BENY new products solve distributed PV power station DC arc and fire safety hazard

Fire safety hazard

In recent years, there have been regular fire accidents at PV plants, not only losing the plant’s property and power generation revenues, but also causing damage to buildings, personal injury, and even spreading to the surrounding area, resulting in a series of secondary disasters.

DC arc is the most common failure phenomenon in PV power plants. Arcs will occur due to contact failure, device aging, insulation breakage and poor grounding. Moreover, the damage of a DC arc is much greater than that of an AC arc because there is no zero crossing point in the DC arc. Once it occurs, it will continue to burn, it is difficult to extinguish and it is very easy to cause fire accidents. According to statistics, more than half of fire accidents in PV plants are caused by DC arcs. As the specifications of PV modules increase, the power and current of the DC side system increases. According to Joule’s law Q=I²Rt, the current doubles and the thermal effect of the short circuit point is increased 4 times, the risk of causing a fire is also greatly increased.

DC Arc Rating

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 errors, while many PV installations are able to operate with typical DC power. voltages that support DC arcs.

There are three main categories of arcs in PV installations:

  • Series arcs can be caused by incorrect wiring or broken series wiring
  • Parallel arcs can be caused by partial short circuits between adjacent lines of different potentials
  • Earth arcs due to insulation fault

Series arc

Series arc, also called a 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 PV plant, there are 2000 pairs of plugs in a 1MW PV plant on the roof. It is difficult to ensure that all plugs are of good quality with so many pairs of plugs. These hazards lead to poor contact and the formation of DC arcs.

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

Parallel arc

Parallel arcs are mainly caused by shorting positive and negative conductors caused by line damage, or shorting between string cables. When the string cables are mechanically compressed 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. If the series arcs in the system are not addressed in time, the heat from the series arcs burns the insulation of the cables and generates parallel arcs.

When a parallel arc develops between the main conductors of the square component, it is more difficult to extinguish, as the arc can get enough energy, which will cause a major fire accident. The series fault arc can be extinguished by disconnecting the DC bus or corresponding string from the PV system, but the parallel fault arc cannot be extinguished and can cause even greater current through the arc path, intensifying the arc.

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

Ground Arch

Aging and damage to components or mechanical damage lead to soil discharge. If the components are laid flat on the color steel tile roof, earth arcs or leakage will occur. This kind of malfunction is not easy to figure out, especially on rainy days. At this point, the solution is to turn off the inverter and wait for the soil to dry before starting it up. This method cannot effectively eliminate hazards and increase the risk of personal electric shock.

DC high voltage

In a PV plant, PV modules are connected in series to form a high voltage DC circuit, generally reaching about 1000V. Even when the system is turned off, there is still a high DC voltage of approximately 1000 volts in the PV module array. Especially for rooftop PV plants, when fire occurs in PV plants and buildings, it is difficult to rescue safely; During routine power plant operation and maintenance or property maintenance, operators and inspectors also run the risk of electric shock.

Risk analysis scenario

Government, school, hospital, roof of homes

  1. Regional control. It is impossible to use drones to scan components for anomalies and cannot find the danger in time;
  2. The population is densely populated. The square array of components has leakage, high risk of electric shock to personnel;
  3. Rescue is limited. In case of emergency, such as fire, the high voltage of the string cannot be turned off, which hinders the rescue;
  4. The influence of public opinion. When an accident happens, public opinion has more impact

Various colors of steel tile roof

  1. It is difficult to inspect. The colored steel tile roof is inconvenient to inspect and the 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 the rescue;
  3. The roof is fragile. And the DC arc spark is easy to burn through the colored steel tile and enter the lower space, causing fire and property damage

Highways, rivers and other areas

  1. Environmental risks. Sporadic arc sparks from cigarette butts and components can easily cause weeds to burn underneath;
  2. Difficult to inspect. The long and narrow area is inconvenient for inspection, operation and maintenance are difficult, and dangers cannot be found in time;
  3. Hard to save. Far away from the urban area, such as fire and other accidents, it is difficult to save;
  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.

National laws and regulations

UNITED STATES:

According to the latest edition of the National Electrical Code NEC2020 document:

Take the distance from the PV matrix 305mm if the limit, beyond the limit, within 30S after the trigger device is started, the voltage drops below 30V; Within the limit, it is required to have a “PV Hazard Control System”, or reduce the voltage to below 80V within 30S after the trigger is started.

Canada:

According to the Canadian Electrical Code 2021 Edition:

When the DC side voltage of the PV system exceeds 80 V, an arc fault interrupting device or other equivalent equipment should be installed.

When the PV system is installed in or on the building, a quick disconnect device must be installed. At 1 meter away from the PV module, after the quick shutdown device is started, the voltage should be reduced to below 30V within 30S.

Germany:

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

In the PV system, if the inverter is switched off or the grid fails, the DC voltage must be less than 120V. Using a tripping device is mentioned to bring the DC side voltage below 120V.

Australia:

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

When the DC voltage is higher than 120Vd.c, an isolating device must be installed between the module and the inverter.

Thailand:

According to section 4.3.13 of Thai Electrical Code-Solar Rooftop Power Supply Installations 2022:

The rooftop PV plant is required to be equipped with a quick shutdown device, and the limit is 300mm from the PV array. The voltage within the limit range is reduced to below 80V within 30 seconds of starting the device and the voltage outside the limit range is reduced to below 30V.

BENY Self-R&D Products

For solar roofs and building fire safety, BENY string level and module level quick shutdown devices regulate the voltage of the panels to a certain safe level in microseconds. Prevent accidents and improve the safety of the solar energy system. BENY fast shutdown solutions are designed according to CE, TUV, UL standard, in accordance with national laws and regulations such as Thai Electrical Code, NEC2020. As a member of the Sunspec alliance, BENY is developing PLC communication RSDs for wider compliance of multiple string inverters. View the products now

Epilogue

The construction of PV plants is in full swing and has close ties with thousands of households. How to ensure the “safety” of PV plants is attracting a lot of attention from the whole industry. To effectively solve this problem, the whole industry should work together to come up with innovative solutions and continuously improve the relevant standards and regulations, and then actually implement the relevant requirements in the subsequent construction of power plants.

As an important energy infrastructure, the safe, stable and efficient operation of PV plants is an important guarantee for economic development.

Sponsored Content by Zhejiang Benyi Electrical Co., Ltd.

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