2. What are the benefits of promoting distributed power generation? Distributed generation can be simply arranged according to the field load, making its layout flexible and power resources effectively allocated; To a certain extent, it has delayed the huge investment required for the upgrading of transmission and distribution networks; It is mutually standby with the traditional large power grid to provide power supply reliability; The introduction of the new electricity reform may even earn some money and experience the boss's feeling; Promote the establishment of a bidding mechanism for power producers.
3. But distributed power generation for so many years seems to be more about the division of slogans and interests. Careful observation will naturally show that distributed generation is directly connected to the power grid, which involves the connection point between distributed generation power supply and power grid-power electronic converter conversion link, as well as related control and protection links. It is estimated that this is a technical difficulty and also a manifestation of enterprise differences.
4. So what are the technical problems of distributed generation? (1) design and planning problems: distributed power generation gradually permeates the power grid, which has strong randomness and needs to consider reliability issues; The types and scales of distributed power generation are varied, and the operation modes are changeable. How to install, where to install and what operation mode will bring different overall evaluation performance. At present and in the future, the carrying capacity of power grid and the distribution of "three publics" affect the grid connection of distributed power generation to some extent, such as wind turbines rotating leisurely in northwest China. (2) Power quality: At present, a small number of distributed power generation devices are basically ignored in the power grid, but after the gradual liberalization, the proportion of new energy will increase, which will bring a series of influences on the voltage form, short-circuit current, voltage flashover, harmonics, DC injection, network loss, power flow and relay protection of the power system. Because many distributed power generation devices are connected to the power grid, the control strategy of converter (inverter) will have an impact on the unbalanced voltage of the power grid. |||| Many distributed power generation grid-connected devices adopt backflow prevention devices, which will not inject power into the power grid during normal operation. However, when the distribution system fails, the current of distributed power supply will be injected into the power grid at the moment of short circuit, which will increase the short-circuit current level of distribution network switches and may make the short-circuit current of distribution network switches exceed the standard. Therefore, when high-power distributed generation is connected to the power grid, it is necessary to analyze and calculate the power grid in advance to determine the influence of distributed generation on the short-circuit current level of the distribution network. |||| Generally, there is no flicker when connected to the grid, and voltage flicker is easy to occur when the energy of the energy storage element is too small when operating in an isolated island |||| Because the power electronic device itself is easy to generate harmonics, it also promotes active and passive harmonic control. |||| Because there is no (high-frequency) isolation transformer in the process of converter grid connection, the overall efficiency of the system can be improved without transformer, so that it has a certain market share. When there is no isolation (high frequency) transformer, there is interaction between DC side of distributed power supply and AC side of power grid (solar power generation can be intuitively imagined). When DC injection exists in the power grid, it will directly cause the magnetic saturation phenomenon of the electromagnetic components (such as transformers) of the system, and at the same time produce torque pulsation. |||| The access of distributed power supply changes the power flow of each branch in the distribution network, and changes the network loss of the system, which is affected by factors such as load, location and capacity of access to distributed power supply. |||| With the access of distributed power supply, the power flow in the system is no longer unidirectional and unpredictable, which greatly affects the voltage regulation. |||| Because the traditional relay protection device of large power grid has been formed, it will not be modified in a short time. On the one hand, we should consider the connection of distributed power supply. Unreasonable (even sometimes reasonable) control strategy and configuration mode will lead to the failure of reclosing, the protection area of relay protection device will be reduced, the power flow will change, and the relay protection will malfunction. || Also pay attention to the island problem. (3) Energy storage configuration, power prediction and smoothing. At present, it is estimated that many people are reluctant to do so. (4) Management, monitoring and maintenance problems. (5) The dispute of beneficial right (this is really a technical job).
The above are just some representative questions. In view of these problems, modeling, forecasting and other means are used to carry out preliminary tests. However, there are still many difficulties in application and field. Since it is so difficult to solve, the power grid has restricted and isolated such an uncontrollable power supply. On the one hand, the performance index of power supply terminal equipment is required, on the other hand, once the power grid fails, the distributed power supply is required to stop running immediately (IEEE 1547).
6. In order to better coordinate the relationship between distributed generation and power grid, the concept of microgrid is introduced. The definition of microgrid has not been unified. Here's one: Microgrid refers to a small power generation and distribution system composed of micro-power supply (distributed power supply), energy storage device, load, monitoring and protection device, and is an autonomous system that can realize self-control, management and protection. Micro-grid can be regarded as a single controllable unit, which is connected to the grid through the static switch at the common * * * coupling point. In actual operation, the access standard of microgrid is only aimed at the common connection point between microgrid and power grid, without considering various (distributed) power sources in microgrid, so as to realize more harmonious coexistence of distributed generation and power grid. At present, the general control strategies of microgrid mainly include master-slave control, peer-to-peer control and hierarchical control based on multi-agents. The internal micro-power control mainly includes constant power control (P/Q), constant voltage and constant frequency control (V/F) and DROOP control (Droop).