In the power supply and distribution system, the distribution transformer generally supplies power to the load, and the current flows from the grid side to the load, which is called forward current. After the photovoltaic power generation system is installed, when the power of the photovoltaic power generation system is greater than the local load power and the load cannot fully absorb it, the remaining power is fed into the grid. Since the current direction is opposite to the normal direction, it is called reverse current.
What is Anti-backflow prevention?
A normal photovoltaic power generation system converts the direct current of photovoltaic modules into alternating current and feeds it into the power grid. A photovoltaic system with backflow prevention only uses the power generated by photovoltaics for local loads, preventing the power generated by the photovoltaic system from being fed into the power grid. The operating principle diagram of the system is as follows:
Anti-backflow demand scenarios
Generally, photovoltaic power stations adopt the operation mode of full grid access or self-use, and the surplus power is connected to the grid. The photovoltaic system is allowed to send power to the grid, so there is no requirement for anti-backflow. The main reasons for installing anti-backflow are as follows:
1. Due to the capacity limit of the upper-level transformer, the local power supply department does not allow new grid connection;
2. It is impossible to connect to the grid due to incomplete procedures and grid connection information;
3. Some areas are not allowed to connect to the grid due to policy reasons;
4. The local area has the ability to absorb most of the power for self-use, and a small part does not need to be connected to the grid.
Principle of Anti-backflow
An anti-backflow meter + CT transformer is installed on the main line of the household incoming line to collect the real-time power, current size and direction on the busbar. When it is detected that there is current flowing to the grid (reverse current), the anti-backflow meter transmits the reverse power data to the inverter through RS485 communication. After receiving the command, the inverter responds in seconds and reduces the inverter output power, so that the current flowing from the photovoltaic power station to the grid is always kept close to 0, thereby achieving anti-backflow and not sending excess electricity to the grid.
Anti-backflow in different scenarios
Growatt provides a variety of flexible solutions according to different application scenarios. For photovoltaic power stations with only one inverter, Growatt smart meters can be used to achieve anti-backflow function. For power stations using multiple inverters, Growatt smart energy managers can be used to achieve anti-backflow function.
Single-machine single-phase anti-backflow system solution
Equipment required for function realization: photovoltaic grid-connected inverter, anti-backflow meter, communication line between meter and inverter
Single-machine three-phase anti-backflow system solution
For household low-power grid-connected inverters, the output current is small, generally less than 80A current models (within 50KW), you can directly use a DC anti-reverse current meter, the inverter AC output terminal wiring is directly introduced into the meter, and then connected to the grid point after coming out of the meter to achieve anti-reverse current.
For high-power grid-connected inverters, the output current is large and exceeds the specification range of the anti-reverse current meter. It is necessary to use another CT transformer to detect the current on the grid bus, and then connect the anti-reverse current meter after reducing the current proportionally through the transformer to achieve current and power measurement at the grid point.
Note: Although the photovoltaic inverter used in some scenarios has a small power, the current of the grid-connected busbar connected is large. At this time, it is also necessary to detect the reverse power of the grid-connected end through the anti-reverse current meter + CT mutual inductor.
The photovoltaic inverter and the anti-reverse current meter have been matched through the protocol. During the on-site installation, the anti-reverse current meter is connected to the RS485 communication port of the inverter through the RS485 line. The installation is simple and saves system costs. Users can choose a direct-connected meter or a CT meter according to the actual situation.
Multi-machine backflow prevention system solution
For scenarios where a photovoltaic power station has more than one model, since a single meter cannot communicate with more than one inverter at the same time, a separate data collector is required to collect data from the backflow prevention meter on the grid-connected side, and perform multi-machine communication and output power control on the inverter side, thereby achieving backflow prevention for the entire photovoltaic power station.
Required equipment: photovoltaic inverter (multiple units), anti-backflow box (including data collector, anti-backflow meter and CT mutual inductor), RS485 communication line.
System wiring: The anti-backflow box is installed between the photovoltaic inverter, user load and power grid. The voltage, current and reverse power of the grid access point are detected by the meter and CT mutual inductor in the anti-backflow box. The output power of the inverter can be adjusted in real time according to the user’s needs and settings, thereby controlling the power of the entire photovoltaic grid-connected system that is ultimately output to the grid, and achieving reverse power close to zero.
Notes:
1. The CT transformer is installed at the busbar of the grid connection point. Before the installation area, its secondary must be connected to the meter in the anti-backflow box to ensure that the transformer secondary is not open.
2. When installing the transformer, no foreign matter such as impurities and dust should fall into the core section to avoid affecting the performance of the transformer.
3. There are silk screens of P1 and P2 on both sides of the current transformer to distinguish the direction. Refer to the figure below for wiring. The P1 side is close to the grid, and the P2 side is close to the inverter and load.
4. The photovoltaic inverter connects the communication signal line to the data collector in the anti-backflow box through the RS485 hand-in-hand series connection. The RS485 communication is affected by factors such as communication distance and signal interference, which will cause delays in the anti-backflow control signal. Generally, it is not recommended to connect more than 20 inverters under the same anti-backflow box to ensure the anti-backflow control accuracy and control effect.
5. Based on the above anti-backflow control principle, it is necessary to first detect whether there is reverse power at the grid connection point and then give a control signal through the RS485 signal line to control the inverter to reduce the output. Affected by factors such as signal delay, a very small amount of current may be sent to the grid by the anti-backflow device during actual operation, which is a normal phenomenon.
Inverter anti-backflow prevention instructions
Currently, all Growatt grid-connected models are equipped with RS485 interfaces as standard, and all can realize the anti-backflow function. In actual project requirements, anti-backflow meters, anti-backflow boxes and other solutions can be flexibly selected according to different scenarios. Among them, anti-backflow meters and anti-backflow boxes involve the problem of communication with photovoltaic inverters, and both must be matched by Growatt. There is no brand requirement for CT transformers, and they can be flexibly selected according to the busbar size and current size on site.
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 that defines the grade characteristics of an inverter. From the literal meaning, 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/identical 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.
Take the conversion of 10KW as an example. If the grid voltage is 400V, the current value required by the inverter 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. Two points need to be noted:
(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 convention 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.
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 function. Imagine: if the inverter allows DC input and AC 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 island occurs, it is when the normal 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.
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 are used: passive or active. Regardless of the detection method, once the power outage is confirmed, the grid-connected inverter will be disconnected from the grid and the inverter will be stopped within the specified response time. The response value currently stipulated by regulations is within 2s.
Does 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, within 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 ultimately leads to efficiency loss; if the string voltage is too high, it is not safe and limits the I-V output curve of the component, making the current smaller and the power fluctuation larger. Taking an 1100V-rated 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 inverter’s performance will be unsatisfactory.
In actual operation, considering the negative temperature coefficient characteristics of the components, the following are recommended for the parameters of the mainstream 182 and 210 modules on the market:
For 182 modules, connect about 16 modules in series, preferably 13 to 17 modules;
For 210 modules, connect about 18 modules in series, preferably 16 to 22 modules.
Of course, the above string recommendations should be determined in combination with specific module parameter values. Currently, various new technologies, new versions, and new specifications of modules are still emerging in the market, and the changes are very fast; while the parameters of the inverter are relatively stable, when matching, the main focus is on the correspondence between the string voltage and the rated/full-load MPPT voltage range of the inverter, and there will be no mistakes.
Note: 1100V is the voltage protection threshold. If it is reached or exceeded, the system will cause irreversible fault errors or safety accidents.
