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At present, the domestic and international power and electric industry is developing and developing micro-power grid (referred to as microgrid) technology. The core of the microgrid is a combination of new power machinery and new power generation technologies, renewable energy generation technologies and energy storage technologies. This new trend is not only important for the power industry, but also presents new challenges and opportunities for the electrical manufacturing industry. This article will give a brief review of this.
And the small-scale gas-fired machine, the Shenzhou micro-grid and the small-scale gas turbine high-speed generator set that may become the main power generation equipment in the micro-grid, illustrate the important role of the micro-grid in the distributed generation system. It is necessary for the development of renewable energy. This article also highlights the construction of high-speed generators and its characteristics.
Turbine, high-speed generator micro-grid technology refers to the decentralized power generation resources (such as self-powered power generation equipment or standby) owned by certain units (such as a certain block, some large buildings) or units in certain enterprises and institutions. Generators, solar power generation units, wind power generation equipment and other renewable energy power generation units are connected to supply power to each unit, and run in parallel with the main power grid through the distribution network to form a system in which large power grids and small power generation equipment operate in conjunction. Each unit can rely on the power supply of a large-scale power grid to obtain a large amount of cheap electric energy, and at the same time, it can return electric energy to a large-scale power grid when the power generation capacity of the unit is sufficient.
Linking small units of distributed power generation equipment into networks and networking with large power grids is a new concept and new technology, and will bring a series of new scientific and technical problems that need to be solved. These problems are largely the control and scheduling of the microgrid itself, the impact of microgrid operation on the large grid, and the economics of the grid and its fault handling. Some people may ask the question that changing the units of decentralized power generation to the unified power supply of the large power grid is an improvement in technology, which can greatly improve the economics of the entire power supply system. Why should we propose measures such as decentralized power supply? Because, with the advancement of technology and the requirements of various objective factors such as environmental protection and reliability, the parallel operation of the microgrid and the large power grid is not only a return to the independent distributed power supply mode, but a solution to the above various problems. way.
Compared with the way that large power grids are powered separately, the advantages brought by the combination of microgrid and large power grid are obvious. In summary, the following aspects can be listed.
The first is to make full use of the capabilities of various power generation equipment, reducing the tension of the main power grid during the peak load period.
In the past few years, power supply in most parts of China has been insufficient, and during the peak load period, power cuts have to be cut, which has adversely affected production and life. In this case, the power generation equipment in the microgrid can provide supplementary power to make up for the lack of power of the backbone network, that is, the microgrid plays a role of peak clipping.
Since the power generation equipment in the microgrid usually includes renewable energy sources, such as solar power generation or several generations of power generation, they generally have energy storage equipment due to their own requirements, which is beneficial to supplement the power supply during the peak load period. This complementary feature is a major feature of the parallel operation of the microgrid and the backbone power grid.
Second is to strengthen the reliability of power supply. Since the electrical load no longer depends solely on the supply of the backbone network in this way, the reliability of the power supply will be greatly improved. The reliability problem of large power grids has always been a problem that has been plaguing people for a long time, and so far has not been solved best. After the Great Blackout in the summer of 2003, the United States proposed to invest $100 billion to renovate its power grid to improve its reliability, but it seems unlikely that it will be realized in the short term. Therefore, for important units, emergency self-supplied generator sets are still an indispensable measure. If each unit uses its own various power generation equipment, including the backup generator set, effectively and forms a microgrid, the reliability of power supply can be greatly improved. At the same time, if the self-supplied unit is merged into the microgrid instead of the one household alone to meet its own needs, the total installed capacity of each unit can be reduced accordingly, which will improve the economics of the standby or self-provided generator set.
The third is to improve the operating efficiency of the entire power grid, while also reducing environmental pollution. When the microgrid and the backbone network are operated in parallel, the microgrid can mainly bear the peak load. In this case, the backbone network carries the base load. This will enable the power station in the backbone network to operate in the most economical way, that is, operating under the highest efficiency conditions, and rarely bear or only bear the sudden change of load in a more balanced manner, so that the boilers, generator sets, etc. of large power plants They all operate under ideal conditions. This not only brings about an increase in operational efficiency, but also reduces the total amount of pollutant emissions, which is conducive to environmental protection. At the same time, the same micro-grid is in the load center, there is no transmission loss caused by long-distance transmission and lifting voltage of the backbone network, thus improving the economics of the entire network.
