Why reduce the spare capacity required for wind farms?

Why reduce the spare capacity required for wind farms?

Against the background of increasingly tense global environmental protection and resource pressures, China’s energy situation is particularly severe. As a new energy technology with great potential and mature technology, wind power has an increasing application scale. The installed capacity of wind power is 150 million kw. The adverse impact of large-scale wind farm grid connection on the stability and power quality of the system is also increasing, which will seriously restrict the access ratio of wind power in the system. When the proportion of capacity reaches more than 20%, the peak regulation capacity and safe operation of the power grid will face huge challenges. At the same time, the areas rich in wind energy resources in China are mostly North China and Northwest China. These areas mainly use large-scale development and centralized grid connection. However, the power grids in these areas are relatively weak, and their ability to resist wind power shocks is limited. , and cross-regional transmission will bring great hidden dangers to the safe operation of China’s large power grid due to its instability.

East China has made great progress in wind power construction in recent years due to its abundant wind energy resources along the southeast coast. In 2008, the new wind power installed capacity of East China Power Grid reached 562.78Mw, and the cumulative total installed wind power capacity has exceeded 1000Mw, reaching 1166.03Mw. Compared with the cumulative wind turbine capacity of 603.25MW in 2007, the growth rate is as high as 93.3%. The newly installed wind power capacity is mainly distributed in three provinces and one city, Jiangsu, Zhejiang, Fujian and Shanghai, and has a strong development trend. Among them, the wind power in Jiangsu is developing rapidly. By the end of 2008, the installed capacity of wind power has reached 648.25Mw, ranking first in the East China power grid. head. According to the preliminary plan, East China Power Grid will also build a 10 million kilowatt-class wind power base in the coastal areas of Jiangsu in 2020. With the integration of large-scale wind farms into the East China Power Grid, how much output of the wind farms can be replaced at the peak load, and how much capacity can be replaced by thermal power plants, that is, how much capacity can be included in the power balance of the East China Power Grid (the capacity of the wind farms is credible degree), which has become a problem that East China power grid planning must face. In addition, after a large-scale wind farm is connected to the East China Power Grid, the impact on the system frequency regulation capacity and reserve capacity is also a concern of the power operation department.

The development of energy storage technology provides an effective solution for the large-scale grid connection of wind power and the improvement of wind power performance. [3]. Due to its fast regulation and long cycle life, vanadium batteries are more suitable for use in wind power soup. In the past decade, the United States, Japan, Europe and other countries have successively used vanadium battery energy storage systems in wind energy/photovoltaic power generation. A large number of demonstration application projects. However, China is still in the stage of trial operation and has not yet achieved commercial application.

The average wind speed of the wind farm roughly determines the capacity factor of the wind farm. According to the results of related research, the capacity factor of the general wind farm is relatively low, ranging from 0.2 to 0.5. If the energy storage capacity is planned according to the daily power generation curve when the wind speed difference is large, the result is too large, which makes the utilization efficiency of the energy storage device not high, and the typical daily power generation curve (which can also be calculated by the typical daily wind speed distribution curve) represents In this section, the typical day is divided into 288 time periods (each time period is 5 minutes), and the effect of smoothing the wind power output curve of the energy storage system and the economy of building the energy storage system are considered and analyzed. An optimization model is established to realize the optimal configuration of wind power and energy storage.

  1. Variation characteristics of wind power

Wind speeds are always changing and this has different effects on the power system. Figure 1 shows the correlation coefficient statistics of output changes of German wind farms in different time and regions. In the figure, r is the output change correlation coefficient, r=AP/PN, AP is the output change capacity, and PN is the rated output , and the abscissa 1 is the distance.

Figure 1-Statistical graph of the correlation coefficient r of the output change of German wind farms
Figure 1-Statistical graph of the correlation coefficient r of the output change of German wind farms

It can be seen from Figure 1 that the smaller the time range, the smoother the wind farm output fluctuation; the larger the wind farm distribution area, the smoother the wind farm output fluctuation. For a wind turbine, second-by-second power changes can already be smoothed out, and the inertia of the huge rotating blades of a variable-speed wind turbine can dampen the effects of fast gusts. Variable speed wind turbine rotor speed can absorb second-by-second power changes. The extreme slope data recorded for a 103MW wind farm are:
(4%~7%) P, /s, (10%~14%) P, /min and (50%~60%) PN/h. For a wind farm that is geographically distributed over a larger area, the second-by-second and minute-by-minute changes will no longer be significant, and the hourly changes will be much smaller than (50%~60%) PN. When the area reaches 200km × 200 km, the maximum hourly fluctuation is about (±30%) P,; when the area reaches 400km × 400 km (such as Germany, etc.), the maximum hourly fluctuation is about (±20%) P , ; and in larger areas covering several countries, the maximum hourly fluctuation is about (±10%) PN, such as the Nordic countries. These are extreme values ​​and most of the time the hourly variation will be within (±5%) P,.

Although the output changes of wind farms vary from region to region, it is closely related to the wind energy resources in the region, which in turn are related to climatic conditions, topography, land and sea, and the size of the region under study. However, the fluctuating pattern and smooth characteristics of wind power generation are common characteristics of large-scale wind farm power generation.

  1. Requirements for system frequency regulation and standby in wind farms

(1) Standby requirements of German wind farms for the system
The German grid is part of the Union of the Coordination of Transmis- sion of Electricity (UCTE) in Western Europe. The organizational structure of UCTE power grid is distributed, and there is no central dispatching agency. Generally speaking, a country is a control area, and the dispatching in each control area is responsible for the power grid in that area. The task of frequency regulation is negotiated by all members of the UCTE grid.

