Analysis of technical difficulties of distributed photovoltaic grid connection

　　Distributed power generation, mainly in the form of solar photovoltaics, is developing rapidly in various countries. Germany’s rooftop photovoltaic installations have grown to more than 10GW. Although government subsidies have been reduced on a monthly basis, this trend has not declined due to skyrocketing electricity prices. The United States has reached the point where a photovoltaic system is installed every four minutes, and rooftop distributed systems play a huge role in this. In Japan, due to the failure of policy support and market model design in the past year, emerging distributed models such as rooftop rentals have not achieved the expected results. However, Japanese local governments strive to improve intermediary methods and increase transaction volume this year, such as using solar power generation as emergency reserve power.

　　China's distributed photovoltaics made further strides in 2013, with a total of 15 supporting policies being introduced. Of the 35GW of photovoltaic power generation capacity in the 12th Five-Year Plan, 20GW will be distributed power generation. From a technical perspective, even in Germany, where the new energy law EEG has been in place for nearly 14 years, distributed photovoltaic grid integration still brings considerable difficulties and extra work to power grid companies. Only 10% of the 900 power system operators It claims to enable large-scale distributed access of photovoltaics (the definition of large-scale access: the installed amount of photovoltaic power supply is greater than the average load value) and operate its network stably. The author has had experience in providing photovoltaic power station grid connection optimization solutions to German distribution network operators. In this process, the grid connection technology game between developers, equipment manufacturers and grid operators can be seen.

　　1. Impact on local voltage stability of the distribution network.

　　Most of the 10kV distribution lines built in China's early stage were single radial distributed power supply, and the system security was low. In the construction and transformation of urban power distribution networks, the establishment of ring network power supply and open-loop operation models is gradually considered. In various distribution network structures, changes in static and dynamic voltages will affect line protection and system operation safety. Under steady-state operation, the voltage theoretically decreases gradually along the power flow direction of the transmission line.

　　After distributed photovoltaic is connected, due to the fluctuation of transmission power and the characteristics of distributed loads, the voltage at each load node of the transmission line is too high or too low, causing the voltage deviation to exceed the technical indicators for safe operation. Figure 1 describes the principle of the impact of distributed photovoltaic access to distribution network on local voltage. After large-scale distributed photovoltaic access, local nodes of the distribution network have the problem of static voltage offset. Distribution networks, especially low-voltage networks, are sensitive to voltage changes. If you want to suppress this effect, you need to use controllable transformers in the selection of medium and low-voltage converters.

　　2. Impact on power grid frequency stability.

　　The experience and lessons of Germany's large-scale development of distributed photovoltaics tell us that small output can still cause frequency stability problems in the power system. As mentioned above, when Germany's distributed, especially rooftop, photovoltaic installation capacity reaches the level of 3GW, the backup power supply in Germany, the so-called primary frequency regulation, will not be able to meet the output loss of distributed photovoltaic power when it is cut out at the same time. The reason is that before the German medium-voltage grid connection guidelines came into effect, the old small photovoltaic inverter design parameters were that when the grid frequency exceeded 50.2Hz, they would be directly disconnected from the grid without participating in grid system services, that is, they would not do anything in the event of a power system failure. Make a contribution. In other countries with a large amount of photovoltaic installations, the emphasis on the frequency safe operation range of grid-connected power supplies and the off-grid time after frequency exceeds the limit are gradually reflected in the grid connection guidelines.

　　3. Contribution to short-circuit current in faults.

　　Traditional synchronous motors have the ability to provide short-circuit current. When superimposed with the short-circuit current provided by the power grid, they can ensure that the line protection is disconnected in 1 to 2 cycles. However, photovoltaic inverters cannot provide high short-circuit current due to limited energy density and limited overcurrent capabilities of power electronic components. Through experiments and dynamic simulations, it is generally believed that the short-circuit current of a photovoltaic inverter is only within 25% greater than the rated current.

　　Even in relevant international standards, the inverter is only required to provide 1 times the rated short-circuit current. This results in that when a short-circuit fault occurs in a transmission line with large-scale access to distributed photovoltaics, the fault on the line cannot be detected and the protection response cannot be made due to the insufficient short-circuit current capability of the photovoltaic inverter. Especially in traditional three-stage protection, instantaneous current quick-break protection may not be recognized.

　　According to the experience of photovoltaic power station grid connection analysis, the short-circuit current at the grid-connected point is mainly provided by the connected main network. Whether the network connected to the grid-connected point is strong or not determines the distributed short-circuit capability as a whole. The short-circuit current contributed by photovoltaic power stations causes problems in the transformation of medium and low-voltage equipment, such as the reselection of components such as current protection, medium-voltage switches and current transformers. Therefore, the short-circuit current contribution of photovoltaic power generation systems should be fully considered in distribution system planning and distributed system design.

　　4. Impact on power quality.

　　Harmonics mainly refer to current harmonics, which are caused by the power electronic components of photovoltaic inverters. Generally, they can only be identified through test analysis. Flicker mainly refers to the human-perceivable effect caused by rapid voltage fluctuations at the power end. Photovoltaic power generation systems are also caused by grid-connected inverters that switch on and off quickly at the same time. If the factory testing of distributed photovoltaic inverters fails to meet standards, poor power quality will, in the worst case, cause damage and interference to nearby power generation systems, sensitive electrical equipment, and signal transmission.

　　In addition to the factors mentioned above, the impact of harmonics and flicker generated by distributed photovoltaics on the power grid and loads also depends on the short-circuit capacity of the grid-connected point and the total amount of distributed power supplies connected to the grid under the same medium-voltage step-up transformer. The power quality of solar photovoltaic power stations should meet relevant national and local grid connection standards, and relevant current harmonics and voltage flicker indicators should be specifically quantified. Figure 2 shows a case of a distributed photovoltaic power station analyzed in accordance with German standards. It can be seen that both low-order harmonics and intermediate harmonics exceed the standards to varying degrees. In this case, the power station needs to be retested or power quality optimized.

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