CN110110486A - A kind of DAB type DC converter high-frequency resistance equivalent modeling method - Google Patents

Abstract

The invention belongs to the technical field of fault location of flexible DC power distribution systems, and in particular relates to a high-frequency impedance equivalent modeling method of a DAB-type DC converter, which includes: analyzing a single DAB module in a DAB-type DC converter, and converting the switching device It is equivalent to a resistive element, and the influence of the switching process is ignored; using the step signal generated when the system fails as a high-frequency source, the high-frequency impedance circuit of the DC converter after the fault is analyzed, and its high-frequency impedance expression is derived. Specifically, it includes: first, the high-frequency transformer in the loop is equivalent, and the parameters are converted to the high-voltage side, and the relationship between the voltage and current between the two coupling coils of the transformer is established; then, according to the structure and parameters of the high-frequency transformer, the transformer is deduced Each circuit parameter in the T-type equivalent circuit is converted to the high-voltage side to obtain its equivalent circuit. The invention solves the nonlinear characteristics brought by the power electronic device to the system, and the obtained model has high precision.

Description

A Modeling Method of Equivalent Impedance of High Frequency Impedance of DAB Type DC Converter

technical field

The invention belongs to the technical field of fault location of a flexible DC power distribution system, and in particular relates to a high-frequency impedance equivalent modeling method of a DAB type DC converter.

Background technique

In recent years, it has become a development trend for large-scale distributed power sources, electric vehicles and other flexible loads to be integrated into the distribution network through power electronic converter equipment. The DC power distribution system has fewer commutation links, simple structure, high conversion efficiency, and The advantages of large radius and capacity and good economy have become a hot topic of research and development at home and abroad. In the flexible DC power distribution system, the DC link presents the characteristics of strong power electronics. Different topological structures and different control strategies of the network will affect the fault characteristics. The fault current decay component is rich and there is no constant fault component, which leads to DC The characteristics after link failure are not clear, and the fault transient process is relatively complex and has strong nonlinear characteristics, so it is difficult to accurately extract and analyze the fault characteristics.

At present, there are few studies and analyzes on the fault characteristics of DC converters at home and abroad. Some scholars have analyzed the fault characteristics of the short circuit between the DC ports of power electronic transformers, and given the expressions of the port voltage and short-circuit current after the fault, but did not DC converter for model equivalence. Most of the fault location methods of the DC system are also carried out by analyzing the electrical quantity after the fault, and have not really solved the nonlinear problem brought by the DC converter to the system. Therefore, it is necessary to study the fault circuit of the DC transformer, analyze the influence of the switching device, solve the nonlinear problem in the system, and propose a linear equivalent model of the DC converter that is not affected by the state of the switching device.

Contents of the invention

Aiming at the above technical problems, the present invention proposes a DAB-type DC converter high-frequency impedance equivalent modeling method, including:

Step 1: Analyze a single DAB module in a DAB-type DC converter, equate the switching device to a resistance element, and ignore the influence of the switching process;

Step 2: Using the step signal generated when the system fails as a high-frequency source, analyze the high-frequency impedance loop of the DC converter after the fault, and derive its high-frequency impedance expression.

Described step 2 specifically comprises:

First, the high-frequency transformer in the loop is equivalent, and the parameters are converted to the high-voltage side, and the relationship between the voltage and current between the two coupling coils of the transformer is established; then, according to the structure and parameters of the high-frequency transformer, the T-type of the transformer is derived, etc. Each circuit parameter in the effective circuit is converted to the high-voltage side to obtain its equivalent circuit.

The step 2 also includes: using the high-frequency electrical quantity generated after the system fault as a source, analyzing the high-frequency signal path in the system after the fault, equating the switching device part in the path to a constant resistance, and converting the The transformer is replaced by the simplified T-shaped circuit, and the high-frequency equivalent model of a single DAB module is obtained, and the high-frequency impedance expression seen from the high-voltage side is obtained.

