Distributed Photovoltaic Station DC Arcs and Fire Safety Security Hazards – Application Scenario Risk Analysis

Under the national dual-carbon policy, photovoltaic power generation will dominate the future energy structure and enter a vibrant new era. In 2021, the newly added installed capacity of distributed photovoltaic stations exceeded centralized power stations for the first time, marking the entry of distributed photovoltaic stations into a stage of rapid scale growth. Resolution point.

“Security” is the lifeblood of all industries and a fundamental issue of veto. The safe operation of photovoltaic stations is also the foundation for the station to achieve investment returns. With the continuous implementation of the “county-wide promotion” project, a large number of government agencies, schools, hospitals, residential roofs, as well as gas stations, various sheds, and industrial and commercial colored steel tile roofs have installed photovoltaic stations; renewable energy building policies will also promote the vigorous development of the BIPV (Building PV Integration) market. For a long time, “preventing DC arcs” and “fire prevention” have been the difficulties of photovoltaic stations, the pain points of rooftop stations, the focus of government attention, and the focus of public opinion.

Fire Safety Hazards

The generation of DC arcs is the most common fault phenomenon in photovoltaic stations. Contact point detachment, device aging, insulation breakdown, poor grounding, etc., can all cause arcs. Moreover, the hazard of DC arcs is much greater than that of AC arcs because DC arcs do not have zero crossing points. Once they occur, they will continue to burn and are difficult to extinguish, which easily leads to fire accidents. Statistics show that more than half of the fire accidents in photovoltaic stations are caused by DC arcs. As the specifications of photovoltaic modules become larger, the power and current on the DC side system also increase. According to Joule’s law Q=I²Rt, when the current doubles, the thermal effect at the short-circuit point increases by four times, greatly increasing the risk of fire.

Classification of DC Arcs

The description of DC arcs in the national standard GB/T 16895.32-2021:

Unlike traditional electrical products, PV modules and their connections do not have complete outer shells to contain arcs and sparks caused by component and wiring faults, and many PV devices can operate at typical DC voltages that sustain DC arcs.

Arcs in photovoltaic devices are mainly divided into three types:

—Series arcs may be caused by incorrect wiring or series wiring disconnection

—Parallel arcs may be caused by local short circuits between adjacent lines with different potentials

—Grounding arcs caused by insulation faults

Series Arcs      

Series arcs, also known as pulling arcs, are usually caused by poor contact between cable plugs of modules, or poor contact between series cables and combiner boxes or inverters. Since rooftop photovoltaic stations have many series-connected plugs, a 1 MW rooftop photovoltaic station has 2000 pairs of plugs. With so many pairs of plugs, it is difficult to ensure that all plugs are of good quality, leading to poor contact and the formation of DC arcs.

Currently, a few inverters integrate arc protection functions, but this protection has two main problems: firstly, if a series string has an arc fault, it will cause the entire inverter to shut down, resulting in significant loss of electricity generation; secondly, there is no arc fault location function, and maintenance personnel cannot timely and accurately locate the arc, which is essentially unsolvable. The only technical reset protection they can do is to keep the inverter running. Therefore, the arc protection function integrated into the inverter cannot effectively solve the problem of arc faults.

Parallel Arcs

Parallel arcs are mainly caused by line damage resulting in short circuits between positive and negative conductors, or short circuits between series cables. When series cables are mechanically squeezed or worn, arcs may occur between positive and negative poles or between different series strings, leading to parallel arc faults. Another situation that may cause parallel arcs is when series arc faults are not handled in time, the heat of the series arc can burn the insulation layer of the cable, resulting in parallel arcs.

When parallel arcs occur between the main conductors of module arrays, because the arc can obtain sufficient energy, it is difficult to extinguish and can cause major fire accidents. Series fault arcs can be extinguished by disconnecting the DC bus or the corresponding series in the photovoltaic system, but parallel fault arcs cannot be extinguished and may even cause larger currents to pass through the arc path, intensifying the arc.

Currently, the arc protection function integrated into inverters cannot detect parallel arcs and grounding arcs, but the destructive power of parallel arcs is often ten times that of series arcs, posing greater safety hazards.

Ground Arcs

Module aging damage or mechanical damage leads to discharge to ground. If modules are laid flat on colored steel tile roofs, ground arcs or leakage may occur. Such faults are not easy to detect, especially on rainy days. The current solution is to first shut down the inverter and then restart it after the ground is dry. This method cannot effectively eliminate hidden dangers and increases the risk of electric shock.

