Double-tuned filter synthesis technology using inductive filter design

1 Introduction

Converter transformers and filter devices are important technical equipment in DC transmission systems. Although traditional converter transformers and filter solutions are widely used, they are not perfect. Traditional filter solutions install filters between the AC bus and the grid-side windings of converter transformers. This requires the harmonic currents and reactive currents generated by the converter to pass through the grid-side and valve-side windings of the transformer. This will inevitably cause strong harmonic magnetic flux to pass through the core and structural parts, increasing the insulation strength of the transformer, losses, vibration and noise [1-2].

In order to solve the above problems, this paper proposes a new type of converter transformer and its filtering system. It is a new harmonic suppression method that uses the principle of electromagnetic induction to achieve harmonic magnetic potential balance between secondary windings, which is called induction filtering [3-5]. The harmonic suppression mechanism of this new filtering method is analyzed. On this basis, a comprehensive design of the valve-side filter of the new DC transmission system platform under construction is carried out [6].

2 Harmonic suppression mechanism of inductive filtering

Taking the single-phase three-winding transformer with a middle lead-out tap connected to a single tuned filter as shown in Figure 1 as an example, a new filtering method using the ampere-turn balance of the transformer coupling winding as the filtering mechanism is described. In the figure, 1 represents the primary winding, 2 represents the secondary extension winding, 3 represents the secondary common winding, and Ih represents the harmonic current source. The arrows show the flow path of the harmonic current in the transformer.

Analysis shows that: under the influence of the harmonic current passing through the Yanbian winding 2, the common winding 2 and the primary winding 1 will induce corresponding harmonic currents to satisfy the following magnetic potential balance relationship:

W2Ih=W3Ih3+W1Ih1 (1)

In the formula: W1 is the number of turns of the primary winding, W2 is the number of turns of the secondary load winding, and W3 is the number of turns of the secondary filter winding.

If the ampere-turns of the extended winding 2 and the common winding 3 can be kept balanced, then Ih1 = 0, and no harmonic current will be induced in the primary winding, thereby isolating the primary winding from the harmonics and achieving the purpose of harmonic shielding.

It can be seen that the implementation of this filtering method requires the following two conditions to be met simultaneously [3]:

(1) Figure 1 The purpose of the transformer secondary winding tapping is to connect the filter to drain the harmonics and provide a prerequisite for the harmonic ampere-turn balance filtering method of the transformer coupling windings 2 and 3. The better the drainage effect, the better the harmonic shielding effect of the coupling winding. Therefore, the filter should strive to achieve resonance.

(2) Whether the ampere-turns of the transformer's secondary coupled windings 2 and 3 can be balanced so that the primary winding 1 does not induce harmonic currents depends on the arrangement of the windings and their impedance relationship. Specifically, the transformer design ensures that the equivalent impedance of the common winding 3 is equal to or approximately equal to 0.

If the above two conditions are met at the same time, the flow path of harmonics in the transformer can be effectively suppressed, so that the harmonics will not be fed back to the grid side through the transformer, thereby playing a role in isolating and shielding the harmonics.

3 Filter Design

3.1 Double-tuned filter characteristics analysis

According to the characteristics of the DC transmission system, an experimental platform is established as shown in Figure 2 to verify the new filtering method and compare and analyze the difference with the traditional passive filtering effect. The rectifier station uses a new converter transformer, the secondary winding has taps connected to DT5/7 and DT11/13, and the primary winding outlet, that is, the grid side, is connected to the second-order high-pass filter HP2 and a parallel capacitor; the inverter station uses a traditional converter transformer, which will not be explained here. The valve side of the new DC transmission system uses a double-tuned filter, and its basic circuit structure can be seen from Figure 2; it has two resonant frequencies and absorbs the harmonics of two adjacent frequencies at the same time, which is equivalent to two parallel single-tuned filters [7].

