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In many countries of the world wind power expands, and covers a steadily increasing part of these countries' power demand. From an environmental point of view this is a favourable development, but there are some technical problems that need to be addressed. Growing wind power has impacts on the power systems into which the wind turbines feed their power. Increasing wind power penetration in a power system means, that wind turbines substitute the conventional power plants that traditionally control and stabilise the power system.
Conventional power plants comprise synchronous generators, which are driven by steam, gas or running water turbines. Their characteristics and their controllability, of both the generator and the prime movers, are well understood and facilitated to their full potential. With such conventional power plants the voltage and the frequency of large interconnected AC power systems, can be controlled and held stable, in steady state, as well as in transient operating conditions. A control task of prime importance is the re-establishing of stable grid conditions after a transient fault, i.e. recovering the grid voltage and damping power system oscillations caused by the fault. Power plants with synchronous generators and conventional prime movers are very suitable for this task.
As wind power penetration in power systems increases, and hence more and more conventional power plants are replaced, the respective power system operators are concerned about the stability and reliability of their power systems. Therefore, more and more power system operators revise their grid connection requirements, and issue grid connection requirements specifically made for wind turbines and wind farms. Until recently, wind turbines were treated by and large as embedded generators, which were not to contribute to power system control. Hence wind turbines were required not to actively attempt to control voltage or frequency. In addition, wind turbines were required to disconnect from the grid when abnormal operating conditions occurred. If, however, wind power substitutes conventional power plants, it also has to take over the power system control and stabilisation tasks, which the substituted conventional power plants were carrying out.
One of these control tasks is to ride through transient disturbances in power systems. This means that generation must not be lost due to temporary excursions in voltage or frequency.
An AC power system is a complex system, which is vulnerable to disturbances. When the voltage or the frequency deviates considerably from its rated operating point, the system might break down, unless the parts of the system, which are the cause of the problem, are disconnected. Voltage and frequency need to be monitored carefully, and controlled accurately at any time, to prevent contingencies with severe consequences.
A transient short circuit fault is a very common disturbance in a power system. It suppresses the grid voltage, and upsets the rotating machines in the vicinity of the fault, causing the speeds of these machines, and the power flows in the network to oscillate. Such sub-synchronous system oscillations have to be damped, to avoid that the system becomes unstable. Traditionally such oscillations are damped by conventional power plants with synchronous generators, which are equipped with power system stabilisers.
Hence, with increasing wind power penetration wind turbines are getting involved in the task of controlling voltage and frequency in steady state as well as transient operation. Although there is a host of aspects related to this problem, in this project the focus is limited to the following:
• Transient fault ride-through: Wind turbines have to be able to ride through transient voltage dips. When the voltage at the terminals of the wind turbine gets suppressed, by e.g. a transient short-circuit, the turbine must not disconnect, and must resume operation as soon as the voltage has recovered. The following aspects are considered:
o Voltage recovery at the wind turbine terminals
o Reactive power demand of the wind turbine
o Active power behaviour of the wind turbine after the fault is cleared
o Prevention of excessive speed excursions of the wind turbine generator
o Damping of torsional drive train oscillations caused by the fault.
• Power system stabilisation: Wind turbines have to be able to dampen power system oscillations, which are caused by e.g. transient faults or switching
events. Power system oscillations caused by a transient fault in the vicinity of a wind turbine is the worst case scenario, as the wind turbine has to ride through the fault, before it can contribute to power system stabilisation. In this project the following aspects are considered:
o Transition from transient fault ride-through to power system stabilisation operation
o Controlled oscillating active power production to counteract power system oscillations
o Optimal control of the relatively slow pitch system
o Damping of drive train oscillations caused by oscillating power extraction
o Evaluation of the impact of wind turbine control on the power system oscillations.
The wind turbine types considered are active-stall turbines and variable speed, variable pitch turbines. The variable speed, variable pitch turbine is one with a conventional drive train, i.e. a gearbox, and a full-scale converter-connected synchronous generator. Although the majority of variable speed turbines operate with doubly fed induction generators, a full-scale converter-connected synchronous generator turbine with gearbox is chosen, as this type currently gains relevance on the market, and as much research has been carried out on doubly fed induction generator turbines already [1,2,3].
The project is based on simulations in the power system simulation tool PowerFactory from DIgSILENT. Different wind turbine controllers and control strategies are developed to enable the two turbine types to ride through transient faults, and to perform power system stabilisation. The controllers developed are of different types, depending on what best suits the control problem at hand. A realistic power system model allows simulating different transient fault situations, and assessing the mutual impacts between the power system and the wind turbines.
2 State of the Art
In this chapter the basis for the project is established. First it is investigated what power system operators require from wind turbines in order to connect to their grids. These requirements are documented in grid connection requirements (GCR), which are issued by the power system operators. The GCR that are already in force constitute a state of the art of the current wind turbine technology. All wind turbines that are to connect to a power system have to comply with the GCR of the respective power system operator.
Further a literature study investigates what other research work has been carried out on this topic. Finally, from the GCR and the literature studied, it is concluded what work has been done in this project.
