The current fault is proportional to short-circuit power of the grid and inverse to short-circuit impedance seen by the fault. You can calculate this impedance determining the Thévenin's impedance viewed from the fault. When you are connected to a electrical grid the short-circuit impedance is very low and current fault is high. In islanded mode, when you are connected to a battery through a power converter, the power converter could react against a short-circuit in different forms. For example, the protection algorithm of this power converter could limit the current to a maximum, in this case, this would generate constant current to the short-circuit. If this power converter is operated as a current source generating constant current, the sort-circuit impedance will be infinite and short-circuit power will be zero and so, current fault very low from the point of view of a protection system. In this case, there are no contribution to remove any fault using classical protection devices. If you have diesel generators connected in islanded mode, the short-circuit power of this machines is also very low if you compare with the electrical grid and you have to take into account this for designing the protection system.

In systems with fault current very closed to nominal current, the protection system has to be revisited, for example, you have to consider new sort-circuit protections by measuring current and under-voltages at the same time or something like that.

The current fault is proportional to short-circuit power of the grid and inverse to short-circuit impedance seen by the fault. You can calculate this impedance determining the Thévenin's impedance viewed from the fault. When you are connected to a electrical grid the short-circuit impedance is very low and current fault is high. In islanded mode, when you are connected to a battery through a power converter, the power converter could react against a short-circuit in different forms. For example, the protection algorithm of this power converter could limit the current to a maximum, in this case, this would generate constant current to the short-circuit. If this power converter is operated as a current source generating constant current, the sort-circuit impedance will be infinite and short-circuit power will be zero and so, current fault very low from the point of view of a protection system. In this case, there are no contribution to remove any fault using classical protection devices. If you have diesel generators connected in islanded mode, the short-circuit power of this machines is also very low if you compare with the electrical grid and you have to take into account this for designing the protection system.

In systems with fault current very closed to nominal current, the protection system has to be revisited, for example, you have to consider new sort-circuit protections by measuring current and under-voltages at the same time or something like that.

Additionally, taking into account the MVA method for power fault analysis, the short circuit power is equal to -> Ssc=Yth. A huge system have more thevenin admittance than a small system have, seen from the fault point. Thus, the more Ssc more fault current.

Hello Eduardo,

Thank You so much for sharing and explaining it so well. However, it appears that since the current magnitudes for both the modes are different, its a challenge to design the protection system for a micro grid.

Hello Odair,

Thanks a lot for explaining, which also goes to state as why its not possible to have higher fault magnitude in a Micro grid operating in an Islanded mode.

Just remember, roughly, because the ratio X/R > 4.

In distribution systems, the protection is often realized with overcurrent relays. The fault location is usually found by visual inspection or by using fault passage indicators. But, these methods can be slow. Further both protection and fault location assume generally a unique path for the fault current, which is no more true with an increased penetration of distributed generation in the network.

Microgrids are special distribution systems that can island from the main supply (e.g. transmission network) in case of a major disturbance, for instance a blackout. Therefore, the fault current is expected to be very different in islanded than in grid-connected mode. Hence there is a need for the study of the protection and fault location in such networks.

since that equivalent Impedance in microgrid copmared with large power system is very small,the protection devises can not identify this fault current and lack protection for this condition leading to system damaged.

Dear Mr. Mehdi ,

Thanks a lot for your response and explanation.

Warm wishes