PV System: how to ensure safety during normal operation

Two particular characteristics of PV generators are their DC voltage levels and the fact they cannot be shut off as long as PV modules are exposed to the sun. The short-circuit current produced by the PV module is too low to trigger the power supply’s automatic disconnect. The most frequently used protective measures do not therefore apply to PV systems. However, as PV modules are installed outdoors they are exposed to the elements. And since they can be installed on roofs, critical attention should be paid to the risk of fire and the protection of fire fighters and emergency services staff.

Protecting people against electric shock

Paragraph 412.1.1 of IEC 60364 states: Double or reinforced insulation is a protective measure in which basic protection is provided by basic insulation, and fault protection is provided by supplementary insulation, or basic and fault protection is provided by reinforced insulation between live parts and accessible parts. NB: This protective measure is intended to prevent the appearance of dangerous voltage on the accessible parts of electrical equipment through a fault in the basic insulation.

IEC 60364-712 stipulates that PV systems whose maximum UOC MAX is higher than 120V DC should use « double or reinforced insulation » as a protection against electric shock.
Switchgear, such as fuses or circuit-breakers on the DC side, do not afford protection against electric shock as there is no automatic disconnect of the power supply.
Overcurrent protection, when used, protects PV cells against reverse current and cables against overload.

Risk of fire: protection against thermal effects

Generally speaking there are three situations that can lead to abnormally high temperatures and the risk of fire in a PV system: insulation fault, a reverse current in a PV module, and overloading cables or equipment.

Insulation fault detection

Double or reinforced insulation is a protective measure against electric shock but it does not exclude all risk of insulation fault. (The assumption here is that the likelihood of an insulation fault and of someone touching an energised part of the installation at the same is very low. Insulation faults in themselves do happen more frequently, however.) DC insulation fault could be more dangerous as arc has less chance to extinguish by itself as it does in AC.

When an insulation fault is detected whatever the solution is, inverter is stopped and disconnected from AC side, but the fault is still present on DC side and voltage between poles is open circuit voltage of PV generator as long as sun is shining. This situation cannot be tolerated over a long period and the fault has to be found and cleared. If not, a second fault may develop on the other pole, causing the current to circulate in the earthing conductors and metal parts of the PV installation with no guarantee that protective devices will operate properly. See “Overcurrent protection”.

The PV generator should be checked to ensure it is insulated from earth.

• When there is no galvanic insulation between the AC side and the DC side:

 - It is impossible to earth one pole.
 - AC protection can be used to detect insulation faults.

• When the AC side and DC side are galvanically separated:

 - An overcurrent protective device (which also detects insulation faults) should be used to trip the grounded conductor in the event of a fault, if the PV cell technology (e.g. thin films of amorphous silicon) requires one of the conductors to be directly grounded.
 - An insulation monitoring device should be used if the PV cell technology requires one of the conductors to be resistance-grounded.
 - An insulation monitoring device should also be used when PV cell technology does not require either conductor to be earthed.

Insulation monitoring device shall be selected taking into consideration both UOC MAX and the capacitance between poles and earth causes leakage current. In addition cables and inverter capacitance should be also considered. An Insulation monitoring device able to handle capacitance up to 500F is suitable for PV system.

The literature provided by manufacturers of photovoltaic modules yield the following figures:

	Maximum power usually developed with a single inverter	Surface necessary to develop such a Power	Usual capacitance by m²	Usual capacitance between lines and earth for a single IT system
Frameless glass-glass module with aluminium frame on an assembly stand (open air)	1 MW	8000 m²	1 nF / m²	8 μF
In-roof glass-glass module with aluminium frame	100 kW	800 m2	5 nF / m²	4 μF
Thin-film PV module on flexible substrate	100 kW	800 m2	50 nF / m²	40 μF

Some measurements made in European plants are giving the following figures:

	Maximum power developed with a single inverter	Surface necessary to develop such a Power	Lowest capacitance measurement	Highest capacitance measurement	Maximum measured capacitance by m²
Frameless glass-glass module with aluminium frame on an assembly stand (open air)	Plant 1: 1 MW	8000 m²	Sunny afternoon: 5 μF	Rainy morning: 10 μF	1.25 nF / m²
	Plant 2: 750 kW	5000 m²	Sunny afternoon: 2 μF	Rainy morning: 4 μF	0.8 nF / m²
In-roof glass-glass module with aluminium frame	Plant 1: 100 kW	800 m²	Sunny afternoon: 2 μF	Rainy morning: 4 μF	5 nF / m²
	Plant 2: 50 kW	400 m²	Sunny afternoon: 0.5 μF	Rainy morning: 1 μF	2.5 nF / m²
Thin-film PV module on flexible substrate	Plant 1: 100 kW	800 m²	Sunny afternoon: 30 μF	Rainy morning: 50 μF	62.5 nF / m²
	Plant 2: 50 kW	400 m²	Sunny afternoon: 15 μF	Rainy morning: 25 μF	62.5 nF / m²

Fig. P8 : Example of leakage capacitance in various PV systems

Protection of PV modules against reverse current

A short circuit in a PV module, faulty wiring, or a related fault may cause reverse current in PV strings. This occurs if the open-circuit voltage of one string is significantly different from the open voltage of parallel strings connected to the same inverter. The current flows from the healthy strings to the faulty one instead of flowing to the inverter and supplying power to the AC network. Reverse current can lead to dangerous temperature rises and fires in the PV module. PV module withstand capability should therefore be tested in accordance with IEC 61730-2 standard and the PV module manufacturer shall provide the maximum reverse current value (IRM)

Fig. P9 : Reverse current

Reverse current into the faulty string = total current of the remaining strings

String overcurrent protection is to be used if the total number of strings that could feed one faulty string is high enough to supply a dangerous reverse current:

1.35 I_RM < (Ns - 1) I_{SC MAX}

where:

• I_RM is the maximum reverse current characteristic of PV cells defined in IEC 61730
• Ns is the total number of strings

There is no risk of reverse current when there is only one string. When there are two strings with same number of PV modules connected in parallel, the reverse current will be always lower than the maximum reverse current. So, when the PV generator is made of one or two strings only there is no need for reverse current protection.

