Transformer and converter valve charging test circuit and method

Abstract

The present invention provides a transformer and converter valve charging test circuit and method, wherein the transformer and converter valve charging test circuit includes: an AC test power supply, a test module having one end coupled to the output end of the test power supply, and a parallel DC bridge arm coupled to the other end of the test module; wherein the test module is used to adjust the output voltage of the AC test power supply, thereby increasing the input voltage of the parallel DC bridge arm from zero to a set voltage within a set time. The present invention provides a test method and a test platform for charging and unlocking transformers and converters using medium and low voltage test power supplies, which can reduce the demand for test power supply capacity. During the converter unlocking process, a method of unlocking the converter under low voltage conditions is used to reduce the impact current. After the converter is unlocked, a method of charging the converter is used by gradually increasing the converter input AC voltage and then raising the DC voltage.

Description

Technical Field

[0001] The present invention relates to the field of circuit technology, and more specifically, to a converter transformer and a converter valve charging test circuit and method. Background Art

[0002] Flexible DC transmission technology is a new type of DC transmission technology based on voltage source converters, turn-off devices and pulse width modulation technology. Compared with the traditional thyristor-based current source DC transmission technology, flexible DC transmission technology has the advantages of high controllability, convenient and environmentally friendly design and construction, small footprint and no need for communication between converter stations. It has obvious advantages in renewable energy grid connection, distributed generation grid connection, island power supply, urban power grid power supply, etc. Flexible DC transmission has developed rapidly in recent years.

[0003] Offshore wind power grid connection is an important application of flexible direct current transmission. Flexible direct current offshore converter stations need to be built on offshore platforms. Considering the limitations of offshore construction conditions, it is impossible to complete the installation and testing of large equipment at sea. It is necessary to install and debug the equipment at the dock and then send the offshore platform as a whole to the designated sea area. If the offshore flexible direct current converter station is found to have quality defects and other problems in large equipment after arriving at the designated sea area, it is necessary to transport the entire platform to the test dock for processing. The cost and time cost are very high. The cost of transporting the offshore platform alone is as high as 100 million yuan. my country's first offshore flexible direct current converter station is scheduled to be completed in 20201. From an engineering perspective, basic tests such as live power tests on large equipment such as converter transformers and converter valves must be completed at the dock.

[0004] The converter transformer and converter valve in the flexible DC converter station have a large impact voltage. The instantaneous current when charging the transformer alone can reach 2 to 4 times the rated current. This places high demands on the strength of the test power supply. The instantaneous charging power of a 1500MW transformer can reach 3000MW. Only large power grids can withstand such impacts. Previous projects all used power grid systems for charging tests (generally 500kV or 220kV). Usually, the terminal can only provide a 10kV low-voltage power supply with a power capacity of only 3 to 5MW, which is completely unable to meet the test needs. Summary of the invention

[0005] In order to solve at least one of the above problems, the first aspect of the present invention provides a transformer and converter valve charging test circuit, comprising:

[0006] An AC test power supply, a test module having one end coupled to the output end of the test power supply, and a parallel DC bridge arm coupled to the other end of the test module; wherein:

[0007] The test module is used to adjust the output voltage of the AC test power supply, thereby increasing the input voltage of the parallel DC bridge arm from zero to a set voltage within a set time.

[0008] In a preferred embodiment, the test module comprises:

[0009] a voltage regulator coupled to an output terminal of the test power supply;

[0010] The controller has one end coupled to the wire between the voltage regulator and the step-up transformer and the other end coupled to the control end of the voltage regulator, and can output a voltage regulation instruction according to the received current or voltage.

[0011] In a preferred embodiment, the test module further comprises:

[0012] a step-up transformer coupled to an output end of the voltage regulator;

[0013] In a preferred embodiment, it further includes: a light-opening cabinet coupled between the test power supply and the voltage regulator.

[0014] In a preferred embodiment, it also includes:

[0015] A converter transformer is coupled between an input end of the converter valve and an output end of the boost transformer.

