Download 9. Grid-Connected of Photovoltaic Systems

9. Grid-Connected of Photovoltaic Systems
H. Boileau Savoie University, FR
Learning outcomes
After reading this chapter, the user should possess knowledge of:
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A core description of PV systems connected to the electrical network
The additional components on the DC part of PV systems
Photovoltaic inverters
The additional components on the AC part of PV systems
System design and forecast of the electrical production
Profitability of photovoltaic installations (TCE method from Bernard
Chabot/ADEME)
Assessing the compatibility between the photovoltaic field and the inverter
Description of PV frames connected to the electrical network
Photovoltaic installations connected to the electrical network represent the majority of the
photovoltaic installations currently installed in the world (in 2015). Indeed, these
photovoltaic installations are the simplest possible, therefore the costs are lower and all of
the produced electricity is injected on the grid to be used. These advantages have these
photovoltaic systems offering a lower cost per produced kWh, which explains the strong
market penetration and commercial interest.
In the absence of radiance, there is, of course no energy production, but the network
compensates. On the other hand, it should be known that, in absence of voltage, the
photovoltaic installation is bypassed for safety reasons, even if there is a strong luminous
radiance.
Figure 1 - Simplified diagram of a photovoltaic installation
Fig. 1 display a simplified diagram of a photovoltaic installation connected to the network
with the main elements
The photovoltaic field, when under solar irradiation, produces electrical energy in the form
of DC current. This DC current is transformed by an inverter into AC to be injected in the
grid, with the same amplitude and phase of the AC voltage of the network (typically an
amplitude of 230 V and a frequency of 50 Hz in Europe). There may be protection devices
installed between the PV installation and the inverter as well, if they are necessary or
compulsory due to local regulations. Protective devices can also be inserted between the
inverter and the network (inevitably necessary but not imposed by the safety standards).
Finally, there usually is an energy meter for the invoicing of produced photovoltaic kWh.
It can be useful to insert a feedback system to check how the installation runs because, for
minor issues or losses, nothing indicates if the system runs correctly or not. A display can be
used for the continuous monitoring of useful information, such as the instantaneous power
and the cumulated energy produced.
The photovoltaic field
The photovoltaic field is the entirety of the photovoltaic modules of a PV installation. The
entirety of these photovoltaic modules can be connected in various ways, to one or several
inverters. The three principal possible configurations are shown in Figure 2.
Figure 2: Principal possible configurations of PV fields.
The explanation of Figure 2 follows:
a) The entirety of the photovoltaic field is connected to one inverter, which is called the
centralized inverter. This configuration is the least expensive but all of the modules must be
of the same type, have the same angular position and direction, for the simple reason that
the current and voltage produced by each chain of modules must be of the same value. If
not, there is a production loss. Each chain obviously needs to have the same number of
modules. The influence of a shade on one or more photovoltaic modules can be rather
important on the electric production, because the modules of each chain are connected in
series and it will change change of current and/or voltage in a chain of modules that is in
parallel with the other chains. Other disadvantages include that a breakdown of the inverter
causes the complete halt of the PV field, the breakdown of a single PV module is difficult to
locate and the voltage of a chain of modules is often high, several hundreds of volts in DC
current, which is dangerous for living organisms.
b), The entirety of the photovoltaic field is divided into chains of PV modules, each one
connected to an inverter. All of the PV modules of the same chain must be of the same type,
have the same angular position and direction. On the other hand, from one chain to another,
the type and position of the modules can be different. For example, this could be a system
on different roofs connected on a single main bus. The influence of shading is less important
than in configuration a, as shaded modules will only affect the chain they are installed on.
Among other advantages, a breakdown of an inverter causes the complete halt of a chain
(but not of the whole PV field) and the breakdown of a single PV module is easier to locate.
However, the voltage of a single chain of modules remain often high, several hundreds of
volts in DC current, which is dangerous for living organisms.
c), In this configuration, each PV module is connected to an inverter, said micro-inverter in
this configuration. Here, all the modules can be of different type and positioned differently
as well, as they are independent. The breakdown of an inverter causes the stop of only one
module, therefore little production loss. The influence of shading is very limited, only to the
modules concerned. The high cost is the primary disadvantage of this solution but offers the
most advantages.
Normative Aspects on the characteristics of PV modules and safety:
For instance, we briefly mention the French norms.
- NF 61215: skill of the design and approval of crystalline PV modules.
- NF 61646: skill of the design and approval of PV modules in thin layer
- NF 61730: Characterization of the performances: flash test, NOCT, coefficients, etc.
- Mechanical Tests: charges, shock, spindly, etc.
- Climatic Tests: hot-cold, UV, etc.
- Electric Tests: dielectric, leakage current, etc.
