01. What is a wind power grid connection?
There are two different types of wind power:
1. Independent operation – off-grid type: Off-grid wind power generation is small in scale, and it is combined with energy storage devices such as batteries or combined with other energy generation technologies (such as wind power/hydropower complementary system, wind power-diesel unit combined power supply system) can solve the problem of power supply in remote areas. Grid-connected wind power generation is a large-scale wind farm with a capacity of several megawatts to hundreds of megawatts, consisting of dozens or even hundreds of wind turbines.
2. Connected to the power system for operation – grid-connected type: grid-connected wind farms can be compensated and supported by the large power grid, and more fully develop available wind resources, which is the main development direction of wind power at home and abroad. In an increasingly open electricity market environment, the cost of wind power will also continue to decrease. If the indirect benefits brought about by factors such as the environment are considered, wind power is also very attractive economically.
Grid-connected wind farms have the following advantages:
(1) Short construction period: The wind turbines and their auxiliary equipment are modular, and the design and installation are simple. The transportation and installation time of a single wind turbine does not exceed three months, and the construction period of a 10MW wind farm does not exceed one year. , and one can be put into production after installing one.
(2) The actual occupied area is small, and the requirements for land quality are low: the construction area of the equipment in the wind farm only accounts for about 1% of the wind farm, and the rest of the site can still be used for agriculture, animal husbandry, and fishery.
(3) The operation management is highly automated and can be unattended. On the other hand, wind power generation is limited by its primary energy source—wind energy.
02. Wind power grid connection – current grid connection method
Generally speaking, the grid connection methods of wind power generation can be roughly divided into three types: asynchronous generators, synchronous generators, and doubly-fed generators.
Grid connection of asynchronous generator sets:
The grid connection methods of asynchronous generators currently used in wind power generation systems are as follows:
Directly connected to the grid
This grid connection method requires that the phase sequence of the generator is the same as that of the grid when the grid is connected. When the speed of the asynchronous generator driven by wind is close to the synchronous speed, it can be automatically connected to the grid; the signal for automatic grid connection is given by the speed device, and then the grid connection process is completed by closing the automatic air switch.
Disadvantages: When directly connected to the grid, there will be a large inrush current and a drop in the grid voltage;
Applicable: The asynchronous generator capacity is below 100 kilowatts and the grid capacity is large.
Step-down grid connection
This grid-connected method is to connect a series resistance or reactor or connect an autotransformer between the asynchronous motor and the grid to reduce the magnitude of the impulse current at the moment of grid-connected closing and the magnitude of the grid voltage drop.
Disadvantages: Because resistors, reactors, and other components consume power, they must be removed quickly after the generator is connected to the grid and enters a stable operating state.
Applicable: Generating units above 100 kilowatts and with larger capacity, it is obvious that this method of grid connection is less economical.
Soft grid connection through the thyristor
This grid-connected method is to connect the stator of the asynchronous generator and the power grid by connecting a bidirectional thyristor in series for each phase. The three phases are all controlled by the thyristor, and the two ends of the bidirectional thyristor are connected in parallel with the moving contacts of the grid-connected automatic switch.
Features: This soft grid-connected connection method can save a grid-connected automatic switch, so the control circuit is relatively simple, and it avoids the phenomenon of contact bouncing, sticking, and wear of the contact automatic switch, which can ensure a higher switching frequency.
Insufficient: It is necessary to select a high back-voltage bidirectional thyristor with a large current allowable value, because the current passing through the bidirectional thyristor needs to meet the rated current value that can pass through the asynchronous generator, and there is a soft grid-connected connection mode with a bypass grid-connected automatic switch. The high reverse voltage bidirectional thyristor can meet the requirements as long as it can pass a current slightly higher than the no-load current of the generator.
Synchronous generator set connected to the grid:
1. Grid-connection of synchronous generators: The synchronous generators driven by wind turbines are connected in parallel with the power grid through frequency conversion devices. The characteristics of parallel operation of this system are as follows:
a. Since the frequency conversion device is used for output control, there is no current impact when connecting to the grid, which has almost no impact on the system.
b. In order to adopt the AC-DC-AC conversion method, the operating frequency of the synchronous generator set and the grid frequency are independent of each other. The speed of the wind wheel and its generator can be changed, so there is no need to worry about the out-of-synchronization problem that may occur when the synchronous generator is directly operated in the well pattern.
c. Since the frequency conversion device adopts a static self-excited inverter, although the reactive power can be adjusted, there is a high-frequency current flowing to the power grid
d. Using impedance matching and power tracking feedback in the wind power system to adjust the output load can make the wind turbine run at the best efficiency and deliver more power to the grid.
2. Grid-connection of direct-drive AC permanent magnet synchronous generator set: the low-speed AC generator is directly driven by the wind turbine, and the working speed is fast. IGBT inverter well pattern with low drive power and reduced conduction voltage. The characteristics of parallel operation of this system are as follows:
a. Since the gearbox is not used, the length of the horizontal axis of the unit is greatly reduced, and the mechanical transmission path for electric energy production is shortened, which avoids the loss and noise caused by the rotation of the gearbox.
b. Since the generator has a large surface, the heat dissipation conditions are more favorable, which reduces the temperature rise of the generator during operation and reduces the fluctuation of the generator temperature rise.
