Stand-alone Grid Systems

Stand-alone Grid Systems

Installing a Stand-alone Grid System

Planning Guidelines

INSELNETZ-PL-UEN105010 | Version 1.0

EN

SMA Solar Technology AG

4

5

7

7.1

7.2

7.3

6

6.1

6.2

6.3

1

2

3

3.1

3.2

3.3

3.4

3.5

3.6

Table of Contents

Normative Framework . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Grid Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Installation and Mounting Locations . . . . . . . . . . . . . . . . 10

Distribution Room . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

PV Inverter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

PV Plant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Wind Turbine System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Designing Cable Cross Sections . . . . . . . . . . . . . . . . . . . . 12

Installation Material. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Laying the Cables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

General Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Installing Cables of a PV Plant . . . . . . . . . . . . . . . . . . . . . . . . . 19

Installing Cables of a Battery System . . . . . . . . . . . . . . . . . . . . 19

Lightning Protection in Off-Grid Systems . . . . . . . . . . . . . 20

Physical Basis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Types of Lightning Strike Risks. . . . . . . . . . . . . . . . . . . . . . . . . . 24

Practical Lightning Protection . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Planning Guidelines INSELNETZ-PL-UEN105010 3


4 INSELNETZ-PL-UEN105010 Planning Guidelines

SMA Solar Technology AG Normative Framework

1 Normative Framework

As with every electrical plant, dangers can arise from off-grid systems for both people and property.

To largely eliminate these dangers, a qualified installation taking current standards into account is indispensable. For this reason, the installation of such plants may only be performed by qualified personnel. These instructions are intended for electrically skilled persons who apply fundamental normative standards in daily practice.

One of the most important standards for the installation and operation of electrical systems is the

German standard DIN VDE 0100 "Erection of power installations with nominal voltages up to

1 000 V". The following table contains parts of this standard which must be adhered to during the installation of a stand-alone grid.

DIN VDE 0100 "Erection of power installations with nominal voltages up to 1 000 V"

Protective measures (Group 400)

Part 410

Part 430

Protection against electric shock

Protection of cables and lines against overcurrent

Part 440

Part 470

Protection against overvoltage

Application of protective measures for safety

Selection and erection of electrical equipment (Group 500)

Part 510 Common rules

Part 520

Part 540

Wiring systems

Grounding, protective conductors and protective bonding conductors

Part 551 Low voltage generating sets

Verification (Group 600)

Part 610 Initial verification

Requirements for special installations or locations (Group 700)

Part 705

Part 731

Part 7-712

Agricultural and horticultural premises

Electrical locations and locked electrical locations

Low-voltage systems

Requirements for special installations or locations - Solar photovoltaic (PV) power supply systems (IEC 60364-7-712:2002, modified);

German implementation HD 60364-7-712:2005 + Corrigendum:2006


Normative Framework SMA Solar Technology AG

Relevant standards also apply outside Germany, which are contained in the international standard

IEC 60364 "Electrical Installation of Buildings". When erecting the stand-alone grid, the following standards must be considered, too:

• DIN VDE 0298 Part 4 for sizing cables and lines

• DIN EN 50272 Part 2 for installing batteries

• DIN VDE 0100-712 for PV plants

Standards dealing with coupling generator systems to the public grid are understandably irrelevant for off-grid systems.

The above-named standards have the status of the year 2008.


SMA Solar Technology AG Grid Forms

2 Grid Forms

A stand-alone grid has generally several feed-in generators, depending on the energy sources available, including diesel generators or PV plants with PV inverters (e.g. Sunny Boy) along with stand-alone grid battery inverters (Sunny Island). All these generators and inverters feed into the AC grid and constitute the source for the consumer's energy supply as a replacement for the main power grid.

The AC grid forms the core area of the off-grid system to be installed. It collects the energy of the feedin generators and distributes and transfers it to the consumers. To allow optimal use of the energy being fed in, transmission losses should be reduced to a minimum. The size (greatest distance between the consumers and sources) of off-grid systems with a nominal voltage of 230 V (single-phase) or

3 x 230 V/400 V (three-phase) is naturally very limited. Large amounts of power can only be economically transmitted over large distances if voltage levels are high.

The required transformers, high-voltage lines and the necessary protective technology cannot be covered in this document.

Parts of the installation must be constructed as a DC grid, for example the battery system or the PV circuits. These circuits should be kept as small as possible, in particular the battery system with its 48 V maximum nominal voltage, which is low compared to the AC system voltage.

Please note that the actual operating voltage of the battery circuit varies across a wide range around the nominal voltage, depending upon whether the battery is being charged or discharged. Battery inverters with 48 V nominal voltage can develop charging voltages of up to 63 V.

