Inverter technology innovation makes photovoltaic power stations more promising

Once upon a time, photovoltaic power generation was restricted because of the impact on the power grid caused by the large fluctuations in inverter output power due to changes in light intensity and temperature. With the advancement of technology, photovoltaic inverters have improved a variety of protection functions, adding low (zero) voltage ride-through, SVG reactive power compensation, anti-PID, inverter energy storage and other functions. Not only are photovoltaic power stations themselves becoming safer, but they can also solve some faults in the power grid. For example, when the power grid oscillates violently for a short period of time, they can support the power grid for a period of time. When the power factor of the power grid is low, they can also generate reactive power to improve the power factor. After adding energy storage batteries, they can absorb electrical energy at the trough and release electrical energy at the peak, playing the role of peak shaving and valley filling. By inputting reverse DC to the components, or grounding the negative pole, the PID of the components can be delayed and the life of the components can be increased.

1. Photovoltaic power stations are safer

Photovoltaic power generation systems include components, brackets, junction boxes, distribution cabinets, inverters, cables and other accessories. The smart inverter has a brain "CPU" with active detection and prevention functions. When a power grid or component fails, the inverter uses current sensors and voltage sampling, and the brain "CPU" makes a judgment and instructs the "actuator" contactor or circuit breaker to disconnect to protect the safety of people, power grids, and equipment. The inverter integrates multiple functions such as leakage current protection, DC component protection, insulation impedance detection protection, and lightning protection.

The AC contactor and DC circuit breaker in the inverter have different installation locations and functions. The AC contactor is responsible for routine power on and off operations. The DC circuit breaker is mainly used to disconnect the connection between the component and the inverter. When the component or DC cable is short-circuited or grounded; the inverter bus capacitor or IGBT is short-circuited, the DC switch must be disconnected.

From a safety perspective, the centralized inverter input DC switch and AC switch both use molded case circuit breakers, which have automatic overcurrent and short-circuit protection functions. When a short-circuit fault occurs in the inverter, the positive and negative poles of the solar cell can be separated in time, cutting off the connection between the grid and the inverter.

Use appropriate fuses. The characteristic of electrical fires is that they burn very quickly and can damage a system of equipment in an instant. In the event of an electrical fire, the first thing to do is to quickly cut off the power circuit within the fire range. Fuses are commonly known as fuses. When a short circuit occurs in the circuit, they can quickly cut off the circuit to avoid greater losses. Therefore, they are widely used in the power industry.

Growatt string inverters have a built-in high-precision RCD (residual current detection) protection module. If a person accidentally touches the PV+ and the leakage current exceeds the limit, the PID suppression module will be cut off to ensure personal safety.

2. Low (zero) voltage ride-through function

The public power grid is connected to tens of millions of households. Once a short circuit occurs at a certain power point, the protection device will immediately cut off the load. However, in this extremely short period of time, the grid voltage will be lowered. If the photovoltaic power station in the grid is also shut down, the grid will be further affected and the recovery time will be longer. The low (zero) voltage ride-through function of the photovoltaic inverter means that when a fault occurs in the grid and the voltage is low for a short period of time, the inverter will not be disconnected from the grid within a specified time, and reactive power will be injected into the grid to help the grid restore voltage.

3. Reactive power compensation of photovoltaic inverter SVG

SVG is a static dynamic reactive power compensation device. By adjusting the voltage amplitude and phase, or controlling the AC current, it can quickly absorb or emit the required reactive power, achieve the purpose of rapid dynamic reactive power regulation, and ensure the stable, efficient and high-quality operation of the power system. How big an SVG does a photovoltaic power station need? The national standard requires that the reactive capacity of a general photovoltaic power station be 25% of the power station capacity, plus a 5% floating capacity, which is 20%-30%.

