FAQ
What is a SolarEdge decentralised system ?
Its operation is simple. Firstly, the MPPT is decentralised, and the search for the maximum power point is carried out individually for each panel by means of a Powerbox. There is also a simplified inverter consisting of an AC/DC converter which is optimised to work at a constant voltage.
The system therefore consists of two important components (the Powerbox and the SolarEdge inverter). In addition, the inverter is equipped with a monitoring component which gathers the data transmitted by the Powerboxes. This data is then analysed by the monitoring component, and can be viewed via a computer.
The advantages of working with a decentralised system are as follows:
- The maximum limit for the number of modules per loop is higher.
- No losses caused by partial shade.
- No losses due to the difference in orientation of the panels.
- Identification of breakdowns is easier for the entire installation.
- The Powerboxes are kept at a safety voltage of 1 Volt during fitting and maintenance of the photovoltaic installation.
- Decreased energy losses, known as “dynamic MPPT loss”, thanks to faster MPP search.
- An anti-theft system which warns the user if a module is stolen. Furthermore, the stolen module is unusable with an inverter other than the one for which it has been registered.
Advices
In an installation where it is difficult to avoid having shadow on the panels, it is advisable to opt for a SolarEdge decentralised system. The diagrams below show the efficiency of a SolarEdge system in comparison with a standard inverter when shadow is cast on one of the modules of the photovoltaic field.
Operating principle of a centralised inverter when affected by shade:
Operating principle of the SolarEdge decentralised inverter and the Powerboxes when affected by shade:
What is the difference between an inverter with and without a transformer ?
In an inverter, a converter enables its voltage to be adapted to that of the network. When the inverter has a transformer, this provides galvanic insulation between the input and the output of the inverter. The galvanic insulation makes it possible to avoid continuous injection of current into the network.
On the other hand, the transformer generates losses. This system therefore has a lower output (loss of output of approximately 1 to 2%) than the system without a transformer.
Inverters with a transformer are heavier and larger, except for inverters with a transformer which works at high frequencies. Smaller and lighter inverters are possible with high-frequency transformers.
Inverters which use technology without a transformer are also permitted on the network, provided that they guarantee never to inject a direct current which is more than 1% of the nominal value, or have a system for monitoring against DC injection. This type of system deactivates the inverter in less than 0.2 seconds when the direct current exceeds the threshold value, which is fixed at 1% of the nominal current.
What is the “derating” of the inverter temperature?
Derating is the reduction of the inverter intake power when its critical temperature is reached. Above the critical temperature the inverter is cut off if its temperature continues to increase.
Derating does not have a negative impact on the inverter. However, certain points must be checked if this happens too frequently.
Advices
If your inverter is regularly reaching its critical temperature:
- Check that the undersizing of the inverter is not too great
- Check that the inverter cooling system is working properly
Why does the inverter have a cooling system ?
A heat removal system makes it possible to protect the inverter’s sensitive semiconductor components against overheating.
There are two types of cooling systems:
• Passive: by means of a convector
• Active: by means of a fan
It must always be possible for air to circulate properly inside and around the inverter.
Example of an installation of several Power-One inverters with a passive cooling system.
The inverters are placed next to one another in order to assist air circulation from bottom to top.
What is the cut-in voltage of the inverter?
When the voltage of the loop is switched on, it is equivalent to the sum of the voltages of each panel, whereas the current is equal to the current which circulates through a single panel.
The cut-in voltage is the minimum voltage below which the inverter will not be able to start operating.
When the voltage supplied by the panels is too low, i.e. when it is below the cut-in voltage, the inverter will not operate. This happens when the temperature of the modules is too high or the irradiance is virtually zero.
What is a multi-tracker inverter ?
It consists of:
- Several MPPTs which enable the photovoltaic field to perform better
- A transformer on the direct current side by means of MPPT
- A converter from direct current into alternating current
- A transformer (depending on the technology)
- Decoupling protection
This diagram shows a multi-tracker inverter without a transformer:
What is a mono-tracker inverter?
It consists of:
- A single system for finding the maximum power point (MPPT) of the photovoltaic field
- A converter from direct current into alternating current
- A transformer (depending on the technology)
- Decoupling protection
This diagram shows a mono-tracker inverter with transformer :
What is the search for the maximum power point or MPPT ?
The tracker contained in the inverter searches for the optimum operating point so as to guarantee production of maximum electric power. The method consists of defining a voltage (UMPP) and an intensity (IMPP) in order to obtain maximum power (PMPP).
Location of the optimum power point on the characteristic curve of the PV panel:
The maximum power point varies constantly according to the quantity of light received (irradiation) and the temperature. The optimum operating point therefore depends on a constant search for this maximum point. This function is carried out by the MPPT (the tracker of the MPP point) included in each centralised inverter.