[0016] In a preferred embodiment, the step-up transformer is a transformer for an offshore flexible direct current station.

[0017] In a preferred embodiment, the set voltage is less than the maximum voltage of the AC test power supply.

[0018] In a preferred embodiment, the regulation rate of the input terminal voltage of the parallel DC bridge arm is lower than a set threshold.

[0019] Another aspect of the present invention provides a method for performing a charging test using the above transformer and converter valve charging test circuit, including:

[0020] During the unlocking process of the parallel DC bridge arm, the output voltage of the AC test power supply is adjusted to a set low voltage through the test module;

[0021] After the parallel DC bridge arm is unlocked, the output voltage of the AC test power supply is gradually increased to a set voltage through the test module.

[0022] In a preferred embodiment, it also includes:

[0023] A voltage regulation rate of the voltage regulator is generated according to the collected voltage and current between the step-up transformer and the voltage regulator.

[0024] Beneficial effects of the present invention:

[0025] The present invention provides a circuit and method for charging test of converter transformers and converter valves in converter stations, which mainly solves the problem of carrying out charging test of converter transformers and converter valves when the voltage level of the test power supply is low and the capacity is insufficient. The present invention uses a medium and low voltage test power supply to perform a test method and test platform for charging and unlocking transformers and converters. During transformer charging, a zero-start voltage boost method is used to reduce transformer excitation inrush current. During converter valve unlocking, a converter valve low-voltage unlocking method is used to reduce impact current, which can reduce the demand for test power supply capacity. Application in the commissioning stage of an offshore converter station can significantly reduce the failure rate of the offshore platform of the converter station after going to sea, and reduce the hidden dangers and huge expenses of later commissioning. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0027] Figure 1 The figure shows a schematic diagram of the structure of a half-bridge MMC converter in the prior art.

[0028] Figure 2 A typical main connection diagram of a symmetrical monopole flexible DC converter station in the prior art is shown.

[0029] Figure 3 A simulation diagram of charging current when the starting resistor is bypassed in the prior art is shown.

[0030] Figure 4 A schematic diagram of the structure of a transformer and a converter valve charging test circuit in an embodiment of the present invention is shown.

[0031] Figure numerals: 1—starting resistor, 2—interface transformer, 3—bridge arm reactor, 4—converter valve, 5—DC reactor, 6—DC isolation switch, 7—DC line, 8—AC incoming line breaker, 9—AC grid bus; Q1~Q5—switches. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] The main structure of the half-bridge MMC converter is as follows: Figure 1 As shown, the converter consists of three phase units, each of which includes two upper and lower bridge arms. The bridge arm branches are composed of a large number of MMC submodules connected in series. Each MMC submodule consists of IGBT, capacitor, diode, thyristor and other components. The capacitor is controlled by the on and off of the IGBT. The voltage of the entire branch is controlled by controlling the number and voltage of the MMC submodules in the phase unit.

[0034] Since a large number of capacitors are connected in series in the MMC converter circuit, when the converter station is charged by AC, if there is no voltage division or current limiting measures, a very large impact current will be generated according to the capacitor di=du / dt, so appropriate measures are needed to control the starting current.

[0035] At present, there is no precedent for offshore flexible direct current stations in China. When the onshore converter stations are officially put into operation, they use the official power supply of the converter station (220kV, 500kV, etc., directly connected to the power system) to charge the converter transformer and converter valve. The capacity of the 220kV or 500kV power system is very large, and can be regarded as an infinite power supply, which can withstand the excitation surge current when the converter transformer is impacted and the impact current when the converter is unlocked.

[0036] In the usual design of the converter main circuit, a starting resistor is connected in series in the DC circuit to limit the charging current of the converter valve. After charging is completed, the starting resistor is removed from the main circuit. Figure 2 It is a basic structural diagram of a symmetrical monopole MMC converter station. In the figure, 1 corresponds to the starting resistor. The circuit breaker connected in parallel with the resistor is the starting resistor bypass circuit breaker, which controls the input and output of the starting resistor.

