Grid Connected Pv Part 2
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Renewable Energy Education Project - REEP COLLEGE OF ENGINEERING – KNUST Kumasi/Ghana
Short course
GRID – CONNECTED SOLAR PV SYSTEM DESIGN AND INSTALLATION
Ahmed Ousmane BAGRE June 2009
TABLE OF CONTENTS
1. GENERAL PRINCIPLE _______________________________________________________ 3 1.1.
SYSTEM WITHOUT BATTERY ____________________________________________________ 3
1.2.
SYSTEM WITH BATTERY ________________________________________________________ 4
1.3.
COMMERCIAL BACKUP SYSTEM _________________________________________________ 5
2. METERING _______________________________________________________________ 6 2.1.
NET‐METERING ______________________________________________________________ 6 2.1.1. System using a Bidirectional net‐meter _______________________________________________ 6 2.1.2. System using a Dual net‐meter ______________________________________________________ 7
2.2.
A FEED – IN TARIFF ___________________________________________________________ 8
3. GRID‐CONNECTED INVERTER ________________________________________________ 9 3.1.
GENERAL ___________________________________________________________________ 9 3.1.1. Inverter classification _____________________________________________________________ 9 3.1.2. Main common functions for grid inverter _____________________________________________ 10
3.2.
TECHNICAL DESIGN OF PV INVERTERS ___________________________________________ 12 3.2.1. Basic inverter design _____________________________________________________________ 12 3.2.2. Different types of inverters technologies _____________________________________________ 13 3.2.3. Technologies in comparison _______________________________________________________ 14
3.3.
CONFIGURATION FOR PV SYSTEM ______________________________________________ 15 3.3.1. Protection technology ____________________________________________________________ 16 3.3.2. Technical parameters ____________________________________________________________ 17 3.3.3. Inverter efficiency _______________________________________________________________ 17
4. GRID –CONNECTED SYSTEM DESIGN _________________________________________ 19 4.1.
GENERAL __________________________________________________________________ 19 Energy efficiency ________________________________________________________________ 19 Documentation _________________________________________________________________ 19 System design steps ______________________________________________________________ 20 System design criteria ____________________________________________________________ 20
4.1.1. 4.1.2. 4.1.3. 4.1.4.
4.2.
DESIGN TO FIT ON AVAILABLE ROOF SPACE ______________________________________ 20
4.3.
DESIGN TO MEET YEARLY ENERGY USAGE ________________________________________ 28 Assessment ____________________________________________________________________ 28
4.3.1.
1. PERFORMANCE PARAMETERS FOR GRID‐CONNECTED PV SYSTEM _________________ 30 1.1.
Standard performances _______________________________________________________ 30
1.2. Monitoring _________________________________________________________________ 31 KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 2
1. GENERAL PRINCIPLE 1.1.
SYSTEM WITHOUT BATTERY
Grid-connected or utility-interactive PV systems are designed to operate in parallel with and interconnected with the electric utility grid. The primary component in grid-connected PV systems is the inverter, or power conditioning unit (PCU). The inverter converts the DC power produced by the PV array into AC power consistent with the voltage and power quality requirements of the utility grid, and automatically stops supplying power to the grid when the utility grid is not energized. This safety feature is required in all grid-connected PV systems, and ensures that the PV system will not continue to operate and feed back into the utility grid when the grid is down for service or maintenance. An interface is made between the PV system AC output circuits and the electric utility network, typically at an on-site distribution panel or service entrance. This allows the AC power produced by the PV system to either supply on-site electrical loads or to back-feed the grid when the PV system output is greater than the on-site load demand. At night and during other periods when the electrical loads are greater than the PV system output, the balance of power required by the loads is received from the electric utility. The system can be with battery (Figure 1a) or battery less (Figure 1b)
AC Load(s) PV array
Inverter
Distribution
Panel
Electric Grid
Figure 1a: Grid-connected system without battery configuration
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1.2.
SYSTEM WITH BATTERY
The grid-connected system with battery bank is necessary where a critical backup power supply is required for critical loads such as computer, lighting, refrigeration and other necessities. Under normal circumstances, the system operates in grid-connected mode, serving the on-site loads or sending excess power back onto the grid while keeping the battery fully charged. In the event the grid becomes de-energized, control circuitry in the inverter opens the connection with the utility through a bus transfer mechanism, and operates the inverter from the battery to supply power to the dedicated loads only. In this configuration, the critical loads must be supplied from a dedicated sub panel. Figure 18 shows how a PV system might be configured to operate normally in grid-connected mode and also power critical loads from a battery bank when the grid is deenergized.
