There are many types of faults that may occur in power systems, among which short-circuit faults are the most common and most serious faults in power systems. A short circuit is a path formed by the insulation damage between phase and phase or between phase and ground due to some reason. The main reason is the insulation damage of the current-carrying part of the electrical equipment. There are two types of short-circuits that may occur in the power system: symmetrical short-circuits and asymmetrical short-circuits.
Single-phase-to-ground short circuit is the most common fault, accounting for more than 80% of all faults. For the neutral point directly grounded system, when single-phase grounding occurs, it is required to quickly remove the hidden point; for the system where the neutral point is not grounded or the neutral point is grounded by the arc suppression coil, when single-phase grounding occurs, short-term live operation is allowed, but it is required to find the grounding point as soon as possible, and take the grounded part out of operation and deal with it. Two-phase grounding faults generally do not exceed 10% of the total fault probability. In neutral point grounding faults, such faults mostly occur at the same location; in the neutral point indirect grounding system, the common situation is that one point is grounded first, and then the other two phase-to-ground voltages rise, forming a second grounding point at the weak insulation. Most of these two points are not the same point. Two-phase short-circuit and three-phase short-circuit are relatively rare, generally not exceeding 5% of the total failure probability, but this kind of fault is relatively serious, and requires faster removal after the fault occurs. When the above types of faults occur, often due to the evolution and expansion of the fault, one fault may be converted to another fault, or two or more complex faults may occur, which account for less than 5% of the total fault probability.
When a transient fault occurs in the power grid, the voltage of the grid connection point of the distributed power supply may drop. Based on the grid connection requirement of not being disconnected from the grid in the event of a system-side fault, the distributed power supply needs to have low voltage ride-through capability. That is, when the voltage of the grid-connected point drops, the distributed power supply can remain connected to the grid, and even provide a certain amount of reactive power to the grid to support grid recovery until the grid returns to normal, thereby “traversing” this low-voltage time (area). Considering that disturbances (lightning strikes, equipment failures, etc.) of the power grid occur frequently during operation, it is very important for the stability of the power grid to use the low voltage ride-through capability to keep the distributed power connected to the power grid.
When the grid-connected distributed power supply is disconnected from the large grid and continues to supply power to the local load and operates independently in the event of a large grid failure, it is called island operation. For the consideration of power safety and power quality, it is necessary to quickly detect the isolated island, and take corresponding control measures for the separation system part and the isolated island, until the system fault is eliminated, and then the grid-connected operation can be resumed. Generally, the distributed power supply needs to have the function of anti-islanding protection. The islanding effect must be detected within a specified time and corresponding measures must be taken.
With the fundamental changes in the distribution network structure including distributed generation and the magnitude, flow and distribution of short-circuit current in the distribution network, various protections of the distribution network will also undergo profound changes. The uncertainty of the power flow of the distribution network will bring certain difficulties to the configuration and action setting of the relay protection and safety automatic devices of the power system, and it is very likely to cause the relay protection and safety automatic devices to malfunction or refuse to operate. In order to ensure that the distribution network with distributed power can restore power supply in a timely and intelligent manner after the fault is eliminated, the distribution network with distributed power also puts forward new requirements for relay protection and control.