CN102611116A - Single-phase electric energy quality controller for electrified railway power supply system - Google Patents

Abstract

The invention relates to a single-phase power quality controller for an electrified railway power supply system, which is characterized in that it includes: a load-side converter, a no-load side converter, a DC unit, a capacitive impedance and a step-up transformer; The side converter and the no-load side converter exchange energy through the DC unit to form a "back-to-back" rectifier inverter; the load-side converter is connected to the capacitive impedance, and the no-load side converter is connected to the step-up transformer. The controller proposed by the invention has the functions of active power control, reactive power compensation and harmonic compensation at the same time, can effectively improve the power quality problem in the electrified railway traction power supply system, and reduce system loss.

Description

Single-phase power quality controller for electrified railway power supply system

technical field

The invention relates to a single-phase power quality controller used in an electrified railway power supply system, belonging to the technical field of power electronics in electrical engineering.

Background technique

China's electrified railways generally use single-phase power frequency alternating current to power railway locomotives. As a large-capacity single-phase load, the electric locomotive will inject negative sequence current into the three-phase power supply system, which will cause the problem of three-phase voltage imbalance and voltage fluctuation in the power supply system. The electric locomotive will generate reactive current and harmonic current at the same time, which will endanger the normal operation of the power system.

In the existing electrified railway power supply system in our country, the scheme of phase sequence rotation and phase split power supply is generally adopted. This scheme can alleviate the impact of three-phase unbalance to a certain extent, but it cannot fundamentally solve the impact of single-phase traction load on the entire public grid. At the same time, the electric phase separation device increases the cost and restricts the running speed of the locomotive, making it difficult to meet the requirements of high-speed and heavy-haul railways.

In "In-phase power supply device for AC traction of railway locomotive based on YN, vd connection transformer" (Chinese invention patent, authorized announcement date: June 24, 2009, announcement number CN100505499C), a method is proposed that can realize the same-phase power supply for the whole railway line without It is a phase-separated power supply device that simultaneously eliminates the impact of railway traction load on the power quality of the public grid. Among them, the rectifier inverter based on power electronic technology plays a key role in the same-phase power supply system. The description of the structure and control method of the rectifier inverter can be found in the following two scientific documents: Document 1: Zhang Xiufeng, Li Qunzhan, Lu Xiaoqin, Power supply system based on active filter and V, v connected in phase, China Railway Science, 2006, 27(2): 98-102; Literature 2: Zeng Guohong, Hao Rongtai, In-phase power supply system based on active filter and impedance matching balancing transformer, Journal of Railway Science, 2003, 25(3): 49-54. Among them, document 1 chooses V, v transformer as the traction transformer, and document 2 chooses the impedance matching balance transformer as the traction transformer. The primary side of both transformers is connected to the three-phase power grid, and the secondary side is converted to two-phase output.

In the same-phase power supply scheme in the above literature, transformers are required on both sides of the rectifier inverter to connect the converter to the traction grid. The simplified principle structure is shown in Figure 1. In "Single-phase unified power quality controller for electrified railway power supply" (Chinese invention patent, authorized announcement date: November 4, 2009, announcement number CN 100557935C), a single-phase power quality controller for electrified railway power supply is proposed. Unified power quality controller (referred to as UPQC), the UPQC uses a large-capacity chain-type H-bridge converter to save the transformer on one side, thereby reducing the cost, floor space and loss of the device. Its structure is shown in Figure 2 Show. But as far as the rectifier inverter itself is concerned, because one side of the converter is directly connected in parallel to the traction grid through an inductor, considering that the traction locomotive is an inductive load, in order to realize compensation for load harmonics, reactive power and unbalanced current, Like the parameters in the patent embodiment, the total DC side voltage of the rectifier inverter must be higher than the magnitude of the secondary side voltage of the traction transformer. The disadvantage is that the high cost of the rectifier inverter limits the application of this scheme.

Contents of the invention

The purpose of the present invention is to provide a novel single-phase power quality controller for electrified railway power supply system; this purpose is achieved by the following technical solutions:

A single-phase power quality controller for an electrified railway power supply system, characterized in that it includes: a load-side converter, a no-load side converter, a DC unit, a capacitive impedance and a step-up transformer; a load-side converter The inverter and the no-load side converter exchange energy through the DC unit to form a "back-to-back" rectifier inverter; the load-side converter is connected to the capacitive impedance, and the no-load side converter is connected to the step-up transformer.

The capacitive impedance includes a set of capacitors and a set of reactors. At the operating frequency of the system, the total impedance of capacitors plus reactors is capacitive.

