I would like to know how negative sequence currents are generated in a generator and how does it lead to heating of the machine.

Also how does negative sequence protections operate??

 

Responding to Electrics’ 12-Jan (20:01) query... this is another of those subjects that is difficult to define. It is, and I used this simile before,"like explaining what a ratchet is without using your hands!" But, I’ll try:

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. In 1918 an analytical tool, Symmetrical Components, was introduced. Fundamentally, it consists of modeling an electrical system to have three, independent circuits: a) the first, responsive to an abcabc... supply-vector (there, I said the V-word) rotation, is called (appropriately enough) the Positive-Sequence; b) the second, responsive to a cbacba... supply-vector rotation, is called (what else) the Negative-Sequence; and c) the third, in which all supply-vectors are in-phase, is called the Zero-Sequence! Now that negative-sequence has been defined let’s move on to Electrics’ questions:

1) How are negative-sequence currents produced?
When subjected to unbalanced conditions, negative-sequence currents produce a counter rotating magnetic-field (with respect to the main-field).

2) How does negative-sequence lead to heating?
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.

3) How does negative-sequence protection operate?
Quite, simply! The monitoring protective-devices are connected to voltage and current transformers in such a way that they “mimic” negative-sequence currents and voltages. They are then set to operate before reaching undesirable conditions!

If additional information is required, please contact me!

Regards,
Phil Corso, PE ([email protected])

 

One question Phil,

I read that negative phase sequence current can be calculated from two of the three phase current measurement? Can you tell how it can be calculated, or even if I have three phase current measurements, how to calculate negative sequence current?

 

Responding to Vedika’s 16-Jan (00:01) question... technically, yes! However, presuming the circuit parameters and configuration are known, then, there are conditions:

a) If one current measurement is known.
This is sufficient if its phase angle is known, as well as the fault condition, i.e., unbalanced parameters, open-phase, phase-to-ground, double-phase-to-ground, fault impedances, etc.!

b) If two current measurements are known.
This is sufficient. One can use the Law-of-Cosines to determine the third value, as well as the corresponding phase angles of all three of the currents!

c) If three current magnitudes are known.
This is, of course, the simplest situation because the vector triangle is now known!

The method of Symmetrical Components is a very effective analytical tool. I used when responding to three topics on the List. Last year, two of the responses addressed problems of an open-phase (blown fuse) in the primary supply to a delta-wye transformer. And, within the last week, the third addressed the problem of "Neutral Interruptus"... my name for the problem of an open neutral in a 3-phase, 4-wire system!

Vedika, I don’t want to leave you with the impression that it’s so easy to use that one need only take a single clamp-on current reading to get a solution. In fact, knowledge of Vector-Analysis and Matrix-Operation (and of course, trigonometry) are pre-requisites. But, if you are interested in additional information, or references, or you have a specific need, feel free to contact me on or off-list!

Regards, Phil Corso ([email protected])

 

I appreciate your support for above answers. Can u please demonstrate it with an example...

Regards
M.Hemanth Kumar

 

Responding to Hemanth's 14-May-09 (08:34)query...
Contact me off-list!

Regards, Phil corso ([email protected])

 

Responding to Phil 12-Jan (14:36)

can u please elaborate that how the monitoring protective-devices are connected to voltage and current transformers so as they "mimic" negative-sequence currents and voltages.

Regards,
M.A.Siddiqui
(siddiqui86 [at] yahoo.com)