As the central controller of the photovoltaic system, the inverter plays a key role in the operation and output of the entire system. When the system has problems such as standby, shutdown, alarm, fault, power generation not meeting expectations, data monitoring interruption, etc., the operation and maintenance personnel always subconsciously start from the inverter to find the cause and solution. In daily communication, it is found that although distributed photovoltaics have been developing rapidly for many years, there are still several typical misunderstandings about inverters. Let's talk about it today.
01 Inverter output voltage?
The parameter "AC output voltage" can be easily found in the specification sheet of each brand of inverter. It is a key parameter for defining the grade characteristics of an inverter. In simple terms, the AC output voltage seems to refer to the voltage value output by the AC side of the inverter. In fact, this is a misunderstanding.
"AC output voltage" is not the voltage output by the inverter itself. The inverter is a power electronic device with current source properties. Since it needs to be connected to the power grid (Utility) to safely transmit or store the generated electric energy, it will always detect the voltage (V) and frequency (F) of the grid it is connected to during operation. Whether these two parameters are synchronized/same with the grid determines whether the electric energy output by the inverter can be accepted by the grid. In order to output its rated power value (P=UI), the inverter calculates whether it can continue to output and how much to output based on the grid voltage (grid connection point) detected at each moment. What is actually output to the grid here is current (I), and the magnitude of the current is adjusted according to the change in voltage.
Taking the need to convert 10KW as an example, if the grid voltage is 400V, the current value required to be output by the inverter at this time is: 10000÷400÷1.732≈14.5A; when the grid voltage fluctuates to 430V at the next moment, the required output current is adjusted to 13.4A; on the contrary, when the grid voltage decreases, the inverter will increase the output current value accordingly. There are two points to note: ① The grid voltage cannot stay at a constant value, it is always fluctuating; ② Therefore, the grid voltage detected by the inverter must have a range. If the actual voltage of the grid fluctuates out of this range, the inverter must detect it in real time and report the fault and stop output until the grid voltage is restored. The purpose of this is to protect the safety of electrical appliances and personnel on the same line in the substation.
In this case, why not change the name of this parameter? The main reason is that the industry has been following the same practice for many years - everyone calls it this way; at the same time, in order to keep it consistent with the output current, it has been called this way.
02 Does the inverter have to be equipped with anti-islanding protection?
The answer is of course yes, no doubt. It can even be said that the reason why an inverter can be called an inverter is because it has anti-islanding protection. Imagine: if the inverter allows the DC side to input and the AC side cannot output, where will the large amount of charge go? The inverter itself is not a storage device and cannot hold a large amount of charge, so it still has to output. When the islanding occurs, it is when the normal power transmission and distribution of the power grid is interrupted for some reason. Once a large amount of charge enters the power grid line along the original path, if there are power maintenance personnel working on it at this time, the consequences will be disastrous. Therefore, if the photovoltaic system is to always keep in sync with the power grid, it must be equipped with anti-islanding protection function (Anti-Islanding).
How to achieve it? The key point to prevent the islanding effect is still the detection of power outages in the power grid. Usually, two "islanding effect" detection methods, passive or active, are used. Regardless of the detection method, once the power grid is confirmed to be out of power, the grid-connected inverter will be disconnected from the grid and the inverter will be stopped within the prescribed response time. The response value currently stipulated by regulations is within 2s.
03 Is the higher the DC string voltage, the better the power generation?
Not really. Within the MPPT operating voltage range of the inverter, there is a rated operating voltage value. When the voltage value of the DC string is at or near the rated voltage value of the inverter, that is, in the full load MPPT voltage range, the inverter can output its rated power value. If the string voltage is too high or too low, the string voltage is far away from the rated voltage value/range set by the inverter, and its output efficiency is greatly reduced. First, the possibility of outputting rated power is excluded - this is not desirable; secondly, if the string voltage is too low, the Boost circuit of the inverter needs to be frequently mobilized to work continuously, and the continuous heating causes the internal fan to work continuously, which eventually leads to efficiency loss; if the string voltage is too high, it is not only unsafe, but also limits the I-V output curve of the component, making the current smaller and the power fluctuation larger. Taking the 1100V inverter as an example, its rated operating voltage point is generally 600V, and the full-load MPPT voltage range is between 550V and 850V. If the input voltage exceeds this range, the performance of the inverter is not ideal.