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INVERTER WITH MULTIPLE MPP TRACKERS: REQUIREMENTS AND STATE OF THE ART SOLUTIONS

ABSTRACT: For most people inverters with multiple MPP Trackers (MPPT) seem to be more flexible compared to inverters with a single MPPT. At first glance this assumption appears to be correct, however, the devil is in the detail. The purpose of this project is to show that not every inverter with multiple MPPT’s is automatically usable for each application. There are high requirements on such an inverter which must be fulfilled to be as flexible as always assumed.

1 Introduction

Inverters with multiple MPP Trackers should be usable for different applications. Firstly, perfect system configuration must be possible. That means each number of PV modules – which results from different system sizes – must be configurable. Secondly, to minimize energy losses, further applications are: shaded PV systems and PV systems with different module orientation. These are in principle the main applications for inverters with multiple MPP Trackers and the crucial question is: “Are inverters with multiple MPP Trackers suitable for all of these applications”?

2 Multistring

The requirements of an inverter with two MPP Trackers are explained in the following text. Figure 1 indicates a simplified block diagram: each string has its own DC/DC converter with a separate MPPT for operating the respective string or string bundle at its maximum power. Figure 1: Block diagram of an inverter with two MPPT’s The key factor for such an inverter to be useful for the mentioned applications in chapter 1 is the DC power capability of each MPPT. As shown in Figure 2, the DC power capability on the main input (MPPT1) must be 100% over an appropriate MPP voltage range – the same as it is for a normal string inverter. If the maximum DC power capability on MPPT1 would be lower than 100% then the installer could be forced to mix up PV modules on the second input (MPPT2) which are not exposed to the same conditions. For example, a maximum provided DC power capability of 70% on MPPT1 in a PV system in which 10% of PV modules are shaded would result in energy losses. However, that should be originally minimized with an inverter with two MPP Trackers.

MPPT

MPPT

=

=

=

=

=

~

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Figure 2: Required DC power capability of MPPT1 As can be seen in Figure 3, for MPPT2 a maximum DC power capability of 50% is sufficient – also in that case the MPP voltage range must be wide enough. 50% DC power capability is required, for example, in an East/West orientated PV system with a 1:1 power separation - 50% DC power on the east roof and 50% DC power on the west roof.

Figure 3: Required DC power capability of MPPT2 Additionally, for both MPP Trackers an extended MPP voltage range with decreasing DC power capability is required to get more flexibility in system configuration and a better performance for shaded systems or systems with different module orientation. With such a design the inverter would be useful for each application but has the disadvantage of high costs due to oversizing.

3 State of the Art Solutions

First of all, there are some inverters with multiple MPPT’s available on the market, which closely meet the requirements explained in the previous chapter. Those inverters have in principle no significant problems concerning the applications mentioned in chapter 1. But there are also some inverters with multiple MPPT’s currently available which do not fulfil the requirements. An example of an inverter with two MPPT’s is shown in Figure 4. As can be seen, both MPP Trackers - MPPT1 and MPPT2 - have the same DC power capability of 58% over a certain MPP voltage range. This means that the requirements on the inverters’ DC power capability aren’t fulfilled. 58% is in fact sufficient for MPPT2 but insufficient for MPPT1. What consequences this has is explained later in chapter 4 and chapter 5. Moreover,

DC power capability – MPPT2

DC power capability – MPPT1

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the minimal MPP voltage (VMPP/MIN) is specified at a lower DC power capability than PMAX which can cause misunderstanding or wrong VMPP calculations. Therefore VMPP/MIN should be specified at PMAX as the arrow shows in Figure 4.

Figure 4: DC power capability of MPPT1 and MPPT2 of an inverter with two MPPT’s In comparison, in Figure 5 the required DC power capability of MPPT1 (100%) over an appropriate input voltage range is shown. As can be seen, the difference between the required DC power capability and the actual one is significant.

