The power factor refers to the ratio of the active power of the power load to the apparent power. In addition to absorbing active power from the power system, the power users' electrical equipment, such as transformers, induction motors, power lines, etc., also absorb reactive power. Reactive power only completes the mutual conversion of electromagnetic energy and does not do work. Reactive power is as important as active power. Without reactive power, the transformer cannot transform, the motor cannot rotate, and the power system cannot operate normally. The consumption of reactive power leads to a decrease in the power factor, thereby occupying the ability of the power system's power generation and supply equipment to provide active power, or increasing the facilities for sending reactive power, and also increasing the active power loss in the power system's transmission process. Therefore, power companies in various countries around the world have requirements for the power factor of power users, and give economic rewards and punishments according to the level of the power factor of users.
With the increasing economic development, the demand for electricity is constantly increasing. The prominent problems that come with it are the huge ineffective consumption of energy and the low utilization rate of resources. The power system is a huge system with considerable power loss. The reasonable allocation of energy is an issue that needs to be solved. The power factor is an extremely important factor in determining the economic benefits of the power generation and supply system. It directly reflects the distribution of active power and reactive power in the system. For the power generation and supply system, the load is required not only to have a high load rate, but also a high power factor.
1. Power factor of power network
In addition to bearing the active power P of the power load, the power network also has to bear the reactive power Q1 of the load. There is the following relationship between the active power P, reactive power Q and apparent power S:
S=P2+Q2 and the ratio of P to S is: PS=cosφ
Defined as the power factor of the power network, its physical meaning is the percentage of active power consumption supplied by the apparent power S of the line. In the operation of the power grid, what we hope is that the power factor is as large as possible. If this can be achieved, most of the apparent power in the circuit will be used to supply active power to reduce the consumption of reactive power.
2. Disadvantages of low power factor
2.1. Large current For a given load, when the supply voltage is constant, the lower the power factor, the larger the current, because; Ic = P3Ucosφ
(1) Where: P is the active load of the power load; U is the line voltage 1
It can be seen that the supply current Ic is inversely proportional to the power factor. We know that the rated capacity of the generator and transformer is proportional to its output current, and thus inversely proportional to the power factor. Therefore, under the condition of supplying the same power and a certain voltage, the lower the power factor, the larger the capacity of the generator and transformer is required, and therefore, the higher the investment in the generator and transformer is.
2.2. Copper loss is large Under a certain load, when the power factor of the transmission line is low, its copper loss is large. The line loss formula is as follows: &P=3I2c×R×10-3(kW)
Substituting (1) into the equation, we get: &P = 3P23UP2cos2φ × R × 10-3 = P2 × R × 10-3U2cos2φ (kW) = K × 1cos2φ
It can be seen that the copper loss of the equipment is proportional to the square of the current, and thus inversely proportional to the square of the power factor. The lower the power factor, the greater the copper loss in the electrical equipment, and the lower the efficiency. Similarly, when the power factor of the system is very low, the current increases for the same power transmission. Therefore, if the wire size is the same, the power transmission system means greater energy loss, or, in other words, for the same energy loss, a thicker conductor is required.
2.3. High investment From formula (1), we can see that for a given load, at a low power factor, the cross-section of the busbar and the conductive area of the protection switch must be increased to pass a larger current, so the investment also needs to be increased.
3 Main reasons for low power factor
3.1 Effect of excitation current
Transformers all carry excitation current, which always lags behind the induced potential. Under normal circumstances, the excitation current will not affect the power factor, but when operating under light load (such as a distribution transformer in a residential area mainly supplies lighting load at night, so the lighting load decreases sharply after midnight, and the distribution transformer enters light load), the primary end power factor will decrease.
3.2 Use of induction motor
The extensive use of induction motors will also cause the system power factor to decrease, because it is impossible for all motors to run at full load. When the motor runs at full load, the power factor can reach 85%; when the motor runs at 75% of the rated load, the power factor is 0.8; and when the motor runs at 50% of the rated load, the power factor is 0.7; if the motor runs at no load, the power factor is 0.2-0.3. It can be seen that when a large number of motors are used and these motors cannot all run at full load, the power factor of the system will inevitably decrease.
3.3 Use of gas discharge lamps Gas discharge lamps are becoming more and more widely used in residential, industrial and commercial lighting, and gas discharge lamps also operate at a low power factor.
3.4 Use of repaired motors Since users inevitably use a large number of repaired motors, the number of turns of the stator windings of these repaired motors is usually less than the original number of turns. Therefore, the leakage flux in these motors increases, causing the power factor of the motor to decrease.
4 The significance of improving power factor
4.1 Can reduce production costs, reduce investment, and improve equipment utilization
The power factor can be expressed as follows:
cosφ =P/S=P/3UI
It can be seen that under certain voltage and current, the higher the cosφ, the greater the output active power. Therefore, improving the power factor is an effective way to give full play to the potential of equipment and improve equipment utilization.
4.2 It can reduce line voltage drop and improve voltage quality
The voltage loss of the power grid can be calculated by the following formula to obtain the line voltage drop:
&U =PR + QX/U
Here, P is the active load of the line; Q is the reactive load of the line; R and X are the line resistance and reactance respectively; U is the line supply voltage. If a capacitor with a capacitive reactance of Xc is used for compensation, the voltage loss is:
&U =PR + Q( X - Xc)/U
Therefore, after using compensation capacitor to improve power factor, voltage loss ΔU is reduced, and voltage quality is improved. From (5) or (6), we can know that the smaller the reactive load, the smaller the line voltage drop.
