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Differential relays, also known as differential protection relays, are used in the relay protection of generators, motors, transformers and busbars in power systems. By comparing the currents at both ends of the protected equipment, when normal, the currents at both ends enter and exit They cancel each other out, and when a fault such as a short circuit occurs inside, it will flow into the inside at the same time, start the relay, and the outlet will trip the switch to protect the generator, motor, and transformer.
As a protective device, a differential relay is fed back by current transformers located at two different locations in the system. The differential relay compares the currents and if there is a difference, there is a fault in the protected area. These devices are often used to protect the coils of generators or transformers.
Transformer differential protection is the main protection of the transformer. It is mainly used to protect various phase-to-phase short-circuit faults that occur inside the double-winding or three-winding transformer windings and their lead-out lines. It can also be used to protect single-phase inter-turn short-circuit faults of the transformer.
Differential protection works by using Kirchhoff's current theorem. When the transformer works normally or has an external fault, it is regarded as an ideal transformer, and the current flowing into the transformer and the outgoing current (converted current) are equal. The differential relay has no action. When the transformer fails internally, both sides (or three sides) provide short-circuit current to the fault point, the sum of the secondary current felt by the differential protection is proportional to the fault point current, and the differential relay operates.
1. Industrial control relay
In industrial control, the main control functions are completed by general-purpose AC relays. Typically a pushbutton or limit switch drives the relay. The contacts of the relay control the solenoid valve, the larger starter motor, and the indicator lights. The voltage is generally 24VDC, 220VAC.
2. Automotive field
The automotive industry is also increasingly using relays. The more common relays are: starting motor starting relay, horn relay, motor or generator circuit breaker relay, charging voltage and current regulation relay, transition signal flashing relay, lighting brightness control relay and air conditioning control relay, automatic opening and closing control of sliding doors Relay; glass window lift control relay.
3. Household appliances
In the field of household appliances, relays are used in washing machines, microwave ovens, air conditioners, electric heaters, etc.
1. High precision: the differential circuit adopts dual-channel amplification, which is beneficial to suppress system failures and faults obstacles to ensure the accuracy of the system;
2. Wide range: the amplification gain of the differential loop can be adjusted, so that the range of the sensor can reach large volume;
3. Good stability: the differential circuit adopts dual amplification, which can effectively suppress the loss of the sensor. spirit and failure, thereby improving the stability of the system;
4. High reliability: the differential circuit adopts two-way amplification, which can effectively suppress the failure of the sensor Spirits and failures, thereby improving system reliability;
5. Wide range of applications: differential circuits can be applied to temperature, displacement, pressure, speed, etc.
Differential protection is protected by the principle of addition (fault) and subtraction (no fault) of current within the protection range. Differential protection works by using Kirchhoff's current theorem. When the transformer works normally or has an external fault, it is regarded as an ideal transformer, and the current flowing into the transformer is equal to the outgoing current, and the differential relay does not operate. Distance protection is to use voltage and current to measure impedance, convert it into distance, and judge whether it is a fault in the area, so as to carry out protection. The action behavior of the distance protection point reflects the distance from the protection installation to the short-circuit point. Compared with current protection and voltage protection, the performance of distance protection is less affected by the operation mode of the system.
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