Critical
Non-Critical
AC Load(s)
AC Load(s)
Critical Load
Main
Sub Panel
Panel
PV array Inverter/ Charger
Battery
Electric
Storage
Grid
Figure 1b: Grid-connected system with energy storage system configuration
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1.3.
COMMERCIAL BACKUP SYSTEM
The Sunny Backup system comprises one or more Sunny Backup 5000s in combination with an Automatic Switch Box (AS-Box-M, AS-Box-L or AS-Box-XL). This system is specially designed for backup applications and enables, in compliance with all standard requirements, continued operation of a grid-connected PV system in the event of grid failure. Thus, this system does not replace the conventional PV inverter (Sunny Boy), but is installed additionally. In the event of grid failure, the Sunny Backup system first ensures safe disconnection of the loads and the PV system from the public grid, and subsequently forms a stable stand-alone grid, into which the Sunny Boy can then feed solar energy. The maximum period of interruption for the loads is approximately 20 ms, which for most loads is equivalent to uninterrupted operation.
The Sunny Backup system is suitable for use in conjunction with all Sunny Boys and Sunny Mini Centrals from SMA Technologie AG. The modular structure allows the suitable construction of systems with a maximum consumer power of approximately 5 kW to 100 kW. The Sunny Backup system can be integrated into new PV system installations, and can also be retrofitted onto existing PV systems. Along with the Sunny Backup 5000 inverter and the automatic switching device, a battery is necessary as a short-term storage device, for reliable operation. During grid failure, the battery has the task of correcting the imbalance between generation and consumption. Whenever less energy is being generated than consumed (e.g. at night), the battery is discharged. Whenever more energy is being generated than consumed (e.g. during the day), the battery is charged. The intelligent battery management built into the Sunny Backup 5000 protects the battery from overcharging and deep discharge. This ensures that the battery service life stipulated by the battery manufacturer can be achieved. KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 5
2. METERING Following the deregulation of electricity supply markets in many countries, many electricity customers in industrialized countries installed their own electricity generating equipment and connected it to the grid. When a customer is generating more electricity than required for his own use, the surplus may be exported back to the power grid. Two main system of metering are used to control the amount of energy consumed by the customer or sent to the grid:
2.1.
NETMETERING
Net metering is an electricity policy for consumers who own (generally small) renewable energy facilities, such as PV system.The net metering principle consists to measure the amount of energy sent to the grid and the amount of energy consumed by the PV system owner and find the net-energy given by: Net Energy = Energy received from the grid – Energy sent to the grid At the end of the month, if the customer uses more energy than the PV generates, the customer pays the difference (positive net-energy). But if the customer has generated more energy than used, the grid company credits the net kilowatt-hours (negative net energy) produced at the power rate. Depending on PV size and electrical consumption, you may produce more or less than you actually use. In order to support the production of PV-generated power, some grid companies offer a better price for the kWh fed into their grid than they charge for the kWh from the grid. In other locations a one-to-one ratio is applied which means the same kWh-price for both flow directions. Net Metering can be done with or without a battery backup. Technologically speaking, two systems of net-metering exist:
2.1.1. System using a Bidirectional netmeter In net metering system using a Bidirectional net-meter, when the PV system produces excess energy, it is sent to the grid system and the meter turns backwards. If the customer uses more power than is being produced, the energy is received from the grid system and the meter turns forwards. Meter 1 (figure 2a) is here just to get precise information on the amount of PV energy produced by the PV system and Meter 2 is the bidirectional net-meter. This option is seen as the most attractive option (no need to buy an additional meter) for the PV system owner. But, for most electricity companies in Europe, it is not satisfactory to allow the meter to simply run backwards for exported unit since this leave the meter open to fraudulent abuse. Furthermore, modern electronics meters can be programmed to run only in one direction to combat meter fraud. For all these reasons, a meter with two separate metering register is required (see figure 2a). KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 6
AC Load(s) PV array
Inverter Utility
kWh
kWh
1
2
Grid
Figure 2a: Bidirectional net-meter configuration
2.1.2. System using a Dual netmeter Most utilities in Europe, have adopted the net metering using a Dual net-meter. This system uses two separate, reverse blocked meters for sold (meter 3) and purchased (meter 2) electric energy. To get precise information on the amount of PV electricity produced by the solar electric system, an additional meter is recommended (meter 1, Figure 2b).
