Photovoltaic inverters

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Photovoltaic inverters differ from inverters for other applications. This is because they must operate the photovoltaic generator at the optimal operating point to generate the maximum power. Grid-connected inverters are therefore often combined with MPP trackers, i.e. DC-DC converters, to set a voltage at the generator that differs from the grid voltage. A battery charge controller is integrated into most stand-alone inverters.

Photovoltaic inverters operate at the rated power PN for only very few hours in any year. Due to the changing solar irradiance, the inverters predominantly operate under part load. Therefore, it is very important that photovoltaic inverters have high efficiencies even when operating under part-load conditions. Furthermore, the inverter should never be oversized. For instance, if a 1-kW inverter operates with a 500-Wp photovoltaic generator, the input power reaches at most 50 per cent of the rated power, and losses due to permanent part-load operation may be very high. Furthermore, the self-consumption of a photovoltaic inverter should be minimal and the inverter should be switched off at night.

Figure 4.58 shows the efficiency characteristics over the range of relative input powers for two commercial inverters. The inverter with the higher rated power has a higher efficiency; however, both inverters show good part-load behaviour even for input power below 10 per cent of the rated value.

A representative efficiency is used to compare different inverters, the so-called Euro efficiency ne, which is given as:

100%

100%

0% 10% 20% 30% 40% 50 % 60% 70% 80% 90% 100% Relative power PIPN

0% 10% 20% 30% 40% 50 % 60% 70% 80% 90% 100% Relative power PIPN

Figure 4.58 Efficiency over a Range of Relative Photovoltaic Generator Powers

It considers the typical part-load operation for irradiation regimes in central Europe.

The ideal energy yield E^^ of a photovoltaic system with photovoltaic area APV, module efficiency nPV and solar irradiation HSokr is given by:

Assuming a 10 m2 inclined area with an annual solar irradiation of 1100 kWh/m2, modules with an efficiency of 10 per cent could generate 1100 kWh per year in the ideal case. In reality, the energy yield of the photovoltaic system is much lower. The average real efficiency of solar modules is lower than the nominal efficiency due to soiling, shading and elevated operating temperatures. Inverter losses reduce the yield further. The so-called performance ratio PR describes the ratio of the real and ideal energy:

Good systems have PRs of more than 0.75 = 75 per cent. This value can be used for designing a new system if no shading losses are to be considered. Very good systems can even reach PRs that are higher than 0.8. Problematic systems can have PRs below 0.6. Inverter failures, MPP tracking problems, module failure or solar generator shading are the main reasons for low PR values.

Figure 4.59 shows the connection of solar modules to a central inverter. A number of modules are connected in series to form a string until the desired voltage is reached. Several identical strings can be connected in parallel to increase the power of the generator. Blocking diodes that were often used in older systems can be omitted because their protection capability is low and they cause permanent forward losses.

Figure 4.59 Photovoltaic System with Parallel Strings and Central Inverter

For partly shaded systems or modules with different power outputs, this connection scheme can cause relatively high losses. Therefore, string inverters with decoupled parallel strings can reduce the losses. Their nominal power is in the range of 1 kW. However, module inverters promise the best performance for partially shaded photovoltaic generators. They can set a different operating voltage for each module. The cabling is simpler because no DC cables are needed. Another advantage is the system modularity. However, the disadvantages of module inverters are lower nominal efficiencies and higher specific costs. Figure 4.60 shows both versions: string and module inverters.

Finally, Table 4.12 shows technical data for several inverters. It is apparent that the efficiency increases slightly with the rated power. The harmonic distortion of available inverters is between 1.5 and 5 per cent. Most of the inverters are designed using the PWM concept.

Modulwechselrichter
Figure 4.60 Photovoltaic Generator with String Inverters (left) and Module Inverters (right)
Table 4.12 Technical Data for Photovoltaic Inverters

Device

Dorfm├╝ller

Fronius

SMA

ACE

Siemens

DMI150/35

Sunrise

SunnyBoy

5001

Ss 4 x 300 kVA

Mirco

2000

Rated DC power

100 W

820 W

1900 W

50 kW

1086 kW

Rated AC power

90 VA

750 VA

1800 VA

50 kVA

1197 kVA

Maximum PV power

150 W

1100 W

2600 W

55 kW

1400 kW

Start of feed-in (W)

2.5

9

7

50

700

Stand-by demand (W)

0

5

7

35

40

Night consumption

0

0

0,1

0

30

DC MPP range (V)

28-50

120-300

125-500

300-420

460-730

Maximum distortion

3%

5%

4%

0.1%

act. Filter

Maximum efficiency (%)

89

92

96

97

97

Euro efficiency (%)

86.6

90.4

95.2

94.2

96.2

Source: data from Photon International, 2001

Source: data from Photon International, 2001

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