Talk about the "safety" of photovoltaic power plants

Add Time：2020/5/7    Views：305

Earlier, reports on "Shanxi household photovoltaic power station inverters caught fire" were reported in major media, and the cause of the accident was confusing: the report just came out was explained as "DC arcing caused by inferior inverters", and subsequent reports It was mentioned that the accident was caused by "Thunderbolt". No matter what the reason is, the gradually disclosed reports of rooftop photovoltaic power station fires have sounded the alarm for all people in the industry, especially those involved in the construction of distributed and household photovoltaic power plants! In distributed, many users do not know , The use of string inverter actually introduces a 600V ~ 1000V high voltage cable on the roof!

According to statistics, according to statistics, more than 80% of fires in power stations are caused by faults on the DC side, so how to avoid the hidden fire hazards caused by DC arcing?

What is a DC arc and what are its causes and destructive power?

DC arc is a gas discharge phenomenon, which can be understood as a high-intensity instantaneous current generated under insulation. According to reports in the literature: when the electric switch is used to disconnect the current or the contact is poor, if the circuit voltage is not less than 20 volts and the current is not less than 80 ~ 100mA, a DC arc will occur between the contacts of the electrical appliance.

Visible DC arc spark

Unlike the AC arc, the DC arc has no zero-crossing, which means that if a DC arc occurs, the trigger location will maintain a stable combustion for a long period of time without extinguishing.

In the actual power station, problems such as poor contact caused by the joints not being tightened, quality problems of the contact parts, and aging of the insulation parts caused by the long running time will directly cause the DC arc phenomenon. It is not difficult to see that as the operating time of the power station increases, the probability of a DC arc will also increase.

The high temperature generated by the DC arc easily exceeds 3000 ° C, which can directly cause a fire. Based on domestic and foreign cases and data, DC arc has become the number one killer in power station fires.

What is the probability of occurrence of a DC arc?

Regardless of other contacts and insulation parts, in a 10 MW distributed power station, there are more than 80,000 light contact points, and there is always the possibility of a DC arc. Even if only 1/1000 of the contact points occur during the 25-year operation time of the power plant, 80 DC arc events will occur in this power station, and the probability of causing a fire is very high.

Can the arc detection device solve the DC high voltage problem?

The arc detection device includes an arc detection function and a shutdown function, but the device can generally only disconnect the entire system. For a string system, the components are connected in series. As long as there is light, the components will generate electricity. The DC high voltage in each string still exists, and it cannot really solve the DC high voltage. Only by adding a shutdown function at the component level can each component be disconnected when the system detects an arc to ensure that the system has no DC high voltage.

In addition, the arc generated in the photovoltaic system can be divided into normal arc and abnormal arc. The arc caused by the normal shutdown of the circuit breaker is a normal arc. The arc caused by wire aging and poor contact is abnormal. This means that the arc detection device needs to correctly distinguish between “good arc” and “bad arc”. Because of such complicated factors, it also brings difficulty to the detection of the arc, and places higher requirements on the detection method and calculation method. The occurrence of arcs and their characteristics are also extremely complex, making it difficult to predict and consider various conditions.

How to solve / improve the problem of DC high voltage caused by DC arcing? At present, there are the following solutions on the market:

Solution 1: Micro inverter solution

At present, in developed countries such as Europe and the United States, more and more rooftop photovoltaic systems are using micro-inverters to replace traditional string inverters. The micro-inverter is designed as a fully parallel circuit. There is no voltage superposition between the components. The DC voltage is less than 60 volts (not higher than the maximum output DC voltage of the components), which completely solves the risk of fire caused by high-voltage DC arcing. Solved the problem that the rescue was hindered by the photovoltaic power plant when the house caught fire.

The initial investment cost of this scheme is relatively high, but due to many foreign safety regulations and labor cost savings, the cost of the micro-inverter system scheme is basically the same as that of the string inverter system, even because of the 10-year warranty and easy The advantages of maintenance and multiple power generation have become the products of choice for European and American civil systems (<10kW).

Solution 2: Optimizer solution

The optimizer system has component-level remote monitoring, shutdown, and MPPT functions. Although it cannot directly solve the DC high-voltage problem, it can shut down the DC output of each component after a fire, without threatening the personal safety of firefighters. When the inverter is disconnected from the power grid, when the AC output is 0V, the DC optimizer will automatically cut off the connection to achieve component-level shutdown, which is equivalent to adding a shutdown to a component, which is eliminated in a true sense DC high voltage formed by components in series.

The overall cost of the optimizer system is lower than that of the micro inverter system, and it is easy to maintain and generate more power in the later period, which is welcomed by the United States and many European countries. In addition, this solution can not only be used in civil systems, but also widely used in commercial and even large ground power stations.

Solution 3: Modified string inverter solution

● Modification of the string inverter design: directly mounted on the component bracket, shorten the size of the DC line, reduce the possibility of DC arcing, and reduce user contact with DC high-voltage cables;

● Extra protective sleeve outside the DC cable of the string inverter, the DC cable is not exposed; additional cost is added;

● If the DC cable is more than 1.5 meters apart, a physical shut-off switch should be installed;

● Increase the DC arc detection device, but as mentioned above, the rapid shutdown of the component level can solve the problem of DC high voltage.