Integration of Wind Energy Resources with Conventional Grids

As we are progressing in time, the non-renewable energy resources are running out and soon they won’t be enough to meet our needs. As the necessity is the mother of invention, this need keeps pushing us to develop alternate energy resources which will eventually replace all the conventional energy sources one day. This development in the Power Generation sector, gives new challenges to Power Engineers each day, and they strive to meet these challenges through the help of latest technologies. In this blog we will be discussing one of the most significant renewable energy resource today, which is being used across the globe. This blog will put some light on the technical challenges associated with generation and the integration of Wind Energy systems into the conventional grids, and how these challenges are conquered through Power Engineering solutions.

Wind Energy

As this self-explanatory term suggests, wind energy means to use wind intelligently in order to generate energy through it. For this purpose, we use wind turbines, which fetch winds kinetic energy and convert it into rotary motion that is further converted into Electrical Energy through a generator. So, the primary components of a wind turbine are Turbine blades, rotor, gear box, generator and sometimes also DC/AC converters. Turbines are categorized as vertical and horizontal axis wind turbines, which can further be categorized into Fixed and Variable speed wind turbines.

Types of Wind Turbines

Horizontal Axis Wind Turbines

Horizontal Axis wind turbines are the most widely used wind turbines around the globe. As they have relatively higher power generation efficiency in regions with consistent wind conditions, and most wind power plants are situated in areas where we can have higher yields of power with as much as possible consistent winds throughout the year. Horizontal Axis wind turbines have their rotational axis parallel to the wind stream and ground, and they are usually set up high above the ground on the tubular structures.

Vertical Axis Wind Turbines

Vertical Axis wind turbines have their fins placed vertically, and the rotational axis of vertical axis wind turbines is perpendicular to the ground. This property provides them the versatility of utilizing the wind from 360 degrees unlike horizontal axis wind turbines and makes them more appropriate for areas with relatively inconsistent wind conditions. Therefore, these are highly applicable in residential areas while on the contrary, horizontal axis wind turbines require more consistent wind conditions which is why they are mostly used in wind power plants.

Fixed Speed Wind Turbines

Fixed speed wind turbines have their speed determined by the gear ratio and generator design according to the Grid Frequency while it is irrespective of the wind speed. These generators are made to work with highest efficiency at a specific speed, and there is a significant decrease in efficiency when the speed of the wind varies from the specified value. Some of these generators can work efficiently around two different values of wind speed as they are built with two sets of windings. One set, with higher number of poles is utilized when the speed is low while the other with lower number of poles is used when the wind speed is comparatively higher. These generators with two sets of windings provide higher overall efficiency than the former ones. Fixed speed wind turbines have lower cost and higher reliability than variable speed wind turbines.

Variable Speed Wind Turbines

Variable speed wind turbines are becoming increasingly common with the passage of time. Their speed varies with the changes in wind speed, and they always tend to achieve the maximum efficiency with respect to the speed of wind. This increases the complexity of the Wind turbine, and the generators are not directly coupled to the grid while they are connected through the Power Inverters. When power is generated through a variable speed turbine, its frequency varies, and therefore we need to insert power inverters between the generator and grids. Some of these Power inverters generate odd harmonics and consume reactive power. To eliminate harmonics from the system, harmonic filters are used while for reactive power compensation different methods can be used, including the addition of capacitor banks in the system. These factors increase the complexity and cost of the system. Therefore, Variable speed turbines are considered technologically advanced and involve high initial costs. But they provide a significant increase in the efficiency of the system which eventually leads to higher power yields, and relatively puts less mechanical stress on the Turbine itself.

The below-mentioned figure shows, the power generated with respect to different Turbine speeds, line ‘T’ represents fixed speed wind turbine while the curve ‘S’ represents Variable speed wind turbine. It can be observed that there is a significant variation in power generation by both the turbines, but the curve ‘S’ follows the maximum point of each speed curve, which means that maximum possible output would be generated through the variable speed wind turbine at different wind speeds, while the constant speed wind turbine would only be generating maximum power at 9 m/s, while during all other wind speeds its output will be lesser than maximum possible output power.

Factors Effecting the Stability of a Wind Integrated Power Grid

For a successful operation of a power generation system and the utilities, it is compulsory to maintain the power quality of the system. Power quality must be maintained while attaining complete balance in between the power being generated, and the power being utilized by the consumer. Among all the considerations regarding the power quality, voltage variations and keeping a smooth sinusoidal wave are a major concern that needs to be focused on throughout the power generation process. Now, we are going to see the factors that are to be looked upon in order to attain these outcomes.

Voltage Instability and Flickering

Flicker or Lighting Flicker are the response of lighting loads to the voltage variations occurring in the power systems. These Flickers can be observed by human eye, and they occur due to fluctuations in voltage. Industries have high active and reactive power consuming loads such as large motor and electric Arc Furnaces, even some home appliances have significant power rating. The switching of these kinds of loads lead to some major variations in active and reactive power in the system, which results in voltage fluctuations and that eventually causes flickering in loads. Voltage instability also occurs due to generation variation in wind farms as wind speed varies timely. Which is maintained using reactive power compensation and voltage regulators. Moreover, Flickering can be a result of minor technical negligence, such as loose wiring connections and nuisance tripping. The graph below shows the voltage fluctuations that result in the form of flickering.

Flickering can be hazardous to the electrical equipment, and electronic devices, moreover it can be a cause of personnel fatigue and disrupted concentration levels.

