Distributed pv power station DC arc and fire "safety" hidden danger --- application scenario risk analysis

         Under the national dual-carbon policy,pvpower generation will play the main role in the future energy structure, and will also enter a new era of vigorous development. In 2021, the newly added capacity of distributedpvpower plants exceeds that of centralized power plants for the first time, marking that distributedpvpower plants have entered a stage of large-scale and rapid growth. the point of resolution.

               "Safety" is the lifeblood of all industries and a principled issue of one-vote veto. The safe operation ofpvpower plants is also the foundation for power plants to obtain investment returns. With the continuous implementation of the "Countywide Promotion" project, a large number of government agencies, schools, hospitals, residential roofs, as well as gas stations, various sheds and industrial and commercial color steel tile roofs have installedpvpower stations; renewable energy building policies will also promote BIPV ( Buildingpvintegration) market is booming. For a long time, the "anti-DC arc" and "fire prevention" ofpvpower plants have been the difficulties ofpvpower plants, the pain points of rooftop power plants, the focus of government attention, and the focus of public opinion.

       

Fire safety hazards

        DC arc generation is the most common fault phenomenon inpvpower plants. Arcs will occur due to contact drop, device aging, insulation rupture, and poor grounding. Moreover, the harm of the DC arc is far greater than that of the AC arc, because the DC arc does not have a zero-crossing point, once it occurs, it will continue to burn, it is difficult to extinguish, and it is very easy to cause a fire accident. According to statistics, more than half of the fire accidents inpvpower plants are caused by DC arcs. As the specifications ofpvmodules become larger, the power and current of the DC side system increase. According to Joule's law Q=I²Rt, the current doubles, and the thermal effect of the short-circuit point increases. 4 times, the risk of causing a fire is also greatly increased.

DC arc classification

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

Unlike traditional electrical products, there is no integral enclosure for PV modules and their wiring to contain arcs and sparks caused by component and wiring faults, whereas many PV installations are capable of operating at the typical DC voltages that sustain DC arcs.

There are three main categories of arcs in PV installations:

--- Series arcs may be caused by incorrect wiring or broken series wiring

--- Parallel arcs may be caused by partial short circuits between adjacent lines of different potentials

--- Earth arc due to insulation fault

Series arc       

        Series arc, also known as drawn arc. Series arcs are usually caused by poor contact of cable plugs between components, and poor connection between string cables and combiner boxes or inverters. Due to the large number of series plugs in the photovoltaic power station, there are 2000 pairs of plugs in a 1MW rooftop photovoltaic power station. It is difficult to ensure that all the plugs are of good quality with so many pairs of plugs. , these hidden dangers lead to poor contact and the formation of DC arcs.

        At present, a few inverters have integrated arc protection function, but there are two major problems with this protection: First, if there is an arc fault in one string, the entire inverter will be shut down, causing great damage. Loss of power generation; second, without the arc fault location function, the operation and maintenance personnel cannot find the location of the arc in time and accurately, which is essentially no solution. The only technical reset protection they can do is to keep the inverter running. From this point of view, the arc-drawing protection function integrated in the inverter cannot really effectively solve the arc-drawing fault problem.

Parallel arc

        Parallel arcs are mainly caused by short-circuiting of positive and negative conductors caused by line damage, or short-circuiting between string cables. When the string cables are mechanically squeezed or worn, arcing will occur between the positive and negative electrodes, or between different strings, which is a parallel arc fault. There is another situation that can also lead to parallel arcs. When the series arcs in the system are not dealt with in time, the heat of the series arcs burns out the insulation layer of the cables, and parallel arcs also occur.

        When a parallel arc occurs between the main conductors of the component square array, since the arc can get enough energy, it is more difficult to extinguish, which will cause a major fire accident. The series fault arc can be extinguished by disconnecting the DC bus or the corresponding string of the photovoltaic system, but the parallel fault arc cannot be extinguished, and may even cause a larger current to pass through the arc path, making the arc more intense.

        At present, the arc protection function integrated in the inverter cannot detect parallel arcs and grounding arcs, but the destructive power of parallel arcs is often 10 times that of series arcs, and the safety hazard is even greater.

Ground arc

        Aging and damage of components or mechanical damage lead to ground discharge. If the components are laid flat on the roof of color steel tiles, there will be ground arcs or leakage. This kind of fault is not easy to find, especially in rainy days. At present, the solution is to shut down the inverter and wait for the ground to dry before starting it up. This method cannot effectively eliminate hidden dangers and increase the risk of personal electric shock.

