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In
electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
, admittance is a measure of how easily a circuit or device will allow a current to flow. It is defined as the
reciprocal Reciprocal may refer to: In mathematics * Multiplicative inverse In mathematics, a multiplicative inverse or reciprocal for a number ''x'', denoted by 1/''x'' or ''x''−1, is a number which when Multiplication, multiplied by ''x'' yield ...
of impedance, analogous to how conductance & resistance are defined. The SI unit of admittance is the
siemens Siemens AG ( ) is a German Multinational corporation, multinational Conglomerate (company), conglomerate corporation and the largest industrial manufacturing company in Europe headquartered in Munich with branch offices abroad. The principal ...
(symbol S); the older, synonymous unit is mho, and its symbol is ℧ (an upside-down uppercase omega Ω).
Oliver Heaviside Oliver Heaviside Fellow of the Royal Society, FRS (; 18 May 1850 – 3 February 1925) was an English Autodidacticism, self-taught mathematician and physicist who invented a new technique for solving differential equations (equivalent to the La ...
coined the term ''admittance'' in December 1887. Heaviside used ''Y'' to represent the magnitude of admittance, but it quickly became the conventional symbol for admittance itself through the publications of
Charles Proteus Steinmetz Charles Proteus Steinmetz (born Karl August Rudolph Steinmetz, April 9, 1865 – October 26, 1923) was a German-born American mathematician and Electrical engineering, electrical engineer and professor at Union College. He fostered the deve ...
. Heaviside probably chose ''Y'' simply because it is next to ''Z'' in the alphabet, the conventional symbol for impedance. Admittance is defined as :$Y \equiv \frac \,$ where :''Y'' is the admittance, measured in
siemens Siemens AG ( ) is a German Multinational corporation, multinational Conglomerate (company), conglomerate corporation and the largest industrial manufacturing company in Europe headquartered in Munich with branch offices abroad. The principal ...
:''Z'' is the impedance, measured in ohms Resistance is a measure of the opposition of a circuit to the flow of a steady current, while impedance takes into account not only the resistance but also dynamic effects (known as reactance). Likewise, admittance is not only a measure of the ease with which a steady current can flow, but also the dynamic effects of the material's susceptance to polarization: :$Y = G + j B \,$ where *$Y$ is the admittance, measured in siemens. *$G$ is the conductance, measured in siemens. *$B$ is the
susceptance In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identi ...
, measured in siemens. *$j^2 = -1$ The dynamic effects of the material's susceptance relate to the universal dielectric response, the power law scaling of a system's admittance with frequency under alternating current conditions.

# Conversion from impedance to admittance

The impedance, ''Z'', is composed of real and imaginary parts, :$Z = R + jX \,$ where *''R'' is the resistance, measured in ohms *''X'' is the reactance, measured in ohms :$Y = Z^= \frac = \left\left( \frac \right\right) \left\left(R - jX\right\right)$ Admittance, just like impedance, is a complex number, made up of a real part (the conductance, ''G''), and an imaginary part (the susceptance, ''B''), thus: :$Y = G + jB \,\!$ where ''G'' (conductance) and ''B'' (susceptance) are given by: :$\begin G &= \Re\left(Y\right) = \frac \\ B &= \Im\left(Y\right) = -\frac \end$ The magnitude and phase of the admittance are given by: :$\begin \left , Y \right , &= \sqrt = \frac \\ \angle Y &= \arctan \left\left( \frac \right\right) = \arctan \left\left( -\frac \right\right) \end$ where *''G'' is the conductance, measured in
siemens Siemens AG ( ) is a German Multinational corporation, multinational Conglomerate (company), conglomerate corporation and the largest industrial manufacturing company in Europe headquartered in Munich with branch offices abroad. The principal ...
*''B'' is the
susceptance In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identi ...
, also measured in
siemens Siemens AG ( ) is a German Multinational corporation, multinational Conglomerate (company), conglomerate corporation and the largest industrial manufacturing company in Europe headquartered in Munich with branch offices abroad. The principal ...
Note that (as shown above) the signs of reactances become reversed in the admittance domain; i.e. capacitive susceptance is positive and inductive susceptance is negative.

# Shunt admittance in electrical power systems modeling

In the context of electrical modeling of transformers and transmission lines, shunt components that provide paths of least resistance in certain models are generally specified in terms of their admittance. Each side of most transformer models contains shunt components which model magnetizing current and core losses. These shunt components can be referenced to the primary or secondary side. For simplified transformer analysis, admittance from shunt elements can be neglected. When shunt components have non-negligible effects on system operation, the shunt admittance must be considered. In the diagram below, all shunt admittances are referred to the primary side. The real and imaginary components of the shunt admittance, conductance and susceptance, are represented by Gc and B, respectively. Transmission lines can span hundreds of kilometers, over which the line's capacitance can affect voltage levels. For short length transmission line analysis, which applies to lines shorter than 80 kilometers, this capacitance can be ignored and shunt components are not necessary in the model. Lines between 80 and about 250 kilometers, generally considered to be in the medium-line category, contain a shunt admittance governed by $Y=yl=j\omega Cl$ where * ''Y'' = total shunt admittance * ''y'' = shunt admittance per unit length * ''l'' = length of line * ''C'' = capacitance of line