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Major: Electrical Engineering and Automation
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abstract
With the continuous development of electric power in China, the requirements for power supply are becoming more and more strict. It is an indispensable part of our daily life and an important part of the whole national economy. It directly affects the development of industrial and agricultural production and the improvement of people's lives, and is an indispensable main energy source for today's social and economic development and people's daily life. For the vast number of power supply enterprises, the power factor of users is directly related to the power loss and electric energy loss in the power grid, the voltage loss and voltage fluctuation of power supply lines, and the power saving and power supply quality of the whole power supply area. This is a well-known truth. Therefore, improving the power factor of power system has become an important topic in the power industry, and to improve the power factor of power system, we must first improve the power factor of each user. This paper briefly introduces the main factors affecting the power factor of power grid, several methods of using low-voltage reactive power compensation, and the general method of determining the reactive power compensation capacity to improve the power factor of power system.
[Keywords:] power factor influencing factors compensation method capacity determination
catalogue
I. Introduction 4
Second, the main content: 6
1, the main factors affecting power factor 6.
1. 1. Inductive devices and power transformers are the main devices that consume reactive power.
1.2, the power supply voltage beyond the specified range will also have a great impact on the power factor of 7.
1.3, the fluctuation of grid frequency will also have a certain impact on the magnetizing reactive power of asynchronous motors and transformers.
2, reactive power compensation of low voltage power grid 8
2. 1, general method of reactive power compensation for low-voltage power grid 8
2. 1. 1, random compensation 8
2. 1.2, satellite compensation 8
2. 1.3, tracking compensation 9
2.2. Take appropriate measures to improve the natural power factor of the system.
2.2. 1, reasonably select the motor 10.
2.2.2 Improve the maintenance quality of asynchronous motors 10
2.2.3. The synchronous operation compensation with synchronous motor or asynchronous motor is 10.
2.2.4. Correct selection of transformer capacity to improve operation efficiency 1 1
3. Manual compensation of power factor 12
3. 1, the most commonly used shunt capacitor bank installed in substation 12.
3.2 The shunt compensation phase-shifting capacitor shall meet the following voltage and capacity requirements 12.
3.3 Split-phase compensation 13
Three. Conclusion 14
Four. Reference 15
I. Introduction
Many electrical equipments work according to the principle of electromagnetic induction, such as distribution transformers and motors. They all rely on the establishment of alternating magnetic field to convert and transfer energy. The electric power required to establish alternating magnetic field and induced magnetic flux is called reactive power, which is a physical quantity with constant energy conversion scale. Therefore, in the power supply system, in addition to active power supply, reactive power supply is also needed, and both are indispensable.
In the power triangle, the ratio of active power P to apparent power S is called power factor COSφ, and its calculation formula is COS φ = P/S.
In the operation of power grid, the power factor reflects the degree to which the apparent power output by power supply is effectively utilized. We hope that the greater the power factor, the better. In this way, the reactive power in the circuit can be minimized, and most of the apparent power will be used to supply active power, thus improving the power of power transmission.
The level of user power factor has a great influence on the full utilization of power generation, power supply and power consumption equipment in power system. Reactive power compensation, also known as local compensation, can not only give full play to the production capacity of power generation and supply equipment, reduce line loss and improve voltage quality, but also improve the working efficiency of users' electrical equipment and save electricity for users themselves. Therefore, for the vast number of power supply enterprises in China, it can not only reduce the compensation pressure of the upper level power grid, improve the power factor of users, but also effectively reduce the power loss and reduce the electricity expenditure of users. Its social and economic benefits will be very significant.
Second, the main content:
1, the main factor affecting the power factor.
1. 1, inductive devices and power transformers are the main devices that consume reactive power.
A large number of inductive devices, such as asynchronous motors, induction furnaces and AC welding machines, are the main consumers of reactive power. According to relevant statistics, among all the reactive power consumed by industrial and mining enterprises, the reactive power consumption of asynchronous motors accounts for 60% ~ 70%; However, the reactive power consumed by asynchronous motors at no load accounts for 60% ~ 70% of the total reactive power. Therefore, in order to improve the power factor of asynchronous motor, it is necessary to prevent the motor from running at no load and increase the load rate as much as possible. The reactive power consumed by power transformer is generally about 10% ~ 15% of its rated capacity, and the no-load reactive power is about 1/3 when it is fully loaded. Therefore, in order to improve the power factor of power system and enterprises, transformers should not run at no-load or low load for a long time.
1.2, the power supply voltage beyond the specified range will also have a great impact on the power factor.
