Renewable energy power generation systems
We describe a modular adjustable power factor renewable energy inverter system. The system comprises a plurality of inverter modules having a switched capacitor across its ac power output, a power measurement system coupled to a communication interface, and a power factor controller to control switching of the capacitor. A system controller receives power data from each inverter module, sums the net level of ac power from each inverter, determines a number of said capacitors to switch based on the sum, and sends control data to an appropriate number of the inverter modules to switch the determined number of capacitors into/out of said parallel connection across their respective ac power outputs.
1. A method comprising:
receiving power data from each of a plurality of inverter modules, each inverter module having a dc power input and an ac power output coupled to a common grid power feed;
tallying a net level of ac power from each of said inverter modules to determine a total net level of ac power provided by said plurality of inverter modules to said common grid power feed;
determining a number of capacitors for each of the plurality of inverter modules to switch into/out of a parallel connection across its respective said ac power outputs based on said total net level of ac power; and
controlling each of said inverter modules to switch the determined number of capacitors into/out of said parallel connections of the inverter module.
2. The method of claim 1 , wherein said inverter system has a maximum rated ac power output from said common grid power feed, the method further comprising determining a fraction of said maximum rated ac power output represented by said total net level of ac power provided by said plurality of inverter modules, and to determine said number of said capacitors responsive to said determined fraction.
3. The method of claim 2 further comprising applying a threshold to said determined fraction such that when said determined fraction is less than a threshold fraction, none of said capacitors are switched into the parallel connection across their respective said ac power outputs.
4. The method of claim 3 , wherein, as said determined fraction increases from said threshold fraction towards unity, both said determined number of capacitors and a number of said inverter modules controlled to switch in said capacitors increase monotonically.
5. The method of claim 4 , wherein said monotonic increase approximates a linear increase in a displacement power factor of said inverter system.
6. The method of claim 1 further comprising using phase control to switch said capacitor into/out of said parallel connection to control a proportion of an ac cycle for which the capacitor is switched-in in order to provide a variable degree of power factor compensation.
7. The method of claim 1 further comprising using a lookup table to look up power factor compensation data defining said number of capacitors to switch into/out of said parallel connection dependent on said total net level of ac power.
8. The method of claim 1 further comprising measuring a sensed current provided by an inverter module coupled to said common grid power feed.
9. The method of claim 1 , wherein said controlling each of said inverter modules to switch the determined number of capacitors employs open loop control of a displacement power factor of said inverter system.
10. The method of claim 1 , wherein said common grid power feed is a three phase ac power feed, wherein the plurality of inverter modules comprises three sets of said inverter modules, one coupled to each phase of said three phase ac power feed.
11. The method of claim 10 , wherein said controlling each of said inverter modules to switch the determined number of capacitors comprises separately controlling a power factor of each phase by receiving said power data, determining said number of capacitors, and sending said control data separately for each set of said three sets of inverter modules.
12. The method of claim 1 , wherein said controlling each of said inverter modules to switch the determined number of capacitors comprises controlling a switching timing of each controllable switching device such that said switching device switches on at substantially a peak voltage point of said ac power.
13. The method of claim 1 , wherein said inverter modules are solar photovoltaic inverter modules, wherein said dc power input receives power derived from a renewable energy source.