[ieee renewable energy conference (inrec) - amman, jordan (2010.03.21-2010.03.24)] 2010 1st...

Proceedings of the 1st International Nuclear and Renewable Energy Conference (INREC10), Amman, Jordan, March 21-24, 2010

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A SURVEY ON THE EFFECT OF DIFFERENT KINDS OF WIND TURBINES ON POWER

SYSTEM STABILITY

Maryam Bahramipanah

University of Tehran Department of computer and

Electrical Engineering Tehran, Iran

[email protected]

Saeed Afsharnia

University of Tehran Department of computer

and Electrical Engineering Tehran, Iran

[email protected]

Zagros Shahooei

Sharif University of Technology Department of Electrical

Engineering Tehran, Iran

[email protected]

ABSTRACT

Due to deterioration of fossil fuel and policy directives on greenhouse gas mitigation, wind power generation has been widely developed in recent years. This has raised many issues in terms of power system operation, dynamics, stability and etc. This paper reviews the effect of different kinds of wind turbines on power system stability. Since there are no existing surveys in this area, it provides a comprehensive overview in transient and dynamic stability. The contribution of this survey is to provide a comparison to determine which kinds of wind turbines is better from the system stability point of view. Furthermore, the effects of wind power generation with energy storage systems are reviewed.

1. INTRODUCTION

Wind power generation systems are recently getting lot of attention because they are most cost competitive, environmental clean and safe renewable power source. In recent years, the desires for electricity production from renewable energy, especially wind energy in all countries around the world have increased. With increasing penetration of wind power generation in power systems, the overall system behavior like system stability is affected. So it is necessary to study the effects of wind power generation on system stability.

This paper is going to review all the studies about evaluating wind farm effects on power system stability. In Section 2 the definition of the power system stability is clarified and its classifications will be gone over. In Section 3 a short introduction of all kinds of wind turbines and the differences among them will be presented. Afterwards the effect of different kinds of wind turbines in wind power generation on power system stability will be stated in next section. Finally in the last section, the effect of energy storage system with wind power generation on voltage stability in power system will be reviewed.

2. POWER SYSTEM STABILITY

The effect of wind power generation on system stability has gained more importance with increasing penetration of wind power generation in power system. Wind power generation can affect the power system stability in two ways: First it is because of the uncertainty of wind energy nature. The next issue is wind turbines instability due to a disturbance on power grids that leads to power system instability. It’s necessary to analyze system stability in case of connecting a large wind farm to the grid. According to [1] and [2]: “Power system stability is the ability of an electric power system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variable bounded so that practically the entire system remains intact”. The most important aspects of system stability can be stated as rotor angle stability, frequency stability and voltage stability. Figure 1 presents the classification of power system stability according to IEEE and CIGRE [2].

Figure 1. Classification of power system stability [2]

In previous years due to the low wind power penetration, rotor angle stability had no significant importance. But nowadays, wind energy penetration level has increased and as a result rotor angle stability or transient stability should be taken to consideration.

Power System Stability

Frequency Stability

Small-Disturbance Angle Stability

Long Term

Voltage Stability Rotor Angle Stability

Transient stabilityLarge-Disturbance Voltage Stability

Small-Disturbance Voltage Stability

Short Term Long Term Short Term Short Term


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(a) (b) (c)

Figure 2. Wind turbine generator (WTG) types [3]

3. WIND TURBINES

Almost all wind turbines installed up to now use either one of the following systems: (a) fixed speed induction generator (squirrel cage induction generator); (b) doubly fed induction generator; (c) direct drive synchronous generator. Figure 2 shows the diagram of all these kinds. As figure 2 depicts, the difference of wind turbines is due to their power generating systems. Direct drive synchronous generator is rarely used in wind farms and as a result most of papers consider fixed speed induction generator (type a) and doubly fed induction generator (type b). The main aspects that characterize these types of induction generators (types (a) and (b)) can be summarized as follows.

3.1. Fixed Speed Induction Generators (FSIG)

According to [4], FSIG consists of squirrel cage induction generator that is coupled through a gearbox to the wind turbine rotor. This type of wind turbine is simple and cheap. But it has some disadvantages such as: (i)lack of control possibilities of both active and reactive power, (ii)gearbox breakdown due to large mechanical loads; that is arisen because of power fluctuations are converted to torque pulsations; (iii)large fluctuations in output power. Due to these reasons, wind turbine manufacturers are increasingly interested in variable speed devices.

