The smart grid is the application of information technology, power electronics and control technology in the power system. Among them, high-power power electronics technology can realize the conversion and control of electrical energy, and is one of the key technologies for achieving more flexible and controllable power grids. This article summarizes the power electronics technology in the smart grid from four aspects: large-scale renewable energy generation in power generation access, HVDC transmission in power transmission and flexible AC transmission, user power technology in power distribution, and energy storage V2G technologies. The application in China gives us a more in-depth understanding of the realization of the smart grid.
China's strong smart grid is based on the strong grid of coordinated development of UHV grids as backbone grids and grids at all levels, supported by information and communication platforms, and characterized by information, automation, and interaction, including all aspects of the power system. A modern power grid that covers all voltage levels to achieve a highly integrated integration of power flow, information flow, and service flow. From large-scale access to renewable energy power generation to high-voltage direct current (HVDC) transmission and flexible AC transmission, from power technologies that improve power quality to energy storage and V2G applications, high-power power electronics are indispensable everywhere. This article is the fourth in a series of articles on Smart Grid Review, which summarizes the four dimensions of renewable energy access in power generation, HVDC transmission in transmission, flexible electricity AC transmission, user power technology in power distribution, and energy storage technologies. The application of power and power electronics in smart grids gives us a more comprehensive and in-depth understanding of the smart grid.
1 Power generation
The large-scale wind power generation, solar power generation in the smart grid, and the frequency conversion of fan and water pump in power plants all rely on power electronics technology.
1.1 Wind Power
Nowadays, mainstream models in the wind power market are variable-speed wind turbines based on doubly-fed induction generators and variable-speed wind turbines based on permanent magnet synchronous generators. The stator of the doubly-fed wind turbine is directly connected to the power grid. The rotor is connected to the power grid through a part of the power inverter. According to the change of the rotation speed of the wind turbine, the excitation current of the variable frequency AC is passed through the rotor to realize the decoupling of the active and reactive power of the generator set. The control makes the wind turbine have the characteristics of variable speed operation and improve the wind energy conversion efficiency of the wind turbine. Variable-speed wind turbines based on permanent magnet synchronous generators are connected to the power grid via full-power inverters. Decoupled control of the frequency converter completely decouples the variable-speed synchronous wind turbines from the power grid. Its characteristics depend entirely on the control system and control of the inverter. Strategy.
1.2 Solar power generation
Solar power generation, also known as photovoltaic power generation, generally consists of photovoltaic arrays, controllers, inverters, and battery packs. The power generated by the PV array is direct current. In addition to the special power load, the inverter needs to convert the direct current into alternating current. The grid-connected photovoltaic power generation system is mainly connected to the grid in the form of a current source. The phase of the output current tracks the phase change of the grid voltage, and the magnitude of the output current amplitude is adjusted to maximize the power of the photovoltaic power generation system injected into the grid. In order to compensate for the fluctuation of photovoltaic power generation, it is also necessary to control the bidirectional charge and discharge of the battery pack through the controller so as to ensure smooth power supply to the load.
1.3 Frequency Control of Fans and Water Pumps in Power Plants
The plant power level of the power plant is 8%, of which the fan water pump power consumption accounts for about 65% of the total power consumption of the auxiliary equipment, and the operating efficiency is low. In the energy-saving and power-reduction of power plants, low-voltage or high-voltage inverters are mainly used to realize the frequency conversion and speed regulation of fans and pumps, which can achieve an energy saving effect of 30%. At present, the low-voltage frequency converter technology has been very mature, and there is a complete series of products, but there are not many enterprises that have the design and production of high-voltage and large-capacity frequency converters.
2. Transmission Links 2.1 HVDC transmission has the advantages of large transmission capacity, good stability, and flexible control and adjustment. For long-distance transmission, submarine cable transmission and networking of different frequency systems, HVDC transmission has unique advantages. In 1970, the world's first thyristor converter valve test project was completed in Sweden, replacing the original mercury arc valve converter, marking the formal application of power electronics technology to DC transmission. Since then, thyristor converter valves have been used in new DC transmission projects in the world.
