Introduction
Sequence component analysis plays an essential role in analyzing power system faults and explaining some power system phenomena. It is very well known that negative sequence current could cause rotor damage, and that damage is highly detrimental to rotating machines such as motors and generators. IEEE Tutorial of the Protection of Synchronous Generators (95 TP 102) has the following statement in section “Current Unbalance Protection”: “During unbalanced conditions, negative sequence current is produced.
The negative sequence current component rotates in the opposite direction from the rotor.” This statement is not quite correct. Positive, negative and zero sequence currents are linear combinations of phase currents; thus, the vector of each sequence current rotates in the same direction as the phase current.
Usually, phase angle is measured with a reference, and the rotation of the negative sequence current generally is ignored. In IEEE Standard for Synchrophasors for Power Systems (C37.118-2005), the absolute phase angle is defined. It is worthwhile to clarify the rotation direction of the negative sequence current to avoid potential confusion in synchrophasor metering.
First, this paper reviews the concept of sequence components. Then, it explains that all sequence components rotate in the same direction. In a rotating machine, the negative sequence current vector rotates in the same direction as the rotor. It is the magnetic flux produced by the negative sequence current that rotates in the reverse direction of the rotor. Thus, the rotor cuts through the flux at twice the synchronous speed, and the induced current in the rotor is twice the line frequency.
I. Phase Rotation
In a power system, phase rotation is defined in the domain of a balanced three phase system. In a balanced system, phasors a, b and c are equal in magnitude and displaced 120° from each other. If phase a leads phase b by 120° and phase b leads phase c by 120°, this system is of abc rotation. If phase a lags phase b by 120° and phase b lags phase c by 120°, this system is of acb rotation.
Computation of Sequence Components of Unbalanced Phasors
With the introduction of sequence components, we will show that any unbalanced set of three phase phasors can be decomposed into three sets of balanced sequence components, i.e., positive sequence components,negative sequence components and zero sequence components. Read entire document here...
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