Probabilistic analysis of dynamic stability of microgrids considering wind turbines

Master's thesis in the field of electrical-power engineering

Abstract

Possible dynamic stability analysis of microgrids considering wind turbines

In recent years, the high penetration of renewable energy sources and especially wind energy in power networks has created new issues. One of the most important issues is the uncertainty in the power produced by wind turbines. The uncertainty created by wind energy in microgrids that have lower power and voltage levels can be much more effective. This issue specifies the need to perform a possible analysis in microgrids that use wind energy to generate power. In this thesis, small signal stability of microgrids will be studied under the influence of production uncertainty by wind energy. For this purpose, Monte-Carlo and quantize methods are used as numerical methods and two-point estimation method and the method based on Gram-Charlier expansion are used as possible numerical analysis methods. The advantages and disadvantages of these methods will be studied. In order to complete the studies in this field, the dynamics of wind turbines will also be examined in this thesis. To achieve this goal, three types of conventional wind turbines in power systems are fully modeled and their dynamic impact on the possibility of system instability is evaluated. Also, to obtain the state equations of the system, a method specific to microgrids will be used, which provides great flexibility for modeling new components.

Keywords:

Small signal stability, probabilistic analysis, microgrid, uncertainty, wind energy

1-1.      Wind energy

1-1-1. An overview of wind energy

The negative and non-negligible effect of burning fossil fuels [1] on the world's climate has been strongly considered in recent years. Reducing the negative effects of these climate changes requires a huge reduction in the production of greenhouse gases [2], which can be achieved by reducing the burning of fossil fuels. According to estimates, it is necessary to reduce these gases by 60-80% by 2050 [1]. For this reason, in many countries, the use of energy production sources that, despite having a high reliability factor, produce little carbon monoxide and are economical in terms of energy has become one of the most important goals of policymakers in the field of energy.

For this purpose, the use of renewable energy sources [3] has been put on the agenda of governments, so that in 2012, the amount of power generation capacity from all renewable sources exceeded 1,470 gigawatts. This amount of production capacity [4] is equivalent to 26% of the global production capacity and 21.7% of the power produced this year [2]. Meanwhile, wind energy[5] has had one of the fastest growth rates compared to other renewable energy sources. In 2012, the amount of power generation capacity from wind energy has reached 282 GW [3]. construction or being planned) and in the "Advanced" scenario, it is assumed that all political solutions are in favor of the production and expansion of the use of wind energy. By examining Figure 1-1, which shows the forecast of the amount of wind power capacity produced in 2004 and comparing it with the actual values ??of wind power capacity in 2012, it can be clearly seen that the best and most optimistic predictions about the future of wind energy are far from reality [4]. Therefore, it can be concluded that in the coming years, wind energy will become one of the most effective and widely used energy sources in the world.

Figure 1-2- World wind speed atlas at a height of 80 meters for 2005

Since the amount of power produced by wind turbines is very dependent on wind speed, it is tried to choose the location of wind power plants in areas with relatively high wind speed. Figure 2-1 shows an example of a wind atlas that can be used for this purpose. In this figure, the wind speed in different regions of the world at a height of 80 meters from the ground is shown.In addition, Figure 1-3 shows Iran's wind atlas at a height of 80 meters above the ground. According to this figure, Iran has a high potential and ability to exploit wind energy [5].

Figure 1-3- Iran wind speed atlas at 80 meters elevation

Almost the working process of all wind turbines is the same, so that the wind energy causes a rotational movement [7] in the turbine blades and this rotation of these blades causes the electric generator axis [8] which is located inside the nozzle [9] to move. Then the rotational speed of the axis is increased by a gearbox [10] so that it is suitable for use by the electric generator. The generator converts rotational kinetic energy into electrical energy with the help of a magnetic field[11]. Finally, the voltage level is converted from about 700V to a suitable voltage for connecting to the grid, for example 20KV, by a transformer.

Figure 1-4- Horizontal axis wind turbine

In Figure 1-4, a wind turbine is shown, which is one of the most common types of turbines for generating electric power. The wind energy observed by the surface of this turbine is obtained from the following equation: In equation 1-1, ? is the air density (approximately 1.225 Kgm-3 at sea level), A is the area that the rotor blades pass through and V is the wind speed facing the turbine. It blows [12]. According to this relationship, the energy produced by a wind turbine is strongly dependent on the wind speed.

One of the biggest problems that stand in the way of expanding the use of wind turbines is the uncertainty [13] that the energy produced by wind power plants brings. This means that the production or non-production of electric energy or in other words the wind blowing and also the wind blowing speed are random variables [14]. Figure 5-1 shows this well. This figure shows the power produced by a wind farm[15] with a capacity of 50 MW. It can be clearly seen that this power has very strong and random fluctuations that can cause problems in planning the production of the entire network.

Figure 5-1- The production power of a typical wind farm with a capacity of 50MW in a week

1-1-2. Different wind turbine technologies

Wind turbines can be examined from several points of view. For example, depending on whether the wind turbine blades rotate around the horizontal axis or the vertical axis, wind turbines are divided into two categories: horizontal axis[16] and vertical axis[17]. and following[18] the wind direction.

No need to place heavy components of the wind turbine (such as the generator, gearbox and other mechanical parts) in the nozzle, which is at a high height from the ground. This issue makes the maintenance [19] of these components easier and also eliminates the need to build a strong structure to maintain these heavy tools.

Disadvantages of horizontal axis wind turbines compared to vertical axis turbines:

The blades in vertical axis turbines are relatively close to the ground. Since the wind speed on the surface of the earth is lower, the power that these turbines absorb from the wind decreases.

The wind on the surface of the earth has strong changes[20], which increases the mechanical stress[21] on the turbine.

Vertical axis wind turbines do not have angle-controlled blades [22] as well and effectively as horizontal axis wind turbines.

The disadvantages and advantages mentioned above, horizontal axis turbines have become much more popular and have become a conventional means of generating electrical energy from wind energy. Generally, today's modern turbines consist of three blades that have a very good dynamic performance. The use of a large number of blades causes the passage of one blade to disrupt the air dynamics and increase the mechanical stress on the next blades. The use of two blades is used only in turbines with high production power. One of the disadvantages of using two blades is the high tension that is created when these blades pass by the tower holding the nozzle.

Other categories of wind turbines are based on how they work and the components of these turbines.