ABB RELION REX640 Technical Manual page 547

Protection and control

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1MRS759142 F

Third harmonic voltage-based differential protection

The voltage generated by a generator is not a perfect sinusoidal wave but contains

triplen harmonics voltages. These triplen harmonics appear in each phase with the

same magnitude and angle, due to which they do not sum to zero and thus also

appear in the neutral side of the generator as a zero-sequence quantity. Among

all the triplen harmonics voltages generated, the third harmonic voltage has the

highest magnitude with the magnitude varying between 1% and 10% of the terminal

voltage, depending on the generator design philosophy. However, for a particular

generator the magnitude of third harmonics on the neutral side and terminal side

depends also on the active power generated.

Figure 315: Typical example of the third-harmonic voltage measured at the

generator neutral and terminals under different conditions

The operating equation of the protection is described in the following equation.

Technical Manual

Protection functions