Keywords 10kV voltage; Power supply system; Static reactive power compensation device
1, Use of Static Var Compensator (SVG)
SVG is mainly composed of capacitors and reactors. As long as it is applied to load compensation and system compensation, it can realize smooth and fast control through the on-off of power electronic switches. In the aspect of load compensation, SVG can effectively suppress the voltage fluctuation and flicker caused by the change of load, and compensate the lack of reactive power when the load lacks reactive power, thus improving the power factor and optimizing the flow of electric energy in the power grid; In the aspect of system compensation, SVG can keep the voltage of line nodes stable and effectively suppress fluctuations; To a certain extent, it increases the transmission capacity of the line to active power and ensures the static stability of the power grid; If the power grid fails, the device can also stabilize the voltage at a constant value in a short time, effectively improving the static stability of the power grid under fault; In addition, SVG with large capacity, fast response, flexible adjustment mode and good economy is more valuable to power system [1].
2. Principle of Static Var Compensator (SVG)
SVG is composed of thyristor controlled reactor and thyristor switching capacitor, and its basic topological structure is shown in figure 1. It can be seen that it is mainly composed of TSC, TCR, filter, step-down device and control part. Its basic function is to control the output of reactive power by controlling the switching of thyristors or adding capacitor banks as needed, so as to effectively control the voltage of reactors and power grid electrical contacts. In this paper, a static var compensator is designed, mainly for load compensation. The following is a specific introduction [2].
3. Compensation principle under three-phase unbalanced load
Figure 2 shows a three-phase unbalanced load, which is mainly supplied by three-phase symmetrical voltage. If the neutral point is ungrounded, it can be expressed as triangle connection through Y-δ transformation, and the load is mainly compensated by 1 SVC.
The three-phase phase voltage and line voltage can be expressed as:
( 1)
Among them, the line current of each phase can be expressed as:
(2)
By selecting the reference phase of phase A, the relationship between the line current and its symmetrical component can be obtained by the following formula:
(3)
The symmetrical component of A-phase load and the symmetrical component of A-phase line current of delta reactive power compensation device can be obtained from the above two formulas. The static reactive power compensation device is added to the system to make the negative sequence component of line current zero and make the three-phase load balance, that is, formula (4). After compensation, the power factor of the system can be increased to 1. At this time, it is necessary to ensure that the imaginary part of the positive sequence current is zero, that is, formula (5).
IA2(l)+IA2(r)=0 (4)
im[ia 1(l)+ia 1(r)]= 0(5)
According to the above three formulas, the three-phase susceptance of reactive power compensation device can be solved as follows:
(6)
4. Design the main circuit
In the main circuit of SVG, the two components of static var compensator are connected in a triangular way, which has many advantages, such as: zero sequence component will not be generated in line current; It can effectively cancel 3N voltage harmonics; Because the junction level of the element is the same as the rated voltage level of the power grid, the corresponding line voltage can be directly taken. There are two connection modes of thyristor valve in the circuit, namely, anti-parallel connection with high-power diode and anti-parallel connection. Because of the former connection mode, the maximum voltage on thyristor and diode is twice the peak line voltage. When the latter connection mode is adopted, because the capacitor is put into operation when the voltage crosses zero, the highest voltage borne by the thyristor valve at this time is only the peak value of the line voltage, which can greatly reduce the number of series thyristor valves and effectively reduce the cost. Therefore, this paper adopts the latter connection mode.
5. Current and voltage sharing of thyristor valve.
5. 1 current sharing of thyristor valve
In the reactive power compensation device, a plurality of thyristors are generally connected in parallel to withstand large current to meet the capacity requirements of a single group of capacitors. Different characteristics of components lead to different currents flowing through components even if the terminal voltages are the same, which affects the current sharing effect. If the series inductor is used for current sharing, the reactance value of the inductor is required to be greater than the internal resistance of the thyristor, so as to effectively reduce the difference of each branch current and achieve good current sharing effect.
5.2 Voltage sharing of thyristor valve
Figure 3 shows the voltage equalizing circuit of thyristor. Similarly, in order to make thyristors withstand higher voltage, it is necessary to connect multiple thyristors in series, because the voltage borne by a single thyristor is relatively low. In the process of series connection, the voltage will also be uneven, which is also caused by the differences in the characteristics of each component. This will cause the voltage distribution of thyristor to be uneven during use. The solution is to install a voltage equalization device to limit the degree of unevenness. The main function of R 1 is to balance the voltage in a stable state to prevent the voltage on each thyristor from being unequal. The resistance value of R 1 is less than the forward and reverse resistance when any series device is blocked. In addition, due to each thyristor, the voltage borne by each thyristor will be uneven. The reason why the resistor R2 is connected in series with the capacitor C is to prevent the capacitor from discharging when the thyristor is turned on and generating excessive di/dt.
6. Concluding remarks
With the rapid development of power electronics technology, the application of reactive power compensation is more and more urgent. Give full play to the advantages of static economy, simplicity, convenience, reliability and energy saving of reactive power compensation, and overcome various limitations brought by immature technology in the past.
refer to
[1] Application of reactive power compensation technology in distribution network. Power network technology, 1999(6)
[2] Liu Huijin et al. 10kV Dynamic Compensation of Fundamental Reactive Power in Distribution Network. Electrotechnics, 200 1(9)