Reactive power support function of dc load system
By introducing tap changers and control systems into DC load systems, the tap position can be dynamically adjusted to minimize reactive power consumption, thus solving the problem that DC load systems cannot actively control reactive power and improving the stability and operational capability of the power grid.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SIEMENS ENERGY GLOBAL GMBH & CO KG
- Filing Date
- 2024-09-16
- Publication Date
- 2026-06-05
AI Technical Summary
Existing DC load systems cannot actively control reactive power consumption, leading to grid instability when voltage drops and failing to meet the stability requirements of modern power systems, especially when renewable energy systems are connected.
By introducing a tap changer and control system into the DC load system, and using outer and inner loop controllers combined with first and second droop controls, the tap position is dynamically adjusted to minimize reactive power consumption. Hysteresis control is also used to reduce tap position changes, thereby ensuring grid stability.
It effectively reduces reactive power consumption, improves grid stability, reduces the need for additional reactive power compensation units, and enhances the grid's operational capability under severe network conditions.
Smart Images
Figure CN122162273A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to a reactive power support method for a DC load system in an AC network. Furthermore, this invention relates to a DC load system with reactive power support functionality for an AC network. Background Technology
[0002] The number of DC load systems (such as electrolysis systems) installed in existing transmission networks continues to increase. To accelerate green hydrogen production, planned electrolysis system capacities reach the GW range. With this in mind, transmission system operators (TSOs) have begun to define requirements for integrating such large-scale electrolysis systems into existing networks. Some fundamental requirements being defined include fault ride-through (FRT) requirements, which define the operating areas where the electrolysis system needs to remain connected to the corresponding adjacent AC network and the operating areas where the electrolysis system can be disconnected from the AC network. Furthermore, initial specifications and requirements such as reactive power / power factor (PF—the power factor of an AC power system is defined as the ratio of active power absorbed by the load to the apparent power flowing in the circuit) and capabilities are also being defined. These new requirements for load systems are based on existing requirements for photovoltaic (PV) and wind farms.
[0003] Given the relatively high DC demand of several thousand amperes in hydrogen electrolysis systems, primarily due to their efficiency and reliability, state-of-the-art thyristor-based rectifiers (B6C configuration, controllable three-phase full-wave bridge rectifier circuit) are utilized. However, for power-consuming units such as electrolysis systems, requirements related to reactive power consumption are being constrained, and considering the converter technology used in such load systems (e.g., thyristor-based rectifiers), active control of reactive power by the rectifier itself is not feasible. The control system for thyristor-based rectifiers used in electrolysis systems can control only the flow of active power by providing the necessary firing angle to the rectifier unit.
[0004] Furthermore, because the electrolysis system is planned to be connected near PV or wind farms for green hydrogen production, the corresponding AC network tends to be weaker compared to networks dominated by conventional power sources. In the event of network disturbances (e.g., voltage drop), the grid support capacity of renewable power generation units alone may be insufficient to ensure stable network operation. Support from load units (e.g., electrolyzer systems) may also be required.
[0005] Thyristor-based rectifiers cannot control reactive power. However, in order to achieve an acceptable power factor at the connection points of the electrolysis system, the system transformer unit of the electrolysis system is equipped with an on-load tap changer (OLTC), which is specifically designed for each project and includes several tap positions specified for different voltage values.
[0006] The position of the tap changer affects the power factor; a higher position results in a lower power factor due to increased reactive power consumption. In current thyristor-based systems, reactive power consumption is not actively controlled, and tap changer control relies on a reference firing angle to influence DC current flow.
[0007] The lack of additional tap changer control logic addresses more severe network conditions where, for example, reactive power reduction would be necessary to provide support in the event of any voltage drops that could occur and destabilize a relatively weak grid. One option for reducing reactive power consumption in existing solutions is to lower the DC current reference value, thereby reducing active power consumption to its minimum operating point. However, operating the system under these conditions is certainly not ideal, and operators will attempt to prevent this operating mode. Summary of the Invention
[0008] Network instability is a major challenge in modern power systems. This instability can manifest as fluctuations in power supply, leading to disruptions and inefficiencies in the operation of various devices and infrastructure. To address these challenges, the present invention aims to provide an innovative solution that better integrates renewable energy sources while ensuring grid stability.