Figure 5: Required DC power capability of MPPT1 with the appropriate MPP voltage range compared to the provided DC power capability Additional to that example, Table I shows some more inverters with two MPP Trackers that are available on the market. As can be seen, all four inverters have the same DC power capability on both inputs. The minimum requirement of 50% DC power capability on MPPT2 is fulfilled by every inverter. However, MPPT1 is always undersized which have consequences regarding the mentioned applications: perfect system configuration, shaded or different orientated PV system respectively. Furthermore, MPPT2 is mainly oversized with the exception of inverter D.

DC power capability – MPPT1, MPPT2

DC power capability – MPPT1

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Table I: Examples of inverters with two MPPT’s - Percentage of the maximum DC power capability of each MPPT

4 Perfect System Configuration

For perfect system configuration a maximal DC power capability of 67% on the main input (MPPT1) and 50% on the second input (MPPT2) is at least necessary. These are the minimum requirements for the installation of even and uneven strings to the inverter. 67% on MPPT1 is demanded for the installation of three strings – two strings on MPPT1 and one string on MPPT2. In the case of even strings the maximum DC power capability necessary on MPPT2 is 50%. Moreover, parallel operation of MPPT1 and MPPT2 must be possible for the installation of 1-string PV systems. As shown in Table I, the required 67% DC power capability cannot be achieved by any inverter which results in restrictions concerning the system configuration. First of all, there are restrictions with standard crystalline PV modules – that means that modules with common 5- and 6-inch solar cells are partly not configurable. And secondly, the system design with three strings is mainly not possible – as mentioned above, for three strings a maximal DC power capability of 67% on any input is required. A comparison between an inverter with two MPPT’s (inverter B – see chapter 3, Table I) and an inverter with a single MPPT is shown in Table II. In that example the system configurations were carried out with a standard crystalline PV module with an MPP current of 7.48A at STC conditions. Both inverters had the same nominal AC power of 10kW and since the installation of the PV modules shall be flexible a system dimensioning from -5% to +20% with regard to PDC/MAX (PDC/MAX = PAC/Efficiency of the inverter) was also considered – from the system dimensioning, results the different number of PV modules. As can be seen, the number of possible system configurations using the inverter with two MPPT’s does not meet the expectations. For both inverter types five different number of PV modules can be installed. Hence, there is no advantage for the inverter with two MPPT’s regarding system configuration compared to the inverter with a single MPPT. At that point it should also be noted that the inverter with two MPPT’s is just partly applicable for the other applications, in fact in three cases only. This will be handled in greater detail in the next chapter. Table II: Comparison between the inverter with two MPPT’s – that maximum DC power capability is 58% for each MPP Tracker - and the inverter with a single MPPT regarding system configuration. Five numbers of PV modules can be installed with each inverter. Separated strings can be installed to the inverter with two MPPT’s with just three numbers of PV modules. The number of strings is higher at the inverter with a single MPPT due to higher open circuit voltage.

Inverter MPPT1 MPPT2

A 62% 62%

B 58% 58%

C 55% 55%

D 50% 50%

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In summary it can be concluded, that an inverter with a single MPP Tracker and with a well suited MPP voltage range has no disadvantages compared to an inverter with two MPPT’s which DC power capabilities do not meet the explained requirements.