4.3 It can improve the transmission capacity of the power grid, increase energy utilization, reduce electricity costs, and increase economic benefits
Since the apparent power and active power are related as follows
P = S cosφ
Q = S2 - P2 = S ×sinφ = S × 1 - cos2φ
Therefore, when transmitting a certain active power P, the higher the cosφ, the smaller the required apparent power. When the active load is constant, if the power factor value is larger, then from (8) it can be seen that the reactive load is smaller, giving full play to the power generation and supply equipment.
production capacity and improved economic benefits.
5 Methods to improve power factor
The way to improve the power factor is mainly to reduce the reactive power required by each part of the power system, especially to reduce the load
Reactive power is the power system that can reduce the reactive current passing through it when transmitting a certain amount of active power.
There are many ways to improve the power factor, but generally speaking they can be classified into two categories:
5.1 Methods to improve natural power factor
The measures to reduce the reactive power required by each electrical equipment to improve its power factor are called methods to improve the natural power factor. The main methods are:
(1) Correctly select the model and capacity of asynchronous motors. According to relevant information, the power load of small and medium-sized asynchronous motors in my country accounts for more than 80% of the total load of the power grid. In several major power grids, the energy consumption of motors accounts for about 60% to 68% of the total industrial power consumption. Therefore, it is of great economic significance to reduce the loss and save energy of motors. Correctly selecting asynchronous motors so that their rated capacity matches the load they carry is very important for improving the power factor. In terms of selection, attention should be paid to selecting energy-saving types and eliminating high-energy-consuming motors. Different models should be selected according to the specific requirements of the motor's mechanical work on starting torque, number of starts, speed regulation, etc. The efficiency η and power factor cosφ of the motor are the main indicators reflecting the economic operation level of the motor, and are closely related to the load rate β. GB/T 12497-90 stipulates the three operating areas of three-phase asynchronous motors as follows:
When the load rate β is between 70% and 100%, it is the economic operation zone;
When 40% ≤β ≤70%, it is the general operation area;
When β < 40%, it is non-economic operation zone 1
(2) Select a matching transformer according to the load. The power factor of the primary side of the power transformer is not only related to the power factor of the load, but also to the load rate. If the transformer is fully loaded, the power factor of the primary side is only about 3 to 5% lower than that of the secondary side. If the transformer is lightly loaded, when the load is less than 0.6, the power factor of the primary side will drop significantly, by 11 to 18%. Therefore, it is more economical to operate the power transformer at a load rate above 0.6, and it is generally more appropriate to operate it at 60% to 70%. In order to make full use of the equipment and improve the power factor, the power transformer is generally not suitable for light load operation. When the load rate of the power transformer is less than 30%, it should be replaced with a transformer with a smaller capacity.
(3) Reasonably arrange and adjust the process flow. Reasonably arrange and adjust the process flow, improve the operating status of motor equipment, and limit the no-load operation of welding machines and machine tool motors. For example, the no-load automatic delayed power-off device process can be used.
(4) Synchronous operation of asynchronous motors. For wound-rotor asynchronous motors with a load factor not exceeding 0.7 and a maximum load not exceeding 90% of the rated power, they can be synchronized when necessary. That is, after the wound-rotor asynchronous motor is started, DC excitation is sent to the three-phase winding of the rotor, which generates torque to pull the asynchronous motor into synchronous operation. Its operating state is similar to that of a synchronous motor. In the case of overexcitation, the motor can send reactive power to the power grid, thereby achieving the purpose of improving the power factor.
5.2 Compensation methods to improve power factor
The use of equipment that supplies reactive power to compensate for the reactive power required by electrical equipment to improve its power factor is called a compensation method to improve the power factor. To improve the power factor by using the compensation method, new equipment must be added and the amount of non-ferrous and ferrous metals required must be increased. In addition, the compensation equipment itself also has power loss, so from an overall perspective, the method of improving the natural power factor of electrical equipment should be used first. However, when the power factor does not reach the value required by the "Electric Power Design Technical Specifications", special compensation equipment must be used to improve the power factor. There are usually three methods for applying artificial compensation for reactive power: using phase-shifting capacitors (i.e. electrostatic capacitors), using synchronous motors, and using synchronous phase regulators.
When the synchronous motor operates in overexcitation mode (0.8 to 0.9 lead), it delivers reactive power to the power system, improving the power factor of the industrial enterprise. Generally, when the process conditions are met, a technical and economic comparison should be made when using or not using synchronous motors to improve the power factor of the enterprise. Usually, for large-capacity motors that work at low speed, constant speed and long-term continuous operation, synchronous motor units are suitable, such as steel rolling motor units, ball mills, air compressors, blowers, water pumps and other equipment. When these equipment use synchronous motors as prime movers, their capacity is generally above 250 kW, and the environment and starting conditions can meet the requirements of synchronous motors, and the downtime is less, so it can play a great role in improving the power factor. However, the synchronous motor has a complex structure and is equipped with a set of starting control equipment. The maintenance workload is large and the price is more expensive than the asynchronous motor. At present, the price of high-voltage phase-shifting capacitors has generally decreased, which correspondingly improves the superiority of the "compensation scheme of asynchronous motors plus phase-shifting capacitors". Phase-shifting capacitors are widely used as artificial compensation devices in industrial enterprises due to their advantages such as low power loss, convenient operation and maintenance, and small short-circuit current.
To sum up, improving the power factor will inevitably promote the country's energy utilization and the economic benefits of enterprises. It is an indispensable condition for ensuring the power quality, voltage quality, reducing network losses and safe operation of the power system. Appropriate measures should be taken according to different situations to improve the power factor, reduce reactive power losses, and thus improve economic benefits.
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