Island operating conditions

Island operating conditions

The general standard stipulates that the distributed grid-connected power generation system must stop outputting power within 2s when the island appears, which affects the utilization rate of the power generation system to a certain extent. With the expansion of distributed generation (DG) scale and capacity and the increase in power generation after DG is connected to the grid, in order to maximize the use of DG’s power generation capacity and improve the power supply reliability of the system, IEEE issued a new set of standards IEEE 1547-2003 to solve the islanding problem. The standard no longer prohibits the existence of conscious islands, but encourages power suppliers and users to continue to operate distributed power supply under island conditions through technical means as much as possible. Therefore, the islanding operation control is to convert the original grid-connected power generation into an independent power generation mode through various control methods when the grid-connected DG is in an island state, and automatically re-connect to the grid when the grid returns to normal.

The premise of the transformation of grid-connected DG into island operation should be the abnormal tripping of the circuit breaker on the system side of the power supply line, and the substation connected to the system power supply and distributed power supply should be separated into two parts without electrical connection. According to different operation modes, it can be divided into short-time power outages to form islands and uninterrupted power supply to form islands.

(1) A mode in which the power supply is cut off for a short time to form an island. It means to quickly disconnect the access switch between the DG and the bus when it is judged that the tie line between the substation and the system connected to the power grid by the DG is faulty. If the system side fault is an instantaneous fault, after the reclosing action is successful, the DG will be reconnected to the grid for operation. If the fault on the system side is a permanent fault, after manually disconnecting the tie line access switch and all load switches, connect the DG to the corresponding no-load bus, and then according to the capacity of DG and its adjustment ability, under the condition that DG can operate stably after forming an island, the corresponding load switches are put into operation one by one. This mode is relatively simple and easy to implement.

When the capacity of DG is quite different from the load capacity, if the island operation mode with short-time interruption is adopted, the conditions that need to be met from the perspective of power supply and load are as follows:

1) The distributed power supply must have a certain adjustment ability and have an appropriate capacity margin, so as to ensure the static stability of the island operation.

2) The sequence of load input should be determined according to the ability of DG capacity to withstand disturbances and the nature of the load. When the load is input, the load with the larger starting current should be input as far as possible, and then the load with the smaller starting current should be input. For loads with a large starting current that may exceed the DG’s regulation capability range, it should be blocked.

3) Under the condition that the distributed power supply has an appropriate adjustment capacity, the load can be put in as much as possible.

(2) Uninterrupted power supply forms an island mode. It means that when judging that the system tie line is faulty, the DG is not disconnected, but the tie line access switch is quickly jumped off, and the power information of the system side and each load before the fault, as well as the rated power and adjustment capacity of each DG. When jumping off the tie line access switch, judge and decide whether to cut off part of the load, so that the DG directly enters the mode of island operation with all or part of the load. This mode can uninterruptedly supply power to all or some users in the isolated island, improve the reliability of power supply, reduce the requirement for system backup, and benefit the grid company, distributed power owners and users. However, in order to ensure that the isolated island can quickly achieve stable operation after decoupling, the capacity of DG should be large enough (close to or greater than the total load capacity), and it should have good adjustment ability.

For DG to form an island of continuous power supply after a system failure, the conditions that need to be met are:

1) The time for DG to undergo disturbance after system failure (the time for DG to run without instability) is greater than the action time of the decoupling point. In order to ensure the transient stability when the island is formed, the DG can also run without instability when part of the load is removed at the same time as the decoupling switch action.

2) When real-time load shedding is required in the process of island formation, to be able to remove part of the load according to the stable performance of the DG and the actual power and load properties in the island, and the DG must have enough spare capacity to ensure the static stability of the island after load shedding.

3) After the island runs stably, if the capacity of the DG allows, it can selectively re-input part of the load. At this time, a certain spare capacity should also be reserved so that the island can operate stably when the load changes.

When there are multiple DGs in an island, in order to ensure the transient stability when the island is formed and the static stability when the island is running, the DG with the largest capacity should have better adjustment ability, and it is best to use biomass energy, waste-to-energy and other steam turbine generator sets. If all DGs are wind turbines or photovoltaic power generation systems connected to the grid through power electronic inverters, it is difficult to maintain the stable operation of the system in the presence of large disturbances due to their own characteristics.