The impedance value of the capacitive impedance at the operating frequency of the system satisfies that the operating frequency component of the output voltage of the load-side converter and the operating frequency component of the compensation current are in the same direction under rated load conditions.

The inductance-capacitance series branch of the capacitive impedance resonates at a certain main harmonic of the traction load current, usually the 3rd, 5th or 7th order.

The step-up transformer is a single-phase isolation step-up transformer, and its transformation ratio is the ratio of the rated voltage of the load-side converter to the system voltage.

Features and beneficial effects of the present invention:

The single-phase power quality controller (Hybrid UPQC for short) used in the electrified railway power supply system proposed by the present invention has the functions of active power control, reactive power compensation and harmonic compensation at the same time, and can effectively improve the performance of the electrified railway traction power supply system. Power quality problems and reduce system losses. Using the Hybrid UPQC proposed by the present invention, the converter is connected to the traction power supply system through the capacitive impedance, and the total DC voltage of the rectifier inverter can be reduced to 50-80% of the existing scheme, thereby reducing the power electronic converter capacity to achieve the purpose of reducing costs and operating losses.

Description of drawings

Figure 1 shows an existing non-phase power supply system based on a double transformer isolation structure.

Figure 2 is a schematic diagram of the existing UPQC structure in which one side uses an isolation transformer and one side is directly integrated into the system through an inductor.

Figure 3 is a schematic structural diagram of a Hybrid UPQC that uses an isolation transformer on one side and incorporates a capacitive impedance into the system on one side provided by the present invention.

Figure 4 is the vector diagram of Hybrid UPQC injecting active current into the system and compensating load inductive reactive power.

Figure 5 is a vector diagram of UPQC injecting active current into the system and compensating load inductive reactive power.

Fig. 6 is a system structure diagram in which the Hybrid UPQC is installed in a traction substation using a V, v transformer to realize the same-phase power supply system in Embodiment 1.

Fig. 7 is a system structure diagram in which the Hybrid UPQC is installed in a traction substation using a single-phase transformer to realize the same-phase power supply system in the second embodiment.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

The overall structure of the single-phase power quality controller (Hybrid UPQC for short) used in the same-phase power supply system for electrified railways provided by the present invention is shown in Figure 3. Converter (referred to as no-load converter), DC unit, step-up transformer and capacitive impedance. The load-side converter and the no-load side converter based on power electronic devices (GTO or IGBT) exchange energy through the DC unit to form a "back-to-back" rectifier inverter. The capacitive impedance connects the traction power supply network and the load-side converter. This impedance is composed of a group of capacitors and a group of inductors. The capacitor and inductor resonate at a certain harmonic of the load, so their total impedance at the system operating frequency is always capacitive sex. The no-load side converter is connected to the system through a step-up isolation transformer.

In the above-mentioned Hybrid UPQC, one side is directly connected to the power supply end of the traction transformer through capacitive impedance, that is, it is connected to the same traction grid in parallel with the traction load of the electrified railway, and the converter on this side is also called the load-side converter; The side is connected to the power supply system through a step-up transformer, and the converter on this side is called the no-load side converter.

Among them, the load-side converter is connected to the traction grid through a capacitive impedance. When its output voltage is lower than the traction grid voltage, it can inject active current, reactive current and harmonic current into the traction grid. The corresponding total DC side voltage of the converter will also be significantly lower than the magnitude of the secondary side voltage of the traction transformer. It can eliminate the influence of reactive power and harmonic current generated by railway traction loads on the power quality of public grids, and at the same time solve the unbalanced problem of public grids generated by single-phase traction loads. Compared with the existing UPQC, this scheme can reduce the capacity of the rectifier inverter in the single-phase power quality controller, and reduce the cost and loss of the power quality controller.

The capacitive impedance includes a set of capacitors and a set of reactors. At the operating frequency of the system, the total impedance of capacitors plus reactors is capacitive. In order to reduce the total voltage of the DC side of the rectifier inverter, the capacitive impedance is The impedance value at the frequency satisfies that under the condition of rated load, the working frequency component of the output voltage of the load-side converter is in the same direction as the working frequency component of the compensation current. In order to facilitate the filtering of the harmonics generated by the railway traction load, the inductance-capacitance series branch resonates at a certain main harmonic of the traction load current, usually the 3rd, 5th or 7th order.

In the above-mentioned Hybrid UPQC, the no-load side converter is connected to the system through a single-phase isolation step-up transformer, and its transformation ratio is the ratio of the rated voltage of the load-side converter to the system voltage. When the transformer is a three-phase to two-phase transformer, such as V, v, impedance matching balance transformer, the system voltage is the effective value of the output voltage of the secondary side of the traction transformer, which is usually the same as the power supply voltage of the traction network; when the traction transformer is a single-phase transformer , the no-load side converter is connected to the three-phase power supply system through a single-phase isolation step-up transformer, and its transformation ratio is the ratio of the rated voltage of the load-side converter to the effective value of the line voltage of the three-phase power supply system.