AC Load(s) PV array
Inverter Utility
kWh
kWh
1
2
kWh
Grid
3
Figure 2b: Dual meter configuration
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2.2.
A FEED – IN TARIFF
A Feed-in Tariff (FiT, Feed-in Law, FiL,) is an incentive structure to encourage the adoption of renewable energy through government legislation. The national electricity utilities are obligated to buy renewable electricity (electricity generated from renewable sources, such as solar photovoltaic at market rates set by the government. The higher price helps overcome the cost disadvantages of renewable energy sources. The rate may differ among various forms of power generation. This system uses two separate, reverse blocked meters for sold (meter 1) and purchased (meter 2) electric energy as shown figure 2c. PV array
Inverter
AC Load(s)
1
kWh
kWh
2
Utility Grid
Figure 2c: Feed-in tariff configuration
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3. GRIDCONNECTED INVERTER 3.1.
GENERAL
The essential device of a grid connected photovoltaic installation is the inverter. It acts as an interface between the solar array and the utility grid. The utility-interactive inverter differs from the stand-alone unit in that it can function only when connected to the grid. This inverter converts direct current produced by the solar array into alternating current that can be fed into the distribution network. The on grid inverter not only conditions the power output of the photovoltaic array, it also serves as the system's control and the means through which the site-generated electricity enters the grid lines. It uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the PV system's output is fully synchronized with the utility power.
3.1.1. Inverter classification Technologically speaking, there are two main types of inverters used in the grid-connected system as shown in Figure 1 The line commutated inverter uses a switching device like a commutating thyristors that can control the timing of turn-on while it cannot control the timing of turn-off by itself. Linecommutated inverters are not suitable for use in standalone systems because AC voltage from the grid is required to turn off thyristors. The self-commutated inverter uses a switching device like IGBT (Insulated Gate Bipolar Transistor), MOSFET or GTO (Gate Turn Off) thyristors and can operate without AC grid voltage. Due to advances in switching devices, most inverters for distributed power sources such as photovoltaic power generation now employ a self-commutated inverter. The Self-commutated inverters are broadly classified as: • Voltage-Source Inverter (VSI), has a capacitor in parallel across the input and the DC
side is made to appear to the inverter as a voltage source • Current –Source Inverters (CSI), has an inductor in series with the DC input and the DC
source appears as a current source to the inverter. According to the inverter control scheme, the following classification can be done: • Current-Controlled Inverters (CCI), • Voltage-Controlled-Inverters (VCI) KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 9
Most inverters are voltage source (VSI) even though the PV is a current source. Because of some advantages in grid-connected inverters in most cases current control scheme is applied. The advantages are higher power factor, better transient current suppression and short circuit current is limited to rated AC current.
Figure 3: Classification of inverters
3.1.2. Main common functions for grid inverter The PV grid-connected inverter has to fulfill the basic functions in order to feed energy form a PV array into the utility grid: • • •
To shape the current into a sinusoidal waveform, To invert the current into an AC current and synchronize with the grid If the PV voltage is lower than the grid voltage, the PV array voltage has to be boosted with a further element.
It also has to fulfill the functions below for security and performance aspect: MPPT function The output characteristics of photovoltaic arrays are nonlinear and change with the module’s temperature and solar irradiance. For a given conditions there is a unique point in witch the array produces maximum output power. This point is called maximum power point (MPP) which varies depending of cell temperature and present irradiation level. To obtain the maximum power KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 10
from a photovoltaic array, a maximum power point tracker (MPPT) is used. The MPPT thus help the inverter to draw peak power from the PV array to maximize the produced energy regardless module temperature and solar irradiation. Many systems have been used and the Perturbation and observation is one of the most commonly used MPPT methods for its simplicity and ease of implementation. It operates by periodically perturbing (incrementing or decrementing) the array terminal voltage and comparing the PV output power with that of the previous perturbation cycle. If the power is increasing the perturbation will continue in the same direction in the next cycle, otherwise the perturbation direction will be reversed.
PV Generator power (W)
P4
P2
P3
P1
PV Generator voltage (V)
Figure 4: Perturb and Observe method
Anti-islanding function Islanding is the continued operation of a grid-connected inverter in cases where the utility grid has been switched off, cut off or the distribution lines have been damaged so that no electric energy is delivered from the utility side. In such a situation the safety of persons and/or the safety of equipment might no longer be guaranteed. Thus, in the event of a loss of mains supply, gridconnected PV systems are required to automatically disconnect from the grid in order to prevent the power network from islanding. A lot of anti-islanding methods have been identified and can be divided into two main groups: KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 11
•
Passive methods: a detection circuit monitors grid parameters (e.g. voltage, frequency, voltage phase jumps, voltage harmonics); these methods do not have any influence on grid quality
•
Active methods: a detection circuit deliberately introduces disturbances (e.g. active or reactive power variation, frequency shift) and deduces from the reaction to these disturbances if the grid is still present.