2.2 Flexible DC transmission based on voltage source converter (VSC) In recent years, there have been new developments in DC transmission technology. The VSC-HVDC based on voltage sourced converter (VSC) is a kind of New DC transmission technology based on VSC and Pulse Width Modulation (PWM) technology, which uses IGBTs and other power electronics that can be turned off to form inverters, and uses pulse width modulation technology for passive inverters to solve the problem of using direct current. The problem of power transmission to the point of no AC power supply, while significantly simplifying the equipment and reducing the cost, can be used for island power supply, urban power distribution network capacity expansion, inter-exchange system interconnection, and large-scale wind power plant grid connection. The first industrial test project for flexible HVDC power transmission using IGBTs as voltage source converters was put into operation in 1997.
2.3 Flexible AC Transmission (FACTS) The concept of FACTS technology was introduced in the late 1980s. It is a transmission technology based on power electronics and modern control technology that can implement flexible and rapid adjustment of impedance, voltage and phase of AC transmission systems. The flexible control of the AC power flow power flow greatly improves the stability of the power system. The existing FACTS equipment and its functions in the power grid are shown in Table 1. Since the 1990s, foreign countries have begun to use FACTS technology for practical power system engineering based on research and development. Among them SVC is to adjust and output the reactive power through the switching of the thyristor control capacitor bank, the apparatus structure is simple, the control is convenient, the cost is lower, so apply earlier. Others such as STATCOM (USA/Japan/China), TCSC (Germany/USA), UPFC (USA), and CSC (USA) also have practical engineering applications.
3 The concept of “Custom Power Technology†in the power distribution segment was proposed by Dr. NGH Oringani of the United States in 1988. This technology integrates high-power power electronic technology and distribution automation technology to provide users with reliable power. Based on power quality requirements, users are provided with specific power supply technologies. User power technology, also known as DFACTS, is an extension of FACTS technology in distribution networks. The current main DFACTS equipment and its functions are shown in Table 2. They can, according to the needs of users, achieve the functions of suppressing system harmonics, eliminating voltage flicker and asymmetry, compensating for power factor and load fluctuations.
In smart power distribution network power electronics, it is worth mentioning that intelligent universal transformer (IUT). Unlike conventional coil transformers, it is a transformer based on power electronics technology (composed of multi-level inverters). As a basic device in the EPRI ADA project in the United States, IUT is already approaching marketization. The IUT is reported to have a rated power of 20 kVA, an input phase voltage of 2.4 kV and an output rated voltage of 120V/240V. In addition to the functions of traditional transformers, the IUT can provide users with selectable service items such as DC or 400 Hz. The power, from single-phase to three-phase conversion, voltage regulation, harmonic filtering, and sagging correction; it can also improve system operation efficiency, such as design standardization (reduced inventory of spare parts), eliminates dangerous liquid medium (oil ), reducing the weight and size, built-in sensors with remote communication capabilities, can assist distribution network remote monitoring, but also as a controllable switch to interrupt the trend.
4 Energy Storage Technology and V2G With the large-scale access to power grids generated by various renewable energy sources, the randomness and volatility of wind power and photovoltaic power generation pose serious challenges to grid dispatch and safe and stable operation. In order to absorb more fluctuations in wind power and photovoltaic power generation, it is imperative that the power grid has a strong regulation capability, and large-capacity centralized energy storage technologies and distributed energy storage technologies such as V2G have emerged.
In addition to pumped energy storage and compressed air energy storage (CSES), battery energy storage (lead-acid batteries, lithium batteries, NaS batteries, vanadium flow batteries, etc.), flywheel energy storage, superconducting energy storage (SMES) and super capacitor storage Can use power electronics technology. All kinds of battery energy storage and large-capacity flow battery storage generate DC power. When the power of the grid is excessive, the controller and the rectifier are used to convert the AC power to DC to charge the battery pack. When the grid power is insufficient, the controller and the inverter convert the DC power of the battery pack into AC power and send it back to the grid. How to achieve rapid charge and discharge of a battery pack, how to achieve balanced charging of multiple batteries, and prolong the battery life have become key issues in intelligent battery charge and discharge management systems. At present, NaS flow batteries and vanadium flow battery packs can reach up to megawatts, and multiple sets of parallel connections can have larger capacity, enabling centralized power regulation of the power grid. This is called a “energy storage power stationâ€.