In the above-mentioned Hybrid UPQC, both the no-load side converter and the load side converter can choose multi-level single-phase voltage source converters, such as single-phase chain-type H-bridge structure converters, which can be controlled based on current tracking Technology (belonging to the conventional technology and not belonging to the content of the present invention) ensures that the present invention realizes compensation for reactive current and harmonic current and controls the transfer of active current, while maintaining the DC side voltage within the set range.

The working principle of the Hybrid UPQC provided by the present invention is briefly described as follows:

In the same-phase power supply system, the load-side converter of Hybrid UPQC compensates the reactive current and harmonic current generated by the electrified railway load. At the same time, when the traction transformer is a three-phase to two-phase transformer, such as V, v, impedance matching balance When the transformer is used, the converter on the no-load side is connected to the other phase winding of the traction transformer; when the traction transformer is a single-phase transformer, the converter on the no-load side is connected to the three-phase power supply system, but it must be selected with the single-phase traction transformer Different line voltage access. The no-load side converter absorbs active power from the system, passes through the DC unit, and then the load-side converter injects the energy into the system, thereby compensating for the unbalance of the three-phase side current. When the no-load side converter absorbs active power, it adopts power factor correction technology, that is, controls the current waveform, and ensures that the measured power factor in the three-phase power supply system is close to unity.

The power factor of the DC electric locomotive is generally about 0.8, and it is an inductive load. It is assumed that the positive direction of the compensation current is injected into the system from the load-side converter of the Hybrid UPQC. When the Hybrid UPQC injects active current into the system and compensates the inductive reactive power of the load, The vector diagram is shown in Figure 4. The compensation current vector includes an active component parallel to the system voltage and a reactive component perpendicular to the system voltage. The direction of the voltage vector generated by the current flowing through the capacitive impedance is that the current vector rotates 90 degrees clockwise. , the sum of the system voltage and the voltage on the capacitive impedance is the output voltage of the Hybrid UPQC load-side converter. Hybrid UPQC can realize the compensation requirement when the output voltage of the load-side converter is lower than the system voltage. When the traditional UPQC injects the same compensation current into the system, the vector diagram is shown in Figure 5. Because the inductive impedance is used to connect the inverter and the system, the direction of the voltage vector generated by the compensation current flowing through the inductive impedance is the direction of the current vector counterclockwise. Ten degrees, the voltage of the UPQC load side converter is the sum of the system voltage and the voltage on the inductive impedance. This voltage must be higher than the system voltage to realize the compensation requirement.

Below are two application examples of the Hybrid UPQC of the present invention:

Example 1: Hybrid UPQC in the same phase power supply system based on V, v transformer

In this embodiment, the Hybrid UPQC is installed in a traction substation using a V, V transformer to realize the same-phase power supply system. The detailed schematic diagram of the same-phase power supply system is shown in Figure 6.

Assume that the public grid, that is, the three-phase voltage on the primary side of the traction transformer is as follows:

In the above formula, V represents the effective value of the phase voltage of the three-phase power supply system connected to the traction substation.

As shown in the wiring diagram in Figure 4, the secondary side voltages of V and V transformers are as follows:

In the above formula, v _α and v _β represent the output voltage of the secondary side of V and v transformer respectively, among which v _α directly supplies power to the traction network, v _β is the no-load side in the same-phase power supply scheme, and V _{sup ply} is the output voltage of the secondary side RMS value of the output voltage.

When the Hybrid UPQC is not connected to the system, the secondary side currents of the V and V transformers are as follows:

$\left[\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\end{array}\right]<span\; class="notranslate">\; =</span>\left[\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}\\ \mathrm{<span\; class="notranslate">\; 0</span>}\\ {<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}\end{array}\right]<span\; class="notranslate">\; .</span>$

i _L is the traction load current of the electric locomotive, which can be decomposed into fundamental active current, fundamental reactive current and harmonic current:

i _L =i _L1p +i _L1q +i _Lh ,

Where i _Lh is the harmonic in the load current, i _L1p and i _L1q represent the fundamental active current and fundamental reactive current respectively, which can be expressed as:

表示电铁负荷供电电压和基波负荷电流之间的相位差。电铁负荷吸收的有功功率可以表示为：Where I _L1 is the RMS value of the load fundamental current,

Indicates the phase difference between the electric railway load supply voltage and the fundamental load current. The active power absorbed by the electric iron load can be expressed as:

When Hybrid UPQC is added to the system, Hybrid UPQC will inject compensation current, and the secondary side current of the traction transformer will be changed to:

$\left[\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; a</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; b</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; c</span>}}\end{array}\right]<span\; class="notranslate">\; =</span>\left[\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; pa</span>}}\\ {<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; pb</span>}}\\ {<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; pc</span>}}\end{array}\right]$

In the above formula, i _L is the load current, i _pa , i _pb and i _pc are the three-phase current injected from Hybrid UPQC to the system respectively.

