GE Multilin L30 Line Current Differential System 9-39 THEORY OF OPERATION 9.1 OVERVIEW99.1.5 DISTURBANCE DETECTIONA disturbance detection algorithm is used to enhance security and to improve transient response. Conditions to detect adisturbance include the magnitude of zero-sequence current, the magnitude of negative-sequence current, and changes inpositive, negative, or zero-sequence current. Normally, differential protection is performed using a full-cycle Fourier trans-form. Continuous use of a full-cycle Fourier means that some pre-fault data is also used for computation – this may lead toa slowdown in the operation of the differential function. To improve operating time, the window is resized to the half-cycleFourier once a disturbance is detected, thus removing pre-fault data.9.1.6 FAULT DETECTIONNormally, the sum of the current phasors from all terminals is zero for each phase at every terminal. A fault is detected for aphase when the sum of the current phasors from each terminal for that phase falls outside of a dynamic elliptical restraintboundary for that phase. The severity of the fault is computed as follows for each phase.The differential current is calculated as a sum of local and remote currents. The real part is expressed as:(EQ 9.3)The imaginary part is expressed as:(EQ 9.4)The differential current is squared for the severity equation:(EQ 9.5)The restraint current is composed from two distinctive terms: traditional and adaptive. Each relay calculates local portion ofthe traditional and restraint current to be used locally and sent to remote peers for use with differential calculations. If morethan one CT are connected to the relay (breaker-and-the half applications), then a maximum of all (up to 4) currents is cho-sen to be processed for traditional restraint:The current chosen is expressed as:(EQ 9.6)This current is then processed with the slope (S1 and S2 ) and breakpoint (BP) settings to form a traditional part of therestraint term for the local current as follows. For two-terminal systems, we have:(EQ 9.7)For three-terminal systems we have(EQ 9.8)The final restraint current sent to peers and used locally in differential calculations is as follows:(EQ 9.9)where: MULTA is a multiplier that increases restraint if CT saturation is detected (see CT Saturation Detection for details);ILOC_ADA_A is an adaptive restraint term (see Online Estimate Of Measurement Error for details)The squared restraining current is calculated as a sum of squared local and all remote restraints:IDIFF_RE_A ILOC_PHASOR_RE_A IREM1_PHASOR_RE_A IREM2_PHASOR_RE_A+ +=IDIFF_IM_A ILOC_PHASOR_IM_A IREM1_PHASOR_IM_A IREM2_PHASOR_IM_A+ +=IDIFF_A( )2 IDIFF_RE_A( )2 IDIFF_IM_A( )2+=ILOC_TRAD_A( )2 max I1_MAG_A( )2 I2_MAG_A( )2 I3_MAG_A( )2 I4_MAG_A( )2 Iq_MAG_A( )2, , , ,( )=If ILOC_TRAD_A( )2 BP 2<then ILOC_REST_TRAD_A( )2 2 S1 ILOC_TRAD_A⋅( )2=else ILOC_REST_TRAD_A( )2 2 S2 ILOC_TRAD_A⋅( )2 S2 BP⋅( )2–( ) 2 S1 BP⋅( )2+=If ILOC_TRAD_A( )2 BP 2<then ILOC_REST_TRAD_A( )2 43--- S1 ILOC_TRAD_A⋅( )2=else ILOC_REST_TRAD_A( )2 43--- S2 ILOC_TRAD_A⋅( )2 S2 BP⋅( )2–( ) 43--- S1 BP⋅( )2+=ILOC_RESTRAINT_A ILOC_REST_TRAD_A( )2 MULT A ILOC_ADA_A( )2⋅+=
