In recent years, with the increasing scarcity of global oil resources and the nuclear power warning brought about by the Japanese earthquake, accelerating the development of safe and clean energy industries, including wind power, has become an inevitable trend. Large-scale wind power generation requires grid connection. Although major wind power countries have accumulated some experience in wind power generation and grid operation, due to the special characteristics of my country's power grid structure, how to coordinate the development of wind power generation and grid operation has become the most important and unavoidable issue in the planning, design, and operation of wind farms.
I. The Impact of Wind Power Generation on Power Grid Operation in my country
In areas of my country rich in wind resources, the power grid is relatively weak. The impact of wind power generation on power grid operation is mainly reflected in power grid dispatch, power quality, and power grid security and stability.
1.1 Impact on power grid dispatching
Regions rich in wind energy resources are characterized by sparse populations, low loads, and weak power grid structures. The input of wind power inevitably alters the power flow distribution of the grid, significantly impacting the node voltages of local power grids. Wind energy itself is an uncontrollable energy source; its generation status and output depend primarily on wind speed. The instability and intermittency of wind speed determine the significant fluctuations and intermittent nature of wind turbine power generation. Grid-connected wind farms act as random disturbance sources to the power grid, exhibiting counter-regulation characteristics. This necessitates the grid side to reserve more backup power and peak-shaving capacity. The instability of wind power generation increases the difficulty of wind power dispatch.
1.2 Impact on Power Quality
The fluctuation in the output power of wind turbines makes them susceptible to turbulence, wake effects, and tower shadow effects during operation, resulting in voltage deviations, fluctuations, flicker, harmonics, and periodic voltage pulsations. Voltage fluctuations and flicker, in particular, have a significant impact on power grid quality. The asynchronous motors in wind turbines lack independent excitation devices and have no voltage before grid connection. Upon grid connection, they experience an inrush current 5 to 6 times higher than their rated current, causing a substantial drop in grid voltage.
The power electronic frequency converters used extensively in variable speed wind turbines generate harmonics and interharmonics, which can cause voltage waveform distortion.
1.3 Impact on power grid security and stability
The initial design and planning of the power grid failed to consider the characteristic that connecting wind turbines to the grid end would alter the unidirectional flow of power in the distribution network, thus changing the direction and distribution of power flow. This resulted in grid voltages near wind farms exceeding safe limits, and even leading to voltage collapse. Large-scale wind power generation injecting electricity into the grid will inevitably affect its transient and frequency stability. Short-circuit currents exceeding the breaking capacity of nearby substation busbars and switches also compromise grid safety.
II. Reasons for the lack of coordination between wind power generation and power grid operation
2.1 The pace of wind power generation and power grid construction is not synchronized.
The preliminary work process for wind power generation projects is relatively simple, with a fast approval process and a relatively short construction period. However, grid connection systems are more complex in terms of project review, scheme determination, and engineering construction. While 220kV grids can be approved by individual provinces (autonomous regions), all grids above 330kV require approval from the National Energy Administration. The grid approval process is complex, requiring supporting documents from each village at the township, county, city, and provincial levels, which is time-consuming. This makes it difficult to synchronize grid connection engineering with wind farm construction, resulting in a mismatch in construction schedules. In my country, the construction period for the first unit of a wind farm is typically 6 months, and the entire wind farm can be completed in just 1 year. In contrast, grid engineering construction takes much longer, especially since transmission line construction often involves cross-regional projects, making coordination difficult. The reasonable construction period for a 220kV transmission project is approximately 1 year, and for a 750kV transmission project, it is approximately 2 years.
2.2 Insufficient peak-shaving capacity for wind power generation
From the perspective of power balance, the power grid cannot fully absorb wind power resources. A sound power supply structure and sufficient reserves are fundamental to the full utilization of wind power. Wind power is characterized by randomness, intermittency, and rapid fluctuations, requiring a certain scale of flexible and adjustable power sources to match it. Taking Inner Mongolia as an example, the power grid has a single power supply structure, with thermal power units accounting for 84% of the region's installed power generation capacity, and heating units accounting for 40% of thermal power units, representing 64% of the grid's peak power load. During the winter heating season, the peak-shaving depth of coal-fired units is around 50%, while that of gas-fired and pumped storage units can reach 100%, with heating units not participating in peak shaving at all. Although the power grid has implemented optimized dispatching, the load margin left for wind power is very small in order to meet heating demand, making it impossible for the grid to meet the requirement of full grid connection for wind power, resulting in difficulties in power balance.
