1. The impact on voltage
Centralized distribution network is generally radial. In steady-state operation, the voltage gradually decreases along the direction of the feeder current. After the access to the photovoltaic power supply, due to the reduction of the transmission power on the feeder, the voltage at each load node along the feeder line is raised, which may result in the voltage offset of some load nodes exceeding the standard. Location and total capacity are closely related. Normally, the voltage deviation of the load node can be controlled within the specified range by setting voltage regulating devices such as a load regulating transformer and a voltage regulator in the medium and low voltage distribution network. For the voltage adjustment of the distribution network, it is very important to set the operation mode of the photovoltaic power supply reasonably. When the midday sunshine is abundant, the output of the photovoltaic power supply is usually large. If the line is lightly loaded, the photovoltaic power supply will obviously raise the voltage of the access point. If the access point is at the end of the feeder line, the voltage of the access point is likely to exceed the upper limit. At this time, the operation mode of the photovoltaic power supply must be set reasonably. For example, the photovoltaic power supply must be involved in voltage regulation to absorb excess reactive power in the line . During heavy nighttime hours, PV power usually has no output, but it still provides reactive power to improve the voltage quality of the line. The impact of photovoltaic power on voltage is also reflected in voltage fluctuations and flicker. As the output of photovoltaic power supply varies with incident solar irradiance, the voltage fluctuation and flicker of local distribution lines may be caused. If it is superimposed with load changes, larger voltage fluctuations and flicker will be caused. Although the actual PV power supply does not cause significant voltage fluctuations and flicker, it is still important to plan the access location and capacity properly when a large number of grid-connected PV power sources are connected.
2. The contribution of short-circuit current
It is generally considered that when a short circuit occurs on the distribution network side, the photovoltaic power supply connected to the distribution network has little contribution to the short-circuit current, and the steady-state short-circuit current is generally only 10% ~ 20% greater than the rated output current of the photovoltaic power supply. The peak current of the PV power inverter with its own energy storage components and output control performance. In the distribution network, short circuit protection generally used over-current protection plus fuse protection. For high-permeability photovoltaic power supply, short-circuit faults on the feeder lines may result in the feeder lines being unable to detect a short-circuit fault due to the vast majority of the short-circuit current provided by the photovoltaic power supply. In 1999, the IEA-PVPS-Task-5 (International Photovoltaic Technology Working Group) was used in Japan to connect a column transformer on a distribution network with four inverters controlling current injection from different manufacturers, The other side of the short circuit test. Test shows that the short-circuit current rise does not exceed 2 times before the fault, 1 to 2 weeks to isolate the fault. In addition, Japan also conducted a short circuit test on a 200kWp photovoltaic power supply system. The study found that after the short-circuit current passes through the transformer, the current decreases and the over-current protection of the transformer does not act. In 2003, NERL (National Renewable Energy National Laboratory) conducted a study on the interaction between distributed generation and distribution networks. Adopting inverter distributed power supply, the simulation prototype is built on the medium-voltage distribution network of 13.2kV. The distributed power supply has a capacity of 5MW, and the research focus is on the fuse protection features. The results show that distributed power supply with inverter access has a small contribution to short-circuit current when single-phase and three-phase faults occur, and the short-circuit current mainly comes from the main network even more than the short-circuit current provided by 5MW induction motor Much smaller. Therefore, it can be concluded that in order to control the current injection of photovoltaic power inverter inverter short-circuit current does not contribute to conclusion.
3. Non-normal island
With the increasing distribution of distributed power in the distribution network, there is an increased likelihood of non-normal islanding. IEC conducted "fault tree theory" in 1998 to analyze the occurrence of non-normal islanding The possibility of electric shock. In 2002, IEA-PVPS-Task-5 used Fault Tree Theory to analyze unusual silos of photovoltaic power. In the extreme case of considering PV power penetration up to six times the night load, it is unlikely that an abnormal island will cause an electric shock with a probability of less than 10-9 bpm. Therefore, as long as properly managed, coupled with the PV inverter itself with anti-islanding function, access to a large number of photovoltaic power supply system does not increase the risk of substantial electric shock. At the same time, the possibility of islanding PV power system in a typical low-voltage residential area distribution network in the Netherlands is studied and it is found that the possibility of non-normal islanding of PV power supply in this area is less than 10-5-10-6 / Year, almost zero. Therefore, it is considered unlikely that a large number of PV power sources will be connected to residential areas to lead to non-normal islanding. In 2006, DISPOWER tested the anti-islanding strategy for grid impedance changes and grid voltage and frequency monitoring of photovoltaic power inverters used in Germany. The results show that the grid works well under normal low impedance conditions. When the power grid is operated under the condition of high impedance is not enough, the accuracy of the change of the grid impedance by the photovoltaic power inverter is relatively poor. At present, there is not a very good solution to meet the German standard of anti-islanding strategy for photovoltaic power. In recent years, a large number of research conclusions show that even if a large number of distributed power sources are connected to the distribution network in the future, as long as the measures are properly implemented, the risk of an abnormal island can be controlled within a reasonable range without causing the system to have an abnormal island Risks of possibility there is a substantial increase, so the occurrence of non-normal islanding will not hinder the access to distributed power such as photovoltaic power supply a technical barrier.
4. Inject current harmonics
Current harmonics have a wide range of effects on the distribution network and users, and usually include changing the voltage averages, causing voltage flicker, causing heating of the rotating machine and generator, heating of the transformer, and saturation of the flux, causing the protection system to malfunction, The system produces electromagnetic interference and system noise. Harmonic sources generated by the photovoltaic power inverter are mainly two: 50Hz harmonic generated by the reference waveform is not good and the harmonics generated by the high frequency switch. Harmonic phase difference between the distribution network line impedance and load can eliminate some harmonics. When the photovoltaic power inverter generates a sine wave, it can partially compensate the voltage waveform distortion of the distribution network, but it will cause the inverter to output more current harmonics. When the photovoltaic power inverter is connected to the weak grid Will obviously appear above phenomenon. When the PV power inverter detects the distribution network voltage to generate the reference fundamental wave, the PV power inverter can output a very good sine wave current, but can not compensate the distribution network voltage waveform distortion. In 1998, IEA-PVPS-Task-5 once tested a residential area in which 80% of Danish homes were installed with photovoltaic power and found that photovoltaic power contributed less than local harmonics, not as much as domestic appliances. Therefore, the researchers believe that this is a common phenomenon in the case of feeder lines with relatively high short-circuit capacity and local high-penetration photovoltaic power supplies. In 1999, IEA-PVPS-Task-5 was tested in Japan for harmonics with multiple PV power sources connected to the same distribution transformer (residential column transformer), using multiple manufacturers and multiple inverters Device. The test results show that the same types of inverters (internal circuits and control strategies) result in a certain number of harmonic superimposes, and different types of inverters cancel each other's harmonic injection. Similar tests have also been conducted in the UK in 1999. The test results show that high-order harmonics attenuate very quickly and the changes of low-order harmonics are complicated. Harmonic distortion in a strong network is generally a constant value, while the weak network in the general harmonic distortion with the access to increase the number of photovoltaic power supply and aggravate. When the feed line impedance is large, the harmonic attenuation can be significant. In order to prevent harmonic generation of a specific number of times, it is necessary to limit the capacity of the photovoltaic power inverter. In actual operation, the harmonic current injected by the photovoltaic power supply can generally meet the requirements of relevant standards.
5. Inject the DC component