Power-One Aurora PLUS and PLUS-HV Series Inverters: guide to the sizing of photovoltaic generators with Aurora Designer and PowerOne String Tool Author: Gianluca Marri Approver: Antonio Rossi Date: 2012/05/03 Purpose Guide to the sizing of photovoltaic generators with the Aurora Power-One PVI-XXX.0 Inverter, using the offline Aurora Designer configurator and the online Power-One String Tool configurator. Both sizing tools are available on the website: www.power-one.com. Field of application The models to which the document refers are the Aurora Power-One PVI-XXX.0 PLUS and PLUS-HV Series inverters, as set out in table 1. PVI-55.0-TL PVI-110.0-TL PVI-165.0-TL PVI-220.0-TL PVI-275.0-TL PLUS Series with isolation transformers (Inverters with 55kW modules) PVI-55.0 PVI-110.0 PVI-165.0 PVI-220.0 PVI-275.0 PVI-330.0-TL PVI-330.0 PLUS Series (Inverters with 55kW modules) PLUS-HV Series (Inverters with 67kW modules) PVI-134.0-TL PVI-200.0-TL PVI-267.0-TL PVI-334.0-TL PVI-400.0-TL Table 1: PVI-XXX.0 PLUS and PLUS-HV Series inverters: types 1
PVI-XXX.0 - PLUS, PLUS-HV: Operating range and output voltage Power-One s centralised inverters in the PLUS and PLUS-HV series have a modular structure and their converter modules have a single-stage topology. This means that the power converter itself handles all the functions for power conversion, maximum power point tracking of the photovoltaic generator (MPPT algorithm) and controlling the current supplied to the grid. This single-stage transformerless topology enables high conversion efficiency levels and demonstrates direct proportionality between the grid voltage at which the inverter works (AC side) and the minimum input voltage at which the inverter can convert and output power to the grid. The relationship which ties the minimum input voltage to operate in MPPT and the grid voltage at which it is possible to transfer energy from the photovoltaic field, is the following: Vin, mpp(min) Vgrid 2 1,03 20 Fig.1: PVI-XXX.0 PLUS and PLUS-HV Series Inverters: Link between Vgrid and Vin The conclusion is, therefore, that the minimum voltage to operate in MPPT and so to output power to the grid depends on the AC working voltage of the converter module and that any (upwards) divergence in the grid voltage value from the nominal value (320V AC for 55kW modules which make up the models in the PLUS series, 380V AC for 67 kw modules which make up the models in the PLUS-HV series) leads to a reduction in the operating interval in MPPT with the module at full power. The figure below sets out the conduct of the converter module as the grid voltage changes. 2
Fig.2: Voltage range for operations in MPPT as the grid voltage changes Once the sizing of the photovoltaic generator has been completed and the typical curve of the photovoltaic generator established, the take-off point of the module may depend on the grid voltage at which the module works. If the grid voltage on which the converter module works is equal to the nominal voltage (320V AC), the voltage range for operation in MPPT extends up to 485V DC. In these conditions the module can intercept the maximum power point and so extract and convert the maximum power which the photovoltaic generator makes available. If the grid voltage at which the converter module works is higher than the nominal voltage (e.g. 330V AC), the voltage range for operation in MPPT falls and does not go below 500V DC. In these conditions the module cannot intercept the maximum power point and works at point (B), i.e. at the minimum voltage which allows power transfer to the grid. In these conditions, the maximum power which the generator makes available is not extracted: the unextracted power is equal to P indicated in figure. The grid connection voltage is therefore very important, since it determines the minimum voltage to operate in MPPT of the converter module. It is necessary to take account of this aspect of the module during the configuration of the photovoltaic generator: a configuration close to the minimum value to operate in MPPT in nominal conditions could lead to a reduction in production for the mechanism described due to the increase in the grid voltage on the output terminals of the converter module. 3