[0009] The objective of this invention is achieved by the independent claims. The dependent claims describe advantageous improvements and modifications to the invention.
[0010] According to the present invention, a method for reactive power support in a DC load system in an AC network is provided. The DC load system includes a DC load unit and a tap changer having a tap position. The tap changer includes a plurality of tap positions. The primary side of the tap changer is connected to the AC network, and the secondary side of the tap changer is connected to the DC load unit via a rectifier for providing DC power to the DC load unit. The input voltage of the rectifier is determined by selecting the tap position and the firing angle of phase trigger control, wherein the tap position is selected such that the firing angle is as small as possible and thus the reactive power is minimized.
[0011] In an advantageous embodiment of the invention, the DC load system further includes a control system having an outer loop controller and an inner loop controller, the sampling time of the inner loop controller being shorter than that of the outer loop controller. The DC load system also includes tap changer control logic implemented in the outer loop controller. The inner loop controller continuously monitors the frequency and voltage on the primary side of the tap changer transformer and sends a signal to the outer loop controller if the frequency or voltage exceeds or falls below a specified limit value. It also determines a tap position based on a first droop control and a second droop control, the first droop control associating the voltage level with the reactive power consumption of the DC load system, and the second droop control associating the active power consumption and reactive power consumption of the DC load system to define a minimum required power factor value. The outputs of the first and second droop controls are compared with values corresponding to the tap position, and the tap changer is switched to the thus determined tap position.
[0012] The basic idea of this invention is to extend the existing control logic that limits the operating conditions of the tap changer by including criteria that are more suitable for providing reactive power support.
[0013] The proposed idea is as follows: In severe network conditions such as voltage drops, different grid specifications may require minimizing reactive power consumed by DC load systems to avoid further exacerbating the network conditions. According to the invention, the lower-level controller of the DC load system is first adjusted to continuously monitor the frequency and voltage level at the transformer input. When severe network conditions are detected by the lower-level control (sampling time typically 250 μs), a signal is sent to the higher-level control (sampling time typically 50 ms), where the tap changer control logic is typically implemented. According to the invention, the current tap changer control logic is overridden, and individual firing angles no longer limit whether the tap changer should be increased or decreased in one or more positions. Here, a new control logic sequence is proposed that always controls the following: a. Voltage level on the primary side of the transformer; b. Reactive power consumed on the primary side of the transformer.
[0014] Using these parameters, control logic can be implemented to limit the operating conditions of the tap changer, thereby reducing reactive power consumption. This can be achieved by limiting two droop controls:
[0015] The first droop control will correlate the voltage level with the reactive power consumption of the DC load system. Voltage and reactive power are closely related in power systems. Reactive power affects the voltage in the network, and voltage regulation is a fundamental task in power transmission and distribution. The relationship between voltage and reactive power can be described as follows: 1) When active power is constant and reactive power increases, the voltage in the grid increases. 2) When reactive power in the network decreases, the voltage decreases.
[0016] The second droop control correlates the active and reactive power consumption of the electrolysis system, thereby limiting the minimum required power factor. Active power (P) and reactive power (Q) are two distinct aspects of electrical power in an AC system. Active power is the power used to drive the load, while reactive power is the power that circulates in inductive and capacitive elements without actually doing work.
[0017] The droop function of both active and reactive power can be used in combined control systems that include both frequency and voltage regulation.
[0018] The output from the droop control is then compared with the value corresponding to the designed tap changer position, and thus a new tap position will be determined, which will correspond to the reactive power consumption required by the electrolysis system for a given severe network condition.