Grid-connected doubly-fed generator set:
Features: A pulse rectifier is added to the generator side and the grid side respectively. In the case of low wind speed, the AC voltage output by the generator is boosted by the motor side pulse rectifier, which can meet the normal operation of the grid side pulse rectifier.
Summary: Grid-connected operation is currently the main form of wind power generation. Various grid-connected schemes have their own advantages and disadvantages. With the increase in wind turbine capacity. The impact on the power grid, when it is stored and connected to the grid, is also increasing. When this impact is severe, it not only causes a large drop in the voltage of the power system. And it may cause damage to the generator and mechanical parts (tower, blade and speed increaser, etc.). If the grid connection impact time is too long, it may also cause the system to collapse or threaten the normal operation of other grid-connected units.
Based on the above analysis, it can be concluded that it is more reasonable to choose a direct-drive AC permanent magnet low-speed synchronous generator. The low-speed AC generator directly driven by the wind turbine is connected to the grid through the IGBT inverter with fast working speed, low driving power, and conduction voltage drop. After the AC-DC-AC conversion method, the alternating current that changes with the wind speed becomes the alternating current that meets the grid-connection requirements, and the wind turbine is integrated into the grid by adopting the quasi-synchronous grid-connection method.
03. Wind power grid connection – classification of grid connection technology
There are two ways of wind farm networking: AC networking and DC networking. The traditional communication networking method has been used for a long time, and it still occupies a dominant position. In the past, the installed capacity of wind turbines was small, and the impact on the power grid was quite limited. At that time, the impact of wind power grid connection on the system mainly included some power quality problems such as voltage fluctuations, flicker, and harmonic pollution. With the increase in the scale of modern wind farms, Continuously expanding, large-capacity wind turbines are integrated into the grid, and the negative impact of wind power interconnection on the system extends to system stability and security.
Wind power grid-connected AC technology (HVAC)
Main advantages: the structure of the transmission system is simple, and its cost is relatively low when the transmission distance is relatively short.
Disadvantages: There are a series of technical obstacles that are difficult to overcome, such as the capacitive power of the line, the stability of the synchronous operation system, and power flow control.
DC grid-connected technology of wind power
Compared with HVAC, it has the following four characteristics:
(1) Economy: The construction cost and operation costs of HVDC lines are lower than those of HVAC, while the construction cost and operation costs of converter stations are higher than those of AC substations. Therefore, for the same transmission capacity, the longer the transmission distance, the better the economy of direct current than alternating current.
(2) Interconnectivity: HVAC capability is limited by the angle stability of synchronous generators, and as the transmission distance increases, the greater the contact reactance between synchronous machines, the more prominent the stability problem, and the HVAC capability is more limited.
(3) Controllability: HVDC has the characteristics of fast and controllable power flow, and can be used for the stability and frequency control of the connected AC system. The HVDC converter is a power control circuit based on power electronic devices, so it can quickly and accurately control the power flow.
(4) Flexibility: It is easy to form a multi-terminal DC transmission network, and then realize the grid connection of multiple wind farms or hybrid distributed power sources.
Due to the above-mentioned series of characteristics of HVDC itself, HVDC has its applicable application fields, such as submarine cable power transmission, long overhead line power transmission, AC system interconnection, and as a countermeasure to limit short-circuit current, it has attracted great attention from people in various countries. At present, HVDC has become the most comprehensive application of power electronics technology in power systems, but it is also the most complex system, and the research on HVDC still has a long way to go. The above-mentioned insurmountable difficulties exist in the interconnection of wind farms through AC. For the interconnection of large-scale wind farms and offshore wind farms, people are increasingly favoring HVDC transmission. HVDC technology is the earliest and relatively mature technology of power electronics technology in the field of power system transmission.
Wind power grid-connected technology using VSC-HVDC system
At present, the main means of interconnecting wind farms are VSC-HVDC, conventional direct current transmission HVDC and alternating current transmission HVAC. Due to its many excellent features, VSC-HVDC is more suitable for wind farm networking, and it also helps the system black start and “island” operation of wind farms.
Features:
a. When DC networking is used to realize long-distance networking of wind farms, the cost increase due to the installation of converter stations can be offset due to the reduction in the cost of DC cables and the reduction in the proportion of losses on the DC line.
b. The DC interconnection can isolate the wind farm from the AC main grid, which is helpful for rapid recovery after system failure. The advantage of AC networking is that the cost is low, but when the distance of the transmission line exceeds a certain value, the proportion of the cost of the AC cable becomes larger, and a large amount of capacitive reactive current will be generated, and a static var compensator (SVC) needs to be added for reactive power compensate. And the AC interconnection must keep the wind farm in sync with the connected AC grid.
c. Due to the operating characteristics of DC transmission, it will not propagate the AC properties of the AC grid (such as voltage phase angle, frequency) when transmitting electric energy, which also isolates the harmonics injected by wind power into the grid and avoids the harmonics of wind power generation on the grid pollution; and because the wind power connected to the grid is affected by the power generation of the wind turbine itself, the voltage usually does not meet the stability requirements. When VSC-HVDC is connected to the grid, it can also play the role of STATCOM and dynamically compensate the wind turbine. The reactive power deficit of the public connection point can stabilize the bus voltage at the machine terminal. In addition, when the wind turbine fails and shuts down, the bus voltage provided by the VSC can also be used to achieve the purpose of self-starting on the grid side.