The relay protection device is a kind of automatic device that can signal timely , make the breaker trip automatically and cut off the fault path when the power grid, power plant, substation and electric equipment are in failure, short  circuit, and have grounding fault, and  they are in abnormal operation of overload and overheating, etc . 

In most cases the relay protection device is a automatic device made up of several relays. According to working principle, there are over current protection, current quick-break protection, over (owe) voltage protection, differential protection, gas protection, and single-phase grounding protection, etc. The basic requirement on relay protection is good selectivity, rapidity of reaction malfunction, sensitivity and reliability and economic efficiency of maintenance. 

Over current protection device: 

The over current protection device is kind of the most basic current protection, which is used to react the short circuit fault and the serious overload fault. Extensive protection objects: Such as line, generator, transformer and various kinds of load. The over current protection generally is provided with a duration of action, which can be divided into two kinds of fixed time limit and inverse time. 

The so-called time over current protection is that no matter how much the fault current exceeds the set values, the action time is certain. If the action time is changing inversely proportional to the fault current, namely the more the fault current exceeds the setting value, the faster the action time is, and then it is called an inverse time over current protection. The following figure is the block diagram of over current protection. In the figure, the links of start, time limit, the signal and outlet are current relays, time relays, signal relay, intermediate relay.

Over current protection: 

When there is short circuit fault in the line, the over current protection will start after the short circuit fault is detected by the current transformer and current relay. After the delay through the time relay, the intermediate relay will make the breaker trip, at the same time, the signal relay sends out the signal of lighting and sound. 

The main advantages of over current protection are simple in structure, reliable and economic, the disadvantage are that along with the increasing of number of power grid extension section , the duration of over current protection device for  the grid will be very long, sometimes  to the degree that the duration that cannot be allowed. In addition, when load current is very big, the sensitivity of protection often cannot meet the requirements. Current instantaneous (quick-break) fault protection: this is another kind of current protection. This kind of protection without delay, it is the instantaneous action. The instantaneous (quick- break) protection is usually used together with over current protection, which forms the so-called "two sections of protection". The outstanding advantages of current instantaneous protection are simple in structure, quick and reliable in action, and economic, the disadvantage is that the influence of the operation mode of the protection scope is very big, the instantaneous protection without time limit cannot protect the total length of the circuit. In order to compensate for this defect, the instantaneous protection with slight time limit is often used, namely the delay quick break protection. 

Differential protection: 

The differential protection acts based on the changes of the current of the protected area. It is widely used to protect the power transformers, substation busbar and high voltage motor with large capacity, etc. 

The area between the current transformers is the differential protective area, when the short circuit fault occurs, namely inside the transformer, the larger starting current will generate in the current relay to make the protective device act, and when the short circuit happens outside the protective area, and only a smaller unstable current flows through the current relay, the protective device will not act. 

Single-phase grounding protection： 

It is also called zero sequence protection, which uses the zero sequence current generating from grounding to make the protection device act and protects the grounding fault mainly by means of the special zero sequence current transformer to act on the relay. The single-phase grounding protection can be divided into zero sequence protection of big current grounding system and the zero sequence protection of small current grounding system. For the big current grounding fault, in fact it is single-phase short circuit, although the over current protection device can react, its sensitivity is lower, the action time is long. So it is required to provide special grounding protection device to increase the sensitivity. For small current grounding system, the single phase grounding is just a kind of abnormal running state, if this kind of fault occurs, according to the requirement, the system still can continue working for 2 hours. So, these grounding protection devices generally only act on signal.