Protection against overcurrent

As in any installation, there should be protection against thermal effect of overcurrent causing any danger.

Short-circuit current depends on solar irradiance, but it may be lower than the trip value of overcurrent protection. Although this is not an issue for cables as the current is within current-carrying capacity, the inverter will detect a voltage drop and stop producing power. It is therefore recommended that the maximum trip current should be significantly lower than I_{STC MAX}.

IEC 60364-712: 712.433.1 Overload protection may be omitted to PV string and PV array cables when the continuous current-carrying capacity of the cable is equal to or greater than 1,25 times ISC STC at any location. 712.433.2 Overload protection may be omitted to the PV main cable if the continuous current-carrying capacity is equal to or greater than 1,25 times ISC STC of the PV generator.

String protection

Where string overcurrent protection is required, each PV string shall be protected with an overcurrent protection device.

The nominal overcurrent protection (Fuse or Circuit breaker) rating of the string overcurrent protection device shall be greater than 1,25 times the string short circuit current I_{sc stc_string}.

Array protection

The nominal rated trip current (ITRIP) of overcurrent protection devices for PV arrays (Fuses or Circuit breaker) shall be greater than 1,25 times the array short-circuit current I_{sc stc_array}

The selection of overcurrent protection rating shall be done in order to avoid unexpected trip in normal operation taking into account temperature. A protection rating higher than 1.4 times the protected string or array short-circuit current I_{sc_stc} is usually recommended.

Circuit breakers or Fuses

Circuit breakers or fuses can be used to provide overcurrent protection. Fuses, usually on the fuse holder or directly connected to bars or cables, do not provide a load-break switch function. So when fuses are used, load-break switches should also be used to disconnect fuses from the inverter in order to allow cartridge replacement. So an array box with fuses on fuse holders as string protection, for example, should also incorporate a main switch.

Circuit breakers offer fine-tuned adjustment and greater accuracy than fuses in order to allow the use of cables, especially for sub-array cables, that are smaller than fuses

Double earth faults

PV systems are either insulated from the earth or one pole is earthed through an overcurrent protection. In both set-ups, therefore, there can be a ground fault in which current leaks to the ground. If this fault is not cleared, it may spread to the healthy pole and give rise to a hazardous situation where fire could break out. Even though double insulation makes such an eventuality unlikely, it deserves full attention.

Fig. P10 : Reverse current

For the two following reasons the double fault situation shall be absolutely avoided: Insulation monitoring devices or overcurrent protection in earthed system shall detect first fault and staff shall look after the first fault and clear it with no delay:

• The fault level could be low (e.g. two insulation faults or a low short-circuit capability of the generator in weak sunlight) and below the tripping value of overcurrent protection (circuit breaker or fuses). However, a DC arc fault does not spend itself, even when the current is low. It could be a serious hazard, particularly for PV modules on buildings.
• Circuit breakers and switches used in PV systems are designed to break the rated current or fault current with all poles at open-circuit maximum voltage (UOC MAX). To break the current when UOC MAX is equal to 1000V, for instance, four poles in series (two poles in series for each polarity) are required. In double ground fault situations, the circuit breaker or switches must break the current at full voltage with only two poles in series. Such switchgear is not designed for that purpose and could sustain irremediable damage if used to break the current in a double ground fault situation.

The ideal solution is prevent double ground faults arising. Insulation monitoring devices or overcurrent protection in grounded systems detect the first fault. However, although the insulation fault monitoring system usually stops the inverter, the fault is still present. Staff must locate and clear it without delay. In large generators with subarrays protected by circuit breakers, it is highly advisable to disconnect each array when that first fault has been detected but not cleared within the next few hours.

Switchgears and enclosure selection

Double insulation

Enclosures on the DC side shall provide double insulation.

Thermal issues

The thermal behaviour of switchgear and enclosures warrants careful monitoring. PV generator boxes and array boxes are usually installed outdoors and exposed to the elements. In the event of high ambient temperatures, high IP levels could reduce air flow and thermal power dissipation. In addition, the way switchgear devices achieve high voltage operation – i.e. through the use of poles in series – increases their temperature. Special attention should therefore be paid to the temperature of switchgear inside outdoor enclosures on the DC side.

Cable protection should comply with requirements of IEC 60364. Part 712 of the standard stipulates that all enclosures on the DC side should meet the requirements of IEC 61439. This standard covers low voltage switchgear and control gear assemblies and sets out requirements that guarantee the risk of temperature rises has been factored into the safe design of DC boxes (generator and array boxes).

Pollution degree of switchgear and enclosure selection

In addition to the standard criteria for selecting enclosures in PV systems with UOC MAX of 1000V, some equipment may show IEC 606947-1 Pollution Degree 2 rather than Pollution Degree 3.

If the switchgear is Pollution Degree 2, the IP level of the enclosure according to IEC 60529 shall be at least IP5x.

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