Planning of grid connected PV systems (PV On-Grid)

Planning of grid connected PV systems (PV On-Grid)

Key Points

 

• System size, that meets the owner's needs?
• If roof mounted, verify additional weight of the PV system (statics)
• Plan the PV generator to minimize shading from vent pipes or adjacent structures
• Plan the PV inverter in cool (if outside, shaded) and dry location
• Plan the distribution of strings and module assignment
• Plan the overvoltage protection and minimum of electrical losses (wiring, fuses, switches)
• Plan the lightning protection, grounding and potential compensation (if needed support by external specialists) 
• Design the system in compliance with national applicable building and electrical codes
• Clarify insurance aspect while system installation, responsibility by installer or system owner? (eg lightning strike, etc)

System components (key points)

DC-Site

• PV Modules (voltage and current rating)
• PV cables and connections (cable management)
• PV strings, switches, fuses and isolation (overvoltage protection)
• Earthing/equipotential bonding and lightning protection (three different things!) 

 

AC-Site

• PV inverter (voltage and current ratings, compatibility with PV generator)
• Cabling, switching devices and protection (cable management, protections)
• kWh meter (energy meter mainly provided by local DNO).

 

System aprovement

• Permissions and DNO approval (Distribution Network Operator)  
• Building regulations (depending on country and national regulations).

  PV Off-grid systems (design of PV off-grid systems)

Safety while mounting PV solar systems

In compliance with applicable safety regulations, assembling and maintenance of PV solar systems (especially within the PV On-Grid systems) requires proven training and experience to work with high voltages and DC/AC components. Before touching any part or component of the PV system, always check for presence of voltage

If it is unavoidable to work simultaneously with positive and negative parts or cables, this must be performed by utilizing insulating gloves, tools, insulating materials for shrouding purposes and appropriate personal protective equipment. 

If needed cover the PV generator or work at night dawn with appropriate lighting, especially mounting thermal solar collectors while hot summer period. The covering must be opaque, well secured and cover the whole generator. A temporary warning signs should be given for periods while PV cables will be installed or connected.

Voltage and Current

When in PV systems (especially in PV Off-grid systems and PV charge controller) the module or PV voltage is mentioned, usually meant the module Voc (Open-Circuit-Voltage). All technical parameters of the PV module or generator, are related to the STC standard (1000W/25C/AM1.5). Under certain conditions e.g. in winter or in higher lying locations (1500m above sea level), the technical parameters of almost any PV module will be surpassed (more than indicated in the product data sheet!). Therefore calculation for voltage and current with factor as:

Voltage:		x1.2		Voc		(V)
Current:		x1.25		Isc		(A)

Example: if in the PV module datasheet 30Voc defined, calculate it as x1.2, thus 36V (possible max occurring). This same if Isc defined as 8A, calculate it as x1.25 and possible occurring may be 10A. If an PV string will be created and in series connected (several modules in section) the voltage and current adds up (!). More below in section: "Cable management". This is especially important while selecting the PV-inverter and the charge controller (its max allowed parameters). In this case of an example PV module with 30V/8A the proper charge controller, should withstand 36V and 10A. In case of PV-inverter (On-Grid) the same rule but multiplied, according to the PV string parameters, number of PV modules and connection type (series/parallel).

In some types of PV modules the temperature coefficients will be different and the effects of increased irradiance may be more pronounced. Eg some thin-film modules have an considerably higher electrical output during the first weeks of exposure to sunlight. Then operation during this period the Voc and Isc above the declared nominal PV power. For this eventuality, to avoid oversizing the inverter PV (actually needed only for the initial period), systems based on thin-film modules in the initial period can be left disconnected from the PV inverter, for few weeks.

Within the PV inverters and charge controllers the max allowed PV input voltage (Voc) and current (Isc) can not be exceeded! The thereby resulting possible damages are not covered by the warranty. 

PV Modules

The electrical aspects for most installers are known by heart. Nowadays very important is the knowledge and experience about solid, high-quality PV material, which you are working with. And that is difficult to recognize from the PV module data sheets.

The problem with comparing providers is, that world-wide there are around 400 manufacturers of photovoltaic modules. In the wake of the economic crisis, bankruptcies and lost guarantees, the important is you work with the well-known and most major manufacturer in the photovoltaic sector, to avoid the risk at least as much as possible.