Regardless of whether the grid is an AC or DC grid, a distinction is made between different grid forms based upon

• the type of connection to ground of the sources, and

• the type of connection to ground of the consumers.

The grid form and the nominal voltage determine which protective measures you can or must use.

The grid form of the AC circuits in a plant connected to the utility grid is largely predetermined, since the sources lie under the control of the grid operator. This is different for a stand-alone grid where the sources and consumers are independent.


Grid Forms SMA Solar Technology AG

The most common grid form used in public utility grids is the so-called TN grid. where the neutral point or center point of the source is grounded. The neutral conductor of the plant is connected to this neutral point. The exposed conductive parts of the connected consumers are grounded via protective conductors in the consumer system, and they are connected to the grounded neutral point of the source. A distinction is made between PEN conductors (TN-C system), which are a combination of protective conductor and neutral conductor, and separate PE and N conductors (TN-S system).

A hybrid form of TN-C system, which is used from the source, and TN-S system, which is used in the consumer system, is common especially in extended grids. This form is known as a TN-C-S system. The

PEN conductor in the TN-C area of the grid is usually grounded again in the house installation and included in the equipotential bonding before the TN-S area begins, where it is split up into separate

N and PE conductors. The N conductor must not be grounded after PEN is split up into N and PE.

Advantages of the TN system are the low-resistance protective connections, which make a reliable execution of simple protective measures possible (automatic disconnection of the power source using fuses in case of error), as well as the fact that only one ground electrode is necessary. A disadvantage is the somewhat more expensive installation due to the protective conductor in every cable. This, however, should be no problem in a stand-alone grid, as it is comparatively small. The TN-S or

TN-C-S system therefore is the preferred grid form for a new off-grid system.

In a TT system, the neutral point of the source is grounded, as it is in TN systems. However, the exposed conductive parts of the consumers are connected to separate ground electrodes of the system using protective conductors. The N conductor must not be grounded in the consumer system.

This grid form has no direct protective conductor connection between the exposed conductive parts of the equipment and the neutral point of the source, as have TN systems. The protective conductor connection is only provided by a connection to ground, which has consequences for the protective measures to be used (use of residual current breakers). Due to the small size of stand-alone grid systems, the TT system has no notable advantages for stand-alone grids.


SMA Solar Technology AG Grid Forms

In an IT system, the neutral point of the source is insulated against ground or has a highly resistive connection to ground. The exposed conductive parts of the consumers are connected to separate ground electrodes of the system using protective conductors, as they do in TT systems. Certain highly sensitive application areas, such as medical rooms, require IT systems, as the consumer system must remain operable in case of a ground fault. This, however, causes additional expense (for example, constant isolation monitoring or all-pole fusing of the cables). Since not all devices are suitable for ungrounded grids for reasons of electromagnetic compatibility (EMC) — in particular PV inverters,

Sunny Island Chargers (charge controllers) and stand-alone grid inverters must be checked — the installation of a stand-alone grid as IT system is not generally recommended .

The complete installation of protective conductors in a stand-alone grid, which is generally a small system, does not significantly increase cost. Due to the advantages in carrying out the necessary protective measures, the TN-S system is recommended for stand-alone grids. If feed-in generators are spatially separated and must be connected using long lines, a TN-C area with combined N and PE conductors can be created for cost reasons.

All grid forms share the characteristic that at least one ground electrode must be present. This is in any case necessary for the equipotential bonding of the electrical system. Very well suited are, for example, building foundation earth electrodes. If such a ground electrode is unavailable, a ground rod can be installed. It is important to achieve the lowest possible transmission resistance to the ground to ensure the greatest possible effectiveness of the protective measure against electric shock

(i.e. fast and safe disconnection in case of error).


Installation and Mounting Locations SMA Solar Technology AG

3 Installation and Mounting Locations

3.1 Distribution Room

The distribution system is the central disconnection point of an off-grid system. The individual components of the off-grid system are electrically connected to the distribution system. Fuses for the individual consumer circuits are also installed here. A counter specified by the network operator is generally not necessary for off-grids. It is only necessary if the operator desires separate settlement from individual consumer groups.

The distribution should be located in a dry, well-lighted room. Make sure that there is free access to the distribution system. Ideal is a dedicated room for the distributor that includes space for the inverters. This is also a good location for at least the Sunny Island inverters, as long as the distance to the batteries is not too great. If the PV plant is mounted in the same building and there are no great distances to the PV array, you can also install the PV inverters (e.g. Sunny Boy) here.

3.2 PV Inverter

Power loss during transmission is one of the most important factors in selecting the mounting location for the PV inverters. It is given by the square of the current which passes through the cable resistance.

