A DC transmission converter station of double-converter series construction

Abstract

The application belongs to the technical field of direct current transmission and particularly relates to a direct current transmission converter station of double-converter series construction, wherein each regional converter is configured with a control host, each region is configured with a station control host, one of the station control hosts is a leading station, the station control host is configured with a direct current station control for realizing bipolar power control, double-station reactive power control and double-station direct current field sequence control, the control host is configured with a pole control for realizing pole layer power control, closed-loop control, voltage balance control and converter switching, and the direct current station control and the pole control are coordinated by a leading station selection logic to corresponding control commands. The application realizes flexible sub-address construction by merging pole layer and converter layer function configuration and adding a set of station control hosts to the structure of the control and protection system of series construction high-voltage direct current transmission, retains the logical part of the original converter layer and pole layer functions, and meets the requirement of high reliability of series construction high-voltage direct current projects.

Description

Technical Field

[0001] This invention belongs to the field of DC power transmission technology, specifically relating to a DC power transmission converter station constructed with two converters connected in series. Background Technology

[0002] Series-connected HVDC transmission systems are a special type of multi-terminal HVDC transmission system. A multi-terminal HVDC transmission system refers to a transmission system composed of three or more converter stations connected in a specific manner. When one converter station is out of service, unlike a two-terminal flexible HVDC transmission system, it does not necessarily require shutdown. Under power coordination control, the other converter stations can still exchange power; only the system's equilibrium operating point shifts. It not only possesses the advantages of two-terminal flexible HVDC transmission but also has its own unique advantages. Multi-terminal HVDC transmission systems can achieve multi-point power supply and multi-point power reception; it is not a simple superposition of two-terminal HVDC transmissions. Common topologies for multi-terminal HVDC transmission include series, mixed, radial parallel, and ring parallel configurations. The series construction scheme is advantageous due to its good economy, flexible operation, and ease of engineering application. In the long term, the series construction scheme will be adopted in the fields of HVDC and UHVDC transmission.

[0003] The primary main wiring diagram of a series-located high-voltage direct current transmission system is as follows: Figure 1 As shown, the main wiring configuration uses two 12-pulse converters connected in series per pole. The two converters can be referred to as converter 1 and converter 2, respectively.

[0004] Because the main circuit of the series-series distributed high-voltage direct current (HVDC) transmission system is different from that of the conventional two-terminal HVDC system, the control and protection configuration scheme of the series-series distributed HVDC transmission system differs from that of the conventional two-terminal DC transmission system in many aspects. The control and protection configuration must meet the requirements of various operating modes of the series-series HVDC transmission project, and adopt necessary technical means to ensure the reliable operation of the series-series distributed HVDC transmission system according to the actual operating mode.

[0005] Based on the existing operational experience of DC transmission systems worldwide, the DC control and protection system is a crucial factor affecting the energy availability and system reliability of DC transmission systems. To improve the flexibility and reliability of series-connected UHVDC transmission control and protection systems, it is necessary to optimize the configuration of series-connected UHVDC transmission control and protection systems based on conventional two-terminal UHVDC control and protection systems. Summary of the Invention

[0006] The purpose of this invention is to provide a DC transmission converter station constructed with two converters in series, in order to meet the high reliability requirements of high-voltage DC projects constructed in series.

[0007] To achieve the above objectives, this invention provides a DC transmission converter station constructed in series with two converters. The series construction involves two converters located in two different regions. Each converter in each region is equipped with a control host, and each region also has a station control host, with one station control host serving as the master station. The station control host is equipped with a DC station controller for implementing bipolar power control, dual-station reactive power control, and dual-station DC field sequential control. The control host is also equipped with pole controllers for implementing pole layer power control, closed-loop control, voltage balance control, and converter activation / deactivation. The DC station controller and pole controller coordinate corresponding control commands through master station selection logic.

