Summary of "Protección de DISTANCIA 📏 (ANSI 21)"

Main ideas / lessons conveyed

Distance protection concept
- Distance relays are named because they use impedance as a proxy for distance along a transmission line.
- The relay measures voltage and current at its location and computes the ratio between voltage and current.
- The relay operates when this measured impedance relationship falls below a predefined characteristic threshold.
- Generation level in service does not significantly affect the distance relay’s behavior during faults.
- Balanced conditions (notably):
  - Three-phase faults
  - Loss of excitation
  - Loss of synchronism
appear similarly to the distance relay → therefore, protections must be designed to avoid unwanted operation in these cases.
- Unbalanced faults have a different appearance, and the apparent speed of the impedance evolution differs → this helps identify faults correctly.
Inputs and fault location
- The relay uses voltage and current inputs from the measurement point.
- From these, it estimates the distance to the fault.
- Modern relays can include fault location algorithms.
XR diagram (impedance plane)
- Relay characteristics are explained in an impedance (XR) diagram:
  - Resistance (r) is plotted on the horizontal axis (abscissa).
  - Reactance (x) is plotted on the vertical axis (ordinate).
  - The origin corresponds to the relay location.
  - The operating region is typically in the first quadrant.
- Any impedance value can be plotted if you know:
  - its resistive/reactive components, or
  - its magnitude/angle.
- Impedances can be shown using primary or secondary resistors.
  - Secondary impedance is derived from:
    - Primary impedance × (CT ratio / PT (PP) ratio) (as stated: CT transformation ratio divided by PP transformation ratio).
Sign/reading conventions for the XR diagram
- The conventions account for active and reactive power flow direction at the measurement node:
  - If the protection node delivers active power → measured value is positive, otherwise negative.
  - If it delivers lagging reactive power → measured value is positive, otherwise negative.
  - If it delivers leading reactive power → measured value is negative, otherwise positive.
- Quantity compared: Magnitude.
Comparator construction (polarizing vs operating quantities)
- The relay forms a linear combination of measured voltage and current to create:
  - polarizing quantities
  - operating quantities
- A comparator is built to compare the magnitude of these quantities.
- Using specific parameter assumptions (stated in the subtitles as k, n2, n4 = 0 with additional constraints), the relay equations reduce to a form used to construct circular characteristics.
- The impedance characteristic described leads to:
  - a circle centered at the origin in the x-plane
  - parametric/circle equations derived using the angle θ of the impedance and the characteristic impedance of the protected line.
Characteristic types: Mho / MUD unit and directionality
- The circle displaced from the origin corresponds to the MUD unit and is inherently directional.
- The circle through the origin corresponds to the other common characteristic (mho-like behavior).
- The subtitles describe how choosing conditions relative to the characteristic impedance (and using decomposition into real/imag parts) yields:
  - specific circle geometry involving:
    - half the characteristic impedance
    - cos(θ) and sin(θ) terms for the real/imag coordinates.
- Coordination is illustrated as part of the characteristic behavior.
Overreach / “barry trip” and setting philosophy
- Distance relays can operate for impedance values larger than their set value → called overreach (also mentioned as “barry trip”).
- Cause (as stated): displacement of the initial boundary (“bs” in the current waveform) changes the measured impedance evolution.
- Mitigation:
  - set distance units to values below the total impedance of the line being protected.

Methodology / protection zones and setting instructions

Distance relay primary protection (with zones)

Two distance units are required for primary protection (as stated).
Zone 1
- Operates immediately.
- Typically set to n × (line impedance).
- Where n < 1, commonly 0.8 to 0.9.
Zone 2
- Set to 100% of the protected line impedance.
- Plus 50% of the shortest adjacent line impedance.
- Includes a time delay.
Zone 3
- Set to 100% of the impedance of two adjacent lines.
- Plus 25% of a third line.
- Includes a time delay.

Communication-aided schemes to avoid overreach

To avoid overreach, a communication channel can be used to cover 100% of the line length.
When communication is present, one of the following schemes is used:
- Transferred trip with direct underreach
- Transferred trip with permissive reach
- Transferred trip with permissive overreach
- Direct compare blocking
- Directional compare unlocking