Said step 2 also includes: further simplifying the high-frequency equivalent impedance _of a single DAB module after ignoring the impedance parameter L12 in the T-type equivalent circuit of the transformer, and finally obtaining the whole DC conversion after the series connection of multiple DAB module equivalent circuits High frequency equivalent impedance of the device.

The equivalent circuit is that the bridge arm resistance, the transformer impedance and the low-voltage side support capacitor are connected in series and then connected in parallel with the high-voltage side capacitor.

Beneficial effects of the present invention:

(1) The analysis shows that the equivalent model is not affected by the switching state of the bridge arm when a bipolar short-circuit fault occurs in the system;

(2) Solve the nonlinear characteristics brought by power electronic devices to the system, linearize the system, and obtain a model with high accuracy, which is beneficial to the analysis of the fault characteristics and fault location technology of the flexible DC system.

Description of drawings

Fig. 1 is the flow chart of DAB type DC converter high-frequency impedance modeling method of the present invention;

Figure 2 is a schematic diagram of a single DAB module topology;

Figure 3 is a structural diagram of a high-frequency transformer and a schematic diagram of its T-shaped equivalent circuit;

Figure 4 is a high-frequency current circuit diagram after a bipolar short-circuit fault occurs at the DC port of the DAB module;

Figure 5 is a single DAB module high-frequency impedance equivalent model;

Fig. 6 is a topological structure diagram of a DAB DC converter;

Fig. 7 is a simulation comparison diagram between the calculated high-frequency impedance of the DC converter and the theoretical value.

Detailed ways

The embodiments will be described in detail below in conjunction with the accompanying drawings.

Fig. 1 is the flow chart of the high-frequency impedance equivalent modeling method of DAB type DC converter of the present invention, the present invention proposes a kind of DAB type DC converter high-frequency impedance equivalent modeling method, comprises the following steps:

Step 1: Analyze a single DAB module in a DAB-type DC converter, equate the switching device to a resistance element, and ignore the influence of the switching process;

Step 2: Using the step signal generated when the system fails as a high-frequency source, analyze the high-frequency impedance loop of the DC converter after the fault, and derive its high-frequency impedance expression.

In the step 1, the DAB type DC converter is composed of multiple DAB modules connected in parallel on the low voltage side and connected in series on the high voltage side. A single DAB module can be analyzed first; The switching frequency is the same, and the diagonal switches are turned on at the same time; based on this, since the current in the DAB module has two directions, according to the control characteristics of the switch, two current loops can be obtained. Regardless of the state, the primary and secondary circuits of the high-frequency transformer will pass through the IGBT or anti-parallel diode twice, the on-resistance of the switching device is the same, and the high-frequency signal energy generated during the switching process is much smaller than the fault transient impact current High frequency, so the switching device can be equivalent to a resistive element, and the influence of the switching process can be ignored.

In the step 2, when a bipolar short-circuit fault occurs in the DC system, the voltage at the fault point will drop to zero within a few milliseconds, and a step signal will be generated, and the frequency spectrum analysis of the step signal can obtain its full frequency domain information, Moreover, the higher the signal frequency, the greater the spectral density; at the same time, due to the requirement of fast protection of the flexible DC grid, the data window length available for fault location analysis is at most several milliseconds, and it is difficult to accurately extract low-frequency information, and the high-frequency signal identification effect is better. Therefore, using the step signal generated when the system fails as a high-frequency source, analyze the high-frequency impedance circuit of the DC converter after the fault, first perform an equivalent of the high-frequency transformer in the circuit, and deduce the transformer T-type according to the structure and parameters of the high-frequency transformer. The circuit parameters L ₁ , L ₂ and L ₁₂ in the equivalent circuit are converted to the high voltage side to obtain the equivalent circuit.