The respect of these standards guarantees the quality of the photovoltaic modules and their
conditions of use as, for example, a voltage of insulation of 1.000 volts specifies the
maximum number of PV modules in a single chain.
The phenomenon of "hot spot''
To explain the phenomenon of hot spot, let us take an example with a photovoltaic module
of 72 cells (characteristic CP = 250Wc, Vmpp = 36Vdc, Voc = 45Vdc, Impp = 7A @ STC).
If a cell of this module is shaded (Icell = 0A) or this module is short-circuited (Figure 3):
Figure 2: PV Module with one shaded cell.
The shaded cell (Icell = 0A, thus it behaves as an open circuit) receives all the voltage in
reverse. The opposite breakdown voltage of a cell PV is typically of 25 Vdc (Zener voltage),
but the operative voltage of the module is Vmpp = 36 Vdc. Therefore, the cell is destroyed,
because it receives a strong voltage and becomes conducting, resulting to destruction by
overload.
Let us consider a more realistic case of a photovoltaic field with three chains (or strings) of
six modules in series, where one cell of one chain is shaded like before (Figure 4).
Figure 3 : Three chains with one shaded cell.
The operating voltage of such a string is 36 Vdc × 6 modules = 216 Vdc. Where the cell is
shaded (Icell = 0A), this chain is an open circuit, with a no load voltage of 45Vdc × 6 modules =
270 Vdc. The reverse voltage of the shaded cell is then 54 Vdc. The Zener effect takes place in
this photovoltaic cell and it becomes conductive.
The power dissipated in the shaded cell PV is 25V × 6A = 150W, but the photovoltaic cell is
not designed to dissipate this levels of power (no more than few Watts). This power will be
converted to thermal where the photovoltaic cell is the most resistive until carbonizing that
point (from where the name of hot spot originates). Figure 5 displays a photograph of a PV
panel destroyed by such an effect.
Figure 4: Damaged PV Panel, hot spot.
The solution to prevent the phenomenon of hot spot is to use diodes, known as bypass
diodes, connected in reverse and per about 20 PV cells. In the example of a 12 Volts PV
module with 36 cells, a bypass diode is connected per PV 18 cells, as shown in Figure 6:
Figure 5 : Operation of a bypass diode.
For this 12 volts module, the shaded cell results to the lack of current generation by the cell.
Without a bypass diode, the PV module produces a lower or no current, with the risk of a
hot spot arising. With the bypass diode, the power generated by the group of 18 unshaded
cells can go through the bypass diode but, in the case of our example, the output voltage of
the module will be two times lower. However, the diode prevents from having a high voltage
in reverse on the shaded cells.
For PV modules of silicon crystalline technology, the bypass diodes are cabled in the
connecting box (Figure 7).
Figure 6 : Connection of a bypass diode.
The number of bypass diodes depends on the number of cells in PV module. Generally, two
bypass diodes for a module with 36 cells, three bypass diodes for a module with 60 cells
(very common type of module) and four bypass diodes for a module with 72 cells.
The influence of the shade on a standard 12 Volts module, according to the point of
operation, is described by the two following figures:
Figure 7 : Current vs voltage for different shades on a cell
Figure 8 : Power vs voltage for different shades on a cell
If a PV cell on a module is shaded, the module will produce an electric current lower than the
rated current. If the load requires a very weak current, the output voltage will be the
nominal voltage, but if the current consumed increases higher than what the shaded cell can
produce, the voltage will decrease by a factor of two because of the bypass diode. Figure 8
shows that if a cell is shaded, then two points of maximum capacity appear, variable
according to the extent of the shading. This phenomenon will thus disturb the operation of
the MPPT (Maximum Power Point Tracking) process of the inverters.
On a PV module where a cell is defective (shades or similar defect), measurements of the noload voltage and short-circuit current are both good. To detect this kind of defect, it is
necessary to generate a current/voltage graph.
Protection against reverse currents:
Reverse currents are another phenomenon that can deteriorate PV modules besides the
phenomenon of hot spot. The phenomenon occurs when the current of a string is reversed
when several strings are connected in parallel. In this case, if one of the chains is shaded, the
cells do not produce any current. The current produced by the parallel chains now may pass
through the chain with the shaded modules, leading to its destruction.
With respect to the norms, a module should withstand in reverse twice the current that it is
able to produce under STC conditions. When there are more than three chains, it is
imperative to put a protective component in series with each chain, like a diode, to avoid
this reverse current.
Figure 9: Wiring example of three strings of six PV modules with the bypass diodes at the
edges of the PV modules and the serial diodes at the end of the strings to protect from
reverse currents.