3.2. Doubly Fed Induction Generators (DFIG)

By definition in [4] doubly fed wind turbines use a wound rotor induction generator, where both the rotor and stator are fed. The advantages of DFIG can be stated as: (i)its controllability of both active and reactive power; (ii)large rotor inertia smoothes the variations of wind speed and as a result it has fewer fluctuations in output power; (iii)and the most important advantage is its ability to get ride through fault by its uninterruptable operation. It has reported in [5], [6], [7] and [8] that how DFIGs connect to grid. With selecting a good control, it has uninterruptable operation and can successfully ride through grid faults. The uninterruptable operation can be achieved by properly arranging the operation and control of the converters and using dynamic reactive compensation. Several types of DFIG controls that result in uninterrupted operation are discussed in [9]-[12]. In [9], a simple power factor control is used to control DFIG and by using crow bar circuit, DFIG can get ride through faults. [10] uses

fuzzy control to enhance robustness of the system. In [11] a stage wise control strategy for grid connection is proposed that lead to system stability improvement. Ref. [12] enhances system stability by use of a new active power control method to adjust magnitude and phase of stator output.

4. EFFECT OF DIFFERENT KINDS OF WIND TURBINES ON POWER SYSTEM STABILITIES

4.1. Frequency Stability

Frequency stability considers the deviation of the frequency in the system after disturbance. Refs. [13] – [16] talk about frequency stability of wind power system. In [13], a control loop is used to reduce frequency deviation and shows two different control strategies. [14] analyzes the frequency dynamics of a system wind farm equipped with DFIG using an Eigen value approach. Ref. [15] probes the influence of fixed speed wind turbine generators on power system fluctuation. A comparison between fixed speed wind turbine generator and the synchronous generator connected to an infinite bus is shown in this reference. It is concluded that fixed speed wind turbine generator has a strong characteristic against power system oscillation. Transmission line between the generator and infinite bus play a significant role in damping ratio of the critical mode. In addition, the influence of load increase on power system oscillation is studied. Load increase in a system with wind farm shows less oscillatory manners than a system without wind generation. Moreover in different configuration like single turbine, wind farm, and spread wind turbines, the effect of increased wind power penetration has been studied. Installing SVC to a system with fixed speed wind turbine generators makes it more stable. So it should be better to add an additional reactive power compensation device. Ref. [16] inspects the effect of wind power on frequency oscillation of the Nordic power system. It concludes that, wind turbines equipped with sufficient control mechanisms can actively take part to damp these oscillations.

4.2. Voltage Stability

The possibility of the system to maintain its voltage within given limits after being subjected to disturbance is named voltage stability. Reference [17]-[30] discuss the effect of wind farms on power system voltage stability. Ref. [17] evaluates the effect of FSIG with pitch regulator on power system voltage stability. A probabilistic voltage stability algorithm of wind turbine generators is developed to detect voltage stability. This result is

Is Vs

Compensating Capacitors

Grid Squirrel Cage

Induction Generator

Gear Box

Rotor Rotor

Ic

Vr

IsVs

Converter

Grid

Gear Box

Doubly Fed Induction Generator

Rotor

Vc

Ic

Vs

Is

Grid Converter

Direct Drive Synchronous

Generator


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used to determine the amount of wind farm capacity that could be used to exactly replace the conventional sources from the voltage stability point of view. In ref. [3], a detailed analysis about fixed speed wind turbines influence on system voltage stability is presented. Furthermore the impact of different penetration levels of wind power on system stability is discussed to determine the acceptable wind power penetration in power system without deteriorating system stability. In addition the effect of SVC on power system stability with wind farm is studied and it is shown that voltage stability is enhanced with the SVC installed. Ref. [18] uses the P-V curves to point out the impact of different contingencies on voltage stability of the system with doubly-fed wind turbines. It is concluded that system voltage stability will be improved by doubly-fed induction generators. Some papers like [19] probe the impact of mixed type of wind turbine generator in the same farm, on power system voltage stability. Ref. [20] makes a comparison between the modeling and control strategies of FSIG and DFIG. According to [21], voltage stability can be divided into small disturbance stability that talks about the problem of flicker emission, and large disturbance stability that highlights the ability of ride through faults.

4.2.1. Small Disturbance Voltage Stability

Uncertainty of wind nature causes flicker emission. In [22]-[27] its influence on power system has been emphasized. In [22] a method in frequency domain is developed to carry out a fast flicker emission analysis of fixed speed wind generator. In [23], simulations and experimental verification of the dynamic responses of a DFIG to voltage sags is presented. Research in [24] studies the performance of doubly-fed wind turbines during grid disturbances and their effects on flicker emission. [25] compares the effect of DFIG and FSIG on this kind of stability. [26] evaluates flicker emission from a wind farm to a grid with high wind power penetration.

