Cause Analysis of Fire Accident at DC Side of PV Power Generation System
Photovoltaic power generation systems are getting closer and closer to our lives. The following figure shows some accident cases of photovoltaic power generation systems, which should arouse the great attention of photovoltaic practitioners.
The reasons are as follows:
1. The Pin Crimping of the PV Cable and the Connector is Unqualified
Due to the uneven quality of construction personnel, or the construction party did not provide professional training to the operators, the unqualified crimping of photovoltaic connector pins is the main reason of poor contact between the PV cable and the connector, but also one of the main causes of accidents in photovoltaic power generation systems. Photovoltaic cable and connector is just a simple connection, nearly 1000V bare cable may fall off the connector at any time on the concrete roof, causing fire accidents.
If you want to know the correct installation order of the MC4 connector, you can read: How to Make MC4 Connectors?
2. The Matching Problem of PV Solar Connectors of Different Brands
In principle, PV solar connectors of the same brand and model must be used for interconnection. Each inverter basically comes with the same number of photovoltaic connectors, please be sure to use the matching connectors to install. As long as it is installed correctly, the connection on the inverter side is generally not a problem. However, there is still a problem on the component side. Due to the variety of brands of photovoltaic connectors on the market, the component factory has not provided matching connectors.
We have three suggestions for this: first, buy pv panel connectors of the same brand as the solar panels; Second, cut out the connector at the end of the string and replace it with a connector of the same brand and type; Third, if you must use PV connectors of different brands, you can cut out a set of them and insert them with the connectors you purchased. If the connector plugging smoothly, perform a blowing action on the inter-plugged connectors. If there is air leakage, this batch of products cannot be used with each other. Then use a multimeter to check whether the inter-plugged connectors are connected. It can’t be used when disconnected. Due to the problem of compatibility, poor contact or water leakage is also one of the causes of fire accidents.
Why is it not recommended that connectors of different brands be used with each other? , the main reason is that various manufacturers may claim that their products can be compatible with Stäubli’s MC4. Even if this is the case, due to the problem of positive and negative tolerances, there is no guarantee that the products of non-Stäubli manufacturers can be compatible with each other. If two different brands of photovoltaic connectors have the inter-mating test report, you can use it with confidence.
3. One or Several Circuit Positive and Negative Poles of the PV String are Reversely Connected
Generally, the inverter consists of multiple MPPTs. In order to reduce costs, it is impossible to carry one MPPT for each circuit. Therefore, under one MPPT, 2~3 sets of photovoltaic connectors are generally input in parallel. An inverter that claims to have a reverse connection function can only guarantee reverse connection protection when one or more channels of the same MPPT are connected in reverse at the same time. If under the same MPP, part of it is reversed, it is equivalent to connecting the positive and negative poles of two completely opposite battery packs with a voltage of nearly 1000V. The current generated at this time will be infinite, there is no grid connection to form the inverter side connector or inverter fire accident.
The key to solving such problems or the construction of the normative issues, after the completion of the laying of the components, according to the DC cable line design drawings, each red PV DC cable all positive identification, to maintain and string identification consistent. Here is a sentence can be used as training: “component positive, the extension line is just an extension of the component positive line, must be positive”. Regarding the marking of the module extension cable, make sure that the different strings at the inverter end are never confused.
4. The Waterproof Performance of the Positive O-Ring of the Connector and the T-Ring of the Tail End is Not Up to Standard
Such problems may not occur in a short period of time, but if it is a rainy season, and the PV cable connectors connector is in a rain-soaked environment. The high-voltage direct current will form a loop with the ground, resulting in an electric leakage accident. This problem is the selection of the connector, and almost no one will pay attention to the real waterproof problem of the connector. The waterproof IP65 and IP67 of the photovoltaic connector are prerequisites, and it must be matched with the photovoltaic cable of the corresponding size. For example, Stäubli’s conventional MC4 has three models of different sizes: 5~6MM, 5.5~7.4MM, 5.9~8.8MM. If the outer diameter of the cable is 5.5, the Stäubli connectors circulating on the market are not a big problem, but If someone chooses the MC4 of 5.9-8.8MM, the hidden danger of leakage accident will always exist. On the issue of positive front O-ring, the general standard photovoltaic connectors and their own manufacturers paired with few waterproof problems, but without testing and other manufacturers to go with the use of waterproof problems are extremely likely.
5. PV DC Connectors or PV Cables are in a Humid Environment for a Long Time
Almost everyone thinks that the conductive parts of photovoltaic cables and photovoltaic connectors are covered with other materials, and PV connectors are claimed to be waterproof. In fact, waterproof does not mean that it can be kept in water for a long time. The IP68 solar connector means that the pre-installed photovoltaic connector with cable is immersed in water, and the top is 0.15~1 meter away from the water surface for 30 minutes without affecting the performance. But what if it was submerged in water for 10 days or more?
PV cables currently on the market including PV1-F, H1Z2Z2-K, 62930IEC131 can also be a short time soak, such as a short drench, or even water accumulation, but the time of water can not be too long, to fast flow and ventilation dry. Photovoltaic cable fire because the construction side of the photovoltaic cable buried in a swampy area, by long-term soaking water, photovoltaic cable in the water penetration caused by the breakdown of the arc burning. In this special emphasis, laying photovoltaic cable through the tube is more likely to fire, the reason is the long-term accumulation of water in the PVC pipe. If you need to lay with PVC pipe casing, remember to let the PVC pipe mouth down, or in the lowest water level of the PVC pipe to punch some holes to prevent water accumulation.
At present, the waterproof photovoltaic cable, foreign selected AD8 waterproof production process, some domestic manufacturers use wrapped around the water barrier, plus aluminum-plastic integrated sheath form of production.
Finally, ordinary photovoltaic cables cannot be soaked in water for a long time, and cannot be handled in a high temperature and high humidity environment for a long time. From this, the construction personnel can do standard work in combination with the actual construction.
6. The PV Cable Skin is Scratched or Excessively Bent During the Laying Process
Scratching the cable skin will greatly reduce the insulation performance and weather resistance of the cable. In construction, cable bending is relatively common. The standard stipulates that the minimum bending diameter should be greater than 4 times the cable diameter, and the diameter of 4 square photovoltaic cables is about 6MM. Therefore, the diameter of the arc at the bend should not be less than 24MM, which is equivalent to the mother The size of a circle formed by the finger and the index finger.
7. In the Grid-Connected State, Plug and Unplug the PV DC Connector
In the grid-connected state, plugging and unplugging the connector will generate an electric arc, which is likely to cause injury accidents. If the arc further ignites flammable substances, it will cause a major accident. Therefore, be sure to perform maintenance after disconnecting the AC power supply, and the photovoltaic system should always be turned off to ensure long-term safety.
8. Any Point in the PV String Loop is Grounded or Forms a Path with the Bridge
The situation that causes any point in the PV string loop to be grounded or form a passage with the bridge is more complicated, including the long-term soaking of the PV cables mentioned above, the installation of PV connectors on the extension lines, and the surface of the cables being scratched during construction or the cable skin may be bitten by the mouse during use, and the lightning will break down, etc.