PV Power Systems

Photovoltaic Power Systems -2

Grid connected PV

Professor Chem Nayar

Curtin University of Technology

Perth , Western Australia

Grid Connect PV Systems

‘

Simplest of systems

‘

No storage

Grid Connect PV Systems

‘

Net Metering – single meter runs in both directions

‘

Can also be with two meters : one to measure energy sold

and the other energy bought

SPECIFICATION (NOMINAL VALUE)

MODEL : PV-MF130EA2

MAXIMUM SYSTEM VOLTAGE

600 V

MAXIMUM POWER (Pmax)

130 W

OPEN CIRCUIT VOLTAGE (Voc)

24.2 V

SHORT CIRCUIT CURRENT (Isc)

7.39 A

MAXIMUM POWER VOLTAGE (Vmp)

19.2 V

MAXIMUM POWER CURRENT (Imp)

6.79 A

ƒ Sine wave output Low harmonic distortion (less

than 4%)

ƒ Input voltage range 160 – 350 VDC

ƒ Output voltage range 187 – 253 VAC

ƒ Single phase, can operate in

frequency range 50 Hz +/- 6%

ƒ Power factor > 0.98

ƒ High efficiency (more than 90%)

ƒ Maximum power point tracking

ƒ Mains and solar generator are galvanically

isolated

ƒ Disconnect from grid line within 1 cycle in

case of abnormal condition

ƒ Computer interface for local and remote

monitoring and data retrieval

Connecting Solar Panels

‘

Series connection to increase voltage

‘

Parallel connection for increasing current

‘

Terminology

– Module

– String

– Sub array

– Array

Blocking Diodes to prevent reverse

current flow

Cable Sizing

‘

Size for volt drop

– Maximum of 5% recommended

‘

Size for current rating

– Note that energy can typically feed from both

the array and the power conditioner

– Current rating of the cable is the rating of the

protective device, not the PV output

– Consider cable exposed temperature when

sizing for current rating

Protection Requirements

‘

Module protection – Bypass diodes

‘

String protection

– Blocking diodes

– Fuses

‘

Array protection

– Overcurrent protection

– disconnection

Australian Requirement

‘

Breaker trip current to be between

– 1.25 x Isc

– 2 x Isc

‘

Isc is for the section feeding through the

trip device

‘

Cable is then sized to the breaker

‘

Note some PV manufacturers recommend

maximum fuse ratings for the modules

Components

‘

Over current protection

– Must be DC voltage rated

– DC arcs are hard to extinguish

‘

Disconnection

– Distinguish between isolators for breaking

down the array and load break isolators for

disconnection under load

– Plugs and sockets can’t be separated under

load

Components cont.

‘

Blocking diodes

– Are not considered a fuse

– Cannot be relied upon to block reverse current

– Make sure they meet the voltage rating requirements

of the system

– They can get hot, keep them cool

‘

Australian requirements include breaker the array

into Extra Low Voltage (ELV) sections and being

able to isolate the inverter for removal

Australian considerations

‘

Australian requirements include

– breaking the array into ELV sections for safe

install and maintenance

– being able to isolate the inverter for safe

removal

GRID CONNECTED INVERTER SYSTEM

‰

Converts DC current from solar panels to AC current and feed to the

grid .

‰

The system uses 50 Hz voltage waveform from grid line as a

reference signal and feed current to the grid line. Before connecting,

the inverter will check property of grid line according to following

conditions

:-• Voltage level

• Frequency range

• Phase of signal

‰

If all conditions are within specified range and synchronized with

internal generating frequency, the inverter will be connected to the

grid

‰

In case there is some abnormal condition with the grid, inverter

should disconnect itself for both safety to human life and safety to

the system.