The serial diode protects effectively but causes a loss in power (loss of 1 volt if connected in
series). In France, UTE C15-712 guideline recommends the use of a fuse, which is causing a
lower power loss but fuses are irreversibly damaged when actuated. It is important to
accurately assess the value of this fuse. Often, the manufacturers of PV modules provide the
recommended value of this serial fuse with the electric characteristics of the PV module.
Additional Components on the DC part of photovoltaic installations
Cables and electric connectors
The electric cables used in photovoltaic installations must meet specific criteria (standard
UTE C 32-500). Some of these criteria are: a double insulation (class II), resistance to 1000V,
resistance to UV and resistance up to a temperature of 90 °C. The cross section of these
wires is standardized. For example, commercial wires may have a cross section of 1.5 mm²,
2.5 mm², 4 mm², 6 mm², etc. The cross section of the wires is selected according to the
intensity of the current and the length of wiring.
Figure 10 : Cables commonly used in PV installations.
The electric connectors are secure (standards UL1703, VDE126-3, etc.). This means that they
offer protection against direct contact, they can be lockable (type MC4) or not (MC3)
depending on their accessibility, with a good behaviour towards UV and bad weather (IP54).
Figure 11: Examples of connectors used in PV installations.
Each photovoltaic module has two connectors, male and female, which facilitates the
connection in series of those. Additional cables are used to make inter-connections
between the strings in parallel and between the PV fields and the inverters.
TAKE HEED, because of the continuous solar irradiation during the day, the voltage at the
boundaries of the PV field has a magnitude of several hundreds of volts. This voltage can be
dangerous during the implementation or the maintenance of a PV installation and, in the
case of a circuit breakdown, a maintained electric arc is being created, because there no
passage through 0 volts like in the case of the AC voltage. For that reason, the use of circuit
breakers specific to photovoltaic is often required (Guideline UTE C 15 712 in France).
Figure 12 : Electric Arc in a PV circuit.
Protection against lightning of PV installations
The photovoltaic field is by default exposed to the sun and subject to all weather conditions,
including direct and indirect lightning impacts. For that, safety standards recommend or
impose the use of lightning protector with suitable installation of the ground cable, with its
specifications depending on the area where the PV installation is installed (in France,
guideline UTE C15 712).
Figure 13:Example of a PV installation grounding (source: Diagram Dehn, online:
http://www.dehn.de/pdf/blitzplaner/BBP_2007_E_complete.pdf)
To protect from direct lightning impacts, there is almost no other solution than a lightning
protector. For indirect lightning impacts, there are various solutions to decrease the
destruction risk of the PV installation components. For example, the wiring of the PV
modules can be done in such a manner so as to decrease the surface of the loops (Figure15),
reducing the electric field induced in the loop by the strong magnetic variation caused by the
amperage of a flash striking the ground in the vicinity.
Figure 14: Wiring examples of four PV modules.
DC Junction Box
A PV field generally is composed of PV modules cabled in series between them, creating
strings. This is done to reach a high enough voltage. When the desired voltage has been
reached, multiple chains can be connected in parallel (as long as their voltage remains the
same), preserving the voltage and increasing the amperage (basic principle of electricity).
Dimensioning requires adaptation through a thoughtful arrangement of the PV modules in
series and parallel to surface available (on a roof, for example), but especially on the voltage
and current intensity specifications of the inverter.
Between the PV field and the inverter(s), a junction box is used to connect the chains of
modules in parallel between them. The junction box also includes the protection
components, such as the lightning protectors, the fuses, DC switches, etc.
Figure 15:Example of a junction box with four chains in parallel, two lightning protectors and
a DC switch.
Photovoltaic Inverters
DC to AC inverters used by PV field convert electrical energy from the DC that the PV field
generates to AC, compatible in terms of voltage and frequency with the network.
The electric symbol of PV inverter is:
Various types of inverters:
Figure 16 : Inverters for PV fields (Source: SMA)
Inverters for PV fields (Figure 17) are typically used for large PV installations on ground or on
mounts, rated for several hundreds or thousands of kWc. Their AC output usually is threephase and the input DC voltage is up to a few hundred volts.
Figure 17 : Inverters for PV installations (Source: SMA)
Inverters for PV installations (Figure 18) are used in small or medium size projects. They are
rated from some kWc up to a few hundreds of kWc. The inverter may be connected to one
or several strings of PV modules, depending on the model and the size of the installation.
Their input DC voltage usually is rated for a few hundreds of volts,, while the AC output may
be single or three phase.
Figure 18: Micro Inverter.