The higher the voltage in a circuit, the lower the loss — with constant transmission power. From the PV array to the consumers, the energy passes through two circuits: the PV circuit (from the array to the PV inverter) and the AC grid. The voltage in the AC grid is — except for load-dependent fluctuations — relatively constant. By contrast, the DC voltage in the PV circuit is subject to large fluctuations. It is therefore not clear if the current is higher on the AC or DC side of the PV inverter.

In principle, there must be a small distance between inverter and PV modules. This ensures that the

DC cables, which are not protected by fuse, are not too long. SMA offers a large selection of models suitable for outdoor installation (degree of protection: IP 65).

SMA Solar Technology PV inverters are designed to be mounted on a solid foundation. Eye level mounting provides optimal readability of the device's operational indicators or display. Open space is also important above and — depending upon the device type — next to the devices to ensure proper heat dissipation. Information on the required device mounting clearances can be found in the installation manual of the Sunny Boy and Sunny Mini Central inverters.

The foundation must provide sufficient support. If possible, do not mount the devices on lightweight walls or on walls that can act as resonance chambers (e.g. thin wooden walls, sandwich type plaster board, etc.). The slight vibrations which the devices can generate during operation could otherwise be amplified and cause the wall to emit sound.

Mounting on a flammable foundation (e.g. wood) is not allowed.

3.3 PV Plant

PV modules are suitable for roof- or ground-mounted installation. Ground-mounted installation requires appropriate mounting supports; roof mounting requires suitable brackets. The module alignment depends on the installation location. It is important that the PV modules are not shaded — not even partially — as this reduces energy output.


SMA Solar Technology AG Installation and Mounting Locations

3.4 Batteries

Depending on their type, batteries can be dangerous because they conduct electricity and contain gases or electrolytes. During the planning and installation of the battery system, keep in mind the

European Standard EN 50272 "Safety requirements for secondary batteries and battery installations", in particular the safety precautions for the handling of batteries.

The installation location of the batteries should be dry, as cool as possible, but also free of frost. Heat and frost significantly reduce the service life of batteries. The room must also be well ventilated. If natural ventilation is not sufficient, artificial ventilation must be installed.

Special racks can be used for battery installation, which provide space-saving storage and good accessibility for maintenance. Note also the battery weight. The floor must be designed to accommodate the weight. When using batteries with liquid electrolyte, an acid-resistant sealing is required so that no battery acid can contaminate the ground. As an alternative to sealing, catch basins can also be used. This problem does not exist with gel or fleece batteries.

3.5 Generator

If you would like to integrate a diesel generator into the off-grid system to be installed, please consider that an installation in potentially flammable rooms is not allowed. The generator should be in a dry but well-ventilated room. The motor exhaust gases must be vented using an appropriate ventilation system. You should consider that the ventilation system can become very hot. This is why you must provide protection against accidental contact.

Good access for maintenance must be considered in the selection of the installation location. Also ensure good illumination, as this makes maintenance easier.

An important aspect when selecting the installation location is the emission of noise. A diesel generator does not work silently and can quickly be considered unpleasant. This is why you should consider if you wish to connect the generator to the electrical system over a short distance or if a somewhat more distant installation location, further away from living and sleeping areas, would not be preferable to saving on cable material. Additional noise protection measures must be taken as needed.

3.6 Wind Turbine System

The optimum installation location for wind turbine systems is especially dependent on local conditions.

Before installing a system, long-term wind speed measurements (over at least one year) should be made at the intended location.

Locations must be far enough away from buildings or trees, as these not only block the flow of air but also cause turbulence that can have a negative effect on the possible energy yield of close-by wind turbine systems. If this results in large distances to the distribution system, please consider that the cable cross section must be large enough to minimize transmission losses.


Designing Cable Cross Sections SMA Solar Technology AG

4 Designing Cable Cross Sections

One frequently asked question which should be answered here in detail is which cable cross section must be installed for the individual circuits of an off-grid system. As a rule, the smallest possible cross section is desired, since larger cable cross sections mean increases in

• cable costs,

• space required for the installed cable (diameter, space in installation ducts or conduits, minimum bending radius of the cable), and

• cable weight.

However, there are also technical aspects which require a minimum cross section for each application. The key measurements which influence cable size are:

• the allowed current capacity of the cable (depending upon the cable insulation material, the preceding fuse or the attached power source, the type of cable installation and the ambient temperature); the larger the cable cross section, the higher the allowed current capacity,

• the voltage drop on the line (depending on the cable length and the maximum possible operating current); the larger the cable cross section, the smaller the voltage drop, and

• the maximum loop impedance which ensures compliance with the prescribed disconnection conditions for the applied protective measures (activation of overcurrent protection devices within the prescribed period of time) (dependent on cable length); the larger the cable cross section, the lower the loop impedance.