4.2.2. Large Disturbance Voltage Stability

Research in [27] describes the results of the investigations on large disturbance voltage stability, carried out on Danish power network. [28] analyze system voltage stability with considering the mechanical torque fluctuation. Furthermore [29], [30] studies the dynamic voltage manners of a DFIG under grid disturbance. These results can be concluded from voltage stability analyses: - FSIG can improve power system voltage stability slightly. - Doubly fed wind generators enhance power system voltage

stability more than FSIG; this is due to DFIG’s ability to stay connected during fault and its reactive power compensation.

- The higher level of generation produce a condition that is less stable because of the proximity of the operating point to the voltage stability limit.

4.3. Rotor Angle Stability (Transient Stability)

With wind farm development and high penetration in recent years, rotor angle stability is going to be more important than

before. This kind of stability is recognized with transient stability. Ref. [31]-[43] peruse this kind of stability. [31] studies system transient stability with considering simple network structure. [32] does the same analyze with considering one wind generator technology and compares the impact of adding a conventional generator and a variable-speed wind power plant to a weak power grid on transient stability and investigates the effects of a variety of wind plant load factors. It selects an area of the Western electricity power system that is electrically far from major generation centers and is weakly connected to the bulk transmission system. The results support the conclusions that wind power plants, equipped with power electronics can be interconnected to weak power grids without reducing stability. [33] analyses specific scenarios in terms of the rotor angle stability of a network. Ref. [3] has evaluated transient stability in power system with fixed speed wind turbines and the effect of SVC on system stability. The satisfactory penetration level of wind power without deteriorating stability is determined and it is shown that transient stability of the system will improve with wind generation and installing SVC leads to more enhancements. Ref. [34] is focusing on transient stability issues and analyzes the impact of generator technology, and connection points separately for getting a thorough understanding about the impact of these aspects on transient stability. Different scenarios have been set-up to analyze the impact of each of the above mentioned aspects on transient stability individually leading to the following conclusions: The location of wind generators can have a large impact on transient stability especially when high wind penetration is located in a particular area. The generator technology has also a considerable impact on transient stability. Ref. [35] looks at transient stability using a general network model and evaluates different generator technologies. This reference probes on differences between places that wind power plants can connect to the network. It is shown that due to low impedance of HV buses, with installing wind farm in the HV system, reactive power absorption is going to reduce; and this leads to power system stability increasing. Furthermore this reference talks about significant differences between two wind generator types (FSIG & DFIG). Ref. [36], [37] do the same analysis too. Ref. [36] studies the different impacts of constant and variable speed wind turbines on New England power system and ref. [37] analyzes transient stability on Southern Italy power system with both constant and variable speed wind turbines. These studies point out that an improvement in transient behavior can be observed when FSIGs are replaced by the DFIG. The contribution of ref. [37] is analyzing the transient stability of a power system where a remarkable number of wind turbine generators are present. Most of researches consider the simple one mass lumped shaft model in wind generators for stability analysis. In ref. [38], a detailed transient analysis has been done with the consideration of two mass and three mass shaft models of wind turbine generators. It is concluded that two mass shaft model is sufficient for transient stability analysis. Ref. [39] shows the effect of wind generation on Danish power system. In ref. [40] transient stability analysis on the New Zealand power system is presented. A comparison between the transient performances of power system with and without wind power generation as a considerable part of the system is shown in this paper. Furthermore the effect of wind generation location is studied and the results imply that any