DC High Voltage

In photovoltaic stations, photovoltaic modules are connected in series to form a DC high-voltage circuit, generally reaching around 1000V. Even when the system is shut down, there is still about 1000 volts DC in the photovoltaic module array. Especially in rooftop photovoltaic stations, when a fire occurs in the photovoltaic station and the building, there are difficulties in safe rescue; during daily operation and maintenance of the power plant or property maintenance, operators and inspectors also face the risk of electric shock.

Scenario Risk Analysis

Government Agencies, Schools, Hospitals, Residential Rooftops

Scenario Risk Analysis:

1. Regional control, unable to use drones to scan module anomalies, unable to detect hidden dangers in a timely manner；

2. High population density, leakage in component arrays, high risk of personnel electric shock；

3. Limited rescue. In emergencies such as fires, the inability to shut down the high voltage of the string hinders rescue efforts；

4. Public opinion impact. If fire and other accidents occur, the impact on public opinion will be greater；

Various Colored Steel Tile Roofs

Scenario Risk Analysis：

1. Difficult patrol inspection, inconvenient inspection of colored steel tile roofs, unable to detect arc safety hazards in a timely manner；

2. Limited rescue. In emergencies such as fires, the inability to shut down the high voltage of the string hinders rescue efforts；

3. Fragile roofs, DC arc sparks can easily burn through colored steel tiles into the lower space, causing fires and property damage；

Coal sheds, Sheds, Material Sheds, Breeding Sheds

1. Difficult inspection, inconvenient ceiling inspection, unable to detect arc safety hazards in a timely manner；

2. Limited rescue. In emergencies such as fires, the inability to shut down the high voltage of the string hinders rescue efforts；

3. Fragile roofs, DC arc sparks can easily burn through the roof into the interior, causing significant property damage；

Gas Stations, Liquor, Chemical, Flour and Other Factory Areas

Scenario Risk Analysis:

1. In high-risk scenarios, it is necessary to detect arcs, shut down, alarm, and locate in a timely manner to effectively eliminate hidden dangers；

2. Flammable and explosive, lack of safety protection measures, easy to cause major safety accidents

Roads, Highways, Rivers and Other Areas

Scenario Risk Analysis:

1. Environmental hazards, cigarette butts and sporadic arc sparks from components can easily cause grass fires below；

2. Difficult patrol inspection, narrow and inconvenient site inspection, difficult operation and maintenance, hidden dangers cannot be detected in time；

3. Difficult rescue, far from urban areas, in case of fires and other accidents, rescue is difficult；

4. Secondary accidents, when vehicles or other accidents damage components, the high voltage of the string cannot be shut down in time, which may cause serious secondary accidents.

Related Legislation:

According to the latest version of the National Electrical Code NEC2020 document requirements：

The distance to the photovoltaic array is limited to 305mm. Exceeding the limit, the device is triggered to reduce the voltage to below 30V within 30 seconds; the voltage is reduced to below 80V.

Canada:

When the DC side voltage of the photovoltaic system exceeds 80V, arc fault interruption devices or other equivalent equipment should be installed.

When photovoltaic systems are installed inside or on buildings, rapid shutdown devices should be installed. The distance from the photovoltaic module is 1 meter. After triggering the rapid shutdown device, the voltage is required to be reduced to below 30V within 30 seconds.

Germany:

According to German standard VDE-AR-E 2100-712 requirements：

In photovoltaic systems, if the inverter is off or the grid fails, the DC voltage needs to be less than 120V. Mentioned using a disconnect device to lower the DC link voltage below 120V.

Australia:

According to the latest AS/NZS 5033:2021 standard section 4.3.3：

When the DC voltage exceeds 120Vdc, a disconnect device needs to be installed between the module and the inverter.

Thailand:

Thailand Electrical Code Section 4.3.13: Solar Roof Power Installation 2022：

It is required that rooftop photovoltaic stations be equipped with rapid shutdown devices, with a limit of 300mm from the photovoltaic array. The equipment should reduce the voltage within the restricted area to below 80V within 30 seconds, and to below 30V outside the restricted area.

Fonrich Safety Protection Solution

Four-in-one Functionality：

Module-level rapid shutdown

Module-level digital management

Module-level real-time arc detection, active protection, and fault localization

Real-time detection, active protection, and fault localization of parallel arcs and leakage

Currently, the arc protection function integrated into inverters is still not sufficiently mature, unable to pinpoint fault locations, and incapable of timely and effective resolution. Only a “module-level” detection and protection scheme can cover series arcs, parallel arcs, and grounding arcs, accurately pinpointing arc fault locations, as well as detecting and locating leakage faults.