PV/Grid Energy System Inverter

Configurations

‘

Large Single Inverter Type (Central

Inverter)

‘

Multiple Small Inverter Type (String

Inverter)

‘

DC Bus (Multi-string Inverter)

‘

“AC” Module

PV/Grid Energy System

Inverter Configurations

Central Inverter Type

‘

Series and Parallel

connection on DC side

‘

All PV panels

connected to single DC

bus

‘

Single Central Inverter

‘

Affected by partial

shading of panels

‘

Only one protection

Kalbarri PV System

in Western Australia (1995)

10kW

10kW

35kVA

(75kVA)

250Vdc

6.6kV

100kVA

415Vac

10kW

10kW

10kW

10kW

35kVA

(75kVA)

250Vdc

35kVA

(75kVA)

250Vdc

6.6kV

100kVA

415Vac

6.6kV

100kVA

415Vac

6.6kV

100kVA

415Vac

String Inverter Type

‘

One inverter per

string

‘

Panels grouped

into smaller

inverter –rated

power of Inverter (

0.7-5kW)

‘

Not so badly

affected by shading

‘

Not badly affected

@ 3.3kW

String Inverter Battery Backup

Controller

Back up Line

AC Grid Line

DC 48 V

AC Line

DC from PV

160 to 240 V

AC Line

DC 48 V

Grid-Connected PV System

with Back up Inverter

Kang Som-Mao, Ratchaburi

PV

CONTROLLER

-BATTERY batteries for S-218C INVERTER APOLLO G –304 And S-218C

DC Linked or Multistring type

‘

Each panel or

group have a

DC-DC step up

converter

‘

High voltage DC

link feeds

transformer-less

converter

DC Linked

AC Modules

‘

One Inverter per

panel

‘

High volume/ low

cost

Inverter characteristics

‘

Efficiency

‘

Response times

‘

Harmonic output

‘

Fault current contribution

‘

Synchronisation

‘

Frequency control

‘

Power factor

Requirement Standard Details

General AS/NZS 3100 Electrical Safety Requirement Compatibility with AS 60038 A.C. Voltage and frequency ratings

electrical installation

Power flow direction N/A Power flow between energy source and grid may be in either direction

Power factor AS 4777.2 Range between 0.8 leading to 0.95 lagging between all outputs from 20% to 100%

of rated volt-amperes Harmonic Currents AS 4777.2

Harmonic current shall not exceed the limits in Table 1. EMC Radio Communications Act Voltage fluctuation AS/NZS

61000.3.3 Rated less that or equal to 16A per a phase and flicker

AS/NZS

61000.3.5 Rated more than 16A per a phase

Impulse protection IEC 60255-5 Withstand a standard lightning impulse of 0.5J, 5kV with 1.2/50 waveform

Transient voltage AS 4777.2 Voltage-duration curve derived from limits measurements taken at a.c. terminal shall

Not exceed the limits listed in Table 2. Direct current N/A Single-phase inverter: the dc output current of the

injection inverter at the a.c. terminals shall not exceed 0.5% of its rated output or 5mA

which ever is greater

Three-phase inverter: the dc output current of the inverter at the a.c. terminals measured between any two phases or between any phase and neutral

shall not exceed 0.5% of its rated output or 5m

which ever is greater Data logging and AS/NZS 60950 Any electronic data logging or communications

communication equipment incorporated in the inverter requires to devices comply with the appropriated requirements

DC-AC ELECTRICAL CONVERSION

EFFICIENCY

‘

Efficiency is the most important parameter for grid-connected PV

generation

‘

Depends on whether galvanic insulation transformer is used

between the AC on the grid side and the DC generated on the PV

side or not.

‘

Transformer can be either 50 Hz LF transformers, or HF

transformers.

‘

The presence or absence of LF or HF transformers in the inverters

influences not only the size, weight, ease of installation and material

costs, but also the earthing and safety measures to be adopted in the

PV system, and the control of DC injection feed into the grid.

