System components can be damaged?

Photovoltaic systems can be damaged by both direct and indirect lightning strikes and overvoltage disturbances caused by electrical switching operations, load switching and so on.

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Lightning strike map

Statistics on damage due to lightning induced over-voltages have shown that electronic installations up to about one kilometre from the strike point may be susceptible to induced or line-carried over-voltages. 

The system designer must take into account the geographic location of the proposed array when looking at surge protection requirements where lightning is concerned.

What happens on a solar ground mount system?

When lightning strikes a solar PV system, it causes an induced transient current and voltage within the solar PV system wire loops. 

These transient currents and voltages will appear at the equipment terminals and likely cause insulation and dielectric failures within the solar PV electrical and electronics components such as the PV panels, the inverter, control and communications equipment, as well as devices in the building installation. 

The combiner box, the inverter, and the MPPT (maximum power point tracker) device have the highest points of failure.

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What form does protection take?

Effectively we use Surge Protective Devices, both on the DC and AC side. The SPD for the DC input to the inverter and solar array must be designed specifically for DC applications. AC SPDs are not suitable because upon failure their disconnect circuitry may not quench the arc. 

AC SPDs rely upon a voltage zero crossing to quench the arc which does not occur with DC so the arc may persist, with the possible hazard of fire.

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How to select an SPD

Select whether it is DC or AC, single or  three phase and then look at the nominal voltage of the system, 230/400 or 1000 V for solar, then look at the voltage level protection.

In, peak value of current that SPD can withstand and function after 15 surges and Imax, this value should be higher than max Isc of the electrical network.

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DC SPD

In the case of a commercial solar system up to 1000 volts DC recommended to select a DC SPD with the following characteristics:

• UCPV Maximum continuous operating voltage, 1000 V
• I max Maximum discharge current (8/20us) 40kA
• In Nominal discharge current (8/20us) 20kA
• Up Voltage protection level (in accordance with AS/NZS1768-2007) 3000V

AC SPD

In the case of AC component recommended to select single or three phase AC SPD with the following characteristics:

• UC Maximum continuous operating voltage, eg 275 V 
• IL Maximum load current, eg 63A
• I max Maximum discharge current (8/20us) 50kA
• In Nominal discharge current (8/20us) 20kA
• Isccr  Short circuit current rating, 25 kA
• Up Voltage protection level (in accordance with AS/NZ S1768-2007) < 800V

Where does the DC SPD go?  

In the case of a commercial solar system the DC SPD goes between the string protection, DC isolators and the string fusing in the inverter. 

The SPD is connected in parallel to your system, and the output is connected to earth.

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DC SPD and cable length relationships?  

There is an intrinsic relationship between cable length and voltage increase:

• Cable length between SPD and load should be kept short as possible
• Damage from overvoltage increase with cable length
• If length exceeds a certain point recommended to use a second SPD

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Conclusion

• ✅   SPD’s are there to divert the potentially harmful effects of lightning and load switching
• ✅   For DC protection only specific DC SPD may be used
• ✅   There are three main types of SPD’s available
• ✅   Whether SPD are used or not depends on geography, geology and the presence of sensitive loads

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If you’d like to see what Greenwood Solutions get up to in the real world of renewable energy, solar, battery storage and grid protection check out our industry and commercial pages:

https://www.greenwoodsolutions.com.au/industry 

https://www.greenwoodsolutions.com.au/commercial