Compliance with the allowed current capacity is particularly important for fire prevention, since an overloaded cable presents a fire hazard. This must be considered in a stand-alone grid just as in a customer's system connected to the public utility grid. DIN VDE 0298 Part 4 is used to determine the allowed current capacity of cables.

The voltage drop along a cable determines which voltage is available at the end of the cable or the cable network to the consumer. The aspect of supply quality is of primary importance. According to

DIN VDE 0100 Part 520, the greatest voltage drop of a customer's system may not exceed 4 % relative to the nominal voltage of the grid. This cannot be directly applied to an off-grid system which includes both the generator and consumer system. It is important that consumers at the end of the line are supplied with sufficient voltage, without it being necessary at the other end for the feed-in generator to produce a voltage that is too high.

Electrical consumers are generally capable of fault-free operation with voltage fluctuations of ±10 % of the nominal voltage.

The voltage drop is associated with a very significant additional variable important for the design of the cables in a stand-alone grid: the transmission losses of the cables. As the system designer, you will have difficulty explaining to your customers that a great deal of money must be invested in the largest possible PV module area and in an advanced and highly efficient inverter, and that, at the same time, a too small cable cross section means a significant part of the power generated will be lost during transmission. 4 % voltage drop also means 4 % power loss in the cable.


SMA Solar Technology AG Designing Cable Cross Sections

Cable losses range over all parts of a stand-alone grid. They can occur in the DC cables of the battery or PV plant, and in the AC cables between the feed-in generators and the distribution, or in the consumer system. Where the greatest losses occur depends upon local conditions, in particular on the required cable lengths. They can therefore not be located in general. However, if different voltage levels are present in a system, the rule applies that with identical cable cross sections, lower losses occur in areas of higher voltage, as the current is lower with identical power. This is why special focus must be put on

• the cables of the battery circuit (12 V DC, 24 V DC or 48 V DC), and

• the DC cables of the PV plant between the generator (i.e. the PV modules) and the inverter.

As a rule, when sizing cable cross sections, the criterion of allowed current capacity is the less significant requirement for the cable. Although compliance with the allowed current capacity ensures safe operation, it generally leads to high cable losses. For this reason, larger cross sections must be installed than are required by the current capacity.

The disadvantage of high cable utilization rates can be presented clearly. Since the square of the current is used in the calculation of cable losses, a doubling of the current means that the power loss increases by four times. Therefore, when you reduce the cable load by installing a second cable with the same cross section and distribute the power to be transmitted across both cables, each of the two cables has only one fourth of the original power loss, and the total power losses are halved. The same effect can also be achieved by doubling the cross section of one cable; the power loss is halved, assuming identical current and length.

The bottom line is that the transmission of power is always associated with losses. There are no ideal, loss-free cables in actual installations. As the planner and installer of a system, you should keep your ability to influence cable losses in mind:

• Install cables across the shortest possible distances and avoid detours.

• Distribute currents across the largest, economically viable cable cross section (larger cable cross sections or parallel connection of cables).

An aid in determining the AC and DC cable losses for PV plants is the Sunny Design program, which you can download free from www.SMA.de/en.

Finally, the cable sizing for the battery cables of an off-grid system will be considered as an example.

The basic rule is to keep the battery cables as short as possible. Since the battery voltages of 12 V,

24 V or 48 V are much lower than the AC voltage of the stand-alone grid, these cables carry correspondingly higher currents.

Additionally, it must also be considered that power is lost twice in the cables from the stand-alone grid inverter to the battery: once when charging the battery and then later when discharging and using the power.

It can roughly be assumed that charging the battery is done as quickly as possible and with a high constant charging current. By contrast, the discharge of the battery depends greatly upon consumer requirements, but generally occurs with only an average of 10 % of the maximum charging current.

This can largely be disregarded in considerations of cable losses when compared to the losses during charging.



Since power storage in batteries is also associated with losses, and inverter losses occur twice in the inverter, high requirements should be placed on minimizing cable losses in the battery cables. This naturally implies higher installation costs. However, it must be considered that the power lost in the cables must be generated (e.g. using diesel fuel) or paid for through a larger module area for the PV plant. Two percent cable losses during battery charging by a Sunny Island 5048 can be calculated as representing approximately 100 Watt power.