3.2.
TECHNICAL DESIGN OF PV INVERTERS
3.2.1. Basic inverter design The basic design of inverters is always very similar no matter what technology is used. The essential feature of a PV inverter is the forming of an AC voltage (grid connection) from DC voltage (PV generator). The figure 6 shows a basic design of a PV inverter where: •
B: Bridge allowing the connection of each of the two input poles via an electronic devise such as transistor. Only two opposite switches can be closed at the same time.
•
C: Capacitor functioning as energy storage and feeding continuously and uniformly the bridge
•
L: Iron core choke as current accumulator helps to get a sine-wave current to feed into the grid
L
+
P
GRID
C L
‐
N
B
Figure 5: Basic design of a PV inverter KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 12
3.2.2. Different types of inverters technologies Initially, at the inverter input, the direct current generated by the solar array is temporarily stored in a capacitor. Thus, the solar current can continually flow with maximum power, and is independent of the switching operation in the inverter. Due to the permanent opening and closing of the electronic switches, the capacitor is repeatedly discharged little by little. This causes the inverter to oscillate perpetually between two states: feeding and not feeding. With the choke as buffer at the inverter’s output, this pulsating direct current is converted to sinusoidal alternating current. However, practical use of this basic inverter is restricted due to the limit input voltage range. To feed current into the grid, the input voltage must always be above the peak of the grid voltage. For example if the grid RMS voltage is 240V, then the minimum voltage to feed a sine wave current to the grid is 240x 2 = 339V . To solve this problem, many options are possible for adjusting or extending the input voltage range:
P
+
GRID -
N Step- up converter
Bridge
Figure 6a: Transformerless inverter block diagram
Bridge 1
Rectifier
Bridge 2
P
+
GRID N
‐ HF Transformer
Figure 6b: Inverter with High Frequency Transformer block diagram KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 13
+
P
L
GRID
C L
‐ Bridge
N LF Transformer
Figure 6c: Inverter with Low Frequency Transformer block diagram
3.2.3. Technologies in comparison Characteristics
50 Hz (LF) transformer ++
Susceptibility (grid)
++ transformer
+
--
Single design/Reliability
++
+
-
Grid compatibility
+
Electromagnetic compatibility
++
Efficiency
+
++
-
+
-
+
-
++
+
Weight
separation
50 Hz
+
-
5/6
High frequency transformer
Semiconductor switches
Galvanic PV/Grid
4
Transformerless --
Electronic
-
-…..0
0
12
electronic
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3.3.
CONFIGURATION FOR PV SYSTEM
If inverter and PV array are treated as a system, four basic configurations can be identified as shown in figure 7 a)
b)
=
=
c)
d)
Figure 7: Configuration for PV System
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System with central inverter: In this topology (Figure 7 a), the PV generator is arranged in many parallel strings that are connected to a single central inverter on the DC side. The advantage of this configuration is doubtless its high efficiency and lowest specific cost of the inverter. However, the energy yield of the PV generator decreases due to the module mismatching and partially shading conditions. Also, the reliability of the system is limited due to the dependence of power generation of a single component, the failure of the central inverter results on the shutdown of the whole plant. The central inverter is used for large PV system (> 10 kW). System with string inverter: Similar to the central single inverter, the PV generator in this concept is divided into several parallel strings (Figure 7c). Each of PV string is connected to an inverter, the so-called “string inverter”. String inverters have the capability of separate MPP tracking of each PV string. This increases the energy yield via the reduction of mismatching and partial shading losses. System with multi-string inverter: Multi-string inverter is an evolution concept of string technology which allows the connection of several strings with separate MPP tracking system (via DC/DC converter) to a common inverter (Figure 7b). This topology allows the integration of PV strings of different technologies and various orientations (south, west and east). The system with multi-string inverter combines the advantages of central and string technologies. The power range for this inverter is between 3 and 10 kW. Module integrated inverter: One inverter is used for each module, which avoid mismatching. The module integrated inverters require a complex cabling on the AC side, due to the fact that each module of the system must be connected to the 230V grid (Figure 7d). This concept is normally only used in PV systems with les power from 50 W up to 400 W