The micro inverter (Figure 19) is connected to a single or up to a few PV modules.
modules These
inverters are not rated for more than a few hundred watts, while their input voltage usually
is not higher than a few tens of volts. The AC output is single phase. Because of the low DC
voltage input, these inverters are interesting because of their safety and can be used for
small, medium or even large photovoltaic fields.
fields
All types of inverters have in common the seeking of the maximum capacity operation point
(MPPT – Maximum Power Point Tracking), a conversion efficiency from DC to AC of about
95% and automatic
tomatic disconnection if no AC network voltage is detected (Standard VDE0126VDE0126
1-1)
1) to avoid the electrocution of workers in cases of network maintenance.
Principle of operation of a PV inverters:
The purpose of the PV inverter is to convert the electric output generated by the PV field to
an AC output compatible with
ith the network.
network To that purpose, the MPPT system seeks the
point of operation where the power is maximum from all of the possible points of operation
o
(DC voltage) at exit of the PV field. Then a second electronic system,, the inverter, converts
the DC voltage into AC, compatible in terms of magnitude and phase with that of the
network. The inverter's efficiency depends on how well it can match the voltage, frequency
frequ
and phase of the network, as the portion of power injected to the network to the produced
power from the PV field determines the electric losses in the inverter. The electronics of the
PV inverter must also take into account the safety standards,
standards such as the disconnection from
the network in the event off temporary absence of AC voltage, or avoid the insertion of
harmonics to the network that can disturb the operation of sensitive electrical appliances.
Part of the performance feedback from a PV installation can also be assured by the inverter,
inverter
by recording or forwarding to a server its operating information.
Electric Characteristics of a photovoltaic inverter:
The photovoltaic inverters have electric characteristics to be consulted for their correct
cor
operation (Figure 20). The main features are:
On the DC (input) side:
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Maximum capacity input: Maximum Pin in Watts
Maximum voltage input: Maximum Vin in Volts
Range of voltage of operation MMPT input: from Vmppt min to Vmppt max
Maximum intensity input: Maximum Iin in Ampere
On the AC (output) side:
•
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Maximum output power: Pout in Watts
Typical voltage and the operating output range: Vac typ, from Vac min to Vac max
Maximum output intensity: Iac max
Conversion yield at the nominal output
Figure 19 : Photovoltaic example of inverter and its characteristic (Source: SMA)
European standardized output of a PV inverter:
The PV inverter consumes a small part of the electric energy generated by the PV field (or
from the network at night), inducing losses. The output of a PV inverter is generally defined
at 100% of its nominal output. However, as solar irradiance during the day varies greatly, the
electric production of the PV field will vary accordingly and, ultimately, the power point of
operation of the PV inverter will also vary from zero (at night) to a value close to its nominal
output under the best conditions of solar irradiance (if correctly dimensioned). To calculate
the electric production of a PV installation in a more realistic way, a European average
output was defined according to various points of operation with a coefficient for each one
of these points of operation.
This European output was defined according to the formula found in Figure 21.
Figure 20 : European standardized output of a PV inverter.
Safety and standard for the photovoltaic inverters:
The PV inverters must conform with various standards, the most important of which
certainly is the standard VDE0126 that obliges the decoupling of the PV inverter when there
is no network voltage. Indeed, if a worker locally disconnects part of the network for
interventions/repairs, it is essential that the PV inverter stops the injection of electricity to
avoid electrocuting the maintenance workers.
Figure 21 : The conditions imposed by standard VDE0126 for France. These conditions can
slightly vary from one country to another.
Additional Components on the AC side of the photovoltaic installations
Network side box with the components of protection:
(C15-100 Standard for France, consumer side)
Figure 22: Network side box with the components of protection.
In this box, we find components such as differential circuit breakers (usually rated at 30 mA)
lightning arrest switches, disconnecting switches (Figure 24).
Figure 23 : Differential circuit breaker (30 mA) lightning arrest switch, disconnecting switch.
It is important to affix a descriptive label indicating the connection of a photovoltaic
installation inside the electric box, network side, to indicate a potential danger to a
technician (Figure 25).
Figure 24 : Descriptive label indicating the connection of a photovoltaic installation
Electric meters and circuit breaker network side:
(Standard C14-100 in France, distributer side of the network)
The production of photovoltaic electricity must be routed through an electric meter to allow
an invoicing of the electrical energy injected in the network, just like the electricity
consumption by an individual. These metering elements, along with protection devices like
the differential circuit breaker, are generally installed within the property but belong to the
electrical energy distribution company.
Figure 25 : Example of an electric meter and differential circuit breaker
The electric meter allows the counting of the kWh produced by the photovoltaic installations
that were injected into the network, as shown in Figure 27:
Figure 26 : Dual electric meter connection.
