The concept of "current relay" is widely used in electrical engineering, and this unit itself is a mandatory component of most protective devices: automatic devices, trip devices, and the like. The design features and the principle of operation of such relays are important to know and understand not only for a novice, but also for an experienced specialist. But first of all, you need to deal with the device of this switching element, as well as with the whole variety of its types and types.
Current relay device
It is most convenient to get acquainted with the design features and the principle of operation of the current relay (TR) on its most common variety - an electromagnetic device.
Unlike induction and electronic analogs, the device of the electronic protective device allows you to visualize how it works.
Any solid state current relay contains the following required elements:
- Magnetic core (core), consisting of 2 parts and having a constant or adjustable air gap.
- Frame with a coil located on a fixed part of the core.
- A spring placed on its moving half and creating a moment of reaction when the relay is activated.
In addition to these nodes, it includes auxiliary elements that increase the functionality of the system.
Operating principle
The electromagnetic device is triggered by the electronic connection created by the passage of alternating current through the coil and causing the attraction of both halves of the core. In this seemingly simple action, some nuances are hidden:
- the spring available on the moving part counteracts the convergence of its two halves;
- It is possible to overcome its resistance only with a certain current strength in the coil;
- this value is the main indicator characterizing the operation of the current relay.
When a current appears in the coil in the core, an EMF is induced, due to which the halves are attracted, but not completely - the spring prevents them from doing this. When it reaches a certain value, the EMF becomes so large that it overcomes its resistance.
To return the system to its original position, the current in the relay will need to be reduced to a certain value, depending on the return coefficient. This indicator is associated with the design features of current and voltage relays and is individually configured for each of them. To do this, it is enough to adjust the spring tension, which can be done independently.
Purpose and connection methods
TR is the main component of all protective devices installed in power circuits. Based on this, it is necessary to consider the features of the application of the device.
Its main purpose is to serve as an executive element in the composition of circuit breakers, residual current circuit breakers and many similar devices. In accordance with this, the scope of their application in conjunction with the above devices is determined.
- Power circuits of high-voltage lines and their protective equipment.
- Switching switchboards, in which TRs are included separately or as part of other devices.
- Household single-phase inputs and distribution (linear) devices installed within the home shields.
In accordance with the purpose of the switching devices, schemes for their inclusion are selected.
Several methods can be used to connect the relay circuit breaker to existing mains or other circuits.They differ in the type of equipment to be protected:
- three-phase asynchronous motors;
- consumers included in 380 Volt power networks;
- loads connected at the output of circuits with a supply voltage of 220 volts.
In accordance with the first of these paragraphs, TPs are used as electronic trip units that disconnect the circuit when the operating currents exceed an acceptable level. When installed in three-phase circuits, they perform the same function, but in a wider range of functionality. As releases, they are part of powerful contactor devices and electronic starters.
Relays installed in introductory (linear) machines and RCDs have a slightly different purpose. Here they fulfill the function of sensitive elements that provide triggering by current cutoff (setpoint). When turned on, they are tuned to extreme operating modes such as overcurrent, short circuit, and leakage.
According to the terminology accepted in electrical engineering, in the first two cases they are logically positioned as a maximum current relay.
In the protective circuits of electric motors, along with disconnecting relays, bi-stable thermal elements are installed on bipolar springs. They provide some delay, which allows not to remove the power supply from the windings in starting conditions.
Types of TR
All known current relay samples are classified according to the following criteria:
- according to the installation method (connection diagram);
- for its intended purpose;
- on execution (modification).
In accordance with the first of these features, existing TP models are divided into direct mounting devices and indirect switching devices (via current transformers). By design, they are divided into built-in devices and executed as a separate module installed on a DYN rail.
As intended, they are produced in the form of products used for the following purposes:
- protection against single-phase short circuits;
- reverse sequence current limitation;
- as a differential protection;
- in the form of remotely controlled independent modules.
For direct and indirect inclusion
Devices intended for direct connection, according to the instructions for use, are installed in the network with an effective voltage of up to 1000 volts and with a limited current value. With its significant amplitude, inclusion in a circuit break is unacceptable, since the relay is not designed for power operation. In this case, a current transformer will be required, allowing to reduce the value of the controlled value by several times. In three-phase networks, such relays are installed in each phase in series with an already connected load.
With this circuit solution, the system operates in a mode close to a short circuit dangerous for operation.
If you need to dismantle the relay, the current transformer may be damaged, and there is a danger to personnel working on the line. Therefore, before operational switching in such circuits, a jumper must be put in place of the device. Another option is to completely disconnect the network and put the equipment into the overhaul mode.
Differential protection and current limitation
The operation of current relays as part of RCDs and circuit breakers is a classic example of the implementation of their features. In this case, they operate in the usual modes for electrical systems associated with responding to the smallest current leakage (RCD) and tripping during overloads in the circuits. The latter function is classified as current limiting, eliminating the failure of the connected equipment and the supply circuit itself.
Modern types of current relays
The "advanced" types of voltage and current relays are known, which according to their capabilities are usually classified as intellectual samples of control equipment.Such devices have a number of auxiliary options that significantly expand their functionality. This is a display by which you can verify the operability of the device, as well as read information on the values of voltage and current (they are displayed on the built-in indicator of the device).
All described features relate to the advantages of current relays. Their disadvantages are determined for each specific type of inclusion separately.
