SVG static var generator vs SVC static var compensator
Reactive power compensation device is essential 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 and reduce the power cost, In addition, in long-distance transmission lines, the installation of 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 at the receiving end and the power grid.
Reactive power compensation equipment has experienced several stages of development. The early typical representative is the synchronous compensator, which is large in size and high in cost and has been gradually eliminated, The second method is the parallel capacitor method. The main advantages are low cost and easy to install and use. However, according to the possible power quality problems such as harmonics in the system, pure capacitors have become rare.
At present, the series reactor capacitor compensation device is a way to improve the power factor. When the user’s system load is continuous production and the load change rate is not high, it is generally recommended to adopt the fixed compensation mode of FC or the automatic compensation mode of step-by-step switching controlled by contactors. This is applicable to the medium voltage and low voltage power supply and distribution systems.
When the load changes rapidly or is an impact load, it needs to be compensated quickly. For example, the internal mixer in the rubber industry, and the demand for the system for reactive power also changes rapidly. However, since the capacitor used in the general reactive power automatic compensation system is disconnected from the operating state and exits the power grid, there is residual voltage between the two poles of the capacitor. The magnitude of the residual voltage is unpredictable and requires 1-3 minutes of discharge time. Therefore, the interval between putting the capacitor into the power grid again can not be put into use for the second time until the residual voltage is consumed below 50V through the discharge resistance inside the capacitor. rapid response cannot be achieved, In addition, since there are a large number of harmonics in the system, the LC tuned filter compensation device composed of capacitors and series reactors needs large capacity investment to ensure the safety of capacitors, but at the same time, it may cause excessive compensation of the system and make the system capacitive.
Static Var Compensator SVC
The static var compensator is composed of TCR (thyristor controlled reactor) + FC (fixed capacitor). thyristor controlled reactor + fixed capacitor bank (usually a certain proportion of reactors need to be connected in series). The importance of static var compensator is that it can continuously adjust the static var compensator by adjusting the trigger delay angle of thyristor in TCR, At present, SVC static var compensator is mainly used in medium and high voltage power distribution systems. It is especially suitable for occasions with large load capacity, serious harmonic problems, impact loads and high load change rate, such as steel plants, rubber, non-ferrous metallurgy, metal processing, high-speed railway, etc.
Static Var Generator SVG
With the development of power electronics technology, especially the emergence of IGBT devices and the improvement of control technology, another kind of reactive power compensation equipment, which is different from the traditional ones based on capacitors and reactors, comes into being. It is static var generator (SVG), which generates reactive power through PWM pulse width modulation control technology and is capacitive, Or it absorbs reactive power and is inductive. Since static var generator SVG does not use a large number of capacitors, but uses multi-level technology of bridge converter circuit or PWM technology for processing, it does not need to calculate the impedance in the system when it is used. At the same time, compared with static var generator SVC, static var generator SVG has the advantages of small size, more rapid continuous dynamic and smooth regulation of reactive power, and capacitive bidirectional compensation.
Comparative analysis of svg static var generator and svc static var compensator
Working principle
SVC static var compensator can be regarded as a dynamic reactive power source. According to the demand of access to the power grid, it can provide capacitive reactive power to the power grid and absorb the redundant inductive reactive power of the power grid. When the capacitor bank is usually connected to the power grid with a filter bank, it can provide reactive power to the power grid. When the power grid does not need too much reactive power, the redundant capacitive reactive power is absorbed by a parallel reactor. The reactor current is controlled by a thyristor valve group. By adjusting the trigger phase angle of the thyristor, the effective value of the current flowing through the reactor can be changed, so as to ensure that the reactive power of SVC at the grid access point can just stabilize the voltage at that point within the specified range, and play the role of reactive power compensation of the grid.
SVG static var generator takes the high-power voltage type inverter as the core, and quickly absorbs or emits the required reactive power 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, so as to achieve the purpose of rapid and dynamic regulation of reactive power.
Fast response speed
Generally, the response speed of SVC 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 on voltage fluctuation and flicker.
Low voltage characteristics
SVG static var generator has the characteristics of current source, and the output capacity is little affected by the bus voltage. This advantage makes SVG have great advantages when it is used for voltage control. The lower the system voltage, the more dynamic reactive power regulation voltage is required. SVG has good low-voltage characteristics, and the output reactive current has no relationship with the system voltage. It can be regarded as a controllable constant current source. When the system voltage decreases, it can still output rated reactive current and has strong overload capacity, SVC static var compensator is impedance type, and its output capacity is greatly affected by the bus voltage. The lower the system voltage, the lower the output reactive current capacity, and it has no overload capacity. 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.
Operation safety performance
SVC static var compensator uses thyristor regulating reactance and multiple groups of capacitors as the main means of reactive power compensation, which is very easy to cause resonance amplification and lead to safety accidents. When the system voltage fluctuates greatly, the compensation effect is greatly affected and the operation loss is large, SVG static var generator matching capacitor does not need to be equipped with filter bank, and there is no resonance amplification phenomenon. SVG is an active compensation device and a current source device composed of IGBT, which can be turned off, thus avoiding resonance phenomenon and greatly improving operation safety.
Harmonic characteristics
SVC uses thyristor to control the equivalent fundamental impedance of the reactor, which is not only greatly affected by system harmonics, but also generates a large number of harmonics. Therefore, filter banks must be used to filter out the harmonic content generated by SVC. SVG adopts three-level single-phase bridge technology, and single-phase can output 5-level voltage waveform. The pulse modulation method of carrier phase shift is adopted, which not only has little influence on system harmonics, but also can suppress system harmonics. Compared with SVC, SVG can greatly reduce the harmonic content in the compensation current after adopting multiple, multi-level or pulse width modulation technologies.
Cover an area
Under the same compensation capacity, the floor area of SVG is 1 / 2 to 2 / 3 less than that of SVC. Since SVG uses fewer reactors and capacitors than SVC, it greatly reduces the volume and floor area of the device. The reactor in SVC not only has a relatively large volume, but also takes into account the installation interval between them, and the overall floor area is large.
Summary: SVG reactive power compensation device has the advantages of fast response speed, low harmonic content and strong reactive power regulation ability, which can greatly improve the power quality of power grid, and has become the development direction of reactive power compensation technology.
