ABSTRACT
What are ‘‘current relays’’ and ‘‘voltage relays’’ and what is the difference between them? These relays are specially designed for current or voltage level control in electric circuits
of high and low voltage, and for generation of certain output signals, when the current or voltage level deviates from a preassigned value. Such relays are also called ‘‘measuring relays,’’ since in the process of operation they constantly measure the level of the actuating value. Very often the output signal of such relays affects the power shutdown device, deenergizing the load and thus protecting it (or themain supply) fromdamages in emergency modes, which is why such relays are also called ‘‘protective relays.’’ Some relays of this type have powerful contacts directly switched to the protected
circuit, or a powerful electromagnet mechanically linked with the power shutdown device. Such relays are called ‘‘direct-action’’ relays (Figure 10.1a). Low-power relays, which only give control signals to an independent power-switching device (a circuit breaker, for example), are called ‘‘indirect-action’’ relays (Figure 10.1b). In the above examples the relays are connected directly to the controlled current circuit. Such relays are called ‘‘primary’’ ones. As a rule, coils of primary current and voltage relays are designed for currents not exceeding 50 to 200 A, and for voltages of not more than 400 V.
And what should be done when one needs to control currents of hundreds and thousands of amperes, or very high voltages? In this case the relays are connected not directly to the strong current or high-voltage circuit but through special matching transformers called ‘‘current transformers’’ (CT) and ‘‘voltage transformers’’ (VTs). Relays connected to the controlled circuit through such intermediate transformers are called ‘‘secondary’’ ones (Figure 10.2). What are CTs and VTs? Like any transformer, a low-voltage CT consists of a primary
winding designed for the current in the controlled circuit, a secondary winding to which relays andmeasuring devices are connected, and also a laminated steel core (Figure 10.3a). Sometimes there is no primary winding in a CT, and the transformer itself looks like a toroid (Figure 10.3b). In this case copper wire or the bus bar is used as a primary winding (it is considered to contain one turn). The primary currents of CT can reach tens of thousands amperes. Standard values for secondary currents are 5 or 1 A. It is for these
