TCR alone can only provide inductive reactive power, so it is often used in conjunction with shunt capacitors. After the capacitors are connected in parallel, the total reactive power is the net reactive power after the reactive power of TCR and parallel capacitors is cancelled, so the total reactive current of the compensator can be biased to the range that can absorb capacitive reactive power. In addition, the small tuning reactor on the parallel capacitor series can also be used as a filter to absorb the harmonic current generated by TCR. By controlling the conduction angle of the anti-parallel thyristor connected in series with the reactor, both inductive reactive current and capacitive reactive current can be transmitted to the system. Because of its fast response time (less than half a cycle), great flexibility and continuous adjustment of reactive power output, this compensation device is the most widely used device in China's transmission system and industrial enterprises.
The basic schematic diagram of TCR+FC SVC is shown in figure 1, and the schematic diagrams of voltage and current before and after compensation are shown in figures 2 and 3. Single-phase TCR is composed of two anti-parallel thyristors and reactors in series, while three-phase TCR is generally connected in triangle. In the figure, QS is the reactive power provided by the system; QL is the reactive power of the load, which changes randomly; The capacitive reactive power provided by QC for the filter is fixed; QR provides adjustable inductive reactive power for TCR.
QS=QL+QR-QC
When the load is disturbed, SVC can adjust the inductive reactive power emitted by TCR by adjusting the trigger angle of thyristor, so that QR can always make up for the change of QL. The integration of such a circuit into the power grid is equivalent to △QS=△QL+△QR=0. This is the principle of TCR+FC static var compensation device to dynamically compensate reactive power.
Connecting this circuit in parallel to the power grid is equivalent to connecting the AC voltage regulating circuit to the inductive load, and the effective phase shift range of this circuit is 90o ~ 180o. When the trigger angle α=90o, the thyristor is completely turned on, and the conduction angle δ= 180o. At this time, the reactive current absorbed by the reactor is the largest. According to the relationship between conduction angle and equivalent admittance of compensator:
BL=BLmax(δ-sinδ)/π
Where BLmax= 1/XL. It can be seen that increasing the conduction angle can increase the equivalent admittance of the compensator, which will reduce the fundamental component of the compensation current, so the reactive power absorbed by the compensator can be changed by adjusting the trigger angle to achieve the purpose of adjusting the reactive power.
Fig. 65438 Basic schematic diagram of TCR+FCSVC.
Fig. 2 SVC is under-compensated before it is put into operation, with voltage leading current of 45 and cosφ=0.707.
Fig. 3 SVC is fully compensated after it is put into operation, and the current and voltage overlap, cosφ= 1.
3 application fields
(1) When the electric arc furnace is connected to the power grid as a nonlinear and irregular load, it will have a series of adverse effects on the power grid. Among them, the main effects are: the three phases of the power grid are seriously unbalanced, resulting in negative sequence current and higher harmonics. Among them, even harmonics of 2 and 4 and odd harmonics of 3, 5 and 7 are common, which makes the voltage distortion more complicated and there is serious voltage flicker.
SVC has the characteristics of fast dynamic compensation and fast response. It can quickly supply reactive current to the arc furnace, stabilize the bus grid voltage, and minimize the impact of flicker. The split-phase compensation function of SVC can eliminate the three-phase imbalance caused by electric arc furnace, and the filtering device can eliminate harmful higher harmonics and improve the power factor by providing capacitive reactive power to the system.
(2) The symmetrical load of large motors such as rolling mills causes voltage drop and voltage fluctuation in the power grid, which makes electrical equipment unable to work normally and reduces production efficiency and power factor; The load will produce harmful high-order harmonics in the transmission device, mainly odd harmonics and side frequencies represented by 5, 7, 1 1 and 13, which will seriously distort the grid voltage. The installation of SVC system can solve the above problems, keep the bus voltage stable and free from harmonic interference, and the power factor is close to 1.
(3) Urban secondary substation (66kv/ 10kv): In the regional power grid, capacitor banks are generally switched in stages to compensate the reactive power of the system and improve the power factor. This method can only provide capacitive reactive power to the system and cannot be adjusted quickly and accurately with the change of load. While ensuring the bus power factor, it is easy to cause reactive power to the system, increase the bus voltage and endanger the stability of electrical equipment and system.
TCR combined with fixed capacitor bank FC or TCR+TSC can quickly and accurately compensate capacitive and inductive reactive power, stabilize bus voltage and improve power factor. In addition, when transforming the old compensation system, only adding thyristor phased reactor (TCR) on the basis of the original fixed capacitor bank can achieve the best effect with the least investment, which has become the most effective method to improve the power supply quality of regional power grid.
(4) Electric locomotive power supply: The transportation mode of electric locomotive not only protects the environment, but also causes serious "pollution" to the power grid. Because the electric locomotive is single-phase power supply, this single-phase load causes serious three-phase imbalance and low power factor in the power supply network. At present, the only way for countries all over the world to solve this problem is to install SVC system at a proper position along the railway, balance the three-phase power grid through the split-phase rapid compensation function of SVC, and improve the power factor through filter devices.
(5) Mine hoist: As a high-power, frequently-started, periodic impact load, the hoist not only endangers the safety of the power grid, but also causes emergency stop failures such as over-current and under-voltage of the hoist, which affects mine production. Therefore, it is of great significance to dynamically compensate reactive power and control higher harmonics in the power supply system of mine hoist to improve the safe operation reliability of mine hoist and power grid and improve the economic benefits of enterprises.
The installed power of hoist is large, which accounts for a large proportion in the total power supply load of mine. With the expansion of coal mine production scale and the deepening of mine, the capacity of supporting lifting equipment is also increasing. The single machine capacity has reached 2000~3000kW, and some even reached 5400kW, and the single bucket lifting capacity has reached 34t. Starting such a large load will cause great impact current to the power grid, with large reactive current component and low power factor. Therefore, high-power hoists require higher capacity and stability of power supply network.
Among them, the main problems of high-power hoist are:
Cause voltage drop and voltage fluctuation of power grid;
Generally, there are even harmonics of order 2 and 4, odd harmonics of order 3 and 5 and other high harmonics, which make the voltage distortion more complicated.
Low power factor;
The way to completely solve the above problems is that users must install dynamic reactive power compensator (SVC) with fast response. The response time of SVC system is less than lOms, which can completely meet the strict technical requirements.
(6) Long-distance transmission: At present, the global power is tending to high power grid, long-distance transmission and high energy consumption, which also forces the transmission and distribution system to be more effective. SVC can obviously improve the transmission and distribution performance of power system, which has been widely proved in the world, that is, SVC can be installed at one or more suitable locations in the power grid to achieve the following purposes when maintaining voltage balance under different power grid conditions:
Stabilizing the voltage of weak current system and reducing transmission loss
Increase transmission power, so that the existing power grid can exert its maximum power.
Increase transient steady-state limit
Increase damping under small interference
Enhance voltage control and stability
Buffer power oscillation
(7) Other general fields
With the energy-saving transformation in oilfield, cement chemical industry and other fields, there are more power electronic devices such as transmission and frequency conversion speed regulation, which produce harmful higher harmonics and endanger other electrical equipment, resulting in lower power efficiency and shorter heating life of other electrical equipment.