Usually, a second electric meter is put in series, but oppositely connected, to meter electric
kWh that may be potentially consumed from the network (by the inverters during night
time, for example). From the aforementioned setup, a consumer of electrical energy can
connect and consume electrical energy locally. In this case, part of the photovoltaic
production will be self-consumed, while the surplus will be injected to the network (Figure
28).
Figure 27 :Dual electric meter connection (self-consuming setup).
It should be noted that to know the quantity of photovoltaic energy self-consumed, it is
necessary to add another electric meter in serial to the photovoltaic installation. This meter
is not essential for the distributer of electrical energy, it only allows the user to know the
electrical energy consumption that the photovoltaic installation provides.
In the case where the producer of photovoltaic electricity wishes to inject all the production
in the electrical network (in the case of an advantageous tariff agreement), it is necessary to
have a second electric network connection for the consumption of electricity from the
electrical network (Figure 29).
Figure 29: nework configuration for injection
Dimensioning or calculation of the photovoltaic yield
Depending on the case and purpose of the PV installation, the designer can either assess the
electric production of the PV system or to size the installation according to certain power
requirements. Very often, especially with PV installations that will be connected to the
electrical grid, the peak power Pc of the installation is limited by external conditions, such as,
for example, the installation surface available. In this case, the peak power Pc of the PV
installation is known and the annual electricity production is what needs to be assessed. To
that end, it is essential to possess data on the annual average irradiation in the vicinity of the
PV installations, and to calculate the optimal tilt and orientation of the PV panels.
The typical data required for the sizing of PV installations is the annual global irradiation
(IGPan) on the horizontal plane. Software, such as Meteonorm, can determine this value for
any site in the world with fair precision, for any given installation tilt and orientation. There
are also free data resources, such as PVGIS (http://re.jrc.ec.europa.eu/pvgis/), which
calculate and offer such data for almost any position in Europe, Africa and Asia by using
climatic maps (Figure 30).
Figure 30 :PVGIS output example for the site of the INES in Le Bourget du Lac in France
The value for the average overall annual irradiation on a plane with a tilt of 30° towards the
south is 3.98 kWh/m² per day (BDD classic PVGIS), which is a global irradiation received by
the modules equal to:
IGPan = 365 × 3.98 = 1452 kWh/m² per annum.
The third important data figure is the performance ratio (PR). The PR represents the entirety
of the electric losses, which include the losses from the Joule effect in the wiring (~1%), the
losses of the PV inverter (~5%) and, most importantly, the losses from the rising
temperature of the PV modules under solar radiance (~10% to ~15%).
Different types of installations affect the airing of the PV modules and, in extent, their
operating temperature under solar irradiation. For example, roof-mounted PV modules may
have higher operating temperatures than ground installations on mounts. Depending on the
location of the installation and the mounting type, it has been empirically estimated that the
PR is close to the following values:
Well ventilated Installation (e.g. PV installation on the ground) PR = 0.8
Fairly badly ventilated (e.g. super-installed on roof): PR = 0.75
Badly ventilated (e.g. fully integrated on roof): PR = 0.70
From these three figures, Pc, IGPan and PR, the annual electricity production Ea of a PV
installation can be assessed:
Ea (in kWh per annum) = Pc (Wc) × IGPan (in kWh/m² per annum) × PR (dimensionless)
This formula appears inhomogeneous but it should be noticed that the peak power Pc
expresses the electric output of a module under an irradiance of 1000W/m2 and is not a SI
unit. As IGPan expresses the annual solar irradiation in kWh/m2, it can be assumed that it
shows the number of hours that the modules will operate under a theoretical irradiance of
1000W/m2. In other words, the PV installation will function this number of hours at its Pc
rating.
Remarks:
1) The performance ratio PR represents the ratio of the electric output of the
installation, or the electrical energy that is supplied to the grid (Ea) divided by the
theoretical electrical energy produced by the PV installation (which is Pc × IGPan)
2) The energy conversion output is calculated through the power peak Pc, which
depends on the surface and the output of the PV module under STC conditions (STC
for Standard Test Condition, i.e. a radiance of 1000W/m², a temperature of 25°C and
a solar spectrum AM1.5)
3) The above formula is usable only if the electric power output of the PV installation is
a linear function of the irradiance, which is not completely true in real-world
conditions, but it is sufficient for a quick assessment. To obtain better precision on
the produced PV energy, it is necessary to use a simulation software, such as PVsyst
or PVSOL that will take into account this nonlinearity.
