voltage level in a specified range. However, the variability of wind power generation is not a low probability; this could result in an increased requirement for reactive power ancillary services to manage voltage control[8].
（3）System Stability
In the power system with high wind power penetration, the transient stability, voltage stability and frequency stability are all influenced by the wind power integration not only because the injection of wind power will change the power flow distribution, transferred power of each transmission line and total inertia of the whole power system, but also because the wind turbine generators perform differently in either steady-state or transient-state compared with the conventional synchronous machine[9].
For current operation of wind farms, protections usually cut off the connections between wind farms and the grid when great disturbances occur. This is equivalent to arouse new generators tripping disturbance after the great disturbances. So the transient stability in such moment is very crucial, especially when large-scale wind farms are integrated. Compared the variable-speed wind turbine based on the doubly-fed induction generator (DFIG) with the fixed-speed wind turbine based on the induction motor, the former is more robust after short-circuit failures and can strengthen system stability with keeping enough stability margin. However, wind power integration may also make the system transient stability worse due to the grid structure. Therefore, transient stabilities of different power systems should be analyzed respectively.
The fixed-speed wind turbine absorbs the reactive power when outputting the active power. The whole demand of a wind farm for the reactive power is considerable, which lead to the decrease of the voltage stability in the area near PCC. On the contrary, the variable-speed wind turbine based on DFIG has certain ability to control the reactive power. According to different operation and control schemes, this wind turbine can absorb or output the reactive power to control the voltage, which benefits the voltage stability. The voltage stability is also related with the short-circuit capacity of PCC, transmission line ratios of R/X and reactive compensation methods utilized of wind farms.
（4）Power Quality
Fluctuations in the wind power and the associated power transport (AC or DC), have direct consequences to the power supply quality. As a result, large voltage fluctuations may
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result in voltage variations outside the regulation limits, as well as violations on flicker and other power quality standards. During the continuous operation and switching operation, wind turbine causes voltage fluctuation and flicker, which are main concerns of unfavorable influence of wind power generation on power quality of the grid. For wind turbine of variable-speed and constant-frequency, harmonic issue caused by converters should also be considered.
Tab. 1 Crid interferences caused by wind turbines and wind farms
The grid interferences of wind turbines or wind farms have different causes, which are mostly turbine-specific. The relevant parameters are listed in Tab.1[10]. Average power production, turbulence intensity and wind shear refer to causes that are determined by meteorological and geographical conditions. All the other causes are attributed not only by the characteristics of the electrical components, such as generators, transformers and so on, but also by the aerodynamic and mechanical behavior of the rotor and drive train. The turbine type (i.e. variable versus fixed speed stall versus pitch-regulated) is of major importance to the power quality characteristics of wind turbines and wind farms.
Flicker is caused by the fluctuation of active and/or reactive power of wind turbines. The main reason for flicker in fixed-speed wind turbines is the wake of the tower while for variable-speed wind turbines, fast power fluctuations are smoothed and the wake of the tower does not affect power output. Therefore, the flicker of variable-speed wind turbines is in general lower than the flicker of fixed-speed wind turbines. In wind farms, power fluctuations are smoothed because of the fact wind turbines are correlated.
（5）Short-Circuit Capacity.
The majority of the wind power farms tend to be constructed remote from the load
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center, which means that electrical distance between them and the other part of the power system is rather long. There is a common sense that long electrical distance makes voltage variation bigger but short circuit problem smaller[11].
However, wind power farms will be able to give more and more significant effects on the calculation of short circuit current in future power system operation. The reason is twofold. One is the above stated fact that wind power generation site is usually apart from the conventional electrical power center. It implies that the distribution of short circuit current might make a drastic change, leading to a completely different short circuit capacity map. The other reason is the fact that more and more wind power generation today are particularly in the form of so called large-scale wind farms (hundreds of megawatts). In wind farms a substantial number of individual units are connected together, and the total generation capacity will greatly rise up.
