Power one-way type high-voltage direct-current transformer and application thereof

Abstract

The invention discloses a power one-way type high-voltage direct-current transformer which comprises a low-voltage side converter, an alternating-current bus, a three-phase controllable capacitor, an alternating-current energy consumption device, a high-voltage side converter and a converter transformer, the low-voltage side converter adopts an MMC, and the high-voltage side converter is formed by connecting diode rectifiers in series. The alternating current bus is used for connecting an alternating current port of the modular multilevel converter, a network side port of the converter transformer, the three-phase controllable capacitor and the alternating current energy consumption device; the three-phase controllable capacitor and the alternating-current energy consumption device both adopt a Y-shaped connection method; the network-side windings of the converter transformer all adopt a Y-shaped wiring mode, and the valve-side windings can adopt three wiring modes, namely a Y-shaped winding, a T-shaped winding and an extended-edge winding. In order to realize stable operation of the power one-way high-voltage direct-current transformer, the invention further provides a low-voltage side converter alternating-current and direct-current side combined networking control strategy, and the power one-way high-voltage direct-current transformer alternating-current and direct-current side combined networking control method is easy to implement, high in applicability and high in use value in engineering.

Description

Technical Field

[0001] The present invention belongs to the technical field of power transmission and distribution of electric power systems, and in particular relates to a power unidirectional high-voltage direct current transformer and an application thereof. Background Art

[0003] The flexible DC grid is a grid-based transmission system, in which the DC ends of the internal converter stations are interconnected. It has the outstanding advantages of multiple power supplies and multiple power receiving points, and can achieve efficient transmission and mutual assistance of power. Therefore, based on the distribution characteristics of my country's new energy, the flexible DC grid is used to bring together the hydropower abundant in the southwest, the wind and solar energy in the three northern regions, and the wind energy in the eastern coastal areas, and form a DC grid interconnected in multiple regions, which can greatly suppress the impact of new energy fluctuations on the system. Furthermore, through the interconnection of power grids, the large-scale electricity collected by the DC grid can be mutually assisted between regions, thereby realizing the nationwide large-scale configuration of renewable energy. The domestic DC grid technology has made certain progress, and the Zhangbei ±500 kV flexible DC transmission demonstration project has been put into operation.

[0004] In general, there are three main technical bottlenecks in the development of DC power grids: ① Rapid detection and isolation technology of DC side faults; ② DC voltage transformation technology; ③ DC line power flow control technology; The core devices corresponding to these three technical bottlenecks are large-capacity, high-voltage, high-speed DC circuit breakers, large-capacity DC transformers, and DC line power flow controllers. At present, large-capacity, high-voltage, high-speed DC circuit breakers have been successfully applied to the Zhangbei ±500 kV flexible DC transmission demonstration project, while large-capacity DC transformers and DC line power flow controllers are still far from actual engineering applications.

[0005] With the development of electric motor technology, high-voltage DC transformers have gradually become a hot topic of concern in academia and engineering. Modular multilevel topology can reduce the voltage stress of power switching devices and avoid the voltage balancing problem caused by the direct series connection of power electronic switching devices. It has been successfully applied to high-voltage DC transmission systems. The DC transformer topology with modular multilevel structure is easy to meet the voltage and capacity requirements in high-voltage and large-capacity scenarios, and has great application potential. In addition, in multi-voltage DC grids, since the high-voltage DC transformer also hopes that the DC transformer can isolate the DC faults of DC grids of different voltage levels, the topology of the isolation structure is adopted. Obviously, a typical topology that meets the above two requirements is that two modular multilevel converters are directly connected on the AC side through the converter transformer. Since modular multilevel converters are applied to both the high and low voltage sides, the economic efficiency of this topology is poor. Considering the unidirectional nature of power transmission in large-scale new energy bases, under the condition of ensuring DC power transmission, the economic efficiency of the high-voltage DC transformer can be improved by using a low-cost power unidirectional high-voltage DC transformer topology. Therefore, it is necessary to conduct a detailed study on the power unidirectional high-voltage DC transformer. Summary of the invention

[0006] In view of the above, the present invention provides a power unidirectional high-voltage direct current transformer, which is simple to implement, has strong applicability, and has great use value in engineering design.