13

14

15

9

10

11

12

7

8

5

6

3

4

1

2

16

0.46 %

0.93 %

1.39 %

1.85 %

2.32 %

2.78 %

3.24 %

3.71 %

4.17 %

4.64 %

5.10 %

5.56 %

6.03 %

6.49 %

6.95 %

25

0.30 %

0.59 %

0.89 %

1.19 %

1.48 %

1.78 %

2.08 %

2.37 %

2.67 %

2.97 %

3.26 %

3.56 %

3.86 %

4.15 %

4.45 %

Cable cross section in mm

2

35 50

0.21 %

0.42 %

0.64 %

0.85 %

1.06 %

1.27 %

1.48 %

1.70 %

1.91 %

2.12 %

2.33 %

2.54 %

2.75 %

2.97 %

3.18 %

0.15 %

0.30 %

0.45 %

0.59 %

0.74 %

0.89 %

1.04 %

1.19 %

1.34 %

1.48 %

1.63 %

1.78 %

1.93 %

2.08 %

2.23 %

0.74 %

0.85 %

0.95 %

1.06 %

1.17 %

1.27 %

1.38 %

1.48 %

1.59 %

70

0.11 %

0.21 %

0.32 %

0.42 %

0.53 %

0.64 %

Table 1: Relative cable losses in a copper cable with a charging current of 100 A and a nominal voltage of 48 V (application: Sunny Island 5048)

The table above lists cable losses relative to charging power with a constant charging current of

100 A for a Sunny Island 5048. For example, if you wish to allow relative cable losses of 1 %, with a cross section of 16 mm², you must maintain a maximum distance from the inverter to the connection terminals of the battery (one-way cable length) of approximately 2 m. If the distance is 9 m, a 70 mm²

(almost four times larger) cross section must be installed to achieve the same losses.

When selecting the correct inverter, consider the maximum cable cross section that can be connected to the device. Cables with a cross section of up to 70 mm² can be connected to Sunny Island 5048.

Cables of up to 95 mm² can be connected to Sunny Island 2012/2224.

The following tables compare the cable losses of a Sunny Island 2224 and a Sunny Island 2012.

The Sunny Island 2224 has a nominal power of 2.2 kW and a nominal battery voltage of 24 V. The

Sunny Island 2012 has a nominal power of 2 kW and nominal battery voltage of 12 V.


SMA Solar Technology AG Designing Cable Cross Sections

A lower nominal battery voltage means a higher charging current at the same charging power. If you compare Table 2 with Table 3, you see that Sunny Island 2012 has significantly higher cable losses resulting from the much higher charging currents.

12

13

14

15

8

9

10

11

6

7

4

5

1

2

3

16

0.74 %

1.48 %

2.23 %

2.97 %

3.71 %

4.45 %

5.19 %

5.93 %

6.68 %

7.42 %

8.16 %

8.90 %

9.64 %

10.38 %

11.13 %

25

0.47 %

0.95 %

1.42 %

1.90 %

2.37 %

2.85 %

3.32 %

3.80 %

4.27 %

4.75 %

5.22 %

5.70 %

6.17 %

6.65 %

7.12 %


2

35

0.34 %

0.68 %

1.02 %

1.36 %

1.70 %

2.03 %

2.37 %

2.71 %

3.05 %

3.39 %

3.73 %

4.07 %

4.41 %

4.75 %

5.09 %

50

0.24 %

0.47 %

0.71 %

0.95 %

1.19 %

1.42 %

1.66 %

1.90 %

2.14 %

2.37 %

2.61 %

2.85 %

3.09 %

3.32 %

3.56 %

1.36 %

1.53 %

1.70 %

1.86 %

2.03 %

2.20 %

2.37 %

2.54 %

70

0.17 %

0.34 %

0.51 %

0.68 %

0.85 %

1.02 %

1.19 %




12

13

14

15

8

9

10

11

6

7

4

5

1

2

3

25

1.90 %

3.80 %

5.70 %

7.59 %

9.49 %

11.39 %

13.29 %

15.19 %

17.09 %

18.99 %

20.89 %

22.78 %

24.68 %

26.58 %

28.48 %

35

1.36 %

2.71 %

4.07 %

5.42 %

6.78 %

8.14 %

9.49 %

10.85 %

12.21 %

13.56 %

14.92 %

16.27 %

17.63 %

18.99 %

20.34 %


2

50

0.95 %

70

0.68 %

1.90 %

2.85 %

3.80 %

4.75 %

1.36 %

2.03 %

2.71 %

3.39 %

5.70 %

6.65 %

7.59 %

8.54 %

9.49 %

10.44 %

11.39 %

12.34 %

13.29 %

14.24 %

4.07 %

4.75 %

5.42 %

6.10 %

6.78 %

7.46 %

8.14 %

8.82 %

9.49 %

10.17 %


4.00 %

4.50 %

5.00 %

5.50 %

6.00 %

6.50 %

7.00 %

7.49 %

95

0.50 %

1.00 %

1.50 %

2.00 %

2.50 %

3.00 %

3.50 %


SMA Solar Technology AG Installation Material

5 Installation Material

In principle, the installation of an off-grid system does not require any special materials. Standard cables, conduits, distribution boxes, circuit breakers, contactors, etc. can be used, as long as they meet the requirements of the different voltage levels (e.g. 400/230 V AC and 12 V DC, 24 V DC or

48 V DC battery voltage).

For contactors operated with battery voltage, please note the operating range of the Sunny Island.

