No power refers to deviation pressure caused by long-term use of two phases (lighting and use of welding machines), that is, one or two phase incoming lines do not work or work less. When the inactive power is too high or too low, it will cause the high-voltage incoming line current deviation pressure, line overheating, and electric fuse blowout, resulting in phase loss and damage to running motor tools. Too high or too low power is a means for the power department to control the correct use of electricity by users!
The usual remedy is to install one or more groups of power compensation cabinets in the power distribution room when using one-phase and two-phase power (usually using multiple welding machines), measure the current, and replace the power connector to make the output voltage uniform.
Active power is the electrical power required for the normal operation of electrical equipment. Reactive power is anything but reactive power. It's very useful. Motors need to build and maintain a rotating magnetic field to spin the rotor, thereby driving mechanical motion. The motor's rotor magnetic field is established by drawing reactive power from the power supply. Transformers also require reactive power to generate the transformer's primary coil magnetic field. Why the power of the secondary coil is fine: 100 units, that is, 100 units of power are transmitted to the device. However, since most electrical systems have inherent reactive losses, you can only use the power of 70 units. Unfortunately, it has to pay for 100 units, although there are only 70 units. In this example, the power factor is 0.7 (if most equipment has a power factor less than 0.9, equipment with a power factor less than 0.9 will be fined), which is mainly found in motor equipment (also called inductive loads. Power factor is the motor efficiency measurement standards.
This article explains the methods and significance of improving power factor, as well as methods of improving natural power factor and artificial compensation.
I. Overview.
In the power supply system, in addition to active power supply and reactive power supply, both are essential. When the perceived load is too large, its power factor is low, affecting the economic operation of lines and distribution transformers. Reactive power compensation equipment must be properly configured to improve the power factor of the system, thereby saving electrical energy and reducing losses.
1The relationship between power factor and reactive power.
The cosine of the phase difference (φ) between voltage and current is called power factor, represented by the symbol cosφ. In value, power factor is the ratio of active power to apparent power, that is, cosφ=P/S.
P2+Q2=S2.
In the operation of the power grid, power factor reflects the effective utilization of power output. We want the power factor to be as high as possible. In this way, the reactive power in the circuit can be reduced to a minimum.
2Relationship between active power loss and power factor of transmission and distribution lines.
Because the wire has resistance, when the current passes through the line, the line itself will produce active power loss, and its active power loss is proportional to the square of the current. Therefore, when a line transmits a certain amount of active power, the active power loss generated by the line itself is inversely proportional to the square of the power factor. Improving the power factor can reduce the active power loss of the line.
3Relationship between transformer copper loss and power factor.
During operation, when the transformer outputs a certain amount of active power, its copper loss is in negative proportion to the transformer's power, and the power is proportional to the power factor of the transformer. Therefore, improving the power factor can reduce the copper losses of the transformer.
4The relationship between the required capacity of the transformer and the power factor.
Since the transformer outputs a certain power in inverse proportion to the power factor of the transformer, when the transformer outputs a certain power, the increase in power factor can reduce the required capacity of the transformer, thereby improving the power supply capacity of the transformer.
2. Improve the power factor.
The main factors affecting power factor.
1. A large number of inductive equipment such as asynchronous motors and induction furnaces are the main consumers of reactive power. Among all the reactive power consumed by industrial and mining enterprises, the reactive power consumption of asynchronous motors accounts for 60% to 70%; the reactive power consumed by asynchronous motors when no-load accounts for 60% to 70% of the total reactive power consumption of the motors. Therefore, in order to improve the power factor of the asynchronous motor, it is necessary to prevent the motor from running without load and increase the load factor as much as possible.
2. The reactive power consumed by the transformer is generally about 10% to 15% of the rated capacity, and its no-load reactive power is about 1/3 of the full load. Therefore, in order to improve the power factor of power systems and enterprises, transformers should not be operated at no load or at low load for a long time.
3. If the power supply voltage exceeds the specified range, it will also have a great impact on the power factor. When the power supply voltage is higher than 10% of the rated value, the reactive power will increase rapidly due to the influence of magnetic circuit saturation. When the power supply voltage is lower than the rated value, the reactive power will also be reduced accordingly, thereby improving its power factor. However, reducing the supply voltage can affect the normal operation of electrical equipment.
The second is the general method of reactive power compensation.
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. It only takes various management or technical means to reduce the reactive power consumed by various electrical equipment. This is the most economical method to improve the power factor:
1) Reasonably select the motor capacity so that it can operate close to full load.
2) Motors whose actual load does not exceed 40% of the rated capacity should be replaced with small-capacity motors.
3) Reasonably arrange and adjust the process flow, improve the operating mode of electrical equipment, and limit the no-load operation of induction motors.
