ENGINEERING ARTICLES

Power flow studies are performed to determine voltages, active and reactive power etc. at various points in the network for different operating conditions subject to the constraints on generator capacities and specified net interchange between operating systems and several other restraints. Power flow or load flow solution is essential for continuous evaluation of the performance of the power systems so that suitable control measures can be taken in case of necessity. In practice it will be required to carry out numerous power flow solutions under a variety of conditions. NECESSITY FOR POWER FLOW STUDIES Power flow studies are undertaken for various reasons, some of which are the following: I. The line flows 2. The bus voltages and system voltage profile 3. The effect of change in configuration and incorporating new circuits on system loading 4. The effect of temporary loss of transmission capacity and (or) generation on system loading and accompanied effects. 5. The effec...

The magnetic rotor field is generated by a field winding F on the rotor which is fed with an adjustable direct current. In addition, the rotor has a short circuited damper winding D at the surface. This winding serves to dampen electrical and mechanical oscillations and to shield the field winding from inverse rotating fields in case of asymmetries or harmonics in the stator currents. (In rotors without an explicitly realized damper winding, eddy currents in the rotor iron can have a similar effect.) Depending on the application of the generator, two different types of rotors are used that are shown in Figure. Figure:  Cross-sections through different rotor types. ROUND ROTOR Round rotors are used with high-speed turbines such as steam or gas turbines. For this reason, generators with round rotors are also called turbo generators. They can have ratings as high as 1800 MVA per unit. Due to the large centrifugal forces, the rotor consists of a long, narrow, solid steel c...

Graph theory has many applications in several fields such as engineering, physical, social and biological sciences, linguistics etc. Any physical situation that involves discrete objects with interrelationships can be represented by a graph. In Electrical Engineering Graph Theory is used to predict the behavior of the network in analysis. However, for smaller networks node or mesh analysis is more convenient than the use of graph theory. It may be mentioned that Kirchhoff was the first to develop theory of trees for applications to electrical network. The advent of high speed digital computers has made it possible to use graph theory advantageously for larger network analysis. ELEMENT OF A GRAPH: Each network element is replaced by a line segment or an arc while constructing a graph for a network. Each line segment or arc is called an element. Each Potential source is replaced by a short circuit. Each current source is replaced by an open circuit. NODE OR VERTEX: The te...

The dc generators and dc motors have the same general construction. In fact, when the machine is being assembled, the workmen usually do not know whether it is a dc generator or motor. Any dc generator can be run as a dc motor and vice-versa. All dc machines have five principal components viz (I) field system (II) armature core (III) armature winding (IV) commutator (V) brushes. Figure 1. (I) FIELD SYSTEM The function of the field system is to produce uniform magnetic field within which the armature rotates. It consists of a number of salient poles (of course, even number) bolted to the inside of circular frame (generally called yoke). The yoke is usually made of solid cast steel whereas the pole pieces are composed of stacked laminations. Field coils are mounted on the poles and carry the dc exciting current. The field coils are connected in such a way that adjacent poles have opposite polarity. The mmf developed by the field coils produces a magnetic flux that passes through the...

A system is single or polyphase depending upon whether the sources feeding it are single or polyphase. A source is single or polyphase according to whether there are one or several driving voltages associated with it. For example, a three-phase source is a source containing three alternating driving voltages that are assumed to reach a maximum in phase order, A, B, C. Each phase driving voltage is associated with a phase branch of the system network as shown in Figure (a). If a polyphase system has balanced voltages, that is, equal in magnitude and reaching a maximum at equally displaced time intervals, and the phase branch impedances are identical, it is called a 'balanced' system. It will become 'unbalanced' if any of the above conditions are not satisfied. Calculations using a balanced polyphase system are simplified, as it is only necessary to solve for a single phase, the solution for the remaining phases being obtained by symmetry. The power system is nor...

In the event of failure or non-availability of the primary protection some other means of ensuring that the fault is isolated must be provided. These secondary systems are referred to as ‘back-up protection’. Back-up protection may be considered as either being ‘local’ or ‘remote’. Local back-up protection is achieved by protection which detects an un-cleared primary system fault at its own location and which then trips its own circuit breakers, e.g. time graded overcurrent relays. Remote back-up protection is provided by protection that detects an un-cleared primary system fault at a remote location and then issues a local trip command, e.g. the second or third zones of a distance relay. In both cases the main and back-up protection systems detect a fault simultaneously, operation of the back-up protection being delayed to ensure that the primary protection clears the fault if possible. Normally being unit protection, operation of the primary protection will be fast and will result ...

A three-phase transformer bank can be easily created by using three single-phase transformers. The two sides of these three transformers can be either connected in a wye or a delta configuration, thus allowing four possible types of connections. These are: • WYE WYE: With the wye-wye (Y-Y) connection, the secondary side is in phase with the primary circuit, and the ratio of primary to secondary voltage is the same as the ratio of turns in each of the phases. A possible connection is shown in Figure 1. Power distribution circuits supplied from a wye-wye bank often create series disturbances in communication circuits (e.g., telephone interference) in their immediate vicinity. One of the advantages of this connection is that when a system is changed from a delta to a four-wire wye to increase system capacity, existing transformers can be used. Figure 1 Y-Y transformer with 0° phase shift between the primary and the secondary sides.  • WYE-DELTA: In the Y-Δ connec...