1. Introduction
There is widespread acceptance that renewable generation is the future of electricity supply. Generation based on fossil fuels is not sustainable as power electricity is being consumed rapidly. On the contrary, wind power has attracted much attention as a promising renewable energy resource. It has potential benefits in curbing emissions and reducing the consumption of irreplaceable fuel reserves when the demand for power electricity has been steadily growing due to the industrial developments and the growth of the economy in most parts of the world.
Wind power generation is becoming more and more popular while the large-scale wind farm (hundreds of megawatts) is the mainstream one. During 2006, the world’s installed wind capacity reached 74 223 MW, up from 59 091 MW in 2005,which include wind energy developments in more than 70 countries around the world. The tremendous growth in 2006 shows that decision makers are starting to take seriously the benefits that wind energy development can bring.
There are no technical, economic or resource barriers to supplying 12% of the world’s electricity needs with wind power alone by 2020, and this against the challenging backdrop of a projected two thirds increase of electricity demand by that date. The report is a crucial tool in the race to cut greenhouse gas emissions as 12% electricity from a total of 1 250 GW of wind power installed by 2020 will save a cumulative 10771 million tons of CO2[1].
Large-scale wind farms connected to power systems have characteristics of high capacity, dynamic and stochastic performance, which challenges system security and reliability. While providing the clean power for power systems, wind farms will also bring about some unfavorable influence on power systems. With the expansion of wind power generation and the increase of wind power ratio in a power system, the influence will likely become the technical barriers for wind power integration. Therefore, the influence should be discussed and the countermeasures to overcome these issues should be proposed.
According to the issues mentioned above, this paper discusses in general terms the problems which are encountered by the developers of wind power generation projects and by utility grids when dealing with projects to integrate wind farms to power systems. Due to the characteristics of high-capacity, dynamic and stochastic performance of wind power generation, the influence includes active and reactive power flow, voltage, system stability,
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power quality, short-circuit capacity, system reserve, frequency and protection. After that, corresponding countermeasures to handle these problems are recommended in order to accommodate wind power generation in power systems.
2. Development situation of wind power generation
From the report of the Global Wind Energy Council (GWEC), the countries with the highest total installed capacity are Germany (20 621 MW), Spain (11 615MW), the USA (11603MW), India(6270 MW) and Denmark (3 136 MW). Thirteen countries around the world can now be counted among those with over 1000 MW of wind capacity, with France and Canada reaching this threshold in 2006. Fig.1 shows the top 10 cumulative installed capacity of the world until December, 2006[2].
Fig. 1 Top 10 cumulative installed capacity of the world until December,2006
China started to develop wind power very late. It stepped into the stage of commercialized development and scale construction only in 1990s. Accumulated and newly added installed generating capacity over the years is shown in Fig.2.The single-unit capacity increased from 100 kW, 200 kW, and 300 kW to 600 kW, 750 kW, and 1500 kW step by step.
China doubled more than its total installed capacity by installing 1 347 MW of wind energy in 2006, a 70% increase from last year’s figure. This brings China up to 2 604 MW of capacity, making it the sixth largest market world wide. the Chinese market has grown substantially in 2006, and this growth is expected to continue and speed up. According to the list of approved projects and those under construction, more than 1 500 MW will be installed in 2007. The goal for wind power in China by the end of 2010 is 5000 MW[3].
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Fig. 2 Accumulative and newly-added installed capacity of wind power in China
3. Characteristics of wind power generation
From the point of view of wind energy, the most striking characteristic of the wind resource is its variability. The stochastic variation of wind farms outputs root mainly in fluctuation of the wind speeds and directions. The wind is highly variable, both geographically and temporally. Furthermore this variability persists over a very wide range of scales, both in space and time.