[0008] Its beneficial effects are as follows: This invention rationally designs the structure of the series-constructed high-voltage direct current transmission control and protection system, that is, by merging the functional configurations of the pole layer and the converter layer (i.e., the control host includes converter layer functions and pole layer functions, and the control host is used to implement converter layer functions and pole layer functions) and adding a station control host (the station control host is used to implement dual-level layer functions), it achieves flexible site-specific construction, while retaining the original logic of the converter layer and pole layer functions, thus meeting the high reliability requirements of series-constructed high-voltage direct current projects.

[0009] Furthermore, either DC station controller in two different regions is also used to achieve bipolar power control, dual-station reactive power control, and dual-station DC field sequential control in both regions when the other DC station controller is out of operation.

[0010] That is, the DC station control systems in the two regions serve as backups for each other. When the dual sets of DC station control systems at either station are out of service, the DC station control system at the other station will perform the relevant functions, and the DC system will continue to operate.

[0011] Furthermore, when any polarity controller exits operation, the converter corresponding to that polarity controller is decommissioned.

[0012] When any pole controller of a high-end or low-end station is out of service, the corresponding converter of the station will be deactivated, and the power of the remaining converters can still be adjusted normally, which does not affect the normal operation of other converters.

[0013] Furthermore, based on the magnitude of the transmission voltage, the region with the higher transmission voltage is designated as a high-end station, and the other region as a low-end station; the dominant station is the station control host of the high-end station; when the high-end station's polar control is out of operation, the low-end station's polar control executes the current command processed by the low-end station's polar control.

[0014] By prioritizing the execution of the current command processed by the end with the higher transmission voltage, polarity control at this end is achieved, avoiding the problem of conflicting current commands between series converters. When the polarity control of the high-end station is out of operation, the corresponding current command is generated through the polarity control of the low-end station, ensuring the normal regulation process of the low-end station.

[0015] Furthermore, when a communication failure occurs between the DC station control of the high-end station and the DC station control of the low-end station, each region coordinates the corresponding control commands through its own region's DC station control selection logic to achieve dual-station reactive power control and dual-station DC field sequential control.

[0016] When a communication failure occurs between the DC station control of the high-end station and the DC station control of the low-end station, the normal adjustment process of both the high-end and low-end stations is ensured through their respective DC station control selection logic.

[0017] Furthermore, when there is a communication failure between the pole controller of the high-end station and the pole controller of the low-end station, the pole controller of the low-end station switches from constant DC current control mode to constant DC voltage control mode, and the pole controller of the high-end station exits the converter voltage balance control function.

[0018] Furthermore, if the communication between the pole controller in any region and the DC station controller in the region where the master station is located fails, the DC station controller will switch the master station to the station control host in another region.

[0019] Furthermore, when the pole controller of the high-end station fails to communicate with the DC station control systems in both regions, the pole controller of the high-end station switches to a single-pole power control mode, receives control commands issued by OWS, and completes power adjustment.

[0020] When the DC station control communication between the pole controller and the master station fails in either region, the master station can be switched to another region. This ensures that the master station can communicate normally with the pole controller in either region, thereby guaranteeing the normal adjustment process of the converters in both regions and ensuring the consistency of the adjustment.

[0021] Furthermore, each region also includes polar protection and converter protection, which are used to realize the protection functions of the polar region and converter region in each region, respectively; each region also includes a monitoring system, which is used to monitor the two regions.

[0022] Furthermore, monitoring servers are configured in both regions, and the monitoring networks of the two regions are interconnected through inter-station communication. Attached Figure Description

[0023] Figure 1 This is a topology diagram of a DC system constructed in series; Figure 2 This is a control and protection configuration diagram of a DC system constructed in series according to the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0025] Example of a DC transmission converter station constructed with two converters connected in series: In this embodiment, the system is configured with a control host for each converter, which includes converter layer and pole layer functions; a station control host is configured for each station, which includes bipolar layer functions; the inter-station communication signals of the two converters on one pole are connected to the opposite station through a switching device; the inter-pole communication signals of the two converters on one station are connected to the opposite pole through a fast bus.

