Distributed power application answers to several questions
introduction
In recent years, attention has been paid to the application and consumption of distributed power sources. The influence of distributed power sources on distribution networks, the short-circuit current characteristics of squirrel-cage asynchronous generators and doubly-fed asynchronous generators are analyzed, and the photovoltaic power generation system is studied. Dynamic simulation model, discusses the grid-connected inverter structure and control strategy of photovoltaic power generation system, studies the method of additional power quality control of photovoltaic inverter, and the short-circuit current of converter-type distributed power supply and motor-type distributed power supply Characteristics and impact on distribution network fault handling.
The above results greatly improve the understanding of the impact of distributed power access to the distribution network, but to improve the distribution network's ability to dissipate the distributed power supply, most of them focus on the coordinated control of the communication network, but Research on the method of consumption that does not rely on communication means is still insufficient. In addition, in terms of solar energy utilization, it is necessary to compare and analyze distributed rooftop photovoltaic power generation with widely used household solar water heaters, and explore how to better utilize solar energy to achieve energy conservation and emission reduction under the current state of the art.
1. Influence of distributed power supply on distribution network
It is generally believed that after the distributed power supply is connected to the distribution network, it will have an impact on relay protection, power distribution automation fault handling and voltage quality.
1.1. Impact on relay protection
The distributed power supply can be divided into two types: motor grid-connected type and inverter grid-connected type. The short-circuit current that the motor-connected distributed power supply can provide does not exceed 10 times its rated capacity, and the inverter is connected to the grid. The short-circuit current that can be supplied by a power supply generally does not exceed 1.5 times its rated capacity.
When the phase-to-phase short-circuit fault occurs on the feeder, the short-circuit current from the main network, the short-circuit current provided by the distributed power supply on the other feeders of the same bus line, and the short-circuit current provided by the distributed power supply on the feeder on the fault will flow to the fault point. In order to maintain the stability of the 10kV bus voltage, the system short-circuit capacity of the distribution network is generally much larger than the capacity of the distributed power supply on the feeder. Therefore, the short-circuit current from the main network is generally much larger than the short-circuit current provided by the distributed power supply, making it easy to implement the main network. The side relay protection is matched, and the distributed power supply side needs to be disconnected from the fault area by anti-islanding measures (such as low voltage off-network).
1.2. Impact on distribution automation fault handling
When the feeder phase-to-phase short-circuit fault occurs, the short-circuit current from the main network, the short-circuit current provided by the distributed power supply on the other feeders of the same bus line, and the short-circuit current provided by the distributed power supply upstream of the fault on the feeder line of the fault will flow to the upstream entry point of the fault area. The downstream output point of the fault area also flows through the short-circuit current provided by the distributed power supply downstream of the fault on the feeder. It is generally believed that if the short-circuit current flowing downstream of the fault zone approaches the short-circuit current flowing through the upstream entry point of the fault zone, the distribution automation fault location strategy will be destroyed. However, as mentioned in the previous section, the short-circuit current from the main network is generally much larger than the short-circuit current provided by the distributed power supply, and the short-circuit current provided by the distributed power supply on the other feeders of the busbar also helps to increase the upstream flow through the faulty area. The short-circuit current of the point, that is, the short-circuit current flowing through the upstream in-point of the fault area will be much larger than the short-circuit current flowing downstream of the fault area, and it is easy to set a fixed value to distinguish them, so generally it will not It has an impact on the fault location of the traditional distribution automation system. In other words, the already established distribution automation system does not need to be reinvented due to the large-scale access of distributed power.
1.3, the impact on voltage quality
The access of the distributed power source has a lifting effect on the voltage of the feeder, and the distributed power source (such as photovoltaic, wind power, etc.) whose output is affected by natural factors also generates voltage fluctuation due to its volatility, and its access point Voltage boosting and voltage fluctuations are the most important. For a feeder connected to multiple distributed power sources, the voltage distribution along the line is as if a small stick is placed at each distributed power source, and a voltage extreme point is formed at each access point of the distributed power source. figure 1. In the figure, S is the substation outlet breaker of the feeder, A, B and C are segmented switches, DG is a distributed power supply, and the longitudinal axis U of the curve represents the voltage along the line.