The configuration of reactive power compensation devices in photovoltaic power stations not only increases the construction cost, but also causes great losses during operation. Inverters also have the ability to generate reactive power. Growatt inverters can achieve a reactive power of plus or minus 0.9 when running at full load. At night, they can even output 100% reactive power, fully tapping the regulation potential of grid-connected photovoltaic inverters, making the system performance indicators meet national standards, and in some cases providing reactive power auxiliary services for the power grid, improving the quality of power supply, and achieving a win-win situation for all parties.

4. Anti-PID technology of inverter

On-site tests of photovoltaic power stations have found that the power of some components has dropped significantly 1 to 2 years after construction, and the power attenuation of some components is as high as 50% or more. There are many causes of component attenuation, such as light-induced attenuation, aging attenuation, hidden cracks, cell rupture, etc. One of the important reasons is the component PID effect.

In order to suppress the PID effect, component manufacturers have done a lot of work in terms of materials and structures and have made some progress; for example, anti-PID materials, anti-PID batteries and packaging technologies can be used. In practice, the prevention and control of PID problems is more carried out from the inverter end. From the inverter perspective, the following three solutions can be adopted:

(1) Negative electrode direct grounding scheme: The negative electrode of the photovoltaic module or inverter is directly grounded through a resistor or fuse, so that the voltage of the negative electrode of the solar panel to the ground is kept at the same potential as the grounded metal frame, eliminating negative bias. This scheme is mostly used in centralized inverters.

(2) Negative electrode virtual grounding scheme, using simulated neutral point device and voltage adjustment device, equivalently raises UN so that U- is greater than 0, eliminates negative bias, and achieves the purpose of negative electrode virtual grounding.

(3) Nighttime anti-PID repair

Utilizing the reversibility principle of component PID, a separate DC source is used to apply reverse voltage to the solar panels when the inverter stops working at night to repair solar panels that have PID phenomenon during the day. The solar panels are repaired only when the inverter is not working, which is a passive solution of "after-the-fact treatment".

Actual power station operation data shows that by integrating the PID protection module in the inverter, the PID phenomenon of the components can be effectively avoided and the power generation loss of the power station can be reduced. At the same time, the PID module has a repair function, which can repair the components that have PID problems and restore the various index parameters of the components to normal.

Shenzhen Growatt PID suppression module adopts the second solution, which is to raise the N line to ground potential to make the PV- to ground potential close to zero or positive, so as to achieve the purpose of PV- virtual grounding, thereby realizing the PID suppression function. The PID module collects inverter data through ShineWebBox to automatically adjust the output voltage, so that the PV- to ground potential of all panels is close to zero or positive, so as to achieve the function of suppressing PID.

5. Photovoltaic energy storage technology

The combination of photovoltaics and energy storage will become an important part of solving the problem of abandoned solar power, smoothing output, and building a smart microgrid system. In the past year, with the development of distributed photovoltaics in full swing, the energy storage industry has also begun to gain momentum. In addition to the well-known energy storage and solution to the problem of abandoned wind and solar power, energy storage can also be used for peak and frequency regulation of power grids, microgrids, and household systems.

Based on the different requirements of users, Growatt has designed a variety of energy storage product lines: (1) SPF series 1-5K off-grid inverter controller, this model adopts high-frequency design, light weight, high efficiency, supports lead-acid batteries and lithium batteries, and supports multiple parallel machines for easy expansion. (2) SP series 1-3K grid-connected energy storage machine, a household grid-connected power station, can be immediately transformed into a photovoltaic grid-connected energy storage system after joining the system, and is also compatible with inverters of any brand. Solar power generation can be provided to the load for immediate use, or it can be stored and discharged within a set time to increase the system's self-generation and self-use ratio and increase revenue. When the power grid is out of power, the controller can continue to work and charge the battery. (3) The medium and large power off-grid integrated machine of the HPS series adopts the architecture of centralized inverter, which is stable and reliable, with integrated solution of controller and inverter, supporting grid-connected power generation, and can also be used off-grid when there is no power grid; (4) The medium and large power bidirectional inverter of the PCS series can form a microgrid system with grid-connected inverter and diesel generator, and can also be used for aging of electric vehicle charging piles and battery pack detection and aging.