Similarly, it is possible to modify this formula for other time intervals, like a month or a day
(in energy):
EM (in kWh per month) = Pc (Wc) × IGPm (in kWh/m ² per month) × PR (dimensionless)
Ej (in kWh per day) = Pc (Wc) × IGPd (in kWh/m ² per day) × PR (dimensionless)
Likewise, it is possible to write this formula for one moment t, therefore the operating
power Pe from the radiance IGP (which is in W/m ²):
Pe (in kW) = Pc (Wc) × IGP (in kW/m ²) × PR (dimensionless)
Or Pe (in W) = Pc (Wc) × IGP (in W/m ²) × PR (dimensionless)
Example of dimensioning assessment
Previously, for the site of INES in Le Bourget du Lac, we calculated the global irradiation on a
30° inclined plane towards south. It is IGPan = 1452 kWh/m² per annum (equivalent to 1.452
hours with a solar irradiation of 1 kW/m²). For a photovoltaic installation with a power peak
of Pc = 3 kWc (surface of 30 m² and 10% energy conversion losses), with the installation
integrated on a roof and the performance ratio considered equal to 0.7.
The annual photovoltaic production Ea estimated is:
Ea = 1.452 × 3 × 0.70 = 3.049 kWh
Remark:
1) The average consumption in specific electricity (without neither heating nor warm water)
of a household in France is approximately 3000 kWh per annum, roughly equivalent to the
electric production of a photovoltaic installation of 3 kWc.
2) The load factor (number of operating hours at nominal output of the installation,
therefore the power peak) is 3049 kWh/3 kW = 1016 hours. This load factor depends on
many parameters, the principal one being the solar irradiation. It varies from 800 hours in
northern Europe to 1500 hours in southern Europe. The load factor is lower compared to
that of wind, approximately 2000 hours, and that of the nuclear power plants, approximately
7000 hours. The better the load factor is, the closer the production of electricity is to the
peak output installed. The load factors mentioned above for PV installations are not
favourable for their economic profitability, which indicates that the initial investment per
power unit will have to be low to remain competitive.
Compatibility study between the photovoltaic field and the inverter
The dimensioning of a photovoltaic installation usually begins by setting a certain
photovoltaic modules number on the supporting mounts (building roof or site on the ground
or structure) with a certain tilt and orientation. This process will determine the number of
photovoltaic modules to be used, according to the available area, but also reveal the various
possible wiring configurations of the modules (number of modules per string and number of
strings). The number of photovoltaic modules per string and the number of strings condition
the electric output, but also the voltage and the current at exit of the photovoltaic
installation. External parameters, such as the solar irradiance and the ambient temperature,
should also be taken into account. The inverter connected to the photovoltaic field has
operating ranges for input and output voltage, current and power, all of which will have to
be compatible with the electric production of the photovoltaic field and the specifications of
the load/grid.
Compatibility in Power:
Due to the fact that solar irradiance under the European latitudes goes up to approximately
1000 W/m² and that the performance ratio generally is about 0.8, the inverter's power is
usually selected to fall between 80% and 100% of the photovoltaic field's power peak. To
ideally estimate the power of the inverter correctly, the power bar chart at exit of the
photovoltaic field is required. This bar chart can be simulated by specialized software, like
PVsyst.
Let us examine two examples:
Example 1. Figure 31 shows the power bar chart of a 3.18 kWc PV field, facing south on a 30°
tilt, in Geneva (Swiss).
Figure 31 Power bar chart of a 3.18 kWc PV field, facing south on a 30° tilt, in Geneva (Swiss).
If the dimensioning of the inverter is equal to 80% of the field's power peak, which is 2.55
kW, the bar chart shows that the plant is under-dimensioned because the power of the field
between 2.55 kW and 3 kW is not fully used. It would be more judicious to choose an
inverter of 3 kW or 3.2 kW (close to the 100% of power peak of this field).
Example 2. Figure 32 shows the power bar chart of a 3.18 kWc PV field, facing south on a 90°
tilt, in Geneva (Swiss).
Figure 32 : Power bar chart of a 3.18 kWc PV field, facing south on a 90° tilt, in Geneva
(Swiss).
If the dimensioning of the inverter is equal to 100% of the power peak of the field, that is to
say 3.2 kW, it can be seen in Figure 32 that the plant is over-dimensioned because the field
does not deliver any power between 2.5 kW and 3.2 kW. It would have been more judicious
to choose an inverter of 2.4 to 2.5 kW that is capable of taking advantage of the field's full
output.
Thus, the ideal is to know the power bar chart, but that is not always possible. Generally, an
inverted equal to 80% of the field's power peak is used when the conditions of tilt and
orientation are unfavourable (vertical, eastern or western orientation), the average ambient
temperatures are rather high and there are conditions of poor solar irradiation, such as in
downtown area (pollution) or at the coastline. Similarly, inverters rated at 100% of the field's
power peak are selected when the conditions of tilt and orientation are favourable, the
average ambient temperatures rather low and there are good solar irradiation conditions,
such as in the countryside. It will be perhaps even necessary to over dimension an inverter in
locations where the sky is very clear, such as high in the mountains (lower atmosphere
density, less pollution, low average temperature and an high albedo).
