ELECTRICAL EASY

We are quite familiar with Synchronous Generator or often called Alternator. There are mainly two types of Synchronous Generator from rotor construction point of view. The two main types of synchronous machine are Cylindrical Rotor and Salient Pole. Saliency simply means projection outward. Therefore from the literal meaning of Saliency one can guess that Salient Pole machines must have poles projecting outward. In general, the Cylindrical Rotor type machines are confined to 2 and 4 pole turbine generators, while salient pole types are built with 4 poles upwards and include most classes of duty. Both classes of machine are similar in Stator construction point of view. Each has a stator carrying a three-phase winding distributed over its inner periphery. Within the stator bore is the rotor which is magnetised by a winding carrying DC current. The main difference between the Cylindrical Rotor and Salient Pole classes of machine lies in the rotor construction. The cylindrical roto...

Transformer Physical Protection refers to the Protections which used the physical quantities to protect the Transformer. Apart from electrical protections which uses the electrical quantities to judge a fault and based upon the judgment, the electrical protections of Transformer isolates the Transformer. On contrary, physical protections continuously measure the physical quantities like oil / winding temperature, gas content in the Transformer Oil etc to judge a fault condition and isolate the Transformer from the fault. There are many physical protections provided in a Transformer, they are as follows: 1)     Transformer Winding Temperature Trip , WTI 2)     Transformer Oil Temperature Trip, OTI 3)     Buchholz Trip 4)     Pressure Relief Device, PRD 5)     Magnetic Oil Level Gauge, MOLG All the above physical protections have already been discussed in earlier posts except Pressure Re...

In this post I will focus on the working principle of the Bipolar Junction Transistor assuming that you are already aware of the construction details of the BJT or simply Transistors. The basic operation of the transistor will be described using the pnp transistor. The operation of the npn transistor is exactly the same if the roles played by the electron and hole are interchanged. In the figure below, the pnp transistor has been drawn without the base-to-collector bias. This situation is similar to that of the forward-biased diode. The depletion region has been reduced in width due to the applied bias, resulting in a heavy flow of majority carriers from the p- to the n-type material. As obvious from the figure above, Emitter to Base junction is forward biased, therefore majority carriers i.e. holes from the Emitter side to the Base side will start flowing and hence a current will set up from Emitter to Base. Let us now remove the Emitter to Base bias of the pnp...

The clamping network is one that clamps a signal to a different DC level. The network contains a capacitor, a diode, and a resistive element, but it can also have an independent DC supply source to introduce an additional shift in voltage level. The magnitude of R and C to be used in the Clamper Circuit must be chosen such that the time constant Ƭ = RC is large enough to ensure that the voltage across the capacitor does not discharge significantly during the interval the diode is non-conducting. In our discussion, we will assume that for all practical purposes the capacitor will fully charge or discharge in five time constants. The network shown in figure below will clamp the input signal to the zero level for ideal diodes. The resistor R is the load resistor or a parallel combination of the load resistor and a resistor designed to provide the desired level of R as determined by Ƭ = RC. The above circuit can be can be well understood in two cases. Case1: When 0...

Programmable Scheme Logic or PSL is a kind of feature provided in Numerical Relay s to implement the protection scheme of a particular type. This feature of Numerical Relays makes it easier to implement many protection schemes in a single Numerical Relay for example, in Distance Relay we can configure Distance protection, over voltage protection, Over Current Protection, Earth Fault Protection etc. Now we will study about PSL. PSL is a logical block which is made from different but suitable DDB. Here DDB stand for Digital Data Bus. There are many DDBs offered in a Numerical Relay. Each DDB perform a unique function. Thus it is very important to have the knowledge of function of DDBs to implement a particular logic. Hope you got some idea of DDB but don’t worry I will go in detail with example to make it crystal clear. Lets us begin with an example. Let us assume that we have an Alstom Relay P442 and we want to implement a protection feature called Local Breaker Back-up (LB...

In this post we will discuss, why electric field inside a conductor is zero. This is very basic but important concept to understand. So we will start will zero and will move further to explain this. Let us assume that a conductor is kept in an external uniform electric field E . The direction of electric field E is shown in the figure. Before starting the discussion, there are two points to know. 1)       Negative charge move in the direction opposite to the direction of electric field. 2)       Positive charge move in the direction of electric field. As we know that, a conductor has a lot of mobile or free electrons, therefore when keep the conductor in an external electric field, electrons will experience a force in the direction opposite to the direction of electric field E and will start accumulating at surface A of the conductor. As electrons are moving opposite to the direction of Electric Field E , pos...

I would suggest to read Transformer Inrush Current before reading this article for better understanding. Any event on the power system that causes a significant increase in the magnetizing voltage of the transformer core results in magnetizing inrush current flowing into the transformer. The three most common events are as follows: Energization of the Transformer . This is the typical event where magnetizing inrush currents are a concern. The excitation voltage on one winding is increased from 0 to full voltage. The transformer core typically saturates, with the amount of saturation determined by transformer design, system impedance, the remnant flux in the core, and the point on the voltage wave when the transformer is energized. The current needed to supply this flux may be as much as 40 times the full load rating of the transformer, with typical value for power transformers for 2 to 6 times the full load rating. Figure below shows the waveform during energization of a trans...

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...