Network A ---i, v---Network B

( Figure 1-1 Power transmission between two networks.)

  ⑵Lowercase letters are used to indicate instantaneous values, 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 on allowable currents and voltages.
  ⑶What 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:

Imax= Jmax A

  ⑷The maximum current density Jmax is determined by the maximum conductor temperature that will not damage the conductor or its insulation system.
  ⑸What are the limiting factors on voltage? The fundamental 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 was 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 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 damaged by a breakdown.
  ⑹The meaning of "ground" is important I we quote from the IEEE standard Dictionary of Electrical and Electronic Terms:
ground (earth ) (electric system). A conducting connection, whether Intentional or accidental, 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 the conducting 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 constituted an equipotential surface when perfectly grounded. Insulation of conductors' from ground is a basic problem.
  ⑺Let us consider some different schemes for implementing the transmission line indicated in Figure 3-1. For a fair comparison we select constraints that all schemes must satisfy. We allow any number of conductors- to be used, as long as each scheme uses the same amount of conducting material. Given that networks A and B are separated by a fixed physical length, this means that in viewing the lines 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 by some maximum current density Jo.
  ⑻We require that at least one conductor be grounded and shall refer to such a conductor as the neutral, designated as "n. H If it is not required to conduct any appreciable 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 1 therefore we allow all schemes to make this assumption. We require that for all schemes, no voltage to ground exceed VO. It is assumed that the reader has a background in basic circuit theory. The adjective "dc" essentially means time invariant or constant with time. Recall that the term n ace, which historically stood for "alternating current", in modern usage means "sinusoidal steady state." These terms are used to describe voltages and currents in time invariant (constant) steady-state and sinusoidal steady-state modes.