[0007] A power unidirectional high-voltage direct current transformer comprises a low-voltage side converter, an AC bus, a three-phase controllable capacitor, an AC energy consumption device, a high-voltage side converter and a converter transformer, wherein:

[0008] The low-voltage side converter is composed of N MMCs (modular multilevel converters) connected in parallel, the DC side of which serves as the low-voltage side DC port of a power unidirectional high-voltage DC transformer, and the AC side is connected to the AC bus, where N is a natural number greater than 0;

[0009] The high-voltage side converter consists of 2N rec The diode rectifier adopts a three-phase six-bridge structure, and its AC side is connected to the AC bus through a corresponding converter transformer. rec After the diode rectifiers are connected in series on the DC side, the positive terminal of the first diode rectifier is used as the positive terminal of the high-voltage DC transformer on the high-voltage side. rec The negative terminal of the diode rectifier is the negative terminal of the high voltage side DC transformer of the power unidirectional high voltage DC transformer. rec is a natural number greater than 0;

[0010] The three-phase controllable capacitor and the AC energy consumption device are connected to the AC bus.

[0011] Furthermore, the grid-side winding of the converter transformer adopts a Y-type connection, wherein the valve-side windings of the No. 1 converter transformer and the No. 2 converter transformer adopt a Y-type connection and a △-type connection respectively, and the voltage phase angle difference between the two is 30°; when N rec ≥2, the valve-side winding of the 2k-1 converter transformer adopts the phase-leading valve-side winding connection method, and the no-load voltage phase of the valve-side winding leads the no-load voltage phase of the grid-side winding by 30°×(k-1) / N rec The valve side winding of the 2k converter transformer adopts a phase-lag valve side winding connection method, and the phase of the valve side winding no-load voltage lags the grid side winding no-load voltage phase by 30°×(k-1) / N rec , k is a natural number and 2≤k≤N rec , the number of the converter transformer corresponds to the serial number of the diode rectifier.

[0012] Furthermore, the three-phase controllable capacitor adopts a Y-type connection method, each branch is composed of two groups of thyristor strings connected in reverse parallel and in series with a high-voltage capacitor, and each group of thyristor strings is composed of multiple thyristors connected in series; the maximum output reactive power of the three-phase controllable capacitor is 0.4 times the rated DC power of the power unidirectional high-voltage DC transformer.

[0013] During normal operation, the measured value of the reactive power on the grid side of the converter transformer is subtracted from the reference value (the reference value is set to 0) and then the trigger angle of the thyristor is obtained through PI control, and then the reactive power control on the grid side of the converter transformer is achieved by controlling the trigger angle of the thyristor.

[0014] Furthermore, the AC energy consumption device adopts a Y-type connection method, each branch is composed of two groups of thyristor strings connected in reverse parallel and in series with an energy consumption resistor, and each group of thyristor strings is composed of multiple thyristors connected in series; the maximum absorbed active power of the AC energy consumption device is the rated DC power of the power unidirectional high-voltage DC transformer.

[0015] During normal operation, the measured value of the average voltage of the submodule capacitor in the low-voltage side converter is passed through a hysteresis comparator to obtain the switching signal of the AC energy consumption device, and then the DC voltage control of the low-voltage side converter is realized by controlling the on-off of the thyristor; the upper limit of the threshold of the hysteresis comparator is 1.1 (per unit value), and the lower limit of the threshold is 1.03 (per unit value). When the measured value of the average voltage of the submodule capacitor exceeds the upper and lower limits of the threshold of the hysteresis comparator, the hysteresis comparator outputs a thyristor opening signal, and the AC energy consumption device is put into operation.

[0016] Furthermore, the low-voltage side converter adopts an AC-DC side joint network control strategy, which includes three parts: a DC voltage control link, an AC voltage control link, and an inner loop control link. The DC voltage control link sets the reference value u of the average voltage of the capacitor of the low-voltage side converter submodule cav_ref(Take 1.0 per unit value) minus the measured value u cav After that, the PI (proportional integral) controller outputs the AC bus q-axis voltage reference value u q_ref ; The AC voltage control link sets the reference value u of the AC bus d-axis voltage d_ref (take 0) minus the measured value u d After that, the PI controller outputs the reference value i of the d-axis component of the AC current of the low-voltage side converter d_ref , the measured value of the AC bus q-axis voltage u q Subtract u q_ref After that, the PI controller outputs the reference value i of the q-axis component of the AC current of the low-voltage side converter q_ref ; The inner loop control link will i d_ref Subtract the measured value i of the d-axis component of the AC current of the low-voltage side converter d Then input it into the PI controller, and then compare its output with (u d -i q L0ω0 / 2) is added to obtain the d-axis modulation voltage U vdref , and at the same time q_ref Subtract the measured value i of the q-axis component of the AC current of the low-voltage side converter q Then input it into the PI controller, and then compare its output with (u q +i d L0ω0 / 2) is added to obtain the q-axis modulation voltage U vqref , where L0 is the bridge arm inductance of the low-voltage side converter and ω0 is the fundamental angular frequency.