[0037] At present, the charging process of the converter under the single-end starting mode of the flexible DC converter valve is as follows:

[0038] The starting resistor shunt circuit breaker QF1 is in the open position, and the AC circuit breaker QF is closed to start charging the converter;

[0039] After the charging current is relatively stable, the starting resistor parallel circuit breaker QF1 is closed to bypass the charging resistor;

[0040] The inverter is unlocked and the DC voltage rises to the rated voltage.

[0041] This solution limits the starting current of the converter pre-charging process by setting a starting resistor.

[0042] The flexible DC converter charging test is a test item during the startup process of the flexible DC converter station. The AC main grid charges the converter transformer and converter valve.

[0043] The offshore flexible direct current converter station is built on the offshore platform, and the construction and commissioning are carried out on the dock. In order to prevent the replacement of equipment on the offshore platform due to problems such as poor equipment quality, the offshore flexible direct current station should be transported to the designated sea area after the commissioning of the AC station system and the DC station system is completed on the dock. The test power supply on the dock generally only has a test power supply with a lower voltage level of 10kV available, and the test power supply capacity generally does not exceed 10MW. Insufficient test power supply capacity requires that the impact current and steady-state current during the test process cannot exceed the power supply capacity, and the test process should not cause the system to lose stability or generate harmonics that may damage the equipment.

[0044] In the existing technical solution, if a low-power test power supply is used for testing, the following problems will be faced:

[0045] (1) When the AC main grid is used to charge the converter transformer and the converter, even if there is a starting resistor in the circuit, the instantaneous power will still reach 1 to 2 times that of the converter transformer. For a transformer with a rated voltage of 500kV and a rated power of 1500MVA (1MVA = 1000kVA), the instantaneous power will reach 1500 to 3000MW (1MW = 1000kW). When the converter transformer is charged, there is a very large excitation inrush current; the converter can be equivalent to a large capacitor. At the beginning of charging, the voltage across the converter suddenly changes. After the voltage across the capacitor suddenly increases in a short time, the charging current will rise sharply, that is, di = du / dt.

[0046] (2) When the starting resistor is bypassed, the resistance in the charging circuit is instantly reduced to zero. The voltage across the converter capacitor suddenly increases, which causes the charging current to rise sharply. The current when the charging resistor is bypassed can reach several hundred amperes (for the above-mentioned transformer with a rated voltage of 500kV and a rated power of 1500MVA, the instantaneous power is 260000kW when the current is 300A).

[0047] The following is a simulation example of a converter station. The rated DC voltage of the converter station is 500kV, and the charging resistor is 6 kilo-ohms. When the bypass circuit breaker is disconnected and the starting resistor is in operation, the DC voltage rises to 321kV; when the bypass circuit breaker is closed and the starting resistor is withdrawn, the AC side inrush current reaches 630A and the instantaneous power reaches 520MW. Figure 3 shown.

[0048] (3) Before the converter unlocking process, the average voltage of the converter valve submodule is low. After the converter unlocks, the capacitor in the submodule quickly rises to the rated capacitor voltage. This charging process will generate a large impact current. For example, if the AC voltage is 500 kV and the AC impact current is 300 amperes, the power reaches 260MW. This is beyond the capacity of a general 10kV test power supply (the capacity of a general 10kV power supply does not exceed 20MVA).

[0049] The embodiment of the present invention provides a test method and a test platform for charging and unlocking a transformer and a converter using a medium- and low-voltage test power supply, which can reduce the demand for the test power supply capacity.

[0050] Specific as Figure 4 The transformer and converter valve charging test circuit shown includes: an AC test power supply, a test module having one end coupled to the output end of the test power supply, and a parallel DC bridge arm coupled to the other end of the test module; wherein the test module is used to adjust the output voltage of the AC test power supply, thereby increasing the input voltage of the parallel DC bridge arm from zero to a set voltage within a set time.

