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Document NA 003 RDVRev. C.Brocca

Answer Drives S.r.l.36054 Montebello Vicentino (VI) Italy

1

Application note

AFE for photovoltaic: MIRO Function

Number Rev. Date Drawn up by Checked by Approved by

NA 003 RDV 03 30/11/2010 NAME S. Moretto E. Rubega C. Brocca

SIGNATURE

NAME x P. Santacà

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CONFIDENTIALITY LEVEL No restrictions

HISTORY OF REVISIONS First issue

TOTAL NUMBER OF PAGES: 13 FILE: NA003RDV_r03EN.doc NUMBERED COPY: NO -------

PROPRIETARYRIGHTS

This document is the property of “Answer Drives” S.p.a. and may not be copied, reproduced or disclosed, in whole or in part,

without written authorization.



2

Paragraph CONTENTS Page

1 Notes on the release .........................................................................................................................................................3

2 “MIRO” function ............................................................................................................................ 3

3 How it works .................................................................................................................. 6

3.1 Transition thresholds .......................................................................................................................... 6

3.2 Transition thresholds recalibration ..................................................................................................... 8

3.3 Shift work ............................................................................................................................................ 9

3.3.1 Partial shift ................................................................................................................................... 9

3.3.2 Total shift ..................................................................................................................................... 9

3.4 Modules enabling/disabling modes..................................................................................................... 9

3.4.1 Enabling/disabling with pause ...........................................................................................................9

3.4.2 Enabling/disabling without pause .............................................................................................. 10

3.5 Digital outputs .................................................................................................................................. 10

4 Parameters .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 Important notes ...................................................................................................................................... 11

6 On the transition thresholds selection...................................................................................................... 12

Other referenced documents

Annexes



3

1 NOTES ON THE RELEASE

This application note refers to the firmware version AG0300E1 dated 30/11/2010 and later. Compared to the previousreleases the following changes were made with regard to the MIRO function:

1. Changed the modules enabling/disabling principle. In the previous version the principle was based on thecomparison of the active power with the thresholds set. In this new version the comparison is made on theactive current.

2. Set at two seconds the time of enabling the modules in parallel. The disabling time is established by theparameter.

2 “MIRO” FUNCTION

In photovoltaic power generation systems it is necessary to minimize the losses in all system components (cables,inverters, transformers, etc.) in order to achieve the maximum conversion efficiency.

With regard to AFE inverters, the efficiency is strongly influenced by the power level delivered in the network. Figure1 shows as an example the efficiency curves of some SVGT inverters in accordance with the power delivered.

From figure 1 it is seen that the efficiency decreases abruptly for values 20% below the nominal power, while forvalues above 40% the efficiency is practically constant.

In conditions of low or medium radiation, this loss in efficiency means that the losses of each inverter becomeappreciable when compared with the incoming power from the photovoltaic modules.

Analysing the data of annual average radiation (Apulia Region, ENEA source) it can be seen that the monthlyaverage power produced by a photovoltaic field is less than 40% for 4 months a year.

From these data we understand how important it is to increase the efficiency of the inverters also for low powers.



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Figure 2

Percentage of average radiation in the period 1994-1999 in Apulia (Monopoli) 100% = 24.2MJ/m2. ENEA source

In large power photovoltaic systems where the inverter(s) are of modular type, i.e. consisting of two or more modulesin parallel, the conversion efficiency can be optimized by enabling only the minimum number of modules required todeliver the power coming from the strings. In this way, the enabled modules work only at high power values andtherefore in the working area with greater efficiency.

The “MIRO” function (Optimized Efficiency Multi-Inverter) has been developed for photovoltaic applications in whichmultiple inverters are used, i.e. consisting of two, three or four modules in parallel.

The strategy of the "MIRO" function is to enable/disable the modules, acting on the line contactor and on the drivingpulses, depending on the power to be delivered with the aim of keeping enabled only the modules needed to convertenergy coming from the photovoltaic field.