Using the high-frequency electrical quantity generated after the system failure as the source, analyze the high-frequency signal path in the system after the failure. Since the DAB module adopts the control method that the diagonal switch tubes are turned on at the same time, there will be two high-frequency currents after the system failure. According to the analysis in step 1, the fault circuit can be obtained, the switching process of the switching device can be ignored, and these two circuits will pass through the switching device twice on the primary side and the secondary side of the DAB, and the equivalent circuit of the current path can be regarded as The same; the switching device in the path is equivalent to a constant resistance, and the transformer in the loop is replaced by a simplified T-shaped circuit to obtain a high-frequency equivalent model of a single DAB module, viewed from the high-voltage side The high frequency impedance is:

In the formula, C ₀ is the outlet capacitance of the high-voltage DC side, C ₁ is the capacitance of the low-voltage DC side, R ₁ and L ₁ are the line resistance and reactance respectively, r is the on-resistance of the switching device of the bridge arm, L ₁ , L ₂ and L ₁₂ is the impedance parameter in the transformer T-type equivalent circuit;

According to the parameters of the transformer T-type equivalent circuit obtained in step ₂ , the calculated value of L12 is about 500 times that _of L1 and L2. _In order to make the circuit more simple, the branch where _L12 is located can be ignored when performing the equivalent , then the high-frequency equivalent impedance of a single DAB module can be further simplified as:

The equivalent circuit is equivalent to the bridge arm resistance, transformer impedance and low-voltage side support capacitor connected in series and then connected in parallel with the high-voltage side capacitor, and all parameters are converted to the high-voltage side;

The DAB type DC converter is composed of multiple DAB modules connected in parallel on the low voltage side and connected in series on the high voltage side. After solving the high frequency equivalent circuit of a single DAB module, looking from the DC line port, the entire DC converter is equivalent to Multiple DAB module equivalent circuits are connected in series, therefore, the high-frequency equivalent impedance of the DC converter can be expressed as:

Z _S ＝N·Z _{S_DAB}

In the formula, Z _{S_DAB} represents the high-frequency impedance of a single DAB module, and N represents the number of DAB modules in the DC transformer.

Figure 2 is a schematic diagram of a single DAB module topology. The DAB module is composed of H-bridges on both sides and high-frequency transformers. The bridge arms of the H-bridge are composed of IGBTs and anti-parallel diodes. At the same time, there are two capacitors on the low-voltage side and high-voltage side of the converter, which play the role of filtering and voltage support. During the working process of the DAB module, the switching frequency of the H bridges on both sides is the same, and the diagonal switches are turned on at the same time. According to the control characteristics of the switch, two kinds of current loops can be obtained. Through analysis, the two loops can be replaced by the same equivalent circuit, thus ignoring the influence of the switch state and the switching process.

Figure 3 is a structural diagram of a high-frequency transformer and a schematic diagram of its T-shaped equivalent circuit. The figure on the left shows the two mutually coupled windings in the high-frequency transformer, L ₁₁ and L ₂₂ represent the self-inductance of the two windings, and L ₁₂ represents the mutual inductance between the windings. According to the principle of the transformer, it can be simplified to the T-shaped circuit shown in the right figure, and all the parameters are converted to the high-voltage side.

Figure 4 is a high-frequency current loop diagram after a bipolar short-circuit fault occurs at the DC port of the DAB module. After a bipolar short-circuit fault occurs on the DC side, the high-frequency quantity decomposed by the step voltage at the fault point can be used as a high-frequency source to analyze the high-frequency fault circuit using it as a power source. Since the control signals of the switching devices are complementary square wave signals, there are two high-frequency loops. Based on the above analysis of the influence of switching devices on the loop, it can be known that the two high-frequency loops can be equivalent to the same circuit, and the switching state and switching process can be ignored.