[0017] A photovoltaic power generation device comprises a photovoltaic power generation unit and a DC transformer connected thereto, wherein the DC transformer adopts the above-mentioned power unidirectional high-voltage DC transformer.

[0018] Based on the above technical solution, the present invention has the following beneficial technical effects:

[0019] 1. For unidirectional high-voltage DC transformers, the present invention fills the gap in the current research on their topological structure and control strategy, and can provide a certain reference for future engineering design.

[0020] 2. The present invention is relatively easy to implement. By using a diode converter on the high-voltage side, it has the characteristics of high reliability, compact size, low loss, low cost, etc., and has obvious technical and economic advantages.

[0021] 3. The present invention has strong versatility and is theoretically applicable not only to ultra-high voltage direct current, but also to other direct current voltage levels. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The topological structure diagram of the power unidirectional high-voltage DC transformer of the present invention is shown in FIG.

[0023] Figure 2 Schematic diagram of the valve-side winding connection of the converter transformer, where (a) corresponds to the phase-leading connection method and (b) corresponds to the phase-lagging connection method.

[0024] Figure 3 It is a schematic diagram of the topological structure of the three-phase controllable capacitor in the present invention.

[0025] Figure 4 It is a schematic diagram of the topological structure of the AC energy consumption device in the present invention.

[0026] Figure 5 It is a schematic diagram of the AC and DC side joint networking control system of the medium and low voltage side converters of the present invention.

[0027] Figure 6 The diagram is a circuit diagram of a simulation system for photovoltaic power generation according to the present invention.

[0028] Figure 7 Schematic diagram of the simulation results of the low-voltage side of the DC transformer of the present invention, wherein (a) corresponds to the DC voltage on the low-voltage side, (b) corresponds to the DC current on the low-voltage side, and (c) corresponds to the DC power on the low-voltage side.

[0029] Figure 8 Schematic diagram of the simulation results of the high-voltage side of the DC transformer of the present invention, wherein (a) corresponds to the DC voltage on the high-voltage side, (b) corresponds to the DC current on the high-voltage side, and (c) corresponds to the DC power on the high-voltage side. DETAILED DESCRIPTION

[0030] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0031] The structure of the power unidirectional high voltage DC transformer of the present invention is as follows: Figure 1 As shown, it includes a low-voltage side converter, an AC bus, a three-phase controllable capacitor, an AC energy consumption device, a high-voltage side converter and a converter transformer thereof, wherein:

[0032] The low-voltage side converter is composed of a single modular multi-level converter, whose DC positive and negative ports serve as the low-voltage side positive and negative DC ports of the high-voltage DC transformer. The low-voltage side positive and negative DC ports are connected to the low-voltage side DC line of the high-voltage DC transformer through smoothing reactors, and its AC port is connected to the AC bus of the high-voltage DC transformer.

[0033] The high-voltage side converter uses 4 sets (N rec=2) The diode rectifiers are directly connected in series on the DC side, the positive terminal of diode rectifier No. 1 serves as the DC positive terminal on the high-voltage side of the high-voltage DC transformer, the negative terminal of diode rectifier No. 4 serves as the DC negative terminal on the high-voltage side of the high-voltage DC transformer, and the positive and negative DC terminals on the high-voltage side are connected to the high-voltage side DC line of the high-voltage DC transformer through a smoothing reactor; the AC terminal of each diode rectifier is connected to the valve side (diode rectifier side) of its own converter transformer, and the grid sides of all converter transformers are connected to the AC busbar of the high-voltage DC transformer.

[0034] The grid-side windings of the four converter transformers are all connected in Y-type. The valve-side windings of converter transformers No. 1 and No. 2 are connected in Y-type and △-type, respectively, and the voltage phase angle difference between the two is 30°. The no-load voltage phase of the valve-side winding of converter transformer No. 3 is 15° ahead of the no-load voltage phase of the grid-side winding. The winding connection is as follows: Figure 2 As shown in (a); the phase of the no-load voltage of the winding on the valve side of the No. 4 converter transformer lags behind the phase of the no-load voltage of the grid side winding by 15°. The winding connection is as follows Figure 2 As shown in (b), please refer to the reference [Qian Zhangfu. Determination of the number of turns and current of the grid-side winding of 24-pulse rectifier and phase-shift rectifier transformers [J]. Transformer, 2001, 38(9): 8-12].

