Types of static relays; merits and demerits

Static relays are relays whose measuring circuits use solid-state components instead of moving parts.

Types of static relays

1. Static overcurrent relays: instantaneous, definite time, inverse/IDMT, very inverse and extremely inverse.
2. Static directional relays: directional overcurrent and directional earth-fault.
3. Static differential relays: for transformers, generators and busbars (with bias and harmonic restraint).
4. Static distance relays: impedance, reactance, mho, offset-mho, quadrilateral.
5. Static voltage relays: over-voltage, under-voltage, negative-sequence voltage.
6. Static frequency relays: under/over-frequency, rate-of-change of frequency (load shedding).
7. Static negative-phase-sequence relays: generator and motor unbalance protection.
8. Static pilot / carrier relays: phase comparison carrier protection.

They can also be classified by measuring principle into amplitude comparator and phase comparator relays, and by the devices used into transistor, IC/op-amp, Hall-effect and rectifier-bridge relays.

Advantages

• Very low burden on CTs/PTs, so smaller CTs can be used.
• Fast operation and quick reset; no overshoot.
• No moving parts, so no friction, contact bounce or wear; long life with less maintenance.
• High sensitivity due to amplification.
• Any desired characteristic can be shaped (mho, quadrilateral, inverse).
• Compact; one unit can perform several functions.
• Not affected by vibration, shocks or mounting position; suitable for earthquake-prone areas and ships.

Disadvantages

• Sensitive to voltage transients and spikes; filters and screening are needed.
• Characteristics change with temperature and component ageing.
• Need a reliable auxiliary DC supply.
• Semiconductors have low overload and surge capacity.
• Higher cost than simple EM relays; repair needs trained staff and test equipment.
• Affected by electromagnetic interference and electrostatic discharge.