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Design and Sizing of on-grid PV

Systems

Firas Alawneh

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Outline

• Grid connected PV systems

• Inverters for grid connected systems

• Characteristics of inverters

• Types of inverters

• Sizing of grid connected PV systems

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GRID CONNECTED PV SYSTEM

Photovoltaic Module

Inverter

User

Grid

Main components of a gridconnected PV system:

•PV generator •Grid connected inverter

The grid connected inverteris different from a stand-alone inverter.A grid connected inverterrequires a grid in order tooperate.

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BLOCK DIAGRAM OF GRID

CONNECTED PV SYSTEMS

Net meteringThe difference between

production-consumption

is measured

Feed-in tariffThe total production of

energy is measured

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Efficiency of inverters

Efficiency of inverter is maximum usually in the range 50-80% of rated power,and drops rapidly at very low power levels.

Measured efficiency curves for a tested inverter of 1500 W

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PV Inverter Design Topologies

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Concepts of inverters

Central inverterOne inverter for the whole PVgenerator. PV strings connectedon a DC bus (DS side).Old PV plants used to utilizecentral inverters.Power range of several kWp.Cheap and efficient technology,based on drive system industry.Drawbacks:Mismatch losses due tocombination of a large group ofPV modulesDifficulties and Losses due to DCwiringPoor expandability and adaptabilityto customer requirementsFailure of inverter causes loss ofwhole PV energy

. . .

. . .

DC

AC

DC BUS

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Concepts of invertersString invertersOne string of PV modulesconnected to one inverter.Connection on AC side.Compromise between central andmodule integrated inverters.Very popular. Power ranges about0.5-5 kWpEasy plant design, one PV string-inverter unit can be repeated forthe total required power by thecustomer.Low cost due to mass production.In case of an inverter failure onlysmall part of PV energy is lost,replacement is easy.Reduced mismatch lossesEase of plant expansion

. . .

. . .

DC

AC

DC

AC

DC

AC

AC BUS

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Concepts of inverters

Multi-String invertersFurther development of stringinverters. Each string of PVmodules uses a small DC/DCconverter, and one larger inverterunit is used for the whole plant.It combines the higher yield ofenergy output of string inverters(due to individual MPP tracking ofsmall strings) with the low cost of acentral inverter.Expansion of system is possible byadding a PV string and an extraDC/DC converter, within a certainpower range.

. . .

. . .

DC

DC

DC

DC

DC

DC

DC

AC

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Concepts of inverters

Module integrated inverter(AC modules)One module uses one inverter.Connection on AC bus.Evolved in mid 90’s, but did notgain acceptance by the market.‘Plug and play’ unit.Mismatch losses and DC wiringminimised.Suitable mainly for smallresidential systems.Drawbacks:Due to low power ratings, lowerefficiencies and higher cost perWp.Difficult and expensive thereplacement of failed inverters.

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Concepts of invertersTeam inverters or master-slave cooperationCombines string technology withthe master-slave concept.At low irradiance levels thecomplete PV array is connected toone inverter only. With increasingsolar irradiation the PV array isdivided into smaller string unitsuntil every inverter operates closeto its rated power.The inverters are controlled in amaster-slave fashion.Increased yield due to invertersoperating closer to maximumefficiency.Communication between invertersand increased complexity of wiringis required.

. . .

. . .

DC

AC

DC

AC

DC

AC

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Matching inverter with PV modules

Design of PV modules – inverter connection

Step 1Connection of modules in series so that the total voltage of the string is within theinput voltage range of the inverter.The open circuit voltage of the string for the minimum expected operationtemperature of the modules should be lower than the maximum allowable inputvoltage of the inverter. Check also for the maximum allowable system voltage ofthe PV module.The optimum power voltage of the string for the maximum expected operationtemperature of the modules should be higher than the minimum allowable inputvoltage of the inverter

Step 2Connection of strings in parallel so that the desired power is obtained. The totaloperation current of the parallel strings should be lower than the maximumallowable current of the inverter. The total PV generator nominal power should beclose to the nominal power of the inverter. The PV generator nominal power maybe up to about 110-115% of the AC maximum power of the inverter.

