power systems loss

What are the disadvantages in using y-y connection? The Y-Y transformer connection was poorly understood in the early days of power engineering and it received a very bad reputation when it was first used; in fact, this connection was avoided for a long time until its limitations were overcome by good engineering practice. Some of the inherent disadvantages of the Y-Y connection are discussed below:

What are the advantages in using y-y connection? Although care must be exercised when using the Y-Y connection , this connection has certain inherent and important advantages over other three-phase transformer connections. The following are some of the advantages of using y-y connection in an electrical system.

What are the principles behind the y-y configuration? The most obvious way of transforming voltages and currents in a threephase electrical system is to operate each phase as a separate singlephase system. This requires a four-wire system comprised of three phase wires plus a common neutral wire that is shared among the three phases. Each phase is transformed through a set of primary and secondary windings connected phase-to neutral. This is commonly referred to as the Y-Y connection, as illustrated in Figure 9.1. The left-hand part of Figure 9.1 shows the physical winding connections as three separate twowinding transformers.

SPEED CONTROL FOR ELECTRIC MOTOR TUTORIAL GUIDE How to control the speed of an electric motor? The relationship between rotational speed, supply frequency, number of poles and slip for induction motors is usually written: n = (2/p) x f x 60 x (1-s) Where: n = speed, r/min             f = frequency, hertz             p = number of poles             s = slip

How is resistor identified according to its color coding? A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's terminals to the intensity of current through the circuit is called resistance. This relation is represented by Ohm's law:

TYPES OF IMPEDANCE FAULT IN DISTRIBUTION SYSTEM Low Impedance and High Impedance fault in the distribution system Low Impedance Faults Low impedance faults or bolted faults can be either very high in current magnitude (10,000 amperes or above) or fairly low, e.g., 300 amperes at the end of a long feeder. Faults able to be detected by normal protective devices are all low impedance faults. These faults are such that the calculated value of fault current assuming a "bolted fault” and the actual are very similar. Most detectable faults, per study data, do indeed show that fault impedance is close to 0 ohms. This implies that the phase conductor either contacts the neutral wire or that the arc to the neutral conductor has a very low impedance. An EPRI study performed by the author over 10 years ago indicated that the maximum fault impedance for a detectable fault was 2 ohms or less. Figure 2, shown below, indicates that 2 ohms of fault impedance influences the level of fault cur

QUICK CURRENT TRANSFORMER FACTS Current Transformer Facts 1) Bushing CTs tend to be accurate more on high currents (due to large core and less saturation) than other types. 2) At low currents, BCT’s are less accurate due to their larger exciting currents. 3) Rarely, if ever, is it necessary to determine the phase-angle error. 4) Accuracy calculations need to be made only for three-phase and single-phase to ground faults. 5) CT burden decreases as secondary current increases, because of saturation in the magnetic circuits of relays and other devices. At high saturation, the impedance approaches the dc resistance. 6) It is usually sufficiently accurate to add series burden impedance arithmetically. 7) The reactance of a tapped coil varies as the square of the coil turns, and the resistance varies approximately as the turns. 8) Impedance varies as the square of the pickup current. 9) Burden impedance are always connected in wye. 10) "Ratio correction