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stability issue in any part of the power system should be identified during the planning stage of the new wind farm development. Immediately disconnecting of wind farms at the fault time in a power system with high wind penetration makes a big problem in power network. [41] presents a novel strategy for ride-through the transient fault. Ref. [42] shows that by limiting the current of DFIG rotor, it can be stay connected during faults. The ability of the DFIG in responding to the fault enables the wind power plant to return to a steady state much faster than the conventional synchronous generators. [18] uses a critical clearing time as a measure of transient stability. It is shown that whatsoever the operating point is close to the voltage stability limit at pre-fault situations, the system is going to be less stable; moreover it has done Time-domain simulations to show the effect of fault location on transient studies. It can be concluded that when a fault occurs near a wind power generator, it doesn’t mean it make the system more unstable than a condition when a fault occur further. For all the fault locations, stability calculation should take consideration individually. Transient stability of power system with wind generations is presented in ref. [43] utilizing pitch controller to preventing the windmill from excess speed increase. It is obtained that the pitch controller has a significant role on the transient stability enhancement. In ref. [16] the effect of wind power on the transient behavior of the Nordic power system during a fault is investigated. The result of all these studies shows that for knowing the impact of wind generation on transient stability margins, it is necessary to be acquainted with system properties, location of wind resources and generator technologies. The problem has to be analyzed individually for each case. It is concluded that doubly fed machine is more robust than fixed speed cage generators in the event of critical faults, furthermore using the DFIG leads to reduction in reactive compensation demands, which helps to avoid other problems as voltage collapse in the power system. The results clearly show that the DFIG improves the stability of the conventional generators in the network. Another important result is about the level of wind penetration. With high wind penetration level without considering any additional power reactive compensator, system stability is degraded. So it is necessary to determine the acceptable level for wind generation without deteriorating power system stability.

5. VOLTAGE STABILITY ANALYSIS IN POWER SYSTEM WITH WIND FARM AND ENERGY STORAGE

SYSTEM

Wind power plants possess two major adverse characteristics: (i) the high demand for reactive power and (ii) the variability in the output power. Many investigations have dealt with the problem of reactive power compensation for wind farms [44-49]. Meanwhile, the problem of wind power variability and intermittency remains nearly unsolved, and therefore requires special attention. A proper way to minimizing the effects of wind farm output variation on voltage stability is utilizing energy storage system (ESS); consequently the intermittency of wind power generation can be controlled. Energy storage system store certain amount of electrical energy, generated from wind power and afterwards it

re-dispatch the power appropriately. Some of the identified ESS applications are postponement of transmission and distribution network expansion, improving stability and power quality, reliability enhancement. In [50], a study is made to reduce wind farm output variation and the adverse stability-related impact with ESS. In this paper the ESS-based application settings are developed to improve power system stability and furthermore it analyzes the ESS effects on voltage stability and load demand mismatch reduction. [51] analyzes voltage stability in the system with wind power generation and ESS; it uses two main criteria: (i) reactive reserve margin, and (ii) smallest Eigen values. The most important issue for using ESS with wind power generations is choosing the correct reference point and that is the point that the ESS starts energy saving. Control system utilizes this point to determine the power direction of ESS; nevertheless it is not straightforward to set this reference point. In [52], [53] the wind farm output power of 0.9 p.u. is the control objective. In [54], the real power is chosen as the reference point and [51] uses a grid-based approach. It selects the reference point based on the overall voltage stability status of the power network. Furthermore the impact of ESS as a backup of the large wind farms on the power system stability is investigated in [55]. The results of these studies show that using ESS with wind farm leads to improve voltage stability of system. The goal to be achieved is to integrate maximum possible amount of wind power into the power system without deteriorating voltage stability. In power systems with large wind power penetration, where the disconnection of a wind generation can lead to system instability, the ESS can play significant role. It can be used as primary and secondary control of large wind farms and it can supply the system with large amount of power in a very time.

6. CONCLUSIONS

Utilizing wind power generation in a power system causes several issues and one of these issues is its effect on power system stability. This paper gives a survey on effect of different kinds of wind turbines on power system stability. From all those studies, the following conclusions can be drawn: 1- Fixed speed wind turbines in a power system can improve

both voltage and transient stability, but in comparison with doubly-fed wind turbines, it is poor. So majority of wind farms is equipped with DFIG because of its ability to rid through faults and its behavior as a power reactive compensator during faults.

2- The nearer of operating point to the voltage stability limit, the less stable of power system will be observed. the stability at low wind penetration levels is less affected. System stability is noticeably degraded at high penetration levels due to the high reactive power absorption of wind generators under transient disturbances. So high penetration levels not only do not improve power system stability but also in these cases stability decrease in comparison with the case which no wind farm exists.

3- Installing FACTS devices with wind farm as the reactive power compensator would lead to better performance of the system in power system stability point of view. This paper


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doesn’t focus on the effect of utilizing FACTS devices with wind farms on power system stability.

4- Utilizing energy storage system with wind farm can lead to power system stability improvement and injecting maximum possible amount of wind power into the system. On the other word ESS moderates the intermittency of wind power so it makes the system more stable.

Large wind turbines can compete with conventional generation to improve power system performance in several aspects like stability. It is obvious that there would be a bright outlook in the wind power in power systems.

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