Grid Connect - damon`s solar training

Session 11 Matching Array and Inverter

DAMON FYSON GRID CONNECT

LESSON 11. COPYRIGHT 1

The design of a grid-connected PV system must take into consideration local operating conditions such that the array and the inverter are matched for those conditions.

The key parameters are:

1) Voltage

2) Current

3) Power



Grid Connected inverters have a Maximum Power

Point Tracking range( MPPT) with a specified mininum and maximum input voltages (DC voltages)

The inverter will track the

MPPT of the PV array to provide the best performance given the prevailing irradance and temperature



• If the array fails to produce the minimum voltage of the inverter voltage window, inverter shuts down

• If the array open circuit voltage exceeds the inverters maximum input voltage window , inverter may be damaged

224v

321 V

MPPT Range

9 modules

480v

443V

200 300 400 500 it is therefore imperative that these calculations are correct



• Fronius IG 35/50/70 inverter data the MPPT range for all inverter shown is 230 – 500 Vdc

• for effective operation the maximum power point voltage should remain in this range

• Max dc voltage that the inverter can handle is 500 Vdc the o/cct voltage of the array should not exceed this figure.



Temperature is a major de-rating factor in calculating yield, a subject for a future session



As temperature increases, the band gap of the intrinsic semiconductor shrinks, and the open circuit voltage (Voc) decreases.

At the same time, the lower band gap allows more incident energy to be absorbed because a greater percentage of the incident light has enough energy to raise charge carriers from the valence band to the conduction band. A larger photocurrent results.

The increase in the current for a given temperature rise however is proportionately lower than the decrease in voltage. Hence the efficiency of the cell is reduced.



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Solar Cell Operating Characteristics

The graph below shows that with constant irradiance the output voltage of a cell or an array of cells falls as it is called upon to deliver more current.

Maximum power delivery occurs the voltage has dropped to about 80% of open circuit voltage.

The Fill Factor (FF) is defined as the ratio between the power at the maximum power point and the product of the open circuit voltage and short circuit current. It is typically better than

75% for good quality solar cells.



The short circuit (SC) current is directly related to the number of photons absorbed by the semiconducting material and is thus proportional to light intensity.

The conversion efficiency is therefore reasonably constant so that the power output is proportional to the irradiance down to fairly low levels, however the efficiency is reduced if the cell temperature is allowed to rise.

The open circuit (OC) voltage varies only slightly with light intensity

Currentά irradiance





The open circuit voltage of PV modules depends on the cell temperature and the solar irradiation. The highest open circuit voltage occurs when the PV modules are at the coldest temperature and in bright sun.

Because PV modules also have a reduction in voltage at high cell temperatures, you must make sure the MPP voltage of the strings will not drop below the minimum inverter DC input voltage of

200V DC in very hot temperature conditions, including wire losses/voltage drop.



Solar Cell Efficiency

The following graphs show the same information as those above but in a slightly different form showing how increased temperature reduces the efficiency.

Output power decreasing

In real outdoor conditions the rated peak power Wp is seldom achieved, since module temperature usually is more in the range of 40

°C - 60°C. Efficiency can be improved by cooling the cells and some systems have been designed to make use of the heat absorbed by the cooling fluid in solar heating applications .



Sunteck temp. coefficients Pmp or Pmax & Voc

Not all manufacture data is so helpful

Voltage temperature may be expressed %/°C, V °C ( or mW/°C) a conversion sheet will assist your calculations when voltage co-effieffent V/ °C is required



Calculating temperature coefficients and changes in voltage due to temperature

When you are doing calculations with respect to temperature, you need to be careful of what units your specifications are in. There are two main types of calculations that are done; one requires the coefficient to be in %/°C and the other in V/°C (or mV/°C).

