Essential components of a static relay

A static relay is a relay in which measurement and comparison are done by electronic circuits (diodes, transistors, ICs, op-amps) with no moving parts; only the output (trip) element may be an electromechanical auxiliary relay or a thyristor.

Block diagram

 CT/PT ┌─────────┐  ┌──────────┐  ┌────────────┐
 ─────►│ Input   ├─►│Rectifier/├─►│ Measuring  │
       │(aux CT, │  │ mixing / │  │ unit       │
       │ PT,     │  │ smoothing│  │(comparator,│
       │ filter) │  └──────────┘  │ level det.,│
       └─────────┘                │ timer)     │
                                  └─────┬──────┘
           ┌─────────────┐        ┌─────▼──────┐
           │ Aux DC      │──────► │ Amplifier  │
           │ supply      │        └─────┬──────┘
           └─────────────┘        ┌─────▼──────┐
                                  │ Output     │──► CB trip
                                  │ (relay/SCR)│     coil
                                  └────────────┘

Function of each block

1. Input (transducer) circuit: auxiliary CTs/PTs and transactors reduce the main CT/PT outputs to small voltages suitable for electronics, isolate the electronics from the power circuit, and allow mixing of phase quantities. Surge-suppression and filters remove transients and harmonics.
2. Rectifier / mixing / smoothing circuit: converts AC to DC (bridge rectifier) for amplitude comparison, or produces square waves for phase comparison; sequence networks extract positive/negative/zero sequence quantities when needed.
3. Measuring unit: the heart of the relay. It contains
   • Comparators (amplitude or phase) that compare operating and restraining quantities,
   • Level detectors that give an output when a quantity crosses a set threshold,
   • Timers / integrators (RC circuits, counters) that provide definite or inverse time delays.
4. Amplifier: raises the low-level output of the measuring unit to a level that can drive the output device; uses transistors or op-amps.
5. Output device: an auxiliary electromagnetic relay (attracted armature) or a thyristor that energises the circuit-breaker trip coil and gives signal/alarm contacts.
6. Auxiliary DC supply: a stabilised supply derived from the station battery (via DC-DC converter) to power the electronics, independent of the fault level.

Features

• Low CT/PT burden, fast operation, no contact bounce or friction.
• Characteristics (inverse, mho, quadrilateral) can be shaped easily with circuits.
• Needs protection against temperature and voltage surges.