Static Relays: Electronic and Solid-State Components

Static Relays

• Static relays use electronic, magnetic, or other components without mechanical motion to develop a response.
• Solid-state components like transistors, diodes, resistors, and capacitors are used.
• Measurement and comparison are done through static circuits.
• Relays combining static and electromagnetic units are considered static if the response is achieved by the static units.
• Electronic, magnetic, or optical components perform the measurement without mechanical motion in static relays.
• Electromechanical relay units can be employed in the output stage as auxiliary relays.
• A protective system is composed of static relays and electromechanical auxiliary relays.

Needs for Static Relays

• The increasing complexity of electrical transmission and distribution systems demands faster and more reliable protective gear.
• Experience indicates that static relays can readily meet the requirements with electronic circuit control functions.
• Transistors enable greater sensitivity and mechanical stability compared to electromechanical relays.
• Replacing existing electromechanical relays with static relays is not always economical solely for maintenance reduction.
• Initial commercial manufacturing of static relays focused on distance and differential protective schemes.
• Static relays enable varied and complex distance protection characteristics not achievable with electromechanical relays.

Essential Components of Static Relays

• A static relay includes a rectifier, measuring unit, amplifier, and output device.
• Relaying quantity (output of CT, PT, or transducer) is rectified.
• The rectified output is fed to a measuring unit with comparators, level detectors, filters, and logic circuits.
• The output is actuated when the dynamic input (relaying quantity) reaches the threshold value.
• The measuring unit's output is amplified and sent to the output unit (usually electromagnetic).
• Static circuits (comparators, level detectors, filters) perform measurements in static relays.
• Conventional electromagnetic relays measure by comparing operating torque/force with restraining torque/force.
• When a measured quantity attains a defined value, the output device is triggered, energizing the circuit breaker trip circuit.
• Static relays can respond to electrical inputs
• Other inputs (heat, light, magnetic field, travelling waves) can be converted into equivalent analogue or digital signals for static relays.
• Multi-input static relays can accept several inputs, with logic circuits determining relay response conditions and event sequence.

Comparison of Static and Electromagnetic Relays

• Conventional electromagnetic relays are robust but require operation under different forces during faults.
• Static relays generally have lower power consumption than electromechanical equivalents.
• Instrument transformers (CTs and PTs) burden is reduced, increasing their accuracy.

Advantages of Static Relays

• Reduced burden on instrument transformers (CTs and PTs) and lower overall costs.
• Quick response, long life, shockproof, fewer maintenance issues and high reliability.
• Absence of moving contacts, preventing arcing, contact bounce, erosion, and replacement.
• Absence of mechanical inertia and thermal storage enables high reset value and quick reset action.
• Insensitivity to gravity allows installation in various environments (vessels, aircrafts).
• Amplification enables greater sensitivity.
• Use of printed or integrated circuits reduces wiring errors and supports batch production.
• Ability to shape operating characteristics closely approaching ideal requirements.
• Combining various functional circuits can substitute several conventional relays with a single static relay.
• Compact design that performs several functions.
• Microprocessor based systems can substitute multiple independent protection and control relay units.
• Accurate and superior characteristics that can be altered as per protection needs.
• Use of power line carriers can enable remote backup and network monitoring.
• Design supports repeated operations.
• Risk of unwanted tripping is low.
• Suitable for earthquake-prone areas, ships, vehicles, locomotives, aeroplanes due to high shock and vibration resistance.
• Self-monitoring, easy testing, and servicing via integrated features.
• Static protection control and monitoring system can perform several functions, like protection, monitoring, data acquisition, measurement, memory, indication, and data-communication.

Limitations of Static Relays

• Requires auxiliary DC supply.
• Sensitive to electrostatic discharges and voltage spikes or voltage transients.
• Characteristics are influenced by ambient temperature and ageing.
• Reliability depends on a large number of small components and their electrical connections.
• Low short-time overload capacity compared with electromagnetic relays.
• Costlier than electromechanical relays for simple single functions, but economical for multi-function protection.
• Static relay characteristic can be affected by the operation of the output device.
• Requires highly trained personnel for servicing.
• Not very robust in construction and easily affected by surrounding interference.
• Conventional electromechanical relays provide an economic and satisfactory choice for simple protective schemes.
• Electromechanical units can be used as components in static relays for auxiliary and output functions.
• Static relays are preferred for complex protective functions needing accurate characteristics and for protection of costly equipment.