Wind farms have great influence on the short circuit capacities of adjacent nodes while the nodes far apart from PCC are little influenced[9]. Therefore, when wind farms with huge capacities are integrated into the grid, capacities of adjacent transformers and switches may need to be increased. It should be studied further how to determine the influence of wind power generation on the short circuit current ratings of existing electrical equipments on the network.
（6）System Reserve.
Meanwhile, system load forecast will potentially become less accurate as the amount of wind power generation increases, which in turn influence power system operational scheme and unit commitment. In the case of generation reserve forecasts, this could translate into higher reserve level requirements to cover uncertainties in the availability of wind power generation[8].
（7）Frequency Regulation.
In order to control power system frequency within defined standards, grid corporations require some power plants to provide frequency control ancillary services. However, as the total amount of wind power generation increases, variation in its output will have a more significant impact on the frequency[8].
（8）Protection.
As for any proposed expansion of a utility grid, there are problems to be addressed
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with each application for connection. For connection to the distribution system, the proposed generation must have a relatively low capacity. In general, that part of the network is radial and the protection would be graded to expect fault current to flow outwards from the hulk supply point. Introduction of a generator into the radial network means that fault current can now be supplied which does not flow in the expected direction. A detailed check of the protection settings with the proposed generator in the network model is therefore required. The results of this check may show that the protection will function quite adequately as it is. At the other extreme, it may show that the existing relays cannot protect the network while the new generation and a redesign of the protection is necessary[7].
The power flows between wind farms and the grid are bidirectional, which should be considered during the protections design and configuration. Whatever kind of generators are adopted in wind turbines, integration of a wind farm will increase the fault level of grid and furthermore affect the relay settings of original protection devices of grid. It is probably necessary to add new protection devices and/or modify the relay settings of original protection devices. Particularly if wind farms are connected into distribution networks, overloading of circuit breakers may occur with the increase of installed capacity of wind farm[8].
5. Countermeasures to mitigate wind power generation influence
The applications of the reactive compensation equipments, such as static var compensator (SVC) and static synchronous compensator (STATCOM) play an important role in wind power generation to mitigate its influence on the power system. In order to maintain the voltage level, the grid corporation may provide additional or upgraded voltage control facilities. Reactive compensation equipments should be installed in the step-up substation of wind farm, which has a fast response characteristic and can be regulated continuously, such as SVC and STATCOM, etc. In order to mitigate voltage fluctuation and flicker caused by wind power generation, both speed control and pitch angle control should be improved so as to minimize the fluctuation of wind turbine output while maximizing the output of wind turbine. Meanwhile, installation of auxiliary devices on wind farm such as SVC and energy storage device can also mitigate both voltage fluctuation and flicker. In most cases, fast-acting reactive-power compensation equipment, including SVC and STATCOM, should be included for improving the transient stability of the network.
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From the wind power generation side, it can improve the voltage stability of the power system and increase wind power penetration by the constant power factor control or constant voltage control. From the grid side, it is of significance to reinforce and change the current network. Voltage source converter (VSC) based HVDC (VSC-HVDC) is a transmission system that does not require any additional compensation, as this is inherent in the control of the converters[13]. It will therefore be an excellent tool for bringing wind power into a network, even at weak points in a network and without having to improve the short-circuit ratio. The active power control capability of VSC-HVDC could then be a perfect tool for handling active power/frequency control. It is capable of handling wind power and of reacting rapidly enough to counteract voltage variations in an excellent way, which can improve the system stability and power quality.
6. Conclusion
Wind energy has come a long way since the prototypes of just 25 years ago, and it will probably continue to advance over the next twenty years. There are a number of issues associated with integration of wind power in system operation and development. Although penetration of wind power generation may displace significant amount of energy produced by conventional plants, concerns are focused on the interaction between wind power generation and the grid. This paper has provided an overview of the influence of wind power generation on power systems and recommended corresponding countermeasures to handle these issues in order to accommodate wind power generation in power systems.
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