Surge Impedance Loading (SIL)

 Assignment  Assignment on Su rge rge I mpedance mpedance Loading Loading (SI L)

Char acteristic acteristic Impedance: Characteristic impedance is also known as natural impedance, and it refers to the equivalent resistance of a transmission line if it were infinitely long. If the transmission is uniform and infinite, the wave in the +z direction will continue indefinitely and never return in the -z direction. If the uniform transmission line is truncated and connected instead to a lumped resistive load =Zo (characteristic resistance) , the entire +z wave is dissipated in the load, which has the same effect as if an infinite line of  characteristic impedance Zo were attached at the same point. Such line is called Flat line or Infinite Line . This matched impedance condition is a unique situation in which all the power of the +z wave is delivered to the load just as if it were an infinite transmission line, with no reflected waves generated in the -z direction. Characteristic impedance is of prime importance for good transmission. Maximum power transfer occurs when the source has the same impedance as the load. Mathematical Explana tion: tion: The rms value of V and I and their phase angles at any specified point along the line in terms of the distance x from the receiving end to the specified point is

V= I=

+

 /

+

 /

conductance are zero and the characteristic impedance reduces to the real number. So,

Here,

Surge Impedance, Zc= Zc =

Zc=

In this case inductive and capacitive var are same. So that, that,

The first term in the equations increases in magnitude and advances in phase as distance x from the receiving end increase. The first term is called incident voltage voltage. The second term diminished in magnitude and is retarded in phase from the receiving end toward the sending end. It is called the reflected  voltage. At any pint along the line the voltage is the sum of the component incident and reflected voltages at that point. p oint. If a line is terminated by Zc , the receiving end voltage is equal to ,thus eliminate the reflected wave part of the equation.

Surge Impedance Loading: impedance loading of a line power delivered by a line to a resistive load equal to its impedance.

Surge Imp edance: edance: In power system work, characteristic impedance is sometimes called surge impedance. If  a tr ansient voltage voltage (a “surge” at high frequency frequ ency or by lightning ) is applied appl ied to the end of a transmission line, the line will draw a current proportional to the surge voltage magnitude divided by the line's surge impedance (I=E/Z). The term surge impedance is usually reserved for the special case of  a lossless line. If a line is lossless, its series resistance and shunt

SIL=

|

Surge is the purely surge

|

The value of the SIL to a system operator is realizing that when a line is loaded above its SIL it acts like a shunt reactor - absorbing Mvar from the system - and when a line is loaded below its SIL it acts like a shunt capacitor - supplying Mvar to the system

Relation with Frequency: Frequency: Because of  the series inductance and parallel capacitance, the characteristic impedance reduces with increasing frequency. At very high frequencies, the characteristic impedance asymptotes to a fixed value which is

resistive. coaxial cables have an impedance of 50 or 75 Ohms at high frequencies. Typically, twisted-pair telephone cables have an impedance of 100 Ohms above 1 MHz Graphic Illustration SIL: This particular line has a SIL of 450 MW. Therefore is the line is loaded to 450 MW (with no Mvar) flow, the Mvar produced by the line will exactly balance the Mvar used by the line.

Some Aspect Aspectss of SIL SIL : 1. The characteristic impedance is applicable for two-wire parallel transmission line, Cotransmission and axial microstrip transmission line(a simple parallel plate approximation). For Coaxial line,

For microstrip line,

b and a are the outer and inner radius of the coaxial line and T and W are the dielectric thickness and conductor width of the microstrip line. 2. The characteristic impedance of a transmission line is determined by the following: (1) Size of the wire use.

(2) Spacing between the wires. (3) Insulation used to separate the wires. The characteristic impedance is not affected by the length of  the line .SIL is dependent only on the kV the line is energized at and the line's surge impedance. The line length is is not a factor in the SIL or surge impedance calculations. Therefore the SIL is not a measure of a transmission line's power transfer capability as it does not take into account the line's length nor does it consider the strength of the local power system. 3. Re-reflections of a reflected wave may also occur at the source end  of a transmission line, if the source's internal impedance (Thevenin equivalent impedance) is not exactly equal to the line's characteristic impedance. This type of reflection may be particularly troublesome as it makes it appear that the source has transmitted another pulse. 4. Loadability Loadability Cur ve: ve: The Loadability curve having extrapolated for longer lines,

are generally accepted in the industry for estimating the maximum loading limits on transmission lines loadability of a line is limited by : (i) Thermal limitation l imitation (ii) Voltage regulation (iii) Stability limitation From the curve, 300 miles transmission line has a Loadability of 1 SIL. Line of  that length can operate without supplying any reactive power from either end. Refer ences: ences: A. http://www.o-t-s.com/sil.htm B. http://en.wikipedia.org/wiki/Char acteristic_impedance act eristic_impedance C. htt http://www.allaboutcircuits.com/  p://www.allaboutcircuits.com/  vol_2/chpt_14/4.html vol _2/chpt_14/4.html D. Elements Elements of power system analysis analysis – William D Stevenson Stev enson

Prepared by, Mahmudul Hassan. EEE,BUET