What is Anti islanding protection in inverter of photovoltaic electricity

Question

What is Anti islanding protection in inverter of photovoltaic electricity

Answer 1

This is a good question about a popular topic.

First, it is important to understand a few things about the ac power grid, how it works, and how it protects itself when faults happen.

Traditionally, the grid was an interconnection of large generators operated by utilities, who had plenty of engineers, operations, and maintenance personnel.  Basically, you could say that all parts of the grid - down to point that it connects to your home or office - were owned and operated by technically skilled people.

"Back then" it was understood that each generator connected to the grid would receive a full engineering review to make sure that it responded as intended when a fault (like a lightning strike) occurred on the grid.

Enter the 'modern' times, in which generators can be solar inverters, wind generators, industrial co-generation, etc.  It was determined by the electric power engineering community that 'small' generators should be required to disconnect from the grid if a fault out on the grid occurs.  This would keep the small generators from 'back-feeding' into a fault and creating an even more hazardous situation.  It also requires that the small generators not operate to serve load in an 'island' separated from the rest of the grid.

In reality, intentional islands are allowed, if the appropriate steps are taken to be sure the grid itself is not being back-energized.  But, unintentional islands are absolutely forbidden.  (They are bad bad juju for the grid.)

Most inverters detect the islanding condition by looking for some combination of the following:

1.  A sudden change in system frequency.

2.  A sudden change in voltage magnitude.

3.  A sudden change in the df/dt (rate of change of frequency).

4.  A sudden increase in active output power (kW) well beyond the expected 'normal' level.

5.  A sudden change in reactive output power (kVAR) well beyond an expected 'normal' level.

Depending on their internal control programming, one or any of these events could indicate that the small generator and some amount of load have become disconnected from the grid.  (For example, it could be just your home with its PV system, or it could be your whole neighborhood disconnected as a block from the larger grid.)

When this island condition is detected, the small generation sources trip offline.

There are specific performance requirements for distributed generation defined in IEEE Std 1547, which cover the time in which anti-islanding protection should operate, and what levels of parameters define abnormal conditions.

As for the internals of anti-islanding protection in inverters, they specifically use a phase locked loop (PLL) with a small amount of positive feedback in its control loop to quickly and continuously check the grid connectivity.  Look for 'anti-islanding' protection papers from Sandia National Labs.