Made in China? the claim, that the Chinese photovoltaic modules have a poor quality, compared to PV modules from European manufacturers is already past. The price differences are obvious and result from the production cost structure and support programs. Generally is underestimated, how strongly and exemplary the Chinese government supports its own industry of renewable technology. That's why in case of well known and major Chinese manufacturers, the price-performance ratio may be attractive.

Nevertheless, PV modules are subject to strict technological requirements, important for an long-term, trouble-free operation. One of the well known quality problems are so called micro-cracks. The microcracks problems are typical for low-quality PV modules. If two visually identical PV modules are compared, an top-quality and an no-name PV module, the difference in quality can be not recognized, by anyone. It would be visible only in a thermovision-test and looks like this:

Other quality faults and failures in PV modules may results from: overloading, overheating, mechanical influences. Typical causes are: reverse current, shading (hotspots), earth faults and short circuits within modules, junction boxes or module wiring.

Cables and connectors

The PV modules are usually mounted on the roof, inverter are often mounted in the basement or closest to the feed-in meter. The PV inverter should be mounted as close as possible to the feed-in meter, because the losses (between OV inverter and energy meter) due to the cable length on the AC side are higher than on the DC side.

PV cables should meet the following requirements:

Weather-/UV resistant according to HD 605/A1

Ozone resistant according to EN 50396

Withstand operating temperature range -40°C to +125 ° C

Withstand voltage up to 1000V (depending on application)

Flame retardant according to IEC 60332-1-2

Halogen free according to EN 50267-2-2 (improved fire and self extinguishing)

Acid/alkali resistant according to EN 60811-2-1

Short circuit resistant at high temperatures (250°C)

Small outer diameter (save space), abrasion resistant, high mechanical strength.

Important in agricultural operations:

Resistant to ammonia gases, oxalic acid, caustic soda and other chemicals

In case of increased risk of damage by martens, raccoon, rodents or termites, please use cable with V4A wire reinforcement.

Ordinary NYM cable (PVC) or rubber-insulated cable, especially for PV on-grid systems, are not allowed to be used!

Cable Cross Section

Use possible large cable cross-section at the shortest possible cable lengths. It is important to know that the cable loss should be not more than 1%. Two different sections have influence on the losses:

DC site (PV modules / PV inverter) and

AC site (PV inverter / feed-in meter).

As above mentioned, the losses due to the cable length on the AC side are higher than on the DC side. Main influence on the losses are: the cable cross-section and the cable length. To calculate the proper cable cross sections, the eventual losses in the given cable line (module-inverter) should be under 1%.

For copper cables the calculation is as follows (goal is to keep losses under 1%):

Pv		=		(2 x L x I²) / (56 x A)					losses [W]
Pv%		=		PV x 100% / P					losses [%]

For aluminum cables due to the higher resistance are higher losses, and it is necessary to provide a larger cable cross section in order to keep the losses under 1%:

Pv		=		(2 x L x I²) / (38 x A)					losses [W]
Pv%		=		PV x 100% / P					losses [%]

L: cable length [m]

I: current (Isc) which flows through the line [A]

A: cross section of solar cable [mm2]

Pv%: loss [%]

P: Total rated power of the string [W].

Available at PVshop.eu and pre-configured complete PV systems, contain 6mm cable cross-section and 50m PVcable per 1-string. Complete PV systems with 2-string inverter includes 100m cable, 3-string inverter 150m etc. An complete set of appropriate connectors (MC4) and dedicated cable clamps (cable/GermanClick profile) are also included.

Life danger! During system operation never disconnect the connectors or cables, may result in significant lightning and fire flame.

Cable management

Types of cable connection (series, parallel) 

Especially important in PV Off-grid systems, where the max. PV voltage may have an significant influence on system efficiency and system design. Connecting several PV modules in string (section), the system voltage can be regulated (minimized) by appropriate connection. In series connection the voltage (V) and current (A) will be added up In a parallel connection only the current (A) will increase and voltage (V) will remain the same. Parallel connections are typical in Off-grid systems, to minimize the PV voltage. Here an example for two PV modules (16V/2.5A):

The max. allowed PV input voltage (at the charge controller) can be regulated by appropriate wiring. In charge controllers (but also in PV inverters) the allowed max input voltage (in PWM controller around 50V and Phocos MPPT 95V) can not be exceeded.