3.3.1. Protection technology All grid-connected inverter have in common the follow protection function: •
Galvanic separation between PV generator and local grid with a transformer or through a Residual Current Detection-Surveillance (RCD) device (Transformerless’ case)
•
Ground fault detection (DC side)
•
Grid voltage surveillance (under and overvoltage)
•
Grid frequency surveillance
•
Short circuit diode to protect against pole interchanging
•
Overvoltage protection with varistors on the DC side
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3.3.2. Technical parameters The most important inverter parameters are rated DC and AC power, MPP Voltage range, maximum DC/AC current and voltage and rated DC/AC current and voltage. Other parameters are power in standby mode, power in sleeping (night) mode, power factor, distortion, noise level etc. The following parameters can usually be found in inverter data sheets: Input data
Output data
Max. DC power (PDC,max)
Max. AC power (PAC,max)
Max. DC voltage (UDC,max)
Nominal AC output (PAC,nom)
PV voltage range, MPPT (UMPP)
Harmonic distortion of the grid current
Max. input current (IPV,max)
Nominal AC voltage (UAC,nom)
DC voltage ripple (Upp)
Nominal AC frequency (fAC,nom)
Max. no of string (parallel)
Power factor Efficiency
3.3.3. Inverter efficiency The output of a grid-connected PV system depends on the PV/inverter sizing ratio (Rs)1, defined as the ratio of PV array capacity at standard test conditions to the inverter's rated input capacity. Properly matching PV and inverter rated capacities improves grid-connected system performance. Under low insolation (incident solar power), a PV array generates power below its rated capacity, leading to inverter operation at partial load. Inverter efficiency drops with partload operation: it also becomes sub-optimal when a significantly undersized inverter is made to operate mainly in conditions of overload, which result in energy loss. Figure 1 illustrates inverter efficiency under both overload and partial-load operation.
η Inv =
Power output AC Power input DC
KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 17
Figure 8: Inverter efficiency
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4. GRID –CONNECTED SYSTEM DESIGN 4.1.
GENERAL
The design of any PV Grid connected system should consider a number of criteria such as: • • • • •
Budget Suitability of the site for safe installation of a PV Array How much roof space is available for system installation Potential for shading of PV Array from nearby buildings, trees or other objects Aesthetics
4.1.1. Energy efficiency An energy audit for the whole house/building/site should be undertaken and recommendations should be supplied to the customer with respect to electrical energy use reductions that can be achieved. For example: • • •
Replacing inefficient electrical appliances with new energy efficient electrical appliances Replacing off-peak electric hot water heaters with a solar hot water system either gas or electric boosted.( if applicable ) Replacing incandescent light bulbs with compact fluorescents
4.1.2. Documentation When providing a quotation for the supply and installation of a PV grid connect system the minimum documentation that should be supplied to the customer with the quotation is: • • • • •
Estimate of yearly output of the PV Grid-Connected system in kWh, based on the tilt Angle and orientation of the installed array. This is the actual energy that is supplied on the output of the inverter. What percentage this represents of the average current yearly energy usage by that building/site. The amount of money savings this represents based on current electrical energy pricing. If possible the savings in CO2 (either tonnes or kg) should also be provided.
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4.1.3. System design steps The design of a PV grid connect system requires a number of steps. A basic design method follows: 1. Determine the main design criteria 2. Obtain relevant information based on critical design criteria. 3. Determining the size of the PV Array 4. Determining the size of the inverter 5. Determining the size of the interconnection cables to minimize system losses
4.1.4. System design criteria A PV Grid Connect system will, in general, be designed in accordance with one of the following limiting design criteria: • • •
Designed to meet yearly energy usage Designed to fit on available roof space Designed to meet a budget
At times the design will be limited by all three criteria and the final size of the PV array and therefore the size of the inverter required will be dependent on which is the dominant limiting criteria. In particular when designing a system to meet a budget or to meet the yearly energy usage, the final PV array must fit on the available roof space.
4.2.
