In Part 1 of this NEW series, we unpacked the definition of a protective relay, the five functional categories of relays, and then ended off by touching on the classification of relays.
We wrap up the series by discussing the types of protective relays, and then move on to transformers, protection zones and end off with the applications of a relay…
What types of protective relays are used?
Digital protective relays
The digital protective relay or numeric relay is a protective relay that uses a microprocessor to analyze power system voltages, currents or other process quantities for detection of faults in an industrial process system.
A digital protective relay’s operating principle ranges from simple to complex. Generally, the digital protective relay manages several protective functions or performance characteristics as well as having the ability of communications, monitoring, recording and programmable logic afforded by microprocessor technology.
Electro-mechanical relays
Electro-mechanical protective relays are the earliest forms of protective relays and operating using electro-magnetic forces and physical range from simple to complex, but generally, they manage only one or two protective functions or performance characteristic.
Electro-mechanical protective relays have been used since the beginning of the electrical power grid and are still in large use today.
Instrument transformers
Protective relays generally do not directly measure the input quantities (current or voltage) they are trying to protect for abnormal conditions. Rather, they require instrument transformers that isolate the relay from the dangerous high voltage and current levels of the power delivery system.
- Current transformers (CT)
Used to measure current in AC circuits. A current transformer reduces the high current level proportionally to a range of 0 to 5 amps.
- Potential transformers (PT) or voltage transformers (VT)
Used to measure voltage (potential difference) in AC circuits. A potential or voltage transformer reduces the high voltage level proportionally to a range of 0 to 120 volts.
Zones of protection
Protective relays are designed to protect zones of the power system. When applying protective relaying, the power system is divided into sections so that the protective relays provide “zones of protection.”
Sometimes it is common for zones of protection to overlap so that multiple layers of protection are afforded to each piece of equipment. This points to the idea of primary and secondary (backup) protection.
Overlapping and backup protection is implemented to avoid the possibility of unprotected areas, especially for critical equipment. This is accomplished by the strategic placement of the instrument transformers (current transformers or potential transformers). Otherwise, simple redundancy of the protective relay scheme provides backup protection.
The zones can be defined as:
- Generators
- Transformers
- Buses and distribution feeders
- Transmission lines
- Motors
Applications of a relay
Wherever electricity is used, there is a high probability relays are involved. Simple relays and limit switches are found in many commercial and residential areas including heating, ventilation and air conditioning (HVAC) systems, stoves, elevators, telephone networks, traffic controls, robotics and many other applications.
Resources:
1. WATCH an educational video on Current Transformers
2. DISCOVER Eaton’s range of Protective Relays
3. CHAT to us for an Eaton solution for your facility
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