The characteristics of modern Switching Transistor are specially suited to the functional requirements mentioned already. These along with the other Semiconductor production of static relays. The properties particularly useful in the realisation of the functional requirements are amplification, switching characteristics, sensitivity, high speeds of response, flexibility of design and application, long life, compact and rugged construction, and simplified power requirements. The development of miniaturisation resulting in the integrated circuit chips has accelerated the pace of development and exploitation to commercial standards of the principles of static relaying.
Practical Non-critical Switching Circuits:
The two main functions of such circuits in relation to protection are the following:
- Provision of a final signal for tripping to the circuit breaker; may also give supplementary functions like inter-tripping, alarm and visual indication.
- Acting as intermediate switching stages within the assembly of functional elements.
Both the above duties have been met in the past by using electromagnetic relays. For (i) e.m. relays have proved to be economical and reliable, of course accounting nevertheless for 10-30 ms of overall tripping time. For (ii) the operating time of e.m. relays may become unduly long, while being reliable, but with unsuitable contact performance sometimes. Conventional e.m. relays have been accepted in association with static relays for non-critical functions if their operating times are tolerable. Progressively efforts have been made to replace e.m. relays by (a) reed relays and (b) static switching circuits using transistors or thyristors.
Reed relays have been found to be reliable with high operating speeds of the order of 1-2 ms even at small multiples of setting. They can therefore be used for the intermediate switching stages giving segregation between input and output. This gives added flexibility and freedom of interconnection. Their operating power may, however, require a preceding stage of transistor amplification with positive feed-back.
Switching Transistor are also suitable for the intermediate switching functions and have been used extensively as such. The rating of trip circuits is generally unsuited to Switching Transistor and if a fully static trip circuit is required for various reasons, then thyristors may be used subject to certain restrictions like interface problems.
Practical Critical Level Detectors:
The inputs in such level detectors are generally at power frequency and may vary over a wide range relative to the critical level. Basic requirements are accuracy, long-term consistency, fast operation and a controllable reset ratio of high magnitude. Obtaining these with e.m. relays has always presented problems. When designing a static equivalent of an e.m. relay; the circuit should be such that it retains the best features of the e.m. relay, but overcomes its disadvantages. An example of such a design is shown in Fig. 1.13. Here the reset ratio is high and snap action is retained. The circuit can be made relatively insensitive to the offset transient conditions present in the fault currents. It may also be made self-energizing since it permits associated units, for which auxiliary supplies may be essential, to be normally energized. The circuit uses two basic elements, as shown in Fig. 1.13. (a), the critical level detector and the pulse integrating circuit. The level detector compares an unsmoothed rectified or an alternating signal against a d.c. datum. For peak inputs below the datum, the output is zero, but at the critical peak input there is a finite output pulse the width of which is determined by the reset ratio.
The output pulse widens with increasing input, but at the critical level the width substantially exceeds the marginal operating level of the second element. This circuit retains snap action, even when the reset ratio inherently exceeds 1.0. With the exception of the datum signals in the level detector; the design parameters are non-critical.
A practical self-energized circuit using these principles is shown in Fig. 1.13 (b). A is the measuring and switching circuit and B is the pulse integrating circuit. The rectified unsmoothed signal feeds two alternative paths ( I1 and I2). Below the critical level, the measuring and switching circuit is fully conducting so that current I2 for all practical purposes is zero. The datum Vz is derived through a zener diode ZD and is substantially d.c. at the critical level input. The loading on the input circuit up to the critical level is controlled solely by R3 which provides the voltage for the measuring and switching circuit through R1,R2. The critical level occurs when the voltage across R1 exceeds Vz by ΔV required to operate the switching circuit, which then switches to a high output impedance and diverts the current I2 into the pulse integrating circuit. The meausuring accuracy depends only on Vz and resistors R1,R2 and R3 if ΔV is small compared to Vz. Temperature compensation can also be incorporated in the circuit. At the instant of switching, the input through R1 increases to a value dependent on the input impedance of the pulse integrating circuit, relative to R3. This provides positive feed-back in the measuring circuit, which controls the instantaneous, reset level. Surge and overload protection are easily incorporated by using non-linear resistors on the input and an electrostatic screen on the interposing transformer.
An alternative simple non-transistor arrangement based on reed relays is shown in Fig. 1.14. The accuracy of the operating level of the first reed relay is important but the reset ratio is not critical. Such a device can provide operating times of 5-10 ms at 2-3 times the setting. The overall reset ratio can be about 0.95.