Relay protection is generally used in generating, transforming, and power distribution places. In fact, the earliest relay protection is a fuse. This kind of protection device that relies on self-heating to disconnect the circuit is often seen so far. But the fuse can only protect the over-current, and is incapable of other faults. Later, there was a relay protection device that controlled the circuit breaker to open and close. The earliest relay protection device was an electromagnetic type, and later developed into an electronic integrated relay protection device. Up to now, the more advanced remote control technology and information Microcomputer-based relay protection device with computer technology in technology combination.
Let us first talk about the concept of relay protection. We hope that the power network will not fail. However, with our current human technology level, the devices we make are not up to the point where it does not fail. Then we are the second best, hoping to notify us in case of abnormal operation, and automatically remove this part of the failure when a failure occurs, so as to ensure the normal operation of other intact equipment.
Our common abnormal operating conditions include: overload, phase loss operation, synchronous generator asynchronous, overvoltage, etc.
And our common faults are: ground fault, short circuit fault, disconnection fault and so on.
In order to ensure the normal operation of the power system, we have some requirements for relay protection devices, generally from four aspects:
First, selectivity. This means that the relay protection device can have selective actions. When the power system fails, the relay protection device can selectively disconnect the faulty part to ensure the continued operation of the normal part. Generally speaking, if the relay protection device cannot correctly select the fault location and branch in the power system, and automatically disconnect the faulty part according to the requirements, then the relay protection device is meaningless.
Second, quick mobility. This means that the relay protection device is required to act quickly. If a short-circuit fault occurs, if the faulty circuit can be removed in the early stage of the short-circuit, the short-circuit current will not rise much, and the impact on the primary circuit may be small. However, if the relay protection device cannot act quickly, and the short-circuit current rises rapidly, it may cause a fire in the primary circuit or burn the primary circuit. This recovery is not only expensive and time-consuming, but also affects the normal operation of the lower-level loop.
When the power generation capacity of the power generation side cannot keep up with the power consumption of the power consumption side, the voltage obtained by the user will decrease and the frequency will decrease. This will affect the malfunction of many electrical equipment. At this time, the relay protection device needs to act quickly to remove the unimportant load, reduce the load of the grid, and make the power generation-electricity consumption tend to be balanced. If the voltage drops and the frequency drops for too long, equipment such as motors may be burned.
Third, sensitivity. As for the relay protection device, as long as the fault is in its protection range, no matter where the fault occurs, the type of the fault, and the operation mode of the power grid, the relay protection device should be able to operate correctly.
Fourth, reliability. This is best understood, that is, when a fault occurs and it needs to be actuated, don't fail to act for its own reasons. When it shouldn't, don't make a mistake.