PV Cables Distribution

In order to minimize the induction surface, within the cable and string management on roof, is to ensure correct installation of the plus-and minus lines. These should be as close as possible to each other. With increasing induction surface, the coupled overvoltage is increased. This point is often during planning and installation of photovoltaic modules ignored. The planning and installation costs (cables) don't increase insignificantly. This aspect of PV cable management is an part of lightning protection.

Connectors

The most used PV modules in grid connected systems use the MC4-Standard from Multi Contact. The MC3 standard is an older one, used in the past in older types of PV modules. The SMA string inverters use the Sunclix standard from Phoenix Contact, the advantage of Sunclix connectors is the possibility of installation without tools, pliers. Both standards can be used simultaneously, the MC4 connectors at the PV modules and Sunclix at the PV inverter.

The MC4 standard is currently definitely the most common PV connector standard.

 

MC4		MC4 (parallel)		MC3		Sunclix

	MC4 standard - mounting instruction and assembly tools.
	Video shows assembling with professional tools. Possible also with hobby tools.

Mismatching

Mismatching means the losses on PV power, mainly within series connected PV modules. Ever bigger the number of installed modules (PV power), the more important is the mismatching-problem. The reason of mismatching can be different performance of an individual module, within the same models of modules. In all modules, connected in series, should flow the same current. But the worst performing module, within the series (string), will impair the power of all other connected PV modules (similar like in: blocked garden hose effect). Are there strong variations in performance, the powerful PV modules can not transfer her best performance. When mismatching PV modules present, it is important to connect modules with low power distribution in a separate PV string or at the beginning of the PV string (-). Quality manufacturer sort out, the modules with mismatching performance. Sorting can also take place individually by the installer, by the current (A) of the modules listed in the flash-list. Other mismatching losses can results from different operating conditions, such different module inclination, orientation or partial shade within the PV string.

Fuses, earthing and overvoltage protection

Photovoltaic modules and inverters represent the most expensive part of the whole PV system, and protection is focused above all on these components. Failure or in extreme case, destruction can be caused by atmospheric or switching overvoltages. Another potential source of failure is an short-circuit of part of the electrical circuit, which can result in overload of certain parts up to their eventual destruction or possibly even in fire. For this reason especial attention is paid to devices from a proven manufacturer.

 

Diagram example of a grid connected PV system and its protections:

 

PV System (grid connected) protection elements

In case of a higher number of strings connected in parallel it is necessary to ensure protection of PV panels against reverse currents and overcurrent protection of cables of the PV generator.

The overvoltage protection is provided at point (1). In case of a longer line between the PV generator and the PV inverter it is appropriate to use surge voltage arresters both at the inverter and close to PV generator. To ensure maintenance of the inverter, it is necessary to meet the requirement for its possible disconnection from both AC and DC side. Therefore DC disconnector (at 2) and an AC disconnector (at 3) are installed at the inverter. In case that it is functionally ensured that switching the DC side off/on always takes place without load, i.e. the AC side will be switched off sooner and switched on later. It is possible to use also a disconnector on the DC side.

Downstream of the AC disconnector there is a surge voltage arrester (4) installed and is above all recommended after a long-line-distance. Furthermore it is possible to connect an internal electricity meter (Wh), which is connected by a protective device with the main switch-board (5).

In case of high-capacity PV source, individual parallel line of the PV source is connected via protective devices to the main switch-board. The switch-board and downstream wiring is protected by a surge voltage arrester (6) on the side of connection to the distribution network.

The electricity meter of the supplied and consumed energy or only of supplied energy (only generation without consumption) is preceded by the main disconnector (7) of the main switch-board. The switchboard, disconnector and the line to the distribution system (grid) are protected against overload and short-circuit by the main protective device (8).

PV Strings (parallel connected)

In case of a higher number of strings connected in parallel in the photovoltaic array (Fig. 2) it is necessary to ensure protection of PV panels against reverse currents and overcurrent protection of cables. Protection of strings (9) is sometimes omitted, because short-circuit current Isc of the PV panel is only by 10 to 20 % higher than its rated current (operating).

In case of application with maximum 3 strings there is no risk of panel damage by reverse current induced by short-circuit. The risk of thermal overload of cables due to the short-circuit can be dealt with by their appropriate overrating. In a higher number of parallel strings it is necessary to take into account the value of possible reverse current with regard to maximum allowable reverse current of the PV module.