When the power supply voltage is higher than the rated value of 10%, the reactive power will increase rapidly due to the influence of magnetic circuit saturation. According to relevant statistics, when the power supply voltage is 1 10% of the rated value, the reactive power will generally increase by about 35%. When the power supply voltage is lower than the rated value, the reactive power is also reduced accordingly, thus improving their power factor. However, the reduction of power supply voltage will affect the normal work of electrical equipment. By Q=UI*Sin? Push away sin? =Q∕UI, so measures should be taken to keep the power supply voltage of the power system as stable as possible.
1.3, the fluctuation of grid frequency will also have a certain impact on the magnetizing reactive power of asynchronous motors and transformers.
To sum up, we know some main factors that affect the power factor of power system, so we should seek some effective and practical methods to improve the power factor of low-voltage power grid, so that the low-voltage power grid can realize the local balance of reactive power and achieve the effect of reducing losses and saving energy.
2. Reactive power compensation of low-voltage power grid
2. 1, the general method of reactive power compensation for low-voltage power grid
There are three main methods of low-voltage reactive power compensation: random compensation, following compensation and tracking compensation. The scope of application of these three compensation methods and the advantages and disadvantages of using this compensation method are briefly introduced below.
2. 1. 1, random compensation
Random compensation is to connect one or more low-voltage capacitor banks in parallel to the electrical equipment in a decentralized way according to the reactive power demand of a single electrical equipment, and it uses a set of circuit breakers together with the electrical equipment. Through the control and protection device, the motor is turned on and off at the same time. Random compensation is suitable for compensating reactive power consumption of individual large-capacity and continuous operation (such as large and medium-sized asynchronous motors), mainly used for compensating excitation reactive power. This method can better limit the reactive peak load of rural power grid.
The advantage of random compensation is that when the electrical equipment is running, reactive power compensation will be put into operation, and when the electrical equipment stops, the compensation equipment will also quit, which will not cause reactive power feedback and does not need to adjust the compensation capacity frequently. It has the advantages of less investment, small floor space, easy installation, convenient and flexible configuration, simple maintenance and low accident rate.
2. 1.2, follower compensation
Adjoint compensation refers to the compensation method of connecting the low-voltage capacitor to the secondary side of the distribution transformer through the low-voltage switch to compensate the no-load reactive power of the distribution transformer. The reactive load of distribution transformer under light load or no load is mainly the no-load excitation reactive power of transformer, and the no-load reactive power of distribution transformer is the main part of the reactive load of rural power grid. For distribution transformers with light load, this part of loss accounts for a large proportion in power supply, which leads to the increase of electricity price, which is not conducive to the same price in the same network.
The advantages of follow-up compensation are: simple wiring, convenient maintenance and management, effective compensation of no-load reactive power of distribution transformers, limitation of reactive power base load of rural power grid, partial balance of this part of reactive power, thus improving the utilization rate of distribution transformers and reducing reactive power network loss, which is one of the commonly used reactive power compensation methods at present.
2. 1.3, tracking compensation
Tracking compensation refers to the compensation method of using reactive power compensation switch device as control protection device to compensate the low-voltage capacitor bank on the 0.4KV bus of large users. It is suitable for special users above 100KVA, and can replace random and slave compensation methods with good compensation effect.
The advantages of tracking compensation are flexible operation mode, less operation and maintenance workload, longer service life and more reliable operation than the first two compensation modes. But the disadvantage is that the control and protection devices are complex and the initial investment is relatively large. However, when the three compensation methods are close in economy, the tracking compensation method should be preferred.
2.2. Take appropriate measures to improve the natural power factor of the system.
Improving the natural power factor does not require any investment in compensation equipment, but only adopts various management or technical means to reduce the reactive power consumed by various electrical equipment, which is the most economical method to improve the power factor. The following briefly introduces the measures to improve the natural power factor.
2.2. 1, reasonably select the motor.
Choose the motor reasonably to make it run at the highest load rate as possible. When choosing a motor, we should not only pay attention to the mechanical characteristics of the motor, but also consider the electrical indicators of the motor. For example, the power factor of a three-phase asynchronous motor (100KW) is only 0. 1 1, and the load of 1/2 is about 0.72, which can reach 0.86 at full load. Therefore, the induction motor with accounting load less than 40% should be replaced by a motor with smaller capacity, and the process flow should be arranged and adjusted reasonably to improve the operation mode and limit the no-load operation. Therefore, from the point of view of saving electric energy and improving power factor, the capacity of motor must be selected correctly and reasonably.
2.2.2, improve the quality of asynchronous motor maintenance.
Experiments show that the changes of stator winding turns and air gap between stator and rotor have great influence on reactive power of asynchronous motor. Therefore, special attention should be paid not to increase the air gap of the motor during maintenance, so as not to reduce the power factor.
2.2.3, using synchronous motor or asynchronous motor synchronous operation compensation.