Advices
If the dimensions, configuration of the locations and of the inverter permit, it is preferable to use all the MPPTs available in order to obtain better performance from the installation.
What is an inverter ?
The inverter is defined as a device which makes it possible to convert direct current into alternating current. Its main functions are:
- Searching for the maximum power point (MPP) relative to the voltage and intensity supplied by the panels;
- Transforming the direct current into alternating current with the same characteristics as the network;
- Synchronising with the network in order to make it possible to inject the current.
- Automatic network decoupling (the inverter is cut off when the network is switched off).
What is the effect of shade ?
Shadow on part of the photovoltaic panel decreases the electrical production of the panel. The photovoltaic cells are connected in series. The cell which is in the shade will impose its low current on the remainder of the photovoltaic module. This cell is in danger of being damaged by overheating. To protect the cells against shade, bypass diodes are used.
Shadow on the modules must be avoided as much as possible. It is therefore necessary to examine the shade in order to arrange the panels so that the fewest possible rows of cells are affected by shadow.
In the diagram below, it is possible to prevent all of the cells’ electricity production from being affected by shade. The panels must be arranged so that the bypass diodes can fulfil their purpose by allowing the current to pass beyond the cells which are in the shade.
In the panel on the left of the diagram, the arrangement means that all the bypass diodes are operating and no current can be produced. On the other hand, at the right of the diagram only the cells in the shade are bypassed.
Behaviour of the bypass diodes when affected by shade:
What are the parameters to be fulfilled during injection into the network?
Before being able to connect up to the network, you must ask for permission from your network manager (GRD – Distribution System Operator) in order to establish the characteristics and the supply mode for your connection.
If you do not know who your GRD is in Belgium, you can find out by following the link for your region:
- Brussels-Capital Region: www.brugel.be
- Flemish Region: www.vreg.be
- Walloon Region: www.cwape.be
PARAMETERS FOR INJECTION INTO THE NETWORK
The inverter must transmit to the network an electric current that complies with the regulations of Standard DIN VDE 0126. If these parameters are not respected, the network decoupling protection is triggered. The inverter verifies the following characteristics of the network at all times:
- The intensity
- The voltage
- The frequency
- The wave form
- The state of the network
The limit values of the voltage and the frequency are specific to each country.
Consult the table below for the required parameters for injection into the network, according to the regulations in force in your country*:
*The above values may differ, which is why it is advisable to contact your network manager (GRD) for further information.
MAXIMUM POWER
For a single-phase connection in Belgium, the maximum power of a decentralised production installation is 5kVA, unless otherwise specified by your GRD.
Again in Belgium, for a three-phase (or multi-phase) connection by an individual, the maximum AC power of this installation cannot exceed 10 kVA. With this type of connection, it is necessary to meet the requirements for distributing the installation’s power between the different phases. The production imbalance between the phases, i.e. the highest current difference between two pairs of phases, must be a maximum of 21.7 Amps (this represents power of 5kW with a cos phi equal to 1).
MAXIMUM LENGTH OF THE AC CABLE
The length of the cable on the AC side, between the inverter and the network, must be as short as possible, and the diameter of this same cable must be adapted to the intensity of the current. This makes it possible to limit the losses by joule effect.
It is advisable to select a cable size such that the loss does not exceed 1% of the nominal power injected by the inverter.
You can consult the table below to determine the maximum length of the cable according to the cross-section of the conductor and the injection power of your inverter. This table applies to inverters which are connected to a single-phase network:
Maximum cable length on the AC side for a power loss which does not exceed 1%:
SELECTING THE INVERTER
Selecting your inverter depends on the type of network to which your photovoltaic installation will be connected.
Consult the following table to determine the compatibility of our inverters with your network:
Advices
If you have problems connecting to the network:
- Ask the network operator if the power is off for maintenance reasons, or if there is a business close to the site which is intermittently absorbing a large amount of current.
- Check the length and diameter of the cable on the AC side.
How does sunshine affect the inverter ?
The power produced by a photovoltaic module is proportional to the irradiance received. The irradiance is the power of the solar radiation received. The power of solar radiation on the earth is approximately 1000 W/m ² in sunny weather. The flow of light varies according to the cloudiness of the sky, the pollution present in the atmosphere, and the dust on the panels.
The intensity of the photovoltaic panels will drop considerably when the sky is overcast, and therefore the panels will emit less power. On the other hand, the voltage decreases slightly when the irradiance drops to very low values.
Behaviour of the 245W DS Solar monocrystalline PV module according to the sunshine, for a temperature of 25°C:
Irradiance according to the cloudiness of the sky:
How does the temperature of the modules affect the inverter?
