Photovoltaic installation architectures
Installation precautions
A PV array is made up of a number of modules in series or parallel, corresponding to the input characteristics of the inverter. However, since these modules are interconnected, the array is very sensitive to shade or differences in terms of the direction faced.
By following a few simple cabling rules, supply can be optimised and any operating problems may be avoided
Position of the panels
If, when installing a PV array on a roof, panels need to face in different directions, it is essential to assemble at least one string per direction and ensure each string is facing in just one direction to ensure optimised supply. Each string must be connected to a specific inverter (or to inputs of a multi-MPPT inverter - see Section 3).
If this instruction is not observed, the array will not be damaged but supply will be reduced, thus increasing the time needed for a return on investment.
Shade
Besides the risk of destruction of shaded modules within a PV array due to the “hot spot phenomenon” as described in Paragraph 2.2 for which manufacturers have devised solutions, research conducted by the Institut National des Energies Solaires (INES – France’s national institute for solar energy) suggests that shading of 10% of the surface area of a string may cause more than a 30% reduction in output!
It is therefore important to eliminate direct shading. However, in many cases this is difficult (trees, chimney, neighbouring wall, pylon, etc.).
If a PV array includes several strings:
- If possible, shaded modules should be included in a single string
- Otherwise, a technology should be chosen which responds better to diffuse light than direct light
Eliminating loops
When connecting components, the first precaution to take is to avoid loops in the cabling within strings.
Even though direct lightning strikes on arrays are relatively rare, currents induced by lightning are much more common and these currents are particularly destructive where there are large areas of looping. Figure P13 shows how to improve an array including a large loop.
Fig. P13: Avoiding loops when cabling strings
Architectures for installations connected to the network
General Rules
Where photovoltaic installations are connected to the network and energy is sold, it is necessary to optimise efficiency and reduce installation costs. With this in mind, a relatively high DC operating voltage of between 200 and 500 V is often used for residential applications, with up to 1000 V being used for applications requiring a higher level of power.
All the modules in a PV array should be identical (same brand and same type) and selected to supply the same level of power. For example, in the PW1700 range, they should all be 180 W, even though there are three power levels (170 W, 180 W and 190 W) in this range manufactured by Photowatt.
In practice, the protection units (DC and AC units) should be positioned close to the inverters for ease of maintenance.
PV array with a single string of modules
This is the simplest configuration (see Fig. P14). It is used for small PV arrays with peak power of up to 3 kWp depending on the modules deployed. In most cases, it is used for residential PV operations.
Fig. P14: Diagram showing a single-string photovoltaic array
Modules are connected in series, supplying direct current of between 200 and 500 VDC in this instance. Optimal efficiency is obtained from the inverter within this voltage range.
A single DC line is fed through to the inverter. The PV array can be isolated from the inverter by means of a load break switch near the inverter.
PV array with several module strings in parallel
This configuration (see Fig. P15), mainly deployed on buildings or in small PV power plants on the ground, is used for PV installations of up to thirty strings in parallel with power output of some 100 kWp. This limit is imposed for technological and financial reasons. If exceeded, the required width of the main DC cable would be impractical.
Direct current can be determined based on the number of modules in series per string and in this instance is between 300 and 600 VDC. By paralleling identical strings, the power required for the installation can be attained. The strings are paralleled in a PV array box. This box includes the safety devices required for paralleling the strings and appliances used to measure the strings’ current. A single DC cable connects these boxes to the inverter. The PV array can be isolated from the inverter by means a load break switch near the inverter.
Fig. P15: Diagram showing a multi-string photovoltaic array with one inverter
As a variation on this diagram, several single-phase inverters can be installed in a three-phase arrangement (see Fig. P16).
Fig. P16: Diagram showing a multi-string photovoltaic array with several single-phase inverters connected in a three-phase arrangement
PV array with several strings divided into several groups
When power levels exceed 50 or 100 kW, photovoltaic arrays are split into subgroups (see Fig. P17) to make it easier to connect the various components. Strings are paralleled on two levels.
- Strings in each subgroup are paralleled in subgroup PV array boxes. These boxes are fitted with safety devices, the necessary measuring equipment and monitoring devices.
- The outputs of these boxes are paralleled in a PV array box near the inverter. This box is also fitted with the required safety devices as well as the measuring and monitoring equipment necessary for paralleling the subgroups.
The array can be isolated from the inverter using a load block switch which may or may not be fitted in the PV array box. The array’s direct current is approximately
1000 VDC.
Fig. P17: Diagram showing a photovoltaic array consisting of several groups
Sizing
Calculating a photovoltaic array
It is absolutely essential to take account of location (geographic location, latitude, altitude, shade, etc.) and installation factors (direction faced, angle, etc.).
Firstly, the approximate power output may be calculated based on the available surface area:
10 m2 = 1 kWp
7140 m² (=football ground) = 700 kWp
The PV array should always be arranged around the inverter. The calculations involved should compare the characteristics of the modules and those of the inverter with a view to identifying the optimal configuration.
- String composition:
NB: Number of modules x Voc (at t° min) < inverter Vmax
The no load voltage of the string (Voc x number of modules in series) at the minimum temperature of the installation location must be lower than the inverter’s maximum input voltage.
=> This must be strictly observed. Otherwise the inverter may be destroyed.
Apart from the aforementioned rule for preventing destruction of the inverter
Number of modules x Voc (at t° min) < inverter Vmax – two other limits must be observed:
- Number of modules x Vmpp (at t° max) > inverter Vmin
The operating voltage (Vm x number of modules in series at all temperatures at the installation location) should fall within the inverter’s MPPT voltage range. Otherwise, the inverter will stall and energy supply will cease.
- Isc strings < inverter I max
The total Isc current for strings in parallel must be lower than the maximum input current for the inverter. Otherwise, the inverter limits the supply of energy delivered to the network.
Inverter specifications
- In Europe, the power level of the inverter must be between 0.8 and 1 times the power of the array:
0.8 < Pinverter / Parray < 1
- Below this (under 0.8 Parray), the inverter limits power significantly. The energy sold to the network will thus be inferior to that which
the panels are capable of supplying and therefore it will take longer to secure a return on investment.
- Above this (over Parray), the inverter is too large for the power level of the array. Again, it will take longer to secure a return on
investment.
- Single-phase or three-phase
A decision should be made over these two options in consultation with the local energy distributor based on the devices available in manufacturers’ product ranges, often within the following limits:
- Inverter Pn < 10 kW => single phase inverter
- 10 kW < Pn < 100 kW => either three-phase inverter(s) or single-phase inverters split between the three phases and neutral. The
management of unbalances between phases needs to be checked in this instance.
- Pn > 100 kW => three-phase inverter(s)
- Configuration software
Manufacturers of inverters help design offices and installers to size strings for residential and service sector installations based on the equipment available by supplying sizing software.
Installation type
The installation type is a factor which should not be neglected since, in countries including France, the purchase price for power supplied is dependent on this. Along with shading, it should be taken into account when choosing a module.
There are three installation types – building integrated, partially integrated and ground-based:
- Building Integrated PhotoVoltaic (BIPV)
This installation type fulfils a dual role (energy supply and roof waterproofing, shading, etc.).
- Partially integrated
This is the simplest assembly to install and, most importantly, does not alter the water resistance of a roof. However, its major drawback is that, in France, operators cannot charge the highest rate for it. This installation type is most commonly used in Germany and Switzerland.
- Ground-based
This installation type is used for power supply plants covering large areas (photovoltaic farms). Again, in France it is not eligible for the highest purchase price.
