Fast impedance protection technique immune to dynamic errors of capacitive voltage transformers
The present invention relates to methods for providing impedance protection differentiating between in-zone and out-of-zone faults based on instantaneous, digitally derived operating and polarizing distance comparator signals. The method uses a pair of fast orthogonal filters to derive D and Q components of the input voltages and currents. Two sets of operating and polarizing signals are derived for better speed of response under varying fault moment with respect to the peaks and zero crossings of power signals. Three stages of comparison between the operating and polarizing impedance terms are used. These comparator stages use half a cycle averaging windows, and three-quarters-of-a-cycle windows. The first stage of comparison is based on energy comparator responding to both magnitude and phase information in the signals. Stages 2 and 3 are of phase comparison type, responding mostly at the phase information and neglecting the magnitude information for better immunity to noise and signal distortions.
1. An article of manufacture that includes a non-transitory computer readable medium useable by a processor, the medium having stored thereon a sequence of instructions which, when executed by the processor, causes the processor to provide impedance protection in transmission lines, wherein the impedance protection is provided by:
interfacing, inputting, and digitizing voltages and currents associated with the transmission lines for a first distance zone;
digitally filtering the digitized currents and voltages, with a pair of orthogonal filters, into derived pairs of orthogonal components;
digitally extracting instantaneous operating signals and instantaneous polarizing signals from the derived pairs of orthogonal components;
digitally comparing a first stage angular and magnitude relationship between the instantaneous operating and polarizing signals during a first half-cycle of a first power cycle in a fault event to determine if a fault is within the first distance zone;
digitally comparing a second stage angular and polarization relationship between the instantaneous operating and polarizing signals during a second half-cycle of the first power cycle in the fault event to determine if the fault is within the first distance zone;
digitally comparing a third stage angular and polarization relationship between the instantaneous operating and polarizing signals during a second power cycle, immediately subsequent to the first power cycle, in the fault event to determine if the fault is within the first distance zone; and
outputting an impedance protection signal indicative of the first, second, and third stage comparison determinations, wherein at least one of the second and third stage comparisons respond to signal polarities defined by exceeding a finite non-zero threshold rather than a mathematical zero (0).
2. The article of manufacture of claim 1 , wherein the derived pairs of orthogonal components are derived using digital filters effectively introducing less than half a cycle delay into the orthogonal components.
3. The article of manufacture of claim 1 , wherein the pair of orthogonal filters are of Finite Impulse Response (FIR) or Infinite Impulse Response (IIR).
4. The article of manufacture of claim 1 , wherein the first, second, and third stage comparisons compare terms created in a time domain as instantaneous combinations of orthogonal voltages and currents.
5. The article of manufacture of claim 4 , wherein at least two sets of terms are used for better speed of response in the first, second, and third stage comparisons.
6. The article of manufacture of claim 1 , wherein the first stage comparison responds to both magnitude and phase information of the instantaneous operating and polarizing signals and is time limited to a fraction of the first power cycle following the fault event.
7. The article of manufacture of claim 1 , wherein the second and third stage comparisons respond primarily to phase information in the instantaneous operating and polarizing signals.
8. The article of manufacture of claim 6 , wherein an energy comparator is used to check the relationship between the instantaneous operating and polarizing signals.
9. The article of manufacture of claim 6 , wherein a phase comparison check approach is used to check the relationship between the instantaneous operating and polarizing signals.
10. The article of manufacture of claim 9 , wherein the phase comparison check approach includes at least one time derivative of the instantaneous polarizing or the instantaneous operating signal, the time derivative being of any order.
11. The article of manufacture of claim 9 , wherein the phase comparison check approach includes at least one time delayed value of the instantaneous polarizing or the instantaneous operating signal.