Frequency Shift Power Control
If PV inverters are connected on the AC side in stand-alone mode, the Sunny Island must be able to limit their output power. This limitation becomes necessary when, for example, the Sunny Island inverter battery is fully charged and the PV power available from the PV system exceeds the power requirement of the connected loads.
To prevent the excess energy from overcharging the battery, the Sunny Island recognizes this situation and changes the frequency at the AC output. This frequency change is monitored by the PV inverter. As soon as the power frequency increases beyond the value specified in f Start Delta, the PV inverter limits its output power accordingly.
Function of the frequency shift power control
|
Designation |
Explanation |
|---|---|
|
f |
Base frequency of the stand-alone grid (50 Hz) |
|
f Delta- to f Delta+ |
Maximum range in relation to the base frequency in which the PV inverter is active. |
|
f Start Delta |
Frequency increase in relation to the base frequency, at which point the power regulation via frequency begins. |
|
f Limit Delta |
Frequency increase in relation to the base frequency, at which point the power regulation via frequency ends. The power of the PV inverter at this point is 0 W. |
If the frequency falls below the limit f Delta- or exceeds the limit f Delta+, the PV inverters disconnect from the stand-alone grid. If a generator is operating in the stand-alone grid, the generator determines the frequency and the PV inverters react to certain frequency changes due to the generator. With generators, the frequency of the output voltage under load is 50 Hz. For this reason, in most cases the PV inverters will feed their entire power into the stand-alone grid, even when the generator is in operation. If the current battery voltage is greater than the rated battery voltage and is also to be synchronized with a generator, the Sunny Island will temporarily increase the frequency and the PV inverters will disconnect from the stand-alone grid via frequency control (overfrequency). The Sunny Island then synchronizes with the generator.