Solid state relay is a new type of electronic device with solid state circuitry in its internal structure. It has the advantages of small size, light weight, high reliability and long life. In the power system, the application of solid state relay is more and more extensive, has been widely used in various switchgear and automatic control devices. Solid-state relays are composed entirely of separated solid-state electronic components (such as optocouplers, transistors, SCRs, resistors, capacitors, integrated circuits, etc.) and have no contacts, so there are no problems such as oxidation or mechanical wear. Its inputs and outputs are electrically insulated from each other, while SSRs are small in size and fast in switching speed compared to ordinary electromagnetic relays. According to the different occasions of use, SSRs can be divided into two categories: AC SSRs and DC SSRs; according to the working principle, they are divided into two types: voltage type and current type; according to the working method, they can be further divided into two categories: temperature-compensated type with positive temperature coefficient and non-temperature-compensated type with negative temperature coefficient.