Relays Types, Applications and Importance

Working and Advantages of Solid State Relay

August 14, 2023 Jessica

A solid-state or static relay is an electrical relay in which the response is developed by electrical/magnetic/optical or other components without mechanical movement of parts.

In the static relay, the comparison or measurement of electrical quantities is made by a fixed circuit. This circuit gives an output signal for the tripping of a circuit breaker. In general, static relays have a DC-polarized relay as a slave relay. The slave relay is an output device and does not function as a comparison or measuring device like a normal relay. It simply closes the contacts. It is preferred because of its low cost and the number of output contacts it provides. An SCR is used instead of an electromagnetic slave relay on a fully static relay. Electromagnetic multi-contact tripping arrangements are much cheaper and simpler than an equivalent group of SCR circuits.

A static or solid-state relay employs semiconductor diodes, transistors, zener diodes, SCRs, logic gates, etc., as components. Currently, ICs are used instead of transistors and are more reliable and compact.

Static relays replace all electromagnetic relays, like induction cup relays for distance and directional protection and disc relays for over-current protection (except the attracted armature relays and DC polarized relays), because these relays control can use many circuits at low costs.

The simplified block diagram of a static relay is shown in the figure below;

Exploring the Advantages of Solid State Relay

As said, in the static relay, the quantities are compared by static electronic circuits. Rectifier converts AC to DC. Under normal conditions, “I” in the relay is zero. During I₁ – I₂ flows through the relay. This flows to the transistor and pulse circuit. When there is a pulse, the transistor acts as a closed switch and opens the contacts of the circuit breaker.

How a static relay works

The output of CTs / PTs / Transducers is rectified in RECTIFIER.
The rectified output is fed into the RELAY MEASURING UNIT.
The output of the measuring unit is then amplified in AMPLIFIER
The amplified result is given to the OUTPUT DEVICE, which energizes the trip coil when the relay operates.

Microprocessor – Simple Block Diagram

With the help of logic circuits and the microprocessor, the integrated protection and control system can perform several data acquisition, processing, transmission, safety, and control functions.

Each function used a separate electro-mechanical or static unit with complex wiring.

Types of Electronic Circuits in a static protection system

Analog Circuits – For simple functions
Digital circuits – For complex functions
Hybrid Circuits – For highly complex functions

A static relay may have one or more programmable units. Such a relay is called a programmable relay of protection and is used at the Generating Station and substation levels for Backup protection, load shedding, auto reclosing of breakers, sequential trappings, etc.

The relay is used at the load dispatch center for load management, Load–frequency control, economic load dispatch, and emergency corrective measures, such as tripping or shedding the load.

Advantages and Limitations of Static Relays (SR)

Static relays (SR) offer several advantages and limitations compared to their electromechanical counterparts.

Advantages of Static Relays

The advantage when compared with Electro Magnetic Relays

Static or Solid State Relays consume less power than Electromagnetic Relays as static relays have less Burden on CTs and PTs than Electro Magnetic Relays. Static Relays’ power consumption is one mill watt. EMR consumes 2 Watts.
In static Relays, there are no moving parts, and hence, the associated problems of arcing, erosion of contacts, and replacement of connections, as in the case of EM relays, do not exist.
A static relay operates without a gravity effect and is hence suitable for use in ships, aircraft, etc. In contrast, EM relays are to be used in a vertical position and will not work under jerks and vibrations.
By incorporating the respective functional blocks, a static relay can perform several functions over current, under or over voltage, single phasing, and short-circuit protection. This is not possible in electromagnetic relays.
A static relay is very compact, whereas an EM relay is not that compact. Highly superior operating characteristics and accuracy are possible with static relays, but this is impossible with EM relays. A speed of 1/2 to 1 cycle of working time is possible with static relays, which is impossible with EM relays.
Using static relays in conduction with Transmitters, converting even non-electrical properties like temperature, pressure, etc., is possible. This is not possible with EM relays.
Static relays can think’ through its logic circuits,’ whereas this is impossible with EM relays.’
Programmable operations are possible with static relays. i.e., sequential instructions that direct the microprocessor in the relay to perform specific functions.
The effect of vibration on the static relay is more or less nil. Static relay is not affected by shocks and hence suitable for earthquake–prone areas, ships, locomotives, etc. EM relays are highly affected by vibrations and shocks.
Static relays allow for simplified testing and servicing. Defective modules can be replaced quickly. Testing of EM relays is comparatively more challenging.

Limitations of Static Relays

Electrostatic Discharges (ESD): Semiconductor components are quite sensitive to electrostatic discharges developed by rubbing of insulating components. Hence, precautions are needed to avoid ESD while manufacturing static relays.
Static relays are sensitive to voltage transients or voltage spikes caused by the operation of the circuit breaker and isolator in the primary circuit of CTs and PTs. In their secondary course, the voltage spike can reach an amplitude of 20KV even, but generally 12KV. Filter circuits and screen cables are adopted to avoid this.
Static relays are temperature-dependent. The characteristics of transistors and diodes used in a static relay change with temperature variants. To avoid this, temperature compensation is provided utilizing Thermocouple circuits.
The price of static is higher than the equivalent electromagnetic relay. But multifunctional static relays provided an economy of cost.
The total reliability of static relays depends upon the various small electronic parts.