Figure 1 shows the voltage distribution along a distributed power supply feeder
Considering that the impedance per unit length of the feeder is large, the influence of the access of the distributed power supply on voltage deviation and voltage fluctuation is obvious, which is a key factor that restricts the distribution power of the distribution network.
2. Distribution network's ability to dissipate distributed power
The key to restricting the distribution network's ability to dissipate distributed power is the voltage deviation and voltage fluctuation generated after the distributed power supply is connected, rather than its impact on relay protection and distribution automation fault handling.
2.1. Three control strategies for distributed power supplies
In the case that the distributed power access capacity is not very large, even if no control measures are taken, the distribution network has a relatively strong capacity to absorb. This method of consumption is called free-selling.
In the case where the distributed power access capacity exceeds the free-capacity capability, it is first considered to camp in the local control strategy in the larger-capacity distributed power source without having to resort to the communication network and coordinated control, but only based on the distributed power source. The collected real-time voltage information of the access point is locally adjusted to the reactive power or active power of the output to meet the requirement that the voltage deviation under light load or heavy load conditions does not exceed the limit. Local control of consumption methods.
In the case that the distributed power access capacity exceeds the local control capacity, it is necessary to consider the coordinated control of several large-capacity distributed power supplies and even controllable loads by means of the communication network to meet the voltage constraint conditions. The way, called the coordinated control of the way of consumption.
In practical applications, the free-selling method should be preferred, and the local control method should be adopted when it cannot fully meet the requirements. The free-selling method and the local control method have strong ability to absorb and should be able to solve the vast majority. The problem is that it is only necessary to adopt the coordinated control method, because the coordinated control method relies on the communication channel, making the distribution network more vulnerable.
Since there are a lot of literature reports on the method of coordinated control and consumption, this article will not repeat them, but only the methods of free consumption and local control.
2.2, the ability to absorb consumption under free consumption
Research and analysis show that do a good job of distributed power access planning (such as appropriate selection of access voltage according to the distributed power supply capacity), try to achieve "big horse car", even if you do not take any control measures on the distributed power supply, The distribution network also has a relatively strong capacity to absorb.
Under the condition that no control measures are taken for the distributed power supply, the distributed power supply access capacity must satisfy three constraints at the same time (that is, the maximum voltage upper deviation value and the maximum voltage fluctuation value caused by the distributed power supply accessing the distribution network are not The limit is exceeded, and the deviation of the maximum voltage caused by the load is not limited when the distributed power supply is not connected to the distribution network. The shaded area enclosed by the three curves is not to take any control measures on the distributed power supply. The range of accessible capacity of distributed power sources [11], see Figure 2. In the figure, PDG and PL represent the capacity and load power of the distributed power source, respectively.
Figure 2 Capacity range allowed by distributed power supplies
For example, for a feeder whose load power is distributed along the feeder line and the distributed photovoltaic power supply capacity is evenly distributed along the feeder line, the YJV-120 type cable is used, and the allowable access capacity of the distributed photovoltaic power source is 75% at a capacity ratio of 75%. The range is shown in the shaded area in Figure 3. In the figure, PPV represents the capacity of the distributed power source. It can be seen from the figure that even if the distributed power source is not controlled, the feeding capacity of the feeder is large.
Figure 3 Capacity range allowed for distributed photovoltaic power supplies
2.3, local control method of consumption
The voltage along the feeder line forms a voltage extreme point at the access point of each distributed power source. Therefore, as long as the local control strategy is adopted to make the voltage of these extreme points meet the voltage constraint, the voltage of the entire feeder can generally satisfy the voltage constraint. Requirements, this is the theoretical basis for the feasibility of local control of consumption methods.
