ELECTRICAL EASY

Dead Machine Protection in a Generator is provided to ensure that Generator is not energized accidentally in standstill condition or when the Generator is on Turning Gear. Accidental energization of Generator when the machine is not running can cause severe damage to the machine. ( How? You will be able to answer after you go through the post.) Suppose the Breaker is closed when the Generator is at standstill condition, the Generator will behave as an Induction Motor with surface of rotor core and rotor winding slot wedges acting as rotor current carrying conductors. This abnormal current in the rotor can cause arcing between the components like slot wedge to core leading to rapid overheating and damage.  Generally, the time to damage the generator stator from the high in-rush currents received during energizing at standstill is in the order of a few seconds. The bearing, however, may be damaged more quickly due to the lack of oil pressure. Thus it is very important to provi...

Plug Setting Multiplier and Time Setting Multiplier are used only for Electromechanical Relays. These terms or parameters are not so used in Numerical Relays but they are conceptually used and incorporated in Numerical Relays too but the way of their implementation is quite different than that of Electromechanical Relays. In this post we will focus on the concept and implementation of Plug Setting Multiplier and Time Setting Multiplier for Electromechanical Relays. As we know that an Electromechanical Relay has a coil which when energized, operates the Relay to have contact changeover. But there shall be some minimum current which when flows through the Relay coil, produces enough magnetic force to pull the lever to make contact change over. Isn’t it? Yes, if you ever get a chance to see electromechanical relay, you will observe that there is a flapper kind of thing which is attached with the lever. The lever in turn is attached with contacts. Thus when a specified current flows...

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...

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...

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...

Before going into the calculation part of Stabilizing Resistor, I will first explain the purpose of Stabilizing Resistor in High Impedance Differential Protection. Stabilizing Resistor in High Impedance Differential Protection is used to prevent the operation of Relay in case of through fault. Through fault is a fault outside the zone of protection. Lets us assume that High Impedance Differential protection is used to protect a Bus bar as shown in figure. It shall be noted here that, in High Impedance Differential Protection, all the CTs are connected in parallel and then the four wires i.e. R, Y, B and N are connected with the Relay as shown in figure above. If there is any fault in the bus, the according to Kirchhoff’s current law, the summation of current will not be zero and a net current will flow through the Relay coil to operate it. In normal condition, the summation of current will be zero and hence no current will flow through the Relay coil and hence the Re...

The concept of Subtransient, Transient and Steady State arises in case of fault in an Alternator. Let us assume a sudden short circuit in three phase of alternator. The fault current will flow in all the three phases of alternator and its waveform will be as shown in figure below. When the alternator is short-circuited, the currents in all the three-phases rise rapidly to a high value of about 10 to 18 times of full load current, during the first quarter cycle. The flux crossing the air gap is large during a first couple of cycles. The reactance during these first two or three cycle is least and the short circuit current is high. This reactance is called subtransient reactance and is denoted by X". The first few cycles come under sub-transient state. After a first few cycles, the decrement in the r.m.s. value of short circuit current is less rapid than the decrements during the first few cycles. This state is called the Transient  State and the reactance in th...

We know that 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. Relay coil when picked up will change its contact status. Let us say Relay Normally Open (NO) contact is wired to the Breaker T...