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Static overvoltage and undervoltage relays

Short answer

Static overvoltage relays trip when rectified voltage exceeds a high setpoint; undervoltage relays trip when below a low setpoint, both using level detectors with timers. They prevent insulation stress from overvoltage and motor overheating/stalling from undervoltage by disconnecting the circuit.

Explain the operating principles of static overvoltage and undervoltage relays. How do these relays detect abnormal voltage conditions and initiate protective actions to prevent equipment damage?

A static over-voltage relay trips when the system voltage rises above a set value (e.g. 110–120% of rated), and a static under-voltage relay trips when the voltage falls below a set value (e.g. 70–85%). Both use electronic level detectors in place of electromagnetic coils.

Block diagram

 PT ─►[Aux PT / isolation]
          │
   [Rectifier + smoothing filter]
          │  V_dc ∝ V_system
   ┌──────┴────────┐
 [Level detector] [Level detector]
  OV: V_dc > V_ref1  UV: V_dc < V_ref2
   │                 │
 [Timer t1]        [Timer t2]
   │                 │
 [Amplifier]       [Amplifier]
   │                 │
 [Output relay]    [Output relay]
   └────► Trip CB / alarm ◄────┘
        [Aux DC supply]

Operating principle

  1. Input stage: the PT secondary (110 V) is reduced by an auxiliary transformer that also isolates the electronics; surge suppressors remove spikes.
  2. Rectification and smoothing: the AC voltage is rectified and filtered to a DC voltage proportional to the rms (or average) system voltage. Some relays use a peak detector.
  3. Level detection:
    • Over-voltage: an op-amp comparator (Schmitt trigger) gives output when V_dc > V_ref1.
    • Under-voltage: the comparator inputs are reversed, giving output when V_dc < V_ref2.
    • V_ref is set by a Zener/potentiometer, which gives the plug setting; hysteresis gives a high reset ratio (≈ 0.95–0.98).
  4. Timer: an RC or digital timer adds a definite or inverse delay so that short swells, dips or motor-starting do not cause tripping. Instantaneous high-set stages are used for severe over-voltage.
  5. Output: amplifier drives an auxiliary relay or thyristor which trips the CB or gives an alarm.

Detecting abnormal conditions and protective action

ConditionCauseRelay action
Over-voltageLoad rejection, AVR failure, Ferranti effect, open-circuit of long lineTrips generator field/CB, line reactors switched in
Under-voltageFaults, overload, loss of supply, motor startingTrips motors (prevents overheating and stalling), starts load shedding or changeover
Phase lossBlown fuse / open conductorThree-phase UV relays sense one low phase
  • Over-voltage stresses insulation and over-fluxes transformers; the relay limits its duration.
  • Under-voltage makes induction motors draw high current and stall; the relay disconnects them and prevents dangerous automatic restart when supply returns.
  • Advantages: accurate settings, high reset ratio, low PT burden and fast operation.

✓ Answer checked by Claude Opus 5.5 (high effort)

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