At equal irradiation, the voltage of a sensor varies according to the temperature. The voltage is inversely proportional to the temperature. In fact, the more the temperature decreases, the more the voltage will increase. Sunshine in cold weather may produce excess voltage for an inverter of an incorrect size. It is therefore important to take this variation into account in order to select the correct inverter.
The photovoltaic field and the system connection must be configured so that the voltage at the input of the inverter is lower than the maximum voltage.
The voltage at the terminals of the photovoltaic panel is therefore dependent on the temperature at the level of this same panel (the voltage increases when the temperature drops). It is necessary to ensure that the estimated minimum temperature for the installation does not make the panels go beyond the maximum voltage limit of the inverter.
Behaviour of the 245W DS Solar monocrystalline PV module according to the temperature, for irradiance of 1000W/m²:
What are the STCs (Standard Test Conditions)?
These are the normal laboratory test conditions. They make it possible to compare the power of different photovoltaic panels..
These conditions are:
• Radiation of 1000 W/m²
• An ambient temperature of 25 °C
• An air mass* (AM) of 1.5
*The air mass is the ratio between the distance that the solar radiation travels through the atmosphere in order to reach the ground, and the distance corresponding to the thickness of the atmosphere directly above the proposed location.
Air mass factor according to the inclination of the sun:
How do I know what size my PV installation should be?
In order to be able to correctly determine the size required for an inverter, it is necessary to take into account the power, the voltage and the current of the photovoltaic field.
- The ratio of the maximum input power of the inverter to the power of the photovoltaic field must be close to 1 (i.e. 100%). The inverter may be undersized when the photovoltaic field is badly orientated and it is clear that the optimum conditions will rarely exist.
- It is important to verify whether the voltage of the photovoltaic module assembly is within the voltage range of the inverter. For this purpose, the maximum and minimum voltages of the panels are calculated on the basis of the information contained in their technical data sheets. The voltage range of the panels is checked for extreme temperatures in the region concerned.
- It must also be ensured that the maximum current of each loop does not exceed the capacity of the inverter.
What equipment must be used for solar wiring?
DC CABLES
The cables for the direct current connect the panels to one another, as well as connecting the panels to the inverter. They are designed for outdoor use and must therefore withstand temperatures ranging from -20°C to 80°C as well as ultraviolet radiation, whilst remaining light and manoeuvrable.
In addition, these cables must guarantee a minimum service life of 30 years, whilst minimising energy losses.
In terms of safety of persons and fire safety, appropriate equipment must be used.
Working with direct current should not be taken lightly. The risks of electrocution by direct current are real, and even more dangerous than alternating current. It is impossible to free oneself from the source of current since, unlike alternating current, direct current does not alternate between polarities several times a second. When direct current passes through the body, the excitation of the muscles is two to three times greater than electrification by alternating current.
There is a greater risk of creating an electric arc between two phases with direct current than with alternating current. It is therefore necessary to ensure the cables are laid carefully without damaging them. This is why each conductor is equipped with double insulation, and cables with different polarity cannot be located in a single sheath.
If a fire breaks out, the cables are fire retardant in order to prevent flames from spreading. Furthermore, they do not emit either toxic or corrosive gases.
DC CONNECTOR
The solar cables are connected to one another by means of special plugs.
We use safe and user-friendly solar connectors, such as Multi-Contact plugs.
When ordering a solar plug, it is important to specify what type of joining piece is needed in order to avoid any confusion.
For example, MC4 plugs consist of a plastic connector, which makes it possible to maintain and insulate the connection between the two cables. If the outer connector is male, the terminal in the plug (connector for the conductor) is female. The other connector, which has an outer female connection, therefore has a male terminal.
What is the power factor (cos phi)?
The power factor represents the angular phase shift between the voltage and the intensity of the current in an alternating circuit.
Power formula for the alternating circuit
P = U x I x cos (phi)
In an alternating network the apparent power is expressed in VA (volt-amps). It consists of the active power and the reactive power. However, it is the active power (wattful power) which provides the device with the energy necessary in order to be transformed into mechanical energy and heat. The reactive power is expressed in VAR, the abbreviation of “reactive volt-amps”. Devices which need a magnetic field will absorb additional reactive intensity in order to create this field. This reactive current is phase-shifted by 90° backwards relative to the active current, and will therefore generate undesirable reactive power which is a source of excessive consumption. Finally, only the apparent power will be visible, and the cos phi is the measurement of the phase shift between the apparent power and the active power which is actually needed to operate the device.
The power factor indicates the quality of this network, and therefore the distribution of these power levels.
The cos phi must be as close to 1 as possible, since the smaller the cos phi, the more apparent power must be supplied to meet a given active power requirement.