Note: As for the PLUS series, when ordering it is possible to choose, in the customisation module, an output voltage for converter modules at 300 or 320 V AC: this enables the voltage range to operate in MPPT to be expanded up to 455V DC and so to obtain a better configuration margin. PLUS Series Module power Maximum input voltage Minimum input voltage to operate in MPPT Output voltage converter modules Maximum efficiency converter module Accessibility PLUS-HV Series 55 kw 1000V DC 55 kw 1000V DC 67 kw 1000V DC 465V DC 485V DC 570V DC 3F / / 300V AC 97.5% Total frontal 3F / / 320V AC 98% Total frontal 3F / / 380V AC 97.7% Total frontal Table 2: PVI-XXX.0 inverters, general features PLUS and PLUS-HV PVI-XXX.0: Possible configurations The Aurora PVI-XXX.0 centralised inverters offer three input configuration possibilities in order to meet the needs of constructing the photovoltaic field. PLUS Series A) MULTI-MASTER: the 55 kw modules which make up the inverter act as separate inverters and manage photovoltaic generators independently from each other. The main advantage of the Multi-Master configuration lies in the configuration flexibility (each module can be configured differently from the others) and in the high number of independent MPPTs which the inverter makes available. The disadvantage lies in the fact that, in the case of breakdown of one of the modules, the whole production associated with the module is lost, due to the complete independence of the converter modules. Version with transformer Version without transformer Number of independent MPPTs in M-M configuration AC nominal power of each of the available MPPTs 4
PVI-55.0 PVI-110.0 PVI-165.0 PVI-220.0 PVI-275.0 PVI-330.0 PVI-55.0-TL PVI-110.0-TL PVI-165.0-TL PVI-220.0-TL PVI-275.0-TL PVI-330.0-TL 1 2 3 4 5 6 55kW 55kW 55kW 55kW 55kW 55kW B) MULTI-MASTER/SLAVE: the 55kW converter modules are connected in parallel in pairs, thus acting as separate 110kW inverters; the configuration is envisaged for machines of power over 110kW. Version with transformer Version without transformer PVI-165.0 PVI-220.0 PVI-275.0 PVI-330.0 PVI-165.0-TL PVI-220.0-TL PVI-275.0-TL PVI-330.0-TL Number of independent MPPTs in configuration M-M/S 2 2 3 3 AC nominal power of each of the available MPPTs 1x55kW + 1x110kW 2x110kW 1x55kW + 2x110kW 3x110kW C) MASTER/SLAVE: the 55kW converter modules are connected in parallel, thus acting as a single inverter with total power equal to the sum of the powers of the modules which make up the inverter. The main advantage of the Master/Slave configuration resides in the presence of the parallel between the converter modules: this connection in parallel enables operational redundancy so that, in the case of a breakdown of one of the converter modules, there is a reduction in production only in the case in which the power available from the photovoltaic generator exceeds the conversion capacity of the modules which remain in service. The disadvantage resides in the reduced configuration flexibility of the photovoltaic generator and in the need for external protection devices for the inverter (ref. inverter installation manual). Version with Version without Number of AC nominal power of transformer transformer independent MPPTs each of the available in M/S configuration MPPTs PVI-110.0 PVI-110.0-TL 1 110kW 5
PVI-165.0 PVI-220.0 PVI-275.0 PVI-330.0 PVI-165.0-TL PVI-220.0-TL PVI-275.0-TL PVI-330.0-TL 1 1 1 1 165kW 220kW 275kW 330kW PLUS-HV Series A) MULTI-MASTER: the 67 kw modules which make up the inverter act as separate inverters and manage photovoltaic generators independently from each other. The main advantage of the Multi-Master configuration resides in the configuration flexibility (each module can be configured differently from the others) and in the high number of independent MPPTs which the inverter makes available. The disadvantage lies in the fact that, in the case of breakdown of one of the modules, the whole production associated with the module is lost, due to the complete independence of the converter modules. Version without transformer PVI-134.0-TL PVI-200.0-TL PVI-267.0-TL PVI-334.0-TL PVI-400.0-TL Number of independent MPPTs in M-M configuration 2 3 4 5 6 AC nominal power of each of the available MPPTs 67kW 67kW 67kW 67kW 67kW B) MULTI-MASTER/SLAVE: the 67kW converter modules are connected in parallel in pairs, thus acting as separate 134kW inverters; the configuration is envisaged for machines of power over 134kW. Version without transformer PVI-200.0-TL PVI-267.0-TL PVI-334.0-TL Number of independent MPPTs in configuration M-M/S 2 2 3 AC nominal power of each of the available MPPTs 1x67kW + 1x134kW 2x134kW 1x67kW + 2x134kW 6
PVI-400.0-TL 3 3x134kW C) MASTER/SLAVE: the 67kW converter modules are connected in parallel, thus acting as a single inverter with total power equal to the sum of the powers of the modules which make up the inverter. The main advantage of the Master/Slave configuration resides in the presence of the parallel between the converter modules: this connection in parallel enables operational redundancy so that, in the case of a breakdown of one of the converter modules, there is a reduction in production only in the case in which the power available from the photovoltaic generator exceeds the conversion capacity of the modules which remain in service. The disadvantage resides in the reduced configuration flexibility of the photovoltaic generator and in the need for external protection devices of the inverter (ref. inverter installation manual). Version without transformer PVI-134.0-TL PVI-200.0-TL PVI-267.0-TL PVI-334.0-TL PVI-400.0-TL Number of independent MPPTs in M/S configuration 1 1 1 1 1 AC nominal power of each of the available MPPTs 134kW 200kW 267kW 334kW 400kW 7
