--- title: "Essential components of a static relay" subject: "Switchgear and Protection" chapter: 8 key: "semester_6/switchgear_and_protection:static-relay-essential-components" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/static-relay-essential-components site: BCT Engineering --- # Essential components of a static relay Switchgear and Protection · Chapter 8: Static and Digital Relays - Asked in: 2070 Bhadra, 8 marks; 2071 Magh, 8 marks; 2072 Magh, 8 marks; 2073 Magh, 8 marks; 2075 Bhadra, 8 marks (asked 5 times) - Video: [Explanation of Static Relay | Advantages and Limitations | by Yuvi-Study Point](https://www.youtube.com/watch?v=RnYzgEyHxVE) — Yuvi-Study Point - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/static-relay-essential-components - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-8 ## Question Explain the essential components (functional circuits) of a static relay with block diagram and the function of each block. ## Short answer Essential components: input circuit (aux CT/PT) scales signals and provides isolation; rectifier/smoothing converts AC for comparison; measuring unit contains comparators, level detectors, timers; amplifier boosts signal; output relay/thyristor trips breaker; auxiliary DC supply powers electronics. ## Answer A **static relay** is a relay in which measurement and comparison are done by electronic circuits (diodes, transistors, ICs, op-amps) with no moving parts; only the output (trip) element may be an electromechanical auxiliary relay or a thyristor. #### Block diagram ```text CT/PT ┌─────────┐ ┌──────────┐ ┌────────────┐ ─────►│ Input ├─►│Rectifier/├─►│ Measuring │ │(aux CT, │ │ mixing / │ │ unit │ │ PT, │ │ smoothing│ │(comparator,│ │ filter) │ └──────────┘ │ level det.,│ └─────────┘ │ timer) │ └─────┬──────┘ ┌─────────────┐ ┌─────▼──────┐ │ Aux DC │──────► │ Amplifier │ │ supply │ └─────┬──────┘ └─────────────┘ ┌─────▼──────┐ │ Output │──► CB trip │ (relay/SCR)│ coil └────────────┘ ``` #### Function of each block 1. **Input (transducer) circuit:** auxiliary CTs/PTs and transactors reduce the main CT/PT outputs to small voltages suitable for electronics, isolate the electronics from the power circuit, and allow mixing of phase quantities. Surge-suppression and filters remove transients and harmonics. 2. **Rectifier / mixing / smoothing circuit:** converts AC to DC (bridge rectifier) for amplitude comparison, or produces square waves for phase comparison; sequence networks extract positive/negative/zero sequence quantities when needed. 3. **Measuring unit:** the heart of the relay. It contains - **Comparators** (amplitude or phase) that compare operating and restraining quantities, - **Level detectors** that give an output when a quantity crosses a set threshold, - **Timers / integrators** (RC circuits, counters) that provide definite or inverse time delays. 4. **Amplifier:** raises the low-level output of the measuring unit to a level that can drive the output device; uses transistors or op-amps. 5. **Output device:** an auxiliary electromagnetic relay (attracted armature) or a thyristor that energises the circuit-breaker trip coil and gives signal/alarm contacts. 6. **Auxiliary DC supply:** a stabilised supply derived from the station battery (via DC-DC converter) to power the electronics, independent of the fault level. #### Features - Low CT/PT burden, fast operation, no contact bounce or friction. - Characteristics (inverse, mho, quadrilateral) can be shaped easily with circuits. - Needs protection against temperature and voltage surges. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What distinguishes a static relay? A. Measurement and comparison use electronic circuits with no moving parts B. It uses only rotating discs C. It has no output element D. It works only on DC Answer: A. Only the output element may be an auxiliary electromagnetic relay or thyristor. (Key point: Definition of static relay) 2. What is the role of the input (transducer) circuit? A. Scale down CT/PT outputs, isolate the electronics, filter surges B. Amplify the trip signal C. Produce the DC supply D. Close the trip coil Answer: A. Auxiliary CTs/PTs reduce signals to electronic levels and isolate the electronics from the power circuit. (Key point: Input circuit role) 3. Which block is the heart of the static relay, comparing operating and restraining quantities? A. Amplifier B. Measuring unit (comparator, level detector, timer) C. Auxiliary supply D. Output relay Answer: B. The measuring unit contains comparators, level detectors and timers that decide whether a fault exists. (Key point: Measuring unit) 4. Why are an amplifier and an auxiliary DC supply needed? A. The measuring unit output is weak, and the electronics need a stable supply independent of fault level B. To increase CT burden C. To make the relay slower D. To replace the CT Answer: A. The amplifier raises the low-level output to drive the output device; the battery-derived supply powers the electronics. (Key point: Amplifier and supply) ## Related in Switchgear and Protection - [Directional static overcurrent relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adirectional-static-overcurrent) — Switchgear and Protection, chapter 8, asked 2× - [Static differential relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-differential-relay) — Switchgear and Protection, chapter 8, asked 3× - [Static distance relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-distance-relay) — Switchgear and Protection, chapter 8, asked 3× - [Static MHO relay](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-mho-relay) — Switchgear and Protection, chapter 8 - [Static vs electromagnetic relays; components](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Astatic-vs-electromagnetic-relays) — Switchgear and Protection, chapter 8, asked 4× - [Digital relay advantages and functional blocks](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Adigital-relay-advantages-blocks) — Switchgear and Protection, chapter 8, asked 2× ## Related in other subjects - [Functional block diagram of excitation control](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-control-block-diagram) — Power Plant Equipment, chapter 3 - [Essential components of an activity diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aactivity-diagram-components) — Object Oriented Analysis and Design, chapter 2 - [Essential components of a DAQ system](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adaq-components) — Instrumentation II, chapter 5, asked 3× - [Armature-controlled DC motor transfer function and block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Atf-armature-dc-motor) — Control Systems, chapter 2