In the case of full compensation, the current of the primary side of the public grid, that is, the traction transformer, will be three-phase balanced and the power factor will be one. At this time, the primary current can be expressed as:

In this case, the public grid only provides the active power of the electric railway load, which can be expressed as:

P _S =3V _A I.

其中K是牵引变压器的变比，完全补偿下，公共电网的电流有效值可以表示为：Assuming that the loss of the traction transformer is not considered, the load power is equal to the output power of the system side, that is, P _S =P _L and

Where K is the transformation ratio of the traction transformer. Under full compensation, the current effective value of the public grid can be expressed as:

Assuming that all harmonics in the load current are compensated on the secondary side of the traction transformer, that is, no harmonic current flows in the traction transformer, the compensation current injected by Hybrid UPQC can be expressed as:

$\left[\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; pa</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; pb</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; pc</span>}}\end{array}\right]<span\; class="notranslate">\; =</span>\left[\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}\\ \mathrm{<span\; class="notranslate">\; 0</span>}\\ {<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; L</span>}}\end{array}\right]<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; K</span>}\left[\begin{array}{c}{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; A</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}}\\ {\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; C</span>}}\end{array}\right]<span\; class="notranslate">\; .</span>$

Based on the previous analysis, the compensation current can be expressed as:

The compensation current is decomposed into parallel and vertical components and harmonic components of the traction network supply voltage.

When the power supply voltage V _supply of the traction network is selected as 27.5kV, the DC voltage of the rectifier inverter is set to 28kV. In the previous solution, the total DC side voltage of the rectifier inverter must be higher than the magnitude of the secondary side voltage of the traction transformer. In this case, the DC voltage must not be lower than 39kV.

In this example, the inductance and capacitance in the capacitive impedance connected to the load-side converter resonate at the fifth harmonic, and the transformation ratio of the step-up transformer connected to the system by the no-load side converter is 1:1.4.

Example 2: Hybrid UPQC in the same-phase power supply system based on single-phase transformers

This example is basically the same as Embodiment 1, the difference is that the traction substation uses a single-phase transformer to supply power to the traction network, the load-side converter of Hybrid UPQC is still connected to the traction network, and the no-load side converter is connected to the traction network through a step-up transformer. into the other two phases of the system, as shown in Figure 7. Assuming that the traction substation is connected to the 110kV public grid, the DC voltage of the inverter is set to 28kV, the inductance and capacitance in the capacitive impedance resonate at the fifth harmonic, and the transformation ratio of the step-up transformer connected to the no-load side converter is Revised to 1:5.5.

Claims (6)

1. A single-phase power quality controller for electrified railway power supply system, characterized in that, comprising: load side converter, no-load side converter, DC unit, capacitive impedance and step-up transformer; load side The converter and the no-load side converter exchange energy through the DC unit to form a "back-to-back" rectifier inverter; the load-side converter is connected to the capacitive impedance, and the no-load side converter is connected to the step-up transformer. 2. The single-phase power quality controller for electrified railway power supply system according to claim 1, characterized in that: said capacitive impedance comprises a group of capacitors and a group of reactors, and at the operating frequency of the system, the capacitor plus The total impedance of the reactor is capacitive. 3. The single-phase power quality controller for electrified railway power supply system according to claim 2, characterized in that: the impedance value of the capacitive impedance at the system operating frequency satisfies that under the rated load condition, the load side becomes The working frequency component of the rectifier output voltage is in the same direction as the working frequency component of the compensation current. 4. The single-phase power quality controller for electrified railway power supply system according to claim 2, characterized in that: the inductance-capacitance series branch resonates at a certain main harmonic of the traction load current. 5. The single-phase power quality controller for electrified railway power supply system according to claim 4, characterized in that: said certain main harmonic is selected as 3rd, 5th or 7th. 6. The single-phase power quality controller for electrified railway power supply system according to any one of claims 1 to 5, characterized in that: the step-up transformer is a single-phase isolation step-up transformer, and its transformation ratio is load The ratio of the rated voltage of the side converter to the system voltage.

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