Figure 5 is a single DAB module high-frequency impedance equivalent model. Based on the equivalent of the high-frequency transformer and the analysis of the high-frequency fault circuit, the transformer in the circuit is replaced by a T-shaped equivalent circuit, and the H bridges on both sides are replaced by on-resistance. At the same time, since the calculated value of L ₁₂ is about L ₁ is 500 times that of L ₂ , so the branch where L ₁₂ is located can be ignored, the parameters of the low-voltage side are converted to the high-voltage side, and the parameters of the same type are combined to finally obtain the equivalent circuit shown in Figure 5.

Figure 6 is a topological structure diagram of a DAB DC converter. The converter is composed of multi-layer DAB modules. The low-voltage side of each module is connected in parallel, and the high-voltage side is connected in series to the medium-voltage DC system. The series structure can effectively divide the voltage and improve the withstand voltage level of the DC converter. Viewed from the medium-voltage DC line port, the entire converter is equivalent to all DAB modules connected in series. Therefore, the high-frequency equivalent impedance value of the DC converter viewed from the high-voltage side is the equivalent impedance of a single DAB module and the module in the converter. product of numbers.

Fig. 7 is a simulation comparison diagram between the calculated high-frequency impedance of the DC converter and the theoretical value. A flexible DC system containing DAB DC converters is built in PSCAD. The low-voltage side of each DAB module is connected to the photovoltaic power supply, and the high-voltage side is connected in series to the medium-voltage DC system. When a bipolar short-circuit fault occurs on the DC side, measure the voltage and current at the outlet of the DC system side of the converter, and perform wavelet transformation to obtain the frequency domain values of the voltage and current, and then obtain the impedance at each frequency. This calculated value is compared with the theoretical value to obtain the results shown in the figure.

The simulation results show that when a bipolar short-circuit fault occurs in the flexible DC system, by analyzing the influence of the converter switching state on the fault circuit, using the step voltage after the fault as the power source, analyzing the circuit characteristics of the high-frequency current, and finally obtaining the DAB type DC converter The high-frequency impedance equivalent model of the device eliminates the nonlinear problem caused by the switching device in the system. In the frequency range of 800-1600Hz, the equivalent model has high accuracy.

This embodiment is only a preferred specific implementation of the present invention, but the scope of protection of the present invention is not limited thereto, any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention , should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims.

Claims (5)

1. a kind of DAB type DC converter high-frequency resistance equivalent modeling method characterized by comprising

Step 1: the single DAB module in DAB type DC converter is analyzed, switching device is equivalent to resistive element, And ignore the influence of switching process；

Step 2: generated step signal analyzes the high frequency of DC converter after failure as high frequency source when using the system failure Impedance path is derived from its high-frequency resistance expression formula.

2. method according to claim 1, which is characterized in that the step 2 specifically includes:

It is first that the progress of circuit medium/high frequency transformer is equivalent, and parameter is all converted to high-pressure side, establish two coupling line of transformer The relational expression of voltage and electric current between circle；Further according to the structure and parameter of high frequency transformer, the equivalent electricity of transformer T-type is derived Each circuit parameter in road, and convert and arrive high-pressure side, obtain its equivalent circuit.

3. method according to claim 1 or claim 2, which is characterized in that the step 2 further include: using being produced after the system failure Raw high-frequency electrical tolerance analyzes the high-frequency signal access in post-fault system, by the switching device part etc. in access as source Constant resistance, and the T-type circuitry instead that the transformer in circuit is come out with abbreviation are imitated into, the height of single DAB module is obtained Frequency equivalent model obtains the high-frequency resistance expression formula entered in terms of high-pressure side.

4. method according to claim 1 or claim 2, which is characterized in that the step 2 further include: ignore the equivalent electricity of transformer T-type Impedance parameter L in road₁₂The high frequency equivalent impedance of single DAB module is further simplified afterwards, finally obtains multiple DAB modules etc. Imitate the entire DC converter high frequency equivalent impedance after circuit is connected in series.

5. method according to claim 2, which is characterized in that the equivalent circuit be arm resistance, transformer impedance and Low-pressure side Support Capacitor is in parallel with high pressure lateral capacitance again after being together in series.