Causes of Harmonics and their Effect on Power Quality

A harmonic is a wave with the frequency equals to the integer multiple of the reference signal. A pure sinusoidal wave does not include any harmonics. The presence of harmonics in a system leads to the distortion of sinusoidal wave. Harmonics are produced due to not-so-perfect working of electrical and mechanical equipment. In wind farms, Harmonics can be introduced into system due to electronic equipment being used in wind turbines for controlling purpose such as inverters and due to huge Power Transformers being used to step-up voltage for the sake of transmission.   These harmonics can be of different orders, while 3^rd and 5^th order harmonics are the most significant in Power Systems, and they tend to affect the power quality the most, in comparison to the other higher order harmonics. The presence of harmonics leads to lower capacitive impedance in the system, which is return produces extensive heating in electronic equipment. These harmonics produce also produce circulating currents in power systems, which increases losses in the system and increases the need of cooling mechanisms for the power equipment. Following figure shows an example of distortion of sinusoidal wave through the addition of 3^rd order harmonic in the system.

In power systems, we use harmonic filter for the sake of eliminating these harmonics and maintaining the power quality. With the technological advancements, such PWM converters are being designed for variable speed wind turbines which are highly efficacious. These PWM converters have very high periodic oscillations, which produces nearly perfect sinusoidal wave, and contain only higher order harmonics, for instance greater than 2000 Hz. These harmonics produce very low distortion and are easier to be filtered from the system. Total Harmonics Distortion is a measure of how distortive the harmonics can be to the sin wave being generated.

The above-mentioned table provides harmonic distortion limits of voltage according to IEEE standard 519.

Reactive Power

The flow of reactive power in the system is due to the reactive components present in a power system. Generally, inductive loads consume reactive power, while the capacitive loads give out reactive power. While the synchronous generators can work both ways, if a synchronous generator is working in an over-magnetized state, then it would be giving out reactive power, on the other hand if it would be under-magnetized then it would be consuming the reactive power. This reactive power that is being introduced in the system; this can lead to a very significant amount of power losses. This value of current is a combination of both active and reactive currents. So, higher the reactive power being flown in the system, higher will be the value of reactive currents which will eventually lead to more losses. So, for compensating this reactive power consumption we install capacitors near the reactive loads in the systems, these capacitors give out reactive power which results in increased values of power factor and less power losses. Some wind turbines have Induction generators, these induction generators are a consumer of reactive power same as an induction motor. Therefore, in such cases we provide reactive power compensation to induction generator, also through capacitor banks as it cannot be compensated through over-magnetization as in synchronous generators.

Other than power losses, flow of reactive power also results in unstable values of voltage. When high reactive currents flow through the transmission lines they produce considerable voltage drops, and that constitutes to unstable voltage values at the distribution or consumer end.

Line impedance and Associated Grid Strength

 A Grid’s Short Circuit Power Level defines the strength of the grid, either the grid can be defined as a strong or a weak grid. A strong grid would be least effected by the variation in the load or production, while a weak grid would be relatively much more effected by these variations.

The SC Power Level is determined through a combination of voltage at some point on the grid, which can be regarded as (V) in figure, and the Line Impedance being offered by the grid, which is regarded as line Impedance (Z) in figure. The voltage at consumer end is highly dependent on these two factors. If we have a high Impedance transmission line and we experience a significant variation in the current due to sudden changes in production or consumption then it would result in high variation in the voltage drops occurring throughout the transmission line, eventually the value of voltage of at the consumer end would be significantly different, which is our major concern. So, for the calculation of Short Circuit Level (Ssc) we use the formula:

Moreover, to have a quantitive calculation of Strength of the grid we can calculate its capacity ratio (Rsc) in order to categorize them as weak or strong grids. Grids with Rsc around 8 to 10 are regarded as Weak Grids, while grids with Rsc around 20 to 25 are regarded as strong. Here ‘p’ regarded as active power.

Maintaining Generation Frequency

In conventional or say, fixed speed wind turbines where there is no involvement of power inverters, the frequency of the system primarily depends on the frequency of generator. Frequency control systems are being used to maintain this frequency. When the load the load varies, it slows the generator down or speeds it up depending on the type of variation. In case, the load would increase, then generator will be slowed down and required to have higher torque while in case load decreases the generator would speed up, and its torque would have to be decreased. This balance of power between the input mechanical power and output electrical power is maintained by the frequency control systems. In Variable speed wind turbines, there is a significant role of Power Electronics where Power inverters are used for the sake of frequency control.

How Power System Studies Increase System’s Stability, and ensure Power Quality?

Power System Design software enables us to Model these grids, and generation resources. A perfectly modelled system would help us simulate and analyze all kinds of scenarios that can take place during the operation of these Generation resources. Through these simulations, we can thoroughly study the response of our system, in all kinds of faults and Source/Load variations. Transient stability analysis would enable us to look for the response of each component at any given instant. We can study these responses and propose the solutions to counter these effects. Moreover, such proposed remedies, like adding capacitor banks for the sake of voltage stability or increasing generator’s torque to counter the over loading. These remedies can be further added to the system through modifying our model, and again we can analyze the results that how far are we able to counter such scenarios.

Engineers at AllumiaX perform Power System Engineering Studies in compliance with NEC, IEC and IEEE Standards. Get in touch with us here and avail our services.

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  About The Author

  Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.