Distributed photovoltaic power generation belongs to renewable energy power generation and has been vigorously developed. However, the distribution of distributed photovoltaic power generation into the distribution network has brought about a certain impact on the protection of traditional distribution networks. Therefore, research on protection mechanisms has been made. Very important practical significance. This paper analyzes the impact of distributed photovoltaic power generation on the protection of distribution networks, proposes specific protection technologies, and on this basis, studies the protection mechanism.
In recent years, as people have been using energy for a long period of time, the use of fossil resources such as coal, natural gas, and oil has only been a short period of time, but the burning of fossil fuels emits carbon dioxide and oxysulfides directly. It has led to the global greenhouse effect and the production of acid rain. The massive development and utilization of fossil energy is one of the major causes of the deterioration of the human living environment. Mankind is facing the dual challenges of economic and social sustainable development. It is historical that fossil energy is replaced by new energy. inevitable. Therefore, under the dual constraints of limited resources and environmental protection requirements, humans should plan ahead and seek alternative energy sources early. This requires that the alternative energy we seek must be renewable clean energy. Solar energy is a kind of renewable energy.
First, the development status of photovoltaic power generation system
With the development of photovoltaic power generation technology, distributed photovoltaic power generation is gradually becoming an effective way to meet the demand for load growth, reduce environmental pollution, increase the comprehensive utilization of energy, and improve the reliability of power supply. Germany started implementing this policy at the beginning of this century and is in the leading position in the world. Relevant European organizations have also estimated the development of this technology: By the 1920s, the total amount of solar power in the world will be one percent of all energy, and by the middle of this century, it will have reached five points. One will be more than half at the end of the century. In this regard, China has also implemented some development strategies. It is required to increase the use of energy by increasing the energy development rate, and to increase the use of energy to reduce the cost of use. Relevant experts believe that by 2025, China’s new energy power generation can account for a quarter of the total power generation. Because solar photovoltaic power generation is more and more frequent, the use of improved power generation rates to reduce costs, and the stability of the power grid has also been widely attention. Today's photovoltaic power generation technology is becoming more and more mature, and it is also moving in the direction of grid-connected power generation and building combined power generation. Of course, there are still some problems in this domestic technology. For example, most of the raw materials still need to be imported to obtain the products. Most of the products are shipped overseas, the materials are bought at high prices, and the products are sold at low prices. The domestic market has been stagnant. Let China's current photovoltaic industry structure has been adjusted.
Second, the effect of distributed photovoltaic power generation on distribution network protection
Generally, it is accessed through 35kV and below voltage level feeders. When the capacity ratio of the two is large enough, it will affect the short circuit current of the distribution network, and then affect the setting and coordination principle of the protection device. The effect of distributed photovoltaic power generation on the protection of distribution networks is mainly reflected in six aspects:
(1) If the fault occurs upstream of the distributed photovoltaic power generation access feeder, the protection device downstream of the feeder may malfunction;
(b) If the fault occurs on an adjacent feeder, then the feeder may malfunction due to protection;
(3) Connecting the distributed photovoltaic power generation to the distribution network will affect the protection sensitivity of the feeder part. Once the sensitivity changes, it will lead to the occurrence of refusal or misoperation;
(4) The feeder itself has a specific scope of protection. Connecting the distributed photovoltaic power generation to the distribution network will affect the protection scope of the feeder and cause it to change.
(5) When distributed photovoltaic power generation is connected to the substation low-voltage bus, the sensitivity of the current protection of each feeder in the distribution network will be greatly improved. This is due to the fact that after the low-voltage bus is connected, the short-circuit capacity of the system will increase a lot. The corresponding short-circuit current also increases, so the sensitivity increases.
(6) After the distributed photovoltaic power generation is connected to the distribution network, once the capacity increases, the protection of the distribution network may be lost.
