Difference between Static Relays and Electromagnetic Relays:
The conventional electromagnetic relays are robust and quite reliable, but are required to operate under different forces under fault conditions. This leads to delicate setting small contact gaps, special bearing systems, special clutch assemblies and several measuring problems. These require instrument transformers (CTs and PTs) with high burden and are bulky in size also. The difference between static relays and electromagnetic relays have many advantages and a few limitations.
Advantages of Static Relays:
1. The power consumption in case of static relays is usually much lower than that in case of their electromechanical equivalents. Hence burden on the instrument transformers (CTs and PTs) is reduced and their accuracy is increased, possibility of use of air-gaped CTs is there, problems arising out of CT saturation are eliminated, and there is an overall reduction in the cost of CTs and PTs.
2. Quick response, long life, shockproof, fewer problems of maintenance, high reliability and a high degree of accuracy.
3. Absence of moving contacts and associated problems of arcing, contact bounce, erosion, replacement of contacts etc.
4. Quick reset action—a high reset value and absence of overshoot can be easily achieved because of the absence of mechanical inertia and thermal storage.
5. There is no effect of gravity on operation of static relays and therefore they can be installed in vessels, aircrafts etc.
6. Ease of providing amplification enables greater sensitivity to be obtained.
7. Use of printed (or integrated) circuits avoids wiring errors and facilitates rationalization of batch production.
8. The basic building blocks of semiconductor circuitry permit a greater degree of sophistication in the shaping of operating characteristics, enabling the practical realization of relays with threshold characteristics more closely approaching the ideal requirements.
9. By combining various functional circuits, several conventional relays can be substituted by a single static relay. For example a single static relay can provide overcurrent, undervoltage, single phasing, short-circuit protection in an ac motor by incorporating respective functional blocks.
10. Static relays are very compact. A single static relay can perform several functions. A single microprocessor based system can substitute several independent protection and control relay units. The space required for installation of protective relays and control relays etc., is reduced.
11. The characteristics of static relays are accurate and superior. They can be altered within certain range as per protection needs.
12. Static relays assisted by power line carrier can be employed for remote backup and network monitoring.
13. Static relays can be designed for repeated operations. This is possible because of absence of moving parts in the measuring circuits.
14. The risk of unwanted tripping is less with static relays.
15. Static relays are quite suitable for earth quake prone areas, ships, vehicles, locomotives, aeroplanes etc. This is because of high resistance to shock and vibration.
16. The static relays are provided with integrated features for self-monitoring, easy testing and servicing. Defective module can be replaced easily.
17. A static protection control and monitoring system can perform several functions such as protection, monitoring, data acquisition, measurement, memory, indication, data-communication etc.
Limitations of Static Relays:
1. Auxiliary dc supply is required. However, this drawback is not very important as auxiliary dc supply can be obtained from station battery supply and conveniently changed as per local needs.
2. Semiconductor components are sensitive to electrostatic discharges. Some components are more sensitive than others. Even small discharges can damage the components and therefore precautions are necessary in the manufacturing of static relays to avoid component failures due to electrostatic discharges.
3. Static relays are sensitive to voltage spikes or voltage transients. Special measures (such as use of filter circuits in relays, screening of cables connected to the relays etc.,) are taken to avoid such problem.
4. The characteristics of static relays are influenced by ambient temperature and ageing. However, temperature compensation can be provided by using thermistor circuits and digital measuring techniques etc., while ageing may be minimized by pre-soaking for several hours at a relatively high temperature.
5. The reliability of the system depends upon a large number of small components and their electrical connections.
6. The static relays have low short-time overload capacity compared with electromagnetic relays.
Both of the above factors [factors (5) and (6)] are the design features of the circuit and careful design of static relay circuitry can compensate for these limitations.
7. Static relays are costlier, for simple and single function, than their equivalent electromechanical counterparts. But for multi-function protection, static relays prove economical. The production technology of plug-in-type static relays on the panel permits the manufacturing of standard relays in mass and the customer’s needs can be met quickly by incorporating required relay units on the panel. Static relays with ICs are cheaper than those with discrete components.
8. Static relay characteristic is likely to be affected by the operation of the output device but this is not so in case of electromagnetic relay because its operation is based on the comparison between operating torques/forces.
9. Highly trained personnel are required for their servicing.
10. Static relays are not very robust in construction and easily affected by surrounding interference.
For simple protective schemes, conventional electromechanical relays provide economic and satisfactory choice. Electromechanical units are also used as components in total predominantly static relays such as for auxiliary relay functions, output functions.
For complex protective functions requiring accurate characteristics for various protection systems and for protection of costly large equipment or machines static relays are preferred technically and economically. These may be hardwired or programmable. As static relays perform protective and monitoring functions, the additional cost is justified on the basis of improved system stability and reliability.
For integrated protection and monitoring systems programmable microprocessor controlled static relays are preferred.