‘

Inverters with an LF transformer can achieve DC-AC efficiency of

92%,while those with an HF transformer typically achieve a

European Efficiency

Normalized efficiency, ηE, and is

valid for irradiance levels in central

Europe. It is defined as a function of

the efficiency at defined percentage

values for nominal AC power. This is

shown in the following equation:

ηE = 0.03η5% + 0.06η10% +

0.13η20% + 0.1η30% + 0.48η50% +

0.2η100%

94.2 92.6 90.8 92.3 η_E 94.2 92.8 90.0 93.3 100 95.0 93.4 90.9 93.8 50 94.6 93.1 92.5 93.1 30 94.2 92.3 92.0 91.0 20 91.5 88.9 90.4 85.8 10 86.7 85.1 84.8 77.5 5 Transformerless LF (new technology) LF (old technology) HF

Efficiency by inverter type (%) AC power

(% of nominal)

Experimental inverter efficiencies for different string inverters; values used are representative of state-of-the-art technology

MAXIMUM POWER POINT TRACKING EFFICIENCY

The DC power input to an inverter depends on which

point in the current-voltage (I-V) curve of the PV array

it is working at. Ideally, the inverter should operate at

the maximum power point (MPP) of the PV array. The

MPP is variable throughout the day, mainly as a

function of environmental conditions such as

irradiance and temperature, but inverters directly

connected to PV arrays have an MPP tracking

algorithm to maximize energy transfer. The MPP

tracking efficiency, η

, can be defined as the ratio

of the energy obtained by the inverter from a PV

array, to the energy obtained with ideal MPP tracking

over a defined period of time.

where P

_DC

is

the DC input

power to the

inverter and P

_M

is the power at

MPP

MAXIMUM POWER POINT

TRACKING EFFICIENCY

‘

Inverters for grid-connected PV

systems must generate energy at a

defined quality

‘

The standards (example:

international Standard IEC

61000-3-2 ) above require a THD of ≤

5% for the harmonic spectra of the

current waveform. nominal.

2 1 1 1 2 1 2 1 100 100 100 %

∑

Total Harmonic Distortion

Table 1 - Harmonic current limits [2]

Harmonic order number Limit for each individual harmonic based on percentage of fundamental

2-9 4% 10-15 2% 16-21 1.50% 22-33 0.60% Even harmonics 25% of equivalent odd harmonics

Total harmonic distortion (to the 50th harmonic) 5%

Power Factor

‘

Traditionally poor due to

– displacement power factor

– harmonics

‘

Present technology is very good

– Maintain close to unity without great difficulty

– Can regulate power factor or reactive power

for voltage control or power factor correction

applications

Example :Current THD and power

factor vs AC power

DC Injection

‘

Is possible if an output transformer is not

present

‘

Control systems can be added to prevent

excessive injection

‘

Is regulated by standards

‘

Limits of 5 mA (0.025% of the rms output

current for a 5 kW system, based on the IEC

61000-3-2) or 0.5% (UL1741) are being

Synchronisation

‘

Performed automatically

‘

Typically uses zero crossing detection on

the voltage waveform

‘

Can be instantaneous on the next zero

crossing

‘

If phase locked loops are used it could take

Frequency Control

‘

Locked to the grid

‘

May have a bias to drift in the event of grid

failure

‘

Lock range may be limited

– Germany 49.8Hz - 50.2Hz

– Australia 48Hz - 52Hz

Prevention of Islanding

‘

An island occurs when the inverter

continues to supply power to a portion of

the grid that has become isolated from the

rest of the system

‘

The power may be unstable during the

island period

Anti islanding methods

‘

Inverters are required to have measures to

protect against this occurring

– Passive methods

• Under/Over voltage

• Under/Over Frequency

– Active Methods

• Frequency drift

• Impedance measurement

• Power Shifting

Earth Leakage Current

In the US, the National Electrical Code, NEC,

requires all PV installations with system

voltages above 50 V DC to be earthed.

Ground fault protection ('GFP') devices are

used to measure the earth leakage current, in

order to disconnect from the ground (that is,

unearth the installation), in the case of fault.

Stray leakage currents may be an issue in the

sensitivity of this protection.

Fault currents

‘

Battery-less systems can only deliver

what the energy source can deliver

– for PV this can be very little to a maximum

of 1.2 times rated current

– wind is extremely variable

‘

If a battery is present the fault current

contribution is limited by the inverter.