[0026] The bipolar layer control primarily receives commands from operators, generates bipolar power reference values, and sends them to the pole control layer via the control bus. Its main functions include: ① Bipolar power control: In bipolar power mode, each pole receives the same PrefDC (steady-state power command), transmitting half of the set bipolar power, with the ground electrode current approaching zero. One pole operates in bipolar power mode, while the other operates in manual current mode. When the current setting of the pole in manual current mode changes, the pole in bipolar power mode will adjust its power setting to ensure the bipolar power value remains constant, the currents of the two poles differ, and current flows through the ground electrode. In bipolar power mode, one pole is in a current-limited state, while the pole without current limitation will increase the transmitted power to ensure a constant bipolar power transmission. If the pole without current limitation is in manual current mode, the constant bipolar power cannot be guaranteed. ② Power limiting: The power limiting function has four levels, which can be enabled or disabled by operators. Each power limiting function is initiated by input from an external stabilization device. When the power limiting function is activated, it is converted to current limiting and applied to each pole. The four power limiting settings are set by the operator, with priorities increasing from 1 to 4. ③ Stabilization Control: The stabilization control function is an additional control function for the DC system. When the AC system is disturbed, the stabilization control function adjusts the transmission power of the DC system to restore stable operation as quickly as possible. The stabilization control function mainly includes power boost, power reduction, frequency limiting, dual-frequency modulation, and damped subsynchronous oscillation. ④ Current Balance Control: The current balance control function (CBC) is used to balance the actual DC current of the bipolar system and compensate for the cumulative error of the current reference value for each pole, minimizing the grounding electrode current as much as possible. ⑤ Low-Load Reactive Power Optimization: The low-load reactive power optimization function increases the firing angle of the converter valve under low load conditions to increase the reactive power absorbed by the converter, reduce the reactive power injected from the DC system into the AC system, and thus alleviate the difficulty of voltage regulation under light load conditions in the AC system. In this mode, the converter transformer tap is adjusted to the highest setting to reduce the valve-side voltage, thereby reducing the DC voltage value. ⑥ Power Reversal. ⑦ Calculation of the Ah value of the grounding electrode.

[0027] The main control unit of a high-voltage direct current (HVDC) system is part of the overall HVDC control system. It provides reference input signals to the control systems of each converter station in the HVDC system, such as constant power control, constant current control, or power flow direction control. The main control unit can be located in the power grid dispatch center, a converter station, or a combination of these locations. However, in actual operation, only one location can execute the main control functions.

[0028] High-voltage direct current (HVDC) system control modes refer to the various modes used to control converter units, poles, or converter stations to maintain various electrical parameters within their set ranges. The main control modes of HVDC systems include: constant DC current control to maintain a constant current during operation; constant DC voltage control to maintain a constant voltage during operation; constant arc-extinguishing angle control to prevent commutation failure due to an excessively small arc-extinguishing angle or excessive reactive power absorption by the converter unit during inverter operation; constant DC power control; control based on AC system frequency; control based on AC system frequency deviation; reactive power control; and subsynchronous oscillation damping control, etc.

[0029] High-voltage direct current (HVDC) converter station control: For a converter station with only one bipolar DC system, its main functions include constant power and other modulation control of the DC system or bipolar system; reactive power control of the converter station and AC bus voltage control of the converter units; power flow direction control; current balance control between the two stages; and protection and recovery strategies for the bipolar system. For converter stations with two or more bipolar DC systems, its main functions also include coordinating the control of each bipolar system. Converter station control generally does not include equipment with control or protection functions such as disconnect switches, circuit breakers, and transformer tap changers.

[0030] Pole control in a high-voltage direct current (HVDC) system is a device used for the control, monitoring, and protection of a single pole within a DC converter station. Its main functions include pole power control, automatic sequence control of the pole, pole addressing, protection and recovery measures, and remote control and communication systems for contacting the opposite converter station. Pole control generally does not include devices with control or protection functions such as disconnect switches, circuit breakers, or transformer tap changers.