III. Measures that power generation companies need to take
3.1 Reduce the impact on power grid dispatch
The variables in power flow distribution in the power grid system and the issues of voltage and frequency regulation can be mitigated through wind power forecasting. In recent years, technologies for forecasting wind farm power output have made great strides. By utilizing reliable historical data from wind farms and forecast data from nearby meteorological stations, changes in wind farm output can be predicted.
3.2 Reduce the impact on power quality
Wind farms need to be equipped with necessary reactive power compensation devices or use doubly-fed variable-speed wind turbines with reactive power control. Alternatively, soft-start devices controlled by bidirectional thyristors can be used. When the wind turbine brings the generator to near synchronous speed, the circuit breaker at the generator output closes, connecting the generator to the grid through a set of bidirectional thyristors. The conduction angle of the bidirectional thyristors is controlled by current feedback. When grid connection is complete, the bidirectional thyristors are short-circuited, achieving a smoother grid connection process and reducing the impact of wind power grid connection on power quality. Through manual intervention, wind turbines in the same area can be started at different times, further reducing the impact of starting and stopping wind turbines on power quality.
3.3 Reduce the impact on power grid security and stability
Reactive power compensation in the system is achieved through group switching of capacitor banks. However, this method cannot achieve continuous voltage regulation, and the number of capacitor switching operations is limited. Therefore, it cannot control voltage fluctuations caused by rapid wind speed changes. Static Var Compensators (SVRs) are installed at the wind farm outlet. The reactive power compensated by the SVR is controlled based on the voltage deviation at the wind farm connection point, stabilizing the wind farm's node voltage, reducing the impact of wind power fluctuations on the grid voltage, and improving the stability of the grid system. At the wind farm outlet, a dual-bridge converter device based on a GTO inverter is used. The superconducting energy storage device utilizes its comprehensive active and reactive power regulation capabilities to reduce wind farm output power fluctuations, while simultaneously achieving synchronous control of voltage and frequency, thus stabilizing the wind farm voltage.
IV. Measures that power grid companies need to take
4.1 Reduce the impact on power grid dispatching
Strengthen research on grid acceptance capacity to improve the grid's ability to absorb wind power generation. Determine the maximum installed wind power capacity that the grid can accommodate as a whole, providing a reference for wind power development planning. Establish a wind power grid connection technology evaluation agency as soon as possible to provide technical support for the rational integration of wind power. Improve grid dispatch and management measures for wind power grid connection, and optimize the power output of other wind turbine units based on wind power forecasts from wind farms.
4.2 Reduce the impact on power quality
Improve the power grid structure. A higher short-circuit ratio at the point of common coupling (PCC) of grid-connected generators results in smaller voltage fluctuations and flicker. A suitable grid line X/R ratio can compensate for voltage fluctuations caused by reactive power, thus reducing flicker. Therefore, the impact of wind power generation on power quality can be reduced by adjusting the appropriate X/R ratio. Increase the grid voltage level, upgrading from high-voltage to ultra-high-voltage transmission, constructing new transmission lines, and installing corresponding automatic control devices can further reduce the impact of wind power transmission and distribution on power quality.
4.3 Reduce the impact on power grid security and stability
This study investigates the impact of wind power generation on grid operation modes, frequency control, voltage regulation, power flow distribution, fault levels, and stability. It also strengthens simulation studies of the grid's ability to accommodate wind power. Through upgrades and modifications, the capabilities of relevant electrical equipment are improved, and the breaking capacity of switches near wind farms is increased. Necessary filters are installed to prevent excessive grid harmonics.
In summary, to achieve coordinated development between wind power generation and grid operation, power generation companies and grid companies must conduct in-depth analysis of the current technological bottlenecks and policy reasons for wind power development, grasp the technological characteristics and policy orientation of wind power development, and strive to solve the impact of wind power generation on grid dispatch, power quality, and grid security and stability, so as to maximize the interests of both wind power generation companies and grid companies.
