If electrical systems were all balanced the expression negative-sequence would never be needed! Fortunately, the solution for balanced systems is to convert all parameters to fixed-impedance single-phase case. Unfortunately, unbalanced systems can not be solved as easily. Although solvable, the use of simultaneous equations is necessary. Easy today, but not so early last century!
In rotating machines, an unbalanced voltage or current in one phase winding due to electrical faults or uneven load further exacerbates the problem. The magnetic coupling between windings becomes uneven, thus it is impossible to use the fixed-impedance single-phase method described above
When subjected to unbalanced conditions, negative-sequence currents produce a counter rotating magnetic-field (with respect to the main-field).
The resultant field, rotating at double-frequency, causes undesirable eddy-currents to flow in various structures such as the machines rotor body, excitation winding, end-bells (if turbo-type), stator end-windings, stator-iron, etc.
How Negative phase sequence relay work
The order in which the voltage of the coil reaches to the maximum value is called the Phase Sequence.POSITIVE PHASE SEQUENCE: If the coil is rotated in anticlockwise direction, the phase sequence will be Positive Phase Sequence, i.e., R-Y-B or A-B-C.NEGATIVE PHASE SEQUENCE: If the coil is rotated in clockwise direction, the phase sequence is called Negative Phase Sequence, i.e., R-B-Y or A-C-B.NOTE: Phase Sequence is of great importance in parallel operation of three phase transformers and alternators.
please see details in this site http://electricalandelectronics.org/2009/03/17/negative-phase-sequence-relay/
4/4+4*4=20 is it correct by any means or not ? No this garbage has got nothing to do with negative phase sequence current in 3 phase electrical systems.
If you reinstall a three-phase motor with two of the supply conductors interchanged, then the resulting negative phase sequence will cause the motor to run in reverse.
Negative Phase Sequence is Phenomena generally found in motors,the formula for positive phase sequence is : Ip= Ir + a * Iy + a^2*Ibthe formula for negative phase sequence is : In= Ir + a ^2* Iy + a*IbAs the name suggest the sequence of current or the mmf is in reverse order, a reverse order current in stator will produce a reverse order mmf, now this reverse order mmf is rotating in direction opposite to the rotor of the motor, thus the mmf cut is very high and so current induced in stator is considerably high; as for normal mmf it is in direction of rotor so mmf cut is very small thus current induced is very small.this high current in your rotor leads to overheating of your motor and finally burning of your windings.CommentNegative phase sequence is not 'generally found in motors'. But one of the effects of negative phase sequence would be to cause a three-phase motor to reverse direction.
Negative sequence current is defined as 3I2 = (phase 1)*(1angle 0) + (phase 2)*(1angle 240) + (phase 3)*(1angle 120) Negative sequence current is seen in three phase power systems due to natural system imbalance. Also during unbalanced fault conditions such as line to line, Line to ground, and line to line to ground faults. It is not seen in purely balanced three phase faults.
The symbol of phase sequence is RYB
A negative sequence relay is looking at unbalanced current, such as what would result from a line to line or line to ground fault. I'm not sure what you're meaning by "mechanism". Please explain if the above doesn't answer your question.
The potential of each line of a three-phase system is displaced from the other two by 120 electrical degrees. The sequence, or order, in which the potential of each line reaches its peak value is termed the 'phase sequence' of the system. The 'normal' phase sequence of an alternator is considered to be the 'positive phase sequence', is arbitrarily assigned colours (e.g. red-yellow-blue, etc.), numbers (e.g. 1-2-3), letters (e.g. A-B-C), or a combination of letters or numbers (e.g. L1-L2-L3). If that alternator should run backwards, or (far more likely) if its terminals were disconnected and then incorrectly reconnected, the the sequence would be A-C-B (etc.), which would be considered to be 'negative phase sequence'. Before connecting two generators (or two complete transmission/distribution systems) together, their phase sequence must be confirmed as being the same. Reconnecting a three-phase motor with the wrong phase sequence would result in it running in the reverse direction and, possibly, damaging its load.
Three coils ( Suppose: A, B and C ) are placed at 120 degree from each other in uniform magnetic field. The coils are rotated at uniform angular speed ( Suppose: omega ) ( unit: radian per second ). Emfs are induced in each coil. As coil B is placed at 120 degree from coil A, emf induced in coil B lags behind that of A by 120 degree. If the three phase voltage are equal in magnitude, the sum of the three phase voltage is zero. This can be shown by different ways. ( e.g. Vector sum ). The order in which the voltage of the coil reaches to the maximum value is called the phase sequence. ( Phase sequence is of great importance in parallel operation of three phase transformers and alternators ). Phase sequence R-Y-B is known as the positive sequence, while R-B-Y is known as the negative sequence. Phase sequence can be checked by using phase sequence indicator. Phase sequence can be reversed by interchanging any two terminals.
These relay sense the Negative Sequence Voltage components of system supply & offers protection against Phase Failure, Phase Sequence Reversal with Under/Over Voltage conditions which are dangerous to motor winding and cause motor burnouts.