[0031] Converter unit control is a device used for the control, monitoring, and protection of an independent converter unit within a converter station. It only controls equipment within its own station and does not have the function of communicating with other stations. The main functions of converter unit control include: converter unit trigger control, converter unit tap changer control, converter unit monitoring, converter unit protection, and converter unit sequence control. The control content mainly includes AC charging and de-energizing of the converter unit, starting and stopping the converter unit, connecting and disconnecting the converter unit from the DC side, switching and controlling between metallic loop and earth loop modes, and switching DC filters. Converter unit control does not include equipment with control or protection functions such as disconnect switches, circuit breakers, or transformer tap changers.

[0032] In this embodiment, both stations are equipped with DC station control and pole control (integrating converter control functions). The DC station control implements bipolar power control, bi-station reactive power control, and bi-station DC field sequential control. The pole control implements pole layer power control, closed-loop control, voltage balance control, and converter activation / deactivation. The DC station control and pole control commands of the two stations are coordinated through the master station selection logic.

[0033] Both stations are equipped with pole protection and converter protection (integrated converter transformer protection), respectively realizing the protection functions of the pole area and converter area of ​​each station. The DC system protection adopts a zoned configuration, and the protection scope and functions are as follows: 1. Converter protection zone: includes the converter and its wiring and other auxiliary equipment; equipped with current differential protection group (valve short circuit protection, commutation failure protection, converter differential protection), overcurrent protection group (DC side overcurrent protection, AC side overcurrent protection), voltage protection group (DC overvoltage protection), etc.

[0034] 2. DC pole and neutral bus protection zones: including smoothing reactors and DC filters, unipolar and bipolar neutral buses, and related equipment and connections; equipped with DC pole current differential protection groups (DC pole bus differential protection, DC neutral bus differential protection, DC pole differential protection), DC filter protection groups (DC filter reactor overload protection, DC filter capacitor unbalance protection, DC filter differential protection), and smoothing reactor protection groups (faults of dry-type smoothing reactors are covered by DC system protection; oil-immersed smoothing reactors, in addition to DC system protection, also have non-electrical protection relays, mainly including gas protection, oil pump and fan motor protection, oil level monitoring, gas monitoring, oil temperature detection, pressure release, oil flow indication, and winding temperature, etc.).

[0035] 3. Grounding electrode lead and grounding electrode protection zone: Equipped with bipolar neutral line protection group (bipolar neutral bus differential protection, station grounding overcurrent protection), transfer switch protection group (neutral bus circuit breaker protection, neutral bus grounding switch protection, earth return transfer switch protection, metallic return transfer circuit breaker protection), metallic return protection group (metallic return transverse differential protection, metallic return longitudinal differential protection, metallic return grounding fault protection), and grounding electrode lead protection group (grounding electrode lead breakage protection, grounding electrode lead overload protection, grounding electrode lead impedance monitoring, grounding electrode lead imbalance monitoring, grounding electrode lead pulse echo monitoring, etc.).

[0036] 4. AC switchyard protection zone of the converter station: This includes the converter transformer and its valve-side connections, AC filters and parallel capacitors and their connections, as well as the converter busbar; it is equipped with converter transformer differential protection groups (converter AC busbar and converter transformer differential protection, converter transformer differential protection, converter transformer winding differential protection), converter transformer overstress protection groups (converter transformer overcurrent protection, converter AC busbar and converter transformer overcurrent protection, converter transformer thermal overload protection, converter transformer overexcitation protection), converter transformer unbalance protection groups (converter transformer neutral point offset protection, converter transformer zero-sequence current protection, converter transformer saturation protection), converter transformer body protection (gas protection, pressure relief, gas detection, oil pump and fan motor protection, oil temperature, oil level detection, winding temperature, etc.), AC switchyard and AC filter protection (converter AC busbar differential protection, converter AC busbar overvoltage protection, AC filter protection, and final circuit breaker protection), etc.