•Calculating the change in voltage due to temperature

When calculating your maximum and minimum voltages for a system, you need your temperature coefficient to be in V/°C. If it is given as %/°C, then you will need to convert it. Have a look at the two examples below, which are calculating the minimum VMP of a module:

•Temperature Coefficient given in mV/°C

VMP= 35.4V

Temperature coefficient = 160mV/°C (Note: 160mV/°C = 0.16V/°C)

If the cell was at 70°C, then the VMP would be 35.4 - [0.16 x (70-25)] = 28.V

•Temperature Coefficient given in %/°C

VMP = 35.4V

Temperature Coefficient = 0.5% is 35.4V.

Then use this figure in your calculations.

35.4 x 0.5 ÷ 100 = 0.177V

If the cell was at 70°C, then the VMP would be 35.4 - [0.177 x (70-25)] = 27.44V



• Calculating a temperature coefficient, ftemp

If you need to calculate ftemp (for example to work out the yearly energy yield of a system), then you need to have your specification in %/ ° C . If it is given as mV/ ° C, then you will need o convert it.

Remember that if you are working out a fraction, you need to divide any percentages by 100 (as 25% =

0.25 etc.). Have a look at the examples below:

•

Temperature Coefficient given in mV/ ° C

Cell Temperature = 55 ° C

Temperature Coefficient = 160mV/ ° C

VMP = 35.4V

Firstly work out, in percentage, how much 160mV is of 35.4V: 0.16 ÷ 35.4 x 100 = 0.45%/ ° C (or 0.0045 as a fraction)

Then use this value in your calculation: ftemp = 1 - [0.0045 x (55 – 25)] = 0.865

• Temperature Coefficient given in %/ ° C

Temperature Coefficient = 0.5%/ ° C

Cell Temperature = 55 ° C f temp would then be: 1

– [0.005 x (55 – c25)] = 0.85



224v MPPT Range

Inverter MPPT range

Figures 1a-d shows examples , matching modules to inverter, cell temp 0 °C to 75° C Fig 1a max. input voltage assumed 480V


LESSON 11. COPYRIGHT

480v

16

224v

205V 8 modules

MPPT Range

324V

200 300 400 500

1b MPP voltage of the array falls below MPPT range/ inverter shuts down

480v voltage



224v

257V

MPPT Range

10 modules

200 300 400 500

480v

492V

1cMPP voltage of the PV array exceeds inverter input voltage with ten modules in series at low temperature



224v

321 V

MPPT Range

9 modules

480v

443V

200 300 400 500

• This voltage range will be determined by the inverter chosen for the job

• 9 modules in series suits the MPPT range of the inverter



• The inverter has a maximum input voltage limit that is higher than the maximum MPPT range

• Max input dc voltage should be used when calculating the maximum allowable array Voc



Solar modules each have different temperature coefficients. These typically range from -0.2%/°C to

0.5%/°C dependant on module technology.

(Refer to the manufacturer’s datasheet for exact values).

The de-rating of the array due to temperature will be dependent on the type of module installed and the average ambient maximum temperature for the location.





The three different solar modules currently available on the market each have different temperature coefficients.

These are:

A) Monocrystalline Modules

Monocrystalline Modules typically have a temperature coefficient of –0.45%/°C. That is for every degree above

25

°C the output power is derated by 0.45%

B) Polycrystalline Modules

Polycrystalline Modules typically have a temperature coefficient of –0.5%/°C.

C) Amorphous Modules

These types of modules have a different temperature characteristic, resulting in a lower coefficient , typically around - 0.2%/ ° C.



The temperature de-rating factors is calculated as follows

Note: The absolute value of temperature is applied

– the formula determines whether the temperature factor is greater or less than 1 due to actual effective temperature of the cell .



For the worked example, assume the average ambient temperature is 25 º C ( Ta.day ) and the module is polycrystalline.