5

Figure 3



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3 HOW IT WORKS

At the start up, the MIRO function enables all the inverter modules. If the active current is less than 35% of the totalnominal current of the inverter, then the MIRO function disables one of the modules.If the active current is less than 35% of the nominal current of enabled modules, then the MIRO function disablesanother module, and so on.If instead the active current is 85% greater than the nominal current of the enabled modules, then the MIRO functionre-enables a module.

The current limits are in turn recalibrated depending on the modules enabled. This precaution prevents that in thetransitional phases, a module is allowed to operate at a power greater than its nominal power.

3.1 TRANSITION THRESHOLDS

The 35% and 85% threshold values for enabling/disabling the modules are fixed. The choice of these thresholds hasbeen made based on the trend of the efficiency according to the power (figure 1).

The condition for disabling a module is that the current must remain below 35% of the nominal current of theenabled modules for a minimum time set by the parameter [32.19].

The condition for enabling a module is that the current must remain above 85% of the nominal current of the enabledmodules for two seconds.

Figure 4

Nenabled Number of enabled modulesNtot Total number of modules in parallelINOM Nominal current of the inverter including all the modules in parallel

According to the two formulas shown in the diagram it is possible to determine the active current values that enableor disable a module.



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2 module inverter (Ntot=2):

1 to 2 modules (nenabled=1) Iactive 42.5%2 to 1 module (nenabled=2) Iactive 35%

3 module inverter (Ntot=3):1 to 2 modules (nenabled=1) Iactive 28.4%2 to 3 modules (nenabled=2) Iactive 56.7%3 to 2 modules (nenabled=3) Iactive 35%2 to 1 module (nenabled=2) Iactive 23.3%

4 module inverter (Ntot=4):1 to 2 modules (nenabled=1) Iactive 21.3%

2 to 3 modules (nenabled=2) Iactive 42.5%3 to 4 modules (nenabled=3) Iactive 63.8%

4 to 3 modules (nenabled=4) Iactive 34.9%3 to 2 modules (nenabled=3) Iactive 26.2%2 to 1 module (nenabled=2) Iactive 17.4%

The efficiency graphs of an inverter composed of four SVGT340 modules depending on the enabled modules are shownas an example. Due to the hysteresis introduced the graphics are different depending on whether the power is increasingor decreasing.

Efficiency at increasing power

Figure 5

Transition 12 modules %3.214

1%.85%.85

Ntot

nactive

Inom

Iactive


2%.85%.85

Ntot

nactive

Inom

Iactive


3%.85%.85

Ntot

nactive

Inom

Iactive



8

Efficiency at decreasing power

Figure 6

Transition 43 modules %354

4%.35%.35

Ntot

nactive

Inom

Iactive


3%.35%.35

Ntot

nactive

Inom

Iactive


2%.35%.35

Ntot

nactive

Inom

Iactive

3.2 TRANSITION THRESHOLDS RECALIBRATION

The transition thresholds described in the previous paragraph are valid as long as the limit of Isd current (parameter[17.04]) is set at 1.0 (or higher).In cases in which the inverter has to work at reduced power (for cooling problems, for example), it is necessary todecrease the Isd current limit (parameter [17:04]).When this limit is less than 1.0, then also the transition thresholds are recalibrated proportionally to the value of thislimit, up to a minimum of 80%.For example, we can consider the following three cases:

Case 1) Current limit [17.04] = 1.1. Since the limit is greater than 1.0 then the transition thresholds calculated in§ 2.1 are not recalibrated.

Case 2) Current limit [17.04] = 0.7. Since the limit is less than 0.8 then the transition thresholds calculated in §2.1 are recalibrated to 80%.