According to the speed of response time, the frequency modulation reserve of UCTE power grid includes secondary, tertiary and hourly frequency modulation reserve. Secondary frequency regulation is mainly to ensure that the power balance in each control area is restored to the agreed value in the programmed power generation plan after a large power imbalance occurs. The secondary frequency regulation is automatically controlled by the central automatic generation control (AGC) in each control area, and its action time ranges from tens of seconds to 15 minutes. When the scale of wind power generation is large, due to mutual offset, short-term The fluctuation of wind power (within the secondary frequency regulation time range of 15min) is not large, so wind power generation has no higher requirements for secondary frequency regulation backup.

The third frequency regulation is actually the rearrangement of the power generation plan, which is usually manually adjusted by the dispatcher in the control area to replace the secondary frequency regulation. The secondary FM reserve capacity occupied in this way can be resupplied. From an economical point of view, for power deviations with a longer duration, three frequency modulations should be used to make up. In conventional power systems, power deviations are caused by power plant trips and load forecast errors. As the proportion of wind power generation in the system increases, the influence of wind power generation prediction error will become more and more obvious. At this time, the grid needs to have positive power backup (when the actual wind power is lower than the predicted value), and there is also a negative power reserve. Power reserve (when the actual wind power is higher than the predicted value).

The German Energy Association concluded the following conclusions in the “Grid connection planning of offshore and onshore wind farms in Germany 2020” (DENA Grid Research section):
1) In 2003, the average value of positive power reserve capacity (including three-time frequency regulation reserve and hourly reserve, the same below) caused by wind power in Germany was 1200Mw, the maximum value was 2000w, and the average value of negative power regulation capacity was 750Mw, The maximum value is 1900Mw.

2) In 2015, the average and maximum value of the positive power reserve capacity to be prepared due to wind power were 3200MW and 7000MW, respectively, accounting for 9% and 19.4% of the total installed wind power capacity in that year; the average value of the negative power regulation capacity was 2 800Mw, the maximum value is 5 500Mw, accounting for 8% and 15.3% of the total installed wind power capacity in that year, respectively.

3) The reserve capacity caused by wind power does not require new conventional power plants in the power grid, and the existing conventional power plants can meet the standby demand of the power grid.

(2) Standby requirements for wind farms in Nordic countries
Due to the smoothing effect of wind power output changes in a very short period of time, in a large area, the increase in primary reserve capacity due to wind farms in the system can be ignored (equivalent to Germany’s secondary frequency modulation reserve); but for 15min The secondary backup (equivalent to the tertiary frequency regulation backup) within the time order of 1 hour needs to be appropriately increased. For a power grid with a wind power penetration of 10% (wind power installed capacity/system maximum load), the increased secondary backup demand is about the wind power rating. 2%~8% of the capacity.

(3) Consideration of system frequency regulation and standby requirements for wind farms in East China Power Grid.
The East China power market has not yet been formed. Different from northern Europe, Germany and other countries, its secondary frequency regulation reserve, that is, the AGC reserve is not obtained through market auction, but operates according to the power grid command. It is separated from the power grid reserve and divided into into the frequency regulation requirements of the grid. Therefore, for East China Power Grid, its frequency regulation requirements mainly include primary and secondary frequency regulation requirements. In addition to the frequency regulation requirements of East China Power Grid, in order to ensure the power quality and the safe and stable operation of the system, the power grid must also ensure a certain reserve capacity. In the power grid planning and design of East China Power Grid, two types of backup are considered: hot backup and cold backup, among which hot backup includes accident hot backup and load backup. Considering that the large-scale wind farm is put into operation, the hot backup of East China Power Grid will be determined by the accident hot backup, load forecast error, and wind farm output forecast error.

  1. Battery energy storage can reduce the benefits of spare capacity required for wind farms

The randomness of wind power leads to a deviation between the predicted output of the wind farm and the actual output after the wind power is connected to the grid, which makes the system need to be equipped with a corresponding rotating reserve capacity, which is determined by the reliability of the wind farm output forecast. The reliability of wind farm capacity is closely related to both wind resources and the distribution area of ​​wind farms. The tens of millions of kilowatts of wind farms in East China are basically distributed in the coastal areas of northern Jiangsu, which is similar to that of Germany, where most of the wind power is located in the seaside of northern Germany. Offshore wind farms (the proportion of offshore wind farms planned in Germany is not large), and offshore wind farms have greatly improved capacity reliability due to the high wind speed over the years. Therefore, referring to the reliability ratio of wind power capacity in Germany, the capacity reliability of wind farms in East China Power Grid in 2020 can be estimated at 5% to 10% of the total installed wind power capacity.

When the wind farm is equipped with an energy storage device of a certain capacity, the energy storage device can adjust the output of the wind farm and reduce the deviation caused by the prediction reliability, thereby reducing the rotating reserve capacity required by the system. When the wind power output is large, the prediction deviation The rated power of the energy storage device may be exceeded, and the effect of reducing the reserve capacity of the energy storage device at this time reaches the upper limit. n=288 is the number of time periods divided in a day, then the battery energy storage system is in the first; the benefit of reducing the wind power rotating reserve capacity in the time period is:

In the formula, e-i- the price of spare capacity in the i-th period (10,000 yuan/MW);
The reliability of wind farm forecasting techniques;
Po,–the output of the wind farm in the first; period;
Pmx – Rated power of vanadium battery energy storage system.

Read more: The role of battery energy storage in the grid