According to the principle of motor, the active power consumed by synchronous motor depends on the mechanical load on the motor, while the reactive power depends on the excitation current in the rotor. Under-excitation state, the stator winding "absorbs" reactive power from the power grid, while under-excitation state, the stator winding "sends" reactive power to the power grid. Therefore, as long as the excitation current of the motor is adjusted to be in an excessive state, the synchronous motor can "transmit" reactive power to the power grid, reduce the reactive power transmitted by the power grid to industrial and mining enterprises, and thus improve the power factor of industrial and mining enterprises. The synchronous operation of asynchronous motor is to properly connect the three-phase rotor windings of asynchronous motor and introduce DC excitation current to make it in the running state of synchronous motor, which is called "asynchronous motor synchronization". Therefore, as long as the DC excitation current of the motor is adjusted to be in an excessive state, reactive power can be output to the power grid, thus achieving the purpose of improving the power factor of the low-voltage power grid.
2.2.4, the correct choice of transformer capacity, improve the operation efficiency.
For transformers with low load rate, methods such as "moving, changing, merging and stopping" are generally adopted to improve their load rate to the optimal value, thus improving the natural power factor of the power grid. For example, transformers with an average load of less than 30% should be disconnected from the power grid, and the load rate should be improved through tie lines.
Through the above description of improving the weighted average power factor and natural power factor, perhaps we have a deeper understanding and understanding of the simple power term "power factor". Knowing that the improvement of power factor has a far-reaching impact on power enterprises, the following is a brief introduction to the manual compensation method of electrical equipment and the determination method of compensation capacity.
3. Manual compensation of power factor
Power factor is a representative and important index of the use and utilization rate of electrical equipment in the factory, and it is also a main index to ensure the safe and economic operation of the power grid. Power supply enterprises can't meet the factory's requirements for power factor only by improving the natural power factor, and the factory itself needs to install compensation devices to compensate the power factor manually.
3. 1, Installation of most commonly used shunt capacitor banks in substations
As can be seen from the above figure, in the original circuit, according to Kirchhoff's law, the inflow current is equal to the outflow current, but capacitors are connected in parallel, which can be seen from the phasor diagram? This angle is obviously smaller than the original angle, so it can improve the power factor, improve the transmission capacity of the line and reduce the loss on the line.
3.2 The shunt compensation phase-shifting capacitor shall meet the following voltage and capacity requirements.
Ue？ c≥Ug？ c
nQg？ c≥Qc
formula
Ue？ C—— Rated voltage of capacitor (KV)
Ug？ C—— working voltage of capacitor (KV)
N- Total number of capacitors connected in parallel.
Qg？ C- working capacity of capacitor (Kvar)
QC- Compensation capacity of capacitor (Kvar)
3.3 Split-phase compensation
Single-phase load is widely used in civil buildings. Because of the randomness of load change, lighting and air conditioning are easy to cause serious imbalance of three-phase load, especially in residential buildings. Because the sampling signal used to adjust and compensate reactive power is taken from any one of the three phases, the undetected two phases are either overcompensated or undercompensated. If it is overcompensated, the voltage of the overcompensated phase will rise, resulting in the damage of electrical equipment such as control and protection components due to overvoltage; If the compensation is insufficient, the loop current of the compensation phase will increase, and the equipment such as lines and circuit breakers will be heated and burned due to the increase of current. In this case, the traditional three-phase reactive power compensation method not only does not save energy, but also wastes resources, so it is difficult to effectively compensate the reactive power compensation of the system. The disadvantages of over-compensation and under-compensation in the compensation process have brought serious harm to the normal operation of the whole power grid.
For three-phase unbalanced single-phase distribution system, it is a better way to solve the above problems by using split-phase capacitor automatic compensation. The principle is that the signal for adjusting reactive power parameters is taken from each of the three phases, and the corresponding compensation is made according to the size of the inductive load and the power factor of each phase, which has no influence on others, so there will be no under-compensation and over-compensation.
Three. Concluding remarks
This paper discusses the influence of power factor on power supply enterprises and how to improve power.
The economic and social benefits brought by factors, especially the most important line loss (the most
The important thing is to reduce the loss (divided into technical loss reduction and management loss reduction), and the factors affecting the electricity quantity are introduced.
This paper introduces the main factors influencing the power factor and the general methods for improving the power factor, and expounds how to determine the power factor.
Reactive power compensation capacity and three specific ways of reactive power artificial compensation. I
Students can only correct their cognitive attitude and sum up the importance of this knowledge well.
To separate a part, do their own essential work, and on this basis, by going up one flight of stairs.
Order, with their own practical actions, to contribute to the cause of power supply.
Four. refer to
1, Yun Xin, electric power supervision, water conservancy and electric power press.
2. Jin, Ding Yushan, practical technology of reactive power compensation in power grid, China Water Resources and Hydropower Press.