The Federation of Electricity and Gas Network Managers in Belgium indicates in its requirements that the power factor must be between 0.8 and 1, unless dispensation is granted by the GRD (Distribution System Operator).
Is my electrical installation in compliance ?
People who want to fit a photovoltaic installation must have an electrical installation which is in compliance. The electrical installation must be checked by an approved body. The inspecting body verifies firstly whether the resistance of the earthing is lower than 30 ohms, and secondly whether the electrical installation has good insulation.
Advices
There is a risk of electrocution when an earthing fault is determined by the inverter! Repairs to the installation must be carried out by a specialised technician.
If an insulation fault is determined by the inverter, it may be caused by two factors in the photovoltaic installation.
a) The wiring:
• Check that the plugs are well fitted.
• Check that no plug is trailing in water.
b) The panel:
• Make sure that the panel does not have any sealing fault (presence of humidity or water in the solar module).
• Make sure that the panel does not have any obvious connection fault (contact between the aluminium frame and the busbar).
How can you recognise the domestic network?
You can easily recognise the type of domestic network by means of a voltmeter. Place your voltmeter between a phase and the earth. If you obtain a voltage of 220 Volts or 0 Volts, your domestic network is a single-phase network. If your voltmeter indicates a voltage which is significantly different from 220 Volts, your network is a two-phase network.
Identification of the domestic network :
What types of electrical networks are there?
There are different types of electrical networks in Belgium. It must be ensured that the inverter is compatible with the network of the building.
1. The three-phase and two-phase network
a) The three-phase network
This consists of three wires. These are phases where the voltage between each phase pair is 230 Volts.
b) The two-phase network
This consists of two phases of the three-phase network which are different from one another.
2. The tetra-polar and single-phase network
a) The tetra-polar network
The tetra-polar network or three-phase network with neutral consists of four wires (3 phases + neutral). Three of the four wires are phases where the voltage between each phase pair is 400 Volts and the voltage between one of the phases and the neutral is 230 Volts.
b) The single-phase network
This always consists of a neutral and one of the phases of the tetra-polar network in order to obtain a voltage of 230 Volts between the cables.
How do you measure a current?
Excessively high currents may destroy the measuring equipment!
- Use only measuring devices with a suitable input current range
Measurement of the electric current is strongly affected by irradiance!
- Measure the intensity of the current when the sky is clear. It is also advisable to measure the irradiance during the current measurement.
Risk of electrocution
- Take the necessary safety measures for handling the cables.
1. Measuring a current ISC
The current ISC is the current which is measured during short-circuit. In order to measure this current, the cables are disconnected from the inverter in order to connect them to the multimeter. Measurement of the direct current side is selected from the device menu. One cable is then connected to the input COM and the other cable to the input A (use of the device as an ammeter). This measurement must be carried out for each input of the inverter.
Method for measurement of the intensity ISC
2. Measuring a stroom current IMPP
The current IMPP is the current which is measured during operation. This current is measured by putting the multimeter into series. Measurement of the direct current side is selected from the device menu. This measurement must be carried out for each input of the inverter.
Method for measuring the intensity IMPP
How do you measure a voltage ?
Excessively high voltages may destroy the measuring equipment!
- Use only measuring devices with a suitable voltage range
Risk of electrocution
- Take the necessary safety measures for handling the cables.
1. Measuring the voltage VOC
The voltage VOC is the voltage measured in an open circuit. In order to measure this voltage, the cables connected to the inverter are disconnected in order to place them correctly on the multimeter. Measurement of the direct current side is selected from the device menu. One cable is then connected to the input COM and the other cable to the input V (use of the device as a voltmeter). This measurement must be carried out for each input of the inverter.
Method for measuring the voltage VOC
2. Measuring the voltage VMPP
The voltage VMPP is the voltage which is measured during operation. In order to measure this voltage, the multimeter and the inverter are connected in parallel by means of shunt pins or on one of the free inputs of the inverter. Note that if this last method is used, the measurement must be carried out on the same MPPT of the inverter. The connections are carried out in the same way on the multimeter as for measuring the voltage VOC. This measurement must be carried out for each input of the inverter.
Method for measuring the voltage VMPP by means of shunt pins
Below are the type MC4 shunt pins for putting the loops into parallel.
Method for measuring the voltage VMPP by putting into parallel on a free input
What is putting into parallel ?
For two loops which are put into parallel, the intensity of the current is the sum of the intensities of the currents which circulate in each loop. On the other hand, the voltage of the loops is equal to the voltage at the terminals of each loop.
What is putting into series?
When the loop is put into series, its voltage is equivalent to the sum of the voltages of each panel. The circulating current is defined by the panel which produces the least electric current.