Due to the obvious effect of adjusting the reactive power on the voltage amplitude, and in order to make full use of natural resources to provide active power and protect the interests of distributed power owners, local control should be within the allowable range of remaining capacity under the premise of ensuring active power. Internally, the reactive power of the distributed power supply is prioritized. When the reactive power is adjusted to the remaining capacity limit, the voltage deviation problem cannot be solved (or the distributed power supply can only provide active power), and then the distributed power supply is Active power is adjusted.
The local control strategy can adopt the fuzzy control method. When the voltage exceeds the limit, it can be repeatedly adjusted according to the real-time voltage information until the voltage constraint requirement is met (note that as long as the allowable voltage deviation range is satisfied, the near-rated voltage cannot be pursued).
It is worth mentioning that the local control of distributed power can not only improve the distribution capacity of the distribution network to the distributed power supply, but also make full use of the distributed power supply to emit inductive reactive power or capacitance as needed. Reactive power, and can continuously adjust the characteristics of reactive power output, realize reactive voltage control of distribution network, solve low voltage and over voltage problems, because it uses the remaining capacity of the converter to provide the required reactive power, so Generally, it does not affect the utilization of natural resources and the output of active power.
3. Comparison of solar water heaters and urban household distributed rooftop photovoltaics
Photovoltaic power generation is not a perfect "green energy", because the preparation of photovoltaic cells is both high pollution and high energy-consuming industries, and there is a serious excess capacity in China.
Solar water heaters have been widely used in China. For a solar water heater, according to the capacity of 60L, the water temperature is raised from 20 °C to 80 °C every day, the daily conversion of thermal energy corresponds to 3600kWs, that is, 1kWh (1 kWh) of energy, will It is converted into electrical energy, and considering 80% conversion efficiency, it can save about 1.3 degrees of energy per day. The available area of ​​a solar water heater is about 1m2. According to the current technical level, a photovoltaic cell with a capacity of 100W is installed. The daily utilization hours are calculated according to 3h, and 300Wh, that is, 0.3kWh (0.3 kWh) of electricity can be issued in one day.
It can be seen that solar water heaters are far more efficient than photovoltaic power generation in terms of solar energy utilization. In addition, since the electric water heater is one of the main loads of the households, the energy saving effect after adopting the solar water heater is obvious; and for the urban families, most of their residences are high-rise buildings, and each household has a very limited roof area. Installation of photovoltaic power generation facilities is far less than installing solar water heaters, and in the case of supporting nighttime electricity use, photovoltaic power generation also needs supporting energy storage devices, making it more uneconomical [12].
Therefore, under the current technical level, for urban households, solar water heaters can be equivalent to a virtual solar power plant, which can play a more active role in energy saving and emission reduction. Of course, the development of photovoltaic power generation in the roof or roof of non-residential buildings, such as industrial enterprises, shops, schools, office buildings, etc., still has certain feasibility and application value.
4, the conclusion
After the distributed power supply is connected to the distribution network, the impact on short-circuit current, relay protection, distribution automation fault processing, harmonics and loss is easier to deal with, and the impact on voltage deviation and voltage fluctuation is obvious, which is the constraint on the distribution network. A key factor in the ability of distributed power consumption.
There are three strategies for dissipating distributed power. As long as it can achieve “big marathonâ€, the free-selling method also has strong ability to absorb, and should be given priority; if it can not fully meet the requirements, it should adopt local control The mode of adoption; it is only necessary to adopt a coordinated control of consumption, because its dependence on communication channels will make the distribution network more vulnerable.
For urban households, the roof area of ​​each household is small, and the efficiency of solar water heaters is higher. Therefore, solar water heaters are more suitable than rooftop photovoltaic power generation. It can be equivalent to a virtual solar power plant, which can play a role in energy saving and emission reduction. positive effect.
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