Fig. 3 Wind spectrum farm Brookhaven based on work by van der Hoven
The wind speed varies continuously as a function of time and height. The time scales of wind variations are presented in Fig.3 as a wind frequency spectrum[4]. The turbulent peak is caused by gusts in the sub second to minute range. The diurnal peak depends on daily wind speed variations and the synoptic peak depends on changing weather patterns, which typically vary daily to weekly but include also seasonal cycles.
From a power system perspective, the turbulent peak may affect the power quality of wind power generation. The influence of turbulences on power quality depends very much on the turbine technology applied. Variable-speed wind turbines, for instance, may absorb short-term power variations by the immediate storage of energy in the rotating masses of
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wind turbine drive trains. That means that the power output is smoother than strongly grid-coupled turbines, fixed-speed wind turbines. Diurnal and synoptic peaks, however, may affect the long-term balancing of power system, in which wind speed forecasts plays a significant role.
Another important issue is the long-term variations of the wind resources. The wind speed up to the height of the hub should be known to calculate the wind farm output. A number of measurements of wind speeds show that wind speeds are mostly mild in a year; their probabilities between 0 and 25m/s are considerable; most of the average annual wind speeds subject to the Wei bull distribution[5], as in formula(1).
(1)
Where: v is average wind speed; k is shape parameter; c is scale parameter.
The relationship between the wind turbine output Pw and the wind speed up to the height of the hub v can be expressed approximately as the curve of wind turbine’s outputs vs. wind speed or a subsection function, as in formula (2).
(2)
Where: Pw is rated output of the wind turbine; v is wind speed up to the height of the hub; VCI is cut-in wind speed; VCO is cut-out wind speed; VR is rated wind speed.
4. Influence of wind power generation on power systems
High penetration of wind power in the power systems faces fundamental technical limits with regard to the integration of large-scale wind farms to the grid. The influence of wind power generation on power systems includes active and reactive power flow, voltage, system stability, power quality, short-circuit capacity, system reserve and infrastructure due to the characteristics of high-capacity, dynamic and stochastic performance of wind power generation. Technically, it influences the gird in the following ways and has to be studied in detail:
（1）Active and Reactive Power Flow
Wind power is a kind of intermittent and stochastic power source, which will complicate the power flow. Because many wind farms are built far away from load centers
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in order to capture more wind energy, there is always some obstacle of transmitting wind power. Some transmission or distribution lines and other electrical equipments may be over-loaded when the additional wind power generation is introduced. So it should be ensured that the interconnecting transmission or distribution lines will not be over-loaded. Both active and reactive power requirements should be investigated. Reactive power should be generated not only at PCC, but also throughout the network, and should be compensated locally[6].
The methods utilized for analysis of conventional generators are certain and ignore the uncertainty of wind speed and load forecasts. Therefore, the probabilistic method is more suitable for wind power generation. This model is based on the wind speed distribution, such as formula (1). The constraints are described by probabilistic forms and the expected values of parameters, such as voltages and powers can be computed.
（2）Voltage Regulation
Once a wind farm has identified its site, the point at which connection to the grid must be identified. Small wind farm can connect at lower voltage, thereby saving on switchgear, cable and transformer costs. If the size of the proposed development is too large to be connected at the local distribution voltage, access to the transmission network at a higher voltage is required[7].
After failures, if the transient unstability does not occur in power systems, some wind turbines shut down due to their low voltage protections. Then outputs of wind farms decrease, which means that the power system lose reactive loads. Therefore the voltage levels climb up, even beyond the upper limits of wind farms buses.
Capacitors are the common reactive power compensation methods. When voltage levels dropdown, the amount of compensation decreases much. However the reactive power demands increase when the asynchronous machines are utilized in wind farms. So voltage levels drop down more, even beyond the lower limits of wind farms buses.
With the increase of wind power installed capacity in power systems, the variability of wind power generation causes variability of voltage level, particularly if integrated into the grid which might not be specifically designed to cater for the significant and possibly rapid load variations (compared with normal customer load variation) caused by highly variable wind power generation. Therefore, the regulatory measures are needed to maintain the
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