Devices with a battery voltage of 48 V (e.g. SI 4248 or SI 5048) have an operating range of 41 V to 63 V. When selecting the contactor coils, you must consider these values, which you will find in the technical data for the devices. The charging current is also an important criterion.

Example:

You are looking for a contactor coil for an inverter with a nominal voltage of 48 V and an operating range of 42 V to 63 V. Contactors for applications in battery circuits can generally be ordered with an "extended voltage range" for the coils, for example 0.7 ... 1.25 * U

N

(U

N

= nominal voltage). This means that a contactor with a nominal voltage of 48 V could only cover an operating range of 33.6 V to 60 V. With a longer charging phase at 63 V, the contactor coil would suffer thermal overload and be destroyed. However, if you select a contactor with a nominal voltage of 60 V, the operating range under the conditions named above is sufficient (0.7 ... 1.25 * U

N

) from 42 V to 75 V.

Accessories

Special accessories for Sunny Island can be obtained from SMA Solar Technology. It makes your selection of such components easier.

The following accessories are available from SMA Solar Technology:

• Batteries

• Fuse boxes for batteries

• Battery shunts (required for installations with DC coupling)

• Distribution boxes for generators

Temperature sensors for the battery are included with the Sunny Island. Use only the included temperature sensors.


Laying the Cables SMA Solar Technology AG

6 Laying the Cables

6.1 General Requirements

When installing cables, take care to protect the cables from external influences.

Conversely, the environment of the cables must be protected from possible hazards. For this reason, only use cables approved for the desired type of installation.

In the area of the distribution system, only surface installations should be used. Compared to flushmounted installations, they have the advantage of making it easier to adapt the system, for example to enhance it.

Make sure to leave sufficient room for later enhancements. Cable channels should therefore not be sized too small. Cables should also not be bundled too tightly, in order to minimize their mutual thermal influence.

There are particular requirements for outdoor installations. The cables must be protected against UV radiation. Standard sheathed cables for moisture-proof installation (NYM) are not suitable. Either special cable material must be used, or the entire length of the cables must be installed in protective conduits.

Installing cables in the ground — for example to a wind turbine system — requires the use of suitable grounding cables. Installation in flexible protective conduits is only allowed over short distances if the cables are replaceable. Ensure that the cable path is marked and maintain good documentation. It helps to avoid damage to the buried cables from later excavation activities.

Installing uninsulated open lines is easier and less expensive than installing an underground cable.

However, special requirements for lightning protection must be met.


SMA Solar Technology AG Laying the Cables

In general, when installing cables, separate the power cables from the control cables (e.g. communication cables) to prevent mutual influence. The use of cable separators in cable conduits is advisable.

In regard to the installation technology, AC circuits of off-grid systems do not differ essentially from systems connected to the public grid. DC circuits of a stand-alone grid are not as familiar to the installer and will therefore be examined more closely here.

6.2 Installing Cables of a PV Plant

There are particular requirements for the cables of a PV plant. They are generally not protected at the

PV array, which means that the cable protection can only be provided by short circuit and ground fault protected installation, which is allowed within power generation systems according to

DIN VDE 0100 Part 430. The usage of single-core sheathed cables with sufficient insulation resistance meets these requirements. However, it is essential to ensure that the cables for the plus and minus poles are laid as close together as possible to prevent the creation of induction loops.

Otherwise, in case of a close-by lightning strike, the highly variable magnetic fields could induce high voltages which are all the higher, the greater the area of the induction loop.

If a short circuit and ground fault protected installation is not possible, you must secure the DC cables as close to the PV array as possible.

6.3 Installing Cables of a Battery System

The same applies to the DC cables of the battery circuit. Short circuit and ground fault protected installation is required up to the fuses. You can omit battery fuses if short circuit and ground fault protected installation is provided throughout, up to the stand-alone grid inverter. When selecting cable material, ensure that this is approved for short circuit and ground fault protected installation

(e.g. cable type NSGAFöu).

Also note that battery acid can corrode the cable insulation. Therefore, use acid-resistant cable material at least for cables close to the batteries, and adhere to the information of the battery manufacturer.


Lightning Protection in Off-Grid Systems SMA Solar Technology AG

7 Lightning Protection in Off-Grid Systems

7.1 Physical Basis

Lightning is an electric discharge during a storm. The path of the lightning only depends upon the charge conditions, which also explains the "zig-zag" path of a lightning bolt. The closer the discharge draws to the surface of the earth, the more influence field concentrations exert on points and curves which promote an arc. "Positive streamers" are produced, particularly on high buildings and components. However, the lightning must not necessarily strike there. It can also select a different path. Since it is impossible to forecast where the lightning will strike, all buildings that one wishes to protect from lightning strike must be equipped with a lightning protection system.