Basic principles of protection from short-circuit and overload are as follows:

Parameters shall be selected with regard to the series-parallel connection of PV panels and their characteristics.

Selection of rated voltage of protective devices:

V_n ≥ 1.2 × V_oc × M

V_n - voltage rating

M - number of panels in series

Voc - open circuit voltage of the PV panel at STC

The factor x1.2 makes provision for voltage increase (~20%) eg. at low ambient temperatures, manufacturing tolerances of PV panels, etc.

Selection of rated current of the fuse-links:

1.4 × I_sc ≤ I_n ≤ 0.85 I_{mod_reverse}

for fuse-links with characteristic gR; gS; gG

I_n ≥ 10A

1.4 ×  I_sc ≤ I_n ≤ 0.7 I_{mod_reverse}

for fuse-links with characteristic gR; gS; gG

I_n < 10A

I_n - fuse current

I_{mod_reverse} - module reverse current rating (maximum permitted return current of the PV panel)

Type of fuse, intended application:

g - general purpose fuse typically meets IEC stdandard

gG - general application

gR - semiconductor devices

The factor 1.4 makes provision for the use at ambient temperature of 60°C, radiation intensity of 1000 W/m2, and influence of cyclic load. If the manufacturer of the PV panel prescribes a maximum protection value, this value shall be accepted.

Note: For collective protection of series-parallel connection of PV panels, the resulting current is proportional to the number of parallel sections.

For strings where no manufacturer rating (Imod_reverse): more than x1.25 and less than x2 Isc (A).

Short-circuit and overload are not, however, the only danger which may damage elements of the PV system. Another threat is formed by overvoltage. Overvoltage is defined as voltage exceeding the largest value of operation voltage in the electrical circuit. There are more types of overvoltage. The most frequent are switching overvoltage and overvoltage caused during a lightning stroke (known as atmospheric overvoltage).

During protection of PV system it is necessary to install, due to outdoor installation a protection from lightning as well. For many applications we can see collection system protecting PV panels and other elements of the PV system from a direct lightning stroke. This protection is, however, not sufficient. It is necessary, to install, together with the collection system, also the overvoltage protections which will ensure equalising of potentials among all wires and thus they will prevent damage to other elements.

FAQ - Fuses, earthing and overvoltage protection

Under what conditions is overcurrent protection necessary on DC side?

Overcurrent protection need not be implemented for PV conductors of strings and PV generator, if conductor rating capacity is equal to or higher than 1.25 × Isc (at STC) in any places. Overcurrent protection need not be implemented for main PV conductors, if conductors rating capacity is equal to or higher than 1.25 × Isc (at STC) of PV generator.

Is it possible to use series connection of fuse-links to achieve higher rated voltage of the fuse group?

In no case. For series combination of fuselinks it is not possible to guarantee uniform distribution of cut-off processes in case of failure. One fuse-link always takes over higher part of cut-off processes, and for this reason it cannot be overloaded above its design characteristic.

When and why to use overvoltage protection on DC side close to both the inverter and PV panels?

In case of application, where PV panels are at a distance from the inverter (more than 10m), it is recommended to use the overvoltage protection both upstream the inverter and at the photovoltaic panels. Due to the fact that voltage in long lines can rise significantly thanks to induction in the line.

Why are surge voltage arresters with gap preferred in PV applications?

In low-voltage networks of 230/400V varistor-based overvoltage protective devices are commonly used. The varistor itself shows a leakage current at operating voltage. In  common applications with one type of varistor-based overvoltage protection the leakage current is negligible. This is not, however, true for the photovoltaic applications, where tens of surge voltage arresters are used. In this case the total leakage current is not negligible, because it decreases the resulting output.

What fuse-link type and characteristic to select for protection of DC side?

The decisive parameter is rated DC voltage of the fuse. For protection of PV applications it is required to use fuse-links with full tripping range, i.e. type „g". Fuse-links with tripping range „a" cannot be used, because their breaking capacity is limited in the range of small overloads.

What protection to use, at current values higher than 20A?

Fuse-links are used advantageously for rated currents up to 20A. For currents higher than 20 A it is possible to use fuselinks for protection of semiconductors (char. gR, gS), with regard to required values of operating DC voltage. These types of fuselinks with characteristic gR are up to rated current of 80A.