[0019] Advantageously, limiting the hysteresis required to return to a higher voltage tap position minimizes the number of tap position changes. Frequent tap position changes lead to wear and tear on tap changer equipment. By implementing hysteresis, the number of changes can be reduced, extending equipment life and lowering maintenance costs. Hysteresis helps prevent rapid oscillations between tap positions, which can cause voltage instability in the grid. By providing a buffer, hysteresis ensures that the tap position is changed only when necessary, contributing to improved voltage stability.
[0020] Furthermore, it is advantageous when the DC load system is an electrolytic system and the DC load is an electrolytic cell. Such a system already has the necessary hardware and can be upgraded with relatively little effort to perform the method according to the invention.
[0021] It is advantageous when the sampling time of the inner loop controller is less than 300 μs. This ensures that even short-term disturbances in the AC network can be recorded.
[0022] It is also advantageous when the tap position corresponding to the required reactive power consumption for a given severe network condition is predetermined. This is the most likely method to quickly determine measures that are meaningful for the power grid and system.
[0023] The objective of this invention for DC load systems is achieved through a DC load system with reactive power support for an AC network. This DC load system includes a DC load unit and a tap changer with several tap positions. The primary side of the tap changer is connected to the AC network, and the secondary side is connected to the DC load unit via a rectifier that provides DC power to the DC load unit. The input voltage of the rectifier is determined by selecting the tap positions and the firing angle of phase trigger control. The control system is adapted to select the tap positions such that the firing angle is as small as possible, and thus minimizes reactive power if the frequency or voltage of the AC network exceeds or falls below a specified limit.
[0024] It is advantageous when the DC load system also includes a control system with an outer loop controller and an inner loop controller, wherein the sampling time of the inner loop controller is shorter than that of the outer loop controller. The DC load system also includes tap changer control logic implemented in the outer loop controller, wherein the inner loop controller is adapted to continuously monitor the frequency and voltage on the primary side of the tap changer transformer and to send a signal to the outer loop controller if the frequency or voltage exceeds or falls below a specified limit value. The outer loop controller is adapted to determine the tap position based on a first droop control and a second droop control, wherein the first droop control correlates the voltage level with the reactive power consumption of the DC load system and the second droop control correlates the active power consumption and reactive power consumption of the DC load system, thereby defining a minimum required value for the power factor. The outer loop controller is adapted to compare the outputs of the first droop control and the second droop control with values corresponding to the tap position and is adapted to switch the tap changer to the thus determined tap position.
[0025] The main advantage of this invention is that, compared with existing tap changer control algorithms that do not directly consider the consumed reactive power (but indirectly via the reference firing angle and the value used for transformer design), this invention proposes a tap changer control logic that can indirectly improve the network voltage by reducing the reactive power consumption of the DC load system in severe network conditions such as voltage drops, thereby significantly reducing the need for additional reactive power compensation units.
[0026] Compared to possible existing solutions that involve reducing active power consumption to its minimum in order to also reduce reactive power consumption, the reactive power consumption generated according to the present invention is several orders of magnitude higher. Attached Figure Description
[0027] Figure 1 An electrolysis system according to the present invention is shown, and
[0028] Figure 2 and 3 The interaction between tap position and firing angle is shown. Detailed Implementation
[0029] Figure 1 A DC load system 1 with reactive power support function according to the present invention is shown, such as an electrolysis system 15. The electrolysis system 15 includes an electrolytic cell 16 as a DC load unit 3 and a tap transformer 4 having a tap changer 5 with several tap positions 6. The primary side 7 of the tap transformer 4 is connected to an AC network 2, and the secondary side 8 of the tap transformer 4 is connected to the electrolytic cell 16 via a rectifier 9 for providing DC power to the electrolytic cell 16. The input voltage of the rectifier 9 is determined by the selection of the tap positions 6 and the firing angle of the phase trigger control 10. According to the present invention, the control system 11 is adapted to select the tap positions 6 such that the firing angle is as small as possible, thus minimizing reactive power.