DESIGN TO FIT ON AVAILABLE ROOF SPACE
Step 1: Peak power (kWp) of the array at STC conditions Parameter 1 a) Find the approximate available roof space (m²) (south facing) b) Choose the type of module (monocrystalline, polycrystalline or amorphous) and estimate its efficiency Then, the nominal power P0 is obtained by the below formula: KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 20
P0 = G 0 x A x η m
(kWp)
Where: G0: Irradiation at Standard Test Conditions (1000 W/m² or 1 kW/m²) A: Available roof space (m²) ηm: Module efficiency
Example 1: Find the nominal power of PV generator to feed a house knowing that: - Available roof space is 20 m² - Polycrystalline module with efficiency of 10%
P0 Array = 1 x 20 x
10 100
= 2 kWp
Step 2: Number of modules for the PV generator Parameter 2: c) Choose the PV module (Nominal peak power Pmpp, voltage at nominal power Vmpp etc.) Example 2: The previous example gave us a Response: total DC power (peak) of 2 kWp. What will be the number of module of Poly 170W from PoArray = Pmmp .N m Schott solar with the characteristics below? Nm = number of modules. Thus Nominal power (Pmpp ) 170 W PoArray Nm =
Voltage at nominal power 35.5 V (Vmpp ) Current at nominal power 4.78 A (Impp ) Open circuit voltage (VOC)
44.0 V
Short circuit current (Isc)
5.30 A
Nm =
Pmpp
2.1000 = 11.76 170
We shall take 12 modules of 170 Wp from Schott. The exact P0Array = 12 x170 = 2040 W or 2,04 kWp
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Step 3: PV array design and inverter check To optimize the annual electric output of the PV generator, it is essential to match the array size and the rated power of the inverter. The peak power of the solar module should not be much lower than the rated power of the inverter to avoid part-load operation (see figure 9). If the array is to large, energy will be lost because of the current limiting function at the inverter overload condition (see figure 9). The sizing rules for PV plants with inverter are thus: • MPP voltage of the plant > Minima inverter input voltage • Open circuit voltage of the plant < Maximum inverter input voltage • Prefer the nominal input voltage of the inverter (maximum efficiency) input power inverter • Power ratio in the range of 90% to 110%. Power ratio = nominal power PVplant • All connecting string must have the same voltage Parameter 3: d) Check the inverter input voltage rating e) Check the inverter MPPT range f) Check the inverter input current range g) Check the inverter power ratio Example 3: Find an inverter for a PV plant of 2.04 kWp using 12 modules of 170 W from Schott Solar (for modules’ characteristics see example 2). Use the inverters given below. Inverter example: SUNNY BOY Input data SB 2100TLL SB 2500 Max. DC power (PDC,max) 2 200 W 2700 Max. DC voltage (UDC,max) 600 V 600 V PV voltage range, MPPT (UMPP) 125 V – 600 V 224V – 600V Max. input current (IPV,max) 11 A 12 A DC voltage ripple (Upp) < 10% < 10% Max. no of string (parallel) 2 3 Output data Max. AC power (PAC,max) 2100 W 2500 W Nominal AC output (PAC,nom) 1950 W 2300 W Harmonic distortion of the grid current < 4% < 4% Nominal AC voltage (UAC,nom) 220V -240V 220V -240V Nominal AC frequency (fAC,nom) 50 Hz 50 Hz Power factor 1 1 96 94.1 Efficiency Transformerless With transformer Technology KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 22
Checking list Check inverter input voltage rating Check inverter MPPT range Check input rating,
SB 2100TLL Calculation 12*Voc = 12*44 V = 528 V
specification < 600 V OK
(125 V – 600 V)/12 (module VMPP = 10.40-50 V = 35.5 V) OK inverter 1 parallel strings current < 11 1 * ISC = 5.30 A OK
Check the inverter 2200/2040 = 1.08 power ratio
0.9 – 1.10 OK
SB 2500 Calculation specification 12*Voc = 12*44 V < 600 V = 528 V OK (224 V – 600 V)/12 module VMPP = = 18.67-50 V 35.5 V) OK 1 parallel strings < 12 1 * ISC = 5.30 A
OK
2700/2040 = 1.32
More than 1.10
96 94.1 Efficiency Transformerless With transformer Technology Conclusion: The inverter SB 2100 TLL achieves all the checking point and has a very good efficiency, it will better suits the project. However, some countries don’t allow the use of Transformerless inverters for grid-connected system. In this case, and in spite of the high power ration (1.38>1.10) of the inverter SB 2500, we are obliged to use it unless we look for another brand.