AC Power Output

‘

The losses in a PV system are due to:

– Inverter losses

– Dust/dirt in the modules

– Mismatch in modules

– Differences in ambient conditions from

Standard Test Conditions (STC) – 1000w/m

_2,

AM 1.5 and 25

0

_C.

1. Select the size of the system to be installed

2. Select main equipment to be installed, calculate

for matching of spec. of

2.1 PV panel

2.2 Grid connected inverter

3. Examine location for PV mounting. There should

be no obstruction of sunlight for whole day or at

least 9.00 a.m. to 4.00 p.m.

4. Consider for tilt angle of panels according to

latitude of that location

5. Select PV mounting structures.

6. Check ampere capacity of each string of inverter, select size

of blocking diode to be 30 % larger than string short circuit

current with diode max voltage more than 2 times of max

system voltage.

8. Select proper wire size so voltage drop for DC side is less

than 3%

8.1 Select wire size between each string to the combiner box

to enable less than 1% voltage drop

8.2 Select wire size between the combiner box to control

box / inverter to enable less than 2% voltage drop

9. Select proper wire size so voltage drop for AC side is less

than 3%

10. Select size of disconnect switch both DC and AC side to

proper rating

1. Select size of system to be around 3 kWp

2. Select main equipments as

2.1 PV panel - Mitsubishi model PV-MF130EA2 - 130 Wp / panel

- 2 strings with 12 panels in each string

- Isc / string = 7.39 amp.

- Total PV power = 130 x 24 = 3,120 Wp - V max = Voc = 24.2 x 12 = 290.4 Vdc

- Oper. volt. at max. power = 19.2 x 12 = 230.4 Vdc - Max DC current = Isc x 2 = 7.39 A x 2 = 14.78 Amp 2.2 Grid connected inverter - Leonics G-303M

- 2.7 kW output

- Max DC voltage = 350 Vdc

- Nominal Operating PV voltage = 230 Vdc

3. Location for PV mounting is on the roof deck with no obstruction of

sunlight for whole day

4. Select hot dip galvanized steel for PV mounting with stainless steel

nuts & bolts

5. Tilt angle of panels is set to 14 deg. facing south as Bangkok locates at

latitude 13.73 deg. North

Case Study : A PV grid connected system in

Bangkok

6. Plan to install control box and inverter in training room , 3 rd floor.

7. Selection of blocking diode

7.1 Min. device rating (I) = Isc x 1.3

= 7.39 x 1.3 = 9.61 A

7.2 Min. device rating (V) = Voc x 2

= 290.4 x 2 = 580.8 V

Then select blocking diode to be 10 ampere 600 V. for each string.

8. Measure cable length of the system

8.1 Cable length between each string to the combiner box = 10 meters

Select wire for each string to be 4 sq.mm. to get voltage drop < 1%

Voltage drop in each string = 11,650 x 10 x 7.39 = 0.86 V

Percentage of volt. Drop = 0.86 / 205 = 0.42 %

8.2 Cable length between combiner box to control box / inverter is 35 m. Select wire size to be 10 sq.mm. to get voltage drop < 2%

Voltage drop = 3,903 x 35 x 7.39 x 2 = 2.02 V

Percentage of volt. Drop = 2.02 / 205 = 0.99 %

9. Cable length between Control Box / Inverter to load panel is 12 meters Select wire size to be 2.5 sq.mm. to get voltage drop < 3%

Voltage drop = 15,695 x 12 x (2,700/238)

