Inverters are looked at by power utilities as “static” devices since they are not of the type classified as rotating machinery such as electric generators. The power companies require users, who put power back onto the grid, to have power factor control to compensate for quick changes in weather conditions that can affect the grid quality of power. This is especially the case with utility-scale inverters in a solar plant when a fast-moving cloud drops the power momentarily being generated by that solar plant facility onto multiple grids.
If one of those grids responds slower than another in trying to keep a 12 kV level steady within a small percentage variance, the slower grid could experience a drop as much as 5%, an unacceptable “voltage flicker”. Magnitude of the “flicker” depends upon the “stiffness” of the line which involves voltage level, distance from the substation, size of the substation transformer and electrical design of a Solar Plant or Wind turbine facility.
Flicker can be a problem because it might affect the human body in some people who are sensitive to light density fluctuations and cause lack of concentration, general feelings of discomfort and even epileptic fits. Flicker can also cause reduction in the quality of welded connections.
Flicker compensation is different than reactive power compensation. During a flicker event, power factor is not targeted as much as keeping the voltage constant during rapid changes in load. Voltage drop during a load change is split into two components: a drop in actual voltage and a drop in reactive voltage.
These components’ influence is described using the ratio of active resistance to reactive resistance of the grid impedance R/X. Capacitive reactive power will increase the voltage and inductive reactive power will decrease the voltage. (Kapp’s triangle or regulation diagram shows the effects in a real transformer like flux mismatch, power losses and finite magnetic permeability of the core that force the voltage source to deliver additional current to compensate for these effects.) See the Ideal Kapp Diagram in Figure 1 and the modified Kapp Diagram (realistic power) in Figure 2.
Figure 1: An ideal Kapp diagram
Figure 2: A more realistic Kapp diagram with losses
The goal of flicker compensation is to handle the mostly inductive, Q so that V=0. This will require a bit of overcompensation to eliminate the summand of the active power? P from the equation (making Qcomp a negative value).
The compensation power can be fed through dynamic compensation systems and/or active mains power filters depending upon how dynamic are the load fluctuations. Another way can be changes to the operating behavior of the load or increases in the short-circuit capacity which will also help decrease flicker. The short-term behavior of the load needs to be measured in each case in order to configure a good flicker compensation.
Modern utility-scale inverters of the current source type on the market today are actually beneficial to the utility grid and help to correct the power quality available on the grid while still maximizing real power generation. These inverters can operate at power factor levels less than unity and still produce 100 percent of real power. The inverter has the ability to provide reactive power based on a function of the entire size of the inverter, not just on the level of generation. So, if cloudy skies drop solar generation from 100 percent to 10%, the inverter can use the other 90% of its remaining capacity to supply reactive power support and enhance utility grid power quality.
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