Classification of Static Relays

• Based on the type of measuring unit or comparator:
  • Electronic relays
  • Transductor or magnetic amplifier relays
  • Rectifier bridge relays
  • Transistor relays
  • Hall effect relays
  • Gauss effect relays

Electronic Relays

• Were among the first static relays developed.
• Used electronic valves for the measuring unit.
• Two basic arrangements includes an amplitude comparator and a phase comparator.
• Suffered from high tension supply requirements, short life, and large power consumption.

Transductor (Magnetic Amplifier Relays)

• Transductor includes a magnetic core with operating and control windings.
• The different groups of windings are not magnetically linked.
• Control windings are energized with DC, and operating windings with AC.
• Transductor presents a variable impedance to currents flowing through the operating windings, varied by the control winding current.
• Sensitivity as an amplitude comparator is limited, and phase comparator use depends on external AC supply.
• Transductor relays are mechanically simple and reliable.
• Due to smoothing and rectifying a signal, a delay is introduced, making the relays slow in operation.

Rectifier Bridge Relays

• Became popular with the development of semiconductor diodes.
• Consist of two rectifier bridges and a moving coil or polarized moving iron relay.
• Can be arranged as either amplitude or phase comparators.

Transistor Relays

• Are the most widely used static relays.
• Can be employed both as an amplifying and switching device.
• Can perform essential relay functions and have necessary flexibility to suit the various relay requirements
• Two basic arrangements are based on transistor comparators.

Hall Effect Relays

• Result from an electric field in a conductor perpendicular to both magnetic field and current directions.
• The voltage developed is enough for measurement with a sensitive moving coil instrument with semiconductors.
• Uses the basic Hall generator.
• Because of high cost, low output, and large temperature error, this comparator is normally not used.

Gauss Effect Relays

• A phenomenon where the resistivity of some metals and semiconductors at low temperature changes when exposed to the magnetic field.
• Involves one AC voltage developing a magnetic field through the crystal, and another AC voltage sends a current radially through the disc.
• Simpler construction and circuitry.
• Its use in static relays is limited because of high cost.

Output Devices

• The thyristor is an excellent means for tripping the circuit breaker from a low power signal of short duration.
• Operates with signals of microsecond duration, and stays in conduction until broken by a switch.
• The measuring circuit of the relay sends a pulse to the SCR gate when a threshold condition is reached.

Comparators

• A protective relay senses any abnormal condition in the system.
• Sends a signal to the breaker which isolates the faulty section of the feeder from the healthy one.
• The relay does all this by comparing two quantities.
• A device that makes such comparison is called the comparator.
• The operating characteristics of any relay are governed by the comparator.

General Equation for Comparators

• General threshold equation for two input signals - tripping is initiated when a phase or magnitude relationship fulfills predetermined threshold conditions.
• Instrument transformers derive input signals from the system.
• Mixing device, such as current voltage transactor, required if the two quantities to be compared are different.

Analysis for Amplitude Comparator

• If the operating criterion is given by | S₁|≥ | S2 |, then at the threshold of operation ISI = IS2.

Phase Comparators

• the operation of the relay takes place when the phase relation between two inputs S, and S2 varies within certain specified limits.
• Inputs must both exist for an output to occur.
• Is represented by two straight lines from the origin of the complex plane and forms a marginal operation.
• Static phase comparators are of two types viz coincidence type phase comparator and vector product phase comparator

Coincidence Type Phase Comparators

• Concept of phase comparison is simpler in that coincidence or non-coincidence is readily measurable.
• Period of coincidence of two sinusoidal signals S, and S,.
• If is the phase angle between S, and S,.