Application Note: String sizing Conext CL Series

Abstract

This application note provides clarification and methodology for string sizing and an introduction to parameters provided in the inverter data sheet. Due to wide variety of solutions offered by Conext CL - IEC or Conext CL-NA inverter model with wiring boxes, understanding of string sizing for the CL inverters is required. This application note also provides information on connecting the PV modules to the CL inverters in a most optimized manner.

Scope

Scope of this document is to elaborate the string sizing methodology for Conext CL inverter IEC and NA models with combined and separate MPPT modes.

Precautions

Application Note: String sizing

Conext™ CL Series

965-0066-01-01 Rev A

DANGER

RISK OF FIRE

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ELECTRIC SHOCK

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EXPLOSION

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AND ARC FLASH This Application Note is in addition to, and incorporates by reference, the installation and operation manual for the Conext™ CL series photovoltaic grid tie inverters. Before reviewing this Application Note you must read the relevant product manuals. Unless specified, information on safety, specifications, installation, and operation is as shown in the primary documentation received with the product. Ensure you are familiar with that information before

proceeding with the information in this application note.

Failure to follow these instructions will result in death or serious injury.

DANGER

RISK OF FIRE

,

ELECTRIC SHOCK

,

EXPLOSION

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AND ARC FLASH String sizing for CL inverters involves high level understanding of PV module and Grid tied inverter behavior. Any person performing this calculation should be qualified and licensed to perform this task.

Related Documents

• Conext CL or Conext CL-NA inverter and wiring box specification • PV Module data sheet

• DC string cable specifications.

Implementation and Configuration

Follow the steps below to calculate the string size. 1. STEP-1: Gather technical information

Technical parameters from PV modules • Model of PV module to include • Maximum Open circuit voltage • Maximum array Short circuit current • Maximum power point voltage and current

• Temperature co-efficients of power, current and voltage. Technical parameters from Inverter

• Full Power MPPT voltage range • Operating voltage range

• Maximum open circuit input voltage

• Absolute max. Short circuit current per MPPT • Single MPPT or multiple MPPT

• Type of Wiring Box. Weather data

• Highest and lowest temperature at the location of installation • TMY or MET data set for location.

2. STEP-2: Understand and ensure the rules of string sizing

• Series connected modules should not have more open circuit voltage than maximum Voc limit of inverter.

Equation 1: Number of modules per string x Voc (at t° min) < inverter Voc. • Parallel connected strings should not conduct more short circuit current

in total than Short circuit current rating of inverter (per MPPT). This should include any derating as required by code for defining maximum Isc.

Equation 2: Isc strings x 125% < inverter I max.

• Series connected modules should not have open circuit voltage lower than the lower limit of MPPT voltage range of inverter.

Equation 3: Number of modules per string x Vmpp (at t° max) > inverter Vmin. 3. STEP-3: Calculate the minimum number of PV modules in Series.

4. STEP-4: Calculate the maximum number of PV modules in Series. 5. STEP-5: Calculate number of strings in Parallel.

Use case example:

PV Module: A typical 310 Wp Poly crystalline PV module parameters are considered.

Inverter: Conext CL 25 kW Inverter.

Weather conditions: Maximum high temperature 36 ºC, Minimum low temperature -5 ºC.

.

By defining:

Ns min= Number of PV modules in series at least

Vmin= Minimum voltage for maximum power point tracking Voc= Open circuit voltage of the panels

ᵠ

= Coefficient of variation of voltage with temperature Vmp= Voltage at the point of maximum power

Tc= Temperature of the cell, Average temperature Tamb= Ambient temperature

Tstc = 25 ºC ambient temperature T= Temperature of cell at 25 ºC

I inc= Incident radiation (maximum annual average) NOCT= Nominal Operating Cell Temperature Isc nec = Isc of module at STC x 125%

Isc = short circuit current of the module at STC

STC = Standard Test Conditions (STC) for measurement

STC conditions define the irradiation conditions and temperature of the solar cell, widely used to characterize the cells, PV modules and solar generators and are defined as follows:

PV module Vmpp/T Vmp Vmp (70 ºC)

310Wp Poly C-Si PV module -0.34%/ºC 36.4 30.83

CL-2500NA Vmpp Min (full power) Voc IscPer MPP / Total

- Irradiance: 1000 W/m2

- Spectral distribution: Air Mass 1.5 G - Cell temperature: 25 ºC

NOCT conditions define the irradiation conditions and temperature of the solar cell, widely used to characterize the cells, PV Modules and solar generators and are defined as follows:

- Irradiance: 800 W/m2

- Spectral distribution: Air Mass 1.5 G - Cell temperature: 20 ºC

- Wind speed: 1 m/S

Minimum number of PV Modules

In order to calculate the Minimum voltage, we must first understand the effect, that a high temperature will have on reducing the PV module Vmp voltage from the data sheet NOCT rating.