The average daily effective cell temperature is:

Tcell.eff = Ta.day + 25

= 25 + 25

= 50

Where :Tcell.eff = average daily effective cell temperature, in degrees C

Ta.day = daytime average ambient temperature

(for the month of interest), in degrees

In the above formula the absolute value of the temperature coefficient [ γ] is applied, this is 0.5%/oC and cell temperature at Standard Test Conditions is 25 ° C [ Tstc) ]

Therefore the effective derating factor due to temperature is:

1 - (50 – 25) x 0.5% = 100 -12.5% = 87.5% = 0.875

The de-rating then for 1,2 and 3 above is 87.5% of 144.4 W, = 126.3 Watts



MINIMUM ARRAY SIZE

The number of modules required in the array = the peak power required by the array divided by the adjusted output of the PV module

In the worked example, the number of 160W modules required is

2.01kW 126.3 W 15.9

always round up to the next full module i.e. 16 in this case

The 16 modules will provide an array with a peak rating of

16 x 160W = 2.56 kW



FINAL ARRAY CONFIGURATION

The array must be matched to the voltage window of the inverter and therefore the final array configuration will be dependent on the inverter selected and the allowable operating voltage window.



MATCHING ARRAY VOLTAGE TO THE MAXIMUM INVERTER

VOLTAGE AND VOLTAGE WINDOW OF THE INVERTER

The output power of a solar module is affected by the temperature of the solar cells. In crystalline PV modules this effect can be as much as 0.5% for every 1 degree variation in temperature. (

NOTE: for other PV cell technologies the manufacturers data must be used).



Many of the inverters available will have a voltage operating window. If the solar voltage is outside this window the inverter will not operate and in the case where a maximum input voltage is specified and the array voltage is above the maximum specified then the inverter could be damaged.





Matching an PV array to Inverter

Worked example shown explains how to size an PV array to specific inverter.

It will follow the following steps

•Minimum number of modules per string

•Maximum number of modules per string

•Maximum number of strings

•Checking the power rating

•Checking the array/inverter match

Follow the handout sheet



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Worked Example

Assume the maximum effective cell temperature recorded at site is

75 °C, and the minimum temperature is 0°C. It is proposed to install a Fronius IG60 inverter and Sunteck 205 PV modules. Assume there is a 2% voltage drop across the dc cables, allowance of a 10% safety margin on the inverter lower input voltage window and 5% on the upper voltage window.



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Aim

to calculate the number of Sunteck Pluto Ade PV modules (in terms of number of modules in each string and the number of strings). required to match the Fronius IG60 inverter in parameters of voltage, current and power.





 Step One: Minimum number of modules in a string (maximum cell temperature)

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V mp_cell.eff

Where:

= V mp-stc

– [

V x ( T cell_eff

- T stc

) ]

V mp_cell.eff

= Maximum Power Point Voltage at effective cell temperature, Volts

V mp-stc

= Maximum Power Point Voltage at STC, Volts

= voltage temperature co-efficient, V per degree Celsius v

T cell_eff

= cell temperature at specified temperature, in degrees Celsius

T stc

= cell temperature at standard test conditions, in degrees Celsius

Calculation Steps

Calculate the difference between the cell temperature and STC 75-25=50 °C

Convert the P

MAX coefficient into V/ °C 0.38%/°C x 38.1 = 0.145 V/°C

Multiply difference in temp by the P

MAX

Temp. coefficient (in V/ °C)

50 X 0.145 = 7.25V

Take this away from the rated VMP

(as the cell temperature is well above 25/ °C) 38.1 – 7.25 = 30.85V

Multiply this by 0.98 allow 2% voltage drop 30.85V x 0.98 = 30.23V

Multiply the inverter min. voltage by 1.1 (10%safety margin) 150 x 1.1 = 165V

Divide the module voltage into the inverter voltage 165 ÷ 30.23 = 5.46 modules

Round this number UP Round up to 6 modules



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Step 2: Maximum number of modules in series (minimum temperature)

V max_ oc

Where:

= V oc_STC

– [

V x ( T min

- T

STC

)]

