Last month, we looked into positive, negative and zero-sequence components, also known as symmetrical components, with virtually no math involved. In summary, any three-phase unbalanced (or nonsymmetrical) voltage or current (or impedance) can be modeled as three sets of symmetrical phasors (magnitude and phase angle between them), one of which rotates in the system’s original direction, one in the opposite direction, and one doesn’t rotate at all where the phase angle between is zero. 


So, back to the original question, why do we care about these three numbers?


A frequent use of them is to more accurately measure the unbalance of a system, rather than the NEMA MG-1 method of taking the maximum deviation from the average of any of the phases divided by the average. The NEMA method doesn’t hold up in distorted systems. The negative sequence divided by the positive sequence and the zero sequence divided by the positive sequence are used as a gauge of the unbalance, especially in the international community. It is also a quick way to see if you have correctly hooked up the measurement leads and current probes of a power quality monitor. Having a negative/positive sequence unbalance near 100 percent means you likely have the rotation reversed, so swap B and C leads, probes or both.


For utility investigations of faults on the distribution system, the negative and zero-sequence components are used to determine the type of fault (line-ground, line-neutral, line-line, line-line-line) as well as its location. Since a generator only generates positive-sequence power, the fault creates the negative and zero-sequence components. 


Con Edison of New York, in conjunction with Electrotek Concepts, has developed a program that can pinpoint underground faults in the thousands of miles of cable under New York City, which saves critical time in getting business back to normal when a fault occurs.


A fault doesn’t have to be just in the distribution system. Faults in the windings of transformers and motors also cause changes in the values of sequence components, which result in changes in the aforementioned unbalance ratios. Turn-to-turn faults can be difficult to detect, but, with negative-sequence values, protection systems can reliably detect such faults. A properly operating delta-connected load or a delta-connected transformer winding cannot have zero-sequence components in the supply lines, nor can a wye-connected load without a neutral conductor. Hence, monitoring for zero-sequence currents will indicate a potential ground-fault problem.


The positive-sequence currents turn the induction motor in the proper intended rotation. The presence of negative-­sequence currents indicates a force trying to oppose that proper rotation, with heat being a byproduct. Excessive heating is a primary source of motor failures, with the life of a motor being almost halved if it is continuously overheated by 10 degrees. Negative-sequence voltages yield a higher proportional impact, where a 1 percent voltage unbalance can cause a 6–10 percent current unbalance. Besides the voltage unbalance, stator-to-stator faults, blown fuses, loose connections and winding faults can be a source of the current unbalance. Unbalance also has a multiplying effect on output; a 5 percent voltage impact reduces output by 25 percent, causing a further derating of the motor. Voltage unbalance also can affect the front end of three-phase rectifier systems used in adjustable speed drives, causing uncharacteristic triplen harmonic line currents.


Harmonics can be characterized as positive, negative or zero sequence, as shown in the table above. The presence of those harmonics labeled as negative sequence cause the same reverse rotation stress on motors as the negative-­sequence currents described before. Zero-sequence harmonics add in the neutral conductors, causing overstress and overheating. In delta-connected transformers, they have no escape path, so they just circulate within the transformer, generating heat.


The reason that the third harmonic is labeled as a zero-sequence characteristic can be seen in the top figure. The positive rotation of the fundamental frequency waveform is seen, as Phase B is lagging in time that of Phase A by 120 degrees, and likewise for Phase C relative to Phase B. But, the third harmonics of each of the phases has no 120-phase rotation. The motor counts on the changing fields from the positive sequence currents to “push” the rotation along with the proper spin. Each of the third harmonic waveforms is in sync with each other. These can’t make a motor rotate. They just make more heat.


One needs not delve into Charles L. Fortes­cue’s “Method of Symmetrical Co-Ordinates Applied to the Solution of Polyphase Networks” to get value from the negative- and zero-sequence unbalance values that most power quality monitors display and trend. They are just a simple pair of numbers used to help find faults before they become failures.