Case 3) Current limit [17.04] between 0.8 and 1.0. The transition thresholds calculated in § 2.1 are linearlyrecalibrated according to the formula:

ThresholdREC = ThresholdNOT_REC* [17.04]



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3.3 SHIFT WORK

As we have seen, within a year, the average power delivered by a photovoltaic inverter is about half of the peak powerfor which the inverter has been designed. In terms of exploitation of parallel modules, this means that there aremodules that always work, whereas others work only a few hours a year. This observation leads to the idea of havingvariable sequences of modules enabling, such that statistically all the modules are used in the same way.For example, if an inverter composed of four modules, numbered from 0 to 3, is taken into consideration, the 16enabling sequences that can be obtained are shown in table 1.

Col.1 Col. 2 Col. 3 Col. 4 Col. 5 Col. 6Line 1 0-1-2-3 0-2-1-3 0-1-3-2 0-2-3-1 0-3-1-2 0-3-2-1

Line 2 1-0-2-3 1-2-0-3 1-0-3-2 1-2-3-0 1-3-0-2 1-3-2-0

Line 3 2-1-0-3 2-0-1-3 2-1-3-0 2-0-3-1 2-3-1-0 2-3-0-1

Line 4 3-1-2-0 3-2-1- 3-1-0-2 3-2-0-1 3-0-1-2 3-0-2-1

Table 1

The first line shows all sequences that always start with the module 0. These sequences are used in cases in whichthere is a master module (see section 3.3.1).

In column 1, the rows 1 and 2 show the sequences for two module inverters (the modules 2 and 3 are never enabled).

In columns 1 and 2, the lines 1, 2 and 3 show the sequences for three module inverters (module 3 is never enabled)

3.3.1 PARTIAL SHIFT

In this type of shift work, only the sequences of the first line of the matrix are generated. Module 0 ("shift master") isalways enabled first. The other modules are enabled by selecting at random one of the sequences of the first row.This mode of operation requires that there is an inverter (master) equipped with a control board and one, two or threemodules in parallel with only the driver board. In this case the module 0 is the master.

3.3.2 TOTAL SHIFT

In this type of shift work, all the sequences of the matrix are generated. There is no "shift master" i.e. the modulethat is enabled first is also random as the other modules.This mode of operation requires that there is no master inverter i.e. that the control card is external to thevarious modules in parallel and that these are all controlled in the same manner.This type of operation for the moment is not applicable.

3.4 MODULES ENABLING/DISABLING MODE

The modules enabling/disabling can occur in two modes to be chosen: with pause or without pause.The behaviour of the MPPT is influenced by the choice of the mode.

3.4.1 ENABLING/DISABLING WITH PAUSE

The mode with pause (recommended) consists in switching off the IGBT driving pulses each time a module is enabledor disabled.This type of enabling/disabling is used when it is necessary to close/open the line contactor upstream of each module.In this mode there is a cumulative feedback signal of the enabled modules contactors closing. This feedback signaltakes the name of "MIRO OK" and it must be brought to the logic input DI2 (terminal XM1-14). After enabling amodule, the MIRO function expects that the DI2 input goes up within 2 seconds. If this does not occur, or if for somereason this signal goes low, the function interprets this as a closing problem of one of the contactors of the enabledmodules therefore it disables all the modules except the first and minimizes the limits of active current. This measureis necessary to prevent that the power incoming from the photovoltaic field is greater than the power delivered by themodules actually enabled.During the period in which the driving pulses of the IGBT are suppressed, the maximum power search function(MPPT) is stopped. As soon as the pulses are rehabilitated, the MPPT function is re-started forcing a fast tracking.



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3.4.2 ENABLING/DISABLING WITHOUT PAUSE

The mode without pause does not require the use of in line contactors and consists only in enabling - disabling theIGBT driving pulse each time a module is enabled or disabled.In this mode of operation, the maximum power search function (MPPT) is not stopped.This type of enabling/disabling cannot be used for the moment.

