SHORT QUESTIONS ABOUT SYMMETRICAL COMPONENTS METHOD?

QUESTION:NAME THE FOUNDER OF SYMMETRICAL COMPONENTS METHOD?

This symmetrical components method was introduced BY  C.L.Fortescue that proposed very optimum solution to the problems occurring in the examination and calculations of unsymmetrical components.

QUESTION:WHAT IS A SYMMETRICAL COMPONENTS METHOD?

It is a special method in which an unbalance system having n corresponding phasors can be broken down into corresponding systems n having balanced phasors ,these phasors are known as original phasors symmetrical components.

QUESTION:DESCRIBE THE MAIN FEATURE OF SYMMETRICAL COMPONENTS ?

The main characteristic for the symmetrical components method is very interesting and helpful in numerical problems especially.The main feature is that all or n phasors of components each set have same length and furthermore adjacent phasors angle of each components set are equal in magnitude.

QUESTION: GIVE ADVANTAGES OF SYMMETRICAL COMPONENTS METHOD?

1) this method is applicable to all orders of unbalance systems.

2) by adopting this method our calculations are extremely easy and interesting.

QUESTION:SYNTHESIS OF UNSYMMETRICAL PHASORS FROM SYMMETRICAL COMPONENTS?

delta and star inerconversions

According to Fortescue's theorem three unbalanced phasors of a three phase system be resolved into three three balanced systems of phasors.The blanced set of components are:

a) Positive sequence components

The positive sequence components consisting of three phasors equal in magnitude,displaced from each by 120 in phase and having the same phase sequence as the original phasors.

b) Negative sequence components

Negative sequence components consisting of three phasors equal in magnitude,displaced from each other by 120 degree in phase and having the phase sequence opposite to that of original phasors.

c) Zero sequence components

Zero sequence components consisting of three phasors equal in magnitude and with zero pahse displacement from each other.

Description

Set of three unbalanced phasors, and the necessary symmetrical components that sum up to the resulting plot at the bottom.
In 1918 Charles Legeyt Fortescue presented a paper which demonstrated that any set of N unbalanced phasors (that is, any such polyphase signal) could be expressed as the sum of N symmetrical sets of balanced phasors, for values of N that are prime. Only a single frequency component is represented by the phasors. However, the credit for the first formal statement should go to L.G. Stokvis who explained the principal and gave experimental verification of its correctness in 1915. In a three-phase system, one set of phasors has the same phase sequence as the system under study (positive sequence; say ABC), the second set has the reverse phase sequence (negative sequence; ACB), and in the third set the phasors A, B and C are in phase with each other (zero sequence, the common-mode signal). Essentially, this method converts three unbalanced phases into three independent sources, which makes asymmetric fault analysis more tractable.

By expanding a one-line diagram to show the positive sequence, negative sequence and zero sequence impedances of generators, transformers and other devices including overhead lines and cables, analysis of such unbalanced conditions as a single line to ground short-circuit fault is greatly simplified. The technique can also be extended to higher order phase systems.

Physically, in a three phase winding a positive sequence set of currents produces a normal rotating field, a negative sequence set produces a field with the opposite rotation, and the zero sequence set produces a field that oscillates but does not rotate between phase windings. Since these effects can be detected physically with sequence filters, the mathematical tool became the basis for the design of protective relays, which used negative-sequence voltages and currents as a reliable indicator of fault conditions. Such relays may be used to trip circuit breakers or take other steps to protect electrical systems.

The analytical technique was adopted and advanced by engineers at General Electric and Westinghouse and after World War II it was an accepted method for asymmetric fault analysis.
. The imbalance between phases arises because of the difference in magnitude and phase shift between the sets of vectors. Notice that the colors (red, blue, and yellow) of the separate sequence vectors correspond to three different phases (A, B, and C, for example). To arrive at the final plot, the sum of vectors of each phase is calculated. This resulting vector is the effective phasor representation of that particular phase. This process, repeated, produces the phasor for each of the three phases.Courtesy of wikipedia...