[0051] The present invention uses a medium and low voltage test power supply to charge and unlock a transformer and a converter. The test method and test platform can reduce the demand for the test power supply capacity. During the converter unlocking process, the converter unlocking method under low voltage is used to reduce the impact current. After the converter is unlocked, the converter charging method is used to gradually increase the converter input AC voltage and then raise the DC voltage.

[0052] Please continue to combine Figure 4 As shown, the test module includes: a voltage regulator coupled to the output end of the test power supply; a controller, one end of which is coupled to the wire between the voltage regulator and the step-up transformer, and the other end is coupled to the control end of the voltage regulator, and can output a voltage regulation instruction according to the received current or voltage.

[0053] In some optional embodiments, the test module further includes: a step-up transformer coupled to the output end of the voltage regulator.

[0054] Specifically, this test scheme uses a voltage regulating device to gradually raise the input voltage of the converter transformer from zero voltage, so that the converter transformer voltage can be slowly raised. The typical test connection is as follows: Figure 4 The test power supply is connected to the voltage regulator through the switch cabinet Q1, and the voltage regulator can adjust its output voltage from zero to the rated voltage; the voltage regulator is connected to the step-up transformer, the step-up transformer is connected to the converter transformer, and the converter transformer is connected to the bridge arm reactor, converter valve and other equipment of the DC field.

[0055] In some embodiments, the test module further includes: a switch cabinet coupled between the test power supply and the voltage regulator.

[0056] Furthermore, it also includes: a converter transformer coupled between the input end of the converter valve and the output end of the boost transformer.

[0057] In some embodiments, the step-up transformer is an offshore flexible direct current transformer. In this embodiment, if it is an offshore flexible direct current, a station transformer can be used as a step-up transformer, and if the output voltage of the voltage regulator can match the input voltage of the converter transformer, the step-up transformer is not required.

[0058] This embodiment can be applied in three ways: one is to perform live testing on a single transformer, the second is to perform live testing on both the converter transformer and the converter, and the third is to directly perform live testing on the converter.

[0059] When the voltage regulator gradually applies voltage to the converter transformer and converter valve starting from zero voltage, there is almost no excitation inrush current in the converter transformer; due to the slow voltage change, the converter charging current du / dt is also very small.

[0060] The controller calculates the overall power and change rate of the test system by collecting the converter transformer voltage and main circuit current, and then controls the voltage regulation rate of the voltage regulator to ensure that the voltage regulation rate does not exceed the set value and the capacity of the test system does not exceed the range of the test power supply.

[0061] (2) Inverter unlocking

[0062] Two methods are used in this scheme to reduce charging power:

[0063] A. Unlock the converter valve submodule under low voltage conditions. The lower the converter valve unlocking voltage, the smaller the unlocking power. The power supply of the submodule driver board is generally obtained from the submodule capacitor. As long as the submodule capacitor voltage can drive the normal operation of the submodule, it can be unlocked. For a submodule with a rated voltage of 4.5kV, the capacitor voltage generally reaches 600V to meet the power supply requirements of the submodule when unlocking.

[0064] B. When unlocking, you can use the method of unlocking sub-modules in batches.

[0065] C. After unlocking the converter, slowly increase the output voltage of the voltage regulator, and the converter will increase the DC voltage output to the rated value. The converter valve needs to calculate the DC voltage control target after unlocking based on the current submodule capacitor voltage and AC voltage to ensure a smooth transition process during unlocking. After the converter valve is unlocked, slowly increase the output voltage of the voltage regulator, and simultaneously increase the DC voltage control target, and finally reach the DC rated voltage.

[0066] In some embodiments, the adjustment rate of the input terminal voltage of the parallel DC bridge arm is lower than a set threshold, thereby protecting the entire circuit by slowly increasing the voltage.

[0067] In some embodiments, the set voltage is less than the highest voltage of the AC test power supply, which can protect the power supply.

