Electrical Connection Winter 2015

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E L E CT R I C AL CONNE CT I ON

W I NT E R 2 01 5

and T3 is on for the negative half-cycle. T1

and T4 switch at high frequency.

The freewheel path during the negative

half-cycle is via T3 and D6, and conversely

through D5 and T2 during positive

half-cycles.

The capacitive divider provides an

almost constant common mode voltage,

thus reducing the possibility of PV leakage

current.

There are many more topologies,

but manufacturers don’t provide much

information on this subject.

EFFICIENCY CONSIDERATIONS

Inverter efficiency is determined by power

loss in the switching transistors and diodes.

The fewer switching components the

better off you are – but, as with all rules, the

proof of the pudding is in the eating.

Thus the full H (4b) inverter has the

highest losses, whereas H5, HERIC and NPC

are substantially lower and close to one

another.

How important is efficiency? Based on

a 20-year life, the opportunity cost of one

percentage point could be in the order of

$16,000. This is in some ways simplistic, the

assumptions being a system with average

power of 2kW over six hours per day and at

18 cents per kW/h.

Inverter efficiency is a matter of weighting

the efficiency levels at various percentage

loadings so as to provide a balanced picture.

There are two accepted ways of specifying

inverter efficiency: the European weighting

and the California Energy Commission (CEC)

weighting. The formulas are shown below.

California Energy Commision (CEC)

weighted efficiency:

CEC

= 0.04.ŋ

10%

+ 0.05.ŋ

20%

+ 0.12.ŋ

30%

+ 0.12.ŋ

50%

+ 0.53.ŋ

75%

+ 0.05.ŋ

100%

European weighted efficiency:

= 0.03.ŋ

+ 0.06.ŋ

10%

+ 0.13.ŋ

20%

+ 0.10.ŋ

30%

+ 0.48.ŋ

50%

+ 0.20.ŋ

100%

Percentages shown in subscript are of

the inverter’s rated output. In practice,

factors such as dirt and grime build-up,

overshadowing, localised heating of the

panels, etc, are likely to swamp energy

efficiency considerations of only the inverter.

POWER FACTOR (IF NOT UNITY)

Before letting go of freewheel current, we

need to look at power factor.

Although there are exceptions, inverters

usually operate at unity power factor for

the obvious reason that they are controlled

in essence by grid voltage. Thus the

inverter switches current in phase with the

grid voltage.

So what happens to loads requiring reactive

current? Well, that’s supplied by the grid.

Think for a moment about an inverter

as used in a variable-frequency drive

supplying an induction motor. The reactive

current of the motor is fed back to the

DC link via the diodes, but that is for an

‘isolated system’.

Power factor is becoming very important.

As explained above, a grid-connected

inverter is a unity power factor device. From

October 1, 2015, Energex in Queensland will

make it mandatory that every solar power

inverter greater than 3kW in size has reactive

power control set to 0.9 lagging.

This has been introduced to minimise

over-voltages on the electricity network. It

will also reduce nuisance tripping of solar

power inverters and interference to other

customers in the local area.

The Energex requirement will mean

an increase in the rating of inverters (3kW

would become 3.33kVA).

Power factor, like spinning reserve, is

an increasing headache for generation,

transmission and distribution people.

Percentage-wise, they have to supply

more and more reactive load as rooftop

solar pushes more and more kilowatts into

distribution system.

When connected to reactive loads,

isolated inverters ‘automatically’ cope with

them, as does the inverter for a motor drive.

Grid-connected inverters can have less

than unity power factor, and some (see

Figure 4d.

Figure 4a.

Figure 4b.

Figure 4c.

FILTER

G-PV

FILTER

G-PV

FILTER

G-PV