Static distance relay

A static distance relay measures the impedance between the relay and the fault using electronic comparators instead of moving discs. It trips when the measured impedance Z = V/I lies inside its set characteristic (impedance, reactance, mho or quadrilateral).

Block diagram

 PT ─►[Aux PT]──────► V ─────┐
                             ▼
                     [Mixing circuit
 CT ─►[Transactor]─► I·Zr ─►  e.g. mho: S1 = I·Zr − V,
                              S2 = V ]
                             │ S1, S2
                     [Comparator: amplitude
                       or phase type]
                             │
                     [Level detector]
                             │
                     [Zone timers T2, T3]
                             │
                     [Amplifier]─►[Output]─►Trip

Working principle

1. Input stage: the auxiliary PT gives a voltage proportional to the line voltage V. A transactor (air-gap CT) gives a voltage I·Z_r, where Z_r is the replica (set) impedance.
2. Mixing circuit forms two signals from V and I·Z_r. Example for a mho relay: S1 = I·Z_r − V and S2 = V.
3. Comparator:
   • Amplitude comparator (impedance relay): trips when |I·Z_r| > |V|, i.e. Z < Z_r.
   • Phase comparator (mho relay): trips when the angle between S1 and S2 is within ±90°, which gives a circle through the origin.
4. Level detector checks the comparator output against a threshold and rejects noise.
5. Timers: zone 1 is instantaneous; the zone 2 and zone 3 outputs pass through timers (e.g. 0.3 s and 1 s).
6. Amplifier and output drive the trip relay/thyristor of the CB.

Characteristics are easy to shape: by changing the mixing networks, the same hardware gives impedance, reactance, mho, offset-mho or quadrilateral characteristics.

Area of application

• Main protection of EHV and HV transmission lines (66–400 kV) with three-zone stepped protection.
• Reactance type for short lines (not affected by arc resistance), mho for long lines (least affected by power swings), and quadrilateral for lines with high fault resistance.
• Carrier-aided schemes (permissive/blocking) for fast clearance over the whole line.
• Back-up for adjacent lines and busbars; loss-of-field protection of generators (offset mho).
• Out-of-step blocking and power-swing detection.