Step 4: Determine the string configuration and therefore the number of modules that will be installed in the available roof space. The final array configuration will be dependent on the inverter chosen. The different inverters available on the market do not have a common string configuration. Some allow a set number of modules in a string, eg 10 while others allow a range in the number of modules in the string. When a range is allowed then select the number that will minimize the number of modules required in the final array. After selecting the inverter and determining the number of modules allowed in the string the number of parallel strings is calculated as follows: Number of parallel strings = the actual maximum number of modules that can fit on roof space ÷ number of modules in each string. KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 23
The number of modules that will be installed on the roof = number of parallel strings x number of modules in each string
Objective 4: Energy EAC output Parameter 4: h) Find the average insolation H [kWh/m2/day], based on site location (Tilt angle Azimuth). H can be obtained from forecast stations in your country or by using some software such as RetScreen, Homer, Meteonorm etc. i) Determine the overall system efficiency h, from PV to AC grid,
η=
E AC E pvpeak
, also
called “DC to AC derate factor”, including: • • • • • •
Derating of PV nominal peak power (e.g. due to tolerances, temperature, or aging) Module mismatches Dirt Shading Losses due to wiring, blocking diodes, connectors Efficiency, and MPP tracking performance of the power electronics (“inverter”) Typical η = 75%.
Therefore, the PV system output approximate annual energy production in kWh can be written by:
E AC = 365. H. P0Array . η
Example 4: Find the annual energy EAC generated by a photovoltaic array with peak power of 2 kWp. The site is in Kumasi/Ghana (latitude = 6.41 N; longitude = 1.28 W). RetScreen simulation tool gives H = 4.35 kWh/m²/day for the global horizontal solar radiation or 4.35 solar hours and η= 75%. EAC = 365 x 4.35 x 2.04 x 0.75 = 2429 kWh/year Note: We didn’t take into account in this example, the PV array orientation and the tilt angle.
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Step 5: Simple cost analysis Parameter 5: j) Find the capital cost of PV systems. For simple analysis, we can use the statistical given by some websites. On May 25th, 2009 the Website the k) Nominal lifetime for PV systems: 20 years l) System cost : System cost = Capital cost x Power peak
Example 5: Find the payback time for a PV grid-connected system of peak power of 2.04 kW and produces 2429 kWh/year. We assume that the lifetime of the system is 20 years and whereas the kWh paid by the National Electricity Company is……… For the simple analysis, we can use the statistical data given by some websites. On May 25th 2009, the website,: http://www.solarbuzz.com estimated PV grid-connected system at $8/Wp and the price comprises: • Modules • BOS (balance of system) - Installation - Wiring - Power electronics “inverter” Simple cost per kWh calculation
kWh cost =
System cost E AC x Life time
System cost: 8 x 2040 = $16320 kWh cost =
16320 = 33.6 ¢/kWh 2429 x 20
Energy production cost in Ghana: • Hydro electric plant : 7 ¢/kWh • Thermal plant : 16 ¢/kWh
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Step 6: Cables sizing The cables in an installation must be sized correctly so that: • •
The current through the cable must be less than the safe current handling capability of the cables. The voltage drop (line losses) in the cable must be less than the value allowed by the standard (5%)
Table 1. Conductor sizes and maximum currents Cable Size, mm2 Maximum current, A
1.5 2.5
4
6
10 16 25 35 50 70 95 120 150 185 240 300
13 21 28 36 46 61 81 99 125 160 195 220 250 285 340 395
From Ohm’s law, the voltage drop in an electrical conductor is V = I R
I = Current, amps
The electrical resistance can be derived from the properties of the conductor material.
ρ = Resistivity, Ω mm2 /m
R=ρL/A
A = Cross sectional area, mm2
Combining Ohms law with the resistance expression, we get V = I. ρ L / A As a electrical DC circuit has 2 conductors, the voltage drop in the cable will be: ∆V = 2 ( L x I x ρ ) / A
R = Resistance, Ohms L = Length, m L = route length of cable in meters I = current in amperes ρ = resistivity of copper wire in Ω / m / mm2 = 0.0183 Ω A = cross section area (CSA) of cable in mm2
For example cable at DC side Max. input current of the inverter (IPV,max) = 11A Max. DC voltage of the inverter (UDC,max) = 600V Cable protection (fuse, circuit breaker) Ipro ≥ IPV,max Assume that we have a DC circuit breaker of Ipro = 15 A under 600 V and the length between the PV array and the inverter is 20 meters. KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 26
•
Choose the cable size (see table 1): For Ipro = 15A, the minima cable size is 2.5 mm² (see table 1)
•
Check the voltage drop using ∆V = 2 ( L x I x ρ ) / A ∆V = 2 (20 x 11 x 0.0183)/1.5 = 5.4 V ∆V% = 100 x ∆V/V = 100 x 5.4/600 = 0.9% which is below 5%
Step 7: System drawing with all the components It is the final step of the PV system design
F2
LOAD
DC breaker
Sunny boy SB 2100
Overvoltage arrestors
Wh
Wh Support structure
F1 Grounding UTILITY ‐
UTILITY GRID
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4.3.
