PV Power Systems
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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 Maximise PV to Grid
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
Components of a Grid Connected System
PV PANELS 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
Sine wave output Low harmonic distortion (less than 4%) Input voltage range 160 – 350 VDC Output voltage range 187 – 253 VAC Single phase, can operate in frequency range 50 Hz +/- 6% Power factor > 0.98 High efficiency (more than 90%) Maximum power point tracking Mains and solar generator are galvanically isolated Disconnect from grid line within 1 cycle in case of abnormal condition 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
PV Array Diagram
Array of sub arrays
Blocking Diodes to prevent reverse current flow
PCU
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 system required
Kalbarri PV System in Western Australia (1995) 10kW
10kW
6.6kV 250Vdc
35kVA (75kVA) 415Vac
100kVA
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 by inverter failure
Grid-Connected PV Inverter (String Type)
@ 3.3kW
String Inverter Battery Backup AC Grid Line
DC from PV 160 to 240 V
DC 48 V
AC Line
Controller
AC Line DC 48 V
Back up Line
Grid-Connected PV System with Back up Inverter Kang Som-Mao, Ratchaburi
PV
75 Wp x 42 modules
CONTROLLER
-
BATTERY
batteries for S-218C
INVERTER
APOLLO G –304 And S-218C
DC Linked or Multistring type
Each panel or group have a DCDC step up converter High voltage DC link feeds transformer-less converter
DC Linked Lboost
S1
S3
S2
S4
S1
S3
HFT
D1
CPV
CDC
Lboost
String # 2
HFT
D2
D4
D1
D3
CPV
CDC
Lboost
String #3
D3
S2
S4
S1
S3
S2
S4
HFT
D2
D4
D1
D3
CPV
CDC D2
D4
S1 Ground
String #1
S2
igrid
Grid
S3
Lgrid S 4
AC Modules One Inverter per
panel High volume/ low cost Plug-and-play
Inverter characteristics Efficiency Response times Harmonic output Fault current contribution Synchronisation Frequency control Power factor DC injection
Requirement
Standard
Details
General
AS/NZS 3100
Electrical Safety Requirement
Compatibility with
AS 60038
A.C. Voltage and frequency ratings
N/A
Power flow between energy source and grid may
electrical installation Power flow direction
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 current shall not exceed the limits in
Harmonic Currents
AS 4777.2
Table 1.
Radio Communications EMC
Act
Voltage fluctuation
61000.3.3
AS/NZS Rated less that or equal to 16A per a phase
AS/NZS and flicker
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
limits
Voltage-duration curve derived from measurements taken at a.c. terminal shall Not exceed the limits listed in Table 2.
Direct current
N/A
injection
Single-phase inverter: the dc output current of the 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 maximum efficiency of 94%.
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%
Experimental inverter efficiencies
Experimental inverter efficiencies for different string inverters; values used are representative of state-of-the-art technology Efficiency by inverter type (%)
AC power (% of nominal)
HF
LF (old technology)
LF (new technology)
Transformerless
5
77.5
84.8
85.1
86.7
10
85.8
90.4
88.9
91.5
20
91.0
92.0
92.3
94.2
30
93.1
92.5
93.1
94.6
50
93.8
90.9
93.4
95.0
100
93.3
90.0
92.8
94.2
ηE
92.3
90.8
92.6
94.2
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, ηMPPT, 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.
MAXIMUM POWER POINT TRACKING EFFICIENCY
where PDC is the DC input power to the inverter and PM is the power at MPP
Total Harmonic Distortion Inverters for grid-connected PV %THD = 100 x
systems must generate energy at a defined quality The standards (example: international Standard IEC 610003-2 ) above require a THD of ≤ 5% for the harmonic spectra of the current waveform. nominal.
I dis I s1
I s2 − I s21 = 100 x I s1 ⎛I ⎞ = 100 x ∑ ⎜⎜ sh ⎟⎟ h ≠1 ⎝ I s1 ⎠
2
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%
AS 4777
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 adopted in the UK and US respectively
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 a up to few seconds
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 – India 47Hz - 53Hz
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. – Typically in the range of 100% to 200%
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/m2, AM 1.5 and 250C.
Pac =Pdc,STCx efficiency
Mismatch in Arrays
Mismatch in Arrays
Mismatch in Arrays
System design 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.
System design 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
Case Study : A PV grid connected system in Bangkok 1. 2.
3. 4. 5.
Select size of system to be around 3 kWp 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 Location for PV mounting is on the roof deck with no obstruction of sunlight for whole day Select hot dip galvanized steel for PV mounting with stainless steel nuts & bolts 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 %
Case Study : A PV grid connected system in Bangkok
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 10. Max DC current Max AC current
= 0.90 %
= 7.39 x 2
= 14.78 A
= 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
Use a modelling package
Verify Does it fit in the area Does it meet budget Does it produce required kWhr Is the CO2 offset met Check it works Re-size if necessary
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. = 130 x 24
= 3,120 Wp
•
Watts STC
•
Watts AC-estimated = 3,120 x 0.7 = 2,184 Watts
•
Watts AC-corrected = 2,010 / 870 x 1000
4.
Watts AC-corrected (2,310) > Watts AC-estimated (2,184)
= 2,310 Watts
*** PASS SYSTEM ACCEPTANCE TEST ***
Generating Power VS Time for 3.12 kWp Grid Connected inverter at Leo Electronics Co., Ltd. (Apr 1, 2004)
2000 1500 1000 500
18
17
16
15
Time
14
13
12
11
10
9
8
0 7
Generating Power
2500
Power generating from Grid Connected System 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
Orientation terminology
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 – Very good for water pumping applications
Tracking Relative Energy Production 160% 140% 120%
Fixed north facing at latitude angle
100%
N-S Axis tracker horizontal
80%
N-S Axis tracker Fixed at latitude angle Dual Axis
60% 40% 20% 0% Albany
34o57"
Geraldton
28o48"
Halls Creek
18o14"
Energy from Power of the Sun 1200 1000
Area = 7500W.hr
800 600 400 200
Time
22:00
20:00
18:00
16:00
14:00
12:00
10:00
8:00
6:00
4:00
2:00
0 0:00
Power
Energy =Power x Time = Area under curve
Peak Sun Hours Equivalent Time at 1 peak sun (1000W/m2) 1200
Area = 7500W.hr
2 1000 1000W/m 800 600 400 200
7.5 hours
22:00
20:00
18:00
16:00
14:00
12:00
10:00
8:00
6:00
4:00
2:00
0:00
0
Solar Irradiance A typical sunny day in Perth 750
500
250
0
18/05/98
0:00
6:00
12:00
18:00
0:00
Time
A Typical cloudy day in Perth 1000
Irradiance S (W/sqm)
Irradiance S (W/sqm)
1000
750
15/05/98
500
250
0 0:00
6:00
12:00
Time
18:00
0:00
Average Daily Solar Radiation, 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
Use a modelling package
Verify Does it fit in the area Does it meet budget Does it produce required kWhr Is the CO2 offset met Check it works Re-size if necessary
Suboptimal orientation – the impact Common in building integrated
applications Roof may be wrong orientation Facade may be vertical Tilt angle may be dictated by aesthetics
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