[0068] Furthermore, an embodiment of the present invention also provides a method for performing a charging test using the above transformer and converter valve charging test circuit, comprising:

[0069] S1: During the unlocking process of the parallel DC bridge arm, the output voltage of the AC test power supply is adjusted to a set low voltage through the test module;

[0070] S2: After the parallel DC bridge arm is unlocked, the output voltage of the AC test power supply is gradually increased to a set voltage through the test module.

[0071] Furthermore, it also includes:

[0072] S3: Generate a voltage regulation rate of the voltage regulator according to the collected voltage and current between the step-up transformer and the voltage regulator.

[0073] In summary, the present invention uses a medium and low voltage test power supply to charge and unlock a transformer and a converter, and a test platform, which can reduce the demand for the test power supply capacity. During the converter unlocking process, the converter unlocking method under low voltage is used to reduce the impact current. After the converter is unlocked, the converter charging method is performed by gradually increasing the converter input AC voltage and then raising the DC voltage.

[0074] It can be seen that the present invention uses a test system composed of a voltage regulator and a step-up transformer to perform a charging test on the transformer when the test power capacity is insufficient. When the test power capacity is insufficient, the test system composed of a voltage regulator and a step-up transformer is used to perform a charging test on the converter. During the converter unlocking process, the method of unlocking the converter under low voltage is adopted to reduce the impact current. After the converter is unlocked, the converter is charged by gradually increasing the converter input AC voltage and then raising the DC voltage.

[0075] Each embodiment in this specification is described in a progressive manner, and the same or similar parts between the various embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the system implementation, since it is basically similar to the method implementation, the description is relatively simple, and the relevant parts can be referred to the partial description of the method implementation.

[0076] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the embodiment of this specification. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example.

[0077] In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction. The above description is only an embodiment of the embodiment of this specification and is not intended to limit the embodiment of this specification. For those skilled in the art, the embodiment of this specification may have various changes and variations. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the embodiment of this specification shall be included in the scope of the claims of the embodiment of this specification.

Claims

1. A transformer and converter valve charging test circuit, It is characterized in that include: An AC test power supply, a test module having one end coupled to the output end of the test power supply, and a parallel DC bridge arm coupled to the other end of the test module; wherein: The test module is used to adjust the output voltage of the AC test power supply, so that the input voltage of the parallel DC bridge arm is increased from zero to a set voltage within a set time; the adjustment rate of the input voltage of the parallel DC bridge arm is lower than the set threshold; The test module includes: a voltage regulator coupled to an output terminal of the test power supply; The controller has one end coupled to the wire between the voltage regulator and the step-up transformer and the other end coupled to the control end of the voltage regulator, and can output a voltage regulation instruction according to the received current or voltage.

2. The transformer and converter valve charging test circuit according to claim 1, It is characterized in that The test module also includes: A step-up transformer is coupled to the output end of the voltage regulator.

3. The transformer and converter valve charging test circuit according to claim 1, It is characterized in that The test module further includes: a switch cabinet coupled between the test power supply and the voltage regulator.

4. The transformer and converter valve charging test circuit according to claim 2, It is characterized in that Also includes: A converter transformer is coupled between an input end of the converter valve and an output end of the boost transformer.

5. The transformer and converter valve charging test circuit according to claim 3, It is characterized in that The step-up transformer is a transformer for offshore flexible direct current stations.

6. The transformer and converter valve charging test circuit according to claim 1, It is characterized in that The set voltage is less than the maximum voltage of the AC test power supply.

7. A method for performing a charging test using the transformer and converter valve charging test circuit according to claim 1, It is characterized in that include: During the unlocking process of the parallel DC bridge arm, the output voltage of the AC test power supply is adjusted to a set low voltage through the test module; After the parallel DC bridge arm is unlocked, the output voltage of the AC test power supply is gradually increased to a set voltage through the test module.

8. The method according to claim 7, It is characterized in that Also includes: A voltage regulation rate of the voltage regulator is generated according to the collected voltage and current between the step-up transformer and the voltage regulator.

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