MULTI-MASTER MULTI-MASTER/SLAVE MASTER/SLAVE Fig.3: PVI-330.0-TL/PVI-400.0-TL: possible configurations Sizing of the photovoltaic generator to be connected to a PVI-XXX.0 inverter In order to illustrate the procedure for sizing a photovoltaic plant with an Aurora Power-One PVI-XXX.0 inverter, a photovoltaic plant is considered with the following design data as an example: - no. 966 Sharp ND-F220A1 modules of 220 Wp, for total power of 212.52 kwp Roof installation Coeff. Panel derating 0.975 (losses on DC side of 2.5%) Maximum ambient temperature of +40 C Minimum ambient temperature of -10 C Example of configuration with PVI-220.0-TL inverter in MULTI-MASTER mode The MULTI-MASTER mode is based on the operational independence of the converter modules which make up the inverter. The photovoltaic generators which make up the plant must be independent from each other and the number of MPPTs is equal to the number of converter modules which make up the inverter. Thus, in the case in question, we have 4 independent MPPTs to be configured. 8
This mode is recommended when the photovoltaic generator is not standard in terms of installation conditions and so makes the divisions into several MPPTs necessary. Considering the design data we can configure the four 55 kw modules. By way of example, we will use 4 different configurations for each module, so as to set out some possible situations. PVI-220.0-TL inverter, configuration 55 kw modules: 1st 55kW module => 21 panels for 12 strings (252 panels) 2nd 55kW module => 23 panels for 10 strings (230 panels) 3rd 55kW module => 20 panels for 11 strings (220 panels) 4th 55kW module => 22 panels for 12 strings (264 panels) Total 966 installed modules To check the configuration we use the offline configurator Aurora Designer. AURORA DESIGNER: Use of the offline configurator The Aurora Designer calculation sheet is a tool with which to size the photovoltaic generator to be associated with the Aurora inverters. The configuration is made on the Input Sheet by the following steps: 1. Language selection; 2. Selection of the panel to be used. Should the panel concerned not be present in the calculation sheet database it is possible : a. To input it through the Add Module, by typing in the requested data in the input fields: i. Brand: panel brand; ii. Modul Typ: panel model; iii. STC Power: nominal power in standard test conditions, expressed in W; iv. Umpp: voltage at the maximum power point in standard test conditions, expressed in V; v. Uoc: open circuit voltage in standard test conditions, expressed in V; vi. Impp: current at the maximum power point in standard test conditions, expressed in A; vii. Isc: short circuit current in standard test conditions, expressed in A; viii. Max Syst. Voltage: maximum system voltage of the panel, expressed in V; ix. Tco Isc: temperature coefficient of the short circuit current, expressed in ma/ K; 9
x. Tco Voc: temperature coefficient of the open circuit voltage, expressed in V/ K. Brand EcoPower Modul Typ EP 230 P60 STC Power Umpp Uoc Impp Isc max. Sys.Voltage Tco Isc Tco Uoc 230 30.72 36.9 7.48 8.01 1000 Wp V V A A V 4.005 ma/k -0.13284 V/K 3. If the aim is for this module to be a permanent part of the database of the configurator, send an email to Power-One support to request input of the panel in the database http://it.power-one.com/renewable-energy/submit-your-inquiry 4. Selection of the inverter: select the inverter model for realisation of the plant. 5. Selection of type of panel mounting: the mounting of the panels impacts on the temperature increase of the cells compared to the ambient temperature. In the configurator it is possible to select one of 3 types of mounting: a. Mounting on solar tracker: in this case the configurator applies +25 C to the cell compared to the maximum ambient temperature set; b. Mounting on a structure: in this case the configurator applies +30 C on the maximum ambient temperature set; c. Mounting on roof: in this case the configurator applies +35 C on the maximum ambient temperature set. 10