[0037] 5. DC line protection zone: Equipped with DC line fault protection group (DC line traveling wave protection, differential undervoltage protection, DC line longitudinal differential protection, restart logic) and DC system protection group (DC undervoltage protection, power reverse protection, DC harmonic protection, etc.).

[0038] In this embodiment, regions with high transmission voltages are designated as high-end converter stations, and other regions as low-end converter stations, with the high-end converter stations serving as the dominant stations. To improve the flexibility and reliability of the series-connected HVDC transmission control and protection system, the system in this embodiment is further configured as follows: Figure 2 : 1) High-end station (i.e.) Figure 2 High-end converter stations and low-end stations (i.e.) Figure 2 The DC station control systems of the low-end converter stations in the middle are mutually redundant. When the DC station control system of any station goes out of operation, the relevant functions are performed by the DC station control system of the other station, and the DC system continues to operate.

[0039] 2) When the high-end or low-end station pole control is out of service, the corresponding converter of this station will be deactivated, and the power of the remaining converters can still be adjusted normally.

[0040] 3) The low-end station polar controller prioritizes the execution of the final current command processed by the high-end station polar controller; when the high-end station polar controller is out of operation, the low-end station polar controller executes the power and current commands of this station.

[0041] 4) In the event of a communication failure between the high-end and low-end DC station control stations, reactive power control and DC field sequential control will switch to independent control of the two stations, without affecting the operation of the DC system.

[0042] 5) Communication failure between high-end and low-end pole controllers causes the low-end pole controller to switch from constant DC current control mode to constant DC voltage control mode, and the high-end pole controller to exit the converter voltage balance control function.

[0043] Constant current control is one of the most basic control methods in DC transmission systems. During normal operation, any deviation in the DC current on the transmission line caused by some reason will be quickly adjusted back to the normal value by this control system. In other words, the task of this control system is to maintain the DC current at a constant value. Therefore, its control characteristic is a vertical line. The basic principle of constant voltage control is similar to that of constant current control, except that the feedback signal is changed to DC voltage. Under the action of the constant voltage control system, the goal is to maintain the DC voltage equal to the set value.

[0044] 6) If communication between the pole controller of any station and the DC station controller of the leading station fails, the DC station controller will switch the leading station to another station; if communication between the pole controller of the high-end station and the DC station controllers of both stations fails, the pole controller will switch to unipolar power control mode and receive control commands issued by OWS (Official Work System) to complete power adjustment; if communication between the pole controller of the low-end station and the DC station controllers of both stations fails, since the low-end station receives current commands from the pole controller of the high-end station, there is no need to switch control modes when communication fails.

[0045] The unipolar control system maintains the DC power at the specified power measurement and control point at the power setpoint set by the operator at the main control station. Unipolar power control is one of the control methods for DC active power transmission during unipolar operation.

[0046] 7) Both stations are equipped with monitoring servers, and their monitoring networks are interconnected through inter-station communication. The control and protection systems communicate with both station monitoring servers. The high-end station is the default primary monitoring system, but monitoring permissions can be switched manually upon application.

[0047] 8) Redundant configuration of the control host: one of the two converters is set as the control converter, and the pole layer control function in the control converter is activated; one of the two stations is set as the control station, and the bipolar layer control function in the control station is activated.

[0048] This embodiment achieves flexible site-specific construction by rationally designing the structure of the series-constructed high-voltage direct current transmission control and protection system. Specifically, it combines the functional configurations of the pole layer and converter layer (i.e., the control host includes both converter layer and pole layer functions) and adds a station control host. At the same time, it retains the original logic of the converter layer and pole layer functions, thus meeting the high reliability requirements of series-constructed high-voltage direct current projects.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. The scope of patent protection of the present invention shall be determined by the claims. Similarly, any equivalent structural changes made based on the description and drawings of the present invention shall also be included within the scope of protection of the present invention.

