever, the Relay picks up, the tripping of Breaker will take place. You may like to Read, Why CT Secondary Shall Never be Kept Open? A simplified Relay circuit is shown in figure below. Figure below shows one...

In earlier posts we discussed, Concept of Symmetrical Components and  Calculation of Symmetrical Components . It is recommended to go through these topics before analyzing Single Line to Ground Fault. You must read,  Concept of Symmetrical Components  and  Calculation of Symmetrical Components Let us consider a three phase system with earthed neutral as shown below. Let us assume that a ground fault takes place in A phase (In many industries and numerical relays, normally the phases are said as A, B and C instead of R, Y and B, though they represent the same thing i.e. A phase means R phase, B means Y phase and C means B phase). E a , E b and E c are the Generator terminal voltage per phase. Bold letters here represent vector form.  Because of ground fault in A phase, the voltage at the point of fault will become zero and current through the other phases i.e. B and C phases will become zero. Therefore we can write as V a = 0 I b = 0 I c = 0 Therefore, from...

Electrical Faults can be classified into two categories: Shunt Faults and Series Faults. Shunt faults include power conductor or conductors to ground or short circuit between the conductors. Series type of fault is basically unbalance in system. Suppose we have used Fuse / Breaker to protect the circuit. If one or two phases open while the third phase remain in circuit, such kind of fault is called Series Fault. Notice that Series Fault may also occur in case of one or two Broken Conductor . Here broken conductor is like breaking of jumper on the tower of transmission line which is not touching the grounded tower body. Shunt faults are characterized by increase in current and decrease in voltage and frequency whereas Series faults are characterized by increase in voltage and frequency and decrease in current in the faulted phase. Shunt faults are classified as: 1)     Line-to-Ground Fault 2)     Line-to-Line Fault 3)     Double Line-to-Groun...

We are now aware of the concept of Sequence components of current / voltage. If you have miss this concept, please read  Concept of Symmetrical Components. Now we are at a stage to calculate the zero, positive and negative sequence components of current / voltage. As already discussed any three phase unbalanced voltage / current can be resolved into three set of balanced vectors. Thus we will use this concept to calculate the positive, negative and zero sequence components of voltages. Mind that the same philosophy is applicable for current also. Before going into the calculation part, let us introduce ourselves with an operator λ. λ is an operator which when multiplied to any vector quantity, rotates the vector by an angle of 120° in anticlock wise direction without changing the magnitude of the vector. This means that λ must have a magnitude unity. From this definition we can write λ as below. λ = e i2π/3    = Cos(2π/3) + jSin(2π/3)    = -0.5 + j0.866 Why no...