To determine the temperature of the cell in any situation, use the following formula:

Equation 4: Tc = T amb + (I inc (w/m2) * (NOCT-20)/800) Where: Tc = 36 ºC + ((1000) * (47 - 20) / 800)) = 70 ºC.

To determine the temperature of the cell at STC, use the following formula: Equation 5: T = Tc - T stc

Where: T=70 ºC - 25 ºC = 45 ºC.

To calculate the Vmp of the module at the site maximum high temperature Equation 6: Vmp (70 ºC) = Vmp (25 ºC) - (T x (Vmp (25 ºC) x∆ Vmpp/T / Irradiance @ STC))

Where: Vmp (70 ºC) = 36.40 V - (45 x (36.40 V x 0.34 / 1000)) = 30.83 V To maintain full nameplate power, calculate the minimum number of PV modules to be connected in series, using the above data.

Equation 7: Ns min = (V min / Vmp min)

Where Ns min = (500 / 30.83) = 16.21 modules~ 16.

This is the minimum amount of PV Modules to be placed in series with each string and ensure the functioning of the inverter at 1000 W/m2and 36 °C ambient temperature.

Maximum number of PV Modules

In order to calculate the Maximum voltage, we must first understand the effect that a low temperature will have on reducing the PV module Vmp voltage from the data sheet NOCT rating.

To determine the temperature of the cell in any situation, use the following formula:

Equation 8: Tc = T amb + (I inc (w/m2) * (NOCT-20)/800) Where: Tc = -5 ºC + ((1000) * (47 - 20) / 800)) = -3.6 ºC

To determine the temperature of the cell at STC, use the following formula: Equation 9: T= Tc - T stc

Where: T= -3.6 ºC - 25 ºC = -28.6 ºC.

To calculate the Vmp of the module at the site maximum high temperature, Equation 10: Vmp (max) = Vmp @ 25 ºC - (T x (Vmp @ 25 ºC x∆ Vmpp/T / Irradiance @ STC)

Where: Vmp (max) = 36.40 V - (-28.6 x (36.40 V x 0.34 / 1000)) = 49.27 V To maintain full nameplate power, calculate the minimum number of PV modules to be connected in series using the above data.

Equation 11: Ns max = (Vmax / Vmax r)

Where, Ns min = (1000 / 49.27) = 20.29 modules~ 20.

This is the maximum amount of PV modules to be placed in series with each string and ensure the functioning of the inverter at 1000 W/m2and -5 °C ambient temperature.

Number of strings in parallel

The maximum number of strings installed in parallel and connected to Conext CL inverters, is calculated.

For Combined MPPT Input to inverter (up to 8 strings)

Equation 12: Number of Strings = Isc Inverter max / (Isc nec)

Where Number of Strings = 72 A / 9.08 A x 125% = 6.34 strings~ 6. Note: 125% oversizing applied for US installations only. Not applicable for IEC installations.

Optimum DC-AC ratio

The DC ratio is based on STC conditions, but doesn't take into account the specific configuration of the project. The performance is a function of location and racking style. For example, a highly optimized system such as a 2-Axis

tracker will have a much higher performance advantage compared to a 5 degree fix tilt. Similarly, a strong solar irradiance region will have much higher energy potential than a weaker region. The amount of clipping will be based on the amount of relevant energy available against the inverter nameplate. As clipping exceeds 3%, there may be diminishing value to higher levels of DC ratio. The Conext CL inverter with its wide oversizing range, can be loaded for very high over paneling. We recommend starting with:

Shallow Fix tilt (roof mount applications)1.30 - 1.50 Steep Fix tilt (ground mount applications)1.25 - 1.35 1-Axis Tracked (ground mount applications)1.20 - 1.30 2-Axis Tracked (ground mount applications)1.10 - 1.20

Schneider Electric recommends a limit of 1.5 as maximum. Higher DC ratios requires review by a Schneider Electric applications engineer.

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Date:October 2015 Revision:Rev A Document Number: 965-0066-01-01 Contact Information:solar.schneider-electric.com.

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