In addition, it is worth mentioning that the development prospects of the microgrid. The decentralized microgrid has a strong vitality and development prospects.
In addition to the reasons, the demand for and use of renewable energy is also an important factor. In general, renewable energy sources are characterized by dispersion and scale. In addition, they are not subject to natural conditions and cannot be stably powered for long periods of time unless they have energy storage devices. These characteristics determine the need to connect renewable energy generation equipment to the microgrid. Because after networking, they can maximize their strengths and avoid their shortcomings. From the perspective of scientific development, the development of renewable energy is the only way to solve China's energy supply. Foreign countries have done a lot of research and practical development and utilization in this regard. China is also vigorously carrying out work in this area. According to research by an authoritative unit in China, by 2050, under the condition of giving priority to environmental protection, China’s renewable energy will account for about 30% of all energy, becoming the most important. Energy supply elements. Even under normal development conditions, the proportion of renewable energy will reach 18%. Therefore, the development of renewable energy power generation equipment and their connection with the microgrid will be an important task for the electrical industry.
In addition to renewable energy power generation equipment, the most popular power generator set in the microgrid is a combination of small gas turbines and high speed generators. This new unit has many features that are superior to existing internal combustion generator sets and small steam turbine generator sets, and will therefore replace them as the main units in the microgrid. A brief introduction will be made below: the composition of the micro gas turbine generator set is as shown. It includes: 1) gas (natural gas) compressors; 2) air compressors; 3) heat exchangers; 4) low-emission combustion chambers; 5) compressor turbines; 6) power turbines; 7) high-speed generators; ) Power electronics (rectifiers and inverters).
The gas compressor pressurizes the gas supplied from the pipe to, for example, 10 atmospheres or more. The high-pressure air compressor pressurizes the clean air for combustion-supporting while compressing and heating to high temperature and high pressure, and then further heated by the heat exchanger and sent to the combustion chamber together with the gas. The combustion chamber is designed for low-emission combustion with Nox emissions below 10Vppm and C0 below 20Vppm to meet environmental requirements. The combusted high temperature and high pressure gas is sent to the turbine of the air compressor to the power turbine that drives the high speed generator. In order to match the rotational speed requirements of the respective units, the two turbines are not on the same mechanical shaft but are respectively coupled to the respective driven units. The gas after the work of the gas turbine can be directly discharged after passing through the heat exchanger to heat and compress the air, but it is more reasonable to send it to the cold and heat supply unit for cooling or heating, so as to utilize the heat energy more effectively. This is another advantage of this system.
Gas turbines operate at high speeds, so the generators they drive should also be high-speed generators. In this case, the generator output is no longer the usual 50 Hz power frequency, but can reach the range of several kilohertz. To this end, the output of the generator is passed through a rectification and inverter circuit, which becomes 50 Hz and then sent to the distribution network for use by the user. In this way, there is an additional rectification inverter in the system, but the step-down transformer can be omitted.
The combination of small gas turbines and high-speed generators has the following advantages over traditional small steam turbine generator sets or internal combustion generator sets: 1. The capital investment is low and the speed is fast. According to foreign data, the investment for small gas turbine generators of the size of 500kW is about US$700/kW, while the small steam turbine generators of the same capacity can reach US$1,500/kW or higher. 2. The operating efficiency of small gas turbine generator sets is generally high. For example, the unit with the above capacity has a thermal efficiency of 34% in the case of a heat exchanger, and 28% for a small steam turbine with the same capacity. The small gas turbine has a higher rotational speed and is suitable for driving high-speed generator sets, especially High-speed generator with axial flux. A brief description of such a high speed generator is given here. High-speed generators for small gas turbines are very different from traditional synchronous alternators. The flux of a conventional synchronous generator is radial, while the flux of such a high speed generator is axial. Its schematic construction is as shown. The rotor is of a disc-like structure with magnetic poles made of neodymium iron boron material embedded thereon to generate axial magnetic flux. The stator coil is wound on the stator, but the iron core is not on the stator as in the conventional motor, so the weight of the generator can be greatly reduced. The rotor and stator can be constructed in pairs as needed, all mounted coaxially on a single mechanical shaft (one stator is shown). There is one iron end plate on each of the rotors at both ends to close the magnetic circuit.
