Protective Relays: Safeguarding Industrial Power Systems

In the power industry, a protective relay monitors electrical parameters—current, voltage, frequency, or other power measurements—either from a generator or to a load. When an abnormal condition is detected, it automatically signals a circuit breaker to open.

Electromechanical Relays as Circuit Breakers

Industrial circuit breakers that handle high power are, in fact, electromechanical relays. Unlike residential breakers that use a bimetallic strip to trip when heated by overcurrent, these large units require an external signal to open.

Each breaker contains two electromagnetic coils: one that closes the contacts and another that opens them. The opening—or trip—coil is energized by protective relays, manual switches, or other devices that supply 125 V DC. Using DC allows a battery backup to operate the breaker’s control circuit during an AC power outage.

Protective Relays Monitoring Large AC Currents

Large‑current protection is achieved through current transformers (CTs) that encircle the conductors feeding a breaker, transformer, or generator.

CTs reduce the primary current to a 0‑5 A secondary, which powers the relay’s internal circuitry. If the monitored current exceeds limits, the relay closes its contact, energizing the breaker’s trip coil with 125 V DC.

Protective Relays Monitoring High AC Voltages

Voltage relays protect against over‑voltage by stepping down the high AC voltage with potential transformers (PTs) to a 0‑120 V secondary range.

The voltage signal powers the relay’s internal mechanism; upon detecting an excessive voltage, the relay closes its contact to energize the breaker’s trip coil with 125 V DC.

Common Characteristics of Protective Relays

Protective relays come in many varieties, some tailored for specific functions, and not all monitor voltage or current. What unites them is a contact‑closure output that can power a breaker’s trip coil, a coil, or an operator alarm panel.

Most protective relay functions have been categorized into an ANSI standard number code. Here are a few examples from that code list:

ANSI protective relay designation numbers

12 = Overspeed 24 = Overexcitation 25 = Syncrocheck 27 = Bus/Line undervoltage 32 = Reverse power (anti-motoring) 38 = Stator overtemp (RTD) 39 = Bearing vibration 40 = Loss of excitation 46 = Negative sequence undercurrent (phase current imbalance) 47 = Negative sequence undervoltage (phase voltage imbalance) 49 = Bearing overtemp (RTD) 50 = Instantaneous overcurrent 51 = Time overcurrent 51V = Time overcurrent -- voltage restrained 55 = Power factor 59 = Bus overvoltage 60FL = Voltage transformer fuse failure 67 = Phase/Ground directional current 79 = Autoreclose 81 = Bus over/underfrequency

• Large electric circuit breakers lack built‑in automatic tripping mechanisms and must be commanded by external devices.
• Protective relays are designed to automatically trigger the actuation coils of these breakers under defined conditions.

RELATED WORKSHEETS:

• Protective Relay Circuits Worksheet