5 PV-Systems and PV-Systems with different Module Orientation

Further applications for inverters with multiple MPPT’s are shaded PV systems and PV systems with differing module orientation. The requirements to be useful for these applications are even higher compared to those explained in the previous chapter. The DC power capability on MPPT1 must be 100% - on MPPT2 50% DC power capability is adequate. All four inverters shown in Table I (see chapter 3) do not fulfil the requirements. Therefore these inverters can be used only to a limited extent for shaded PV systems or systems with differing orientations. An example with inverter C is shown in Table III. The following factors were considered to calculate possible system configurations: IMPP/STC of the PV module = 8.5A; AC power of the inverter = 10kW; system dimensioning from -5% to +15% with regard to PDC/MAX (PDC/MAX = PAC/Efficiency of the inverter). As can be seen, four different numbers of PV modules can be installed. Since MPPT1 is undersized, the following restrictions are obvious concerning shaded PV systems and different orientated PV systems: Firstly, the usage of the inverter in a shaded PV system or PV systems with different module orientation is only possible in just two cases - with 46 PV modules and with 48 PV modules. Secondly, the inverter with two MPPT’s is not as flexible as expected because a 3:1, 3:2 or 4:1 string separation is not possible. Moreover, with the shown system design for 46 PV modules and 48 PV modules mismatching losses can occur if just one string or a few PV modules are shaded or of differing orientation.

System dimensioning

(# PV modules)

46

47

48

49

50

51

52

53

54

55

56

System dimensioning

(# PV modules)

46

47

48

49

50

51

52

53

54

55

56

4x12

-

5x10

-

-

5x11

4x14

4x13

-

3x18

-

-

Inverter with a single MPPT

-

-

Inverter B with two MPPT's

2x23 // 1x23 + 1x23 // 1x24 + 1x22

1x24 + 1x23

2x24 // 3x16 // 1x24 + 1x24

-

-

3x17

-

-

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Table III: Possible system configurations using inverter C with two MPPT’s

Consequently, the last two chapters have shown that not every inverter with two MPP Trackers is automatically useful for every application as is always assumed. In comparison, in Table IV the possible system configurations with a 10kW AC inverter with a single MPPT are shown – the factors used for the system configurations were similar to those before. As can be seen, the numbers of PV modules which can be installed are similar compared to the inverter with two MPPT’s. Of course, a string separation is not possible but this is not always necessary. The installation of inverters with single MPP Tracker in east-west orientated PV systems is a good option and can provide an attractive alternative compared to inverters with multiple MPPT’s [1][2]. Table IV: Possible system configurations with an inverter with single MPPT

6 Conclusion

The findings of several analyses of inverters with multiple MPP Trackers are briefly summarized in the following paragraphs. Investigated state of the art solutions have shown that inverters with multiple MPP Trackers have restrictions regarding:

/ Perfect system configuration

/ Shaded PV systems

/ PV systems with different module orientations

System dimensioning

(# PV modules)

46

47

48

49

50

51

52

53

54

55

-

-

-

-

5x11

Inverter C with two MPPT's

2x12 + 2x11

-

4x12 // 2x12 + 2x12

-

5x10

System dimensioning

(# PV modules)

46

47

48

49

50

51

52

53

54

55

-

4x13

-

-

5x11

Inverter with a single MPPT

-

-

4x12

-

5x10

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Inverters with a single MPPT have often no disadvantages regarding system configuration compared to inverters with multiple MPPT’s. Inverters with multiple MPP Trackers must fulfil high requirements. Misleading datasheet information could lead to poor system configurations. In conclusion, many inverters with multiple MPPT’s do not keep their promises regarding system configuration, shaded PV systems and PV systems with different module orientations.

7 Reference

/ [1] Efficient East-West orientated PV Systems with one MPP-Tracker, 25th European Photovoltaic Solar Energy Conference, Spain, 2010

/ [2] Auslegung von PV-Anlagen im Polystring-Betrieb –Eigenverbrauchsoptimierung vs. Mismatch-Verlust, 27. Symposium Photovoltaische Solarenergie, Bad Staffelstein, 2012

Characters (no spaces): 9,894 Words: 1,930 Enquiries: Editor: Dietmar Staudacher, +43 (664) 8502427, staudacher.dietmar@fronius.com, Froniusplatz 1, 4600 Wels, Austria. Trade press: Andrea Schartner, +43 (664) 88536765, schartner.andrea@fronius.com, Froniusplatz 1, 4600 Wels, Austria.