The valve-side double-tuned filter is composed of a series resonant circuit C1, L1 and a parallel resonant circuit C2, L2. The two circuits have their own frequency-impedance relationship and resonance point, and the two circuits are connected in series to form the impedance-frequency relationship of the double-tuned filter. The impedance characteristics of the parallel circuit C2L2 are shown in Figure 3(a). In the two impedance characteristic curves in Figure 3(a), the dotted line part indicates that the impedance of the filter is capacitive; the solid line part indicates that the impedance of the filter is inductive; it can be seen from Figure 3(a) that for the fundamental frequency, the impedance of the parallel circuit is very small, that is, the voltage borne by the parallel circuit is very low, and the impedance of the series circuit is large and is capacitive. Therefore, for the double-tuned filter, the series capacitor mainly bears the connected bus voltage.

At the intersection of the impedance characteristic curves of the series and parallel circuits, ω1 and ω2, the capacitive reactance and the inductive reactance cancel each other out, and their corresponding harmonic orders are n1 and n2. The blocking frequency characteristics of the double-tuned filter need to combine the blocking frequency characteristics of the series circuit and the parallel circuit. At this time, the blocking frequency characteristic curve of the double-tuned filter is shown in Figure 3(b), and it is obvious that it resonates at the harmonic orders n1 and n2.

3.2 Double-tuned filter parameter design[8]

According to formula (4), using MATLAB image processing tools, the following relationship curve between DT11/13 safety factor and tuning point can be obtained, as shown in Figure 4.

The following conclusions can be drawn from the curve in Figure 4: (1) The safety factor must be greater than 1, so that each tuning point can be slightly moved forward to avoid resonance with the system, and DT11/13 can be inductive at the 11th and 13th tuning frequencies to avoid harmonic amplification. (2) The safety factor must have an upper limit, so that the filter will not be over-deflected, affecting the filtering effect and the reactive power compensation for the system.

By repeatedly calculating formula (4), we can get the most suitable range of DT11/13 safety factor: 1.0

4 Simulation Analysis

This paper simulates the system in Figure 2 based on the power system toolbox Simpowersystem under Matlab dynamic simulation tool Simulink. The parameters of the AC and DC system of the experimental platform used in the simulation, as well as the parameters of the new converter transformer and the traditional converter transformer are omitted due to space limitations. It can be clearly seen from Figures 5(a) and 5(b) that the new converter transformer and its filtering system can effectively suppress the main sub-characteristic harmonics on the valve side, and the distortion rate of the grid-side winding current is greatly reduced. Comparing the current spectrum diagrams on the valve side and the grid side, the valve-side filtering scheme under the new system can effectively suppress the 5th, 7th, 11th and 13th main sub-characteristic harmonics generated by the converter on the valve side, and the harmonic content of the primary output terminal of the transformer is far lower than the national standard, which further illustrates the superiority and feasibility of the filter based on the induction filtering mechanism in this paper.

5 Conclusion

(1) This paper proposes a new harmonic suppression method, induction filtering, which uses the electromagnetic induction principle of transformer to achieve harmonic magnetic potential balance between secondary windings. By analyzing its harmonic suppression mechanism, the impedance requirements of induction filtering on transformer impedance and matching filter devices are given;

(2) This paper takes the new DC transmission system platform established in the laboratory as the research object, designs the parameters of the valve-side double-tuned filter under the new system, verifies the effectiveness of the new filtering method through experiments, and reveals the superiority of the new filtering method and the DC transmission system based on the new converter transformer.

The author's innovations are as follows: 1. A new harmonic suppression method, inductive filtering, is proposed to achieve harmonic magnetic potential balance between secondary windings using the electromagnetic induction principle of transformers; 2. Since the double-tuned filter is installed in the busbar on the AC grid side, considering its influence on the system impedance, the concept of safety factor is introduced to correct the filter component parameters, so that the double-tuned filter is slightly inductive at the 11th and 13th harmonic frequencies, avoiding resonance with the system and preventing harmonic amplification.