ANU College ofEngineering & Computer Science

Uncontrolled levels of reactive power on the grid leads to sagging (melting) wires, brownouts and ultimately, to large area blackouts.

Why? Because reactive power stays on the line reverberating back and forth the full distance from the load to the generator.

Active power doesn’t do that; it travels one direction: into the load, causing useful work. But reactive power isn’t “useless”, it’s needed to make electrical motors turn and to help power converters change DC to AC. Modern life wouldn’t be the same without it.

Wouldn’t it be better, then, to get reactive power off the main transmission lines by supplying reactive power locally? We could do that with residential, commercial, and utility grade PV systems, as long we could control the amount of reactive power the panels produce.

This is easily done using a 2-power micro-inverter, with an internet browser interface, on eachpanel. There is the extra advantage that each panel’s energy output can then be monitored independently of all the others.

My PhD topic: Examines the impact and effect of 2-power mirco-inverters on the electrical grid.

Business and Social questions:

• Do they provide an effective way for the utility to control reactive power buildup on the grid?

• Will the utilities be willing to control regional areas of PV systems via the internet?

• Will residential and commercial building owners allow the utilities access to their systems?

• Will solar farm operators accept contracts which require them to follow “real-time” utility instructions on power production?

• What is the additional cost of micro-inverters over regular string inverters? Will people pay these extra costs?

Technical questions:

• What is the mathematical and engineering theory behind the operation of micro-inverters?

• How much additional energy do they need to supply the extra reactive power?

• How are micro-inverters tied into the grid? What are the technical TUV and IEEE requirements?

• How do micro-inverters form the output waveforms to meet the stringent harmonic requirements?

• How do micro-inverters disconnect rapidly when grid connection drops out?

• How efficient are they compared with standard systems?

• Can PV systems with micro-inverters produce sufficient reactive power to provide the benefits expected?

The Standard “String” Inverter System: the grid generates the Reactive Power a long distance away.

The Micro-Inverter System: the PV system produces the Reactive Power locally.

Reactive Power weighs down the grid,

• by increasing current on the wires, thereby requiring heavier wires, larger (more expensive) generators, and more fossil fuels;

• by dropping voltage at the load below the standard 120 or 240 volts, thereby requiring costly contracts with “third party” owned and operated reactive power generators.

The standard string inverter is typically set to provide a power factor very close to 1.0. Hence all large inverters currently connected to the grid do not produce reactive power. Nothing prevents them from being modified to “float”, however, so that they produce whatever active and reactive ratio their load might require at any given moment. But if you want them to be controllable, then they need to be re-designed, since the output stages are voltage source stages, rather than current source stages. And there’s the rub.

But, the utilities don’t pay for reactive power produced by PV systems. Not yet, anyway. There are however, rules going onto the books in California, which will allow PV systems to act as ancillary “third-party” producers of power, and this will eventually allow residential and commercial PV owners to participate in benefits now enjoyed only by large solar farm owners.

2-Power Micro-inverters are designed with current controlled output stages and hence can produce any ratio of active to reactive power up to 1:1. The ratio can be controlled using an embedded micro-chip with software algorithms. Moreover, since one inverter goes on one panel, each panel operation can be optimized distinctly and separately from the string, something a combiner box with one MPPT per string cannot do.

Monitoring each panel via the internet also becomes exceedingly easy, since the chip is already on-board and has the necessary software algorithms to make energy monitoring easy and direct.

A 2-Power Micro-inverter offered by Apparent.

(www.apparent.com)

The Mathematics of Power Flow

Total power consists of two types of power:

• Active power, which is given by that portion of total power where the voltage and current are “in-phase”.

• Reactive power, which is given by that portion where the voltage and current are 90 degrees out of phase.

This is represented by a right triangle in which the base is the active power and the side is the reactive power. The total power is the hypotenuse.

As the voltage and current are taken out-of-phase by the controller (form point A to point C), the active power is constrained to remain the same. The reactive power and total power therefore increase. The micro-inverter reaches internal limits at about 45 degrees, at which time the active power decreases along the micro-inverter’s maximum power curve (from point C to point D).

The amount of energy required to produce this out-of-phase power is very small. It is the area under half the sine curve. That energy stays in the transmission line alternating back and forth; first the generator makes it, then the load makes it, and so it goes, back and forth.

By Arnold F. McKinley, College of Engineering and Computer Sciences,

Centre for Sustainable Energy Systems, E207

Advisors: Kylie Catchpole, Sudha Mokkapati, Tom White