[0035] The structure of three-phase controllable capacitor is as follows Figure 3 As shown, it adopts a Y-type connection method. Each branch is composed of two strings of reverse parallel thyristor strings and a high-voltage capacitor in series. Each thyristor string contains multiple thyristors in series. The maximum output reactive power of the three-phase controllable capacitor is selected as 0.4 times the rated DC power of the DC transformer. During normal operation, the reactive power control of the converter transformer grid side is achieved by controlling the trigger angle of the thyristor.

[0036] The structure of AC energy consumption device is as follows Figure 4 As shown, it adopts a Y-type connection method, each branch is composed of two anti-parallel thyristor strings and an energy-consuming resistor in series, and each thyristor string contains multiple thyristors in series; the maximum absorbed active power of the AC energy-consuming device is selected as the rated DC power of the DC transformer; in normal operation, all low-voltage side converter DC voltage control is achieved by controlling the opening and closing of the thyristors.

[0037] The low voltage side converter adopts Figure 5 The AC / DC side joint network control strategy shown in the figure, the control system includes three parts: DC voltage controller, AC voltage controller and inner loop controller, two or three of which contain two control dimensions of d-axis and q-axis, and have the function of controlling the AC bus voltage and the low-voltage side DC voltage at the same time. Specifically: the input of the DC voltage controller is the measured value (unit value) of the average capacitor voltage of the low-voltage side converter submodule cav and reference value u cav_ref(taken as 1.0), after subtracting the two, the PI controller outputs the AC bus voltage q-axis AC voltage reference value u q_ref The input of the d-axis AC voltage controller is the measured value u of the d-axis component of the AC bus voltage. d and reference value u d_ref (taken as 0.0), after subtracting the two, the PI controller outputs the reference value i of the d-axis component of the AC current of the low-voltage side converter d_ref ; The input of the q-axis AC voltage controller is the measured value u of the q-axis component of the AC bus voltage q and reference value u q_ref After subtracting the two, the PI controller outputs the reference value i of the q-axis component of the AC current of the low-voltage side converter q_ref The inputs of the d-axis and q-axis AC voltage controllers are the measured values ​​of the d-axis component of the AC current of the low-voltage side converter, i d , reference value i d_ref and the measured value of the q-axis component i q , reference value i q_ref , after subtracting the two, they are passed through the PI controller and then added (u d -i q L0ω0 / 2) and (u q +i d L0ω0 / 2), we can get the d-axis and q-axis voltage modulation waves U vdref and U vqref , where L0 and ω0 are the bridge arm inductance and fundamental angular frequency respectively.

[0038] The following is based on Figure 6 The circuit simulation model is built. The ±20kV photovoltaic power generation unit is boosted to ±200kV through the ±20kV / ±200kV power unidirectional DC transformer, and then connected to the ±200kV ideal voltage source through a 20km DC cable. The main parameters of the ±20kV / ±200kV power unidirectional DC transformer are shown in Table 1. In the simulation, it is assumed that the system reaches a stable operating state before 0.8s, and the DC power emitted by the photovoltaic unit is 100MW. At 1s, the photovoltaic unit has a power step to 50MW, and at 1.5s, the photovoltaic unit power steps back to 100MW.

[0039] Table 1

[0040]

[0041]

[0042] Figure 7 and Figure 8The simulation waveforms of the high voltage side and the low voltage side of the DC transformer in this embodiment are respectively given; as can be seen from the figure, the simulation results show that the power unidirectional high voltage DC transformer of the present invention can operate stably, proving the effectiveness of the present invention.

[0043] The above description of the embodiments is to facilitate the understanding and application of the present invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art to the present invention based on the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A power unidirectional high voltage DC transformer, characterized in that: It includes low-voltage side converter, AC bus, three-phase controllable capacitor, AC energy consumption device, high-voltage side converter and converter transformer, among which: The low-voltage side converter is composed of N MMCs connected in parallel, the DC side of which serves as the low-voltage side DC port of a power unidirectional high-voltage DC transformer, and the AC side is connected to the AC bus, where N is a natural number greater than 0; The high-voltage side converter consists of 2N rec The diode rectifier adopts a three-phase six-bridge structure, and its AC side is connected to the AC bus through a corresponding converter transformer. rec After the diode rectifiers are connected in series on the DC side, the positive electrode port of the first diode rectifier is used as the DC positive electrode port on the high-voltage side of the power unidirectional high-voltage DC transformer, and the positive electrode port of the second N diode rectifier is used as the DC positive electrode port on the high-voltage side of the power unidirectional high-voltage DC transformer. rec The negative terminal of the diode rectifier is the negative terminal of the high voltage side DC transformer of the power unidirectional high voltage DC transformer. rec is a natural number greater than 0; The three-phase controllable capacitor and the AC energy consumption device are connected to the AC bus.