Third, the new protection technology of distributed photovoltaic power generation access distribution network
(I) Overcurrent protection
In PV inverters, the maximum power point of the solar PV array must be tracked, and sampling is also required. There are three sampling objects: DC-side voltage, DC-side current, and AC current. This method has been adopted for the judgment of overcurrent protection of the circuit, and since the photovoltaic inverter has a high resolution characteristic, the sensitivity is very high when the circuit is overcurrent-protected. In the circuit over-current protection, the use of photovoltaic inverter method, the protection method consists of three parts: overcurrent protection main circuit, overcurrent protection drive circuit, over-current protection control circuit.
(B) Over-voltage protection
For small photovoltaic power stations, when the grid voltage is abnormal, over- and under voltage protection will play a role, according to different circumstances with different stopping power generation time. However, for large-scale photovoltaic power stations, the ability to resist grid disturbances is enhanced through low-voltage ride-through capability.
For the prevention of lightning overvoltage and overvoltage, the method adopted is overvoltage protection of photovoltaic power station. There are two types of overvoltages: vertical and horizontal, so when implementing protection, it is important to consider both types of overvoltage protection. Specifically, there are two overvoltage protection measures for photovoltaic power stations:
1, single-stage protection, mainly for lightning protection and other transient voltage protection, the protection method is to use varistors and gas discharge tube;
2, multi-level protection, suitable for single-stage protection can not play a good role in protection, and the inverter voltage level is low.
(c) Over-frequency protection
In a small photovoltaic power plant, the specific frequency range is 49.5 Hz to 50.2 Hz. If the frequency of distributed photovoltaic power generation to the distribution network exceeds this range, over-frequency protection needs to be initiated. In large and medium-sized photovoltaic power stations, the capability must be to resist abnormal disturbances in the frequency of the system.
(IV) Anti-island protection
When an abnormal situation occurs in the external large power grid, it will become a power outage. At this time, the distributed photovoltaic power generation that accesses the distribution network will continue to supply power to a part of the power distribution system. This is called an island phenomenon, and the island phenomenon includes planning. And non-planned two. Anti-island protection means that when an islanding phenomenon occurs, the connection between the photovoltaic power station and the island system is disconnected, and the conditions for the operation of the island phenomenon are eliminated, so that the island phenomenon does not harm the maintenance work of the user's equipment and equipment.
IV. Research on the Protection Mechanism of Distributed Photovoltaic Generation to Distribution Network
(I) Calculation method of short-circuit current
In the traditional distribution network, there are four short-circuit current calculation methods: equivalent voltage source algorithm, superposition algorithm, phase zero loop current algorithm, transient simulation algorithm. The first three are used to calculate the steady-state fault current and the fourth is used to calculate the transient fault current. When distributed photovoltaic power generation is connected to the distribution network, it has a great impact on the traditional distribution network: the power distribution system has multiple power supplies, and the quantity and direction of active power and reactive power transmitted on the line change. In this way, new requirements have been put forward for the protection of the distribution network, and while the protection of the distribution network is being studied, new short-circuit current calculation methods have to be developed. The new short-circuit current calculation method can be based on the traditional basis. On the amendment, combined with the characteristics of distributed photovoltaic power generation.
(B) Protection setting and configuration principles
When distributed photovoltaic power generation is connected to the distribution network, the protection of feeders can be divided into protection of upstream main feeders, protection of upstream branch feeders, protection of downstream main feeders, protection of downstream branch feeders, and protection of distributed feeders at different locations. The influence of the capacity of photovoltaic power stations is also different. When a fault occurs, the fault occurs upstream of the protection, and occurs within the protection scope and downstream of the protection. The impact on the protection of the distribution network is also not the same. It is necessary to implement specific protection according to the location of the specific fault.
In recent years, distributed photovoltaic power generation has been rapidly developed. Distributed photovoltaic power generation relies on solar energy and belongs to renewable energy. The distribution of distributed generation into the distribution network has a certain impact on the protection of the traditional distribution network, mainly reflected in the reliability, sensitivity and selectivity, in order to better achieve the protection of the distribution network, New distribution network protection mechanisms have been researched to ensure the normal operation of distribution network transmission.
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