Difference of reactive power compensation device installed on high and low voltage side of transformer
Writer： Hengfeng you electric Time：2023-04-07 views：times
Where the reactive power compensation equipment compensates depends entirely on the compensation target. A capacitor is used to compensate the reactive power consumed by the asynchronous machine, which means that the motor consumes too much reactive power when it is turned on, and the voltage is pulled too low, so that other equipment can't work normally. It needs to be compensated by installing an electric container. When the motor is running, the capacitor is hung on the power grid as a system equipment, which must have an impact on all power grid equipment. It is only a big and small problem, whether it is appropriate or not.
For example, in order to compensate a 10mvar inductive reactive power, a 100mvar capacitive compensation device is installed. It is difficult to say that the purpose of installing this compensation device is 10mvar.
1. The transformer does not need compensation
The purpose of reactive power compensation must be clear. It is not that I need compensation, but because of the voltage. As we all know in power flow calculation, reactive power is balanced locally, and reactive power mainly affects voltage. Therefore, in order to ensure local voltage, reactive power compensation needs to be carried out locally. The voltage should not be too high or too low. If it is too high, the insulation breakdown will damage the equipment, and if it is too low, it will not meet the working conditions of the equipment. The purpose of transformer is to turn high voltage into low voltage. It itself has many voltage regulation means, such as changing tap (transformation ratio), changing winding design and changing iron core design.
2. The reactive power of the magnetic field established by the transformer itself is provided by the power grid at the charging side
Do not run the long line with no-load main transformer, because the long line is capacitive and the main transformer is inductive. Series resonance is easy to occur when connected in series, resulting in huge high voltage at the resonance point and failure.
3. The secondary side can compensate the reactive power of the primary side
Reactive current is caused by load. If the secondary side is connected with pure resistive load, the power factor of the secondary side is 1, and the primary side cannot compensate the reactive power of the secondary side.
4. The transformer needs to consume reactive power to establish magnetic field
In other words, the operation of the transformer will produce reactive power, that is, the primary side can be equivalent to inductive load. When the transformer is unloaded, the transformer can be regarded as inductive load.
When the secondary side is connected to the load, the angle of the load current relative to the voltage will be reflected in the primary side. Passing through the transformer does not change the angle of the current relative to the voltage (why?), Therefore, the secondary side can compensate the primary side.
In addition to the current converted from the secondary side, the current at the primary side also includes the current (inductive) required to establish the magnetic field. Therefore, if the secondary side compensates the capacitive current, the magnetic field current will be offset at the primary side of the transformer, that is, "eaten by the transformer", and the inductive current compensated at the secondary side will be superimposed. The greater the current, the more reactive power required to establish the magnetic field, and the greater the inductive current required to provide.
As for whether the reactive power at the primary side needs to be compensated, it is another problem. The electricity charge of the power consumption enterprise is to assess the reactive power. If the reactive power is too low, it will be fined. Therefore, the power consumption enterprise will compensate the power factor higher than a certain value. The generator side pays more attention to the voltage, but does not care much about the power factor.
5. Code for design of power supply and distribution system GB 50052-95
Article 5.0.2 when the measures to improve the natural power factor still fail to meet the reasonable operation requirements of the power grid, parallel power capacitors shall be used as reactive power compensation devices. When the technical and economic comparison confirms that it is reasonable to use synchronous motor as reactive power compensation device, synchronous motor can be used. The first
Article 5.0.3 when the power capacitor is used as the reactive power compensation device, it should be balanced and compensated locally, and the reactive power of the low-voltage part should be compensated by the low-voltage capacitor; The reactive power of the high-voltage part should be compensated by the high-voltage capacitor. The reactive power of electrical equipment with large capacity, stable load and frequent use should be compensated separately and locally. Capacitor banks for compensating basic reactive power should be compensated centrally in the distribution substation. In workshops with normal environment, low-voltage capacitors should be compensated separately.
Article 5.0.4 reactive power compensation capacity should be determined according to reactive power curve or reactive power compensation calculation method.
Article 5.0.5 for the switching mode of reactive power compensation device, manual switching of reactive power compensation device should be adopted under one of the following circumstances.
1、 Capacitor bank for compensating low-voltage basic reactive power.
2、 Perennial stable reactive power.
3、 Transformers that are often put into operation or high-voltage motors and high-voltage capacitor banks with less switching times in distribution and substation.
Article 5.0.6 automatic reactive power compensation device should be installed under any of the following circumstances.
1、 Avoid over compensation and install automatic reactive power compensation device when it is economically reasonable. 2、 Avoid excessive voltage during light load, resulting in damage to some electrical equipment, and when it is economically reasonable to install automatic reactive power compensation device.
2、 Only when the automatic reactive power compensation device is installed can the allowable value of voltage deviation under various operating loads be met.
Article 5.0.7 when the high-voltage and low-voltage automatic compensation devices have the same effect, the low-voltage automatic compensation device should be used.
Article 5.0.8 the regulation mode of automatic reactive power compensation should be determined according to the following principles:
1、 When the compensation is mainly based on energy saving, the reactive power parameter adjustment is adopted; When the three-phase load is balanced, the power factor parameter can also be adjusted.
2、 Those who provide reactive power necessary to maintain the voltage level of the power grid and compensate mainly by reducing voltage deviation shall be adjusted according to the voltage parameters, except those who have adopted automatic voltage regulation by transformer.
3、 When the reactive power changes steadily with time, it shall be adjusted according to the time parameters.
Article 5.0.9 when grouping capacitors, the following requirements shall be met:
1、 Resonance shall not occur during switching of grouped capacitors.
2、 Appropriately reduce the number of grouping groups and increase the grouping capacity.
3、 It shall adapt to the technical parameters of supporting equipment.
4、 The allowable range of voltage deviation shall be met.
