In this article we will discuss about:- 1. Meaning of Static Relays 2. Need for Static Relays 3. Essential Components 4. Advantages 5. Limitations.
Contents:
- Meaning of Static Relays
- Need for Static Relays
- Essential Components of Static Relays
- Advantages of Static Relays
- Limitations of Static Relays
1. Meaning of Static Relays:
A static relay refers to a relay in which there is no armature or other moving element and response is developed by electronic, magnetic or other components without mechanical motion. The solid-state components used are transistors, diodes, resistors, capacitors and so on. The functions of comparison and measurement are accomplished by static circuits.
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A relay using combination of both static and electromagnetic units is also called a static relay provided that the response is accomplished by static units.
In static relays, the measurement is performed by electronic, magnetic, optical or other components without mechanical motion. However, additional electromechanical relay units may be employed in output stage as auxiliary relays. A protective system is formed by static relays and electromechanical auxiliary 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 static relays in comparison to the corresponding electromagnetic relays have many advantages and a few limitations.
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2. Need for Static Relays:
With the rapid growth of electrical transmission and distribution systems during last forty years and with the advent of much larger power stations and interconnected systems, the duty imposed upon protective gear became more and more severe. Thus relay’s functions became more and more complicated and many types of relays tend to become very complex mechanically and hence costly and difficult to test and maintain.
The basis of the so called static relaying is the use of the circuits and components to obtain a variety of functions and operating characteristics which for protection purposes have traditionally been obtained using electromechanical devices. The need of fast and reliable protective schemes was realized because short-circuit levels, circuit ratings and complexity of interconnection have increased. Shorter operating times have become more essential for preserving dynamic stability of the system as the character and loading approach design limits.
The satisfaction of these requirements has left little scope for further improvements in conventional electromechanical relays. Experience shows that such requirements can readily be met by using static relays, which are capable of performing electronic circuit control functions in a manner similar to that of an electromagnetic relay without using moving parts or elements.
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The transistors have made it possible to achieve greater sensitivity and simultaneously excellent mechanical stability which would have never been possible with electromechanical relays. The noteworthy point is that it is usually not economical to replace existing electromechanical relays with their static counterparts just to reduce maintenance.
It is interesting to note that the static relays have first been commercially manufactured for the distance and differential protective schemes while the much simpler overcurrent relays have not been developed. This is because the distance and differential schemes are more amenable to mathematical analysis whereas the overcurrent characteristics are more of empirical nature. With the use of static relays it has been possible to achieve many varied and complex distance protection characteristics which are impossible with the conventional electromechanical relays.
3. Essential Components of Static Relays:
The relaying quantity i.e., the output of a CT or PT or a transducer is rectified by a rectifier. The rectified output is supplied to a measuring unit comprising of comparators, level detectors, filters, logic circuits. The output is actuated when the dynamic input (i.e., the relaying quantity) attains the threshold value. This output of the measuring unit is amplified by amplifier and fed to the output unit device, which is usually an electromagnetic one. The output unit energizes the trip coil only when relay operates.
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In a static relay the measurement is carried out by static circuits consisting of comparators, level detectors, filters etc., while in a conventional electromagnetic relay it is done by comparing operating torque (or force) with restraining torque (or force). In individual relays there is a wide variation. The relaying quantity such as voltage/current is rectified and measured. When the quantity under measurement attains certain well defined value, the output device is triggered and thereby the circuit breaker trip circuit is energized.
Static relays can be arranged to respond electrical inputs. The other types of inputs such as heat, light, magnetic field, travelling waves etc., can be suitably converted into equivalent analogue or digital signals and then supplied to the static relay. A multi-input static relay can accept several inputs. The logic circuit in the multi-input digital static relay can determine the conditions for relay response and sequence of events in the response.
4. 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.
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2. Quick response, long life, shock proof, 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, under-voltage, 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 earthquake 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.
5. 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 minimised 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 hard-wired 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.