The Institute of Electrical and Electronic Engineers (IEEE) defines a relay as an electric device is designed to respond to input conditions in a prescribed manner and, after specified conditions are met, to cause contact operation or similar abrupt change in associated electric control circuits. Often, the Input signals are electric, but could also be mechanical, thermal, or other quantities or a combination of quantities. Relays differ from Limit switches and similar simple devices.
Relays are used in all aspects of activity: home, communication, transportation, commerce, and industry, to name a few. Relays are electronic devices primarily used for switching and in automation applications. Depending on the operating principle, application, function, input signal, and performance characteristics, relays are classified into five types:
- Protective relays: Applied to all parts of power systems (CTs, generators, buses, transmission and distribution lines, reactors, motors, and capacitor banks), they operate on intolerable power system conditions. These relays are widely used in electrical systems to detect defective lines, components, or other system conditions (IEEE 100).
- Regulating relays: Regulating relays do not respond to systems faults unless faults are left for far too long. They are associated with tap changers on transformers and voltage regulators of power generators.
- Reclosing, Synchronism check, and Synchronizing relays: These relays are used to interconnect pre-energized parts while restoring lines after an electrical outage
- Monitoring relays: These are used to verify the conditions of power or protective systems such as fault detectors, checking the voltage or directional sensing units which confirm the power system’s health. They indicate but cannot identify the fault or trouble.
- Auxiliary relays: These relays are further divided into two types – contact multiplication and circuit isolation. Auxiliary relays are used in a protective system and have multiple applications.
Protective Relays – Applications:
Six protection zones are required on any given electrical system, and protective relays are specifically designed based on the application as the primary protection for the system. The six protection zones are:
- Generators and generator-transformer units
- CTs (Current Transformers)
- Buses (bus bars)
- Electrical Lines (transmission, sub-transmission, and distribution)
- Utilization equipment (motors, static loads, or other)
- Capacitor or reactor banks (when separately protected)
Types of Relay Testing: Conventional Vs Secondary Relay Tests
Conventional Testing and commissioning of relays (also called Staged fault test) involve the following steps and are carried out as a part of periodic or scheduled maintenance of electrical systems:
- Relay bench testing to assure proper operation of protective relays.
- Application of system-specific settings
- Verification of electrical wiring and connections
- Function and specification tests of all protection and control systems to ensure that relays operate as per their design limits.
- Commissioning or in-service test
Of the steps mentioned above, the first four are carried out before energization. The final step is to confirm and validate that proper values and phase angles exist for all quantities supplied to the relaying and metering. These tests ensure that the relays operate under allowable design limits and capable enough of tripping the system within the specified time limit. Though the conventional approach provides comprehensive testing of relays, verifying the effectiveness of protection, these are often time-consuming, complex, and error prone.
Secondary Injection Testing (Test Sets):
Secondary injection tests involve simulating computerized faults instead of being field-tested. The procedure involves fault-generating quantities created as files and are injected into the relays using sophisticated test sets. Though these tests provide the complete condition of the electrical systems (which a primary injection test can do), they offer accurate testing of protective relays. Due to advancements in relay test sets, GPS clock receivers, sequential event recorders, secondary injection testing has been widely used for testing protective relays.
Advantages of Secondary Injection tests to Conventional tests:
- Test preparation and planning is less extensive than staged fault test and only requires a database of test studies to determine the test conditions, and hence less expensive
- Most test sets do not require line outages or circuit breaker clearances.
- The ease of simulating external faults into protective relays is better than staged tests.
- Secondary test kits are flexible, whereas staged tests require a complex setup.
- Accuracy and data logging capabilities for analysis are possible on test sets.
- Safer to conduct the test as these are do not need physical testing
Secondary Relay Test Set Applications:
With the advancements in computational capabilities, relay test kits are considered a standard protocol for protective relay testing. Few applications of test kits are as follows:
- Commissioning of transmission line protection systems of (newly) installed relays
- Verification of communication channels
- Effectiveness of relay settings and it’s operation according to design limits
- Troubleshooting relay malfunctions
- Evaluation of new relay protection systems
- Condition of relay wirings
Relay Test Set Calibration – Why is it Essential?
Following errors are set into relay test set due to prolonged usage at various operating conditions. Operators should measure and eliminated these errors using calibration and resetting:
- Ammeter and Voltmeters: Built-in components such as ammeter and voltmeter require calibration due to operation at different test sets. Calibration ensures a zero setting and linear correlation amongst these components.
- Knob maintenance and calibration: Knobs are used to set current output from a source. Due to the loosening of knobs, errors set into the system.
- Errors in Tripping time and tripping level measurements: The accuracy of Test kits depend on several internal components. Short circuit or cable damages may permanently induce errors and must be filtered from the test kits.
- Operational Error: Due to lack of skill and training, operational errors set in. These errors affect the performance and accuracy of the system.
Cumulative errors often result in operational uncertainties, affecting the accuracy of measurement. Calibration avoids these errors, ensures the test kit’s accuracy for sustained and repeated use, and enhances service life. Following a regular and timely calibration schedule ensures accuracy of measurement and improves process accuracy.
e2b calibration offers industry-leading ISO-certified protective relay test kit calibration services. Our labs are ISO/IEC 17025 accredited and operated by a team of qualified calibration experts to test and calibrate your test kits. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for all your equipment calibration needs.
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