V max_ oc

V oc_STC

= Open Circuit Voltage at minimum cell temperature, Volts

= Open Circuit Voltage at STC, Volts

T cell_eff

T

STC

= voltage temperature co-efficient, V per degree Celsius

= expected minimum daily cell temperature, in degrees Celsius

= cell temperature at standard test conditions, in degrees Celsius

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Calculation Steps

Calculate the difference between the cell temperature and STC 0 – 25 =25°C

Convert the V

OC coefficient into V/ °C 0.29%/°C X 45.8V = 0.133V/°C

Multiply the difference in temp. by

V

OC temp. coefficient (in V// °C)

Add this figure to original voltage`

( as the cell temp is below 25 °C)

25 x 0.133 = 3.32V

45.8 + 3.32 =49.12 V

No voltage drop (maximum o/cct, no current no Vd)

Multiply the max. inverter voltage

49.12 V by 0.95 to give a 5% safety margin 500 x 0.95 =475V

Divide the max. inverter voltage (incl. safety margin) 475 ÷ 49.12 = 9.67 modules by the max module voltage

Round this figure down Round down to 9 modules



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Step 3: Maximum number of Strings

I

SC cell eff

= I

SC-STC +

[ I

SC X

( T cell eff

- T

STC

)]

Where:

I

SC cell eff =

I

SC at effective cell temp, Amps

I

SC-STC =

I

SC at STC Amps

I

SC

= I

SC temperature coefficient, A/ °C

T cell eff

= cell temperature at specified temperature, in °c

T

STC

= cell temperature at standard test conditions, in °c

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Calculation Steps:

1. Calculate the difference between the cell temperature and STC 0 – 25 =25°C

2.Convert the I sc coefficient into A/°C 0.046%/°C X 5.73A = 0.0026A/°C

3. Multiply the difference in temp. by ISC temp. coefficient (in A//°C) 50 x0.0026 = 0.132A

4. Add this figure to the rated I

SC

( as the cell temp is above 25°C) 5.73A + 0.132 = 5.86A

5. Divide this current into the maximum dc inverter input current 35.8/5.86 = 6.11 strings

6.Round this figure down Round down to 6 strings



 Step 4: Checking the Power Ratings

 Inverter manufactures may give a number of ratings for their inverter

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Maximum (Recommended ) PV array rated power: recommendation from the manufacture on how much power the inverter can process. Usually rated in Watt/ kWs

Maximum dc input power : maximum amount of dc power that the inverter can convert into AC (this rating is lower than dc array power as there are PV array losses)

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 Maximum AC output power : Maximum rated AC power that the PV array can deliver

 Calculation must ensure

voltage, current

and

power

match a module/array to the inverter.



From the example, primary calculations suggest 6 parallel strings of 9 modules in series. This is 54 modules at 205 watts = 11070 watts, however this well above the recommended inverter power range 4600 to 6700 watts.

Therefore the number of modules that can be used in the system based on power would be:

Minimum No . = 4600/205w = 22.44 modules = 23 modules

Maximum No. 6700/205w = 32.68 modules = 32 modules

NB: if 4 strings of 9 modules were used this would mean 36 modules and 7380w which will not fit within the power rating of the inverter, even according the earlier calculations such an array may be ok.

From the table highest output would be 4 strings of eight modules and output wattage 6560 watts.



Grid Connect - damon`s solar training

Session 11 Matching Array and Inverter

(Refer to the manufacturer’s datasheet for exact values).

NOTE: for other PV cell technologies the manufacturers data must be used).

Matching an PV array to Inverter

Worked Example

Aim

voltage, current

power

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Grid Connect - damon`s solar training

Session 11 Matching Array and Inverter

(Refer to the manufacturer’s datasheet for exact values).

NOTE: for other PV cell technologies the manufacturers data must be used).

Matching an PV array to Inverter

Worked Example

Aim

voltage, current

power

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