Electrical Energy TransmissionConsider the situation shown in Fig.1.The rate of electrical energy flow(power)from network A to network B isfig1 Power transmission between two networksLowercase letters are used to indicate instantaneous valuesm, that is, that p, v, and I may vary with time. High power levels require high voltage and current values. For a given value of current, higher power flows may be obtained by increasing the voltage, and vice versa. Unfortunately, the existing technology sets practical upper limits onm allowable currents and voltages.What are the limiting factors for current? We fabricate power conductors using materials with high conductivity, appropriate mechanical characteristics, and that are economical: aluminum is the most common choice, with copper used for some applications. The current-carrying capacity of a conductor is related to its maximum allowable current density and its cross=sectional area:IJAmaxmaxThe maximum current densityJ is determined by the maximum conductormaxtemperature that will not damage the conductor or its insulatinon system.What are the limiting factors on voltage? The vundamental consideration is to provide electrical isolation(or insulation)between adjacent parts that can conduct current-that is, to confine current to the paths through which it waw intended to flow. When the voltage exceeds the breakdown strength for a given insulation system, undesirable conduction paths will be created and the system will be created and the system whll be either temporarily or permanently disabled. Fluid insulation tends to be “self-healing”(the system will recover from a breakdown if it is de-energized for a short time and then re-energized), whereas solid insulation is permanently damagedby a breakdown.The meaning lf “ground”is important; we quote from the IEEE Standard Dictionary of Electrical and Electronic Terms:ground (earth)(electric system). A conducting connection, whether intentional oraccidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth. Note:It is used for establishing and maintaining the potential of the earth (or of theconducting body) or approximately that potential, on conductors connected to it ,and for conducting ground current to and from the earth(or the conducting body).We understand this to mean that at a given location in the power system,accessible parts of power apparatus and earth constitute an equipotential surface when perfectly grounded. Insulation of conductors from ground is a basic problem.Let us consider some different schemes for implementing the transmission lineindicated in Figure 1. For a fair comparison we select constrainst that all schemes must satisfy we allow any number of conductors to be used ,as long as each schemeuses the same amount of conducting material, Given that networks A and B areseparated by a fixed physical length this means that in viewing the limes in cross section we must observe the same cross-sectional conducting area (A) for all schemes.Also, we argue that no conductor shall carry current greater than that constrained bysome maximum current densityJ.We require that at least one conductor be grounded and shall refer to such aconductor as the neutral, designated as “n”. If it is not required to conduct anyappreciable current, we will not include its cross-sectional area in A. This condition is achieved under certain symmetrical loading conditions, referred to as “balanced”loading ,and can be maintained in a practical situation; therefore we allow all schemesto ground exceedV.It is assumed that the reader has a background in basic circuit 0theory. The adjective “dc”, essentially means time invariant or constant with time.Recall that the term“ac”, which historically stood for “alternating current “, in modernusage means “sinusoidal steady state.”These terms are used to describe voltages and currents in time invariant (constant) steady-state and sinusoidal steady-state modes.Transformers come in many sizes. Some power transformers are as big as ahouse. Electronic transformers, on the other hand, can be as small as a cube ofsugar. All transformers have at least one coil; most have two although they may have many more.The usual purpose of transformers is to change the level of voltage. But sometimes ghey are used to isolate a load from the power source.TYPES OF TRANSFORMERSStandard power transformers have two coils. These coils are labeled PRIMARY and SECONDARY. The primary coil is the one connected to the source. The secondary is the one connected to load . There is no electrical connection between the primary and secondary. The secondary gets its voltage by induction.The only place where you will see a STEP-UP transformer is at the generating satation. Typically, electricity is generated at 13,800 volts. It is stepped up to 345,000 volts for transmission. The next stop is the substation where it is stepped down to distribution levels, around 15,000 volts. Large substation transformers have cooling fins to keep them from overheating. Other transformers are located near poinst where the electric power is used.TRANSFORMER CONSTRUCTIONThe coils of transformer are electrically insulated from insulated from each other. There is a magnetic link, however. The two coils are wound on the same core. Current in the primary magnetizes the core. This produces a magnetic field in the core . The core field then affects current in both primary and secondary.There are two main designs for cores;1.The CORE type has the core inside the windings.2.The SHELL type has the core outside.Smaller power transformers are usually of the core type. The very large transformers are of the shell type.There is no difference in their operation,however.Coils are wound with copper wire. The resistance is kept as low as possible to keep losses low.IDEALIZED TRANSFORMERSTransformers are very efficient . the losses are often less than 3 percent . This allows us to assume that they are perfect in many computations.Perfect means that the wire has no resistance. It also means that there are no power losses in the core.Further, we assume that there is no flux leakage.That is, all of the magnetic flux links all of the turns on each coil.EXCITA TION CURRENTTo get an idea of just how small the losses are,we can take a look at the EXCITA TION CURRENT. Assume that nothing is connected to the secondary .If you apply rated voltage to the primary, a small current flows. Typically, this excitation current is less than 3 percent of rated current.Excitation current is made up of two parts.One part is in phase with the voltage. This is the current that supplies the power lost in the core. Core losses are due to EDDY CURRENTS and HYSTERESIS.Eddy currents circulating in the core result from induction. The core is ,after all, a conductor within a changing magnetic field.Hysteresis loss is caused by the energy used in lining up magnetic domains in the core. The alignment goes on continuously first in one direction,then in the other.The other part of the excitation current magnetizes the core. It is this magnetizing current that supplies the “shuttle power is power stored in the magnetic field and returned to the source twice each cycle.Magnetizing current is quadrature(90 degrees out of phase) with the applied voltage.电能输送考虑图1所示的情况。