= 2.14 V

Percentage of volt. Drop = 2.14 / 238 = 0.90 %

10. Max DC current = 7.39 x 2 = 14.78 A

Max AC current = 2,700 / 232 = 11.64 A

Select both DC and AC breaker to be 20 A

Calculate annual energy output

‘

Use data source and get annual daily average energy

available

‘

Adjust down for losses

– Inverter 7%

– Temperature 15%

– Cable 3%

– Dirt 2%

– Orientation 1%

– Total about 25%-30%

‘

Multiply by the size of the array to get the electrical kWhr

output

– OR

Verify

‘

Does it fit in the area

‘

Does it meet budget

‘

Does it produce required kWhr

‘

Is the CO

₂

offset met

‘

Check it works

System Acceptance Test

1. Sum total module ratings at STC (Standard Test Condition) : Watts _STC

2. Estimate inverter AC output to be 70% of Watts _STC : Watts _AC-estimated

3. Measure real AC output and irradiation, then define

Watts _AC-corrected = Real AC output / irradiation x 1000

4. Compare that Watts _AC-corrected is more than Watts _AC-estimated

Result from the installation

Generating power and irradiation is measured on Mar 26, 2004 at 11.25 p.m.

• Watts _STC = 130 x 24 = 3,120 Wp

• Watts _AC-estimated = 3,120 x 0.7 = 2,184 Watts

• Watts _AC-corrected = 2,010 / 870 x 1000 = 2,310 Watts

4. Watts _AC-corrected (2,310) > Watts _AC-estimated (2,184)

Generating Power VS Time for 3.12 kWp

Grid Connected inverter at Leo Electronics Co., Ltd. (Apr 1, 2004)

0

500

1000

1500

2000

2500

7

8

9

₁₀

₁₁

₁₂

₁₃

₁₄

₁₅

₁₆

₁₇

₁₈

Time

Ge

ne

ra

tin

g P

ow

er

Date

Gen. Power

Date

Gen. Power

Date

Gen. Power

1/4/2004

14.30

24/3/2004

13.73

16/3/2004

12.85

31/3/2004

12.79

23/3/2004

12.12

15/3/2004

10.88

30/3/2004

12.13

22/3/2004

10.94

14/3/2004

12.53

29/3/2004

12.33

21/3/2004

8.02

13/3/2004

12.02

28/3/2004

13.49

20/3/2004

7.22

12/3/2004

11.67

27/3/2004

13.51

19/3/2004

8.57

11/3/2004

13.21

26/3/2004

13.14

18/3/2004

11.87

10/3/2004

11.34

25/3/2004

13.01

17/3/2004

14.68

9/3/2004

10.15

Max. Generating Power/day

14.68

kWh/day

Min. Generating Power/day

7.22

kWh/day

Average Generating Power/day

11.94 kWh/day

Tracking Array

‘

The PV array may either be fixed, sun-tracking

with one axis of rotation, or sun-tracking with

two axes of rotation.

‘

Generally fixed arrays are used though

significant increase in energy yield is possible

with single axis tracking with an additional small

gain using duel axis tracking

‘

Trackers

– add cost but offset by PV savings

– require some maintenance

Tracking Relative Energy

Production

Albany Geraldton Halls Creek

Fixed north facing at latitude angle NS Axis tracker -horizontal NS Axis tracker -Fixed at latitude angle Dual Axis

34

_57"

28

_{48" 18}

_14"

Energy from Power of the Sun

Time

Power

Energy =Power x Time

Area = 7500W.hr

Peak Sun Hours

Equivalent Time at 1 peak sun (1000W/m

₎

7.5 hours

Area = 7500W.hr

0 250 500 750 1000 0:00 6:00 12:00 18:00 0:00 Time Irra dia n ce S (W /sq m ) 18/05/98

A typical sunny day in Perth

0 250 500 750 1000 0:00 6:00 12:00 18:00 0:00 Time Ir ra di an ce S ( W /s qm ) 15/05/98

A Typical cloudy day in Perth

Calculate annual energy output

‘

Use data source and get annual daily average energy

available

‘

Adjust down for losses

– Inverter 7%

– Temperature 15%

– Cable 3%

– Dirt 2%

– Orientation 1%

– Total about 25%-30%

‘

Multiply by the size of the array to get the electrical kWhr

output

– OR

Verify

‘

Does it fit in the area

‘

Does it meet budget

‘

Does it produce required kWhr

‘

Is the CO

₂

offset met

‘

Check it works

Suboptimal orientation – the

impact

‘

Common in building integrated

applications

‘

Roof may be wrong orientation

‘

Facade may be vertical