DC high voltage

        In a photovoltaic power station, photovoltaic modules are connected in series to form a DC high voltage circuit, which generally reaches about 1000V. Even when the system is shut down, there is still a DC high voltage of about 1000 volts in the photovoltaic module matrix. Especially for rooftop photovoltaic power plants, when a fire occurs in photovoltaic power plants and buildings, it is difficult to rescue them safely; during daily operation and maintenance of power plants or property maintenance, operators and inspectors are also at risk of electric shock.

Scenario Risk Analysis

Government offices, schools, hospitals, residential roofs

Scenario risk analysis:

1. Regional control, it is impossible to use drones to scan components for abnormality, and hidden dangers cannot be found in time;

2. The population is dense, and the component square array has leakage, and the risk of electric shock to personnel is high;

3. Rescue is limited. In case of emergency such as fire, the high voltage of the string cannot be turned off, hindering rescue;

4. The influence of public opinion. If an accident such as a fire occurs, the public opinion will have a greater impact;

Various color steel tile roofs

Scenario risk analysis:

1. It is difficult to inspect, the color steel tile roof is inconvenient to inspect, and the hidden danger of arc safety cannot be discovered in time;

2. Rescue is limited. In case of emergency such as fire, the high voltage of the string cannot be turned off, which hinders rescue;

3. The roof is fragile, and the DC arc spark is easy to burn through the color steel tile and enter the lower space, causing fire and property damage;

Coal shed, car shed, material shed, breeding shed

1. The inspection is difficult, the ceiling is inconvenient to inspect, and the hidden danger of arc safety cannot be discovered in time;

2. Rescue is limited. In case of emergency such as fire, the high voltage of the string cannot be turned off, which hinders rescue;

3. The roof is fragile, and the DC arc sparks are easy to burn through the roof and enter the interior, causing heavy property damage;

Gas station, alcohol, 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 time to effectively eliminate hidden dangers;

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

Highways, highways, rivers and other areas

Scenario risk analysis:

1. Environmental risks, sporadic arc sparks from cigarette butts and components can easily cause weeds to burn below;

2. It is difficult to inspect, the long and narrow area is inconvenient for inspection, the operation and maintenance is difficult, and hidden dangers cannot be found in time;

3. It is difficult to rescue, away from the urban area, such as fire and other accidents, it is difficult to rescue;

4. Secondary accident, when the vehicle or other accident damages the components, the high voltage of the string cannot be turned off in time, which is likely to cause a serious secondary accident.

Related legislation：

U.S:

As required in the latest edition of the National Electrical Code NEC2020 document:

The distance to the photovoltaic matrix is 305mm as the limit. Outside the limit, the voltage drops below 30V within 30S after the triggering device is started; The voltage drops below 80V.

Canada:

When the DC side voltage of the photovoltaic system is greater than 80V, an arc fault interrupting device or other equivalent equipment shall be installed.

When the photovoltaic system is installed in the building or on the building, a quick shutdown device should be installed. 1 meter away from the photovoltaic module, after the quick shutdown device is triggered, it is required to reduce the voltage to below 30V within 30S.

Germany:

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

In the photovoltaic system, if the inverter is turned off or the grid fails, the DC voltage needs to be less than 120V. The use of a shut-off device is mentioned to bring the DC link voltage below 120V.

Australia:

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

When the DC voltage is greater than 120Vd.c, a disconnecting device needs to be installed between the module and the inverter.

Thailand:

Section 4.3.13 of the Thai Electrical Code: Solar Rooftop Power Supply Installations2022:

It is required that the rooftop photovoltaic power station must be equipped with a quick shutdown device, and the limit is 300mm from the photovoltaic matrix. The voltage within the limit range is reduced to below 80V within 30 seconds after the device is started, and the voltage outside the limit range is reduced to below 30V.

Fonrich "safety protection" scheme

Summary of core requirements of regulations	Fonrich scheme
Realize intelligent detection of DC arcs	Intelligent detection of component-level arcs, early warning and precise positioning
Quick shut-off function	Manual and various automatic quick shut-off functions
DC voltage is lower than safe voltage 30V/80V/120V	DC voltage is lower than safe voltage 30V/80V/120V
Not involved	Intelligent detection, early warning, precise positioning and rapid protection of parallel arc, leakage, and ground faults

Four-in-one function:

Component-level fast shutdown

Component-level digital management

Component-level arcing real-time detection, active protection and fault location

Parallel arc and leakage real-time detection, active protection and fault location

The arc protection function integrated on the current inverter is not complete enough, and the fault location cannot be located, and it cannot be dealt with in a timely and effective manner if it cannot be accurately located. Only the "component-level" detection and protection scheme can cover series arc, parallel arc and ground arc, and can accurately locate the arc fault location, and can also realize the detection and location of leakage faults.