The lightning strike has several effects:

The acoustic effect is caused by electrodynamic forces. It causes a very tight concentration of the lightning channel. The quickly decreasing electrodynamic internal pressure in the lightning channel during lightning current increase is estimated to be several tens of bars. The hot lightning core implodes. Near by, this is perceived as a bang and at greater distances as rolling thunder.

The thermal effect follows Joule's law W = i² · t · R, where W is heat, i² the square of the current, t duration of current flow and R ohmic resistance. According to the formula above, objects can only be heated to melting temperature if cable cross sections are small or the specific resistance is high. For this reason, poor connections are a hazard. A great deal of energy is released in the form of heat when current flows through poor conductors. The water content of wood and masonry, for example, is intensely heated and vaporizes. The overpressure of this fast process causes explosion-like blasts.

Electrodynamic effects occur when parts of the lightning path are arranged in such a way that the magnetic fields overlap. A force is exerted on the conductor carrying the lightning current that increases, the smaller the distance. If current flows in the same direction, attractive forces are created.

According to Faraday's law, a lightning charge of 100 As causes 30 mg of iron or a similar amount of zinc or lead to be decomposed at the current output, which is an electrochemical effect .

However, arresters, which are often struck by lightning, are not expected to develop any electrochemical decomposition; much more significant is corrosion.


SMA Solar Technology AG Lightning Protection in Off-Grid Systems

The following values are typically associated with lightning:

Lightning current values

Maximum value of the lightning current i max

Charge Q

This value is decisive for the voltage drop at the grounding resistor or for the voltage increase compared to the distant environment.

This value determines the energy released at the impact point. The charge results from the product of the lightning current and the duration of the lightning.

Current squared impulse i² · t This value determines the heating and the dynamic load. It is calculated from the product of the square value of the lightning current and the duration of the lightning.

Rate of lightning current rise di/dt

This value is decisive for the level of induced (generated) voltage in all open and closed installation circuits in the vicinity of lightning conductors.

The grounding system must be connected to the equipotential busbar by the shortest path possible.

For lightning protection systems without lightning protection equipotential bonding, it is required that the grounding resistance R is less than or equal to five times the smallest distance in meters between above-ground lightning protection and large metal parts or a high voltage system. A ring-shaped ground electrode must be laid at least 0.5 m deep and as a closed ring around the outside foundation. Depending on the discharge, individual ground electrodes must either have a length of

20 m when used as surface ground electrodes, or 9 m when used as deep ground electrodes. Deep ground electrodes have to be separated by approximately 1 m.

The internal lightning protection includes all measures against the effects of a lightning current and its electrical and magnetic fields on metal installations and electrical systems. The heart of the internal lightning protection is the lightning protection equipotential bonding.

This includes bonding of

• ducts (not gas lines),

• metal installations within the building,

• grounded parts of high voltage and IT systems, and

• all active wires of power and information technology cables and cords leading into and out of the building to the grounding system.

Two device types are available to protect live cables connected to the equipotential bonding:

• Lightning current arresters (class B arresters) can carry direct lightning currents and lead them to a grounding system.

• Overvoltage arresters (class C arresters) can limit overvoltage and carry the induced surge currents to a grounding system.



Another possible protection are shielded cables, where the shielding is connected to the grounding system. Sufficiently large shielding can both carry the partial lightning currents and prevent the induction of overvoltages in the cable.

While the external lightning protection is primarily designed to prevent fire hazard, the internal lightning protection limits the effects of the lightning current and its electrical and magnetic fields on living creatures, metal installations, electrical consumers and electronic devices. Therefore, both are necessary for effective lightning protection.

Additional information on protection from the effects of lightning, prepared by the lightning protection and lightning research committee (Ausschuss für Blitzschutz und Blitzforschung, ABB) of the VDE

Association for Electrical, Electronic and Information Technologies e.V., can be found in German on the VDE website, www.vde.com/ABB.

Minimum cross section for protective bonding conductors

Conductor material Required cross section in mm

2

Copper 10

Aluminum

Steel

16

50

For reasons of standardization, 16 mm² coarse Cu wire should be used as a rule.

The decision if and to what extent lightning protection is necessary in a stand-alone grid is largely determined by the location of the system components and the requirements on the operating safety of the overall system.

For example, a risk analysis can be made using the software developed by AixThor

(www.aixthor.com) "Risk management: Assessment of risk for structures" (VDE 0185 Part 2:11-2002).