DESIGN TO MEET YEARLY ENERGY USAGE
Step 0: It is necessary to obtain from the customer copies of their most recent electricity accounts. As a minimum, obtain records of their last 12 months and if possible obtain records for the last few years. Use these records to estimate the average yearly energy usage in kWh.
Step 1 to Step 7: See “Design to fit on available roof space
4.3.1. Assessment Design a grid-connected PV system for a residential house which energy consumption is estimated at 7200 kWh/year. The residential house is in Ouagadougou/Burkina where the daily solar radiation estimated is 5.5 kWh/m²/d. Use modules SQ85-P from Shell and inverters from Xantrex. The characteristics of the devices are given below: Module SQ85-P Shell Nominal power (Pmpp )
85 W
Voltage at nominal power (Vmpp ) 17.2 V Current at nominal power (Impp )
4.95 A
Open circuit voltage (VOC)
22.2 V
Short circuit current (Isc)
5.45 A
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1. PERFORMANCE PARAMETERS FOR GRIDCONNECTED PV SYSTEM
1.1.
Standard performances
The use of appropriate performance parameters facilitates the comparison of grid-connected photovoltaic (PV) systems that may differ with respect to design, technology, or geographic location. Three of the IEC standard 61724 performance parameters may be used to define the overall system performance with respect to the energy production, solar resource, and overall effect of system losses. These parameters are the final PV system yield, reference yield, and performance ratio: The final PV system yield Yf is the net energy output EAC divided by the nameplate DC power P0 of the installed PV array. It represents the number of hours that the PV array would need to operate at its rated power to provide the same energy. The units are hours or kWh/kW, with the latter preferred by the authors because it describes the quantities used to derive the parameter. The Yf normalizes the energy produced with respect to the system size; consequently, it is a convenient way to compare the energy produced by PV systems of differing size:
Yf =
E AC P0
(kWh/kW) or (hours)
The reference yield Yr is the total in-plane irradiance H divided by the PV’s reference irradiance G. It represents an equivalent number of hours at the reference irradiance. If G equals 1 kW/m2, then Yr is the number of peak sun-hours or the solar radiation in units of kWh/m2 The Yr defines the solar radiation resource for the PV system. It is a function of the location, orientation of the PV array, and month-to-month and year-to-year weather variability.
Yr =
H G
(hours)
The performance ratio PR is the Yf divided by the Yr. By normalizing with respect to irradiance, it quantifies the overall effect of losses on the rated output due to: inverter inefficiency, and wiring, mismatch, and other losses when converting from DC. to AC. power; PV module temperature; incomplete use of irradiance by reflection from the module front surface; soiling; system down-time; and component failures: KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 30
PR =
Yf Yr
(dimension less)
PR values are typically reported on a monthly or yearly basis. Values calculated for smaller intervals, such as weekly or daily, may be useful for identifying occurrences of component failures. Decreasing yearly values may indicate a permanent loss in performance
1.2.
Monitoring
Most simple way to perform monitoring is display available on inverter or on inverter control unit. Other possibilities of local monitoring includes monitoring by local PC via RS232 connection and/or remote display located in living room for example (connection between inverter and display is usually wireless). For the performance analysis, the following parameters have to be monitored: •
Array voltage - VDC (V)
•
Grid voltage - VAC (V)
•
Array current - IDC (A)
•
Grid (injected) current - IAC (A)
•
Array power - PDC (W)
•
Grid (injected) power - PAC (W) Module temperature - Tmodule (°C)
•
Ambient temperature - Tamb (°C)
•
Solar radiation - G (W/m2)
•
Daily/monthly solar insolation - H (J/m2)
•
Wind speed - v
Data can be stored in inverters memory or in external units (data loggers). Different solutions are available on the market. Some producers offer also additional memory increase or upgrade. Better data loggers offer monitoring functions (data logging) for various environmental and additional system related parameters what can give the user detailed overview about the whole system. Additional parameters that can be monitored are module- and ambient temperature, solar radiation, solar irradiation, wind speed etc. Temperature sensors are usually PT100 sensors. Solar radiation is measured with reference solar cells (Si-mono), analog inputs of the control equipment are usually standard 4-20 mA or 0-10 V inputs. Digital inputs are also available and can be used for net-meter/control equipment interconnection. KNUST_Reep Short Courses June 2009 /Grid –connected Solar PV system‐Design and Installation 31
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