6. Set-up of the minimum and maximum temperatures: the ambient temperatures must be input and not the minimum and maximum temperatures of the cells of the panels of the photovoltaic generator. The configurator will consider: a. To calculate the level of the minimum ambient temperature, the value of minimum ambient temperature set (in other words if a minimum ambient temperature of -10 C is input, the configurator will calculate the string voltages considering a cell temperature of -10 C). b. To calculate the level of the maximum ambient temperature, the value of the maximum ambient temperature set plus the increase in temperature linked to the type of mounting chosen (in other words if a maximum ambient temperature of 40 C is input with a roof mounting, the configurator will evaluate the parameters with a cell temperature of 40 C+35 C=75 C). c. To calculate the level of Vmp,typ@25 C ambient temperature, the value of the ambient temperature of 25 C plus the rise in temperature due to the type of mounting chosen (in other words if a roof mounting is chosen, the configurator will evaluate the parameters with a cell temperature of 25 C+35 C=60 C). 7. Set-up of the derating coefficient of the panels The Derating Coefficent of the Panels (DCP) is an estimate of the losses in the DC section of the plant. This value represents the ratio between the power which is available for conversion at the input terminals of the inverter and the nominal power installed. In other words, it takes account of the losses of the DC circuit: we may speak of the efficiency of the DC circuit, which must not be confused in any case with the efficiency of the PV panels. The losses which occur in the DC section of the plant ensure that, under standard test conditions, the nominal power installed may not be wholly available for conversion at the input terminals of the inverter. Among the losses in the DC circuit we may include: a. Losses for conduction in connection cables; b. Losses for conduction in fuses, disconnecting switches and other protection devices that may be present; 11
c. Losses linked to dirt which deposits on the panels; d. Losses for mismatching or losses linked to the imperfect parity among the electric characteristics of the panels. Nominally, in fact, the panels are all equal to each other and have the same electric characteristics. However, as revealed by flash tests, the panels have (albeit minimal) differences in their electric values. Since the photovoltaic generator is a collection of panels that are collected in various ways in series/parallel, if no sorting of the panels is made or no choice of the panels with which to build the various strings, it is possible to incur losses connected to these differences (in terms of current as regards panels in the same string and in terms of voltage between the various strings as regards strings in parallel). 8. Configuration of the photovoltaic generator: choice of the number of panels in series and therefore of strings in parallel for each MPPT (in the case of configuration with independent channels) or for the individual MPPT (in the case of configuration with parallel channels). AURORA DESIGNER: Configuration of the individual 55 kw modules In the section Step 1 Input General Data we select the brand and the model of the panel planned for the project. Let us select PVI-55.0 (or 55kW module), type of mounting and maximum / minimum ambient temperature. Note: In the case of configuration of a PVI-XXX.0 Multi-Master inverter with more than 2 modules, with Aurora Designer it is necessary to configure a 55/67 kw module each time. Let us configure the 1st module and set a configuration with 21 panels for 12 strings, for a total of 252 panels. 12
Fig.4: Configuration 1st 55kW module with Aurora Designer In the section Results Configuration of the system let us check the total photovoltaic power installed in the 55kW module ( Total PV Power STC ) and total input power to the inverter (. Total DC Input Power ). The second value is duly corrected on the basis of the value set for the Coeff. of Panel derating. In the case of a coefficient equal to 1, Total PV Power STC and Total DC Input power are the same. On the basis of these values, the configurator returns an estimate of the output power to the Inverter ( Estimate of output power to Inverter ) and a percentage ( Total PV Power STC / Nom-Max AC Power to Inverter ) given by the ratio between the power installed and the nominal-maximum power of the Inverter. (in the case of PVI-XXX.0 centralised inverters, the nominal power and maximum power values correspond) In the section Results Voltages and currents we can check the correctness of our configuration on the basis of the temperatures which we have set, on the basis of the inverter s working range. It can be noted that the configuration of the strings, chosen by us in the example, is indicated as optimal, this is because the voltage Vmp,typ@25C Ambient Temperature is equal to 556V DC, in other words around the nominal input voltage of the inverter. 13