2.1 Grid Connection Requirements
In the past there was usually no wind power connected to the power system, or the level of wind power penetration was extremely low compared to the power production from conventional power plants. Therefore, GCR for wind turbines or wind farms were originally not necessary. As wind power started to be developed more actively at the end of the 1980s, network companies that faced increasing numbers of wind turbines in their systems elaborated their own connection rules. During the 1990s, those connection rules were harmonized on national levels, e.g. in Germany and Denmark. This harmonization process often involved national network associations, as well as national wind energy associations, which represented the interests of wind turbine manufacturers, as well as wind farm developers and owners.
While other authors, like e.g. Santjer and Klosse  have analysed single GCR, here a comparison of different relevant GCR is conducted.
Since GCR are subject to frequent revision, it is difficult to make a comparison that is always up to date. Hence in this thesis the state of GCR in force, or published as proposals, in the beginning of 2004, i.e. the time when this study was carried out, is considered. Changes that happened in the meantime are not taken into account. It has
to be noted that it is not the intention of this comparison to present a status of current GCR. It has to be acknowledged that the GCR discussed in this thesis are outdated at the time this thesis is written (spring 2006). The comparison presented in this thesis is relevant nonetheless, as it provided the basis for this project.
In this thesis GCR of several countries which are proactively meeting the challenge of considerable wind power penetration are analysed. The countries considered are Denmark [5,6], Germany , Ireland , Sweden  and Scotland . For the sake of comprehensibility the selection of countries is not complete. Equally, the selection of power system operators in the respective countries is not complete either.
As discussed above, this thesis deals with transient fault ride-through and power system stabilisation. Therefore only the relevant parts of the GCR are taken into account here. Requirements that deal with unrelated aspects like power quality, modelling and verification, communication etc. are beyond the scope of this work.
The following discussion shall not reproduce specific numbers from the individual GCR, but intends to give a general idea of the content and background of GCR. More details are given in Publication 1.
2.1.2 Active Power Control
From a power system operator's point of view, the ability to control active power is important for two reasons: during normal operation to avoid frequency excursions; and during transient fault situations to obtain transient and voltage stability. The analysed GCR do generally not distinguish between active power control for normal operation and for transient fault operation.
Power control is especially important for transient stability and voltage stability in case of faults. If the power can be reduced efficiently as soon as a fault occurs, the turbine can be prevented from going into overspeed . Considering turbines with directly grid-connected induction generators, the reactive power demand is less after the fault is cleared, if the power can be reduced effectively, which helps re- establishing the grid voltage . Another concern, from the viewpoint of the power system operators, is the rate at which power is ramped up after a fault is cleared. The requirement for ramp rates is made to avoid power surges on the one hand, and to avoid that generation is missing, because generators ramp up too slowly on the other
hand. Both cases would mean power imbalance, which could lead to instability, even though the initial fault has been cleared.
Power control is required in all considered GCR. The requirements vary greatly and depend, among other factors, mainly on the short-circuit power of the system considered. The lower the short-circuit power, the more demanding is the power control necessary for keeping the system stable during and after a fault.
2.1.3 Frequency Operating Range
Wind turbines need to tolerate frequency deviations during steady state operation. Some GCR even require the participation in primary and secondary frequency control. More interesting in this project, are the frequency requirements related to transient events. A transient fault in an interconnected power system can lead to oscillations in the system frequency. It is desirable that the frequency tolerance of wind turbines is as wide as possible; to avoid that under such post-fault conditions the situation gets worse, because wind turbines disconnect and hence generation is lost. Extensive frequency operating ranges, however, have effects on the operation of wind turbines. The speed of fixed speed wind turbines depends directly on the grid frequency. The aerodynamic properties of wind turbine blades is non-linearly dependent on the tip speed ratio, and hence on the speed of the turbine . The operation of variable speed turbines with doubly fed induction generators, is to a large extend independent of the grid frequency, while the operation of variable speed turbines with full-scale converters is fully independent of the grid frequency.
2.1.4 Voltage Control and Reactive Power Compensation
Utility and customer equipment is designed to operate at a certain voltage rating. Voltage regulators and control of reactive power at the generator and consumer connection points are used in order to keep the voltage within the required limits, and to avoid voltage stability problems. In some GCR also wind turbines are required to contribute to voltage control. In Figure 1 the reactive power requirements are compared in terms of power factor. Note that 'lagging' refers to production of reactive power, and 'leading' to absorption of reactive power. In Figure 1, only the operating limits are considered, i.e. it is not taken into account under which voltage conditions the respective amount of reactive power is demanded. In Figure 1 the requirements for
the Danish transmission system  and the Swedish system  are not shown, as they were relatively slack. They only demanded a neutral power factor (power factor = 1) over the whole range of active power.
The main reason for reactive power requirements is that generators can actively control the voltage at their terminals, by controlling the reactive power exchange with the grid. Especially during transient faults the voltage has to be supported, since the reactive power demand of induction generators increases when the voltage drops [14,15]. Generators with voltage source inverters can support the system voltage at their terminals by exporting reactive power . By doing so they boost their active power export during the fault, and hence mitigate the problem of acceleration.