Claims

1. A DC transmission converter station constructed in series with two converters, wherein the series construction involves the two converters in the station being located in two different regions, characterized in that, Areas with high transmission voltage are designated as high-end stations, while other areas are designated as low-end stations. Each region's converters are equipped with a control host, and each region also has a station control host. The station control host at the high-end station is the master station. The station control host is equipped with a DC station controller for implementing bipolar power control, dual-station reactive power control, and dual-station DC field sequential control. It is also equipped with pole controllers for implementing pole layer power control, closed-loop control, voltage balance control, and converter activation / deactivation. The DC station controller and pole controller coordinate corresponding control commands through master station selection logic. The DC station controllers at the high-end and low-end converter stations serve as backups for each other; if the DC station controller at one station is deactivated, the other station's DC station control system will take over. Currently, the DC system continues to operate under the relevant functions. When the pole controller of a high-end station or a low-end station is out of service, the corresponding converter of that station will be deactivated. The pole controller of the low-end station will prioritize executing the final current command processed by the pole controller of the high-end station. When the pole controller of the high-end station is out of service, the pole controller of the low-end station will execute the power and current commands of that station. If the pole controller of any region fails to communicate with the DC station control of the master station, the DC station control will switch the master station to the station control host of another region. If the pole controller of the high-end station fails to communicate with the DC station control of both regions, the pole controller of the high-end station will switch to single-pole power control mode, receive control commands issued by OWS, and complete power adjustment. If the pole controller of the low-end station fails to communicate with the DC station control of both stations, there is no need to switch control modes.

2. The DC transmission converter station constructed with dual converters in series as described in claim 1, characterized in that, The control of either DC station in two different regions is also used to realize bipolar power control, reactive power control and DC field sequential control in both regions when the other DC station control is out of operation.

3. The DC transmission converter station constructed with dual converters in series according to claim 1, characterized in that, Both stations are equipped with pole protection and converter protection to realize the protection functions of each station's pole area and converter area respectively.

4. The DC transmission converter station constructed with dual converters in series according to claim 1, characterized in that, When the polar controller of the high-end station is out of operation, the polar controller of the low-end station executes the current command processed by the polar controller of the low-end station.

5. The DC transmission converter station constructed with dual converters in series according to claim 4, characterized in that, When a communication failure occurs between the DC station control of the high-end station and the DC station control of the low-end station, each region coordinates the corresponding control commands through its own region's DC station control selection logic to achieve dual-station reactive power control and dual-station DC field sequential control.

6. The DC transmission converter station constructed with dual converters in series according to claim 4, characterized in that, When a communication failure occurs between the pole controller of the high-end station and the pole controller of the low-end station, the pole controller of the low-end station switches from constant DC current control mode to constant DC voltage control mode, and the pole controller of the high-end station exits the converter voltage balance control function.

7. The DC transmission converter station constructed with dual converters in series according to claim 3, characterized in that, The DC system protection adopts a zoned configuration, and the protection scope includes: converter protection zone, DC pole and neutral bus protection zone, grounding electrode lead and grounding electrode protection zone, converter station AC switchyard protection zone, and DC line protection zone.

8. The DC transmission converter station constructed with dual converters in series according to claim 7, characterized in that, The converter protection zone includes the converter and its connecting lines and other auxiliary equipment, and is equipped with a current differential protection group, an overcurrent protection group, and a voltage protection group.

9. The DC transmission converter station constructed with dual converters in series according to claim 1, characterized in that, Each region also includes polar protection and converter protection, which are used to realize the protection functions of the polar area and converter area in each region, respectively; each region also includes a monitoring system, which is used to monitor the two regions.

10. The DC transmission converter station constructed with dual converters in series according to claim 9, characterized in that, Each region is also equipped with a monitoring server, and the monitoring networks of the two regions are interconnected through inter-station communication.

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