Let us pass on now to the 2nd module and set a configuration with 23 panels for 10 strings, for a total of 230 panels. Fig.5: 2nd Configuration 55kW module with Aurora Designer In this case the configuration of the strings is not indicated as optimal, but if we check the section Results Voltages and currents it is important to highlight that the configuration of the 2nd module is, nonetheless, correct and functional, since the voltage Vmp,typ@25C Ambient Temperature is equal to 609V DC, broadly in the work range of the inverter. The advantage of a higher DC voltage is that, on a constant installed power basis, of reducing the number of strings in parallel on the DC side, thus simplifying cabling, reducing distribution losses and offering the possibility of monitoring the individual string. Finally the voltage value Voc,Max@-10C (951V DC) verifies the chosen configuration, since it is less than 1000V. For the 3rd module, we can set a configuration with 20 panels for 11 strings, for a total of 220 panels. 14
Fig.6: 3rd Configuration 55kW module with Aurora Designer The configuration of the strings is indicated as optimal (Vmp,typ@25C Ambient Temperature = 529V DC). In this specific case let us check, in the section Results Voltages and currents, also the voltage value Vmp,Min@40C on the basis of the minimum work ranges of the inverter. In this configuration, the voltage Vmp,Min@40C is 497V DC, higher than the Minimum Voltage to operate in MPPT of the Inverter (485V DC). Note: In the sizing stage of a PVI-XXX.0 inverter, it is advisable to maintain a margin of 15V DC between the Vmp,Min and the Minimum Voltage to operate in MPPT of the Inverter, in order to prevent any fall in performance in terms of Voltage of the PV generator. Finally for the 4th module of the PVI-220.0-TL inverter, we can set a configuration with 22 panels for 12 strings, for a total of 264 panels. 15
Fig.7: 4th Configuration 55kW module with Aurora Designer The configuration of the strings is indicated as optimal (Vmp,typ@25C Ambient Temperature = 529V DC). In the section Results Voltages and currents the configurator in the value of installed PV power (58080 w) indicates too high?. This is because we have chosen to install a PV power that is higher than the nominal power of the inverter, in fact the percentage ( Total PV Power STC / Nom-Max AC Power to Inverter ) is over 100%. In addition, in the section Results Configuration of the system under the heading Estimate of output power to Inverter, the configurator indicates 55000w -power limit reached and the Total Number of Panels is higher than the Maximum Number of Panels/Inverter. It is important to highlight that the nominal power installed should not cause alarm, but only provides indication of a possible power limitation of the inverter. In fact the inverter can work with limited power, but it is not cost effective because it entails a reduction in the plant s energy production. 16
It is therefore necessary for this operation to be done at a design level, paying due consideration to the condition of the PV generator. Example of configuration with PVI-220.0-TL inverter in MULTI-MASTER /SLAVE mode In this configuration the inverter acts as many separate 110 kw inverters and the number of MPPTs is equal to the Framework number (pair of 55 kw modules). This mode is a good compromise between the independence of the photovoltaic generator connected to each pair of modules and coverage of production in the case of an individual breakdown of the module. In MULTI-MASTER /SLAVE mode, the inverter under consideration has 2 independent MPPTs to be configured. With the design data, we may imagine the following configuration: PVI-220.0-TL inverter, 110kW framework configuration 1st 110kW framework => 21 panels for 23 strings (483 panels) 2nd 110kW framework => 21 panels for 23 strings (483 panels) Total 966 installed modules To check the configuration we can use the online configurator POWER-ONE STRING TOOL (http://stringtool.power-one.com/) POWER-ONE STRING TOOL: Use of the online configurator Step 1 Location Select language and installation location 17
Fig.8: Selection of location, temperatures and PV panels with Power-One String Tool Notes on View Standard: in the configuration matrices allowed for the photovoltaic generator no indications are provided as to the soundness of the configuration User Friendly: the configurator, through colour mapping, indicates the soundness of the configuration in regard to the characteristics of the inverter. Step 2 Temperatures Select the type of mounting Set the minimum ambient temperature (this is also used as a minimum cell temperature to calculate the string voltages) Set the maximum ambient temperature (the maximum cell temperature at which the string voltages are assessed is calculated by considering the type of mounting selected) 18