If this analysis reveals that lightning and overvoltage protection measures are necessary, execution can be planned using the requirements described in Part 3 and 4 of VDE 0185.

The manufacturers of lightning and overvoltage components also offer planning assistance for the implementation of protective measures. For example, the companies DEHN (www.dehn.de) or

Phoenix Contact (www.phoenixcontact.de) should be mentioned.

When planning off-grid systems, due to the often very exposed locations of the feed-in components such as wind turbine systems or PV arrays, one will usually come to the conclusion that lightning and overvoltage protection is necessary.

DIN and VDE standards on lightning / overvoltage protection:

• VDE V 0185-1 2002-11 Protection against lightning - General principles

• VDE V 0185-2 2002-11 Protection against lightning - Risk Management: Assessment of risk for structures

• VDE V 0185-2 Supplement 1 2004-06 Protection against lightning Part 2: Risk management -

Supplement 1: Calculation assistance for assessment of risk for structures

• VDE 0185-3 2004-09 Protection against lightning Part 3: Physical damages to structures and life hazard



• VDE V 0185-3/1 2005-06 Protection against lightning Part 3: Physical damages to structures and life hazard

• VDE V 0185-4 2002-11 Protection against lightning - Electrical and electronic systems within structures

• VDE 0845-4-1 2000-07 Lightning protection Telecommunication lines - Fibre optic installations

(IEC 61663-1:1999 + Corrigendum 1999)

Additional interesting/useful publications on lightning protection can be found at the VDE Publishing

House's website, www.vde-verlag.de/english.



7.2 Types of Lightning Strike Risks

Direct strike

The lightning current flows through the PV array, the connected system components and the building services equipment, which generally is destroyed. Severe mechanical damage can also not be ruled out.

Indirect strike

A strike occurs close to the generator or the distribution room. Parts of the lightning current can flow across the PV array and/or enter the cabling. The voltage is boosted in the system cables.

Close strike

Within a radius of approximately 500 m of the lightning discharge channel, magnetic (inductive) effects influence cable loops of all kinds. With each discharge, large overvoltages develop in the loops, which can lead to errors or destruction.

Distant strike

At greater distances to the system, capacitive effects occur. This means that an electric field is created between cloud or lightning channel (high potential) and electrically conductive parts of the plant, which is separated by an induced charge and therefore creates voltage within the system. Upon a sudden collapse of the field by the strike, the charges equalize through a surge current. If a surge current flows through the line, the current and the characteristic impedance cause a surge voltage in the line.

Two protection goals must be met:

1. Protection from the effects of direct lightning strikes in the system components themselves or the system buildings.

2. Protection from lightning overvoltages which can also be caused by indirect lightning strikes.

According to VDE 0185 Part 1, two protective measures are available: the installation of an external lightning protection system (insulated or not insulated, i.e. with or without lightning currents across system components) and the installation of an internal lightning protection system (overvoltage protection measures).



7.3 Practical Lightning Protection

External lightning protection consists of all of the devices for capturing and discharging the lightning current to the grounding system:

1. Capture devices

A mesh is installed on the roofing with a mesh size of 10 m x 20 m. No point of the roof surface may be more than 5 m from the next capture device. Roof structures made of non-conducting material are considered protected if they do not extent beyond the level of the mesh by more than 0.3 m. If they extend further out, they must be equipped with a capture point, which is connected with the mesh.

Roof structures made of metal are not connected to the mesh if they

• protrude no more than 0.3 m above the level of the mesh,

• are at least 0.5 m away from a capture device, and

• have no more than a 1 m² enclosed area or are max. 2 m long.

2. Dischargers

Dischargers are electrically conductive connections between capture devices and the ground. The required number of dischargers is given by the circumference of the outer roof edge. Up to max. 20 m, one discharger is sufficient. For more than 20 m, the roof circumference is divided by 20.

This yields the number of dischargers. Decimals are generally rounded up. The dischargers should be placed on the corners and junctions of the mesh.

3. Grounding system

A grounding system must be installed for lightning protection if, for example, no foundation earth electrode is present. The system must be operative without the use of metallic water pipes, ducts or grounded conductors.


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Planning Guidelines 27 INSELNETZ-PL-UEN105010

Stand-alone Grid Systems

Stand-alone Grid Systems

Installing a Stand-alone Grid System

1 Normative Framework

2 Grid Forms

3 Installation and Mounting Locations

4 Designing Cable Cross Sections

5 Installation Material

Accessories

6 Laying the Cables

7 Lightning Protection in Off-Grid Systems

Two protection goals must be met:

1. Capture devices

2. Dischargers

3. Grounding system

Exclusion of liability

SMA Factory Warranty

Trademarks

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Stand-alone Grid Systems