Compatibility in current intensity:
The solar irradiation from a clear sky is about 800 to 900 W/m², but under certain conditions
with a strong direct radiance and an overcast sky of white clouds (important diffusion), the
solar radiance can reach 1300 W/m² for a few minutes some times per annum. Although
that is not detrimental for the inverter if it features over current protection, it is
recommended to have a safety margin on the acceptable maximum current intensity of the
inverter (attention to the fuses of protection that also have to be over dimensioned to avoid
having to change them too often. Usually, the ideal is to have a 30% margin (according to the
country's legislative recommendations) compared to the STC operation conditions of the
photovoltaic modules.
Compatibility in no-load voltage:
Without any grid voltage, the inverter is disconnected and therefore the current intensity is
zero. However, the inverter remains connected to the PV field, where now the voltage is the
highest possible. The worst case scenario for the maximum voltage at the input of the
inverter needs to take into account that the solar irradiance might reach up to 1300 W/m².
This voltage must remain lower than the maximum acceptable voltage by the PV inverter, or
there is a risk of damaging it. This is one of the leading causes of inverters breakdowns,
especially when the high no-load voltage has not been checked in mountain installations).
For that reason, it is recommended to have a safety margin on the maximum no-load
voltage, ideally 15% compared to the photovoltaic modules STC conditions (according to the
countries legislative recommendations).
The photovoltaic modules usually have a maximum operating voltage, often about 1000 volts
(check the characteristics of the photovoltaic modules). This value allows to calculate how
many modules it is possible to put in a single serial string.
Compatibility in operating voltage:
Under operation, the inverter adjusts the point of operation at the boundaries of the
photovoltaic field to seek the point where the power is maximum (MPPT), selecting the best
possible pair of voltage and current intensity. The selected pair varies according to the solar
radiance and the cell temperature of the photovoltaic modules. The current intensity under
operation varies between zero and the maximum, whereas the operating voltage will vary
between a Upvmin value (as soon as the irradiance reaches some tens of W/m²) and a
maximum Upvmax value (for a strong irradiation). For good compatibility between the
photovoltaic field and the inverter, these two voltage values must be within the Vmppt-min and
Vmppt-max voltages of the photovoltaic inverter (see the electric characteristics of PV inverter).
Software such as PVsyst or PVSOL make it possible to simulate all of the operation points of
the voltage, current intensity (and, thus, power) over a year. The output can be hourly, using
an average weather file, allowing to check compatibility between the PV field and the
photovoltaic inverter.
For a summary check out, it is possible to take the following conditions:
Vmppt-min inverter < 80% of the typical STC voltage of the PV field
Vmppt-max inverter > 115% of the typical STC voltage of the PV field
Explanation: 80% of typical STC voltage of the PV field represents the voltage of the PV field
for a radiance of 100 W/m² (for a silicon crystalline PV field) and 130% of the typical STC
voltage of the PV field represents the voltage of the PV field for a radiance of 1300 W/m²
(for a silicon crystalline PV field).
Dimensioning example of a small photovoltaic installation
Let us assume that we have a roof upon which we want to install PV modules. Eight
Photowatt PW2350-235 modules (Figure 33) can fit on the roof.
Figure 33 :Photowatt PW2350-235 module characteristics.
These eight photovoltaic modules are connected to a SMA Sunny Boy 1700 inverter (Figure
34).
Figure 34 : SMA Sunny Boy 1700 inverter characteristics
By choosing a wiring of two strings in parallel with four modules in series per string attached
to the inverter (Figure 35),, let us see if this combination is compatible or not.
Figure 35 :Two
Two strings in parallel with four modules in series per string attached to an
inverter.
points
Let us check the various points:
Power check:
The photovoltaic
voltaic installation includes eight Photowatt PW2350 - 235Wc modules, so the
total peak power is 1880 Wc. The inverter SMA Sunny Boy 1700 has a maximum power input
of 1850 Watts.
The ratio inverter/field power is equal to 0.98, is between 0.8 and 1 (adapted for a
photovoltaic field in good conditions, typically tilted to 30° and directed
irected towards the south).
Thus, the selection is OK in terms of power.
Current intensity check:
The amperage output of the PV installation in the worst case scenario is equal to the typical
intensity (STC conditions)) multiplied
multipl
by two (because there are two strings in parallel) and
again multiplied by 1.3 (assuming
assuming a radiance
r
of 1300 W/m²). This equals to 7.86
7
A × 2 × 1.3 =
20.43 A. The maximum current input of the inverter is 12.6 A, which is lower than the
current produced by the photovoltaic field. Thus there is a problem with the current
intensity.