Flywheel energy storage is the mechanical energy that converts electrical energy into a flywheel through an electric motor. When needed, the motor turns into a generator, converting the mechanical energy of the flywheel back into electrical energy and feeding it into the grid. The flywheel is made of high-strength material such as glass fiber, rotates at more than 40,000 rpm, and is suspended in a vacuum through a pair of magnetic bearings. There is almost no energy loss, and the entire device can achieve an operating efficiency of 98%. The flywheel is driven by a bidirectional power converter. The energy storage device is decoupled from the grid to ensure that the electrical energy generated by the device satisfies the power quality requirements of the grid. Through the modular design, multiple flywheel devices can be operated in parallel, using the mobile container application, which can be used as a regulating power station and backup power supply. At present, the United States has built a 20MW mobile flywheel energy storage system to carry out industrial application tests.
With the development of electric vehicles, a large number of on-board batteries have become a natural distributed energy storage system, providing a new way to enhance the adjustment capabilities of the power grid. Statistics show that an electric vehicle is in a stopped state 95% of the time. The owner can charge when the electricity price is low, and when the grid needs it, the energy stored in the battery is fed back to the grid to obtain the difference. Between the owner and the system dispatcher, intelligent charging and discharging management is achieved through real-time electricity prices and smart meters. This is the V2G (Vehicle to Grid) technology. In the United States, for example, the United States has 176 million vehicles, which is 24 times the total installed capacity of the U.S. power system. Assuming that 1/4 of them are 414 million electric vehicles, their on-board batteries are sufficient to store the output power of all wind farms in the United States. The huge electric vehicle energy storage effectively regulates the fluctuation of the output power of renewable energy power generation and increases the effective reserve capacity of the system. Hundreds of thousands of electric vehicles can also constitute microgrid operation, and can also provide power grids under emergency conditions. Effective support has improved the safe operation of the power grid. At present, more than 20 cities in the United States are conducting V2G pilots and an electric vehicle running in V2G mode. The owners can make a profit of 4000-5000 US dollars a year, which is equivalent to the total mileage of 1/3-1/6 of the whole year. cost. China has also introduced a new energy vehicle development plan, and each purchase of an electric vehicle can receive a subsidy of 3,000 yuan from the national finance. China is developing a series of standards for charging stations. The Olympic Games charging stations developed by the School of Electrical Engineering at Beijing Jiaotong University, the World Expo Charging Station and the Expo V2G demonstration project have also been put into operation.
5 Conclusions The application of power electronics in smart grids can be summed up in the following aspects: (1) Improve the power grid resource allocation capabilities. FACTS technology can make full use of the transmission capacity of the power grid without major changes to the existing equipment and achieve long-distance, large-capacity, low-loss transmission of large hydropower, large coal power, large nuclear power, and large renewable energy sources, effectively alleviating China's Uneven distribution of energy and load; (2) Improve the safety and stability of the power grid. For example, FACTS and VSC-HVDC have faster response speeds, better controllability, and stronger control functions than traditional transmission schemes, providing the most effective means for fast, continuous, and flexible control of smart grids. 3) Improve clean energy grid-connected operational control capabilities. Wind power and photovoltaic grid-connected converters have multiple functions such as soft grid connection, soft decoupling, decoupling control of active and reactive power, and power quality control. Large-capacity centralized energy storage and V2G distributed energy storage are adopted More renewable energy sources provide reliable protection; (4) Improve grid service capabilities. The power electronics technology guarantees different characteristics. Power users can reliably access and use electrical energy. Through the combination of low-voltage converters and low-cost energy storage, the two-way flow of electrical energy between the distributed power generation units and the power grid can be realized. Users can lower the storage height. Buy, get a certain economic benefits, while DVR and APF as a representative of the custom power equipment to meet the user's requirements for power quality; (5) increase the capacity of urban distribution network transformation. On the one hand, the electricity load of large and medium-sized cities has grown rapidly. The transmission capacity of the original overhead distribution network has been unable to meet the demand for electricity load. another
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