2. The power unidirectional high-voltage DC transformer according to claim 1, characterized in that: The grid-side winding of the converter transformer adopts a Y-type connection, wherein the valve-side windings of the No. 1 converter transformer and the No. 2 converter transformer adopt a Y-type connection and a △-type connection respectively, and the voltage phase angle difference between the two is 30°; when N rec ≥2, the valve-side winding of the 2k-1 converter transformer adopts the phase-leading valve-side winding connection method, and the no-load voltage phase of the valve-side winding leads the no-load voltage phase of the grid-side winding by 30°×(k-1) / N rec The valve side winding of the 2k converter transformer adopts a phase-lag valve side winding connection method, and the phase of the valve side winding no-load voltage lags the grid side winding no-load voltage phase by 30°×(k-1) / N rec , k is a natural number and 2≤k≤N rec , the number of the converter transformer corresponds to the serial number of the diode rectifier.

3. The power unidirectional high-voltage DC transformer according to claim 1, characterized in that: The three-phase controllable capacitor adopts a Y-type connection method, each branch is composed of two groups of thyristor strings connected in reverse parallel and connected in series with a high-voltage capacitor, and each group of thyristor strings is composed of multiple thyristors connected in series; the maximum output reactive power of the three-phase controllable capacitor is 0.4 times the rated DC power of the power unidirectional high-voltage DC transformer.

4. The power unidirectional high-voltage DC transformer according to claim 3, characterized in that: During normal operation, the measured value of the reactive power on the grid side of the converter transformer is subtracted from the reference value and then the trigger angle of the thyristor is obtained through PI control, and then the reactive power control on the grid side of the converter transformer is achieved by controlling the trigger angle of the thyristor.

5. The power unidirectional high-voltage DC transformer according to claim 1, characterized in that: The AC energy dissipation device adopts a Y-type connection method, each branch is composed of two groups of thyristor strings connected in reverse parallel and connected in series with an energy dissipation resistor, and each group of thyristor strings is composed of multiple thyristors connected in series; the maximum absorbed active power of the AC energy dissipation device is the rated DC power of the power unidirectional high-voltage DC transformer.

6. The power unidirectional high-voltage DC transformer according to claim 5, characterized in that: During normal operation, the measured value of the average voltage of the sub-module capacitor in the low-voltage side converter is passed through a hysteresis comparator to obtain the switching signal of the AC energy consumption device, and then the DC voltage control of the low-voltage side converter is realized by controlling the on-off of the thyristor; the upper limit of the threshold of the hysteresis comparator is 1.1, and the lower limit of the threshold is 1.

03. When the measured value of the average voltage of the sub-module capacitor exceeds the upper and lower limits of the threshold of the hysteresis comparator, the hysteresis comparator outputs a thyristor opening signal, and the AC energy consumption device is put into operation.

7. The power unidirectional high-voltage DC transformer according to claim 1, characterized in that: The low-voltage side converter adopts an AC-DC side joint network control strategy, which includes three parts: DC voltage control link, AC voltage control link and inner loop control link. The DC voltage control link sets the reference value u of the average voltage of the low-voltage side converter submodule capacitor to cav_ref Subtract the measured value u cav After that, the PI controller outputs the AC bus q-axis voltage reference value u q_ref ; The AC voltage control link sets the reference value u of the AC bus d-axis voltage d_ref Subtract the measured value u d After that, the PI controller outputs the reference value i of the d-axis component of the AC current of the low-voltage side converter d_ref , the measured value of the AC bus q-axis voltage u q Subtract u q_ref After that, the PI controller outputs the reference value i of the q-axis component of the AC current of the low-voltage side converter q_ref ; The inner loop control link will i d_ref Subtract the measured value i of the d-axis component of the AC current of the low-voltage side converter d Then input it into the PI controller, and then compare its output with (u d -i q L0ω0 / 2) is added to obtain the d-axis modulation voltage U vdref , and at the same time q_ref Subtract the measured value i of the q-axis component of the AC current of the low-voltage side converter q Then input it into the PI controller, and then compare its output with (u q +i d L0ω0 / 2) is added to obtain the q-axis modulation voltage U vqref , where L0 is the bridge arm inductance of the low-voltage side converter and ω0 is the fundamental angular frequency.

8. A photovoltaic power generation device, comprising a photovoltaic power generation unit and a DC transformer connected thereto, characterized in that: The DC transformer adopts a power unidirectional high-voltage DC transformer as described in any one of claims 1 to 7.

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