Step 3 Selection of PV panels Select the brand and model of the panel chosen to realise the plant (Possibility of changing the parameters of the panel should the desired panel not be present in the data base) Step 4 Inverter selection Select the model of Inverter Fig.9: Selection of Inverter with Power-One String Tool Step 5 Results The proposed tables contain all the possible configurations that are acceptable for the selected inverter, within a load factor between 40% and 130%. All the proposed configurations are compatible, choose the box with the desired configuration In the case of PVI-xxx.0 inverters, three configurations are proposed: - Multi-Master - Multi-Master/Slave - Master/Slave In the view User Friendly the cells are coloured and mapped as follows: Orange cells: the configuration is admitted but there may be one (or more) of the following conditions: Installed power lower than 70% of the Pdc,max of the inverter Installed power higher than the Pdc,max of the inverter Yellow cells: the configuration is admitted and the typical take-off point of the photovoltaic generator (Vmp@Tamb,media) is not around the nominal input voltage of the inverter (Vin,nom). Green cells: the configuration is admitted and the typical take-off point of the photovoltaic generator (Vmp@Tamb,media) is around the nominal input voltage of the inverter (Vin,nom). 19
In the table Multi-Master /Slave configuration let us select then the configurations desired for the two MPPTs. Fig.10: Selection Multi-Master /Slave configuration with Power-One String Tool Step 6 - Report Once the configuration for the plant has been identified, it is possible to create the configuration report in which the detailed results of the selected configuration are given. 20
Fig.11: Configuration report with Power-One String Tool A consideration on the Use factor, in this configuration is 94.2 %. A value close to 100% indicates a good sizing of the system, but it is worth highlighting that in no case may the nominal power installed generate alarm, damage or warranty problems. In fact, in certain situations it can be useful to have a slight oversizing of the system (also around 110115%) to take account of the losses due to the derating of the DC system. 21
On the other hand, a system which is excessively oversized would not have damaging consequences for the inverter but would lead it to work at limited power, i.e. not to transfer to the grid all the power available on the panels and so make poor use of them. Of course, it is possible to check a Multi-Master/Slave system also using the offline configurator Aurora Designer. Let us select PVI-220.0 as the Inverter and Independent as the configuration for the MPPT channels. Fig.12: Configuration of PVI-220.0 inverter Multi-Master/Slave with Aurora Designer 22
Example of configuration with PVI-220.0-TL inverter in MASTER/SLAVE mode The MASTER/SLAVE mode is based on the parallel (DC side) of all the converter modules which make up the inverter. This configuration guarantees a high level of immunity against individual breakdowns thanks to the redistribution of the power among the modules making up the inverter, it is necessary, however, to maintain the same number of panels per string and include a general external DC interrupter switch to cut off the whole field. In MULTI-MASTER /SLAVE mode, the inverter considered has a single MPPT to be configured. With the design data, we may imagine the following configuration: PVI-220.0-TL inverter, Master/Slave configuration => 21 panels for 46 strings (966 panels) Total 966 installed modules To check the configuration let us use the online configurator POWER-ONE STRING TOOL. We then repeat the previous steps up to Step 5 Results and in the table Master/Slave configuration let us select the configuration desired for the MPPT1. 23
Fig.13: Selection Multi-Master configuration with Power-One String Tool As for the power, the mode with an individual MPPT permits automatic redistribution of the power among the modules which make up the inverter. This is the strength of this configuration, which is best used with large photovoltaic generators installed in standard conditions, where total cover of breakdowns is required. Note: In the case of grounded photovoltaic generators the configuration MUST be Master/Slave; no other configurations are allowed. The presence of the earth connection for all the generators entails the loss of the prerequisite for the Multi-Master and Multi-Master/Slave configurations, which require the photovoltaic generators to be independent. 24
Of course, it is possible to check a Master/Slave system also using the offline configurator Aurora Designer. Let us select PVI-220.0 as the Inverter and Parallel as the configuration for the MPPT channels. Fig.14: Configuration PVI-220.0 inverter Master/Slave with Aurora Designer 25