[0030] The control system 11 includes an outer loop controller 12 and an inner loop controller 13, the sampling time of which is shorter than that of the outer loop controller 12. The electrolysis system 15 also includes tap changer control logic 14 implemented in the outer loop controller 12. According to the invention, the inner loop controller 13 is adapted to continuously monitor the frequency and voltage at the primary side 7 of the tap changer 4. If the frequency or voltage exceeds or falls below a specified limit, the inner loop controller 13 sends a signal to the outer loop controller 12. The outer loop controller 12 is adapted to determine the tap position 6 based on a first droop control and a second droop control, the first droop control associating the voltage level with the reactive power consumption of the electrolysis system 13, and the second droop control associating the active power consumption and reactive power consumption of the electrolysis system 15, thus defining a minimum required power factor value. The outer loop controller 12 is adapted to compare the outputs of the first and second droop controls with the values corresponding to the tap position 6 and switch the tap changer 5 to the thus determined tap position 6.
[0031] Figure 2 and Figure 3 This section explains phase angle control used as a power controller for electrical loads operating on AC voltage. In phase angle control, the current is typically controlled by a bidirectional thyristor (a circuit of two thyristors connected in anti-parallel). After the zero-crossing 17 of the AC voltage (and current), the bidirectional thyristor does not conduct current until it receives a trigger pulse 18; from this time (the phase of the AC signal), energy is supplied to the load until the next zero-crossing 17. The later the bidirectional thyristor is triggered, the lower the average power.
[0032] Figure 2 and Figure 3An example of tap changer 5 using tap transformer 4 is shown, illustrating how it operates via phase angle control in two operating states. For 100% DC current I... dc The full load operating point below, Figure 1 The tap changer 5 is designed to output a voltage level corresponding to the rated secondary AC voltage level U1 of the transformer, resulting in a firing angle at the first angle α1. Assume the resulting power factor is 0.9.
[0033] However, as Figure 2 As shown, if tap changer 5 should be in a higher tap position corresponding to a voltage level U2 higher than U1, the firing angle increases to a value α2 greater than α1 (because the input voltage of the rectifier increases, and a higher firing angle is sufficient to achieve the necessary DC current specified in DC current control), and the power factor will decrease to a lower value of approximately 0.8. The reason for the decrease in the power factor is that even if the active power consumption remains constant, the reactive power consumption has increased.
[0034] Regarding reactive power consumption, active control is not implemented in current thyristor-based systems. Reactive power consumption is limited during transformer unit design to achieve, for example, a minimum power consumption (PF) of 0.9. Furthermore, tap changer control 14 typically depends only on a reference firing angle. For example, if the firing angle would be lower than, for example, α1, for a tap position corresponding to a voltage level U1 on the transformer secondary side, tap changer 5 would be triggered to move to a higher tap position 6, resulting in a higher output voltage U2. This is because, for angles lower than α1, the control cannot significantly increase the DC current to reach the desired reference value, and the DC current limit would be reached due to the input AC voltage supplied to the rectifier. By changing to a higher tap position 6, the firing angle increases, and the controller has greater leeway to influence the DC current.
[0035] List of reference numerals 1 DC load system 2 AC network 3 DC load units 4 tap changer 5-tap converter 6-point receiving position 7 Primary Side 8 secondary sides 9 rectifiers 10-phase trigger control 11 Control System 12 outer loop controller 13 Inner Loop Controller 14-Tap Converter Control Logic 15 Electrolysis System 16 electrolytic cells 17 crossed zero 18 trigger pulses.
Claims
1. A method for supporting reactive power in a DC load system (1) in an AC network (2), the DC load system (1) comprising a DC load unit (3) and a tap transformer (4) having a tap changer (5), the tap changer (5) comprising a plurality of tap positions (6), the primary side (7) of the tap transformer (4) being connected to the AC network (2), and the secondary side (8) of the tap transformer (4) being connected to the DC load unit (3) via a rectifier (9) for providing DC power to the DC load unit (3), the input voltage of the rectifier (9) being determined by the selection of the tap positions (6) and the firing angle of a phase trigger control (10), characterized in that, If the frequency or voltage of the AC network (2) exceeds or falls below a specified limit, the tap position (6) is selected such that the trigger angle is as small as possible, thereby minimizing the reactive power.
