Reactive power compensation device is indispensable in the power system, its main function is to improve the power factor of the power supply and distribution system, so as to improve the utilization rate of transmission equipment and substation equipment, improve the power efficiency, reduce the cost of electricity; In addition, in long-distance transmission lines, installing dynamic reactive power compensation devices at appropriate locations can also improve the stability of the transmission system, improve the transmission capacity, and stabilize the voltage of the receiving end and the grid
SVC
Static Var Compensator: (Static Var Compensator)
Its typical SVC representation is composed of TCR (Thyristor Controlled Reactor) + FC(Fixed Capacitor), that is, thyristor controlled reactor + fixed capacitor bank (usually a certain proportion of reactors need to be connected in series). The importance of the static reactive power compensation device is that it can continuously adjust the reactive power of the compensation device by adjusting the trigger delay Angle of the thyristor in TCR. SVC compensation form is currently mainly used in medium and high voltage distribution system, especially for large load capacity, serious harmonic problems, impact load, high load change rate of the occasion, such as steel mills, rubber, nonferrous metallurgy, metal processing, high-speed iron and so on.
SVG
Static Var Generator: (Static Var Generator)
It uses PWM pulse width modulation control technology to make it emit reactive power and be capacitive. Or absorb reactive power, inductive. Because SVG does not use a lot of capacitors, but adopts the bridge converter circuit multilevel technology or PWM technology to deal with, so there is no need to calculate the impedance in the system when using. At the same time, compared with SVC, SVG also has the advantages of smaller size, faster continuous dynamic smooth adjustment of reactive power, and tolerance inductive bidirectional compensation.
Comparative analysis of SVG and SVC reactive power compensation devices
- It works differently
(1) SVC can be regarded as a dynamic reactive power source. According to the needs of access to the grid, it can provide capacitive reactive power to the grid, it can also absorb the excess inductive reactive power of the grid, the capacitor bank is usually connected to the grid as a filter bank, it can provide reactive power to the grid, when the grid does not need too much reactive power, these excess capacitive reactive power, by a parallel reactor to absorb. The reactor current is controlled by a thyristor valve group. By adjusting the thyristor trigger phase Angle, the effective value of current flowing through the reactor can be changed, so as to ensure that the reactive power of SVC in the grid access point can just stabilize the voltage of the point within the specified range, and play the role of reactive power compensation.
(2) SVG takes high-power voltage-type inverter as its core. By adjusting the amplitude and phase of the output voltage of the inverter, or directly controlling the amplitude and phase of the current on the AC side, it can rapidly absorb or emit the required reactive power and realize the purpose of rapid dynamic adjustment of reactive power. - Fast response speed
General SVC response speed is 20-40ms; The response speed of SVG is no more than 5ms, which can better suppress voltage fluctuation and flicker. Under the same compensation capacity, SVG has the best compensation effect for voltage fluctuation and flicker. - Good low voltage characteristics
SVG has the characteristics of a current source, and the output capacity is little affected by the bus voltage. The lower the system voltage, the more the need for dynamic reactive power regulation voltage. SVG has good low-voltage characteristics, and the output reactive current has no relationship with the system voltage, so it can be regarded as a controllable and constant current source. When the system voltage is reduced, it can still output rated reactive current and has strong overload capability. While SVC is an impedance characteristic, the output capacity is greatly affected by the bus voltage, the lower the system voltage, the output reactive current capacity is proportionally reduced, and there is no overload capability. Therefore, the reactive power compensation capability of SVG is independent of the system voltage, while the reactive power compensation capability of SVC decreases linearly with the decrease of the system voltage. - High operation safety performance
SVC uses thyristor to adjust the reactance and multiple sets of capacitors as the main means of reactive power compensation, which is very prone to resonant amplification, resulting in safety accidents. When the system voltage fluctuates greatly, the compensation effect is greatly affected and the running loss is large. SVG supporting capacitors do not need to set filter banks, there is no resonance amplification phenomenon, SVG is an active compensation device, is the use of turn-off device IGBT current source device, thus avoiding the phenomenon of resonance, operation safety performance is greatly improved. - Harmonic characteristics
SVC uses thyristor to control the equivalent fundamental impedance of the reactor, which is not only greatly affected by the harmonics of the system, but also produces a large number of harmonics by itself, so the harmonic content generated by SVC must be filtered by a filter bank. SVG adopts three-level single-phase bridge technology, single-phase can output 5-level voltage waveform, and adopts the pulse modulation method of carrier phase shift, which is not only less affected by system harmonics, but also can suppress system harmonics. Compared with SVC, SVG greatly reduces the harmonic content in the compensation current by adopting multiple, multilevel or pulse width adjustment techniques. - Small footprint
With the same compensation capacity, the footprint of SVG is reduced by 1/2 to 2/3 compared to that of SVC. Because SVG uses fewer reactors and capacitors than SVC, it greatly reduces the size and footprint of the device. The reactor in SVC is not only relatively large in size, but also takes into account the installation interval between each other, and the overall footprint is larger.
To sum up, SVG reactive power compensation device has the advantages of fast response speed, less harmonic content, strong reactive power regulation ability, etc., improve the power quality of the grid, and has become the development direction of reactive power compensation technology.