No-load voltage check:
The no-load voltage of a module is 37.2 V (STC conditions). With a radiance of 1300W/m²,
this no-load voltage is 37.2 V × 1.15 = 42.78 V. Knowing that the maximum voltage of the
circuit cannot exceed 1000 VDC, thus 1000 V/42.78 V = 23.37, so we can have a maximum kf
23 PV modules in series. In our case, we have only four modules in series.
With four PV modules in series, the maximum voltage of the photovoltaic field is 37.2 V ×
1.15 × 4 = 171.12 V. This value is lower than the maximum input voltage of the inverter that
is 400V. Thus, the maximum voltage no load voltage is OK.
Typical voltage operation (MPPT point) check:
The typical voltage (not MPPT) of a module is 29.9V (STC conditions). With four PV modules
in series and a radiance varying from 100 W/m² to 1,300 W/m², the voltage under operation
of the photovoltaic field varies from 29.9V × 4 × 0.8 = 95.68 V to 29.9 × 4 × 1.15 = 137.54 V.
As the operating range of the inverter varies between 147V with 320V, the output voltage
of the photovoltaic field is too low for the inverter. Thus, there is a problem with the MPPT
operation voltage.
To conclude this example of wiring in parallel two strings of four modules each and
connecting them to a specific SMA inverter, this combination is not compatible. The voltage
is too low and the current too high. A solution would be to seek another inverter or to
change the type of the photovoltaic modules, but an obvious solution would be to change
the wiring of two strings of four modules into a single string of eight modules in series,
increasing the voltage and reducing the current output of the PV field. Let us investigate
whether this association is compatible.
Power check:
The same as before. Thus, the selection is OK in terms of power.
Current intensity check:
The amperage output of the PV installation in the worst case scenario is equal to the typical
intensity (STC conditions and only one string) multiplied by 1.3 (Radiance of 1300 W/m²),
which is 7.86 A × 1.3 = 10.21 A. The input current of the inverter is 12.6 A, which is higher
than the current produced by the photovoltaic field. Thus, the current intensity is OK.
No-load voltage check:
The no-load voltage of a module is 37.2 V (STC conditions). With a radiance of 1300W/m²,
this no-load voltage is 37.2 V × 1.15 = 42.78 V. Knowing that the maximum voltage of the
circuit cannot exceed 1000V DC, thus 1000 V/42.78 V = 23.37, so we can have a maximum kf
23 PV modules in series. In our case, we have only eight modules in series.
With eight PV modules in series, the maximum no load voltage of the photovoltaic field is
37.2V × 1.15 × 8 = 342.24 V. This value is lower than the maximum input voltage of the
inverter that is 400V. Thus, the maximum voltage no load voltage is OK.
Typical voltage operation (MPPT point) check:
The typical voltage (not MPPT) of a module is 29.9V (STC conditions). With eight PV modules
in series and a radiance varying from 100 W/m² to 1300 W/m², the voltage under operation
of the photovoltaic field varies from 29.9V × 8 × 0.8 = 191.36 V to 29.9 × 8 × 1.15 = 275.08V.
As the operating range of the inverter varies between 147V with 320V, the output voltage
of the photovoltaic field is compatible with the inverter. Thus, there is no problem with the
MPPT operation voltage.
Therefore, with the wiring of the eight modules Photowatt 2350 - 235 in series at the input
of an SMA Sunny Boy 1700 inverter, this configuration is compatible.
The most important point to check is not to exceed the maximum voltage at the input of the
inverter because that can be destructive for the inverter. For the other cases, generally the
risk is only to have a lower production than that expected.
In this example, compared to the point of operation under STC conditions (radiance of 1000
W/m², spectrum AM1.5 and temperature of 25°C), the maximum intensity is taken with a
factor of 1.3, the maximum voltage is taken with a factor of 1.15, and the minimum voltage
with a factor of 0.8. These factors are relatively arbitrary but make it possible to simplify the
study. For different studies, these variations are calculated by using the temperature
coefficients of the photovoltaic module while varying the temperature (e.g. from 0°C to
70°C). This method gives results rather similar to the factors described before. On the other
hand, an experienced engineer should be able to realistically adapt these factors for various
conditions, such as a desert or the mountain landscape with extreme temperatures and
radiances.
For the compatibility check between the PV field and the inverter, inverter manufacturers
often provide free software..
Figure 36 : PV field to inverter compatibility check software (Source: SMA Sunny Design
software)
Bibliography
Weiss, Johnny. "Photovoltaics Design and Installation Manual." (2007): 52-54.
Balfour, John R., and Michael Shaw. Advanced photovoltaic system design. Jones & Bartlett Publishers, 2011.