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Vin

ve

rter,d

c,m

ax

Vin

ve

rter,m

pp

,ma

x

Vin

ve

rter,m

pp

,min

V

I

MPP

Vp

v,s

yste

m,m

ax

Iinverter,dc,max

MPP Tracking for a certain PV String at different solar irradiance levels

MPP Adjustment

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Vin

ve

rter,d

c,ma

x

Vin

ve

rter,m

pp

,ma

x

Vin

ve

rter,m

pp

,min

1 Under-sized String

1’ Adjusted MPP

2 Well-sized String

3 Over-sized String

3’ Adjusted MPP

4 Over-sized String

4’ Adjusted MPP

V

I

MPP

1

2

3

Vp

v,sy

stem

,ma

x

Iinverter,dc,max

4

1’

3’

MPPT Tracking for different PV String Configurations at STC

4’

Sunny Boy 1200 (SB1200) Made by SMA (Germany)

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Technical Specifications - Nominal

Efficiency Curve of Inverter (Dependence on Input PV Voltage)

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=

~

PV Generator Inversion Stage

DC

Power

AC

PowerUtility Grid

The main purpose of grid connected PV system sizing is to match the number of the chosen PV modules with the chosen inverter.

To correctly match the inverter with the PV modules, follow the following steps:

1. The nominal Peak power of the PV array should match the maximum allowed DC input power of the inverter. Increasing PV peak power above the allowed level will yield to power loss in the PV produced power in case of high irradiance levels as the inverter will limit maximum power to its rated power. The designer should follow the PV inverter rules when selecting the number of PV modules to avoid power loss due to oversizing.

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2. The open circuit voltage (Voc) of the PV array should not be higher than the maximum PV system voltage allowed by the PV module’s manufacture; otherwise the PV guarantee will be lost.

3. The open circuit voltage (Voc) of each PV string (series connected PV modules) in the PV array should not be higher than maximum or absolute DC voltage at the input of the inverter to avoid overvoltage at the input of the inverter (overvoltage is not included usually in the inverter warranty). Voc should be calculated in low irradiance and low temperature case according to the following equation:

Voc (G, Tamb) = Voc (G) + Voc (G) × αVoc × (Tpv - 25°C)

Where:G is solar irradiance (e.g. 100 W/m2)Tamb is the ambient temperature (e.g. -10 °C)αVoc is the temperature coefficient for VocTpv is the PV operating temperature which is calculated as follows:

Tpv = Tamb + G × (NOCT – 20 °C) / 800

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NOCT is the Normal Operating Cell Temperature which is rated at the following conditions:

G = 800 W/m2Tamb = 20 °CWind speed = 1 m/s

To calculate the number of PV modules in each string (series connected PV modules), the calculated Voc of the PV array should be divided by the nominal open circuit voltage of one PV module.

4. The Vmpp range of the PV string should be within the allowed Vmpp range of the inverter, in order to operate the inverter in an efficient way. The Vmpp range of the PV string is calculated in two cases:

Case 1: Low irradiance (e.g. 100 W/m2) and ambient temperature (e.g. -10 °C) levelsCase 2: High irradiance (e.g. 1000 W/m2) and ambient temperature (e.g. 50 °C) levels.

The following equation is used to calculate Vmpp (100, -10) and Vmpp (1000, 50):

Vmpp (G, Tamb) = Vmpp (G) + Vmpp (G) × αVmpp × (Tpv - 25°C)

Vmpp (100, -10) and Vmpp (1000, 50) should be within the allowed Vmpp range of the chosen inverter. Number of PV modules allowed in the PV string is then calculated.

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5. The Impp current of the PV array should be greater than the maximum allowed current of the inverter at its input. The Impp current of the PV array is calculated using the following equation:

Impp (G, Tamb) = Impp (G) + Impp (G) × αImpp × (Tpv - 25°C)

Impp should be calculated at one case when the solar irradiance and ambient temperature levels are high, Impp(1000, 50). This value for parallel strings should not exceed the limit of the inverter. To calculate the number of PV strings (parallel connected PV strings), the calculated Impp of the PV array is divided by the nominal Impp of one PV module.