2. The reactive power support method for a DC load system (1) in an AC network (2) according to claim 1, wherein the DC load system (1) further comprises a control system (11) having an outer loop controller (12) and an inner loop controller (13), wherein the sampling time of the inner loop controller (13) is shorter than the sampling time of the outer loop controller (12), and the DC load system (1) further comprises tap changer control logic (14) implemented in the outer loop controller (12), characterized in that, The frequency and voltage at the primary side (7) of the tap changer (4) are continuously monitored by the inner loop controller (13), and if the frequency or voltage exceeds or falls below a specified limit, a signal is sent to the outer loop controller (12), and a tap position (6) is determined according to a first droop control and a second droop control, wherein the first droop control associates the voltage level with the reactive power consumption of the DC load system (1), and the second droop control associates the active power consumption and reactive power consumption of the DC load system (1), thereby limiting a minimum required value for the power factor, wherein the outputs of the first droop control and the second droop control are compared with values corresponding to the tap position (6), and wherein the tap changer (5) is switched to the tap position (6) thus determined.
3. The method according to claim 1 or claim 2, wherein, Hysteresis is defined to return to the tap position with higher voltage (6) in order to minimize the number of tap position changes.
4. The method according to any one of the preceding claims, wherein, The DC load system (1) is an electrolysis system (15), and the DC load (2) is an electrolytic cell (15).
5. The method according to claim 2, wherein, The sampling time of the inner loop controller (13) is less than 300µs.
6. The method according to the preceding claim, wherein, The tap position (6) corresponding to the required reactive power consumption for a given severe network scenario is predetermined.
7. A DC load system (1) for an AC network (2) with reactive power support, the DC load system (1) comprising a DC load unit (3) and a tap transformer (4) having a tap changer (5), the tap changer (5) comprising a plurality of tap positions (6), the primary side (7) of the tap transformer (4) being connected to the AC network (2), and the secondary side (8) of the tap transformer (4) being connected to the DC load unit (3) via a rectifier (9) for providing DC power to the DC load unit (3), the input voltage of the rectifier (9) being determined by the selection of the tap positions (6) and the firing angle of a phase trigger control (10), characterized in that, The control system (11) is adapted to select the tap position (6) such that the trigger angle is as small as possible when the frequency or voltage of the AC network (2) exceeds or falls below a specified limit, thereby minimizing the reactive power.
8. A DC load system (1) with reactive power support function for an AC network (2) according to claim 7, wherein the DC load system (1) further comprises a control system (11) having an outer loop controller (12) and an inner loop controller (13), wherein the sampling time of the inner loop controller (13) is shorter than the sampling time of the outer loop controller (12), and the DC load system (1) further comprises tap changer control logic (14) implemented in the outer loop controller (12), characterized in that, The inner loop controller is adapted to continuously monitor the frequency and voltage at the primary side (7) of the tap transformer (4), and to send a signal to the outer loop controller (12) if the frequency or the voltage exceeds or falls below a specified limit, wherein the outer loop controller (12) is adapted to determine the tap position (6) according to a first droop control and a second droop control, the first droop control associating the voltage level with the reactive power consumption of the DC load system (1), and the second droop control associating the active power consumption and reactive power consumption of the DC load system (1) to define a minimum required value for the power factor, the outer loop controller (12) is adapted to compare the outputs of the first droop control and the second droop control with values corresponding to the tap position (6), and is adapted to switch the tap converter (5) to the tap position (6) thus determined.
9. The DC load system (1) according to claim 8, wherein, The DC load system (1) is an electrolysis system (15), and the DC load (3) is an electrolytic cell (16).