3.5 DIGITAL OUTPUTS

The modules enabling/disabling controls are given by the SCADA PLUS control board to the "GIMPA" board usingthe programmable logic outputs.The GIMPA board acts on the enable signal of the pulses of each module and which is enabled when thecorresponding logic output is low. For this reason it is necessary that the outputs are programmed as "Inverted"(see parameter [32.20]).Although the digital outputs are programmable, and therefore it is possible to assign as desired the drivers output foreach module, the following default settings are used:

DO4 terminals XM1-21/25 MIRO 1DO5 terminals XM1-22/25 MIRO 2DO6 terminals XM1-23/25 MIRO 3

The MIRO 0 control should be used only in the case in which there is no master (see § 3.3.2)



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4 PARAMETERS

The MIRO function parameters become visible in the PV Control family when the parameter [06.05] is set updifferently from "Single".

L32.17] “Miro Enable”Disabled MIRO function disabled. All modules are always enabled.

Paused The various modules enabling/disabling occurs with disabled pulses. See section 3.4.1

Continued The various modules enabling/disabling occurs with pulsed on. See section 3.4.2

L32.18] “Miro Shift Work”Disabled Shift work disabled. The modules enable sequence is fixed: 0-1-2-3.

Partial Shift work partially enabled. The modules enable sequence is random with the exception ofthe module 0 that is always the first to start. See section 3.3.1.

Total Shi f t work complete ly random. The various modules enabling/disabling occurs withrandom sequences. Unlike the "Partial" case, in this case the first module to be enabled isitself random.

L32.19] “Miro Switch delay”Time in seconds in which the module disabled condition must persist (see Chapter 2).

L32.20] “Miro Rev outputs”Miro outputs inversion. Default=Inverted. The use of the GIMPA board requires that thedigital outputs configured as MIRO are reversed.

L32.21] “Miro Test Power ref”Parameter that simulates the active current (in% of the [02.02] parameter) and that is usedto test the MIRO function. If this parameter is different from zero, then the MIRO functionuses the value set in this parameter to enable or disable a module. At the end of the test, itis necessary to return this parameter to zero. The values of this parameter that make amodule enable/disable are (assuming that the parameter [17:04] = 100%):2 module inverter:

42.5%35%

3 module inverter:28.4%56.7%

35%23.3%

4 module inverter:21.3%42.5%63.8%34.9%26.2%

17.4%

5 IMPORTANT NOTES

Note 1) The parameters values of the clean power filter [03.01] and [03.02] are calculated for the size of theindividual module, i.e. referring to the parameter [06.01].



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6 ABOUT TRANSITION THRESHOLD CHOICES

The current thresholds established for enabling and disabling the various inverter modules were chosen based on theefficiencies progress shown in figure 1.A constraint on the choice of the transition thresholds is given by the fact that there must be a certain hysteresis toavoid the oscillation between the enable and the disable of a module around a certain value of current.In formulas, this constraint is expressed as

NOM

tot

activeINFNMOM

tot

activeupper I

N

nhysteresisThresholdI

N

nThreshold .

1().(..

where the meaning of the terms has already been specified in Chapter 2.

ThresholdUPPER Power threshold for enabling a moduleThresholdLOWER Power threshold for disabling a modulenenabled Number of enabled modulesNtot Total number of modules in parallelINOM Nominal current of the inverter including all the modules in parallel

It follows that

tot

activeLOWERupper

N

nhysteresisThresholdThreshold

1().(

This constraint must be satisfied for 1 5 nenabled 5 Ntot-1 therefore the most restrictive condition is for nenabled = 1.So it must be

).(2 hysteresisThresholdThreshold LOWERupper (1)

Another constraint that should be fulfilled is Threshold UPPER 5 90% - hysteresis showing that the following relationmust be fulfilled

hysteresisThresholdhysteresisThreshold upperLOWER %90).(2 (2)

From (2) it results that it must be

3

.2%90 LOWERThresholdhysteresis

3

From the graph of figure 1 it is shown that the work area with greater efficiency is above the inverter 35% nominal powerand therefore ThresholdLOWER = 35% was chosen.From (3) it follows that hysteresis 5 6.7%. Therefore hysteresis = 5% was chosen so from the formula (2) isobtained 80 5 ThresholdUPPER 5 85% therefore ThresholdUPPER = 85% was chosen.

rev.

03

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