When the rotor rotates, the magnetic flux of the different magnetic poles successively cuts the stator coil to generate induction heat.
Since the number of rotor and stator pairs can be increased or decreased according to power requirements, it is flexible and convenient for the design and mass production of such motors. Since there are no cogging on the stator and rotor, it is ideal for operation at high speeds. In addition, no matter whether the stator or the rotor has an indispensable end winding on a conventional generator, the failure of the common end winding is completely eliminated, thereby greatly increasing the reliability. The advantages of this type of generator are obvious. To sum up, there are roughly the following aspects: 1. Light weight and small volume, the weight is only 20% to 50% of conventional generators. For example, a 400kW high-speed generator weighs about 57kg, has a length of about 0.55m, and has a diameter of about 0.35m, plus rectifier inverter and switchgear, etc. weighs about 675kg, and the length, width and height of the chassis are 0.45, 1.2 and 2.5m respectively. In comparison, the traditional generator with the same capacity can weigh up to 3000kg. The diameter and length are about 1.5m and 0.9m respectively. It can be seen that the difference is very large. In general, high-speed generators have a power density that is about 2.2 to 4.8 times that of conventional generators. 2. Direct coupling to high-speed gas turbines without the need for reduction gears, which helps reduce costs, reduce noise, improve reliability, and reduce maintenance. 3. In the case of using the inverter, the frequency stability can reach 0.05 Hz, which is close to the requirement of 0.03 Hz for the large backbone network, and is much higher than the standard that can be achieved by small power plants. 4. Even when the prime mover speed fluctuates greatly (for example, 10%), the voltage regulation rate can be kept in a small range, for example, ±0.5%. This is because the rectifier inverter circuit can quickly respond to changes in output voltage and automatically Adjustment. 5. There is basically no excitation current and iron loss, which can improve efficiency and reduce heat generation. 6. Can withstand large unbalanced loads, such as up to 50%, which is unacceptable for large motors, but high speed generator systems can still maintain less than 3% of total harmonic components and 20 under these conditions. % unbalanced voltage. 7. Since the air gap of the motor is planar, and the stator coil is directly exposed to the air gap, there is no cogging core around, so the condition of air cooling is much better than that of the conventional motor. In addition, the rotor has no excitation coil, which generates much less heat, so the temperature rise is lower, which is beneficial to prolong the life. 8. Since the inverter isolates the generator from the load, the generator is not susceptible to nonlinear load harmonics.
9. The overload current limit allowed by the inverter can be specified by parameter design, so it can take into account the requirements of relay protection and the limitation of generator overload current. 10. It is convenient to produce generators with different capacities, because within a certain range, as long as the number of pairs of rotors and stators on the shaft is increased, the capacity can be increased. 11. It has a large capacity to withstand the large-capacity motor starting current. This is because the rotor at high speed has a greater moment of inertia and can withstand greater transient load shocks.
The above is a brief overview of the microgrid and the small gas turbine high-speed generator set that may become the main power generation equipment in the microgrid. In fact, microgrid power generation equipment can include a wider range of new power sources. Microgrid technology is closely related to distributed generation systems. The distributed generation system is a method in which a power supply system based on a large power station is currently changed to a power supply device of a small and multi-variety type distributed at each load center. This will help to make full use of multiple sources, especially renewable energy and low-pollution energy. For example, the use of hydrogen energy has also been mentioned on the agenda, and the fuel cell fueled by hydrogen and natural gas reforming has reached a practical level. These power generation methods are small and scattered, and distributed power generation systems were born under this premise. Although it is still in its infancy, it has broad prospects. This has put forward new tasks and new requirements for the electrical industry, which should attract our attention. sundial
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