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Compatibility check between PV Inverter and modules Module Specifications:

Yingli Solar YL235-29b

Pdc,maxTotal Nominal PV

Peak Power

Nominal Power

Ratio (NPR)Vdc,max Vmpp,min Vmpp,max Impp,max Remarks

Pmpp 235 Wp

(W) (kWp) (%) (V) (V) (V) (A) Voc 37 V

Inverter SMA SB-1200 1320 - 80-120% 400 100 320 12.6 1 MPPT (2 string inputs) Vmpp 29.5 V

String 1 4 Vdc,max 41.3 V

String 2 0 Vmpp,min 23.2 V

String 1 5 Vmpp,max 33.0 V

String 2 0 Isc 8.54 A

String 1 6 Impp 7.97 A

String 2 0 Impp,max 8.2 A

String 1 7 αV -0.37% /°C

String 2 0 αI 0.06% /°C

String 1 8 NOCT 46 ±2 °C

String 2 0

String 1 9

String 2 0 Site Conditions:

String 1 10 Tamb,low -10 °C

String 2 0 G_low 100 W/m2

Tpv,low -6.75 °C

Tamb,high 50 °C

G_high 1000 W/m2

Tpv,high 82.5 °C

2.115

2.35

0.94

1.175

1.41

1.645

1.88

Compatible

140% 165.4 92.9 131.9 8.2 non-compatible, NPR >120%

112% 206.7 116.1 164.8 8.2

Compatible

80% 289.4 162.6 230.8 8.2 Compatible

94% 248.1 139.3 197.8 8.2

non-compatible, NPR <80%

62% 372.1 209.0 296.7 8.2 non-compatible, NPR <80%

70% 330.8 185.8 263.7 8.2

Over-voltage danger56% 413.5 232.2 329.7 8.2

Matching inverter with PV modules – Example 1

Matching inverter with PV modules – Example 2

Pm 125 Wp Max. System Voltage 600 V

Voc 32.3 V Temper. Coeff. of Voc -0.313%

Isc 5.46 A Temper. Coeff. of Isc 0.03%

Vmp 26 V

Imp 4.80 A

Max AC power 3300 Wac

Input voltage range

(MPP Range)

125-750 V

Max input current 11 A

PV module Sharp NE-L5E2E Inverter SMA SB 3300TL HC

Voc change with temperature :--0.101V/oC. At -5oC: Voc=35.33V. At 70oC: Vmp=21.5 V approx.

For the input voltage range of inverter, 6 to 21 modules could be connected in series. Due to

module max. system voltage (600V) only up to 16 modules can be connected in series (565 V).

Possibilities

Series Parallel PV power Comments

16 2 4 PV power too high. Energy will be lost due to overload of inverter.

15 2 3.75 Acceptable

14 2 3.5 Acceptable

10 2 2.5 Acceptable, but not recommended. PV power too low

10 3 3.75 Not acceptable. PV power OK, but current higher than max allowable

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For grid-connected photovoltaic (PV) systems, two main performance

indicators are used: the yield and the performance ratio:

Performance Evaluation of Grid Connected PV Systems

Y = Measured AC energy production in AC kWh/year ÷ Total PV STC Peak Power in kWp [AC kWh/kWp/year]

It is also called Annual Equivalent Hours (Hrs/Year), which represents how man y hours the PV system

generates annually AC kilowatt-hours at its rated PV peak power.

PR = Measured AC kWh/year ÷ ideal DC kWh/year (ideal = STC)

ideal DC kWh/year = Measured in-plane Global Solar Radiation in kWh/m2/year

× Total PV Area in m2 × STC PV Efficiency

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Example - Yield Comparison Between Jordan and Japan

Question: calculate the PV peak power needed to cover an annual AC electric load consumption of 200,000 kWh / year in both Jordan and Japan? (Assume, Y = 1800 kWh/year for Jordan and assume that Y = 900 kWh/year for Japan). Answer:

Nominal PV Power = 200,000 kWh/year = 111.11 kWp Jordan 1800 kWh/kWp/year

Nominal PV Power = 200,000 kWh/year = 222.22 kWp Japan900 kWh/kWp/year

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Example – Performance Ratio Calculation

Question: Calculate PR for a PV system in a certain year, given the following:

- Total measured AC electricity generated or injected into the grid: 3182 kWh- Total measured in-plane global solar radiation: 1795 kWh/m2

- Total PV surface area: 14.2 m2

- Standard or STC PV efficiency: 15.1%

Solution:

Performance Ratio (PR) = AC kilowatt-hours / year × 100 %DC kilowatt-hours at STC / year

= 3182 kWh / year × 100 %1795 kWh/m2/year ×14.2 m2 × 15.1%

= 3182 kWh / year × 100 %3849 kWh / year

= 82.7 %

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End