4) Select the transformer correctly and increase the transformer load rate (generally 75% to 80% is more appropriate). Transformers with load rates below 30% should be replaced.
5) For a wound rotor asynchronous motor with a load rate of 60% to 90%, it can be synchronized if necessary, and the motor can output reactive power to the power system.
2 Manual compensation power factor.
The requirement is met simply by increasing the natural power factor. Therefore, manual compensation is necessary. There are three main methods of reactive power compensation: low-voltage individual compensation, low-voltage centralized compensation and high-voltage centralized compensation.
1) Individual low voltage compensation.
Low-voltage individual compensation refers to dispersing single or multiple low-voltage capacitor banks in electrical equipment according to the reactive power requirements of individual electrical equipment, and sharing a set of circuit breakers with the electrical equipment. Through control, the protection device and motor cut simultaneously. Random compensation is suitable for compensating the reactive power consumption of individual large-capacity and continuous operations (such as large and medium-sized asynchronous motors), mainly excitation reactive power. The advantage of low-voltage individual compensation is that when the electrical equipment is running, reactive power compensation is invested, and when the electrical equipment is shut down, the compensation equipment is also started, which will not cause reactive power transfer. It has the advantages of low investment, small floor space, convenient installation, convenient configuration, simple maintenance, and low accident rate.
2) Low voltage centralized compensation.
Low-voltage centralized compensation refers to connecting low-voltage capacitors to the low-voltage bus side of the distribution transformer through a low-voltage switch, using the reactive power compensation cutting device as a control and protection device, and directly controlling the cutting of the capacitor according to the reactive load on the low-voltage bus. The capacitor is cut as a whole and cannot be adjusted smoothly. The advantages of low-voltage compensation: simple wiring, small operation and maintenance workload, balanced reactive power, and improved power distribution utilization.
3) High voltage centralized compensation.
High-voltage centralized compensation refers to the compensation method in which parallel capacitor banks are directly installed on the 6~10kV high-voltage busbar of the substation. It is suitable for users who are far away from the substation or the end of the power supply line; the compensation device automatically cuts according to the load, reasonably improving the user's power factor and avoiding an increase in electricity bills caused by a reduction in power factor.
The third is reactive power.
In addition to synchronous motors, the reactive power sources of power systems also include electrostatic capacitors, static reactive power compensators and static reactive power generators. In addition to capacitors, several other types can absorb capacitance and inductance.
1) Synchronous motor.
Synchronous machines include generators, motors and camera regulators. Synchronous generator is the most active power source and the most basic reactive power source. It mainly operates with lagging power factor to provide reactive power to the system. However, when necessary, it can also reduce the excitation current to make the power factor advanced, that is, The so-called phase-advancing operation is used to absorb the excess reactive power of the system. A synchronous camera is a synchronous motor that runs without load. The advantage is that it can absorb or output the reactive power of the system. The synchronous motor of the excitation device can smoothly adjust the input or output reactive power according to the voltage. However, it suffers from large active power loss, complicated operation and maintenance, slow impact speed, and gradually withdraws from power grid operation.
2) Parallel capacitor.
Parallel capacitor compensation is one of the most widely used reactive power supplies. Since the alternating current of the capacitor just exceeds the voltage on the capacitor plate, the parallel capacitor itself has low power consumption, flexible installation, and saves investment; it provides reactive power to the system, which can improve the power factor and reduce the reactive power provided by the generator.
3) Static reactive power compensator.
The static VAR compensator consists of a thyristor-controlled cutting reactor and a capacitor. Since the thyristor reacts very quickly to the control signal, the number of disconnections cannot be controlled. When the voltage changes, the static compensator can be adjusted quickly and smoothly to meet the needs of dynamic reactive power compensation, while achieving phase-separation compensation; effective adaptability to three-phase unbalanced loads and impact loads; however, due to the thyristor control during the cutting process High harmonics will be generated, so a special filter needs to be added.
4)Static var generator.
Its main body is a voltage source inverter, which converts the DC voltage on the capacitor into a three-phase AC voltage synchronized with the power system voltage through appropriate disconnection of the switchable thyristor, and then connects to the power grid through a reactor and a transformer. Appropriate control of the inverter's output voltage can flexibly change its operating conditions to make it in a capacitive, inductive or zero-load state. Compared with the static reactive power compensator, the generator responds faster and has less harmonic current. When the system voltage is low, it can still inject larger reactive power into the system.
3. Conclusion.
To sum up, improving the power factor of national energy utilization and promoting the economic benefits of enterprises are indispensable conditions for ensuring the power quality and voltage quality of the power system, reducing network losses and safe operation. Corresponding measures should be taken according to different situations to improve the power factor. Reduce reactive power losses and improve economic benefits.